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T. Sinnige
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68 records found
1
Journal article
(2026)
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Ramon R. Duivenvoorden, André F.P. Ribeiro, Tomas Sinnige, Leo L.M. Veldhuis
Propeller–wing–flap configurations exhibit highly complex flow patterns due to the aerodynamic interactions among their components. To facilitate effective design of such configurations, a fundamental understanding of these interactions is essential. This paper characterizes the deformation of a propeller slipstream caused by nacelle, wing, and flap interference at various angles of attack and flap deflections, based on validated numerical simulations. We examine different stages and mechanisms of deformation, from immediately behind the propeller to the wake downstream of the wing. Our findings reveal that significant deformation occurs in high-lift conditions before the slipstream physically interacts with the wing, profoundly impacting subsequent deformation. Furthermore, the root vortex system rolls into a strong nacelle vortex that induces crossflow on the wing surface behind the nacelle and dominates the slipstream deformation in the wake. Additionally, the tip vortices stretch around the leading edge and roll into vortex systems aligned with the wing surface, dominating local flow features. These insights improve the understanding of slipstream deformation, helping to improve the aerodynamic performance of propeller–wing–flap systems.
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Propeller–wing–flap configurations exhibit highly complex flow patterns due to the aerodynamic interactions among their components. To facilitate effective design of such configurations, a fundamental understanding of these interactions is essential. This paper characterizes the deformation of a propeller slipstream caused by nacelle, wing, and flap interference at various angles of attack and flap deflections, based on validated numerical simulations. We examine different stages and mechanisms of deformation, from immediately behind the propeller to the wake downstream of the wing. Our findings reveal that significant deformation occurs in high-lift conditions before the slipstream physically interacts with the wing, profoundly impacting subsequent deformation. Furthermore, the root vortex system rolls into a strong nacelle vortex that induces crossflow on the wing surface behind the nacelle and dominates the slipstream deformation in the wake. Additionally, the tip vortices stretch around the leading edge and roll into vortex systems aligned with the wing surface, dominating local flow features. These insights improve the understanding of slipstream deformation, helping to improve the aerodynamic performance of propeller–wing–flap systems.
Conference paper
(2026)
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Geovana Neves, Tiago Barbosa de Araújo, Tomas Sinnige, Roberto Gil Annes da Silva
This study extends the interchangeable aircraft framework introduced in prior work by investigating thrust-dependent aeropropulsive effects in the SMI-L2 rear-mounted tractor configuration, a layout previously shown to exhibit inherently increased longitudinal stability due to rear pylon contributions established at a single cruise thrust condition. Employing a panel method solver and a decomposition methodology based solely on 3D aerodynamic coefficients, the analysis characterizes component-level aeropropulsive contributions across representative thrust levels from initial climb to cruise at fixed advance ratio. Addressing the sensitivity question, the slipstream dynamic pressure and swirl angle at the pylon grow approximately linearly with thrust, while the propeller in-plane force depends primarily on the circumferentially non-uniform inflow at the installed propeller disk. Addressing relevance, the in-plane force dominates static margin across the full thrust range while the slipstream dynamic pressure at the pylon emerges as the dominant driver of pitch damping at high thrust, revealing a hierarchy inversion between the two stability metrics. The net slipstream contribution to static margin is destabilizing due to partial cancellation between its swirl and dynamic pressure components, remaining secondary to the in-plane force. Addressing the sizing limits, a scissor plot analysis demonstrates that propeller effects shift the critical forward center-of-gravity condition from trim to the pull-up maneuver, while the rear pylon transforms the aft center-of-gravity constraint from stability-limited to control-authority-limited, with direct consequences for horizontal tail sizing in early design.
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This study extends the interchangeable aircraft framework introduced in prior work by investigating thrust-dependent aeropropulsive effects in the SMI-L2 rear-mounted tractor configuration, a layout previously shown to exhibit inherently increased longitudinal stability due to rear pylon contributions established at a single cruise thrust condition. Employing a panel method solver and a decomposition methodology based solely on 3D aerodynamic coefficients, the analysis characterizes component-level aeropropulsive contributions across representative thrust levels from initial climb to cruise at fixed advance ratio. Addressing the sensitivity question, the slipstream dynamic pressure and swirl angle at the pylon grow approximately linearly with thrust, while the propeller in-plane force depends primarily on the circumferentially non-uniform inflow at the installed propeller disk. Addressing relevance, the in-plane force dominates static margin across the full thrust range while the slipstream dynamic pressure at the pylon emerges as the dominant driver of pitch damping at high thrust, revealing a hierarchy inversion between the two stability metrics. The net slipstream contribution to static margin is destabilizing due to partial cancellation between its swirl and dynamic pressure components, remaining secondary to the in-plane force. Addressing the sizing limits, a scissor plot analysis demonstrates that propeller effects shift the critical forward center-of-gravity condition from trim to the pull-up maneuver, while the rear pylon transforms the aft center-of-gravity constraint from stability-limited to control-authority-limited, with direct consequences for horizontal tail sizing in early design.
This study quantifies the viscous interaction between propeller tip vortices and a turbulent boundary layer developing over a semi-elliptic leading-edge plate, located downstream. The experimental wind-tunnel set-up is designed to be representative of the tractor-propeller-wing configuration. Using stereoscopic particle image velocimetry and static wall-pressure measurements, the near-wall flow topology is resolved over the plate, semi-immersed in the propeller slipstream. The results show that the interaction exhibits high spatio-temporal coherence and is dominated by a coupling between primary and secondary vortical structures. Two distinct interaction regions are identified relative to the tip-vortex core: on the inboard side, towards the slipstream interior, the boundary-layer flow experiences strong velocity gradient transitions and amplified near-wall vorticity. The flow on the outboard side, moving out of the slipstream, exhibits wall-parallel velocity deficits and vorticity lift-up consistent with unsteady vortex-induced separation mechanisms. Spanwise velocity induced by the wall-normal component of the primary vortex connects these two regions, with the secondary vortex structure identified as enhancing boundary-layer lift-up on the outboard side. Although no local flow reversal occurs under the tested conditions, localised shear amplification and vorticity roll-up indicative of separation-like behaviour were observed. These findings advance the understanding of viscous slipstream-boundary-layer interaction and its implications for tractor-propeller-wing integration.
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This study quantifies the viscous interaction between propeller tip vortices and a turbulent boundary layer developing over a semi-elliptic leading-edge plate, located downstream. The experimental wind-tunnel set-up is designed to be representative of the tractor-propeller-wing configuration. Using stereoscopic particle image velocimetry and static wall-pressure measurements, the near-wall flow topology is resolved over the plate, semi-immersed in the propeller slipstream. The results show that the interaction exhibits high spatio-temporal coherence and is dominated by a coupling between primary and secondary vortical structures. Two distinct interaction regions are identified relative to the tip-vortex core: on the inboard side, towards the slipstream interior, the boundary-layer flow experiences strong velocity gradient transitions and amplified near-wall vorticity. The flow on the outboard side, moving out of the slipstream, exhibits wall-parallel velocity deficits and vorticity lift-up consistent with unsteady vortex-induced separation mechanisms. Spanwise velocity induced by the wall-normal component of the primary vortex connects these two regions, with the secondary vortex structure identified as enhancing boundary-layer lift-up on the outboard side. Although no local flow reversal occurs under the tested conditions, localised shear amplification and vorticity roll-up indicative of separation-like behaviour were observed. These findings advance the understanding of viscous slipstream-boundary-layer interaction and its implications for tractor-propeller-wing integration.
For the next generation of transport aircraft, boundary-layer ingestion (BLI) is being proposed as a promising technology to reduce energy consumption. However, the aerodynamic interaction between the propulsor and the fuselage boundary layer has received little attention. In this study, an experimental approach is used to study the effect of a fuselage aft-cone-mounted propeller on the flow around a fuselage aftbody. An idealized fuselage model with an integrated rear propeller is tested in a low-subsonic wind tunnel. The loads on the propeller were measured directly with the use of a rotating shaft balance. Integration of the aft-fuselage pressure field allowed for a complete force decomposition. Operation of the propeller was shown to significantly increase pressure and friction drag on the fuselage. Furthermore, hot-wire measurements show that the turbulence characteristics of the fuselage boundary layer upstream of the propeller were altered by the propeller. Compared to the propeller-off measurement, a clear deviation from the universal log law was observed. Phase-locked hot-wire and embedded microphone data reveal small in-phase fluctuations with the propeller blade passage. Despite their persistence throughout the boundary layer, the fluctuations are not believed to significantly impact the mean inflow to the propeller or affect its performance. Despite their insignificant impact on propeller performance, the fluctuations could still be relevant in terms of noise and vibrations.
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For the next generation of transport aircraft, boundary-layer ingestion (BLI) is being proposed as a promising technology to reduce energy consumption. However, the aerodynamic interaction between the propulsor and the fuselage boundary layer has received little attention. In this study, an experimental approach is used to study the effect of a fuselage aft-cone-mounted propeller on the flow around a fuselage aftbody. An idealized fuselage model with an integrated rear propeller is tested in a low-subsonic wind tunnel. The loads on the propeller were measured directly with the use of a rotating shaft balance. Integration of the aft-fuselage pressure field allowed for a complete force decomposition. Operation of the propeller was shown to significantly increase pressure and friction drag on the fuselage. Furthermore, hot-wire measurements show that the turbulence characteristics of the fuselage boundary layer upstream of the propeller were altered by the propeller. Compared to the propeller-off measurement, a clear deviation from the universal log law was observed. Phase-locked hot-wire and embedded microphone data reveal small in-phase fluctuations with the propeller blade passage. Despite their persistence throughout the boundary layer, the fluctuations are not believed to significantly impact the mean inflow to the propeller or affect its performance. Despite their insignificant impact on propeller performance, the fluctuations could still be relevant in terms of noise and vibrations.
Journal article
(2026)
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P. Comunian, G. Cafiero, J. Serpieri, F. Avallone, R. Zamponi, D. Ragni, T. Sinnige
This work investigates the unsteady aerodynamic interaction that arises from the impingement of a propeller slipstream on a wing. To this end, an innovative measuring device for unsteady pressure is deployed, comprising a flexible printed circuit board sleeve embedded with MEMS pressure sensors and microphones. The device performance is validated against conventional measurement techniques. The wing is a benchmarked NACA 633018 airfoil-based model, and the propeller is the TUD-XPROP-S. In addition to pressure measurements, oil flow visualizations are performed to elucidate the flow pattern on the wing when the propeller operates at advance ratios of 0.8 and 1.8, and nominal blade pitch angles of 30° and 45°. The measurements reveal the formation of a laminar separation bubble on the portion of the wing not washed by the propeller slipstream. The flow is seen to remain attached on the advancing blade side, at least for the tested angles of attack. The microphone measurements capture the trace of the propeller’s tip vortices over the wing and the deformation of the slipstream over the wing. This work serves a dual purpose. Firstly, presenting an innovative measuring device for unsteady pressure, as the sensor-embedded sleeve requires minimal installation efforts and allows for a comprehensive measurement of the unsteady surface pressure field. Secondly, discussing the complex spatio temporal interaction that is formed from the impingement of a propeller slipstream onto a wing.
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This work investigates the unsteady aerodynamic interaction that arises from the impingement of a propeller slipstream on a wing. To this end, an innovative measuring device for unsteady pressure is deployed, comprising a flexible printed circuit board sleeve embedded with MEMS pressure sensors and microphones. The device performance is validated against conventional measurement techniques. The wing is a benchmarked NACA 633018 airfoil-based model, and the propeller is the TUD-XPROP-S. In addition to pressure measurements, oil flow visualizations are performed to elucidate the flow pattern on the wing when the propeller operates at advance ratios of 0.8 and 1.8, and nominal blade pitch angles of 30° and 45°. The measurements reveal the formation of a laminar separation bubble on the portion of the wing not washed by the propeller slipstream. The flow is seen to remain attached on the advancing blade side, at least for the tested angles of attack. The microphone measurements capture the trace of the propeller’s tip vortices over the wing and the deformation of the slipstream over the wing. This work serves a dual purpose. Firstly, presenting an innovative measuring device for unsteady pressure, as the sensor-embedded sleeve requires minimal installation efforts and allows for a comprehensive measurement of the unsteady surface pressure field. Secondly, discussing the complex spatio temporal interaction that is formed from the impingement of a propeller slipstream onto a wing.
Conference paper
(2025)
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G. Margalida, T. Sinnige, Reynard de Vries, Joaquin Exalto, Rob E. Wolleswinkel
Distributed propulsion systems, characterized by multiple propellers, represent a promising approach for full-electric aircrafts, offering several advantages but also introducing technical challenges. The main objective of this paper is to quantify how the propeller performance and noise emissions of an eight-propeller full-electric aircraft configuration compare to that of a conventional fuel-based turboprop. In both cases, the key parameters driving the trade-off between noise emissions and aerodynamic performances are analysed as well as the benefits of each configuration. The propeller noise emissions are analysed in terms of the perceived noise emissions at the three certification points: approach, take-off, and flyover. Optimizations are performed as a function of blade count to investigate the performance and noise trends for different propeller configurations. The results show a promising performance for the battery-electric aircraft with distributed propulsion, achieving a propeller efficiency between 83% and 88% in cruise without incurring a major noise penalty compared to the reference turboprop aircraft, despite the large increase in aircraft size and weight.
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Distributed propulsion systems, characterized by multiple propellers, represent a promising approach for full-electric aircrafts, offering several advantages but also introducing technical challenges. The main objective of this paper is to quantify how the propeller performance and noise emissions of an eight-propeller full-electric aircraft configuration compare to that of a conventional fuel-based turboprop. In both cases, the key parameters driving the trade-off between noise emissions and aerodynamic performances are analysed as well as the benefits of each configuration. The propeller noise emissions are analysed in terms of the perceived noise emissions at the three certification points: approach, take-off, and flyover. Optimizations are performed as a function of blade count to investigate the performance and noise trends for different propeller configurations. The results show a promising performance for the battery-electric aircraft with distributed propulsion, achieving a propeller efficiency between 83% and 88% in cruise without incurring a major noise penalty compared to the reference turboprop aircraft, despite the large increase in aircraft size and weight.
Accurately determining experimental blade loading distributions is crucial for analyzing rotor performance but challenging due to the limitations of conventional measurement techniques. This paper presents a so-called wake-informed lifting line model that estimates blade loading distributions from phase-locked velocity measurements in the slipstream, eliminating the need for blade instrumentation. The model is evaluated against computational fluid dynamics (CFD) simulations under both attached and separated flow conditions. For the attached flow condition, the model achieves excellent agreement with CFD, with errors in the peak value of thrust distribution below 1%. In the separated flow condition, the model captures radial gradients and the shape of the thrust distribution but exhibits discrepancies in absolute values, with a 10% error in the peak value. These differences arise from the inherent limitations of the potential flow model, the increased significance of drag, and the heightened influence of the spinner’s presence in separated flows. Incorporating profile drag through external polar data improves the model prediction, reducing the error to 4%. The model cannot reliably predict power distributions without external polar data for both attached and separated flows due to the crucial role of drag in the torque direction. The application of the model to experimental flowfield data shows a performance similar to that of the validation case. Therefore, the wake-informed lifting line model offers a promising approach for obtaining experimental blade loading distributions, overcoming the limitations of traditional methods.
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Accurately determining experimental blade loading distributions is crucial for analyzing rotor performance but challenging due to the limitations of conventional measurement techniques. This paper presents a so-called wake-informed lifting line model that estimates blade loading distributions from phase-locked velocity measurements in the slipstream, eliminating the need for blade instrumentation. The model is evaluated against computational fluid dynamics (CFD) simulations under both attached and separated flow conditions. For the attached flow condition, the model achieves excellent agreement with CFD, with errors in the peak value of thrust distribution below 1%. In the separated flow condition, the model captures radial gradients and the shape of the thrust distribution but exhibits discrepancies in absolute values, with a 10% error in the peak value. These differences arise from the inherent limitations of the potential flow model, the increased significance of drag, and the heightened influence of the spinner’s presence in separated flows. Incorporating profile drag through external polar data improves the model prediction, reducing the error to 4%. The model cannot reliably predict power distributions without external polar data for both attached and separated flows due to the crucial role of drag in the torque direction. The application of the model to experimental flowfield data shows a performance similar to that of the validation case. Therefore, the wake-informed lifting line model offers a promising approach for obtaining experimental blade loading distributions, overcoming the limitations of traditional methods.
Conventional propellers operating at negative thrust conditions, even at 0 deg angle of attack, are characterized by flow separation and significantly different noise emissions than at positive thrust conditions. Operating the propeller at nonzero angles of attack at negative thrust conditions can further impact aerodynamic performance and far-field noise emission. This paper studies these effects using lattice-Boltzmann very large eddy simulations coupled with the Ffowcs Williams and Hawkings analogy. At positive thrust, operation at 10 deg angle of attack increases thrust along the freestream direction by approximately 3% compared to operation at 0 deg angle of attack, while efficiency remains constant. Conversely, the negative thrust condition shows approximately a 7% decrease in thrust magnitude and a 10% reduction in regenerated power. In this condition, the positively cambered blade sections exhibit dynamic stall, resulting in broadband fluctuations of up to 10% of the mean loading near the blade tip. The nonzero angle of attack induces opposite variations in absolute blade loading between positive and negative thrust conditions, resulting in opposite changes in the noise directivity. At positive thrust, noise increases in the region from which the propeller is tilted away (i.e., below the propeller at a positive angle of attack), while the opposite occurs at negative thrust. The varying blade loading over the azimuth results in destructive interference between loading and thickness noise for the negative thrust case at the 10 deg angle of attack. These findings highlight the crucial role of considering nonzero angles of attack in propeller design and optimization analyses.
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Conventional propellers operating at negative thrust conditions, even at 0 deg angle of attack, are characterized by flow separation and significantly different noise emissions than at positive thrust conditions. Operating the propeller at nonzero angles of attack at negative thrust conditions can further impact aerodynamic performance and far-field noise emission. This paper studies these effects using lattice-Boltzmann very large eddy simulations coupled with the Ffowcs Williams and Hawkings analogy. At positive thrust, operation at 10 deg angle of attack increases thrust along the freestream direction by approximately 3% compared to operation at 0 deg angle of attack, while efficiency remains constant. Conversely, the negative thrust condition shows approximately a 7% decrease in thrust magnitude and a 10% reduction in regenerated power. In this condition, the positively cambered blade sections exhibit dynamic stall, resulting in broadband fluctuations of up to 10% of the mean loading near the blade tip. The nonzero angle of attack induces opposite variations in absolute blade loading between positive and negative thrust conditions, resulting in opposite changes in the noise directivity. At positive thrust, noise increases in the region from which the propeller is tilted away (i.e., below the propeller at a positive angle of attack), while the opposite occurs at negative thrust. The varying blade loading over the azimuth results in destructive interference between loading and thickness noise for the negative thrust case at the 10 deg angle of attack. These findings highlight the crucial role of considering nonzero angles of attack in propeller design and optimization analyses.
This paper presents an improved approach for fast numerical modeling of the mutual aerodynamic interactions between a wing and tractor propellers for preliminary design purposes. Vortex methods are used to model the propeller and wing aerodynamic performance. The blade element momentum (BEM) method, which is used to model the propeller performance, is extended to allow for a nonuniform inflow field, such that the upstream effects of the wing can be included in the propeller performance modeling. The circulation distribution over the propeller blades is then used in the slipstream tube model (STM) to determine the time-averaged propeller slipstream velocities. Finally, an improved vortex lattice method (VLM) is used to model the wing’s spanwise lift distribution. The method includes an often overlooked correction for the finite slipstream dimensions experienced by the wing segments, in both vertical extent and spanwise extent. This physics-based correction, based on the image vortex technique, partially offsets the large discrepancies in the lift augmentation found in previous low-order numerical analyses of propeller–wing interactions, while keeping the analysis routine computationally very cheap. The BEM and STM are validated against experimental data, while the VLM and complete propeller–wing system are validated against high-fidelity numerical data, confirming the accuracy of the used models. Discrepancies are found in regions or operating conditions where viscosity becomes more relevant, such as separated flows. The numerical model derived in this paper can be used for a quick and accurate first-order estimation of the aerodynamic performance of new conceptual distributed propeller aircraft.
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This paper presents an improved approach for fast numerical modeling of the mutual aerodynamic interactions between a wing and tractor propellers for preliminary design purposes. Vortex methods are used to model the propeller and wing aerodynamic performance. The blade element momentum (BEM) method, which is used to model the propeller performance, is extended to allow for a nonuniform inflow field, such that the upstream effects of the wing can be included in the propeller performance modeling. The circulation distribution over the propeller blades is then used in the slipstream tube model (STM) to determine the time-averaged propeller slipstream velocities. Finally, an improved vortex lattice method (VLM) is used to model the wing’s spanwise lift distribution. The method includes an often overlooked correction for the finite slipstream dimensions experienced by the wing segments, in both vertical extent and spanwise extent. This physics-based correction, based on the image vortex technique, partially offsets the large discrepancies in the lift augmentation found in previous low-order numerical analyses of propeller–wing interactions, while keeping the analysis routine computationally very cheap. The BEM and STM are validated against experimental data, while the VLM and complete propeller–wing system are validated against high-fidelity numerical data, confirming the accuracy of the used models. Discrepancies are found in regions or operating conditions where viscosity becomes more relevant, such as separated flows. The numerical model derived in this paper can be used for a quick and accurate first-order estimation of the aerodynamic performance of new conceptual distributed propeller aircraft.
This paper explores the influence of the characteristics of the helical vortex system of a propeller slipstream on the resulting propeller–wing interaction, with a particular focus on how variations in helix angle impact slipstream deformation. Slipstream characteristics are changed by controlling the thrust and torque coefficient of the propeller through adjustments in blade pitch, advance ratio, and blade count. We conducted experimental measurements of a propeller–wing–flap model using seven-hole pressure probes, oil flow visualization, and infrared thermography in both cruise and high-lift configurations (with deployed slotted flap). The results presented in this paper demonstrate the dominance of the torque coefficient, and thereby longitudinal vorticity in the slipstream, on the slipstream deformation. The paper also underscores the role of the nacelle integration in the development of the slipstream, as well as the flow on the wing surface. The insights into the slipstream deformation provided in this work are essential for future closely coupled propeller–wing designs, particularly when it comes to high-lift configurations.
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This paper explores the influence of the characteristics of the helical vortex system of a propeller slipstream on the resulting propeller–wing interaction, with a particular focus on how variations in helix angle impact slipstream deformation. Slipstream characteristics are changed by controlling the thrust and torque coefficient of the propeller through adjustments in blade pitch, advance ratio, and blade count. We conducted experimental measurements of a propeller–wing–flap model using seven-hole pressure probes, oil flow visualization, and infrared thermography in both cruise and high-lift configurations (with deployed slotted flap). The results presented in this paper demonstrate the dominance of the torque coefficient, and thereby longitudinal vorticity in the slipstream, on the slipstream deformation. The paper also underscores the role of the nacelle integration in the development of the slipstream, as well as the flow on the wing surface. The insights into the slipstream deformation provided in this work are essential for future closely coupled propeller–wing designs, particularly when it comes to high-lift configurations.
This study deals with the comparison of different numerical fidelity levels to predict the noise of an isolated propeller in positive and negative thrust conditions. For this purpose, unsteady Reynolds-Averaged Navier-Stokes (URANS) and Improved Delayed Detached-Eddy Simulations (IDDES) were carried out and compared with Lattice Boltzmann (LBM) very large eddy simulations and wind tunnel data measured at TU Delft. It was found that the aerodynamic behavior with respect to the propeller loads, flow field in the slipstream, and surface pressure was well predicted by all methods. In the subsequent acoustic propagation, according to the Ffowcs Williams - Hawkings analogy, it was found that the noise directivity differed when using different CFD methods for the case of negative thrust due to increased broadband noise, while all CFD methods showed a similar noise directivity at the first blade passing frequency in the positive thrust condition. In the negative regime, the URANS simulation did not take into account the broadband fluctuations, which led to a significant underestimation of about 40 dB in streamwise directions in the overall noise, while IDDES and LBM showed similar trends, but still deviated about 5 dB in the prediction of the overall noise. Finally, a study was conducted with a low-fidelity acoustic evaluation based on Hanson’s model, which enabled a direct comparison of noise generated only by propeller loads, where a good agreement in tonal noise was achieved compared to the FW-H formulation with all CFD methods exhibit a similar noise directivity in both operation conditions.
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This study deals with the comparison of different numerical fidelity levels to predict the noise of an isolated propeller in positive and negative thrust conditions. For this purpose, unsteady Reynolds-Averaged Navier-Stokes (URANS) and Improved Delayed Detached-Eddy Simulations (IDDES) were carried out and compared with Lattice Boltzmann (LBM) very large eddy simulations and wind tunnel data measured at TU Delft. It was found that the aerodynamic behavior with respect to the propeller loads, flow field in the slipstream, and surface pressure was well predicted by all methods. In the subsequent acoustic propagation, according to the Ffowcs Williams - Hawkings analogy, it was found that the noise directivity differed when using different CFD methods for the case of negative thrust due to increased broadband noise, while all CFD methods showed a similar noise directivity at the first blade passing frequency in the positive thrust condition. In the negative regime, the URANS simulation did not take into account the broadband fluctuations, which led to a significant underestimation of about 40 dB in streamwise directions in the overall noise, while IDDES and LBM showed similar trends, but still deviated about 5 dB in the prediction of the overall noise. Finally, a study was conducted with a low-fidelity acoustic evaluation based on Hanson’s model, which enabled a direct comparison of noise generated only by propeller loads, where a good agreement in tonal noise was achieved compared to the FW-H formulation with all CFD methods exhibit a similar noise directivity in both operation conditions.
The far-field acoustic emissions of an isolated scale-model propeller ingesting turbulent inflow are experimentally investigated in wind-tunnel measurements. Phase-averaging and phase-shifting signal processing techniques are used to separate the deterministic (tonal) and random (broadband) components of the recorded sound signals. It is reported that a combination of the two techniques prevents tones unrelated to the shaft rotational frequency from contaminating the estimate of the broadband part of the signal. Scaling and directivity analyses of the obtained broadband component highlight the presence of two noise generation regimes depending on the considered frequency range. In particular, a quasi-omnidirectional directivity pattern is observed for frequencies for which the Helmholtz number is much lower than unity when turbulence is ingested. On the other hand, a dipole-like pattern with minima close to the propeller’s rotational plane gradually appears for higher frequencies. A time- and frequency-domain analysis through the Continuous Wavelet Transform (CWT) method shows how the increase in broadband noise is due to a large number of short-duration pulses linked to the ingestion of turbulence.
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The far-field acoustic emissions of an isolated scale-model propeller ingesting turbulent inflow are experimentally investigated in wind-tunnel measurements. Phase-averaging and phase-shifting signal processing techniques are used to separate the deterministic (tonal) and random (broadband) components of the recorded sound signals. It is reported that a combination of the two techniques prevents tones unrelated to the shaft rotational frequency from contaminating the estimate of the broadband part of the signal. Scaling and directivity analyses of the obtained broadband component highlight the presence of two noise generation regimes depending on the considered frequency range. In particular, a quasi-omnidirectional directivity pattern is observed for frequencies for which the Helmholtz number is much lower than unity when turbulence is ingested. On the other hand, a dipole-like pattern with minima close to the propeller’s rotational plane gradually appears for higher frequencies. A time- and frequency-domain analysis through the Continuous Wavelet Transform (CWT) method shows how the increase in broadband noise is due to a large number of short-duration pulses linked to the ingestion of turbulence.
The application of an acoustically absorbent material (melamine foam) is investigated for the treatment of turbulence grids in an anechoic open-jet wind tunnel facility featuring an axisymmetric contraction. A comparative study of both the generated turbulence and the grids’ self-noise is performed. It is found that the application of melamine foam on the downstream side of the grids marginally affects the produced turbulence, while providing an efficient suppression of tonal peaks in the grids’ self-noise spectrum. Broadband noise levels instead show opposing trends depending on the frequency range considered. On the one hand, a general decrease due to the acoustic treatment is observed for Strouhal numbers lower than unity. On the other, an increase, in the form of broad peaks, is seen to occur over certain higher frequency ranges.
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The application of an acoustically absorbent material (melamine foam) is investigated for the treatment of turbulence grids in an anechoic open-jet wind tunnel facility featuring an axisymmetric contraction. A comparative study of both the generated turbulence and the grids’ self-noise is performed. It is found that the application of melamine foam on the downstream side of the grids marginally affects the produced turbulence, while providing an efficient suppression of tonal peaks in the grids’ self-noise spectrum. Broadband noise levels instead show opposing trends depending on the frequency range considered. On the one hand, a general decrease due to the acoustic treatment is observed for Strouhal numbers lower than unity. On the other, an increase, in the form of broad peaks, is seen to occur over certain higher frequency ranges.
A 3D unsteady RANS simulation utilizing the Spalart-Allmaras (SA) turbulence model was conducted to investigate aerodynamic interactions within a propeller-wing-flap system. The research specifically examines the complex flow field around a slotted flap, highlighting the interaction between the propeller slipstream and the main wing and flap during powered high-lift conditions. Operating conditions include a chord-based Reynolds number of 2 million, with thrust and flap settings reflecting take-off conditions (J = 0.765, Tc = 1.267, 5/= 15°) at an angle of attack a = 8.3°. Chordwise pressure distributions and surface shear stress contours show strong agreement with previous experimental measurements and oil flow visualizations of the same geometry. Findings indicate that a portion of the propeller slipstream transfers from the pressure side of the main wing to the upper side of the flap through the cove, dominating the flap flow field. Although the upper side of the main wing experiences fluctuating flow originating from the propeller slipstream, this flow does not induce unsteadiness or penetrate the flap upper side boundary layer along the wing span. Furthermore, it is shown that the shedding of vortices from the propeller root, along with the resulting vortices on the lower side of the geometry, weakens the flap boundary layer as this flow is transferred through the cove area, consequently inducing flap flow separation. Overall, the findings provide valuable insights into propeller-wing-flap interactions, which had not been visualized before in this detail, yet emphasizing the need for further research to confirm and expand on these results.
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A 3D unsteady RANS simulation utilizing the Spalart-Allmaras (SA) turbulence model was conducted to investigate aerodynamic interactions within a propeller-wing-flap system. The research specifically examines the complex flow field around a slotted flap, highlighting the interaction between the propeller slipstream and the main wing and flap during powered high-lift conditions. Operating conditions include a chord-based Reynolds number of 2 million, with thrust and flap settings reflecting take-off conditions (J = 0.765, Tc = 1.267, 5/= 15°) at an angle of attack a = 8.3°. Chordwise pressure distributions and surface shear stress contours show strong agreement with previous experimental measurements and oil flow visualizations of the same geometry. Findings indicate that a portion of the propeller slipstream transfers from the pressure side of the main wing to the upper side of the flap through the cove, dominating the flap flow field. Although the upper side of the main wing experiences fluctuating flow originating from the propeller slipstream, this flow does not induce unsteadiness or penetrate the flap upper side boundary layer along the wing span. Furthermore, it is shown that the shedding of vortices from the propeller root, along with the resulting vortices on the lower side of the geometry, weakens the flap boundary layer as this flow is transferred through the cove area, consequently inducing flap flow separation. Overall, the findings provide valuable insights into propeller-wing-flap interactions, which had not been visualized before in this detail, yet emphasizing the need for further research to confirm and expand on these results.
The electrification of aircraft is strongly coupled with the use of propellers as a propulsion system because of their high efficiency and their convenient integration with electric motors. Due to the operational flexibility of electric motors, the propeller can also be used in alternative operations, such as negative thrust and power mode. By operating the propeller at negative inflow angles at the blade segments, the torque and thrust are in the opposite direction compared to the conventional positive thrust conditions. This can be useful for control purposes or for energy harvesting. An experimental investigation was carried out to explain the physics behind the aerodynamic performance of a propeller at both positive thrust and energy-harvesting operation. Next to the measured integrated forces and moments on the propeller, stereoscopic particle image velocimetry was used to analyze the flowfield around the blades as well as the slipstream behind the propeller disk to identify the dominating flow phenomena that drive the energy-harvesting operation. The highly cambered blade sections for this typical aircraft propeller do not operate efficiently in energy-harvesting mode due to the associated negative angles of attack. The thin tip blade sections experience separated flow in these conditions, reducing the useful output power, compared to wind turbines, which feature opposite camber. To maximize the output power in the energy-harvesting conditions, a low pitch setting is required in combination with a relatively high advance ratio. However, this also comes at a cost of large negative thrust (drag) values.
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The electrification of aircraft is strongly coupled with the use of propellers as a propulsion system because of their high efficiency and their convenient integration with electric motors. Due to the operational flexibility of electric motors, the propeller can also be used in alternative operations, such as negative thrust and power mode. By operating the propeller at negative inflow angles at the blade segments, the torque and thrust are in the opposite direction compared to the conventional positive thrust conditions. This can be useful for control purposes or for energy harvesting. An experimental investigation was carried out to explain the physics behind the aerodynamic performance of a propeller at both positive thrust and energy-harvesting operation. Next to the measured integrated forces and moments on the propeller, stereoscopic particle image velocimetry was used to analyze the flowfield around the blades as well as the slipstream behind the propeller disk to identify the dominating flow phenomena that drive the energy-harvesting operation. The highly cambered blade sections for this typical aircraft propeller do not operate efficiently in energy-harvesting mode due to the associated negative angles of attack. The thin tip blade sections experience separated flow in these conditions, reducing the useful output power, compared to wind turbines, which feature opposite camber. To maximize the output power in the energy-harvesting conditions, a low pitch setting is required in combination with a relatively high advance ratio. However, this also comes at a cost of large negative thrust (drag) values.
Propellers have gained traction in recent years and are subsequently being integrated into unconventional airframe configurations as part of conceptual designs for developing sustainable aircraft. Close coupling of the propeller and airframe enhances the non-uniformity of the inflow to the propeller. This non-uniform inflow introduces unsteady loading, affecting the propeller's performance. This paper investigates the impact of accounting for installation effects during propeller blade design within an optimization toolchain. This toolchain is applied to a propeller mounted at the rear of a fuselage utilizing Boundary Layer Ingestion (BLI). The proposed design methodology utilizes existing analysis methods to assess the installed aerodynamic and structural performance of the propeller. A gradient-based optimizer is coupled to these analysis methods to design two distinct blades for a multi-segment mission with the objective to maximize the aerodynamic performance. The geometry of the first propeller blade is optimized for the uniform inflow and consequently optimized to operate in the non-uniform inflow. The second blade is from the outset designed for the non-uniform inflow condition. The implementation of the proposed design methodology captures the sensitivity of the inflow field within the design loop, resulting in two distinct blade designs. Upon comparing these blades in BLI inflow conditions to meet identical performance requirements, it was observed that the blade designed to account for installation effects featured a higher inboard chord distribution and consumed 1.58% less energy compared to the blade optimized for uniform inflow for the considered mission profile.
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Propellers have gained traction in recent years and are subsequently being integrated into unconventional airframe configurations as part of conceptual designs for developing sustainable aircraft. Close coupling of the propeller and airframe enhances the non-uniformity of the inflow to the propeller. This non-uniform inflow introduces unsteady loading, affecting the propeller's performance. This paper investigates the impact of accounting for installation effects during propeller blade design within an optimization toolchain. This toolchain is applied to a propeller mounted at the rear of a fuselage utilizing Boundary Layer Ingestion (BLI). The proposed design methodology utilizes existing analysis methods to assess the installed aerodynamic and structural performance of the propeller. A gradient-based optimizer is coupled to these analysis methods to design two distinct blades for a multi-segment mission with the objective to maximize the aerodynamic performance. The geometry of the first propeller blade is optimized for the uniform inflow and consequently optimized to operate in the non-uniform inflow. The second blade is from the outset designed for the non-uniform inflow condition. The implementation of the proposed design methodology captures the sensitivity of the inflow field within the design loop, resulting in two distinct blade designs. Upon comparing these blades in BLI inflow conditions to meet identical performance requirements, it was observed that the blade designed to account for installation effects featured a higher inboard chord distribution and consumed 1.58% less energy compared to the blade optimized for uniform inflow for the considered mission profile.
A tightly coupled aeroelastic design code for composite propeller blades was developed, verified, and used to perform design sensitivity studies. The design code features a structural model that accounts for geometric nonlinearities through the application of a corotational framework, nonlinear responses to loads, and a cross-sectional modelling approach to accurately represent the detailed 3D blade structure as a reduced-order Timoshenko beam element model. Blade element momentum (BEM) theory was used to evaluate aerodynamic loads, which are mapped onto the structural mesh. The nonlinear aeroelastic analysis routine uses Newton’s method to converge on a solution, with analytical derivatives for all applied loads. Excellent agreement with other analysis methods was shown during verification studies for all developed models. During validation, performance trends obtained from BEM were consistent with experimental results, with a maximum error of 20% at operating conditions under consideration during this research. The use of either symmetric-unbalanced or symmetric-balanced laminates was considered during sensitivity studies. Small variations in performance compared to the rigid propeller were observed from blades constructed out of symmetric-balanced laminates, as the minimal amount of bend-twist and extension-shear coupling resulted in small twist deformations. Conversely, propellers constructed out of symmetric-unbalanced laminates were shown to yield a noticeable variation in thrust and power compared to the rigid propeller due to the presence of bend-twist and extension-shear coupling, which results in coupling between twist and blade axis deformations. The presence of an aerodynamic wash-out effect was also found to alleviate blade loads, resulting in a lower power requirement at a given thrust setting, and an opposite trend was observed in the presence of a wash-in effect. The proposed analysis framework may be applied towards more extensive design studies or optimization in future projects.
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A tightly coupled aeroelastic design code for composite propeller blades was developed, verified, and used to perform design sensitivity studies. The design code features a structural model that accounts for geometric nonlinearities through the application of a corotational framework, nonlinear responses to loads, and a cross-sectional modelling approach to accurately represent the detailed 3D blade structure as a reduced-order Timoshenko beam element model. Blade element momentum (BEM) theory was used to evaluate aerodynamic loads, which are mapped onto the structural mesh. The nonlinear aeroelastic analysis routine uses Newton’s method to converge on a solution, with analytical derivatives for all applied loads. Excellent agreement with other analysis methods was shown during verification studies for all developed models. During validation, performance trends obtained from BEM were consistent with experimental results, with a maximum error of 20% at operating conditions under consideration during this research. The use of either symmetric-unbalanced or symmetric-balanced laminates was considered during sensitivity studies. Small variations in performance compared to the rigid propeller were observed from blades constructed out of symmetric-balanced laminates, as the minimal amount of bend-twist and extension-shear coupling resulted in small twist deformations. Conversely, propellers constructed out of symmetric-unbalanced laminates were shown to yield a noticeable variation in thrust and power compared to the rigid propeller due to the presence of bend-twist and extension-shear coupling, which results in coupling between twist and blade axis deformations. The presence of an aerodynamic wash-out effect was also found to alleviate blade loads, resulting in a lower power requirement at a given thrust setting, and an opposite trend was observed in the presence of a wash-in effect. The proposed analysis framework may be applied towards more extensive design studies or optimization in future projects.
A wind-tunnel experiment was performed at the DNW Low-Speed Tunnel with a powered propeller-wing model to prove the concept of energy-harvesting with propellers and assess its impact on the wing performance. By separating the contributions of the propeller and wing to the overall system forces, both for positive and negative thrust settings improved understanding was obtained of the propeller-wing interaction. A tip-mounted propeller configuration was simulated. At positive thrust settings, the operation of the propeller increased the lift gradient and improved the aerodynamic efficiency of the wing (L/D) by 10-35% compared to the propeller-off configuration. At CL = 0.5 and net zero force in streamwise direction the benefit was 12%, while at CL = 1.0 and a net force in streamwise direction of approximately three times the wing drag the benefit was 32%. At negative thrust, the propeller operation decreased the lift gradient, but the wing aerodynamic efficiency was still higher than that of the propeller-off configuration. This was an unexpected result, which was explained by the reduction in friction drag on the wing immersed in the propeller slipstream due to the lower dynamic pressure, and a possible reduction in wing induced drag due to downwash on the outboard part of the wing. The aileron effectiveness was decreased by about 10% when switching from positive to negative thrust operation. However, for angles of attack up to approximately 14 degrees even at negative thrust, the aileron effectiveness was still higher than for the clean wing.
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A wind-tunnel experiment was performed at the DNW Low-Speed Tunnel with a powered propeller-wing model to prove the concept of energy-harvesting with propellers and assess its impact on the wing performance. By separating the contributions of the propeller and wing to the overall system forces, both for positive and negative thrust settings improved understanding was obtained of the propeller-wing interaction. A tip-mounted propeller configuration was simulated. At positive thrust settings, the operation of the propeller increased the lift gradient and improved the aerodynamic efficiency of the wing (L/D) by 10-35% compared to the propeller-off configuration. At CL = 0.5 and net zero force in streamwise direction the benefit was 12%, while at CL = 1.0 and a net force in streamwise direction of approximately three times the wing drag the benefit was 32%. At negative thrust, the propeller operation decreased the lift gradient, but the wing aerodynamic efficiency was still higher than that of the propeller-off configuration. This was an unexpected result, which was explained by the reduction in friction drag on the wing immersed in the propeller slipstream due to the lower dynamic pressure, and a possible reduction in wing induced drag due to downwash on the outboard part of the wing. The aileron effectiveness was decreased by about 10% when switching from positive to negative thrust operation. However, for angles of attack up to approximately 14 degrees even at negative thrust, the aileron effectiveness was still higher than for the clean wing.
Using propellers in negative thrust conditions can potentially result in many benefits, such as a steeper descent, a reduced landing run, reduced community noise, energy regeneration, etc. However, the aerodynamics and aeroacoustics of propellers in this regime are not well understood. This paper presents an aeroacoustic analysis of an isolated propeller operating in both positive and negative thrust conditions, using scale-resolved lattice-Boltzmann very large eddy simulations and the Ffowcs Williams & Hawkings analogy. The propeller was operated at a constant tip Mach number so that any differences in tonal noise between positive and negative thrust conditions were due to changes in blade loading. Results showed that the flow separation around the blades in the negative thrust case led to a 2 to 6 times higher standard deviation in integrated thrust compared to the positive thrust case. The blade loading in the negative thrust case shows the amplitude of fluctuations up to 18% for inboard sections and up to 30% near the blade tip compared to the time-averaged loads. The noise in the propeller plane is 10 dB higher in the positive thrust regime than in the negative thrust regime at a given absolute thrust level of |T_C = 0.08|. The lower noise at negative thrust is caused by two factors: the lower magnitude of the negative torque compared to the positive torque at a given thrust level and the shift of the blade loading inboard in the negative thrust condition due to the stall of the blade tip. Along the propeller axis, the negative thrust regime has 13-15 dB higher noise because of the increased broadband noise generated by the flow separation. In the negative thrust case, the noise along the propeller axis (89 dB) and propeller plane (92 dB) are comparable. However, this is not the case for the propulsive case. The comparison of noise in the vicinity of the propeller plane showed that using the propellers in negative thrust conditions allows for a steeper and quieter descent compared to a conventional descent; as long as the magnitude of the negative torque produced is equal to or less than the torque required to operate the propeller in a conventional landing.
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Using propellers in negative thrust conditions can potentially result in many benefits, such as a steeper descent, a reduced landing run, reduced community noise, energy regeneration, etc. However, the aerodynamics and aeroacoustics of propellers in this regime are not well understood. This paper presents an aeroacoustic analysis of an isolated propeller operating in both positive and negative thrust conditions, using scale-resolved lattice-Boltzmann very large eddy simulations and the Ffowcs Williams & Hawkings analogy. The propeller was operated at a constant tip Mach number so that any differences in tonal noise between positive and negative thrust conditions were due to changes in blade loading. Results showed that the flow separation around the blades in the negative thrust case led to a 2 to 6 times higher standard deviation in integrated thrust compared to the positive thrust case. The blade loading in the negative thrust case shows the amplitude of fluctuations up to 18% for inboard sections and up to 30% near the blade tip compared to the time-averaged loads. The noise in the propeller plane is 10 dB higher in the positive thrust regime than in the negative thrust regime at a given absolute thrust level of |T_C = 0.08|. The lower noise at negative thrust is caused by two factors: the lower magnitude of the negative torque compared to the positive torque at a given thrust level and the shift of the blade loading inboard in the negative thrust condition due to the stall of the blade tip. Along the propeller axis, the negative thrust regime has 13-15 dB higher noise because of the increased broadband noise generated by the flow separation. In the negative thrust case, the noise along the propeller axis (89 dB) and propeller plane (92 dB) are comparable. However, this is not the case for the propulsive case. The comparison of noise in the vicinity of the propeller plane showed that using the propellers in negative thrust conditions allows for a steeper and quieter descent compared to a conventional descent; as long as the magnitude of the negative torque produced is equal to or less than the torque required to operate the propeller in a conventional landing.
Multi-Element Airfoil in Jet Flows
Identifying Dominant Factors and Interactions
Propeller–wing–flap systems are subject to complex aerodynamic interactions between each part of the system. Although the propeller–wing interaction in cruise conditions is well defined, the high-lift condition is relatively unexplored. Effective analysis of the complex aerodynamic relationship between propeller, wing, and flap is being impeded by a lack of understanding of the underlying mechanisms. In this paper, we therefore investigate the effects of a 2D jet impinging on a multisection airfoil. We quantify which factors that define a jet–wing–flap configuration dominate lift, drag, and moment responses. We further investigate interactions between these factors and discuss how they affect the flow. We find that the jet velocity ratio is by far the dominant factor in lift, drag, and moment responses, but it does not have strong interactions with other factors. The sensitivities of the multi-element airfoil do not change significantly when impinged upon by a jet, except when critical Mach numbers are exceeded. This strongly affects the aerodynamic response and dominant sensitivities. We furthermore conclude that the immersion of the flap is a key aspect when it comes to augmenting the lift by increasing the dynamic pressure in the flowfield. The conclusions from this paper can provide key insights for propeller–wing–flap flows.
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Propeller–wing–flap systems are subject to complex aerodynamic interactions between each part of the system. Although the propeller–wing interaction in cruise conditions is well defined, the high-lift condition is relatively unexplored. Effective analysis of the complex aerodynamic relationship between propeller, wing, and flap is being impeded by a lack of understanding of the underlying mechanisms. In this paper, we therefore investigate the effects of a 2D jet impinging on a multisection airfoil. We quantify which factors that define a jet–wing–flap configuration dominate lift, drag, and moment responses. We further investigate interactions between these factors and discuss how they affect the flow. We find that the jet velocity ratio is by far the dominant factor in lift, drag, and moment responses, but it does not have strong interactions with other factors. The sensitivities of the multi-element airfoil do not change significantly when impinged upon by a jet, except when critical Mach numbers are exceeded. This strongly affects the aerodynamic response and dominant sensitivities. We furthermore conclude that the immersion of the flap is a key aspect when it comes to augmenting the lift by increasing the dynamic pressure in the flowfield. The conclusions from this paper can provide key insights for propeller–wing–flap flows.