Building-generated turbulence can significantly influence the propagation of noise from advanced air mobility (AAM) vehicles operating in urban environments, yet its impact on acoustic variability remains poorly quantified. In this study, the effect of an isolated building wake on sound propagation is investigated using time-resolved Lattice-Boltzmann very-large-eddy simulations. A simplified tonal acoustic source representative of an AAM vehicle is placed downstream of the building, and the resulting unsteady sound field is analyzed within and downstream of the turbulent wake. The results show that wake-induced turbulence produces pronounced temporal fluctuations in the received sound pressure level, with variability exceeding 3 dB in localized regions. These fluctuations extend beyond the physical extent of the wake due to interference effects and reflected propagation paths from the building and ground. Analysis along selected propagation directions indicates a strong correlation between turbulence-induced velocity fluctuations and acoustic variability along direct propagation paths, while this correlation weakens in regions dominated by multiple reflections. The findings emphasize the importance of accounting for unsteady, building-induced flow effects when evaluating AAM noise in urban environments. ... Read more

Turbulence from densely built urban structures alters the acoustic signature of advanced air mobility (AAM) vehicles, complicating prediction of noise impact. Ray tracing, using instantaneous frozen velocity-field snapshots from time-resolved simulations, provides an efficient approach for estimating turbulence-induced acoustic variability. Comparisons of the equivalent sound level (⁠ ⁠), variability metric (⁠ ⁠), and transient sound-level fluctuations show good agreement with a time-resolved reference solution, where discrepancies are mainly near the source and ground. Correlation analysis confirms that dominant temporal variability trends are reproduced at most observer locations, demonstrating that the method provides a reliable and computationally efficient framework for assessing AAM noise in urban environments. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

Future regional and short to medium-range aircraft are expected to use propellers for sustainable aviation, possibly with (hybrid-)electric propulsion systems. This will enable innovative integration of aero-propulsive systems, potentially increasing the overall aerodynamic efficiency of the airframe and propellers, resulting in a lighter airframe and increased passenger comfort. The presence of electric motors and propellers in such configurations, in addition, offers a unique opportunity to leverage propellers as airbrakes by operating them at negative thrust. This approach offers multiple potential advantages, including shorter landing runs, enhanced landing maneuverability, energy harvesting during braking operations, faster aircraft turnaround times, and reduced community noise exposure.

Despite these potential advantages, the aerodynamic and aeroacoustic characteristics of propellers operating in negative thrust mode remain largely unexplored. This dissertation addresses this knowledge gap through computational analysis, comparing the performance of isolated propeller configurations in negative thrust mode with the well-understood positive thrust mode. The results reveal that the distinct aeroacoustic characteristics of propellers operating in negative thrust conditions, compared to conventional positive thrust conditions, offer a promising avenue for reducing community noise, not only through the possibility of steeper descents but also through changes in the noise emissions from the propeller itself.
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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. ... Read more

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. ... Read more

Operating a conventional propeller at negative thrust results in the operation of positively cambered blade sections at negative angles of attack, leading to flow separation. Consequently, accurately simulating the aerodynamics of propellers operating at negative thrust poses a greater challenge than at positive thrust. This study offers a comprehensive assessment of the aerodynamic modeling capabilities of numerical methods, spanning low to high fidelity, for computing propeller performance across both positive and negative thrust regimes. Low-fidelity methods, namely, blade-element momentum and lifting line theories, effectively predict propeller performance trends at positive thrust. However, they fail to capture trends at negative thrust beyond the maximum power output point due to the neglect of three-dimensional flow effects. Both steady and unsteady Reynolds-averaged Navier–Stokes (RANS) simulations with y ⁺ < 1 perform well across both positive and negative thrust conditions, with errors below 2%for both thrust and power magnitudes near the maximum power output point. Lattice-Boltzmann very-large-eddy simulations (LB-VLESs) with y ⁺ ≤ 10 exhibit excellent agreement with experimental data with less than 1% error near the maximum power output point but with significant computational costs. Conversely, LB-VLESs with y ⁺ ≥ 15 offer a more economical approach to capture general trends with the computational cost of the same order as unsteady RANS. However, wall models introduce errors in modeling flow separation, leading to a 16% overestimation of power magnitude near the maximum power output point. The results highlight the necessity of using tools with increased fidelity levels when considering propeller operation at negative thrust compared to the conventional positive thrust regime. ... Read more

This paper studies the effect of operation at non-zero angles of attack on the aerodynamic performance and far-field noise emissions of an isolated propeller operating at positive and negative thrust conditions. To achieve this, scale-resolved lattice-Boltzmann very large eddy simulations coupled with the Ffowcs Williams & Hawkings analogy have been used. The results show that when the propeller operates with a 10◦ angle of attack at the positive thrust condition, the blade loading increases on the advancing side and decreases on the retreating side, leading to a 9.6% increase in integrated thrust (when computed along the propeller axis) and a negligible increase (0.1%) in propeller efficiency. Conversely, at the negative thrust condition, the operation at 10 deg angle of attack results in a 7.9% decrease in thrust magnitude and an 11.1% reduction in energy-harvesting efficiency. In this condition, the positively cambered blade sections exhibit dynamic stall at the 10◦ angle of attack, resulting in broadband fluctuations of up to 10% of the mean loading. As a result of the opposite change in absolute blade loading in the negative thrust condition compared to the positive thrust condition at the 10◦ angle of attack, the change in the noise directivity is also the opposite. Whereas in the positive thrust case, the noise increases in the region from which the propeller is tilted away (i.e., below the propeller at a positive angle of attack), in the negative thrust case, it is the other way around. This study highlights the need to account for non-zero angles of attack in propeller design and optimization analyses. ... Read more

Regenerative propellers offer many potential benefits such as improved maneuverability, reduced landing run, and decreased community noise, besides the potential to reduce energy consumption by recovering energy during descent and landing. Since the blade loading in regenerative mode will be opposite to that in the conventional propulsive mode, the aerodynamic and aeroacoustic performance of the propeller will be markedly different in both modes of operation. This paper analyzes the performance of an isolated propeller at positive and negative thrust using multi-fidelity numerical approach and validation experiments to identify the most relevant flow phenomena and resulting tonal-noise mechanisms. The results show that the low-fidelity BEM model does not perform well in energy-harvesting conditions mainly due to polar data limitations near the stall conditions at negative angles of attack. The study of tonal noise sources reveals that at a given advance ratio, the loading noise depends upon the relative level of thrust and torque noise in the upstream direction of the propeller, whereas it is lower or similar to the propulsive case in the downstream direction. ... Read more