Contrary to conventional wisdom favouring smooth surfaces for drag reduction, the last decades have brought forth different textured surfaces showing drag reduction in turbulent boundary layers, of which riblets are the most studied.
Sirovich and Karlson (1997) introduced a different textured surface named chevron-shaped protrusions, indicating a drag reduction of 10% in turbulent channel flows.
Later studies questioned their efficacy as no credible reproduction of the results was obtained throughout the years.
The last credible reproduction study by Carrasco Grau et al. (2023) suggested that the reasons for this discrepancy could reside in the difference in model and test section size.
All reproduction studies were performed in facilities with external boundary layers instead of internal and with a covered area no longer than 0.8 meters, instead of the 8-meter-long channel flow which was fully covered in these chevrons from the original study.
This work will investigate just that, using a channel flow facility with dimensions more akin to that used by Sirovich and a model size of 2.4 meters.

The study assembles and characterises an improved channel flow facility with dimensions identical to those used by tay et al. (2011) at the National University of Singapore (NUS), measuring a total length of eight meters and a test section of 2.4 meters.
This facility used an array of 29 static pressure taps to determine the skin friction via the mean pressure gradient method, and utilised hot wire measurements in the midpoint of the test section for investigation of the flow mechanics of the chevrons.
The measurements performed on a flat plate were compared to the results obtained at the NUS to characterise the facility and validate its suitability for single-point drag measurements in the order of 5-10% increase or decrease.
Once this was confirmed the different configurations of chevron-shaped protrusions were investigated.

The key findings of this study are twofold.
Firstly, the developed channel flow facility at DUT proves proficient in generating canonical boundary layer profiles, enabling accurate skin friction measurements of textured surfaces.
The facility provides good hot-wire measurements without sensor vibrations capable of investigating the boundary layer characteristics in this facility.
Secondly, the study establishes that chevron-shaped protrusions are likely unsuited for reducing turbulent skin friction in turbulent channel flows.
Despite an inability to definitively disprove the working hypothesis, the observed increase in drag suggests that the technique's efficacy is not determined by the facility type and model size.

It is not considered worthwhile to conduct additional research into chevron-shaped protrusions as a potential technique for reducing drag in turbulent boundary layers.
However, the minimal drag penalty associated with this technique opens new possibilities for the application of this technique in the aviation sector, mainly in aiding with separation control and heat transfer.

This research addresses the challenge of improving aerodynamic performance by delaying flow separation using deployable vortex generators (VGs) actuated by shape memory alloys. These VGs combine passive and active elements to enhance mixing within the boundary layer, restoring momentum near the wall and minimizing separation in an Adverse Pressure Gradient (APG). A literature review identified a gap in understanding the dynamics of shape-adaptive VGs. Through Computational Fluid Dynamics (CFD) and experimental studies, the research optimized VG designs for aerodynamic efficiency. Results showed that counter-rotating vanes were most effective in controlling flow, while side load and yaw moment challenges necessitated robust actuation mechanisms. A novel design using rigid vane actuators and shape memory alloys for precise adjustments was proposed to address these issues. This study significantly advances the understanding and application of shape-adaptive VGs in flow control.

With an unprecedented surge in the number of individuals utilizing electric vehicles for commuting across the globe, it is becoming critical to immediately deliver an adequate and range-friendly degree of thermal comfort in car cabins to help drivers remain focused and attentive. The research presented in this thesis is part of the development work carried out by the Aerothermal team at Tesla. The team aims to better understand high aspect ratio subsonic jets, particularly intersecting twin jets, to ventilate the vehicle's passenger compartment.

The intersecting twin jet has recently captured significant attention from the Aerothermal team for several reasons. Firstly, it provides the ability to actively control the flow angle from the HVAC unit into the vehicle cabin without requiring direct interaction with the vent. Additionally, it facilitates the flushing of the vent outlets with the A-class surfaces in the vehicle interior, helps achieve a simple and minimalist design for the instrument panel and other interior surfaces that align with Tesla's design language

The objective pursued in this thesis is to investigate the physics represented by the bulk flow characteristics of intersecting twin jets, and to scrutinize the installation effect on the merging jet, focusing on the influence of the installation surface offset height. This objective is achieved by characterizing the evolution of large-scale flow structures in High AR intersecting twin jets at moderate Reynolds numbers experimentally using planar Particle Image Velocimetry (PIV) in both installed and uninstalled conditions. Once accomplished, the experimental results are then used to help select, validate, and calibrate the appropriate RANS turbulence model.

The experimental results show that the intersecting jets converge before merging and forming a single jet. This jet behaves like a single jet emanating from a more recessed origin. The variation of the flow ratio between the two nozzle outlets results in varying the jet angle. The study reveals that the placement of the surface impacts the jet angle and velocity and introduces a circulation zone that affects the jet's behavior and direction. The surface's impact depends on the offset distance, length, and profile of the surface. The results also suggest that accurate computational prediction of the jet characteristics requires high mesh resolution and an appropriate selection of turbulence intensity. Among the four turbulence models assessed, the k-omega SST model is found to be adequate for the application, although it exhibits some numerical hysteresis. Overall, the findings provide insights into the dynamics of jet-surface interactions and can guide the design of systems involving such flows.

In recent years, research for turbulent boundary layer (TBL) has shifted towards developing novel approaches to reduce drag. This shift is driven by new industrial standards emphasizing fuel-efficient and environmentally friendly air transportation. As wall-bounded turbulence plays a significant role in the overall drag experienced by an aircraft, any understanding and control authority on associated flow structures becomes vital for more optimized and efficient flow control techniques to reduce drag. In this context, conventional passive flow control techniques have been widely used as they are simple and do not require any external power supply. However, active flow control techniques have demonstrated the potential for greater effectiveness, which offers more substantial drag reduction. Existing active control methods, such as wall oscillation, traveling wave, blowing, and suction, have been previously studied to reduce the viscous drag in the TBL. However, the efficacy of such control strategies tuned to outer layer large-scale motions (LSMs) at higher Reynolds number (Re) flows has emerged as a promising avenue for achieving significant drag reduction. In contrast, control techniques targeting the inner layer small-scales encounter limitations due to restricted control authority and the difficulties associated with miniaturizing hardware at higher Re flows.

LSMs convecting in the logarithmic region (log region) of TBL play a critical role in the dynamics of wall-bounded turbulence, as they carry a significant portion of the turbulent kinetic energy (TKE) [Abbassi et al., 2017]. The active wall oscillation technique tuned to the frequency of the LSMs has proven to be energy-efficient, as drag reduction increases with the frictional Reynolds number (Reτ ) and the contribution of LSMs to the TKE and wall shear stress increases with the Reτ [Marusic et al., 2021]. In this thesis work, the receptivity of LSMs to active large-scale control strategy is assessed via a spanwise array of wall-normal jets in a TBL. Multiple wall-normal jets manipulate the flow over a flat plate by introducing a spanwise traveling wave, which aims to mimic wall oscillation tuned to the large scales in the log region of the TBL.

To assess the efficacy of the suggested control strategy with spatial and temporal tuning, particle image velocimetry (PIV) and hot-wire anemometry (HWA) are employed to quantify second-order turbulence statistics and analyze the organization of the coherent patterns introduced in the flow via two-point correlations. In addition, the effect of different control cases derived from different tuning and control strategies is analyzed at Reτ = 2227. The TKE production plots show that control cases targeting the LSMs effectively attenuate the TKE production near the wall. Furthermore, based on the analysis of the spectral energy plot of streamwise velocity fluctuations, it is observed that the energy content associated with larger length scales convecting in the middle of the log region decreases in the controlled case compared to the uncontrolled base case. This finding suggests that certain control cases notably impact the organization of LSM and second-order turbulence statistics far downstream of the actuation point. Additionally, the reduction in TKE production near the wall represents a reduction in wall shear stress and viscous drag. However, additional research must be conducted in the future based on the current findings to reach a definitive conclusion regarding the efficacy of individual control cases.

An experimental investigation is presented in which an array of pulsed jet actuators is used to control a turbulent separation bubble formed on a curved backward facing ramp. The array is positioned upstream of detachment and consists of wall-normal high aspect ratio skewed rectangular jets which generate streamwise vortices in the boundary layer increasing momentum transfer and delaying separation. While similar systems have shown promise in previous research, this work considers a pressure-induced separation of a relatively high Reynolds number (Re_τ=4600) turbulent boundary layer (TBL), where the large turbulent structures of the separating BL are of similar scale and magnitude as those generated by actuation and significantly affect the dynamics of detachment.

Both steady blowing and periodic pulsing actuation strategies are tested and compared. Preliminary jet velocity and pulsing frequency sweeps are carried out to identify optimal actuator operating parameters, relying on wall static pressure measurements to evaluate control effectiveness. Select cases of interest are then investigated using two-dimensional two-component particle image velocimetry and compared against the uncontrolled baseline which is characterized using PIV and hot wire anemometry. Additional PIV-derived metrics are utilized to assess system performance.

For steady blowing, a jet-to-crossflow velocity ratio VR>1 was required to produce a separation delay, while diminishing improvements in control effect with increasing jet velocity started at VR=1.6 (actuation momentum ratio of C_μ=2.3%). This nominal velocity ratio was adopted for all further investigation. The actuator was found to produce alternating strong and weak downwash regions in the TBL resulting in an artificial sweep/ejection pattern at detachment. Periodic forcing with the same nominal velocity ratio was able to achieve better or comparable results to steady actuation, while requiring less input momentum (C_μ=1.2-1.8%). The optimum actuation frequency was determined to be the natural frequency of the uncontrolled bubble, with the performance of higher frequency actuation tending towards steady blowing levels. As shown by an analysis of flow dynamics based on phase-averaged PIV velocity fields, actuation at the bubble time scales produces significant flow oscillation in phase with actuation. This resonant behaviour results in transient high momentum sweeps between actuation pulses that boost actuator performance, achieving double the performance benefit afforded by steady actuation according to multiple metrics. In comparison, actuation at time scales multiple times shorter than that of the bubble produces a quasi-steady flow and performance comparable to that of steady actuation.

Additionally, a novel alternating actuation strategy is tested, in which the period of active blowing is composed of high frequency alternation between two inverted actuator rows. This aimed to produce a quasi-2D periodic control effect using 3D actuators, which Squire's theorem suggests could excite the separated shear layer instability more than conventional 3D perturbation. While high frequency alternation did achieve a quasi-2D effect, it also prevented the sweep/ejection pattern characteristic of 3D perturbation from forming, thus significantly limiting the actuator performance.

A flying aircraft consumes fuel to overcome air resistance during its motion. A significant part of this consumption (55%) is due to turbulent skin friction arising at the interface of the aircraft surface and the air. The current work aims to develop and investigate relevant turbulent skin-friction reduction techniques and assist in advancing technologies that will contribute to achieving a reduction in turbulent skin-friction drag and, consequently, fuel consumption and the associated emissions. The thesis examines active flow control techniques derived from the spanwise wall oscillation concept. The latter involves introducing a time-dependent spanwise motion to the wall over which a turbulent boundary layer is present. The current work relies on the experimental investigation using particle image velocimetry to quantify the effect of the active control techniques.....@en

A Tomographic Background-Oriented Schlieren (TBOS) technique is developed to aid in the visualization of compressible flows. An experimental setup was devised around a sub-scale rocket nozzle, in which four cameras were set up in a circular configuration with 30° angular spacing in azimuth. Measurements were taken of the overexpanded supersonic jet plume at various nozzle pressure ratios (NPR), corresponding to different flow regimes during the start-up and shut-down of rocket nozzles. Measurements were also performed for different camera parameters using different exposure times and f-stops in order to study the effect of measurement accuracy. Density gradients and subsequently two-dimensional line-of-sight integrated density fields for each of the camera projections are recovered from the index of refraction field by solving a Poisson equation. The results of this stage are then used to reconstruct two-dimensional slices of the (time-averaged) density field using a tomographic reconstruction algorithm employing the filtered back-projection and the simultaneous algebraic reconstruction technique. By stacking these two-dimensional slices, the (quasi-) three-dimensional density field is obtained. The accuracy of the implemented method with a relatively low number of sparse cameras is briefly assessed and basic flow features are extracted such as the shock spacing in the overexpanded jet plume.

TOP contoured nozzles, with large area-ratios, are commonly employed in rocket propulsion systems as they feature an excellent thrust-to-weight ratio. A significant shortcoming to this design is that, during the startup and shutdown transients of a LRE, the internal nozzle flow progresses through a series of overexpanded flow states - Free Shock Separation (FSS) and Restricted Shock Separation (RSS), which produce critical loads associated with SWBLI and asymmetric flow separation. Exacerbated by FSI, this operational phase is known to generate the highest vibroacoustic loads at which payload and vehicle structures are subjected to when the engine is operated at off-design conditions.

Motivated by the importance of understanding how the interaction between the developing flow and the vibrating nozzle walls has an effect on supersonic noise generation and propagation, this work has studied the effect that wall compliancy has on the vibroacoustic loading of TOP contoured nozzles. This is demonstrated by means of cold flow tests carried out in the High Speed Laboratories of the Delft University of Technology on a stiff-walled aluminum nozzle, which serves as a baseline test case, and on a urethane-based compliant walled nozzle. Tests are conducted under comparable flow conditions and test parameters are measured by means of acoustic and optical techniques. Simultaneous recordings are performed and include the nozzle-wall deformation, by means of stereoscopic tracking of tracers on the nozzle lip, the imprint of the near-field acoustic signature, by means of arrays of pressure-microphones, and Schlieren imaging of the jet plume.

Measurement data allows for a Fourier decomposition of the nozzle lip displacement and of the acoustic pressure field in azimuth. Reconstruction of the instantaneous plume development enables the identification of the main flow structures responsible for noise generation.

Comparison of results between the two test articles highlights a different spectral content and directivity pattern. Correlation between the structural displacements and the acoustic signal, together with the use of DMD, quantitatively aids the investigation of how FSI has an impact on the generation of an aeroelastic tone at 180 Hz. Findings suggest that its production is the result of the periodic thickening and thinning of the shear layer owing to the heightened flapping motion of the nozzle lip preceding RSS transition, driven by an intensified shock foot instability.

In the last decades, the prevailing belief that smooth surfaces offer the lowest drag has been challenged often. Scholars have, for example, introduced rough and modified surfaces to reduce turbulent skin friction. One of the technologies proposed in the literature is an array of chevron-shaped protrusions; however, there is no academic consensus on the drag performance of this technique. Furthermore, although the theoretical working mechanism has been documented well, there is no experimental evidence in the literature to support the hypothesis around this mechanism.

In this study, the experiments from the literature are replicated, and new array configurations are tested to characterise the effect of individual parameters on the drag performance. The test plates are manufactured by applying vinyl protrusions of roughly 100 μm in thickness to an aluminium base plate. This thickness corresponds to 5δ_ν - 6δ_ν for the design Reynolds number of Re_τ= 1270. Direct force measurements are performed in the M-tunnel at a Reynolds number range of approximately 630 < Re_τ < 1850 to determine the drag performance. Furthermore, the coherent structures are characterised by means of 2D-2C PIV of a wall-parallel plane at a minimum distance of 17δ_ν from the wall.

The drag reduction reported in the literature could not be replicated, and the balance measurement results offer relevant insights into the drag performance of chevron-shaped protrusions. The results consistently show that the added roughness due to the presence of the protrusions is not the only parameter that determines the drag performance, confirming a meaningful interaction between the protrusions and the flow. Moreover, the results are found to be highly sensitive to the randomisation of the array. No substantial effect of the protrusions on the coherent structures close to the wall has been observed. In particular, no evidence has been observed that supports the working mechanism as proposed in the literature.

An analysis of possible causes to explain the discordance between this study and the literature is performed. Based on this analysis and the results from the aforementioned parametric study, an improved design is proposed, and recommendations for future research are postulated. This technology has inherent benefits for real-world implementations as it can easily be (retro)fitted to aircraft by means of a foil. Further research into this flow control technique is thereby deemed relevant due to the combination of the large drag reduction reported in the literature, the advantages in practical applications, and the novel opportunities for additional investigations.

To make propellers more viable as a greener alternative to jet engines, the issue of propeller noise needs to be addressed. Previous propeller design optimization studies rarely take into account the effects on aerodynamic and aeroacoustic performance due to non-uniform inflow, the type of flow the propeller would experience if it was installed on an aircraft. This study compares propeller designs optimized in uniform flow with propeller designs optimized in an inflow under an angle of attack of 5 degrees, and compares their aerodynamic and aeroacoustic performance when both propellers operate under an angle of attack of 5 degrees. The objective was to investigate whether it makes a difference in terms of performance if the non-uniform inflow is included and accounted for in the optimization routine. It was found that for the non-uniform inflow studied in this work, there was no significant improvement in aerodynamic and aeroacoustic performance when comparing the design optimized in the non-uniform flow with the design optimized in uniform flow.

The fundamental research of cavitation has been a hot topic for decades, while a particle image velocimetry (PIV) measurement on single bubble dynamics is uncommon due to many technical hassles. The involved flow field is small in space and highly unsteady in time, so it challenging the spatial and temporal resolution that current PIV techniques is able to achieve. In present study, single cavitation bubbles of hundreds of micrometers are seeded at near a solid wall by laser techniques. At the mean time a high-speed photography experiment up to 200,000 Hz, a planar PIV experiment combined with shadow method, and the first tomographic PIV experiment in the field, are meticulously designed and implemented. The images taken are firstly calibrated by a high-order anti-distortion algorithm before signal-to-noise ratio enhancement processing. According to the bubble morphology data, two simple formulas are fitted to describe the bubble collapse time and the bubble radius as functions of the wall-stand-off distance. A planar PIV shows the flow fields during the bubble oscillation, which helps to explain the mechanism of the formation of the two types of vortex left by a cavitation bubble. The consistence between the results of planar PIV and the 2D slices of tomographic data as well as the axial symmetry proved by the 3D velocity field data validates the sufficiency of a planar PIV measurement for flow fields induced by a single cavitation bubble. A principle named Proof-of-concept indeed guides the tomographic PIV experiment where the technical difficulties encountered and the resulting data may serve as a valuable starting point for future tomographic PIV experiments in the field of bubble dynamics. A SPCC PIV evaluation algorithm and an AI PIV approach, which are able to achieve a single-pixel-resolution velocity field, are implemented to resolve the wall shear rates exerted by a cavitation bubble. In the case when a bubble does not touch the wall the SPCC algorithm succeeds, while it is effort-consuming and badly influenced by unsteadiness among experiments. In particular, the flow fields are dominated by the many sources of instabilities for the moment long after the bubble generation, in which case the SPCC algorithm cannot deal with the images ensemble from different flow conditions. Although the AI deep learning method in present work is failed in obtaining the correct velocity gradient at the near-wall region, the there is a very bright way out. The governing law NS equations, the physical constrains non-slip conditon, and high accuracy PIV cross-correlation velocity data can be together considered within the loss function of a convolutional neural network. In such way the AI method is supposed to be capable to get the correct cavitation flow field up to single-pixel-resolution.

During the start-up and shutdown sequence of rocket engines, immense forces are generated that are required to propel the payload into earth's orbit or outer space. The bulk of the generated forces are in the form of thrust. However, forces are also generated that act perpendicularly to the thrust direction. These side-loads are unwanted byproducts of the complex flow features present in the nozzle and as such, research has been conducted in the past to address this issue and discover their origin. Conventionally, research into side-loads has been conducted using subscale nozzles that don't deform as the flow passes through them. Due to the rigidity of these models, the nozzles don't deform to any degree that would cause it to interact with the flow. This fluid-structure interaction, present in full-scale rocket nozzles due to their sheer size, is therefore not taken into account when testing on subscale nozzles. To this end, a rocket nozzle testing facility was designed and built to investigate the fluid-structure interaction present in rocket nozzles using flexible subscale rocket nozzles. Due to the laborious nature of designing and manufacturing the testing facility, no working subscale flexible nozzles could be manufactured within the given time.

To continue building upon this previous work, subscale rocket nozzles were poured using various types of polyurethane. To cast these nozzles, multiple mould parts were manufactured. To guarantee a smooth surface finish for the inner nozzle wall, an inner mould was CNC machined. To enable the casting of multiple nozzles with different wall thicknesses, outer moulds were 3D-printed. After having determined the correct variant of polyurethane, Smooth-On's 'Smooth-Cast 66D', three different nozzles were cast on which tests were conducted. The nominal nozzle was designed with a wall thickness of 2mm, with two additional nozzles being cast with a wall thickness of 1.5mm and 2.5mm.

Having manufactured the nozzles, they were mounted to the test facility. The test facility consists of a stand connected to an air tank capable of containing air at pressures of up to 40 bar. Due to pressure losses throughout the valves, the maximum attainable nozzle pressure ratio, or NPR, amounted to slightly over 30. Various tests were performed during which the NPR was either altered during the test, done by manually opening or closing the control valve contained in the system. Further tests were conducted at a number of constant values of NPR at which excessive vibrations occurred to gain further insight into the flow phenomena present at these critical values of NPR. Lip tracking was done as well as schlieren photography. For the lip tracking, correction fluid was applied to the nozzle lip as well as spray painting the outer wall of the nozzle black. The area surrounding the nozzle was made dark such that the predominant feature photographed would be the circular nozzle lip. Once the images were taken, the nozzle lip was discretised in 180 points and was compared to an image of the nozzle taken when the wind tunnel was closed. The end result is a discretised function of 180 points displaying the lip deflection at each azimuthal location. With the help of the discrete Fourier transform, the dominant eigenmodes could be extracted from the nozzle vibrations in the form of their Fourier coefficients. In addition to lip tracking, schlieren images were taken simultaneously with the aim of visualising the flow downstream of the nozzle exit. The brightness of a set of pixels was tracked and their spectral make-up was determined.

Along the entire scale of NPR, two regions were found in which most of the vibrations occurred. The first region of activity, was purported to be linked to the transition of separation regime from free shock separation, FSS, to restricted shock separation, RSS. The second region of NPR, in which the deflection was often even more excessive than the first, is highly likely due to the presence of a recirculation bubble passing over the nozzle lip. Out of the first three eigenmodes, named the breathing, bending and ovalisation mode respectively, the ovalisation mode accounted for the largest proportion of the deflection by far. Based on these transient tests, further tests were conducted at a constant NPR of NPR = 21.7 and NPR = 27.7. These were chosen as all three nozzles showed large deflections at these NPR. For the nominal 2mm nozzle, two additional values were chosen of NPR = 22.7 and NPR = 27.5 to conduct further tests at. The energy present in each of the first three eigenmodes was determined by calculating the variance of the Fourier coefficients. The energy in the modes during the constant NPR runs were compared to the energy in the modes during the transient tests. These were obtained by determining the energy of an NPR-envelop spanning 1 unit of NPR centered around the NPR at which the constant run was performed. By and large, the energy storage in the first three modes remained similar when comparing transient tests with constant NPR runs. The bending mode showed the largest deviation from this trend, mainly when in RSS. Frequency analyses showed the nozzle to oscillate at rates predominantly within 130Hz-400Hz. Wavelet power spectra showed no clear trend between the NPR and the frequency with which the nozzles vibrated. Strouhal calculations further confirmed that NPR would not have a drastic effect on the unsteadiness frequency of the flow. The testing helped provide insight into the mechanical vibrations of nozzles of various wall thicknesses in different flow conditions. However, due to the absence of quantitative flow visualisation and measured flow unsteadiness frequency, no concrete link could be made to the aeroelastic coupling and the fluid's role in the forcing of the vibrations.

An experimental study is presented towards inner-scaled Helmholtz-Resonators (HRs) as a passive turbulent boundary layer (TBL) flow control method. Using acoustic pressure-velocity coupling at the orifice of the HR, it is aimed to attenuate the kinetic energy of the grazing turbulence to reduce the mean skin friction. The HRs are tuned towards the spatial and temporal scales of the near-wall cycle turbulence events that play a major role in the production of turbulence. The adopted HR design strategy has a spatial tuning towards the most efficient attenuation of sweep events, based on recent studies on micro-cavity arrays. The temporal tuning is based on the streamwise wavelength of the most energetic wall-normal velocity fluctuations. These interact with the wall to create pressure fluctuations and form the primary excitation source of the HRs.

A parameter study on HRs under a fully developed grazing TBL at Re_τ ≈ 2200 was performed. A strong pressure-velocity coupling was found between the HR and the grazing TBL when the HR design frequency either matched the frequency of the most energetic wall-normal velocity fluctuations or when the HR design frequency was below this frequency. The pressure-velocity coupling extends to y⁺ ≈ 25, in which phase-interlocking of the grazing TBL occurs. Clear inflow and outflow regions were identified with a streamwise width related to the HR resonance frequency. The inflow and outflow regions cause an increase and decrease in streamwise velocity, respectively. These velocity fluctuations appear as an increase in spectral energy around the wavelengths of the HR resonance and are accompanied by a reduction in spectral energy at higher wavelengths. While quadrant analysis indicates an increase in turbulence production from an increased relative strength of Q2 and Q4 events, variable-interval time averaging indicates no significant changes to the intensity and duration of the near-wall cycle turbulence events as a result of the HR resonance. To date, no significant changes to the mean boundary layer statistics were found that could directly be attributed to the achieved pressure-velocity coupling. Note that only a single HR has been the focus of the current study, with valuable information about HR scaling, resonance and domain-of influence on the TBL flow. Future arrays of HRs may be able to show a more pronounced global effect on the mean flow.

Supersonic flows cause thin panels to flutter, which is characterized by high-amplitude self-sustained oscillations, increasing the risk of fatigue failure. Flutter is known to be exacerbated when a shock wave impinges on the panel, creating a shock wave/boundary-layer interaction (SWBLI) which promotes separation of flow and leads to increased aerodynamic and thermal loading on the panel. This novel fluid-structure interaction (FSI), known as shock-induced panel flutter, poses a risk to the structural integrity of components used in high-speed aerial vehicles, such as rocket nozzles and supersonic engine inlets.

Due to the complex coupled interaction between flow and structural dynamics in flutter, experiments provide a better option to investigate the underlying physical mechanisms, rather than computational studies, which usually have to compromise between computational cost and resultant accuracy. The ST-15 supersonic wind tunnel facility at TU Delft is employed to conduct experimental measurements of shock-induced panel flutter, using high-speed Schlieren imaging to capture flow structures and stereographic Digital Image Correlation (DIC) to record panel displacements in separate campaigns. Tests are done at Mach 2 with fully-clamped thin panels, and the flutter response is excited using different shock strengths and impingement locations.

In both campaigns, accelerometers are used to measure spurious vibrations around the wind tunnel test section. This helps reveal the existence of external vibrations inherent to the facility, which are also found to drive the frequency of the panel flutter: at 760-770 Hz without an impinging shock, and 605-640 Hz with an impinging shock. The latter frequency is detected in both - shock motion and panel displacements – which establishes coupling between flow and structure despite measurements being non-simultaneous. Changing the shock impingement location does not have a significant effect on the degree of flow separation caused over the thin fluttering panel, which is always higher than the separation on a rigid plate at the same shock strength, thus proving that fluttering panels are not viable means of shock-induced separation control. A stronger impinging shock produces increased shock-induced flow separation but results in less energetic panel flutter, which is attributed to the higher post-shock pressure rise suppressing the panel. Flutter is found to be most energetic, and consequently, the panel is most susceptible to fatigue failure, when the shock impinges at 60% of the panel length.

The growing demand for efficient propulsion systems that operate primarily in hover, has led the ducted rotor configuration to emerge as an increasingly popular design solution. One of the major design parameters for the ducted-rotor assembly is the radial distance between the blade tip and the circumferential wall of the duct, referred to as the tip gap. How a variation in the tip gap affects the performance of the ducted-rotor assembly is addressed in this current research work.

The aerodynamic characteristics and performance of the ducted-rotor assembly are experimentally investigated for a parametric variation of the tip gap. The aerodynamic performance of both the rotor and the duct are characterized through force measurements with load cells. These types of measurements reveal that, for typical operating conditions, the duct generates more than half of the total thrust of the assembly. The thrust performance of the duct was also characterized with a detailed mapping of the static pressure distribution over the duct inner-wall, confirming that the thrust-generating mechanism of the duct is a direct consequence of the relatively low pressure over the inlet area of the duct. Even though the diffuser of the duct generates a small amount of pressure drag, its presence is critical for the below-the-rotor pressure recovery and for establishing a high axial throughflow. From detailed flow measurements using the particle image velocimetry method it was furthermore found that the performance deterioration—with an increase of the tip gap—is associated with a contraction of the rotor slipstream in the duct-diffuser, since the less rapid breakdown of the tip vortices reduce the axial momentum near the diffuser wall. A set of preliminary acoustic experiments were also conducted and show that the noise radiated by the system is also strongly dependent on the size of the tip gap.

Over the last few decades, the bypass ratio and nacelle diameter of modern turbofans have been increasing to achieve higher efficiencies. This trend has forced the underwing-mounted engines to be coupled more closely to the wing surface and the flap system, which significantly enhances the low-frequency noise, known as jet installation noise (JIN).

In this project, lobed nozzles are proposed and tested as a passive flow control technique for the reduction of the JIN. Measurements were performed using hot-wire anemometry and a linear array of microphones to investigate the effects of lobed nozzles on the near-field structure of a jet. The lobed nozzles were found to alter the convection velocities of the pressure fluctuations and the transition to turbulence of the jet. Moreover, far-field pressure measurements were performed inside the anechoic wind tunnel to examine the effects of the near-field structure on the perceived far-field noise. These findings can be used to assess the effectiveness of lobed nozzles for the reduction of the JIN in specific jet installation configurations.

The reduction of turbulent skin-friction drag has the potential to generate numerous societal benefits. In large-scale industrial applications, the largest share of energy consumption goes into overcoming the friction drag produced by turbulent boundary layers formed on the outside wall. Estimates assess that over 50% of the total fuel burn is due to overcoming friction drag, highlighting how reducing it would result in large environment and economic savings. In this work, an active technique for friction drag reduction in a turbulent boundary layer is studied in TU Delfts windtunnel laboratory. The flow over a stationary wall is modified by the steady rotation of flush-mounted discs - aiming to mimic a motion of wall-oscillation perpendicular to the flow, a well-documented turbulent drag reduction technique. The effect of the disc motion on turbulent drag is investigated at friction Reynolds numbers of 912 and 1483. The chosen rotational velocity of the discs, the disc diameter and disc spacing represent the optima defined in literature, and a further parameter exploration was conducted around these optima. To assess the performance of the disc array, high resolution planar PIV is employed to measure skin-friction drag reductions at the wall, as well as to quantify second order statistics and other phenomena within the turbulent boundary layer. The results reveal drag reductions in the order of 50%, with Reynolds stresses and peaks of turbulent kinetic energy production decreasing by similar amounts. Analysis of turbulent velocity fluctuations reveals that the prevalence of turbulence producing events is significantly decreased close to the wall, and two-point correlations of vorticity contours show that vortical events are remarkably hindered by the disc rotation. Further analysis of a wall-parallel plane reveals that the mean velocity increases over the discs and decreases in between. This suggest that the detrimental effects of such a disc array occur at spanwise locations between each disc row, in accordance with literature. By using an assumption of outer flow similarity, the performance of the array is then quantified as being 10% less than the local optimum in the disc center.

The turbulent boundary layer development under the influence of an air cavity is studied experimentally using planar PIV, with the aim of gaining insight and building upon the flow physics typically encountered in the application of air layer drag reduction. A detection technique based on correlation values is implemented to obtain an approximate shape of the air cavity and the location of the air-water interface. The technique was successful in identifying the maximum cavity thickness with sufficient accuracy. The leading and trailing edges of the cavity however, were harder to identify, the former owing to a limitation of the developed technique and the latter due to the dynamic nature of the flow and a slightly limited FOV. The ratio of the initial boundary layer thickness to the maximum thickness of the air cavity is 6.7, and as a consequence the boundary layer did not separate at the leeward side of the air cavity. The turbulent boundary layer is observed to feel the presence of the air cavity up to 8.5-9.5 cm upstream due to an adverse pressure gradient. Alternating streamwise pressure gradients are generated due to the curvature of the air cavity: from an adverse to favourable and back to adverse. Compared to solid bump studies in literature, additional perturbations due to a free-slip boundary condition and the unstable nature of air cavity increase the complexity of the current flow. The mean velocity profile and stresses are able to capture the effects of alternating streamwise pressure gradients and air injection, with variations mostly restricted to the inner region. Effects of streamline curvature in the outer region are found to be minimal, while potential effects of the free-slip condition were much harder to identify separately and further research would be needed to appropriately assess them. The mean velocity profile is found to deviate from the classic logarithmic behaviour at the apex of the air cavity, although the flow does not seem to relaminarise. Quadrant analysis shows differences in Reynolds stress producing events compared to the baseline turbulent boundary layer case hinting at possible alteration to coherent structuring of the turbulent boundary layer developing below the air cavity.

This research studies the fluid-structure interaction (FSI) of compliant surfaces in air flows, with an objective of finding possible turbulent viscous drag reduction. A compliant surface is a thin layer of viscoelastic material drawing inspiration from dolphin epidermis, which was thought to have drag-reducing capabilities by Gray (1936). Research into the drag reduction capabilities of compliant surfaces has been long going for more than five decades, starting with Kramer (1960), yet no firm conclusion has been reached. In addition, most of the experimental research has been focused on water flows, with air flows regarded as incapable of inertially forcing a compliant surface to deform. This research attempts to disprove this assumption by applying the proper inertial scaling to the FSI between the compliant surface and air flows. Compliant surfaces are characterised by the stiffness and thickness, which are presumed to be of first-order influence on the FSI. A parameter sweep of these compliant surface properties was conducted by drag delta measurements and flow visualisation by planar particle image velocimetry (PIV) in the M-Tunnel at the Low Speed Laboratory (LSL) Drag delta results confirm the possibility of turbulent viscous drag reduction by compliant surfaces, with a measured drag delta of -3.43 %. This result is further supported by the decrease in 1D turbulence intensity at the test plate's trailing edge from hot-wire measurements, and a smaller decay of the shape factor H from PIV. Quadrant analysis of the PIV data found evidence of a reduction in combined Q2 and Q4 events, further supporting the drag delta measured. Correlation between the compliant surface's viscoelastic properties and the drag delta found a negative correlation between the magnitude of complex shear modulus and a positive correlation between the loss tangent and drag reduction. The high loss tangent for the drag-reducing compliant surface and its subtle positive correlation with the drag reduction indicates that the viscoelasticity might have a greater influence on the FSI than expected.