Browsing by Author "Borgoltz, Aurelien"

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    Advanced Boundary Simulations of an Aeroacoustic and Aerodynamic Wind Tunnel
    Szőke, Máté; Devenport, William J.; Borgoltz, Aurelien; Roy, Christopher J.; Lowe, K. Todd (2021-05-25)
    This study presents the first 3D two-way coupled fluid structure interaction (FSI) simulation of a hybrid anechoic wind tunnel (HAWT) test section with modeling all important effects, such as turbulence, Kevlar wall porosity and deflection, and reveals for the first time the complete 3D flow structure associated with a lifting model placed into a HAWT. The Kevlar deflections are captured using finite element analysis (FEA) with shell elements operated under a membrane condition. Three-dimensional RANS CFD simulations are used to resolve the flow field. Aerodynamic experimental results are available and are compared against the FSI results. Quantitatively, the pressure coefficients on the airfoil are in good agreement with experimental results. The lift coefficient was slightly underpredicted while the drag was overpredicted by the CFD simulations. The flow structure downstream of the airfoil showed good agreement with the experiments, particularly over the wind tunnel walls where the Kevlar windows interact with the flow field. A discrepancy between previous experimental observations and juncture flow-induced vortices at the ends of the airfoil is found to stem from the limited ability of turbulence models. The qualitative behavior of the flow, including airfoil pressures and cross-sectional flow structure is well captured in the CFD. From the structural side, the behavior of the Kevlar windows and the flow developing over them is closely related to the aerodynamic pressure field induced by the airfoil. The Kevlar displacement and the transpiration velocity across the material is dominated by flow blockage effects, generated aerodynamic lift, and the wake of the airfoil. The airfoil wake increases the Kevlar window displacement, which was previously not resolved by two-dimensional panel-method simulations. The static pressure distribution over the Kevlar windows is symmetrical about the tunnel mid-height, confirming a dominantly two-dimensional flow field.
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    Advanced Instrumentation and Measurements Techniques for Near Surface Flows
    Cadel, Daniel R. (Virginia Tech, 2016-09-20)
    The development of aerodynamic boundary layers on wind turbine blades is an important consideration in their performance. It can be quite challenging to replicate full scale conditions in laboratory experiments, and advanced diagnostics become valuable in providing data not available from traditional means. A new variant of Doppler global velocimetry (DGV) known as cross-correlation DGV is developed to measure boundary layer profiles on a wind turbine blade airfoil in the large scale Virginia Tech Stability Wind Tunnel. The instrument provides mean velocity vectors with reduced sensitivity to external conditions, a velocity measurement range from 0ms^-1 to over 3000ms^-1, and an absolute uncertainty. Monte Carlo simulations with synthetic signals reveal that the processing routine approaches the Cramér-Rao lower bound in optimized conditions. A custom probe-beam technique is implanted to eliminate laser flare for measuring boundary layer profiles on a DU96-W-180 wind turbine airfoil model. Agreement is seen with laser Doppler velocimetry data within the uncertainty estimated for the DGV profile. Lessons learned from the near-wall flow diagnostics development were applied to a novel benchmark model problem incorporating the relevant physical mechanisms of the high amplitude periodic turbulent flow experienced by turbine blades in the field. The model problem is developed for experimentally motivated computational model development. A circular cylinder generates a periodic turbulent wake, in which a NACA 63215b airfoil with a chord Reynolds number Re_c = 170, 000 is embedded for a reduced frequency k = (pi)fc/V = 1.53. Measurements are performed with particle image velocimetry on the airfoil suction side and in highly magnified planes within the boundary layer. Outside of the viscous region, the Reynolds stress profile is consistent with the prediction of Rapid Distortion Theory (RDT), confirming that the redistribution of normal stresses is an inviscid effect. The fluctuating component of the phase- averaged turbulent boundary layer profiles is described using the exact solution to laminar Stokes flow. A phase lag similar to that in laminar flow is observed with an additional constant phase layer in the buffer region. The phase lag is relevant for modeling the intermittent transition and separation expected at full scale.
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    Aerodynamic Effects of Painted Surface Roughness on Wind Turbine Blade Performance
    Joseph, Liselle AnnMarie; Borgoltz, Aurelien; Kuester, Matthew; Devenport, William J.; Fenouil, Julien (Virginia Tech, 2015-06-09)
    This paper briefly examines the aerodynamic effects of typical wind turbine blade roughness by investigating an appropriate scaling criteria which best relates the roughness configuration to the resulting changes in aerodynamic forces and transition. The wind tunnel test results of two wind turbine blade sections tested with three different roughness samples are presented. The two models, consisting of a 457mm-chord and 800mm-chord airfoils using the DU96-W-180 profile, were tested in the Virginia Tech Stability Wind Tunnel at free-stream Reynolds number based on the chord between 1.5 and 3M. Preliminary analysis of the lift and drag scaling are presented as well as a sample of the transition results.
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    Aerodynamic Validation of Wind Turbine Airfoil Models in the Virginia Tech Stability Wind Tunnel
    Kuester, Matthew; Brown, Kenneth; Meyers, Timothy; Intaratep, Nanyaporn; Borgoltz, Aurelien; Devenport, William J. (Virginia Tech, 2015-06-09)
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    Anomaly Detection in Aeroacoustic Wind Tunnel Experiments
    Defreitas, Aaron Chad (Virginia Tech, 2021-10-27)
    Wind tunnel experiments often employ a wide variety and large number of sensor systems. Anomalous measurements occurring without the knowledge of the researcher can be devastating to the success of costly experiments; therefore, anomaly detection is of great interest to the wind tunnel community. Currently, anomaly detection in wind tunnel data is a manual procedure. A researcher will analyze the quality of measurements, such as monitoring for pressure measurements outside of an expected range or additional variability in a time averaged quantity. More commonly, the raw data must be fully processed to obtain near-final results during the experiment for an effective review. Rapid anomaly detection methods are desired to ensure the quality of a measurement and reduce the load on the researcher. While there are many effective methodologies for anomaly detection used throughout the wider engineering research community, they have not been demonstrated in wind tunnel experiments. Wind tunnel experimentation is unique in the sense that many repeat measurements are not typical. Typically, this will only occur if an anomaly has been identified. Since most anomaly detection methodologies rely on well-resolved knowledge of a measurement to uncover the expected uncertainties, they can be difficult to apply in the wind tunnel setting. First, the analysis will focus on pressure measurements around an airfoil and its wake. Principal component analysis (PCA) will be used to build a measurement expectation by linear estimation. A covariance matrix will be constructed from experimental data to be used in the PCA-scheme. This covariance matrix represents both the strong deterministic relations dependent on experimental configuration as well as random uncertainty. Through principles of ideal flow, a method to normalize geometrical changes to improve measurement expectations will be demonstrated. Measurements from a microphone array, another common system employed in aeroacoustic wind tunnels, will be analyzed similarly through evaluation of the cross-spectral matrix of microphone data, with minimal repeat measurements. A spectral projection method will be proposed that identifies unexpected acoustic source distributions. Analysis of good and anomalous measurements show this methodology is effective. Finally, machine learning technique will be investigated for an experimental situation where repeat measurements of a known event are readily available. A convolutional neural network for feature detection will be shown in the context of audio detection. This dissertation presents techniques for anomaly detection in sensor systems commonly used in wind tunnel experiments. The presented work suggests that these anomaly identification techniques can be easily introduced into aeroacoustic experiment methodology, minimizing tunnel down time, and reducing cost.
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    Design and Analysis of a Deterministic Disturbance Generator
    Palanganda, Shaheen Thimmaiah (Virginia Tech, 2023-08-30)
    This thesis introduces the Deterministic Disturbance Generator (DDG) and its development process. The DDG performs two motions and five pitch rates. The flap motion, which rotates the airfoil from 0◦ to 20◦ and back, and the ramp motion, which rotates it from 0◦ to 20◦ with a dwell of 1s before returning to 0◦. To determine the angle of attack, a Matlab function converted thrust rod displacement into the assumed angle, validated against true angle of attack measurements on the DDG. Mean angular displacements were plotted, and standard deviations of the 95% confidence intervals were calculated within ±1.3◦ for all motions. The mechanical force on the actuator was computed to be 77N. Aerodynamic forces on the DDG were determined to be 15N and 19N for flap and ramp motions respectively. The total force on the system did not exceed 100N in any case, staying below the peak force capacity, while acceleration reached its limit. Flow velocimetry in the Virginia Tech Stability Wind Tunnel (VTSWT) employed a time-resolved Particle Image Velocimetry (PIV) to study the effects of 20◦ flap and ramp motions, with mean actuation times of 63ms and 37ms. Flap motion showed a significant deficit in mean streamwise velocities, and the ramp motion exhibited similar behavior until its dwell position, generating a large wake region due to airfoil stall after its peak. Comparison of data from the Goodwin Hall Subsonic Tunnel (GHST) with VTSWT data for overlapping domains revealed similar flow field features when normalized based on the boundary layer velocity (43mm plane from wall) of the latter. Considering actuation time differences, the freestream normalized GHST data was combined with VTSWT data. The cohesive PIV domain offered a broader perspective on the missing flow features.
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    The Development of Remote Laboratory Sessions at the Stability Wind Tunnel of Virginia Tech During the Coronavirus Pandemic
    Szőke, Máté; Borgoltz, Aurelien; Kuester, Matthew; Intaratep, Nanyaporn; Devenport, William J.; Katz, Andrew (2021-01-01)
    This paper discusses the remote delivery of wind tunnel experiments performed at the Stability Wind Tunnel of Virginia Tech, in April 2020, during the early stages of the coronavirus pandemic. The originally in-person laboratories were transformed to entirely remote sessions, on a time-frame of a few weeks, to ensure the delivery of the laboratory sessions and the safety of all participants via social distancing and the use of widely-available video conferencing software. The paper outlines the structure of the laboratory sessions, comprising the tour of the facility, data acquisition, and data visualization alongside with all information technology components used to ensure the successful remote delivery of the laboratory sessions. After the two-week-long experimental campaign, participating students provided feedback on the efficacy of the laboratories via a detailed questionnaire. It was found that the students were highly satisfied with the remote delivery of the laboratory sessions but showed a preference for in-person laboratories.
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    Direct Assessment and Investigation of Nonlinear and Nonlocal Turbulent Constitutive Relations in Three-Dimensional Boundary Layer Flow
    Gargiulo, Aldo (Virginia Tech, 2023-07-12)
    Three-dimensional (3D) turbulent boundary layers (TBLs) play a crucial role in determining the aerodynamic properties of most aero-mechanical devices. However, accurately predicting these flows remains a challenge due to the complex nonlinear and nonlocal physics involved, which makes it difficult to develop universally applicable models. This limitation is particularly significant as the industry increasingly relies on simulations to make decisions in high-consequence environments, such as the certification or aircraft, and high-fidelity simulation methods that don't rely on modeling are prohibitively expensive. To address this challenge, it is essential to gain a better understanding of the physics underlying 3D TBLs. This research aims to improve the predictive accuracy of turbulence models in 3D TBLs by examining the impact of model assumptions underpinning turbulent constitutive relations, which are fundamental building blocks of every turbulence model. Specifically, the study focuses on the relevance and necessity of nonlinear and nonlocal model assumptions for accurately predicting 3D TBLs. The study considers the attached 3D boundary layer flow over the textbf{Be}nchmark textbf{V}alidation textbf{E}xperiment for textbf{R}ANS/textbf{L}ES textbf{I}nvestiagtions (BeVERLI) Hill as a test case and corresponding particle image velocimetry data for the investigation. In a first step, the BeVERLI Hill experiment is comprehensively described, and the important characteristics of the flow over the BeVERLI Hill are elucidated, including complex symmetry breaking characteristics of this flow. Reynolds-averaged Navier-Stokes simulations of the case using standard eddy viscosity models are then presented to establish the baseline behavior of local and linear constitutive relations, i.e., the standard Boussinesq approximation. The tested eddy viscosity models fail in the highly accelerated hill top region of the BeVERLI hill and near separation. In a further step, several nonlinear and nonlocal turbulent constitutive relations, including the QCR model, the model by Gatski and Speziale, and the difference-quotient model by Egolf are used as metrics to gauge the impact of nonlinearities and nonlocalities for the modeling of 3D TBLs. It is shown that nonlinear and nonlocal approaches are essential for effective 3D TBL modeling. However, simplified reduced-order models could accurately predict 3D TBLs without high computational costs. A constitutive relation with local second-order nonlinear mean strain relations and simplified nonlocal terms may provide such a minimal model. In a final step, the structure and response of non-equilibrium turbulence to continuous straining are studied to reveal new scaling laws and structural models.
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    The Effects of Curvature on Turbulent Boundary Layers Over a 3D Bump Geometry: An Experimental Study Using BeVERLI Hill
    Chen, Fangzhou (Virginia Tech, 2025-01-23)
    This thesis presents an experimental investigation of the effects of curvature on turbulent boundary layers using the Benchmark Validation Experiment for RANS and LES Investigations (BEVERLI) Hill setup. The study focuses on analyzing the flow behavior over a three-dimensional bump geometry that incorporates both concave and convex surfaces, with the aim of improving the understanding of the complex interactions among curvature, pressure gradients, and turbulence characteristics. The study examines the mean velocity, Reynolds shear stresses, pressure gradient, turbulence intensity, and pressure coefficient variations in relation to the bump curvature. The results are consistent with prior studies on the destabilizing influence of concave curvature with observations such as increased turbulence intensity, a decrease in mean velocity relative to the free-stream velocity U∞, and higher Reynolds stresses normalized by U2∞ throughout entire turbulent boundary layer, particularly in the near-wall region. Convex curvature results are consistent with prior study as well, which exhibits a stabilizing effect, shown to reduce turbulence intensity, an increase mean velocity relative to the free-stream velocity U∞, and lower Reynolds stresses normalized by U2∞ throughout entire boundary layer. This study also highlights the influence of pressure gradient effect, which acts with the curvature effect, impacts the boundary layer stability. This interaction is observed in amplification of turbulence with increasing of turbulence intensity and boundary layer growth. This stability particularly reflects on the embedded shear layers with inflection points which can create conditions for linear instabilities to grow, thus enhancing coherent turbulent motions. Furthermore, the thesis discusses the challenges in separating the influence of curvature from pressure gradient effects in current model, and proposes future research directions to address this issue. By conducting experiments under controlled pressure gradient flow conditions over concave and convex curvature, researchers can analyze the contributions of curvature effect separately from pressure gradient effect. Alternatively, using a hybrid RANS-LES model, will lead to a more precise understanding of flow dynamics over curved surfaces.
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    The Effects of Upstream Boundary Layers on the NGV Endwall Cooling
    Mao, Shuo (Virginia Tech, 2022-06-03)
    Modern gas turbine designs' ever-increasing turbine inlet temperature raises challenges for the nozzle guide vane cooling. Two typical endwall cooling schemes, jump cooling and louver cooling, result in different interactions between the injected coolant and the mainstream, leading to different cooling effects. This study investigates these two cooling schemes on the endwall cooling experimentally and numerically. Wind tunnel tests and the CFD simulations are carried out with engine-representative conditions of an exit Mach number of 0.85, an exit Reynolds number of 1.5×10^6, and an inlet Turbulence intensity of 16%. The jump cooling scheme experiments investigate two blowing ratios, 2.5 and 3.5, two density ratios, 1.2 and 1.95, and three endwall profiles with different NGV-turbine alignments. Four coolant mass flow ratios from 1.0% to 4.0% are tested for the louver cooling. The results show that the cavity vortex, the horseshoe vortex, and the passage vortex are the main factors that prevent the upstream coolant from reaching the NGV passage. The jump cooling scheme generally provides high momentum to the cooling jets. As a result, the coolant at the design case density ratio of 1.95 and blowing ratio of 2.5 is sufficiently energized to penetrate the horseshoe vortex. It then forms a relatively uniform coolant film near the NGV passage inlet, leading to a minimum adiabatic cooling effectiveness of 0.4 throughout the passage. Reducing the coolant density or increasing the blowing ratio leads to higher coolant momentum, so the coolant jets can further suppress the horseshoe vortex. However, high momentum may cause coolant lift-off, mitigating the coolant reattachment. Therefore, the density ratio needs to be carefully balanced with the blowing ratio to optimize the cooling effect. This balance is also affected by the combustor-NGV misalignment, as a higher step height requires higher coolant momentum to overcome the step-induced vortices. On the contrary, the louver cooling scheme provides less momentum to the coolant. The results showed that only by exceeding a coolant mass flow rate of 1~2% can the coolant form a uniform film which provides good coverage upstream of the NGV passage inlet. As for the cooling of the NGV passage, the mass flow rate ratio of the range investigated is not sufficient for desirable cooling performance. The pressure side endwall proves most difficult for the coolant to reach. In addition, the fishmouth cavity at the combustor-NGV passage causes a three-dimensional cavity vortex that transports the coolant in the pitch-wise direction. Moreover, the coolant transport pattern is dependent on the coolant blow rate. Overall, the more-energized coolant film generated by the jump cooling tends to survive longer, but it is also more prone to lift-off. At the same time, the less-energized coolant film caused by the louver cooling is more susceptible to vortices and the discontinuity of the endwall geometry. However, it develops faster, especially in the lateral direction. The two schemes could be applied simultaneously for an ideal cooling system. The jump cooling can provide enough momentum for the coolant to persist in the NGV passage. Meanwhile, the louver cooling covers the upstream region before the jump cooling coolant reattaches to the endwall.
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    Fluid Dynamics of Inlet Swirl Distortions for Turbofan Engine Research
    Guimaraes Bucalo, Tamara (Virginia Tech, 2018-04-25)
    Significant effort in the current technological development of aircraft is aimed at improving engine efficiency, while reducing fuel burn, emissions, and noise levels. One way to achieve these is to better integrate airframe and propulsion system. Tighter integration, however, may also cause adverse effects to the flow entering the engines, such as total pressure, total temperature, and swirl distortions. Swirl distortions are angular non-uniformities in the flow that may alter the functioning of specific components of the turbomachinery systems. To investigate the physics involved in the ingestion of swirl, pre-determined swirl distortion profiles were generated through the StreamVane method in a low-speed wind tunnel and in a full-scale turbofan research engine. Stereoscopic particle image velocimetry (PIV) was used to collect three-component velocity fields at discrete planes downstream of the generation of the distortions with two main objectives in mind: identifying the physics behind the axial development of the distorted flow; and describing the generation of the distortion by the StreamVane and its impact to the flow as a distortion generating device. Analyses of the mean velocity, velocity gradients, and Reynolds stress tensor components in these flows provided significant insight into the driving physics. Comparisons between small-scale and full-scale results showed that swirl distortions are Mach number independent in the subsonic regime. Reynolds number independence was also verified for the studied cases. The mean secondary flow and flow angle profiles demonstrated that the axial development of swirl distortions is highly driven by two-dimensional vortex dynamics, when the flow is isolated from fan effects. As the engine fan is approached, the vortices are axially stretched and stabilized by the acceleration of the flow. The flow is highly turbulent immediately downstream of the StreamVane due to the presence of the device, but that vane-induced turbulence mixes with axial distance, so that the device effects are attenuated for distances greater than a diameter downstream, which is further confirmed by the turbulent length scales of the flow. These results provide valuable insight into the generation and development of swirl distortion for ground-testing environments, and establishes PIV as a robust tool for engine inlet investigations.
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    Generation of Downstream Vorticity Through the Use of Modified Trailing Edge Configurations
    Worrall, Benjamin Nida (Virginia Tech, 2010-05-05)
    Detailed measurements were taken downstream of several modified trailing edge configurations designed to impart streamwise velocity into the flow behind a cascade of GE Rotor B fan blades. These measurements were conducted in the Virginia Tech Low Speed Linear Cascade wind tunnel. The trailing edge configurations tested utilized passive techniques for producing streamwise vorticity, which in turn causes downstream wake diffusion and increased mixing. A more diffuse wake, when it impinges on the downstream stator, will produce lower noise levels as a result of this rotor-stator interaction. Furthermore, increased mixing in the flow will reduce the levels of turbulence kinetic energy observed downstream of the blade trailing edge. Thus, this project seeks to identify which passive techniques of imparting streamwise vorticity are most effective at improving the flow characteristics responsible for some of the noise production in modern jet aircraft. The three trailing edge configurations tested in detail for this project showed significant ability to widen and stretch the downstream wake by utilizing vorticity generation techniques. The TE-8 configuration was the most effective at increasing the wake width downstream of the trailing edge. Additionally, each configuration was able to successfully reduce some of the turbulence kinetic energy levels observed downstream when compared to the baseline blade, the most effective configuration being TE-8. Finally, the momentum thickness of each configuration was measured. When compared to the baseline, the TE-1 configuration showed an increased momentum thickness, TE-8 showed little change, and TE-7 actually showed an improved momentum thickness value.
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    Laser Displacement Sensors for Wind Tunnel Model Position Measurements
    Kuester, Matthew; Intaratep, Nanyaporn; Borgoltz, Aurelien (MDPI, 2018-11-22)
    Wind tunnel measurements of two-dimensional wing sections, or airfoils, are the building block of aerodynamic predictions for many aerodynamic applications. In these experiments, the forces and pitching moment on the airfoil are measured as a function of the orientation of the airfoil relative to the incoming airflow. Small changes in this angle (called the angle of attack, or α ) can create significant changes in the forces and moments, so accurately measuring the angle of attack is critical in these experiments. This work describes the implementation of laser displacement sensors in a wind tunnel; the sensors measured the distance between the wind tunnel walls and the airfoil, which was then used to calculate the model position. The uncertainty in the measured laser distances, based on the sensor resolution and temperature drift, is comparable to the uncertainty in traditional linear encoder measurements. Distances from multiple sensors showed small, but statistically significant, amounts of model deflection and rotation that would otherwise not have been detected, allowing for an improved angle of attack measurement.
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    Modifications of Coherent Structures in Fan Blade Wakes for Broadband Noise Reduction
    Borgoltz, Aurelien (Virginia Tech, 2007-10-29)
    The effects of trailing edge flow control on the wakes of a linear cascade of idealized fan blades was investigated experiments with a view to the likely effects on broadband aircraft engine interaction noise. Single and three-component hotwire velocity measurements were made downstream of the cascade for a chord Reynolds number of 390,000 and a Mach number of 0.07. Measurements of the two-point velocity correlation were used extensively to evaluate the impact of various flow control strategies on the organization of the coherent structures of the wakes and their potential to generate noise. A baseline flow was established by measuring the wake downstream of unmodified GE-Rotor-B blades. Four sets of serrated trailing edge blades (with two different serration sizes and with two trailing edge cambers) and three sets of blades with trailing edge blowing (a simple rectangular slot, rectangular slot with Kuethe-vane vortex generators, and rectangular slot with serrated lips) were tested. The serrated trailing edges introduce corrugations into the wake, increase the wake decay and width as well as turbulence levels (possibly because of the blunt trailing edge created at the serration valley). The serrated trailing edges also increase the turbulence scales in the direction perpendicular to the plane of the wake because of the injection of streamwise vorticity. In almost all cases the serrations reduce the spanwise and streamwise turbulence scales. Serrations do not, however, affect the apparent time scale of quasi-periodic structures in the wake, and this appears to limit the potential of this trailing edge treatment to reduce broadband noise. The analysis of the characteristic eddies (obtained from proper orthogonal decomposition combined with linear estimation) revealed that the serrations do not change the qualitative form of the eddies. Trailing edge blowing was found to significantly decrease the wake deficit and width as well as the turbulence levels at all blowing rates. Blowing through the simple rectangular slot, at mass flow rates between 1.4 and 2.0% of the total passage through flow, was shown to significantly affect the size, the organization and the strength of the coherent structures. For small blowing rates the strong spanwise eddies near the trailing edge actually appear to be enhanced. For larger blowing rates, however, the turbulent scales are reduced in all directions. The addition of Kuethe vanes on the suction side of the blowing blade results in a low momentum region just downstream of the vanes that may result from flow separation there. This further enhances the shedding and increases the blowing rate needed to overcome it. The serrated blowing blades show the greatest potential to reduce broadband noise as they reduce the turbulence levels and scales without creating potentially detrimental structures. While no acoustic measurements were made, analysis of hypothetical perpendicular and parallel interactions of blades with these wakes has made possible to characterize for the first time the impact of the changes in the eddy structure of these wakes on their potential to generate broadband noise. The serrated trailing edges (especially the larger serrations) actually increase the potential of the wake to generate broadband noise (a direct consequence in the overall increase in turbulence scale and intensity). In contrast, every trailing edge blowing configuration was found to produce large reductions in the potential noise (a maximum of 6dB reduction was obtained at 2.0% blowing). The addition of Kuethe vanes on the suction side of the blowing blades significantly reduced the efficiency of the simple blowing configuration (a result of the increased coherency associated with the shedding of streamwise vorticity by the vanes). The serrated blowing configuration was found to yield reductions similar to the simple blowing configuration.
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    Pressure Fluctuations in a High-Reynolds-Number Turbulent Boundary Layer over Rough Surfaces of Different Configurations
    Joseph, Liselle AnnMarie (Virginia Tech, 2017-10-12)
    The pressure fluctuations under a high Reynolds Number, rough-wall, turbulent, boundary layer have been studied in the Virginia Tech Stability Wind Tunnel. Rough surfaces of varying element height (1-mm, 3-mm), shape (hemispheres, cylinders) and spacing (5.5-mm, 10.4-mm, 16.5-mm) were investigated in order to ascertain how the turbulent pressure fluctuations change with changes in roughness geometry. Rough surfaces which contain two types of elements are investigated and relationships between the combination surface and the individual surfaces have been uncovered. Measurements of the wall pressure fluctuations were made using pinhole microphones and hotwire measurements were made to obtain the velocity and turbulence field. Among the principal findings is the development of two scaling laws for the low frequency pressure fluctuations. Both of these are based on the idea that the defect between the edge velocity and some local boundary layer velocity sustains the pressure fluctuations in the outer regions of the flow. The first scaling uses the broadband convection velocity as the local velocity of the large scale pressure fluctuations. The second scaling uses the mean boundary layer velocity. Both these scalings appear more robust than the previously proposed scalings for the low frequency region and are able to scale the pressure spectra of all the data to within 3.5-dB. In addition, it was proven that the high frequency shear friction velocity scaling of Meyers et al. (2015) is universal to rough surfaces of different element shape and density. Physical insights into the shear friction velocity, on which this scaling is based, have been revealed. This includes an empirical formula which estimates the element pressure drag coefficient from the roughness density and the Reynolds number. The slopes in the mid-frequency region were found to vary with element density and microphone location such that a useful scaling could not be determined for this region. The possibility of an overlap region is explored and the expectation of a -1 slope is disproved. It is hypothesised that an evanescent decay of the mid-frequency pressure fluctuations occurs between their actual location and the wall where they are measured. A method for accounting for this decay is presented in order to scale the pressure fluctuations in this region. Lastly, a piecewise interpolation function for the pressure spectrum of rough wall turbulent boundary layers was proposed. This analytical function is based on the low frequency scaling on mean velocity and the high frequency scaling of Meyers et al. (2015) The mid-frequency is estimated by a spline interpolation between these two regions.
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    Prototype Development and Feasibility Assessment of a Vertically Mounted Floating Element Skin Friction Balance
    Raza, Muhammad (Virginia Tech, 2025-01-23)
    Wall shear stress is one of the most essential scaling parameters used in fluid dynamics. It is significant because it helps us compare results in different experimental studies. The accurate measurements of wall shear stress will be instrumental in improving the existing empirical models and validating CFD models. Wall shear stress is also vital in improving fuel efficiency, heat transfer efficiency, and aerodynamic efficiency in real-world applications. This work discusses the design and implementation of a prototype floating element balance — a direct method of wall shear measurement. The direct measurement methods are robust and can significantly improve the validity of experimentation when perfected. In this work, a prototype floating element balance is designed and developed to estimate the wall shear stress in a smooth wall pilot facility to assess its feasibility for large-scale development. The floating element balance utilizes a strain gauge to estimate the wall shear stress. The preliminary tests show promising results, revealing potential design improvements. A strain measurement study is conducted to investigate the force-strain relationship and the reliability of the balance, which highlights the long-term stability and consistency in the strain measurement. However, further investigations are required into the drift response of the floating element balance. The strain measurements are also employed to calibrate the balance using a linear curve fit with a coefficient of determination of R^2 = 0.99, indicating a satisfactory linear estimation.
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    Sensitivity of Wind Turbine Airfoil Sections to Geometry Variations Inherent in Modular Blades
    Brown, Kenneth; Molinaro, Nick; Meyers, Timothy; Borgoltz, Aurelien; Devenport, William J.; Luedke, Jonathan; Pesetsky, David (Virginia Tech, 2015-06-09)
    In the ongoing work to increase the efficiency of large-scale, horizontal-axis wind turbines, the modular blade concept has been proposed. The aerodynamic performance of modular blades, whose baseline profile remains unchanged from conventional blades but who are susceptible to a larger degree of variation in both manufacturing tolerances and fabrication materials, is yet unknown. This paper works towards quantifying the aerodynamic effects of variations to a baseline wind turbine section, specifically, examining the effects of offsets in the leading edge of the profile and the use of a tensioned fabric as a flow surface over the aft of the profile. Wind tunnel tests were performed on a modified DU91-W2-250 section with an offset in the leading edge and cavities in the aft that were alternatively fitted with fabric-covered panels and rigid aluminum panels. Measurements included lift, drag, airfoil surface pressures, and surface deflection of the fabric material. Preliminary results show the modifications have a noticeable impact on the aerodynamics of the section, including altered surface pressure distributions and wake characteristics.
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    Stereoscopic Particle Image Velocimetry Measurements of Swirl Distortion on a Full-Scale Turbofan Engine Inlet
    Nelson, Michael Allan (Virginia Tech, 2014-10-08)
    There is a present need for simulation and measuring the inlet swirl distortion generated by airframe/engine system interactions to identify potential degradation in fan performance and operability in a full-scale, ground testing environment. Efforts are described to address this need by developing and characterizing methods for complex, prescribed distortion patterns. A relevant inlet swirl distortion profile that mimics boundary layer ingesting inlets was generated by a novel new method, dubbed the StreamVane method, and measured in a sub scale tunnel using stereoscopic particle image velocimetry (SPIV) as a precursor for swirl distortion generation and characterization in an operating turbofan research engine. Diagnostic development efforts for the distortion measurements within the research engine paralleled the StreamVane characterization. The system used for research engine PIV measurements is described. Data was obtained in the wake of a total pressure distortion screen for engine conditions at idle and 80% corrected fan speed, and of full-scale StreamVane screen at 50% corrected fan speed. The StreamVane screen was designed to generate a swirl distortion that is representative for hybrid wing body applications and was made of Ultem*9085 using additive manufacturing. Additional improvements to the StreamVane method are also described. Data reduction algorithms are put forth to reduce spurious velocity vectors. Uncertainty estimations specific to the inlet distortion test rig, including bias error due to mechanical vibration, are made. Results indicate that the methods develop may be used to both generate and characterize complex distortion profiles at the aerodynamic interface plane, providing new information about airframe/engine integration.
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    A Study of Aerodynamics in Kevlar-Wall Test Sections
    Brown, Kenneth Alexander (Virginia Tech, 2014-07-03)
    This study is undertaken to characterize the aerodynamic behavior of Kevlar-wall test sections and specifically those containing two-dimensional, lifting models. The performance of the Kevlar-wall test section can be evaluated against the standard of the hard-wall test section, which in the case of the Stability Wind Tunnel (SWT) at Virginia Tech can be alternately installed or replaced by the Kevlar-wall test section. As a first step towards the evaluation of the Kevlar-wall test section aerodynamics, a validation of the hard-wall test section at the SWT is performed, in part by comparing data from NACA 0012 airfoil sections tested at the SWT with those tested at several other reliable facilities. The hard-wall test section showing good merit, back-to-back tests with three different airfoils are carried out in the SWT's hard-wall and Kevlar-wall test sections. Kevlar-wall data is corrected for wall interference with a panel method simulation that simulates the unique boundary conditions of Kevlar-wall test sections including the Kevlar porosity, wall deflection, and presence of the anechoic chambers on either side of the walls. Novel measurements of the boundary conditions are made during the Kevlar-wall tests to validate the panel method simulation. Finally, sensitivity studies on the input parameters of the panel method simulation are conducted. The work included in this study encompasses a wide range of issues related to Kevlar-wall as well as hard-wall tunnels and brings to light many details of the performance of such test sections.
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    The Testing and Verification of a Nanomembrane Based Pressure Sensor for Small-Scale Underwater Pressure Measurements
    Talaksi, Omar (Virginia Tech, 2023-07-06)
    A MEMS piezoresistive pressure sensor provides a low-cost and accurate means of detecting and quantifying small-scale disturbances in underwater environments. A highly sensitive MEMS pressure sensor has been developed that can be packaged in two different ways – one in a cylindrical housing, and the other in a flexible, yet robust, strip configuration – enabling more freedom for the user to choose an option that fits their needs. The sensing element of each consists of four piezoresistive elements in a Wheatstone Bridge configuration arranged on a deformable buried-oxide layer, which is then bonded to a Silicon base layer with a hollow cavity carved using reactive-ion etching. Previous work has shown the survivability of these sensors in an underwater environment and also measurements of low frequency pressure changes due to flow and varying turbulence intensities. The present work is focused on evaluating these pressure sensors and testing the limits of the sensing element in the low, medium, and high frequency regime (<100Hz to >1kHz) to gain further insight into the performance. Five experimental tests were developed and conducted to guide this research objective. The sensor responses under different flow conditions were measured and analyzed with selected filtering and resampling techniques to eliminate background noises. First, the sensors were calibrated to ensure their linearity and to determine their pressure sensitivities. Then, using bench-top testing rigs and a water tunnel, the sensor performance was evaluated in submerged environments when subjected to multiple small-scale flow disturbances across the tested frequency regime. It was found that the present sensors are capable of providing more accurate measurements across a tested frequency regime of 0 to 20,000Hz when compared to other off-the-shelf products. Testing in submerged environment showed that the sensors are capable of detecting small-scale pressure fluctuations as a result of eddies which are evident in a Von Karman vortex street and a turbulent flow. Despite the presence of EMI noise within a water tunnel, the sensors demonstrated a decay of pressure fluctuations that is consistent with previous research in the field. Overall, the present work increases understanding of the sensors' performances across a broad range of frequencies and provides insight into potential uses and future work.