Browsing by Author "Szőke, Máté"
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- Advanced Boundary Simulations of an Aeroacoustic and Aerodynamic Wind TunnelSzőke, 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.
- Comparative Assessment of Sound Generated Using Laser-Induced PlasmaSzőke, Máté; Bahr, Christopher; Cattafesta, Louis; Rossignol, Karl-Stéphane; Ura, Hiroki; Zhang, Yang; Zigunov, Fernando (2021-08-10)
- The Development of Remote Laboratory Sessions at the Stability Wind Tunnel of Virginia Tech During the Coronavirus PandemicSzőke, 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.
- Experimental Investigation of Bio-inspired Unidirectional CanopiesNurani Hari, Nandita; Szőke, Máté; Devenport, William J.; Glegg, Stewart A. L.; Priddin, Matthew; Ayton, Lorna J. (2022-02-08)An analytical approach has been developed to model the rapid term contribution to the unsteady surface pressure fluctuations in wall jet turbulent boundary layer flows. The formulation is based on solving Poisson’s equation for the turbulent wall pressure by integrating the source terms (Kraichnan, 1956). The inputs for the model are obtained from 2D time-resolved Particle Image Velocimetry measurements performed in a wall jet flow. The wall normal turbulence wavenumber two-point cross-spectra is determined using an extension of the von Kármán homogeneous turbulence spectrum. The model is applied to compare and understand the baseline flow in the wall jet and to study the attenuation in surface pressure fluctuations by unidirectional canopies (Gonzales et al, 2019). Different lengthscale formulations are tested and we observe that the wall jet flow boundary layer contributes to the surface pressure fluctuations from two distinct regions. The high frequency spectrum is captured well. However, the low frequency range of the spectrum is not entirely captured. This is because we have used PIV data only up to a height of 2.3𝜹, whereas the largest turbulent lengthscales in the wall jet are on the order of 𝒚𝟏/𝟐≈𝟔𝜹. Using the flow data obtained from PIV and Pitot probe measurements, the model predicts a reduction in the surface pressure due to canopy at low frequencies.
- Flow Field Analysis Around Pressure Shielding StructuresSzőke, Máté; Nurani Hari, Nandita; Devenport, William J.; Glegg, Stewart A. L.; Teschner, Tom-Robin (2021-02-08)The flow field around a series of streamwise rods, referred to as canopies, is investigated using two-dimensional two-component time-resolved particle image velocimetry (PIV) and large eddy simulations (LES) to characterize the changes in the flow field responsible for reducing the low and high-frequency surface pressure fluctuations previously observed. It was found that an axisymmetric turbulent boundary layer (ATBL) develops over the rods, whose thickness grows at a greater rate above the rods than below. This boundary layer reaches the wall below the rods at a point where previously a saturation was found in the low-frequency noise attenuation, revealing that the ATBL is responsible for the low- frequency noise attenuation. The flow is displaced by the presence of the rods, particularly above them, which offset was primarily caused by the blockage of the ATBL. The flow below the rods exhibits the properties of a turbulent boundary layer as its profile still conforms to the logarithmic layer, but the friction velocity was found to drop. This viscous effect was found to be responsible for the high-frequency noise attenuation reported previously.
- Investigating the Aeroacoustic Properties of Kevlar FabricsSzőke, Máté; Devenport, William J.; Nurani Hari, Nandita; Alexander, W. Nathan; Glegg, Stewart A. L.; Li, Ang; Vallabh, Rahul; Seyam, Abdel-Fattah M. (2021-02-08)The aeroacoustic properties of porous fabrics are investigated experimentally in an effort to find a porous fabric as an ideal interface between wind tunnel flow and quiescent conditions. Currently, the commercially available Kevlar type 120 fabric is widely used for similar applications, such as side-walls in hybrid anechoic wind tunnels or as a cover of phased microphone arrays. A total number of 8 fabrics were investigated, namely, four glass fiber fabrics, two plain weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores clogged. Two, custom-made rigs were used to quantify the transmission loss and self-noise of all eight fabrics. It was found that the pores serve as a low-resistance gateway for sound waves to pass through, hence enabling a low transmission loss. The transmission loss was found to increase with decreasing open area ratio while other fabric properties had a minor impact on transmission loss. The self-noise of the fabrics has also been evaluated and it was found that the thread density (thread per inch) is a primary factor of determining the frequency range of self-noise with the open area ratio potentially playing a secondary role in the self-noise levels. For both metrics, the mass per unit area seemed to play a minor role in the aeroacoustic performances of the fabrics. Finally, surface pressure measurements revealed that the commercially available plain Kevlar (type 120) has no quantifiable effect on the hydrodynamic pressure field passing over the fabric, sug- gesting that Kevlar behaves as a no-slip wall from the flow's perspective when no pressure difference is present on the two sides of the fabric.
- Mean Flow Characteristics and Turbulent Structures of Turbulent Boundary Layers in Varying Pressure Gradients and Reynolds NumbersSrivastava, Surabhi (Virginia Tech, 2023)Turbulent boundary layers flowing over a smooth surface were studied to understand the influence of varying pressure gradients and flow Reynolds number on the boundary layer growth and mean turbulent properties. The test was conducted in the Virginia Tech Stability Wind Tunnel with a 0.914 m chord length, NACA 0012 Airfoil in the test section. This airfoil was rotated to different angles of attack to induce varying pressure gradients on the boundary layer developing on the test section walls. Mean pressure measurements, boundary layer pressure measurements, and time-resolved, wall-normal, stereoscopic particle image velocimetry (TR-PIV) measurements were made. The TR-PIV data was acquired at a chord-based Reynolds number of 1.2 million, 2 million, and 3.5 million, at a sampling rate of 1 kHz, in two different camera configurations. The boundary layer pressure measurements were acquired at different flow Reynolds numbers ranging between 0.76 million and 3.5 million. Both adverse and favorable pressure gradients of varying intensities were imposed on the boundary layer by rotating a 0.914 m chord NACA 0012 airfoil to angles of attacks between -{10}^o and {12}^o. Measurements at varying streamwise locations enabled the study of boundary layer flow development under changing pressure gradients. The pressure gradient influences were observed in the boundary layer characteristic properties, on the mean velocities, and on the Reynolds stresses present in the flow. The pressure gradient influences were found to be consistent at varying Reynolds numbers, but the intensity of their effects was influenced by the flow Reynolds number. Moreover, the influence of pressure gradients and flow Reynolds numbers was evident in both outer and inner scales. The test data acquired was also validated with previous works.
- Propagation characteristics of laser-induced acoustic sources in hybrid anechoic wind tunnelsSzőke, Máté; Devenport, William J. (Academic Press-Elsevier, 2021-10-13)The propagation characteristics of an acoustic point source generated using laser-induced plasma (LIP) were investigated experimentally. Experiments were performed in a Kevlar-walled hybrid anechoic wind tunnel (HAWT) where the sound of the LIP was measured using a 251-element microphone array, while the flow speed in the empty test section was varied. The time instant of the LIP formation was also captured. The far field sound pressure was assessed through arrival times (source to microphones) and pressure correction levels, and these quantities were compared against a commonly used shear layer refraction model. A detailed uncertainty assessment is presented on the arrival times and pressure levels. It was found that the time domain analysis was limited by the sampling rate of the analog-to-digital converter regardless of the flow speed. The uncertainty of the pressure levels was limited by the uncertainty of the microphones at low flow speeds, while they increased with flow speed at shallow observer angles. The high-speed Schlieren imaging of the LIP was performed, which revealed that the sound of the LIP reaches the far field microphones over a shorter time duration than modeled because the wave speed was initially supersonic. The discrepancy was found to be comparable to the temporal resolution of the aeroacoustic experiments. The discrepancy between the experimental and theoretical arrival times was found to increase with flow speed, and they were nearly independent of the azimuth angles. The discrepancy between the experimental and theoretical pressure correction ratio was found to be uniform for most observer locations. With an increase in flow speed, the discrepancy became positive at large, and negative at low polar angles. The sound refraction at the Kevlar wall did not change the frequency content of the sound over the investigated range of frequencies (1–10 kHz).
- Trailing-edge serrations: improving theoretical noise reduction modelsAyton, Lorna J.; Szőke, Máté; Paruchuri, Chaitanya; Devenport, William J.; Alexander, William (2021-08-02)This paper discusses the elements that make up a theoretical model for predicting the noise generated by a serrated trailing edge, and in particular, the required input to such a model; the wavenumber frequency spectra of the turbulence at the edge. It is proposed that this input, which is often modeled empirically, varies between a straight edge and a serrated edge and this fundamental difference in turbulence structure at the trailing edge leads to inaccurate theoretical predictions of noise reduction. Experimental measurements are therefore taken for straight and serrated trailing edges, with a particular focus on measuring the quantities which arise in typical wavenumber-frequency spectra models such as the TNO model. Whilst elements like the boundary layer thickness, shear profile, and skin friction velocity are found to be similar across the straight and serrated edges, the normal velocity wavenumber spectra is observed to vary substantially between the straight edge, and different locations along the serrated edge. In particular, the high-frequency decay rate of this spectra is reduced for a serrated edge versus a straight edge. This observed difference is then incorporated into a simple Chase-style empirical wavenumber-frequency spectrum; theoretical predictions which take account of a weaker high-frequency decay rate for a serrated edge agree much better with the experimental far-field noise predictions than theoretical predictions which assume an identical turbulent structure for both straight and serrated edges.
- Understanding Pressure Shielding by CanopiesNurani Hari, Nandita; Szőke, Máté; Devenport, William J.; Glegg, Stewart A. L. (2021-01-01)Previous studies have demonstrated that structures such as a canopy or finlets placed within a boundary layer over an aerodynamic surface can attenuate pressure fluctuations on the surface without compromising aerodynamic performance. This paper describes research into the fundamental mechanisms of this pressure shielding. Experiments and analysis are performed on elemental canopy configurations which are arrays of streamwise rods placed parallel to the wall in order to eliminate the confounding effects of a leading edge support structure. Experiments show that such a canopy produces attenuation in three distinct frequency ranges. At low frequencies, where convective scales are much greater than the canopy height, attenuation spectra scale on the canopy height Strouhal number, but at high frequencies, a dissipation type frequency scaling appears more appropriate. There is mid-freqeuncy region which shows reduction in attenuation and is observed for all canopy structures tested. Attenuation in this region appears to scale with Strouhal number based on canopy spacing.