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1. Development and Calibration of a new Anechoic Wall Jet Wind Tunnel

   https://doi.org/10.2514/6.2019-1936  

   Kleinfelter, Austin W.; Repasky, Russell; Hari, Nandita; Letica, Stefan; Vishwanathan, Vidya; Organski, Lee; Schwaner, Jon; Alexander, William N.; Devenport, William J. (January 2019, AIAA Science and Technology Forum and Exposition 2019)

   A new Anechoic Wall Jet Wind Tunnel was built at Virginia Tech. A detailed design based on the old wall jet tunnel was done to improve the quality of the resultant flow. Aerodynamic and acoustic calibrations were performed in order to understand properties and characteristics of the flow generated by this new facility which can be used for various aeroacoustic studies. Far-field acoustics were measured using half-inch B&K microphones in a streamwise array to characterize and reduce the background noise. Sound pressure levels were lower by 10 dB for frequencies up to 700Hz in comparison to the old facility. The turbulent surface pressure fluctuations of the wall-jet flow were studied using Sennheiser microphones placed along streamwise and spanwise locations to record surface pressure fluctuations. Comparison of the autocorrelation plotted for microphones along the same span indicate uniform flow features. A decay in the turbulence levels is observed along the downstream direction as expected. Aerodynamic calibrations included mean velocity measurements along different spanwise locations, wall-jet boundary layer profiles and streamwise cross-sections. Spanwise and cross-sectional velocity profiles show good uniformity of the flow. Detailed boundary layer analyses were performed with the parameters obtained from the experiments. 

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2. Flow Field Analysis Around Pressure Shielding Structures

   M. Szoke, N. Hari (August 2021, AIAA Aviation 2021)

   null (Ed.)

   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. 

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3. Assessment of Turbulence Models over a Curved Hill Flow with Passive Scalar Transport

   https://doi.org/10.3390/en15166013  

   Paeres, David; Lagares, Christian; Araya, Guillermo (August 2022, Energies)

   An incoming canonical spatially developing turbulent boundary layer (SDTBL) over a 2-D curved hill is numerically investigated via the Reynolds-averaged Navier–Stokes (RANS) equations plus two eddy-viscosity models: the K−ω SST (henceforth SST) and the Spalart–Allmaras (henceforth SA) turbulence models. A spatially evolving thermal boundary layer has also been included, assuming temperature as a passive scalar (Pr = 0.71) and a turbulent Prandtl number, Prt, of 0.90 for wall-normal turbulent heat flux modeling. The complex flow with a combined strong adverse/favorable streamline curvature-driven pressure gradient caused by concave/convex surface curvatures has been replicated from wind-tunnel experiments from the literature, and the measured velocity and pressure fields have been used for validation purposes (the thermal field was not experimentally measured). Furthermore, direct numerical simulation (DNS) databases from the literature were also employed for the incoming turbulent flow assessment. Concerning first-order statistics, the SA model demonstrated a better agreement with experiments where the turbulent boundary layer remained attached, for instance, in Cp, Cf, and Us predictions. Conversely, the SST model has shown a slightly better match with experiments over the flow separation zone (in terms of Cp and Cf) and in Us profiles just upstream of the bubble. The Reynolds analogy, based on the St/(Cf/2) ratio, holds in zero-pressure gradient (ZPG) zones; however, it is significantly deteriorated by the presence of streamline curvature-driven pressure gradient, particularly due to concave wall curvature or adverse-pressure gradient (APG). In terms of second-order statistics, the SST model has better captured the positively correlated characteristics of u′ and v′ or positive Reynolds shear stresses ( > 0) inside the recirculating zone. Very strong APG induced outer secondary peaks in and turbulence production as well as an evident negative slope on the constant shear layer. 

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4. Effect of wall cooling on boundary-layer-induced pressure fluctuations at Mach 6

   https://doi.org/10.1017/jfm.2017.212  

   Zhang, Chao; Duan, Lian; Choudhari, Meelan M. (July 2017, Journal of Fluid Mechanics)

   Direct numerical simulations of turbulent boundary layers with a nominal free-stream Mach number of $$6$$ and a Reynolds number of $$Re_{\unicode[STIX]{x1D70F}}\approx 450$$ are conducted at a wall-to-recovery temperature ratio of $$T_{w}/T_{r}=0.25$$ and compared with a previous database for $$T_{w}/T_{r}=0.76$$ in order to investigate pressure fluctuations and their dependence on wall temperature. The wall-temperature dependence of widely used velocity and temperature scaling laws for high-speed turbulent boundary layers is consistent with previous studies. The near-wall pressure-fluctuation intensities are dramatically modified by wall-temperature conditions. At different wall temperatures, the variation of pressure-fluctuation intensities as a function of wall-normal distance is dramatically modified in the near-wall region but remains almost intact away from the wall. Wall cooling also has a strong effect on the frequency spectrum of wall-pressure fluctuations, resulting in a higher dominant frequency and a sharper spectrum peak with a faster roll-off at both the high- and low-frequency ends. The effect of wall cooling on the free-stream noise spectrum can be largely accounted for by the associated changes in boundary-layer velocity and length scales. The pressure structures within the boundary layer and in the free stream evolve less rapidly as the wall temperature decreases, resulting in an increase in the decorrelation length of coherent pressure structures for the colder-wall case. The pressure structures propagate with similar speeds for both wall temperatures. Due to wall cooling, the generated pressure disturbances undergo less refraction before they are radiated to the free stream, resulting in a slightly steeper radiation wave front in the free stream. Acoustic sources are largely concentrated in the near-wall region; wall cooling most significantly influences the nonlinear (slow) component of the acoustic source term by enhancing dilatational fluctuations in the viscous sublayer while damping vortical fluctuations in the buffer and log layers. 

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5. Accurate near-wall measurements in wall bounded flows with optical flow velocimetry via an explicit no-slip boundary condition

   https://doi.org/10.1088/1361-6501/acf872  

   Jassal, Gauresh Raj; Schmidt, Bryan E. (September 2023, Measurement Science and Technology)

   Abstract High fidelity near-wall velocity measurements in wall bounded fluid flows continue to pose a challenge and the resulting limitations on available experimental data cloud our understanding of the near-wall velocity behavior in turbulent boundary layers. One of the challenges is the spatial averaging and limited spatial resolution inherent to cross-correlation-based particle image velocimetry (PIV) methods. To circumvent this difficulty, we implement an explicit no-slip boundary condition in a wavelet-based optical flow velocimetry (wOFV) method. It is found that the no-slip boundary condition on the velocity field can be implemented in wOFV by transforming the constraint to the wavelet domain through a series of algebraic linear transformations, which are formulated in terms of the known wavelet filter matrices, and then satisfying the resulting constraint on the wavelet coefficients using constrained optimization for the optical flow functional minimization. The developed method is then used to study the classical problem of a turbulent channel flow using synthetic data from a direct numerical simulation (DNS) and experimental particle image data from a zero pressure gradient, high Reynolds number turbulent boundary layer. The results obtained by successfully implementing the no-slip boundary condition are compared to velocity measurements from wOFV without the no-slip condition and to a commercial PIV code, using the velocity from the DNS as ground truth. It is found that wOFV with the no-slip condition successfully resolves the near-wall profile with enhanced accuracy compared to the other velocimetry methods, as well as other derived quantities such as wall shear and turbulent intensity, without sacrificing accuracy away from the wall, leading to state of the art measurements in the $y^{+}<1$region of the turbulent boundary layer when applied to experimental particle images. 

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