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1. Advanced Wind Tunnel Boundary Simulation for Kevlar Wall Aeroacoustic Wind Tunnels

   Szoke, Mate; Devenport, William; Borgoltz, Aurelien; Roy, Christopher; Lowe, Todd (May 2021, In Advanced Wind Tunnel Boundary Simulation II)

   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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2. Experimental Investigation of Supercritical Carbon Dioxide in Horizontal Micro Pin Arrays with Non-Uniform Heat Flux Boundary Conditions

   https://doi.org/10.1615/TFEC2019.fnd.027547  

   Jajja, Saad A.; Sequeira, Jessa M.; Fronk, Brian M. (April 2019, 4th Thermal and Fluids Engineering Conference)

   This study is part of the preliminary experimental investigations designed to assess the feasibility of using supercritical carbon dioxide (sCO2) in the vicinity of its critical point for thermal management applications. In the present study, sCO2 was used as a working fluid in a diffusion bonded 316/316L stainless steel test section having staggered micro pin fin array flow passages of hydraulic diameter 679 µm (0.679 mm). The test section was subjected to a single wall non-uniform heat flux boundary condition and was operated in a horizontal orientation. The primary objective was to characterize the heat transfer performance of sCO2 as it flows through the staggered pin fin array for experimental conditions that span its critical and pseudocritical point. Data analysis methods employing 2-D and 3-D heat transfer models of the test section were used to calculate the average heat transfer coefficients for a given set of experimental conditions. Experiments were conducted by varying the inlet temperature (18 ≤ T_in ≤ 50 °C) and for fixed mass flux (300 kg m-2 s-1), heat flux (40 W cm-2), and reduced pressure (1.1). Experimental data were also compared against the predictions of a correlation proposed for single phase flows in microchannel staggered diamond pin arrays. The correlation predicted the data within 4.3 % when the ratio, T_Bulk/T_PC exceeded 1. It was also found that the enhancement in the heat transfer, a result of employing staggered pin array flow geometry instead of microchannels, carries a commensurate penalty in pressure drop. 

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3. On the origins of transverse jet shear layer instability transition

   https://doi.org/10.1017/jfm.2020.127  

   Shoji, Takeshi; Harris, Elijah W.; Besnard, Andrea; Schein, Stephen G.; Karagozian, Ann R. (May 2020, Journal of Fluid Mechanics)

   This experimental study explores the physical mechanisms by which a transverse jet’s upstream shear layer can transition from being a convective instability to an absolute/global instability as the jet-to-cross-flow momentum flux ratio $$J$$ is reduced. As first proposed in computational studies by Iyer & Mahesh ( J. Fluid Mech. , vol. 790, 2016, pp. 275–307), the upstream shear layer just beyond the jet injection may be analogous to a local counter-current shear layer, which is known for a planar geometry to become absolutely unstable at a large enough counter-current shear layer velocity ratio, $$R_{1}$$ . The present study explores this analogy for a range of transverse jet momentum flux ratios and jet-to-cross-flow density ratios $$S$$ , for jets containing differing species concentrations (nitrogen, helium and acetone vapour) at several different jet Reynolds numbers. These studies make use of experimental data extracted from stereo particle image velocimetry as well as simultaneous stereo particle image velocimetry and acetone planar laser-induced fluorescence imaging. They provide experimental evidence for the relevance of the counter-current shear layer analogy to upstream shear layer instability transition in a nozzle-generated transverse jet. 

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4. Oscillatory flows in compliant conduits at arbitrary Womersley number

   https://doi.org/10.1103/PhysRevFluids.8.124102  

   Pande, Shrihari D.; Wang, Xiaojia; Christov, Ivan C. (December 2023, Physical Review Fluids)

   We develop a theory of fluid--structure interaction (FSI) between an oscillatory Newtonian fluid flow and a compliant conduit. We consider the canonical geometries of a 2D channel with a deformable top wall and an axisymmetric deformable tube. Focusing on the hydrodynamics, we employ a linear relationship between wall displacement and hydrodynamic pressure, which has been shown to be suitable for a leading-order-in-slenderness theory. The slenderness assumption also allows the use of lubrication theory, and the flow rate is related to the pressure gradient (and the tube/wall deformation) via the classical solutions for oscillatory flow in a channel and in a tube (attributed to Womersley). Then, by two-way coupling the oscillatory flow and the wall deformation via the continuity equation, a one-dimensional nonlinear partial differential equation (PDE) governing the instantaneous pressure distribution along the conduit is obtained, without \textit{a priori} assumptions on the magnitude of the oscillation frequency (\textit{i.e.}, at arbitrary Womersley number). We find that the cycle-averaged pressure (for harmonic pressure-controlled conditions) deviates from the expected steady pressure distribution, suggesting the presence of a streaming flow. An analytical perturbative solution for a weakly deformable conduit is obtained to rationalize how FSI induces such streaming. In the case of a compliant tube, the results obtained from the proposed reduced-order PDE and its perturbative solutions are validated against three-dimensional, two-way-coupled direct numerical simulations. We find good agreement between theory and simulations for a range of dimensionless parameters characterizing the oscillatory flow and the FSI, demonstrating the validity of the proposed theory of oscillatory flows in compliant conduits at arbitrary Womersley number. 

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5. Characterization of the critical lift-off of a single flat-plate microchip particle in straight rectangular microchannel flows

   https://doi.org/10.1016/j.ijmultiphaseflow.2025.105355  

   Yeung, Raymond; Sainz, Cynthia; Mandala, Jason; Brisk, Philip; Grover, William H; Rodgers, Victor GJ (December 2025, International Journal of Multiphase Flow)

   Hydrodynamic sorting of microchip particles in microchannels is essential in microfluidic systems used for applications requiring particle-based multiplexing. Understanding the forces acting on the particle, as well as the dependencies of the forces on channel and fluid flow parameters, allows for prediction of the flow conditions needed to initiate particle movement, or lift-off. This study presents the experimental characterization of the lift-off of a single, flat-plate, non-neutrally buoyant microchip particle initially sedimented near the inlet of straight, rectangular microfluidic channels of different channel sizes and solvents at moderate Archimedes number of 191 to 2820. The critical shear Reynolds number, corresponding to the minimum required for lift-off, was found to increase with larger Archimedes number and the relationship was found to exhibit particle-channel size dependency. The observed critical lift-off for the flat-plate particle was lower than that predicted using a previous generalized lift-off model based on modified particle Reynolds and Archimedes numbers which may be explained by entrance effects and fluid film lubrication pressure under the particle. Numerical evaluations of the hydrodynamic forces acting on the particle revealed that electrostatic forces are significant. A remodified Archimedes number, based on the channel width, particle diameter, and solvent relative permittivity, is introduced as a correction to the generalized lift-off model to account for hydrodynamics and electrostatics affecting the lift-off of a flat-plate particle. This model is in good agreement with the generalized particle lift-off model and allows for prediction of flat-plate particle lift-off in microfluidic channels for a moderate range of Archimedes numbers. 

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