Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T11: Boundary Layers: Surface Effects 
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Chair: Raúl Bayoán Cal, Portland State University Room: 143A 
Monday, November 20, 2023 4:25PM  4:38PM 
T11.00001: Scaling relations in stably stratified channel flow with secondary motions induced by heterogeneous surface temperature Thijs Bon, Johan Meyers, Raúl Bayoán B Cal Recent studies have demonstrated that large streamwisealigned secondary circulations can be generated in stratified flows over thermally heterogeneous surfaces with spanwise length scales in the order of the boundary layer depth. These mean secondary flow structures significantly affect timeaveraged profiles of velocity and temperature in turbulent boundary layers, through the dispersive or coherent fluxes. In order to investigate this, we performed a series of Direct Numerical Simulations of stably stratified channel flow with spanwise heterogeneous temperature strips of both finite and infinite length, at Re_{τ}=180 and Re_{τ}=550. We analyzed the penetration depth of the heterogeneity into the outer layer and assessed accuracy of existing fluxprofile scaling relations. Although the blending height strongly depends on the geometry of the surface temperature patches, mean velocity and temperature profiles of all cases collapse if an outerlayer scaling based on displacement thickness is applied. Comparison of the highReynolds cases with temperature patches of finite length to local MoninObukhov Similarity Theory (MOST) reveals that the similarity functions for temperature and velocity gradients agree with empirical linear relations in the region far above the surface, where dispersive fluxes are negligible. Profiles of the canonical case with infinitely long spanwise heterogeneity, which has been mostly studied in the context of secondary motions, do not follow local MOST at any height. 
Monday, November 20, 2023 4:38PM  4:51PM 
T11.00002: Effect of Surface Fluid Entrapment in a Laminar Flat Plate Boundary Layer Paul S Krueger, Abigail E Hays, David A Willis, Haosen Tan For streamlined objects in laminar flow, the drag is dominated by friction drag. To counteract this, the boundary condition of the object can be changed to introduce an apparent slip at the surface. This work investigates a novel approach to providing slip via fluid entrapment in a perforated domain embedded in the surface. The fluid entrapment system uses a cavity underneath the perforated domain to allow entrapment of fluid of the same type, or entrapment of gas (in a liquid flow) via gas supply to the cavity. Either case provides a fluid at the surface perforations and relaxes the noslip condition. As a model case, fluid entrapment was used for flow over a flat plate with the perforated domain at various downstream locations. Flow over the flat plate was provided by a miniature water tunnel. Velocity profiles were measured using Particle Image Velocimetry (PIV) for a Reynolds number (Re) range of 4,0006,000. Boundary layer profiles with air entrapment were analyzed and compared with the results for water entrapment and for a solid (control) flat plate. The results show slip flow at the surface for the air entrapment and water entrapment cases, with the air entrapment case producing larger slip flow, in terms of slip velocity and slip length, than the water entrapment case. 
Monday, November 20, 2023 4:51PM  5:04PM 
T11.00003: Direct measurement of slip length operator and eddy viscosity fields for turbulent flows over superhydrophobic surfaces Kimberly Liu, Ali Mani Superhydrophobic surfaces (SHS) are textured hydrophobic surfaces which have the ability to trap air pockets when immersed in water. This can result in significant drag reduction, due to substantially lower viscosity of air resulting in large effective slip velocity at the interface. Past studies model this slip velocity in terms of a homogenized Navier slip boundary condition with a slip length relating the wall slip velocity to the wallnormal velocity gradient. In this study, we seek to understand the effects of superhydrophobic surfaces in the context of momentum mixing. We use the macroscopic forcing method (Mani and Park, Phys. Rev. Fluids, 2021) to compute the eddy viscosity of a turbulent channel over SHS, implemented as both a patternresolved boundary condition and homogenized slip length boundary condition. We present key differences in the mixing behavior of both boundary conditions through quantification of their implications on the nearwall eddy viscosity. In addition, we compute the fully generalized slip length operator to assess the nonlocal effects of SHS patterning. 
Monday, November 20, 2023 5:04PM  5:17PM 
T11.00004: Direct numerical simulations of turbulence over compliant walls Tianrui Xiang, Joseph Katz, Tamer A Zaki The interaction of turbulence with compliant walls leads to the generation of Rayleigh waves in the material and qualitative changes in the flow in the nearwall region (Esteghamatian et al., J. Fluid Mech. 942, A35, 2022). The Rayleigh wave speed sets the height of a critical layer whose role will be examined using data from direct numerical simulations. The computations adopt an EulerianEulerian approach with a levelset method that distinguishes the fluid and compliant wall, and the results are analyzed in a wavefitted coordinate in order to highlight the influence of the wave motion. Our findings are consistent with and supplement recent observations from experiments by Katz and coworkers. Even when the wall deformation is on the order of a few viscous units, the flow dynamics between the critical layer and the wall are qualitatively altered compared to rigidwall turbulence, in particular the vorticity flux and the phase speed of propagation of perturbations. The analogy to windwave interaction is explored, specifically by evaluating the validity of Lighthill's theory on the role of the critical layer in the energy exchange between the wave motion and the flow above it. 
Monday, November 20, 2023 5:17PM  5:30PM 
T11.00005: Thermal and Momentum Profile in Turbulent Boundary Layers with Surface Mass Transfer Rozie Zangeneh, James Chen, Paul E DesJardin Direct numerical simulations have been carried out to investigate the effects of surface mass transfer on the 
Monday, November 20, 2023 5:30PM  5:43PM 
T11.00006: Utilizing Curved Channels to Either Enhance or Diminish Diffusion Dylan D Bruney Solute transport in diverse fluid systems (e.g., medicine in blood, chemicals in microchannels, river pollutants) involves significant boundary layer formation, dispersing solute concentration and creating gradients impacting diffusion dynamics. We develop and employ an effective diffusion equation with a diffusivity parameter to model this phenomenon. This study focuses on sinusoidal boundary perturbations' role in the long time limit using homogenization, specifically exploring the lubrication approximation and Stokes flow regimes. By using the lubrication approximation, we identify a threshold where perturbations transition from enhancing to diminishing diffusion relative to a flat wall. However, in Stokes flow, any wall perturbation only reduces diffusivity compared to a flat wall. Our model is validated through Monte Carlo simulations and full advectiondiffusion simulations. 
Monday, November 20, 2023 5:43PM  5:56PM 
T11.00007: Abstract Withdrawn

Monday, November 20, 2023 5:56PM  6:09PM 
T11.00008: Abstract Withdrawn 
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