Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T20: Flow Control: Structured Surfaces |
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Chair: Daniel Chung, University of Melbourne Room: 146C |
Monday, November 20, 2023 4:25PM - 4:38PM |
T20.00001: Turbulence suppression and skin-friction drag reduction with hairy surfaces Rayhaneh Akhavan, Jae Bok Lee A new method for turbulence suppression and skin-friction drag reduction using hairy surfaces is presented. The studies are based on direct numerical simulation, using lattice Boltzmann-immersed bounday (LB-IB) methods, performed in turbulent channel flows at a bulk Reynolds number of Re_{b}=7200 (Re_{τ0}≈221), with a uniform carpet of hairy surfaces implanted on both channel walls. Hairy surfaces with initial hair filament heights of 4 ≤ h_{0}^{+0} ≤ 16 in base flow wall units, hair filament height to spacing ratios of 1/4 ≤ h_{0}/s ≤ 2, hair filament diameters of d^{+0 }≈ 0.5, density ratios of 30 ≤ ρ_{r} ≤ 1000 and Cauchy numbers of Ca = 0,10, 20, 40, 60, 80 were investigated. It is found that the magnitude of drag reduction scales primarily with the ratio of the characteristic timescale of the hair filaments to the eddy turnover time of the large-scale turbulent eddies in the base turbulent channel flow, T_{fil}u_{τ0}/H, reaching its optimal value at T_{fil}u_{τ0}/H ≈ 1.5. Drag reductions of ∼ 5.5% were achieved with sparse, flexible hairy surfaces with initial filament heights of h_{0}^{+0} ≈ 8, diameters of d^{+0 }≈ 0.5, filament height to spacing ratios of 1/2 ≤ h_{0}/s ≤ 1, density ratio of ρ_{r}=700 and Ca=40, for which T_{fil}u_{τ0}/H ≈ 1.5. The mechanism of drag reduction is found to be a disruption of the pressure-velocity-gradient correlations in the presence of hairy surfaces, which, in turn, leads to an accumulation of turbulence kinetic energy in the streamwise component of turbulent velocity fluctuations, leading to concomitant reductions in the Reynolds shear stress and turbulence production, and a drop in the streamwise turbulent vorticity fluctuations. Examination of the pre-multiplied spectra reveals that these effects are due to modifications of the largest turbulent scales in the presence of hairy surfaces, and extend to wall-normal distances far beyond the initial height of the hair filaments. These large-scale effects give optimism that the magnitude of drag reduction can be further enhanced as the Reynolds number of the flow increases. These results suggest that flexible hairy surfaces show promise for design of novel "functional surfaces" for turbulent skin-friction drag reduction, acting both as sensor and actuator without the need for any external power input. |
Monday, November 20, 2023 4:38PM - 4:51PM |
T20.00002: Modeling Boundary Layer Separation Over Bio-Inspired Organized Surface Roughness Elements Cristiano Cabrera, Abigayle E Moser, Antonio Esquivel-Puentes, Luz Sotelo, Luciano Castillo Passive flow control techniques are utilized to mitigate turbulence generation, drag, and boundary layer separation. Utilization of organized surface roughness elements as passive flow control methods allows for the manipulation of boundary layer development to reduce drag without needing to put energy into the system while in use. The current study utilizes hydrodynamically smooth bio-inspired surfaces modeled after shark skin (dermal denticles) to delay the boundary layer separation. Simulations were performed using a multi-physics modeling software (COMSOL) to explore various streamwise and spanwise riblet spacing to observe how various packing densities influenced the boundary layer separation point and near-wall Reynolds stresses. By normalizing the lengths and spacings relative to the radius of the structure, a relationship was derived between the spacings and lengths of structures and the boundary layer detachment, as well as the structure spacing's influence on boundary layer development. |
Monday, November 20, 2023 4:51PM - 5:04PM |
T20.00003: Rapidly characterizing drag reduction when riblets are yawed or tip-rounded Christopher J Camobreco, Jeremy Wong, Nicholas Hutchins, Daniel Chung The drag reduction that riblets provide in the viscous regime can be rapidly quantified with two-dimensional Stokes flow computations. Wong et al. (A viscous vortex model for predicting the drag reduction of riblet surfaces, J.Fluid Mech., under review) recently improved the accuracy of these computations for flow-aligned riblets by modifying the boundary forcing to represent a nearby quasi-streamwise vortex. However, setting the vortex wavelength constrained the riblet spacing to its integer factors. This constraint is relaxed herein by employing a Floquet–Bloch formulation, which redefines the boundary forcing across a single riblet period. Yawed flow conditions are also efficiently addressed, as the computations remain two-dimensional even when the Bloch wave is not aligned with the riblets. Modeled drag reduction trends agree well with direct numerical simulations for both yawed and tip-rounded riblets. |
Monday, November 20, 2023 5:04PM - 5:17PM |
T20.00004: Why do only some riblets generate spanwise rollers? Daniel Chung, Christopher J Camobreco, Sebastian Endrikat, Ricardo Garcia-Mayoral, Mitul Luhar Spanwise-coherent rollers form over certain riblet-treated surfaces, increasing skin-friction drag. Acute-angled triangular riblets are particularly susceptible to this instability. Previous linear stability analyses have struggled to correctly predict the streamwise wavelength of these rollers, nor determine which riblets generate rollers. This is rectified by imposing an empirical boundary condition which captures the streamwise-scale dependence of the riblet admittance, as measured in direct numerical simulations. The rollers are then predicted to be marginally stable at streamwise wavelengths matching those observed in direct numerical simulations (≈ 150 viscous units) and at similar wall-admittance magnitudes. The admittances required for roller growth are much lower than previously believed, explaining how changes to the riblet tip angle, and thereby the wall-admittance, can impact the likelihood of rollers being generated. |
Monday, November 20, 2023 5:17PM - 5:30PM |
T20.00005: Passive flow control by metamaterial-based phononic subsurfaces Mahmoud Hussein, Armin Kianfar, Adam R Harris, David Roca Flow control over surfaces is a many-decades old engineering problem of a multi-disciplinary nature. It is concerned with devising passive or active means of intervention with the flow field and its underlying mechanisms in a manner that causes desirable changes in the overall flow behavior. For streamlined bodies cruising through a flow, such as air or water, there is a key interest in the control of flow instabilities which manifest as fluid waves. These are perturbations in the flow velocity field that if left to grow are likely to trigger transition of the flow from laminar to turbulent, which in turn causes significant increases in skin-friction drag. A rise in drag reduces the efficiency in air and water vehicles, wind turbines, and pipelines. It is therefore desired to device intervention methods to impede the growth of these instabilities. Alternatively, in some scenarios, the objective may be to speed up the growth of the instabilities to prevent or delay flow separation. |
Monday, November 20, 2023 5:30PM - 5:43PM |
T20.00006: Sub-laminar skin friction drag over butterfly inspired grooves due to the ‘roller-bearing effect’ Amy W Lang, Sashank Gautam, Leonardo M Santos The scales found on butterfly wings are known to perform fluid dynamic functions. This study investigates the aerodynamic function to reduce skin friction drag. A previous flight test study with live monarch butterflies showed that removing scales from the wings reduced flight efficiency (measured in energy change or Joules/flap) on average by 38%. Butterfly scales covering monarch wings have a typical size of 0.1 mm in length, where the tips of the scales curve upwards to create microscopic grooves within the roof-shingle patterned surface. This DPIV study tested the hypothesis that flow passing transverse to the grooves would result in the formation of trapped or embedded vortices that result in a partial slip condition to the outer flow passing over the wing surface. This flow control method to decrease skin friction drag is designated the 'roller-bearing effect'. Tow tank studies, using high viscosity mineral oil to lower the Re, measured the flow over various butterfly scale inspired groove, or transverse cavity, models. Five of the six models tested confirmed the presence of sub-laminar drag at low Reynolds numbers. A maximum value of 26.3% drag reduction was measured for the grooves with a 45-degree wall angle and 2:1 aspect ratio for a Re = 8.5, which is dynamically similar to the air flow over butterfly scales. Results showed that drag reduction is lost as the Re increases by one order of magnitude as the embedded vortices become unstable. |
Monday, November 20, 2023 5:43PM - 5:56PM |
T20.00007: Experimental Analysis of Mixed Configuration Riblets for Turbulent Boundary Layer Control Md. Rafsan Zani, Nir S Maor, Pengyao Gong, Dhanush Bhamitipadi Suresh, Emmanuvel Joseph Aju, Yaqing Jin This study investigates the characteristics of mixed and uniform blade riblets in a turbulent boundary layer under various flow conditions, represented by different non-dimensional spacing (s+), with results compared to those of a flat plate. Particle Image Velocimetry (PIV) and Micro Particle Image Velocimetry (micro-PIV) techniques are employed to examine the near-wall flow behavior of both riblets and flat plate. The log-law method is used to determine shear velocity using regular PIV data, while data from micro-PIV were utilized to calculate near-wall viscous and Reynolds shear stress and vorticity. For regular blade riblets, an increase in s+ beyond the optimum s+ leads to elevated near-wall shear stress and shear velocity, resulting in increased drag. In contrast, mixed riblets configuration demonstrates lower shear velocity and drag reduction across a broad range of s+ values when compared to the flat plate. Additionally, at high s+, the regular blade riblets exhibit high vorticity close to the wall, while the mixed riblets configuration shows the lowest vorticity compared to both the flat plate and regular blade riblets. |
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