Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session R33: Drag Reduction III: Textured Surfaces |
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Chair: Yaqing Jin, The University of Texas at Dallas Room: 255 E |
Monday, November 25, 2024 1:50PM - 2:03PM |
R33.00001: On the mechanism of turbulence suppression and turbulent skin-friction drag reduction with hairy surfaces Rayhaneh Akhavan, Jae Bok Lee, Yuting Cao Both DNS studies (Akhavan & Lee 2023) and experiments (Takata et al. 1996) have shown that by implanting a uniform carpet of flexible filaments, of appropriate characteristics, on the inner walls of a pipe or a channel, one can suppress the production of turbulence and get skin-friction drag reduction. In DNS studies performed in turbulent channel flows at a bulk Reynolds number of Reb=7200 (Reτ0≈221) with filaments of height 4 ≤ h+0 ≤ 16 and diameter of d+0≈0.5 (in base flow wall units), filament height to spacing ratios of 1/4 ≤ h/s ≤ 2, canopy solidities of 0.01 ≤ λ= (hd/s2) ≤ 0.25, filament to fluid density ratios of 30 ≤ ρr= (ρs/ρfε) ≤ 1000, and Cauchy numbers of Ca= (ρf uτ02 d h3 / Kb) ≈ 0, 10, 20, 40, 60, 80, it was found that the magnitude of drag reduction is determined primarily by the ratio of the characteristic timescale of the filaments, Tfil, to the eddy turnover time of the largest turbulent eddies, H/uτ0. Highest drag reductions of DR ≈ 5.5% were observed at Tfiluτ0/H≈1.5 (for filaments with h+0≈8, d+0≈0.5, 1/2 ≤ h/s ≤ 1, ρr=700 and Ca≈20). It has been suggested that such filamentous structures suppress the turbulence through a spectral shortcut mechanism, in which the energy of the largest turbulent eddies is redirected to the motion of the filaments. However, we find no such spectral shortcut mechanism at play. Instead, the filaments suppress the turbulence by disrupting the pressure-strain correlations, which results in accumulation of the turbulence kinetic energy in the streamwise component of the velocity, and a concomitant reduction in the Reynolds shear stress, turbulence production and streamwise vorticity fluctuations. Examination of the pre-multiplied spectra reveals that these effects are most pronounced at the largest turbulent scales and extend to wall-normal distances far beyond the height of the filaments. Reexamination of the mechanism of DR with polymer additives and spanwise wall oscillations reveals that a similar disruption of the pressure-strain correlations is also present in DR with these agents. These results suggest that targeting the pressure-strain correlations may be the path to more effective passive DR technologies. |
Monday, November 25, 2024 2:03PM - 2:16PM |
R33.00002: Broadband flow instability attenuation via coiled locally resonant phononic subsurfaces Adam Harris, Armin Kianfar, David Roca, Mahmoud I Hussein Phononic materials such as locally resonant elastic metamaterials have become increasingly relevant over the past few decades for their applications in a wide range of disciplines in applied physics. One application that emerged in recent years is the use of phononic materials for passive flow control through the notion of a phononic subsurface (PSub). A PSub is an elastic structure designed to intervene with flow instabilities in a desired manner, enabling favorable effects such as delay of flow transition and reduction of skin-friction drag. Practical applicability requires PSubs to exhibit these effects over a broad range of flow instability frequencies. Here, broadband flow instability attenuation is proposed through the design of a “coiled” metamaterial-based PSub which enables flow stabilization effects over a range of a few thousand hertz. To test our design, we construct a coupled fluid-structure and implement direct numerical simulations. In the simulation, a Tollmien–Schlichting (TS) instability wave is introduced in the inflow conditions, and its kinetic energy is tracked throughout the simulation. We consider several cases involving multiple TS modes spanning a relatively wide frequency range and demonstrate simultaneous passive stabilization of all the modes in the region in the flow neighboring the application of the PSub. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R33.00003: Downstream passive flow control by phononic subsurfaces Mahmoud I Hussein, David Roca, Adam Harris, Armin Kianfar Flow control over surfaces is a hereditary 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. An increase in drag reduces the efficiency in air and water vehicles, pipelines, and other applications involving fluid-structure interactions. 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 25, 2024 2:29PM - 2:42PM |
R33.00004: Abstract Withdrawn
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Monday, November 25, 2024 2:42PM - 2:55PM |
R33.00005: Uncertainty Quantification of the Drag Reduction Effect of Superhydrophobic Structure Spacing through Direct Numerical Simulation with Nek5000/NekRS. Byeong-Cheon Kim, Kyoungsik Chang, Sang-Wook Lee, Jaiyoung Ryu, Minjae Kim, Jaemoon Yoon The drag reduction technologies have been investigated and show promise for preventing global warming by reducing energy consumption. One representative technology is the superhydrophobic surface (SHS). SHS structures are classified into two types: ridge (longitudinal groove) type and post type[1]. The hydrodynamic features of SHS vary depending on the type. Consequently, recent research has focused separately on ridge types[2] and post types[3]. In this study, the drag reduction effect of SHS was quantified irrespective of the SHS type. By utilizing uncertainty quantification (UQ) and direct numerical simulation (DNS), the effects of different SHS types were investigated. The streamwise and spanwise distances between the solid posts were treated as random input variables, assumed to follow a Gaussian distribution. The mean values(μ1, μ2) of each input variables were 20μm and the standard deviation(σ1, σ2) were 0.33μ. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R33.00006: Evaluating dynamic pressure measurements within the turbulent boundary layer of drag reducing surfaces Frank A Mier, Connor Wilkinson, Rhett Cook, Jonathan W Naughton, Pourya Nikoueeyan Laboratory scale evaluation of various drag reduction surface morphologies can provide valuable insight on their capabilities for both design optimization and as-built performance verification. Specifically, the primary drag reduction technique evaluated here is the implementation of riblet surfaces which refers to small scale, streamwise structures which tailor the turbulent boundary layer to reduce viscous drag. Various techniques, including velocity profile surveys and direct measurement of shear force, have been developed but often involve significantly complex calibration procedures. In an effort to streamline small-scale testing of candidate surfaces, a boundary layer pressure rake has been implemented along with unique remote-pressure-measurement reconstruction techniques to understand dynamic pressure fluctuation behavior. Results of these measurements are compared to direct measurement of shear via force balance. |
Monday, November 25, 2024 3:08PM - 3:21PM |
R33.00007: Modeling turbulent flow over sharp riblets via change of coordinates Mohammadamin Naseri, Armin Zare Numerical and experimental studies have revealed the drag reducing trends of small-sized riblets in wall-bounded flows. Recent efforts in reduced-order modeling have demonstrated the efficacy of the eddy-viscosity-enhanced linearized Navier-Stokes equations in capturing such trends. In these works, the immersed boundary method was used to capture the effect of riblets on the flow momentum. Such methods, which are complemented by constructive turbulence modeling procedures exhibit deficiencies in capturing intricate flow mechanisms that gain strength above large and sharp riblet structures. We promote the use of an alternative method in accounting for riblet-shaped boundaries that involves a transformation of the coordinate system to the generalized curvilinear coordinates. While the change of coordinates reflects the spatial periodicity of the boundary conditions onto the differential operators and thereby convolutes the structure of the linearized dynamics, it allows for an accurate representation without the need for a stretched mesh or excessive parametric tuning of the immersed boundary. We further demonstrate how perturbation analysis in the height of riblets yields a sequence of computationally efficient equations that can be solved to capture the flow physics. We use this simulation-free approach to predict drag reduction and energy suppression due to semi-circular riblets, as well as the emergence of flow mechanisms such as Kelvin-Helmholtz instability for larger riblets. |
Monday, November 25, 2024 3:21PM - 3:34PM |
R33.00008: Impact of multi-scale riblet design on turbulent boundary layer control and drag reduction effectiveness Yaqing Jin, Md. Rafsan Zani, Nir S Maor, Dhanush Bhamitipadi Suresh This research explored the impact of incorporating secondary blade riblet structures on flow statistics and the effectiveness on friction drag reduction. Turbulent flow behaviors and drag reduction capabilities were assessed in a flow visualization channel with varying non-dimensional riblet spacing s+. The findings indicated that although both riblet surfaces showed comparable drag reduction at low s+, the secondary riblet blade structure notably extended the drag reduction effectiveness to s+=32, achieving approximately 10% lower friction drag compared to the single-scale surface when s+ increases to 44.2. Additionally, the average number of uniform momentum zones on the multi-scale blade riblet surface decreased by approximately 9% compared to the single-scale riblet, indicating a reduction in strong shear layers within the turbulent boundary layer. Near-wall flow analysis revealed that at higher s+, the multi-scale riblet surface resulted in reduced wall-normal velocity fluctuations and Reynolds shear stresses. Quadrant analysis further showed that this design suppressed both sweep and ejection events. These experimental results illustrate that surfaces with spanwise riblet height variations can sustain drag reduction efficiency over a broader range of flow speeds. |
Monday, November 25, 2024 3:34PM - 3:47PM |
R33.00009: Suppression of crossflow-induced boundary layer transition on a swept wing by sinusoidal roughness elements Makoto Hirota, Yuki Ide, Yuji Hattori The crossflow instability is one of the main causes of the laminar-to-turbulent transition of the boundary layer around swept wings of aircrafts. Because it is dominant near the leading edge, the suppression or avoidance of this crossflow-induced transition is important for making a wide area of the upstream boundary layer laminar and reducing the total frictional drag on the wing. In this study, assuming the JAXA technical reference aircraft TRA2012A flying at Mach number 0.781, a DNS analysis of boundary layer transition is performed locally at a position on the airfoil where the pressure and velocity distributions are obtained by a global RANS analysis. Stability analyses of primary and secondary instabilities have been performed for this boundary layer, and the DNS reproduces turbulent transitions in a similar process. To suppress this transition, a sinusoidally deformed artificial roughness is placed near the leading edge. The linear growth rate of crossflow instability is strongly suppressed because of the mean flow distortion by the roughness, which indicates the possibility of avoiding the transition over over a wide area of wing surface. |
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