Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q42: Boundary Layers: Superhydrophobic Surfaces and Drag Reduction |
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Chair: Julien Landel, University of Manchester Room: 6e |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q42.00001: Model-based analysis of turbulent drag reduction in channel flow over corrugated surfaces Wei Ran, Armin Zare, Mihailo Jovanovic We develop a model-based approach to quantify the effect of spatially periodic surface corrugation on skin-friction drag in turbulent channel flow. The effect of surface corrugation is modeled as a volume penalization on the Navier-Stokes equations and the dynamics of velocity fluctuations around the resulting base velocity profile are studied using the eddy-viscosity enhanced linearized equations. We utilize the second-order statistics of velocity fluctuations resulting from the stochastically forced linearized model to compute a correction to the turbulent viscosity of flow over the corrugated surface. This correction in turn influences the turbulent mean velocity and modifies skin-friction drag. For triangular surface corrugation, our simulation-free approach reliably predicts drag-reducing trends observed in high-fidelity simulations and experiments. We investigate the effect of height and spacing of triangular riblets on these drag-reducing trends and demonstrate similar trends for the turbulent kinetic energy of velocity fluctuations. Finally, we examine the flow structures that are extracted from a modal decomposition of the velocity covariance matrix and uncover the influence of periodic surface corrugation on the energetic near-wall cycle. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q42.00002: Retention of infused liquid for sustenance of drag reduction of turbulent flow over liquid infused surface Martand Mayukh Garimella, Edgardo Javier Garcia Cartagena, Stefano Leonardi Liquid infused surfaces (LIS) are composed of functionalized surface textures wetted with an immiscible, chemically-matched liquid lubricant. It has been experimentally demonstrated that grooved LIS configurations can reduce turbulent drag up to 35\%. However, in practical configurations, due to high shear exerted by the flow, the infused liquid may get washed away. Previous studies have considered longitudinal bars. However, this texture cannot hold the lubricant-gas in the cavities. An alternative would be to place transversal bars to retain this lubricant. Direct numerical simulations of the flow in a channel with a rectangular mesh on the lower wall have been carried out. The aspect ratios of the cavities, the Reynolds and Weber numbers were varied. Also, two viscosity ratios between the two fluids, N=0.02 and N=0.4 were used to mimic idealized superâ€“hydrophobic and liquidâ€“infused surfaces. In comparison to the flow over longitudinal bars, the addition of transverse bars reduces drag reduction. However, it was observed that the flow recirculates within the cavities with an expected reduction in drainage. In addition, at a higher Weber number, the interface above the mesh texture is more stable than that over longitudinal bars. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q42.00003: Investigation of turbulent flows over superhydrophobic surfaces with oscillatory slip length Kimberly Liu, Ali Mani Superhydrophobic surfaces have been widely studied for the purposes of drag reduction; the formation of an air film on the surface allows for a slip boundary condition, greatly decreasing the wall friction and thus the overall friction drag. Experimental results of patterned superhydrophobic surfaces, with dynamic pressure control allowing for oscillation of the individual air films, show an even further drag reduction. In addition, it has been observed that under such conditions, strong oscillatory velocities are induced in the streamwise direction, reminiscent of a Stokes boundary layer (Wang and Gharib, Bull. Am. Phys. Soc. 2018). We present a numerical study that isolates two possible factors that we believe may be contributing to the significant drag reduction. We utilize direct numerical simulations (DNS) to study the effects of (1) slip boundary condition with slip length oscillating in time and (2) oscillating streamwise wall boundary condition with nonzero bulk flow, revealing the foundational interplay between effective slip length, oscillations of slip length, and reduction of drag. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q42.00004: Impact of surfactant on the drag-reduction potential of superhydrophobic surfaces in turbulent flows Julien R. Landel, Scott Smith, Fernando Temprano-Coleto, Francois Peaudecerf, Frederic Gibou, Paolo Luzzatto-Fegiz Recent studies, Peaudecerf et al. (PNAS 2017) and Song et al. (PRF, 2018), have shown the negative effect of surfactant on the drag-reduction performance of superhydrophobic surfaces (SHS) in laminar flow conditions. As SHS could have a large impact in reducing energy utilisation for many internal and external flow applications, particularly under turbulent flow regimes (e.g. ships, pipelines), it is important to understand and predict how surfactant-Marangoni stresses affect turbulent flows over SHS. This is crucially important, since surfactants are present in normal environmental conditions for most applications. Our existing model for SHS inclusive of surfactant (Landel et al. arXiv:1904.01194, 2019) captures the effect of surfactant for two-dimensional laminar flow. Using a technique inspired by Belyaev {\&} Vinogradova (J. Fluid Mech. 652, 2010), we adapt our two-dimensional model to flows above three-dimensional SHS with longitudinal gratings. Then, we use the results of Fukagata et al. (Phys. Fluids 18, 2006) to relate the effect of the surfactant-affected slip length on the drag reduction of SHS in turbulent flows. We discuss the impact of the main parameters: the gas fraction, the surfactant concentration, and the Reynolds number on the drag reduction. Finally, we compare the results of our model with experimental results from the literature. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q42.00005: Effect of Interface Deformation and Contact Line Motion on Drag Reduction with Superhydrophobic and Liquid-Infused Surfaces in Laminar and Turbulent Flow Rayhaneh Akhavan, Amirreza Rastegari Effect of interface deformation and contact line motion on drag reduction (DR) with super-hydrophobic (SH) and liquid-infused (LI) surfaces is investigated in laminar and turbulent flow by direct numerical simulation (DNS) using a two-phase, single relaxation time, free-energy lattice Boltzmann method. In this method, the dynamics of a diffuse interface is incorporated into the governing equations using a Peng-Robinson free-energy functional. This obviates the need for interface tracking. DNS studies were performed in channel flows with longitudinal microgrooves of width $0.16 \le g/H \le 0.64$ in laminar flow and $15 \le g^{+0} \le 64$ in turbulent flow, at solid fractions of $\phi_s =1/16$ or $1/2$. Viscosity ratios of $\mu_{ext}/\mu_{int} = 10$, $20$ and $55$ were studied at Weber numbers of $10^{-2} \le We =\rho U_{bulk} \nu/\sigma \le 10^{-1}$ in laminar flow and $10^{-3} \le We_{\tau_0} = \rho u_{\tau_0} \nu/\sigma \le 10^{-2}$ in turbulent flow. The results show that, in both laminar and turbulent flow, interface deformation and contact line motion can significantly modify the magnitude of DR compared to results obtained with `idealized', flat SH or LI interfaces. The conditions under which the contact line depins and the interface breaks down are identified by DNS. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q42.00006: A direct numerical simulation study of heat transfer over superhydrophobic and liquid-infused surfaces Umberto Ciri, Stefano Leonardi Recently, superhydrophobic (SHS) and liquid-infused surfaces (LIS) have been proposed as a wall treatment to achieve drag reduction in turbulent flows. Conceptually, these surfaces consist of a textured substrate with a secondary fluid filling the texture cavities over which the primary fluid flows. In the case of SHS, water flows over air trapped in the cavities, while for LIS a liquid lubricant is used instead of air. Turbulent drag reduction is possible because the second fluid creates a slippery interface with the primary fluid, thus reducing friction drag. While several studies have shown potential in terms of drag reduction, less attention has been dedicated to the heat transfer. The objective of this work is to study heat transfer characteristics of these surfaces and the correlation between the velocity and thermal fields (Reynolds analogy). Direct numerical simulations of turbulent flow and heat transfer are performed using different textured geometries (modeled with the immersed boundary method) and varying the viscosity ratio and interfacial tension between the two fluids. The level-set method is used to couple the dynamics of the interface between the two fluids to the Navier-Stokes equations. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q42.00007: Drag reduction variation with respect to changes in cavity geometry for a butterfly-inspired surface Sashank Gautam, Amy Lang Monarch butterfly wings are covered with small scales arrange in a pattern that resembles roof shingles, with the tips pointing up forming low-profile cavities. As the flow passes over the wing, the skin friction drag depends on the direction of flow with respect to scale orientation. When the flow is transverse to the cavity orientation, a single vortex forms inside each of the cavity. Previous studies have documented this phenomenon as roller bearing effect which results in sub-laminar drag for very low Re. Previous work focused on rectangular cavities with an aspect ratio 2:1. This study aims to replicate the butterfly-inspired geometry with slanted wall cavities, specifically cavity models of AR 2:1 and AR 3:1 with cavity wall inclination angles of 22, 45 and 90 degrees, with the hope of optimizing the drag reduction. We hypothesize that models with AR 2:1 with 45 degrees cavity inclination angle and AR 3:1 with 22 degrees inclination angle will result in higher drag reduction. As all solid surfaces exert a no-slip condition, because of the formation of secondary vortices in the corners the primary embedded vortex will be in less contact with the cavity walls and thus be able to maintain a higher partial slip velocity as it interacts with the boundary layer. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q42.00008: Drag reduction inhibited by hydrophobic roughness distribution in the laminar regime Pierre-Yves Passaggia, Marco Castagna, Nicolas Mazellier, Azeddine Kourta Super-hydrophobic treatments of surfaces allow for minimizing friction by mean of a thin air layer separating the wall from the surrounding liquid. This interface induces a partial slip which decreases friction at the wall. However, the randomly distributed roughness in the case of a settling sphere in the laminar regime was found to produce no significant drag reduction. We begin by ruling out detrimental effects to super hydrophobic surfaces such as Marangoni-induced stresses, adsorption/desorption kinetics of surfactancts and air/liquid interface deformation using a time-scale analysis. Instead, we propose a new mechanism accounting for losses induced by the motion of the gas encapsulated around the sphere within the surface corrugations. This flow is modeled using an isotropic porous medium approach and uses the surface tortuosity and porosity as geometric parameters. Results from the model compare favorably with experiments performed by our group in glycerin and data from Modak \& Bauhmik (2017) who considered honey and syrup as working fluids. Preliminary results of spheres manufactured to control both the tortuosity and the porosity show promising drag-reduction. These results are finally discussed in the context of turbulent wall-bounded flows. [Preview Abstract] |
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