Session Q06: Boundary Layers: Superhydrophobic Surfaces (3:55pm - 4:40pm CST)

Fluid retention in liquid infused surfaces: A direct numerical simulation study

Liquid infused surfaces (LIS) are surface textures wetted with infused liquid lubricant and can reduce turbulent drag up to 35%. However, for practical use, these surfaces must be designed to withstand the shear of the external flow. In previous studies, ideal texture geometries such as infinite longitudinal bars have been used. However, these geometries fall short in retaining the lubricant and sustaining drag reduction. Therefore, in this study, we have tried to model texture geometries which can retain the lubricant. For this purpose, we performed direct numerical simulations of a turbulent channel flow with a texture made of rectangular cavities. A viscosity ratio between the lubricant and the main stream of fluid, m=0.4 is defined. The aspect ratios of the cavities, and the Weber number are varied. Compared to the flow over longitudinal bars, the rectangular mesh has additional transverse bars to close the cavity. This increases the drag but helps in retaining the lubricant. For the finite surface tension cases, a rebounding capillary pressure wave propagation is observed for the mesh configuration altering the flow dynamics close to the wall. Overall, this texture sustains the drag reduction and decreases the turbulent intensities showing promise for further studies.   

Effect of interface deformation and contact line motion on turbulent skin-friction drag reduction with superhydrophobic surfaces

Effect of interface deformation and contact line motion on turbulent skin-friction Drag Reduction (DR) with SuperHydrophobic (SH) surfaces is investigated by DNS using a two-phase free-energy lattice Boltzmann method. DNS studies were performed in turbulent channel flows at $Re_{\tau_0} \approx 222$ with SH longitudinal microgrooves of width $15 \le g^{+0} \le 64$ at solid fractions of $\phi_s=$1/16 \& 1/2 on both walls. Simulations were performed at viscosity ratios of $\mu_{ext}/\mu_{int}=50$, Weber numbers of $We_{\tau_0} \equiv \rho u_{\tau_0} \nu/\sigma \approx 2 \times 10^{-3}$ and dynamic contact angles of $\theta_{adv} = 112^{\circ}$ and $\theta_{rec} = 106^{\circ}$. Two initial configurations of SH interfaces were investigated, corresponding to contact angles of $\theta_c = 90^{\circ}$ and $120^{\circ}$. Contact line motion was found to magnify the apparent wetted surface area of the microgrooves, thus reducing the effective streamwise slip velocities by 7-50\%. Interface deformation was found to enhance the effective spanwise slip velocities by up to 200\% with initially curved interfaces. These combined effects lead to drops of 7-32\% and 16-50\% in the magnitude of DR with initially flat and curved interfaces, respectively, compared to `idealized' flat SH walls.   

Analysis of wall-normal jets induced by bubble oscillations on superhydrophobic surfaces

Superhydrophobic surfaces (SHS) have received significant attention for achieving drag reduction by reducing skin friction drag. Experimental results of patterned SHS have shown that pressure control can sustain wall-attached air films and that the dynamic modulation of air film height can lead to even further drag reduction. It has been observed that, under such conditions, rapid change in the height and shape of the air film can induce substantial wall-normal velocities (Wang and Gharib, J. Fluid Mech. 2020). In this work, we numerically characterize these jet-like flows structures in a laminar setting. We present an assessment of the effects of the free shear boundary condition, which corresponds to the dynamic slip condition of the experimental air films, and the effects of an otherwise no-slip boundary condition, which corresponds to unsteady wall deformation. Interaction of the induced near-wall flow structures with turbulent crossflow and implications on possible drag reduction of the turbulent boundary layer will be discussed.   

Stability Limits of Liquid-Infused Surfaces and Their Effects on Turbulent Drag

Liquid-infused surfaces (LIS) have shown great potential in decreasing drag for turbulent flows. They consist of surface structures infused with another liquid, and are relatively robust against failure due to turbulent pressure fluctuations, in contrast to superhydrophobic surfaces. However, their stability depends on the surface tension and the surface chemistry of the surface. We investigate the stability limits for the case of square longitudinal grooves with infused liquid, using direct numerical simulations at friction Reynolds numbers around $Re_\tau = 180$. The interface is described using a volume-of-fluid (VOF) framework, allowing large interface deformations as well as moving contact lines. The viscosity ratio is kept at the order of 1, representing realistic values of oil-water systems. A large contact angle causes the contact line to depin and move into the groove. For our geometry, however, mass conservation is a stabilizing effect, because if the interface depins on one position, it is raised elsewhere. Due to the finite surface tension, the surface creates an apparent slip, but damps only parts of the wall-normal velocity fluctuations. A too low surface tension causes large capillary waves to form, increasing drag dramatically.