Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session W14: General Fluid Dynamics: Drag Reduction (10:00am - 10:45am CST)Interactive On Demand
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W14.00001: Wetting and slippage on irregularly nanostructured superhydrophobic surfaces Clarissa Sch\"onecker, Xin Zhao, Andreas Best, Kaloian Koynov, Hans-J\"urgen Butt Irregularly nanostructured superhydrophobic surfaces are widely employed for applications such as drag-reduction, control of wetting, ant-biofouling, and many more. While they show a significant drag reduction in applications, this cannot be explained by microscopic models or experiments which are based on regularly structured surfaces. As an example of an application-relevant surface, we investigated wetting and slippage on silicone nanofilaments. For this purpose, we developed an evaluation method for Fluorescence Correlation Spectroscopy that allows us to measure velocity profiles down to about 0.4micrometer close to the surface. We found that the velocity profiles are still nonlinear below 1micrometer close to the surface, which is important for an accurate slip measurement. Additionally, we found that the irregularity of the surface may lead to large air inclusions. These inclusions possess a large slip length and may therefore explain the high drag-reduction observed in applications of irregularly nanostructured superhydrophobic surfaces. [Preview Abstract] |
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W14.00002: Energy budget in lubricated drag-reduced turbulent channel flow Alessio Roccon, Francesco Zonta, Alfredo Soldati We use direct numerical simulation to study the problem of drag reduction in a lubricated channel, a flow instance in which a thin layer of a lubricating fluid (density $\rho_1$, viscosity $\eta_1$, thickness $h_1$) is injected in the near-wall region of a plane channel, so to favor the transportation of a primary fluid (density $\rho_2$, viscosity $\eta_2$, thickness $h_2$). The primary and lubricating fluids have the same density but different viscosity, such that a viscosity ratio $\lambda=\eta_1/\eta_2$ can be defined. Building on a sound flow characterization, we show that significant drag reduction (DR) can be achieved. Reportedly, the observed DR is a non-monotonic function of $\lambda$ and, in the present case, is maximum for $\lambda = 1.00$ ($\simeq 13\%$ flow rate increase). For the cases $\lambda \le 1.00$ (low-viscosity lubricating fluid), and confirming previous investigations, we show the existence of two different DR mechanisms: when the two fluids have the same viscosity, DR is purely due to the effect of the surface tension. When the viscosity of the lubricating layer is reduced, turbulence can be sustained in the lubricating layer and DR is simply due to the smaller viscosity of the lubricating layer that acts to decrease the corresponding wall friction. [Preview Abstract] |
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W14.00003: Ventilated supercavitation around a moving body in a still fluid Yeunwoo Cho, Jaeho Chung This experimental study examines ventilated supercavity formation in a free-surface bounded environment where a body is in motion and the fluid is at rest. For a given torpedo-shaped body and water depth (H), depending on the cavitator diameter (d$_{\mathrm{c}})$ and the submergence depth (h$_{\mathrm{s}})$, four different cases are investigated according to the blockage ratio (B$=$d$_{\mathrm{c}}$/d$_{\mathrm{h}}$, where d$_{\mathrm{h}}$ is the hydraulic diameter) and the dimensionless submergence depth (h*$=$h$_{\mathrm{s}}$/H). Cases 1--4 are no cavitator in fully submerged (B$=$0, h*$=$0.5), small blockage in fully submerged (B$=$15{\%}, h*$=$0.5), small blockage in shallowly submerged (B$=$1.5{\%}, h*$=$0.17) and large blockage in fully submerged (B$=$3{\%}, h*$=$0.5) cases. In case 1, no supercavitation is observed and only a bubbly flow (B) and a foamy cavity (FC) are observed. In cases 2 and 3, a twin-vortex supercavity (TV), a reentrant-jet supercavity (RJ), a half-supercavity with foamy cavity downstream (HSF), B and FC are observed. In case 4, a half-supercavity with a ring-type vortex shedding downstream (HSV), double-layer supercavities (RJ inside and TV outside (RJTV), TV inside and TV outside (TVTV), RJ inside and RJ outside (RJRJ)), B, FC and TV are observed. The body-frontal-area-based drag coefficient for a moving torpedo-shaped body with a supercavity is measured to be approximately 0.11 while that for a cavitator-free moving body without a supercavity is approximately 0.4. [Preview Abstract] |
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W14.00004: A comparison of polymer and surfactant drag-reduced flows Lucas Warwaruk, Sina Ghaemi Small concentrations of polymers and surfactants can cause drag reduction in turbulent liquid flows by as much as 80{\%}. Whether different additives inhibit drag in a similar manner, remains an enigma. We directly compared the steady shear viscosity, extensional rheology, and velocity statistics in a turbulent channel flow, for solutions of a flexible polymer, a rigid polymer and a surfactant. The rigid polymer had the largest shear viscosity, while the flexible possessed the largest extensional characteristics. Lagrangian three-dimensional particle tracking velocimetry was used to measure the turbulence statistics. Measurements were performed at a constant high drag reduction (HDR) of 55{\%}, and maximum drag reduction (MDR), of 75{\%}. The surfactant and the flexible polymer solutions showed similarities in their mean velocity and Reynolds stresses at both HDR and MDR. The mean velocity and Reynolds stresses for the rigid polymer did not overlap with the other additives. However, the discrepancy was small and it was associated with the higher shear viscosity of the rigid polymer solution. [Preview Abstract] |
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W14.00005: Effect of Wetted Microtexturing on Hydrodynamic and Thermal Characteristics in Microchannel Flow Nastaran Rabiei, Grace McDonough, Carlos Hidrovo Microscale duct flow is characterized by large laminar pressure drop. Due to the wide applications of microchannel flow in different areas, such as drug delivery and microelectronics cooling, exploring new methods to manipulate their hydrodynamic and thermal behavior can result in improved performance and energy saving benefits. Our goal here is to obtain a better understanding of the flow physics inside microchannels with microstructures on the walls. We are working on investigating the combined effect of flow and heat transfer when there are square trenches with different dimensions, both experimentally and numerically. The microstructures on the surfaces increase the wetting surface area which is expected to increase the friction (skin drag) induced by the shear forces, but the recirculation generated inside the grooves can reduce this effect. Conversely, the recirculation can cause a negative pressure difference opposing the flow direction (pressure drag). The textures disturb the thermal boundary layer and can potentially improve heat transfer through recirculation mixing. However, low conductivity of stagnant fluid trapped inside the grooves can adversely impact the total heat transfer. In this ongoing research, we are interested in figuring out if any combination of the geometrical parameters of the trenches can result in the lowest drag while having the highest heat transfer. [Preview Abstract] |
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