Session F22: General Fluid Dynamics: Obstacles, Constrictions, and Drag Reduction

Investigation of flow characteristics in the vicinity of a sediment embedded vertical retaining wall

Global measurements of turbulent flows at the leading edge a vertical retaining wall were conducted to examine the intricate flow physics behind the local scouring process. Three laboratory setups were considered: one with an immobile, permeable, rough boundary and a fixed channel bank, one with a mobile gravel bed, but a static channel bank, and one with a mobile bed and an erodible bank. The measurements were obtained using stereo particle image velocimetry (SPIV) in a plane perpendicular to the approach flow direction over a granular bed under a clear-water scour condition. Time-averaged flow topology, turbulence statistics, and instantaneous fields associated with the in-plane and out-of-plane velocity components were examined. Investigation of instantaneous streamline topology indicated the intermittent development of vortices within the area under study. It was also demonstrated that, in the presence of a scour hole, streamwise vorticity tends to diffuse along the boundary rather than concentrate close to the wall and bed. Additionally, the results indicated that development of scour hole increases the values of turbulent kinetic energy (TKE) and turbulence intensity near the bed. These, in turn, provide more energy to enhance grain mobility and sustain the transport of sediment particles and consequently deepen the scour hole.   

Sliding Friction on Liquid-Infused Surfaces

Slippery porous liquid-infused surfaces (SLIPS) are well-known for their ability to stably minimize the hysteresis of a wide variety of liquids. However, whether SLIPS could also reduce the sliding friction of solid objects has not been given much consideration. Here, we measure the friction force associated with dragging an aluminum cube across an array of ordered silicon micropillars impregnated with silicone oil. The solid fraction of the micropillars was either 0.025 or 0.25, while the viscosity of the silicone oil was 10, 100, or 1,000 cSt. Non-intuitively, it was observed that the sliding friction decreased with increasing lubricant viscosity or increasing solid fraction. These findings suggest that the key parameter is therefore the hydraulic resistance of the alleys between the micropillars, which should be as large as possible to minimize lateral oil drainage from underneath the sliding body. This would indicate that scaling down to nano-roughness would be optimal for minimizing the sliding friction, which was confirmed by additional experiments on a disordered nanostructured substrate.   

Superhydrophobic Drag Reduction in Various Turbulent Flows

Superhydrophobic surfaces (SHSs) have been studied exhaustively in laminar flow applications while interest in SHS drag reduction in turbulent flow applications has been increasing steadily. In this discussion, we will highlight recent advances of SHS applications in various high-Reynolds number flows. We will address the application of mechanically robust and scalable spray SHSs in three cases: fully-developed internal flow; a near-zero pressure gradient turbulent boundary layer; and an axisymmetric DARPA SUBOFF model. The model will be towed in the University of Michigan's Physical Model Basin. Experimental measurements of streamwise pressure drop and the near-wall flow via Particle Image Velocimetry and Laser Doppler Velocimetry will be discussed where applicable. Moreover, integral measurement of the total resistance of the SUBOFF model, with and without SHS application, will be examined. The SUBOFF model extends 2.6 m and is 0.3 m in diameter, and will be tested at water depths of three to six model diameters. Previous investigation of these SHSs have proven that skin-friction savings of 20{\%} or more can be attained for friction Reynolds numbers greater than of 1,000.   

Superhydrophobic and polymer drag reduction in turbulent Taylor-Couette flow

We use a custom-built Taylor-Couette apparatus (radius ratio $\eta = 0.75$) to study frictional drag reduction by dilute polymer solutions and superhydrophobic (SH) surfaces in turbulent flows for $15000 < \mathrm{Re} < 86000$. By monitoring the torque-speed scaling we show that the swirling flow becomes fully turbulent above $\mathrm{Re} = 15000$ and we focus on measurements in this regime. By applying SH coatings on the inner cylinder, we can evaluate the drag reducing performance of the coating and calculate the effective slip length in turbulent flow using a suitably modified Prandtl--von~K\'{a}rm\'{a}n analysis [1]. We also investigate drag reduction by dilute polymer solutions, and show that natural biopolymers from plant mucilage can be an inexpensive and effective alternative to synthetic polymers in drag reduction applications, approaching the same maximum drag reduction asymptote. Finally we explore combinations of the two methods -- one arising from wall slip and the other due to changes in turbulence dynamics in the bulk flow -- and find that the two effects are not additive; interestingly, the effectiveness of polymer drag reduction is drastically reduced in the presence of an SH coating on the wall.\\ {}[1] S.~Srinivasan et al., Phys.~Rev.~Lett. 114, 014501 (2015).   

Frictional Torque Reduction in Taylor-Couette Flows with Riblet-Textured Rotors

Inspired by the riblets on the denticles of fast swimming shark species, periodic surface microtextures of different shapes have been studied under laminar and turbulent flow conditions to understand their drag reduction mechanism and to offer guides for designing optimized low-friction bio-inspired surfaces. Various reports over the past four decades have suggested that riblet surfaces can reduce the frictional drag force in high Reynolds number laminar and turbulent flow regimes. Here, we investigate the effect of streamwise riblets on torque reduction in steady flow between concentric cylinders, known as Taylor-Couette Flow. Using 3D printed riblet-textured rotors and a custom-built Taylor-Couette cell which can be mounted on a rheometer we measure the torque on the inner rotor as a function of three different dimensionless parameters; the Reynolds number of the flow, the sharpness of the riblets, and the size of the riblets with respect to the scale of the Taylor-Couette cell. Our experimental results in the laminar viscous flow regime show a reduction in torque up to 10{\%} over a wide range of Reynolds numbers, that is a non-monotonic function of the aspect ratio and independent of Re. However, after transition to the Taylor vortex regime, the modification in torque becomes a function of the Reynolds number, while remaining a non-monotonic function of the aspect ratio. Using finite volume modelling of the geometry we discuss the changes in the Taylor-Couette flow in presence of the riblets compared to the case of smooth rotors and the resulting torque reduction as a function of the parameter space defined above.   

Anisotropic particles in highly turbulent Taylor-Couette flow

In industry and nature, particle-laden turbulent flows consist mostly, if not always, of anisotropic particles. Examples of such flows are plankton distributions in the oceans, and pumping of concrete. In these flows, the suspended particles often distribute inhomogeneously, thereby affecting the drag and the flow properties significantly. Despite their widespread occurrence, a good understanding of how such particles affect the flow is still missing. Here we performed Particle Tracking Velocimetry and global torque measurements for a suspension of rigid fibers (or rods) in the Twente Turbulent Taylor-Couette facility. The fibers are density matched with the fluid, and we used particle volume fractions up to $\alpha=2\%$ of fibers with aspect ratio $\lambda= L / d=5$, where $L=5$ mm is the length and $d=1$ mm the diameter. The global torque measurements were performed for Reynolds numbers up to $2.5 \times 10^5$ and showed similar values of drag reduction as was obtained for spherical particles ($\lambda=1$). Using PTV we have extracted the orientation, the rotation rate, and the translation velocity and acceleration for the fibers. The fibers do not show a clear alignment with the main velocity gradient. We do, however, observe occasional large rotation rates for the fibers.   

Three-dimensional hydrodynamics of a suspended cylindrical canopy patch

Three-dimensional large eddy simulations (LES) are carried out to determine the local hydrodynamics of a suspended canopy patch impinged by a uniform incident flow. The patches are circular (with bulk diameter $\mathrm{D})$ and are made of rigid circular cylinders (height $\mathrm{h}$ and diameter $\mathrm{d})$. Four different patch densities ($\mathrm{\phi =}\mathrm{N}_{\mathrm{c}}\mathrm{d}^{\mathrm{2}}\mathrm{/}\mathrm{D}^{\mathrm{2}})$ and four different patch aspect ratios ($\mathrm{AR=h/D})$ are considered by varying the number of cylinders in the patch ($\mathrm{N}_{\mathrm{c}})$ and the height of the patch ($\mathrm{h})$, respectively. Based on a volumetric-flux budget through the patch surface, the bleeding dynamics inside and in the vicinity of the patch was found to be controlled not only by $\mathrm{\phi }$, but also remarkably by $\mathrm{AR}$. The relative longitudinal bleeding normalized by the total flux entering the patch ($\hat{Q}_{\mathrm{x}}\mathrm{=}\mathrm{Q}_{\mathrm{x}}\mathrm{/}\mathrm{Q}_{\mathrm{influx}})$ was observed to be inhibited by increasing $\mathrm{\phi }$ but insensitive to the variation of $\mathrm{AR}$; the relative lateral bleeding ($\hat{Q}_{\mathrm{y}}\mathrm{=}\mathrm{Q}_{\mathrm{y}}\mathrm{/}\mathrm{Q}_{\mathrm{influx}})$ increases with either increasing $\mathrm{\phi }$ or $\mathrm{AR}$; and the relative vertical bleeding ($\hat{Q}_{\mathrm{z}}\mathrm{=}\mathrm{Q}_{\mathrm{z}}\mathrm{/}\mathrm{Q}_{\mathrm{influx}})$ increases with increasing $\mathrm{\phi }$ while decreases with increasing $\mathrm{AR}$. However, for patches with a constant $\mathrm{\phi }$, an increase in $\mathrm{AR}$ contributes to enhance the absolute strength of vertical bleeding ($\mathrm{Q}_{\mathrm{z}})$ at the patch free end.   

Turbulence annihilation in surface tension stratified flow

In this work we use Direct Numerical Simulation (DNS) together with a Phase Field technique to study the turbulent Poiseuille flow of two immiscible liquid layers inside a channel. A thin liquid layer (fluid 1) flows on top of a thick liquid layer (fluid 2), such that their thickness ratio is $h_1 = 9h_2$. The two liquid layers have the same density but different viscosities $\eta$. In particular, we consider the case $\eta_2 < \eta_1$. The problem is described by the shear Reynolds number ($Re_{\tau}$), by the Weber number ($We$, which quantifies surface tension effects) and by the viscosity ratio $\lambda$ between the two fluids. Compared to a single phase flow at the same shear Reynolds number ($Re_\tau=300$), in the two phase flow case we observe an increase of the flow rate of fluid 1 and a strong modification of the turbulence structures near the liquid-liquid interface. Alltogether, these observations support the presence of a significant Drag Reduction (DR), whose efficiency depends strongly on the interface deformability ($We$) and on the viscosity ratio between the two fluids ($\lambda$).   

Wind-Induced Reconfigurations in Flexible Branched Trees.

Wind induced stresses are the major mechanical cause of failure in trees. We know that the branching mechanism has an important effect on the stress distribution and stability of a tree in the wind. Eloy in PRL 2011, showed that Leonardo da Vinci's original observation which states the total cross section of branches is conserved across branching nodes is the best configuration for resisting wind-induced fracture in rigid trees. However, prediction of the fracture risk and pattern of a tree is also a function of their reconfiguration capabilities and how they mitigate large wind-induced stresses. In this studies through developing an efficient numerical simulation of flexible branched trees, we explore the role of the tree flexibility on the optimal branching. Our results show that the probability of a tree breaking at any point depends on both the cross-section changes in the branching nodes and the level of tree flexibility. It is found that the branching mechanism based on Leonardo da Vinci's original observation leads to a uniform stress distribution over a wide range of flexibilities but the pattern changes for more flexible systems.