Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F26: General Fluid Dynamics: Drag Reduction, Obstacles and Constrictions |
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Chair: Jonathan Clausen, Sandia National Lab Room: Georgia World Congress Center B314 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F26.00001: Bubbly drag reduction using a superhydrophobic inner cylinder in Taylor-Couette turbulence Pim A. Bullee, Ruben A. Verschoof, Dennis Bakhuis, Rodrigo Ezeta, Sander G. Huisman, Chao Sun, Rob G. H. Lammertink, Detlef Lohse We investigate the drag of a highly turbulent flow over a non-wetting surface of micro-scale roughness. The Taylor-Couette geometry is used, allowing accurate drag and flow measurements. The inner cylinder is coated with a rough, hydrophobic material, whereas the outer cylinder is kept smooth. We vary the void fraction of air α present in the working fluid to introduce bubbles to the flow. For smaller volume fractions of air, up to α ≤ 2%, we observe that the increased surface roughness from the coating increases the drag. For larger fractions of air, α > 2%, the drag decreases compared to a smooth hydrophilic, uncoated cylinder using the same volume fraction of air. This suggests that two mechanisms play a role: the roughness invokes a shift in the log-layer – resulting in an increase in drag – and the more effective drag-reducing mechanism of the superhydrophobic surface. The balance between these two effects determines whether bubble drag reduction is more effective when using a superhydrophobic surface compared to using a smooth hydrophilic surface. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F26.00002: Investigation of Drag in Viscoelastic Turbulent Channel Flow With Slip Ethan Davis, Jae Sung Park The problem of reducing skin-friction drag has endured and is one of the long-stated motivations for research into energy savings. Many investigations have looked into various approaches to achieve skin-friction reduction. One of the most successful approaches is to add a small amount of long-chain, linear polymers to a fluid. The polymer additives can lead to significant drag reduction of upwards of 80%. Another successful approach involves superhydrophobic surfaces, allowing for slip at the wall. Superhydrophobic surfaces have been shown to achieve a skin-friction drag reduction of 20-30% in the turbulent regime. The polymer additive and superhydrophobic surface approaches give different and important insights to drag reduction. Therefore, identifying a link between these two distinct approaches would be an important advancement in our understanding of turbulence flow control. In the present study, direct numerical simulations of a turbulent channel flow were performed with the inclusion of both surface slip and polymer additives. When using the combined techniques, preliminary results show an increase in drag when compared to each respective technique. Detailed mechanisms responsible for increased drag will be explored and discussed further. |
Monday, November 19, 2018 8:26AM - 8:39AM |
F26.00003: Drag reduction via a spanwise body force and its connections to turbulent dynamics Ethan Davis, Thomas Hafner, Jae Sung Park An external body force has been successfully used to achieve turbulent drag reduction. In this study, we investigate the effects of a spanwise body force on skin-friction drag reduction in a turbulent channel flow and its connection to turbulent dynamics. A form of a traveling wave is considered for the spanwise body force, which consists of four control parameters that are the magnitude of force, penetration depth, period of oscillation, and wavelength. Direct numerical simulations (DNS) were performed to investigate the effect of these parameters on drag reduction. The current DNS results show that the skin friction is reduced by as much as 20%. An optimal condition of the spanwise body force for drag reduction is further discussed. Interestingly, it is observed that in low-drag intervals, where the wall shear stress is much less than its mean value, the spanwise body force appears to significantly affect turbulent dynamics to make the wall shear stress not as chaotic as in high-drag events. The power analysis of the body force for actual energy savings will be performed to clarify the roles of the body force for the drag reduction process. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F26.00004: The study of Taylor-Couette flow structure under the influence of macroscale corrugated surface Md Abdur Razzak, Boo Cheong Khoo, Kim Boon Lua, Tee Tai Lim, Yin Jen Lee Taylor-Couette flow with a stationary corrugated (a longitudinal groove, amplitude to wavelength ratio 0.25 and amplitude to average gap ratio 0.5) outer cylinder and a rotating smooth inner cylinder has been studied using Direct Numerical Simulation (DNS) for the radius ratio 0.5 and Reynolds number range of 60 to 650. There are six distinct flow regimes observed in this study where, at the 1st critical Reynolds number, axisymmetric stationary Taylor-Vortex (ASTV) is observed. As the Reynolds number is increased to its 2nd critical value, a pair of axisymmetric stationary secondary vortices (ASSTV) are observed in the minimum gap region of the inner cylinder. Following that, axisymmetric periodic secondary axial flow (APSAF) appears at the 3rd critical Reynolds number. As the Reynolds number is increased further, APSAF turns into a non-axisymmetric periodic secondary axial flow (NAPSAF) at the 4th critical Reynolds number. NAPSAF transforms into a non-axisymmetric complex periodic flow (NACPF) at a critical value denoted as the5th critical Reynolds number. Finally, at the 6th critical Reynolds number, non-axisymmetric non- periodic random flow (NANPRF) appears. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F26.00005: Phase averaged Turbulent statistics in drag reduced pipe flow at low and moderate Reynolds number with Transverse Wall Oscillations Daniel Coxe, Ronald J Adrian, Yulia Peet Transverse wall oscillations is a drag reduction mechanism studied in this work. We investigate the turbulent statistics in a pipe flow with transversely oscillated walls as a function of the phase angle of the wall oscillations through Direct Numerical Simulation at two different Reynolds Numbers. At Reτ = 340, a total drag reduction of 36% is achieved. At this Reynolds number, the Stokes solution to the Navier-Stokes equations matches perfectly with the mean azimuthal velocity up to the Stokes layer. The Reynolds shear stress exhibits a characteristic decrease all throughout the domain. Similarly, for the Reτ = 720 case, 34% drag reduction is achieved and a Stokes solution matches reasonably well with the mean azimuthal velocity albeit with a larger deviation. Likewise with the Reynolds stresses, the mean shear stress exhibits a decrease. Phase averaging indicates that, phase to phase, the stress is always reduced compared to the non-oscillated pipe, a Stokes solution is a valid representation of the mean azimuthal velocity below the Stokes layer, and the streamwise fluctuations exhibit reduced peak rms values. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F26.00006: Turbulent drag reduction by compliant surface tension active wall layer Alfredo Soldati, Alessio Roccon, Francesco Zonta 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 h1/h2 = 0.075. The two liquid layers have the same density but different viscosities η, specifically we consider the case η1 < η2. The problem is described by the shear Reynolds number (Reτ), by the Weber number (We, which quantifies surface tension effects) and by the viscosity ratio λ (ratio between the viscosity of the two fluids). Compared to a single phase flow at the same shear Reynolds number (Reτ = 300), in the stratified case we observe an increase of the flow rate of fluid 2 and a strong modification of the turbulence structures near the liquid-liquid interface. Altogether, these observations support the presence of a significant Drag Reduction (DR), whose efficiency depends strongly on the viscosity ratio (λ). |
Monday, November 19, 2018 9:18AM - 9:31AM |
F26.00007: The Influence of Polymer Additives on Heat and Momentum Transfer in Turbulent Flows Charles Alfred Petty, Andre Benard Turbulent drag reduction by small amounts of high molecular weight linear macromolecules has been an intense area of research for more than 80 years. A quantitative understanding of this incredible phenomenon requires a closure model for the Cauchy stress for a viscoelastic fluid and a closure model for the normalized Reynolds stress, which must be a symmetric and non-negative operator for all turbulent flows in inertial and in non-inertial frames. Unlike the Reynolds stress, the Cauchy stress must be frame invariant inasmuch as the active forces at the molecular scale are assumed to be objective vector fields derived from gradients of thermodynamic scalar potentials. However, turbulent fluctuations at the continuum scale are not objective. Thus, the Reynolds stress is not an objective operator. In this presentation, a recently developed closure for the Reynolds stress is used to explore the impact of polymer additives on the friction factor and the Nusselt number for fully-developed pipe flows. The analysis shows that the finite propagation of mean momentum associated with the Cauchy stress decouples the space-time velocity correlation near the solid/fluid interface. This phenomenon accounts for the onset of drag reduction as well as the maximum extent of drag reduction. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F26.00008: A parametric study on drag reduction using engineered microtextures in viscous laminar flow Pooyan Tirandazi, John Healy, Carlos H Hidrovo Friction reduction has been studied over years for many engineering applications that involve internal and external flows. Inspired by the natural surface structure of different creatures, engineered microtexturing of surfaces is one of the effective ways to reduce the drag. By introducing texture geometries, the flow behavior close to the wall can be manipulated towards achieving a reduced net drag force on the surface. Most works have focused on optimizing the surface texturing for reducing the friction and minimizing the pumping power requirements, while less attention has been paid to characterizing the flow and boundary layer near the wall, especially in laminar regime. We study the role of microtexturing on flow behavior under low to moderate Re. We numerically study the configuration of the textures and investigate the boundary layer and streamline behavior as well as the local shear stress distribution along the solid-fluid interface under different wetting states and flow conditions. The outcomes of this work will provide a guideline for optimal design of artificial textures with major implications for many engineering applications such as microfluidic systems used in thermal management and biochemical diagnostics. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F26.00009: Hydrodynamic object identification using artificial neural networks Sreetej Lakkam, B T Balamurali, Roland Bouffanais Passive object sensing has evolved in aquatic animals to enable them to recognize hydrodynamic objects. This unique capability can be used in autonomous underwater vehicles to gain better awareness of the marine environment. Here, we present a data-driven model that uses artificial neural networks to identify the shape of an obstacle placed in potential flow using data from a stationary sensor array. Specifically, the machine learning framework is used to solve the inverse problem of estimating the body shape from the measured velocity field. The ability of neural networks to deduce the complex underlying relationships without explicit mathematical description is used for parametric fitting of velocity flow data to accurately predict object shape characteristics. Synaptic weights obtained using a gradient-descent based optimization are used to obtain relations between the shape coefficients and the velocity field. Large data sets corresponding to flows with varying object shapes are generated and used to train and validate the performance of this machine learning approach. Finally, this data-driven method is easy to train owing to the analytical nature of the forward problem and is found to accurately estimate object shapes from limited and localized data acquired at a distance. |
Monday, November 19, 2018 9:57AM - 10:10AM |
F26.00010: Energy harvesting with a rotating cylinder pair in a free-stream flow Wim M van Rees We present numerical simulations of a cylinder pair placed orthogonally to a uniform parallel flow at a Reynolds number of O(100). The cylinder pair's rotation is driven by the incoming flow and energy harvesting is modeled by applying an external torque proportional to each cylinder's rotational velocity. For a given cylinder spacing, this problem is described by two non-dimensional parameters: the Reynolds number and the non-dimensional damping coefficient. We perform a set of simulations to compute the average power generated as a function of these two parameters, and compare against other approaches. The results provide insight into the use of viscous-based power generation mechanisms at intermediate Reynolds numbers. |
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