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 B26: Drag Reduction I |
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Chair: Javier Jimenez, E.T.S. Ingenieros Aeronauticos Room: 608 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B26.00001: Active control of deterministic turbulent spots for drag reduction Kwing-So Choi, Yaxing Wang, Michael Gaster, Chris Atkin, Yury Kachanov, Vladimir Borodulin A series of experiments was carried out using Gaster's wind tunnel at City, University of London, where the freestream turbulence level in the test section was extremely low (0.006{\%} between 2 Hz and 2 kHz). With weak excitations applied from spanwise-periodic 19 miniature speakers located downstream of a flat-plate leading edge, the boundary-layer development was studied in detail using a hot-wire anemometer at laminar, transitional and turbulent stages. Careful velocity measurements revealed an appearance of turbulent spots, which were precisely reproducible in both time and space each time the pseudo-random signal was applied. The emergence of turbulent spots, which bypassed a full development of T-S waves, was deterministic at least in the lower frequency range of velocity signals, enabling us to examine their structure that was not possible before. Opposition control was then carried out by issuing wall-normal jet on the high-speed region of turbulent spots with a view to achieve a skin-friction reduction by delaying transition to fully-developed turbulence. This was done without sensors as all boundary-layer structures were ``deterministic''. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B26.00002: ABSTRACT WITHDRAWN |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B26.00003: Active Drag Reduction in Turbulent Airfoil Flow Wolfgang Schröder, Marian Albers Drag reduction in turbulent boundary layers is key for substantial energy savings in aerodynamics. Large parts of the flow over the wing of modern aircraft are turbulent such that even net energy savings of a few percent lead to high cost savings. Active drag reduction methods have shown to be capable of significantly reducing the drag in generic external turbulent wall-bounded flows. Based on the knowledge of previous studies for flat-plate turbulent boundary layer flow the technique of spanwise traveling transversal surface waves is applied to 74 percent of the surface of a NACA4412 wing section at a chord-based Reynolds number of $Re_c = 400,000$. Different parameter combinations are tested for maximum drag reduction and maximum net power saving. The results show a reduction of the total drag of up to 8.5 percent and a decrease of the viscous drag by up to 12.9 percent. Note that this includes all actuated and non-actuated parts of the surface, i.e., locally a much higher decrease of the wall-shear stress is achieved. Additionally the lift is slightly increased by up to 1.4 percent. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B26.00004: Active flow control of the logarithmic layer Anna Guseva, Miguel P. Encinar, Javier Jimenez Active flow control for years has been a vivid topic of fluid dynamics research. It is of especial importance for wall-bounded turbulent flows, where intense dissipation at the wall can produce undesirable effects. One successful control approach is to apply at the boundary a velocity field opposite to the observed in the buffer layer. The focus of this work is on creating a control strategy that can be reproduced in experimental facilities. By acting on the flow from the wall, we aim to affect the eddies of relatively large wavelengths (λ/h > 0.1) at $Re_\tau = 1000$. We reconstruct the wall-normal velocity in the log-layer ($y^+ \geq 100$) with the linear stochastic estimation method. Preliminary implementation of opposition control on the large scales results in substantial drag increase, indicating that we are able to significantly affect those scales and have plenty of control authority. In the conference we present further development of this strategy. We compare the control efficiency of applying different wavelength bands at the wall and check the impact of imposed boundary conditions on the statistics of velocity fluctuations, as well as new structures created in the flow. Finally, we assess the general applicability of our control to the existing measurement techniques. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B26.00005: Drag Reduction in the Flow around a Cylinder: A Bayesian Optimization Approach Anthony Larroque, Miguel Fosas de Pando, Luis Lafuente Bayesian Optimization has recently gained popularity as an effective optimization method to deal with expensive black-box objective functions. Advantages of this method include the ability to determine the global minimum at a reduced number of iterations, and the possibility to include uncertainty in the evaluation of the objective function. Recent developments also include parallel function evaluations or multiple sources of information with varying fidelity. All these features render Bayesian Optimization a promissing tool for Direct Numerical Simulations or Large Eddy Simulations, where the computational cost of determining the cost function, such as the drag coefficient, is typically very high, and the computational resources are very limited. In this work, we consider the three dimensional flow around a cylinder and apply Bayesian Optimization to determine the optimal blowing and suction strategy at the wall that leads to a minimum drag coefficient. The efficiency of the resulting optimization scheme is compared to alternative methods. Finally, we discuss the optimal parameters of the velocity profile, the underlying physical mechanisms, and the influence of the Reynolds numbers on the optimal solutions. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B26.00006: Drag reduction of a sphere with oscillation in pseudo-plastic fluid Xianping Zhang, Kazuyasu Sugiyama, Minoru Iwamuro, Tomoaki Watamura A force acting on a spherical particle or bubble moving in pseudo-plastic fluid is numerically investigated. The sphere motion is characterized by prescribed translational and oscillating velocities. The unsteady Stokes equation together with the power-law viscosity $\mu=K\dot{\gamma}^{n-1}$ (here, $K$ is the consistency factor, $\dot{\gamma}$ is the shear rate, and $n$ is the power index) is solved by a finite-difference approach with varying $n$ and the oscillation amplitude $A$. With increasing $A$, the time-averaged drag force reduces due to the enhanced shear-thinning effect. Such a drag reduction is more remarkable with decreasing $n$, and is arranged by two scaling relations for small $A$ and for large $A$. Examining the instantaneous and time-averaged velocity distributions, we discuss the relevance of the Stokes boundary layer near the sphere surface and the nearly irrotational velocity in the bulk. [Preview Abstract] |
Saturday, November 23, 2019 5:58PM - 6:11PM |
B26.00007: Turbulent drag reduction by compliant lubricating layer Alessio Roccon, Francesco Zonta, Alfredo Soldati We propose a physically-sound explanation for the drag reduction (DR) mechanism in a lubricated channel, a flow configuration in which an interface separates a thin layer of fluid (viscosity $\eta_1$) from a main layer of fluid (viscosity $\eta_2$). To single out the effect of surface tension, we focus initially on two fluids having same density and viscosity, and we then consider a wide range of viscosities of the lubricating layer: from $\lambda=\eta_1/\eta_2=0.25$ (less viscous) up to $\lambda=\eta_1/\eta_2=4.00$ (more viscous). A database comprising DNS of two-phase flow channel turbulence is used to study the physical mechanisms driving DR, which we report between 20\% and 30\% for $\lambda \leq 1$, 10 \% for $\lambda=2.00$ and absent for $\lambda=4.00$. The maximum DR occurs when the two fluids have the same viscosity ($\lambda=1$), and corresponds to the relaminarization of the lubricating layer. Decreasing the viscosity of the lubricating layer ($\lambda<1$) induces a marginally decreased DR, but also helps sustaining strong turbulence in the lubricating layer. This led us to infer two different mechanisms for the two drag-reduced systems, each of which is ultimately controlled by the outcome of the competition between viscous, inertial and surface tension forces. [Preview Abstract] |
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