Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D7: Flow Control: Drag Reduction 
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Chair: Mohamed GadelHak, Virginia Commonwealth University Room: B115 
Sunday, November 20, 2016 2:57PM  3:10PM 
D7.00001: Active flow control for a NACA0012 Profile: Part II H. Oualli, M. Makadem, H. Ouchene, A. Ferfouri, A. Bouabdallah, M. GadelHak Active flow control is applied to a NACA0012 profile. The experiments are conducted in a wind tunnel. Using a highresolution visiblelight camera and tomography, flow visualizations are carried out. LES finitevolume 3D code is used to complement the physical experiments. The symmetric wing is clipped into two parts, and those parts extend and retract along the chord according to the same sinusoidal law we optimized last year (B.\ Am.\ Phys.\ Soc., vol.\ 60, no.\ 27, p.\ 247, 2015) for the same profile but clipped at an angle of 60 deg, instead of the original 90 deg. The Reynolds number range is extended to 500,000, thus covering the flying regimes of microUAVs, UAVs, as well as small aircraft. When the nascent cavity is open and the attack angle is 30 deg, the drag coefficient is increased by 1,300\%, as compared to the uncontrolled case. However, when the cavity is covered and Re $\leq 10^5$, a relatively small frequency, $f \leq 30$~Hz, is required for the drag coefficient to drop to negative values. At the maximum Reynolds number, thrust is generated but only at much higher frequencies, $12 \leq f \leq 16$~kHz. [Preview Abstract] 
Sunday, November 20, 2016 3:10PM  3:23PM 
D7.00002: Direct numerical simulation of turbulent channel flow over a liquidinfused microgrooved surface Jaehee Chang, Taeyong Jung, Haecheon Choi, John Kim Recently a superhydrophobic surface has drawn much attention as a passive device to achieve high drag reduction. Despite the high performance promised at ideal conditions, maintaining the interface in real flow conditions is an intractable problem. A nonwetting surface, known as the slippery liquidinfused porous surface (SLIPS) or the lubricantimpregnated surface (LIS), has shown a potential for drag reduction, as the working fluid slips at the interface but cannot penetrate into the lubricant layer. In the present study, we perform direct numerical simulation of turbulent channel flow over a liquidinfused microgrooved surface to investigate the effects of this surface on the interfacial slip and drag reduction. The flow rate of water is maintained constant corresponding to Re$_{\tau}$$\sim$180 in a fully developed turbulent channel flow, and the lubricant layer is sheardriven by the turbulent water flow. The lubricant layer is also simulated with the assumption that the interface is flat (i.e. the surface tension effect is neglected). The solid substrate in which the lubricant is infused is modelled as straight ridges using an immersed boundary method. DNS results show that drag reduction by the liquidinfused surface is highly dependent on the viscosity of the lubricant. [Preview Abstract] 
Sunday, November 20, 2016 3:23PM  3:36PM 
D7.00003: Drag reduction of boattailed bluff bodies through transverse grooves. Part I: experiments Alessandro Mariotti, Guido Buresti, Maria Vittoria Salvetti The reduction of the aerodynamic drag of elongated axisymmetric bluff bodies is interesting for several applications. One wellknown method to reduce the drag of this type of body is a geometrical modification called boattailing, consisting in a gradual reduction of the body crosssection before a sharpedged base. We combine boattailing with properly contoured transverse grooves to further delay boundarylayer separation and reduce drag. The considered geometry is axisymmetric with an elliptical forebody and a cylindrical main body followed by a circulararc boattail. The effectiveness of the contoured grooves was assessed through experiments and simulations. In this talk the experimental investigation is presented. Pressure measurements show that the introduction of a single transverse groove leads to a significant increase of the pressure on the body base and, consequently, to a reduction of drag compared with the boattail without the groove. Velocity measurements and flow visualizations highlight that this is due to a delay of flow separation over the boat tail. A steady local recirculation is present inside the groove and downstream its reattachment the boundary layer is thinner and has higher momentum than in the case with no groove, allowing separation to be delayed. [Preview Abstract] 
Sunday, November 20, 2016 3:36PM  3:49PM 
D7.00004: Drag reduction of boattailed bluff bodies through transverse grooves. Part II: largeeddy simulations Maria Vittoria Salvetti, Alessandro Mariotti, Guido Buresti The present work focuses on strategies for aerodynamic drag reduction of elongated axisymmetric bluff bodies, which can be viewed as simplified models of a road vehicles. We combine boattailing, i.e. a gradual reduction of the body crosssection before a sharpedged base, with properly contoured transverse grooves. The effectiveness of this strategy was assessed through experiments and simulations. Experiments showed that the introduction of a single groove leads to a further delay of boundarylayer separation and to a reduction of drag compared with the boattail configuration without grooves. In this talk, we present LargeEddy Simulations (LES). LES results agree with the experimental findings. The success of the proposed flow control strategy is due to the relaxation of the noslip condition in the small recirculation region inside the groove, which reduces the momentum losses near the wall and thus delays boundary layer separation. The effects of the introduction of the groove on the mean topology and on the dynamics of the near wake are also highlighted. Finally, a sensitivity analysis of the proposed control strategy efficiency to the groove location and to the boattail geometry is shown. [Preview Abstract] 
(Author Not Attending)

D7.00005: Drag reduction of a streamlined body at incidence using rotating cylinders James Schulmeister, Michael Triantafyllou The flow past a streamlined body at incidence is characterized by crossflow separation that induces large forces and moments. We investigate the use of counterrotating control cylinders to delay separation and reduce the drag on a streamlined body at incidence in water tunnel experiments. A streamlined body model with rotating control cylinders was fixed at angles of incidence up to 30 degrees in a water tunnel while the forces and moments were monitored. The control cylinders have diameters equal to 10\% of the maximum diameter of the streamlined body and are embedded in the model such that part of the circumference is exposed to the flow. The control cylinders are counterrotated so that the moving surface imparts momentum to the flow, encouraging the delay of crossflow separation and the reduction of drag. [Preview Abstract] 
Sunday, November 20, 2016 4:02PM  4:15PM 
D7.00006: Turbulent drag reduction with liquidinfused surfaces Alexander Smits, Tyler Van Buren We present turbulent skin friction reduction over liquidimpregnated surfaces in TaylorCouette flow. The surface of the inner cylinder of the facility contains square grooves, with widths from 100 $\mu$m to 800 $\mu$m and a fixed liquid area of half the total area. Alkane liquids are infused in the surface with viscosities from $\sim$1/3 to 2 times that of water. For Reynolds numbers up to $Re_d$=10,500 corresponding to a flow shear of $\tau$=50 Pa, we achieve drag reduction exceeding 30\%, three times higher than ever reported for liquidinfused surfaces. [Preview Abstract] 
Sunday, November 20, 2016 4:15PM  4:28PM 
D7.00007: Large Structures of DragReducing Pipe Flow by Surfactant Additives. Yuki Kishita, Yoshitsugu Naka, Yuki Minamoto, Masayasu Shimura, Mamoru Tanahashi Characteristics of dragreducing turbulent pipe flows with surfactant additives have been investigated using stereoscopic particle image velocimetry. Measurements have been performed for a case with surfactant solution of 150 ppm at different Reynolds numbers: ${Re}_{d}=31254,\thinspace 58268,\thinspace 85556$, around the maximum dragreduction. Two distinct peaks are observed in streamwise velocity fluctuations around $y\mathrm{/}R\mathrm{=0.07,\thinspace 0.25}$ and weak peaks are observed in radial velocity fluctuations at the same locations, where the Reynolds shear stress is negative. The deviations toward $u_{z}^{'}\mathrm{>0}$, $u_{r}^{'}\mathrm{>0}$ are observed at $y\mathrm{/}R=0.215$, and these components are proved to contribute to the negative Reynolds stress. Drag reducing turbulent structures are investigated by means of snapshot POD analysis. The most energetic POD modes show flat periodic structures along the wall, and such structures indicate the relation with these fluctuation peaks and negative Reynolds shear stress. [Preview Abstract] 
Sunday, November 20, 2016 4:28PM  4:41PM 
D7.00008: Reynolds number dependence of largescale friction control in turbulent channel flow Jacopo Canton, Ramis \"{O}rl\"{u}, Cheng Chin, Nicholas Hutchins, Jason Monty, Philipp Schlatter The present study reconsiders the control scheme proposed by Schoppa \& Hussain (Phys. Fluids 10, 10491051 1998), using new direct numerical simulations (DNS). The DNS are performed in a turbulent channel at friction Reynolds number ($Re_\tau$) between 104 (employed value in original study) and 550. The aim is to better characterise the physics of the control, investigate the optimal parameters and $Re$ dependence. The former purpose lead to a redesign of the method: moving from imposing the mean flow to the application of a volume force. Results show that the original method only provides transient drag reduction (DR) but actually increases the drag for longer times. The forcing method, instead, leads to sustained DR, and is therefore superior for all wavelengths investigated. A DR of 18\% is obtained at the lowest $Re_\tau$ for a viscousscaled spanwise wavelength of the vortices of 230; the optimal wavelength increases with $Re_\tau$, but the efficiency is reduced, leading to a zero DR for $Re_\tau=550$, confining the method to low $Re$ for internal flows. Although the findings by Schoppa \& Hussain are invalidated, the forcing method is currently implemented in a spatially developing boundary layer to check whether it might lead to a different conclusion in external flows. [Preview Abstract] 
Sunday, November 20, 2016 4:41PM  4:54PM 
D7.00009: Turbulent Boundary Layer Drag Reduction by Active Control of Streak Transient Growth Flint Thomas, Thomas Corke, Fazle Hussain, Alan Duong, Ryan McGowan, Chrisotpher Jasinski, Daniel Simmons Experiments are reported employing a novel method of largescale active flow control that was designed to intervene in Streak Transient Growth (STG) which was first postulated by Schoppa and Hussain (Phys. Fluids 1998, JFM 2002) as the primary mechanism in the production of streamwise vorticity in wallbounded turbulence. We term the actuator SLIPPS (``Smart Longitudinal Instability Prevention via Plasma Surface). It consists of a new pulsedDC plasma actuator array that is mounted flush with the wall in a zero pressure gradient, high Reynolds number turbulent boundary layer. The array induces a nearwall spanwise mean velocity component that is comparable in magnitude to the local friction velocity. This prevents the liftup of lowspeed streaks, thereby limiting their flanking wallnormal vorticity, which has been shown to be a critical parameter is STG. Experiments demonstrate friction drag reduction of up to 68{\%}. Measured drag reduction is found to scale with the actuator array spanwise interelectrode spacing, with the maximum drag reduction corresponding to the simultaneous control of approximately 810 lowspeed streaks. Due to the unique voltagecurrent characteristics of the pulsedDC actuator, the drag reduction is obtained with net power savings. [Preview Abstract] 
Sunday, November 20, 2016 4:54PM  5:07PM 
D7.00010: A relation between velocityvorticity correlations and skin friction in wallbounded turbulent flows Min Yoon, Junsun Ahn, Jinyul Hwang, Hyung Jin Sung The relationship between the skin friction and the velocityvorticity correlations in wallbounded turbulent flows is derived from the mean vorticity equation. A formula for the skin friction coefficient ($C_{f})$ is proposed and evaluated with regards to three canonical wallbounded flows: turbulent boundary layer, turbulent channel flow, and turbulent pipe flow. The skin friction coefficient can be derived from the mean spanwise vorticity at the wall. Double integration with respect to the wallnormal direction (from 0 to $y)$ is needed to derive $C_{f}$ from the second derivative of the mean spanwise vorticity in the mean spanwise vorticity equation. One more integration is needed to find the contribution of each component to $C_{f}$ from the wall to the boundary layer edge (from 0 to $\delta )$. The present formula encompasses four terms: advective vorticity transport, vortex stretching, viscous, and inhomogeneous terms. Dragreduced channel flow with the slip condition is used to test the reliability of the formula. The advective vorticity transport and vortex stretching terms are found to dominate the contributions to the frictional drag. [Preview Abstract] 
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