Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D17: Flow Control: Drag Reduction I 
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Chair: Ayumu Inasawa, Tokyo Metropolitan University Room: 205 
Sunday, November 22, 2015 2:10PM  2:23PM 
D17.00001: Experimental study of flow in a channel with a periodically heated wall Ayumu Inasawa, Keinosuke Taneda, Jerzy M. Floryan Flows in a channel with spatially periodic wall heating are examined experimentally at the Reynolds numbers below Re $=$ 20 and at the Rayleigh number based on the amplitude of the periodic heating and the channel half width Rap $=$ 3500, to realize the superthermohydrophobic effect leading to a significant drag reduction (Floryan, 2012). The periodic heating is applied at the lower wall while the temperature of the upper wall is uniform and controlled. The results show that steady separation bubbles are created by periodic heating, which separate the main stream from the wall and, thus, the net friction drag is reduced. It is also found that the separation bubbles are strengthened when the average temperature of the lower wall exceeds that of the upper wall. Comparisons between the experiments and the theoretical results are presented. [Preview Abstract] 
Sunday, November 22, 2015 2:23PM  2:36PM 
D17.00002: On the Analysis of Flows in Vibrating Channels Sahab Zandi, Alireza Mohammadi, Jerzy Maciej Floryan Pressure losses in channels with vibrating walls have been analyzed. Surface vibrations were assumed to have the form of travelling waves. The waves can have arbitrary profiles. The spectrally accurate immersed boundary conditions (IBC) method based on the Fourier expansions in the flow direction and the Chebyshev expansions in the transverse direction has been developed. The results show dependence of the pressure losses on the phase speed of the waves, with the waves propagating in the downstream direction reducing the pressure gradient required to maintain a fixed flow rate. A drag increase is observed when the waves propagate with a phase speed similar to the flow velocity. Analytical solution demonstrates that the drag changes result from the nonlinear interactions and vary proportionally to $A^{\mathrm{2}}$ for small enough $A$, where $A$ stands for the wave amplitude. [Preview Abstract] 
Sunday, November 22, 2015 2:36PM  2:49PM 
D17.00003: Turbulent boundary layer control at moderate Reynolds numbers by means of uniform blowing/suction Yukinori Kametani, Koji Fukagata, Ramis Orlu, Philipp Schlatter The effect of uniform blowing or suction from the wall on a spatially developing turbulent boundary layer has been studied in order to use them ultimately for flow control on the surface of highspeed vehicles. In the present study, a series of large eddy simulations is performed to investigate the effects of uniform blowing/suction on the skin friction drag as well as the scale of turbulent structures at moderate Reynolds numbers up to \textit{Re}$_{\theta } \quad =$ 2500, based on freestream velocity, $U_{\mathrm{\infty }}$, and momentum thickness, $\theta $. The amplitude of blowing or suction is fixed to 0.1{\%} of $U_{\mathrm{\infty \thinspace }}$with different streamwise ranges of the control region. While the Reynolds shear and normal stresses and their spectral energy distributions are increased by blowing and decreased by suction, in particular, in the outer region, the FIK identity reveals that drag reduction (DR) or enhancement (DE) are mainly linked to changes in the spatial development of the mean wallnormal convection term rather than the contribution from the Reynolds shear stress. Despite the weak amplitude of the control, over 10{\%} of DR and DE are achieved by blowing and suction, respectively. In case of blowing, the mean DR rate increases as the blowing region extends because the local reduction rate grows in the streamwise direction. [Preview Abstract] 
Sunday, November 22, 2015 2:49PM  3:02PM 
D17.00004: Reactive Control of Boundary Layer Streaks Induced by Freestream Turbulence Using Plasma Actuators Kevin Gouder, Ahmed Naguib, Philippe Lavoie, Jonathan Morrison Over the past few years we have carried out a systematic series of investigations aimed at evaluating the capability of a plasmaactuatorbased feedforwardfeedback control system to weaken streaks induced ``synthetically'' in a Blasius boundary layer via dynamic roughness elements. This work has been motivated by the delay of bypass boundary layer transition in which the streaks form stochastically beneath a freestream with turbulence of intensity of more than approximately 1{\%}. In the present work, we carry forward the knowhow from our previous research in a first attempt to control such naturally occurring streaks. The experimental setup consists of a turbulencegenerating grid upstream of a flat plate with a sharp leading edge. At the freestream velocity of the experiment, turbulent spot formation is observed to start at a streamwise location of $x \quad \approx $ 350 mm from the leading edge. The control system is implemented within a streamwise domain stretching from $x \quad =$ 150 mm to 300mm, where the streaks exhibit linear growth. At the upstream and downstream end of the domain a feedforward and a feedback wallshearstress sensors are utilized. The output from the sensors is fed to appropriately designed controllers which drive two plasma actuators providing positive and negative wallnormal forcing to oppose naturally occurring high and lowspeed streaks respectively. The results provide an assessment of the viability of the control approach to weaken the boundary layer streaks and to delay transition. [Preview Abstract] 
(Author Not Attending)

D17.00005: Drag reduction through wavecurrent interactions with a marine hydrofoil Ignazio Maria Viola, Susan Tully, David Ingram A hydrofoil exposed to oscillating flow experiences a reduction in drag due to the KnollerBetz effect. This is experimentally identifiable by an increasingly inverted von Kármán wake and a corresponding thrust force on the foil. The rate of drag reduction, dependent on plunge amplitude and frequency, reduces with unsteady flow phenomena at higher reduced frequencies. For experimental ease, investigations of this effect have relied on actively plunging/pitching a foil within a steady current. However, one potential application is to drag reduction in highspeed ships adopting submerged foils. In this case the foil is travelling through wavecurrent induced oscillatory flow, resulting in an additional dynamic variation of hydrostatic pressure across the chord; a phenomena not fully addressed in previous experiments. Here we investigate the effects of this pressure gradient on drag reduction for a stationary foil in combined waves and current, through a combination of force measurements and particle image velocimetry. [Preview Abstract] 
Sunday, November 22, 2015 3:15PM  3:28PM 
D17.00006: Rotational Stabilization of Cylinder Wakes Using Linear Feedback Control Jeff Borggaard, Serkan Gugercin, Lizette Zietsman We demonstrate the feasibility of linear feedback control to stabilize vortex shedding behind twin cylinders using the cylinder rotations. Our approach is to linearize the flow about a desired steadystate flow, use interpolationbased model reduction on the resulting linear model to generate a lowdimensional model of the inputoutput system with inputindependent error bounds, then use this reduced model to design the feedback control law. We then consider the practical issue of limited state measurements by building a nonlinear compensator that is computed from the same linear reducedorder model an constructed through an extended Kalman filter with a proper orthogonal decomposition (POD) model. Closedloop simulations of the NavierStokes equations coupled with controls generated through flow measurements demonstrate the effectiveness of this control strategy. [Preview Abstract] 
Sunday, November 22, 2015 3:28PM  3:41PM 
D17.00007: Bluntbody drag reduction through base cavity shape optimization Manuel LoriteD\'Iez, Jos\'e Ignacio Jim\'enezGonz\'alez, C\'andido Guti\'errezMontes, Carlos Mart\'InezBaz\'an We present a numerical study on the drag reduction of a turbulent incompressible flow around two different blunt bodies, of height $H$ and length $L$, at a Reynolds number $Re=\rho U_{\infty} H/\mu=2000$, where $U_{\infty}$ is the turbulent incompressible freestream velocity, $\rho$ is their density and $\mu$ their viscosity. The study is based on the optimization of the geometry of a cavity placed at the rear part of the body with the aim of increasing the base pressure. Thus, we have used an optimization algorithm, which implements the adjoint method, to compute the twodimensional incompressible turbulent steady flow sensitivity field of axial forces on both bodies, and consequently modify the shape of the cavity to reduce the induced drag force. In addition, we have performed three dimensional numerical simulations using an IDDES model in order to analyze the drag reduction effect of the optimized cavities at higher Reynolds numbers.The results show average drag reductions of 17 and 25$\%$ for Re=2000, as well as more regularized and less chaotic wake flows in both bodies. [Preview Abstract] 
Sunday, November 22, 2015 3:41PM  3:54PM 
D17.00008: Power loss minimizing blowing and suction profiles for drag reduction on a circular cylinder Pritam Giri, Ratnesh Shukla Active and passive flow control strategies that facilitate drag reduction at low energetic costs are of considerable fundamental and practical relevance. Here, we investigate the efficacy of a zero net mass transpiration blowing and suction flow control strategy based on intake and expulsion of fluid from the boundary of a circular cylinder placed in a uniform cross flow of a viscous incompressible fluid. We find this control strategy to be most effective when the blowing and suction profile is such that the fluid intake and expulsion occur over upstream and downstream portions of the circular cylinder, respectively. With increasingly strong intake and expulsion, the vorticity production at the cylinder surface diminishes significantly and the unsteady vortex shedding is suppressed entirely. We find that for sufficiently strong blowing and suction strengths the net power consumption attains a minimum for a significantly reduced net drag force. At a Reynolds number of 1000 the drag is reduced by a factor of over 15 from its base value for a stationary cylinder with zero mass transpiration. We show that a selfpropelling state with zero drag force is achieved for a configuration that corresponds to an irrotational flow with vanishing tangential but finite normal surface velocity. [Preview Abstract] 
Sunday, November 22, 2015 3:54PM  4:07PM 
D17.00009: Feedback Control of a SquareBack Ahmed Body Flow for FormDrag Reduction Olga Evstafyeva, Aimee Morgans Road transport accounts for roughly 22\% of $CO_2$ emissions worldwide, and at highway speeds two thirds of usable energy is consumed overcoming aerodynamic drag. For squareback vehicles, aerodynamic drag is dominated by form drag, originating from pressure difference between the front and the back face (base) of the vehicle. This study explores using feedback control to increase mean base pressure and thus reduce the formdrag of 3D Ahmed body flows at low (laminar) and medium (transitioning to turbulence) Reynolds numbers. Using Large Eddy Simulations as a testbed, a linear control strategy to attenuate basepressure force fluctuations is investigated. Bodymounted sensing and actuation is used: sensing of the base pressure force fluctuations, and actuation of a zeromean slot jet just ahead of the base. The dynamic linearity of the response to actuation is tested and a feedback controller then designed using frequency domain harmonic forcing system identification data. Recent advances in understanding of the Ahmed body wake dynamics such as toptobottom and lefttoright bistable behaviour, are considered in the feedback control implementation. [Preview Abstract] 
Sunday, November 22, 2015 4:07PM  4:20PM 
D17.00010: Simulation, Modeling and Feedback Control of the flow around a SquareBack Bluff Body Laurent Dalla Longa, Aimee Morgans Because of capacity, aesthetic and comfort requirements, most road vehicles are not streamlined but blunt bluff bodies. The flow exhibits a large wake recirculation area leading to high pressure drag, which at highway speeds, represents the main source of energy loss. In this work, Large Eddy Simulations of the flow past a squareback bluff body with interacting shear layers are performed with the aim of reducing aerodynamic drag. A linear feedback control strategy is applied to increase the back face pressure and therefore obtain drag reduction. Synthetic jets located along the perimeter of the back face are used for actuation while body mounted sensors record the base pressure. System identification, via harmonic actuator forcing, is used to characterize the flow response to actuation, which is assumed to be dynamically linear. Based on the identified frequency response, a feedback controller is designed in the frequency domain which aims to either attenuate or amplify base pressure fluctuations by shaping of the sensitivity transfer function. This is first done for a Dshaped body. Current work extends this strategy to a simplified lorry geometry on which experiments were carried out recently. [Preview Abstract] 
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