Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L36: Flow Control VI: Systems and Mechanisms |
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Chair: John Kuhlman, West Virginia University Room: 407 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L36.00001: An active flow control theory of the vortex breakdown process Zvi Rusak, Joshua Granat, Shixiao Wang An active flow control theory of the vortex breakdown process in incompressible swirling flows in a finite-length, straight, circular pipe is developed. Flow injection distributed along the pipe wall is used as the controller. The flow is subjected to non-periodic inlet-outlet conditions. A long-wave asymptotic analysis results in a nonlinear model problem for the dynamics and control of both inviscid and high Reynolds number flows. The approach provides the bifurcation diagram of steady states and the stability characteristics of these states. In addition, an energy analysis of the controlled flow dynamics suggests a feedback control law which relates the flow injection to the evolving maximum radial velocity at the inlet. The feedback control cuts the natural feed forward mechanism of the breakdown process. Computed examples based on the full Euler and NS formulations at various swirl levels demonstrate the evolution to near-steady breakdown states when swirl is above a critical level. Moreover, applying the proposed feedback control law during flow evolution, shows for the first time the successful elimination of the breakdown states and flow stabilization on an almost columnar state for a wide range of swirl, up to 30 percent above critical. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L36.00002: Flow control using ferrofluids Francois Cornat, David Beck, Ian Jacobi, Howard Stone A novel flow control technique is proposed which employs a ferrofluidic lubricant infused in a micro-patterned substrate as a ``morphing surface'' for control of wall-bounded flows. Traditionally, morphing surfaces produce dynamic changes in the curvature and roughness of solid substrates for active control of high Reynolds number flow features such as boundary layer separation and turbulent streaks. We show how these surface modifications can be achieved with a thin liquid layer in the presence of a normal magnetic field. By impregnating a chemically-treated, micro-patterned surface with a fluorinated ferrofluid, the fluid is maintained as a thin super-hydrophobic film and can be redistributed on the substrate by magnetic forces to dynamically reveal or conceal the underlying surface roughness. Moreover, the surface topography of the ferrofluid film itself can be modified to produce an enhanced roughness, beyond the scale of the underlying substrate pattern. Both types of ferrofluidic surface modifications are studied in micro- and macro- scale channels in order to assess the feasibility of flow modification at low to moderate Reynolds numbers. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L36.00003: Vibration Amplitude of a Flexible Filament Changes Non-Monotonically with Angle of Attack H. Dogus Akaydin, Cees J. Voesenek, David Lentink Certain animals exploit the interaction of their flexible, foil-like appendages with vortices to propel themselves effectively in surrounding fluids. The interaction is reciprocal because the fluid forces deform the appendage while the appendage alters the flow. As the flow separates from a foil-like structure, it rolls-up into a vortex with a low-pressure core, which deforms the structure until the vortex is shed into the wake. A control over this interaction can provide a robust aerodynamic performance. To identify the salient parameters that control fluid-structure interactions on a deformable structure, we varied the thickness, length and angle of attack of a rubber filament in a quasi-two-dimensional flow generated using a soap-film tunnel. We resolved fluid-structure interaction by simultaneously measuring deformation of the filament and motion of particles in the fluid using high-speed cameras. We show that increasing length or decreasing diameter of the filament increases its vibration amplitude monotonically. In stark contrast, the angle of attack of the filament may alter the amplitude of vibrations in a non-monotonic way: Within a certain range of angle of attack, the filament motion and vortex shedding lock-in, i.e. become synchronized, and result in a violent flapping behavior. This response can therefore be ceased not only by decreasing but also by increasing the angle of attack at the leading edge. Such an insight can help us engineer more effective bio-inspired robots, energy harvesters, and flow control devices with vibration control. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L36.00004: Control of a Separation bubble at Low Reynolds Numbers Using Electro-Active Polymers Haley Dell'Orso, Lucia Chang, Sarah Zaremski, Edward DeMauro, Chia Leong, Michael Amitay An experimental investigation was performed to study the effects of electro-active polymers (EAPs) on a 3-dimensional separation bubble on a two-dimensional NACA0009 airfoil at a Reynolds number of 20,000 and an angle of attack 5 deg. A single row of EAPs was placed at 20{\%} chord and activated at 1500V and 50Hz, corresponding to the Kelvin-Helmholtz frequency of the separated mixing layer. Stereoscopic Particle Image Velocimetry data were collected in the vicinity of the EAPs for three cases: baseline (no EAP present), EAP present but not actuated, and EAP present and actuated. Data demonstrated that the presence of the EAP slightly reduced the magnitude of the separation bubble. When the EAPs were actuated at the chosen frequency and voltage, the separation bubble was almost completely mitigated. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L36.00005: Effect of Boundary Layer Thickness on Secondary Structures in a Short Inlet Curved Duct Jeremy Gartner, Michael Amitay The flow pattern in short ducts with aggressive curvature can lead in some cases to an asymmetric flow field. In the current work, a two dimensional honeycomb mesh was added upstream of the curved duct to create a pressure drop across it, and therefore an increased velocity deficit in the boundary layer profile. This velocity deficit led to a stronger streamwise separation, overcoming the flow mechanisms that result in the asymmetric flowfield. Experiments were conducted at \textbf{\textit{M}} $=$ 0.2, 0.44 and 0.58 in an expanding aggressive duct with square cross section with an area ratio of 1.27. Pressure data, together with Particle Image Velocimetry (PIV), verify the symmetry of the incoming flow field. Steady pressure distributions along the lower surface of the curved duct were obtained, as well as steady and time dependent total pressure distributions at the aerodynamic interface plane, enabling the analysis of the flow characteristics throughout the duct length. The effect of inserting a honeycomb was tested by increasing its height from 0 to 2.2 times the baseline flow boundary layer thickness upstream of the curve. Crosstream flow symmetry was achieved for specific geometrical configurations with a negligible decrease in the pressure recovery. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L36.00006: Experimental sensitivity analysis of a hydrodynamically self-excited low-density axisymmetric jet Larry Li, Matthew Juniper We report preliminary findings from an experiment on the passive control of a hydrodynamically self-excited low-density axisymmetric jet. The control element that we use is a thin axisymmetric ring. We position this ring at different locations around the jet wavemaker and measure the response with a hot wire. We present our findings via sensitivity maps of the global frequency and the limit-cycle amplitude, and compare these to predictions from linear global instability analysis. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L36.00007: Control of fully turbulent pipe flow Jakob Kuehnen, Bjoern Hof We present a novel, very simple passive control technique, where a local modification of the flow profile by means of a stationary obstacle leads to full relaminarisation downstream. Relaminarisation is achieved about 50 diameters downstream of the control point. Since, in a smooth straight pipe, the flow remains laminar from that position significant reduction in skin friction can be accomplished. High-speed stereoscopic particle image velocimetry (S-PIV) has been used to investigate and capture the development of the transitional flow downstream the obstacle. We will present S-PIV measurements as well as pressure drop measurements and videos of the development of the flow during relaminarisation. The guiding fundamental principle behind our approach to control the velocity profile will be explained and discussed. [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L36.00008: Nonlinear switched models for control of unsteady forces on a rapidly pitching airfoil Scott Dawson, Steven Brunton, Clarence Rowley The unsteady aerodynamic forces incident on a pitching flat plate airfoil at a Reynolds number of 100 are investigated through direct numerical simulation. Linear state-space models, identified from impulse response data via the eigensystem realization algorithm, are used to accurately track rapid changes in lift coefficient through either feedback or feedforward control, even in the presence of gust disturbances. We develop a technique to project between states of linear models obtained at different angles of attack using primal and pseudo-adjoint balanced POD modes. This allows for the formation of a nonlinear switched model that is accurate over a wide range of angles of attack, in both pre- and post-stall regimes. We additionally investigate phenomena that are not captured by linear models, such as an increase in mean lift that occurs when vortex shedding frequencies are excited. The effect of changing the pitch axis is also investigated, where it is found that pitching aft of the mid-chord results in right half plane zeros that increase the difficulty of the control problem. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L36.00009: Transformation of steady fluid flow, in porous media, into a pulsed fluid flow; experiment findings and mathematical modeling Hasson M. Tavossi In this paper we present experimentally and mathematical model for the conditions under which a steady fluid flow passing through the porous media can transform itself into a pulsed fluid flow. Our experimental findings show that a pulsed fluid flow in the porous media can result from a steady flow, under certain conditions. This paper describes experimental conditions under which such a pulsed flow can result. The experimental setup is presented and a mathematical model is obtained using analogous mechanical oscillator of a mass-spring system, and an electrical oscillator consisting of the inductor-capacitor circuit. The proposed model shows the effects of fluid parameters such as; flow-rate, pressure drop, fluid density, viscosity, pore ratio and pore shape, on the resonance frequency of this pulsed flow in the porous media. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L36.00010: A dynamic observer to capture perturbation energy in noise amplifiers Juan Guzman, Denis Sipp, Peter Schmid We aim at building a reduced order model of a fluid system which accurately predicts the dynamics of a flow from a local wall measurement. This is particularly difficult in the case of noise amplifiers where the upstream noise environment triggering the receptivity of the flow is not known, which rules out classical Galerkin approaches to build reduced-order models. Here, we propose a methodology to obtain such a model from simultaneous time-resolved PIV and wall-shear stress measurements. The technique will be illustrated on the case of a transitional flat-plate boundary layer, where the snapshots of the flow are obtained with a DNS simulation. Yet, the considered approach is meant to be tractable in experiments so that special care has been taken to only use data available in an experiment. The proposed approach combines a reduction of the degrees of freedom of the system by a projection of the PIV snapshots onto a POD basis together with a system-identification technique to obtain a state-space model. Comparisons of velocity measurements at various places in the boundary layer between the DNS simulation and the obtained dynamic observer demonstrates the accuracy of the resulting model. Such a model may be used in a feedback control framework to delay transition. [Preview Abstract] |
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