Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session L25: Flow Control: Boundary layer |
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Chair: Clarence Rowley, Princeton University Room: 31A |
Monday, November 19, 2012 3:35PM - 3:48PM |
L25.00001: Numerical investigation of the AFRODITE transition control strategy Simone Camarri, Jens H.M. Fransson, Alessandro Talamelli The generation of properly distributed and shaped velocity streaks is a method to delay the Tollmien-Schlichting (TS) transition scenario in a boundary layer. Indeed, it is shown in the literature that stable velocity streaks in a Blasius boundary layer (BL) may lead to a damping of TS waves. This idea is explored in the AFRODITE\footnote{Advanced Fluid Research On Drag reduction In Turbulence Experiments, project funded by the European Research Council.} project, where streaks are generated experimentally in a Blasius BL by placing ad-hoc miniature vortex generators (MVGs) on the plate wall. In this presentation we show representative results obtained by the numerical setup that has been designed to support the experiments of the AFRODITE project. The DNSs are carried out using an open-source tool, Nek5000, which is a spectral-element code for incompressible flows. A 3D BL solver and its adjoint version are also used as additional numerical tools. The present DNSs include the simulation of the flow around the MVGs, so that the simulated streaks are those effectively generated by the considered devices. Details of the interactions between incoming TS waves and the MVGs are also investigated by DNS. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L25.00002: Controller Selection and Placement in a Compressible Boundary Layer Daniel Bodony, Mahesh Natarajan A method for estimating the optimal location and type of flow control to use in compressible, turbulent flows is developed. Through linearizing the compressible Navier-Stokes equations about an unstable equilibrium point, the forward and adjoint equations of motion are used to estimate the structural sensitivity of the flow using a generalized ``wavemaker'' concept. Matrix optimization is used to enhance the structural sensitivity over possible controller types (e.g., mass or energy sources) and locations, with optimal solutions identifying advantageous control strategies. The algorithms and theory are applied to a high-subsonic separating boundary layer appearing in an S-duct and compared with more traditional optimal methods through adjoint-based gradient information. It is found that optimum locations for mass and energy sources are typically located upstream of those momentum sources, and use different physical mechanisms for affecting the flow. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L25.00003: Exploring the Potential of Turbulent Flow Control Using Vertically Aligned Nanowire Arrays Sean Bailey, John Calhoun, Christopher Guskey, Michael Seigler, Aneesh Koka, Henry Sodano We present evidence that turbulent flow can be influenced by oscillating nanowires. A substrate coated with vertically aligned nanowires was installed in the boundary wall of fully-developed turbulent channel flow, and the substrate was excited by a piezoceramic actuator to oscillate the nanowires. Because the nanowires are immersed in the viscous sublayer, it was previously unclear whether the small scale flow oscillations imparted into the bulk flow by the nanowires would influence the turbulent flow or be dissipated by the effects of viscosity. Our experiments demonstrated that the nanowires produced perturbations in the flow and contributed energy throughout the depth of the turbulent layer. A parallel investigation using a dynamically scaled surface of vertically aligned wires in laminar flow found that, even at low Reynolds numbers, significant momentum transport can be produced in the flow by the introduction of a travelling wave motion into the surface. These findings reflect the potential for using oscillating nanowires as a novel method of near-wall turbulent flow control. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L25.00004: Effects of actuators and sensors on feedback control of transition in the 2D Blasius boundary layer Brandt Belson, Clarence Rowley, Onofrio Semeraro We examine the effects of different types and positions of actuators and sensors on the performance and robustness of controllers designed to reduce the growth of Tollmien-Schlichting waves in the 2D Blasius boundary layer. We perform direct numerical simulations, and in order to facilitate controller design, we find reduced-order models with the Eigensystem Realization Algorithm. Good performance is obtained with the sensor upstream of the actuator, as in previous work. We categorize this as feedforward control, whereas sensors downstream of the actuator correspond to feedback control. The performance of feedforward controllers can be degraded by disturbances and perturbations, so we examine feedback controllers. The original choice of actuator and sensor results in ineffective feedback controllers (both for H2-optimal and simple classical controllers) due to weakly observable structures and a strict tradeoff between performance and robustness. Another choice of actuators and sensors is better suited for feedback control, and a PI feedback controller with the sensor slightly downstream of the actuator has good performance and robustness. Sensors farther downstream of the actuator cause inherent time delays that limit performance and robustness. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L25.00005: Shock Wave Boundary Layer Interaction Control Using Pulsed DBD Plasma Actuators Alexandre Likhanskii, Kristian Beckwith Flow separation in the shock wave boundary layer interaction (SWBLI) region significantly limits the development of supersonic inlets or scramjets. For past decades, scientists and engineers were looking for a way for active flow control of SWBLI. We will present our recent results of comprehensive simulations of SWBLI active control using pulsed nanosecond DBD plasma actuators at M=3. In the first part of simulations, we computed heat release from the ns pulse driven DBD plasma actuator to the flow using Tech-X plasma code Vorpal. This information has been consequently used in the simulations of SWBLI problem using Tech-X CFD code Nautilus. We compared baseline case with plasma actuators OFF to the case when plasma actuators were ON. We demonstrated strong perturbations in the region of SWBLI, suppression of flow separation and overall downstream increase of mass flow by ten percent when actuators are ON. We investigated the dependence of the results on the choice of different turbulence models and compared them to the laminar boundary layer case. We also performed parametric studies for different pulse repetition rates, pulse operation modes and DBD placement. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L25.00006: Stability analysis of Boundary Layer in Poiseuille Flow through a modified Orr-Sommerfeld equation Jean Bio Chabi Orou, Vincent Monwanou, Cl\'ement Miwadinou For applications regarding transition prediction, wing design and control of boundary layers, the fundamental understanding of disturbance growth in the flat plate boundary layer is an important issue. In the present work we investigate the stability of boundary layer in Poiseuille flow. We normalize pressure and time by inertial and viscous effect. The disturbances are taken to be periodic in the spanwise direction and time. We present a set of linear governing equations for the parabolic evolution of wavelike disturbances. Then, we derive modified Orr-Sommerfeld equations that can be applied in the layer. We find that Squire's theorem is applicable for the boundary layer. We find also that normalization by inertial or viscous effects leads to the same stability or instability. We find through the graphs that transition from stability to instability or the opposite can occur according to the Reynolds number and the wave number. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L25.00007: Time-resolved PIV of a turbulent boundary layer over a spanwise-oscillating surface Kevin Gouder, Jonathan Morrison This work reports measurements of a turbulent boundary layer at Re$_{\theta } \approx $ 2500, over a resonant spanwise-oscillating surface driven by a linear electromagnetic motor. Time-resolved PIV measurements of velocity are presented and supplemented by hot-wire measurements of velocity and direct drag measurements of friction drag using a drag balance. A maximum of 16{\%} surface friction reduction, as calculated by the diminution of the wall-normal streamwise velocity gradient was obtained. The PIV laser beam was parallel to the plane of the oscillating surface at a height of $y^{+} \approx $ 15, hence, top-down views of the near-wall turbulence activity and the effect of the surface oscillation on its evolution were obtained. It has been shown that the imposition of a spanwise Stokes-like layer at a non-dimensional period of $T^+={Tu_\tau ^2 } \mathord{\left/ {\vphantom {{Tu_\tau ^2 } \nu }} \right. \kern-\nulldelimiterspace} \nu $ $\approx $ 100 at peak-peak oscillation amplitudes equal to or larger than the mean streak spacing enabled the direct manipulation of the quasi-streamwise near-wall structures and caused fundamental changes in their evolution leading to reductions, for example, in the near-wall values of the mean-square of the streamwise fluctuating velocity component. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L25.00008: Closed-Loop Control of Unsteady Transient Growth Disturbances in a Blasius Boundary Layer using DBD Plasma Actuators Philippe Lavoie, Ronald Hanson, Kyle Bade, Ahmed Naguib, Brandt Belson, Clarence Rowley Plasma actuators have recently been shown to negate the effect of the transient growth instability occurring in a Blasius boundary layer for the purpose of delaying bypass transition. Specifically, during steady operation, the energy of a disturbance introduced via an array of static cylindrical roughness elements was reduced by up to 68\%, as shown by Hanson et al (Exp.\ Fluids, 2010). In the present work, the actuators used in the aforementioned study were integrated into a complete closed-loop control system capable of negating unsteady transient growth disturbances induced in a Blasius boundary layer established in a wind tunnel. Shear stress measurements from an array of hot-wires mounted just above the surface of the boundary-layer plate downstream of the actuators are used to provide feedback information about the state of the boundary layer. The effectiveness and robustness of the closed-loop controller are rigorously established based on both control-model simulations and experiments. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L25.00009: Numerical study of linear feedback control for form-drag reduction Jeremy Dahan, Aimee Morgans The present work is a numerical investigation of linear system identification and model-based feedback control methods for form-drag reduction. Large-Eddy Simulation (LES) is used to represent the flow over a simple bluff body with a sharp trailing edge, with a turbulent separation. For actuation, two types of perturbations are considered: a model of zero-net-mass-flux slot jets and momentum sources. Pressure measurements distributed over the base of the body provide the sensor information. The first part of the study will focus on the open-loop characterization of the flow. The base pressure field will be studied in relation to the wake dynamics. The effect of key actuation and flow parameters, such as actuation type, actuation location and Reynolds number, will be investigated. A black-box model of the flow response, obtained via system identification, will be examined. The second part will look at the design of robust controllers. It will be shown that uncertainties in the model and inflow conditions can be partially mitigated by the robustness of the controller. The behaviour of the feedback-controlled flow will be compared with the results achievable using open-loop forcing to draw conclusions about the success of the flow response model and the controller synthesis. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L25.00010: Linear modeling of turbulent skin-friction reduction due to spanwise wall motion Carlos Duque-Daza, Mirza Baig, Duncan Lockerby, Sergei Chernyshenko, Christopher Davies We present a study on the effect of streamwise-travelling waves of spanwise wall velocity on the growth of near-wall turbulent streaks using a linearized formulation of the Navier-Stokes equations. The changes in streak amplification due to the travelling waves induced by the wall velocity are compared to published results of direct numerical simulation (DNS) predictions of the turbulent skin-friction reduction over a range of parameters; a clear correlation between these two sets of results is observed. Additional linearized simulations but at a much higher Reynolds numbers, more relevant to aerospace applications, produce results that show no marked differences to those obtained at low Reynolds number. It is also observed that a close correlation exists between DNS data of drag reduction and a very simple characteristic of the ``generalized'' Stokes layer generated by the streamwise-travelling waves. [Preview Abstract] |
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