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 G15: Flow Control: General |
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Chair: Stephane Dorbolo, University of Liège GRASP Room: 203 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G15.00001: Taming a flow with a string Stephane Dorbolo, Nicolas Vandewalle, Baptiste Darbois-Texier The speed of a liquid jet out of a pipe is a function of the flow and of the pipe section. Consequently, the trajectory of the liquid jet is governed by the flow and the geometry of the pipe (section and angle with respect to the gravity). We propose to regulate the trajectory of the jet by introducing a flexible wire in the outflow. According to the flow and according to the length of the wire, three regimes can be obtained: (i) no change, (ii) the control of the trajectory, (iii) the guide of the jet direct downwards the vertical. We also show that the wire acts as a free pipe. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G15.00002: Active flow control for a NACA-0012 profile H. Oualli, M. Mekadem, M. Boukrif, S. Saad, A. Bouabdallah, M. Gad-el-Hak Active flow control is applied on a NACA-0012 profile. The experiments are carried out in a wind tunnel, and flow visualizations are conducted using high-resolution visible-light and infrared cameras. Numerical LES finite-volume 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 for a circular/elliptical cylinder (B.\ Am.\ Phys.\ Soc., vol.\ 59, no.\ 20, p.\ 319, 2014). The Reynolds number varies in the range of 500--100,000, which is typical of UAVs and micro-UAVs. The nascent cavity resulting from the oscillatory motion of the profile segments is kept open allowing the passage of fluid between the intrados and extrados. The pulsatile motion is characterized by an amplitude and frequency, and the airfoil's angle of attack is changed in the range of 0--30~deg. For certain amplitude and frequency, the drag coefficient is increased over the uncontrolled case by a factor of 300. But when the cavity is covered to prevent the flow from passing through the cavity, the drag coefficient becomes negative, and significant thrust is produced. The results are promising to achieve rapid deceleration and acceleration of UAVs. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G15.00003: The Flow Field Downstream of a Dynamic Low Aspect Ratio Circular Cylinder: A Parametric Study Samantha Gildersleeve, Clingman Dan, Michael Amitay Flow past a static, low aspect ratio cylinder (pin) has shown the formation of vortical structures, namely the horseshoe and arch-type vortex. These vortical structures may have substantial effects in controlling flow separation over airfoils. In the present experiments, the flow field associated with a low aspect ratio cylinder as it interacts with a laminar boundary layer under static and dynamic conditions was investigated through a parametric study over a flat plate. As a result of the pin being actuated in the wall-normal direction, the structures formed in the wake of the pin were seen to be a strong function of actuation amplitude, driving frequency, and aspect ratio of the cylinder. The study was conducted at a Reynolds number of 1875, based on the local boundary layer thickness, with a free stream velocity of 10 m/s. SPIV data were collected for two aspect ratios of 0.75 and 1.125, actuation amplitudes of 6.7{\%} and 16.7{\%}, and driving frequencies of 175 Hz and 350 Hz. Results indicate that the presence and interactions between vortical structures are altered in comparison to the static case and suggest increased large-scale mixing when the pin is driven at the shedding frequency (350 Hz). [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G15.00004: Solution to Shape Identification of Steady-state Viscous Flow Fields to Prescribe Flow Velocity Distribution Eiji Katamine, Ryoma Kanai This paper presents a numerical solution to shape identification problem of steady-state viscous flow fields. In this study, a shape identification problem is formulated for flow velocity distribution prescribed problem, while the total dissipated energy is constrained to less than a desired value, in the viscous flow field. The square error integral between the actual flow velocity distributions and the prescribed flow velocity distributions in the prescribed sub-domains is used as the objective functional. Shape gradient of the shape identification problem is derived theoretically using the Lagrange multiplier method, adjoint variable method, and the formulae of the material derivative. Reshaping is carried out by the traction method proposed as an approach to solving shape optimization problems. The validity of proposed method is confirmed by results of 2D numerical analysis. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G15.00005: Limitations of Adjoint-Based Optimization for Separated Flows J. Javier Otero, Ati Sharma, Richard Sandberg Cabin noise is generated by the transmission of turbulent pressure fluctuations through a vibrating panel and can lead to fatigue. In the present study, we model this problem by using DNS to simulate the flow separating off a backward facing step and interacting with a plate downstream of the step. An adjoint formulation of the full compressible Navier-Stokes equations with varying viscosity is used to calculate the optimal control required to minimize the fluid-structure-acoustic interaction with the plate. To achieve noise reduction, a cost function in wavenumber space is chosen to minimize the excitation of the lower structural modes of the structure. To ensure the validity of time-averaged cost functions, it is essential that the time horizon is long enough to be a representative sample of the statistical behaviour of the flow field. The results from the current study show how this scenario is not always feasible for separated flows, because the chaotic behaviour of turbulence surpasses the ability of adjoint-based methods to compute time-dependent sensitivities of the flow. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G15.00006: Input-output dynamic mode decomposition Jennifer Annoni, Mihailo Jovanovic, Joseph Nichols, Peter Seiler The objective of this work is to obtain reduced-order models for fluid flows that can be used for control design. High-fidelity computational fluid dynamic models provide accurate characterizations of complex flow dynamics but are not suitable for control design due to their prohibitive computational complexity. A variety of methods, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), can be used to extract the dominant flow structures and obtain reduced-order models. In this presentation, we introduce an extension to DMD that can handle problems with inputs and outputs. The proposed method, termed input-output dynamic mode decomposition (IODMD), utilizes a subspace identification technique to obtain models of low-complexity. We show that, relative to standard DMD, the introduction of the external forcing in IODMD provides robustness with respect to small disturbances and noise. We use the linearized Navier-Stokes equations in a channel flow to demonstrate the utility of the proposed approach and to provide a comparison with standard techniques for obtaining reduced-order dynamical representations. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G15.00007: A novel control strategy for a Taylor--Couette flow A. Bouabdallah, H. Oualli, M. Mekadem, M. Boukrif, S. Saad, M. Gad-el-Hak Advancing transition is desired in applications where heat, mass, or momentum transfer needs to be augmented. On the other hand, delaying transition is imperative in crystal growth devices, where all instabilities are to be avoided in order to prevent the appearance of geometrical irregularities in the resulting crystal. The hydrodynamic stability of a viscous flow in a closed, fully filled Taylor--Couette system is considered in the present numerical study. The fluid evolves in an annular cavity between the rotating inner cylinder and the outer fixed one. The base flow is axis-symmetric with two counter-rotating vortices each wavelength. The Taylor number varies in the range of 0--50. Numerical simulations are implemented on a finite-volume CFD code. The control strategy involves a pulsatile motion superimposed separately on the inner and outer cylinder's cross-section, with maximum amplitude of, respectively, 5\% and 15\% of the radius. The frequency varies in the range of 0--100~Hz. The objective is to localize the transition and to assess the flow's response to the imposed boundary motions. Substantial advancement of transition is found when the inner cylinder's cross-section is varied, while this transition is delayed when the outer cylinder's cross-section is pulsating. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G15.00008: Controlling turbulence Jakob Kühnen, Björn Hof We show that a simple modification of the velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarises. The annihilation of turbulence is achieved by a steady manipulation of the streamwise velocity component alone, greatly reducing control efforts. Several different control techniques are presented: one with a local modification of the flow profile by means of a stationary obstacle, one employing a nozzle injecting fluid through a small gap at the pipe wall and one with a moving wall, where a part of the pipe is shifted in the streamwise direction. All control techniques act on the flow such that the streamwise velocity profile becomes more flat and turbulence gradually grows faint and disappears. In a smooth straight pipe the flow remains laminar downstream of the control. Hence a reduction in skin friction by a factor of 8 and more can be accomplished. Stereoscopic PIV-measurements and movies of the development of the flow during relaminarisation are presented. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G15.00009: Modification of shear layer characteristics using local periodic heating Chi-An Yeh, Phillip Munday, Kunihiko Taira Motivated by the recent development of carbon-based thermophone membranes, we examine their use as a flow control actuator by performing 2D DNS of a compressible subsonic shear layer downstream of a splitter plate for a plate thickness based Reynolds number of 4000. Time varying heat flux boundary condition is utilized as the membrane actuator model on the elliptic nose of the splitter plate. A range of boundary layer thicknesses $\theta$ and actuation frequencies are chosen to study the effectiveness of the actuator in modifying the shear layer physics through changing vortex rollup and vortex merging dynamics. For incoming boundary layer with large $\theta$, the heat injection does not shift the rollup frequency when using actuation frequencies between the baseline rollup frequency and its first subharmonic. However, vortex merging is observed to occur earlier downstream. When a positive mean heating is introduced at the same frequency, the early occurrence of the vortex merging is still observed even if the fundamental rollup is delayed due to increased viscosity from the local heating near the nose. For shear layers with small $\theta$, the rollup occurs earlier than the baseline and is locked onto the actuation frequency, but no significant change in the merging is observed. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G15.00010: Coupled Modification of Body-Wake Flow on an Axisymmetric Moving Platform Thomas Lambert, Bojan Vukasinovic, Ari Glezer The unsteady interactions between fluidic actuators and the cross flow over the aft end of a moving bluff body are exploited for modification of the global unsteady aerodynamic loads in wind tunnel experiments using a moving axisymmetric model. The present study focuses on the effects of actuation by an azimuthally-segmented array of four aft-facing synthetic jet modules around the tail end of the model on the coupling between the moving body and its near wake. The model is supported by eight servo-controlled wires, each including a miniature inline force transducer for measurements of the time-resolved tension during the time-dependent six degrees of freedom motion along a prescribed trajectory. In the present investigations the model's motion is described by parameterized Lissajous rotation (combined pitch and yaw), which is designed to mimic the natural unstable motion of a similar airborne platform in the absence of roll. Enhancement and suppression of stabilizing aerodynamic loads on the model are each investigated using coupled force and moment measurements and particle image velocimetry in the near wake at reduced frequencies of up to 0.259. [Preview Abstract] |
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