Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R21: Instability: Control |
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Chair: Pavel Popov, University of California, Irvine Room: 2010 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R21.00001: Suppression of wake's instabilities by optimal streaks Gerardo Del Guercio, Carlo Cossu, Gregory Pujals Wakes can sustain large transient energy growth. Optimal perturbations are computed for the cases of parallel, weakly non-parallel and the circular cylinder wakes. Streaks are found to be the optimal amplified structures produced by the non normal energy amplification. The level of energy increases with the spanwise wavelength of the perturbations except in the circular cylinder wake where the optimal is reached for $\lambda_z \approx 6\,D$. In parallel wakes these streaks are shown to suppress the absolute instability. Furthermore the global instability of the weakly non-parallel and the circular cylinder wakes can be completely annihilate with moderate streaks amplitudes. The comparison of these spanwise periodic (3D) optimal perturbations with the spanwise uniform (2D) control showed that the energy required to stabilize the wake is always smaller for the 3D control. Moreover the sensitivity of the global mode growth rate is discovered to be quadratic for 3D perturbations while being linear for 2D ones meaning that usual first order sensitivity analysis is unable to predict their larger efficiency. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R21.00002: Control of Tollmien-Schlichting Waves on a Flat Plate Using a Piezoelectric-Driven Oscillating Surface Haley Dell'Orso, Burak Tuna, Edward MeMauro, Michael Amitay Micro-air vehicles operate in the regime of low Reynolds numbers where the drag associated with skin friction is significant. One proposed method for drag reduction is to control the transition from laminar to turbulent flow by using active surface modification to either excite or suppress instabilities within the flow. To do so, the Piezoelectric-Driven Oscillating Surface (PDOS) actuator was developed and quantified. Two PDOS actuators were placed on a flat plate at two stream wise locations in a low Reynolds number flow. The upstream PDOS was actuated at a characteristic frequency appropriate to phase-lock Tollmien-Schlichting waves within the flow while the downstream PDOS was actuated at the anti-phase to reduce the magnitude of the T-S waves. Particle image velocimetry data were obtained along the centerline of the flat plate at different streamwise locations. Data showed that the upstream PDOS successfully locked-in to the instabilities in the flow and the growth of T-S waves was recorded over the increasing streamwise locations from the leading edge of the flat plate. Finally, the anti-phase (at the proper amplitude) was applied using the downstream PDOS and yielded substantial attenuation of the magnitude of the T-S waves. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R21.00003: ABSTRACT WITHDRAWN |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R21.00004: Three-dimensional characterization and control of Tollmien-Schlichting waves on a flat plate Burak Tuna, Michael Amitay Tollmien-Schlichting (T-S) waves are instability waves inside the boundary layer which are the prime mechanism for the transition from laminar to turbulent flows. The T-S waves grow in amplitude and develop three-dimensionality as they advect downstream. At sufficiently large amplitude they break up into turbulent spots, followed by a turbulent flow, which yields a drag increase. The present work aims to identify the T-S waves and reduce their amplitude to delay transition to turbulence. For that propose, Piezoelectric-Driven Oscillating Surface (PDOS) actuator was developed; Two PDOS actuators were used are two stream wise locations. The upstream PDOS was used to excite and phase-lock the T-S waves, and the downstream PDOS was used to cancel the T-S waves by applying an anti phase disturbance at the proper amplitude. Stereoscopic particle image velocimetry (SPIV) was used to identify the three-dimensional development of the T-S waves along the flat plate. Moreover, the SPIV results showed that reduction of peak values of velocity fluctuations due to the T-S waves could be achieved, and this reduction corresponds to a delay of laminar to turbulent transition. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R21.00005: Optimal transient growth in flow past a slanted surface Marco Martins Afonso, Philippe Meliga, Eric Serre We investigate numerically and theoretically the flow past a slanted surface inclined at 25 degrees, mimicking the rear part of a simplified ground-vehicle geometry of the bluff-body type. We are interested in how to reduce the flow separation by the upward generation of contra-rotating longitudinal vortices developing into longitudinal streaks through the lift-up effect. Upon introducing a volumic forcing or a wall velocity perturbation, we use gradients computed with the adjoint method to optimize the energy gain in the domain. Such an analysis is undertaken as a function of the Reynolds number, the wavelength of the perturbation in the transverse direction, and the size and location of the domain over which the gain is estimated. If time permits, we will also perform nonlinear numerical simulations of the controlled flow to assess the nonlinear interaction of the induced perturbation with the natural flow instabilities. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R21.00006: Streak amplification and self-sustenance of turbulence in pipe flow Baofang Song, Marc Avila, Ashley Willis, Bj\"orn Hof We propose that a minimum transient growth, i.e. a measure of the linear amplification of disturbances to the base velocity profile, is needed for pipe flow turbulence to be self-sustained. Transient growth in shear flows accounts for the generation of strong velocity streaks via streamwise vortices and increases linearly as the Reynolds number increases. The instability of sufficiently amplified streaks causes streak breakdown and further regeneration of streamwise vortices. Here we show that if the transient growth can be greatly reduced by forcing the velocity profile (i.e. streaks amplification suppressed), the turbulence self-sustaining cycle is intercepted and turbulence relaminarises. For this modified flow profile, after further increase in Reynolds number turbulence only becomes sustained once the same level of transient growth has been regained. We show that for a variety of different flow profiles and Reynolds numbers turbulence always first arises at the same level of transient growth (approx. 15). Besides, this value of transient growth is also demonstrated to be the minimum for turbulent puffs to be self-sustained in normal (unforced) flow at low Reynolds numbers. Finally, we show how this mechanism can be exploited on turbulence control. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R21.00007: Sensitivity of aerodynamic forces in laminar and turbulent flow past a square cylinder Philippe Meliga, Edouard Boujo, Fran\c{c}ois Gallaire, Gregory Pujals We use adjoint-based gradients to analyze the sensitivity of the drag force on a square cylinder. At Re $=$ 40, the flow settles down to a steady state. The quantity of interest in the adjoint formulation is the steady asymptotic value of drag reached after the initial transient, whose sensitivity is computed solving a steady adjoint problem from knowledge of the stable base solution. At Re $=$ 100, the flow develops to the time-periodic, vortex-shedding state. The quantity of interest is rather the time-averaged mean drag, whose sensitivity is computed integrating backwards in time an unsteady adjoint problem from knowledge of the entire history of the vortex-shedding solution. Such theoretical frameworks allow us to identify the sensitive regions without computing the actually controlled states, and provide a relevant and systematic guideline on where in the flow to insert a secondary control cylinder in the attempt to reduce drag, as established from comparisons with dedicated numerical simulations of the two-cylinder system. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R21.00008: Fluid-plasma interaction in compressible unstable flows Luca Massa The receptivity of the boundary layer discrete modes to dielectric barrier discharge (DBD) actuation is studied to improve the understanding of the interaction between non-equilibrium plasma and fluid in convectively amplified vortical layers. The momentum transfer induced by a DBD patch at various Reynolds numbers is evaluated using an adaptive mesh refinement computational solver in the Mach number regime 0.8-2.0. The energy of the induced modal perturbation is determined by weighting such a source term with the corresponding adjoint eigenfunctions. Conditions of maximum overlapping between the adjoint and the source term define the regimes of maximum receptivity and the locations of optimal placement of the DBD patch at different Mach and Reynolds numbers. The interaction between non-equilibrium plasma and the jet in cross flow is also being studied to determine the ability of DBD patches to influence mixing in the compressible regime, thus improving flame-holding in plasma assisted ignition and combustion. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R21.00009: Sensitivity analysis applied to stalled airfoil wake and steady control Gustavo Patino, Rafael Gioria, Julio Meneghini The sensitivity of an eigenvalue to base flow modifications induced by an external force is applied to the global unstable modes associated to the onset of vortex shedding in the wake of a stalled airfoil. In this work, the flow regime is close to the first instability of the system and its associated eigenvalue/eigenmode is determined. The sensitivity analysis to a general punctual external force allows establishing the regions where control devices must be in order to stabilize the global modes. Different types of steady control devices, passive and active, are used in the regions predicted by the sensitivity analysis to check the vortex shedding suppression, i.e. the primary instability bifurcation is delayed. The new eigenvalue, modified by the action of the device, is also calculated. Finally the spectral finite element method is employed to determine flow characteristics before and after of the bifurcation in order to cross check the results. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R21.00010: Second-order sensitivity of parallel shear flows and optimal spanwise-periodic flow modifications Francois Gallaire, Andrea Fani, Edouard Boujo We perform a second-order sensitivity analysis of the linear temporal stability of a parallel flow subject to small spanwise periodic modification. The need for a second-order analysis results from the fact that spanwise-periodic flow modifications have a quadratic effect on the stability properties of parallel flows (i.e. the first-order eigenvalue variation is zero). From a simple one-dimensional (1D) calculation we compute the second-order sensitivity operator, which allows us to predict the effect on stability of any small modification without computing the eigenmode correction. Comparisons with two-dimensional (2D) stability calculations of modified flows show excellent agreement and validate the method. From the second-order sensitivity operator we optimise the growth rate variation and compute optimal flow modifications, providing lower and upper bounds for the growth rate variation induced by any spanwise-periodic modification of given amplitude. We finally discuss under which conditions a spanwise periodic modulation is more efficient to stabilize/destabilize the flow in comparison to a spanwise homogeneous flow modification. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R21.00011: Non-linear dynamics of viscoelastic liquid trilayers subjected to an electric field George Karapetsas, Vasilis Bontozoglou The scope of this work is to investigate the non-linear dynamics of the electro-hydrodynamic instability of a trilayer of immiscible liquids. We consider the case of a polymer film which is separated from the top electrode by two viscous fluids. We develop a computational model and carry out 2D numerical simulations fully accounting for the flow and electric field in all phases. For the numerical solution of the governing equations we employ the mixed finite element method combined with a quasi-elliptic mesh generation scheme which is capable of following the large deformations of the liquid-liquid interface. We model the viscoelastic behavior using the Phan-Thien and Tanner (PTT) constitutive equation taking fully into account the non-linear elastic effects as well as a varying shear and extensional viscosity. We perform a thorough parametric study and investigate the influence of the electric properties of fluids, applied voltage and various rheological parameters. [Preview Abstract] |
Tuesday, November 25, 2014 3:28PM - 3:41PM |
R21.00012: Uncertainty Quantification of Non-linear Oscillation Triggering in a Multi-injector Liquid-propellant Rocket Combustion Chamber Pavel Popov, Athanasios Sideris, William Sirignano We examine the non-linear dynamics of the transverse modes of combustion-driven acoustic instability in a liquid-propellant rocket engine. Triggering can occur, whereby small perturbations from mean conditions decay, while larger disturbances grow to a limit-cycle of amplitude that may compare to the mean pressure. For a deterministic perturbation, the system is also deterministic, computed by coupled finite-volume solvers at low computational cost for a single realization. The randomness of the triggering disturbance is captured by treating the injector flow rates, local pressure disturbances, and sudden acceleration of the entire combustion chamber as random variables. The combustor chamber with its many sub-fields resulting from many injector ports may be viewed as a multi-scale complex system wherein the developing acoustic oscillation is the emergent structure. Numerical simulation of the resulting stochastic PDE system is performed using the polynomial chaos expansion method. The overall probability of unstable growth is assessed in different regions of the parameter space. We address, in particular, the seven-injector, rectangular Purdue University experimental combustion chamber. In addition to the novel geometry, new features include disturbances caused by engine acceleration and unsteady thruster nozzle flow. [Preview Abstract] |
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