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 R12: Flow Control: Vortices and Turbulence |
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Chair: Mohamed Gad-el-Hak, Virginia Commonwealth University Room: 3018 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R12.00001: Scaling the Response of Separating Turbulent Boundary Layer to Pulsed Excitation Seifert Avraham, Vitali Palei The talk will start by offering the presenters view of active flow control current status, main challenges and future directions. Then recent experimental results of turbulent separating boundary layer, subjected to pulsed excitation will be presented and discussed. A search for instability mechanism did not result in any disturbances that were amplified. Therefore, pulsed excitation that intermittently enhances the skin friction with optimal time lag was sought. Scaling the response of the excited flow leads to dimensionless optimal magnitude and repetition rates. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R12.00002: Ekman and Taylor Vortices' Destruction and Mixing Enhancement in a Taylor--Couette System H. Oualli, M. Mekadem, A. Bentsabet, M. Abada, A. Bouabdallah, M. Gad-el-Hak Suppression of Ekman and Taylor vortices is sought in several industrial processes such as cylindrical crystal growth and osmotic/photonic water purification. Last meeting, we investigated experimentally and numerically an active flow control strategy to obliterate vortices in a Taylor--Couette flow. The control consists of effecting minute radial pulsatile motion of the rotating inner cylinder's cross-section. The results showed that destruction of either type of vortices occurs at different pulsatile frequencies, requiring one order of magnitude higher frequency to obliterate the Ekman type. This problem is revisited with identical parameters and conditions for the controlling strategy but the Taylor--Couette system is now inclined relative to the horizontal direction in such a way that gravitational effects are no longer negligible. It is found that body forces contribute to the complete destruction of Taylor and Ekman vortices, reducing the optimum frequency by more than 50\% for even a modest inclination angle of $\theta=15^{\circ}$. Furthermore, the axial and azimuthal velocity fluctuations are increased by one order of magnitude, thus yielding substantial enhancement in flow mixing. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R12.00003: Estimation of the global modes in the wake of a low-aspect-ratio pyramid Zahra Hosseini, Robert J. Martinuzzi, Bernd R. Noack A pressure sensor based estimation technique is proposed to extract the most energetic global modes in the turbulent wake of a wall-mounted square-based pyramid with apex angle of $\zeta=60^{\circ}$ immersed partially in a thin turbulent boundary layer. A modified Extended Proper Orthogonal Decomposition (EPOD) technique is presented which exploits extracting the maximum pressure-velocity correlations for the optimal velocity estimation. The method is assessed based on the planar stereoscopic Particle Image Velocimetry data taken simultaneously with fluctuating pressure at the pyramid surface and the wall. The proposed modifications enable to recover significant dynamics that are otherwise lost in the EPOD estimation and greatly reduce the residual of the estimated coherent kinetic energy. The method will be used to estimate the three-dimensional coherent structures reconstructed from the dominant modes: mainly the fundamental harmonics associated to the periodic shedding and a slow-drift mode capturing the base flow modulations. Such three-dimensional description of the coherent structures helps to understand complex couplings between slow drift and harmonic fluctuations in tapered body wake. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R12.00004: The effect of butterfly-scale inspired patterning on leading-edge vortex growth Jacob Wilroy, Amy Lang, Redha Wahidi Leading edge vortices (LEVs) are important for generating thrust and lift in flapping flight, and the surface patterning (scales) on butterfly wings is hypothesized to play a role in the vortex formation of the LEV. To simplify this complex flow problem, we designed an experiment to focus on the alteration of 2-D vortex development with a variation in surface patterning. Specifically we are interested in the secondary vorticity generated by the LEV interacting at the patterned surface and how this can affect the growth rate of the circulation in the LEV. For this experiment we used rapid-prototyped longitudinal and transverse square grooves attached to a flat plate and compared the vortex formation as the plate moved vertically. The plate is impulsively started in quiescent water and flow fields at Re $=$ 1500, 3000, and 6000 are examined using Digital Particle Image Velocimetry (DPIV). The vortex formation time is 0.6 and is based on the flat plate travel length and chord length. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R12.00005: Interaction of a vortex ring and a bubble Narsing K. Jha, Raghuraman N. Govardhan Micro-bubble injection in to boundary layers is one possible method for reducing frictional drag of ships. Although this has been studied for some time, the physical mechanisms responsible for drag reduction using microbubbles in turbulent boundary layers is not yet fully understood. Previous studies suggest that bubble-vortical structure interaction seems to be one of the important physical mechanisms for frictional drag reduction using microbubbles. In the present work, we study a simplification of this problem, namely, the interaction of a single vortical structure, in particular a vortex ring, with a single bubble for better understanding of the physics. The vortex ring is generated using a piston-cylinder arrangement and the bubble is generated by connecting a capillary to an air pump. The bubble dynamics is directly visualized using a high speed camera, while the vorticity modification is measured using time resolved PIV. The results show that significant deformations can occur of both the bubble and the vortex ring. Effect of different non-dimensional parameters on the interaction will be presented in the meeting. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R12.00006: Machine learning control (MLC) --- a novel method for optimal control of complex nonlinear systems Bernd R. Noack, Laurent Cordier, Vladimir Parezanovic, Kai von Krbek, Marc Segond, Markus W. Abel, Steven Brunton, Thomas Duriez We propose a model-free closed-loop control strategy for complex nonlinear systems with a finite number of sensors and actuators (MIMO). This strategy yields a feedback law which optimizes a cost functional with machine learning methods. Thus, no dynamical model of the plant is required in contrast to model-based approaches, In addition, no working open-loop control is necessary in contrast to adaptive approaches. The approach is illustrated for strongly nonlinear dynamical systems which are not accessible to linear control design. Control studies of several shear-turbulence experiments will be presented in the talks of T.\ Duriez and V.\ Parezanovi\'c. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R12.00007: Closed-loop control of experimental shear flows using MLC Thomas Duriez, Vladimir Parezanovi\'c, Kai von Krbek, Laurent Cordier, Bernd R. Noack, Jean-Paul Bonnet, Marc Segond, Markus W. Abel, Nicolas Gautier, Jean-Luc Aider, C\'edric Raibaudo, Christophe Cuvier, Michel Stanislas, Antoine Debien, Nicolas Mazellier, Azeddine Kourta, Steven Brunton We employ a novel closed-loop control strategy for turbulent flows using machine learning methods in a model-free manner (see MLC talk of B.~R.\ Noack). MLC yields in-time control of experimental shear flows and has advantages over the state-of-the-art control. In this talk, MLC is applied to four different experimental closed-loop control setups: (1) the TUCOROM mixing layer tunnel (talk of V.\ Parezanovi\'c), (2) the G\"ortler PMMH water tunnel with a backward facing step, (3) the LML Boundary-Layer wind tunnel with a separating turbulent boundary layer, and (4) the Malavard wind tunnel with the SepaCoDe ramp. In all cases, MLC finds a control which yields a significantly better performance with respect to the given cost functional as compared to the best previously tested open-loop actuation. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R12.00008: Closed-loop control of an experimental mixing layer using MLC Vladimir Parezanovi\'c, Laurent Cordier, Bernd R. Noack, Andreas Spohn, Jean-Paul Bonnet, Thomas Duriez, Marc Segond, Markus W. Abel, Steven Brunton A novel framework for closed-loop control of turbulent flows is tested for an experimental mixing layer flow. This framework, called Machine Learning Control (MLC), provides a model-free method of searching for the best control law (see talk of B.~R.\ Noack). Here, MLC is benchmarked against classical open-loop actuation of the mixing layer. Results show that this method is capable of producing sensor-based control laws which can rival or surpass the best open-loop forcing, and be robust to changing flow conditions. Additionally, MLC can detect non-linear mechanisms present in the controlled plant, and exploit them to find a better type of actuation than the best periodic forcing. Other experimental shear-flow control studies with MLC will be presented in a talk by T.\ Duriez. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R12.00009: Symmetries, multistability and stochastic dynamics of turbulent wakes Georgios Rigas, Aimee Morgans, Jonathan Morrison The dynamics of a turbulent wake generated behind a bluff three-dimensional axisymmetric body are investigated experimentally at a diameter based Reynolds number $\sim 2\times 10^5$. Proper orthogonal decomposition of base pressure measurements indicates that the most energetic coherent structures retain the structure of the symmetry-breaking laminar instabilities and manifest as unsteady vortex shedding with azimuthal wavenumber $m=\pm1$. In a rotating reference frame, the wake preserves the reflectional symmetry, as observed in the laminar and transitional regimes. Due to a slow and random rotation of the symmetry plane around the axis of the body, the turbulent wake explores an infinite number of metastable states and statistical axisymmetry is recovered in the time average. A simple dynamical model, where the deterministic part describes the broken symmetries of the flow and the stochastic part accounts for the incoherent fluctuations, shows excellent agreement with the experimental results for the spatiotemporal evolution of the turbulent wake. Finally, we show how these models can be obtained directly from the governing Navier-Stokes equations. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R12.00010: Subcritical Transition in Channel Flows Joseph Maestri, Philip Hall Exact-coherent structures, or colloquially non-linear solutions to the Navier-Stokes equations, have been the subject of great interest over the past decade due to their relevance in understanding the process of transition to turbulence in shear flows. Over the past few years the relationship between high Reynolds number vortex-wave interaction theory and such states has been elucidated in a number of papers and has provided a solid asymptotic framework to understand the so-called self-sustaining process that maintains such structures. In this talk, we will discuss this relationship before talking about recent work on solving the vortex-wave interaction equations using numerical techniques in order to propose laminar-flow control techniques. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R12.00011: Vortex-Wave interaction in plane Poiseuille flow Liam Dempsey, Andy Walton, Philip Hall Our main interest is in the process of transition to turbulence at high Reynolds numbers in the flow in a plane channel. We will consider the basic flow to be driven by a uniform streamwise pressure gradient (plane Poiseuille flow). We will formulate a high Reynolds number asymptotic structure in the form of a nonlinear vortex-Tollmien-Schlichting-wave interaction (VWI)- or so-called self sustaining process. We look for starting solutions for the VWI by performing a weakly nonlinear analysis close to the lower branch neutral point and seek equilibrium solutions of the resulting nonlinear amplitude equation. We provide numerical solutions of the interaction equations which are localised in the spanwise direction at large enough amplitudes. [Preview Abstract] |
Tuesday, November 25, 2014 3:28PM - 3:41PM |
R12.00012: Linear Mechanisms and Pressure Fluctuations in Wall Turbulence Kamthon Septham, Jonathan Morrison Full-domain, linear feedback control of turbulent channel flow at $Re_\tau$ $\leq$ 400 via $vU'$ at low wavenumbers is an effective method to attenuate turbulent channel flow such that it is relaminarised. The passivity-based control approach is adopted and explained by the conservative characteristics of the nonlinear terms contributing to the Reynolds-Orr equation (Sharma $et\ al.\ Phys.\ Fluids$ 2011). The linear forcing acts on the wall-normal velocity field and thus the pressure field via the linear (rapid) source term of the Poisson equation for pressure fluctuations, $2U'\frac{\partial v}{\partial x}$. The minimum required spanwise wavelength resolution without losing control is constant at $\lambda_{z}^{+}$ = 125, based on the wall friction velocity at $t = 0$. The result shows that the maximum forcing is located at $y^{+} \approx 20$, corresponding to the location of the maximum in the mean-square pressure gradient. The effectiveness of linear control is qualitatively explained by Landahl's theory for timescales, in that the control proceeds via the shear interaction timescale which is much shorter than both the nonlinear and viscous timescales. The response of the rapid (linear) and slow (nonlinear) pressure fluctuations to the linear control is examined and discussed. [Preview Abstract] |
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