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 E20: Flow Control: General |
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Chair: Mihailo Jovanovic, University of New Mexico Room: 2008 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E20.00001: Three-dimensional simulations of flow around cylinders with fairing to suppress VIV Fangfang Xie, Yue Yu, Yiannis Constantinides, George Karniadakis Three-dimensional simulation of stationary and moving cylinders with free-to-rotate fairings are conducted at Reynolds number 100 $\le $ Re $\le $ 10,000. Fairings are nearly-neutrally buoyant devices, which are fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIV) and reduce the drag force. The effect of gap between fairings along the cylinder axis on the hydrodynamic forces (C$_{d}$, C$_{l})$ and the translational and rotational motion of fairings (x$_{rms}$, y$_{rms,}\theta_{rms})$ are investigated. With the increase of Re, the drag coefficient C$_{d\, }$of fairing decreases. Compared to the plain cylinder case, fairings without gap can reduce C$_{d\, }$by 15{\%} while the fairing with gap can reduce C$_{d}$ by almost 50{\%}. The lift force (C$_{l})$ and angular momentum of fairing (M$_{fh})$ for different gaps are also decreased. Correspondingly, the vibration (y$_{rms})$ and rotation ($\theta_{rms})$ amplitudes of fairing are also reduced. We also investigate the change in flow structure induced by the fairing gaps. A pair of stream-wise vortices are generated in the gap region, which extracts energy from the flow in the cross-flow direction hence causing decrease of the lift force. As Re increases, pressure recovery in the wake of the fairing is observed, which is the main reason for the substantial decrease of drag force. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E20.00002: Receptivity of the Boundary Layer to Vibrations of the Wing Surface Tomass Bernots, Anatoly Ruban, David Pryce In this work we study generation of Tollmien-Schlichting (T-S) waves in the boundary layer due to elastic vibrations of the wing surface. The flow is investigated based on the asymptotic analysis of the Navier-Stokes equations at large values of the Reynolds number. It is assumed that in the spectrum of the wing vibrations there is a harmonic which comes in resonance with the T-S wave on the lower branch of the stability curve. It was found that the vibrations of the wing surface produce pressure perturbations in the flow outside the boundary layer which can be calculated with the help of the piston theory. As the pressure perturbations penetrate into the boundary layer, a Stokes layer forms on the wing surface which appears to be influenced significantly by the compressibility of the flow, and is incapable of producing the T-S waves. The situation changes when the Stokes layer encounters an roughness; near which the flow is described using the triple-deck theory. The solution of the triple-deck problem can be found in an analytic form. Our main concern is with the flow behaviour downstream of the roughness and, in particular, with the amplitude of the generated Tollmien-Schlichting waves. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E20.00003: Relaminarisation of fully turbulent flow in pipes Jakob Kuehnen, Bjoern Hof Drag reduction still remains one of the most alluring applications of turbulence control. We will show that flattening the streamwise velocity profile in pipes can force turbulent flow to decay and become laminar. Two different experimental control schemes are presented: one with a local modification of the flow profile by means of a stationary obstacle and one with a moving wall, where a part of the pipe is shifted in the streamwise direction. Both control schemes act on the flow such that the streamwise velocity profile becomes more flat and turbulence gradually grows faint and disappears. Since, in a smooth straight pipe, the flow remains laminar from that position a reduction in skin friction by a factor of 5 can be accomplished. We will present measurements with high-speed particle image velocimetry, measurements of the pressure drop 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] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E20.00004: Model-based and adaptive laminar-flow control via dielectric-barrier-discharge plasma actuators: an experimental comparison Nicol\`o Fabbiane, Bernhard Simon, Sven Grundmann, Shervin Bagheri, Dan S. Henningson This work compares two of the mostly investigated reactive-control techniques in delaying the laminar-to-turbulence transition in boundary-layer (BL) flows: a Linear Quadratic Gaussian (LQG) regulator and a Filtered-X Least Mean Squares (FXLMS) algorithm. The two compensators are compared on damping 2D TS-waves excited via both single-frequency and white-noise disturbances in a zero-pressure-gradient BL flow. Surface hot-wire sensors are used to detect the incoming waves and measure the effectiveness of the control action that is provided by a dielectric-barrier-discharge plasma actuator positioned between the two sensors. Based on DNS of the experimental set-up a linear reduced order model is built using the Eigensystem Realization Algorithm and used for the LQG design. The two control techniques show comparable performances when tested at their design condition. However, when tested off-design the LQG compensator shows a stronger sensitivity to model variations. If the free-stream velocity is changed, the LQG regulator estimates a wrong phase information of the incoming disturbance resulting in a less effective control action. The FXLMS compensator, instead, is capable to adapt to the new condition and prescribe the correct phase information with no significant performance loss [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E20.00005: Estimation and feedback control of vortex shedding Simon Illingworth We consider estimator-based control of the cylinder wake in low-Reynolds-number simulations. There are two parts to the study. In the first part, we show that feedback control with a single sensor becomes increasingly difficult as Reynolds number increases. This is because of a larger region of absolute instability. The convective nature of the flow means that the single sensor is unaware of what is happening downstream of it, making feedback control very difficult. This motivates the second part of the study, where we consider estimator-based feedback control. Keeping with a single sensor measurement, we investigate how well one can estimate the entire flow field using only this single sensor. To do so we use a Kalman filter, and excellent results are seen. We then combine this Kalman filter with suitable feedback control laws to suppress vortex shedding. This control strategy still uses only a single sensor but, crucially, the control actions are based on (the estimate of) what is happening in the full domain. This control strategy achieves much better suppression in the far wake when compared with the single-sensor controller without estimator. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E20.00006: ABSTRACT WITHDRAWN |
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