Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session BD: Flow Control II |
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Chair: David Ashpis, NASA Glenn Research Center Room: 101D |
Sunday, November 22, 2009 10:30AM - 10:43AM |
BD.00001: The breakdown of the viscous regime in riblets Ricardo Garc\'Ia-Mayoral, Javier Jim\'enez We investigate the mechanisms involved in the breakdown of the viscous regime in riblets, with a view to determining the point of optimum performance, where drag reduction ceases to be proportional to the riblet size. This occurs empirically for a groove cross-section $A_g^+ \approx 120^+$. To study the interaction of the riblets with the overlaying turbulent flow, we systematically conduct DNSes in a ribbed turbulent channel with increasing riblet size. The conditionally averaged crossflow above and within the grooves reveals a mean recirculation bubble that exists up to the point of viscous breakdown, isolating the groove floor from the overlying crossflow, and preventing the high momentum fluid from entering the grooves. We do not find evidence of outside vortices lodging within the grooves until $A_g^+ \approx 400$, which is well past the drag minimum, and already into the drag increasing regime. Interestingly, as the bubble breaks down, we observe that quasi-two-dimensional spanwise structures form just above the riblets, similar to those observed above porous surfaces and plant canopies, which appear to be involved in the performance degradation. [Preview Abstract] |
Sunday, November 22, 2009 10:43AM - 10:56AM |
BD.00002: Linear Control of Turbulent Channel Flow and the Role of Pressure Jonathan Morrison, Ati Sharma, Beverley McKeon The response of fully turbulent channel flow to global, linear control is examined. Through full-domain sensing on the wall- normal velocity, control is designed to ensure that the perturbations decay monotonically. The physical effect is such that $vdU/dy$ is countered directly. The control is shown to work for flow disturbances of any size and not those small enough to permit linear approximation. When the forcing bandwidth is progressively reduced, control in terms of drag reduction remains effective. The response of the near-wall flow at $y^+=15$ to full-domain actuation is examined in detail, with particular emphasis on the pressure field. It is shown that the near-wall pressure fluctuations are attenuated more quickly than those associated with both the velocity and vorticity fields. Reasons for this result are examined, in particular the pressure- gradient fluctuations, which drive the momentum field, are examined. [Preview Abstract] |
Sunday, November 22, 2009 10:56AM - 11:09AM |
BD.00003: Control of transition in Poiseuille flow using streamwise traveling waves. Part 1: Receptivity analysis Rashad Moarref, Mihailo R. Jovanovi\'{c} We assess the efficacy of a zero-net-mass-flux blowing and suction in the form of streamwise traveling waves for transition control in Poiseuille flow. As shown by Bewley (2009), the net efficiency is always negative if the uncontrolled flow stays laminar. We show, however, that a positive net efficiency can be achieved in situations where the uncontrolled flow becomes turbulent but the controlled flow remains laminar. Starting from this observation, we develop a framework for selection of traveling wave parameters for control of transition with a positive net power balance. Our detailed parametric study shows that, relative to the uncontrolled flow, the velocity fluctuations around the upstream traveling waves at best exhibit similar receptivity to background disturbances. In contrast, the properly designed downstream traveling waves can significantly reduce receptivity which makes them well-suited for preventing transition. Our theoretical predictions are confirmed by direct numerical simulations of the Navier-Stokes equations in Part 2 of this paper. [Preview Abstract] |
Sunday, November 22, 2009 11:09AM - 11:22AM |
BD.00004: Control of transition in Poiseuille flow using streamwise traveling waves. Part 2: Direct numerical simulations Binh Lieu, Rashad Moarref, Mihailo R. Jovanovi\'{c} This work builds on and confirms the theoretical findings of Part 1 of this paper. We use direct numerical simulations (DNS) of the Navier-Stokes (NS) equations to assess the efficacy of blowing and suction in the form of streamwise traveling waves for transition control in Poiseuille flow. We highlight the effects of the modified base flow on the dynamics of velocity fluctuations and the net efficiency. Our simulations verify theoretical predictions of Part 1 that the upstream traveling waves promote turbulence even when the uncontrolled flow stays laminar. On the other hand, the downstream traveling waves with parameters selected in Part 1 are capable of reducing the fluctuations' kinetic energy and maintaining the laminar flow. For this choice of control, a positive net efficiency of $26 \, \%$ compared to the turbulent uncontrolled flow can be achieved. The DNS results of this paper elucidate the predictive power of the method developed in Part 1 and suggest that the linearized NS equations with uncertainty may serve as an effective control-oriented model for preventing transition. [Preview Abstract] |
Sunday, November 22, 2009 11:22AM - 11:35AM |
BD.00005: Shape Optimization of Peristaltic Pumping Shawn Walker, Michael Shelley Transport is a fundamental aspect of biology and peristaltic pumping is a fundamental mechanism to accomplish this; it is also important in many industrial processes. We present a variational method for optimizing peristaltic pumping in a two dimensional periodic channel with moving walls to pump fluid. No a priori assumption is made on the wall motion, except that the shape is static in a moving wave frame. Thus, we pose an infinite dimensional optimization problem and solve it with finite elements. Sensitivities of the cost and constraints are computed variationally via shape differential calculus and $L^2$-type projections are used to compute quantities such as curvature and boundary stresses. Our Optimization method falls under the category of sequential quadratic programming (SQP) methods. As a result, we find optimized shapes that are not obvious and have not been previously reported in the peristaltic pumping literature. Specifically, we see highly asymmetric wave shapes that are far from being sine waves. Many examples are shown for a range of fluxes and Reynolds numbers up to $\mathrm{Re}=500$ which illustrate the capabilities of our method. [Preview Abstract] |
Sunday, November 22, 2009 11:35AM - 11:48AM |
BD.00006: Computational Sensitivity Analysis of Low-Reynolds Number Turbulent Channel Flow Richard Kirkman, Meredith Metzger Computational sensitivity analysis has been performed for low-Reynolds number turbulent flow in a plane channel. Two methods: (i) the continuous sensitivity equation method (CSEM) and (ii) complex step differentiation (CS), have been implemented in the context of direct numerical simulations to determine the sensitivity derivatives (or coefficients) of the primitive variables to changes in the Reynolds number. Simulations were performed at Reynolds numbers of 100 and 180, based on the friction velocity and channel half-width. Turbulent velocity statistics compare very well to others in the literature (Kim et al., 1987; Kuroda et al., 1989). The sensitivity results correctly predict the expected change in both the mean streamwise velocity and Reynolds shear stress profiles to changes in Reynolds number. Furthermore, the mean sensitivity results correctly predict the local slope of the skin friction coefficient versus Reynolds number. The \textit{instantaneous} sensitivity results also reveal that regions of high magnitude sensitivity correlate to regions containing coherent structures in turbulent channel flow. The additional computational expenses incurred in order to run the computational sensitivity simulations in this context are also discussed. [Preview Abstract] |
Sunday, November 22, 2009 11:48AM - 12:01PM |
BD.00007: Prediction and Manipulation of Skin Friction in High-Reynolds Number Flows Yulia Peet, Pierre Sagaut Skin friction, which depends on the local shear rate, becomes difficult to predict when Reynolds number of the flow exceeds its critical value, and the flow becomes disorderly. We have developed an analytical approach which relates skin friction coefficient to statistical information in the flow above the surface in a general case of high-Reynolds numbers and complex wall shapes. Current approach allows separating the contribution of different dynamical effects into skin friction, thus providing the basis for understanding how skin friction can be manipulated by passive and active flow control methods. In the current presentation, we show the effects of organized surface nonuniformities on skin friction and compare the results of theoretical analysis with numerical simulations. [Preview Abstract] |
Sunday, November 22, 2009 12:01PM - 12:14PM |
BD.00008: Hybrid Manipulation of Streamwise Vorticity in a Turbulent Boundary Layer Abraham N. Gissen, Bojan Vukasinovic, Ari Glezer Manipulation of streamwise vorticity in a turbulent boundary layer is investigated experimentally at high subsonic speeds ($M$~=~0.5) along converging-diverging duct wall designed to provide an adverse pressure gradient that mimics the pressure gradient in a typical offset diffuser. Counter-rotating vortex pairs and single-sense vortices are formed and characterized using conventional passive sub-boundary layer micro-ramps and micro-vanes, respectively. Fluidic analogues of these passive devices are established by using surface-mounted synthetic jet actuators. Hybrid manipulation of streamwise vorticity within the boundary layer is demonstrated by simultaneous combination of passive and active actuation which enables robust, controllable ``fail-safe'' operation that requires no net mass injection. [Preview Abstract] |
Sunday, November 22, 2009 12:14PM - 12:27PM |
BD.00009: ABSTRACT WITHDRAWN |
Sunday, November 22, 2009 12:27PM - 12:40PM |
BD.00010: Spanwise Varying Open-Loop Control of a Backward-Facing Step Flow Aaron Baugh, Stuart Gilbert, Marc Schostek, David Breakey, Lorenz Sigurdson We are experimentally investigating active control of the reattaching shear layer downstream of a backward-facing step in water. Transitional and turbulent separation bubbles are studied. Control is achieved using 128 hydraulic suction-and-blowing actuation ports along the span at the corner of the step where the flow first separates, inspired by Sakakibara and Anzai's design for a plane jet [\textit{Phys. Fluids} 13, 1541 (2001)]. Perturbation magnitudes vary in space across the span of the step, and each port's perturbation is periodic with zero-net mass flux. This actuation technique is the physical manifestation of some of the numerical simulations of Kang and Choi [\textit{J. Fluid Mech.} 463, 201 (2002)]. We use backlit dye to track the evolution of vorticity in the reattaching shear layer. Tufts, which have been more commonly employed in aerodynamic studies, are adapted here for use in water. An array of approximately 1000 tufts is in place downstream of the step to examine the effects of the control schemes on the length of the recirculation bubble. [Preview Abstract] |
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