Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session AT: Turbulent Flow Control I |
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Chair: Taegee Min, University of California, Los Angeles Room: Hilton Chicago Stevens 5 |
Sunday, November 20, 2005 8:00AM - 8:13AM |
AT.00001: Drag Reduction in a Radially Pulsating Cylinder at Moderate Reynolds Number H. Oualli, H. Hanchi, A. Bouabdallah, R. Askovi\c{c}, M. Gad-el-Hak The early separation and wide wake behind a bluff body are responsible for the large pressure drag. Passive, active and reactive control strategies center for the most part on modulating the wake and its omnipresent K\'arm\'an vortices. Here, we numerically investigate the uniform flow around a two-dimensional, radially pulsating circular cylinder at Reynolds number of 300, where, for a constant-diameter cylinder, the three-dimensional instability mode A transitions to mode B. A second-order finite-difference discretization of the unsteady, two-dimensional vorticity--streamfunction equation is used, and time is advanced via a second-order Adams--Bashforth scheme. A Smagorinsky eddy viscosity model is employed for closure. The small-amplitude pulsation of the cylinder changes the dynamics of the K\'arm\'an vortex street as well as the secondary vortices. The interplay between the primary and secondary vortices modulates the flow field and results in a dramatic reduction of the total drag. Drag monotonically decreases as the oscillation frequency increases beyond a certain threshold, and thrust is produced at sufficiently high frequency. [Preview Abstract] |
Sunday, November 20, 2005 8:13AM - 8:26AM |
AT.00002: Role of Dimples on Golf Ball Jin Choi, Woo-Pyung Jeon, Haecheon Choi It is an open question why the drag coefficient on golf ball remains nearly constant with increasing Reynolds number after its sharp decrease. In order to investigate this interesting phenomenon, we measure the drag, separation angle, wall pressure and streamwise velocity inside/outside dimples before main separation. When drag reduction occurs with dimples, the separation angle measured is nearly constant even with increasing Reynolds number. Also, the wall pressure distributions outside dimples are nearly the same at different Reynolds numbers, although those inside dimples vary depending on the Reynolds number. From the streamwise velocity measurement, it is found that dimples located at the angles of $65^o \sim 90^o$ (three rows of dimples exist in our experimental setup) make an important role in changing flow characteristics. Inside one or two rows of dimples located at those angles, a small separation bubble exists and flow becomes quickly transitional and turbulent with reattachment. After the reattachment, the flow does not separate at the downstream dimples owing to fuller velocity profile. With dimples, the main separation occurs further downstream at 110 degree. This main separation angle does not change even with increasing Reynolds number, because downstream dimples do not make an important role in changing flow characteristics any more. [Preview Abstract] |
Sunday, November 20, 2005 8:26AM - 8:39AM |
AT.00003: Suboptimal feedback control for drag reduction in flow over a sphere Seung Jeon, Haecheon Choi The objective of the present study is to propose a method of controlling the wake behind a sphere for drag reduction using suboptimal control theory. The present numerical method is based on an immersed boundary method in a cylindrical coordinate. The Reynolds numbers considered are 100, 250, 300 and 425, at which the base flows are steady axisymmetric, steady planar-symmetric, unsteady planar-symmetric, and unsteady asymmetric, respectively. Several cost functions to be minimized or maximized from blowing and suction on the sphere surface with zero-net mass flow rate are considered for each Reynolds number. Some of them successfully provide drag reduction. Among them, the most effective cost function is the square of the difference between the target pressure (potential-flow pressure) and real pressure on the sphere surface. With this cost function, the flow becomes steady axisymmetric for all the Reynolds numbers considered, and about 20\% drag reduction is obtained at $Re=425$. [Preview Abstract] |
Sunday, November 20, 2005 8:39AM - 8:52AM |
AT.00004: Sublaminar skin-friction drag in controlled channel flow Sung Kang, Taegee Min, Jason Speyer, John Kim We investigated the conjecture proposed by Bewley\footnote{Bewley \emph{Prog. Aerospace Sci.}, {\bf 37}, 2001} that the lowest sustainable drag of an incompressible constant mass-flux channel flow, when controlled via a distribution of zero-net mass-flux blowing/suction over the no-slip channel walls, is exactly that of the laminar flow. Numerical experiments in a two-dimensional channel flow show that below-laminar drag could be sustained with surface blowing/suction in the form of upstream traveling waves. This open-loop control produces Reynolds shear stress opposite to its typical form in a channel flow, resulting in a sustained drag below the laminar value. A linear mechanism contributes to this favorable distribution, allowing the use of the linearized Navier-Stokes equations to relate the convection speed of the traveling wave and the reduction in drag. It shows that downstream traveling waves have the opposite effect, resulting in higher drag. Applied to a fully turbulent flow, an upstream traveling wave also reduces the drag below that of a laminar flow with the same mass flux. Details of the controlled flow field will be presented. [Preview Abstract] |
Sunday, November 20, 2005 8:52AM - 9:05AM |
AT.00005: Turbulence suppression in channel flows by transverse wall oscillations Mihailo Jovanovic Although several numerical and experimental studies indicate that properly designed transverse wall oscillations can lead to a turbulence suppression, an obstacle to fully utilizing this sensorless flow control strategy is the absence of a theoretical framework for its design, optimization, and evaluation. We model and analyze the influence of transverse wall oscillations on the evolution of flow perturbations in channel flows. The amplitude and frequency of periodic oscillations enter as coefficients, and the free-stream turbulence or surface roughness enter as stochastic external excitations into our models. We quantify the effect of these excitations on velocity perturbation energy and develop a system theoretic paradigm for the optimal selection of transverse oscillation parameters for turbulence suppression. We demonstrate that depending on the wall oscillation frequency the energy of velocity perturbations can be increased or decreased compared to the uncontrolled flow. Our results provide a first compelling theoretical explanation as to why properly designed transverse wall oscillations can suppress turbulence in the wall-bounded shear flows. [Preview Abstract] |
Sunday, November 20, 2005 9:05AM - 9:18AM |
AT.00006: Skin-friction reduction by multiple cavities Chulkyu Kim, Woo-Pyung Jeon, Haecheon Choi A possibility of reducing the skin-friction drag in a turbulent boundary layer with a two-dimensional cavity (elongated in the spanwise direction) has been investigated in the literature. In our previous numerical study (Hahn et al. 1999), we have shown that a single cavity indeed reduces the skin friction by about 10\% in a turbulent boundary layer. Now, the question is whether or not the skin friction is decreased by a series of cavities distributed all over the flat plate. In order to answer this question, we perform an experiment for flow over multiple cavities. The Reynolds numbers based on the free- stream velocity and momentum thickness ($\theta$) are from 5300 to 9300. The drag is measured directly with a free floating balance. The cavity configurations considered are in the range of $d/\theta = 0.25 \sim 1, l/d = 1 \sim 4$ and $s/d = 5 \sim 20 $, where $d$ is the cavity depth, $l$ is the cavity length and $s$ is the streamwise spacing between two consecutive cavities. So far, the case of $d/\theta =0.25, l/d = 1$ and $s/d = 11$ produces about 2\% drag reduction among the cavity configurations investigated. Direct drag measurements are being performed for more cavity configurations, and the drag- reduction results and some boundary layer measurements will be given in the final presentation. [Preview Abstract] |
Sunday, November 20, 2005 9:18AM - 9:31AM |
AT.00007: DNS of Drag Reduction by Dilute Polymer Solutions at MDR Dong-Hyun Lee, Rayhaneh Akhavan The phenomenon of Maximum Drag Reduction (MDR) by dilute polymer solutions is investigated by DNS. The objective is to establish the flow features and critical parameters needed to reach MDR. Simulations are performed in turbulent channel flows at $Re_{\tau_o} \approx 230$ \& $570$ using a pseudo-spectral mixed Eulerian/Lagrangian scheme with either the FENE-P dumbbell or the FENE-LSMR chain models of the polymer and realistic polymer parameters. It is observed that the Weissenberg number ($We_{\tau_o}$) is the critical parameter for achieving high drag reduction, with the other parameters, such as concentration, having little effect. Results to date suggest that a $We_{\tau_o} \sim Re_{\tau_o}$ is required to achieve MDR. At $Re_{\tau_o} \approx 230$, Virk's MDR asymptote is reached at $We_{\tau_o} \sim 100$. The flow statistics here agree with the experimental data of Ptasinski \textit{et al.} [{\it Flow Turbulence Combust.} {\bf 66}, 159--182 (2001)]. At higher $We_{\tau_o}$, DNS shows a relaminarization of the flow. The flow statistics during the intermediate states leading to relaminarization agree with the experimental data of Warholic \textit{et al.} [{\it Exp. Fluids.} {\bf 27}, 461--472 (1999)]. At $Re_{\tau_o} \approx 570$, the highest $We_{\tau_o}$ we have so far achieved in DNS ($We_{\tau_o} \sim 300$) is not sufficient to achieve MDR. These results suggest that in poly-disperse polymer solutions, it is the trace amount of the highest molecular weight polymers which contribute most to drag reduction. [Preview Abstract] |
Sunday, November 20, 2005 9:31AM - 9:44AM |
AT.00008: Effects of hydrophobic surface on stability and transition Taegee Min, John Kim Effects of hydrophobic surface on stability and transition in wall-bounded shear flows are investigated. Hydrophobic surface is represented by a slip-boundary condition on the surface. Linear stability analysis with slip-boundary conditions shows that the critical Reynolds number increases with streamwise slip. Effects of slip-boundary conditions on transient growth of initial disturbances are investigated through the singular value decomposition (SVD) analysis of the linearized Navier-Stokes equations. The maximum transient growth (i.e., the amplification factor for the optimal disturbance) is reduced with streamwise slip, indicating that non-normality of the linearized Navier-Stokes equations is reduced with streamwise slip. Spanwise slip, on the other hand, increases the non-normality. Finally, it is shown that transition to turbulence is delayed significantly with streamwise slip, whereas spanwise slip induces earlier transition. The present results suggest that it is desirable to develop a hydrophobic surface with specified directional sensitivity in order to meet a particular need for specific applications. [Preview Abstract] |
Sunday, November 20, 2005 9:44AM - 9:57AM |
AT.00009: Investigation and Control of the Lateral Expansion of Turbulent Region in a Flat Plate Boundary Layer Ayumu Inasawa, Seiichiro Izawa, Yu Fukunishi A spanwise interface between laminar and turbulent regions is investigated and an attempt to control the vortical structure is carried out in a wind-tunnel experiment. The laminar-turbulent interface is monitored by a rake of single hotwire probes and the vortical structure at the interface is captured by two types of X-type probes using a conditional sampling and ensemble-averaging technique. The tendency that the turbulent region sticks out into the laminar region at the middle height of the boundary layer is found. A pair of longitudinal vortices aligned in the wall-normal direction is found to be the key structure at the interface. It is found that the vortex pair pushes the turbulent fluid into the laminar region, which contributes to the expansion of the turbulent region. It is also shown that the vortical structure can be weakened using a thin vertical plate inside the boundary layer, which results in a suppression of the lateral expansion of the turbulent region. [Preview Abstract] |
Sunday, November 20, 2005 9:57AM - 10:10AM |
AT.00010: Resonances in modulated turbulence Willem van de Water, Carl Tipton The question is whether there exist preferred frequencies with which to periodically excite turbulence. The possibility of such a resonance is intriguing as one may object that turbulence does not have a single dominating timescale but a continuum of strongly fluctuating times. We present evidence for such a resonance in windtunnel experiments where turbulence is modulated using an active grid. This grid can be controlled such that both the temporal frequency and the symmetry of the imposed spatial pattern can be changed. The results highlight the importance of the spatial structure of the modulation and the way in which the turbulent response is characterized. These experiments have been inspired by numerical studies of simple turbulence models, but resonances are now also found in direct numerical simulations which completely resolve the velocity field. [Preview Abstract] |
Sunday, November 20, 2005 10:10AM - 10:23AM |
AT.00011: The stabilizing role of anisotropy in the free stream on boundary layer development Bettina Frohnapfel, Jovan Jovanovic, Edin Skalic, Milenko Jovanovic An experimental study on the transition of a flat plate boundary layer was conducted in the large wind tunnel of the chair of fluid dynamics (LSTM) in Erlangen, Germany. Although this is not an especially designed transition tunnel it was possible to maintain a stable laminar regime up to Re$_{x}$=4x10$^{6}$, one of the highest transition Reynolds numbers achieved in a flat plate boundary layer. It is argued that this was possible due to a stabilizing effect originating from the high anisotropy level in the free stream disturbances that exists in the tunnel. Based on a statistical analysis of the dynamical equations for small axisymmetric disturbances, the influence of anisotropy on the dynamics of those disturbances in a laminar boundary layer was studied. The derived transition criterion is formulated in terms of a transition Reynolds number - based on intensity and Taylor length scale of the disturbances - that shows a dependency on the anisotropy level of the free stream disturbances. In this respect all available existing measurements on natural boundary layer transition at high Reynolds numbers were analyzed. [Preview Abstract] |
Sunday, November 20, 2005 10:23AM - 10:36AM |
AT.00012: An optimal feedback controller for the reduction of turbulent energy in 3D plane-duct flow Paolo Luchini, Thomas Bewley, Maurizio Quadrio Feedback control of 3D turbulence was demonstrated by Choi, Moin \& Kim (1994) in the form of a simple opposition control with an empyrically adjusted coefficient. H\"ogberg \& Bewley (2000) applied modern optimal control to the linearized Navier-Stokes equations in order to develop a spatially localized convolution kernel and tested the controller so obtained on a turbulent DNS with encouraging results. Quadrio \& Luchini (2002) suggested that a larger amount of physical information could be embodied in the controller if the linearized problem were replaced by a linear response of the full turbulent flow to external disturbances, and presented preliminary results for the computation of such linear response from a DNS. Here the optimal controller based on the linear response of the turbulent flow will be presented. A numerical version of the Wiener-Hopf method allows us to quickly compute the controller convolution kernel corresponding to any given pair of physical quantities chosen as actuator and sensor. [Preview Abstract] |
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