Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session AL: Turbulent Boundary Layers I |
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Chair: Dale Pullin, California Institute of Technology Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 8 |
Sunday, November 19, 2006 8:00AM - 8:13AM |
AL.00001: Dynamical Contributions to the Skin Friction in Polymer Drag Reduced Wall-Bounded Turbulence Christopher White, Vijay Somandepalli, Yves Dubief, Godfrey Mungal The friction drag in polymer drag reduced turbulent boundary layer flow is decomposed into four dynamical contributions. It is found that drag reduction is achieved by either, or both, an attenuation of the Reynolds stress and a reduction in the total stress gradient near the wall. Particle image velocimetry (PIV) measurements made in a turbulent developing boundary layer flow, with and without polymer drag reduction, demonstrate that polymers produce both these effects. [Preview Abstract] |
Sunday, November 19, 2006 8:13AM - 8:26AM |
AL.00002: Streamwise Development of Turbulent Boundary Layer Drag Reduction with Polymer Injection Godfrey Mungal, Yongxi Hou, Vijay Somandepalli Zero pressure gradient turbulent boundary layer drag reduction by polymer injection has been studied with PIV from low to maximum drag reduction. A previously developed technique - the (1-\textit{y/$\delta $}) fit to the total shear stress profile - is used to evaluate the skin friction, drag reduction and polymer stress. The mean velocity is seen to respond quickly to the suddenly reduced wall shear stress due to polymer injection, unlike the entire Reynolds shear stress profile. The Reynolds shear stress profiles (in wall units) can be greater than unity and this unique feature is used to judge if the flow is in equilibrium. Downstream drag reduction magnitude is used to categorize the flow into three zones: development, steady-state and depletion regions. The polymer stress is found to be proportional to drag reduction in the depletion region but not necessarily so in the other two regions. The dynamical contributions of the various stresses in the boundary layer to the overall stress balance shows that the polymer stresses can sometimes account for up to 25{\%} of the wall shear stress. [Preview Abstract] |
Sunday, November 19, 2006 8:26AM - 8:39AM |
AL.00003: Drag reduction in turbulent pipe flow by applied electric potentials Magne Waskaas, Vytautas Daujotis, Kjell Wolden, Rimantas Raudonis, Deivis Plausinaitis A novel approach to drag reduction is presented on the basis of applied positive electric potentials to a pipe. This has been studied by measuring the pressure drop over a 13.1 m epoxy-coated pipe made of carbon steel, through which water was flowing under conditions of constant flow rate. Potentials were applied between the pipe and the counter electrode located at the pipe inlet. The results show a decrease in the pressure drop (up to 2\%) when positive electric DC-potentials in the range 0.6 -- 1.6V were applied to the pipe. However, no significant changes was obtained for applied potentials in the ranges of 0 to 0.6 V, 1.6 to 2.0 V or 0 to -2.0 V. Waterflow through an epoxy coated turbine pipe (length 1562 m, diameter 1 m, total fall 380 m) in a hydroelectric power plant has also been studied. A 1.1 V potential was applied between the pipe and the manlock (made of stainless steel and electrically insulated from the pipe). Results show that the head loss decreased from 45.9 m to 39.8 m at maximum flow rate, which corresponds to a 1.8\% increase in the electricity production. Although small, the effect represents the possibility of significant cost savings. The mechanism by which the drag is reduced is not currently understood. [Preview Abstract] |
Sunday, November 19, 2006 8:39AM - 8:52AM |
AL.00004: The mechanism of drag reduction in turbulent pipe flow with spanwise wall oscillation Andrew Duggleby, Kenneth Ball, Mark Paul The results of a comparative analysis between turbulent pipe flow and drag reduced turbulent pipe flow by spanwise wall oscillation based upon a Karhunen-Lo\`{e}ve expansion are presented. The turbulent flow is generated by a direct numerical simulation at a Reynolds number $Re_\tau = 150$. The spanwise wall oscillation is imposed as a velocity boundary condition with an amplitude of $A^+ = 20$ and a period of $T^+ = 50$. The flow is driven by a constant pressure gradient, resulting in a 27\% mean velocity increase with wall oscillation. The peaks of the Reynolds stress and root-mean-squared velocities shift away from the wall and the Karhunen-Lo\`{e}ve dimension of the chaotic turbulence attractor is reduced from $2453$ to $102$. The coherent vorticity structures are pushed away from the wall into higher speed flow, causing an increase of their advection speed of 34\% as determined by a normal speed locus. The mechanism of drag reduction by spanwise wall oscillation is discussed. [Preview Abstract] |
Sunday, November 19, 2006 8:52AM - 9:05AM |
AL.00005: Power law and log law velocity distributions in the wall bounded turbulent flows on transitional rough walls: New approach to universal scaling Noor Afzal An alternate two layers theory, based on four new scalings for transitional wall roughness variables, is presented for large appropriate roughness Reynolds numbers. For velocity profile the matching of inner and outer layers in the overlap region, by Izakson-Millikan-Kolmogorov hypothesis (Afzal, N. 2005 Proc. Royal Society A: PME 461, 1889-1910) leads to functional solutions that are universal log laws, as well as universal power laws, that explicitly independent of transitional wall roughness, having same constants as in smooth wall case. The universal log or power laws velocity profile and skin friction, if expressed in terms of traditional Reynolds numbers also yield log law and power laws that depend on surface roughness. The skin friction, in traditional variables, is predicted by a single relation for inflectional type of Nikuradse roughness for sand grain type roughness data and Colebrook commercial monotonic roughness. The extensive experimental data for various types of wall transitional roughness provide very good support to present theory of universal log laws as well as new predictions in traditional log laws . The experimental data from various sources (Osaka and Mochizuki, Kameda et al, Antonia and Krogstad, Smalley et al, Schultz and Flack and Leonardi et al for boundary layers and Nikuradse, Shockling and Bakken for pipes/Channels) provide strong support to the new scaling for log and power laws. Moody type diagram for inflectional roughness for boundary layer and pipe flows are presented. [Preview Abstract] |
Sunday, November 19, 2006 9:05AM - 9:18AM |
AL.00006: Development of Roughness Scaling Parameters Karen Flack, Michael Schultz, Ralph Volino The most important unresolved question regarding surface roughness is to identify suitable roughness length scales that can be used to predict the frictional drag of a body covered with any generic roughness. Parameters such as the density, the mean slope, and the texture of the roughness elements have all been identified as important parameters; however, correlations using these parameters have not proved useful for a wide range of surface roughness. The focus of this research is to identify length scales of the surface topography, obtained from laser profilometry, that correlate with the roughness functions, obtained from detailed boundary layer velocity profiles and towing tank tests. [Preview Abstract] |
Sunday, November 19, 2006 9:18AM - 9:31AM |
AL.00007: Power Law for Rough Favorable Pressure Gradient Turbulent Boundary Layers Katherine Newhall, Luciano Castillo Current studies on rough favorable pressure gradient (FPG) boundary layers are very challenging particularly since it is difficult to obtain values of skin friction as a function of roughness, pressure gradient strength and $Re_\theta$. This study presents a new modified form of the power law from George and Castillo (1997) developed for smooth zero pressure gradient boundary layers. The new form accounts for mild pressure gradients, and aids in the account for rough surface boundary layers. Emphasis will be given to its application to rough FPG flows. The values of the skin friction for smooth FPG boundary layers are obtained within 3\%. Moreover, the composite profile for the mean velocity accurately describes both FPG and APG flows. The modified power law solution has the advantage of being a continuous solution for smooth and rough profiles subject to external pressure gradients. In addition, by using the Navier Stokes equation the Reynolds shear stress is accurately calculated from the composite description of the mean velocity. [Preview Abstract] |
Sunday, November 19, 2006 9:31AM - 9:44AM |
AL.00008: Large-eddy simulation of wall-bounded turbulent flows using a virtual-wall model Daniel Chung, Dale Pullin We will present results obtained from large-eddy simulation (LES) of incompressible channel flow using an extended form of the stretched-vortex subgrid-scale model together with a closure developed specifically for the near-wall region of wall-bounded turbulent flows. Our wall model combines filtering of the Navier-Stokes equations over the near-wall region with an assumption of local inner scaling in a way that couples a wall-closure equation directly to the outer-flow LES. This provides a fluctuating, slip-flow boundary condition at a ``virtual wall'' near the bottom of the log-law region. LES results will be presented and compared with both DNS and experiment at large $Re_\tau$. [Preview Abstract] |
Sunday, November 19, 2006 9:44AM - 9:57AM |
AL.00009: Self-similar vortex clusters over rough walls Oscar Flores, Javier Jim\'enez, Juan C. del \'Alamo The properties of vortex clusters in rough-walled turbulent channels are analysed using direct numerical simulations at $Re_\tau \approx 600$, in which roughness is simulated by distributions of velocity disturbances at the walls. The results are compared with data coming from smooth channels (del \'Alamo {\em et al} 2006 JFM 561). As in the smooth case, the present clusters separate into wall-detached objects and wall-attached ones. The detached clusters from rough and smooth walls are similar, consistent with their definition. Also expected, the attached clusters inside the roughness sublayer are quite different to those in the near-wall region over smooth walls. However, the attached clusters that reach above the buffer region are quite independent of the nature of the wall. The same result is obtained for the average velocity field conditioned to these tall attached clusters. This suggests that the dynamics of the logarithmic and outer regions depends only on the mean velocity gradient, even for vastly different velocity profiles (Flores \& Jim\'enez, 2006, JFM in press). [Preview Abstract] |
Sunday, November 19, 2006 9:57AM - 10:10AM |
AL.00010: The dynamics of relaminarization in turbulent pipe flow Kenneth Ball, Andrew Duggleby, Mark Paul Propagating structures, based upon a Karhunen-Lo\`{e}ve decomposition, undergoing reverse transition from turbulent to laminar flow are investigated. The turbulent flow is generated by a direct numerical simulation starting at a fully turbulent Reynolds number of $\mathrm{Re}_\tau=150$, and is slowly decreased until $Re_\tau=95$. At this low Reynolds number the high frequency modes decay first, leaving only the slower decaying streamwise vortices. The flow undergoes a chugging phenomena, where it begins to relaminarize and increase its mean velocity. The remaining propagating modes then destabilize the streamwise vortices, rebuild the energy spectra, and eventually the flow regains its turbulent state. A simulation capturing three chugging cycles before the flow completely relaminarizes is presented. The high frequency modes present in the outer layer decay first, establishing the importance of the outer region in the self-sustaining mechanism of wall bound turbulence. [Preview Abstract] |
Sunday, November 19, 2006 10:10AM - 10:23AM |
AL.00011: ABSTRACT WITHDRAWN |
Sunday, November 19, 2006 10:23AM - 10:36AM |
AL.00012: On the Lamb vector divergence as a momentum field diagnostic employed in turbulent channel flow Curtis W. Hamman, Robert M. Kirby, Joseph C. Klewicki Vorticity, enstrophy, helicity, and other derived field variables provide invaluable information about the kinematics and dynamics of fluids. However, whether or not derived field variables exist that intrinsically identify spatially localized motions having a distinct capacity to affect a time rate of change of linear momentum is seldom addressed in the literature. The purpose of the present study is to illustrate the unique attributes of the divergence of the Lamb vector in order to qualify its potential for characterizing such spatially localized motions. Toward this aim, we describe the mathematical properties, near-wall behavior, and scaling characteristics of the divergence of the Lamb vector for turbulent channel flow. When scaled by inner variables, the mean divergence of the Lamb vector merges to a single curve in the inner layer, and the fluctuating quantities exhibit a strong correlation with the Bernoulli function throughout much of the inner layer. [Preview Abstract] |
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