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 HR: Turbulent Boundary Layers: Drag Reduction |
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Chair: Joseph Klewicki, University of Utah Room: Hilton Chicago Stevens 3 |
Monday, November 21, 2005 1:20PM - 1:33PM |
HR.00001: Reynolds number effect on drag reduction in a microbubble-laden spatially-developing turbulent boundary layer S. Elghobashi, A. Ferrante Direct simulations of a microbubble-laden spatially developing turbulent boundary layer (SDTBL) were performed to compare the amounts of skin friction reduction due to the bubbles' presence for two Reynolds numbers: $Re_{\theta}=1430$ and $Re_{\theta}=2900$. The results show that increasing the Reynolds number {\em decreases} the percentage of drag reduction. Increasing $Re_{\theta}$ {\em `squeezes'} the quasi-streamwise vortical structures toward the wall, whereas the microbubbles {\em `push them away'} from the wall. The net result of these two {\em opposing effects} determines the amount of skin friction reduction by the microbubbles. The displacement of the vortical structures by the microbubbles is a result of the local positive velocity divergence, $\nabla \cdot {\bf U}$, created by the bubbles' concentration gradients. Thus, the volume fraction of bubbles that is responsible for the reduction of skin friction in a SDTBL at a given Reynolds number is not sufficient to produce the {\em same} amount of reduction in skin friction at higher Reynolds numbers. [Preview Abstract] |
Monday, November 21, 2005 1:33PM - 1:46PM |
HR.00002: Analysis of Mass Transfer in Polymer Turbulent Boundary Layers Anshuman Roy, Ales Alajbegovic, Ronald Larson In this work, we address the problem of drag reduction in a turbulent boundary layer over a flat plate by polymer injection. To predict the mean concentration of a constant flux of injected polymer both in the gradient and streamwise directions, we have constructed a mean flow model that faithfully reproduces the mean velocity profiles seen in experiments and direct numerical simulations. The constructed mean velocity gradient is dependent on the local polymer concentration, friction factor and the distance from the wall. To analyze polymer mass transfer, we integrate the momentum equation in the wall normal direction, reducing it to a one-dimensional equation for momentum thickness (or equivalently, friction factor). The mass balance equation for this problem can be reduced to a turbulent advection-diffusion equation by invoking the Chilton-Colbourn analogy between turbulent mass and momentum transport and using a turbulent diffusivity that is analogous to eddy momentum diffusivity. A reduction in turbulent drag results in reduced eddy momentum diffusivity and consequently, reduced turbulent mass diffusivity. Finally, we compare the rates of polymer mass transfer predicted by our theory and those measured experimentally. [Preview Abstract] |
Monday, November 21, 2005 1:46PM - 1:59PM |
HR.00003: Concentration Measurements in a Polymer Drag Reduced Boundary Layer Godfrey Mungal, Vijay Somandepalli, Yongxi Hou The addition of dilute polymer solutions to turbulent wall bounded flows can cause a significant reduction in the skin friction drag. This drag reducing effect of polymers, called the Tom's effect, has been well known for more than 50 years now. However, there is limited understanding of the physics behind this phenomenon of drag reduction. In our work, PLIF and PIV are used to study the distribution of a slot-injected polymer solution along the length of a developing flat plate boundary layer for various drag reductions and flow conditions. PLIF-based concentration measurements of the injected polymer solution will be presented for various polymer concentrations. Turbulent fluxes obtained from combining PLIF measurements with simultaneous PIV measurements are used to study the streamwise evolution and distribution of the polymer across the boundary layer and along the length of the tunnel. These statistics help in improved understanding of the dispersion of the polymer, its effects on the boundary layer and, as a consequence, the physics of polymer drag reduction. [Preview Abstract] |
Monday, November 21, 2005 1:59PM - 2:12PM |
HR.00004: Direct Numerical Simulation of Drag Reduction in Turbulent Pipe Flow with Spanwise Wall Oscillation Using a Spectral Element Method Andrew Duggleby, Kenneth Ball Results of a direct numerical simulation of turbulent pipe flow with spanwise wall oscillation, using NEK5000, a spectral element Navier-Stokes solver, are presented. The polar-cylindrical coordinate singularity at the pipe axis is avoided by solving the flow in Cartesian coordinates with a stadium-like element cross-section. Near the center of the pipe, a Cartesian configuration is used, while near the wall, the elements are mapped to a polar configuration. Each element uses 10th order Legendre Lagrangian interpolants in each direction, with a local Jacobi/Conjugate Gradient solver and a global Schwarz Multigrid solver. Validation with previous DNS and experiments is performed for $Re_{\tau}=180$ using 960 elements and a length of 10 R, and the drag reduction studies are performed at $Re_{\tau}=150$ using 2560 elements and a length of 20 R. Comparisons will also be made with previous DNS and drag reduction studies. Results showing better correlation with experiments using a spectral method compared to a 2nd order finite difference radial discretizations will be presented. [Preview Abstract] |
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