Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session GE: Turbulence: Boundary Layers IV |
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Chair: David Dowling, University of Michigan Room: Salt Palace Convention Center 151 D-F |
Monday, November 19, 2007 10:30AM - 10:43AM |
GE.00001: Effect of Freestream Turbulence over Rough, Favorable Pressure Gradient Turbulent Boundary Layers Sheilla Torres-Nieves, Jose Lebron-Bosques, Brian Brzek, Luciano Castillo, Raul Bayoan Cal, Charles Meneveau Laser Doppler anemometry measurements are performed downstream of an active grid in the Corrsin wind tunnel at The Johns Hopkins University to study the effect of freestream turbulence (\textit{Tu}$\le $7{\%}), surface roughness and external favorable pressure gradient. Overall, the effect of freestream turbulence has proven to be dominant over pressure gradient and roughness. Mean profiles show that freestream turbulence effects alter the entire boundary layer including the inner flow. A reduction in the wake is also seen. Moreover, freestream turbulence increases the Reynolds stresses, making the values near the edge of the boundary layer to be non-zero. For the streamwise fluctuations, turbulence intensity affects the inner and outer regions, while the wall-normal and shear stress only change in the outer flow. Also, it is seen that roughness prevents the streamwise fluctuations from increasing near the wall, mainly because of the destruction of the viscous regions. Furthermore, a 20{\%} increase in the skin friction is reported, 25{\%} more than the increase obtained over smooth surfaces. [Preview Abstract] |
Monday, November 19, 2007 10:43AM - 10:56AM |
GE.00002: Scaling the Rough Favorable Pressure Gradient Turbulent Raul Bayoan Cal, Brian Brzek, Gunnar Johansson, Luciano Castillo Laser-Doppler anemometry measurements of the mean velocity and Reynolds stresses are carried out for a rough surface favorable pressure gradient turbulent boundary layer. The experimental data is compared with smooth favorable pressure gradient and rough zero pressure gradient data. The mean velocity deficit profiles collapse, but to different curves when normalized using the free-stream velocity for constant upstream conditions. The friction velocity scaling shows the effects of the favorable pressure gradient condition, but tends to group the rough data. The effects of the pressure gradient and roughness are clearly distinguished in the outer region of the boundary layer. When scaled with $U_\infty^2$, the $\langle u^2 \rangle$ component of the Reynolds stress augments due to the rough surface up to about 50\% despite the imposed favorable pressure gradient; while when using the $u_*^2$ scaling, it is dampened more than 100\%. This influence is the most evident where the shape of the profile completely changes and becomes `flatter' in the inner region. Similarly, the pressure gradient imposed on the flow changes the magnitude of the Reynolds stress profiles especially on the $\langle v^2 \rangle$ and $\langle uv \rangle$ components for the $u_*^2$ or $U_\infty^2$ scalings. The Reynolds stress profiles augment and possess a Reynolds number dependence due to the pressure gradient condition when scaled with $u_*^2$. [Preview Abstract] |
Monday, November 19, 2007 10:56AM - 11:09AM |
GE.00003: ABSTRACT WITHDRAWN |
Monday, November 19, 2007 11:09AM - 11:22AM |
GE.00004: Dynamics of Hairpin Vortices and Friction Drag Reduction in Turbulent Flow of Dilute Polymer Solutions Kyoungyoun Kim, Ronald Adrian, S. Balachandar, R. Sureshkumar We portray, for the first time, the nonlinear auto-generation of new vortices and formation of hairpin packets in the presence of polymer stress by performing a series of dynamic simulations and explain the effect of such dynamics on the reduction in turbulent stresses and hence, drag reduction. In the dynamical simulations, an initially isolated vortical structure is evolved in the viscoelastic flow where the polymer stress is modeled by the FENE-P model (finitely extensible nonlinear elastic-Peterlin). The initial conditions are given by the conditionally averaged flow fields for Reynolds-stress-maximizing Q2 event obtained from fully turbulent channel flow at $Re_\tau =395$ with drag reduction of 0{\%}, 18{\%} and 61{\%}. We found that the threshold of initial vortex strength for the auto-generation of new hairpins increases as the viscoelasticity increases, especially in the buffer layer. The result suggests that the auto-generation of new vortices is suppressed by the polymer stresses, thereby the coherent as well as incoherent Reynolds stress decrease and ultimately turbulent drag is reduced. [Preview Abstract] |
Monday, November 19, 2007 11:22AM - 11:35AM |
GE.00005: Experimental Measurements of Turbulent Drag Reduction Using Ultrahydrophobic Surfaces with Periodic Microfeatures Robert Daniello, Jonathan P. Rothstein The experimental results of fully-developed turbulent channel flow past a series of ultrahydrophobic surfaces will be presented. We have shown previously that these surfaces can produce significant drag reduction in laminar channel flow by supporting a shear-free air-water interface between hydrophobic microridges or microposts. In this talk, we will experimentally demonstrate that it is possible to utilize these micropatterned surfaces as a passive technique for achieving significant drag reduction in fully-developed turbulent flows. Two-dimensional velocity profiles as well as shear and Reynolds stress fields generated from particle image velocimetry will be presented. These measurements clearly demonstrate a reduction in drag along the ultrahydrophobic wall when compared to a smooth surface.~Pressure drop measurements along the channel will also be presented. Discussion will include the influence of Reynolds number and surface geometry on the velocity profiles, Reynolds stresses and the resulting drag reduction. [Preview Abstract] |
Monday, November 19, 2007 11:35AM - 11:48AM |
GE.00006: The Effects of Superhydrophobic Surfaces on Turbulent Skin Friction and Flow Structure Charles Peguero, Charles Henoch, Kenneth Breuer The application of superhydrophobic surfaces to the reduction of skin friction in turbulent flows is examined through experiments conducted in two facilities: the low-speed turbulent water channel at Brown University and the moderate speed (U = 8m/s) boundary layer facility at the Naval Undersea Warfare Center in Newport, RI (NUWC). High resolution PIV measurements are taken in the water channel at Brown University for both baseline (hydrophilic) and superhydrophobic surfaces. The mean and fluctuation velocity statistics are compared between the two surfaces. The friction velocity, u*, is estimated from the velocity fields using several independent methods. Direct drag and LDV measurements are taken for both the hydrophilic and superhydrophobic surfaces in the water tunnel at NUWC and will be discussed. [Preview Abstract] |
Monday, November 19, 2007 11:48AM - 12:01PM |
GE.00007: ABSTRACT WITHDRAWN |
Monday, November 19, 2007 12:01PM - 12:14PM |
GE.00008: Effect of Surface Roughness on Polymer Drag Reduction with a High-Reynolds-Number Turbulent Boundary Layer Brian Elbing, David Dowling, Michael Solomon, Sherry Bian, Steven Ceccio A recent experiment at the U.S. Navy's Large Cavitation Channel (LCC) investigated the effect of wall roughness on wall-injection polymer drag reduction (PDR) within a high-Reynolds-number (10$^{7}$ to 2x10$^{8}$ based on downstream distance) turbulent boundary layer (TBL). Testing was performed in two parts: 1) PDR experiment on a 12.9 m long, 3.05 m wide hydro-dynamically smooth flat plate and 2) PDR experiment on the same model with the entire surface roughened. The roughness was produced by blowing glass beads into epoxy paint that was applied to the entire model. The roughened model had an average roughness height ranging between 307 and 1154 $\mu $m. Drag reduction was determined using six, stream-wise located integrated skin-friction balances. In addition to skin-friction measurements, sampling was performed at three stream-wise located ports. The sampling ports were used to determine the amount of degradation, if any, caused by the turbulent flow on the polymer. Both the skin-friction measurements and sampling analysis indicates that wall roughness in a turbulent boundary layer significantly increases degradation of the polymer solution. [Preview Abstract] |
Monday, November 19, 2007 12:14PM - 12:27PM |
GE.00009: ABSTRACT WITHDRAWN |
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