Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session M20: Turbulent Boundary Layers X: Drag Reduction |
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Chair: Jim Brasseur, Pennsylvania State University Room: 30A |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M20.00001: Effects of Superhydrophobic Surface on Laminar and Turbulent Flows Hyunwook Park, John Kim The recent development of superhydrophobic surfaces (SHSs) has attracted much attention as it leads to a possibility of achieving substantial skin-friction drag reduction at high Reynolds number (Re) turbulent flows. A SHS, consisting of a hydrophobic surface combined with micro- or nano-scaled topological features, can yield an effective slip length on the order of several hundred microns. In this presentation, the effects of SHSs on both laminar and turbulent channel flows are investigated numerically. A SHS is modeled through no-slip boundary condition on top of micro-scaled features and stress-free boundary condition on gas-liquid interfaces. In laminar flows, the effective slip length depends on the geometry only independent of Re, consistent with the analysis of Lauga and Stone (2003)\footnote{E. Lauga and H.A. Stone, JFM 489 (2003) 55-77}, while in turbulent flows it depends on Re, thus indicating its dependence on flow conditions near the surface. The resulting drag reduction is much larger in turbulent flows and near-wall turbulence structures were significantly modified. We conclude that this indirect effect plays a more significant role in reducing drag in turbulent flows than the direct effect of the shear-free condition that led to drag reduction in laminar flows. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M20.00002: Direct measurement of turbulent skin-friction reduction on superhydrophobic surfaces Hyungmin Park, Guangyi Sun, Chang-Jin ``CJ" Kim Recent advances in superhydrophobic (SHPo) surfaces have spurred a great interest in fluid mechanics because their large slip may result in a significant reduction of skin friction in turbulent flows. However, experimental confirmation of the reduction has been sporadic (only internal flows) and equivocal because most times the surface slip was small and the drag measurement indirect. Here we present a direct measurement of the drag on large-slip surfaces in a turbulent boundary-layer flow. The silicon-micromachined sample has a SHPo (microgrates) next to a reference (smooth) surface, each suspended by identical micro flexure beams. Monolithically fabricated in a batch process and sharing all the variations, the two surfaces shift differently only by the difference in the drag. The drag reduction was measured optically (directly) in a turbulent boundary layer in a water tunnel experiment at a moderate Reynolds number ($Re_{\tau}\sim250$) over a gas fraction (fraction of the shear-free surface area) of $30\%-90\%$. Unlike other reports, the drag reduction clearly increased with the gas fraction. More than $50\%$ skin-friction reduction was achieved with 90\% gas fraction. During the flow tests, the SHPo surfaces were visually confirmed to contain the air without any loss. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M20.00003: Direct numerical simulation of turbulent flows over superhydrophobic surfaces with gas pockets using linearized boundary conditions Jongmin Seo, Sanjeeb Bose, Ricardo Garcia-Mayoral, Ali Mani Superhydrophobic surfaces are shown to be effective for surface drag reduction under laminar regime by both experiments and simulations (see for example, Ou and Rothstein, Phys. Fluids 17:103606, 2005). However, such drag reduction for fully developed turbulent flow maintaining the Cassie-Baxter state remains an open problem due to high shear rates and flow unsteadiness of turbulent boundary layer. Our work aims to develop an understanding of mechanisms leading to interface breaking and loss of gas pockets due to interactions with turbulent boundary layers. We take advantage of direct numerical simulation of turbulence with slip and no-slip patterned boundary conditions mimicking the superhydrophobic surface. In addition, we capture the dynamics of gas-water interface, by deriving a proper linearized boundary condition taking into account the surface tension of the interface and kinematic matching of interface deformation and normal velocity conditions on the wall. We will show results from our simulations predicting the dynamical behavior of gas pocket interfaces over a wide range of dimensionless surface tensions. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M20.00004: Pressure Drop Measurements for Turbulent Channel Flow over Superhydrophobic Surfaces with Superimposed Riblets Richard Perkins, Joseph Prince, Julie Vanderhoff, Daniel Maynes We consider the combined drag reducing mechanisms of riblets and superhydrophobicity. Pressure drop measurements were acquired for turbulent channel flow over riblet surfaces, superhydrophobic surfaces, and surfaces with both drag reducing mechanisms. The riblets were nominally 80 $\mu$m tall, 16 $\mu$m wide, and spaced with a period of 160 $\mu$m. The superhydrophobic structuring was composed of alternating microribs (15 $\mu$m tall and 8 $\mu$m wide) and cavities (32 $\mu$m wide), aligned parallel to the flow. The channel consisted of a control section and a test section comprised of smooth and patterned wafers, respectively. In all cases, the test section walls were structured on top and bottom while the side walls were left smooth. The channel had a hydraulic diameter of 7.3 mm and an aspect ratio of 10:1. Seven pressure ports were precision machined into the walls of both the control and test sections. The pressure drop measurements were acquired simultaneously over both sections to eliminate uncertainty associated with the flow rate. The drag reduction for all test sections was then computed directly and data were obtained over a Reynolds number range of 11000 to 15000. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M20.00005: Dual-plane stereoscopic PIV measurements of turbulence statistics in drag-reducing pipe flows with surfactant additive Yoshitsugu Naka, Shuhei Nuka, Masayasu Shimura, Naoya Fukushima, Mamoru Tanahashi, Toshio Miyauchi Drag-reducing turbulent pipe flows with surfactant additive are investigated by dual-plane stereoscopic PIV (DPSPIV). The DPSPIV system gives two parallel planes of three component velocity with a good spatial resolution. Measurements are undertaken in a fully developed pipe flow for 21 different conditions, i.e., the combinations of 3 concentrations of the surfactant additive and 7 Reynolds numbers. Two stereo PIV planes are settled at 50$D$ downstream from the pipe inlet and are arranged perpendicular to the mean flow with a small spacing in the streamwise direction. Within the present $Re$ range, the drag-reduction ratio takes its maximum value 63\% at $Re_D$=56 000. Noticeable differences between drag-reducing and water flows are observed on their statistics. The peaks of the rms values of the velocity fluctuations are shifted away from the wall since the thicknesses of the viscous sub-layer and the buffer region increase compared to the water flow case. In the shear stress profile, the Reynolds shear stress is much reduced in the near wall region, and the viscous shear stress contributes more. Velocity gradient characteristics such as profiles of vorticity components and the balance of the transport equation of the turbulent kinetic energy are also described. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M20.00006: New approaches to turbulent skin-friction drag reduction in wall flows based on the mechanism of drag reduction of polymer additives Rayhaneh Akhavan, Dong-Hyun Lee Polymer additives provide one of the most effective means of skin-friction DR in wall flows. Recent insights into the DR mechanism of polymer additives reveals that the cornerstone of high DR with polymer additives is redirection of a small fraction of TKE from energy-containing eddies into a path other than the normal turbulent energy cascade. This redirection of energy initiates a self-amplifying sequence of events within the turbulence dynamics, through which the turbulence loses its three-dimensional structure and turbulence production is suppressed. This mechanism suggests that the same dramatic DRs observed with polymer additives can be reproduced through {\em any} mechanism which redirects a small fraction of TKE from energy-containing eddies into a path other than the normal turbulent energy cascade. This hypothesis has been tested using DNS in turbulent channel flow, where the molecular viscosity in a small band of wavenumbers, corresponding to $0.01< k/k_d< 0.1$, was increased from $\nu$ to $4 \nu$. Drag reductions of up to 54\%, comparable to that observed with polymers at MDR, were observed, and many of the flow features observed in DR with polymer additives were reproduced. These results open up new possibilities for devising novel DR strategies for wall flows. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M20.00007: The spectral link for frictional drag in non-uniform turbulent soap-film flows Chien-Chia Liu, Rory Cerbus, Walter Goldburg, Gustavo Gioia, Pinaki Chakraborty The spectral link provides a novel relation between the frictional drag ($f$) and the exponent $\alpha$ of the turbulent energy spectrum ($E(k) \sim k^{-\alpha}$): $f \sim \rm{Re}^{(1-\alpha)/(1+\alpha)}$, where Re is the Reynolds number. The spectral link has been verified experimentally in uniform turbulent soap-film flows where the same $\alpha$, which is either 3 (the enstrophy cascade) or 5/3 (the inverse energy cascade), prevails across the width of the flow. We perform experiments on non-uniform turbulent soap-film flows where $\alpha$ switches between 3 and 5/3 across the width of the flow. Our measurements of $f$ vs. Re are in excellent accord with the localized version of the spectral link. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M20.00008: Drag reduction in Turbulent Channel Flow with Longitudinal Arrays of Slip/no-slip Stripes on the Walls Amirreza Rastegari, Rayhaneh Akhavan Drag reduction in channels covered with longitudinal arrays of slip/no-slip stripes on the walls has been investigated using DNS with the lattice Boltzmann method. Computations were performed in channels of size $5h \times 2.5h \times 2h$ at a $Re_{b}=3600$ ($Re_{\tau 0} \approx 230$) with stripes of size $0.02 \le g/h=w/h \le 0.56$ corresponding to $4 \le g^{+0}=w^{+0} \le 128$ where $g=w$ denotes the widths of the slip/no-slip stripes and $h$ is the channel half-width. Unlike in laminar flow, where the magnitude of DR is controlled by geometrical parameters $g/h$ and $w/h$, in turbulent flow the magnitude of DR is found to scale with $g^{+0}=w^{+0}$, independent of Reynolds number. DRs of $5\%,11\%,18\%,23\%,38\%,47\%$, and slip velocities of $U_{s}/U_{b}=0.06, 0.10, 0.15, 0.23, 0.32, 0.37$ were observed for $g^{+0}=w^{+0}= 4, 8, 16, 32, 64, 128$, respectively. Analysis of the mechanism of DR reveals that in the LDR regime ($DR<25\%$, $g^{+0} \le 32$, $U_s/U_b<0.25$), DR is due to a combination of wall-slip and change in the anisotropy structure of turbulence near the wall, while in the HDR regime ($DR> 30\%$, $g^{+0} \ge 64$, $U_s/U_b>0.3$), DR is primarily due to cessation of turbulence production over the slip stripes due to the large slip velocities over these regions. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M20.00009: Experimental investigation for drag reduction effect by traveling wave-like wall deformation in turbulent channel flow Hiroya Mamori, Yuho Ishiwata, Kaoru Iwamoto, Akira Murata A traveling wave-like control for wall-turbulence is known to have a potential to relaminarize a turbulent flow to a laminar flow, which has been confirmed by means of a direct numerical simulation. Especially, a wall deformation is one of the methods to realize this traveling wave-like control. The objective of the present study is an investigation for the drag reduction effect due to the traveling wave-like wall deformation experimentally. An elastic rubber is used for a vibrating plate and an oscillator is composed by an amplified piezoelectric actuator. The parameter of the traveling wave is preliminary determined by the direct numerical simulation, which induces the relaminarization at low-to-moderate Reynolds numbers. The drag coefficient is measured in the range of $2000 < \rm{Re}_b < 10000$, where $\rm{Re}_b$ is the bulk Reynolds number. The traveling wave-like wall deformation is found to decrease the drag below the uncontrolled turbulent flow level. The dependency of the drag coefficient for the frequency of the vibrating of the actuator is also investigated. The flow field is visualized by a Particle Image Velocitmetry measurement, and the drag reduction mechanism is discussed by using the turbulent statistics of the controlled flow. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M20.00010: Equilibrium turbulent boundary layers with wall suction/blowing and pressure gradients Saurabh Patwardhan, O.N. Ramesh Conditions for the equilibrium conditions in turbulent boundary layers with suction or blowing across a no slip wall and pressure gradients are derived from the governing equations. It is also shown that under these conditions the governing equations show self similarity in the conventional inner co-ordinates as well as ``laminar-like'' co-ordinates. The only turbulent boundary layer in ``perfect equilibrium'' known as sink flow turbulent boundary layer forms a subset of this more general equilibrium concept. Direct numerical simulations were carried out to investigate this hypothesis for the case of favourable pressure gradient with small blowing at the wall. Reynolds number invariance and complete self similarity of mean velocity profile and second order turbulence statistics is observed along the flow direction similar to the sink flow boundary layer. A comparison between the case with wall blowing and imposed favourable pressure gradient and the sink flow case for same value of pressure gradient parameter reveals a shift in log law in mean velocity profile and increase in peak turbulence intensities. [Preview Abstract] |
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