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 AE: Turbulence: Boundary Layers I |
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Chair: Ellen Longmire, University of Minnesota Room: Salt Palace Convention Center 151 D-F |
Sunday, November 18, 2007 8:30AM - 8:43AM |
AE.00001: Cross-spectral analysis of cross-plane data from 143 hot-wires in zero-pressure gradient turbulent boundary layer at R$_\theta =$ 9800 and 19,100 Murat Tutkun, William K. George Zero pressure gradient flat plate experiments have been performed using a hot-wire rake of 143 single wire probes. The experiments were conducted in the large (21.6 m in length, 2 m in width and 1 m in height) LML boundary layer facility of Laboratoire de M\'{e}canique de Lille as part of the European WALLTURB program (http://wallturb.univ-lille1.fr/). Experiments were conducted at freestream velocities of 10 m/s and 5 m/s, respectively. For both, $\delta_{99}$ $\approx$ 0.3 m. 13 rakes of 11 probes each were distributed approximately logarithmically over a 0.3$\times$0.3 m$^{2}$ cross-section. (The probes and wires, 0.5 mm long, were contributed by J. Delville and co-workers at LEA/Poitier.) For each block, 200 integral time scales of the streamwise velocity were recorded at 30 kHz. In total, 2200 blocks of data were obtained for each Reynolds numbers. Simultaneously, several SPIV planes were recorded upstream by M. Stanislas and co-workers at LML/Lille. This talk focuses only on the results of the hot-wire measurements, with particular attention to their implications for turbulence structures and their temporal evolution, and how these vary with Reynolds number. [Preview Abstract] |
Sunday, November 18, 2007 8:43AM - 8:56AM |
AE.00002: The wall signature of hairpin packets in turbulent boundary layers Clara O'Farrell, Stephan Priebe, Pino Martin We use a direct numerical simulation database (Martin, 2006;\footnote{Martin, M.P., J. Fluid Mech., Vol. 570, pp. 347-364, 2006.} Martin, 2004\footnote{Martin, M.P., AIAA Paper 2004-2337}) of turbulent boundary layers, statistical tools (Brown \& Thomas, 1977\footnote{Brown, G.L. \& Thomas, A. S. W., Phys. Fluids, Vol. 20, No. 10, Pt. II, pp. 243-251, 1977}) and pattern recognition and tracking algorithms (Wang \& Silver, 1997\footnote{Wang, X. \& Silver, D., IEEE Transactions on Visualization and Computer Graphics, Vol. 3, No. 2, pp. 129-141, 1997}; Richdale, 2007\footnote{Richdale, G.C., Senior Thesis, Princeton University, 2007}) to identify hairpin packets and their wall signature. The visualization algorithms are validated against the statistical analyses. We investigate the variation of time scales and length scales associated with coherent structures and the role of hairpin packets on the generation of skin friction, wall-pressure loading and heat transfer. [Preview Abstract] |
Sunday, November 18, 2007 8:56AM - 9:09AM |
AE.00003: Statistics on Near Wall Structures and Shear Stress Distribution from 3D Holographic Measurement. J. Sheng, E. Malkiel, J. Katz Digital Holographic Microscopy performs 3D velocity measurement in the near-wall region of a turbulent boundary layer in a square channel over a smooth wall at Re$_{\tau }$=1,400. Resolution of $\sim $1$\mu $m over a sample volume of 1.5x2x1.5mm (x$^{+}$=50, y$^{+}$=60, z$^{+}$=50) is sufficient for resolving buffer layer and lower log layer structures, and for measuring instantaneous wall shear stress distributions from velocity gradients in the viscous sublayer. Results, based on 700 instantaneous realizations, provide detailed statistics on the spatial distribution of both wall stress components along with characteristic flow structures. Conditional sampling based on maxima and minima of wall shear stresses, as well as examination of instantaneous flow structures, lead to development of a conceptual model for a characteristic flow phenomenon that seems to generating extreme stress events. This structure develops as an initially spanwise vortex element rises away from the surface, due to local disturbance, causing a local stress minimum. Due to increasing velocity with elevation, this element bends downstream, forming a pair of inclined streamwise vortices, aligned at 45$^{0}$ to freestream, with ejection-like flow between them. Entrainment of high streamwise momentum on the outer sides of this vortex pair generates streamwise shear stress maxima, 70 $\delta _{\nu }$ downstream, which are displaced laterally by 35 $\delta _{\nu }$ from the local minimum. [Preview Abstract] |
Sunday, November 18, 2007 9:09AM - 9:22AM |
AE.00004: The minimal logarithmic region. Oscar Flores, Javier Jim\'enez We present a `minimal' DNS of a turbulent channel with smooth walls at $Re_\tau \approx 2000$. The simulation is performed in a periodic domain with streamwise and spanwise lengths $\pi h/2 \times \pi h/4$. Below $y/h \approx 0.2$, it reproduces the mean and fluctuating velocity profiles and spectra of the logarithmic region of full-sized turbulent channels, although the largest scales are absent. The simulation is minimal in the sense that it essentially fits a pair of the characteristic wall-attached eddies identified in the logarithmic region by del \'Alamo {\em et al} (2006). As in full-channels, these eddies are associated with large low-momentum streaks $(\Delta z^+\approx 500)$, here of infinite length, which break up after sinuose meandering. The break-up results in a wall-normal velocity ejection, and in a sweep behind it. While the former transports low-momentum fluid across the logarithmic region, the sweep interacts with the remaining part of the streak, and reorganizes it into a ramp that eventually decays by what appears to be a different, less violent, process. [Preview Abstract] |
Sunday, November 18, 2007 9:22AM - 9:35AM |
AE.00005: Orientation and circulation of vortices in a turbulent boundary layer Qi Gao, Cecilia Ortiz-Due\~nas, Ellen Longmire The strengths of individual vortices are important in determining the generation and development of surrounding vortices in turbulent boundary layers. The dual-plane PIV data at z$^{+ }$= 110 and z/$\delta $ = 0.53 in a turbulent boundary layer at Re$_{\tau }$=1160 obtained by Ganapathisubramani et al. (2006) were investigated. 3D swirl strength was used to identify vortex cores. The eigenvector of the velocity gradient tensor was used to determine the orientation of each core, and the resulting eigenvector direction was compared with the average vorticity direction. Circulation of the cores was calculated using the vorticity vector only and using the vorticity vector projected onto the eigenvector. The probability distribution of the angle between the eigenvector and the vorticity vector indicated a peak at 15-20 degrees. The eigenvector angle distributions indicate that at z$^{+}$=110, more hairpin legs cross the measurement plane while at z/$\delta $ = 0.53, more heads are evident. Details of the orientation and circulation distributions will be discussed in the presentation. [Preview Abstract] |
Sunday, November 18, 2007 9:35AM - 9:48AM |
AE.00006: Azimuthal correlation in the outer layer of turbulent pipe flow Sean Bailey, Marcus Hultmark, Alexander Smits, Michael Schultz Two point hot-wire measurements of streamwise velocity were performed in the log-layer and wake region of turbulent pipe flow for Reynolds numbers ranging from 8$\times $10$^{4}$ to 8$\times $10$^{6}$ at four wall-normal positions with azimuthal probe separation. The azimuthal correlations were found to be consistent with the presence of very large scale coherent regions of low-wavenumber, low-momentum fluid observed in previous studies of wall-bounded flows. Within the log-layer the azimuthal scale determined from the correlations was found to be similar to that observed for channel flows. As wall-normal position increased outside the log-layer, there was a decrease in azimuthal scale relative to that of channel flow. Using cross-spectral analysis, high-wavenumber motion was found to grow azimuthally with wall-normal distance at a faster rate than the low-wavenumber motions. [Preview Abstract] |
Sunday, November 18, 2007 9:48AM - 10:01AM |
AE.00007: Near-wall PTV measurements in a high-Reynolds-number flat-plate turbulent boundary layer Ghanem F. Oweis, Eric S. Winkel, Marc Perlin, Steven L. Ceccio, David R. Dowling We report on near-wall particle-tracking velocimetry (PTV) measurements in the turbulent boundary layer that forms on a hydraulically smooth flat plate. The experiments were performed on a 13-m-long test model in the US Navy's W.B. Morgan Large Cavitation Channel at flow speeds up to 20 m/s for downstream-distance-based Reynolds numbers up to and exceeding 200 million. These measurements where made with three custom-built (hardware and software) PTV systems that nominally cover 0.1 $<$ y+ $<$ 200 ($\sim $1 mm), and were deployed 1.96 m, 5.94 m, and 10.68 m from the test model's leading edge. The main design features of the PTV system are described including improved sensitivity to the vertical velocity component and accurate pinpointing of the wall's vertical location. Profiles of mean velocity and turbulence quantities are presented in addition to the wall shear stresses derived from the mean stream-wise velocity gradient near the wall, (0.5 $<$ y+ $<$ 5.5). These PTV results are compared with traditional velocimetry and shear stress measurements made with other instrumentation.. [Preview Abstract] |
Sunday, November 18, 2007 10:01AM - 10:14AM |
AE.00008: Influence of High Freestream Turbulence on Smooth, Favorable Pressure Gradient Turbulent Boundary Layers Jose Lebron-Bosques, Sheilla Torres-Nieves, Brian Brzek, Luciano Castillo, Raul Bayoan Cal, Charles Meneveau Experiments were conducted at the Corrsin Wind Tunnel at The Johns Hopkins University to understand the effects of high freestream turbulence on a smooth, favorable pressure gradient turbulent boundary layer. Freestream turbulence (\textit{Tu}$\le $6{\%}) was generated using an active grid, and values of \textit{Re}$_{\theta }\le $2570 were obtained. Measurements of the streamwise and wall-normal components of the mean velocity deficit and Reynolds stresses have been performed using Laser Doppler Anemometry. It is shown than none of the existing scales are able to collapse the mean velocity profiles and strong evidence of multiple scales is observed. Furthermore, a reduction in the wake region caused a decrease in the mean velocity gradient near the edge of the boundary layer. Moreover, turbulence production will be evaluated to study if this is the cause of an increase in the Reynolds stresses. It is also found that high freestream turbulence causes the skin friction to increase due to a higher velocity gradient at the wall. [Preview Abstract] |
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