Session MA - Turbulent Boundary Layers V.
MIXED session, Tuesday afternoon, November 23
Grand I, Westin Seattle

[MA.001] Near wall structure of the velocity field in high Reynolds number boundary layers.

The present investigation used Particle Image Velocimetry (PIV) to interrogate the near wall turbulent motions at the SLTEST site in western Utah. The data were recovered over a 40 minute period in which thermally neutral conditions existed with steady winds. The field of view extended 0.5m in the surface normal direction, and 1.0 meters in the streamwise direction. The surface roughness was approximately 18 wall units. The data indicate the existence of an inclined shear layer in 40 percent of the observed images. The mean angle of the shear layer was found to be 18 degrees, with a standard deviation of 10 degrees. Implications of this result are discussed along with conditional averages based on vorticity and swirl strength.

[MA.002] SLTEST Velocity/Vorticity Measurements at y+=2600 and 1100

Successful measurements (6/03) of (u,v,ømega _z) and (u,w,ømega _y) at y=15cm were obtained at the SLTEST site (Dugway) Utah. A neutral atmosphere for the measurement periods: I-30 and II-25 min. was experienced. A relatively high velocity: \bar u_I =4.9m/s and a low velocity \bar u_II =1.86m/s resulted in y+=2600 and 1100 respectively; k^+\approx 60. (The uncertainty in y+ is estimated at \pm 20%). The ^ \bar u^+,\;\tilde u^+,\;\tilde v^+ and \tilde ømega ^+ values were 18, 2.69, 1.27, 2.07 and 15.8, 2.29, 0.93, 1.61 for the two periods. The influence of the wall is strongly expressed in the vorticity fluctuations: [\tilde ømega _y /\tilde ømega _z ]=0.91 for I and 0.3 for II. The spectra show that the ``loss'' of ømega _y intensity is a result of the steep decrease for k values smaller than from the peak at k\eta =0.2. Similarly, the rhs terms of \-\partial øverline u'v' /\partial y=øverline v'ømega '_z -øverline ømega 'ømega '_y \ change significantly. The ratio: øverline ømega 'ømega '_y /øverline v'ømega '_z is -1.8 for I and -0.043 for II (øverline v'ømega '_z <0). The spectral peaks of these correlations are similar; they show that the peak intensities occur close to the vorticity peaks (k\eta \approx 0.2) vs. the small (2-6\times 10^-3) k\eta velocity peaks.

[MA.003] Investigation of large-scale structures in turbulent boundary layers using PIV in multiple planes

Stereo-PIV measurements were made on multiple planes in a turbulent boundary layer, including inclined cross-stream planes at \pm45^\circ to the streamwise direction, together with streamwise-wall-normal and streamwise-spanwise planes. The results show clear evidence of large-scale organization with long streamwise low-momentum zones consistent with the scenario of spatially coherent packets of hairpin vortices in the logarithmic region of the flow. Statistical correlation analysis across the boundary layer indicates the occurrence of a distinct two-regime behavior, in which streamwise-velocity-fluctuation correlation contours either appear to be coupled to the buffer region, or decoupled from it. The demarkation between these two regimes is found to scale well with outer variables. The results are consistent with a coherent structure that becomes increasingly uncoupled (or decorrelated) from the wall as it grows beyond the logarithmic region, providing additional support for a wall-wake description of turbulent boundary layers.

[MA.004] Axisymmetric turbulent boundary layers at small a+.

High resolution DPIV measurements were made over axisymmetric bodies of various diameters. The goal of the research was to test the hypothesis that bodies with non-dimensional radius, a+, less than 28 would not sustain a turbulent boundary layer. Using cables and pipe from 0.24 to 6.0 cm in diameter, an a+ range from 10 to 300 was examined. In all cases, mean and fluctuating velocity statistics clearly indicate the existence of a turbulent boundary layer. For large a+, the turbulence is similar to the canonical flat plate boundary layer. However, for decreasing a+, there are significant departures from the canonical TBL. Variations in turbulence quantities as a function of a+ will be presented along with insights into how decreasing radius alters turbulent structure.

[MA.005] PIV Measurements of the Characteristics of a Turbulent boundary Layer Above and Within a Mature Corn Canopy

Particle Image Velocimetry (PIV) measurements were performed in the turbulent boundary layer just above and within a mature corn canopy. Data were acquired at several heights, each at 4 Hz for 17min. The Kolmogorov length \textit\eta is about 0.4mm while the Taylor length scale is about 40mm. The Taylor microscale Reynolds numbers range between 2000-3000. The sample area is 18x18 cm, and the vector spacing is 2.8 mm. The mean flow profiles, Reynolds stresses, and turbulent flux (triple correlation) terms are consistent with those found in the literature. By combining the time series with the instantaneous velocity distributions we construct energy spectra that extend over 6 orders of magnitude of length scales. They exhibit an inertial range extending over three decades of wavenumbers. The small-scale spatial spectra reveal local isotropy for \textit\kappa _1\textit\eta > \quad \sim 10^-3. Quadrant-Hole analysis shows that the contribution of sweeps to the Reynolds stresses, dissipation and production are consistently higher than those of the ejections. Furthermore there is a strong correlation between conditionally sampled vorticity magnitude and dissipation. On the other hand, dissipation is only weakly correlated with the Reynolds stresses.

Sponsored by NSF.

[MA.006] Development of a micro-PIV/ LIF System for the Study of High Reynolds Number Turbulent Boundary Layers

Skin friction constitutes a considerable proportion of the total drag on a ship. The flow mechanisms giving rise to friction drag occur mostly in a thin region near the solid surface. A HIgh Reynolds number, smooth, flat PLATE (HIPLATE) model is used to conduct equilibrium boundary layer experiments at scales approaching prototype applications. The plate measures 3 m wide by 12.9 m long, and Reynolds numbers (based on downstream distance) of 200 million have been achieved. This talk reports on a friction-drag-reduced flow study by means of injecting long-chain, water-soluble polymer into the boundary layer. We describe the details of a particle imaging velocimetry (PIV) system to measure the mean and fluctuating velocity profiles in the inner region of the boundary layer (y < 2 mm, or y+ < 400), with a vector spacing of approximately 40 microns. A laser induced fluorescence (LIF) system is also used to measure the mean and fluctuating concentration profiles of the injected polymer. Preliminary results are discussed. [Sponsored by DARAPA]

[MA.007] Investigation of Boundary Layer Structure by Dual-Plane PIV

Dual-plane PIV was employed in a turbulent boundary layer at Re_\tau \sim 1100 to study the nature of the vortical structures there. Laser sheets separated by 1 mm were aligned in streamwise-spanwise (x,y) planes, and the scattered light was captured by three cameras: two in a stereo configuration and one in a normal configuration. All velocity gradient components were determined for fields in the log (z^+ = 125) and outer (z/\delta = 0.5) regions. Three-dimensional swirl strength was used to isolate vortex cores, and the vorticity direction of individual swirl centers was determined. Instantaneous fields in the log region reveal signatures of hairpin vortex packets consistent with previous results. The packets contain evidence of smaller hairpin heads embedded within the long low-speed regions surrounded by larger hairpins. The data set at z^+ = 125 yielded a most probable hairpin inclination angle of 32^\circ and an average inclination angle of 57^\circ. In the presentation, these results will be contrasted with those at z/\delta = 0.5.

[MA.008] Satellite sensing of submerged fossil turbulence and zombie turbulence

Surface brightness anomalies from a submerged municipal wastewater outfall trapped by buoyancy in an area 0.1 km^2 are surprisingly detected from space satellites in areas > 200 km^2. How is this possible? Microstructure measurements near the outfall diffuser reveal enhanced turbulence and temperature dissipation rates above the 50 m trapping depth. Near-vertical radiation of internal waves by fossil and zombie turbulence microstructure patches produce wind ripple smoothing with 30-50 m internal wave patterns in surface Fourier brightness anomalies near the outfall. Detections at 10-14 km distances are at 100-220 m bottom boundary layer (BBL) fossil turbulence scales. Advected outfall fossils form zombie turbulence patches in internal wave patterns as they extract energy, vorticity, turbulence and ambient vertical internal wavelength information as their density gradients are tilted by the waves. As the zombies fossilize, patterned energy radiates near-vertically to produce the detected Fourier anomalies. Zombie turbulence patches beam extracted energy in a preferred direction with a special frequency, like energized metastable molecules in a chemical maser. Thus, kilowatts to produce the submerged field of advected fossil outfall turbulence patches are amplified by beamed zombie turbulence maser action (BZTMA) into megawatts of turbulence dissipation to affect sea surface brightness on wide surface areas using gigawatts of BBL fossil turbulence wave energy available.

[MA.009] THICK AXISYMMETRIC TURBULENT BOUNDARY LAYER MEASUREMENTS IN THE NEAR WALL REGION

High resolution Stereo-PIV measurements were made on long, small diameter cylinders in the High Speed Tow Basin at NSWCCD. The cylinders were approximately neutrally buoyant and towed through a stationary laser sheet oriented perpendicular to the tow direction. The objective of the study was to determine the streamwise development of the axisymmetric turbulent boundary layer in the near wall region for cases where d \gg the cylinder radius, a. Results for a=0.45 mm, 1.25 mm, and 18 mm and lengths of approximately 150 m will be presented. Tow speeds of 3.9, 7.7, 12.9 and 15.5 m/sec were tested with images acquired over the entire length of the cylinders. Approximately 40 instantaneous vector fields were obtained for each location. Mean and fluctuating streamwise and cross-stream velocities and vorticity were computed. Results show that transverse curvature significantly effects boundary layer development for the tested speeds. Boundary layer parameters for the different cases will be compared.

[MA.010] EXPERIMENTAL MEASUREMENTS OF TRANSVERSE CURVATURE EFFECTS ON THICK AXISYMMETRIC TURBULENT BOUNDARY LAYERS

Experiments involving drag and high resolution stereo-PIV measurements were made on long, small diameter cylinders in the High Speed Tow Basin at NSWCCD. The cylinders were approximately neutrally buoyant and were towed at speeds ranging from 3.9 to 15.5 m/s. The objective of the investigation is to determine the effect of transverse curvature on axisymmetric turbulent boundary layers for cylinders with radius a=0.45 mm, 1.25 mm and 18 mm and lengths up to 152 m. Results will be presented for values of \delta /a ranging from 1 to 150. Images were acquired over the entire length of the cylinders with approximately 40 instantaneous three-dimensional vector fields obtained for each location. The mean wall shear stress \tau_w and the momentum thickness \theta are computed from mean velocity profiles. In addition, the total drag on the cylinder is used to determine the total mean \tau _w on the cylinder and the maximum \theta using an expression derived from a standard control volume analysis. Results show that \tau _w and \theta are highly dependent on the transverse curvature for the tested speeds

Part M of program listing