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 KR: Turbulent Boundary Layers: General |
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Chair: Ronald Panton, University of Texas Room: Hilton Chicago Stevens 3 |
Monday, November 21, 2005 4:10PM - 4:23PM |
KR.00001: Characterizing Coherent Structures in Supersonic, Turbulent Boundary Layers Matthew Ringuette, M. Pino Martin, Alexander Smits Using a direct numerical simulation database, we present a preliminary characterization of the properties of coherent structures in turbulent boundary layers at Mach numbers from 0.3 to 7. The attributes of organized turbulent motions, such as angle, length scale, convection velocity, and internal structure, as well as their variation with Mach number, are the subject of ongoing research. Moreover, there are little quantitative data on how the length scales of the streaky structures at the wall vary with Mach number. We describe a strategy for characterizing the instantaneous properties of coherent structures that are captured using various identification criteria. Structures that have been implicated in the production of turbulent stresses, such as the ``legs'' and ``heads'' associated with horseshoe vortices, as well as the wall streaks, are investigated. [Preview Abstract] |
Monday, November 21, 2005 4:23PM - 4:36PM |
KR.00002: A Compressible ``Poor Man's Navier--Stokes'' Equation Discrete Dynamical System C.B. Velkur, J.M. McDonough Starting from the 3-D compressible Navier--Stokes equations we outline the derivation of a discrete dynamical system (DDS) known as the ``poor man's Navier--Stokes (PMNS) equation'' which we propose to use as the fluctuating component in synthetic-velocity forms of sub-grid scale models for large-eddy simulation. The DDS is obtained directly from the governing equations via a Galerkin procedure followed by decimation of all wave vectors but a single, arbitrary one that is incorporated into the bifurcation parameters; these are related to Reynolds, P\'{e}clet and Mach numbers, or the velocity gradients, and thus to flow physics. We provide computational results in the form of regime maps (bifurcation diagrams) to show that the DDS can produce essentially any temporal behavior observed either experimentally or computationally in compressible Navier--Stokes flows as the bifurcation parameters are varied over their ranges of effective behaviors analogous to results reported by McDonough \& Huang ({\it Int.\ J.\ Numer.\ Meth.\ Fluids} {\bf 44}, 545, 2004) for the 2-D incompressible PMNS equations. [Preview Abstract] |
Monday, November 21, 2005 4:36PM - 4:49PM |
KR.00003: Large-eddy simulation of accelerating boundary layers Giuseppe De Prisco, Anthony Keating, Ugo Piomelli, Elias Balaras Large-eddy simulation of flat-plate boundary layers in favorable pressure gradients (FPG) are performed for two different acceleration parameters. The high-acceleration case is in good agreement with the experimental data by Fernholz and Warnack [JFM, vol. 359, 329 (1998)]. Substantial reduction in turbulent kinetic energy and shear stress production, strong decorrelation of u and v fluctuation, and a reduction of the bursting frequency indicate that the accelerated boundary layer is in a laminar-like state when the pressure-gradient parameter K exceeds a threshold value. Near the wall, the turbulent shear stress becomes negligible compared with the viscous stresses. In the region of peak acceleration the pressure gradient is larger than the turbulent momentum transport and balances the viscous stress; in the low-K case, on the other hand, turbulent transport remains dominant over the pressure gradient. Downstream of this region, the boundary layer has a fast re-transition to turbulence. In the low K case, the boundary layer does not depart significantly from equilibrium. Research supported by the AFOSR. [Preview Abstract] |
Monday, November 21, 2005 4:49PM - 5:02PM |
KR.00004: Validation of Dual Plane PIV Measurements in Wall Turbulence Using DNS Data Neelakantan Saikrishnan, Ivan Marusic, Ellen Longmire Experimental dual plane particle image velocimetry (PIV) data from a zero pressure gradient flow over a flat plate at friction Reynolds number $Re_\tau = 1160\ $ is compared with direct numerical simulation (DNS) data from a fully developed channel flow at $Re_\tau = 934$. An averaging scheme is implemented to reduce the resolution of the DNS data to that of the PIV data and thus study the effects of averaging inherent to PIV. A vortex core identification algorithm is implemented on all the datasets using the three dimensional swirl $\lambda^{+}_{3D}$, and statistical distributions are computed of the projection angles of vortical structures in the boundary layer. The close match between the PIV, the raw and the averaged DNS data suggest that PIV can be a reliable and accurate technique for statistical analysis and identification of vortex structures in the turbulent boundary layer. In the talk, details of the statistical distributions and the averaging effects will be given. [Preview Abstract] |
Monday, November 21, 2005 5:02PM - 5:15PM |
KR.00005: Population Trends of Small-Scale Spanwise Vortices in Wall Turbulence Y. Wu, K.T. Christensen The population trends of prograde and retrograde (counter to the sense of the mean shear) spanwise vortex cores are studied via detailed PIV measurements in the streamwise--wall-normal plane of turbulent channel flow at $\mathrm{Re}_\tau=566, 1184$ and 1759 and in a zero-pressure-gradient turbulent boundary layer at $\delta^+=1401$ and 2347. A vortex extraction algorithm is used to isolate individual small-scale spanwise vortex cores from the background turbulence and the population trends of these vortices are studied as a function of Reynolds number and wall-normal position in both flows. Substantial numbers of prograde spanwise vortices with structural signatures consistent with the heads of hairpin-like vortices are found to populate the inner boundary of the log layer. In addition, a significant number of retrograde vortices also exist, sometimes appearing as isolated structures but often forming counter-rotating vortex pairs with the remaining prograde vortices. Retrograde vortices are found to be most prominent near the outer edge of the log layer of \underline {both} turbulent channel flow and the turbulent boundary layer, indicating that they may be generated locally within the log layer, advected into this region from more-distant wall-normal locations, and/or may be the byproduct of vortex merging. Of particular significance is the observation that the fractions of prograde and retrograde spanwise vortices collapse irrespective of Reynolds-number and flow in the log layer of wall turbulence. [Preview Abstract] |
Monday, November 21, 2005 5:15PM - 5:28PM |
KR.00006: Measurements of Instantaneous Wall Shear Stresses and Near-wall Structures Using Digital Holographic Microscopy J. Sheng, E. Malkiel, J. Katz Flow measurements are conducted near the wall of a square channel at Re$_{h}$=60,000 using Digital Holographic Microscopy. Instantaneous 3D velocity distributions are obtained over a volume of 1.5 x 2.5 x 1.5 mm$^{3}$, corresponding to x$^{+}$=50, y$^{+}$=83, z$^{+}$=50, y being the wall normal direction. The (pixel) displacement resolution is 0.7$\mu $m in the streamwise and spanwise directions and 10$\mu $m in the wall-normal directions. Using PIV guided particle tracking, each reconstructed hologram provides 2000 -- 6000 vectors. The distributions of 2 $\mu $m particles are not uniform, and they tend to cluster in layers at 2$<$y$^{+}<$5, and at 20$<$y$^{+}<$50. Local distributions of wall shear stresses are computed directly from the instantaneous velocity gradients in the viscous sub-layer (0$<$y$^{+}<$5). Preliminary analyses reveal clear correlations between the distribution of local wall-shear stresses and the presence of streamwise flow structures in the buffer layer (5$<$y$^{+}<$50). Current on-going analysis examines the effects of these buffer-layer structures, the local 3-D vorticity distribution and alignment of the strain tensor eigenvectors on the distribution of wall-shear stresses. [Preview Abstract] |
Monday, November 21, 2005 5:28PM - 5:41PM |
KR.00007: On the measurement of wall shear stress in turbulent boundary layers Gunnar Johansson, Faraz Mehdi, Jonathan Naughton Wall shear stress is an important parameter for turbulent boundary layers, both theoretically and practically. Yet highly accurate measurements have proven to be difficult. In this study, we carry out a comparative study of three methods for its measurement, which are all characterized by being exact in principle: the measurement of the mean velocity gradient at the wall, measurement of all terms in the integrated momentum equation, and the oil film interferometry method. All three methods are applied under identical conditions in a number of stream-wise positions in a wall jet facility. Each of the methods exhibits their own unique difficulties. These difficulties, the resulting inaccuracies and some means to minimize them are discussed. [Preview Abstract] |
Monday, November 21, 2005 5:41PM - 5:54PM |
KR.00008: High accuracy skin friction measurements demonstrated in a wall jet Jonathan Naughton, Faraz Mehdi, Gunnar Johansson The demand for high accuracy mean skin friction $C_f$ measurements has increased recently due, in part, to the need to have accurate friction velocities for scaling wall bounded flows. Other areas that will benefit from accurate $C_f$ values are validation cases for computational fluid dynamics and wall-bounded flow control studies. Here we consider a $C_f$ measurement technique capable of high accuracy, oil film interferometry (OFI). Specifically, the steps required for quality measurements in general flows are discussed including image registration and image analysis with a focus on one, two, and multiple image analysis approaches. The methods are applied to interferograms taken in a wall jet flow using OFI, and the results are applied to scaling wall jet velocity profiles obtained using laser Doppler anemometry. [Preview Abstract] |
Monday, November 21, 2005 5:54PM - 6:07PM |
KR.00009: Mean-Velocity Profile of Turbulent Boundary Layers Approaching Separation Thomas Indinger, Matthias H. Buschmann, Mohamed Gad-el-Hak Turbulent boundary layers approaching separation are a common flow situation in many technical applications. Numerous theoretical, experimental and numerical attempts have been made to find the proper scaling for the mean-velocity profile of this type of wall-bounded flow. However, none of these approaches seems to be completely satisfactory, and controversy still persists regarding the behavior of the mean velocity profile of turbulent boundary layers approaching separation. In this talk, we present new water-tunnel experiments of adverse-pressure-gradient turbulent boundary layers that clearly show the breakdown of the logarithmic law. Using these data and experimental results from several independent research groups, we analyze the classical scaling for ZPG TBL and the scaling by George \& Castillo and Zagarola \& Smits for APG TBL. Only the latter can be applied successfully for the outer region of the mean-velocity profile close to separation. It is shown that Zagarola \& Smits' scaling is consistent with the classical two-layer approach, and can be applied to collapse the different data. Analyzing the Reynolds shear stress, the George \& Castillo's scaling shows a reasonably good collapse of the data in the outer region. [Preview Abstract] |
Monday, November 21, 2005 6:07PM - 6:20PM |
KR.00010: Flow Separation Control for Low-Pressure Turbine Blade using Vortex Generator Jets Amit Kasliwal, Karman Ghia, Urmila Ghia Numerical study of flow separation control is conducted employing Vortex-Generator Jets. This strategy is first tested for the flow past a cylinder at Reynolds number (Re) of 13,400, and then applied to flow in a low-pressure turbine (LPT) cascade for the PAK-B blade geometry at Re = 25,000. A fourth-order accurate compact-difference scheme is used along with sixth-order filtering (C4F6). FDL3DI, a research code developed at WPAFB, is used as the flow solver. A blowing ratio of 2.0 with a skew angle of 90$^{\circ}$ and a pitch angle of 30$^{\circ}$ is employed in the simulations for the aforementioned configurations. The control jets are pulsed with F$^{+}$ = 1.0 for the case of the cylinder, and with F$^{+}$ = 2.33 for the LPT case. The results show a significant decrease in drag on the cylinder after the jets are turned on. The total-pressure loss is calculated in the wake region, at x/D = 3.0, and a reduction of 10{\%} is observed. For the LPT case, the implemented flow separation control strategy totally eliminates the separation and leads to 27.5{\%} reduction in wake total-pressure loss. [Preview Abstract] |
Monday, November 21, 2005 6:20PM - 6:33PM |
KR.00011: Numerical study on the largest scales of fully developed turbulent pipe flow by LES Makoto Tsubokura Large Eddy Simulations of fully developed turbulent pipe flows up to $Re_\tau=2360$ were conducted using a very long streamwise analysis region, which amounts to a hundred times longer than a pipe radius, to investigate the longest streamwise motions typically observed in the logarithmic layer of wall turbulence. In the previous study we conducted an intensive grid resolution study to properly reproduce outer large-scale structures in plane channels and found that sufficiently fine grid resolutions of around $h_x^+\sim 30$ and $h_z^+\sim 20$ for streamwise and spanwise directions in wall coordinate are required in the near wall region. Following this grid resolutions, our new pipe LES were carried out based on the fully conservative finite difference scheme in cylindrical coordinates along with the novel pole treatment developed recently by Morinishi et al. (2003). The structural difference of the obtained large scales between the pipe and the plane channel flows is discussed regarding the universality of the outer large scales, and the $k^{-1}$ spectrum of the streamwise velocity in the context of self-similar structure having been reported in experimental measurements of pipe flows will also be mentioned. [Preview Abstract] |
Monday, November 21, 2005 6:33PM - 6:46PM |
KR.00012: Relaminarization under stationary vortices Robert Breidenthal Flow visualization reveals that a turbulent boundary layer is relaminarized when stationary streamwise vortices are introduced. Following a suggestion of Balle, the vortices are stabilized by large streamwise ``Karman'' grooves in a wavy wall. In a water tunnel, upstream vortex generators place a large streamwise vortex in the middle of each groove, at the stationary point where Prandtl's vortex force vanishes. According to a theory by Cotel, the wall fluxes of a turbulent boundary layer should decline to laminar values under such ``persistent'' vortices. The observed relaminarization is consistent with this theory and with previous measurements of heat transfer by Touel and Balle. However, the structure of the transverse flow resembles the cats-eye pattern of a temporal shear layer rather than the anticipated von Karman wake. The cats-eye pattern corresponds to the forced shear layers of Oster-Wygnanski and Roberts, who found that the Reynolds stresses and mixing rate also decline to laminar values. [Preview Abstract] |
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