Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R11: Turbulence: Shear Layers II |
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Chair: Jerry Westerweel, Delft University of Technology Room: 314 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R11.00001: Scaling Law for the Onset of Turbulence in Channel Flow Gr\'egoire Lemoult, Jean-Luc Aider, Jose Eduardo Wesfreid We are presenting experiments about the onset of turbulence in channel flow. We perturbed the base plane Poiseuille flow with continuous injection of water normally to the flow. We performed Particle Image Velocimetry experiments to measure the mean velocity profile with respect to the Reynolds number, $Re$ and the amplitude of the perturbation $u_{jet} /u_{cl}$. Due to the unstable nature of the flow with respect to finite amplitude perturbation, it should exist a minimal amplitude $\varepsilon $, which trigger the transition to the turbulence. In order to find this critical value, we define a new experimental criterion using the value of $\tilde {u}=\bar {u}_{cl} /\bar {u}_{cl,unperturbed}$, giving the deformation of the mean velocity profile. We found a power law for the onset of turbulence $\varepsilon =O(Re^{-3/2})$ and compared it with different models and previous experiments. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R11.00002: Experimental Evolution of Local and Global Variables in the Subcritical Transition in Channel Flow Jose Eduardo Wesfreid, Gregoire Lemoult, Jean-Luc Aider We perform experiments on the subcritical transition to the turbulence in a water channel with plane Poiseuille flow, perturbed by controlled injection of water normally to the wall. For different values of Reynolds number $Re$ and different amplitude of the perturbation $u_{jet} /u_{cl} $, we observed different states from laminar to turbulent. Using Particle Image Velocimetry, we study the dynamics of a local variable of the velocity field as the transverse magnitude and simultaneously we follows a global one, as the deformation of the mean velocity profile $\tilde {u}=\bar {u}_{cl} /\bar {u}_{cl,unperturbed} $. We discuss the evolution in the phase space of those variables as a function of the strength of the perturbation, and compare it with predictions made from low dimensional models. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R11.00003: The low-frequency undulation mechanism in a open cavity shear layer flow Xiaofeng Liu, Joseph Katz The unsteady flow and pressure field in an open cavity shear layer has been investigated using time resolved PIV measurements. Here, we focus on the closed-loop feedback mechanism that causes low-frequency undulation in the location of the shear layer relative to the cavity trailing corner. It is found that impingement of high momentum fluid on the forward-facing surface of this corner, when the shear layer is low, periodically increases the pressure there, and the inward flow along this wall. The latter re-circulates back to the leading corner of the cavity, increasing the pressure there, and imposing adverse pressure-gradients on the boundary layer upstream of the cavity. The resulting increase in boundary layer thickness starts an upward shift in the elevation of the shear layer, which reduces the momentum, pressure and jetting into the cavity at the downstream corner. This reduced backflow decreases the pressure at the inlet corner and the incoming boundary layer thickness, causing a downward shift in elevation of the shear layer. This low-frequency undulation has substantial impact on the turbulence and noise generations by the cavity corner. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R11.00004: Simultaneous measurement of pipe flow downstream and upstream of 90 degrees bend by using stereo PIV Jun Sakakibara, Nobuteru Machida We measured velocity vector distribution in cross sections of a fully developed turbulent pipe flow upstream and downstream of a 90 degrees bend by using stereo PIV and single camera PIV simultaneously. Reynolds number was $Re$=27,000, and ratio of inner diameter $d$ (=50mm) of the pipe and radius of the centerline of the bend was 1.0. Instantaneous flow field downstream of the bend represented the unsteady motion of the anti-symmetric counter-rotating Dean vortices. Stagnation point, which was created by the flow induced by the two vortices, was observed at both inner and outer side of the bend. The stagnation point moves unsteadily within roughly 30 degree above and below the symmetry plane. In order to clarify the origin of such unsteady motion of the stagnation point, conditional average of the upstream velocity vector field conditioned by the azimuthal location of the stagnation point downstream the bend was computed. Under condition where the inner side stagnation point stays above (below) the symmetry plane, the conditional streamwise velocity upstream the bend exhibited a positive peak below (above) the plane on the outer side. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R11.00005: ABSTRACT WITHDRAWN |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R11.00006: The turbulent wake of a submarine model at varying pitch and yaw angle Miloud Alaoui, Anand Ashok, Alexander Smits The objective of the present study is to understand how the pitch and yaw angle affect the mean flow and turbulence in the wake of an axisymmetric submarine model (DARPA SUBOFF model). Measurements in the wake were performed at a Reynolds number based on the length of 2.37 $\times$ 10$^{6}$. Mean velocity and two-component turbulence measurements were performed using Pitot probes and hot wires in the span-wise plane at three different downstream positions: 5, 7.5 and 10 diameters downstream of the trailing edge. The range of measured angles of attack and yaw angles were limited to between 0 and 10\r{ } in part to avoid wind tunnel interference effects. Work supported by ONR Grant N00014-09-1-0263. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R11.00007: Mixing length and scale-to-scale kinetic energy transfer in the wake of a fractal tree Kunlun Bai, Charles Meneveau, Joseph Katz To study the dynamics of turbulence interacting with multi-scale objects, we measure turbulence structure in the wake of a fractal tree-like object in a water channel, using PIV. The eddy viscosity is obtained from the correlation of mean shear and Reynolds shear stress distributions across the wake. We address the question whether a mixing length-scale can be identified in this flow, and if so, how it relates to the geometric length-scales in the pre-fractal object. One approach is based on spectral distributions. Another more practical approach is based on length-scale distributions evaluated using fractal geometry tools. These models agree well with the measured mixing length indicating that information about multi-scale clustering of branches has to be taken into account. To explore the energy transfer at different scales, data are spatially filtered at various scales and the subgrid-scale flux among scales is evaluated. In contrast to regions characterized by a single length scale (bottom of the tree) where a classical inertial range cascade behavior is observed with scale-independent flux, at heights where multiple branch generations interact with the flow, we find that the energy flux depends strongly on the filter size, increasing with smaller filter sizes. The results can be explained by production at multiple-scales. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R11.00008: Heated Jets Emitting From a Rectangular Stack into a Cross Wind B.E. Johnson, G. Elliott, K.T. Christensen A detailed analysis of jets in cross-flow is performed where jets heated to a centerline temperature $T_\circ = 425$\,K emit from a rectangular stack ($AR = 3.76$) into $T_{\infty} = 300 $\,K cross-wind at area-averaged momentum flux ratio $r = 3.3$. Cross-flow and jet centerline velocities are $U_{\infty} = 10$\,m/s and $V_\circ = 50$\,m/s, respectively. Injection is normal to the bounding wall from a raised stack such that the initial incidence of jet interaction with cross-flow occurs well outside of the boundary layer. Rake-mounted thermocouple measurements of mean temperature and cross-plane stereoscopic PIV measurements of the turbulent flow field are performed at multiple stations downstream of the stack in spanwise--wall- normal regions of interest. Stack yaw angles of $0^{\circ}$, $45^{\circ}$, and $90^{\circ}$ comprise a set of key orientations from which inferences can be made of real-world heated jet in cross-flow behavior where cross-flow directionality may vary under shifting mean wind direction. From the measurements made under each of these stack orientations, the downstream dispersion of the heated jet fluid is characterized as is the downstream evolution of the turbulence and associated vortical structures. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R11.00009: Turbulent structures of impinging circular jet Vesselina Roussinova, Ram Balachandar Turbulent impinging jets are used in various engineering applications due to their ability to provide superior heat and mass transfer. In hydraulic engineering, impinging jet flows have a detrimental effect due to their ability to scour and erode sand beds. In order to gain a better insight into the mean flow, turbulence and coherent structures in impinging jet flows, we performed high resolution particle image velocimetry (PIV) measurements of a round normally impinging jet issuing from a nozzle with diameter d = 0.01 m at Reynolds number Re =20 000 and at the jet-to-plate distance H = 20d. This configuration was chosen to match previously reported experiments and to verify results obtained from numerical simulations in which several phenomena have been noted, but the underlying turbulence dynamics remained obscure. PIV velocity fields are measured in the streamwise - spanwise (x-z) planes in the free jet and stagnation regions while streamwise - wall normal (x-y) planes are probed in the radial wall jet region in the immediate proximity of the impinging plate. The focus of this study is to investigate in details mean velocities, various turbulent quantities and vorticity. Analysis of the coherent structures is also documented through the analysis of swirling strength and proper orthogonal decomposition (POD). [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R11.00010: Half-loop and full-loop shedding in the wake of wall-mounted square cylinders due to boundary layer-wake interaction Jason Bourgeois, Zahra Hosseini, Robert Martinuzzi The vortical flow in the finite wall-mounted obstacle wakes can be important in heat transfer devices, turbomachinery components, and flame stabilizer devices, and is of fundamental importance since it displays fully three-dimensional states of K\'{a}rm\'{a}n vortex streets that are found in 2D bluff body wakes. The turbulent state of the wake of a finite square cylinder (height-to-width ratio $h/w$=8) has been found to be sensitive to the conditions of the approach boundary layer. The energetic quasi-periodic vortical structure topology is found to change between two topological states. Boundary layer thicknesses $\delta/d$=0.7 and 1.6 yield half-loop and full-loop structures, respectively. This modification of the structure topology has significant consequences for modifying the mixing, momentum transfer, turbulence production, and Reynolds stresses in the wake. Using synchronized particle image velocimetry (PIV) and surface pressure measurements for these two boundary layers, the coherent structures in the wake of the cylinder are reconstructed and analyzed. Vortical connector strands which tie together subsequently shed structures lead to high incoherent Reynolds stresses, streamwise vorticity, vortex stretching, and turbulence production in their neighbourhood, but do not appear in the lower regions of the wake for the half-loop topology. [Preview Abstract] |
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