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 H27: Separated Flows III |
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Chair: Sanjay Kumar, University of Texas at Brownsville Room: 31C |
Monday, November 19, 2012 10:30AM - 10:43AM |
H27.00001: A numerical study of the unfluence of wall effects on the onset of unsteadiness in the three dimensional flow over a backward-facing step Nikolaos Malamataris In three dimensional separated flows, a flow component is developed in the spanwise direction that permits the flow to laterally escape [1]. This work shows for the first time how this flow situation occurs in the three dimensional, backward-facing step flow in a numerical experiment that mimics actual laboratory conditions (expansion ratio 1:2, aspect ratio 1:40). To this purpose, the full three dimensional Navier Stokes equations are solved directly with finite elements up to the highest Reynolds number (Re = 950) where the flow is stable. The wall effects are studied thoroughly, by showing how the recirculation regions vary close to the lateral wall and how the limiting streamlines are related to the spanwise flow in terms of their direction depending on the magnitude of the Reynolds number. It is shown how this spanwise flow goes all the way to the lateral wall and bounces back in a manner that it is impossible to be sustained at Reynolds numbers higher than 950 [2]. It is argued that this flow is responsible for the early onset of unsteadiness for this flow as has been observed in laboratory experiments and never fully understood so far.\\[4pt] [1] J.D\'{e}lery, Onera on-line lessons, Part I, 2011\\[0pt] [2] N.A.Malamataris, DOI: 10.1002/fld.3699 [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H27.00002: Direct Numerical Simulation of laminar separation bubbles O.N. Ramesh, Saurabh Patwardhan, Abhijit Mitra This work presents the DNS of laminar separation bubbles (LSB) that formed over a flat plate due to an imposed pressure gradient. Mean flow parameters such as mean velocity, static pressure distribution and the geometric parameters, such as aspect ratio of the LSB, over the plate closely corresponds to those found in experiments and literature. The locus of the inflection point of the mean velocity profile was found to lie outside the dividing streamline and this is expected to correspond to a convectively unstable bubble. A closer look of the LSB as when advects along the reverse flow streamline adjacent to the wall suggest that turbulence progressively decayed as one moved upstream. This is indicative of the phenomenon similar to relaminarisation in this region, presumably due to the decrease in pressure along the reverse flow streamline. The energy budget inside the dividing streamline showed interesting trends and these will be discussed during the presentation. Furthermore, the dynamics of free shear layer and nonlinearity will also be presented. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H27.00003: Negative production of turbulent kinetic energy in a turbulent separation bubble Hiroyuki Abe, Yasuhiro Mizobuchi, Yuichi Matsuo, Philippe R. Spalart DNS data are used to examine the behavior of turbulence in the boundary layer separating from a flat plate, and reattaching. Particular attention is given to a region of negative production of turbulent kinetic energy. The inlet Reynolds number $R_\theta$ based on momentum thickness is equal to 300, 600 and 900. In all cases, the production $P_k$ is weak across the bubble and goes negative with a smaller magnitude than the dissipation at the top, where the streamline curvature is convex. An indicator of streamwise curvature $U_{2,1}$, which comes from a rapid pressure-driven change of the mean strain rate, is indeed associated with negative $P_k$. That is, the budget term arising from $U_{2,1}$ yields negative Reynolds shear stress ($-\overline{uv}<0$), and then the product of $-\overline{uv}$ and $U_{1,2}$ contributes to negative $P_k$. There is no one-to-one correspondence in a region between negative $-\overline{uv}$ and negative $P_k$. The correspondence is however excellent when the Reynolds shear stress is defined in the streamline orthogonal coordinate system, i.e., $\overline{ab}\equiv\left((\overline{vv}-\overline{uu})U_1 U_2+\overline{uv}(U_1^2-U_2^2))\right/(U_1^2+U_2^2)$, which underlines that the streamline curvature is an important ingredient for negative $P_k$. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H27.00004: Direct measurement of wall shear stress in a backward facing step flow by using a photonic wall shear stress sensor Ulas Ayaz, Tindaro Ioppolo, Volkan Otugen We report direct wall shear stress measurements in a reattaching channel flow. The sensor used to perform measurements is a photonic wall shear stress sensor based on the morphology dependent resonances (MDR) of dielectric microspheres. The wall shear stress acting on a circular movable plate with 1 mm diameter, is mechanically transmitted to a Polydimethylsyloxane (PDMS) microsphere. The applied shear force on the microsphere leads to a shift in the MDRs, thus, by monitoring the MDR shifts, the magnitude as well as the direction of the shear stress are measured. The sensor is calibrated in a two dimensional channel with air flow. For flow separation and reattachment, a backward facing step is introduced into the channel and shear stress measurements have been performed at various distances from the step. Frequency and the magnitude of the shear stress fluctuations at the reattachment region have been recorded and compared to the reported measurements in literature. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H27.00005: Unsteady separation in a forward-facing step flow David Pearson, Paul Goulart, Bharathram Ganapathisubramani The structure and behaviour of the separation region upstream of a forward step is investigated using time-resolved 2D Particle Image Velocimetry. Conditional averages of the flow-field based on the amount of reverse flow present are used to determine the shape and size of the separated flow in relation to the separation point. It is shown that the separation is of 'open' form with no reattachment point for approximately 50\% of the time. When a reattachment point forms on the step face the separation region can become unstable and expand up and over the step corner. This transfer of mass occurs approximately 10\% of the time and is postulated to be caused by large-scale transverse motions at the step face. The conditional averages can be traced backward in time to investigate the upstream flow field prior to such events. It is found that the large scale separations are preceded by a region of low momentum flow convecting toward the step. This momentum deficit creates the conditions under which the separation expands. The size and shape of the momentum deficit, and the timescales over which it acts, is consistent with the large boundary layer structures observed in the literature. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H27.00006: Evolution of coherent vortical structures in turbulent flow over backward-facing step Pankaj Nadge, Raghuraman Govardhan The flow over a backward-facing step represents a geometrically simple flow that exhibits both boundary layer separation and reattachment. In the present work, we use detailed PIV measurements downstream of the step to help understand the evolution of vortical structures in this flow. In particular, velocity field measurements are done in a plane parallel to the lower wall (streamwise-spanwise plane). Upstream of the step, instantaneous velocity fields in this plane show counter-rotating vortical structures that are signatures of the three dimensional hairpin vortex structures present in turbulent boundary layers. These counter-rotating structures can be identified by the low streamwise velocity that exists between them. Conditional averaging of velocity fields gives clear counter-rotating structures whose length-scales can be measured. For the present case of a backward-facing step, the flow is evolving with streamwise distance after the step. Using similar techniques, we identify such counter-rotating structures downstream of the step; starting from near the step in the separated shear layer, all the way until well after shear layer reattachment on to the lower wall. Details about these structures and their evolution with streamwise distance will be presented at the conference. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H27.00007: Experimental investigation of the leading edge vortex on vertical axis wind turbine blades Reeve Dunne, Beverley McKeon A NACA 0018 airfoil is pitched about the leading edge over a large angle of attack range ($\pm \sim 40^\circ$) at a chord Reynolds number of 110,000 to simulate the flow over a single blade in a vertical axis wind turbine (VAWT). Particle image velocimetry (PIV) measurements are made to investigate the effects of pitching on leading edge vortex (LEV) development and separation. Time resolved experiments are performed to track vortex formation and convection over the airfoil for sinusoidal pitching motions corresponding to a VAWT trajectory as well as impulsive pitch up and pitch down motions. These results are compared to the wake of steady, post stall, high angle of attack airfoils ($\alpha=20^\circ-30^\circ$). The characteristics of the leading edge vortex development and subsequent separation from the airfoil are discussed, with a view to characterizing its effect on power generation with VAWTs and future flow control strategies for turbine performance improvement [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H27.00008: Unsteady Cavity Induced Vibrations of Flexible Hydrofoils Deniz Tolga Akcabay, Eun Jung Chae, Yin Lu Young The objective of this study is to investigate the dynamic interplay between elastic foil deformation and unsteady sheet/cloud cavitation. Recently, there is increasing interest in the use of light and flexible materials in marine propulsion devices and controlled surfaces, which can deform/vibrate due to un-intentional overload when operating in off-design conditions, or due to intentional passive/active controlled response of the hydrofoil. Numerical studies are conducted by applying a new hybrid coupling approach to efficiently and stably couple a URANS solver with a simplified structural model of the cantilevered, rectangular foil represented by a two degree-of-freedom system. The numerical model is first validated with experimental measurements of a rigid and a plastic NACA 66 hydrofoil. Next, numerical results are shown for plastic NACA 66 hydrofoil with varying mass and stiffness in turbulent subcavitating and cavitating flows. The influence of varying mass and stiffness on the cavitation patterns, vorticity contours and flow streamlines, bending and twisting deformation, and hydrodynamic load coefficients are presented. In particular, results are shown for un-locked and locked-in response due to unsteady sheet/cloud cavitation induced vibration. [Preview Abstract] |
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