Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G4: Separated Flows I: Diffusers and Massively Separated Flows |
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Room: 326 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G4.00001: Asymmetric Separation and Perturbation Sensitivity in an Annular Diffuser Jesse Coffman, Scott Morris, Aleksander Jemcov, Joshua Cameron When an annular diffuser stalls, the separation can take many forms. Experiments show that one type of separation appears to be asymmetric and periodic. This asymmetry appears to be influenced by upstream and downstream components and inlet flow conditions. By understanding the changes effected at the exit of the diffuser by the inlet perturbations, the diffuser performance can be more accurately predicted within a system. This work aims to understand the influence of velocity perturbations at the inlet of the diffuser on the overall duct performance. This is done by application of the Euler equations and a RANS simulation for various circumferential wavenumbers. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G4.00002: Multiple local recirculations to reduce flow separation and increase efficiency of diffusers Alessandro Mariotti, Guido Buresti, Maria Vittoria Salvetti Boundary layer separation causes a decrease of performance in diffusers. We analyze a passive method for flow control in diffusers, based on the introduction of single and multiple contoured cavities in the walls. The proposed control strategy is investigated through numerical simulations of the flow in plane diffusers having an area ratio of 2 and different divergence angles. The location and geometry of the cavities are numerically optimized to maximize the pressure recovery in the diffuser. In all cases, the introduction of the optimal cavities leads to a strong increase in the pressure recovery and, when present, to a significant reduction of the main flow separation zone. The flow separates at the cavity upstream edge and rapidly reattaches, forming a small closed recirculation region within and immediately downstream of the cavities. These recirculation zones lead to both a favorable local modification of the virtual shape of the diffuser and to a reduction of the dissipation in the near-wall region due to the relaxation of the no-slip condition. A generic optimization of the diffuser shape is also carried out; if the number of degrees of freedom is large enough, the presence of small local recirculations is again found in the optimized configuration. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G4.00003: Unsteady Structure of Three-Dimensional Stall Cells Kevin Disotell, James Gregory A description of three-dimensional flow separation encountered on wings operating just above their maximum lift condition remains a critical link toward increased payload capability for air vehicles. In particular, the development of spanwise-periodic separation zones or ``stall cells'' observed in the surface streamline pattern appear to result from the amplification of a spanwise instability in the separated shear layer for certain stall types. Time-averaged point measurements and steady simulations have largely been used to characterize the geometry of stall cells, although unsteady motion due to possible shear layer flapping has been reported by Yon and Katz (1998). Details of the flow topology remain unclear especially under turbulent flow conditions. The development of stall cells on a rectangular NACA 0015 airfoil of aspect ratio 2.5 at chord Reynolds numbers above 750,000 is investigated in the current work using planar, time-resolved particle image velocimetry measurements. Flow visualization with miniature surface tufts and high-speed imaging revealed intermittent cellular patterns at incipient stall conditions which are explored further. Understanding the behavior of cellular separation can help inform flow control strategies aimed at mitigating stall. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G4.00004: Turbulence characteristics of separated boundary layer flow under unsteady pressure gradients using direct numerical simulation Junshin Park, William Bromby, Donghyun You To understand turbulence characteristics of separated boundary layer flow under unsteady pressure gradients, a direct numerical simulation study is performed. Steady and unsteady blowing-suction velocity distributions are imposed along the upper boundary of the computational domain to introduce steady and unsteady adverse pressure gradients leading to steady and unsteady separated turbulent boundary layers, respectively. Time averaged and phase averaged turbulence statistics such as velocity, vorticity, kinetic energy budgets, Reynolds stress budgets, wall pressure fluctuations and skin friction distributions are examined in detail with aims of gaining understanding of flow physics for unsteady separated turbulent boundary layer and the sources of incapability of the conventional Reynolds-averaged Navier-Stokes models in predicting unsteady separation. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G4.00005: ABSTRACT WITHDRAWN |
Monday, November 25, 2013 9:05AM - 9:18AM |
G4.00006: Spanwise correlation lengths of unsteady surface pressure behind a backward facing step Michael Bilka, Mark Paluta, Scott Morris The flow over a backward facing step is a canonical test case used for the study of separated flow problems. Study of this configuration has led to deeper understanding of the behavior and structure of separated flows. For many practical applications, it is not only necessary to understand the flow behavior but also the generated unsteady surface pressure. This information can provide a basis for understanding the unsteady loading experienced by a body, which can generate unwanted flow-structure interactions, such as vibration and sound radiation. The present work investigates the unsteady surface pressure generated by the separated flow behind a backwards facing step. Using an extensive array of surface pressure sensors, measurement is made not only of the unsteady surface pressure but also of the spanwise correlation of the surface pressure fluctuations at various locations of separation and reattachment. The spanwise correlation has received limited treatment in the literature for separated flows and is important for the modeling and prediction of fluid-structure interactions. [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G4.00007: Flow over a backward-facing step: Mean separation bubble and evolution of coherent structures Pankaj Nadge, Raghuraman Govardhan We present PIV measurements downstream of a backward-facing step at large step based Reynolds numbers. The structure of the mean separation bubble is mapped in detail, and the effect of Reynolds number and expansion ratio (ER) on it is studied; the ER being the primary geometrical parameter for this configuration. These measurements show that there exists a mean separation bubble structure that is nearly independent of ER at large Re. Further, these measurements permit evaluation of the forces acting on the mean separation bubble in the streamwise direction due to the Reynolds stresses. Towards understanding the coherent structures in the flow downstream of the step, time-resolved PIV measurements have been performed in a plane parallel to the lower wall. These show the presence of counter-rotating vortical structures, which may be thought of as signatures of three-dimensional hairpin-like structures. These counter-rotating pairs are observed to evolve as they convect downstream. Conditional averaging of these counter-rotating structures show that their length-scale increases with streamwise distance. Details about these structures and their evolution will be presented at the conference. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G4.00008: Data Reduction Methods to Identify Characteristic Scales in Transient, Inhomogeneous Flows John Dantonio, Joshua Cameron, Scott Morris Identification of meaningful spatial and temporal scales in transient, inhomogeneous flows is challenging as many data reduction methods favor spatial over temporal resolution (or vice versa). Spectra of state variables often contain significant broadband content and may be non-stationary. Measurements may be over an inhomogeneous grid or have unknown convergence properties. Correlation and wavelet based methods are presented to identify both spatial and temporal scales from pseudo-random data. These methods are relevant to many separated industrial flows; an application to axial compressor rotating stall will be presented. [Preview Abstract] |
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