Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session H37: Separated Flows |
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Chair: Anastasios Liakos, United States Naval Academy Room: Sheraton Back Bay A |
Monday, November 23, 2015 10:35AM - 10:48AM |
H37.00001: Topological study of steady state, three dimensional flow over a backward facing step Anastasios Liakos, Nikolaos Malamataris The topology and evolution of flow over a backward facing step in three dimen- sional channel flow is examined for low to moderate Reynolds numbers. Direct nu- merical simulations were performed via a home made parallel finite element code. The computational domain has been designed according to actual laboratory experiment conditions. Analysis of the results is performed using the three dimensional theory of separation. Results indicate that a (primary) vortex is present for all Reynolds numbers immediately downstream from the step. Frictional stresses along the lateral wall create a novel vortical structure similar to a horseshoe vortex. A vortex along the top wall appears at Re = 400. As the Reynolds number increases, the top vortex increases both in spanwise and streamwise length and stunting the growth of the primary vortex. The downstream motion of the top vortex releases frictional stress thus destroying some critical points upstream while creating new ones downstream. Finally, at Re = 900 and 950, the primary and top vortices are twisting severely, which may indicate the onset of instability. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H37.00002: Fluid force and static symmetry breaking modes of 3D bluff bodies. Olivier Cadot, Antoine Evrard A cavity at the base of the squareback Ahmed model at Re $=$6.10$^{6}$ is able to reduce the base suction by 18{\%} and the drag coefficient by 9{\%}, while the flow at the separation remains unaffected. Instantaneous pressure measurements at the body base, fluid force measurements and wake velocity measurements are investigated varying the cavity depth from 0 to 35{\%} of the base height. Due to the reflectional symmetry of the rectangular base, there are two Reflectional Symmetry Breaking (RSB) mirror modes present in the natural wake that switch from one to the other randomly in accordance with the recent findings of Grandemange \textit{et al.} (2013). It is shown that these modes exhibit an energetic 3D static vortex system close to the base of the body. A sufficiently deep cavity is able to stabilize the wake toward a symmetry preserved wake, thus suppressing the RSB modes and leading to a weaker elliptical toric recirculation. The stabilization can be modeled with a Langevin equation. The plausible mechanism for drag reduction with the base cavity is based on the interaction of the static 3D vortex system of the RSB modes with the base and their suppression by stabilization. There are some strong evidences that this mechanism may be generalized to axisymmetric bodies with base cavity.. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H37.00003: Direct numerical simulations of a turbulent separation bubble over a wide Reynolds-number range Hiroyuki Abe, Yasuhiro Mizobuchi, Yuichi Matsuo, Philippe R. Spalart Direct numerical simulations (DNSs) of a turbulent boundary layer separating from a flat plate and reattaching have been performed with inlet data generated by rescaling-recycling at $Re_\theta$=300, 600 and 900. The focus is put on massive separation and the set-up close to those of Spalart \& Coleman (1997) and Na \& Moin (1998) at lower Reynolds number. This extends the work of Abe et al. (CTR Annual Brief, 2012) but removes the stagnation point, present over the bubble and due to strong blowing and suction $V_{top}$ at the upper boundary. The new simulations have a reduced $V_{top}$, compensated by a smaller ceiling height. The overall agreement with the earlier DNSs is satisfactory. A small difference appears in the recovery region, in which turbulence is reduced slightly in the present DNSs. At all three $Re_\theta$, separation and reattachment locations are nearly identical. Also, the mean spanwise vorticity is conserved to a large extent along the bubble. We associate this inviscid transport with the high peak in Reynolds shear stress known to appear after reattachment, and to be a challenging phenomenon for turbulence theories and models. In the latter region, a significant $Re$ dependence is found for the skin friction due to the weak development of near-wall turbulence. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H37.00004: Mitigation of Helical Vortex Separation of a 5:1 Prolate Spheroid at an Angle of Attack James Schulmeister, Michael Triantafyllou Ocean and air vehicle maneuvering performance is determined by the balance between a vehicle's capacity to exert large lateral forces and its fluid dynamic resistance. The flow past a 5:1 prolate spheroid at an angle of attack is representative of many maneuvering hull forms where cross-flow separation creates helical vortices that dramatically increase drag during maneuvers. We investigate a shape modification to a 5:1 prolate spheroid that streamlines the spheroid in the cross-flow direction in order to mitigate vortex separation. We conduct a series of experiments with models towed at angles of attack between 5 and 30 degrees at a Reynolds number by length equal to 100,000. Reconstructions of the helical vortex separation from cross-flow particle image velocimetry measurements show that modest streamlining significantly reduces the size and circulation of separated vortices, indicating drag reduction. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H37.00005: Numerical study of turbulent flow separation over a wall mounted circular cylinder Taejong Yu, Donghyun You Flow over a wall-mounted circular cylinder with a finite span and a free end is numerically studied at a range of Reynolds numbers. Separated flow behind a wall-mounted cylinder is characterized by dominant vortical structures developed around and behind the cylinder: {\it i.e.}, Karman vortices and tip-shedding vortices. The formation and interaction among the vortices are found to be distinct depending on the aspect ratio of the span length to the diameter of the cylinder as well as the Reynolds number. It is also found that drag and lift forces on the cylinder show different dominance of Karman vortices and tip vortices for different span-to-diameter ratios. A detailed analysis of the mean and fluctuating velocity, pressure fields, and spectral characteristics of separated flow is presented for laminar-to-transitional flows over cylinders with different aspect ratios. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H37.00006: Flow structure and unsteadiness in the supersonic wake of a generic space launcher Anne-Marie Schreyer, S{\"o}ren Stephan, Rolf Radespiel At the junction between the rocket engine and the main body of a classical space launcher, a separation-dominated and highly unstable flow field develops and induces strong wall-pressure oscillations. These can excite structural vibrations detrimental to the launcher. It is desirable to minimize these effects, for which a better understanding of the flow field is required. We study the wake flow of a generic axisymmetric space-launcher model with and without propulsive jet (cold air). Experimental investigations are performed at Mach 2.9 and a Reynolds number $Re_D = 1.3 \cdot 10^6$ based on model diameter D. The jet exits the nozzle at Mach 2.5. Velocity measurements by means of Particle Image Velocimetry and mean and unsteady wall-pressure measurements on the main-body base are performed simultaneously. Additionally, we performed hot-wire measurements at selected points in the wake. We can thus observe the evolution of the wake flow along with its spectral content. We describe the mean and turbulent flow topology and evolution of the structures in the wake flow and discuss the origin of characteristic frequencies observed in the pressure signal at the launcher base. The influence of a propulsive jet on the evolution and topology of the wake flow is discussed in detail. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H37.00007: Flow past a finite cylinder of constant curvature Jessica Shang, Howard Stone, Alexander Smits Wake visualization experiments were conducted on a finite cylinder whose plane of curvature is aligned with the freestream, at $230 \leq Re_D \leq 916$. The curvature of the cylinder is constant and ranges from a straight cylinder to a quarter-ring. The wake characteristics depend on the curvature, aspect ratio, and Reynolds number. The wake of a cylinder whose stagnation face is concave to the flow exhibited regions of non-shedding separated flow due the spanwise flow induced by the free end, in addition to regions of oblique and/or parallel shedding. In some cases, vortex shedding was entirely suppressed. In the laminar wake regime, increasing the curvature or decreasing the aspect ratio restricted vortex shedding to smaller regions along the span of the cylinder. The spanwise location where vortex shedding occurred was self-similar across cylinders of the same aspect ratio and varying curvature. After the wake transitioned to turbulence, the vortex shedding extended along most of the cylinder span. In contrast, a cylinder convex to the flow always exhibited vortex shedding in this $Re_D$ regime. The shed vortex lines were oblique to the flow at lower $Re_D$ and became more normal to the flow with increasing $Re_D$. The Strouhal number also increased with $Re_D$. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H37.00008: Optimal control of an asymptotic model of flow separation Ubaid Qadri, Peter Schmid In the presence of surface imperfections, the boundary layer developing over an aircraft wing can separate and reattach, leading to a small separation bubble. We are interested in developing a low-order model that can be used to control the onset of separation at high Reynolds numbers typical of aircraft flight. In contrast to previous studies, we use a high Reynolds number asymptotic description of the Navier--Stokes equations to describe the motion of motion of the fluid. We obtain a steady solution to the nonlinear triple-deck equations for the separated flow over a small bump at high Reynolds numbers. We derive for the first time the adjoint of the nonlinear triple-deck equations and use it to study optimal control of the separated flow. We calculate the sensitivity of the properties of the separation bubble to local base flow modifications and steady forcing. We assess the validity of using this simplified asymptotic model by comparing our results with those obtained using the full Navier--Stokes equations. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H37.00009: A numerical experiment that provides new results regarding the inception of separation in the flow around a circular cylinder Nikolaos Malamataris, Anastasios Liakos The exact value of the Reynolds number regarding the inception of separation in the flow around a circular cylinder is still a matter of research. This work connects the inception of separation with the calculation of a positive pressure gradient around the circumference of the cylinder. The hypothesis is that inception of separation occurs when the pressure gradient becomes positive around the circumference. From the most cited laboratory experiments that have dealt with that subject of inception of separation only Thom has measured the pressure gradient there at very low Reynolds numbers (up to Re=3.5). For this reason, the experimental conditions of his tunnel are simulated in a new numerical experiment. The full Navier Stokes equations in both two and three dimensions are solved with a home made code that utilizes Galerkin finite elements. In the two dimensional numerical experiment, inception of separation is observed at Re=4.3, which is the lowest Reynolds number where inception has been reported computationally. Currently, the three dimensional experiment is under way, in order to compare if there are effects of three dimensional theory of separation in the conditions of Thom's experiments. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H37.00010: 3-D Stall Cell Inducement Using Static Trips on a NACA0015 Airfoil Haley Dell'Orso, Michael Amitay Stall cells typically occur at high angles of attack and moderate to high Reynolds numbers (10$^{\mathrm{5}}$ to 10$^{\mathrm{6}})$, which are applicable to High Altitude Long Endurance (HALE) vehicles. Under certain conditions stall cells can form abruptly and have a severe and detrimental impact on flight. In order to better understand this phenomenon, stall cell formation is studied using oil flow visualization and SPIV on a NACA0015 airfoil with AR $=$ 2.67. It was shown that there is a critical Reynolds number above which stall cells begin to form, and that Re$_{\mathrm{crit\thinspace }}$varies with angle of attack. Zig-zag tape and balsa wood trips were used to induce stall cells at lower Reynolds numbers than they would otherwise be present. This will aid in understanding the formation mechanism of these cells. It was also demonstrated that, in the case of full span trips, stall cells are induced by the 3-D nature of zig-zag trips and did not appear when balsa wood trips were used. This suggests that the formation of the stall cell might be due to 3-D disturbances that are naturally present in a flow field. [Preview Abstract] |
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