Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session GF: Separated Flows I |
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Chair: Sanjay Kumar, The University of Texas at Brownsville Room: Long Beach Convention Center 103A |
Monday, November 22, 2010 8:00AM - 8:13AM |
GF.00001: Wake modes of rotationally oscillating cylinders Prabu Sellappan, Tait Pottebaum Vortex shedding from bluff bodies is a fundamental problem in fluid mechanics and is important in applications such as vortex-induced vibration, heat transfer, and as a test for control strategies. Prior work has focused on vortex shedding from cylinders in cross flow and cylinders undergoing transverse or streamwise oscillations. Vortex shedding from a rotationally oscillating cylinder and the different wake modes that are produced have been investigated in this study. Experiments were carried out in a water tunnel at Re=750 for various amplitudes and frequencies of rotational oscillations. DPIV was used to study and map the different wake modes within the parameter space. Results show the mapping of wake modes to regions of the parameter space ranging from 0.7 to 3 times the natural shedding frequency and peak-to-peak rotational oscillation amplitudes from 5\r{ } to 160\r{ }. The wake modes and the regions in which they occur are compared with scalar measurements previously reported in the literature. Further utilization of the wake mode map will include understanding heat transfer from rotationally oscillating cylinders. [Preview Abstract] |
Monday, November 22, 2010 8:13AM - 8:26AM |
GF.00002: The relationship between Strouhal number and Reynolds number for a heated cylinder in the shear layer instability regime Joseph Laurienti, Tait Pottebaum The wake structure of a circular cylinder in isothermal cross flow has been extensively studied, and general agreement exists in the literature on the relationship between Strouhal number (St) and Reynolds number (Re) for parallel vortex shedding. However, no such consensus relationship exists for a heated cylinder in cross flow. Some recent studies have examined the St--Re relationship for heated cylinders in the laminar vortex shedding regime and have successfully collapsed the data from various temperature ratios (T$^{\ast }$ = T$_{cyl}$/T$_{\infty })$ using an effective Reynolds number (Re$_{eff})$ that evaluates fluid viscosity at a temperature between the free stream and cylinder temperatures. The present work focuses on higher Re, where the separated shear layers become unstable. Water tunnel experiments were performed on parallel vortex shedding from a heated cylinder in the range 250 $<$ Re $<$ 800 for various T$^{\ast }$. Long duration DPIV data sets were used to measure both vortex shedding frequency and detailed wake structure as functions of Re and T$^{\ast }$. St--Re curves will be presented for each T$^{\ast }$ and the use of Re$_{eff}$ to collapse the data will be evaluated. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GF.00003: An experimental study of flow past two rotating cylinders Sanjay Kumar, Benito Gonzalez, Oliver Probst Flow past two uniformly rotating cylinders in a side-by-side configuration is studied experimentally at Reynolds numbers, \textit{Re}, varying from 100 to 500 and the ratio of surface speed of cylinder to the free stream velocity, $\alpha $, varying from 0 to 5. The center-to-center spacing between the cylinders, $T$, normalized by the cylinder diameter, $D$ are 1.8, 2.5, 4.0, and 7.5. Two possibilities of rotations are considered with the cylinder surfaces in between the two cylinders moving upstream in one case (inward rotation case) and downstream in the other (outward rotation case). The diagnostics is done by flow visualization and particle-image-velocimetry. Vortex shedding is found to be suppressed in the inward rotation cases for \textit{Re} = 200 to 500 and all spacing ratios at $\alpha =\alpha _{s}\sim $2.0. The value of $\alpha _{s}$ for \textit{Re} of 100 in this case increases from 1.2 to1.7 as $T/D$ increases from 1.8 to 4.0 and does not increase further with $T/D$. For outward rotation cases, vortex shedding suppression is observed for \textit{Re} of 100 and for all values of $T/D$; however, for higher \textit{Re}, suppression is observed for $T/D$ of 4.0 and 7.5 only. The measurements of $\alpha _{s}$ in this case showed a decreasing trend with increasing $T/D$. Symmetry breaking in the wake is reported for inward rotation case near $\alpha _{s}$ = 1.35 for the case of $T/D$ = 2.5 at \textit{Re} of 200. [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GF.00004: Secondary Instability in the Flow past Two Aligned Square Cylinders Choon-Bum Choi, Yong-Jun Jang, Kyung-Soo Yang, Hyunjun Jeon Interference of the wakes behind two nearby bluff bodies is important in many engineering applications. In this investigation, secondary instability (SI) in the flow past two square cylinders in side-by-side or tandem arrangements has been numerically studied via a Floquet analysis. An immersed boundary method was employed to implement the cylinders in the computational domain. The distance between the neighboring faces of the two cylinders (G) is the key parameter which affects SI under consideration. In this presentation, we report the critical Reynolds number for SI and the corresponding spanwise wave number of the most unstable (or least stable) wave for each of the selected Gs. Several distinct modes were identified in both arrangements, and described in detail. The representative three-dimensional vortical structure of each mode was depicted with vorticity contours. We also attempted to explain the underlying mechanisms of the key features of the secondary instability from the view points of flow physics. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GF.00005: Characteristics of the flow over a sphere at subcritical Reynolds numbers Jungil Lee, Kwangmin Son, Haecheon Choi The characteristics of turbulent flow over a sphere at subcritical Reynolds numbers is investigated using the mode analysis. The Reynolds numbers considered are $Re=3700$, $10^4$ and $10^5$. The flow fields are generated from large eddy simulation with a dynamic global subgrid-scale model based on the Germano identity [Park et al., Phys. Fluids (2006); Lee et al., Phys. Fluids (2010)]. The flow statistics are in excellent agreement with previous experimental and numerical ones. The mode analysis is conducted on the axial velocity fluctuations integrated over the radial direction at each streamwise location. The axisymmetric mode (mode 0) represents cylindrical vortex rings or sheet that envelop(s) the recirculation region, and the helical mode (mode 1) is related to hairpin vortex or wavy vortical structure in the wake. The energy at each mode is maximum near the end of the recirculation region and decreases downstream. At $Re=3700$, mode 0 is dominant within the recirculation region but mode 1 becomes dominant in downstream locations. On the other hand, at $Re=10^4$ and $10^5$, mode 1 is most dominant throughout the flow field. These features are also manifest from instantaneous vortical structures. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GF.00006: Turbulent Boundary Layer Separation Induced over a Flat Plate by a Rotating Cylinder Farhana Afroz, Emily Jones, Drew Smith, Jennifer Wheelus, Amy Lang A novel technique to generate and control an adverse pressure gradient (APG) over a flat plate was implemented by using a rotating cylinder for the purpose of studying turbulent boundary layer (TBL) separation. For this experiment, a flat plate and a fixed diameter cylinder were mounted vertically in a water tunnel to investigate the flow field where the boundary layer was tripped to the turbulent state. Variability in the strength of the APG induced on the plate was achieved using the rotation speed of the cylinder. Digital Particle Image Velocimetry (DPIV) was used to investigate the nature and extent of TBL separation induced by the cylinder rotation. Moreover, a theoretical, inviscid flow calculation of the pressure coefficient induced by the rotating cylinder on the flat plate was performed to predict the strength of the APG. Location of separation, percentage mass flow reversal, and length of the separated flow region were all analyzed as a function of the Reynolds number and strength of the APG. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GF.00007: Flow Instability in Baffled Channel Flow Changwoo Kang, Kyung-Soo Yang, Kyongjun Lee Flow instability of baffled channel flow, where thin baffles are mounted on both channel walls periodically in the direction of the main flow, has been numerically investigated. Flow in a baffled channel is regarded as a simple model for flow in finned heat exchangers, including micro channels. In baffled channel flow, flow characteristics are significantly affected by geometrical configuration of the baffles. Two key parameters were considered, namely baffle interval ($L)$ and Reynolds number (\textit{Re}) of the main flow. The baffle height is fixed as one quarter of the channel height ($H)$. By using a parametric study, we elucidate dependency of the primary instability, a Hopf bifurcation from steady to a time-periodic flow, on $L$. It turned out that the most unstable flow is obtained with $L/H$=3. Transition of two-dimensional (2D) time-periodic flow to three-dimensional (3D) flow is initiated by a secondary instability (SI). Floquet stability analysis was performed to identify the critical Reynolds number of SI for some selected baffle intervals. Several distinct modes were identified, and dependency of SI on $L$ was elucidated. A 3D simulation was finally carried out to confirm the Floquet analysis. The current results shed light on understanding flow characteristics of a finned heat exchanger. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GF.00008: An Experimental Study of Flow Separation over a Flat Plate with 2D Transverse Grooves Emily Jones, Amy Lang, Farhana Afroz, Jennifer Wheelus, Drew Smith It has been hypothesized that flexible shark scales may aid in controlling boundary layer separation in that the scales bristle when encountering a localized flow reversal, thereby forming cavities within the skin that trap vortices between the scales. The formation of the embedded vortices can lead to the creation of a partial slip condition over the surface as well as turbulence augmentation in the boundary layer. In an attempt to replicate and study these effects on flow separation, a simplified model of the shark skin consisting of a plate with square 2D transverse grooves was utilized. Separation over the plate was induced via the placement of a rotating cylinder above the surface, and the experiments were carried out in a water tunnel with a tripped turbulent boundary layer. Using DPIV to analyze the flow, the results were compared to separation occurring over a flat plate. The effects on the location of separation and length of the separated flow region were all analyzed as a function of the Reynolds number and strength of the adverse pressure gradient induced by the rotating cylinder. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GF.00009: Investigating Separated Shear Layer Development over an Airfoil with an Imbedded Microphone Array Serhiy Yarusevych, Ryan Gerakopulos At low Reynolds numbers, laminar boundary layer separation on an airfoil often leads to deterioration in airfoil performance and noise emissions. The development of a separated shear layer is governed by laminar to turbulent transition, involving formation of coherent structures. This study highlights the design of a time-resolved surface pressure measurement system capable of estimating salient flow characteristics based on the analysis of surface pressure fluctuations. Wind tunnel experiments were performed for a symmetric NACA 0018 aluminum airfoil model equipped with a total of 95 static pressure taps and 24 microphones. Tests were performed for a range of angles of attack and Reynolds numbers to investigate two flow regimes common to airfoils operating at low Reynolds numbers, namely, flow separation without subsequent reattachment and separation bubble. Experimental results show that the microphones can be utilized to estimate the extent of the separation region and study the development of flow disturbances in the separated shear layer. Using hot wire measurements for validation, it is demonstrated that the microphones can detect the frequency signature of disturbances amplified in the separated shear layer. Further statistical analysis is employed to estimate such important characteristics of the disturbances and coherent structures as spanwise correlation, propagation speed, and phase. [Preview Abstract] |
Monday, November 22, 2010 9:57AM - 10:10AM |
GF.00010: Experimental study of unsteady turbulent boundary layer separation under conditions relevant to dynamic stall David Schatzman, Flint Thomas An experimental investigation focused on the study of the physics of unsteady turbulent boundary layer separation under conditions relevant to the dynamic stall process is presented. A flat boundary layer development plate allows for the growth of a turbulent boundary layer of thickness sufficient for high spatial resolution measurements. Downstream of the flat plate, a convex ramp section imposes a streamwise adverse pressure gradient that gives rise to boundary layer separation. In order to impose an unsteady pressure gradient, an airfoil section is located above the convex ramp. Leading edge plasma flow control is used to alternately attach and separate the airfoil flow which gives rise to unsteady turbulent boundary layer separation on the convex ramp. Measurements of the resulting unsteady turbulent boundary layer separation via phase-locked two-component PIV, unsteady surface pressure measurements, and wall-mounted hot-films quantify the dynamics of the separation process at the wall and throughout the unsteady boundary layer. Two-component LDA measurements are used to characterize the motions of ejection and sweep events within the unsteady boundary layer using a quadrant splitting technique. [Preview Abstract] |
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