Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session LU: Separated Flows I |
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Chair: Jean-Paul Dussauge, Universite d'Aix-Marseille Room: 200I |
Monday, November 23, 2009 3:35PM - 3:48PM |
LU.00001: Dependence of the unsteady behavior of shock wave boundary layer interactions on the Reynolds number and on the state of separation Pierre Dupont, Louis Souverein, Jean-Paul Dussauge The mean spatial organization and the unsteady behavior of a plane shock wave impinging on a turbulent boundary layer is investigated experimentally. Several parameters are considered: flow deviation, Mach and Reynolds numbers. Particular attention is paid to the incipient separation cases obtained at two Reynolds numbers with an order of magnitude of difference, obtained in different wind tunnels. The measurement technique is two component planar particle image velocimetry. The low frequency unsteadiness of the reflected shock is deduced by means of conditional statistics based on the existence of reverse flow. Upstream as well as downstream influences are considered. The results indicate that common mechanisms with the separated cases exist in the incipient cases, but with more or less importance, depending on the probability to observe reverse flow in the interaction. The effect of the Reynolds number will be shown to be negligible. [Preview Abstract] |
Monday, November 23, 2009 3:48PM - 4:01PM |
LU.00002: Compressible turbulence properties in a shock wave boundary layer interaction Jean-Paul Dussauge, Sebastien Piponniau, Pierre Dupont Shock induced separations are known to develop unsteadiness. Recently, a simple model was proposed to relate this unsteadiness to the dynamics of the mixing layer formed downstream of the separation shock. Such layers are known to be sensitive to compressibility. We will present turbulence measurements in the case of a Mach 2.3 shock reflection, performed by 2-component PIV. Turbulent data accuracy is tested in the upstream boundary layer, where no significant compressibility effects are expected on velocity fluctuations. The same turbulent data are shown in the interaction, where compressibility effects are likely. Results will be compared with data obtained in plane compressible mixing layers and in subsonic separated flows. It will be shown that, despite curvature effects and proximity of the wall, very similar behaviors are observed in the case of shock reflection. The main effect of compressibility is found to modify turbulence anisotropy, through a drastic reduction of vertical fluctuations, as in compressible mixing layers. [Preview Abstract] |
Monday, November 23, 2009 4:01PM - 4:14PM |
LU.00003: Spanwise modulations in shock-induced boundary layer separations Lionel Larcheveque, Lionel Agostini, Jean-Paul Dussauge Large-eddy simulations are carried out in order to clarify the origin of the large-scale spanwise modulations found experimentally within the separation bubble induced by a shock impinging on a turbulent boundary layer. Structures of various spanwise lengthscales are highlighted by means of short time averaged flowfield and spatial Fourier transforms. The individual influences of each of these modulations on the global dynamics of the flow are investigated by varying the spanwise extent of the computational domain. The effects of the spanwise boundary condition, ranging from periodicity to no-slip, are also investigated. Joint time-space spectral analyses are performed to quantify the timescale associated with each of the spanwise lengthscales. These timescales are compared with the timescales of the phenomena found in two-dimensional shock-boundary layer interaction with separation such as the shock motion, the mixing layer developing over the separated bubble and the turbulent structures stemming from the incoming boundary layer. The results are used to draw some conclusions on the possible origins of the spanwise modulations. [Preview Abstract] |
Monday, November 23, 2009 4:14PM - 4:27PM |
LU.00004: The leading-edge vortex and quasi-steady vortex shedding on an accelerating plate Kevin Chen, Tim Colonius, Kunihiko Taira A computational inquiry focuses on leading-edge vortex (LEV) growth and shedding during acceleration of a 2-D flat plate at a $30^{\circ}$ angle of attack and at low Reynolds number. The plate accelerates from rest with a velocity given by a power of time ranging from 0 to 5. Comparison with Wagner's inviscid theory of airfoils reveals a lift coefficient peak across all powers during the initial LEV growth. The peak universally lasts four to five chord lengths of translation. A pattern of leading- and trailing-edge vortex shedding follows the shedding of the initial LEV. Plotted against Reynolds number, the nondimensional frequency, lift coefficient maxima, and lift coefficient minima of the shedding pattern closely match their values in the absence of acceleration. This leads to the support of a quasi-steady theory of vortex shedding, where dynamics are determined primarily by Reynolds number and not acceleration. Finally, the time scale of the lift augmentation matches well with the half-stroke of a flying insect, and supports a nondimensional vortex formation time close to 4. [Preview Abstract] |
Monday, November 23, 2009 4:27PM - 4:40PM |
LU.00005: An Experimental Investigation of Compressible Dynamic Stall on a Pitching Airfoil Katie Thorne, Patrick Bowles A new facility has been designed and constructed at the University of Notre Dame to investigate dynamic stall on a 2-D pitching airfoil at high subsonic Mach numbers. This work is motivated by the need to investigate dynamic stall at conditions relevant to military helicopters. One focus of the experiments is to characterize the role of shock/boundary layer interactions during the pitching cycle. The new dynamic stall facility is integrated into a closed-loop, low turbulence wind tunnel capable of achieving test section Mach numbers in excess of M = 0.6. The design of the dynamic stall test section was focused on achieving reduced pitching frequencies of up to k = 0.2 and chord Reynolds numbers up to $5 \times 10^6$. The facility has the unique ability to execute non-harmonic pitching motions through the use of an actuated pitch link mechanism. Optical access is provided to allow the use of high-speed and Schlieren imaging. Thirty-one flush mounted Kulite dynamic pressure transducers provide the instantaneous unsteady surface pressure distribution over the airfoil. Initial dynamic stall measurements obtained in the new facility will be described. [Preview Abstract] |
Monday, November 23, 2009 4:40PM - 4:53PM |
LU.00006: Measurements of Unsteady Turbulent Boundary Layer Separation under Conditions Relevant to Helicopter Rotors David Schatzman, Flint Thomas A unique experimental facility was developed to study unsteady turbulent boundary layer separation under conditions relevant to helicopter rotors. The facility provides the capability for unsteady turbulent boundary layer separation measurements of high spatial and temporal resolution. Leading edge plasma flow control on a stalled airfoil is used as a tool to impose an unsteady pressure gradient on turbulent boundary layer flow over a convex ramp section. 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. Phase locked PIV measurements are utilized to capture the dynamics of the unsteady turbulent boundary layer separation. High speed digital imaging of smoke flow visualization and simultaneous unsteady wall pressure records are used to track events that occur in the outer part of the boundary layer and propagate toward the wall. Joint hot-wire and unsteady wall pressure measurements are used to quantify these events during the unsteady separation process. [Preview Abstract] |
Monday, November 23, 2009 4:53PM - 5:06PM |
LU.00007: The Effects of Geometry on Trailing Edge Flows Scott Morris, David Stephens The flow field in the near wake of an airfoil is strongly influenced by the shape of the trailing edge. The shape will have and effect on the mean lift and drag, as well as the unsteady surface pressure that can lead to undesired aeroelastic and aeroacoustic phenomena. This talk will describe recent results that include unsteady wall pressure, PIV measurements of the velocity field, and radiated sound measurements from 9 different edge geometries with varied bluntness. The results will focus on the effects of the approach boundary layers on the separation point and the overall unsteadiness of the wake. The boundary conditions that lead to large scale vortex shedding will also be examined. [Preview Abstract] |
Monday, November 23, 2009 5:06PM - 5:19PM |
LU.00008: Role of the recirculation region in the transition of a laminar separation bubble O Ramesh, Sourabh Diwan The role of the recirculating region in the transition of a laminar separation bubble is explored in this work. The primary instability of a separation is convective and this inflexional mode is active along the mean inflection point locus located outside the mean dividing streamline. The region inside the dividing streamline and close to the wall -- called the wall mode- however is seen to display a small region of negative production of turbulent kinetic energy. This has an interesting stabilizing role on the bubble dynamics. The negative production region is seen to increase in extent with increased values of maximum reversed flow velocity of the bubble. This is shown by doing a linear stability analysis of a mean velocity distribution of a model separation bubble. This negative production region is shown to have two important implications for the separation bubble: (a) The upper branch dynamics of the neutral stability curve is significantly affected by the negative production (b) while there is onset of absolute instability in the bubble with increased values of maximum reversed flow, it is seen that relatively higher values of reversed flow required for this onset are necessitated by the presence of the negative production region. [Preview Abstract] |
Monday, November 23, 2009 5:19PM - 5:32PM |
LU.00009: Study on three-dimensional separation in diffusers Elbert Jeyapaul, Paul Durbin A family of three-dimensional diffusers having the same adverse pressure gradient but varying inlet aspect ratios have been studied by detached eddy simulation and Reynolds averaged computation. They provide a geometry for fundamental study of three-dimensional, turbulent separation. The diffuser has a rectangular cross section with two flared walls. RANS computations used the the standard $k-\omega$ and SST turbulence models. Experimental results of Cherry et al provide a reference against which the DES was validated. The SST model has been observed to be overly sensitive to inlet channel aspect ratio. Separation is observed to switch from top to the side wall at higher expansion ratios, while DES predicts separation to stay on the top wall. The recirculating region predicted by the model changes sides even for very small side angles. In a symmetrically--sloped diffuser the model does not predict a symmetric separation at higher side angles. The Reynolds stress models predict symmetric separation, so some of the shortcomings to the SST formulation emerge from the eddy viscosity assumption. [Preview Abstract] |
Monday, November 23, 2009 5:32PM - 5:45PM |
LU.00010: Numerical Study of Flow past a Square Cylinder with an Angle of Incidence Dong-Hyeog Yoon, Kyung-Soo Yang, Choon-Bum Choi A parametric study has been carried out to elucidate the characteristics of flow past a square cylinder inclined with respect to the main flow for \textit{Re}$\le $300. Reynolds number and angle of incidence (\textit{$\theta $}) are the key parameters which determine the flow characteristics. There exist two kinds of critical Reynolds numbers; flow becomes unsteady at a lower critical Reynolds number (\textit{Re}$_{c1})$ and the two-dimensional time-periodic wake becomes unstable to three-dimensional disturbances at an upper critical Reynolds number (\textit{Re}$_{c2})$. We present the two critical Reynolds numbers as a function of \textit{$\theta $} by using the Stuart-Landau equation for a steady flow and a fully-resolved Floquet stability calculation for a time-periodic wake, respectively. In particular, there are two different instability modes for a time-periodic wake, namely, one (mode A) associated with a long spanwise wavelength and the other (mode B) with a short spanwise wavelength. The spanwise wave numbers of the most unstable (or least stable) wave for the critical case are presented for each \textit{$\theta $} considered. We also report flow-induced forces on the cylinder, and flow patterns past the cylinder for the range of \textit{Re} considered in this investigation, and attempt to present physical explanations about them. [Preview Abstract] |
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