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 CE: Instability: Boundary Layers II |
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Chair: Xiaohua Wu, Royal Military College of Canada Room: Long Beach Convention Center 102C |
Sunday, November 21, 2010 1:00PM - 1:13PM |
CE.00001: Design of a DBD Plasma Actuator Array to Control Stationary Cross Flow Modes in a Supersonic Boundary Layer Chan-Yong Schuele, Eric Matlis, Thomas Corke, Stephen Wilkinson The control of cross flow dominated laminar turbulent transition is crucial for the improvement of efficiency of supersonic aircraft. Passive methods such as distributed micron sized roughness elements have proven to work efficiently as laminar flow control devices in subsonic and as we could recently show in supersonic flows. This study describes the replacement of micron sized roughness elements with an array of dielectric barrier discharge (DBD) plasma actuators in order to excite less amplified stationary cross flow modes. These are intended to suppress the growth of the naturally occurring most amplified stationary modes. The use of DBD plasma actuators allows for a dynamic control that can respond to changing flight conditions, which is difficult to achieve with traditional roughness elements. Experiments have been performed in the 0.5\,m Mach 3.5 NASA LaRC Supersonic Low Disturbance Tunnel on a $7^{\circ}$ half angle sharp cone at a $4.3^{\circ}$ angle of attack, and a unit Reynolds number of 250000/in. [Preview Abstract] |
Sunday, November 21, 2010 1:13PM - 1:26PM |
CE.00002: Hypersonic boundary-layer instability with localized roughness Olaf Marxen, Gianluca Iaccarino, Eric Shaqfeh Understanding the process of laminar-turbulent transition in supersonic flows has important implications for the design of thermal protections systems for hypersonic vehicles. A localized three-dimensional roughness element inside the boundary layer on the surface of such a vehicle may profoundly alter the instability of the boundary layer, and hence the transition process. However, our understanding of this alteration is far from comprehensive. A numerical investigation of a laminar flat-plate boundary layer with a localized 3-D roughness is carried out. The roughness height is on the order of half the boundary layer thickness. Streamwise vortices behind the roughness deform the boundary layer, leading to a strong low speed and high temperature streak behind the roughness. Moreover, wall-normal as well as spanwise gradients result from these streaks and are responsible for creating additional instabilities of the deformed boundary layer. Simulations have been performed for spanwise symmetric and asymmetric roughness shapes. Excitation of small-amplitude disturbance upstream of the roughness in the simulation allows to trigger the instabilities. Growth rates and amplitude functions will be compared for the different roughness shapes. [Preview Abstract] |
Sunday, November 21, 2010 1:26PM - 1:39PM |
CE.00003: Stability Analysis of a Mach 10 Boundary Layer with Nonequilibrium Chemistry Shirin Ghaffari, Olaf Marxen, Gianluca Iaccarino, Eric Shaqfeh, Thierry Magin High temperature conditions in high Mach number flights can invalidate the assumption of a calorically perfect gas. As temperature rises, thermodynamic and transport properties of the gas mixture become not only a function of temperature but also of the chemical composition. If chemical nonequilibrium exists, additional transport equations for the species densities should be solved. Chemical nonequilibrium in the bulk can strongly affect boundary layer stability and transition to turbulence and thus it is an important capability to have in direct numerical simulation of high Mach number flows. At present, not many high-order numerical methods are capable of handling the high temperature regime. We examine boundary layer stability of a Mach 10 flow over a flat plate for the bulk in chemical nonequilibrium but thermal equilibrium. We carry out a high-order numerical integration of the Navier-Stokes equations via direct coupling to a library that computes gas properties based on the kinetic theory. Spatial amplification of small disturbances that may lead to transition on an isothermal or adiabatic flat plate are investigated. [Preview Abstract] |
Sunday, November 21, 2010 1:39PM - 1:52PM |
CE.00004: Boundary-layer transition on a flared cone in a Mach 6 quiet wind tunnel Jerrod Hofferth, William Saric The Mach 6 Quiet Tunnel at Texas A\&M is a low-disturbance blowdown facility suitable for boundary-layer stability and transition research. Following its reactivation in 2009, initial testing confirmed the presence of low-disturbance ($< 0.1\%$ $P_t^{\prime}/P_t$) freestream flow at select locations on the centerline of the nozzle for settling chamber pressures up to 10 atm, and a fully-traversed freestream flow-quality assessment is currently underway. As a third performance benchmark to complement these direct measurements, the present work measures the transition location on the NASA Langley 93-10 flared-cone model. This model has a 0.5m length, beginning as a $5^{\circ}$ half-angle circular cone. At the $X=254$mm station, a flare of surface radius 2.35m begins which is intended to induce transition within the quiet test core. Boundary-layer transition is detected on the thin-walled model by an observed surface temperature rise using an array of 51 embedded thermocouples. Transition data are presented for a sharp (2.5 $\mu$m) nose-tip radius case for comparison with the Lachowicz \& Chokani (1996 data). Data for larger-radius nose-tips are also presented. [Preview Abstract] |
Sunday, November 21, 2010 1:52PM - 2:05PM |
CE.00005: Hypersonic boundary layer instabilities affected by various porous surfaces Xiaowen Wang, Xiaolin Zhong Hypersonic boundary layer instabilities of a Mach 5.92 flow over a flat plate affected by various porous surfaces are studied by numerical simulations. Steady base flow is obtained by solving compressible Navier-Stokes equations with a fifth-order shock-fitting method and a second-order TVD scheme. Stability simulations consist of two steps: (1) disturbances corresponding to a single boundary layer wave (mode F or mode S) are superimposed at a cross-section of the boundary layer near the leading edge to show spatial development of the wave; (2) porous coatings are used downstream of the superimposed wave to investigate its effect on boundary-layer instabilities. The results show that porous coating only has local effects on the instabilities of mode S and mode F. In porous region, Mack's first mode is destabilized whereas Mack's second mode and Mode F are stabilized. For felt-metal porous coating, destabilization of Mack's first mode is so significant that disturbances are slightly destabilized when porous coating are put on the whole flat plate. At approximately the same porosity, regular structure porous coating is weaker in first mode destabilization and second mode stabilization than felt-metal porous coating. [Preview Abstract] |
Sunday, November 21, 2010 2:05PM - 2:18PM |
CE.00006: Investigation of a Turbulent Spot in a Hypersonic Cone Boundary Layer Jayahar Sivasubramanian, Hermann Fasel Direct Numerical Simulations (DNS) were performed to investigate the growth and breakdown of a localized disturbance into a turbulent spot in a sharp cone boundary layer at Mach 6. In order to model a natural transition scenario, the boundary layer was pulsed through a hole on the cone surface. The pulse disturbance developed into a three dimensional wave packet which consisted of a wide range of disturbance frequencies and wave numbers. The dominant waves within the resulting wave packet were identified as two dimensional second mode disturbance waves. In addition, weaker oblique waves were observed on the lateral sides of the wave packet. The developing wave packet grows linearly at first before reaching the nonlinear regime and eventually leads to localized patches of turbulent flow (turbulent spot). The wall pressure disturbance spectrum showed strong secondary peaks at the fundamental frequency for larger azimuthal wave numbers. This development indicates that fundamental resonance might be the dominant nonlinear mechanism for a cone boundary layer at Mach 6. [Preview Abstract] |
Sunday, November 21, 2010 2:18PM - 2:31PM |
CE.00007: Boundary-layer transition over aerodynamically-significant rotating bodies Stephen Garrett, Zahir Hussain, Alistair Barrow, Paul Towers For practical reasons rotating-disk flow has served as the foremost model problem for studying transition in fully 3D incompressible boundary layers for over six decades and has a huge body of associated literature. However, continuing developments in spinning projectiles and aeroengines has led to the need to understand the onset of transition over rotating cones and spheroids as objects in their own right. Although numerous experimental observations have been published, these geometries received only little theoretical attention prior to 2002 when Garrett and co-workers commenced their work. In this paper we give a comparative study of the instability characteristics of the flows over these distinct geometries, discussing their similarities and differences with each other and the rotating-disk paradigm. The rotating-cone flow in particular is found to demonstrate significantly different characteristics as the half-angle is reduced below 40degs. This observation has led to the hypothesis and ultimate identification of an alternative instability mode which is expected to dominate for slender cones. Theoretical studies using numerical and asymptotic techniques are discussed. Comparisons are made to existing experimental data, and in many cases excellent agreement is observed for measurable properties. Where close agreement is not seen, we discuss possible reasons why. [Preview Abstract] |
Sunday, November 21, 2010 2:31PM - 2:44PM |
CE.00008: The centrifugal instability of the boundary layer on a slender rotating cone in a forced free-stream Zahir Hussain, Stephen Garrett, Sharon Stephen The laminar-turbulent transition of flow within the boundary layer over a slender rotating nose cone (for example a spinning missile) can lead to increases in drag, with negative implications for control and targeting. However, continuing developments on spinning projectiles, which have furthered understanding of the onset of laminar-turbulent transition over rotating cones, may lead to design modifications and significant cost savings. Experiments in the literature have shown that increasing the incident free-stream has a stabilizing effect on these spiral vortices. Furthermore, Kobayashi (1981) has calculated the stability diagram for a slender cone of half-angle $15^{\circ}$ using the Orr-Sommerfeld approximation. In this study, we provide a new mathematical description of the onset of counter-rotating spiral vortices observed for a $15^{\circ}$ rotating cone placed in forced free-streams of varying strength. In particular, we resolve appropriate scalings in order to include variations in the basic-flow profiles, accounting for the influence of streamline curvature. A combined large Reynolds number and large vortex wavenumber analysis is used to obtain the asymptotic branch of neutral stability. Our results capture the effects of the governing centrifugal G\"ortler instability mechanism, and lead to favorable comparisons with existing numerical neutral stability curve results. [Preview Abstract] |
Sunday, November 21, 2010 2:44PM - 2:57PM |
CE.00009: Sidewall boundary layer instabilities in a rapidly rotating cylinder driven by a differentially co-rotating lid Juan Lopez, Francisco Marques The flow in an enclosed completely filled rapidly rotating cylinder that is driven by the differential co-rotation of the top lid is studied numerically. Although the flow is in a very simple geometry, the fast background rotation and large differential rotation of the lid lead to very thin boundary layers with a variety of instability modes with very fine spatial scales as well as inertial waves that are sustained in the fast rotating interior flow and which interact with the viscous modes in the sidewall boundary layer leading to complex spatio-temporal dynamics. The numerical simulations are compared and contrasted to experimental visualizations of the sidewall boundary layer instabilities reported by Hart and Kittelman (1996), that include axisymmetric rolls propagating down the sidewall layer, backwards tilted diagonal rolls that precess slightly retrograde with respect to the rotating sidewall, forward tilted rolls with prograde precession significantly faster than the sidewall rotation, and a wavy turbulent state that has backwards tilted structures erupting from deep within the sidewall layer into the interior and are riding on the forward tilted diagonal rolls in the deep layer. [Preview Abstract] |
Sunday, November 21, 2010 2:57PM - 3:10PM |
CE.00010: Influence of a recent Transition Model on Complex Nonsteady Boundary Layer Flows with Dynamic Stall and Multiple Phases Adam Lavely, Michael Kinzel, Ganesh Vijayakumar, James Brasseur, Eric Paterson, Jules Lindau Computational fluid dynamics (CFD) simulations are prone to inaccuracies associated with incorrectly formulated physical models. Common in CFD is the spurious treatment as locally laminar flow regions as turbulent, resulting in incorrect turbulent-boundary-layer profiles, separated-flow behavior, and local skin-friction coefficients. The combined effects impacts global measures like drag, lift coefficient, and wake intensity. Recently, Menter {\&} Langtry (AIAA \textbf{47} 2009) developed a transition model applicable to unsteady three-dimensional CFD codes that shows promise to improve the prediction of local laminar regions. Our aim is to evaluate the accuracy of this model with the additional complexities of unsteady flow around rotating wind turbine blades and multiphase flows using codes designed within OpenFOAM. We investigate how transition and locally laminar flow regions impact various complex problems of interest including: (1) stationary S809 airfoil through stall, (2) an oscillating S809 airfoil in dynamic stall, and (3) a ventilated gaseous cavity in a liquid flow. We will evaluate the efficacy of the model by comparing with experimental results, and shall evaluate the impact on integral measures and flow details. \textit{Supported by NSF {\&} DOE.} [Preview Abstract] |
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