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 R4: Instability: Boundary Layers I - Surface Topography |
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Chair: Pierre Ricco, The University of Sheffield Room: 326 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R4.00001: Distributed Roughness and Transient Growth in a Flat Plate Boundary Layer Matthew Kuester, Edward White Very few experiments or numerical simulations have deliberately studied how distributed surface roughness affects boundary layer transition through the transient-growth mechanism. In this experimental work, transient growth initiated by randomly distributed, streamwise-extended surface roughness is experimentally studied in the Klebanoff--Saric Wind Tunnel at Texas A\&M University. Two distributed roughness surfaces with the same surface topography, but different amplitudes, were manufactured and mounted flush with the wall in a flat-plate boundary layer. Detailed measurements of the roughness wake were made using hotwire anemometry at three sub-critical Reynolds numbers. Transient growth was observed at multiple spanwise wavelengths. Multiple wavelengths underwent robust growth but only weak growth was observed near the so-called ``optimal'' wavelength. Comparing cases with different Reynolds numbers and roughness heights reveals how the steady velocity disturbance scales and gives futher insight into the receptivity mechanisms of distributed surface roughness. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R4.00002: Appraisal of boundary layer trips for landing gear testing Philip McCarthy, Graham Feltham, Alis Ekmekci Dynamic similarity during scaled model testing is difficult to maintain. Forced boundary layer transition via a surface protuberance is a common method used to address this issue, however few guidelines exist for the effective tripping of complex geometries, such as aircraft landing gears. To address this shortcoming, preliminary wind tunnel tests were performed at Re = 500,000. Surface transition visualisation and pressure measurements show that zigzag type trips of a given size and location are effective at promoting transition, thus preventing the formation of laminar separation bubbles and increasing the effective Reynolds number from the critical regime to the supercritical regime. Extension of these experiments to include three additional tripping methods (wires, roughness strips, CADCUT dots) in a range of sizes, at Reynolds number of 200,000 and below, have been performed in a recirculating water channel. Analysis of surface pressure measurements and time resolved PIV for each trip device, size and location has established a set of recommendations for successful use of tripping for future, low Reynolds number landing gear testing. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R4.00003: Discrete surface roughness effects on a blunt hypersonic cone in a quiet tunnel Nicole Sharp, Edward White The mechanisms by which surface roughness creates boundary-layer disturbances in hypersonic flow are little understood. Work by Reshotko (AIAA 2008-4294) and others suggests that transient growth, resulting from the superposition of decaying non-orthogonal modes, may be responsible. The present study examines transient growth experimentally using a smooth 5-degree half-angle conic frustum paired with blunted nosetips with and without an azimuthal array of discrete roughness elements. A combination of hotwire anemometry and Pitot measurements in the low-disturbance Mach 6 Quiet Tunnel are used for boundary layer profiles downstream of the ring of roughness elements as well as azimuthal measurements to examine the high- and low-speed streaks characteristic of transient growth of stationary roughness-induced disturbances. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R4.00004: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R4.00005: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R4.00006: Linear and nonlinear receptivity of the boundary layer in transonic flows Anatoly Ruban, Marina Kravtsova, Tomass Bernots This paper is concerned with the phenomenon of the generation of Tollmien-Schlichting waves in laminar boundary layer on an aircraft wing in transonic flow regime. A particular form of the boundary layer receptivity is considered when the boundary layer encounters a local roughness on the wing surface in the form of a gap, step or a hump. We assume that the boundary layer is exposed to acoustic noise and study the interaction of the acoustic waves with the flow perturbations produced in the boundary layer by the roughness. Two approaches are used. The first one is theoretical; it is based on large Reynolds number asymptotic analysis of the Navier-Stokes equations leading to the transonic version of the triple-deck theory. Under assumptions that the acoustic noise level is weak, and the roughness height is small, and analytic formula for the amplitude of the generated Tollmien-Schlichting wave is deduced. In the second, numerical, approach the restriction on the roughness height is lifted, which allows for the flows with local separation near the roughness to be considered. The calculations have been perform for different value of the Karman number, and we found that in the flow separation always leads to a significant enhancement of the receptivity process. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R4.00007: Numerical investigation of boundary layer receptivity to free-stream disturbances and surface excrescences Adrian Sescu, Miguel Visbal, Donald Rizzetta In this study, the receptivity of boundary layers to surface imperfections and free-stream disturbances is analyzed in two-dimensions using a high-fidelity Navier-Stokes solver based on high-order compact spatial schemes, and implicit time integration. The surface imperfection is an idealized form of typical excrescences that exist on the surface of aircraft wings, while the free-stream disturbances mimic real perturbations existing in the atmosphere or in wind tunnels. The geometry consists of an superelliptic-leading-edge flat plate with a forward or rearward excrescence on its surface. Acoustic and vortical waves are generated using a source term, as opposed to using inflow boundary conditions, to avoid spurious waves that may propagate from boundaries. The results show that the acoustic waves are very efficient in exciting the Tollmien-Schlichting (TS) waves downstream of the step, as expected, and that the wavelength of TS waves scales linearly with the wavelength of the acoustic waves. The vorticity waves are less likely to excite the TS waves, but when they do so the TS waves are grouped in wave packets that are consistent with the wavelength of the vorticity waves. Other relevant results will be included and discussed. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R4.00008: Effect of porous surface on pre-transitional supersonic boundary-layer disturbances generated by free-stream vortices Pierre Ricco A supersonic laminar flat-plate boundary layer at Mach number M$=$6 flowing over a porous surface is studied numerically and by asymptotic methods. The flow is perturbed by small-amplitude free-stream vortical disturbances of the convective gust type. These external agents generate streamwise-elongated low-frequency disturbances of the kinematic kind, i.e. compressible streaky Klebanoff modes, and of the thermal kind, i.e. thermal streaks, which grow algebraically downstream. For boundary layer fluctuations with a spanwise wavelength comparable with the boundary layer thickness, the porous surface has a negligible effect on the growth and evolution of the streaks. When the spanwise wavelength is instead larger than the boundary layer thickness, the disturbances are effectively attenuated by the porous surface. For a specified set of frequency and wavelengths, the streaky structures evolve into oblique Tollmien-Schlichting waves through a leading-edge-adjustment receptivity mechanism. The growth rate of these waves increases slightly over the porous set, thus confirming previous results obtained through stability analysis. Our receptivity analysis allows us to calculate the wave amplitude, which is attenuated by the porous surface. Further asymptotic analysis based on triple-deck theory confirms the numerical results. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R4.00009: Experimental Investigation on the Effects of Free-Stream Turbulence on Swept-Wing Transition Tariq Saeed, Jonathan Morrison This study focusses on the experimental investigation of the high-frequency secondary instability development of roughness-induced stationary crossflows on a swept wing in moderate levels of free-stream turbulence. The first phase of this study is to determine the magnitude of the vortices produced for comparison with the predictions. The evolution of the disturbances are measured as they evolve downstream. The ``AERAST'' model used in this study is designed to enhance the growth of the crossflow instability, and has a sweep angle of $40^\circ$. The model has been tested in the Department's 10' x 5' wind tunnel which has a top speed of 40 m/s, corresponding to $Re_c = 2.2 \times 10^6$. The measured streamwise and lateral free-stream turbulence levels of the facility are 0.13\% and 0.21\%, respectively; an additional turbulence reduction screen to be installed would give 0.05\% and 0.12\%, respectively. A sublimation flow-visualisation technique is used to assess transition location, and detailed hot-wire measurements are conducted to assess disturbance evolution. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R4.00010: Interpretations of Incompressible Continuous Spectrum Receptivity Curves for Transient Growth Jason Monschke, Edward White Receptivity of transient disturbances to distributed surface roughness is not representable as a single value but is instead a complex-valued function with a different value for each continuous spectrum mode of the Orr--Sommerfeld/Squire equations. Specific characteristics of the curves give rise to streamwise vorticity of varying strength and at different locations within the boundary layer. The various combinations of streamwise vorticity and the initial streamwise velocity disturbance result in the many types of energy evolution seen in experiments and DNS. Following the work of Tumin [Phys. Fluids 15, 2525 (2003)], Denissen \& White [Phys. Fluids 21, 114105 (2009)] developed a technique to decompose experimental measurements made downstream of a roughness array into the constituent continuous spectrum modes. These techniques provide for significant data reduction because receptivity curves encode the complete downstream evolution of the laminar boundary layer. Even though the decomposition uniquely characterizes receptivity to roughness, the small set of measured curves has hindered a systematic understanding of the physical meaning of roughness receptivity curves. Our findings help to associate specific receptivity-curve shapes with physically observable behavior. [Preview Abstract] |
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