Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D21: Instability: Boundary Layers II |
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Chair: Stephen Garrett, University of Leicester Room: 2010 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D21.00001: The centrifugal instability of the boundary-layer flow over slender rotating cones Stephen Garrett, Zahir Hussain Existing experimental and theoretical studies are discussed which lead to the clear hypothesis of a hitherto unidentified convective instability mode that dominates within the boundary-layer flow over slender rotating cones. The mode manifests as G\"ortler-type counter-rotating spiral vortices, indicative of a centrifugal mechanism. Although a formulation consistent with the classic rotating-disk problem has been successful in predicting the stability characteristics over broad cones, it is unable to identify such a centrifugal mode as the half-angle is reduced. An alternative formulation is developed and the governing equations solved using both short-wavelength asymptotic and numerical approaches to independently identify the centrifugal mode. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D21.00002: Rimming flows and pattern formation inside rapidly rotating cylinder Denis Polezhaev, Veronika Dyakova, Victor Kozlov The dynamics of fluid and granular medium in a rotating horizontal cylinder is experimentally studied. In a rapidly rotating cylinder liquid and granular medium coat the cylindrical wall under centrifugal force. In the cavity frame gravity field performs rotation and produces oscillatory fluid flow which is responsible for the series of novel effects of pattern formation, namely, axial segregation of heavy particles and pattern formation in the form of sand regular hills extended along the axis of rotation. At least two types of axial segregation are found: a) patterns of spatial period of the same order of magnitude as fluid layer thickness which induced by steady flows generated by inertial waves; b) fine patterns which manifests Gortler - Taylor vortices developing as a consequence of centrifugal instability of viscous boundary layer near the cylindrical wall. Under gravity, intensive fluid shear flow induces partial fluidization of annular layer of granular medium. The oscillatory motion is followed by onset of regular ripples extended along the axis of rotation. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D21.00003: Role of Spatio-Temporal Wave-front in causing Flow Transition Swagata Bhaumik, Tapan Sengupta Theoretically, boundary layer transition has been identified to occur via K- and H- or N-type routes (Y. S. Kachanov, Ann. Rev. Fluid Mech., 26, 1994) depending upon the arrangement of $\Lambda$-vortices in the transitional zone. While the aligned pattern of these vortices are identified with the first type, a staggered arrangement is attributed to the latter. Subsequently, the H-type breakdown is explained due to triad resonant interaction between a monochromatic spatial 2D TS wave and its two oblique 3D sub-harmonic counterparts. We show via high accuracy DNS of receptivity of the zero-pressure gradient boundary layer that both K- and H-types of transition can be noted for monochromatic deterministic wall-excitation due to growth of spatio-temporal wave-front, which in Bhaumik \& Sengupta (Phys. Rev. E, 89, 043018, 2014) has been established as the precursor of flow transition. In addition to the high accuracy dispersion-relation preserving numerical schemes, computations are also carried out over a significantly longer computational domain. The H-type transition is noted for lower frequency of excitation cases, while K-type is seen to occur for higher frequency cases which is in contrast to current theoretical view-point, particularly for H-type transition. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D21.00004: Flat-plate boundary-layer receptivity to free-stream vortical disturbances with roughness Richard Bosworth, Jonathan Morrison This study focuses on the experimental investigation of the roughness-induced generation of Tollmien-Schlichting (TS) waves in a flat-plate boundary layer, exposed to free-stream vortical disturbances. Experiments are taken in the department's low-speed, low-turbulence wind tunnel where streamwise and lateral free-stream turbulence intensities are below 0.07{\%}. Repeatable, harmonic, 2D free-stream disturbances are created using a metal ribbon placed upstream of a metal plate, with a leading edge designed specifically for receptivity experiments. The ribbon is forced to vibrate at a frequency conducive to the generation of TS waves within the boundary layer. It is shown that, without roughness present on the plate, the vortical disturbances decay into the boundary layer and that TS waves are not generated. The addition of roughness strips, with heights on the order of the inner deck scaling from Triple Deck Theory, clearly initiate a boundary layer response with the characteristic TS wave profile. This further confirms the theoretical predictions that a scale conversion process is required to generate TS waves from free-stream disturbances in a flat-plate boundary layer. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D21.00005: Numerical Study of Boundary Layer Receptivity to Periodic Vortical Disturbances in Freestream Yu Nishio, Seiichiro Izawa, Yu Fukunishi A flow passing a flat plate with an elliptic leading edge whose aspect ratio is 5 is simulated to investigate its receptivity to periodic vortical disturbances in the freestream. The disturbance consists of vortex pairs aligned in the spanwise direction, whose rotational directions are opposite. When they successively collide to the leading edge of the flat plate, streamwise vortices appear in the boundary layer downstream. The tilting and stretching of oncoming vortex columns with their axes normal to the flat plate was believed to be the cause of the generation of the streamwise vortices. However in this study, it is shown that the streamwise vortices in the boundary layer is generated by a combination of the spanwise velocity induced by the parts of vortices outside the boundary layer and the no-slip condition at the wall. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D21.00006: Experimental study of crossflow instability on a Mach 6 yawed cone Stuart Craig, William Saric Boundary-layer stability and transition represents a key challenge for the designer of hypersonic vehicles, which typically feature highly-swept and conical features inclined to the free stream. The transition process on each of these geometries is typically dominated by the three-dimensional crossflow instability. In order to advance the goal of a physics-based transition prediction method, crossflow experiments were undertaken in the Mach 6 Quiet Tunnel at Texas A{\&}M University. Detailed boundary-layer measurements were performed on a 7-degree cone at a 6-degree angle of incidence using constant-temperature hot-wire anemometry (CTA) to produce boundary-layer contours at constant axial location. These contours illustrate the characteristic streamwise vortex pattern and mean-flow distortion characteristic of crossflow-dominated flows. Additionally, the high frequency response of the CTA system allows for analysis of the spectral content of the flow. These measurements show a high degree of qualitative agreement with analogous studies performed in low-speed flows. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D21.00007: Linear stability of steady flow in a precessing sphere -- Global and local disturbances Shigeo Kida It is known by the linear stability of the steady flow in a precessing sphere that the critical curve behaves as Po=7.9/Re**0.5 for global disturbances at large Re, where Po is the Poincare number and Re is the Reynolds number (Kida 2013). Here we perform the linear stability analysis of the local disturbances localized in the critical regions and the conical shear layer emerging wherefrom, and show that the asymptotic form of the critical curve is Po=21.25/Re**0.8 and that this result agrees with the corresponding labolabory experiment. On the other hand, the critical curve for the global disturbances is not observed in the experiment of a precessing sphere but in the experiment of a slightly elongated spheroid, the minor-to-major axis ratio being 0.9 (Goto et al. 2014). These correspondences between theory and experiment can be understood by noting that conical shear layers exist stably only for a spherical cavity. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D21.00008: Numerical study of absolute instability on a rotating disk flow Keunseob Lee, Yu Nishio, Seiichiro Izawa, Yu Fukunishi Numerical simulation is carried out aimed at investigating the absolute instability of the three-dimensional boundary layer flow on a rotating disk. An artificial random disturbance is given at the wall to a laminar flow whose Reynolds number is higher than the critical value for the absolute instability. The disturbance first grows up into spiral vortices aligned regularly in the circumferential direction, and after that, an onset of turbulence takes place. The process is similar to what takes place at the convectively unstable region of much lower Reynolds number. The region of spiral vortices expands not only outward but also toward the center of the disk, and so does the turbulent region. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D21.00009: Effect of Wall Suction on Cross-Flow Absolute Instability of a Rotating Disk Boundary Layer Joanna Ho, Thomas Corke, Eric Matlis The effect of uniform suction on the absolute instability of Type I cross-flow modes on a rotating disk is examined. Specifically it investigates if wall suction transforms the absolute instability into a global mode as postulated in the numerical simulations of Davies and Carpenter (2003). The experiment is designed so that a suction parameter of $a = \bar{W}_0/(\nu \omega)^{1/2} = 0.2$ locates the absolute instability critical Reynolds number, $R_{c_{a}} =650$, on the disk. Uniform wall suction is applied from R = 317 to 696. The design for wall suction follows that of Gregory and Walker (1950), where an array of holes through the disk communicate between the measurement side of the disk and the underside of the disk in an enclosure that is maintained at a slight vacuum. The measurement surface is covered by a 20 micron pore size Polyethylene sheet. Temporal disturbances are introduced using the method of Othman and Corke (2006), and the evolution of the resulting wave packets are documented. The present results indicate a rapid transition to turbulence near $R_{c_{a}}$. [Preview Abstract] |
Sunday, November 23, 2014 4:12PM - 4:25PM |
D21.00010: Global stability and frequency response of boundary layers developing over shallow cavities 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 study the flow over a shallow rectangular cavity at Reynolds numbers at which the boundary layer is unstable to Tollmien-Schlichting waves. We obtain steady two-dimensional solutions to the incompressible Navier-Stokes equations and study the growth of three-dimensional perturbations on top of these steady base flows. We use the linearized Navier-Stokes operator to identify how the dominant modes of instability vary with the thickness of the upstream boundary layer and with the cavity aspect ratio. We calculate the global frequency response and optimal forcing to map out the influence of the cavity on the growth of TS-waves. Finally, we compare the results with those for boundary layers developing over backward-facing and forward-facing steps. [Preview Abstract] |
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