Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session E2: Instability in Boundary Layers I |
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Chair: William Saric, Texas A&M University Room: 302 |
Sunday, November 20, 2011 4:40PM - 4:53PM |
E2.00001: Boundary-layer instability \& transition on a flared cone in a Mach 6 quiet wind tunnel Jerrod Hofferth, William Saric Measurements of boundary-layer transition location and instability growth on a sharp-tipped 5$^{\circ}$-half-angle flared cone were conducted in a low-disturbance Mach 6 wind tunnel at a freestream unit Reynolds number of 10 $\times$ 10$^6$/m. Under quiet flow at these conditions, the boundary layer becomes transitional near the base of the cone, where significant second-mode instability growth is evident. Transition location is determined using an array of embedded thermocouples, and instability development is observed in mean and fluctuating mass flux data using hotwire anemometry. The present work seeks to reproduce and build upon previous experiments which used the same test article and similar diagnostics in the facility's former installation at NASA Langley. Together with comprehensive measurements of the freestream disturbance environment, these baseline cone data characterize the facility's performance relative to that in its previous installation. In addition, the current campaign establishes experimental readiness for future research, which will study the effects of periodic surface roughness and controlled-input disturbances. [Preview Abstract] |
Sunday, November 20, 2011 4:53PM - 5:06PM |
E2.00002: Plasma Roughness for Transition Control in a 3-D Supersonic Boundary Layer Chan-Yong Schuele, Eric Matlis, Thomas Corke, Stephen Wilkinson The design and use of patterned ``plasma roughness" for control of transition to turbulence of the boundary layer with a supersonic free-stream is presented. The plasma roughness consisted of an azimuthal array of 20\,nm thick electrodes that were equally spaced around the cone tip, just upstream of Branch I for cross-flow instability growth. The electrodes were part of a DBD arrangement that produced an azimuthally periodic stationary body force that acted on the flow. The azimuthal spacing of the electrodes was designed to either enhance the most amplified stationary mode growth ($m=45$ in this case), or to excite a sub-critical mode number ($m=68$) that was designed to suppress the most amplified mode. The experiment was performed on a $14^{\circ}$ right-circular cone placed at a $4.3^{\circ}$ angle of attack in the NASA LaRC SLDT. Measurements consisted of azimuthal profiles of the total pressure just above the cone surface. These documented the mean flow distortion produced by the growing stationary cross-flow modes. Comparisons were made with and without the plasma roughness, as well as against passive patterned roughness with the same azimuthal mode numbers. The results indicated that the stationary cross-flow modes were receptive to the patterned plasma roughness, and that $Re_{trans}$ was increased. [Preview Abstract] |
Sunday, November 20, 2011 5:06PM - 5:19PM |
E2.00003: Nonlinear wavepackets in unstable boundary layers: experiments and modeling Marcello Medeiros The work involved wind tunnel experiments and weakly nonlinear modeling of modulated TS waves. It covered from wavetrains emanating from a harmonic point source to the wavepackets from pulses. The nonlinear regime of these waves exhibit three stages. The first one was governed by direct nonlinear forcing and produced longitudinal streaks. The second was governed by K- type instability. Both stages were triggered by the spanwise modulation. The third nonlinear stage was governed by subharmonic, H - type instability and was exclusive of streamwise modulated waves. The phase of the waves relative to the modulation envelope was also found to be very important for the third stage only. In general, it was found that the modulation did not affect the kernel of the secondary instabilities observed. The effect of both streamwise and spanwise modulation was on the deterministic production of seeds for these instabilities, via quadratic interaction of the existing modes. The weakly nonlinear model explained many features that at first seemed surprising. For instance, the effect of phase on the subharmonic stage was explained by the production of subharmonic seeds from the direct nonlinear interaction of both the primary modes and the fundamental instability modes. [Preview Abstract] |
Sunday, November 20, 2011 5:19PM - 5:32PM |
E2.00004: The Effect of Moderate Freestream Turbulence on Crossflow Instability and Transition Robert Downs, Edward White Previous research indicates that the generation of stationary and traveling crossflow modes is dependent on the level of freestream turbulence, \textit{Tu}. Recent experiments in low turbulence environments have also shown that the initial amplitudes of stationary crossflow disturbances arising from surface roughness are influenced by small variations in \textit{Tu}. Preliminary results are presented concerning experimental investigation into the role of moderate levels of freestream turbulence (\textit{Tu }= 0.025{\%} - 0.2{\%}) in boundary layer receptivity to surface roughness. Transition induced by crossflow instability is studied using a 45-degree swept wing model in the low turbulence Klebanoff-Saric Wind Tunnel. Naphthalene flow visualization is used to assess transition location and detailed hotwire anemometry measurements are made to quantify these initial disturbance amplitudes. [Preview Abstract] |
Sunday, November 20, 2011 5:32PM - 5:45PM |
E2.00005: Natural by-pass boundary layer transition Shahab Shahinfar, Jens H.M. Fransson The present measurement campaign on the free-stream turbulence induced boundary layer transition scenario has provided a unique set of experimental data, with potential to enhance the understanding of the effect of the free-stream turbulence characteristic length scales on the transition location and not only the turbulence intensity, which has been the focus in most previous studies. Recent investigations where the turbulence intensity has been kept essentially constant, while the integral length scale has been changed, show that the transition location is advanced for increasing length scale. However, the present data show that the integral length scale has a relatively small influence on the transition location as compared to the turbulence intensity and data analyses are now directed towards enhanced understanding of how the different parts of the incoming energy spectrum affects the energy growth inside the boundary layer. [Preview Abstract] |
Sunday, November 20, 2011 5:45PM - 5:58PM |
E2.00006: An experimental study of the edge effect on transition of the rotating-disk boundary-layer flow Shintaro Imayama, R.J. Lingwood, P. Henrik Alfredsson Lingwood [J. Fluid Mech., 299, 17 (1995)] showed that the flow instability in the rotating-disk boundary layer is not only of convective nature but also that the flow becomes absolutely unstable. Furthermore, in the absence of bypass mechanisms, the absolute instability triggers nonlinearity and transition to turbulence at a fixed Reynolds number ($Re$). Healey [J. Fluid Mech., 663, 148 (2010)] suggested that the observed spread (albeit small) in transition $Re$ in different experiments is an effect of the $Re$ at the disk edge and provided a nonlinear model to take this effect into account. Here, we further investigate this problem experimentally with hot-wire measurements on a rotating polished glass disk with a diameter of 474mm and a total imbalance and surface roughness less than 10$\mu$m. To investigate the influence of the disk edge, we vary $Re$ at the disk edge by changing the rotational speed and map the development of the disturbance velocity in the radial direction. Furthermore, the effect of a stationary annular plate around the edge of the rotating disk is also investigated. Our experiments show no effect of the disk edge $Re$ on the stability and transition, however there was a shift of both the growth curve and the transition $Re$ by about 10 units with and without the outer stationary plate, with the lower $Re$ observed with the plate. [Preview Abstract] |
Sunday, November 20, 2011 5:58PM - 6:11PM |
E2.00007: Surface acoustic wave induced flow in micro channels: peculiar flow instability observed in experiment and advances in theory Ofer Manor, James Friend, Leslie Yeo Challenges in microfluidics have initiated a renewed interest in surface acoustic waves (SAW) and in particular Rayleigh SAW for generating high fluid velocities and mixing in microchannels. SAW are generated by piezoelectric actuators that transfer electric to kinetic energy and give rise to different types of flow regimes -- Eckart, Rayleigh and Schlichting streaming. Here we extend the theory for the Schlichting boundary layer. We show that due to the complex nature of the Rayleigh SAW, the leading order result for the steady flow in the Schlichting layer is of greater magnitude than previously considered in studies on stationary planar acoustic waves. We confirm this result by measuring the behavior of a dilute suspension of particles in 80 micron thick PDMS channels. In the presence of only a stationary planar acoustic wave in the liquid we observe the weak accumulation of particles in the vicinity of acoustic nodal lines. These structures are easily interrupted by bulk flow. In the presence of Rayleigh SAW, however, particle structures are found to maintain much better integrity and to occupy much larger area under dispersive flow effects. [Preview Abstract] |
Sunday, November 20, 2011 6:11PM - 6:24PM |
E2.00008: Squire's transformation and 3D Optimal Perturbations in Bounded Parallel Shear Flows Jean-Marc Chomaz, J. John Soundar Jerome The aim of this short communication is to present the implication of Squire's transformation on the optimal transient growth of arbitrary 3D disturbances in parallel shear flow bounded in the cross-stream direction. To our best knowledge this simple property has never been discussed before. In particular it allows to express the long-time optimal growth for perturbations of arbitrary wavenumbers as the product of the gains from the 2D optimal at a lower Reynolds number itself due to the Orr-mechanism by a term that may be identified as due to the lift-up mechanism. This property predict scalings for the 3D optimal perturbation well verified by direct computation. It may be extended to take into account buoyancy effect. [Preview Abstract] |
Sunday, November 20, 2011 6:24PM - 6:37PM |
E2.00009: The role of the frozen relative Mach number on the stability of boundary layers in chemical non-equilibrium Jill Klentzman, Erman Ulker, Anatoli Tumin The stability of boundary layers in chemical non-equilibrium is investigated in the inviscid limit. The flow of binary mixtures of oxygen and nitrogen over a flat plate is considered and a parametric study varying the edge temperature and pressure while keeping the frozen edge Mach number constant is conducted. In contract to the case of a calorically perfect gas, in which little to no effect on the stability would be expected, varying these parameters in the case of a gas in chemical non-equilibrium produces significant changes in the stability results. Both cold and adiabatic, non-catalytic walls are considered and the results compared. It is found that wall cooling leads to an increase in the maximum growth rate of the second mode instability and a shift in the second mode to higher wave numbers. The trends observed due to real gas effects reveal that the frozen relative Mach number may play a significant role similar to what has been found in perfect gas flows. [Preview Abstract] |
Sunday, November 20, 2011 6:37PM - 6:50PM |
E2.00010: The effect of chemistry and transport models on the inviscid stability of boundary layers in binary mixtures of oxygen and nitrogen Erman Ulker, Jill Klentzman, Anatoli Tumin An inviscid stability analysis of boundary layers in binary mixtures of oxygen and nitrogen in chemical non-equilibrium is conducted. Cold and adiabatic, non-catalytic walls are considered and two different chemistry and transport models are used: the reaction rates and transport models of Gupta et al. (1990) as well as Blottner's transport model (1971) with Park's reaction rates (1989). The results show that changing the chemistry and transport models has a major impact on the boundary layer profiles and the perturbation growth rates when dissociation is significant in the boundary layer. In the model of Gupta et al., the reaction rates do not change with respect to the collision partner in binary mixtures of oxygen, which is in contrast to Park's reaction rates. For binary mixtures of nitrogen, the reaction rates change according to the collision partner for both models. The results of the comparison of different chemistry and transport models show that the flow in binary mixtures of oxygen is more affected by the choice of the model than in binary mixtures of nitrogen. [Preview Abstract] |
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