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 AP: Instability: Boundary Layers I |
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Chair: William Saric, Texas A&M University Room: 200D |
Sunday, November 22, 2009 8:00AM - 8:13AM |
AP.00001: Global optimal disturbances using time-steppers Antonios Monokrousos, Luca Brandt, Dan S. Henningson The global linear stability of boundary-layer flows subject to three-dimensional disturbances is studied by means of Lagrange optimization. We consider the optimal initial condition leading to the largest growth at finite times and the optimal harmonic forcing leading to the largest asymptotic response (pseudo-spectrum). Both optimization problems are solved using a Lagrange multiplier technique, where the objective function is the kinetic energy of the flow perturbations and the constraints involve the linearised Navier-Stokes. Whereas the computation of optimal initial condition is known in the time-stepper context, the formulation of the optimal forcing problem is novel. The approach proposed here is particularly suited to examine convectively unstable flows, where single global eigenmodes of the system do not capture the downstream growth of the disturbances. For spanwise wavelengths of the order of the boundary layer thickness finite-length streamwise vortices exploit the lift-up mechanism to create streaks. For long spanwise wavelengths the Orr mechanism combined with the amplification of oblique wave packets are responsible for the disturbance growth. The latter mechanism is found to be dominant for the relatively long computational domain and high Reynolds number considered here. The use of matrix-free methods enables us to extend the present framework to any geometrical configuration. [Preview Abstract] |
Sunday, November 22, 2009 8:13AM - 8:26AM |
AP.00002: Receptivity of G\"{o}rtler Flow Lars-Uve Schrader, Luca Brandt, Dan Henningson, Tamer Zaki The flow over a concave surface, e.g. the lower side of a turbine blade, is subject to centrifugal forces which may destabilize the boundary layer. The instabilities appear as streamwise aligned counter-rotating vortices and may be steady or traveling, depending on the perturbation source. We consider the boundary layer on a concave wall with constant radius of curvature and expose the flow to two different disturbance sources: streamwise-localized, spanwise-sinusoidal roughness elements and free-stream vortical disturbances modeled by continuous-spectrum modes for the Blasius inflow. Results from numerical simulations using the Spectral Element Method (SEM) will be shown. The SEM provides spectral accuracy while allowing for geometries beyond the scope of global spectral methods based on Fourier modes. Owing to the non-parallel nature of the G\"{o}rtler vortices, three-dimensional simulations are in particular appropriate to characterize the receptivity of the G\"{o}rtler boundary layer. [Preview Abstract] |
Sunday, November 22, 2009 8:26AM - 8:39AM |
AP.00003: Boundary-layer transition in the wake of surface irregularities Jeffrey Crouch, Vladimir Kosorygin, Lian Ng Aerodynamic surfaces designed for laminar flow inevitably have geometric imperfections. These imperfections impact the unsteady processes in the boundary layer and may accelerate the laminar-turbulent transition. An experimental study is conducted to investigate the steady and unsteady disturbances in the wake of protruding and recessed surface irregularities, and to link these disturbances to the initial movement of the transition location. The steady disturbance field on the centerline of the irregularity is characterized by a region of velocity deficit followed by a much longer region of velocity surplus. Unsteady disturbances in the wake of the irregularity, measured prior to transition, have increased magnitudes and display a shift toward higher frequency. Local stability analysis is shown to capture many of the features of the pre-transitional flow. The transition Reynolds numbers collapse reasonably well when plotted in terms of the roughness height (non-dimensionalized by the boundary-layer displacement thickness). The initial movement of the transition can be represented by a reduction in the critical N-factor, consistent with a linear-amplitude based transition criterion. [Preview Abstract] |
Sunday, November 22, 2009 8:39AM - 8:52AM |
AP.00004: Calculating boundary layer receptivity to transiently growing roughness-induced perturbations using experimental data Nicholas Denissen, Edward White The receptivity problem is of great interest in perturbations generated by surface roughness. To quantify non-modal receptivity, continuous spectrum amplitude distributions are calculated for transiently growing roughness-induced perturbations in a flat-plate boundary layer. Complex, realistic, surface roughness is beyond the scope of direct numerical simulation (DNS) currently. This makes analyzing the receptivity of experimental results essential. A method using regularizing functionals is shown for calculating the distributions when only partial experimental data is available. These results are validated against DNS results. These amplitude distributions provide a way of rigorously characterizing the boundary layer receptivity to surface roughness. Extracting the continuous spectrum amplitudes using the partial data technique reveals the underlying vortex behavior that creates transient growth that is too difficult to measure experimentally. The method described is amenable to future work with realistic distributed roughness and complex surface geometries, and is applied to cases currently beyond the scope of DNS. [Preview Abstract] |
Sunday, November 22, 2009 8:52AM - 9:05AM |
AP.00005: Control of Stationary Crossflow Modes in a Supersonic Boundary Layer using Distributed Roughness Chan-Yong Schuele, Eric Matlis, Thomas Corke, Stephen Wilkinson, P. Balakumar, Lewis Owens Passive methods like distributed micron sized roughness elements have proven to work efficiently as subsonic laminar flow control devices. Attempts to experimentally extend the principle of suppression of the most amplified stationary cross flow modes to supersonic boundary layers have not been successful until now. This study presents evidence for the receptivity of a supersonic boundary layer with transition dominated by stationary cross flow modes to patterned roughness with different wave numbers. Experiments have been performed at the Mach 3.5 NASA LaRC Supersonic Low Disturbance Tunnel on a $7\deg$ half angle sharp cone at $4.3\deg$ angle of attack and a unit Reynolds number of $2.5x10^5in^{-1}$. Pitot tube pressure measurements as well as surface flow visualization were used to detect the occurence of stationary crossflow modes and transition. Based on these two measurement approaches we conclude that the stationary cross-flow mode was receptive to the passive patterned roughness, indicating that control of transition to turbulence in cross-flow dominated conditions should be possible. [Preview Abstract] |
Sunday, November 22, 2009 9:05AM - 9:18AM |
AP.00006: Effect of Surface Thermal Perturbations on Compressible Boundary Layer Stability Christopher Alba, Datta Gaitonde High-speed laminar-turbulent boundary layer transition is a critical issue for re-entry and sustained hypersonic cruise vehicles. Turbulent wall heating rates can increase several orders of magnitude compared to laminar rates and skin friction drag can become a major component of the overall drag. We analyze approaches to modulate transition by altering the stability features of the boundary layer through the use of thermal perturbations. To this end, high-fidelity numerical simulations to generate basic states for Mach 1.5 and Mach 5.6 flat plate boundary layers with and without thermal bumps. Linear Parabolized Stability Equations (PSE) are solved using the STABL software suite to establish the flow stability characteristics under baseline (no excitation), constant and pulsed bump cases for each freestream Mach number. The effects are described in terms of neutral curves showing amplification for various frequencies versus Reynolds number. The three-dimensional flow structure is also examined near the breakdown to turbulence flow region to gain insight into the final stages of transition. [Preview Abstract] |
Sunday, November 22, 2009 9:18AM - 9:31AM |
AP.00007: Experiments and NPSE of roughness receptivity in swept-wing boundary layers William Saric, Matthew Woodruff, Helen Reed New data are presented on 3-D boundary-layer receptivity to roughness in low-disturbance environments. The measurements include infra-red thermography with calibrated and temperature-compensated hotfilms to study roughness-related issues of boundary-layer transition in flight. A swept-wing model is mounted on the wing of a Cessna O-2 aircraft where nonlinear parabolized stability equations (NPSE) correlate the stability measurements and transition locations. The laminarization scheme of spanwise-periodic discrete roughness elements (DRE) is investigated at chord Reynolds numbers of 7.5 million. Flight experiments were conducted where the surface roughness amplitude was varied from 6 to 50 microns while the disturbance shear-stress was measured with calibrated hotfilm gauges in two locations: $x/c $= 15{\%} and 30{\%}; the former in the linear range and the later in the nonlinear range. In this way, the disturbance velocity amplitude was calculated as a function of roughness Reynolds number. These data were then used as initial conditions for the NPSE calculations to determine the efficacy of the DREs. The work was supported by: AFOSR Grant FA9550-05-0044, AFRL, and NASA Langley Research Center. [Preview Abstract] |
Sunday, November 22, 2009 9:31AM - 9:44AM |
AP.00008: Uncertainty quantification of the instability in a supersonic boundary layer with roughness Olaf Marxen, Gianluca Iaccarino, Eric Shaqfeh Knowledge of the location of laminar-turbulent transition on the surface of vehicles (re-)entering a planetary atmosphere is important for heat-shield design. However, due to the heat-shield material itself or as a result of ablation during flight, the surface of a heat shield is often not smooth. Instead, surface roughness occurs, but the height of this roughness may not be known beforehand. A numerical investigation of disturbance amplification in a laminar compressible flat-plate boundary layer with a localized 2-D roughness is carried out. Both linear and weakly non-linear disturbance evolution are considered. The non-linear case exhibits a secondary subharmonic resonance. In addition to deterministic simulations, a stochastic approach is applied to quantify uncertainties. The random parameter is chosen to be the height of the roughness in the linear case, while in the non-linear case the amplitude of the primary disturbance is considered a random parameter. Deterministic simulations show that the 2-D roughness acts as an amplifier for convective disturbances, and the resulting increased disturbance amplitude can enhance a secondary instability. The stochastic approach allows to quantify the probability for an increased or decreased amplification. [Preview Abstract] |
Sunday, November 22, 2009 9:44AM - 9:57AM |
AP.00009: ABSTRACT WITHDRAWN |
Sunday, November 22, 2009 9:57AM - 10:10AM |
AP.00010: Classification of the Flow Produced by an Oscillating Fence in a Laminar Boundary Layer Michael Hind, William Lindberg, Jonathan Naughton Flow visualization has revealed that an oscillating fence produces a range of vortical structures in a flat plate laminar boundary layer. The structure can be classified by the ratio $\phi_0$ of the fence oscillation frequency to the fundamental shedding frequency of the static fence. Particle image velocimetry was used to quantitatively investigate the flow structures of each classification regime. Fences operating in the subcritical flow regime ($\phi_0<0.1$) shed vortices due to vortex saturation behind the fence. The vortices of the critical flow regime ($\phi_0 \sim 1$) strengthen during the fence upstroke and are forced to shed once the fence begins to descend. The vortices of the supercritical flow regime ($\phi_0>1$) are shed once per fence oscillation cycle and coalesce to form larger vortices at the fundamental shedding frequency of the static fence. For the transitional flow regime ($\phi_0 \sim0.1-1$), the structures are two-dimensional during the fence upstroke that become three-dimensional once the fence begins to descend. Through this classification system, it is possible to determine the frequency required for a given flow to produce the desired type of structure. By varying the fence frequency, the structure can be made to change dramatically. [Preview Abstract] |
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