Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FN: Instability: Boundary Layers I |
Hide Abstracts |
Chair: Patrick Huerre, CRNS Ecole Polytechnique Room: Salt Palace Convention Center 251 B |
Monday, November 19, 2007 8:00AM - 8:13AM |
FN.00001: Receptivity of Three-Dimensional Boundary Layers Lars-Uve Schrader, Luca Brandt, Dan Henningson Transition to turbulence of a boundary layer on swept aircraft wings, for instance, originates in external disturbances acting on the boundary layer and entering into it. These outer disturbances may arise from e.g. surface roughness or from the turbulent motion of the incoming flow upstream of the wing. This DNS study deals with the linear receptivity mechanisms associated with a spanwise periodic, chordwise localized roughness element on the one hand and a single vortical Orr-Sommerfeld free-stream mode on the other hand. Both types of external disturbances have been employed slightly downstream of the leading edge, exciting a steady or a travelling crossflow-instability wave, respectively. The efficiency of the crossflow eigenmode excitation has been measured in terms of a receptivity coefficient. Further, the nonlinear interaction between the roughness-induced disturbance and the disturbance due to the vortical free-stream mode has been investigated. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FN.00002: Computations for laminar flow control in swept-wing boundary layers. Helen Reed, Richard Rhodes, William Saric The laminarization of a swept-wing boundary layer by the introduction of passive spanwise-periodic roughness elements (DRE) near the leading edge is modeled by linear stability theory and nonlinear parabolized stability equations. Studies predict that, for chord Reynolds numbers of 8 million and with an appropriate pressure coefficient design, the crossflow instability can be stabilized and laminar flow achieved. Sensitivity to element placement and height is studied, and it is shown that the optimum location for the control elements is at the Branch I neutral point of the control wavelength. This work serves as a companion to flights tests of a swept-wing model mounted below the wing of a Cessna O-2 aircraft at Texas A and M's Flight Research Laboratory. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FN.00003: Flight experiments on laminar flow control in swept-wing boundary layers. William Saric, Andrew Carpenter, Helen Reed Data are presented on boundary-layer transition to turbulence in low-disturbance environments. The measurements include infra-red thermography 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 an Euler code is used calculate the aircraft flowfield a nonlinear parabolized stability equations correlate the stability measurements and transition locations. The laminarization scheme of spanwise-periodic distributed roughness elements (DRE) is investigated at chord Reynolds numbers of 8 million. Measurements were made to determine the transition locations for clean configurations and transition locations for enhanced surface roughness that simulates an operational surface finish. For clean configurations, natural laminar flow was achieved over 80{\%} of the surface of a 37\r{ } swept-wing model at chord Reynolds numbers of 8.1 million. With a background surface roughness of 1.1 $\mu $m rms, transition moved forward to 30{\%} chord. The DRE moved transition to 60{\%} chord. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FN.00004: Optimal spatial disturbances along the swept attachment line Alan Guegan, Peter Schmid, Patrick Huerre Optimal spatial perturbations are described in the context of swept Hiemenz flow. The two parameters are the Reynolds number $Re$ based on the flow stretching rate and spanwise sweep velocity, and the width $L$ of the chordwise energy weight that defines the region of interest around the attachment line. At Reynolds numbers $Re\sim 500$ typically found in commercial aircrafts optimal spatial disturbances take the shape of counter-rotating spanwise vortices. Energy is amplified by the so-called lift-up mechanism in the spanwise direction. Amplification is stronger at higher Reynolds numbers but is mostly unchanged as $L$ increases, although more counter-rotating spanwise vortices are observed. A physical interpretation is suggested that accounts for the shape of the perturbations and energy amplification scalings with $L$ and the Reynolds number $Re$. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FN.00005: Analysis of the Effect of Hump on the Stability of Boundary Layer by using Parabolized Stability Equations Bing Gao, Seung O. Park A linear PSE is applied to stability analysis of incompressible boundary layer flow over a hump. A spatial relaxed technique is introduced to overcome the marching difficulty as the marching step is required to be rather small to catch the effect of surface roughness on the stability of T-S waves. Results shows that the streamwise velocity profiles of disturbances exhibit the third peak inside the boundary layer as the profiles of mean flow have an inflection point. The third peak can retain a rather long distance after reattachment point. There exists a resonance effect which makes instability waves more unstable when the wavelength of disturbances is comparable to the width of hump. The critical layer plays an important role in deciding whether the hump has a large effect on the stability of T-S waves. Keywords: spatial relaxed, parabolized stability equations, hump, stability analysis, resonance effect, critical layer. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FN.00006: Transient growth in the asymptotic suction boundary layer Thomas Kurian, Jens H.M. Fransson Recent experimental studies on roughness induced transient growth have shown that suboptimal perturbation theory may predict the evolution of energy in the streamwise direction. However there is a fundamental difference between the experimentally generated streaks using roughness elements and the ones arising from the theory. Knowledge in this area is sought after so that transition originating from transient growth may be predicted accurately. In the current experimental investigation we have performed a geometrical parameter analysis in order to study the transient energy growth behind a spanwise array of cylindrical roughness elements. We have chosen to work with the asymptotic suction boundary layer (ASBL), which has the advantage that we may change the boundary layer thickness and the Reynolds number independently. Five different heights of roughness elements were used along with three different free stream velocities. The applied pressure drop across the porous material gave a constant displacement thickness $d = 1.45$ mm. We will present the energy evolution of the individual modes triggered by the roughness array using spatial mode decomposition. Furthermore, the experimental results will be compared with suboptimal perturbation theory on the ASBL. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FN.00007: The Effects of Wall-Permeability on the Asymptotic Suction Boundary Layer Nils Tilton, Luca Cortelezzi Experimental studies have demonstrated that transition to turbulence in boundary layers can be delayed by applying constant suction through spanwise slots, discrete holes, or porous panels on the wall. Linear stability results, however, tend to over predict the stabilizing effects of suction. One possible explanation for this discrepancy is the common assumption that the wall-tangential base-flow and the velocity perturbations vanish at the wall. This study examines the effects of wall-permeability on the linear stability of the asymptotic suction boundary layer by realistically modelling the wall as the interface between a fluid region and a fluid saturated, rigid, homogeneous, isotropic, porous material. As a result, the wall-tangential base-flow and the velocity perturbations no longer vanish at the wall. We restrict our study to sufficiently small permeabilities in order to neglect inertial effects in the porous region and we couple the disturbance fields in the adjacent fluid and porous regions using interface conditions derived by Ochoa-Tapia and Whitaker (1995). We perform a fully coupled, three-dimensional, linear stability analysis of the laminar flow in the fluid/porous domain. We find that the critical Reynolds number which accounts for permeability is lower than those reported in previous linear stability analyses. Hence, the overall stability of the asymptotic suction boundary layer is dictated by the competing effects of suction and wall-permeability. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FN.00008: Flow Instability Induced by Surface Thermal Perturbation Hong Yan, Datta Gaitonde, Joseph Shang A three-dimensional simulation was performed to study the pulsed thermal bump and its induced kinetic response on the flow structures of a Mach 1.5 laminar boundary layer. The pulsed thermal bump is characterized as a surface heating element with $ \Delta T_w=$ 500K and frequency of 100 kHz and duty cycle of 0.5, and is centered on a flat plate to mimic spanwise periodic series of heating element. The spectrum analysis shows that the primary frequency mode dominates, but higher harmonics are observed far downstream of the heating element. The vortex generation depends mainly upon the spanwise velocity variation induced by the thermal bump. The vortex shedding is significant in the pulsed heating, while it is not observed in the steady heating. The generalized inflection points are observed during the heating process for both steady and pulsed heating, while they are not seen in the no heating condition. Results indicate that the spanwise disturbance is a dominant factor in heating induced flow instability. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 9:57AM |
FN.00009: Three-dimensional stability by global modes in the flat plate boundary-layer flow Espen {\AA}kervik, Luca Brandt, Dan S. Henningson The stability of the flat-plate boundary-layer flow is studied by means of three-dimensional eigenmodes of the linearized Navier--Stokes equations obtained by linearization about the steady state. The disturbance variables are approximated using a Fourier--Chebyshev collocation technique in inhomogeneous directions. Given the large size of the generalized eigenvalue problem we employ Arnoldi iterations using \verb|ARPACK|. By expanding the flow disturbance variables in the basis of eigenmodes the growth potential is revealed by the computation of the optimal initial condition. This yields a low-dimensional model of the flow and a unified view on its stability characteristics. We discuss three different mechanisms associated with the non-normality of the operator: The {\it lift-up} mechanism is a componentwise non-normality where momentum is transferred from the spanwise to the streamwise velocity component. The {\it Orr} mechanism provides, through structures leaning against the shear, an efficient way of obtaining short time growth while borrowing energy from the mean flow, transferring momentum from the streamwise component to the wall-normal component. The {\it TS} mechanism is related to the streamwise non-normality where initial disturbances are located upstream and wavepacket propagation leads to a large energy gain downstream. [Preview Abstract] |
Monday, November 19, 2007 9:57AM - 10:10AM |
FN.00010: Experimental Study of Transient Growth Instabilities Jonathan F. Morrison, Philippe Lavoie, Ahmed Naguib Transient growth of instabilities in laminar boundary layers has attracted significant attention in recent years. Theoretical work on the subject provides predictions regarding the evolution of disturbances arising from optimal perturbations. While experiments have provided qualitative verifications of the theory, the non-optimal nature of the disturbances introduced in experiments does not always provide a conclusive validation. An extensive parametric study was undertaken to study the transient growth of instabilities introduced by a spanwise periodic array of roughness elements in a wind tunnel. A distinctive aspect of these experiments is that the effects of Reynolds number, and the roughness height and spacing relative to the boundary layer thickness are investigated independently of each other. A particularly novel and important aspect of the present study is that the three components of the perturbation velocity field generated by the by the roughness elements are measured. This allows a complete quantification of the sub-optimal perturbations present in such experiments and the transient growth of the associated disturbances. The perturbation velocities are presented for optimal and a range of suboptimal roughness distributions. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700