Observations on streamwise vortices in laminar and turbulent boundary layers

The frequent but often unsuspected presence of streamwise vortices in nominally two dimensional laminar and turbulent boundary layers and some of their consequences are described. Since there is no body of systematic information on streamwise vortices imbedded in boundary layers, a number of issues concerning their occurrence and behavior are discussed in the form of a set of succinct observations. Desirable experimental and numerical research to remedy the current lack of knowledge is recommended

Bypass transition to turbulence and research Desiderata

Bypass transitions are seldom mentioned in texts or meetings on instability and transition to wall turbulence. The nature of a number of bypass transitions is illustrated by example. Until this characteristics is truly understood predicting transition on the basis of theory or statistically inadequate correlations (as they all are) entails risks that should be considered in justifying any design involving transition. A historical overview of bypass transition identified on blunt bodies is given

On the question of instabilities upstream of cylindrical bodies

In an attempt to understand the unsteady vortical phenomena in perturbed stagnation regions of cylindrical bodies, a critical review of the theoretical and experimental evidence was made. Current theory is revealed to be incomplete, incorrect, or inapplicable to the phenomena observed experimentally. The formalistic approach via the principle of exchange of instabilities should most likely be replaced by a forced-disturbance approach. Also, many false conclusions were reached by ignoring that treatment of the base and perturbed flows in Hiemenz coordinate eta is asymptotic in nature. Almost surely the techniques of matched asymptotic expansions are expected to be used to capture correctly the diffusive and vorticity amplifying processes of the disturbances regarding the mean-flow boundary layer and outer potential field as eta and y/diameter approach infinity. The serious uncertainties in the experiments are discussed in detail

Report of conference evaluation committee

A general classification is made of a number of approaches used for the prediction of turbulent shear flows. The sensitivity of these prediction methods to parameter values and initial data are discussed in terms of variable density, pressure fluctuation, gradient diffusion, low Reynolds number, and influence of geometry

Optimal streaks in a Falkner-Skan boundary layer

This paper deals with the optimal streaky perturbations (which maximize the perturbed energy growth) in a wedge flow boundary layer. These three dimensional perturbations are governed by a system of linearized boundary layer equations around the Falkner-Skan base flow. Based on an asymptotic analysis of this system near the free stream and the leading edge singularity, we show that for acute wedge semi-angle, all solutions converge after a streamwise transient to a single streamwise-growing solution of the linearized equations, whose initial condition near the leading edge is given by an eigenvalue problem first formulated in this context by Tumin (2001). Such a solution may be regarded as a streamwise evolving most unstable streaky mode, in analogy with the usual eigenmodes in strictly parallel flows, and shows an approximate self-similarity, which was partially known and is completed in this paper. An important consequence of this result is that the optimization procedure based on the adjoint equations heretofore used to define optimal streaks is not necessary. Instead, a simple low-dimensional optimization process is proposed and used to obtain optimal streaks. Comparison with previous results by Tumin and Ashpis (2003) shows an excellent agreement. The unstable streaky mode exhibits transient growth if the wedge semi-angle is smaller than a critical value that is slightly larger than $\pi/6$, and decays otherwise. Thus the cases of right and obtuse wedge semi-angles exhibit less practical interest, but they show a qualitatively different behavior, which is briefly described to complete the analysis

Etude DNS de la transition déclenchée par rugosité à Mach 6

International audienceIn hypersonic flows, it is useful to be able to trip the transition to turbulence upstream of air intakes for example. In the present work, direct numerical simulations have been performed of the flow past an isolated roughness element at Mach 6. First, the capability of two solvers to compute laminar and transitional flow involving freestream disturbances was demonstrated. A series of simulations was then carried out without acoustic perturbation of the freestream. The Reynolds number was increased from 14,000, to 28,000 and then to 40,000. The first two cases remain laminar within the computational domain, whereas the last case undergoes a self-sustained transition to turbulence. A response to a perturbation impulse shows the presence of a varicose mode at the intermediate Reynolds number and a sinuous mode at the largest Reynolds number.Dans les écoulements hypersoniques, il est nécessaire de pouvoir déclencher la transition vers la turbulence en amont des prises d'air par exemple. Dans cette étude, des simulations numériques directes de l'écoulement autour d'une rugosité isolée à Mach 6 ont été réalisées Dans un premier temps la capacité de deux solveurs à prédire l'écoulement laminaire ainsi que transitionnel impliquant des perturbations acoustiques a été démontrée. Une série de simulations a ensuite été réalisée à Mach 6 avec perturbations acoustiques. Le nombre de Reynolds a été augmenté de 14,000 à 28,000 puis à 40,000. Les deux premiers cas demeurent laminaires, alors que le dernier cas expérience une transition auto-entretenue vers la turbulence.Une étude de la réponse à une perturbation impulsionnelle montre la présence d'une instabilité variqueuse au nombre de Reynolds intermédiaire, et une instabilité sinueuse au plus fort nombre de Reynolds

Investigation of three-dimensional shock wave/turbulent-boundary-layer interaction initiated by a single fin

Three-dimensional shock wave/turbulent-boundary-layer interaction of a hypersonic flow passing a single fin mounted on a flat plate at a Mach number of five and unit Reynolds number 3.7×10^7 was conducted by a large-eddy simulation approach. The performed large-eddy simulation has demonstrated good agreement with experimental data in terms of mean flowfield structures, surface pressure distribution, and surface flow pattern. Furthermore, the shock wave system, flow separation structure, and turbulence characteristics were all investigated by analyzing the obtained large-eddy simulation dataset. It was found that, for this kind of three-dimensional shock wave/turbulent-boundary-layer interaction problem, the flow characteristics in different regions have been dominated by respective wall turbulence, free shear layer turbulence, and corner vortex motions in different regions. In the reverse flow region, near-wall quasi-streamwise streaky structures were observed just beneath the main separation vortex, indicating that the transition of the pathway of the separation flow to turbulence may occur within a short distance from the reattachment location. The obtained large-eddy simulation results have provided a clear and direct evidence of the primary reverse flow and the secondary separation flow being essentially turbulent

Reflected Shock Tunnel Noise Measurement by Focused Differential Interferometry

A series of experiments is conducted where a quantitative non-intrusive optical technique is used to investigate disturbances in the free-stream of T5, the free-piston driven reflected shock tunnel at Caltech. The optical technique, focused laser differential interferometry (FLDI), measures fluctuations in density. In the test matrix, reservoir enthalpy is varied while the reservoir pressure is held fixed. The results show the perturbations in density are not a strong function of the reservoir enthalpy. During one experiment, exceptional levels of noise were detected; this unique result is attributed to non-ideal operation of the shock tunnel. The data indicate that rms density fluctuations of less than 0.75% are achievable with attention to tunnel cleanliness. In addition, the spectral content of density fluctuation does not change throughout the test time

Transition Prediction in Hypersonic Boundary Layers Using Receptivity and Freestream Spectra

Boundary-layer transition in hypersonic flows over a straight cone can be predicted using measured freestream spectra, receptivity, and threshold values for the wall pressure fluctuations at the transition onset points. Simulations are performed for hypersonic boundary-layer flows over a 7-degree half-angle straight cone with varying bluntness at a freestream Mach number of 10. The steady and the unsteady flow fields are obtained by solving the two-dimensional Navier-Stokes equations in axisymmetric coordinates using a 5th-order accurate weighted essentially non-oscillatory (WENO) scheme for space discretization and using a third-order total-variation-diminishing (TVD) Runge-Kutta scheme for time integration. The calculated N-factors at the transition onset location increase gradually with increasing unit Reynolds numbers for flow over a sharp cone and remain almost the same for flow over a blunt cone. The receptivity coefficient increases slightly with increasing unit Reynolds numbers. They are on the order of 4 for a sharp cone and are on the order of 1 for a blunt cone. The location of transition onset predicted from the simulation including the freestream spectrum, receptivity, and the linear and the weakly nonlinear evolutions yields a solution close to the measured onset location for the sharp cone. The simulations over-predict transition onset by about twenty percent for the blunt cone

N-acetylcysteine relieves neurologic signs of acute ethanol hangover in rats

Orally administered NAC before acute ethanol intoxication led to a decrease in the severity of neurological deficiency in rats and reduced the amnesic effect of ethanol. This could be due to an improvement of ethanol metabolism and a decrease in the severity of disorders associated with oxidative stress and liver dysfunctio