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The influence of high-temperature gas-effects on hypersonic flat-plate boundarylayer flows is presented in this investigation. Chemical reactions are taken into account and the temperature dependent thermodynamic properties are modelled. The present work investigates for the differences in the spatial disturbance development in a flat-plate boundary-layer flow at M= 20 with ideal-gas properties and for a chemical non-equilibrium case. The comparison of DNS data with linear stability results for the ideal-gas case show good agreement. The non-equilibrium results show a decrease in disturbance levels mostly for the three-dimensional disturbance waves whereas two-dimensional (second mode) disturbances are almost unchanged in their linear amplitudes.

An experimental study is done to confirm the existence of a new instability due to the curvature of external streamlines in a three-dimensional boundary layer. Monochromatic-wave excitation from a tiny hole near the attachment line of a yawed circular cylinder is used to separate unsteady disturbances due to the streamline-curvature in-stability from traveling waves of the cross-flow instability. Experimen-tal results show that a point-source disturbance evolves into a wedge-shaped distribution and that amplitude and phase distributions in the spanwise direction definitely include both modes arising from the two instabilities. Observed characteristics and behavior of those disturbances are shown to be in excellent agreement with the latest results of a linear stability theory based on the complex characteristic method.

Receptivity of the attachment line boundary layer on an infinitely long cylinder, whose axis is inclined relative to the flow direction, to free-stream vortical disturbances is studied. The perturbations are maximal at the upper edge of the boundary layer and its amplitude is proportional to 蜰 . This may be a reason for subcritical laminar-turbulent transition.

Low speed experiments were conducted to study features of crossflow transition on the windward surface of a wing swept at 40°. Surface pressure and wall shear stress fluctuation measurements and flow visualization by chemical sublimation conducted at different incidence angles revealed both stationary vortices and TS waves. Interesting results from the hot-film gages are discussed.

Direct Numerical Simulations (DNS) of transitional boundary layer separation with and without background fluctuations have been performed. In the absence of explicitly added free-stream fluctuations, unstable Kelvin-Helmholtz (KH) modes are found to be triggered by small scale numerical truncation error. Addition of uniformly distributed free-stream fluctuations causes a stronger triggering of unstable KH modes such that the location of transition moves upstream. When replacing the uniformly distributed free-stream fluctuations by fluctuations concentrated in periodically passing wakes the location of transition is found to move alternately upstream and downstream. Compared to the uniformly distributed fluctuations, the large scale fluctuations carried by the wakes are much more effective in reducing the size of the separation bubble.

The effect of a systematic variation of the sweep angle on the disturbance amplification and onset of transition is studied in a generic family of swept laminar separation bubbles (LSB) by means of direct numerical simulation. The detailed analysis of a transition scenario with fundamental resonance in a 30°-LSB shows, that the saturation of background disturbances is the key event, after which a rapid breakdown of transitional structures to smaller scales and thus turbulent flow occurs. The stages of transition are similar to unswept LSB, but two-dimensional disturbances lose their dominance for sweep angles larger than 15°. Instead, oblique Tollmien-Schlichting waves which travel approximately along the direction of the potential streamline experience the maximal amplification in the linear stage and stimulate the strongest growth of background disturbances after saturation.

Traditionally, laminar separation bubbles have been characterised as being ‘long’ or ‘short’ on the basis of a two parameter ‘bursting’ criterion involving a pressure gradient parameter and Reynolds Number at separation. In the present work we suggest a refined bursting criterion, which takes into account not just the length of the bubble but also the maximum height of the bubble, thereby shedding some light on the less understood phenomenon of ‘bursting’ in laminar separation bubbles.

In this paper spatial Direct Numerical Simulation is used to study the nonlinear interaction between planar Tollmien Schlichting waves and Goertler Vortices. The results show the development of lambda vortices typical of K-type and Htype break down. Two different structures have been found which are shifted with respect to each other by π in the spanwise direction.

A detailed experimental study on the formation of crossflow vortex mode packets and their secondary instability in a swept wing boundary layer was carried out. As a result, two modes of nonstationary perturbations traveling along the vortices were found.

In the present experimental work, which utilizes a hot-wire visualization method, an influence of external pressure gradient on the development of secondary instabilities in the straight wing boundary layers has been demonstrated for the first time. The comparison of experimental data and direct numerical simulation results of this process has been given.