Dirk Heitmann

Simulation of the interaction of a laser generated shock wave with a hypersonic conical boundary layer

Flow simulations with the DLR-TAU code of the interaction of a laser induced disturbance with a conical boundary layer at Mach 5.85 are shown. The simulations start 0.2μs after an optical breakdown from an initial disturbance determined from Taylor’s intense explosion similarity law and this initial disturbance is imposed on the flow field of a 7half-angle cone. The initial disturbance parameters such as energy were obtained from comparisons to experimental data. The disturbance was imposed some distance above the model and the boundary layer so that mainly the propagating shock wave interacts with the boundary layer. This interaction results in the formation of a second mode wave packet and its generation and development are described. Furthermore, the influences of disturbance intensity and excitation with single and double laser pulses are treated and a comparison to experimental data is given.

Investigation of the response of a hypersonic 2D boundary layer to controlled acoustic disturbances

Surface mounted high-frequency pressure sensors were used to investigate the response of a hypersonic 2D boundary layer to acoustic disturbances. The disturbances were generated by focussing a high-power laser slightly above the model surface. When a critical intensity is exceeded, a plasma is produced and a spherical shock wave propagates from this point. Since the disturbance is rather weak, the shock wave changes into an acoustic wave after a few microseconds. Measurements of the interaction of this disturbance with the boundary layer were made on a smooth wall and on a wall with a 2D roughness. It is shown that the disturbance interacting with a boundary layer results in the formation of a second mode like instability wave, although the amplitudes remain very small. In the presence of a roughness element as receptivity site, the wave’s amplitude increases and the behavior of such instability waves is compared to naturally occurring waves in Mach 6 boundary layers on a flat plate and on a 7-half-angle cone.

Numerical and experimental investigation of laminar-turbulent boundary layer transition on a blunt generic re-entry capsule

Numerical and experimental results on laminar-turbulent transition in the boundary layer of a blunt Apollo-like capsule at 0° and 24° angle of attack are presented. Local stability analyses have been performed and a measurement campaign in the Hypersonic Ludwieg tube Braunschweig at a Mach number of 5.9 was carried out. Infrared thermography showed laminar and transitional surface heating in the unit Reynolds number range of Re_infinity = 6x10^6 /m to Re_infinity = 20x10^6 /m at a surface mean roughness of Ra = 10 micrometer, whereas for a mean roughness of Ra = 0.5 My micrometer no indications for a transitional boundary layer was noted. PCB and Kulite sensors used to measure pressure fluctuations inside the boundary layer do not show any peaks in the frequency spectra which might be related to boundary layer disturbances. The only relevant peak in the spectra does not change with unit Reynolds number and is currently attributed to a bow shock oscillation. Consistent with the experimental findings, the modal instability analysis does not provide any modal boundary layer disturbance growth at windtunnel conditions. Therefore, a scaling ansatz for the laminar boundary layer is introduced and evaluated in order to estimate the unit Reynolds numbers required for the onset of modal disturbance growth on Apollo-like capsules. Results for both first-mode and cross-flow instability are presented.

Investigation of Laser-Generated Flow Perturbations in Hypersonic Flow over a Flat Plate

Controlled localized perturbations are generated to analyze the transition mechanism in hypersonic flow. The experiments are performed at the Hypersonic Ludwieg Tube (HLB) at Technische Universitat Braunschweig on a flat plate model. The perturbations are gen- erated optically with lasers. Four lasers are used to generate a wave packet. Parameters such as frequency and amplitude can be varied. The position can be changed as well. Furthermore this method offers the advantage of being non-intrusive, i.e. no roughness element is created. This paper describes the setup of the perturbation system. Afterwards measurements of transition are described by means of infrared thermography and mea- surements of second mode instability waves in the boundary layer using high-frequency pressure sensors. Finally measurements with application of the perturber are presented. Here parameters like disturbance shape and single/multi-pulse perturbations are varied and their influence on the flow pattern is described.

Investigation of Laser Generated Perturbations for Boundary Layer Stability Experiments

This paper describes boundary layer stability experiments with controlled perturbations at hypersonic flow around a flat plate. Laser pulses are used to generate pressure and temperature disturbances. By using multiple laser pulses a wave packet with adjustable amplitude and frequency can be created. The development of such artificial waves is measured with high frequency pressure sensors. The possibility to generate and excite second mode instability waves with this setup is demonstrated.

Hypersonic Ludwieg Tube

Top-level requirements for a hypersonic research facility call for a Mach number high enough that typical hypersonic flow behaviors are achieved for blunt and slender configurations. Furthermore, in order to enable testing of laminar and turbulent flows, the facility should provide a Reynolds-number of 20 Million, based on the model length. The test time should be in the order of 100 ms in order to allow for a range of useful flow measurement techniques. Continuous progress in wind tunnel engineering and in instrumentation available for flow measurements in hypersonics over the last decades has resulted in a specific wind tunnel configuration that meets these requirements. This is the hypersonic Ludwieg tube. Several of these gas dynamics facilities are in operation worldwide. The present contribution describes one of the successful designs of this kind, along with data that characterize the quality and capabilities of this facility.