Heinrich Lüdeke

Numerical Study of Mach 6 Boundary-Layer Stabilization by Means of a Porous Surface

Direct numerical simulations are carried out of boundary layer flow at Mach 6 over a porous surface, in which a Mack mode of instability is excited. The pores are resolved rather than modeled, allowing an evaluation to be made of the accuracy of simplified analytical models used in previous investigations based on linear stability theory. It is shown that the stabilizing effect of porosity is stronger in the simulations than in the corresponding theory for both two- and three-dimensional pores. From comparisons of spanwise grooves, streamwise slots and square pores it appears that the detailed surface structure is not as important as the overall porosity and the hydraulic diameter is able to collapse the results for different pore shapes to good accuracy. When the porous surface consists of fewer larger pores, the flow is noisier, with sound waves generated at the pore edges. Also the influence of a random distribution of 2D grooves was investigated and the former statements were shown to remain valid. Finally the influence of rarefied gas behavior of the flow inside the pores was taken into account by introducing a slip-boundary condition with finite Knudsen number. The additional damping effect at moderate Knudsen numbers, investigated by other groups, was confirmed.

Time-Resolved Visualization of Instability Waves in a Hypersonic Boundary Layer

LAMINAR-TURBULENT transition in hypersonic boundary layers remains a challenging subject. This is especially true of the hypervelocity regime, in which an intriguing phenomenon is the possible damping of second-mode disturbances by chemical and vibrational nonequilibrium processes. To generate flows with sufficiently high enthalpy to investigate such effects, the use of shock-tunnel facilities is necessary; furthermore, it is now generally accepted that direct measurements of the instability mechanisms active within the boundary layer, together with a characterization of the freestream disturbance environment, are required, as simple measurements of transition locations can lead to ambiguous conclusions. However, as difficult as the accurate measurement of instability waves in conventional hypersonic facilities can be, in shock tunnels it is appreciably more so. For identical unit Reynolds numbers, the higher stagnation temperature in a shock tunnel means that the dominant second-mode disturbances lie at even higher frequencies (typically hundreds of kHz or higher); moreover, because of the destructive testing environment, hot-wire techniques, a staple for instability measurements in conventional tunnels, cannot be used. Fast-response pressure transducers are an obvious alternative, but recent experiments have highlighted the challenging nature of interpreting data from mechanically sensitive sensors in the high-noise environment of a shock tunnel, especially without accompanying stability computations. Measurements with recently developed atomic-layer thermopile (ALTP) heat-flux sensors show promise, though their use has yet to be demonstrated in shocktunnel facilities.

Numerical Investigation of Hypersonic Boundary-Layer Stabilization by Porous Surfaces

The acoustic second-mode instability predicted by linear stability theory is compared with direct numerical simulation for a hypersonic flow over various porous walls. The damping effect of the micropores on the second mode is shown by comparison of the two different approaches. In addition to investigating the effect of pore size, the influence of the pore shape is studied by using spanwise grooves and cylindrical pores. Specifically, the comparability of different pore shapes by two definitions of hydraulic diameter is analyzed. The influence of rarefied gas behavior of the flow inside the pores is also investigated by comparing a slip boundary condition with finite Knudsen numbers with a nonslip boundary condition for different radii and pore depths.

On the flow physics of a low momentum flux ratio jet in a supersonic turbulent crossflow

A Detached Eddy Simulation (DES) study of a low momentum flux ratio jet, J=(ρu2)jet/(ρu2)∞=0.35, in the HyShot II scramjet system is carried out. The flow structure near the injector, shock pattern in the symmetry plane as well as the instantaneous coherent structures are presented and explained in the paper. Different from most previous studies, over-expanded and under-expanded flow states occur simultaneously at the exit of the current jet porthole. The shock system near the injector is therefore a combination of a detached normal shock and a small three-dimensional barrel shock terminated by a Mach disk. Further insight into the flow physics is conducted by visualizing instantaneous coherent structures. The formation of Ω-shaped vortices, which was observed in experiments previously, but never well-studied numerically, is discussed in detail. A new understanding of the key flow physics and mixing patterns in the low momentum flux ratio jet in supersonic crossflow is finally schematically provided.

Stability analysis of hypersonic boundary layer flow over microporous surfaces

Hypersonic boundary-layer-transition is dominated by so-called Mack modes, second mode instabilities that can be damped passively by acoustic absorptive coatings. Those coatings are realised in practise by porous walls. In the present paper a second mode stability analysis is performed for a boundary layer flow at Mach 6 over a smooth wall and various porous walls. The influence of the porosity, the radius and the thickness of the pores is investigated. For this study three dierent codes are used: The linear stability code SLST of the University of Southampton in comparison with the DLR linear stability code NOLOT and finally direct numerical simulations including the modelling of the pores through the wall. Good agreement for a smooth wall as well as for dierent porous wall cases is demonstrated.

Launch Vehicle Base Flow Analysis Using Improved Delayed Detached Eddy Simulation

During the ascent of carrier rockets, the massively separated flow in the base region generates highly unsteady wall pressure fluctuations. The resulting dynamic loads can induce a response of the launcher structure called buffeting. To study the characteristics of the base flow behind three different configurations of the Ariane 5 launcher, improved delayed detached-eddy simulation in combination with unstructured grids is used. The objective of this work is to demonstrate the ability of improved delayed detached-eddy simulation to describe the impact of the connecting struts between central stage and boosters and to analyze the efficiency of a buffeting reduction device in the base region. Comparisons of the numerical results with time-resolved and time-averaged surface pressure data and particle-image-velocimetry measurements obtained from the German–Dutch High Speed Wind Tunnel are carried out in good agreement.

Numerical Investigation of Transition Control by Porous Surfaces in Hypersonic Boundary Layers

The present numerical investigation of the effect of porous surfaces on transition in hypersonic boundary layers is intended to improve understanding of the physical mechanisms and to provide numerical tools for the prediction of the associated delay in transition. Direct numerical simulations are carried out by a 4th order version of the DLR-Flower code, compared with the results of linear stability theory. Good agreement of both approaches and an accurate prediction of the damping of the Mack-mode instability which is responsible for supersonic transition is demonstrated.

Direct Numerical Simulation of Tollmien-Schlichting Waves to Support Linear Stability Analysis

In the present study direct numerical simulations (DNS) of Tollmien-Schlichting waves in attached boundary layers will be shown in comparison with local linear stability theory (LST). The goal of the investigation is a simulation of such modes in separated flow where LST is limited due to its basic approach. The method provides an improved understanding of the physical mechanisms behind the transition scenario in the linear as well as in the non-linear regime.

Direct Numerical Simulation of the Transition Process in a Separated Supersonic Ramp Flow

Despite intensive theoretical and experimental research, transition to turbulence in separated hypersonic ramp flows is still a challenge to predict. One of the most successful approaches to model the dominant mechanisms is direct numerical simulation, which has demonstrated the capability to generate reliable datasets. In the present study, different supersonic test cases are chosen from literature and compared with simulations using a high-order version of the DLR FLOWer code. The results using higher-order Pade-filter approaches are encouraging. With this validation as a background, a supersonic ramp case was chosen for study. Different ramp-angles and Reynolds numbers were investigated to determine a transitional test case, for which turbulence can be resolved behind re-attachment. A hypersonic ramp with 12° angle of attack was then chosen for three-dimensional DNS of the transition process. These DNS were carried out with different grid extents in the wall-normal direction and different sweep angles of the incoming flow with good success. Simulations of laminar-turbulent breakdown are presented using various perturbation spectra at the inflow.

Simulation of Streamwise Vortices on Turbulent Hypersonic Ramps

Purpose of this investigation is the simulation of separated turbulent ramp flow as a simple generic model for re-entry vehicle flaps under three dimensional disturbances. Parameter variations of cross flow, disturbance intensity and wave length of the disturbances are tested. Comparison with wind tunnel data of the Ludwieg tube facility in Gottingen (RWG) was used to validate the numerical code and getting a deeper insight into the flow topology. The dependence of the vortices from surface disturbances and a tendency of circular vortex shape at reattachment has been shown experimentally and numerically in good agreement. Finally the damping influence of cross flow in the separation region was investigated.