A typical challenge for re-entry vehicles during flight is an effective control of hypersonic transition and the laminar/turbulent state of the boundary layer. The state of the boundary layer is of high importance since skin friction drag and heat transfer rates in a turbulent boundary layer can be several times higher than those of a laminar one. A lot of different strategies are used to delay or prevent the transition process. One possibility to manipulate the transition is the use of porous surfaces to influence the growth of the second mode in a passive way. The second mode or so called Mack mode is the dominant mode for the transition process at hypersonic Mach numbers .
Figure 1 shows the first practical result on this working field: A wind tunnel experiment of a 5° sharp cone of Rasheed . The figure in the top on the right side shows transition over a smooth wall while over a porous surface a laminar flow can be seen (right side).
The DLR uses two simulation techniques to investigate this effect numerically: Direct numerical simulation, which is performed DLR FLOWer 4th order code that provides complete solutions for flow over and in the pores, and a stability code, called NOLOT, which is a low-cost method to predict the growth rates of the Mack Modes. Figure 2 shows a boundary layer flow at Mach 6: On the left the normal velocity over a smooth wall can be seen while the right figure shows the reduction of the Mack mode amplitude due to the pore effect by absorbing a part of the disturbance energy. This is visible by comparing the legends of both figures: The values for calculations with pores are one order of magnitude smaller.
For these cases (smooth wall / porous wall with 16 pores) figure 3 shows the eigenfunction shape of the perturbation calculated with DNS in comparison with the stability code NOLOT. In both cases a good agreement is visible.
To illustrate the whole porous wall effect a movie of the boundary layer flow at Mach 6 over a porous wall with 8 pores is generated: The Mack modes are moving over the porous wall. The pores absorb a part of the disturbance energy and thus the growth of the modes can be reduced.
A generic wind tunnel model has been built in the scope of the internal DLR research project IMENS-3C to investigate the potential of a porous carbon/carbon material to delay hypersonic boundary layer transition. The model is a 7 degree half-angle blunted cone with an overall nominal length of 1100 mm and an exchangeable nose tip. It is equipped with an insert supporting an ultrasonically absorptive carbon fibre reinforced carbon (C/C) material with a natural porosity. The below figure provides an image of the model in the manufacturing process in combination with a microscopic view of the porous surface.
The model was designed and built in cooperation with the DLR Institute of Structures and Design in Stuttgart providing the C/C material. The fully instrumented model was tested at Mach 7.4 in the High Enthalpy Shock Tunnel Göttingen (HEG).
The extensive studies revealed a damping effect on the high frequency instabilities in the boundary layer leading to a significant delay of the transition onset. A wavelet analysis of the time resolved boundary layer fluctuation measurements confirm the damping of the second mode instabilities above the porous surface.
1 Mack, L.M. : "Boundary layer linear stability theory". AGARD Special course on stability and transition of laminar flow, 1984.
2 Rasheed, A., Hornung, H.G., Fedorov, A.V., Malmuth, N.D.: "Experiments on passive hypervelocity boundary-layer control using an ultrasonically absorptive surface". AIAA Journal, Vol. 40, No. 3, pp. 481-489, 2002.
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Futher Publications by Viola Wartemann and Alexander Wagner .