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 [1].

Figure 1 shows the first practical result on this working field: A wind tunnel experiment of a 5° sharp cone of Rasheed [2]. 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).

Figure 1: Hypersonic transition at a Mach 5 conical flow [2]

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.

Figure 2: DNS (contours of the normal velocity): (a) Mack mode developing above a smooth wall and (b) a porous surface with a porosity of 0.25 (16 pores) at the same time step.

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.

Figure 3: Eigenfunction shape of the wall normal disturbance velocity: (a) smooth wall and (b) porous wall with 16 pores.

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.

Figure 4

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.

Figure 5: Photographic view of the model during manufacturing and microscopic view of the porous material

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.

Figure 6: Wavelet analysis showing 2nd mode activities above the model 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.

Publication and Conferences:

Sandham, N.D., Lüdeke, H.: "A numerical study of Mach 6 boundary layer stabilization by means of a porous surface," AIAA-Journal Vol. 47, No. 9, September 2009

Wartemann, V., Lüdeke, H., Sandham, N.D.: "Stability analysis of hypersonic boundary layer flow over microporous surfaces", 16th AIAA / DLR / DGLR International Space Planes and Hypersonic Systems and Technologies Conference, Bremen, Germany, 19-22 October 2009

Lüdeke, H.; Wartemann, V.; Sandham, N.: "Investigation of Transition Control by Porous surfaces in Hypersonic Boundary Layers", New Results in Numerical and Experimental Fluid Mechanics VII, Vol. 112, Springer-Verlag, 2010

Wartemann, V.; Lüdeke, H.: "Investigation of Slip Boundary Conditions of Hypersonic Flow over microporous Surfaces", V European Conference on Computational Fluid Dynamics, ECCOMAS CFD, Lisbon, Portugal, 2010

Wartemann, V.; Lüdeke, H.; Willems, S.; Gülhan, A.: "Stability analyses and validation of a porous surface boundary condition by hypersonic experiments on a cone model", 7th Aerothermodynamics Symposium, Brugge, Belgium, 9 - 12 May, 2011

Wagner, Alexander und Laurence, Stuart und Martinez Schramm, Jan und Hannemann, Klaus und Wartemann, Viola und Lüdeke, Heinrich und Tanno, Hideyuki und Ito, Katsuhiro (2011) Experimental investigation of hypersonic boundary-layer transition on a cone model in the High Enthalpy Shock Tunnel (HEG) at Mach 7.5. 17th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, 11. - 14. Apr. 2011, San Francisco, USA. DOI: 10.2514/6.2011-2374

Wagner, Alexander und Hannemann, Klaus und Wartemann, Viola und Tanno, Hideyuki und Ito, Katsuhiro (2012) Free piston driven shock tunnel hypersonic boundary layer transition experiments on a cone configuration. RTO-MP-AVT-200: Hypersonic Laminar-Turbulent Transition , 16. - 19. April 2012, San Diego, California, USA

Wagner, Alexander und Hannemann, Klaus und Kuhn, Markus (2012) Experimental investigation of hypersonic boundary-layer stabilization on a cone by means of ultrasonically absorptive carbon-carbon material. 18th AIAA/3AF International Space Planes and Hypersonic Systems and Technologies Conference, 24. Sept. - 28. Sept. 2012, Tours, France. DOI: 10.2514/6.2012-5865

Wagner, Alexander und Hannemann, Klaus und Eggers, Thino (2012) Recent Aerothermodynamic Studies in the High Enthalpy Shock Tunnel Göttingen (HEG). International Symposium on Hypersonic Aerothermodynamics - Recent Advances, 09. - 13. Dec. 2012, Bangalore, India

Wagner, Alexander und Wartemann, Viola und Hannemann, Klaus und Giese, Tobias (2013) Hypersonic boundary-layer stabilization by means of ultrasonically absorptive carbon-carbon material, Part 1: Experimental Results. 51st AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 07. - 10. Jan. 2013, Grapevine, Texas, USA. DOI: 10.2514/6.2013-270

Wagner, Alexander und Wartemann, Viola und Hannemann, Klaus und Eggers, Thino (2013) Experimental Investigations of Hypersonic Boundary-Layer Stabilization by means of Porous Surfaces. Texas A&M University Seminar, 11.01.2013, College Station, Texas, USA

Wagner, Alexander und Wartemann, Viola und Kuhn, Markus und Martinez Schramm, Jan und Hannemann, Klaus (2013) Experimental investigation of hypersonic boundary layer transition delay by means of ultrasonically absorptive carbon-carbon material in the High Enthalpy Shock Tunnel Göttingen (HEG). Lecture Series on high-speed laminar-turbulent transition, 30.-31.05.2013, Brüssel, Belgien