1 44 th AIAA Fluid Dynamics Conference & Exhibit Laminar, transitional and turbulent boundary layers Hypersonic Laminar Flow Control Using a Porous Coating of Random Microstructure A. Maslov 1 , A. Fedorov 2 , A. Shiplyuk 1 , V. Kozlov 2 , A. Sidorenko 1 , P. Polivanov 1 and N. Malmuth 3 1 Institute of Theoretical and Applied Mechanics, Novosibirsk, 630090, Russia maslov@itam.nsc.ru 2 Department of Aeromechanisc and Flight Engineering, Moscow Institute of Physics and Technology, Zhukovski, 140180, Russia afedorov@pt.comcor.ru 3 Rockwell Scientific Company, Thousand Oaks, California 91360, USA nmalmuth@rwsc.com Extended Abstract 1. Introduction For essentially two-dimensional (2-D) hypersonic flows, the initial phase of laminar-turbulent transition is associated with amplification of the second mode. 1,2 This mode belongs to the family of trapped acoustic waves propagating in the wave-guide, which is formed within the boundary layer. 2,3 Fedorov and Malmuth 4,5 assumed that a thin porous coating may suppress the second and higher acoustic modes and, at the same time, may not trip the boundary layer due to roughness of the porous surface. The stability analyses 4,5 showed that such an ultrasonically absorptive coating (UAC) reduces the second-mode growth rate dramatically. Calculations based on the N e -method predicted significant increase of the laminar run due to this stabilization effect. Rasheed et al. 6 tested this concept of passive laminar-flow control in the GALCIT T5 shock tunnel of California Institute of Technology (Caltech). The experiments were carried out at freestream Mach numbers 4.6-6.4 and the total enthalpy 0 4.18 13.34 H ≤ ≤ MJ/kg on a sharp cone, which had half of its surface solid and the other a porous sheet perforated with blind cylindrical holes of 60 µm diameter and 500 µm depth (the average spacing between the holes was 100 µm). The model was instrumented by thermocouples, and the transition loci were was determined from the Stanton number distributions measured simultaneously on both sides of the model for each run. These experiments demonstrated that the UAC of regular microstructure delayed transition by a significant amount that qualitatively confirmed the theoretical predictions. 5 However a quantitative comparison was not feasible because the cone was not long enough to measure the transition locus on the porous surface. Since the boundary-layer disturbances were not measured, the experiments 6 did not give direct evidence of the second-mode instability. Whether the second mode was involved in the transition process was not clear. This motivated stability measurements on a sharp cone covered by UAC of various microstructures and direct comparisons of experimental data with theoretical predictions. The first series of stability experiments 7,8 was conducted in the Mach 6 wind tunnel T-326 of the Institute Theoretical of Applied Mechanics (ITAM, Novosibirsk Russia) on a 7 degree half-angle sharp cone whose longitudinal half surface was solid and other half surface was covered by a thin porous coating of random structure, namely, a fibrous absorbent material (felt metal). Hot-wire measurements of “natural” disturbances and artificially excited wave packets were performed on both solid and porous surfaces. Stability analysis for 2-D and 3-D disturbances showed that the felt-metal UAC strongly stabilizes the second mode and marginally destabilizes the first mode. These results are in a qualitative agreement with the experimental data for natural disturbances. The theoretical predictions are in a good quantitative agreement with the stability measurements for artificially excited wave packets associated with the second mode. The second series of stability experiments 9 was conducted in the T-326 wind tunnel on a 7 degree half-angle sharp cone with the UAC of regular microstructure, which is similar that tested at Caltech. 6 Spectra of natural disturbance indicated that the second mode is a dominant instability. The UAC stabilized the second mode and weakly affected the first mode. Measurements of artificially excited wave packets showed that the porous coating