Direct TS-wave Simulation in a laminar boundary layer over suction-slots

Abstract

Hybrid laminar flow control of TS-waves in attached boundary-layers is generally carried out by suction through a micro-porous surface and a subsequent stabilization of the laminar flow. This micro porousness is usually realized by laser drilled holes with diameters below 100 micron. More advanced manufacturing methods will allow the application of suction by continuous slots in a thin metal-foil, applied on a supporting sub-structure. These slots will allow the same suction-rate at much smaller streamwise slot-dimension, less equivalent surface roughness, and a better production-quality. The use of slots for transition control is not a new approach, it was already proposed by Pfenninger in the 1970's to reduce attachment-line instabilities. The slot-geometry allows 2D-simulations of TS-waves by DNS in given boundary layers and investigations of the interaction between amplified modes and perturbations from the suction-channels. By this way, an applicable number of slots per wavelength and the allowed slot-diameter which does not substantially perturb the TS-modes can be determined. The occurrence of such interactions at deep gaps, resulting from acoustic resonances, was recently shown by H. Zahn for TS-waves on laminar wings. For the following study, direct TS-wave simulations are carried out in a prescribed Blasius boundary-layer over suction-slots. The development of acoustic waves, generated by the perturbation-flow of the slots, has been demonstrated by using a high-order implicit Navier-Stokes solver in compressible formulation. The interactions of these perturbations with amplified TS-waves will be investigated by subsequent time-accurate simulations of the flow-field. In addition, the damping of the chosen TS-modes can be directly provided from a post-processing of the results, without an approximation of distributed suction-flow. Furthermore the time-resolved physics inside the slots, including resonance phenomena, are a direct outcome of the study, as demonstrated by Zahn. Simulations at different grid resolution and slot-number are planned for realistic Reynolds-numbers on a flat plate with fully resolved slot-geometries. Slot diameter and corresponding wall-thickness from wind-tunnel models will be modified to demonstrate the impact of these parameters on the interaction-scenario. Comparisons of the suction-rate with steady calculations of cylindrical 3D-holes will be carried out as well.