Experimental study of a Tollmien–Schlichting wave interacting with a shallow 3D roughness element

Abstract

The paper is devoted to an experimental study of the effect of a shallow 3D roughness element on the evolution of a 2D Tollmien–Schlichting wave in a Blasius boundary layer. The experiments were carried out under controlled disturbance conditions on an airfoil section which could provide a long run with zero pressure gradient flow. A pneumatically driven slit source was used to introduce the Tollmien–Schilichting wave upstream of the lower branch of the neutral stability curve. A few wavelengths downstream, the T–S wave interacts with a cylindrical roughness element. The height of the roughness was slowly oscillating in time, which allows a continuous measurement of the T–S wave response downstream the roughness. The oscillation frequency was approximately 1500 times lower than the frequency of the studied Tollmien–Schlichting wave and therefore, behaved as a steady roughness with respect to the T–S wave. Hot wire anemometry was used to measure wall normal profiles and spanwise scans close to the maximum of the eigenfunction of the T–S wave. The oscillation of the roughness and the synchronization of all-equipments permitted the use of ensemble average techniques. Two different amplitudes of T–S waves with a non-dimensional frequency of F120E-06 were studied. They show a strong amplification of the disturbances in a small spanwise wave number range. The analysis of the wall normal T–S profiles suggests the growth of oblique modes.

Keywords:

• Boundary layer
• transition
• Blasius
• isolated roughness
• fundamental resonance

Acknowledgements

This project was financially supported by FAPESP and CAPES from Brazil. The authors thank Dr. Mendonça, M.T., from CTA, Brazil, for their contributions.