A multi-sensor hot-wire anemometer system for investigation of wall-bounded flow structures

This work describes a hot-wire anemometer system and an experimental procedure designed for mapping of the flow structure in disturbed boundary layers. It also briefly presents results from periodically disturbed wing boundary layers. The hot-wire system consists of a number of individual anemometer units, which can be combined to operate multi-sensor probes for simultaneous measurement of velocities in many points. In the present paper, however, only a single probe was used mounted in a dedicated traverse system in order to demonstrate how ensemble averaging can provide knowledge about the growth and decay of organised flow structures, as long as they are periodic in both time and space. The information from this flow structure can then be used in the design of multi-array hot-wire probes for further studies of the non-deterministic stages of the boundary layer transition

Experimental study of the K-regime of breakdown in straight and swept wing boundary layers

Experiments on the stability of three-dimensional boundary layers on the straight and swept airfoils have been conducted. Detailed measurements of the streamwise velocity field in (Y, Z) planes, as well as the 3D frequency-wavenumber spectra have revealed linear and non-linear evolutions of the disturbances generated by an external acoustic field in the airfoil boundary layers. The Tollmien-Schlichting waves, excited by the sound, were dominant in both configurations, and the disturbance flow field was found to remain highly deterministic and periodic both in time and space until the latest stages of the transition. To control a spanwise variation of the disturbance flow field roughness elements were placed on the surface and equidistantly aligned along the airfoil leading edge. The K-type transition was identified with the aligned order of so-called Lambda-patterns at non-linear stage of the transition. The experiment was conducted in a closed-circuit wind-tunnel at the department of Thermo and Fluid Dynamics, Calmers. This work was supported by the Royal Swedish Academy of Sciences (KVA)