Experimental investigation on boundary layer transition induced by distributed roughness elements

An experimental study was undertaken to understand the problem of roughness-induced boundary layer transition and the sensitivity of boundary layer to the dimensions of DRE(distributed roughness elements). The distances between the DRE and the transition locations in different Reynolds numbers are considered. Experiments were conducted in the rotor blade whirl tower at AVIC ARI. Infrared thermography was employed to detect boundary layer transition on the rotor blade. In order to investigate the impact of the parameters of roughness elements on transition position, a new method was developed to measure the boundary layer transition position quantitatively. A method based on the ratio of turbulent and laminar areas was used to estimate the downstream transition locations. Seven rotational speeds between 300rpm and 600rpm were tested with leading to tip speeds between Vtip=25m/s and Vtip=40m/s respectively. The results showed that, the IRT technique can be used to measure boundary layer transition on the rotor blade of a helicopter or rotor wings. The transition detection method based on the ratio of turbulent and laminar areas is accurate and reliable. The relationship of transition length induced by different hight of DRE and Reynolds number is gained preliminary, and as the height of roughness elements increase, the transition length is decreased.