An Experimental Investigation of the Effects of Surface Defects on the Laminar-Turbulent Transition of a Boundary Layer with Wall Suction

The projected increase in air traffic volume has led to a renewed interest in drag reduction research to reduce aviation’s environmental impact. One solution is wall suction, which can effectively postpone the laminar-turbulent transition of a boundary layer developing over an aircraft’s wetted area. Since a boundary layer in the laminar regime has lower skin-friction coefficient than in the turbulent regime, a delayed transition results in lower drag and reduced fuel consumption. However, implementing a suction system is likely to introduce surface defects, especially at the junction between the suction and solid panels. Additionally, surface defects generally tend to promote transition, and could therefore cancel any drag reduction achieved by wall suction.The aim for the present research is to study the combined effects of surface defects and wall suction on the transition of a Blasius boundary layer in two-dimensional incompressible flow. First, an experimental protocol was developed and implemented to verify the quality of the aerodynamic conditions in the test facility, and establish a reference for the smooth case with different suction distributions. As expected, wall suction always delayed transition, compared to the configuration without suction, and had varying effectiveness depending on the suction configuration. Concurrently, porous panels without suction were found to destabilize the boundary layer. Subsequently, three types of surface defects (wires, forward-facing steps and gaps) were tested with wall suction. No significant differences between configurations with and without suction were observed. In particular, the critical defect dimensions (height and/or width), for which transition occurs at the defect location, were identical regardless of the suction configuration. For subcritical defects (where transition is not triggered immediately) however, wall suction could still delay transition, albeit less effectively than in the smooth case.