Separation Postponement by means of Periodic Surface Excitation

Abstract

Flow separation on an airfoil can pose serious consequences. Almost always a decrease in lift and an increase in drag can be seen. To counteract ow separation, many separation control devices have been investigated. The control devices can be subdivided into passive and active methods. The difference is that passive methods do not add energy to the ow whereas active methods do. The passive device mostly applied and investigated is the VG. A relatively new active ow control method is periodic surface excitation. A periodically moving surface influences the ow such that separation is postponed. However the question is how effective periodic surface excitation is with respect to other methods. Furthermore it is still unclear how the postponement of separation is established. The aim of this report is to compare the effectiveness of periodic surface ex- citation with the effectiveness of Vortex Generators. Furthermore in this report an explanation is given why periodic surface excitation can produce postponement of separation. The comparison is based on experimental research. The model used in the experiment is a ramp. On the back side separation was provoked. In the given setup separation was triggered at the top of this ramp. The Vortex Generator and the actuator that provided surface excitation were placed in front of the separation point. By testing different configurations, the reduction of the separation area was determined. The reduction was measured by means of pressure, tuft and PIV measurements. An explanation why periodic surface excitation could reduce the separation area was investigated by PIV measurements.