Turbulent boundary layer noise mitigation by geometrical surface treatments and near wall passive flow control devices

Abstract

By way of experimental investigation, this thesis outlines the effects of drag reducing riblet wall surface and outer layer large energy break-up (LEBU) device on the broadband noise of the turbulent boundary layer and the potential implications on hypothetical radiated noise from a sharp trailing edge. Through hot wire measurements on a stable time-invariant artificially tripped boundary layer, the flow conditions of the baseline turbulent boundary layer and the turbulent boundary later over the drag reducing riblets has been investigated. From there, the fluctuating pressure field of the turbulent boundary layer has been measured and characterized by way of wall embedded microphone sensors. Turbulent spots have been employed as a tool and visual aid into the spatial and temporal effect that the passive flow control devices have on the turbulent flow structures. It has been found that there is potential to affect trailing edge noise predictions when using a hybridised method of passive flow control, where both the near wall and outer boundary layer are targeted simultaneously.