<p>Boundary layer separation is usually an unwanted phenomenon in most technical applications as for instance on airplane wings, on ground vehicles and in internal flows such as diffusers. If separation occurs it leads to loss of lift, higher drag and results in energy losses. It is therefore important to be able to find methods to control and if possible avoid separation altogether without introducing a too heavy penalty such as increased drag, energy consuming suction etc.</p><p>In the present work we study one such control method, namely the use of vortex generators (VGs), which are known to be able to hinder turbulent boundary layer separation. We first study the downstream development of streamwise vortices behind pairs and arrays of vortex generators and how the strength of the vortices is coupled to the relative size of the vortex generators in comparison to the boundary layer size. Both the amplitude and the trajectory of the vortices are tracked in the downstream direction. Also the influences of yaw and free stream turbulence on the vortices are investigated. This part of the study is made with hot-wire anemometry where all three velocity components of the vortex structure are measured. The generation of circulation by the VGs scales excellently with the VG blade height and the velocity at the blade edge. The magnitude of circulation was found to be independent of yaw angle.</p><p>The second part of the study deals with the control effect of vortex generators on three different cases where the strength of the adverse pressure gradient (APG) in a turbulent boundary layer has been varied. In this case the measurements have been made with particle image velocimetry. It was found that the streamwise position where the VGs are placed is not critical for the control effect. For the three different APG cases approximately the same level of circulation was needed to inhibit separation. In contrast to some previous studies we find no evidence of a universal detachment shape factor<i> H</i>_12,that is independent of pressure gradient.</p>

Boundary layer separation is usually an unwanted phenomenon in most technical applications as for instance on airplane wings, on ground vehicles and in internal flows such as diffusers. If separation occurs it leads to loss of lift, higher drag and results in energy losses. It is therefore important to be able to find methods to control and if possible avoid separation altogether without introducing a too heavy penalty such as increased drag, energy consuming suction etc. In the present work we study one such control method, namely the use of vortex generators (VGs), which are known to be able to hinder turbulent boundary layer separation. We first study the downstream development of streamwise vortices behind pairs and arrays of vortex generators and how the strength of the vortices is coupled to the relative size of the vortex generators in comparison to the boundary layer size. Both the amplitude and the trajectory of the vortices are tracked in the downstream direction. Also the influences of yaw and free stream turbulence on the vortices are investigated. This part of the study is made with hot-wire anemometry where all three velocity components of the vortex structure are measured. The generation of circulation by the VGs scales excellently with the VG blade height and the velocity at the blade edge. The magnitude of circulation was found to be independent of yaw angle. The second part of the study deals with the control effect of vortex generators on three different cases where the strength of the adverse pressure gradient (APG) in a turbulent boundary layer has been varied. In this case the measurements have been made with particle image velocimetry. It was found that the streamwise position where the VGs are placed is not critical for the control effect. For the three different APG cases approximately the same level of circulation was needed to inhibit separation. In contrast to some previous studies we find no evidence of a universal detachment shape factor H12, that is independent of pressure gradient. / QC 20101119

The object of the present work is to experimentally study the case of a turbulent boundary layer subjected to an Adverse Pressure Gradient (APG) with separation and reattachment. The effect of unsteadiness on turbulent boundary layer separation by means two different methods were explored viz. the effect of local forcing by acoustic waves and effect of wakes on separation bubble. The experiments were conducted in a low speed open circuit blower type wind tunnel. The turbulent separation bubble was created on the test plate by a contoured ceiling which created the adverse pressure gradient. The velocities were measured using single element hot wire and X-wire. Limited studies on quasi shear stress were also conducted using surface mounted hot film probes. Static pressure was measured using a projection manometer. Boundary layer is tripped near the leading edge of the flat plate to ensure a turbulent boundary layer. Surface pressure distribution and flow visualization were conducted as part of diagnostics. In the case of laminar separation bubble, lot of investigations have been done on the effect of unsteady wake and the most important conclusion was that the wake induces `bypass' transition to turbulence and since the turbulent boundary layer is more resistant to separation, it remains attached. In the case of turbulent separation bubble, laminar-turbulent transition is not relevant and if the bubble is suppressed, it should be by some other mechanism. This is what we seek to unravel in this study. A closer look at the mean velocity profiles reveal the occurrence of inflection point before separation as in the case of laminar separation bubble and the peak values of turbulence intensities correspond to the location of point of inflection. Turbulent separation correlations proposed by various investigators were compared with the present results and are found to be in good agreement. Surface flow visualization pictures are used to get qualitative information. The wall forcing on the separation bubble was done using a speaker which blows a small amount of air when the diaphragm moves up and sucks in when the diaphragm moves down. The blowing effect seems to be more effective in suppressing the separation compared to suction. The interaction with wake is studied using an unsteady bar which is moving up and down. The inflection point in the mean velocity distribution seems to move closer to the wall with the impingement o the wake. Also the turbulence intensities have increased and seem to move closer to the wall. The displacement and momentum thickness have increased and the shape factor has decreased which indicates suppression of the bubble. The quasi shear stress in the separated region also increased which indicates suppression of separation. While the oncoming unsteady wake might be a parcel of fluid with defect velocity when seen in isolation, in comparison to the velocity defect in the separation bubble, it is a region of velocity excess. As a result, one can expect the impingement of the unsteady wake on the TSB to transport momentum thereby contributing to separation reduction. But the mechanism of separation is different from laminar separation bubble affected by wakes. The suppression in the case of turbulent separation bubble is partly due to the entrainment of turbulence and partly due to the kinematic impact of the wake on the bubble.