# Licentiate seminar

## PIV measurements in a separating turbulent APG boundary layer

 Defendant Main Advisor Extra Advisor Date Kristian P. Angele Barbro Muhammad- Klingmann Michel Stansilas 2000-12-20 Opponent Evaluation committee

### Abstract

The present study reports measurements in high Reynolds number turbulent boundary layers using Particle Image Velocimetry (PIV). Two different experiments were performed. A zero pressure gradient (ZPG) case was studied where the near wall region was resolved down to $y^{+}$=8 and the turbulence statistics were evaluated from a data set consisting of 2700 image pairs. By comparison with hot wire measurements, the PIV accuracy in the Reynolds stresses was found to be in the order of 10-15\%. In terms of spatial resolution PIV can be made to perform better than X-wires. PIV and LDV was used investigating a separating adverse pressure gradient (APG) turbulent boundary layer, where the shape factor $H_{12}$ varied between 1.41 and 4.12. The boundary layer separated at approximately $H_{12}$=3.65$\pm$0.35 forming a shallow separation bubble in near-equilibrium, with $H_{12}$=4. The mean velocity profiles were self-similar in Perry-Schofield coordinates and the Reynolds stresses were found to scale with the displacement thickness, $\delta$$^{*}, and their maximum value was located at y/\delta^{*}=1, corresponding to approximately y^{+}=400. This coincidence between the inner and outer scale was found at all x-positions, indicating that there is only one length scale governing the flow. Measurements in the wall-parallel plane reveiled low speed streaks with a spanwise spacing of \lambda^{+}=370, which is approximately equal to the distance between the wall and the center of turbulence production. This lends itself to the conclusion that turbulent energy is transported from the middle of the boundary layer to the wall by large scale vorticies at a length of \delta$$^{*}$ under the effect of APG. In ZPG at $y^{+}$=100 low speed streaks with a spacing of $\lambda^{+}$=620 were detected. This is substantially larger than the $\lambda^{+}$=100 seen around $y^{+}$=20 suggesting that merging of sub-layer streaks may take place in the logarithmic region.