Investigation of turbulent puffs in pipe flow with time-resolved stereoscopic PIV C.W.H. van Doorne, B. Hof, F.T.M. Nieuwstadt, J. Westerweel, B. Wieneke * Delft University of Technology Laboratory for Aero & Hydrodynamics Leeghwaterstraat 21, 2628 CA Delft, The Netherlands * La Vision GmbH, Anna-VandenHoeck-Ring 19, D-37081 G¨ottingen, Germany Abstract Time-resolved stereoscopic particle image velocimetry (SPIV) was used to study the 3D flow field and the flow structure of a turbulent spot, or puff, at low Re in a pipe. The high sampling frequency of the SPIV system (500 Hz) makes it possible to obtain time- resolved velocity measurements over the entire circular cross-section of the pipe. When time is converted into a spatial coordinate with help of the bulk velocity, i.e. assuming frozen turbulence, the result is the first quasi-instantaneous 3-D velocity measurement of a turbulent puff. The 3-D plots of the iso-contours of the streamwise vorticity, and various cuts of the 3-D vector fields show the complex structure of the flow in the turbulent puff. At the trailing edge of the puff, where the laminar flow undergoes transition to turbulence, pairs of counter rotating streamwise vortices result in large mushroom-like structures as seen in flow visualizations. Integration of the velocity fields over the cross-section of the pipe further shows that very large spikes occur in the energy of the radial and azimuthal velocity components. These spikes appear to be related to the presence of hairpin vortices. (d) (c) (a) (b) (d) (b) (a) (c) 3D visualization of the iso-contours of streamwise vorticity (± 3.5 s −1 ) in the puff. The bottom figure shows the flow in the lower half of the pipe (y< 0) and the structures close to the wall appear on top (viewing in the positive y-direction). 1