Turbulent Boundary Layer PIV

Identification of Lagrangian coherent structures in the turbulent boundary layer
PAN Chong, WANG JinJun & ZHANG Cao
Institute of Fluid Mechanics, Beijing University of Aeronautics and Astronautics, Beijing

Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional velocity field obtained by time-resolved 2D PIV measurement.

Simple Microvec TR PIV system was used in this experiment (similar to educational system 2). CCD Camera with 640 x 480 pixels and 200fps was acting as a high speed camera and the 1.5W SSD Laser was set in continuous mode. See the setup on the left.

Particle images are analyzed by MircroVec with interrogation windows of 16×8 pixels and overlap rate of 50%. The software package MANGEN was used to calculate FTLE field. The fluid trajectories are obtained by advecting particles in PIV-measured velocity field with a 4th-order Runge-Kutta-Fehlberg integration algorithm.

A snapshot of FTLE distribution in the (t, y) plane inside the turbulent boundary layer.
Comparison of the capability of three vortex identification techniques, the FTLE method, the velocity vector method and the vorticity method.

Probability density distribution of inclination angle θ of typical LCS at three different height.