Time-resolved scanning stereo PIV measurement of three-dimensional velocity field of highly buoyant jet

The time-resolved scanning stereo PIV system, which consists of a CW Nd:YAG laser, two high-speed CMOS cameras, a Galvano mirror and a pulse controller, is developed for time-sequential three-dimensional velocity field measurement of liquid flow. This experimental technique is validated by the measurement of a laminar non-buoyant jet, and is applied to the 3D velocity measurement of a highly buoyant jet at Froude number 0.3 and Reynolds number 200, which satisfy the inflow condition due to the unstable density gradient. The measurement of statistical properties of velocity field shows the growth of turbulence intensities near the nozzle exit, which indicates the presence of inflow motion near the nozzle exit due to the unstable density gradient. The vorticity contours about the horizontal axis shows the presence of counter-rotating vortices on both sides of the jet centerline, while the vorticity contour about the vertical axis indicates the presence of spiral growth of a pair of vortices along the jet centerline. The growth of the spiral structure along the jet centerline is recognized in the iso-vorticity contours obtained from the time-resolved measurement of 3D velocity field.