An experimental study was carried out to investigate the turbulent flow field of confined vortical flow in a narrow annulus. The test section consists of two concentric annuli that are separated by a tube with four equally spaced rectangular apertures. Water is passing from the inner annulus to the outer annulus and the main region of interest is near aforementioned rectangular holes.
Flow velocity was measured using a stereoscopic particle image velocimetry (PIV) system. The small size of the outer annulus required development of multiple improvements to the PIV system. Stereoscopic calibration within the curved and narrow annulus required development of a calibration method that ensured accurate alignment of the laser sheet to provide an accurate polynomial mapping of the physical coordinates to the pixel location on the ICCD chips of the cameras. Macro lenses were used to provide the desired field of view. Test section dimensions required accurate vertical repositioning of the cameras and the laser within the measurement plane. This was implemented using a two-axis positioning system. Reflections within the outer annulus required development of a polyurethane based paint that reflected the laser light with a shifted wavelength. The paint survived high shear stresses and pressure gradients during testing.
Three dimensional flow field was assembled based on measurements taken, providing insight into the behavior of the flow as it exits the aperture and mixes in the outer annulus. A vortex core was identified.
The results of measurements are compared to CFD results and serve as a tool validating the CFD model. Both the CFD and the measured results predict similar trends in flow behavior, showing strong radial and tangential components of velocity at the plane coincident with the edge of the hole.