High-speed scanning stereoscopic PIV for 3D vorticity measurement in liquids

Abstract

A scanning stereo particle image velocimetry (SSPIV) system was developed to measure the three-dimensional (3D) distribution of three-component (3C) velocity in a turbulent round jet. A laser light sheet produced with a high-repetition-rate pulsed Nd:YLF laser was scanned by an optical scanner in a direction normal to the sheet. Two high-speed mega-pixel resolution C-MOS cameras captured the particle images illuminated by the light sheet, and the stereoscopic PIV method was adopted to acquire the 3D-3C velocity distribution of turbulent water flow. A water jet formed by a round nozzle with an exit diameter of D = 5 mm was diagnosed by the current technique. The jet Reynolds number was set at Re ≈ 1000, and the streamwise location of the measurement was fixed at approximately x = 45D. A measurement volume (∼100 × 100 × 100 mm 3 ) containing 50 velocity planes was scanned in 0.22 s, which was sufficiently short to capture the instantaneous vortical structures. The residue of the continuity equation (divergence) was approximately 7% of rms vorticity on the centreline of the jet. The iso-vorticity surfaces clearly depict vortical structures in the jet shear layer.

Three-dimensional evolution of flow structures in transitional circular and chevron jets

“…Time resolved PIV measurements were conducted by Wernet 16 on high-speed jet to characterize the turbulence by means of space-time correlation functions. The first instantaneous 3D vorticity field of a jet at Re ¼ 1000 was performed by Hori and Sakakibara, 17 based on a high speed scanning stereo PIV system. 18 The development of tomographic PIV (TOMO-PIV) 19 enables the measurement of the instantaneous velocity vector field over a three-dimensional domain, with the advantage that the velocity and the complete velocity gradient tensor and the associated quantities (e.g., the vorticity vector and the second invariant) can be obtained independently of the flow speed.…”

Scanning tomographic particle image velocimetry applied to a turbulent jet

“…[1][2][3][4] Earlier methods, based on holographic reconstruction 5,6 required much more elaborate optical setups. Another method, scanning stereoscopic PIV, 7,8 is simpler to establish, but its application is limited to low speed flows.…”

“…42 For our fully turbulent inlet conditions these vortex structures will not be as dominant as for a transitional flow but we do expect the flow features in our downstream experimental domain to be highly vortical. 7 Using vorticity as a first choice for a characterization criterion, the vorticity magnitude field is thresholded to isolate the most intensely vortical regions. Figure 14(a) shows isosurfaces of this thresholded field for an instantaneous snapshot of flow, which appears to highlight a complex network of tube-like vortical structures oriented both axially and azimuthally.…”

A matching pursuit approach to solenoidal filtering of three-dimensional velocity measurements

“…Defocusing digital particle image velocimetry has been applied to single-phase and bubbly flow [8]. Scanning-light-sheet PIV has been able to describe complex flow patterns in relatively low velocity flows [5]. Holography is three-dimensional in nature and can be very effective in small measurement volumes [4].…”