Shestakov M.V.   Tokarev M.P.   Markovich D.M.  

Visualizing the Evolution and Interaction of Vortices in a quasi two-dimensional jet: Time-resolved Tomographic PIV Measurements

Reporter: Shestakov M.V.

Anisotropic jet flows find their application in modern technological devices and heat ex-changers. This kind of jet flows are formed by jet flowing into a narrow channel. Jet confinement in one of three dimensions leads to complex oscillating jet motion – meandering of the jet (Dracos et. al. 1992). Jet meandering occurs as a result of development of quasi two-dimensional large-scale vortex structures in the mixing area of the jet. Our recent experiments showed that in quasi two-dimensional turbulent jet complex multiscale flow structure is formed in which two types of coherent vortex structures are presented (Shestakov et. al. 2014).
In present work we experimentally investigated the 3D structure and evolution of a quasi-two dimensional turbulent jet in a narrow channel. This work focuses on characteristics of 3D vortex structures in the flow. We investigated development of the secondary streamwise vortices, their interaction with each other and evolution into large-scale quasi-2D vortices. Time-resolved tomographic PIV technique (with repetition rate up to 10 kHz for 1 Mpx CMOS cameras) was used to measure subsequent 3D velocity distributions. We analyzed two ratios of the nozzle width to depth, viz., 1 and 2.5.
The experimental setup consisted of a closed hydrodynamic contour, including a tank, a pump, flow meter, and measurement section. The measurement section was a narrow channel formed by two parallel plates made of organic glass (size: 307×270 mm2, thickness: 20 mm), located at a distance 4 mm from each other.
The measurement system consisted of a high-repetition Nd: YAG laser (Photonix DM-532-100 with 10 mJ at 10 kHz), four high speed CMOS cameras (Photron FASTCAM SA5 with resolution of 1024×1024 px of 12 bit images at 7 kHz), and synchronizing device Berkeley Nucleonics BNC 575. The thickness of the laser sheet was 4 mm. We used SIGMA AF 105 mm f/2.8 EX DG MACRO lenses for the optical setup.
Using time-resolved tomographic particle image velocimetry (TR-TomoPIV) experimental technique we studied a vortical organization of a quasi-two-dimensional turbulent confined jet. The results show that the flow is governed by two types of coherent vortices which are well-known quasi-two-dimensional large-scale eddies growing in both shear layers in checkerboard order and long streamwise eddies, observed for the first time. The latter are lined up along the flow representing very long vortex tubes meandering together with the flow.

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