10 th Pacific Symposium on Flow Visualization and Image Processing Naples, Italy, 15-18 June, 2015 Paper ID:189 1 3D Dynamics of Vortex Structures in a Quasi Two-dimensional Jet Maxim V. Shestakov 1,* , Mikhail P. Tokarev 1 , Dmitriy M. Markovich 1,2 1 Institute of Thermophysics, Siberian Branch of RAS, Novosibirsk, Russia 2 Department of Physics, Novosibirsk State University, Novosibirsk, Russia *corresponding author: mvsh@itp.nsc.ru Abstract The work focuses on investigation of spatial-temporal 3D vortex structure of a quasi two-dimensional turbulent jet. Time-resolved tomographic PIV technique with repetition rate up to 10 kHz was used to measure 3D velocity distributions. It was shown that in quasi two-dimensional turbulent jet two types of coherent vortex structures exist. On the basis of instantaneous distributions of Q criterion 3D dynamics of vortex structure is studied. Longitudinal secondary vortex structures in far field of the quasi two-dimensional turbulent jet were detected for the first time. Keywords: time-resolved tomographic PIV, quasi two-dimensional jet, dynamics of vortex structures 1 Introduction The interest to quasi two-dimensional turbulent bounded jet flows is caused by wide variety of such flows in nature and industry, in particular because of presence of large-scale coherent vortex structures. Large-scale coherent vortex structures play important role in mixing and mass transfer, e.g. spread of contamination, momentum and energy transfer in shallow flows including rivers, lakes, channels and oceans. Quasi two-dimensional turbulent jet flows are highly complex for numerical and experimental study. The complexity is determined by presence and interaction of disparate scales of turbulence. On the one hand this is three-dimensional small-scale turbulent motion of a scale h (thickness of fluid layer), on the other hand this is large-scale turbulent eddies induced by growth of shear instability in horizontal direction. One of the first studies related to large-scale quasi two-dimensional vortex structures is [1]. In the work the structure of far field of quasi two-dimensional jet spreading in a narrow channel was studied in a wide range of channel depth values h. The authors emphasized two flow areas: near field of the jet (three-dimensional flow) where secondary flows have a great impact and far field of the jet (quasi two-dimensional flow), which is characterized by large-scale quasi two-dimensional vortices. The authors showed that beyond a distance of approximately ten times the depth of the bounded fluid layer the jet starts to meander around its axis. Large vortical structures develop with axes perpendicular to the bounding surfaces of the fluid layer. With increasing distance, the size of these structures increases by pairing. These features of the jet are associated with the development of quasi two-dimensional turbulence. By the present time, studies of large-scale quasi two-dimensional vortex structures and meandering effect have been resumed as evidenced by a number of numerical and experimental research works. For historical reasons studies of near field and far field of quasi two-dimensional bounded jet were carried out independently and intended to different goals. Initial interest to study of bounded jets was determined by development of jet fluidics for space and defense industry [2-6]. These works referred to walls influence on a flow structure in near field of a jet and directed to development of jet flow control techniques. It was shown that in the wall bounded jet secondary flows appear in mixing layer. In the works [3,4] the model of secondary flows development was proposed on the basis of vortex filament re-orientation. Turbulent properties of secondary flows in bounded jets with different aspect ratio were studied in [5,6]. In [5] a spatial vortex structure was described based on visualization, which confirms assumptions from [3,4]. In the work [1] the near field of quasi two-dimensional jet flow is also partially studied and a secondary flow existence area was defined. In order to confirm hypothesis of secondary flow development [3,4] in the work [7] measurements of three velocity components in the flow volume were done in the near field by means Tomo-PIV technique. Analyzing instantaneous velocity fields in flow volume reconstructed from first 10 POD modes the authors managed to determine longitudinal vortices, which are responsible for secondary flow development. However, insufficient spatial and temporal resolution did not allow them to answer a number of questions concerning secondary flow formation and development.