10th International Symposium on Turbulence and Shear Flow Phenomena (TSFP10), Chicago, USA, July, 2017
Dynamics of Large-Scale Vortices in Non-Swirling and Swirling Turbulent Jets.
Time-Resolved Tomographic PIV Measurements
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V.M. Dulin *, S.S. Abdurakipov , D.M. Markovich , Kemal Hanjalić
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1: Kutateladze Institute of Thermophysics, Siberian Branch of Russian Russia
2: Physical Department, Novosibirsk State University, Russia
3: Faculty of Applied Science, Delft University of Technology, The Netherlands
* Corresponding author: [email protected]
The present paper reports on a parallel study of the dynamics of large-scale vortical structures in non-swirling, low-swirl and
high-swirl turbulent jets by means of the time-resolved tomographic PIV technique. 3D realizations of the instantaneous velocity
and velocity gradients, local strain and rotation rates, local helicity and pressure fluctuations are analyzed. Deformations of the
large-scale vortices in the jet flows are studied by estimating local vortices stretching/compression rates and by local values of the
Lamb vector divergence. Linear stability analysis is also applied to the mean velocity profiles in order to predict the most unstable
azimuthal modes. Besides, local pressure fluctuations (at the scales resolved by PIV) are evaluated based on the 3D PIV data and
Navier−Stokes equations. Smagorinsky model for the effective viscosity is used to account for small-scale velocity fluctuations
during linear stability analysis and evaluation of the local pressure fluctuations.
The waster jet flow was produced by a contraction nozzle inside a plexiglas test section. The nozzle exit diameter d was 15 mm.
The Reynolds number Re = U0d/v (where U0 is the bulk flow velocity of the jet; v is the water kinematic viscosity), was 8 900. A
vane swirler was mounted inside the nozzle to organize jets with swirl. Using swirlers with different inclination angle of the blades,
the geometrical swirl rate S (defined as the ratio between the angular momentum flux and the axial momentum flux, normalized by
the nozzle exit radius) was 0.41 and 1.0.
Near the nozzle exit of the non-swirling (S = 0) and low-swirl (S = 0.41) jets, the linear stability analysis predicted the highest
growth rate for the axisymmetric mode m = 0. Downstream 0.5d from the nozzle exit, growth rate of the azimuthal mode m = +1
for the low-swirl jet became higher than that for the mode m = 0. Modes m = −2 and −1 were found to be most unstable for the core
region of the low-swirl jet. Toroidal vortices were also detected in 3D PIV data for the mixing layer of the non-swirling and lowswirl jets. In the latter case they broke apart and formed longitudinal vortex filaments. Core of the low-swirl flow was featured by a
local decrease of the axial momentum due to the expansion of the swirling jet. The high-swirl jet (S = 1.0) was featured by a
bubble-type vortex breakdown with central recirculation zone. The stability analysis predicted helical mode m = +1 (counterwinding to the direction of the jet swirl) to be the most unstable. Despite the flow was already turbulent near the nozzle exit,
similar structure was found in the 3D velocity snapshots in the outer mixing layer.
2p
ρU 02
2p
ρU 02
Vortex core
of swirling jet
Counter-winding
helical vortex
Vortex core
of swirling jet
Figure. Large-scale vortices in (left) low-swirl jet (right) high-swirl jets, visualised by Q-criterion. The grayscale colour indicates
estimated local pressure at the iso-surfaces and cross-planes
session.paper