Abstract
Abstract. A novel dual-plane dye laser particle image velocimetry (PIV) technique used to analyze helicity and energy dissipation in an unexcited turbulent swirling jet of pressurized cold air has established that regions within the flow field of the jet exhibiting high helicity are correlated regions of high turbulent kinetic energy dissipation. This PIV configuration provides estimates of all components of the velocity gradient tensor, facilitating calculation of the helicity from the vorticity components. Application of this novel dual-plane PIV technique is extended in this study to investigate helical structures in a turbulent swirling jet where the underlying shear flow is subjected to external acoustic sinusoidal forcing in a plane perpendicular to the central axis of the jet. It was found that acoustic excitation had a significant effect on the mean velocity profile parallel to the direction of the jet. The horizontal forcing resulted in the generation of vorticity that was skewed with a pitch that favored a distribution of angles around 90° with respect to the velocity vector. The distribution of the time-averaged helicity angle indicated organized helical activity, but such activity is not dominated by large-scale coherent structures of maximal helicity.
Highlights
Helical structures are a common phenomenon in geophysical flows, for example, in magnetohydrodynamics (Moffatt, 1978), storm systems (Lilly, 1986), underwater gravity currents (Sumner et al, 2014), and atmospheric inertia-gravity waves (Viúdez and Dritschel, 2006)
It is well established that helical coherent structures play a crucial role in turbulent flows
The dual-plane dye laser particle image velocimetry (PIV) technique was used to illuminate the vertical planes aligned along z = −1 mm and z = 0 mm that permitted measurements of the velocity vector field to be made
Summary
Helical structures are a common phenomenon in geophysical flows, for example, in magnetohydrodynamics (Moffatt, 1978), storm systems (Lilly, 1986), underwater gravity currents (Sumner et al, 2014), and atmospheric inertia-gravity waves (Viúdez and Dritschel, 2006). It is well established that helical coherent structures play a crucial role in turbulent flows. The role of helicity in three-dimensional turbulent flows is still enigmatic. Kraichnan (1973) investigated analytically the interaction of two helical waves, concluding that the presence of a non-zero mean helicity in a turbulent flow inhibits the energy transfer to small scales. One of the main effects of helicity in neutral isotropic turbulence is the inhibition of energy transfer at large eddy sizes. This was confirmed numerically by André and Lesieur (1977). Homogeneous turbulence with significant mean helicity exhibits a much slower rate of decay of turbulence and its corresponding energy than its zero-mean helicity counterpart
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