Abstract
The near field evolution of a low Reynolds number free jet and associated turbulent fluctuations are investigated experimentally, in the presence of a grid placed at nozzle exit. The jet spreading characteristics without grid are validated first with available data in the literature and the well known features of vortex roll-up and pairing phenomena in the shear layer region are confirmed. The inclusion of a grid at the nozzle exit appears to reduce velocity fluctuations in the near field, but it accelerates the transition to a full-grown turbulent flow field with respect to axial distance, by the introduction of finer scales. Grids also cause mean velocity overshoot in the radial profile by distorting the approach flow in the boundary layer region ahead of the grid. Distinct vortex roll-up and pairing phenomena in the shear layer region could not be observed with grids. However, weak shear mode instabilities originating in the shear layer for 0 ≤ z/D ≤ 3 appear to interact with the potential core, leading to the formation of a “unified shear/preferred mode” instability. These features are dominantly seen for grid-mounted jet flows having large displacement thickness to grid opening ratio. Power spectra indicate a progressive evolution of turbulent flow features with respect to axial distance, starting with the development of inertial sub-range into a −5/3 slope exponent; followed by the establishment of full range of turbulent scales, and the eventual attainment of isotropy. The establishment of inertial sub-range is attributable to vortex-pairing and turbulent fragmentation in free jets and to the introduction of fine scales at the jet inlet in the cases with grids.
Published Version
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