I am trying to measure the axial turbulent intensity decay with a hot wire anemometer in an open jet issued from a profiled nozzle having a grid in-flush with its opening. I want to know whether the decay law is valid - at least in the potential cone region.
In the case of standard jets the turbulence level increases along relatively contained axial distances from the nozzle, because of the interaction with stagnant air, reaches a maximum and then decreases further downstream. Instead, in case a grid is positioned at the nozzle exit, the turbulence rapidly increases at first, then decreases according to the degeneration law and finally increases again due to the interaction with the stagnant air. If the distance from the jet exit is high enough, the grid generated turbulence is completely dissipated thus turbulence levels are about the same whether the grid is present or not.
yes. My measurements shows the same. But about 40 mesh size (M) to 100 M there is a decay and this zone is within the potential cone. And all my experiments are going to be within this field. Moreover , the presence of grid has modified the structure of the jet and final intensity reached in the shear layer interaction region seems to depend on the mesh solidity (only prelim experiments are over; still working).
The velocity fluctuations in an open jet are made up from two independent contributions (a) the vorticity coming from inside the nozzle (or generated by the grid in your case) and from (b) solenoidal fluctuations related to the fluctuating pressure near field of the vortices in the jet's shear layer.
(a) vorticity-related fluctuations:
- are convected with the flow speed,
- decay with increasing distance, decay law valid if grid uniformity is perfect,
- lateral fluctuations are larger than longitudinal ones (due to the contraction in the wind-tunnel nozzle),
- lateral and longitudinal fluctuations should be more uniform in grid-generated turbulence,
- length scale is small.
Beware: non-uniform grids generate large scale vortical structures, which tend to decay very slowly and may induce instabilities when accelerated in a nozzle. Best grids are made from sheet metal with regularly punched rectangular holes.
(b) pressure field induced fluctuations:
- phase speed smaller than tunnel speed (roughly 60 % of tunnel speed),
- level increases with distance from the nozzle as the thickness of the shear layer increases in the downstream direction,
- longitudinal fluctuations larger than lateral fluctuations,
- length scale is large.
The pressure field can be measured with microphones (with nose cones). Hot wire measurements cannot separate the two contributions.
Read
U. Michel, "Lower limit of the velocity fluctuation level in wind tunnels."
J. Böttcher , E. Wedemeyer, "The flow downstream of screens and its influence on the flow in the stagnation region of cylindrical bodies."
P.John Finley, "The Berlin axisymmetric boundary layer facility"
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