Self-similarity of coflowing jets: The virtual origin

From round and plane jet experimental data, it can be inferred that the existence of the virtual origin can be attributed to variations in thickness of the internal boundary layer at the orifice. Based on this, and further support from large eddy simulations (LES) of axisymmetric coflowing round jets with various inlet conditions, we propose that the local value of the variance in the thickness of the boundary layer and thus the velocity distribution at the jet inlet can be used to predict the location of the virtual origin. This, in turn, indicates the inadequacy of the traditionally used length scale, the jet half-width/radius, and suggests the local variance is a length scale with better physical significance. It appears unlikely that the turbulence intensity distribution at the orifice plays a significant role in the determination of the virtual origin or the axial spread of the effective jet width. (c) 2007 American Institute of Physics.