Preferred modes and the spreading rates of jets

For about a decade it has been noticed from the measurements of many jets that the value of the preferred mode and the spreading rate vary within a range of about 100%. In the present paper it is suggested that extremely low-level spatially coherent disturbances in individual facilities change the initial conditions of a laminar shear layer. The various initial conditions are able to cause different downstream developments of the jet and result in the discrepancies reported in the literature.