Statistical analysis of the velocity field in a mechanical precessing jet flow

An experimental investigation of a precessing jet issuing from a mechanically rotating nozzle directed at an angle of alpha=45degrees relative to the axis of rotation is reported. Both conventional and conditional statistics of the velocity field of the jet were measured using a combined hot-wire and cold-wire (to identify any reverse flow) probe. Three distinct values (approximate to0.005, 0.01, and 0.02) of the precession Strouhal number St(p) (= rotation frequency x nozzle diameter / jet exit bulk velocity) were used to assess the effect of varying St(p). The measurements reveal that the Strouhal number in general has significant influence on the entire mixing field generated by a precessing jet. The occurrence of precession at all the Strouhal numbers of investigation produces a central recirculation zone at xless than or equal to7d, where x is a distance measured from the rotating nozzle exit. A critical Strouhal number, i.e., St(p,cr)approximate to0.008 for the present case, is identified: at St(p)greater than or equal toSt(p,cr) the core jet converges to the axis of rotation while at St(p)greater than or equal toSt(p,cr) it does not. The characteristics of the turbulent flow in the near and intermediate regions are quite different and depend upon the magnitude of St(p). The near-field region, x/dless than or equal to10-15, is dominated by a regime of global precession of the entire jet. As a result, the large-scale entrainment of the ambient fluid is substantially enhanced while the fine-scale turbulent mixing is suppressed. Under the supercritical regime (i.e., St(p)greater than or equal toSt(p,cr)), the jet in the far field resembles some features of the nonprecessing counterpart. Nevertheless, significant differences still retain in the statistical properties. (C) 2005 American Institute of Physics.