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

Abstract

<jats:p>An experimental investigation of a precessing jet issuing from a mechanically rotating nozzle directed at an angle of α=45° 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 (≈0.005, 0.01, and 0.02) of the precession Strouhal number Stp (≡ rotation frequency × nozzle diameter / jet exit bulk velocity) were used to assess the effect of varying Stp. 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 x⩽7d, where x is a distance measured from the rotating nozzle exit. A critical Strouhal number, i.e., Stp,cr≈0.008 for the present case, is identified: at Stp⩾Stp,cr the core jet converges to the axis of rotation while at Stp⩾Stp,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 Stp. The near-field region, x∕d⩽10–15, 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., Stp⩾Stp,cr), the jet in the far field resembles some features of the nonprecessing counterpart. Nevertheless, significant differences still retain in the statistical properties.</jats:p>