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

Abstract

An experimental investigation of a precessing jet issuing from a mechanically rotating nozzle directed at an angle of ␣ = 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 (Ϸ0.005, 0.01, and 0.02) of the precession Strouhal number St p (ϵ rotation frequency ϫ 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 x ഛ 7d, where x is a distance measured from the rotating nozzle exit. A critical Strouhal number, i.e., St p,cr Ϸ 0.008 for the present case, is identified: at St p ജ St p,cr the core jet converges to the axis of rotation while at St p ജ St 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 / d ഛ 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., St p ജ St p,cr ), the jet in the far field resembles some features of the nonprecessing counterpart. Nevertheless, significant differences still retain in the statistical properties.

The influence of inlet flow condition on the frequency of self-excited jet precession

“…Using a mechanically precessing jet (MPJ) nozzle, Mi and Nathan (2005) and Schneider (1996) demonstrate that St p is the primary parameter controlling the downstream development and mixing characteristics of the non-reacting MPJ flow. In comparison, the influence of Re is negligible.…”

Artificial intelligence control of a turbulent jet

“…The twin-peak behavior is evident along the z-axis due to the flapping motion, but not so along the y-axis. The u rms /U j distributions under combined forcing are asymmetrical about the centre at the jet exit (figure 19c), and again resemble the precession jet (Mi & Nathan 2005). Unlike the case under flapping forcing, the u rms /U j profiles under combined forcing display marked twin peaks along the y and z axes, where the right peak is more pronounced than the left, which is probably linked to the helical motion.…”

“…All the features have been observed in the precession jet. Mi & Nathan (2005) investigated the streamwise development of u rms in a precession jet. As shown in their figure 18, the u rms of the precessing jet is asymmetric compared with a non-precession jet and exhibits two peaks, located tangentially 'in front of' and 'behind' the jet centre.…”

The naturally oscillating flow emerging from a fluidic precessing jet nozzle

“…However, above a critical value, the precession generates a helical flow in the near field associated with a low-pressure core (Schneider et al 1997a). Further increases in St p produce a gradual tightening of the helix (Mi & Nathan 2005), but provide no substantive changes to the qualitative features. These studies, along with Schneider et al (1997b) also reveal that St p has a much greater influence on the flow than does the Reynolds number (Re d = ud/ν), where ν is the kinematic viscosity of air.…”