The effect of initial conditions on the exit flow from a fluidic precessing jet nozzle

The fluidic precessing jet (FPJ) is a member of a family of self-excited oscillating jet flows that has found application in reducing oxides of nitrogen (NO_x) from combustion systems in the high-temperature process industries. Its flow field is highly three-dimensional and unsteady, and many aspects of it remain unresolved. Velocity data, measured close to the exit plane, are presented for a variety of FPJ nozzles with three different inlet conditions, namely, a long pipe, a smooth contraction and an orifice. The results indicate that jet inlets that are known to have nonsymmetrically shedding initial boundary layers, namely those from the orifice or long pipe, cause jet precession to be induced more easily than the smooth contraction inlet, which is known to have a symmetrically shedding initial boundary layer. The nature of the exit flow is dominated by the degree to which a given configuration generates precession. Nevertheless, the three different inlet conditions also produce subtle differences in the exit profiles of mean velocity and turbulence intensity when the flow does precess reliably. Roman symbols d diameter of inlet (m) - D₁ diameter of FPJ chamber (m) - D₂ diameter of FPJ chamber exit lip (m) - E expansion ratio D₁/d - f frequency (Hz) - f_p precession frequency (Hz) - h step height (D₁-d)/2 (m) - n power law index to describe pipe inlet jet (dimensionless) or nth sample passing through LDA probe volume - N total number of bursts sampled (dimensionless) - r radial distance from FPJ chamber axis (m) - rms root-mean-square or fluctuating velocity component, (m/s) - R₁ radius of FPJ chamber (m) - R₂ radius of exit lip (m) - Re Reynolds number u_id/ (dimensionless) - S(f) arbitrary power spectrum (m²/s) - St Strouhal number, f_ph/u_i (dimensionless) - t_n residence (or transit) time of a particle moving through the LDA probe volume (s) - u axial component of mean velocity (m/s) - u_cl axial component of mean centreline velocity (m/s) - u_i bulk inlet velocity near the inlet plane (m/s) - u_n velocity of the nth particle through the LDA probe volume (m/s) - u_vc axial component of mean velocity in the region of the vena contracta (m/s) - u axial component of rms velocity (m/s) - v radial component of mean velocity (m/s) - v radial component of rms velocity (m/s) - w tangential component of mean velocity (m/s) - w tangential component of rms velocity (m/s) - x axial distance from FPJ chamber inlet plane (m) - x axial distance from FPJ chamber exit plane (m)Greek symbols kinematic viscosity of air at 21°C, 14.7×10^-6 m²/s