Influences of initial velocity, diameter and Reynolds number on a circular turbulent air/air jet

This paper assesses the suitability of the inflow Reynolds number defined by Re_o ≡ U_oD/\nu (here U_o and D are respectively the initial jet velocity and diameter while \nu is kinematic viscosity) for a round air/air jet. Specifically an experimental investigation is performed for the influences of U_o, D and Re_o on the mean-velocity decay and spread coefficients (K_u, K_r) in the far field of a circular air jet into air from a smoothly contracting nozzle. Present measurements agree well with those previously obtained under similar inflow conditions. The relations K_u ∝ U_o and K_r ∝ 1/U_o for U_o < 5 m/s appear to work, while each coefficient approaches asymptotically to a constant for U_o > 6 m/s, regardless of the magnitudes of Re_o and D. It is revealed that Re_o may not be an appropriate dimensionless parameter to characterize the entire flow of a free air/air jet. This paper is the first paper that has challenged the suitability of Re_o for turbulent free jets.