SELF-SIMILARITY OF PASSIVE SCALARS IN A DECELERATING JET

Scalar mixing is investigated in a decelerating turbulent round jet using direct numerical simulation. The mass fraction of jet fluid and the fluid residence time, measured by the mass-weighted age of the jet fluid, both exhibit self-similar radial profiles in statistically-stationary turbulent jets. Upon stopping the inflow, a deceleration wave passes through the jet, behind which a new self-similar state is observed for the two scalar variables. The self-similar state during the jet deceleration is different from that in the statistically-stationary jet. Opposite to its behaviour in the steady-state jet, the jet fluid mass fraction exhibits a linear increase with downstream distance in the decelerating jet, whereas the centreline mass-weighted age of jet fluid remains proportional to downstream distance. The dynamics of the scalar mixing in the transient self-similar state are discussed through analysis of the scalar transport budgets.