Development and scaling of turbulent, twin round jets

Abstract

The velocity field of stationary, turbulent, twin round jets has been found to scale with an intrinsic velocity $U_0$ and length $L_0$ , both depending linearly on inflow plane parameters – jet velocity $U_j$ , diameter $d$ and distance between jets $S$ . Flow fields were obtained from large-eddy simulations at these conditions in two experiments: (1) at Reynolds number ${Re}=230\,000$ based on $U_j$ and $d$ , and $S/d=5$ ; and (2) at ${Re} = 25\,000$ , $S/d = 2, 4, 8$ . Each jet develops independently and then merges into a single jet with an elliptic cross-section. Downstream, the jet becomes circular after a mild overshoot. Close quantitative agreement with experiment was obtained in all cases. As the merged jets develop, fluctuation levels over a central half-width are nearly uniform and scale with the local maximum mean velocity. In all cases, the mean streamwise velocity along the centreline of the configuration, $U_c$ , rises to a peak $U_0$ at a distance $L_0$ from the inflow plane. The velocity $U_0$ decreases and $L_0$ increases with $S$ . For all nozzle spacings, a similar development was observed: $U_c/U_0$ is a function of distance $x/L_0$ only, and is essentially independent of $S/d$ and ${Re}$ . Further, these intrinsic and input quantities are connected by simple relations: $U_0 = U_j/(1.02S/d + 0.44)$ and $L_0/d = 5.58S/d - 1.16$ . The far field development of the merged jet can also be scaled with $U_0$ and $S$ , analogous to round jet scaling with $U_j$ and $d$ . Thus all twin round jets may be described by these new intrinsic scales.