Development and scaling of turbulent, twin round jets

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. 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 = 5.585/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.

Automated summary

This study investigates the velocity field characteristics of stationary, turbulent twin round jets. Large-eddy simulations were conducted to obtain the flow fields, revealing that the two jets develop independently and merge into a single jet with an elliptic cross-section downstream. The merged jet becomes circular after a certain distance. The fluctuation levels of the merged jet scale with the local maximum mean velocity, and the mean streamwise velocity reaches a peak at a certain distance from the inflow plane. The velocity and length scales of the merged jet are connected to the input parameters through simple relations, and the far field development can be scaled using intrinsic scales.