IMPINGEMENT OF AXISYMMETRIC DEVELOPING JETS

The impingement of turbulent jets on solid boundaries has been studied extensively in the past. It has been established that flow parameters in the impingement region are governed by the impingement height, H and the initial kinematic momentum flux of the jet. This results from the fact that the jet can be considered as a point source of momentum, whenever it is fully developed before entering the impingement region. Obviously, this finding is suspect in cases of “small” values of H. An experimental and analytical study of developing jets, impinging on a smooth wall, is presented. Measurements of the velocity field, static pressure in excess of ambient, and wall shear stress are reported. Analysis of the results shows that impingement region flow parameters arc governed by the diameter, d and the intial jet velocity, U() but are independent of H. The impingement region was found to extend 1.2d above the wall and 1.4d radially from the stagnation point. Semi-empirical methods for predicting the velocity field, static pressure and wall shear stress are developed. The radial growth of the jet from the stagnation point, including the wall jet region, has also been analyzed. Using the quantitative relation governing the stagnation pressure as a criterion, it is proposed tha values of H less than approximately 5.5d be considered as “small”: greater than about 8.3d as “large”, the range 5.5d ≲ H ≲ 8.3d being transitional.