Measurements, Computation and Analysis of a Semi-Constrained, Axisymmetric Impinging Jet

Experiments, closed-form analysis and computation were conducted to investigate flow in a semi-constrained, axisymmetric impinging jet. The aspect ratios (i.e. plate separation divided by jet diameter, S/D) were 0.25 and 0.59. Reynolds numbers based on jet diameter were approximately 7,600 and 17,700. Data collection was via hot-wire anemometry and flow visualization. Data consisted of a series of mean velocity profiles across the wall-jet boundary layer in the attached-flow region, power spectral distributions at selected points within the boundary layer and flow visualization. Spectra were used to help identify transition to turbulence of the wall layer. Flow visualization was conducted by seeding the flow with neutrally-buoyant bubbles, illuminating the bubbles with a laser sheet and capturing pictures with a standard 35mm camera. An approximate closed-form analysis is presented. It gives theoretical values of shear stress of a stagnation flow which approximately models the target plate flow and yields a set of parameters for generalizing the experimental and CFD results. Numerical simulations of the flow based upon solution of the RANS equations and various turbulence closure models are presented. Results of analyses with three different closure models are discussed in this paper: a) all laminar (no turbulence), b) k-ω turbulence closure model and c) k-ε turbulence closure model. The results from the k-ω model are generally in better agreement with the experimental results than are the k-ε model results. The performance of the three models is discussed. A need for proper modeling of transition within the flow is shown. This study serves to support computation of flows which have impingement, acceleration, adverse pressure gradients, transition to turbulence and flow separation using the low-aspect-ratio, semi-constrained, steady impinging jet.