An investigation of a low Strouhal number oscillatory jet submerged in a thin rectangular cavity

Laser Doppler anemometry (LDA) and cinematic particle image velocimetry (PIV) measurements of a low Strouhal number oscillatory jet are compared to two-dimensional computational fluid dynamic (CFD) model predictions. The LDA and PIV measurements are recorded from a water model consisting of a jet generated from a submerged entry nozzle (SEN) immersed in a rectangular cavity. The cavity has a width to depth ratio (W/H) ranging from 0.1< W/H < 1.0 and width to length ratio (W/L) ranging from 0.1 < W/L < 0.5. In the CFD model, the flow emerging from the nozzle is represented as an internal mass source, and the flow past the region occupied by the nozzle (the crossflow region) is incorporated using a flow resistance. The CFD model is developed using the commercial fluid flow software CFX4. Analysis of the temporal LDA and PIV data has found the jet to have sustained oscillations about the broad face of the cavity with Strouhal numbers based on nozzle diameter ranging from 0.001-0.011. LDA analysis of the crossflow between the SEN and cavity walls allowed more detailed characteristics of the jet to be studied and compared with the CFD model predictions. Further analysis of the experimental data indicated that for self-sustaining oscillations to be present there must be a feedback loop through the crossflow region that links the recirculation cells bounding the jet. With this loop present, the Strouhal number of the jet was found to be independent of Reynolds number for a fixed geometry but highly dependent on the cavity width to length ratio. In addition, changing the area available for crossflow did not significantly affect Strouhal number. The crossflow amplitude was found to peak in the range 0.25 < W/L < 0.38 for the water model and 0.38 for the numerical model. Jet oscillation was also stable for a particular cavity geometric range as shown in figure 1. Comparisons of experimental Strouhal number with values derived from the numerical model were found to match to within 10% in most cases. Qualitative comparisons with full field experimental data were found to give good agreement with all the main flow features present in the numerical model. Figure 1. Stability map for jet oscillation in cavity with fixed characteristic length of L = 1050mm 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 0.2 0.4 0.6 0.8 1 1.2 H / W di / W Stable Oscillation Unstable Oscillation