Laser Doppler anemometry measurements of a confined turbulent water jet with a uniform background flow

An axisymmetric, turbulent water jet with several very slow, coflowing external streams was measured with a frequency shifted laser Doppler anemometer. The objective was to approximate a jet submerged in an ambient fluid of infinite domain by using a confined jet in a uniform coflow. The coflow prevents flow reversal outside the jet, but if the coflow velocity is not small compared to the jet velocity, the jet will no longer be self-preserving. Thus, the objective is reached in the limit as the coflow approaches zero, but in the absence of reverse flow. In the present study, a jet with several slow coflows was examined to investigate this behavior, the data was extrapolated to the limit (U_s/U_j) to obtain the free jet results and reduce uncertainty in earlier data.

The Reynolds number based on the jet diameter and exit velocity was 32100. Conservation of momentum of the jet was demonstrated up to the measurement limit of x/d = 100. Its distribution suggests that the near fleld axial pressure variation has significant effects on the momentum flux. The results also indicate that momentum flux measurements require accurate data to the edge of the jet. The similarity of mean and rms velocity profiles suggest the existence of a region of self-preservation.

The entrainment rate, centerline velocity decay rate and spreading rate of the jet were determined and compared to previous measurements with and without a coflowing stream. The variation of these jet parameters with respect to the veIocity ratios was obtained. The limiting values of the jet parameters were determined by extrapolation to zero velocity ratio.

This study indicates that a slow coflowing stream is an ideal way to eliminate the recirculating zone present outside jets without coflows. By reducing the coflow to a negligible velocity with constant Craya-Curtet number, researchers can greatly reduce the wide experimental variation in jet entrainment and spreading rates found in different facilities. The results also indicated that a confined jet with a very slow coflow without recirculation can asymptotically approach the conditions of a free jet. An estimate of the variation of the duct size versus the velocity ratio is obtained. It suggests that it is not possible to reduce the velocity ratio to an arbitrarily small value without backflow because the duct would become impractically large.