Laser Velocimeter Measurements in Highly Turbulent Recirculating Flows

Abstract

This paper presents the results of an extensive study of subsonic separated flows using a laser Doppler velocimeter. Both a rectangular rearward facing step and cylindrical (axisymmetric) sudden expansion geometry were studied. The basic objectives were to resolve the question of whether a velocity bias error does, in fact, occur in LDV measurements in highly turbulent flows of this type and, if so, how it may be eliminated; map the velocity field (mean velocity, turbulence intensity, Reynolds stress, etc.) including the entire recirculation zone; and compare experimental results with numerical predictions based on the k-ε turbulence model. Measurements were carried out using a one-dimensional LDV operating in forward scatter with signal processing by means of a commercial counter-type processor. Results obtained show that velocity bias does occur in turbulent flows and that it can be overcome by proper data acquisition procedures. The results also indicate that the important mean velocity and turbulence quantities can be obtained with reasonable accuracy using a one-dimensional LDV system. Although the k-ε turbulence model provides a good qualitative picture of the flow field, it does not yield a completely adequate quantitative description. Results obtained here illustrate the discrepancies to be expected and provide a basis for further model development.

Investigation of turbulent transport in an axisymmetric sudden expansion

“…3, although the centerline velocity is first underpredicted and then overpredicted. This was also found in previous studies by Stevenson et al 20 and Durrett et al 10 The large discrepancy between the predicted and measured axial velocity near the centerline at x/H > 10 does not imply a mass balance error since the flow area for r/R <0.2 accounts for only 4% of the total flow area. The model also predicts higher negative axial velocities in the recirculation zone than the experimental measurements indicate.…”

“…Typically, less than 50 data points out of the 6400 were discarded. This is higher than the theoretical number of discards for a 3a cutoff (i.e., 20) because the accompanying sample was discarded if either one of the two simultaneous measurements was rejected.…”

PIV measurements in a turbulent wall jet over a backward-facing step in a three-dimensional, non-confined channel

“…In our experiments, the turbulence intensity of the incoming wall jet reaches approximately 24% of the reference velocity U m . This level was higher than that of the duct flow in the studies of Stevenson et al [58], Vogel and Eaton [61] and Ötügen [46] and strongly enhances the crossstream diffusion momentum [28]. Finally, it is necessary to note that the turbulent wall jet over a backward-facing step in this study is characterized by three length-scales, i.e., the vertical height y m of the inner region, the jet half width Y 1=2 for the wall jet flow, and the step height h for the backward-facing step flow.…”

The effect of expansion ratio for creeping expansion flows of UCM fluids

“…Most of these Newtonian turbulent works are concerned with the corresponding axisymmetric geometry [2,3] given their easier construction and inherent symmetry. Limited results for viscoelastic fluids in turbulent flow have also been reported in both planar [4] and axisymmetric geometries [5][6][7].…”