EXPERIMENTAL-STUDY OF PLANE TURBULENT WAKES IN A SHALLOW-WATER LAYER

Shallow two-dimensional turbulent wake flows have been studied experim entally on a large water table. In the experiments, the ambient Reynol ds number Re-n = UaH/nu, in which U-a is the depth-averaged ambient ve locity, H the water depth, and nu the kinematic viscosity, is large, w ell above a lower critical value of the order of 500 for open-channel flows so that the ambient base flow is fully turbulent. Different type s of blunt bodies extending over the full depth are inserted in that b ase flow, including cylinders and flat solid and porous plates oriente d transversely to the ambient flow. In all cases, the transverse body dimension D greatly exceeds the water depth, D/H much greater than 1. With that condition, the wake Reynolds number Re-d = UaD/nu is very la rge, greater than 10(4). The shallow near-wake characteristics of plan e wakes from blunt bodies extending over the full water depth have bee n found to fall into one of three classes: (i) the vortex street (VS) type with an oscillating vortex shedding mechanism, (ii) the unsteady bubble (UB) wake type with flow instabilities growing downstream of a recirculating bubble attached to the body, and (iii) the steady bubble (SB) wake type with an attached bubble followed by a turbulent wake t hat contains no growing instabilities. When Re-h > 1500, the flow clas sification is uniquely dependent on a shallow wake parameter, S = c(f) D/H in which c(f) is a quadratic law friction coefficient. For circula r cylindrical bodies the VS-UB transition is characterized by a critic al value, S-ca approximate to 0.2, and the UB-SB transition by S-cc ap proximate to 0.5. Solid plates, oriented transversely, differ by a fac tor of 1.25. The shallow far-wake behavior has been investigated with a special variable porosity wake device that reduces the wake velocity deficit and completely suppresses the VS instabilities in the near-fi eld. Thus, only UB and SB wake types are found in that case. Furthermo re, the shallow plane wake is observed to ''stabilize'' for large down stream distances, x/H, in the sense that the growth and maintenance of the large scale structures in the wake flow become suppressed and the wake collapses into a more ordered flow that, however, still contains small scale (of scale H) turbulence. This wake stabilization is contr olled by two factors: first, the usual evolution in a turbulent wake t hat reduces the velocity deficit while increasing the wake parameter S , and secondly, the exponential loss of the momentum deficit flux in t he wake due to bottom friction.