Measurement and Calculation of Developing Turbulent Flow in a U-Bend and Downstream Tangent of Square Cross -

Experimental measurement and numerical modeling has been performed on the low in a 180(DEGREES) bend of square cross-section, preceeded and followed by straight ducts of the same cross-section. Measurements of mean velocities and their associated turbulent stresses along the streamwise ((THETA)) and the gapwise or radial (r) directions have been made at several longitudinal planes, using the nonintrusive laser-Doppler velocimeter technique in backscatter mode. Mean flow data reveal features in qualitative agreement with results obtained from inviscid flow analysis. Measurements of the turbulent stresses display previously undocumented anisotropic characteristics arising from shearing motions induced in the core of the flow. In the downstream tangent, measurements show that drastic reductions of the secondary motion take place in less than 5 hydraulic diameters. From then on, however, the flow recovers only slowly from the effects of the bend. Numerical predictions using a k-(epsilon) model of turbulence have been performed for the experimental configuration. Model deficiencies are more clearly revealed by the higher order accuracy of the finite differencing scheme used and the implementation of a partially-parabolic calculation algorithm. It is found that, with the problem of numerical diffusion relieved, false physical diffusion and the isotropic characteristics inherent in the model are the main causes for the differences observed between measurements and computations. Additional predictions with an algebraic stress model (ASM) show poor agreement with the measurements. As a result, two "experimental" tests have been carried out to check indirectly the predicting ability of the ASM closure. It is found that the proposed model is capable of resolving the anisotropy of the turbulence correctly, provided that the mean velocity field is known accurately.