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Streamline curvature in the plane of the mean shear produces large changes in the turbulence structure of shear layers, usually an order of magnitude more important than normal pressure gradients and other terms in the mean-motion equations for curved flows. The effects on momentum and heat transfer in boundary layers are noticeable on typical wing sections and are very important on highly-cambered turbomachine blades: turbulence may be nearly eliminated on highly-convex surfaces, while on highly-concave surfaces momentum transfer by quasi-steady longitudinal vortices dominates the ordinary turbulence processes. The greatly enhanced mixing rates of swirling jets and the characteristic non-turbulent cores of trailing vortices are also consequences of the effects of streamline curvature on the turbulence structure. A progress report, comprises a review of current knowledge, a discussion of methods of predicting curvature effects, and a presentation of principles for the guidance of future workers.
A technique for improving the numerical predictions of turbulent flows with the effect of streamline curvature is developed. Separated flows, the flow in a curved duct, and swirling flows are examples of flow fields where streamline curvature plays a dominant role. A comprehensive literature review on the effect of streamline curvature was conducted. New algebraic formulations for the eddy viscosity incorporating the kappa-epsilon turbulence model are proposed to account for various effects of streamline curvature. The loci of flow reversal of the separated flows over various backward-facing steps are employed to test the capability of the proposed turbulence model in capturing the effect of local curvature. The inclusion of the effect of longitudinal curvature in the proposed turbulence model is validated by predicting the distributions of the static pressure coefficients in an S-bend duct and in 180 degree turn-around ducts. The proposed turbulence model embedded with transverse curvature modification is substantiated by predicting the decay of the axial velocities in the confined swirling flows. The numerical predictions of different curvature effects by the proposed turbulence models are also reported. Cheng, Chih-Hsiung and Farokhi, Saeed Unspecified Center DUCTED FLOW; EDDY VISCOSITY; FLOW DISTRIBUTION; K-EPSILON TURBULENCE MODEL; MATHEMATICAL MODELS; SEPARATED FLOW; TURBULENT FLOW; BACKWARD FACING STEPS; COMPUTATIONAL FLUID DYNAMICS; CURVATURE; SWIRLING...
Concave curvature has a relatively large, unpredictable effect on turbulent boundary layers. Some, but not all previous studies suggest that a large-scale, stationary array of counter-rotating vortices exists within the turbulent boundary layer on a concave wall. The objective of the present study was to obtain a qualitative model of the flow field in order to increase our understanding of the underlying physics. A large free-surface water channel was constructed in order to perform a visual study of the flow. Streamwise components of mean velocity and turbulence intensity were measured using a hot film anemometer. The upstream boundary was spanwise uniform with a momentum thickness to radius of curvature of 0.05. Compared to flat wall flow, large-scale, randomly distributed sweeps and ejections were seen in the boundary layer on the concave wall. The sweeps appear to suppress the normal mechanism for turbulence production near the wall by inhibiting the bursting process. The ejections appear to enhance turbulence production in the outer layers as the low speed fluid convected from regions near the wall interacts with the higher speed fluid farther out. The large-scale structures did not occur at fixed spanwise locations, and could not be called roll cells or vortices. (Author).
Turbulent transport of momentum, heat and matter dominates many of the fluid flows found in physics, engineering and the environmental sciences. Complicated unsteady motions which mayor may not count as turbulence are found in interstellar dust clouds and in the larger blood vessels. The fascination of this nonlinear, irreversible stochastic process for pure scientists is demonstrated by the contributions made to its understanding by several of the most distinguished mathematical physicists of this century, and its importance to engineers is evident from the wide variety of industries which have contributed to, or benefit from, our current knowledge. Several books on turbulence have appeared in recent years. Taken collectively, they illustrate the depth of the subject, from basic principles accessible to undergraduates to elaborate mathematical solutions representing many years of work, but there is no one account which emphasizes its breadth. For this, a multi-author work is necessary. This book is an introduction to our state of knowledge of turbulence in most of the branches of science which have contributed to that knowledge. It is not a Markovian sequence of unrelated essays, and we have not simply assembled specialized accounts of turbulence problems in each branch; this book is a unified treatment, with the material classified according to phenomena rather than application, and freed as far as possible from discipline-oriented detail. The approach is "applied" rather than "pure" with the aim of helping people who need to under stand or predict turbulence in real life.