3D turbulent flow over irregular bed surfaces

摘要

Features of 3D velocity field in an open channel with smooth and regular rough bed are sufficiently known and understood by civil engineers, whereas flows over irregular bed surfaces are still not fully known. The paper deals with 3D rough turbulent flows in straight open channel with irregular bed roughness. The bed of the channel is characterized by roughness elements which can be uniform or non-uniform in sizes, and which can form regular or irregular surfaces of channel bed. It is assumed that the turbulence structure above the viscous sublayer, over regular and non-regular bed roughness is the same and the logarithmic law (Log-Law) describes this structure. This structure is scaled with height (distance from bed), friction velocity, the absolute size of the elements forming the bed roughness and the coefficient designated by letter "B". An open channel turbulent flow is described by the Reynolds equations with simple turbulent model, which is based on the assumption of isotropic but not homogeneous eddy viscosity at a cross-section. The eddy viscosity is described by an enhanced mixing length hypothesis. The model also takes into account the non-isotropic character of normal turbulent stresses at the channel cross-section. The Reynolds equations with the continuity equation for steady, parabolic 3D turbulent flow in open channel are solved for irregular and different height of rough elements forming the channel bed. It is shown how to define the boundary conditions for this bed and how to estimate the parameters defining the turbulence structure over this bed. The numerical simulations show the disturbances in velocity distribution due to the boundary irregularities. To smooth these disturbances the spatial average operation is applied to get the equivalent Log-Law velocity profile, i.e. the equivalent shear velocity, the equivalent size of roughness elements and equivalent coefficient B. It is shown that the boundary conditions defined by this equivalent velocity give reasonable good prime velocity distribution as well as the secondary flow pattern.