Turbulence Under Various Influences and Physical Circumstances

As mentioned in chapter 1, there are many factors and influences which cause a real turbulent flow in nature and technology to deviate from the idealized homogeneous and isotropic state, sometimes strongly. In the latter case, turbulent flows may lose most of their resemblance to the three-dimensional homogeneous isotropic flow, but can be quite similar to the (quasi-) two-dimensional one. Here we are inevitably back also to the origins of turbulence but with a different focus: the source sustaining the turbulence, such as mean shear, buoyancy, electromagnetic forces, shock waves. In other words there are different kinds of influences. The ‘simplest’ influences are ‘one way’¹. They modify the turbulent flow in some way, but are neither influenced by the turbulent flow nor sustain it, e.g., rotation under some conditions. Another kind of influences are ‘two way’. They are truly interactive in the sense that along with modifying the turbulent flow they are changed by the flow too, e.g., magnetic field, density stratification and other active scalars. Under certain circumstances these influences contribute also to production and sustaining of turbulent flows, such as in the case of turbulent convection. Turbulent flows with mean shear (strain) belong to this latter category.

What follows in the sequel is an overview of turbulent flows under various influences and physical circumstances, which include shear, buoyancy, rotation, (electro-) magnetic field, compressibility and additives. For obvious reasons the material of this chapter is limited by only the most important essential features, simple examples and qualitative aspects with references on comprehensive sources and some early and latest papers.