In previous chapters, we have discussed the kinematic structure of a flow, but made no reference to the external action that is necessary for establishing the flow or to the physical mechanism that is responsible for sustaining the motion. Consideration of these issues requires an under-standing of the hydrodynamic forces developing in a fluid as a result of the motion. In this chapter, we discuss the physical origin of these forces and introduce constitutive equations relating the stresses developing at the surface of infinitesimal fluid parcels to the parcel deformation. In subsequent chapters, we shall use these constitutive equations to build an integrated theoretical framework that will allow us to compute the structure of a steady flow and the evolution of an unsteady flow from a specified initial state.
KeywordsShear Rate Stress Tensor Wall Shear Stress Surface Force Edge Force
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