Fundamentals of Turbulent Flows

In turbulent free-shear flows, fluid streams interact to generate regions of turbulence that evolve without being limited or confined by solid boundaries. Such interactions create mean shear, which is a source of turbulent kinetic energy that results in enhanced flow mixing. Far downstream, the flow retains little memory of its origins and exhibits self-similar behavior. Its mean velocity profile, turbulence intensities, and Reynolds stresses, when scaled appropriately, become independent of downstream distance as it freely expands into its surroundings. Free-shear flows occur in combustors, vehicle wakes, and jet engine exhaust. We focus our attention on three canonical categories of such flows: jets, wakes, and mixing layers. A detailed similarity analysis of the plane jet is provided alongside summarized results for the plane wake and mixing layer. We introduce examples involving turbines in wind farms and drag on wake-generating bodies. The notion of entrainment, which is central to the expansion of free-shear flows, is discussed. We also examine the scales and structural features of turbulent free-shear flows, including streamwise rib vortices and spanwise rollers.