Steady streaming

The time-average of a fluctuating flow often results in a nonzero mean. Such steady streaming may result directly from the action of an oscillatory nonconservative body force or, if such a force is conservative, indirectly through the action of Reynolds stresses in the main body of the fluid or in thin boundary layers at no-slip boundaries. A theory for such streaming flows is developed for homogeneous fluids. For flows with a free surface it is demonstrated that a mechanism for steady streaming exists even in an inviscid fluid. Diverse areas in which steady streaming flows may be important are discussed.