Steady streaming

The time-average of a fluctuating flow often results in a nonzero mean. Suc h steady streaming may result directly from the action of an oscillatory no nconservative body force or, if such a force is conservative, indirectly th rough the action of Reynolds stresses in the main body of the fluid or in t hin boundary layers at no-slip boundaries. A theory for such streaming flow s is developed for homogeneous fluids. For flows with a free surface it is demonstrated that a mechanism for steady streaming exists even in an invisc id fluid. Diverse areas in which steady streaming flows may be important ar e discussed.