On laminar wall jets

Abstract

The name “wall jet” is applied to a jet of fluid which spreads out over a surface, the fluid outside the jet being at rest. It may, for example, be produced by allowing a free jet to impinge normally on a plane surface. A discussion of the phenomenon on the basis of boundary layer theory, for both laminar and turbulent incompressible flow, has recently been given in a paper [1] by the author (1956). For laminar flow it was shown that a similarity solution exists in which the velocity profile does not vary along the jet length, and is the same for both radial and two-dimensional wall jets. In the present paper this similarity solution is obtained in a generalised form, with an additional length parameter governed by the distance which the jet profile takes to assume its final similar shape. Mr. N. Riley has shown that the similarity solution is also applicable when the fluid is compressible or heated, if it is assumed that the coefficient of viscosity is proportional to the temperature, and his results are briefly discussed, with his permission. Finally, consideration is given to perturbations of the basic similarity solution due to a small external flow, as might arise as a consequence of entrainment of fluid into the wall jet.