Abstr:The plane wall jet
Semi-Confined Flows
Underlying Flow Regime 3-10
Abstract
The underlying flow regime (UFR) – the plane wall jet - documented here is of major importance in many engineering applications, such as film cooling and the automobile demister, but it is also an important generic flow in itself.
After a review of the existing literature, starting with the work of Launder and Rodi (1981, 1983), where the deficiencies of the experimental data for the turbulent wall jet were obvious, a test case for this UFR is chosen, which has been specifically designed to produce high-quality turbulence data for CFD model validation (Karlsson et al. (1993a,b) and Eriksson et al. (1997, 1998, 2002). A synthesis of the work is given in the PhD thesis of Eriksson (2003). The resulting data set is included in the ERCOFTAC data base and has been a test case at two ERCOFTAC/IAHR Workshops on Refined Turbulence Modelling (Paris 1996, Delft 1997). Proceedings from the workshops are available from the editors and through the homepage www.ercoftac.org.
A wall jet may be formally defined as "a shear flow directed along a wall where, by virtue of the initially supplied momentum, at any station, the streamwise velocity over some region within the shear flow exceeds that in the external stream" (Launder and Rodi, 1981). The most well-known everyday example of a wall jet is probably the automobile demister where it is used for heat and mass transfer modifications. (That is, to keep the windscreen free from mist and/or ice.) There are numerous other engineering applications of the wall jet in essentially different areas. Some examples are the film-cooling of the liner walls of gas-turbine combustion chambers and of the leading stages of the turbine itself, submerged bottom outlets in hydropower dams,the flow from the main circulation pumps of internal type in boiler water nuclear reactors and room ventilation concepts.
The wall jet is usually thought of as a two-layer shear flow, where the inner layer (from the wall to ym, the position of maximum velocity) is qualitatively similar to the conventional turbulent boundary layer, while the outer layer (extending from ym to the outer edge of the flow) resembles that of a free jet. The flow situation is sketched in figure 1. The interaction of the two layers naturally modifies them as compared to the generic flows. Some examples are: The growth rate of the wall jet is considerably lower (~ 30%) than that of the plane free jet. The displacement of the position of zero shear stress from the position of maximum velocity, where it would occur in a free jet, to about two-thirds ym. The shear stress in the wall region drops off much more quickly than in a boundary layer. The turbulence intensities are higher in the inner layer of the wall jet, including the limiting values at the wall. How this basic combination of wall-bounded flow and free shear flow behaves, and how the two layers interact to determine the development of the wall jet, is of interest also from a more fundamental point of view.
The wall jet also provides the experimentalist with all the challenges he may wish for. There one finds small scales, large gradients, wall effects, high local turbulence intensity and large anisotropy of the turbulence in the inner layer, and large turbulence intensities (asymptotically - infinite) in the outer layer.
Contributors: Jan Eriksson; Rolf Karlsson - Vattenfall Utveckling AB