Laser Doppler Velocimeter Measurements in 3-D Impinging Twin-Jet Fountain Flows

Abstract

Mean velocity and turbulence measurements were conducted on the three-dimensional fountain flow-field generated by the impingement of two axisymmetric turbulent jets on a ground plane; application was to vertical-take-off and landing (VTOL) aircraft. Three configurations with different jet spacing and height above the ground were considered resulting in different characteristics of the opposing wall jets that form the fountain. Basic instantaneous velocity data were obtained by use of a two-component laser Doppler velocimeter (LDV) in a plane connecting the nozzle centerlines at different heights above the ground. The jet impingement region and the fountain upwash region formed by the collision of the wall jets were emphasized. The distributions of mean velocity components and turbulence quantities, including the turbulence intensity and the Reynolds shear stress, were derived from the basic velocity data. Detailed studies of the characteristics of the fountain revealed self-similarity in the mean velocity and turbulence profiles across the fountain. The spreading of the fountain was found to be linear and the spreading rates equal for all three jet impingement configurations. The mean velocity decay was found to be inversely proportional to the height above the ground; the rate of decay was equal for the three configurations. The spread and mean velocity decay characteristics closely resemble those of radial jets. The fountain was observed to be highly turbulent with maximum turbulence intensities in the range of 50–60%.