Turbulent plane jet with and without confining end walls

Abstract

THE published data on turbulent plane jets show variations in both the mean and turbulence quantities that in most instances have been attributed to differences in experimental conditions. The major influencing factors are the inlet geometry and aspect ratio of the nozzle, the nature of the exit velocity profile and the associated magnitude of the turbulence intensity, the Reynolds number at the nole exit, and the state of the ambient environment.1'2 However, one fundamental difference among plane jet apparatus that is not well documented concerns the use of confining end walls and/or a wall section at the plane of exit. This paper describes the characteristics of a plane jet issuing from a wall with and without confining end walls. The objectives are to provide a perspective for the comparison of existing data under these conditions and to identify the relative merits of the two configurations for the design of future experiments. Contents The plane jet flow facility consists of a centrifugal blower that supplies air via a 1.0-m diffuser section to a 400 mm x 600 mm x 12.5 mm settling chamber, followed by a corner section containing 90-deg turning vanes, a second shorter settling chamber, and finally a 40:1 two-dimensional contraction (based on a cosine curve) that ends as a 600-mm long x 10-mm wide (D) slot. The design of the nozzle insures that the exit turbulence intensity is low [(u2)l/2/U0 = 0.008] and that the wall boundary layers at the exit will be laminar. The jet issues vertically from a horizontal wall section extending along the length of the slot and 1.1 m to each side and may be confined by removable vertical walls located at each end of the slot and extending 1.1 m to the sides and 1.85 m downstream, and vertical screens of open air ratio 0.8 spanning the ends of the vertical walls. The jet exit velocity U0 was 11.0 m/s for all of the test runs, and the Reynolds number based on U0 and the slot width D was 7230.3 Mean velocities were measured independently with a pitot tube (connected to an electronic micromanometer) and also simultaneously with a pitot tube and a normal hot-wire (HWA) separated by 5 mm in the spanwise (z) direction. In addition to near-field pitot tube measurements that confirmed the symmetry of the exit profile, detailed lateral velocity profiles were obtained in the far-field regions of the free and the confined jets to identify any structural differences and to determine the extent of two-dimension ality. The far-field profiles for the free jet show considerably more twisting and variability along the length of the slot than those of the confined jet. The walls of the confined jet achieved the expected effect of extending the region of two-dimension ality. For the streamwise region between x/D = 50 and 100, the standard deviation of the growth rates of the confined jet, based on