Axisymmetric Jet Control Using Passive Grids

The structure of a circular jet, modified by placing grids of two different configurations near the nozzle exit plane, was investigated by using a single hot wire probe. An annular grid which perturbed mainly the axisymmetric shear layer and a circular disk grid which covered most of the potential core were used to modify the jet passively. Behind the annular mesh, the width of the shear layer is reduced and a significant decrease in turbulence intensity is observed. Spectra of the velocity fluctuation show the suppression of the formation of organised structures in the jet mixing layer. In contrast to the annular grid, the influence of the disk grid on the mean velocity and turbulence intensity is not as significant. Evidence for the presence of large scale structures downstream of the edges of the grids is presented. Introduction Turbulence control or management in different flows is a rapidly expanding field of research because of potential benefits in industry. In particular, in plane and axisymmetric jets, extensive investigations have been carried out by using active techniques for turbulence enhancement as well as suppression, but investigations using passive techniques are not as extensive. The main objective in jet turbulence control is to target and interfere with the organised vortical structures that are formed due to Kelvin–Helmholtz instability in the mixing layer. Tong and Warhaft [11] and Parker et al [8] obtained a significant reduction of turbulence intensity by placing a thin wire ring in the shear layer near the nozzle exit plane. The width of the shear layer was reduced and the spectra of velocity fluctuations indicated the suppression of organised large structures in the mixing layer of the jet. Stephens [9] and Stephens et al [10] used grids and honeycombs to completely cover the nozzle exit and obtained similar results. Burattini et al [2] suggested that the grids suppress the shear layer instability and damp the jet column instability. Another passive technique employed in a plane jet is to place a bluff body like a cylinder in the potential core and use the vortex shedding behind it as a feed back mechanism to control the shear layer instability [6, 3, 5, 1]. Antonini and Romano [1] disturbed a circular jet by placing a long, thin cylinder parallel to the nozzle exit plane and obtained an increase in mixing. In jet turbulence control using a grid, apart from the influence of the mesh size and wire diameter of the grid, the circular edge is also likely to have an impact on the jet development. By using wire screens to partially cover the test section of a wind tunnel, Oguchi and Inoue [7] produced a mixing layer similar to that developing behind a splitter plate. Smoke flow visualisation indicated the presence of large scale structures downstream of the grid in the mixing layer. When a thin wire was attached to the screen edge, vortex shedding behind the wire also dominated the flow. Use of a narrow rectangular mesh or a small diameter disk grid yielded a wake. Spectra of velocity fluctuations in the wake produced by a rectangular screen (Zhou and Antonia [12]) clearly showed the presence of organised vortical structures even though there was no primary vortex street. Hydrogen bubble flow visualisation (Finlay [4]) in the wake of a rectangular grid (Figure 1) shows the formation of large structures downstream. Figure 1: Hydrogen bubble flow visualisation of structures behind a grid (from [4]). The main aim of the present work is to investigate the effect of placing an annular grid and a disk grid at the nozzle exit plane on the development of a circular jet by using a single hot wire probe. The annular grid perturbs the initial shear layer and a small peripheral part of the potential core; the disk grid covers most of the potential core. Experimental Conditions The jet was produced by a 55 mm diameter (d) circular nozzle to which air was supplied by a variable speed centrifugal blower via a diffuser, settling chamber and contraction which had an area ratio of 85:1. Two annular and two disk grids were used in this Case Grid Wire dia. (mm) Aperture size (mm mm) Outer dia. (mm) Inner dia. (mm) 1 Annular 0.5 1.25×1.25 100 30 2 Annular 0.2 0.54×0.54 100 30 3 Disk 0.5 1.25×1.25 30 − 4 Disk 0.2 0.54×0.54 30 − Table 1: Summary of mesh geometry for disturbed flow conditions. study and Table 1 gives the details of the grid geometry, mesh and wire size. Hereafter, measurement conditions will be referred to as Cases 1, 2, 3 and 4. The grid was isolated from the body of the jet by mounting it on a separate frame which was placed at a distance of 2 mm downstream of the nozzle exit plane. Figure 2 shows the nozzle, the coordinate axes and the four grids. Measurements of the mean velocity (U) and the longitudinal velocity fluctuation (u) were made by using a single hot wire probe operated in a constant temperature mode. The hot wire, made of 90% Pt-10% Rh, had diameter was 2.5 μm and the wire length to diameter ratio was nearly 200. A height gauge with a resolution of 0.01 mm was used to move the probe in the ydirection. The hot wire signals were digitised using a 12 bit A/D converter and stored on a PC for further processing. Annular Grid Disk Grid