Experimental Investigations of the Effect of Reynolds Number on a Plane Jet

The effect of Reynolds number, μ / Re , H U c o = , where Uo,c is the nozzle exit centreline velocity, H is the slot-opening width and n is the kinematic viscosity of air) on the velocity field of a turbulent plane jet from a radially contoured nozzle of aspect ratio 60 is investigated. Measurements are conducted using a single wire anemometer over an axial distance of 160h. The Reynolds number is varied between 1,500 and 16,500. Results show that the Re affects various flow properties such as the velocity decay rate, half-width and turbulence intensity. The significant dependence on Re of the mean flow field persists till Re = 16,500, while the Re effect on the turbulent properties becomes weaker above Re = 10,000. The present investigation also suggests that an increase in Re leads to a higher rate of mixing in the near field but a lower rate in the far field. Introduction A true plane jet issues from a rectangular nozzle of high aspect ratio, w/H (where w is the short side and H is the long side of the rectangular nozzle). The jet is confined within two parallel side walls, attached to the short sides of the nozzle. This configuration enables the jet to spread in the lateral (y) direction only. Some applications of a plane jet include engineering environments, e.g. in propulsion units and lift-producing devices, Quinn [1], air curtains and some combustion applications. It is well established experimentally that, at sufficiently high Reynolds number, the normalized mean velocity profiles and the spreading rates of a round jet are almost independent of Reynolds number (Panchapakesan and Lumley [1], Hussain et al. [3]). The visualizations of Mungal and Hollingsworth [4] for their jet at very high Reynolds number (Re ~ 2 × 10) showed that its spreading rates and normalized mean velocity profiles are close to those of a laminar jet. In contrast, at low Reynolds numbers, both the mean and turbulent flow fields depend significantly on Re. This is evident, from Oosthuizen [5] for round jet, measured at moderate Re. Likewise, both Lemieux and Oosthuizen [6] and Namar and Otugen [7], studied the effect of Reynolds number in a plane free jet, over the Re ranges of 700 – 4200 and 1000 – 7000, revealed that Re influenced the entire mixing field. When Lofdahl et al. [8] investigated a plane wall jet, they found the jet spreading rates to remain variable even up to Re = 20,000. An investigation by Stanley et al [9] and Klein et al [10] at Re = 3,000 and Re ~ 6,000, respectively, was conducted only in the near and transition fields (x/H ≤ 15 and x/H ≤ 20) even though Klein et al [10] claimed they covered the far field. Clearly, wide range of different conditions makes it impossible to isolate the effect of Reynolds number from that of other variables. Hence conclusive quantitative data describing the Re dependence of a plane jet is not available. To address this issue, we have carried out an experimental investigation into a plane jet at Re between 1,500 and Re = 16,500, using a radial contraction nozzle with aspect ratio of w/H = 60, bounded by side walls, over a wider Re-range, and flow region up to x/H = 160. Experimental Details The overall jet facility, shown in Figure 1, has been described in detail in Deo [11]. Figure 1: Schematic diagrams of the plane jet facility, showing the wind tunnel, plane nozzle arrangement and side-walls. The present plane nozzle is referred to as a radial contraction nozzle r/H ≈ 2.14, where r is the nozzle exit radius), to differentiate it from the smooth contraction nozzles used in most previous investigations (e.g. Gutmark and Wyngnanski [11], Namar and Otugen [7], Bradbury [13] and Antonia et al [14]). For a comprehensive description of the radially contoured nozzle, see Deo [11] and Deo et al [15]. A constant temperature hot-wire anemometer (CTA) was employed to undertake measurements. A copper plated tungsten single hot wire probe, length lw ~ 1 mm and diameter dw ~ 5 μm was mounted parallel to the z-axis of the traverse. Hot wire calibration was performed in the jet potential core (turbulence intensity ~ 0.5%. Control of the jet Reynolds number, μ / Re , H U c o = , was achieved by varying the speed of the wind tunnel fan. The maximum achievable Reynolds number was 16,500, without reducing the nozzle aspect ratio. The range of Reynolds numbers was selected to be 1,500 ≤ Re ≤ 16,500. The extent of measurements along the axial direction was 0 ≤ x/H ≤ 160, to include the near, transition and far fields. Results and Discussion