The effect of mechanical tabs on the vortical and turbulent structures in the near fields of jet mixing flows is investigated in the present paper. In order to compare the changes of the vortical and turbulent structures of jet flows with and without mechanical tabs, flow visualization and instantaneous quantitative concentration field measurements were conducted in a water channel by using a laser induced fluorescence (LIF ) technique. The flow visualization confirmed the existence of a pair of counter-rotating streamwise vortices produced by each mechanical tab in a jet flow. The generated streamwise vortices can cause an inward indentation of ambient flow into the core jet flow and an outward ejection of core jet flow into the ambient flow. It also showed that the process of Kelvin-Helmholtz vortex pairing was accelerated, the small-scale vortical structure appeared earlier and a large-scale helical coherent structure was found in the near fields of tabbed jet flows. Based on the flow visualization and instantaneous quantitative concentration field measurements, two aspects of the effect of the streamwise vortical pairs induced by mechanical tabs on the jet mixing flows were suggested.speeds and high temperatures. Ahuja [3] reported that, C06297

Experimental research for seven exhaust ejector system combinations, including four two-dimensional (2-D) lobed nozzles and three 2-D mixing tubes, has been conducted to investigate the effect of the lobe structure on the aerodynamic performances of the ejector systems. Test results show that: 2-D eligned lobed nozzles have better pumping and mixing enhancement abilities than the 2-D staggered lobed nozzles, but they also cause bigger pressure losses; the 2-D lobed nozzle with equal lobe width have better aerodynamic performances than the 2-D lobed nozzle with bigger inner lobe width. 2-D aligned scalloped lobed nozzle has the best pumping and mixing enhancement performances, but it also has the biggest pressure loss coefficient. Among the tested combinations, the combination of 2-D staggered lobed nozzle with equal lobe width and 2-D mixing tube with multiple-ring cooling structure has the best combined aerodynamic performances.

The jet axial velocity decay and velocity distributions of two 2D Lobed Nozzles (2DLN), and three baseline nozzles, one circular and two rectangular with different aspect ratios (AR), were measured and compared at low velocity (M0 <0.35) and ambient temperature conditions. The five nozzles have the same exit area. Test results show: 1) The jet axial velocity decay of the 2DLN may be characterized by three distinct regions; i.e., “potential core region”, where the jet axial velocity almost remains constant; “rapid decay region”, where streamwise and normal vortices play major roles for enhanced mixing; and, “smooth-down decay region”, where the mixing process is dominated by viscous shear layer spreading and small-scale turbulence. 2) The jet potential core length of the two 2DLN tested is only half to one-third that of the baseline rectangular nozzle (AR=4.37), and about one-fourth to one-sixth compared with the baseline circular nozzle. The length, in which the jet mixed with surrounding air is nearly uniform, is one to two times that of the 2DLN equivalent exit diameter, and depends on lobe design.

No abstract

Full Navier Stokes Analyses have been conducted for the flows behind the trailing edge of a lobed forced mixer. The governing equations are derived from the time-dependent compressible Navier-Stokes equations and discretized in the finite-difference form. A simple two-layer eddy viscosity model has also been used to account for the turbulence. Computed results are compared with some of the velocity measurements using a laser-Doppler anemometer (Yu and Yip (1997)). In general, good agreement can be obtained in the streamwise mean velocity distribution but the decay of the streamwise circulation is underpredicted. Some suggestions to the discrepancy are proposed.