Experimental Research of Turbulence etcI4 Generation in Complex Jet Flows JIANG Yun-xing', XIE Xi-lin2, YU Yu-xuan2, MA Wei-wei lege of Science, Donghua University, Shanghai, CHINA an University, Shanghal, CHINA
TEMPLATE DESIGN © 2007 www.PosterPresentation s.com Introduction Visualization in the test section Nozzles which show in Figure.2 were used to enable flow visualization. The results of the flow visualization study are given in Figure.5 for the rolling up process of vortex rings. Images in Figure.5 are the lower half of the symmetrical pattern when the middle nozzle outlet is the shape of big rectangular. Experimental Research of Turbulence Generation in Complex Jet Flows JIANG Yun-xing1, XIE Xi-lin2, YU Yu-xuan2, MA Wei-wei1 1. College of Science, Donghua University, Shanghai, CHINA 2. Department of Mechanics & Engineering Science, Fudan University, Shanghai, CHINA Some spatial evolution of vortices has been observed and analyzed by self-developed flow facility (Figure.1). De De Table 1. Six middle nozzle shapes with their AR and equivalent diameter Figure 3. The schematic diagram of the experiment. Flow measurements are carried out by constant-temperature hot-wire anemometry. The mean velocity and turbulence measurements are make with DANTEC Streamline and 55P11 hot-wire probes which are calibrated by the calibration equipment. The probes arrangement is shown in Figure.4. Probes 1 and 2 are fixed on the nozzles which are used to perform correlation analysis. Probes 3 to 5 are the moving probes to surveying the test section. Figure 4. The probes arrangement for dynamic surveying. Figure 5. The rolling up process of vortex ring. We observed two types of vortex rings merging. One type (Figure.6) is neighbor vortex rings merged, termed briefly as NVRM hereinafter. Images in Figure.6 are the lower half of the symmetrical pattern when the middle nozzle outlet is the shape of big elliptic. The other (Figure.7) is neighbor vortex rings first merged on the same shear layer respectively, then merged on the tail of the core region, termed briefly as NVRMA hereinafter. Images in Figure.7 are the symmetrical pattern when the middle nozzle outlet is the shape of big rectangular. The leftto-right sequence can be viewed as an idealized instantaneous picture of the mixing layer, or alternatively as a progression in time while riding with the mean speed. Some typical phenomenon are observed directly by using the flow visualization. Furthermore, we research the typical flow field which we studied with dynamic surveying technology. The schematic diagram of the experiment is shown in Figure.3. The airs pass through the nozzle, mix up the smoke and led it to the test section. The laser source is set on the left, which is parallel to the nozzles. The laser beam goes through the cylindrical lens generate a laser light plane and an image acquisition system in the flow field which could be observed obviously. Figure 1 . The sketch of flow facility. We changed the middle nozzle outlet into different geometric configurations for the purpose of qualitative and quantifying the characteristics of the flows. Outlet geometries investigated are square, circular, elliptical and rectangular (Figure.2). Figure 2. The different geometries of the middle nozzle outlet. Different jet configurations with their aspect ratio AR and equivalent diameter in this study are given in Table 1. Figure 6. Neighbor vortex rings merged. Figure 7. Neighbor vortex rings first merged on the same shear layer respectively, then merged on the tail of the core region Figure.8 (a), (b), (c) we have observed that perfect spiraling is restored, which indicates its dynamical stability. The decrease of the separation distance is fairly independent of the Reynolds number. The onsets of the instability leading to the struction of the regular spiral structure are shown in Figure.8 (d), (e), (f), these spirals are stretched and are more and more entangled together by differential rotation. Both models demonstrate very close dynamical behavior. Figure 8. Six structures of helical vortexes. Visualize shown helical vortexes merged and non-merged phenomenon. As shown in Figure.9, nozzle outlet are big rectangular. Figure.9 (a) is the helical vortexes nonmerged phenomenon which found in middle layer Reynolds number is 2542, shear layers Reynolds number are 3812. Figure.9 (b) is the helical vortexes merged at the edge of the core reign which found in both middle layer and shear layers Reynolds number are 4660. Figure 9. The non-merging and merging phenomenon of helical vortexes. The vortex evolution process specifically depends on the middle nozzle outlet geometric configurations. Figure.10 reflects the sensitivity of the vortex evolution to change in Reynolds number. With another word, the curvature of the nozzle outlet in a mixing layer can be greatly influence the evolution of the coherent structures in turbulent shear flows. We can observed directly in the Figure.10, the Reynolds number of vortex rings turn to helical vortex increase with the middle nozzle outlet curvature and aspect ratio. Vortex evolution in the shear layer Figure 10. Vortex evolution with Reynolds numbers (the middle nozzle outlet geometric configurations). Dynamics serving in the test section Figure.11 shows the turbulence main flow velocity profiles for different vortex pattern with different middle nozzle outlet. Figure.11 (a) is the situation of NVRM. Lines 1 to 5 represent the middle nozzle outlet is circular, small elliptic, big elliptic, big rectangular and square, respectively; (b) is the situation of NVRMA. Lines 1 to 2 represent the middle nozzle outlet is small elliptic and big rectangular; (c) is the situation of helical vortices merged. Lines 1 to 4 represent the middle nozzle outlet is circular, small elliptic and big rectangular with different velocities, respectively; (d) is the situation of helical vortices non-merged. Lines 1 to 3 represent the middle nozzle outlet is small rectangular and big rectangular with different velocities, respectively. Our measure show the longest core region happened in the helical vortex merged situation when the middle nozzle outlet is big rectangular (line 3 in Figure.11 (c) and its aspect ration show in Table.1) which Reynolds number is 4660. The second long core region happened in the helical vortex non-merged situation (Figure.11 (d), line 2 reverse direction because of some rotate around the vortex). That is to say helical vortex can significantly improve the length of the core region. Figure 11. The turbulence main flow velocity profiles for different vortex pattern Energy spectra The power spectrum analysis is obtained by fast Fourier transform (FFT) of the velocity fluctuations. Figure.12 (a) to (g) show the turbulence main and shear layers flow frequency profiles for different vortices patterns with different middle nozzle outlet based on spatial FFT analysis of datasets for six thousand successive times. Figure 12. The power spectrum analysis of different vortices patterns. Table.2 summary the main information about Figure.12, including of characterization of frequencies, spatial evolution of dominant frequencies and leading vortices of different vortices patterns. Table 2. Characterization of frequencies, spatial evolution of dominant frequencies and leading vortices of different vortices patterns Cross-spectrum spectra based on two distinct turbulence components recorded at the same point in space. The results of Evolution of cross-spectrum spectra in this study are summarized in Figure.13. (b) and (c) show the broadband frequencies looks like (a) type helical vortex shown in Fig.8. The mechanism of this phenomenon have not clear. Figure 13. Evolution of cross-spectrum spectra in helical vortices merged patterns. These multiple experimental results shown that the curvature of the nozzle outlet in a mixing layer can be greatly influence the evolution of the coherent structures in turbulent shear flows. It also provide an opportunity for some researchers to compare and analyze with the numerical stimulation. 2111329@mail.dhu.edu.cn
