A Study On Fractal Characteristics Of Open Channel Flow Turbulence Based On PIV Technology

The liquids mixed with each other in the turbulence motion, and keep astatic andirregular motion. It is a complicated, unordered and random unpredictable system.For along period of time, we have not breakthrough on the analysis of turbulence motion byclassical physics viewpoint which adopts traditional linear mathematical analysismethod, as the turbulence shows significant nonlinear characteristics.There is a nestedstructure in the turbulence. The turbulence is comprised by vortexes. The ones are in thebig ones, while the smaller ones are in the small ones.There is a dissipative structureduring the procedure of turbulence energy attenuation. The vortexes would occur thephenomenon of cascade division. The driving force of liquid motion produces thebiggest vortex. And this vortex would cascade step by step and finally come into beingsmaller ones during the development of the turbulence. This process could have severallevels. The vortexes will stop splitting until the force of intermolecular could inhibit thefurther cascade. It means the scale of the vortex has reached the dissipative scale η. Thenest and dissipation is a significant self-similar structure.The turbulence motion accordswith two characteristics of fractal theory subjects. On the one hand, it has nocharacteristic length, on the other hand, the part and whole has self-similarity.This paper adopts pmma launder simulates open channel turbulence motion. Inallusion to the randomness and self-similarity of turbulence motion, we measure itsvelocity field with PIV velocity measuring technique. The fractal theory and statisticaltheory have been used to analysis the fractal characteristics and turbulence intensity ofopen channel turbulence, and also analysis the transmission and dissipation ofturbulence energy.The measurement of the velocity vector field of open channelturbulence shows that the turbulence velocity is considerable out-of-order. It will showirregular and mutative fluctuation. The result of isovel graph shows turbulence hasvortex structure. The small vortexes are in the big ones, while the smaller ones are in thesmall ones, and these vortexes have large scale span.Turbulence velocity signals sequence is another reflection of turbulence. Solvingthe fractal dimension of turbulence velocity time signals is1.6376, which is closed totheoretical fractal dimension5/3. While the fractal dimensions of velocity in space areD_X=1.5137，D_Y=1.6159. Solving correlation of fractal dimensions are all above0.88,which declares signals of turbulence velocity surely have self-similarity in some scales.With the development of turbulence, the fractal dimensions of turbulence velocitywould not change over time, it is a invariant metric of constant turbulence. However, thefractal dimensions of velocity will diminishing with the space development, whichreflects the procedure of gradual dissipation and attenuation of turbulence energy.With solving of the intensity of open channel turbulence by statistical theory, wefind that the turbulence intensity distribution also shows randomness and irregularity,and it also shows a gradually dissipation trend with the development of turbulence space.Both fractal dimension and turbulence intensity can indicate the intensity ofinstantaneous velocity fluctuation, which is similar to the research result of fractaltheory. Otherwise, the fluctuation tends to be gentle is the performance of turbulenceenergy dissipation.We analysis the turbulence energy dissipation with the development of turbulencespace with the measurement results of PIV, it indicates that the attenuation of turbulenceenergy mostly attributes to the kinetic energy dissipation.For high Reynolds number, thepulsation deformation rate S’ is much bigger than time-averaged pulsation deformationrate S. Therefore, the turbulence kinetic energy dissipation is unlimited by time-averageand boundary conditions. It shows a significant decreasing tendency. Dimensionalanalysis indicates that turbulence energy dissipation rate ε=Au~3/l. We also analysis thevortex cascade model of turbulence. The driving force of fluid motion produces thebiggest vortex. The big ones will cascade into smaller vortexes with development, untilit reaches the dissipative scale η. This kind of energy dissipative cascade model is Fmodel which fractal dimension is2.5~2.6. It is similar to fractal sets. Analysis showsthat dissipation rate is not in direct proportion to turbulence kinetic energy and intensity.We still need demonstrations with a plenty of tests and theory models.