INVESTIGATION OF THE TURBULENT RANKINE VORTEX IN THE PIPE BEHIND THE AXIAL FAN IMPELLER USING OPTICAL (PIV AND LDA) MEASUREMENT METHODS AND VISUALIZATION

Đorđe S. Čantrak

Abstrakt: Turbulence, as one of the unsolved problems of classical physics, attracts the attention of many researchers. At this point, the study of the turbulent swirling flow, with a circumferential velocity profile characteristic for the combined forced-free Rankine vortex, in a pipe using two optical measurement methods for measuring the flow velocity PIV (particle image velocimetry) and LDA (laser Doppler anemometry) is presented. Experimental results, as well as the text of the publication, for the most part, represent excerpts from the author's doctoral dissertation, as well as his authorial and co-authored scientific papers.

Analyzes of the statistical characteristics of the studied turbulent swirling flow in the pipe behind the axial fan impeller, which is designed to generate a Rankine vortex, are based on the optical measuring techniques PIV and LDA. Stereo PIV anemometry was used, as well as stereo PIV anemometry with fast cameras and lasers (stereo time-resolved PIV), as well as one-component and two-component LDA systems. Some of the obtained experimental results are presented. Their operating principles are, also, discussed in short. Numerous possibilities of analysis of the obtained experimental results are presented.

The used mathematical apparatus for complex analyzes is introduced. With the help of the correlation-spectral theory of turbulence, a mathematical interpretation of the structural analysis of turbulence is given. Physical interpretations of complex interactions of mean and fluctuating velocity fields that characterize turbulent transfer processes are presented.

The phenomenon of the vortex core precession is studied in particular. The influences of Reynolds and vortex numbers, as well as the axial fan impeller rotation speed on these phenomena are analyzed. The physical interpretation of the experimental data indicates important structural properties of the vortex turbulent core and shear layer. Experimental-correlation analysis investigates the evolution of statistical characteristics and correlation moments, on the basis of which it is concluded that there is a significant inhomogeneity and anisotropy of turbulence. The measured distributions of turbulent stresses using the stereo TR PIV measuring technique enabled the formation of invariant anisotropy maps, so that the anisotropy and the structure of turbulence in the core, shear layer and ground flow were discussed. Additional physical information on the structure of turbulence was obtained from experimentally determined autocorrelation functions and integral scales of turbulence, as well as by analyzing the spectral function of circumferential fluctuation velocities.

 Key words: turbulence, Rankine vortex, axial fan, anisotropy, invarants, LDA, PIV.