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Sreda, 11. januar 2011. u 18 sati:
Lecture No 1177
Teaching and Research Assistant Đorde Cantrak, Hydraulic Machinery and Energy Systems Department, Faculty of Mechanical Engineering, University of Belgrade
Investigation of the turbulent swirl flows by use of stereo PIV, LDA and classical techniques
Abstract: Investigation of turbulent swirl flows, thanks to its presence in nature and science, as well as to its complexity of the three-dimensional velocity field, has fundamental and practical significance. Experimental investigations have been performed behind the axial fan impeller in a straight pipe in three measuring sections x/D=2.96, x/D=21.1 and x/D=25.92, where ? denotes distance from the test rig inlet ?nd D is average test rig inner diameter. Measurements by use of optical techniques stereo PIV (Particle Image Velocimetry) and LDA (Laser Doppler Anemometry) have been performed in the first and the last measuring section. Measurements with classical probes have been conducted in the second section. These experiments have been conducted for three angle positions of the axial fan and five regimes defined by the rotation number of the axial fan. Application of the stereo PIV offers good spatial, up to 160mm ? 120mm dimension, and acceptable, up to 7Hz, time resolution of the turbulent swirl flow in pipe. Three-dimensional picture of the velocity field is obtained in defined sections. Excellent time resolution of the LDA measurements, over 30 kHz, offers good basis for Reynolds statistics application. However, only one-dimensional system was employed. Thee components were successively measured, without time correlation. Combining stereo PIV and LDA for investigation of unsteady swirl flows, what is here case in the core region allows a more detailed interpretation of velocity fluctuations than that from LDA alone. This is of the significance for turbulence models and numerical simulations which try to resolve fluid flow unsteadiness. Measurements with classical probes for two-dimensional flows in this work, as well in  confirmed difficulties in determination of the pressure and velocity field in the core region. HWA measurements in  confirmed the same conclusion with velocity field. On the other side, good results were obtained in the outer regions. The most recent investigations show existence of the velocity radial component in the core region, i.e. three-dimensional character of the turbulent swirl flow . Investigation of the vortex dynamics in large-scale turbulence is possible with visualization and quantification of smaller vortices and understanding of their contribution to the fluid flow [4, 5]. Reynolds equations are of the great significance for statistical analysis . Visualization and quantification of the vortex structure by use of PIV offers additional possibilities for results interpretation, especially by use of optical systems of great time resolution Time-Resolved PIV (TR PIV). In this lecture will also been shown part of the newest results obtained by stereo TR PIV behind the axial fan impeller. In these experiments were employed high repetition rate laser with maximum frequency of 10 kHz and two speed cameras with 20.000 frames per second each. This technique and obtained results offer excellent basis for application of the different velocity field decompositions which is of importance in the process of resolving turbulence. Keywords: turbulence, swirl flow, PIV, LDA, TR PIV. Acknowledgement This work was, with its great part, supported by the Ministry of Science and Education Republic of Serbia No. ?R 35046, Head Prof. Dr Milan Lecic and Bilateral Project between the Ministry of Science and Education Republic of Serbia and German Academic Exchange Service (DAAD) where the national project leaders are Prof. Dr.-Ing. Svetislav Cantrak and Prof. Dr.-Ing. Martin Gabi. References
1. Beniek ?. (1979): Investigation of the Turbulent Swirling Flows in Straight Pipes, Ph.D. thesis, Faculty of Mechanical Engineering, University of Belgrade, Belgrade, Serbia.
2. Lecic M. (2003) Theoretical and Experimental Investigations of the Turbulent Swirling Flows, Ph.D. thesis, Faculty of Mechanical Engineering, University of Belgrade, Belgrade, Serbia.
3. Protic Z.D., Nedeljkovic M.S., Cantrak Đ.S., Jankovic N.Z. (2010): Novel Methods for Axial Fan Impeller Geometry Analysis and Experimental Investigations of the Generated Swirl Turbulent Flow, Thermal Science, Vol. 14, pp. S125-S139.
4. Adrian R.J., Christensen K.T., Liu Z.-C. (2000): Analysis and interpretation of instantaneous turbulent velocity fields, Experiments in Fluids, No. 29, pp. 275-290.
5. Derksen J.J. (2005): Simulations of confined turbulent vortex flow, Computer & Fluids, No. 34, pp. 301-318.
6. Cantrak S. (1981) Experimental Investigation of the Statistical Properties of Swirling Flows in Pipes and Diffusers, Dr.-Ing. thesis, Faculty of Mechanical Engineering, Technical University Karlsruhe, Karlsruhe, Germany.
SREDA, 18. januar 2012. u 18 sati:
Lecture No 1178
Mr Zvonko Rakaric teaching assistant of Faculty of Technical Sciences University of Novi Sad
The Free and Forced Oscillations of Nonconservative Systems with Fractional Order Restoring Force
Abstract: In this work free and forced oscillations of the systems with a non-negative real power restoring force are studied. In order to analyze free oscillations, a novel perturbative method is developed with a Jacobi elliptic function as a generative solution. The parameters of the Jacobi elliptic function are determined by using the energy conservation law and Hamiltons variational principle. The method is further developed for forced oscillations. Applications of both methods to the systems with different damping mechanisms are given. In addition, the corresponding phenomena are also investigated.
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