FVV Reports

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A preceding research project on the lubrication flow of journal bearings had shown that a three-dimensional simulation of the lubrication film flow may improve the cavitation prediction for radial journal bearings by 50 %. The aim of the follow-up project was to extend the newly developed substructure model to the requirements of radial journal bearings under unsteady load and to predict cavitation caused by shaft displacement. The extended substructure model is based on a three-dimensional two-phase numerical model computing the Navier-Stokes as well as the Rayleigh equations. It was carefully validated: The computed three-dimensional flow field was compared with the data of a flow experiment; unsteady cavitation phenomena were compared with literature data and, for the first time, validated against a filmed vapour cavitation process of a radial journal bearing-like flow. Improvements on the three-dimensional substructure model include the shaft travelling in the radial journal bearing. With the aid of the new model all known forms of cavitation in radial journal bearings described by literature can be predicted, i.e. flow and shock cavitation as well as suction-induced and exit cavitation. The numerical results were found in excellent agreement with experimental findings documented in literature. Its high performance can be shown using the example of the constructional design of the oil grooves: The minimum oil cavitation can be identified by parameter variation. The combined effort of standard 2-D-methods in conjunction with a 3-D-substructure can obtain both an acceptable computational effort and the necessary resolution in critical areas to predict cavitation properly. Good simulation results always depend on the correct description of the processes at the edge of the simulation area. In the case of journal bearings, it would be necessary to evaluate the flow directions at the bearing side, where a still little known oil collar influences the lubrication film flow. Investigating into a better description of the boundary conditions and the characteristics of that oil collar would help to improve the precision of both models.

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