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With the progress of super computers in recent years, a number of studies on “Computational Fluid Dynamics” (CFD) have been carried out, and various schemes for Navier-Stokes equations have been presented. Similar methods have also been applied to automotive engineering - aerodynamics, for exampre - in order to determine flow phenomena. In this paper, the application of numerical simulations to the flow cavitation that occurs in some part of orifices in the vehicle hydraulic system, will be discussed. Authors have developed a CFD program for the clarification of flow phenomena in such orifices. Using the relationship between calculated results and measured results of noise levels in such orifices, a new method for estimation of the occurrence of flow cavitation has also been developed. As a result, a new orifice configuration capable of preventing the cavitation has been designed.

On diesel vehicles, we frequently experience the phenomenon that low frequency fore and aft vibration of a vehicle, occurs at acceleration, does not decline in amplitude easily and rather increases finally. The phenomenon has recently attracted great concern in driveability problems on diesel vehicles. This phenomenon can not be explained well by the simple torsional vibration model of the powertrain with one node, which has been used so far successfully to analyze low frequency fore and aft vibration of a vehicle. So, we have assumed that the undamped vibration occurs through the interaction of the engine and the powertrain. Taking this interaction into consideration, we have constructed a simulation model, with which the undamped vibration can be simulated accurately. By this simulation model, we have estimated the order of magnitude of the effects of various design parameters affecting the undamped vibration.