Transient and Steady Flow Computations for an Electro-Mechanical System

The computation of transient turbulent flows has considerable engineering significance. During the period from the mid 1980's until now Computational Fluid Dynamics (CFD) has been used in a wide range of industries. The use of CFD in electronic/mechatronic system design is just one of many examples. The flows in these systems effect component operating temperatures and also the deposition of contaminants. Both of these factors are strongly related to product reliability. As with many other engineering systems, most forced convection flows in electronics/mechatronics are transitional or turbulent. A significant amount of numerical work has been motivated by the need to predict fluid flow in electronic systems. Much of this is reviewed by Tucker (1997), being mostly for steady flows. However, Reindl et al. (1991) study transient laminar flow in a square enclosure with a heated wall. Also, Shy and Rao (1993) predict transient laminar free convection around an enclosed vertical channel. The application of CFD to mechatronics is discussed by Tucker and Hewit (1996). The present work attempts to investigate the performance of turbulence models when predicting both transient and steady flows for the geometry presented in Figure 1. The flow inlet area shown has a suddenly opening shutter, giving rise to an in rush of air from an external air source. The geometry is quite specific being part of an electronic/mechatronic Automatic Teller Machine (ATM). However, the results produced here have a wider context. They are suggestive of the choice of turbulence models when predicting flows in systems with similar flow features. These include streamline curvature, large vortical structures and stagnation resulting from an impinging rectangular jet. The turbulence models tested include the standard k-ε model (Launder and Spalding, 1974), low (Wolfshtein, 1969) and high Reynolds number k-l models, and a standard mixing length (ml) model. Also, two zonal models are tried using the k-ε model away from walls with the k-l and ml models applied elsewhere.