Geoff Cunningham

A numerical investigation of the flow structures and losses for turbulent flow in 90° elbow bends

Abstract Computational fluid dynamic modelling was carried out on a series of pipe bends having R/ r values of 1.3, 5, and 20, with the purpose of determining the accuracy of numerical models in predicting pressure loss data from which to inform one-dimensional loss models. Four separate turbulence models were studied: the standard k—ε model, realizable k—ε model, k—ω model, and a Reynolds stress model (RSM). The results are presented for each bend in the form of upstream and downstream pressure profiles, pressure distributions along the inner and outer walls, detailed pressure and velocity fields as well as overall loss values. In each case, measured data were presented to evaluate the predictive ability of each model. The RSM was found to perform the best, producing accurate pressure loss data for bends with R/r values of 5 and 20. For the tightest bend with an R/r value of 1.3, however, predictions were significantly worse due to the presence of flow separation, stronger pressure gradients, and high streamline curvature.

Camshaft Design for an Inlet-Restricted FSAE Engine

ABSTRACT Restricting the flow rate of air to the intake manifold is a convenient and popular method used by several motor sport disciplines to regulate engine performance. This principle is applied in the Formula SAE and Formula Student competitions, the rules of which stipulate that all the air entering the engine must pass though a 20mm diameter orifice. The restriction acts as a partially closed throttle which generates a vacuum in the inlet plenum. During the valve overlap period of the cycle, which may be as much as 100 volume of exhaust gas therefore increasing high-spe degrees crank angle in the motorcycle engines used by most FSAE competitors, this vacuum causes reverse flow of exhaust gas into the intake runners. This, in turn, reduces the amount of fresh air entering the cylinder during the subsequent intake stroke and therefore reduces the torque produced. This effect is particularly noticeable at medium engine speeds when the time available for reverse flow is greater than at the peak torque speed. The objective of the study described in this paper was to mitigate the reverse flow effect by reducing the duration of the valve overlap period. A thermodynamic model of the Yamaha YZF R6 engine was developed for this purpose and validated using cycle-averaged and crank-angle-resolved test data. The resulting model was then used to find the optimum values of lift, duration and timing for both the intake and exhaust valves. The camshafts required to give these valve lift profiles were designed using valve train analysis software. This process included a consideration of the dynamic forces encountered by the valve train and ensured that the resulting stresses remained within safe limits. The new camshafts increased the torque output by up to 30% at medium engine speeds, without reducing the high-speed torque, and therefore significantly improved the vehicle drivability.