Flow in a turbocharger turbine is highly unsteady in nature as it responds to the exhaustudmanifold of an internal combustion (IC) engine. Despite this it is conventionaludto use quasi-steady turbine models in one-dimensional turbocharged engine simulations,udeven though they cannot reproduce the known hysteresis of turbine mass flow and performanceudcharacteristics recorded under pulsating flow conditions. Using filling-and-emptyingudmodels improves the situation by permitting mass accumulation in the turbineudvolute. Depending on the unsteadiness level, this approach may still be insufficient toudcapture true turbine operation since neither method can resolve unsteady effects due toudpressure wave action in the flow. It is unclear when transition occurs between filling-and-udemptying and wave action modes.udTo this end, a proprietary computational gas dynamics code in C++ is presented toudsimulate the unsteady, compressible flow inherent to IC engine exhaust manifolds. TheudEuler equations for one-dimensional inviscid flow are discretized to provide second-order,udconservative, shock-capturing finite difference schemes able to resolve wave propagationudin ducts with area variation, wall friction and heat transfer. A wave action turbineudvolute model is constructed using bespoke boundary conditions. Validation against experimentaluddata shows satisfactory agreement for pulse frequencies up to 40 Hz, andudimproved instantaneous swallowing capacity prediction at all tested frequencies comparedudto quasi-steady calculations.udFourier series characterization of on-engine pulse waveforms reveals multiple harmonicudcomponents, causing significant regions of divergence between filling-and-emptying andudwave action predicted hystereses. Comparison of concurrent wave action and filling-and-emptyingudsimulations applying simpler sinusoidal waveforms allows development of theudunsteadiness measures FSt and FSt(p). An approximate guideline to ensure a filling-and-emptyingudmode stipulates FSt [Symbol appears here. To view, please open pdf attachment]0.15 and FSt(p) [Symbol appears here. To view, please open pdf attachment]0.02. Evaluation of FSt and FSt(p)udfor an example on-engine case indicates certain wave action already by 1600 rev/min,udborne out by subsequent inspection of the swallowing capacity traces.
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