Accuracy of transient versus steady state forces on a rudder operating in a propeller slipstream
In computational fluid dynamics (CFD), a transient simulation is in general more costly than computing the steady state of the system, if such a state exists. The velocity field produced by the propeller blades upstream of a rudder is transient in nature, and rudder design using CFD may therefore become very time-consuming. If a steady solution could accurately predict the performance of the rudder, such an approach would be favourable. The aim of the present study was to assess the possibility to accurately predict the performance of a rudder operating in a propeller slipstream using steady state simulations, e.g. an actuator disk model (ADM). For this reason, the performance of the two-dimensional NACA 0021 rudder section submitted to a sinusoidal transverse gust, representing a transient propeller slipstream, was simulated using ANSYS Fluent. The predicted force coefficients are presented for a number of gust amplitudes, mean angles of attack and reduced frequencies of the transverse gust. The simulations have shown that the modelling error introduced when predicting the performance in a steady state is highly dependent on all these parameters of the actual transient flow, and that the steady result may be a severe over- or under-prediction of the real performance of the rudder. Heavily loaded propellers are suspected to be less suitable for ADM modelling in rudder performance prediction. The predicted unsteady lift coefficient was compared to the linear theories of Horlock and Sears, and the agreement was fair at zero mean angle of attack but poor at a mean angle of attack of 10°. It was also found that the predicted performance of the rudder was significantly altered when the chord based Reynolds number was increased by a factor of 10, which has implications on the validity of model-scale simulations. The effect of including turbulent transition modelling for some of the simulations was also investigated, and the discrepancy in predicted performance was found to be considerable. Due to the formation of a laminar separation bubble the predicted trailing edge separation and viscous stress on the rudder were significantly decreased, leading to better overall performance.
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