Transom Flow Elevations in the Partially Ventilated Condition

Predicting separation from a transom stern presents a problem for potential flow calculations. As the vessel speed increases, the transom flow elevation progressively deviates from the static condition. In the absence of viscosity, potential theory is not able to adequately capture this process. Without a priori information regarding the transom flow elevation, potential flow calculations either under or over-predict the resistance. The under-prediction results from placement of the wake sheet at the static condition waterline, and over-prediction results from placement of the wake sheet at the transom/keel intersection (the fully ventilated condition). This paper expands the investigation of transom flow elevations by experimentally measuring the deflection of the flow from the static condition for five different transom configurations ranging from round bilge to deep-vee sections. The models use a common forebody of the vessel. The models were effectively fixed in sinkage and trim. High definition video was used to capture the flow elevations for a range of speeds. This paper presents the findings in terms of the transom Froude number and ventilation factors, and makes comparisons to prior empirical equations. Model data is also provided.

Propeller Wake Sheet Roll-up Modeling in Three Dimensions

An algorithm for predicting the complete three-dimensional vortex sheet roll-up is developed. A higher order panel method, which combines a hyperboloidal panel geometry with a bi-quadratic dipole distribution, is used in order to accurately model the highly rolled-up regions. For given radial circulation distributions, the predicted wake shapes are shown to be convergent and consistent with those predicted from other methods. Then, a previously developed flow-adapted grid and the three-dimensional wake sheet roll-up algorithm are combined in order to estimate the propeller loading/trailing wake interaction. The complete wake geometry is determined by the method without the need of any experimental information on the shape of the wake. Predicted forces and tip vortex trajectories are shown to agree well with those measured in experiments.

An Approximate Theoretical Approach for the Determination of Oscillatory Aerodynamic Coefficients for a Helicopter Rotor in Forward Flight

SummaryA method is described for the determination of the oscillatory aerodynamic coefficients for a helicopter rotor in forward flight. The method treats the blades and the recirculating wakes on a three-dimensional basis, but certain approximations are made so that the problem is tractable. In particular it is assumed that the downwash effect of a wake sheet beneath the rotor can be represented by concentrating the doublet distribution in the sheet in a single line of doublets.