Comparison of performance and wake systems between conventional and contra-rotating propellers in thrust similitude

Performance and wake flow of contra-rotating and isolated propellers are compared, based on the data from high-fidelity computations using a large eddy simulation approach on a cylindrical grid consisting of 4.6 × 109 points. Simulations are conducted for working conditions of isolated propellers producing the same overall thrust as the contra-rotating system. Results show the achievement of values of efficiency of propulsion about 10% higher by the contra-rotating propellers. The rear propeller of the contra-rotating system is indeed able to recover most of the azimuthal momentum gained by the flow through the front propeller, which is instead wasted in the case of conventional propellers working alone. This is in line with the improved performance in the former case. In addition, weaker minima of pressure and maxima of turbulent stresses are generated both on the surface of the blades of the contra-rotating propellers and in their wake, with potential benefits to cavitation and acoustic signature. All these advantages are even reinforced moving from design conditions to higher loads, since the achievement of the same levels of overall thrust requires a smaller displacement from the optimal working condition in the case of the contra-rotating system, where the required increase in thrust is split between its front and rear rotors.