Effects of a duct before a propeller on propulsion performance

We numerically investigated the effect of a duct on propeller performance. Numerical simulations were performed for a wide range of diameters (0.7D≤DD≤1.0D) and angles of attack of the duct (0°≤θ≤20°), where D is the propeller diameter. The duct profile is NACA0015 whose chord length is 0.3D and trailing edge is located 0.2D upstream from the center of the propeller. The propeller force coefficients presented the same increasing pattern and the rate of the propeller efficiency with the duct to the bare propeller augments with increasing θ, irrelevant to DD. However, the net thrust reflecting the total drag, including the duct, depends on the duct size. Hence, there is a critical value of θ such that the magnitude of the total drag becomes maximum for a relatively large duct size. A certain size of duct leads two regions of the inflow into the propeller, which is originated by the duct. One is the inner region of the duct where the high-speed flow exists because of the contraction of the duct. Another is the outer region of the duct where the flow decelerates due to the duct wake. The circumferential locality of the axial velocity becomes weaker with increasing θ due to the accelerated flow along the axial direction. Eventually, lower and higher pressure coefficients on the suction- and pressure-side surfaces, respectively, cover a much wider area than those of the case without the duct, leading to greater thrust and torque.