Presently, several techniques for analyzing ducted propellers are available; they include: the vortex lattice method (VLM) on the propeller and the boundary element method (BEM) on the duct, the BEM on both the propeller and the duct, the Reynolds-averaged Navier–Stokes (RANS) method on the duct and the BEM or the VLM on the propeller and finally, the RANS method on both the propeller and the duct. Regardless, the analysis of ducted propellers is still a non-trivial task due to the complex flow interactions between the propeller and the duct. Fortunately, the RANS method has shown some promise as the flow separation can be captured.
However, simulation costs using this approach are astronomical and the situation gets dire owing to the time requirements (for meshing and post-processing). As such, more efficient analytical tools are needed for ducted propellers, particularly in the design stage. Moreover, the panel method does not apply to hydrofoils, propellers and ducts with blunt trailing edges due to the flow separation downstream. To this end, numerous publications have been presented regarding the subject matter; however, improvements of the most commendable designs are still necessitated.
In a recent research paper published in Journal of Fluid Mechanics, scientists at University of Texas at Austin Weikang Du (PhD candidate) and Professor Spyros Kinnas developed an improved extension model to represent the local flow separation downstream of blunt trailing edges in hydrofoils and propeller ducts. They focused on adopting the criteria of zero lift and zero moment to determine the end of the extension zone, and flow separation criteria to determine the starting points on either side of the section.
They considered incompressible, fully wetted and steady flow, i.e. without any cavitation or vortex shedding in their numerical approach. They coupled the flow separation model with the viscous/inviscid interaction (VII) method, and the numerical results obtained from their model were compared and contrasted with those obtained from the Reynolds-averaged Navier–Stokes (RANS) method and the experimental procedures.
The authors observed that the pressure distributions and skin frictions along the hydrofoils and ducts correlated well with those from the Reynolds-averaged Navier–Stokes method. In addition, it was seen that the thrust and torque of the propeller agreed much better with experimental measurements when the extension was determined from their model rather than choosing random locations.
In summary, the Weikang Du-Spyros Kinnas study presented a numerical model to predict the mean steady flow separation that could facilitate the integration of the panel method to hydrofoils, propellers and ducts with blunt trailing edges. Generally, the presented model was seen to demand much less in terms of simulation time and computational resources while at the same time preserving high accuracy. Altogether, the presented model can serve reliably and efficiently in the design and analysis of hydrofoils and propeller ducts with blunt trailing edges. Moreover, this flow separation model can be extended from 2D to 3D to so that the viscous/inviscid interaction method can be applied to propellers with blunt trailing edges.
Weikang Du, Spyros A. Kinnas. A flow separation model for hydrofoil, propeller and duct sections with blunt trailing edges. Journal of Fluid Mechanics (2019), volume 861, page 180–199.
Weikang Du, Spyros A. Kinnas. A 3D flow separation model for open propellers with blunt trailing edges. 6th International Symposium on Marine Propulsors, Rome, Italy, May 2019.Go To Journal of Fluid Mechanics