WakePropRudd - Characterization of the wake of a propeller-rudder system

The interaction between an upstream propeller and a downstream wing or rudder is typical in aeronautical and naval applications. Propellers generate a very complex wake, featuring a wealth of coherent structures, especially the tip and hub vortices, whose evolution is substantially affected by the interaction with downstream appendages. Such interaction increases pressure fluctuations, which define vibrations, cavitation phenomena and the acoustic signature of the overall system. The above phenomena have an adverse impact on the structural integrity of devices (fatigue), the comfort of passengers and crew members on board of ships and airplanes and on the environmental impact of marine and air transportation via noise emission. To date, studies dealing with propeller-rudder interaction are very limited, because of experimental and computational challenges. In particular, numerical simulations require a substantial computational effort to handle bodies in relative motion, achieve high levels of resolution in both space and time, resolve turbulent fluctuations with minimal modeling assumptions and produce a statistical sample large enough to achieve convergence of the statistics of turbulence. In this project we propose high-fidelity computations to capture the complex wake features of a propeller operating upstream of a hydrofoil at incidence, aimed at investigating the impact of this configuration on the topology of the overall wake. Our earlier studies on a similar configuration, but with the downstream hydrofoil aligned with the propeller wake, demonstrated a substantial spanwise shift of both tip and hub vortices. Such shift reinforces their mutual interaction, leading to earlier destabilization and triggering high level of turbulent fluctuations. Downstream of the overall system the wake topology becomes even more complex, since the several components of the wake, especially the branches of the tip and hub vortices coming from the two sides of the hydrofoil and the shear layers shed from the trailing edge of the latter, keep significant values of cross-stream velocity. This promotes further shear, producing a non-monotonic evolution of turbulence, affecting the wake signature of the overall system. The configurations considered in this project introduce significant pressure gradients across the hydrofoil, because of the orientation of the latter at non-zero incidence. Such pressure gradients are expected to produce an even more complex physics and wake signature, mimicking the working conditions of the propeller-rudder system within a scenario of a maneuvering ship. Computations will be carried out using a viscous fluid dynamic solver with optimal conservation properties via a high-fidelity technique (Large-Eddy Simulation, LES), where all important scales of turbulence are fully resolved, instead of being modeled, making the approach also suitable to generate an unprecedented database for future hydro-acoustic studies. The solver has parallel capabilities and its scalability was demonstrated on several distributed memory clusters in the framework of a number of practical flow problems involving turbomachinery, wind turbines and naval hydrodynamics. Results will be compared against those from our earlier studies on the same propeller in isolated conditions and operating upstream of the same hydrofoil at zero incidence, to assess the influence of its incidence angle on the properties of the wake flow.