Coupling of an unsteady aerodynamics model with a computational fluid dynamics solver. Momentum source methods are an efficient means of representing airfoils in Navier-Stokes CFD simulations. Momentum source terms are added to the Navier-Stokes equations instead of resolving the solid boundary of the airfoil with a mesh. These source terms are calculated using an aerodynamics model. This approach is useful where the overall performance and midto far-field influence of a wing or rotor are desired and details of the flow field near the blade are not the objective of the simulation. One example is simulation of rotorcraft operations where the objective may be to assess an operation's feasibility in terms of control margins, rather than to inform rotor design decisions. Coupling an aerodynamics model to a CFD solver is straightforward in cases where the airflow relative to the blade is steady. Unsteady conditions require an unsteady aerodynamics model, complicating the coupling with the CFD solver. A coupling method is proposed whereby the incident velocity is extracted from the CFD solution and corrected using a theory based approximation for the unsteady induced velocity. The steady-state momentum source method is demonstrated for 2D and 3D simulations and the unsteady coupling method is validated against experiments on a pitching airfoil and verified for blade-vortex interactions. The unsteady coupling method enables meaningful incident velocities to be extracted from unsteady flow-fields, as shown by agreement with experiments and simulations using analytical expressions for the incident velocity in place of the CFD solver.

The application of viscous-flow solvers to calculate the forces on ship hulls in oblique motion has been studied for a long time. However, only a few researchers have published work in which the flow around ships in steady turns was studied in detail. To predict ship manoeuvres, an accurate prediction of the loads due to rotational motion is also required. In a collaborative CFD exercise, the Submarine Hydrodynamics Working Group (SHWG) performed calculations on the bare hull DARPA SUBOFF submarine to investigate the capability of RANS viscous-flow solvers to predict the flow field around the hull and the forces and moments for several steady turns. In the study, different commercial as well as bespoke flow solvers were used, combined with different turbulence models and grid topologies. The work is part of a larger study aiming to improve the knowledge and understanding of underwater vehicle hydrodynamics. In this paper, the results of the exercise will be presented. For several cases, verification studies are done to estimate the uncertainties in the results. Flow fields predicted by the different members of the SHWG are compared and the influence of the turbulence model will be discussed. Additionally, the computed forces and moments as a function of the drift angle during the steady turns will be validated. It will be demonstrated that using sufficiently fine grids and advanced turbulence models without the use of wall functions will lead to accurate prediction of both the flow field and loads on the hull.

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The ship airwake is one of the most important components of a ship-helicopter dynamic interface simulation as the airwake determines many of the operating limits during dynamic interface operations. As part of the development of a dynamic interface simulation we have compared different Large Eddy Simulation (LES) methods for simulating ship airwakes using two test cases: a cube in a channel and a simplified frigate shape. The cube case was used to compare a compressible Implicit Large Eddy Simulation (ILES) solver (Flamenco) with compressible and incompressible standard LES solvers (OpenFOAM). The simplified frigate shape case compares LES with and without wall modelling with ILES and provides validation over a range of wind-over-deck angles. The results are presented as comparisons of time-averaged velocity and pressure profiles with experimental data and flow field visualizations. The comparisons between LES techniques may be used to inform simulation design choices for flow around ships and other sharp-edged bluff-bodies.

One of the major problems in computational aero-acoustics is the disparity in length scales between the flow field and the acoustic field. As a result, a mapping function is normally used to achieve a non-uniform grid distribution. In this paper, a B-spline collocation method with an arbitrary grid placement capability is proposed. This capability not only allows an optimum grid distribution but also avoids the numerical complexities associated with the mapping function. The B-spline collocation method is applied to the case of spinning co-rotating vortices. The result agrees well with the matched asymptotic solution.