Francis X. Caradonna

Hybrid CFD for Rotor Hover Performance Prediction

This paper presents recent advances in a fully coupled hybrid free vortex wake and computational fluid dynamics (CFD) solution methodology for hovering rotor performance calculations. The hybridization process effectively separates the tasks of computing the blade surface flow and the wake convection. The blade loading solution is obtained using a Reynolds-averaged Navier-Stokes (RANS) solver on a relatively small grid surrounding the blade to minimize numerical diffusion, while the far-field wake region is solved using a vorticity-embedding (VE) potential flow solution with a force-free vortex wake. The VE methodology is an unusual type of CFD-based vortex-lattice method, which eliminates the need for expensive Biot-Savart law computations by solving the full-potential equations with an auxiliary velocity field representation of the Lagrangian vortex wake. The VE methodology alone can be applied to the hover problem and is shown to provide robust and accurate predictions, at least for attached flow cond itions. The hybrid approach is more general and can predict a detailed flow field around the rotor blades. The acc uracy of these methods is demonstrated by comparison with UH-60A model rotor performance, wake and loads data.

Development of a New Potential Flow Based Model for Hover Performance Prediction

This paper presents recent developments in hover performance prediction using the potential-based wake method, known as ivorticity embedding.i The method uses a Computational Fluid Dynamics (CFD) solution of the potential equation and captures the vortex-induced velocities as part of the total o w eld. The method uses an auxiliary vortical velocity eld that describes the wake structure and acts as a forcing function to the potential equation. This eld is, in turn, constructed from circulation-carrying wake markers, whose location is determined by a Lagrangian convection. Various aspects of the solution process, and their effects on convergence, are discussed in-depth. Two modes of operation of the method are discussed. The rst of these is a istand-alonei mode, useful for preliminary design, in which the rotor is described as a ilifting surface.i The other, intended for detailed analysis, is a hybrid mode that couples a vorticity-embedding solution for the wake with a conventional Reynolds-Averaged Navier-Stokes (RANS) solver for the blade. These methods are demonstrated by comparison with UH-60A performance, wake and loads data.

A Contour Coupling Methodology for Helicopter Hover Performance Analysis

This paper introduces a novel contour coupling methodology within a hybrid free vortex wake and Computational Fluid Dynamics solution procedure for helicopter rotor blades in hover. Coupling between the outer vorticity-embedded potential flow solution and the inner Reynolds-Averaged Navier-Stokes solver is achieved through the radial distribution of circulation along the rotor blade. The most common approach to obtain the blade circulation distribution is to apply the classical Kutta-Joukowski lift theorem with the blade lift obtained from surface pressure integration. In the present approach, the circulation is determined by integrating around closed sectional contour paths of particular geometry. This is a generalized form of the Kutta-Joukowski theorem, however without the associated flow assumptions. The accuracy of both the new and traditional coupling approaches is demonstrated by comparison against model rotor performance data for the UH-60A blade and the tapered-tip variant under attached/separated flow conditions. The differences in rotor performance predictions between both methods are found to be small and this is attributed to the effect of slightly differing circulation distributions on sectional thrust/torque and wake data. In general, the classical Kutta-Joukowski appears to perform well even with the occurrence of modest amounts of separation.

A Generalized Potential Method for Modeling Rotor Wake Flows

This paper presents a generalized and fast potential method for the computation of rotor wake flows. The potential solver is one that combines an Eulerian description of arbitrary vortical wake structures using the Vorticity-Embedding concept with a Lagrangian free wake advection scheme. Original implementation of the vorticity embedding for hover treated the wake as a single sheet from root to tip and used a cylindrical shaped grid to allow locally two-dimensional calculations for embedding. This approach was shown to be useful for engineering analysis by comparison against performance data for the model UH-60 rotor. A new formulation of embedding further decomposes these vortex sheets into smaller wake patches, each of which corresponds to a dipole. The advantage of this formulation is that it does not require the two-dimensionality assumption and can allow for arbitrary distortions of the wake sheets. This approach is first applied to a simple vortex ring problem and is compared to the original embedding formulation. Following this, the roll-up of vortex sheets is computed for the case of a single vortex ring sheet, a pair of vortex ring sheets, and for an elliptically loaded wing to demonstrate both the capability and the generality of the new embedding process. The paper concludes with a hover-type wake solution in an axisymmetric flowfield using the new embedding process, and gives an outlook on future developments that will extend the method to forward flight and full-vehicle configurations.