On the Numerical Prediction of Rotor Wakes Using Linear and Non-Linear Methods

Flow fields around helicopter rotors are extremely complex. Thus, the solution of the governing equations can usually only be achieved by numerical methods. Different levels of approximation are used in the industrial design process. The rotor wake system must in any case be represented appropriately. Linear and non-linear methods as well as the coupling of both approaches are discussed. The linear methods are based on the solution of the Laplace equation. The corresponding integral equation is solved via boundary element methods including free wake representation. Sophistication towards non-linearity and compressibility calls for the implementation of field panel methods, where the boundary element approach is extended by using spatial distributions of non-linear source terms, thus solving the Poisson equation. Euler methods are capable of describing transonic effects. The resulting gain in accuracy is, however, achieved at the expense of substantial increase in computational effort. Due to the complex kinematics of the system, computation of a single grid is nearly impossible. The Chimera approach provides a possible workaround procedure. The coupling of Euler and free wake panel methods is used to significantly reduce computational requirements. Special attention is devoted towards wake generation and wake capturing procedures within the aforementioned methods.