A new analysis tool, an unsteady Hybrid Navier-Stokes/Vortex model, for a horizontal axis wind turbine (HAWT) has been developed for yawed flow by coupling a prescribed wake Vortex Line Method (VLM) with an unsteady Navier-Stokes solver, and its convergence and computational cost have been studied for 10 and 20 degrees of yaw. In this study, a steady viscous solution of a hybrid method is compared in detail with a full-scale Navier-Stokes simulation as a validation. Furthermore, the unsteady hybrid solver is applied to the NREL Unsteady Aerodynamics Experiment (UAE) Phase VI rotor. A test case under 10 degrees of yaw shows that the global power output agrees well with the NREL experiment, and 10 cycles of computation require less than three days using a work station under a serial CPU simulation. The same simulation performed using a super computer is used as reference, and it is estimated that the equivalent case can be obtained about 8 times faster using the work station with the present method, while keeping the same level of accuracy, than a full-domain Navier-Stokes simulation. To treat high yaw cases, new distorted prescribed vortex sheets are modeled with the VLM code. To see the difference between the base helix and the distorted helicoidal wake, unsteady hybrid VLM/Navier-Stokes solutions are examined for selected azimuth angles, and results are contrasted to a free wake BEM and NREL experimental data. For low yaw angles, the base helix approximation agrees well with the distorted helix, and yield better prediction than a free wake model, whereas for high yaw angles of more than 30 degrees, the present distorted wake gave a lower estimation of rotor torque than the free wake solver.
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