چكيده به لاتين
Appropriate design of flying vehicles can be obtained specifying the relations between aero/hydrodynamic loads and the vehicles kinematic parameters which are expressed as stability derivatives. Computational Fluid Dynamics (CFD) can be applied as an efficient and economical alternative to obtain the static stability derivatives. Also, to accomplish maneuverability study and dynamic analysis, dynamic stability derivatives can also be obtained finding the body responses to some specified periodic time variant motions. Despite improvements in steady flow simulations, efficient modelling of unsteady flow has been yet a challenge in CFD. In numerical simulation of unsteady flow, the solution time history must be resolved accurately. Consequently, the added dimension (time) whould increase the computational cost effectively. In time-accurate solvers by decaying the initial transient state, some unsteady flows reach a periodic steady state solution. Since, the time which is needed to reach the periodic solution is usually much larger than the steady state period, most of the computational time is spent on resolving the initial transient solution.
In order to reduce the long computational time of solving the periodic unsteady flow field, Non-Linear Frequency Domain approach (NLFD) can be proposed. Considering the assumption of solution periodicity, the efficient NLFD algorithm uses Fourier series representation in time. Thus, the periodic solution can be calculated directly while the transient decay is not a component of the solution.
All previous researches have used non-linear reduced frequency approach in compressible form of Navier-Stokes equations. The innovation of this research is to develop a CFD code which simulates unsteady periodic incompressible flows using non-linear reduced frequency approach using incompressible form of Navier-Stokes equations.
The NLFD code validation is performed for some analytical periodic test cases. The capability of the developed NLFD code in predicting the details of the unsteady periodic incompressible flow field is then investigated. The computational cost is also compared for the NLFD and a time-accurate method which shows that the NLFD is approximately 2-6 times faster (based on the harmonics number used) than the time-accurate method.
Afterward the NLFD code is used to calculate the dynamic stability derivatives of a 2-D body in incompressible flows. The results show that NLFD leads to good estimations of the longitudinal and lateral forces and moment coefficients. For all cases, good agreement between the NLFD code results, using a limited number of time varying modes, and the time-accurate solution, confirms the NLFD capability to accurately resolve the flow field.
