The grid quality is very critical issue when one try to analyze the characteristics of fluid with CFD technique. There is no way to get good enough numerical result without high quality grid system. With that reason, many different types of grid have been developed up to now and many related research are ongoing today also.
There are some specific cases which usual single grid system is not proper to apply to. When the shape of given body is so complex such as aircraft landing gear, it is almost not possible to apply just single kind of grid such as rectangular type or O type grid. If there are several number of bodies in a domain, one should depart the domain as sub domains anyway to apply computational scheme. And if bodies are moving with time going, In this time, the body fitting grid should follow the moving body without any time delay.
In 1981, a brilliant idea had been published. That was overset grid or overlapped, chimera grid. Overset grid does not mean a kind of grid type but group of sub grids which can be different kind of types. When there is a airfoil with flap, two independent grid could be created with overset grid technique. And at the intersection of th sub-grids, data exchanging process exists to pass the fluid information to each other by means of interpolation. Overset grid is also applicable to moving body, because sub-grid can move to anywhere independently from background grid. The area the overset grid can cover is so huge.
However overset system needs a few additional algorithm which is not necessary to general single type grid. The first one is domain connectivity technique. Since independent calculations are conducted at each sub-domain, the information of each grids should be exchange after each time marching. This process determines the quality of overset grid technique. How much the interpolation error can be reduced is dominant issue. The second thing is the hole points treatment. Hole point which is the grid point existing inside the body should be removed at the time marching because there are no fluid exist.
Up to now days, sub-grid has been used as body fitting grid which is attached to moving body or separated body usually. But sub grid can be used as dense mesh which covers the fluid domain where high characteristics gradient exist such as vortex. In this case, there are nobody which can be the base of sub-grid. so automatic gradient detecting algorithm should be added.
In this thesis, high oder overset grid technique has been developed which can be applied to high order CAA(Computational AeroAcoustics) technique. CAA is a branch of CFD which is focus on the acoustics instead of the fluid itself and needs higher oder of accuracy and higher oder of resolution scheme than conventional CFD scheme. The results of several acoustic propagation and scattering problems are included in this thesis. And finally vortex-airfoil interaction problem is calculated with moving overset grid technique. In this problem a dense sub-grid moves with the vortex by means of vortex detecting and tracking technique. By using vortex tracking overset grid, good enough result has been obtained with small number of mesh points compare to other researcher.
CAA(Computational AeroAcoustics)는 CFD의 한 부류로서 유체 그 자체보다는 음파의 거동에 더 초점을 두고 해석하는 학문이다. 유동에 비해 상대적으로 크기가 작은 음파를 해석하기 위해서 CAA는 일반적인 CFD기법보다는 고차 정확도의 해석기법을 사용한다. 그러나 고차 정확도의 해석기법을 사용하게 됨으로써 격자의 성능에 매우 민감하여 복잡한 형상을 대상으로 한 문제에 적용시키는데 한계가 있어왔다. 본 논문에서는 CAA에 사용되는 고차 고해상도 해석기법에 적합한 중첩격자 기법이 제시되었다. 중첩격자기법을 사용할 경우, 여러 개의 물체가 존재하는 경우나 복잡한 형상에 대해서 비교적 쉽게 해석을 수행할 수 있으며, 전체적인 격자의 수도 줄일 수 있다. 오차 해석을 통해 4차 정확도의 라그랑지 내삽법을 제안하여 사용하였다. 또한 와류를 따라 움직이는 이동 중첩 격자 기법이 사용함으로서, 낭비되는 격자의 수를 줄였다. 제안한 이동중첩격자기법을 기반으로 검증 문제를 해석하였으며, 이를 통해 음파의 전파와 산란 현상을 성공적으로 해석할 수 있음을 보였다. 또한 와류-에어포일 간섭 현상을 해석하여 에어포일 표면에서의 물성치는 물론 원거리에로 전파되는 음파를 해석하였다.
