ANSYS comes with its own meshing software, ICEM CFD. It can import STL geometry. Once you've imported your geometry to ICEM CFD, you will need to create a material point that is internal to geometry, that is, it is inside of your orta. Then, for example, if you plan to simulate flow through the aorta, you will assign aorta walls the "Wall" boundary condition, as well as define inlet and outlet. Also you will need to set meshing parameters such as minimal and maximal cell size and parameters that determine the meshing of curved geometrical entities. The best idea is of course to refer to ICEM CFD documentation. I hope this helps. Good luck!