1. A computer-readable, non-transitory storage medium storing a program for causing a computer to execute a process, the process comprising: identifying an axis that is a straight or curved line inside of a space; first generating plural surface regions that are orthogonal to the identified axis; second generating, for each of plural unstructured grids disposed inside the space, a first vector at a point, based on a second vector at each vertex of the unstructured grid, wherein the point is included in the unstructured grid and is located on one of the plural surface regions; determining whether or not a certain arrow corresponding to the generated first vector intersects with another surface region or a tip of the certain arrow corresponding to the generated first vector is tangent to the another surface region; upon determining that the certain arrow corresponding to the generated first vector intersects with the another surface region or the tip of the certain arrow corresponding to the generated first vector is tangent to the another surface region, third generating plural second surface regions that are orthogonal to the identified axis; fourth generating, for each of the plural unstructured grid disposed inside the space, a third vector at a point, based on the second vector at each vertex of the unstructured grid, wherein the point is included in the unstructured grid and is located on one of the plural second surface regions; and displaying an arrow corresponding to the generated third vector.