In the laminar flow past a cylinder, the net force acting on the cylinder is zero. Will it be still be zero if it is replaced by another symmetric shape (for instance a triangular section prism)?
Interesting question! I enjoy thinking outside the box from well-defined/common theories, such as where else they can be applied.
I would say yes, the net force would be zero for a symmetrical shape. As long as the streamline pattern produced by the body is completely symmetrical, no net force can be present (see D'Alembert's paradox).