A manifold structure for re-directing a fluid stream between first and second ends of a manifold cavity for delivering or discharging a fluid to or from a corresponding fluid transmitting device, such as a heat exchanger is disclosed. In particular, a manifold structure formed within a flow box enclosing a fluid transmitting device is disclosed wherein the manifold structure re-directs an incoming or outgoing fluid to or from the fluid transmitting device, such as a heat exchanger, housed within the flow box. In one embodiment, the manifold structure comprises a first curved surface having a concave curvature for redirecting a fluid stream from a first direction to a second direction or vice versa. In another embodiment, the manifold structure further comprises a second curved surface having a convex curvature in facing, spaced-apart relationship to the first curved surface, the corresponding first and second curved surfaces inducing swirling movement into the fluid stream.