A CFD simulation of an heat exchanger e.g. a radiator, considers two working fluids - usually air and water/coolant. CFD is used to determine the efficiency of the heat exchanger which is directly a function of the velocity of the fluids at the wall. CFD also predicts whether the coolant flow is uniform within the heat exchanger. Coolant flows of water jacket in engine block can also be analyzed, as the mechanism is principally the same as heat exchanger.