A dual potential decomposition of the velocity field into a scalar and a vector potential function is extended to three dimensions and used in the finite-difference simulation of steady three-dimensional inviscid rotational flow and viscous flow;The finite-difference procedure has been used to simulate the flow through the 80- by 120-Foot Wind Tunnel at NASA Ames Research Center. Rotational flow produced by the stagnation pressure drop across vanes and screens which are located at the entrance of the inlet is modeled using actuator disk theory. Results are presented for two different inlet vane and screen configurations. The numerical predictions are in good agreement with experimental data;The dual potential procedure has also been applied to calculate the viscous flow along two and three-dimensional troughs. Viscous effects are simulated by injecting vorticity which is computed from a boundary-layer algorithm. For attached flow over a three-dimensional trough, the present calculations are in good agreement with other numerical predictions. For separated flow, it is shown from a two-dimensional analysis that the boundary-layer approximation provides an accurate measure of the vorticity in regions close to the wall; whereas further away from the wall, caution has to be exercised in using the boundary-layer equations to supply vorticity to the dual potential formulation.