The stirred tank reactor is one of the most commonly used devices in industry for achieving mixing and reaction. We consider a combined experimental/computational approach for the simulation of flow inside a stirred tank. Two sets of experiments are performed to measure the velocity field in the neighborhood of the impeller. The first set of PIV measurements is on six different r-z planes phase locked at 0 degrees, 10 degrees, 20 degrees, 30 degrees, 40 degrees and 50 degrees to the blade location. The second set of PIV measurements is on a curved theta -z plane whose radial location is just outside the impeller blade tip radius. Measurements indicate that the impeller-induced flow is dominated by three flow components: a circumferential flow, a tangential jet and pairs of tip vortices. A simple theoretical model is developed for each flow component and their superposition is observed to provide a good description of the impeller-induced flow. The theoretical model is used as a velocity boundary condition for numerical simulation. The impeller-induced boundary condition is fully three dimensional, an important aspect that significantly enriches the mathematical representation of the primary source of motion. The results of two- and three-dimensional simulations are compared with measurements in the interior of the tank.