Until today, no ideal heart valve prosthesis for the replacement of a diseased natural valve or for use in ventricular assist devices exists. Valves still cause thromboembolic complications originating from thrombus formations in the valve's stagnant zones. Optimization of valve design involves avoiding stagnation zones and zones of high shear stresses. This requires detailed flow field investigations. Usually, the regions which are more prone to thrombus formation can be estimated using a dye washout experiment. The method allows an assessment of regions with a high or low residence time that may in turn predict regions with a corresponding thrombus risk. This successful experimental method was simulated using numerical methods with a combination of the computational fluid dynamics program FLUENT (Fluent Inc., Lebanon, NH, USA) and of the visualization tool AMIRA (TGS Inc., San Diego, CA, USA). The numerical dye washout visualization was applied to four monoleaflet valves with varying valve housing geometries. The results show a significant difference in the washout processes of the examined valves. The dye washout was characterized by a time course of the gray value averaged over a defined region of interest. Finally, these curves were quantified by a half dye time. The half dye time in the best optimized valve was only 0.2753 s. The same time in the original valve was 0.6834 s. This study shows that the proposed numerical method of dye washout visualization can be used as an additional tool of the flow characterization in artificial organs.