Visualization has developed a tendency to use mathematical analysis to obtain and present important data properties. In three-dimensional fluid flows, engineers are interested in several important features. One type are recirculation zones where the fluid stays for a long time. This plays a key role in combustion problems since recirculation allows a completion of chemical reactions which usually have a smaller time scale than fluid dynamics. Strong indicators for such recirculation zones are looping streamlines in a steady vector field or in the time steps of unsteady data. The article presents a method for the detection of such loops by analyzing streamlines approaching them.