For a given screen, either of the above conditions for elimination of disturbances in mean velocity could only be satisfied at one tunnel speed, and the arrangement of screens is usually decided by the need to reduce the turbulence generated in the return circuit. The jets coming through the holes in the screen stick together in irregular patterns by mutual entrainment, and actually produce flow nonuniformities of their own, triggered by otherwise-negligible imperfections of weave. Therefore screens need cleaning at intervals, and some tunnels have a liquid-level manometer or other pressure transducer permanently installed to monitor the pressure drop across the first screen or the whole set of screens. A significant increase in the pressure drop indicates that the screens need cleaning. A honeycomb naturally produces some turbulence of its own, with eddy sizes of the same order as the cell diameter, which decays very much more slowly than the small-scale turbulence produced by screens.
Most modern tunnels have both honeycomb and screens.
For present purposes the simpler 1945 analysis is adequate. From this odd way of looking at the problem we can see that the x -component velocity induced by this trailing vorticity must be half as large at the screen as it is far downstream.