Fluid dynamic gauging (FDG) is an experimental technique that exploits the behaviour of a suction flow through a nozzle located near a fouling deposit to determine the thickness of the layer. The nozzle does not contact the layer but the shear stress imposed by the flow could cause it to deform. The effect of the external shape of the nozzle on the stresses on the surface being gauged was investigated using computational fluid dynamics (CFD) simulations. Different nozzle shapes were studied using a 2D axi-symmetric computational domain. CFD results were validated by comparison with (i) a set of experimental values for the normal stresses acting on the gauged surface, and (ii) an analytical solution for the limiting case approached under certain conditions. The results obtained for different simple geometries show that certain desirable pressure and surface shear stress distributions can be obtained by selection of the appropriate external nozzle shape. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.