Explicit equations to describe the leak rate of a single phase fluid through a narrow crack under a low pressure gradient have been developed and are presented. Four distinct flow regimes, which change with crack opening displacement, have been previously identified and are the basis of this model. The fluid flow is governed by the pressure gradient and the tortuosity of the crack, which is particularly important when the opening displacement is small.The equations have been developed by considering the pressure forces created when the fluid flows down an idealised zig-zag channel. The nature of the flow, and hence the governing equations, change as the crack aperture increases.The power of this approach is clearly seen when the flow rates predicted using this model are compared both to the flow rates predicted from computational fluid dynamics analyses and those found by experimentation. The agreement between these sets of data is good, showing that the major effects governing the flow rate have been identified and accounted for.

The problem of fluid seepage through the cracked walls of pressure vessels has been examined. A simplified two-dimensional computational fluid dynamics (CFD) model of water passing through a narrow crack has been constructed and the results compared with published data. The CFD modelling process was validated by experimental work on two geometrically identical idealized crack bodies. The flowrate of air through the crack as a function of opening displacement was measured using the smaller apparatus, and the corresponding flow patterns were observed using smoke particles in the larger air flow apparatus. Four distinct flow regimes have been identified when the opening displacement of the crack is less than 3 times the surface roughness of the crack faces.