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Abstract This paper discuss the conditions for applicability of cubic law，it is pointed out that the cubic law express with average aperture is approximately valid only at the case of fracture with very small fluctuations in apertures. For rock fractures，such condition only appears in the local segment of rock fracture，which is the local cubic law. And in the regions of apertures with big fluctuations，the influence of geometric roughness on flow should be considered. For easily using the local cubic law，a series-parallel discrete equivalent model for natural rough fractures is introduced；and the equivalent aperture and flow formula of rough fracture are derived. Variation analysis of fluid pressure in equivalent model unit caused by big-aperture-fluctuation is carried out with numerical software；and then an excess pressure loss coefficient is defined to modify the calculation method of rough fractures flow.
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Received: 30 October 2012
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2013, Vol. 32 
