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Experimental Studies of Wetting Front Instability Induced by Gradual Change of Pressure Gradient and by Heterogeneous Porous Media¹

ABSTRACT

The stability of infiltration flows in Hele-Shaw cells was examined. Flows were perturbed either by gradually increasing G, the pressure gradient behind the wetting front, or by heterogeneities in the cells. Two methods were used to increase G gradually through zero: (i) by allowing the air ahead of the front to be compressed by the flow, and (ii) by monotonically increasing the hydraulic conductivity of the cell with depth. In the other class of experiments, the flow was instantaneously perturbed by a step-increase in conductivity at selected depths.

It was found that Philip's (1975a, 1975b) stability analysis adequately predicted both the onset of instability and the wavelength of the initial disturbances. For quadratic increase in conductivity with depth the observed growth rate of disturbances was faster than expected from the linear theory.

Examination of the data of Peck (1965) and Hill and Parlange (1972) showed that Philip's analysis is also adequate for coarse sands.

¹ Contribution from CSIRO, Australia.

² Research Scientist, Experimental Officer, and Chief, respectively, CSIRO Div. of Environ. Mech., P.O. Box 821, Canberra City, A.C.T. 2601 Australia.

Received for publication November 15, 1976. Accepted for publication February 14, 1977.

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