University of California
San Diego, California, U. S. A.
The displacement of fluid within a horizontal pipe by the introduction of fluid with a different density is produced by a combination of the rate at which new fluid is introduced and by the buoyancy forces acting on the two fluids. At low input flowrates the buoyancy-driven flow dominates and the input fluid flows along the pipe as a gravity current. As the flowrate increases the depth of this current increases until it reaches a maximum depth (determined by the shape of the pipe). For higher flowrates the input fluid can not be carried as a gravity current and the input fluid then displaces all of the resident fluid within the pipe. Expressions are given for the movement of the input fluid over the whole range of input flowrates and momentum fluxes, and the effects of mixing produced by a high momentum flux source input are discussed. The results of small scale laboratory measurements using salt solution introduced into a circular pipe of fresh water, and large scale flows produced by the introduction of methane or propane into air-filled circular pipes, are compared with the theoretical predictions.