Self excitation of concentration fluctuations in dredge pipelines
Type:
Presented during:
WODCON XVII: "Dredging in a Sensitive Environment" - 2004, Hamburg, Germany
Authors:
A.M. Talmon
Abstract: High concentration sand-water mixtures that are conveyed by long distance dredge pipelines are known to develop high amplitude concentration fluctuations. The booster pumps in such a pipeline experience the passage of major density waves. This causes the boosters to run inefficiently and pumping capability might become critical. End-of-pipe sand production varies strongly.
From a continuity-based linear stability analysis, that was published five years ago by the author, it was concluded that these density waves originate from hindered settling. According to the theory, concentration variations develop whenever there is solids exchange with a sand layer in the invert of the pipe. This might be a stationary bed layer or a sliding bed layer. Further theoretical development, which included the dynamics of the sliding bed layer, led to the conclusion that the amplification in the sliding bed regime will be weakened compared to the stationary bed regime.
These self-excitating density waves have been purposely aroused in a laboratory circuit. The, all important, internal structure of the flow is monitored by means of concentration profile measurements. The experiments show that the presence of a bed layer is indeed a necessity for density wave development. The nearbottom suspension is highly saturated. A thin bed layer is sufficient for the density waves to feed upon, much in agreement with the theory.
A concise summary of the developed theory is given and some typical results from the laboratory tests are reported.
Keywords: pipelines, self-excitation, sand transport, physical model testing.