Aninvestigationisdescribedforinstabilityproblemofflowthroughapipeofcircularcross-section.Asadisturbancemotion,weconsiderageneralnon-axisymmetricmode.Anassociatedamplitudeormodulationequationhasbeenderivedforthisdisturbancemotion.Thisequationbelongstoadiffusiontype.ThecoefficientofitcanbenegativewhileReynoldsnumberincreases,becauseofthecomplexinteractionbetweenmoleculardiffusionandconvection.Thenegativediffusivity,whenitoccurs,causesaconcentrationandfocussingofenergywithindecayingslugs,actingasaroleofreversingnaturaldecays