Experimental Study On The Critical Froude Number Of Subaqueous Gravity Currents

It is generally believed that the densimetric Froude number(Frd)or bulk Richardson number of subaqueous gravity flows is the same unity as for open channel flows.Recently there is suggestion that the Critical Frd of subaqueous gravity currents can be non-unity or non-existent depending on extent of the entrainment of the gravity currents on the environmental fluid and the sediment exchange at the bed based on the theoretical analysis research of the specific energy.There is no clear experimental data to support this conclusion at present.In this study,an subaqueous gravity flow experiment was down with a flume with 5 m long 3 degree slope and then transition to a 10 m long horizontal bed.The velocity was measured using an Acoustic Doppler velocimeter(ADV),the concentration measured using a siphon and a conductivity ammeter with two methods adopted at tail of the flume to control the backflow.With a comprehensive analysis of depth-averaged velocity,depth-averaged concentration,thickness of gravity currents,densimetric Froude number,bulk Richardson number,kinetic energy correction coefficient,specific energy of the experiment flow,the following conclusion are reached.Frd for subaqueous gravity flows lost the meaning of Froude number for open channel flows.In consistent with a previous theoretical analysis considering discharge and concentration variation of gravity flows,the critical Frd of the experiment flow is 0.49.Even though the Frd of experiment flow is less than unity along all its path,by considering its sudden change of depth-averaged velocity,flow thickness at slope transition,especially its specific energy diagram which is drastically different from that open channel flows with only a short region around slope transition showing sign of supercritical flow,we may conclude that a weak internal hydraulic jump happened around slope transition for the experiment gravity flow.