Experimental Investigation of Entrance Hydraulic Condition Effects on Flow Structure in Turbidity Currents

Water-storage capacity of reservoir reduces mainly due to sediment laden. Turbidity current has an important role on sediment transfer in reservoir. It is necessary to study sediment interaction and flow in order to predict mechanism of turbidity current. In this paper effects of changes in entrance hydraulic condition of turbidity current on head velocity، layer-average thickness، layer-average velocity، body velocity and turbulent structure have investigated experimentally. The front velocity of the head of turbidity current was determined by video recording and body velocity and turbulence parameters measured by Vecterino. When the initial Froude number decreases the maximum velocity increases in body and head. Positive shear Reynolds stress near bed indicates that major contributor in this region is sweep or ejection while major contributor near interface is inward interaction or outward interaction. Entrainment is dominated at interface. The investigation shows that head velocity depends on inlet Froude number and inlet Reynolds number. Variation of head velocity along channel is exponential. The maximum reduction of head velocity takes place at whereas variation of head velocity at is negligible. Driving forces at are inertial force and gravity force. Driving force decreases after hydraulic jump and only gravity force remains as driving force. Therefore head velocity is constant at. Head velocity increases when inlet Reynolds number increases. Body velocity increases when inlet Froude number decreases، as gravity force increases when inlet Froude number decreases. Effects of inlet Froude as number on body velocity is negligible at the end of channel. Negative value of body velocity at the interface of turbidity current and ambient fluid indicates entrainment phenomenon at this region. When inlet Froude number decreases، vertical component of velocity increases too،then maximum velocity approaches to the bed. Elevation of maximum velocity increases along the channel due to sedimentation of particles and decreases of vertical component of velocity. Body velocity decreases along the channel due to decrease of inertial force. Vertical Reynolds stress decreases when inlet Froude number decreases. Because of increase in particle turbulence dissipates and therefore vertical Reynolds stress decreases. Oscillation of vertical Reynolds stress is due to turbulence at this region. The maximum of vertical Reynolds stress tacks place near bed and interface of turbidity current and ambient fluid and minimum of vertical Reynolds stress tacks place near maximum velocity elevation. Shear Reynolds stress have two maximum values. One is near the bed and the other one is near the interface of turbidity current and ambient fluid. Maximum Reynolds shear stress is positive near bed and negative near interface. Minimum of Reynolds shear stress take place near maximum velocity elevation.