Experimental study of submergence effect on turbulent parameter around spur dike located in a 90 bend

abstract In this research the statistical parameter of the flow around T shaped spur dike located in a 90 bend was investigated. the experiments were conducted in a 90 bend at tarbiat modares university. the velocity was measured using ADV apparatuse. the frequency of the ADV set to 50 HZ this frequency was used by many previouse researchers. Two different submerged spur dike with submergence ratio equal to 5% and 50% were used. The submergence ratio is the ratio of the flow depth on crest of the spur dike to spure dike height. The spur dikes were located in 45 degree respect to the beginning of the bend and the experiments were done in freeze bed condition. The flow around the spur dike were investigated using parameters such as: Probability of the events (the ratio of the number of the events to the total of the events during the velocity measurment), variation of the events during the velocity measurments, shear Reynolds stresses in streamwise and lateral direction and angle of the events (the ratio of the vertical velocity fluctuation to the streamwise velocity fluctuation). The results showed that two secondary flow formed in lateral direction of the channel and the secondary flow affect on the upstream of the bend. These flow affect on sediment transport mechanism in lateral direction. In the upstream toe of the spur dike the Reynolds stresses in 5% submergence spur dike is greater compared to the 50% submergence spur dike since stronger horse shoe vortex due to the stronger downflow. The scour hole propagation along the shear layer in longitudinal direction may be due to the increasing the probability of the sweep and ejection events and variation of the interaction events, decreasing the angle of the events and increasing the Reynolds stresses. The Reynolds stresses along the shear layers located in 5% spur dike is greater that the 50% spure dike and larger scour process may be expected in this region. the stability of the events are more pronounced compared to the instability of the events. the angle of the sweep and ejection were minimum near the bed surface and maximum in the middle of the flow depth. Two different zone with maximum angle of sweep and ejection were observed that showed the fluid parcels deviate to the downstream recirculating zone. The angle of the events has different value near the over topping flow that passed from the crest of the spur dike. the flow measurments and analysis of the statistical parameters confirmes the previous researchers conclusions about larger scour hole around the spur dikes with lesser submergence. This may related to the difference of the flow structures around the submerged sour dikes especially near the upstram toe of the spur dike and sround the shear layer.