EXPLORATORY STUDIES ON SCOUR REDUCTION USING TAIL FINS

Abstract

The problem of Local scour around any structure built across an alluvial channel is of great importance to hydraulic engineers for a safe and economical design. The presence of an obstruction like a blunt-nosed pier in an alluvial channel leads to formation of horseshoe vortex at upstream and sides of the pier. and also wake vortices at the downstream side of pier because of flow separation at the sides. These vortices result in large shear stress on the boundary and cause scour. In the case of alluvial channels, the grip length of the pier below the channel bed is to be reckoned not from the original bed of the channel but from the scoured bed around the pier. For minimising scour around the pier, additional arrnagements can be considered to prevent the formation or reduction of the scour hole, thereby effecting considerable reduction in foundation depth. The objective of the investigation was to review the impact of tail fin located centrally behind the circular cylindrical pier on the local scour. A fixed bed masonary flume 8.30 m long, 0.50 m wide and 0.45 m deep with working section 2.0m long, 0.50 m wide and 0.24 m deep located at 3.5 m from the flume entrance was used. The experiments were restricted to the case of clear-water flow past circular bridge piers. Only uniform sediment was used. Three tail fins of height equal to that of the cylindrical pier and lengths equal to 3.0 b, 5.0 b and 7.0 b were used for two cylindrical piers of diameter 48 mm and 32 mm. Here b is the diameter of the pier. Sediment used was 1.74 mm in size. The angle of attack of approach flow was maintained as zero degree. (1) The effect of presence of tail fin on pressure distribution and drag for cylindrical pier was observed. The pressure at the junction of the tail fin and the pier for two different flow conditions were compared with those without tail fin. The comparison was also made for pressure coefficient at nose of the pier i.e. at 0° to the approach flow. It was found that there was a reduction in the magnitude of the average base pressure due to the introduction of a tail fin on the pier. As such, the oscillatory wake is expected to reduce due to the introduction of the tail fin. However, the corresponding scour reduction was not appreciable, apparently because the flow near the nose of the pier is largely responsible for scour and this is not affected much by introduction of tail fin.