unidirectional flow

Pipelines passing through the seas and rivers as a means for transferring fluids (oil, gas, water, sewage) have a great importance since the pipelines change flow pattern, increase turbulence and intensify the bed shear stress around submerged pipes, which create scour holes under the pipes. Development of scour hole under pipelines leads to instability and failure of pipelines, which may cause severe environmental and economic damages. In this paper, the results of laboratory research on the effect of different attack angle of the approaching flow on the maximum scour depth under submerged pipes have been provided. Experiments were conducted in two modes: 1) scour under pipe with 90° angle of attack, 2) scour under pipe with less than 90° angle of attack. The pipe was also placed in 2 positions, i.e. on the bed and at a distance of D/4 from the bed. Three (45°, 60° and 90°) angles of attack were employed in the experiments. The results showed that the angle of attack is an effective parameter on the location of the maximum scour depth and performance of vortices. For angles smaller than 90 degrees, maximum scour depth occurred in the right hand side of the flume.

Scour under the pipelines laying on the sea and river beds occurs and as a result the scour hole develops. Occurrence of scour hole under the pipelines leads to instability, bending and even failure of the pipeline. This causes fluid inside the pipeline spread into the sea and river and in addition to water pollution endangers the human and marine life. In this paper, the results of laboratory research on the effects of placing submerged plate to reduce the maximum scour depth under submerged pipelines are presented. Experiments were conducted in two modes: 1) scour under pipelines without submerged plate, 2) scour under pipelines with submerged plate. The results showed that when a submerged plate was placed in the downstream of the pipeline, scour in the front of the pipelines was reduced about 70 percent. By using the submerged plate and creating gap between the pipeline and bed better results can be achieved to reduce scour under the pipeline.

In this research, the impact of macroalgal mats on sediment pattern was studied experimentally. To achieve this object, a series of experiments were conducted in a large flume for different flow conditions and bed covers. The Power Spectral Density (PSD) method was used to separate wave-induced velocities from turbulence. The results showed that the separation of wave-induced orbital velocity from turbulence is possible by using the power spectral density (PSD) method. The sediment results revealed that the bed level variations in bare bed were more severe than in algal bed. So that, the range of its variations in bare bed was three times of algal bed especially in wavy flow conditions. In addition, the average number of ripple crests per meter decreased and the relative ripple wave length increased with increasing of flow discharge. Also the bedload quantity in presence of algae decreased by 80% in unidirectional flow (current) as well as 15% in wave-induced flow (combination of wave and current) conditions.