Research On The Drag Reduction Performance Of A Circular Flow Around A Surface Texture

Riser is an important channel for offshore oil and gas development to penetrate of the offshore drilling platform and the surface, and its length will greatly increase with the step towards to deep ocean of Offshore Oil and Gas Development. Accordingly, the area of the force act on riser also increases while currents flow through the riser. In such a large length, the vortex induced vibration caused by the drag and lift forces, which risers suffer from the currents, will influent the safety and reliability of risers more and more. And a huge tension generate, it put forward a higher requirement to the tension device performance and the body and joints of riser boundary. Further, It will certainly cause adverse performances on the stability, safety and service life of riser system. Therefore, deepwater riser flow around drag reduction becomes very important. The riser flow around model can be approximately considered as a flow around cylinder model, thus the flow around drag reduction research can be transformed into the study of the drag reduction of a circular cylinder flow around.Textured surface drag reduction technology as a new drag reduction method has been highly regarded by scholars and a series of studies has been carried out. This article will applies the textured surface drag reduction technology to the study of the drag reduction in a cylinder flow around, that is to explore the feasibility and design method of surface texture buoyancy material for reducing the flow resistance of deep water riser. Details as follows:(1) A large number of literature at home and arbroad about the risers and the drag reduction of non plane surface has been investigated, from which the goove and dimple have been comfirmed as the main texture form in the study of the drag reduction performace of flow around cylinder.(2) A exploration research about numerical calculation method of flow around a cylinder which was arranged texture on its surface has been developed. Through the comparison between numerical analysis and experimental results, the correct numerical model, suitable grid partition for texture cylinder have been found. And also the selection of important parameter, such as span-wise length and time step size, has been completed.(3) The simulation and analysis of flow around a cylinder with 6 kinds of groove depth,4 kinds of grooves and 2 kinds of groove shape texture were carried out. the results showed that with the depth of 0.01D (D:cylindrical diameter) and the number of arc-shaped grooves at 32, the relatively smooth cylinder drag coefficient decreased by 16.1% and lift coefficient decreased by 5.6%, presence of the vortices of a circular cylinder flow around wake of a trench didn't have much performance on the shedding frequency and less Strouhal number changes. Further, change of the trench structure parameters were analyzed, the results showed that:the drag coefficient first decreased then increased with the depth of the trench increased, and decreased with the increase of the number of grooves, triangular grooves would have better drag reduction performance compared with the arc-shaped grooves.(4) The simulation and analysis of flow around the cylinder is carried out, which is arranged with 4 kinds of dimple depth,2 kinds of dimple inner shape, and 2 kinds of dimple distribution form. The results showed that with the depth of 0.01 D and the diameter of 0.1 D, and the number of the circumferential distribution pits at 20 row, the relatively smooth cylinder drag coefficient decreased by 16.8% and lift coefficient decreased by 6.6%. Further, change of pit structure parameters were analyzed, the results showed that:the drag coefficient first decreased then increased with the depth of the pit increased, cylindrical pits would have better drag reduction performance compared with the spherical pits, and pits by diamond distribution would have better drag reduction performance compared with which distributed by rectangle, little performance on the vortex shedding frequency also, and Strouhal number scarcely changed.(5) According to the fluid similarity theory, a new test bench of a cylinder flow around had been designed and built out, flow around cylinder experiment could be performed under a plurality of Reynolds number, which has get rid of the dependence of flow around a cylinder experiment for the tunnel.(6) By using the experimental bench, the flow around the cylinder with 4 kinds of groove structure and 4 kinds of dimples structure were carried out. The experimental results show that the drag and lift coefficients of the two kinds of texture are decreased, which confirmed truth of drag reduction with reasonable groove or dimple texture. Based, through comparing the results of the flow around cylinder with different structure groove and dimple, the influence law of the texture structure parameters to drag reduction performance has been found out. For the groove, drag reduction performance becomes better with the increase of the quantity of groove. For the dimple, cylindrical dimple has a better drag reduction performance compare to spherical dimple, and diamond distribution is better than rectangular distribution. Also, the drag reduction performance becomes worse with the increase of the depth of groove or dimple. Then, the experimental results are consistent with the simulation results, which furtherly verifies the reliability of simulation results.In this paper, the good drag reduction performance of the texture, especially the groove texture and the dimple texture, to flow around cylinder has been revealed, and still the influence law of the different structure parameters to flow around the cylinder. The research results of this paper have important reference value for the study of drag reduction on non planar surface, and provide a new idea for the study of drag reduction of deep-water riser.