Yucheng Li

Numerical investigations of lift suppression by feedback rotary oscillation of circular cylinder at low Reynolds number

This article describes a strategy of active flow control for lift force reduction of circular cylinder subjected to uniform flow at low Reynolds numbers. The flow control is realized by rotationally oscillating the circular cylinder about its axis with ω ( t ) = − λ C L ( t ) , where ω ( t ) is the dimensionless angular speed of rotation cylinder, λ is the control parameter and C L ( t ) is the feedback signal of lift coefficient. The study focuses on seeking optimum λ for the low Reynolds numbers of 60, 80, 100, 150, and 200. The effectiveness of the proposed flow control in suppressing lift force is examined comprehensively by a numerical model based on the finite element solution of two-dimensional Navier–Stokes equations. The dependence of lift reduction on the control parameter λ is investigated. The threshold of λ , denoted by λ c , is identified for the Reynolds numbers considered in this work. The numerical results show that the present active rotary oscillation of circular cylinder is able to reduce the amplitude of lift force significantly as long as λ ≤ λ c , at least 50% for the laminar flow regime. Meanwhile, the present active flow control does not result in the undesirable increase in the drag force. The Strouhal number is observed to decrease slightly with the increase of λ . As for a specific Reynolds number, the larger λ gives rise to the larger amount of lift reduction. The lift reduction reaches the maximum at λ = λ c . The mechanism behind the present lift reduction method is revealed by comparing the flow patterns and pressure distributions near the active rotationally oscillating circular cylinder and the stationary circular cylinder. It is found that the critical value λ c generally increases with Reynolds number. Two types of lift shift are observed in the numerical results for the cases with λ > λ c . The first is characterized by the regular fluctuation of lift coefficient but with nonzero mean value, while the second is associated with the sustaining increase of lift coefficient. The phenomenon of lift shift is found to be related closely to the evolution of vortex pattern in the near wake of circular cylinder.

Numerical simulation of a submerged gravity cage with the frame anchor system in irregular waves

When typhoon or extreme wave conditions occur, submergence under water may be an effective way for the net cage to avoid the attack. In this paper, a numerical method is developed to simulate the hydrodynamic behavior of net cage, which has been verified in our previous paper. Herein, by the numerical model the mooring line force and cage motion are calculated when the net cage is both in floating and submerging conditions. According to the simulated results, it can be found that the decrease of the mooring line force and cage motion is obvious when the net cage is submerged. The results of this study will give a good reference for better knowledge of dynamic behavior of submersible net cage.

THE WAVE-CURRENT FORCE SPECTRUM ON INCLINED CYLINDERS

Abstract A linearization method is proposed for the vectorial Morrison equation which enables the force spectrum on an inclined cylinder to be calculated in a relatively simple manner from the wave spectrum in the presence of a current. To test the method some random wave tests were carried out in a laboratory flume with forces measured on a cylinder over a range of inclinations, currents and angle of inclination to the current direction. Comparisons show the measured force spectra to be sufficiently close to the calculated ones for the method to be applicable to engineering practice.

Research on the Float Collar of a Gravity Fish Cage

Abstract The development of the fish cage has become more and more important for human beings, and a lot of achievements have been obtained in the field of aquacultural engineering. However, due to the environmental protection and the coastal utilization problems, moving marine aquaculture from near-shore region into deep sea has become imperative. Generally speaking, the advantages of developing the deep-sea aquaculture outweigh its disadvantages. However, some issues have become very critical, such as the higher waves and/or stronger current forces on the whole system of the fish cage. Thus a systematic and thorough research into the fish cage system in deep water is a task of top priority. By selecting a typical gravity fish cage with the fold-line or straight-line mooring system, this paper focuses on the research on the movement of the generalized float collar and the tension of the mooring ropes. The comparison between the experimental and computational results shows that the current computational method can correctly predict the movement of the float collar and the tension of the mooring ropes.

Mechanism of Local Scour Around Submarine Pipelines Based on Numerical Simulation of Turbulence Model

In this paper, the mechanism of local scour around submarine pipeline is studied numerically based on a Renormalized Group (RNG) turbulence model. To validate the numerical model adopted in this paper, the equilibrium profiles of local scour for two cases are simulated and compared with the experimental data. It shows that the RNG turbulence model can give an appropriate prediction for the configuration of equilibrium scour hole, and it is applicable to this situation. The local scour mechanism around submarine pipeline including the flow structure, shear stress distribution and pressure field is then analyzed and compared with experiments. For the further comparison and validation, especially for the flow structure, a numerical model of Large Eddy Simulation (LES) is also developed in this paper. The numerical results of RNG demonstrate that the critical factor governing the equilibrium profile is the seabed shear stress distribution in the case of bed load sediment transport, and the two-equation RNG turbulence model coupled with the law of wall is capable of giving a satisfying estimation for the bed shear stress. Moreover, the piping phenomena due to the great difference of pressure between the upstream and downstream parts of pipeline and the vortex structure around submarine pipeline are also simulated successfully, which are believed to lead to the onset of local scour.Copyright