Study On Fluid-Solid Coupling Characteristics Of Deep Sea Gravity Sampling Device

At present,the exploration of the ocean is advancing into the deep sea,so it is very important to acquire the sediment layer samples of the seabed,which is of great significance for exploring seabed mineral resources,marine engineering geological surveys and safeguard of state rights and interests.As the primary equipment for obtaining surface soil samples of the seabed,the gravity sampler has the advantages of simple structure,simple working principle and low cost.However,it's uncontrollable operation can also lead to lean,falling down and even break of the sampling tube,which will seriously affect the efficiency of sampling work and causes the loss of time and economic cost.In order to improve the efficiency of deep-water gravity sediment sampler under the effect of external flow field and cable,this paper simulated the process of sampler free falling and its motion under the effect of cable traction based on OpenFOAM procedures and dynamic overset grid method,and explored the influence of initial angle of the free falling,cable release velocity on sampling rate.Firstly,the dynamic overset grid method was applied,Newmark method and six degrees of freedom module are used to iteratively solve the motion equation of the structure in the fluid.The free-falling process of the sphere and cylinder in the static water area was calculated,and the time-history curve and terminal velocity of the falling velocity were obtained.The results were compared with the literature results or theoretical solutions.Meanwhile,the force curves of the spherical and two-dimensional sampler models under the external flow field were calculated,the advantages and disadvantages of the two methods were compared,and the feasibility and accuracy of the dynamic overset grid method in solving the fluid-solid coupling problem of the large range of falling objects were verified.Secondly,the time-domain coupling calculation model of the interaction between the sampler and the fluid was constructed.The free-falling attitude of the sampler was simulated in the time-domain,and the relationship between the falling distance,falling speed and the deflection angle of the sampler were analyzed.The results showed that the sampler should have enough velocity to get a high sampling rate in the ideal vertical state,and the free fall distance should not be too long when it has the initial angle.Finally,the program code of the cable constraint was added in the six degrees of freedom constraint condition to add the pulling force of which the size and direction change with the motion of the sampler.This paper studied the motion state of the sampler under the effect of water body and cable when cable speed was different,and influence of cable speed on the sampling rate.In the case of vertical cable,with the increase of the lowering speed,the effect of water flow force and transverse mooring force become increasingly obvious,the deviation angle and lateral deviation of the sampler increase gradually,but the value is so small that nearly has no influence on the sampling work.When the sea condition is good and the cable breaking force meets the requirements,the lowering speed can be appropriately increased to improve the sampling efficiency.