Numerical Study On Underwater Explosion Based On Ghost Fluid Methods

The numerical research on UWE (underwater explosion) has a wide application prospect in military realm and engineering field. In the region of military, it is necessary to simulate the propagation of underwater blast wave and compute the peak pressure for the development of torpedo and depth charge. In the region of engineering, it is important to do numerical study for the development of port and shipping lane. The numerical simulation of UWE can provide references for experimental research. It can also point out especial theoretical direction and has very important referential significances when the experiment cannot be implemented, In addition, the research and improvement on computational method of UWE has important academic significances. The current approaches to numerical simulation of UWE include Finite Element Method, Smoothed Particle Hydrodynamics, Boundary Element Method, GFM (Ghost Fluid Method) and some Euler-Lagrange combination methods. One of these methods, GFM, has been divided into some kinds such as OGFM (Original GFM), NGFM (New Version GFM), MGFM (Modified GFM), RGFM (Real GFM) and SMGFM (Solid MGFM) etc. In GFM, to solve one medium, the domain on the other side of material interface is supposed to be the same ghost medium and defined to be with appropriate flows states for the nodal points. Hence, the status at the grid nodes next to material interface can be solved. Since ghost assumption can simplify complicated problem, GFM has a more widely application for simulation complicated problem than other methods. GFM is employed to do numerical research for UWE in this thesis because of the lack of literatures on numerical study on UWE using GFM by so far. For different specific problems of UWE, different kinds of GFM algorithm are used. This dissertation presents the numerical simulation of the propagation of underwater blast wave, the calculation on peak pressure of the flow field of UWE and also provides improvements on some steps of GFM for numerical simulation of UWE.The main research contents and innovation work of this dissertation include:1) This thesis presents a more accurate numerical simulation of column charge UWE by Level set method and some improvements on numerical accuracy. Column charge is common in UWE.Three-dimensional cylindrical problem is often cut into2D axis problem on the condition that the computational region can be cut into a2D axis domain. However, this approach is not applicable in some complex computational region. Thus, we need to solve initial values of Level set function. By far, the re-initial technique is always used to define initial values of Level set function. This technique can lead to non-physical movement of material interface. In this thesis, an accurate method based on dividing and respectively solving computational region is developed to solved the initial values of Level set function to reduce the numerical error.2) In this thesis, the interpolation method is coupled into ARPS (Approximate Riemann Problem Solver) to reduce the numerical error. Predicting the interfacial status using ARPS is an important step of GFM. For original ARPS, minimum angle algorithm is used to select two relative grid nodes just bordering interface. The minimum angle algorithm always leads to large numerical errors. In this thesis, we replace the minimum angle algorithm by interpolation method and compare the differences between the results by the way coupled and original ARPS. It is verified that combination of ARPS and interpolation method has the property of reducing numerical errors by comparing to the results without the employment of interpolation method.3) This thesis presents the numerical simulation of underwater blast wave in complex computational region by two different methods and comparison between the respective results. One method is using arbitrary coordinate system method in body-fitted grids. The other is using NGFM in uniform Cartesian grids. The research demonstrates that NGFM can present more stable results and its weakness is incapability on computing the pressure of grid nodes on rigid wall. In calculation, we need to use the pressure of grid nodes just bordering the rigid wall. The results show that the pressure of grid nodes just next to the rigid wall is very close to the value of grid nodes on the wall if the grid size is very small. In contrast, using arbitrary in uniform grid can present the pressure on the rigid wall. Nevertheless, the numerical oscillation often occurs in large area of flow field because Tait equation is used to describe water. On the other side, since we need to solve the grid derivative, the numerical result is not accurate.4) This thesis presents a fluid-solid ARPS based on Euler equations and fundamental equations of elastodynamics to expand the application range of SMGFM. Predicting the flows states via fluid-solid ARPS is an important process in SMGFM. The present fluid-solid ARPS is deduced based on Euler-Naviers equations with respective employment of Euler equation to solve flow field and Naviers equation to solve the grid nodes of solid medium. Nevertheless, Naviers equation cannot work efficiently when applied in simulation of solid. In this dissertation, a fluid-solid ARPS is deduced based on Euler-basic elastodynamics equations. The comparison among the results by new ARPS, rigid wall assumption and compressible assumption is also presented. The numerical results demonstrate the correctness of the deduced ARPS. Meanwhile, the numerical results show that there is little discrepancy between the respective results of blast wave propagation of flow field and pressure history by different approach abovementioned are very close to each other.