| When an aircraft passes through the atmosphere at high speed, its surface structure will encounter severe aerodynamic heating. In this case, thermal protection should be applied to the exterior of the aircraft, though the thermal protection structure will be ablated to certain extent. Change of the aircraft's surface will be caused due to ablation and its numerical simulation is of great significance in the design of the thermal protective structure for the aircraft. Common simulation methods for ablative contour recession are adaptive mesh technique, birth-death element method, etc. However, all these methods have their own drawbacks. Adaptive mesh technique can ensure the smoothness of the ablative surface, but it can only be applied to simulation of small ablation quantity. When the ablation quantity grows bigger, the mesh will be seriously distorted, leading to divergence and blowing up in computation. Birth-death element method achieves the ablative recession via deleting elements. It can simulate large ablation quantity but the formed ablative surface may be jagged, which will induce distortion during the second loading.Based on ABAQUS, this thesis simulates the ablation of thermal protection structure through programming mesh reconstruction. The program is able to realize continuous regression of ablative surface via continuously generating new inp files. New positions of the ablative surface can thus be obtained based on the results of heating analysis in every time step. Subsequently, methods of element reconstruction, boundary recovery are resorted to in order to fit a new ablative surface. This result is then calculated with new thermal load and the calculating procedure is iterated successively.Aiming to solving the ablative problem of thermal protection structure of aircrafts, this thesis proposes a new method to simulate ablative recession. It contains reconstruction of hexahedral element and boundary recovery of tetrahedral mesh and can deal with the moving boundary problem in ablation precisely. Besides, this method is not subject to the amount of ablation quantity and is able to reconstruct a mesh with smooth surface. In the light of temperature of element nodes, reconstruction of hexahedral element is classified into nine categories, which are further categorized into dozens of subcategories. Based on this classification, node elimination and reconstruction are carried out. As for irregular tetrahedral meshes, new elements are generated and new ablative contours are fitted by birth-death element method.Mesh elements in the newly ablated surface are calculated according to ablation recession rate. Temperature interpolation is applied to elements that stretch across the ablation surface so as to set new nodes. These new nodes are thereby regarded as basic points to determine the new ablation boundary surface. Subsequently, the mesh reconstruction of elements near the ablative contour is carried out using the local mesh reconstruction method. In this way, the ablative recession of the surface in every time step can be predicted precisely and smoothly.In addition to the ablation type of temperature related ablation model, another method dealing with regression of ablative surface is also proposed in the thesis. In the method, the fitting is reconstructed by the generation of "virtual temperature", which can be applied to both convex and concave surfaces.Numerical examples show that the numerical method and developed codes in this thesis can quickly simulate the whole process of ablative surface regression and treat problems based on large ablation quantity. The calculated regression surface of ablation is precise and smooth, qualified for coupling iterative computation with aerodynamic heating analysis. Besides, in terms of the features of ablation, original mesh is used as the basic grid to reconstruct the local elements around the ablative surface while elements in other places remains unchanged, resulting in a high computational efficiency. |
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