Improvement of FFD parametric approach in the application of a lifting body

J. Leng, Z.-g. Wang, W. Huang, Y. Shen, K. An
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Abstract

FFD (free-form deformation method) is one of the most commonly used parameterisation methods at present. It places the parameterised objects inside the control volume through coordinate system transformation, and controls the control volume through control points, thus realising the deformation control of its internal objects. Firstly, this paper systematically analyses and compares the characteristics and technical requirements of Bernstein, B-spline and NURBS (non-uniform rational b-splines) basic functions that can be adopted by FFD, and uses the minimum number of control points required to achieve the specified control effect threshold to express the control capability. Aiming at the problem of discontinuity at the right end in the actual calculation of B-spline basis function, a method of adding a small epsilon is proposed to solve it. Then, three basic functions are applied to the FFD parameterisation method, respectively, and the differences are compared from two aspects of the accurate expression of the model and the ability of deformation control. It is found that the BFFD (b-spline free-form deformation) approach owns better comprehensive performance when the control points are distributed correctly. In this paper, the BFFD method is improved, and a p-BFFD (reverse solution points based BFFD) method based on inverse solution is proposed to realise the free distribution of control points under the specified topology. Further, for the lifting body configuration, the control points of the p-BFFD method are brought closer to the airframe forming the EDGE-p-BFFD (edge constraints based p-BFFD) method. For the case in this paper, the proposed EDGE-p-BFFD method not only has fairly high parameterisation accuracy, but also reduces the expression error from 1.01E-3 to 1.25E-4, which is nearly ten times. It can also achieve effective lifting body guideline constraints, and has the ability of local deformation adapting to the configuration characteristics. In terms of the proportion of effective control points, the EDGE-p-BFFD method increases the proportion of effective control points from 36.7% to 50%, and the more control points, the more obvious the proportion increase effect. The new method also has better effect on the continuity of geometric deformation. At the same time, this paper introduces the independent deformation method of the upper and lower surfaces based on the double control body frames, which effectively avoids the deformation coupling problem of the simultaneous change of the upper and lower surfaces caused by the movement of control points in the traditional single control framework.
FFD参数化方法在提升体应用中的改进
自由变形法(FFD)是目前最常用的参数化方法之一。通过坐标系变换将参数化对象置于控制体内,并通过控制点控制控制体,从而实现对其内部对象的变形控制。本文首先系统地分析比较了FFD可采用的Bernstein、b样条和NURBS(非均匀有理b样条)基本函数的特点和技术要求,并用达到规定控制效果阈值所需的最小控制点数来表达控制能力。针对b样条基函数在实际计算中出现的右端不连续问题,提出了一种添加小ε的方法来解决。然后,将三种基本函数分别应用于FFD参数化方法,并从模型的准确表达和变形控制能力两方面比较了三者之间的差异。结果表明,当控制点分布正确时,b样条自由变形法具有较好的综合性能。本文对BFFD方法进行了改进,提出了一种基于逆解的p-BFFD (reverse solution points based BFFD)方法,实现了控制点在指定拓扑下的自由分布。此外,对于升力体配置,p-BFFD方法的控制点更靠近机身,形成edge -p-BFFD(基于边缘约束的p-BFFD)方法。对于本文的案例,提出的EDGE-p-BFFD方法不仅具有较高的参数化精度,而且将表达式误差从1.01E-3降低到1.25E-4,降低了近10倍。该方法能够实现有效的提升体导轨约束,并具有适应结构特点的局部变形能力。在有效控制点比例方面,EDGE-p-BFFD方法将有效控制点比例从36.7%提高到50%,控制点越多,比例增加效果越明显。新方法对几何变形的连续性也有较好的效果。同时,本文引入了基于双控制体框架的上下面独立变形方法,有效避免了传统单控制框架中由于控制点运动引起的上下面同时变化的变形耦合问题。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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