Journal of Numerical Simulations in Physics and Mathematics
ISSN: pending (Online) | ISSN: pending (Print)
Email: [email protected]
The numerical simulation, also known as computer simulation, is one of most effective technique that uses electronic computers in combination with some numerical methods such as the FE method, the FD scheme, and the FVE method, and through numerical calculation and image display methods, to study engineering problems, physical problems, and various problems in nature. Numerical simulation technology was first introduced in 1953 by Bruce et al. [1] to simulate one-dimensional (1D) gas-phase unsteady radial and linear flows. Due to the limited computational capabilities at the time, it was initially applied only to single-phase 1D problems. In 1954, West et al. [2] extended the approach to two-dimensional (2D) problems, presenting a method to simulate unsteady two-phase flow in oil reservoirs.
With the development of computer technology, the numerical simulation has made significant progress and has been applied in more fields. For example, in the field of aerospace, the numerical simulation can be used as an alternative to wind tunnel tests, thereby saving a significant amount of costs; in nuclear test field, by using the numerical simulation to conduct the tests on a computer, nuclear pollution can be avoided and more reliable data can be obtained.
The Journal of Numerical Simulations in Physics and Mathematics serves mainly as a platform for numerical simulation scholars to showcase their numerical calculation and simulation results.
Of course, the development for the numerical simulations technology is inseparable from the progress of the calculation methods such as the FE method, the FD scheme, and the FVE method. In other words, the progress of the calculation methods such as the FE method, the FD scheme, and the FVE method is the source for the development for the numerical simulations technology. Therefore, promoting the development and advancement of computational methods is the main central task, which is main purpose for the Journal of Numerical Simulations in Physics and Mathematics.
Hence, in the next Section 2, we will review the development of the FE method, the FD scheme, and the FVE method as well as their merit and demerit. Then, in Section 3, we will introduce some reduced-dimension approaches for the FE method, the FD scheme, and the FVE method. Finally, we summary main conclusions for this editorial in Section 4.
The FE method, originally proposed by Turner et al. [3], has been widely used to solve structural problems. It has found extensive applications in real-world engineering computations and has become an effective approach for solving various steady and unsteady partial differential equations (PDEs), including hydrokinetic equations.
The FD method has a longer history than the FE method, which is originated from the work of Newton, Euler, and others. They once used the difference quotient instead of the derivative to simplify the calculation. In 1928, Courant et al. [4] proved the convergence theorem of the typical FD schemes of the three typical PDEs, providing a foundation for modern FD theory. Meanwhile, they also applied the FD method to find the numerical solutions of PDEs and developed the FD method. Because the FD method is universal and easy to implement numerical calculations and numerical simulations in electronic computers, it has developed greatly and been widely applied in scientific engineering computing.
The FVE method was developed in Imperial College mainly to solve fluid dynamics problems in 1980, whose emergence was much later than that of the FD scheme and the FE method (see [5]). The core idea of the FVE method is based on the law of conservation. By partitioning a continuous physical domain into a series of control volumes and then applying the law of conservation to each control volume, the PDEs are transformed into systems of algebraic equations. The FVE method is not only applicable to fluid mechanics, but also gradually extends to other fields, including elastic mechanics, for solving the stress, strain, and displacement of structures.
Nevertheless, when the FE method, the FD scheme, and the FVE method are applied to solving the PDEs in the actual engineering, they all generally contain hundreds of thousands or even tens of millions of unknowns. Even when solved on advanced computers, it still takes several days or even dozens of days to calculate the FE, FD, and FVE solutions. Especially, as a result of the FE method, the FD scheme, and the FVE method in the actual engineering computations containing a large number of unknowns, the calculating errors during the actual calculations could be quickly accumulated. This leads to significant differences in the obtained FE, FD, FVE solutions and fails to achieve the expected results. Therefore, how to reduce the unknowns in the FE method, the FD scheme, and the FVE method to slow down the accumulation of computing errors in the calculation, save CPU running time, mitigate the calculation load, and improve the accuracy of the FE, FD, and FVE solutions is a key issue.
Therefore, we will review the reduced-dimension methods for the FE method, the FD scheme, and the FVE method in the next section, which can greatly reduce the unknowns in the FE method, the FD scheme, and the FVE method so as to slow down the accumulation of computing errors in the calculation, lessen CPU running time, mitigate the calculated load, and improve the accuracy of the FE, FD, and FVE solutions.
Lots of examples for the numerical simulations, for example, those examples in [6], have shown the proper orthogonal decomposition (POD) is one of the most effective approaches to reduce the dimension for the FE, FD, and FVE equations. The POD method has a long history. It is actually principal vector analysis in optimization. Therefore, it is still applied in data mining at present. The POD method is essentially to find a set of orthogonal bases for a set of known data under a certain least squares optimality, that is, to find an optimal low-dimensional approximation for the set of known data. It was originally proposed by Pearson [7] in 1901 for extracting targeted main components from a large amount of data. Pearson's sample analysis and data processing are still used in the data mining at present. The fashionable term for this kind of data is known as "Big Data". The term for the POD method was proposed by Sirovich [8] in 1987 and is mainly used for analyzing the characteristics of fluids.
The reduced-dimension approaches for the FE method, the FD scheme, and the FVE method are first proposed by Luo's team, they are specifically introduced as follows.
There are two dimensionality reduction methods for the FE method. One is the dimensionality reduction of the FE subspace, and the other is the dimensionality reduction of the unknown coefficient vectors in the unknown FE solutions. They are stated as follows.
The POD-based FE subspace dimension reduction method was first gradually proposed by Luo's team internationally since 2007 (see [9]), whose basic idea is stated as follows.
Let be a bounded and simply connected region, and be the most maximum time limit. Consider the following unsteady PDE:
where is an integer, is a differential operator with respect to spatial variables, and is a known function.
Step 1. By using the FE method to discretize spatial variables and difference quotient to discretize time derivatives, we get a fully discrete FE equation:
here span, are the FE basis functions, is related to the number of spatial nodes and the degree of the basis function of the piecewise interpolation polynomial, is a positive definite bilinear functional for given , , and is a linear continuous functional determined by .
step 2. Find the first (, usually take ) FE solutions from (3) and use the continuous POD method in [6, Chapter 4] to construct (generally ) POD basis functions containing the main information.
step 3. By replacing the FE space in (3) with span spanned by the POD bases , the FE equation (3) with tens of millions of unknowns is simplified into the following FE POD reduction model with only unknowns:
The POD-based dimension reduction method for the unknown FE solution coefficient vectors was first gradually proposed by Luo's team internationally since 2020 (see [10]), whose basic idea is stated as follows.
Step 1. Represent the unknown FE solutions in (3) into some linear combinations of the FE basis function vector and the unknown solution coefficient vectors as folllows:
Step 2. Substitute the linear combinations into (3) to obtain the following matrix form:
where and are determined by the positive definite bilinear functional and in (3), respectively.
Step 3. Find the first (, usually take ) FE solution coefficient vectors from (11) and construct matrix .
Step 4. Find the (usually ) standard orthogonal eigenvectors corresponding to the largest eigenvalues for the and form the POD basis .
Step 5. Replace in (11) with to obtain a dimensionality reduction format with only unknowns:
where are the -dimensional unknown vectors and are the first solution coefficient vectors in (11).
The POD-based FD dimension reduction method was also first gradually proposed by Luo's team internationally since 2007 (see [11]), whose basic idea is stated as follows.
Step 1. The PDE (1) is discretized by difference quotient and is written into a vector form FD scheme:
where is a positive definite matrix for the obtained and are determined by .
Step 2. Find the first FE solution coefficient vectors , usually take ) from (17) and construct matrix . The (usually ) standard orthogonal eigenvectors corresponding to the largest eigenvalues for the to form the POD basis . Replace in (11) with to obtain a dimensionality reduction FD scheme as follows:
or
where are the -dimensional unknown vectors and are the first solution coefficient vectors in (17).
The POD-based FVE dimension reduction method was first gradually proposed by Luo's team internationally since 2011 (see [12]), whose basic idea is also stated as follows.
Step 1. Discretize the PDE (1) into a fully discrete FVE equation:
where , , , and are the same as those in (3), is a linear interpolation operator, and is a piecewise polynomial space one order lower than .
step 2. Find the first (, usually take ) FVE solutions from (29) and use the continuous POD method in [6, Chapter 4] to construct (generally ) POD bases containing the main information.
step 3. Replace the FE space in (29) with span spanned by the POD bases , the FVE equation (29) with tens of millions of unknowns is simplified into the following POD-based FVE reduction formulation with only unknowns:
In this editorial, we have stated the significance for the numerical simulations and the creating the Journal of Numerical Simulations in Physics and Mathematics. We have also reviewed the origin and development of three commonly used computational methods: the FE method, the FD scheme, and the FVE method, as well as their dimension reduction methods, which are often used in the numerical simulations. It is worth noting that these methods, especially the dimensionality reduction methods based on the POD method, still have considerable room for development.
Journal of Numerical Simulations in Physics and Mathematics
ISSN: pending (Online) | ISSN: pending (Print)
Email: [email protected]
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