Finite Element Analysis. Barna Szabó

Figure 1.13 The ratio

If we approximate the eigenfunctions using a uniform mesh consisting of 5 elements, and increase the polynomial degrees uniformly then we get the curves shown in Fig. 1.14. The curves show that only about 40% of the numerically computed eigenvalues will be accurate. The error increases monotonically for the higher eigenvalues and the size of the error is virtually independent of p.

      It is possible to reduce this error by enforcing the continuity of derivatives. Examples are available in [32]. There is a tradeoff, however: Enforcing continuity of derivatives on the basis functions reduces the number of degrees of freedom but entails a substantial programming burden because an adaptive scheme has to be devised for the general case to ensure that the proper degree of continuity is enforced. If, for example, μ would be a piecewise constant function then the continuity of the first and higher derivatives must not be enforced in those points where μ is discontinuous.

From the perspective of designing a finite element software, it is advantageous to design the software in such a way that it will work well for a broad class of problems. In the formulation presented in this chapter continuity is a requirement. Functions that lie in where are also in . In other words, the space is embedded in the space . Symbolically: . The exact eigenfunctions in this example are in .

Figure 1.14 The ratio

corresponding to the p version. Uniform mesh, 5 elements.

Table 1.6 Example: p‐Convergence of the 24th eigenvalue in Example 1.16.

p	5	10	15	20
ω 24	194.296	100.787	98.312	98.312

Example 1.16 Let us consider the problem in Example 1.15 modified so that μ is a piecewise constant function defined on a uniform mesh of 5 elements such that on elements 1, 3 and 5, on elements 2 and 4. In this case the exact eigenfunctions are not smooth and the exact eigenvalues are not known explicitly.

At there are 24 degrees of freedom. Suppose that the 24th eigenvalue is of interest. If we increase p uniformly then this eigenvalue converges to 98.312. The results of computation are shown in Table 1.6.

      Any eigenvalue can be approximated to an arbitrary degree of precision on a suitably defined mesh and uniform increase in the degrees of freedom. When κ and/or are discontinuous functions then the points of discontinuity must be node points.

      Observe that the numerically computed eigenvalues converge monotonically from above. This follows directly from the fact that the eigenfunctions are minimizers of the Rayleigh quotient.

Exercise 1.21 Prove eq. (1.143).

      Exercise 1.22 Find the eigenvalues for the problem of Example 1.15 using the generalized formulation and the basis functions , (). Assume that κ and

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