3.3. Dissipation and dispersion analysis
Although we can devise wavelet upwind schemes based on our proposed method, the above numerical tests suggest that some of the wavelet schemes are unstable even for the linear scalar advection problems. Therefore, it is crucial to investigate the dissipation and dispersion properties and provide an instruction to choose “good” wavelets for stable wavelet upwind schemes.
We examine the dissipation and dispersion properties of the schemes base on the Fourier analysis method [2, 26]. Suppose that
, then we can obtain the numerical derivative:
where
is the corrected wavenumber which is a complex variable and
. Therefore,
can be rewritten as
. The exact derivative is given by
Comparing the above relation, we can obtain that
kr = 0 and
ki =
kΔ
x for ideal numerical schemes. When conducting Fourier analysis on Equation (19), we can calculate the following numerical solution:
where
kr is the dissipation coefficient and
ki is the dispersion coefficient. The exact solution for Equation (19) can be computed by
Then we can give the physical meaning of kr and ki. kr depicts the attenuation of the wave amplitude at t induced by the numerical error, and ki describes the change in propagation speed of the wave caused by the numerical method. It can be noted that the wave amplitude will be amplified over time if kr is a negative value. The corresponding scheme shows the instability when solving the advection problems. Therefore, kr for stable numerical schemes should be non-negative. Here kΔx is defined as effective wavenumber and denoted by α. When ki / α > 1, the numerical wave will run faster than the exact one. And the numerical wave will fall behind the exact wave for ki / α < 1. We remark that only the spectral method has the ideal dispersion property, which means that ki /α = 1.
We compuate the dissipation coefficient of the different wavelet schemes as shown in
Figure 5. For specified parity of
N, it can be seen that
kr decreases with the increase of
N.
kr of the scheme with
N∈odd is smaller than that with
N∈even, indicating that the wavelet upwind schemes with
N∈odd show better dissipation property. To seek the reason for the instability of several schemes, we analyze the
kr for all the above schemes. We find that the negative
kr exists for schemes with
N∈even. To clarify this fact, the locally enlarged version of the
Figure 5b is illustrated in
Figure 6. It can be observed that
kr are all negative when
α < 0.8. This suggests that the schemes with
N∈even will show a negative diffusion phenomenon when
J is large enough, and the error will be amplified over time. This actually induces the instability of the wavelet schemes. We also find that the local extreme value approaches to 0 as
N increases for
N∈even, which means that the negative diffusion process is weakened, and the stability of the schemes are improved. This explains that the numerical tests for the schemes with
N∈even in subsection 3.1 and 3.2 are still stable. But for longer time integration and some
α near the local extreme value, the results might be divergent.
Based on the above analysis, we can conclude that only
N∈odd schemes provide the correct implicit viscosity for solving the hyperbolic conservation laws. However, numercial tests in subsection 3.1 show that the scheme with
N = 7 and BM = 3 is still unstable. As has been discussed, the asymmetry of the scaling function is another factor influencing the stability of the wavelet schemes. We further evaluate the dissipation coefficients of the schemes with
N = 7, BM = 3 and
N = 9, BM = 3 as shown in
Figure 7. It can be found that
kr is negative when
α is smaller than the specified value. Therefore, the schemes with BM = 3 are unstable. Now we can achieve a basic instruction that
N∈odd, BM = 1 schemes are with non-negative dissipation coefficients and stable for hyperbolic conservation laws.
Then, we will explore the dispersion property related to the resolution of the schemes. For a specified wavelet scheme, an effective node number is defined by the point per wavelength abbreviated as
PPW, which can be computed by the following relation
The spectral method has the optimal resolution corresponding to
α =
π and
PPW = 2. It can be seen from Equation (26) that
PPW is inversely proportional to
α. The length of the interval
α that
ki /
α is approximately equal to 1 depicts the ability of the scheme to trace the wave accurately. We choose the interval of
α that satisfies
and
to measure the maximum
α which reflects the resolution of the schemes directly. The dispersion coefficients against
α are plotted in
Figure 8. It can be observed that
N < 5 schemes have a large dispersion error and a low resolution. The maximum
α that
ki /
α meets the tolerance relation increases with an increment in
N for the specified parity. To clarify the resolution more clearly, we list the maximum
α in a tolerance range in
Table 4. It can be seen that the maximum
α gradually tends to
π as
N increases, and the schemes with
N∈even behave better in resolution when
N > 6. On the basis of the above analysis, we can obtain that the wavelets are more applicable to design the high-order schemes, and the scheme with larger
N has the higher accuracy and better resolution.
Next, we conduct a numerical test with a smooth initial distribution to verify the theoretical analysis of the dissipation and dispersion performance. The test is the advection of a sine wave described in subsection 3.1 with
u(
x, 0) = sin (5
πx) as the initial condition. The
PPW is approximately equal to 6 at the resolution level
J = 4. The numerical results obtained by the schemes with
N∈odd and BM = 1 at
t = 2 are shown in
Figure 9. It can be seen that the higher order schemes show less dissipative, approach to the exact solution more accurately and reveal higher resolution for the wave. Therefore, the numerical results are in accordance with the stability and resolution analysis.
Finally, a numerical test of the advection of a multi-scale function is devised to show the capability of the wavelet schemes in recognizing smooth and discontinuous solutions in the high-speed flows. The initial condition consists of step functions, saw-tooth function, sine waves in different frequencies, which is shown as
For this numerical test, we apply the adaptive wavelet upwind schemes proposed in our previous study [
24]. The main idea of adaptive node generation is to recognize trouble nodes based on the wavelet coefficients that are larger than a threshold parameter
ε = 1.0
−5, and insert nodes in the adjacent zones near the trouble nodes. Moreover, an integration reconstruction method is designed based on the Lebesgue differentiation theorem to suppress the spurious oscillations. We choose the basic resolution level
J0 = 6, the maximum resolution level
Jmax = 12, the same adaptive and reconstruction parameters for different schemes. The numerical results obtained by the adaptive wavelet schemes with
N∈odd and BM = 1 at
t = 2 are compared with that of the classic fifth-order finite difference WENO scheme (WENO-5) proposed by Jiang and Shu [
34] as illustrated in
Figure 10. It can be found that the higher order wavelet scheme can capture the discontinuities without spurious oscillations and distinguish different scale structures accurately with less nodes, which also verifies the better resolution of the higher order scheme. For the WENO-5 scheme, a uniform node distribution with
N1 = 2048 is required to depict all the details of the solution. The nodes required in the adaptive wavelet upwind schemes are about half of the WENO-5 scheme, showing that the adaptive wavelet schemes with the integration reconstruction can capture discontinuities free from the numerical oscillations and distinguish complex solutions efficiently.