1. Introduction
Natural convection (NC) equations for buoyancy-driven fluid often appear in practical problems. The stationary NC equation is a coupling equation for the incompressible flow and heat transfer process of viscous fluid, in which the incompressible fluid can be characterized by Boussinesq’s approximation. In atmospheric dynamics, it is an important forced dissipative nonlinear system. It contains the velocity field, pressure and temperature, which we can analyze from the thermodynamic point of view. The motion viscosity of the fluid produces a certain amount of heat, so the motion of the fluid must be accompanied by the conversion of temperature, velocity and pressure. Therefore, the study of this nonlinear system is of great significance. Christie and Mitchell [
1], Boland and Layton [
2,
3] and others have extensively researched the numerical analysis and numerical results of NC equations (see [
4,
5,
6,
7,
8,
9]). In recent years, there has been continuous research on NC equations, such as in [
8,
10,
11,
12,
13], which studied the natural convection of cavity-filled nanofluids. Ahmad [
14] studied the effect of viscosity and thermal conductivity on natural convection in exothermic catalytic chemical reactions on curved surfaces, and in [
10,
15,
16,
17,
18,
19], Singh et al. considered the effect of factors such as Lorentz force on natural convection.
Combined with previous research results, the dimensionless parameter
(Rayleigh number) plays an important role in NC equations. It is well known that the buoyancy force is the driving force of NC equations, where the buoyancy force is the density difference caused by the temperature difference. The Rayleigh number
represents the ratio of the buoyancy force to the viscous force, which characterizes the relative strength of the buoyancy-driven inertial force to that of the viscous force [
20,
21]. When
, the buoyancy force is much larger than the viscous force, and the convection caused by the buoyancy force is significant, which leads to the convection dominance problem [
22]. Therefore, there are a number of effective numerical techniques dedicated to solving this difficulty: the variational multiscale method in [
23], the defect-correction method in [
24,
25,
26,
27], etc. Among them, defect correction is one of the commonly used methods to address large
number problems.
The defect-correction method (DCM) is an iterative improvement technique for improving the accuracy of computational solutions without introducing mesh refinement [
28,
29,
30,
31]. In general, the basic idea can be briefly described as follows. Assume that
M is a mapping
, where
X and
Y are typical linear spaces, and assume that our goal is to find a good approximation of
, where
. Assume that we actually solve a related problem
, whose solution we denote by
. Then, we find the residual or defect
and use it to solve the error or correct
, either by nonlinearly updating
or by linearly updating
to obtain the corrected solution
, setting
; obviously, this process can be repeated. If the mapping
M is nonlinear, then the linearization of
is usually chosen in the correction phase to simplify the computation. In the case of solving linear systems, the well-known iterative refinement method is an example of a defect-correction technique.
For computational practicality, the minimum-order coordinated velocity, pressure and temperature finite elements with
are used to discretize NC equations. The
finite element approximation of the pseudo-stress tensor
is the discontinuous slice constant of
We use the superconvergent slice to recover the amount of continuous space
The approximate solution computed by the local recovered error estimator
is closer to the true solution than the approximate solution of the general finite element method. The a posteriori error has been extensively studied (see [
32,
33,
34,
35,
3637)] Therefore, we use the adaptive defect-correction method to solve NC equations.
This paper is divided into four parts. First, it introduces the main properties of NC equations and the classical results. The second part proposes the defect-correction method and presents the analysis of its error estimation. The third part is combined with the restorative error estimator to solve NC equations. Finally, the validity and reliability of our method are verified by numerical experiments.
2. Preliminaries
Suppose that
is a bounded, convex open area whose boundary
is Lipschitz continuous, so
, where
is a regular open subset of
. We consider the following NC equations [
7,
9]:
where
is fluid velocity,
is fluid pressure,
is temperature,
is the external force function,
and
are the Prandtl number and Rayleigh number, respectively,
is the thermal conductivity parameter, and
is the two-dimensional unit vector. Next, we introduce some Hilbert spaces:
We denote the inner product and norm of
by
and
. We define the inner product in space
X and
W.
and its norm
are written as
and
. In addition, the dual space for
is
. The dual space of
X is
[
2].
For simplicity, we define
u and
v such that they belong to the same finite element space
X. We define two continuous bilinear forms
and
and a trilinear form
:
In addition, we need to define two bilinear forms
and a trilinear form
in
and
, respectively.
According to the bilinear form defined above and the trilinear form, the following two known conclusions are obtained [
2,
3]:
where
are two fixed constants that depend only on
.
The variational form of the NC Equation (
3) is as follows: solving
,
:
Then, there exists a unique result for the following solution [
2,
3].
Theorem 1. There exists at least a solution pair that satisfies (2) and where ; moreover, if , , κ and γ satisfy the following uniqueness condition: then the solution pair of problem (2) is unique. 4. Recovery Error Estimator for NC Equations
4.1. Recovery Error Estimator
In this section, we construct a recovery error estimator and analyze its properties. Assuming that is the numerical result, we consider the pseudo-stress tensor and its finite element approximation .
I is
-identity matrix. The main idea of the Zienkienwicz–Zhu estimator is to restore the discontinuous finite element gradient to a continuous recovery term (see [
37]).
In , N, , and are respectively defined as the set of all vertices in , the set of all vertices within in and the base function of i () that shares a collection of all units of a vertex ().
Combined with the superconvergent piece recovery technique in the literature [
37], the piecewise constant tensor
is restored to continuous, and a recovery pseudo-stress tensor is obtained.
For any vertex
and its patch
, the following is defined:
where
is the area of triangulation
K. Details can be found in [
37].
A recovery error estimator is constructed. The local estimator and global estimator are defined as
The recovery error estimator
has the following important properties. The proof is similar to that in the literature ([
37]) and is not presented in detail here.
Before moving to the next subsection, we illustrate the connection between
and
, where
is the unit outward normal vector on edge
e, which plays an important role in the a posterior error of this recovery-based error estimator. In two dimensions,
denotes the unit tangential vector along
e.
denote the jumps of the normal and tangential component of
on side
e, respectively. Since
, we have
From the above relation, we have
Lemma 1. Based on the definitions of and , there are two grid-independent constants and . Lemma 2. According to the definitions of and , there is a constant C independent of the grid, and the following and error estimation can be obtained. Proof of Lemma 2. According to the Brezis–Gallonet inequality in [
37] and the method in [
37], the conclusion can be easily obtained. □
4.2. The Reliability Analysis
A basic framework of posteriori error estimation for nonlinear stability problems is proposed, and the basic equivalence of the error to residual error is established. We introduce the main results and apply them to estimate the restorative error estimator.
Theorem 4. Suppose that and are the solutions of (2) and (5), respectively, Proof of Theorem 2. According to (
12), with the Cauchy–Schwarz inequality, we have
According to the definition of
, the approximate solution
satisfies
At the same time .
With the above lemma, we obtain
Theorem 4 is proved. □
4.3. Effectiveness Analysis
From Lemma 1, the following can be obtained:
In order to prove the validity of the recovery error estimator , we first need to estimate and
Lemma 3. Suppose that and are the solutions of (2) and (5), respectively. There is a normal number C that is not related to the grid: Proof of Lemma 3. From the definition of
, the following can be obtained:
Specific details can be found in [
37]. □
Next, we define the operator
:
.
Theorem 5. Suppose that and are the solutions of (2) and (5), respectively, and C is a grid-independent constant. Proof of Theorem 3. Combined with the above estimates, Lemma 1 and Lemma 3 are applied.
The proof of validity is complete. □
5. Numerical Experiment
In this section, four numerical examples are given to verify the effectiveness of the proposed method combined with the adaptive method.
For the sake of convenience, a few definitions are given below:
the number of degrees of freedom of triangulation ;
denotes the relative value of norm;
denotes the relative value of global recovery-based estimator;
denotes the convergence rate of the error;
denotes the convergence rate of the error;
denotes effectivity index for the global recovery-based estimator ,
where the validity index denotes the ratio of the estimator error to the true error , and if tends to 1, it means that our estimator error is asymptotically equivalent to the true error, thus verifying our validity and reliability.
5.1. Smooth True Solution
The purpose of the first example is to solve a smooth true solution problem in the
region and verify our method, which is effective for the smooth true solution. We define the true solution
, pressure
p and temperature
T as follows:
Figure 1 shows the initial mesh of the adaptive method and the adaptive grid. From
Table 1 and
Table 2, if only the defect-correction method is used to solve the NC equations, more degrees of freedom are needed to achieve better accuracy, while the adaptive method can yield better accuracy with less vertex information. For example, the accuracy of 5000 degrees of freedom in
Table 2 is similar to that of 2773 degrees of freedom in
Table 1. This shows that the adaptive method is more economical.
Table 3 presents the comparison of the results of our method (DCM) and the adaptive algorithm without the defect-correction method (NDCM). We selected the values of the error estimator
for similar degrees of freedom with different
. From the table, we can see that the error results without the defect-correction method when
are very large, and when
is greater than
, the results are not counted. Our method, on the other hand, is still relatively stable at
. It can be seen that the posteriori error estimator based on the defect-correction method is applicable.
tends to 1, which indicates that our estimators and real errors are effective and progressive, indicating that our method is effective.
5.2. L-Shape Domain Problem
The second example is a flow problem in the L-shape domain
, with
,
and 1000,
. The NC equations (
1) is determined by exact velocities
and
, pressure
p and temperature
T:
We note that both velocity u and pressure p are continuous in the domain. However, it is clear that u and p are singular at the point and along the line , respectively.
To show the efficiency of the error estimator, we compare numerical results for adaptive refinements with those for uniform refinement.
Figure 2 presents the initial mesh (left), the final uniform mesh (middle) and the final adaptive mesh (right).
Table 4 and
Table 5 show the numerical results for uniform/adaptive refinements with the recovery-based estimator
.
According to in
Figure 2c, we can see that near the singular point
of
u and the line of
with the singularity of
p, mesh refinement is carried out effectively, and the desired results are obtained, which shows that the adaptive mesh refinement is effective.
As can be seen from
Table 4,
Table 5 and
Table 6, our adaptive method can obtain better accuracy with less vertex information, which is better than the uniform grid. For example, when
, to achieve accuracies of 0.1631 and 0.1665, the adaptive method only needs 1337 degrees of freedom, while the uniform grid requires 4282 degrees of freedom. When
, the error is 0.2778 and 0.2602; the adaptive method only needs 3622 degrees of freedom, while the uniform grid needs 6172 degrees of freedom. This shows that our method is more efficient. Furthermore, as
becomes larger, our method calculates results that are superior to FEM.
5.3. Thermally Driven Flow
In the second numerical example, we consider the problem of square cavity flow without a true solution. As shown in
Figure 3, the definition of the calculation area is
, with
and
, and the boundary definition is as follows: the left boundary and the bottom are
, and the bottom edge is
. The rest is
, and the speed is the 0 Dirichlet condition.
Figure 4 shows the initial mesh and the adaptive encryption result grid.
Figure 5 and
Figure 6 are the velocity streamline and isothermal chart for different Rayleigh numbers:
and
.
Figure 7 and
Figure 8 present the results of the streamline of the velocity and the isotherm diagram calculated without the defect-correction method at different
numbers, and it can be seen that as
increases, the calculated results become more unstable [
38]. In
Figure 9, we also show the vertical velocity distribution at
and the horizontal velocity distribution at
, which are very popular graphical illustrations in experimental studies of thermally driven cavities. We see that the profiles increasingly vary as the Rayleigh number increases, which is consistent with previous studies [
5,
7,
39].
Table 7 and
Table 8 present the peak values of vertical velocity at
and horizontal velocity at
for different Rayleigh numbers, respectively, where the number ’m’ in parentheses corresponds to the degree of freedom of the used mesh. It is easy to see that our results are concordant with benchmark data [
5,
7,
39].
5.4. Bernard Convection Problem
In this experiment, we consider the domain
with the forcing
and
.
Figure 10 displays the initial and boundary conditions for velocity
u and temperature
T:
on
,
on the lateral boundaries, with a fixed temperature difference between the top and bottom boundaries.
The results of numerical streamlines and numerical temperature are shown in
Figure 11 and
Figure 12 with
,
and
. The results coincide with the phenomenon described in the literature [
38], indicating that our adaptive DCM is suitable.