1. Fluent iterative process
After the modeling is meshed, through a series of settings, click Calculation, and Fluent starts calculation. The process is as follows:
(1) Based on the initialization value, it is assumed that the initial value of the flow field physical quantity is Q0;
(2) On the basis of the initial value, calculate the flow field according to the conservation equation, and obtain the new flow field physical quantity value Q1;
(3) Comparing the values of Q0 and Q1, |Q1-Q0| is called the absolute error, and |Q1-Q0|/Q1 is the relative error.
(4) If the error is less than the set value P, convergence is achieved; otherwise, convergence is not achieved, take a new iteration value Q2 for calculation, and repeat the above steps.
2. The concept of residual
The residual is the sum of the fluxes on all surfaces of the unit. The physical quantities detected mainly include velocity, mass, energy, turbulence parameters, etc. Theoretically, the sum of the transport of each physical quantity within the unit should be zero after convergence. The smaller the residual, the better, but due to numerical accuracy issues, it is impossible to get 0 residuals. For single-precision calculations, it should generally be lower than the default initial residual 1e-03. Of course, the convergence of each item should also be considered (for example, continuous items are not easy to converge but energy items are easy). For transient simulation, the values of each physical quantity always change, so the transient problem is considered to be steady state at each time step, and its residual plot is always a wavy line.
Generally, the inlet and outlet flows can be monitored in Fluent. When the residual converges to a certain extent, the convergence can be determined by checking whether the inlet and outlet flows are stable and balanced (the balance of lift and drag must be monitored during aircraft calculations).
The residual oscillates at a higher level. It is necessary to check whether the boundary conditions are reasonable. Secondly, check whether the initial conditions are reasonable, such as the flow field of the shock wave. Improper initial conditions will cause oscillation of the flow field. Sometimes there may be separation or backflow in the flow field, which is an unsteady phenomenon in itself. The residual will oscillate to a certain extent during calculation. At this time, if the inlet and outlet flows reach a stable balance, the flow field can also be considered to have converged. In addition, Fluent uses multi-grid by default. Setting multi-grid to 0 in the later stage of calculation can avoid the residual errors of some wavelengths from oscillating on the fine grid.
3. What is the meaning of continuity in the residual plot? And the reason why it is difficult to converge?
Continuity is the mass residual, that is, the continuity equation residual, which is the embodiment of mass conservation.
If other parameters converge during the calculation process, but only continuity does not converge, this may be related to the solution method SIMPLE of the Fluent program. SIMPLE derives a pressure correction equation based on the continuity equation to solve the pressure. Since the flow field coupling term in the continuity equation is oversimplified, the pressure correction equation cannot accurately reflect changes in the flow field, resulting in slow convergence of the equation.
4. Fluent’s method of judging convergence
1. Monitoring residual value
During the iterative calculation process, when the residual values of each physical quantity reach the convergence criterion, the calculation will stop. Fluent's default convergence standard is: except for the residual value of energy, when the residual value of all variables drops below 1e-03, the calculation is considered to have converged, and the convergence standard of the energy residual value is below 1e- 06.
2. The calculation results no longer change as the iteration proceeds.
Sometimes, because the convergence criteria are set inappropriately, the residual values of physical quantities never meet the convergence criteria during the iterative calculation process. However, in some cases, even if the residual error does not reach the set value, as long as the physical quantity of concern is stable, it can be considered to be converged. For example, mass flow, you can calculate the mass flow difference between the inlet and outlet in the flux option in the report. If it is less than 1%, and the inlet and outlet flow continues to be stable, it can also be considered to be convergent.
3. The mass, momentum and energy of the system are all conserved
Check whether mass, momentum, and energy flowing into and out of the entire system are conserved in the Flux Reports dialog box. Conservation then the computation converges. If the imbalance error is less than 0.1%, the calculation can also be considered to be converged.
5. FLUENT does not converge. Common solutions
1. Improving grid quality is the most important method. For simple models, structured grids can be used to make them as structured as possible. For parts where the physical quantities change more drastically, the grid needs to be locally refined, such as the boundary layer. (Adaptable grids can be used to automatically capture grids with large gradients).
2. Adjust the Courant Number appropriately . Courant Number actually refers to the relative relationship between time step and space step. When there is a computational domain with a sharp shape, the system will automatically reduce the Courant Number. However, errors will occur when the local flow rate is too large or the pressure difference is too large, and local mesh refinement is required.
In Fluent, Courant Number will appear during coupled solution and can be used to adjust the stability and convergence of the calculation. Generally speaking, as the Courant Number changes from small to large, the convergence speed gradually accelerates, but the calculation stability gradually decreases. Therefore, during the calculation process, it is best to set the Courant Number from a small size and observe the convergence of the iterative residuals. If the convergence speed is slow and stable, the Courant Number size can be appropriately increased.
3. Appropriately reducing the relaxation factor will make it easier to converge, but at the cost of slower convergence. Generally, if the residual curve of a certain physical quantity is relatively high, the relaxation factor of this physical quantity can be reduced accordingly. For example, if the speed convergence is not good, the relaxation factor of momentum can be reduced.
4. For transient problems, the steady-state calculation can be converged first and then the transient problem can be solved . For example, for the problem of falling film evaporation, the flow can be calculated in the steady state first, and then the transient state can be turned on to add evaporation after the flow is stable, so that convergence can be achieved quickly. However, it cannot be used in some cases, such as considering the working conditions under a certain transient time.