1. Before simulation
1. What is initialization? What impact does the initialization method in Fluent have on the calculation results? How do you understand the "patch" in initialization?
The initialization of the problem is to give the flow field an initial value when doing calculations, including pressure, speed, temperature, turbulence coefficient, etc. Theoretically, the given initial field will not have an impact on the final result, because as the number of iteration steps increases, the calculated flow field will be infinitely close to the real flow field. However, due to the discrete format accuracy of calculation software such as Fluent, (will produce discrete errors) and truncation errors. If the initial field deviates too far from the actual physical field, it will be difficult for the calculation to converge, or even diverge at the beginning of the calculation. Therefore, during initialization, the initial value should still be given as close to the actual physical phenomenon as possible. This requires us to have a relatively clear understanding of the physical fields to be calculated.
The patch in the initialization is a supplement to the initialization. For example, when encountering a multi-phase flow problem, the parameters of each phase need to be more finely restricted to maximize the closeness to the real physical field. These can be achieved through patches, which can initialize flow field partitions, and can also initialize specific areas by writing simple functions.
When the number of iteration steps is completed and the flow field has not yet converged, you can click the "Calculate" button again to continue calculating the same number of iterations without re-initializing and setting a larger number of steps.
2. Various pressure concepts of Fluent.
The following pressures will appear in Fluent: Static pressure, Dynamic pressure, and Total pressure. These pressures are aerodynamic concepts, and the relationship between them is:
Total pressure = Static pressure + Dynamic pressure
The stagnation pressure is equal to the total pressure (because the stagnation pressure is the pressure when the speed is 0, and the dynamic pressure is 0 at this time). Static pressure is the value measured by the pressure gauge.
In Fluent, four pressures are defined: Absolute pressure (absolute pressure), Relative pressure (reference pressure), and: Operating pressure (operating pressure), Gauge pressure (gauge pressure). The relationship between them is:
Absolute pressure = Operating pressure + Gauge pressure
Some of the above pressures are actually one-to-one correspondence, but the difference in expression, for example: Static pressure (static pressure) and Gauge pressure (gauge pressure) have the same meaning.
For the operating pressure, if it is an incompressible flow, the default value is generally 101325 Pa. If it is a compressible flow, you can set the operating pressure to 0 and regard the gauge pressure as an absolute pressure. The specific recommended settings are as follows:
| Density relation | Mach number | operating pressure |
|---|---|---|
| ideal gas law | >0.1 | 0 or approximately equal to the average pressure of the flow field |
| ideal gas law | <0.1 | Approximately equal to the average pressure of the flow field |
| function about temperature | incompressible | Do not use |
| constant | incompressible | Do not use |
| incompressible ideal gas | incompressible | Approximately equal to the average pressure of the flow field |
3. The difference between pressure far field and pressure outlet boundary.
The pressure far field refers to the pressure value far away from the outlet boundary. It has a very weak influence on the outlet boundary. The pressure value on the surface or edge of the outlet boundary does not need to be constant, but can change. This boundary can only be compressed when compressed. Used on the move. The pressure value on the surface or edge of the pressure outlet boundary is a constant value. Since the pressure distribution often cannot be determined during solution, but the pressure on the boundary generally does not change much, the pressure outlet boundary is used in most cases. However, in a few cases when the pressure change on the boundary surface may be large, the pressure far field needs to be used.
4. When defining the speed entrance in FLUENT, what is the applicable scope of the speed entrance? What are the methods for defining turbulence parameters? What are the differences between each?
Boundary conditions for velocity inlets apply to incompressible flows, requiring a given inlet velocity and all scalar values that need to be calculated. The velocity inlet boundary condition is not suitable for compressible flow, otherwise the inlet boundary condition will cause certain fluctuations in the total temperature or total pressure at the inlet.
Regarding the definition method of turbulence parameters, there are different turbulence parameter combinations according to the selected turbulence model. For details, you can refer to the relevant chapters of the Fluent user manual, or you can refer to Wang Fujun's book "Computational Fluid Dynamics Analysis—CFD Software Principles and Applications" 》pages 214-216.
5. Several situations in which free outflow cannot be used.
(1) Includes pressure inlet conditions;
(2) Compressible flow;
(3) Unsteady flow with changing density.
6、axis and symmetry。
axis is axial symmetry (unit radian), and the two-dimensional symmetry axis must be the X-axis. Symmetry is mirror symmetry (plane symmetry, unit thickness). Symmetry reduces the problem of plane symmetry by half and can be three-dimensional. The axis must be in the X direction, and the calculation area must be above the X axis.
7. How to choose single or double precision solver?
Fluent's single- and double-precision solvers are suitable for all computing platforms. In most cases, the single-precision solver can well meet the calculation accuracy requirements and has a small amount of calculation. However, there are some cases where it is recommended to use a double-precision solver:
(1) If the geometry contains features of completely different scales (such as a long, thin-walled tube), use double precision;
(2) If there are multiple closed areas connected by small-diameter pipes in the model, and there is a large pressure difference between different areas, use double precision.
(3) For problems with higher thermal conductivity or for meshes with larger aspect ratios, use double precision.
8. What are the general practices for grid independence verification?
Grid independence verification, that is, determining whether the number of grids has nothing to do with the simulation results. During numerical simulation, the number of meshes directly affects the accuracy of the calculation results. Too few grids will lead to inaccurate simulation results, and too many grids will make the calculation too slow, so a suitable number of grids should be selected.
First, divide the grid sizes in order from large to small, and divide the number of different grids from small to large. After that, import Fluent separately to simulate under the same conditions. After the simulation is completed, compare the simulation results of a certain physical quantity, or the distribution of the physical quantity in a certain direction. Generally, as the number of grids gradually increases, the simulation results of a certain physical quantity will become closer and closer, and the distribution of the physical quantity will gradually become consistent. At this time, if the error is allowed, select a smaller number of grids that can obtain accurate results and conduct subsequent simulations. This is the general approach to grid independence verification.
2. Simulation
1、"turbulent viscosity limited to viscosity ratio of 1.000000e+005 in *** cells"的问题。
The turbulent viscosity ratio is too large, indicating that the turbulence level during the calculation process is extremely high and has reached a situation that is impossible in actual working conditions, and Fluent limits it. There is a high probability that it is caused by the divergence of the calculation process, so the calculation needs to be converged.
This problem can occur due to: (1) poor mesh quality (skewness >0.85 for tetrahedral/hexahedral meshes, >0.9 for triangular/quadrilateral meshes); (2) improper setting of turbulence boundary conditions , or there is no good initial value.
When this warning occurs, generally speaking, the most likely problem is the quality of the grid, especially the problem of the Y+ value; when dividing the grid, it should be noted that the height of the first layer of grid is very important, you can use NASA's Viscous Grid Space Calculator to calculate the first layer grid height.
If there are no obvious problems with the mesh quality, you can try to reset the turbulent boundary conditions instead of using the default turbulent kinetic energy, turbulent viscosity ratio and other parameters (the default values are all 1, this value may appear to be very large under some working conditions, thus prompt this question).
2. The "reversed flow..." problem occurs when running calculations.
This problem means that backflow has occurred. This problem is more relaxed than the warning of turbulent viscosity ratio. In some cases, this warning may only appear at the beginning of the calculation, and will disappear as the iteration proceeds. If the calculation occurs for a period of time, the warning disappears, it will have no impact on the calculation results. If this warning persists, the following processing may be required:
(1) If the external flow is simulated, the reason for this warning may be that the boundary conditions are not far enough away from the object. If the boundary condition is set far enough, there may indeed be a backflow phenomenon there during the calculation process; for compressible flow, the boundary is best set at 10 times the characteristic length of the object; for incompressible flow, the boundary is best set at 4 times the characteristic length of the object.
(2) If this warning occurs, whether for external flow or internal flow, the pressure-outlet boundary condition can be used instead of the outflow boundary condition to improve this problem.
3. Determine simulation convergence through flow.
In report → flux → mass flow rate, select all inlets and outlets. After calculation, the difference between the inlet and outlet flows will be displayed. If its value is less than 1% of the total inlet flow rate, and other detection quantities will basically disappear after continuing the iteration. If fluctuation occurs, it can be considered that the simulation solution has converged.
3. Post-processing
1. The Y+ value of the grid and its function.
After the simulation, go to "Report" → "Drawing" → "XY Plot" → "Y-Axis Function" in the upper right → drop-down box "Turbulence" and "Wall Yplus", select the wall in "Surface" to view the grid Y+ value.
Y+ is a parameter related to the wall function. Through the study of the boundary layer, the boundary layer can be divided into three regions: viscous bottom layer (0 < Y+ < 5), buffer layer (5 < Y+ < 30) and completely turbulent layer (Y+ > 30).
Here, two dimensionless physical quantities u+ and y+ are used to define the laws within the boundary layer:
u+ represents the dimensionless velocity, u represents the fluid velocity in the boundary layer, and τw is the wall shear stress.
y+ represents the dimensionless distance to the wall, y represents the distance from a certain point in the boundary layer to the wall, and v represents the kinematic viscosity of the fluid m^2/s.
A large number of experiments have been conducted on the three regions of the boundary layer, and the results show that the rules of u+ and y+ in these three regions are different. For the viscous bottom layer (0<y+<5), the relationship between u+ and y+ is approximately linear; for the fully turbulent layer, the relationship between u+ and y+ is approximately logarithmic, which is called the logarithmic law; for the buffer layer, the linear relationship curve and the logarithm The law curve has an intersection point in the buffer layer, and the y+ value corresponding to the intersection point is around 11.
Fluent uses wall functions to calculate fluid flow, heat and mass transfer within the boundary layer, and other issues. The wall function is a semi-empirical formula derived from experimental rules and is used to connect the viscosity-influenced region between the wall and the fully turbulent region. It calculates the boundary layer law based on the logarithmic law and ignores the viscous bottom layer and buffer layer. Therefore, when we draw the boundary layer mesh, we cannot draw the viscous bottom layer and buffer layer, but must directly draw the completely turbulent layer. That is to say, when using the wall function, not only does it not need to refine the grid in the boundary layer, but it must ensure that the first layer of grid is in the range where the logarithmic law can be applied. Generally speaking, Y+ = 15 is used as the dividing line where the logarithmic law can be used, so the first layer of grid should ensure that y+ > 15 as much as possible.
In order to leave a certain margin and ensure the accuracy of the calculation results, Fluent requires that Y+ must be greater than 15. If y+ is less than 15, Fluent cannot guarantee the accuracy of the solution. The lower limit of y+ is 15, and the upper limit of y+ depends on the Reynolds number:
For high Reynolds number: such as ships, airplanes, etc., the range of logarithmic law is expanded, and the upper limit of y+ can be taken to several thousand, reducing the number of grids; for low Reynolds number: such as turbine blades, etc., the upper limit of Y+ can be taken to 100; for very low Reynolds number: the range of the logarithmic law is very narrow. In order to ensure Y+ > 15, the number of grid layers in the boundary layer may be very small, and the calculation results will be deteriorated. Therefore, it is not recommended to use the wall function.
For a more detailed understanding of boundary layer theory, please refer to:
One article clearly explains Fluent wall function (Y+) and near wall processing - Zhihu (zhihu.com)
2. What are the common file format types in FLUENT: dbs, msh, cas, dat, trn, jou, profile, etc.?
In the Gambit directory, there are three files, namely default_id.dbs, jou, and trn files. Running save on Gambit will save these three files in the working directory: default_id.dbs, default_id.jou, and default_id.trn.
The jou file is a Gambit command record file. You can batch Gambit commands by running the jou file;
The dbs file is Gambit's default file for storing geometry and mesh data;
The trn file is a file that records command line window (transcript) information. After the Fluent program exits, a text file in TRN format will be generated, and the file name will be named in the format of "fluent-date-time-random number". The TRN file records all text information displayed in the Fluent command line. This file can be generated by unchecking "Automatic Transcript" in "Preference - General - Fluent settings."
The msh file can select the msh file output format in the export after Gambit divides the grid and sets the boundary conditions, and the file can be read by the Fluent solver.
The case file includes the mesh, boundary conditions, solution parameters, user interface and graphics environment.
The data file contains the flow values for each grid cell and the convergence history (residual values). Fluent automatically saves file types, defaulting to date and case files.
The profile file boundary profile is used to specify the flow conditions in the boundary area of the solution domain. For example, they can be used to specify the velocity field at the entrance plane.
To read the profile file, click the menu File/Read/Profile... to pop up the file selection dialog box, and you can read the boundary profile file. Write a contour file. You can also create a contour file based on the conditions of a specified boundary or surface. For example: you can create a profile file in the outlet condition of one study, then read the profile file in other studies, and use the outlet profile as the inlet profile of the new study. To write a profile file, you need to use the Write Profile panel (Figure 1), menu: File/Write/Profile.
The bat file is a script file generated after Fluent is started. The file name generally starts with "cleanup-...". The purpose of this script file is to ensure that after the Fluent program exits normally (use the close button at the top right of the interface to exit), all corresponding processes can be terminated, thereby releasing system resources. The script file is automatically executed during the exit phase of the Fluent program. This file can be deleted after closing Fluent.
4. Others
1. Define the volume heat source or component mass source in a certain surface (2D) or a body (3D) in the calculation area. How to define this zone? And this zone is still fluid.
First define the required zone in Gambit. For flowing with the fluid, I think this can be handled with a dynamic grid. In the dynamic grid setting interface, set the zone that flows with the fluid as a rigid body so that it can be used as a zone without affecting the fluid. Circulation can also flow with fluids. It’s just that its moving UDF is difficult to define. It’s best to edit the grid UDF according to its flow rules.
2. 2D aircraft airfoil - typical compressible external flow simulation
In Fluent, when the Mach number is above 0.3, the compressibility of the gas cannot be ignored. So for this type of compressible simulation, the density-based solver should be selected and the energy equation turned on.
For external wing flow problems, the single-equation Spalart-Allmaras turbulence model is recommended. In the material settings, set the density term of air to "ideal-gas", the viscosity term to Sutherland's equation, and the coefficients are all default.
Regarding operating pressure, in Fluent, for flows with a convective Mach number greater than 0.1, the operating pressure is recommended to be 0.
After the simulation is completed, the y+ value of the wing surface mesh is displayed to determine whether the mesh is acceptable. For the Spalart-Allmaras model, the requirement for the y+ value is either y+ = 1, or y+ greater than or equal to 30. If most of the grid is greater than 30, it means that the grid is acceptable.
3. What is the PDF method? What are the methods for simulating pulverized coal combustion in Fluent?
The probability density function transport equation method (PDF method) is a new model method gradually established in recent years to describe turbulent two-phase flow. The so-called Probability Density Function (PDF) method is based on the randomness and probabilistic statistical description of the turbulence field. It uses the velocity, temperature, component concentration and other characteristic quantities of the flow field as random variables to study the probability density function in the phase. Methods for studying the transport behavior of space. The PDF model is between macroscopic simulation and mesoscopic simulation. It starts from the molecular dynamics theory of random motion and the basic conservation law of two-phase turbulence to explore the laws of two-phase turbulence. Therefore, it can be used as a two-phase model within the framework of the two-fluid model. Theoretical foundations of turbulence models. It is essentially a bridge between the EL model and the EE model. Lagrange analysis of particle motion can be used to establish a closed EE two-phase turbulence model through statistical theory, that is, the integration of the PDF equation.
Correct simulation of non-premixed turbulent combustion processes requires simultaneous simulation of mixing and chemical reaction processes. FLUENT provides four reaction simulation methods: finite rate reaction method, mixture fraction PDF method, imbalance (flame element) method and premixed combustion method. The flame element method is a special case of the mixed fractional PDF method. This method is based on unbalanced reactions. Unbalanced phenomena that cannot be simulated by the mixed fraction PDF method, such as flame suspension and extinguishing, and the formation of NOx, can be simulated by this method. However, since this method is not yet perfect, it can only be applied to adiabatic models in FLUENT.
For many combustion systems, radiation is the main energy transfer mode, so when simulating combustion systems, the simulation of radiation energy transfer is also very important. In FLUENT, the model for simulating this process is also very comprehensive. Including DTRM, P-1, Rosseland, DO radiation models, and the WSGG model to simulate the absorption coefficient.