CFD STEPS & MESH GENERATION :
(1) First of all, you must create or choose a computational domain, and then a mesh generation (also called a grid); after that, the computational domain is divided into many small elements called cells (mesh). For 2-D domains, the cells are areas, while for 3-D domains the cells are volumes. You can assume that each cell behaves like a tiny control volume in which conservation equations are solved. The quality of a CFD solution is highly dependent on the quality of the grid. Therefore, it must be necessary that the mesh should have high quality before proceeding to the next step.
Source- Fluid mechanics by cengel |
(3) After that, the numerical parameter and solution models or algorithms selected based on whatever you analyze in your problem. In ANSYS, There are many models available for laminar and turbulent fluid flow as shown. From that the most popular & widely used models are Laminar, K-epsilon, & K-omega.
various model available in Ansys |
(4) After that Boundary conditions are specified on each edge of the computational domain (2-D flows) or on each face of the domain (3-D flows).The boundaries of a 2-D domain are called edges, while the boundaries of a 3-D domain are called faces.
(5) For solving problems in ANSYS, you need to specify residual. Residual is the function of error. As you increase residual the result may not obtain accurately but at the same time, solution converged in less time. another hand if you decrease the value of residual, the solution may take more time to converged and give an accurate results.
(6) After specified residual, model or problem is solved iteratively. An iteration is a number of steps to solve the problem on the different boundary conditions. For example, if you give 100 iterations to solve the problem means your solution equation is repeated 100 times the same equation will calculate on the different boundary condition. Sometimes hundreds or even thousands of iterations are required to converge on a final solution. As the iteration is solved continuously, the residuals may decrease by several orders of magnitude. For better understanding, you can refer to numerical methods in engineering mathematics. it is similar to the bisection, newton rapshon method.
(7) Once the solution has converged, flow field parameters such as velocity and pressure are plotted and analyzed graphically. Most commercial CFD software has built-in post processors, designed for quick graphical analysis of the flow field. ANSYS is also stand-alone post processor software packages available for this purpose. Since the graphics output is often displayed in vivid colors, that's way CFD is also known as the nickname "colorful fluid dynamics". For example, the below image shows the velocity analysis of the nozzle in which the magnitude of velocity at different points at any location is indicated by different colors.
(8) And finally, Global properties of the flow field, such as pressure drop, integral properties, such as forces (lift and drag), surface integral properties and volume integral properties like pressure, velocity, density, temperature, area and moments acting on a body, are calculated from the converged solution.
Mesh type :
- Structured mesh
- Unstructured mesh
Structured mesh
Fig - a |
Fig - b |
the structured mesh consists of planar cells with four edges (2-D) fig (a) or volumetric cells with six faces (3-D) fig (b).
Unstructured mesh
Fig - a |
Fig - b |
An unstructured grid consists of cells of various shapes, but generally, triangles or quadrilaterals (2-D) fig (b) and tetrahedrons or hexahedrons (3-D) are used fig (a).
Mesh Generation :
Fig - c |
As we show earlier the 1st step to perform CFD is to create a proper mesh that defines the cell on which solution equation and flow variable like pressure, velocity, etc..are calculated throughout the computational domain. ANSYS can run with either structured or unstructured mesh. The cells can be separated from rectangular, each cell is numbered according to (i, j, k). A 2-D structured mesh is shown in Fig - c
- To construct this grid, nine nodes are specified on the top and bottom edges; these nodes correspond to eight intervals along these edges. Similarly, five nodes are specified on the left and right edges, corresponding to four intervals along these edges.
- The intervals i = 1 to 8 and j= 1 to 4 are marked in Fig. The mesh is generated by joining nth i node corresponding to nth j node. In a 2-D structured mesh, each cell is easily identified by an index pair (i, j). For example, the shaded cell in Fig. is at (i =4 , j= 3).
Fig - d |
The two unstructured mesh for the same domain as that of the above fig - d are generated, using the same interval distribution on the edges; these grids are shown in Fig. b. In unstructured grid, cells cannot identify by indices i and j.
Both mesh type, structured mesh, and unstructured mesh have their own advantages and disadvantages.
- For complex geometries, an unstructured mesh is usually much easier for the user to the generation of mesh. In ANSYS, you can create both types of mesh.
- Do a comparison between structured mesh and unstructured mesh ( show above images fig-c and fig-d), you can notice that the right-hand side edge indicates in red curves is not identically generated.
- At boundary layers, where flow variables change rapidly near to the wall and highly resolved and finer mesh are required near to the wall, structured mesh enables much finer resolution than do unstructured mesh for the same number of cells.
- In particular, you must always avoid highly skewed cells, as this can lead to convergence difficulties and inaccuracies in the numerical solution.
Fig - e |
Fig - f |
The shaded cell in fig -e is an example of a highly skewness cell. Skewness is defined by the following equation
Source- Fluid mechanics by cengel |
- Where Î¸max is the maximum angle between any two edges of the cell and Î¸min is the minimum angle between any two edges of the cell. Î¸equal is the angle between any two edges in the ideal cell. For quadrilateral cells, Î¸equal = 90° and for triangular cells Î¸equal =60° as shown in fig-f.
- Generally, the structured mesh has low or zero skewness and the unstructured mesh has high skewness compare to structured mesh.
- Generally, high skewness occurs mostly near to a sharp corner in bodies. To avoid this high skewness just chop off or apply little fillet at a sharp corner as shown in fig below. This does not affect very much in a result of the analysis.
Reduce skewness |
Hybrid mesh
- Sudden changes in cell size can may lead to convergence difficulties in the CFD. For complicated geometries in which boundary analysis is important, in this condition the goal is to create a mesh in such a way that no cells in the mesh are highly skewed. To avoid a sudden change in mesh and highly skewed cells, we can use hybrid mesh with a combination of structured mesh and unstructured mesh. fig show example of hybrid mesh.
- In hybrid mesh, we create finer (structured) mesh near to boundary or most focusing area of analysis and course (unstructured) mesh in a computational domain far from boundary. This technique has two advantages (1) sudden change in meshing is avoid so the probability of formation of a highly skewed cell is reduced and (2) Computational time is reduced as we create finer mesh only at near to boundary which is our focusing analysis area.
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