ADVANCED PRE-PROCESSING WITH ANSA FOR A STRUCTURAL ANALYSIS WITH A TURBOCHARGER

JORGE LOZANO LUCIO

Advanced CFD Meshing using ANSA

Challenge - 8

ADVANCED PRE-PROCESSING WITH ANSA FOR A STRUCTURAL ANALYSIS WITH A TURBOCHARGER

Fig.1 Turbocharger modeled in Solid Edge

Objective

Perform a pre-processing mesh by applying PID’s and cleaning the geometry errors, the Turbocharger must be with a target element size of 0.25mm for Structural Analysis. In Fig.1 the turbocharger ready to mesh is seen being able to recognize all elements and obtain better understanding of the CAD model to perform the pre-processing.

 

Theory

Mesh generator for structured solid objects: A solid is represented in the structured mesh by the volumetric material cells of the inside volume of a closed unstructured surface in the input mesh. Because both solid and surface objects are represented by a surface input mesh, it is necessary to distinguish among these using control parameters for each group of input objects. Thus, the input mesh for a solid is required to be a closed surface with a well-defined inside that can be identified by the occurrence of an even number of intersections between ray and object.

Application of a structured mesh in any aspect of grid generation has certain advantages and disadvantages. The advantage of such a mesh is that the points of an elemental cell can be easily addressed by double indices (i,j) in two dimensions or triple indices (i,j,k) in three dimensions. The connectivity is straightforward because cells adjacent to a given elemental face are identified by the indices, and the cell edges form continuous mesh lines that begin and end on opposite elemental faces. In two dimensions, the central cell is connected by four neighboring cells. In three dimensions, the central cell is connected by six neighboring cells.

Graph 1 shows how the CPU time increases with the number of output elements. Analyzing models with different numbers of input elements from 240 to 101,158, a significant

Graph 1 The performance of the mesher with different numbers of input and output elements.

Structured mesh also allows easy data management and connectivity occurs in a regular fashion, which makes programming easy. Nevertheless, the disadvantage of adopting such a mesh, particularly for more complex geometries, is the increase in grid non-orthogonality or skewness that can cause unphysical solutions due to the transformation of the governing equations. The transformed equations that accommodate the non-orthogonality act as the link between the structured coordinate system (such as Cartesian coordinates) and the body-fitted coordinate system, but contain additional terms, thereby augmenting the cost of numerical calculations and difficulties in programming. Because of this, such a mesh may also affect the accuracy and efficiency of the numerical algorithm that is being applied.

 

Procedure

The primary objective of this ANSA pre-processing challenge is to obtain an executable CFD Turbocharger model ready for a simulation.

There will be two types of mesh generation, for which a comparison of the procedure and in the first case the possibility to perform a transient thermal simulation is obtained, by exporting the mesh to OpenFOAM files.

The first will be a constant generated surface wrap mesh with the 5mm target mesh element due to the initial conditions of the challenge (the element size will be modified for the following mesh), in Fig. 2 the original turbocharger geometry model is shown containing single cons, unchecked errors, and other geometry inconsistencies, The first activity is to clean the external surface.

Fig. 2 Initial Turbocharger model with geometry errors.

In Fig. 3 when all surfaces are cleaned or adjusted to the proper geometry, the following is to release all coons having only single coons, to be able to fill all holes as the inlet and oulet mainly having only external surfaces.

Fig. 3 All coons released Turbocharger model.

 

Once all holes or openings in the geometry are filled, is proceeded to select the constant wrap surface wrap mesh with a 5mm target element size, in Fig. 4 is shown a block preview, to represent an overview of the element size.

Fig. 4 Constant surface mesh preview.

 

A quality criterion is given to adjust the elements to the wanted mesh quality, this is due to the results conservation and precision while performing the analysis, in Fig. 5 a table with all properties is shown being proper specifications to mesh the model.

Fig. 5 Quality criteria.

 

In Fig. 6 a first wrap mesh generated is shown being represented by each of the original surface PIDs, by hiding the generated mesh and seeing only the original geometry model we’re able to delete all unused of to select the clean up tool to delete all unwanted properties.

Fig. 6 Surface mesh generated.

The following step is to arrange all PID’s according to the purposed simulation to perform, in this case transient thermal simulation will be performed and a volume is created according to the tetrahedral interior mesh criterion, being an only material volume which will have a specific diffusion value (non-real case).In Fig. 7 a pre—processed mesh is obtained to simulate in any external processor, having less than 100,000 elements.

Fig. 7 PID’s selected for OpenFoam analysis.

As mentioned, the quality mesh criteria must be arranged to the given parameters, by selecting the negative view, as in Fig. 8, is easier to modify any element according to the offset elements.

Fig. 8 Negative view, volumen and surface with correct quality criteria.

 

The following codes shown are the system, constant and 0 files exported by ANSA to an OpenFOAM readable file, ready to simulate, and as seen a video is generated having the temperature being dissipated through the turbocharger. This is only a theoretical simulation not similar as in reality, but as result seeing the capabilities of ANSA as a pre-processing software.

 

Initial Temperature conditions code:

/*----------------------------------------------------------------------------------*\
|                                                                                    | 
|    ANSA_VERSION: 17.1.0                                                            | 
|                                                                                    | 
|    file created by  A N S A  Sat Oct 05 21:31:07 2019                              | 
|                                                                                    | 
|    Output from: D:/TUTORIALS ANSA/ENGINE-0/ASSEMBLY-TOTAL.ansa                     | 
|                                                                                    | 
\*----------------------------------------------------------------------------------*/



FoamFile
{
	version 2.0;
	format binary;
	class volScalarField;
	location "";
	object T;
}
/*---------------------------------------------------------------------------*/
/*---------------------------------------------------------------------------*/


dimensions [0 0 0 1 0 0 0];

internalField uniform 273;


boundaryField
{
	SURFACE-1
	{
	type		zeroGradient;
	}

	FRONT-SURFACE
	{
	type		zeroGradient;
	}

	TOP-SURFACE
	{
	type		zeroGradient;
	}

	SURFACE-2
	{
	
	type            fixedGradient;
        gradient	300;
	value uniform	473;
	}

	SURFACE-3
	{
	type		zeroGradient;
	}

	PartBody
	{
	type		zeroGradient;
	}


}

fvSchemes code:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  6
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

ddtSchemes
{
    default         Euler;
}

gradSchemes
{
    default         Gauss linear;
    grad(T)         Gauss linear;
}

divSchemes
{
    default         none;
}

laplacianSchemes
{
    default         none;
    laplacian(DT,T) Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}


// ************************************************************************* //

ControlDict code:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  6
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application     laplacianFoam;

startFrom       latestTime;

startTime       0;

stopAt          endTime;

endTime         600;

deltaT          0.05;

writeControl    runTime;

writeInterval   10;

purgeWrite      0;

writeFormat     ascii;

writePrecision  6;

writeCompression off;

timeFormat      general;

timePrecision   6;

runTimeModifiable true;


// ************************************************************************* //

Video 1. Transient thermal simulation over turbocharger in OpenFOAM.

 

For the following mesh generation (structured mesh) the first step is to perform a cleaning mesh of the model, eliminating all inconsistencies and constantly checking on the geometry manager checks.

As in Fig. 9 the turbocharger model is cleaned and ready for a solid structured mesh.

Fig. 9 Clean Turbocharger model geometry

As always for every surface mesh generated, we must check on the negative view to see once the quality criteria are selected the offset elements of the model, in this case the target element is 0.25mm to obtain a well defined mesh elements along which will be easier to accommodate to a structured mesh, As in Fig. 10 a negative view is shown and all elements are generated according to the quality criteria.

Fig. 10 Negative unstructured mesh generated.

 

The following will be to identify every component to a known PID name, in Fig. 11, showing the element number, type and PID generated by the software.

Fig. 11 PID property tab.

As mentioned, this will be a structured mesh, and the tab in Fig. 12 will be used, to obtain a structured (and unstructured) mesh, being the structured mesh more time consuming but a resulting in a greater quality mesh according to the turbocharger model.

Fig. 12 Volume tab for structured and unstructured mesh.

 

In Fig.13 a structured mesh view is obtained, seeing how fine and aligned the elements are having in this image perfect squares or non-skewed elements, this give us the possibilities and assurance of a good result and great convergence while processing.

Fig. 13 Structured surface mesh closed up.

In Fig. 14 a side (left) and a front (right) view are obtained, having a proper quality mesh, and structured mesh elements.

Fig. 14 Side and front model meshed closed up.

 

As result, a complete turbocharger mesh was performed as in Fig. 15, for optimizing the mesh file, the geometry could be deleted and a clean up can be selected as well, the file is ready to export to any processor, due to the element number, over 5 million a simulation isn’t performed but it could be and comparing it to the wrap surface mesh, a better more realistic thermal analysis could be performed.

Fig. 15 Completed turbocharger surface mesh.

 

Conclusion

Two types of meshes are performed; a wrap surface constant mesh and a structured mesh, a thermal simulation using a wrap mesh is performed in OpenFOAM, although the geometry quality was lost as seen in the video, the results may give a general idea of the effect of the temperatures and the volume properties, but for a company or an good engineer the same geometry must be stable having structured mesh elements without affecting any model quality, and still obtaining an almost perfect mesh quality criteria, as seen in the second mesh performed, the element quantity increased up to 5 million, securing the stability, precision and resulting with real life data.

 

 

References

1.- “Structured Mesh Generation”, Michael K. Berens, Ian D. Flintoft, and John F. Dawson, IEEE ANTENNAS & PROPAGATION MAGAZINE, JUNE 2016.

2.-“ https://www.sciencedirect.com/topics/computer-science/structured-mesh”.


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