Week 3 Challenge : CFD meshing on Turbocharger

REPORT

CFD Meshing on Turbocharger

THEORY

Turbocharger

In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gasses. It uses this energy to compress the intake gas, forcing more air into the engine in order to produce more powder for a given displacement.

Like other forced induction devices, a compressor in the turbocharger pressurizes the intake air before it enters the inlet manifold. In the case of a turbocharger, the compressor is powered by the kinetic energy of the engine’s exhaust gases, which is extracted by the turbocharger’s turbine.

The main components of the turbocharger are:

• Turbine – usually a radial turbine design
• Compressor – usually a centrifugal compressor
• Center housing hub rotating assembly

Application

Turbochargers have been used in the following applications:

• Petrol-powered car engines
• Diesel-powered car engines
• Motorcycle engines
• Diesel-powered truck engines
• Aircraft piston engines
• Marine engines
• Locomotive and diesel multiple unit engines for trains
• Stationary/industrial engines

Safety

• Turbocharger failures and resultant high exhaust temperatures are among the cause of car fires.
• Failure of the seals will cause oil to leak into the cylinders causing blue-grey smoke. In diesel engines, this can cause an over speed, a condition known as diesel engine runaway.

MODEL

Surface meshing is performed with the given target lengths as per PIDs.

1. Blade stage-1 = 1 mm
2. Blade stage-2 = 1 mm
3. Impeller = 2 mm
4. Shaft rotor = 1mm
5. Turbo casing = 5 mm
6. Compressor casing = 5 mm
7. Inlet casing cover = 5 mm

PROCEDURE

GEOMETRY CLEAN-UP

• When reading geometry in ANSA one should first check the topology.
• Switch to PID colour view mode. This displays the faces coloured according to their Property ID.
• Before one starts cleaning up the geometry of the model it is useful to perform a number of checks that are incorporated in ANSA. ANSA can identify and isolate on the screen the problematic areas that require the user’s attention. Check to locate geometrical problems of the model such as Unchecked Faces, Collapsed Cons, Triple Cons, Needle Faces, Overlap Faces, Cracks, and Single Cons. A number of errors are shown by the check manager, it is required to clean up the geometry before stepping up for meshing. These errors are shown in the geometry with highlighted red lines. The errors found on the model can be visualized by selecting the appropriate option from the focus toolbar. The FOCUS group contains all the functions that are used to isolate and control the visibility of selected entities in order to obtain a clear image of the working area of the model.
• De-activate SHADOW and DOUBLE CONS visibility and see if there are any red or cyan CONS which indicate single or triple connectivity.
• Check if these features are intentional or topological errors.
• Use the TOPO functions to fix such problems.

Area Selection Mode

• ENT: selection of individual entities; one by one.
• PID: selection of all the entities of one selected PID.
• PID Region: selection of the entities of an unconnected region of one PID.
• LOOP: Automatic selection of closed loops of edges/ CONS etc (useful for hole closure)

TOPO Deck

• To patch the problematic surfaces, choose a new face from the TOPO deck. Choose the surface type according to the surface shape required to be formed.
• Fill holes in the CONS section can also be used to patch a surface. It can be used for multiple patches at once.
• Inserting and projecting hot points are also used for complicated sections to be patched. Using hot points smaller less complicated geometry shape is created so that fitted faces can be used.
• Extra faces can be deleted using the Delete function from the Utilities Toolbar. To activate the delete function. Select the two faces with the left mouse button. Confirm with the middle mouse button. A warning window appears. Press OK to proceed with the deletion of the selected faces. The advantage of the Delete function of the Utilities Toolbar in comparison with the Faces > Delete function, is the fact that one can select more than one Face, without being prompted each time for confirmation).

TOPO menu

From this menu, the user can create, modify and clean up the geometry. The menu is divided into groups of functions according to the entities that they refer to.

HOT POINTS – These functions are used to create or delete Hot Points or Weld Spots.

• Hot points > Parametric

Activate the HOT PNTs>PARAM function. Select the red CONS. The input window opens. Keep the 0.5 value and press Enter. A Hot Point is created. Without exiting the function use the right mouse button to select the three other Double CONS, to create additional Hot Points at the same parametric distance.

• Hot points > Release

Activate the function and select with box select the area. All topological conditions are released. This function can always be used to correct improperly pasted CONS.

As a result all CONS now appear in red, and distinct Hot Points are visible.

CONS - Functions that are applied on CONS (Curve ON Surfaces).

FACES – Functions to create or modify Faces.

• Faces > Topo

To activate this function, select with a box selection the CONS and confirm with the middle mouse button. ANSA perform topology connection. Now the boundary appears in cyan indicating that three Faces are connected at each CONS. One can delete the interior excess of the larger Face.

Auxiliaries > Fine

Due to a relatively large resolution length, the protrusion appears quite coarse and it becomes hard to distinguish its proper shape. Activate the FINE function from the geometry group of functions. Left click a couple of times on the crosshatches of the faces in order to reduce the Resolution length. (One can use the right mouse button to do the opposite, i.e. increase the Resolution length and make entities appear coarser). Now the geometry will appear clearer.

Note:

• To select an existing surface one can only select by clicking on cross edges.
• For CFD analysis the surface should be closed for volumetric mesh.
• The four errors showing the Triple Cons present in the model are warning signs which should be cross-checked in the model if it is required or can be avoided.

Creating new PIDs

PID function

• Select with the left mouse button the small triangular Face from its crosshatch. Confirm with the middle mouse button. The Properties window appears, so one can select the PID one wants to assign to the selected face. One can now view the faces coloured according to their Property ID.
• For this model, a number of PIDs are created Blade Stage_1, Blade Stage_2, Shaft Rotor, Turbo Casing, Inlet Casing Cover, Impeller, Casing Inlet, Compressor Casing, Casing Outlet, Bottom Casing Inlet, and Bottom Casing Outlet.
• Now assign mesh length and mesh each component one by one by considering one PID separated in the screen.

Shell Mesh

• Switch to the MESH menu. In the MESH menu, the Faces are now Macros Areas and the CONS are Perimeter Segments. The hot points are displayed as white dots.
• First, assign the mesh length. Mesh lengths are chosen according to the size of PIDs in order to attain the best possible mesh result.
• To assign mesh length make sure the value input will be done in four places. Firstly, in the Perimeter Module buttons, the inside length gives the desired mesh value in Perimeter and Macro respectively. Then, in Mesh Parameter and Quality Criteria, here we also have to insert values for maximum and minimum target length and also mention element type in the Mesh parameter section.

Note:

1. The quality of the resulting mesh can be greatly improved by joining Macros together, prior to applying Perimeter lengths and spacing.
2. ANSA will only fix the mesh if it violates the current settings of the Quality Criteria window.

Mesh Parameters

The target length in CFD mode is Local. This means that ANSA will reconstruct the mesh using the same underlying size of the elements it improves.

Length

Activate the Perimeters>Length function. Select all the Macros Areas with the left mouse button making a box selection. Confirm with the middle mouse button. The Length Parameters windows open. Type in the value as mentioned in the model description, leave the other fields empty and press Enter. The new element length is applied to all the Perimeter Segments of the selected Macro Areas.

Meshing

After assigning mesh length, generate mesh using the Spot Mesh option in Mesh Generation Module. Here, it is preferred to create mesh after selecting an individual PID module rather than meshing the entire model at once.

Unmeshed macros in meshing

• There appeared to be unmeshed Macros if some extreme Joining had been performed, for example, this model showed that as the meshing for two adjacent PIDs with different mesh lengths was performed, the unmeshed macros appeared.
• Any macros that have not meshed are reported in the legend on the right. Use the right-click Show Only to isolate them on the screen.
• To resolve this issue, re-meshing is performed. Setting the min and max element length and allowing these constrained values to put proximities of element length to the area and further not specifying the element length to perimeters and macros will do the work.
• It is suggested to make a Macros>Cut to isolate the problematic area.

Check

Lastly, check for any unmeshed surfaces and measure the meshed surfaces before moving forward to volume meshing.

Volumetric Mesh

• The ANSA batch meshing manager is a powerful tool for versatile, controllable and predictable high-quality meshing with shell (surface, CFD, wrapping of 3D parts), or volume (solid components, CFD, HEXA interior) elements.
• Having first created the boundary layer elements, consider the definition of the Volume entities. This can be easily done with the function Volumes>Define [Auto Detect].
• All the Volumes defined manually or detected by ANSA are listed in the Volumes>List window.
• Check this list to monitor the type, Name, PID and status of the Volumes.
• One can always manage the Volumes in the model using the Volumes>List window, edit the volume, and delete the PID showing outer structure of the model.

Meshing the volume

• ANSA provides different meshing algorithms for volume meshing.
• Here, Tetra CFD is chosen.
• Tetra CFD: Old tetra mesh generation for CFD models with large variation in element length on the surface. Significantly slower than Tetra Rapid, it should only be used if Tetra Rapid does not perform well in a specific case.

LEARNING OUTCOMES

Error Faced

Collapsed cons

• Check geometry option showed four collapsed cons attached to compressor casing triple cons. It is fixed it using the fix option shown in the check geometry box after right-clicking on the error shown.
• The other procedure can also be used to manually release them, delete the extra hot points, use BREAK, and then TOPO.

RESULT

Meshed individual parts according to the specified Mesh Length

Compressor Casing, Casing Inlet, Casing Outlet

Turbo Casing, Bottom Casing Inlet, Bottom Casing Outlet, Inlet Casing Cover

Impeller

Blade Stage_1, Blade Stage_2

Shaft Rotor

Volume body generated shown in TOPO Mode

Volume Mesh

Small cut showing Tetra elements

Half body displaying volume meshing

CONCLUSION

• 915864 tria elements (2D elements) have been bonded to form a surface mesh over the geometry.
• 10108286 tetra elements (3D elements) have been bonded to form a volume mesh inside the geometry.

Link: https://drive.google.com/file/d/1IZw4aRrcpJmJJ4NcBC4nE8DWXtkiYn8-/view?usp=sharing