Week-11 : Discretization of 3D intake manifold using GEM-3D

Aim:-> To perform discretization of intake manifold with converting it in 1D circuit model from 3D model and observing effect of discretization length on result. 

Theory:

Discretization is the splitting of large parts into smaller sections to improve a model's accuracy. There are two ways in which a fluid system is discretized. The first is to break the system up into several different components such as several pipes and/or flow-splits. The second is by discretizing a 'Pipe' part in to multiple sub-volumes, each performing their own calculations. When a pipe is discretized (by selecting discretization length to be shorter than the pipe length) the result will be the same as if several shorter single-volume pipes would have been used instead of one longer pipe.

Coarse discretization (larger discretization lengths) will normally result in faster simulation time, but sometimes this is at the expense of accuracy. Finer discretization may result in better accuracy and resolution, but with longer computational times. There is a limit at which decreasing the discretization length provides little or no increase in accuracy, and only adds computational time. The ideal solution is to find that limit to obtain the best accuracy with good computational time.

The discretization lengths will affect computational time in a slightly different way for each solution method. For both the explicit and implicit solution methods, computational time will be higher for smaller discretization lengths, because there will be more sub-volumes in the system that require calculation of pressure, temperature, etc. In other words, there are more solution variables. In the explicit solution method, the discretization length also affects the simulation time step. The time step is proportional to the discretization length due to the Courant condition. Smaller discretization lengths will require smaller time steps, and thus more computational time. For the implicit solution method, the time step is imposed as a constant value, and therefore simulation time is just a function of the number of sub-volumes in the system. 

• Explicit solution method: an increase of computational time by a factor of ~4. The time step will decrease by a factor of 2 due to the Courant condition and there will be twice as many sub-volumes which require calculation.
• Implicit solution method: an increase of computational time by a factor of ~2. Remember that the time step in the implicit method is imposed as a constant value, so it is not affected by the discretization length. Since there are twice as many sub-volumes, there will be twice as many solution variables, resulting in an increase of computational time by a factor of ~2.

• The study is done on the Single Cylinder Diesel Engine.

• It's a CI, single-cylinder engine, with two pipes for the intake system and the other two pipes for the exhaust system., SOHC, and direct-injection, naturally aspirated. The heat transfer model is the Woshni GT, and the cylinder geometry is shown below:

Create a parameter for Discretization Length:

Discretization Length:

• Approximate length of each sub-volume where solution quantities (pressure, temperature, mass fractions, etc.) are calculated.
• The discretization length does not need to be an even fraction of the entire pipe length; the code will adjust to divide the pipe appropriately into an integer number of sub-volumes. The discretization length will affect the time step when the explicit flow solver is used.
• The case setup is set for two discretizing lengths i.e. 40mm, 0.1mm. to study how results and time taken for simulation are effected.

Comparison of Results and Plots:

Cylinder Predictions:

• Its clearly visible from the above tables that the difference between the two cases in terms of Brake torque and BSFC is very small.
• Even the difference of the maximum cylinder pressure is quite small.
• Emissions is also quite low in both cases the Nox emissions in case1 is 1110.21 and in case 2 is 1095.12.The Carbon oxides is almost similiar in both cases.
• The Carbon dioxide emissions is lightly difference in between them.

• We can clearly observe from above tables that there is variation in values i.e. error is found if discretizing length is more. This owes to in accuracy of solver assuming the flow and changes to be similar across varying lengths of intake pipe. Hence, discretizing is must when accuracy is required.
• Further, there is significant increase in time taken to perform simulation as discussed earlier , the discretized element length shouldn't be unnecessarily small as it increases the time step and shouldn't be of higher value to not capture the results accurately.

Part-2 Explore Tutorial 2  (GEM 3D):

Building an Intake Manifold from an STL File:

• Create the file and import the geometry. Launch GEM3D and create a new model file by clicking the New icon in the home tab, select GEM3D and click finish.

• The first step is to import the intake manifold from an STL file that is provided with the GT-SUITE installation. To import the shell, select Import 3D from the File group in the Home tab. This will launch the Import 3D Wizard, shown below

• Since STL files do not contain unit information, it is very important to select the correct unit when importing STL files using the surface method. Since our example STL file was created using mm as the unit, make sure the unit is set to mm, then click Finish.

• GEM3D will create a mesh shape component ('GEMMeshShape') from the imported shape and add it to the model, which should look like the image below (with maybe the exception of color). Now showing the required geometry.
  

Separating the section by Curves:

• The inlet and the ends of the 4 intake runners are closed. We need to remove these caps so the ends are open and available for flow. To do this, we will use the Separate by Curves operation. This will separate the geometry into parts based on changes in the surface of the object. This process will also separate the intake runners into two different sections near the area change at the top of the runners. Ensure the part is selected and choose Separate by Curves from the Shape Operations group in the Convert tab.
• Delete the end sections of the runners and plenum. Use Ctrl + Left click to select all the end sections and press the delete tab. The following window will appear.

• Now Click Yes and the model should look like the image below, where the inner surfaces are now partly visible through the open ends where the caps were removed.

Using the Cutting Plane:

• Now that the end caps have been removed, we can continue separating the shape into sections that can be represented by pipes and flowsplits. We will start by separating off the 4 runners from the manifold since the runners can be represented by pipes. To do this we will make use of the cutting plane operations available in the Cutting Planes group in the Convert tab.
• To do this Go to Convert Tab>Cutting plane group>3 point cutting plane and select 3 points on the other side of the Manifold block. To translate the plane by dragging the left click on the plane such that only runners are cut not the manifold. Select the manifold with left click,and click on clip to separate the runners from the manifold.

Converting Sections into Flow Components:

• Now its time to convert all of the pipes into the flow components. We do this by right-clicking on the first section of the first runner (blue coloured bent section in the image above) and select Convert Shape to Component. This will open the convert mesh window, shown in the image below. Then we do it for all the pipes as seen below. This window appears once we right-click and select Convert Shape to Component. The port numbering suggests the flow of the air.

• After doing the NEXT, For the geometry, Multiple Bends is selected and Linear interpolation between ends is been selected for Cross Section Variation. End measurement Location is selected and user-defined Diameter of 51.6mm is selected. The same diameter is kept at port2.

• Then we do it for all pipes and we convert all the pipes into 8 different components as shown below:

Adding a Custom Connection:

• In the conversion process for the runners, information regarding the curved entry from the manifold (fillets) was lost. This curved entry would represent a better flow transition than a default orifice connection, so we need to manually add an orifice connection to represent this. To add a custom orifice to a flow component right-click on the first section of runner 1 (blue component in the image above, "run1-1") and choose Add Connection. This will open the port connection window, shown in the image below, that allows the selection of which port to add the connection to and what type of connection to use.

• Now select the port 1 and then select the orificeconn in Connections and press ok and then the new window will open and open theOrificeConn Template as shown below.

• With the connection template open, we can give it a name of "ori-run1". Since all 4 runners likely have the same curved entry shape, they should all get the same discharge coefficient. Therefore, define a parameter named [runner-cd] for the Forward Discharge Coefficient attribute.
• Give the parameter a value of 0.95, which should represent a nice transition since an abrupt area change would normally result in a discharge coefficient of approximately 0.8. Click OK to Case Setup to save the value and return to the connection window. Click OK to the connection window to save that connection. Repeat the above procedure for the rest of the runners using connection names of "ori-run2", "ori-run3", and "ori-run4".

Adding a Subassembly Connection to Impose Connection Direction:

• In this step we specify the end (boundary) connections to the runner. To do this right click on the end section of the first runner component (runl-2) and choose Add Connection.
• We want to select port 2 (the port not connected to the rest of the runner) and the subassembly connection ('GEMSubAssExtConn'). Click OK and the connection will be added and the appropriate template window will open.

• Use "SAconn1" for the name of the component and a Port ID of 1. Set the Flow Direction to "Outlet" since we expect flow to be from the manifold through the runner and out this port.Click OK to save this connection.

• Repeat this procedure for the rest of the runners using names of "SAconn2", "SAconn3", and "SAconn4" along with Port ID values of 2, 3, and 4

Dividing Pipe Sections with Area Changes:

• On the other end of the manifold, the entrance appears to consist of a pipe section with a round and D-shape, then a 90 degree turn into the manifold. The straight section can be converted into 2 pipes, one representing each effective diameter. Therefore we need to cut and separate these two sections so they can be converted. Convert Tab > Cutting Planes Group > Pipe Normal Cutting Plane > Then click on the rounded outer wall of the model very close to the entrance Move the cutting plane by clicking and dragging to a location just before the shape starts to change then press Clip to seperate the two regions.

• Next, we need to separate the D-shaped pipe section from the flowsplit section entering the manifold. To do this place another pipe normal cutting plane on the D-shaped section and drag it all the way to the end (where it joins the manifold). The model should now have 2 separate pipe-like sections like in the image below.

• The next step is to convert both of these sections into flow components. Let's start with the D-shaped section by right-clicking on it and selecting Convert Shape to Component. In the convert mesh window that opens choose the Pipe option. Let's switch ports 1 and 2 since they are opposite the expected flow direction. To do this click on Exclude All Ports. Than right-click on the blue outline of the inlet and choose Include Port. This will assign port 1 to the opening. Do the same for the outlet and click Next.

• We will model this section as a Straight pipe type with the larger area rather than a taper because the area change is fairly abrupt. Therefore, for the Diameter for cross section 1 select the checkbox Define from Port 2. Press the Next button to create the straight pipe. Press Next again and give it a name of 'pipet' and click Finish to save the pipe.
• Next, let's convert the round shaped section. Right-click on that section and select Convert Shape to Component. In the pipe conversion – geometry selection window choose the Straight as bend type option. We will model this section as a straight pipe, so for the Diameter for cross section 2 select the checkbox Define from Port 1. Press the Next button to create the straight pipe. Press Next again and give it a name of "pipe1" and click Finish to save the pipe. After finishing the conversion of these 2 pipes, the model should look like the images below

• Now we want to specify a subassembly connection on this last pipe since it has an end boundary. To do this right-click on "pipe1" and choose Add Connection. This will open the port connection window, shown in the image below.

• We want to select port 1 (the port opposite the manifold) and the subassembly connection ('GEMSubAssExtConn'). Click OK and the connection will be added and the appropriate template window will open. Use "SAconn5" for the name of the component and a Port ID of 5. Set the Flow Direction as an "Inlet", as shown below, since we expect flow to be toward the manifold from this port. Click OK to save this connection.

Separate the manifold from the 90 degree entrance flow split:

• The next step is to separate the manifold from the 90 degree entrance flow split. This must be done with the cutting plane operation, but will require use of some advanced features of that operation. The goal is simple, cut off the top flow split section without cutting through the manifold itself. This will be a multiple step process as we will need.
• To do this Go to Convert tab > Cutting Planes group > Select 3 Points Cutting Plane. To place the plane, pick 3 points on the top of the manifold. Graphical operations are allowed so the model can be rotated, panned, and/or zoomed to make it easier (Do not to click on the shape during those operations or it will select one of the points). Then orient the plane is such a way that it does not cut the manifold or the flowsplit part. To orient it use the translate and rotate X, Y and Z slide bars. And adjust the Precision for translating and rotating the plane as required.

• After perfectly orienting the plane select the manifold and press clip to separate the flow split and the manifold. After Spilting the manifold it should be look like this.

Converting Sections into General Flowsplits:

• Now that the entrance to the manifold is separated, we can convert it into a flow component. To do this, right-click it and select Convert Shape to Component. This will open the convert mesh window

• Select the flow volume as Flow split and change the port numbers .Port number 1 includes the inlet and port number 2 as outlet

• Now flow split using the exact geometry of the mesh shape will be created and drawn in the graphical window.

• To finish click Next and give the component a name of "man1" and click the Finish button to save the conversion and close the convert mesh window.

Dividing a Large Volume into Smaller Sections (Manual Discretization):

• The only mesh shape left to be converted is the manifold section. Rather than converting the entire thing into a flowsplit, we want to manually divide it to get better resolution and accuracy in the GT-SUITE flow model. Manual discretization involves making cuts to divide up the large mesh shape into smaller mesh shapes and converting those into flowsplits.

• The first step is to use a cutting plane to cut off the first end section toward the first runner. Enable the cutting plane by selecting 3 Points Cutting Plane from the Cutting Planes group in the Convert tab. Select 3 points on the end of the manifold such that they form a plane nearly perpendicular to the manifold section.
• Once the plane is ready we have to translate the plane such that it is between the 1st runner and 2^nd runner as shown in below figure. Then select the manifold button and press the clip button.

Restoring (Re-using) a Cutting Plane:

• We need to make one more cut between the 3rd and 4th runners to finish dividing the manifold. Rather than picking 3 points again, we can restore the last used cutting plane. To do this, select Restore Cutting Plane from the Cutting Planes group in the Convert tab. This will place a cutting plane at the same location and orientation as the last one used and open the control window.

• Translate this cutting plane by dragging it with the mouse until it is between the 3rd and 4th runners, again being careful not to cut through the port openings. Once in place, select the mesh shape, then click the clip button. This will separate the end section from the center section, leaving 3 manifold sections like the image below.

• Now convert each section into a flow component. Select the end section near runner 1. right-click and select Convert Shape to Component. Select Flowsplit from the select the conversion geometry pane. Since flow is expected to enter from the side rectangular port, make sure that is assigned port number 1. Then click Next twice to convert and create the flowsplit.

• Click next give the new component a name man-end1 and click the finish to save the conversion.

Determining Orientation During Conversion:

• Next, we will convert the center section into a flowsplit. Right-click on it and select Convert Shape to Component. Select Flowsplit from the select the conversion geometry pane. Again, all open ports will be identified. Let's make sure the port numbers start with the entrance flow (port 1), then go to the manifold end sections (ports 2 and 3), then to the runners (ports 4 and 5).

• Click the Next button and the conversion should now look like the image below.

• Click Next and name the component "man-center" and click Finish to save the component.

• With all the mesh shapes now converted to flow components, the model should look like the image below.

Adding Flow Connections:

• During the process of converting meshes to components, we created several pipes which do not align perfectly. To remedy this, we can use the flow connection operation to join the runner sections at the top of the manifold. To do this, select Flow Connection > Flow Connection from the Connections group under the tab Flow. This will show available surfaces for adding flow connections, as shown below. To join the runners together, click on the flow connection face at the end of the first intake runner, then click on the flow connection face at the end of the second intake runner. The sections should join together, as shown in the screenshot below.

After connection

• Now, we will need to add a flow connection between the plenum and first intake runner pipes. To add a flow connection while preserving the geometry that we have displayed in GEM3D, we can add an Extruded Connection. To do this, select Extruded Connection from the Connections group in the Flow tab. All the faces which are available for adding extruded connections appear, just as with flow connections. Click the face on the runner first, then click the face on the plenum. You should now see a small section of extruded pipe filling in the previous gap, as shown in the screenshot below.

After Extruded

• We will also need to add flow connections between the intake pipes and flowsplits upstream of the plenum. To add flow connections, select Flow Connection > Flow Connection from the Connections group in the Flow tab. This will show all available faces for flow connections. Join the end inlet pipe (named "pipe1") to the next pipe (named "pipe2"). Next, join pipe2 to the D-shaped flowsplit named "man1". When complete, your manifold should look like the image below, with the only faces available for adding flowsplits being between the sections of the plenum and at the ends of the runners and inlet. Note that we don't need flow connections between the plenum flowsplits, as they are a series of flowsplits derived from one piece.

Discretizing the Model:

• To discretize a model, select Export GT Model ( ) from either the Home tab or the toolbar.

• This will open the discretization window that contains the available discretization options. Let's use a pipe discretization length of 40 mm and make sure the output file name is "IntMan.gtsub". Once these are correct, we need to choose an orientation for the model.

• When GEM3D discretizes the model, it will need to place each component on a 2D map in GTISE. It will do this based on the current graphical orientation of the model by trying to match the parts placement. Since the GEM3D placements are 3D and the GT-ISE map is 2D, the orientation must be carefully chosen as some orientations will result in messy model files. Once the model is in this orientation, click the Discretize button and GEM3D will create a GT-SUITE model file (.gtsub). You will get a message saying the discretization completed successfully.

• Press the Open in GTise button to open the newly created model file in GT-ISE. The model file map will look like the image below

• Reorganise the components the circuit looks like this