Conversion of 3D Intake manifold to 1D GT-suite model using GEM3D

Objective 1: To convert 3D intake manifold (plenum.stl) to 1D GT model using GEM3D

Step 1: Importing plenum.stl to GEM 3D

first, open file to open plenum.stl model. Select Next and then make sure the Surface radio button is selected before selecting Next for the second window. 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.

Step 2: Removal of caps using selection by curves

Notice that 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.

The tolerance controls how sensitive this operation is to changes in the surface area of the part. A Tolerance of 1.0 should be used for this operation.

Now that the end sections of the runners and plenum are separated from the main shape, we can delete them from the model.

Step 3: Separation of runners from intake manifold

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 enable the cutting plane, select 3 Points Cutting Plane from this group. A cutting plane can then be created by clicking on 3 points on the model to define a plane. Since we want to cut off the runners, let's pick 3 points on the underside of the manifold.

Once the plane is created, translate the plane such that it separates runners and manifold as shown in figure below

Step 4: Converting runners to flow components(pipes)

Once we have sections that can be represented by pipes or flowsplits we need to convert them into flow components. This is done with the convert mesh operation. To begin the conversion, right-click on the first section of the first runner and select Convert Shape to Component.

The first thing we need to do in this window is select the conversion geometry type we would like to convert to. Since this particular shape is a pipe, select Pipe. Click next. Select Multiple Bends since it has more than 1 bend.

Repeat the above procedures and convert the first section (next to the manifold) of the other 3 runners into flow components. Use names of "run2-1", "run3-1", and "run4-1"

Next we need to convert the end sections of each runner into components. Select the end section of the first runner , right click and select Convert Shape to Component.

As shown in figure below, port 1 should be assigned to the pipe inlet and port 2 shound be assigned to the pipe outlet

To accurately capture the tapered shape of this section, we need to convert this section into a bent pipe so the inlet and outlet diameters can be different. Therefore, select the Multiple Bends option as bend type. For a bent pipe we can select 2 different cross sections. Port 2 is fine, and since we used Separate by Curves to separate the intake runners, the cut was placed directly at the area change, making port 1 accurate. confirm that the Wall Thickness is set to 1 mm.

Repeat the above procedures and convert the middle section of the other 3 runners into flow components. Use names of "run2-2", "run3-2", and "run4-2".

Step 5: Adding a orifice 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 ( "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.

We want to select port 1 (the port at the manifold) and the orifice connection ('OrificeConn'). With the connection template open, we can give it a name of "ori-run1". To account for the curved entry we want to modify the forward discharge coefficient (essentially make it higher than it would normally be for an abrupt area change). Since all 4 runners likely have the same curved entry shape, they should all get the same discharge coefficient. Forward discharge coefficient must be set to 0.95.

Repeat the above procedure for the rest of the runners using connection names of "ori-run2", "ori-run3", and "ori-run4".

Step 6: Adding a subassembly connection to impose a flow direction

The next thing we want to do is specify the end (boundary) connections, meaning the connections that do not connect to anything. In our model the ends of each runner need an end connection specified. This is done with a special subassembly connection. To add the subassembly connection right-click on the end section of the first runner component ("run1-2") and choose Add Connection. This will open the port connection window, shown in the image below.

We want to select port 2 (the port not connected to the rest of the runner) and the subassembly connection ('GEMSubAssExtConn').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. This allows us to control the flow direction at this boundary, while GEM3D can determine the rest of the directions from any specified values. 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.

Step 7: Dividing manifold intake pipe and converting into flow components

Now that we have the runners converted, let's move to 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. Since they are pipe sections, we can utilize the pipe normal cutting plane operation. From the Cutting Planes group in the Convert tab, choose Pipe Normal Cutting Plane. To place the cutting plane, click on the rounded outer wall of the model very close to the entrance. This will place the pipe normal cutting plane at that location and in an orientation that is normal to the flow direction, as shown in the image below, as

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 plane can only be translated so far before it interferes with the adjacent flowsplit section.

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.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.

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').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.

Step 8: Separating 90 deg flow split from manifold

The next step is to separate the manifold from the 90 degree entrance flowsplit. This must be done with the cutting plane operation. The goal is simple, cut off the top flowsplit section without cutting through the manifold itself. This will be a multiple step process as we will need to place the cutting plane, rotate it to the correct orientation, and translate it into the correct location.

Start by selecting the 3 Points Cutting Plane from the Cutting Planes group in the Convert tab. To place the plane, pick 3 points on the top of the manifold.Once the cutting plane has been created, it must be rotated slightly so that it does not cut through either the manifold or the flowsplit part.Once the cutting plane is in the correct orientation and location (shown above), select the mesh shape to be cut (it should display in magenta), and press the clip button in the control window. After the cut the model should look like the image below

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. The first thing we need to do is select Flowsplit from the select the conversion geometry options on the right side. Flow will be expected to enter this flowsplit at the D-shaped port, so the entrance being port 1 and the exit being port 2.

Step 9: Dividing the manifold into smaller sections and convertion into flow components(flowsplits)

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.
In our case each end of the manifold has a runner connection. Since the center inlet is wide enough to cover both middle runners, the center section of the manifold should cover both middle runners. That means we want to divide the entire manifold into 3 sections; a center section and 2 end sections.

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 formed, we need to translate it along the manifold so that it is between the 1st and 2nd runners where we want to make our cut.

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 that the manifold is divided, we can convert each section into a flow component. Select the end section near runner 1 (red component in the image above), right-click and select Convert Shape to Component.
Select Flowsplit from the select the conversion geometry pane.Click Next, give the new component a name "man-end1" and click Finish to save the conversion. Repeat the above procedure with the other end of the manifold (near runner 4) using a name of "man-end2".

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 Next and name the component "man-center".

Step 10: Adding flow connections

You will notice that 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.

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.

Step 11: Discretizing the model

Now that the model consists entirely of flow components, we can discretize the model. First, save the model again using the save toolbar button or choosing Home/Save. 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. 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 GT-ISE. 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. Rotate the model to an orientation with the entrance pointed down and the runners to the right.

Once the model is in this orientation, click the Discretize button and GEM3D will create a GT-SUITE model file (.gtsub).The 2D gt suite model of intake manifold is shown below

Objective 2: To observe the effect of discretization length of intake runner on the DI engine model

Model: 1 Cyl DI model

Case setup:

To observe the effect of change in discretization length, two cases have been selected with discretization length of 40 and 0.1 mm.

Results:

Observations:

1. The average run time for case 1(40 mm) is 4 sec and for case 2(0.1 mm) is 13609.7.

2. The factor of real time in case 2 have increased by 12500 which very high.

3. Negligible change in values can be observed such as, brake power(kW), brake torque(Nm), BSFC, maximum pressure and maximum temperature.

4. Since there are no significant change in values due to reduction in discretization length, in order to save simulation time, discretization length in case of intake systems should be 0.44 times bore and for exhaust should be 0.55 times the cylinder bore.