Discretized model building from CAD model .STL file using the GEM3D application.

Objective: To build a discretized model from CAD STL file using the GEM3D application and to explore the single-cylinder DI model with different discretizing length.

Following parameters, we have to compare from the model

• Parameters-Torque, BSFC, max cylinder pressure
• Simulation time

Given:

Tutorial 1CyliDI-final present in GT-power to compare the results of different discretization lengths.

Path: `"Tutorials "->"Engine Performance "->"1CylDIorSI "->"1cylDI-final "`

Example plenum.stl present in GT-power to convert GT model into a discretized model

Path:`" Examples "->"Acoustics "->"Non-Linear_(standard) "->"plenum.stl "`

Theory: 

                                                                                                                           Single Cylinder DI engine

We are going to simulate a single cylinder 4 stroke Direct Injection (DI) engine for two different cases of discretization length in order to realize the impact of discretization length of intake runner on case results. This is the non-predictive DI combustion model where we have specified the values of combustion attributes i.e. Ignition delay, pre-mixed fraction, tail fraction pre-mixed duration, etc. inside the combustion object of cylinder template. The heat transfer model we are going to use in this model is 'WoschniGT' where the in-cylinder heat transfer will be calculated by a formula that closely emulates the classical Woschni correlation without swirl. The initial condition for fluid we are going to consider is Pressure 2.4 bar, Temperature 350 K, the composition is air. The wall temperatures are defined as Head temperature 550K, Piston temperature 590 K, cylinder temperature 450 K.

Di Injection:

The fuel we are going to inject has defined in the fluid object as diesel2-combust. The injected mass is 80 mg, injected fluid has kept at room temperature of 300K. We are considering here advancement of fuel injection from -5 degrees which will last till injection duration of 18 degrees.

Engine:

Since this is the Direct injection engine the compression ratio at which the engine will operate will be higher than that of the compression ratio of the SI engine. The compression ratio is 16.5 for this DI engine model.

Intake runner:

The diameter of the intake runner is 40mm and the length is 120mm. Since we are interested to know the effect of discretization length of engine parameters, simulation run time, etc we have defined discretization length as a parameter. The imposed wall temperature is 300K and the heat transfer correlation is 'Colburn'. The default roughness we have selected as Cast iron.

Results:

We can see here, the engine is running at 3600 rpm and the discretizing length we have to consider here as 40mm for case 1 and 0.1mm for case 2. All other operating parameters of the engine are the same for both the cases.

Engine Performance prediction comparison:

As we have run the simulation by considering two different discretization lengths, the engine performance predictions are presented in the table below.
 

Negative sign indicates a percentage increase and Positive sign indicates the percentage decrease in the parameter value.

We have considered all parameters to compare the results. As we know in CFD, the accuracy of the solution is very much dependent on the mesh element here in the 1D simulation discretization length plays a major role in the same. We can observe above even we have change the discretization length by 99.75% i.e. from 40mm to 0.1mm, the percentage change in the final solution either increase or decrease is nearly 1.5%. 
Some of the important parameters we are focusing here are Brake torque which corrected to 95.9 N-m, BSFC is corrected to 238.9 g/kW-hr which is increased by 0.294%, maximum in-cylinder pressure is 115.18 bar.

Simulation time:

In this case setup, the initialization state of individual case has been set as 'User_imposed' in Run setup to know the exact simulation runtime taken by individual case right from initialization till completion of the simulation.

Timestep comparison:

This plot gives us the idea about timestep restriction imposed by all elements of the engine like intake port, intake runner, cylinder, exhaust port, exhaust runner during runtime based on the discretization length.

CASE-1: Discritization length- 40mm

CASE-2: Discritization length- 0.1mm

The above two plots show the variation of timesteps during the simulation, we can observe here that in case 1 timestep reaches 0.8 during exhaust because of the higher velocity of gases after combustion takes place to maintain the courant number it lowers the timestep. But in case 2 to it reaches to 0.00362 during intake this is because of the discretization length we have set as 0.1mm which is comparatively quite less than the discretization length of other engine components and the inlet air velocity.

From the above plots, we can observe the average time steps taken by case 1 are 0.000045106 sec compare to 1.9425E-7 sec taken by case 2. The percentage of CPU time used to complete the simulation by case 1 is 59.5414 % and 94.4575 % of CPU time used by case 2. The nos. of timestep cumulatively taken by case 1 is 3696 and 851019 taken by case 2. 
The timesteps per cycle taken by case 1 are 739 and 171598 are taken by case 2.

The figures that we got from the simulation itself show that lowering the discretization length drastically decreases the simulation timesteps which ultimately impacted simulation completion time because of restriction to increase in timestep as we are solving the CFD equations explicitly.

                                                                                                       "Discretization of Intake Manifold using GEM-3D"

GEM3D is a tool that can be used to build 3D models of flow, thermal, and mechanical systems that can be discretized and made into model files for use with GT-SUITE. It provides the ability to build the model in a 3D environment so that the full details of the model can be included. It also includes sophisticated discretization logic that is able to transform the 3D model into a model file that is compatible with the GT-SUITE software. GEM3D can be used to build any system that contains components like pipes, mufflers, manifolds, air boxes, heat exchangers, underhood, cabin, batteries, crankshafts, etc.

In this session, we are going to convert the CAD model of intake manifold into discretized form so that it will be used for simulation of any engine system using that discretized model.
Following is the step by step procedure e are going to implement to convert CAD into the discretized model.

STEP-1: Lunch the GEM-3D application, Create new document choosing GEM3D option, import CAD file 'Plenum.stl' from path `" Examples "->"Acoustics "->"Non-Linear_(standard) "->"plenum.stl "`

Select "Surface",next "split by correction", finish. In this way we can import the "Plenum.stl" file into GEM-3D tool.

STEP-2: Separating sections by curves

As we can see above figure all intake and outlet ends of geometry are closed. So we need to remove that 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.

Now that the end sections of the runners and plenum are separated from the main shape, we can delete them from the model. Select all the ends by holding the Ctrl key. We can see below.

STEP-3: Using the cutting plane.

Now we can see all the caps are removed and now we have separate 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.

Adjust the cutting plane by using plane control such that it should not cut plenum and select the plenum and hit "Clip" from the plane control. 
GEM3D will separate the mesh shape along the cutting plane and create the new mesh shapes, as shown in the image below.

STEP-4: Converting sections into Flow components.

We have converted geometry into sections we need to convert them into flow components. To do this, select the first runner, right-click on it, and select "Convert shape to component". 

The first thing we need to do in this window is to select the conversion geometry type we would like to convert to. Since this particular shape is a pipe, select Pipe. This is the window we will get after selecting "Convert shape to component" here we have to make sure that port numbering should be as per flow takes place means 1 for intake and 2 for the outlet in this case. Click next. Select Multiple Bends since it has more than 1 bend as we can notice it also bends in another plane.

We need to choose which diameter to use for this cross-section using the Measurement Location -> Custom Plane for Port 1. We want to choose a location that is far enough away from the end where the effective diameter becomes fairly constant. Place the plane in a location near that shown in the image below. Assume that the diameter stays constant throughout the pipe. Therefore select for Port 2 the check box Define from Port 1.

Once in place, press Next and you will see the created component. Confirm that the Wall Thickness is set to 1 mm in the Geometry tab of the Calculated Geometry window and press Next again.

Go to the main tab and type init for the Initial State Name. We can define the initial state directly in GEM3D by double-clicking on the name "init". This will open a 'FluidInitialState' template where we can define the Pressure as 1 bar and the Temperature as 300 K. We can get the properties of air from the template library by right-clicking in the cell for Composition and choosing value selector. In the value, selector scroll down and choose the "air" 'FluidMixture' object. Click OK to close the fluid and initial state objects. Go to the Thermal tab and type in 300 K for the Imposed Wall Temperature and the conversion is complete. Press the Finish button at the bottom of the conversion window to save the part. The model should now look like the image below.

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" (in order with runner 2 next to runner 1).

Next, we need to convert the end sections of each runner into components. Select the end section of the first runner (left-side blue colored bent section in the image above), right-click and select Convert Shape to Component. This will open the convert mesh window, shown in the image below.

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. Once again, confirm that the Wall Thickness is set to 1 mm and then click Next. Give the component a name of "run1-2". Press the OK button at the bottom of the conversion window to save the part. The model should now look like the image below.

You should notice that there is an extra surface created by the Separate by Curves operation that we performed earlier (shown below). This surface is the vertical face of the intake runner where the diameter abruptly changed. Since this surface is not really physical, we can easily delete this surface by simply clicking on it, and pressing the delete key.

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 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 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.

STEP-6: Using Parameters

Give the object name as 'ori_run1'. In order to lower the flow losses caused by sudden changes in the area, the entry has been modified to curved entry hence 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. Therefore, define a parameter named [disc_coeff] for the Forward Discharge Coefficient attribute.

Set the value for [disc_coeff] in case setup as 0.95.

Repeat the above procedure for the rest of the runners using connection names of "ori_run2", "ori_run3", and "ori_run4".

STEP-7: Adding a Subassembly Connection to Impose Connection Direction

In this step, we want to specify the end connection, meaning the connection that is not connected to anything. In this model, the end of each runner needs end connection specified. This done by selecting runner, right click on it, select 'Add connection', select 'GEMSubAssExtConn'.

Use "SAconn1" for the name of the component and a Port ID of 1. Set the Flow Direction to "Outlet" since we expect the flow to be from the manifold through the runner and out of this port. This allows us to control the flow direction at this boundary. 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.

STEP-8: Dividing Pipe Sections with Area Changes

Till now we have converted all the runner now move to other ends 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.

After choosing appropriate bend angle and diameter, hit 'Clip' to create two different pipes.

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. First, select D shaped pipe, right-click on it select "Change 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. Then 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 the name of "pipe2" 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 – the geometry selection window chooses 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 click Finish to save the pipe. After finishing the conversion of these 2 pipes, the model should look like the image 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.

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.

STEP-9: Making More Difficult Cuts

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.

STEP-10: 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. The first thing we need to do is select Flowsplit from the select the conversion geometry options on the right side.

 STEP-11: Dividing a Large Volume into Smaller Sections (Manual Discretization) and Restoring (Re-using) a Cutting Plane

Manual discretization involves making cuts to divide up the large mesh shape into smaller mesh shapes and converting those into flowsplits. 

We have made a cut in between runner 1 and runner 2 using 3 point cutting plane and adjusting in between them. 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.

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 Flowsplit from the select the conversion geometry pane.

STEP-12: 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).

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

STEP-13: Adding Flow Connections.

As we can notice that during the process of converting meshes to components, we created several pipes that 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.

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.

Repeat the process for the other three runners. When you are finished creating extruded connections, press the Esc key.

Note that we don't need flow connections between the plenum flowsplits, as they are a series of flowsplits derived from one piece.

STEP-14: 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.

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. Notice how the parts are placed on the map to match the 3D placement. If the map placement is not very good, it is very easy to go back to GEM3D, rotate the model to a new orientation, and discretize the model again.

In this way, we have converted step by step the CAD intake manifold model into discretized model which will be suitable to use for GT-SUITE to perform the simulation.