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

Discretization of Intake Manifold using GEM-3D

Objective:

This study is divided into 2 parts. In the first part, the influence of the discretization length on the simulation is evaluated. In the second part, a 3D CAD model of an intake manifold is converted into 1D model using the graphical application software, GEM 3D

Section 1: Impact of discretization length on simulation:

In this section, the impact of the discretization length on the simulation time was evaluated. For this purpose, a single cylinder diesel engine model, as shown in the figure, was utilised

In this model, the discretization length on the inlet side (intake runner and intake port) was changed from a constant value to a parameter. The settings for the intake runner (left) and the intake port (right) are shown in the figure below.

The discretization length was changed to 40, 1, 0.1mm and its influence on the simulation time was studied. The case setup used in this study is shown in the following figure

As we are interested in the total simulation, we need to start the simulation of each case from the correct starting point. To achieve this, the initialization in the run-setup was changed to user_imposed as shown below

Results:

The results obtained are summarized in the following table.

Three parameters were chosen for comparison. It can be observed from the table that a reduction in the discretization length did not change the accuracy of parameters to considerable extent.

The following figure shows the simulation time for the three cases.

It can be observed that the simulation time for the third case with a discretization length of 0.1 mm takes 12500x longer than the first case with a discretization length of 40 mm. The same inference can be observed from another plot shown below.

From the above results, it can be stated that decreasing the discretization length can only be justified when it results in a considerable increase in the accuracy of the parameters of interest.

In this case, the decrease in discretization length did not increase the accuracy of the parameters, but resulted in a considerable increase in the simulation time.

Section 2: Conversion of 3D model to 1D

In this section, a 3D model of an intake manifold was converted into 1D model. The 3D model used in this study is shown in the figure below

The 3D represents the fluid volume. In case the fluid volume is not readily available, it is recommended to extract the fluid volume from the solid model using the GT-SpaceClaim.

Next the geometry was imported into GEM 3D as a surface model as shown in the figure below

As the next step, the geometry was divided into different parts with the “Separate by Curves” operation. Then, the inlet and the outlet caps were removed to facilitate the flow. The resulting model is shown in the figure below

The model was separated further into sections which can be represented by pipes and flowsplits. First the 4 runners were separated from the manifold since they can be represented as pipes. This action was accomplished using the 3-point cutting planes under the convert tab. Care should be taken such that the cutting plane does not intersect the manifold. The plane was translated to avoid such interferences. The resulting model is shown in the figure

Next, the sections were converted into flow components. This was performed by right-clicking on the screen and selecting the “convert shape to component” option. The parameters used for this conversion are shown in the following figures

First, the runners were converted into pipes with multiple bends. The numbers 1 and 2 in the above figure points to the inlet and outlet. Care should be taken so that the numbers correctly point to the right inlet and outlet of our model.

The inlet end of the pipe was approximated using a diameter of 48.6 mm and the outlet end was calculated from the inlet end. The pipe wall thickness was selected as 1 mm.

Next the initial state object was defined using the following values

In a similar way, all the 8 runners were converted into pipe components. The resulting model is shown in the figure below

During the conversion process, the connection between the runners were lost. A default orifice connection can not capture the flow here. Therefore an orifice connection was added between the two runners of the same curved pipe.

To account for the curved entry, a forward discharge coefficient of 0.95 was specified for the orifice connection. The procedure was repeated for all the 4 pipes.

Next, the boundary conditions were defined using the custom connection as shown below

The same procedure was followed and the 1 inlet and 4 outlets were defined.

Next, the manifold was converted to flow components. Before that it should be divided into sections. The entrance consists of round pipe, D-pipe and a 90 degree bend. The entrance was divided using the “cutting plane normal to pipe.” This option also provided a graph showing variation of pipe diameter. The pipe was then clipped at points where there was a sudden changed in the pipe diameter or angle. This is shown in the figure below.

The above process was repeated for the entire intake region of manifold. The intake region was divided into two pipes and one flowsplit. The modified model is shown in the figure below.

Next, the 90 degree bend was converted into flowsplit. Before that, the 90 degree bend was separated from the manifold using a “3-point cutting plane”

Next, the manifold was divided into 3 parts, each part was then converted to a flowsplit

The two outer regions were converted into flowsplits using the values specified in the following figure

In the process of converting the meshes to components, different pipes were created which did not align properly with each other. To remedy this, additional flow connections were created between the different flow components.

The corresponding planes were selected to create the connection. In addition to the general flow connection, extruded flow connection was also used to create the flow connections. The final model is shown in the figure below. The flow connections can be seen as grey surfaces in the following figure.

Once all the flow components were created, a GT-Model was created by clicking the “Export GT-Model” option. The following figure shows the options used when creating the GT-Model

THe final 1D model obtained is shown in the following figure

Conclusion:

In this study, the influence of discretization was studied successfully using a 1D CI Engine model. The conversion of 3D model to 1D was also explored in this case-study.