Discretization in GT-Power

INTRODUCTION TO DISCRETIZATON:

GT Power computes its problems in 1D form ie., the properties keep changing along the flow and is constant at a particular cross section. To solve the governing N-S equations, the discretization has to be done along the direction of flow. The level of discretization that can be done is based on CFL condition which limits the timestep for explicit solvers. Smaller the discretization length, more the time taken by the solver but higher the accuracy achieved and the physics can be captured more accurately.

For an engine simulations in GT-Power, the following is recommended:

1. Discretization length of approximately 0.4 times bore diameter is recommended for intake systems and 0.55 times the bore diameter for exhaust systems. This is because of the differences in speed of sound due to the temperature difference.
2. The discretization length must be decreased for any simulations that requires higher frequency analysis such as Acoustics or Engine Performance above 6000 RPM.

COMPARISON OF DISCRETIZATION LENGTHS:

A Single Cylinder CI Engine is run with discretization lengths of 40 mm and 0.1 mm for intake runner and compared. The plant layout of the engine is shown below.

Plant Layout:

Case-Setup:

Outputs and Results:

1) Performance Comparison

2) Cylinder Predictions:

3) Simulation Time:

INFERENCE:

1. As the discretization length is decreased, the simulation time increases mutifold and the higher accuracy of results are obtained.
2. The Brake Power is predicted at 48.5 kW from an initial of 48.6 kW. The new torque obtained is 95.9 Nm from 96.2 Nm earlier. The new BSFC is predicted at 238.9 g/kWh from the original value of 238.2 g/kWh.
3. The new predicted max in-cylinder temperature is 16 degrees higher than the original value of 2042 K. ..The new predicted max in cylinder pressure is 2 bar higher than the original value of 117 bar.
4. The graph of simulation time showns that the realtime factor has incrased by a factor of ~4000 for the discretization of 0.1 mm when compared to the original discretization length of 40 mm.

GEM-3D Tutorial: Building Intake Manifold from STL File Model:

GEM3D is a graphical 3D tool used to build and discretize flow systems for use with GT-SUITE. It provides the ability to build models using component templates, import shapes from an STL file, connect complete flow systems, dimension the model, and automatically discretize.In this tutorial a 3D model (STL file) of an Intake Manifold is converted to a usable model in GT Suite (1D model) using GEM-3D. GEM 3D converts it into a flow model where acoustics can be applied form the solid model.

Initial Model of Intake Manifold (STL FILE):

The given initial geometry is complex and has a lot of curves. Initially to boundary flag the model, the option 'Separate by Curves' is used. Separate by Curves asks for relative tolerance (which controls how sensitive this operation is to changes in the surface area of the part). The tolerance is chosen as 1. The original model and the flagged model is shown below. Each flagged model can be selected separately.

Now, the entry of the manifolds must be separated from the Rail. For this, the 'Cutting Plane by 3 points' option is used. A plane is created parallel to the surface of the rail and the geometry to be sliced is selected as shown below (Bottom Left). Now, the first half of the manifolds can be selected separately as shown below (Highlighted in Pink: Bottom Right)

Next, each half manifold can be converted to a flow component by right click and selecting the 'Convert Shape to Component' Option. This option allows the user to convert the selected part to any flow component as listed below. According to the function of the shape, the manifold is selected as a 'Pipe'. Further the flow direction, the diameter, the thermal model, friction model, etc are selected as shown below.

The Conversion preview of a manifold and the complete geometry after this step is shown below.

The flow in the manifold is now approximated as a circular jet with constant diameter. When the air enters the manifold from the rail to the line due to sudden contraction in area, there will be losses. But in reality, the air enters smoothly and has a high discharge coefficient. This can be modelled by right clicking the manifold pipe and using the 'Add Component' option to add an orifice at the entry with a high discharge coefficient from the rail to the pipe.

Next, the second half of the manifold is converted to pipe using 'Convert shape to component' option similar to the first half of the manifold. The only change in this is that, the diameter of the second half is not kept constant as it might affect the flow patterns. The original diameters of the ends of the pipe are retained. The obtained model is shown below.

Next, the air intake pipe and the rail is discretized. To make the initial cutting plane for the manifold rail, the '3 Point Cutting Plane' is used. An image demostrating the same is shown below (Immediate Bottom). After the manifold has been divided into 3 components, it is discretized into 3 flowsplits. Similarly, the entry pipe is discretized to two straight pipes and a flowsplit using the option 'Convert shape to component'. The final discretized model is shown below (far bottom).

Now, during the conversion of model, there are many spaces generated at the joints of two components. These have to be filled up. To check for the spaces, under the flow tab, the 'flow connection' option must be selected. This will display all the planes of the pipe where flow is discontinuous. The pairs of planes must be selected manually to make the flow continuous. This will automatically identify the flow patterns and the spaces. Image of a space at the joint is shown below (Bottom right). The pairs of planes must be selected manually to make the flow continuous.

At this stage, the discritization process is complete. Now this can be loaded in GT-ISE using the option Home > Export GT Model. This will create the plant layout of the Intake Manifold. The plant layout is shown below. Here ends the tutorial.

Plant Layout of the Intake Manifold:

CONCLUSION:

A study on discretization was done by comparing the results obtained from a single cylinder CI Engine. Smaller the discretization length, higher the accuracy but more the simulation time. A tradeoff has to be done between the simulation time and the accuracy of the results based on the application and requirement. Also a 3D model of an Intake system was converted to 1D form and discretized using GEM-3D.