Simulation of Bird Strike in aero-engine using LS-Dyna

OBJECTIVE:

To perform a bird strike in the given aero-engine model using LS-Dyna. The following objectives are to be satisfied.

1. To assign the given material for blades. To use elastic material for the bird(2000MPa) and the casing(200GPa).

2. To follow a consistent numbering approach.

3. The different parts of the file should be defined in a separate file. Also, the boundary conditions and control cards should be defined in a separate file. The main file should be called which includes all the necessary files.

PROCEDURE FOR CASE SETUP:

In the problem statement, it is clear that separate files have to be defined for all the parts and also for control cards and boundary conditions. The below steps give the procedure for defining the part files.

For Bird:

1. Open the given file and delete all the parts except the bird.

2. Assign the following material and section card to the part file.

3. Save the file with a proper file name with a .k extension.

For Blade:

1. Open the given file and delete all the parts except blades.

2. Include the material file in the current file using the *INCLUDE option.

3. Assign the material and section to the part as shown in the figure below.

4. Save the file with a proper name with a .k extension.

For Hub:

1. Open the given file and delete all the parts except the hub.

2. Assign the following material and section card to the part file. 

3. Save the file with a proper name with a .k extension.

For Engine Casing:

1. Open the given file and delete all the parts except the engine casing.

2. Assign the following material and section card to the part file. 

3. Save the file with a proper name with a .k extension.

Boundary Condition Definition:

The boundary conditions are shown in the figure below.

There are two boundary conditions applied to the model.

• Engine casing has to be arrested without moving in all directions.
• The initial velocity generation defined for bird and blades.

The file is saved with a proper name with a .k extension.

Contact Interface Definition:

The following contact interfaces are defined for the problem.

1. If the bird strikes the blades, the deformed blades might come in contact with each other. So, the automatic surface to surface contact has to be defined.

2. Also, after the bird strike the blades might come in contact with the engine casing. Therefore, the automatic single surface to surface contact has to be defined.

3. The important contact definition is between bird and blade. It is explained in the image below.

4. The contact definition between hub and blades is established using a tied contact card. This is shown in the image below.

The file is saved with a proper name with a .k extension.

Control Card Definition:

The following control cards are defined. The control timestep card is defined for the purpose of mass scaling. Without mass scaling, the simulation is considerably more which is discussed in the results section.

The following *DATABASE cards are defined for the simulation.

The file is saved with a proper name with a .k extension.

Main File Definition:

As shown in the above figure, in the include file the path of the file must not be given. This helps in referencing the file inside the current folder. Therefore, it can be run across all system folder paths.

RENUMBERING:

The renumbering is done as per the problem statement as shown in the figure below.

TRIALS AND DEBUGGING:

1. The following error was observed in the simulation.

This is due to the referencing of the material card in the blade file which is shown below. 

As shown in the above figure, the material has been called twice. In the bird file, the material is called, and also in the include file, the material is called. Due to this, the error has occurred. After excluding the material card, the above error was fixed.

2. The following error was encountered while running the simulation.

This error has occurred due to improper section assignment to the bird. In the bird file, the section assigned was solid. But the correct section is the shell. 

After changing the section card, as shown above, the error was fixed.

3. The following error was encountered after running the simulation.

The above error was encountered due to improper boundary conditions assigned to the casing. The degrees of freedom of the casing must be fixed. 

After defining the above boundary conditions, the error was fixed.

4. The following error was encountered after running the simulation.

This is due to the large rotational velocity assignment to the blades. Due to the higher centrifugal force, the blades tend to move out. By reducing the value of initial velocity to blades, the above error was fixed.

5. The following plot was obtained.

The above energy plot is not consistent with the simulation. The total energy increases after the impact. But in the general case, it should remain constant. The inconsistency has happened due to the following reasons.

For the bird, the prescribed motion set was defined instead of the initial velocity generation card. So, because of this, the bird tries to move in the prescribed motion even after hitting the blades. Due to this reason, the above plot was obtained.

After defining the above card for bird velocity, the global energy plot was consistent with the simulation. 

RESULTS AND DISCUSSION:

1. The total run time obtained without and with mass scaling is shown in the figure below.

As shown above, the total simulation run time is 7 hours 18 mins. This is without mass scaling.

After adopting mass scaling, the total run time is 10 mins with a 6% mass error which is well within the accepted range. Increasing the dt2ms beyond this value would cause an increase in mass error. However, the run time highly depends upon the system usage and configuration.

2. The following simulation is obtained.

From the above simulation, it can be inferred that the bird strikes the blade at 0.5 ms.

3. The Von-Mises contour is shown below.

As shown in the above contour, there is initial stress between the blades and the hub. This is due to the contact definition. As soon as the bird hits the blade, there is a spike in the effective stress value.

4. The deformation in the blade is shown in the figure below.

As soon as the bird strikes, the blades are deformed with an increase in the value of stress over them. This is clearly shown in the above figure.

5. The effective plastic strain contour is shown below.

As shown in the above figure, after the bird strike the strain is generated on the blade which is slowly transferred to the rest of the blades. The maximum strain obtained is 0.983

6. The global energy plot is shown below.

As shown in the above figure, after the bird strikes the kinetic energy decreases which causes an increase in the internal energy of the system. The hourglass energy remains close to zero during the entire simulation.

7. The effective plastic strain is shown below.

The element corresponds to the element near the deformation region. The plastic strain increases as the bird strike the blade and after which it remains constant.

Note:

There might be a change in the value of stress and strain in the case of the simulation without mass scaling. This is because the added mass might change the values. However, a huge difference in value won't be observed.

CONCLUSION:

All the objectives given in the problem statement are satisfied. From this challenge, the following concepts are learnt.

1. Renumbering in accordance with the standards.

2. Usage of include keyword.

3. Mass scaling and its significance without compromising the stability of the model is learnt.

4. Running the simulation and debugging the error.

Drive Link: https://drive.google.com/file/d/1UiCY1gWlP8qRWavQ77L7ZUzOdSK2cTil/view?usp=sharing