Bird Strike Simulation

 AIM: 

To simulate a non-linear transient dynamic problem of a Bird Strike with a Jet Engine Blade and evaluate the results.

OBJECTIVES:

1. The given file contains all the 4 parts which as to separate and save part wise.

2. The suitable material is to be created and saved in a subfolder.

3. The node number, Element number, and parts number are assigned as mentioned above.

4. Create the main file that should have only the *include file and run the explicit simulation.

5. Study the mass scaling by changing the DT2MS.

THEORY:

INTRODUCTION: 

Here, the meshed model of Aero-engine is given which is to be separated as per the above condition. This is a classic nonlinear transient dynamics problem similar to a car crash and mobile drop simulation. This problem can be solved using a generic explicit solver. The blades should rotate at a constant velocity but the casing should remain stationary. Here, the boundary condition is applied for casing so that it should not move in any direction. The cylindrical bird model should travel along its own axis and hit the blades hence velocity is given and assigned in a particular direction and constrained in all directions. For elastic material, Young's modulus for the bird(2000MPa) and the casing(200GPa) are given. The material model for the blades is given as M024 i.e., MAT_PIECEWISE_LINEAR_PLASTICITY. The velocity of the engine blades and the birds can be chosen so that blade failure can be seen within a short span of time. The material for the bird part is considered according to the literature. After all the conditions applied, create the main file and use the *INCLUDE command to involve all the parameters and run the simulation. 

Gas Turbine Working and Types:

Both Piston (reciprocating) engines and Gas turbine engines are internal combustion engines. They have a similar cycle of operation that consists of intake, compression, combustion, expansion, and exhaust. In a piston engine, each of these events is a separate distinct occurrence in each cylinder. Also, in a piston engine, an ignition event must occur during each cycle, in each cylinder. Unlike reciprocating engines, in gas turbine engines these phases of power occur simultaneously and continuously instead of one cycle at a time. Additionally, ignition occurs during the starting cycle and is continuous thereafter.

The basic gas turbine engine contains four sections: intake, compression, combustion, and exhaust.

Fig: Basic components of a gas turbine engine.

To start the engine, the compressor section is rotated by an electrical starter on small engines or an air-driven starter on large engines. As compressor r.p.m. accelerates, the air is brought in through the inlet duct, compressed to high pressure, and delivered to the combustion section (combustion chambers). Fuel is then injected by a fuel controller through spray nozzles and ignited by igniter plugs. (Not all of the compressed air is used to support combustion. Some of the compressed air bypasses the burner section and circulates within the engine to provide internal cooling.)

The fuel/air mixture in the combustion chamber is then burned in a continuous combustion process and produces a very high temperature, typically around 4,000°F, which heats the entire air mass to 1,600 – 2,400°F. The mixture of hot air and gases expands and is directed to the turbine blades forcing the turbine section to rotate, which in turn drives the compressor by means of a direct shaft. After powering the turbine section, the high-velocity excess exhaust exits the tailpipe or exhaust section. High-pressure exhaust gases can be used to provide jet thrust as in a gas turbine engine. Once the turbine section is powered by gases from the burner section, the starter is disengaged, and the igniters are turned off. Combustion continues until the engine is shut down by turning off the fuel supply.

Bird-Strike Impact

Bird strike is the collision between a bird and an aircraft. Bird strikes have always been a cause of worry in the aeronautical industry: Aircraft both old and new have suffered from bird strikes. Jet engine ingestion occurs when the bird hits the jet engine of an aircraft and gets sucked in. Given that the fan blades rotate at a high rpm, a bird strike on a fan blade causes its displacement into the adjacent blade. This leads to a cascading failure, wherein the entire system fails, thereby resulting in a lot of damage.

Several bird models were proposed, each more effective than the other. A bird could be thought of as behaving like a fluid when undergoing high-speed impact. Some of the models propose an elastic-plastic deformable body with a low value of yield point and small hardening, while others consider a hyper-elastic (rubber-like) body, the jelly-like body which easily splatters over the whole blade, a liquid body, or even particles of a solid body.

The Bird-Strike damage can be very severe and can shut down the jet engine. as shown below. This is the behavior we are trying to simulate for the damage done from the bird strike.

  

Fig: Bird-Strike Impact                           

Steps followed in this simulation

1. Save the different parts of the engine into different (.k) files
2. Numbering the elements, nodes, and other keywords as per the given standard
3. Assigned the section for each part card for a separate component of an engine
4. Define the boundary conditions to the associated parts
5. Define the contacts between the parts and the database within a single keyword file.
6. Assign the Material card for each components.
7. Defined the Control and Database card for the relevant postprocessing result.
8. Include all the card into the main file and run the solver after saving it 
9. Results and conclusion

PROCEDURE:

1. Open the Project file:

The given project file contains four different parts, hub, blades, engine casing, and bird respectively. So, these parts are separated and placed into a single input file. Here we are going to simulate the effect of the classic bird strike impact on the gas turbine fan blade.

The various engine parts along with control, contact, and database cards are included in the main file. The elements, nodes, and parts are renumbered in an organized way as per industry standards.

The condition we carry on with this project is blades to rotate at a constant velocity and the engine casing should remain stationary. The cylindrical bird should travel along its own axis toward the engine and hit the blades. At last, all the separate parts files should be in a single input file (.k file) and are called into the main file through the *INCLUDE keyword. The unit system chosen for this analysis is kg-mm-ms. and should have consistent numbering.

Note: The Young’s modulus for the bird is given as 2000MPa and for the casing is given as 200GPa.

Note: Both bird and casing are modeled by an elastic material,  the elastic material properties defined for the bird with the density and Poisson’s ratio values are taken similar to human muscle values. Similarly, the engine casing is defined with properties that are similar to the metal.

1. Save the different parts of the engine into different (.k) files

We will save the different components of the engine under different file names (such as shown below in the figure) so that they can include in the Main file along with boundary conditions.

Fig: Different files for these Engine components.

There will be a single Bird_strike.k file containing all the part numbers in an unorganized way, that we need to save them under a different name so that they can be numbered easily and later included in the Main file.

Fig: Saved the parts under differently named files 

2. Numbering the elements, nodes, and other keywords as per the given standard

Individual saved files are now opened and with help of Renumber option available one by one node, element, the part can be renumbered as per the given standard.

Fig: Using Renumber option 

3. Assigned the section for each part card for a separate component of an engine

a. Engine Hub

Similarly, for the hub, we will assign the Section(solid), and part to the individually saved Hub.k file. 

The Hub is represented as a solid element with element formulation of equals 10 for the tetrahedron. The E value assigned is 145 Gpa and a high FAIL value. 

Fig: Section card defined

Fig: Hub part card updated

b. Engine Blade

Here we will assign the section(shell), and part to the individually saved blade.k file.

Fig: Section card defined

Fig: Blade part card updated

c. Engine casing

We will assign the section(shell), and part to the individually saved engine_casing.k file.

Fig: Section card defined

Fig: Casing part card updated

d. Engine_Bird

For the bird model, we will create the shell section thickness 6 mm and part to the individually saved bird.k file.

Fig: Section card defined

Fig: Bird part card updated

4. Define the boundary conditions to the associated parts

a. Engine casing

Fixing all the DOF of the casing nodes as shown below using a single point constraint

Fig: Boundary Spc card defined

Fig: A view of the casing part after providing boundary condition

b. Engine_Blade

We will assign the Initial velocity generation of Omega(w)=  0.5 rad/ms.

Assume the speed of the blade is nearly 5000rpm then 

`(2*pi*N)/60=(2*pi*5000)/60` = 523 rad/s = 0.523 rad/ms = 0.5 rad/ms.

Fig: Initial velocity (rotational) card defined for blade part

Fig: A view of the blade part after providing Initial velocity (rotational) condition 

c. Engine_Bird

Assign the Initial velocity Generation of 20 mm/ms(72 kmph) in the X-direction

Fig: Initial velocity (linear) card defined for bird part

Fig: A view of the bird part after providing Initial velocity (linear) condition 

5. Define the contacts between the parts and the database within a single keyword file.

a. Contact between Hub and Blade

A Tied node to surface contact is defined between hub and blade as it is an integrated unit and should rotate along with the blade at a similar angular speed. The master is assigned to Hub and the slave to Blades.

Fig: Contact card defined for the Blade and Hub

b. Contact between Blade and bird

Automatic surface to surface contact is defined between the blade and bird interface with the master node as bird and blade as slave nodes. As we need to observe stress and strain developed in the fan blade, therefore, it is defined as a slave.

Fig: Contact card defined for the Blade and Bird

c. Contact between blade and casing 

Automatic surface to surface contact is defined between the blade and casing interface with the master node as casing and blade as slave nodes.

d. Contact for blade ie, self contact

The Automatic single surface contact is defined for the blade self contact. with the master as 0 and slave as the blade.

Fig: Contact card defined for the Blade

6. Define the Material Card for each parts.

a. Engine Hub

For the hub, we will assign the material(MAT_024), 

Fig: Elasto-plastic material card defined

b. Engine Blade

Here we will assign the material(MAT_024), but here we will include the material file given question assign that material to the part card.

Fig: Include keyword used to input the material model for blade

As we can see that E= 68.94 Gpa and FAIL = 0.2723 which means the material will fail when it reaches 27.23 % of the plastic strain value. the thickness assign to the shell section is 3mm for the blade.

Fig: Elasto-plastic material card defined

Note: Here we will only mention the material id which is the same as above in the material card for the blade material definition.

Load curve defined

Below is the data of the material that we have input to the blade material card through LSID (12000)

 Fig: Load curve defined for blade 

c. Engine casing

We will assign the material(MAT_024),

Fig: Elasto-plastic material card defined

d. Engine_Bird

For the bird model, we will use the MAT_Elastic (001) and Young modulus as 200Mpa or 2Gpa and poison ration 0.15 and low-density value as we need to replicate a bird into the simulation.

Fig: Elastic material card defined

Note: It would be better if we represent the Bird with the smooth particle hydrodynamic (SPH) as it is a soft body impact in structural analysis the bird impact can be considered as a fluid material.

7. Defined the Control and Database card for the relevant postprocessing result.

a. Control cards

Define control cards for Energy, termination, and Timestep as shown below.

Termination time setup is 4 ms  and Energy card to take into account hourglass energy with hgen=2 (as shown below)

Fig: Control Energy card defined for the simulation

Fig: Control Termination card defined for the simulation

Fig: Control Timestep card defined for the simulation

The Control Timestep card plays an important role in reducing the runtime. we can have a different combination of DT2MS and TSSFAC to check the estimated runtime and % mass added during mass scaling. Here we will try to keep the percentage of mass added below 7% and reduce the runtime. 

a. In 1st trial, we did this by keeping DT2MS = -6.0E-05  and TSSFAC=0.9 the runtime estimated is 2hrs 17 min but actually it took 17 min 2 sec with 0% mass addition.

Fig: Estimated data before the simulation run

Fig: Final data after the simulation run

b. In the 2nd trial, we did this by keeping DT2MS = -6.0E-04  and TSSFAC=0.9 the runtime estimated is 19 min but actually it took 1 min 52 sec with 24.2% mass addition.

Fig: Estimated data before the simulation run

Fig: Final data after the simulation run

c. In the 3rd trial, we did this by keeping DT2MS = -4.0E-04  and TSSFAC=0.9 the runtime estimated is 29 min but actually it took 3 min 11 sec with 6.6% mass addition. (Acceptable)

Fig: Estimated data before the simulation run

Fig: Final data after the simulation run

b. Database Card

Among the Database card, we will include ASCII, D3PLOT, EXTENT_BINARY (for strain data in post-processing)

Fig: Database ASCII card defined for the simulation

Fig: Database Binary card defined for the simulation

Fig: Database Extent_Binary card defined for the simulation

8. Include all the card into the main file and run the solver after saving it 

The Main_Bird_Strike.k file will include all the file i.e components, material, section, part, boundary condition, initial velocity,contact, control, and database.

Fig: Include card defined for the main file

Fig: Include file in the Main simulation file 

RESULTS:

Energy plots:

• The 1st level of check to validate the accuracy of the simulation is done from the energy plot, we can see the Total energy is almost constant over the period of simulation which indicates the result simulation run well.
• Hourglass energy is zero throughout the simulation.
• The kinetic energy and internal energy shows a very similar pattern during the simulation where K.E reduces and I.E increases after the impact.  

Von-mises stress plot

This stress plot is taken on one of the element of the blade where the impact takes with the bird at every time, but the stress peaks and reaches a value of 0.0796 Gpa at the (time of 2.2 ms )moment of impact and drop to a value of 0.01 Gpa at the end of the simulation. 

Plastic strain plot

This strain plot is taken on one of the elements of the blade where the impact with the bird. At t= 2.3ms the strain values peak to 0.0935 i.e. 9.35% strain then is constant throughout the simulation. 

Contact plot(blade_Bird)

The energy plot between the blade and bird contact is shown below, that before the interaction of the surface the energy was zero but nearly from 1ms the energy within the contact starts to develop. The energy conversion between the slave(blade) and master(bird) is symmetric. the maximum value of energy developed is 7.86 units at t= 1.20 ms.

Von- Mises Stress animation for Bird- Strike Impact.

 

Effective plastic Strain animation for Bird- Strike Impact.

 

 

CONCLUSION:

                   - All the components are following a consistent number pattern for each element, node, and part, as followed in many industries, and all the files such as for parts, Boundary conditions, contact, Material card, control, database, are included in the Main file also the Boundary conditions and initial velocity conditions are deployed in the same real-world scenario. 

                  - From the energy plot, we can observe that total energy is constant over the simulation which represents the simulation runs well.

                  - The max V-M stress produced in the fan blade is 0.0796 Gpa at nearly 2.2ms of simulation 

                  - The max strain developed is 0.0935 at 2.3ms of simulation and remains constant over the simulation 

                  - For Mass-scaling, the mass added is 6.67% which below the accepted range ie, 7%.