Bird-strike Explicit Analysis on a gas-turbine engine fan blade using LS-Dyna solver

AIM: To simulate the Non-linear transient dynamic problem of a bird strike into a Gas turbine engine blade.

ABSTRACT: In this project, we are going to simulate the effect of the classic bird strike impact on the gas turbine fan blade. 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.

THEORY:

Gas Turbine Engine

The gas turbine is a machine that burns fuel to provide energy to create a moving flow of air and to extract valuable power or generate useful thrust from that movement. A jet engine employs Newton’s laws of motion to generate force, or thrust as it is normally called in aircraft applications. It does this by sucking in the air slowly at the front and then blowing it out quickly at the back. In the gas turbine, the burning is continuous and the expanding gas is ejected from the engine. This is the action applied in Newton’s third law, to generate thrust as the reaction.

                                                                       

Working Principle

Brayton Cycle

1-2 Process: Isentropic compression(Compressor work input)

2-3 Process: Constant pressure heat addition

3-4 Process: Isentropic expansion( Turbine Work output)

4-1 Process: Constant pressure heat rejection

                                                                       

The basic principle of a gas turbine is as shown in the diagram below. First, the air is compressed by a compressor, and this compressed air is guided into the combustor. Here, fuel is continuously combusted to produce gas at high temperatures and pressure. What a gas turbine for the industry does is the gas produced in the combustor is expanded in the turbine (a vaned rotor made by attaching multiple blades to a round disk), and as the result, the rotational energy, which operates the compressor at the previous stage, is produced. The remaining energy is delivered with an output shaft.

 

                              

                                                                                 

Components of Engine                                                              

FAN [Driving force]

The fan module is the assembly of the fan disc, the low pressure (LP) fan shaft, and the fan blades.

Its primary jobs are drawing air into the engine, compressing the bypass stream to produce 80 percent of the engine’s thrust, and feeding air to the gas turbine core. The fan system must be strong, light, and quiet. Modern fan blades are hollow for lighter weight and have a wide chord for better stiffness and strength. The engine must pass intense and rigorous testing before being certified as safe to fly. One such test is the fan blade-off test, which is conducted to demonstrate the ability of the fan casing to contain a fan blade should it become detached during engine running. Another test is the bird ingestion test(bird-strike), which examines the ability of the fan to withstand the impact of birds during flight. As fan intake diameter increases so too do the number of birds used to demonstrate that the engine retains its integrity and the capability to continue supplying thrust until ultimately, it can be shut down safely.

The tip of the fan blade may be travelling at a speed of 1000mph

                                            

Compressor [Applying the pressure]

The compressor is made up of the fan and alternating stages of rotating blades and static vanes. 

The primary purpose of the compressor is to increase the pressure of the air through the gas turbine core. It then delivers this compressed air to the combustion system. The pressure rise is created as air flows through the stages of rotating blades and static vanes. The blades accelerate the air increasing its dynamic pressure, and then the vanes decelerate the air transferring kinetic energy into static pressure rises.

Blisks, a single component comprising both blades and a disc. These reduce the weight and improve the efficiency of the compressor by removing the need for blade roots and disc slots

                                           

 

Combustion Camber [Injecting the Energy]

The annular combustion chamber is located within a casing structure. Kerosene is introduced through fuel injectors into the front of the chamber.

It burns fuel with air received from the compressor, sending hot gas downstream to the turbine. The gas temperatures within the combustor are above the melting point of the nickel alloy walls. Cooling air and thermal barrier coatings are therefore used to protect the walls and increase component lives. Dilution air is used to cool the gas stream before entering the turbines. 

                                              

The Turbine [Harvesting the power]

The turbine is an assembly of discs with blades that are attached to the turbine shafts, nozzle guide vanes, casings, and structures. The turbine extracts energy from the hot gas stream received from the combustor. In a turbofan, this power is used to drive the fan and compressor. Turbine blades convert the energy stored within the gas into kinetic energy. Like the compressor, the turbine comprises a rotating disc with blades and static vanes, called nozzle guide vanes. The gas pressure and temperature both fall as it passes through the turbine.

                                               

Bird-Strike Impact

Bird strike, also known as avian ingestion, 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 shutdown the jet engine. as shown below. This is the behaviour we are trying to simulate for the damage done from the bird strike 

                                                                

 

METHODOLOGY: 

1. Save the different parts of the engine into different (.k)file
2. Renumber the elements, nodes, and parts as per the given standard
3. Assign the section and material for defining the part card for a separate component of an engine
4. Define the boundary conditions to the associated parts
5. Define the contacts and interaction between and within the components
6. Define the Control and Database card for the following to get the relevant post results
7. Include all the card into the main file and run the solver after saving it 

PROCEDURE:

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.

There will be a single Bird_strike.k file containing all the part numbered 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.

Renumber the elements, nodes, and parts 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

Assign the section and material for defining the part card for a separate component of an engine

Engine casing

We will assign the material(MAT_024), section(shell), and part card for the component inside the individually saved files i.e. casing material data inside the birdstrike_casing.k 

As given the casing material young modulus has to be kept 200Gpa with all other data assumed near to steel with FAIL value very large tending, not to fail. the thickness defined in the shell card is 5 mm.

Engine Blade

Very similar to the casing case we will assign the material(MAT_024), section(shell), and part to the individually saved blade.k file, but here we will include the material file given question assign that material to the part card.

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.

Below is the data of the material that we have input to the blade material card through LSID (1002)

Engine_Hub

Similarly, for the hub, we will assign the material(MAT_024), Section(solid), and part to the individually saved Hub.k file. The Hub is represented as a solid element with element formulation 10 point tetrahedron. The E value assigned is 145 Gpa and a high FAIL value. 

Engine_Bird

For the bird model, we will use the MAT_Elastic (001) and shell section thickness 6 mm 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. 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.

Define the boundary conditions to the associated parts

Engine casing

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

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.5000)/60= 523 (Rad)/(Sec) = 0.523 (Rad)/(ms) ~ 0.5(Rad)/(ms)`

 

 

Engine_Bird

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

Define the contacts and interaction between and within the components

1)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 slave to blades.

2) 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 slave. the SSTYP and MSTYP are by part ID selection.

3)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.

 

4)Contact between blade self impact 

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

Define the Control and Database card for the following to get the relevant post results

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)

Control Timestep card play a important role in reducing the runtime. we can have different combination of DT2MS and TSSFAC 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. 

1st trial we done by keeping DT2MS = -6.0E-05  and TSSFAC=0.9 the runtime estimated is 5hrs 26 min but actual it took 35 min with 0% mass addition.

2nd trial we done by keeping DT2MS = -6.0E-04  and TSSFAC=0.9 the runtime estimated is 40 min but actual it took 3 min with 24.6% mass addition.

3rd trial we done by keeping DT2MS = -4.0E-04  and TSSFAC=0.9 the runtime estimated is 1hrs 32 min but actual it took 5.6 min with 6.6% mass addition.(Acceptable)

Database Card

among the Database card we will include ASCII, D3PLOT, EXTENT_BINARY (for strain post analysis)

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

The ENGINE_MAIN.k file will include all the file i.e components, material, section, part, boundary condition, contact, control, and database( As shown below)

RESULTS:

Energy plots:

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. This is the 1st check to validate the accuracy of the simulation done. Hourglass energy is zero through out 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 1st incidents. At nearly 1 ms the stress peaks and reaches a value of 0.075 Gpa just at the moment of impact then reduces and stays at a value of 0.02 Gpa throughout the simulation. 

 

Plastic strain plot

This strain plot is taken on one of the element of the blade where the impact 1st incidents. At t= 1ms the strain values peaks to 0.08 i.e. 8% strain then is constant throughout the simulation. 

Contact plot(blade_Bird)

The energy plot between the blade and bird contact show below is well depict that before the interaction of the surface the energy was zero but nearly from 0.75 ms the energy within the contact starts to develop. The of energy conversion between the slave(blade) and master(bird) is symmetric. the maximum value of energy developed is 8 unit at t= 1.25 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.
• All the files such as for component, BC's, contact, control, database are included in the Main file.
• The BC and load conditions are mimicking the real-world scenario. 
• For Mass-scaling, the % mass added is 6.67% which below 7%, and accepted.
• From the energy plot, we can observe that total energy is constant over the simulation which represents the simulation runs well(1st check).
• The max V-M stress produced in the fan blade is 0.075 Gpa at nearly 1ms of simulation 
• The max strain developed is 0.085 at 1ms of simulation and remains constant over the simulation 
• The analysis would be more promising to represent real behaviour if we model the bird as an SPH (way ahead)