Bird Strike - Project - 2

Aim:

• To perform and simulate the non-linear case of Birdstrike in Aero-engine using explicit analysis in LS-Dyna

Objective: 

• The FE model consists of 4 parts that need to be separated and save part-wise. The suitable material is created and saved in a subfolder
• The node number, element number, and part number are assigned as mentioned above. Also, to conclude the final simulation create the main file with the *include file and run the explicit simulation using mass scaling

Bird Strike

Bird strikes are a major threat to aircraft structures, as a collision with a bird during flight can lead to serious structural damage. Computational methods have been used for more than 30 years for the bird-proof design of such structures, being an efficient tool compared to the expensive physical certification tests with real birds. At the velocities of interest, the bird behaves as a soft body and flows in a fluid-like manner over the target structure, with the high deformations of the spreading material being a major challenge for finite element simulations.

Bird strike simulations on aircraft structures have been performed and improved since the late 1970s. A large variety of bird impactor geometries, materials, masses, densities, and modeling methods exists in the literature. There are three established techniques for numerical bird impactor modeling are used: Lagrangian, Eulerian, and SPH. The best approach depends on the specific application, software package, computational resources, and parameter setup

Procedure:

Case setup

• The given keyword file BIRD_STRIKE  consists of nodes and elements for all the four parts collectively i.e, Engine casing, Blade, Hub, and Blade

• Separate the given parts by assigning the keywords file. Those parts are distributed and saved .k keyword file

                                                   BIRD                                                                                                        BLADE

                                        ENGINE CASING                                                                                                   HUB

For this FE model the unit system followed here will be Kg, mm, ms

Material

• For a bird, *MAT_ELASTIC card material is selected and properties are assigned from the journal "https://www.researchgate.net/publication/258630404_Bird-Strike_Modeling_Based_on_the_Lagrangian_Formulation_Using_LS-DYNA" 
• The given properties are, Density-2.000$e^{-6}$, Young's Modulus-2.000, Poisson Ratio-0.4700 

• For blades, *MAT_24_Piecewise_Linear_plasticity card is given with properties of Aluminium, Density-2.713$e^{-6}$, Young's Modulus-68.946999, Poisson Ratio-0.3300,
  Yield stress-0.0133600, Failure flag-0.2723, and Tangent modulus is given as Load curve ID by plastic strain rate behavior

• For turbine casing, *MAT_ELASTIC card material is selected again and the steel properties are assigned Density-7.8000$e^{-6}$, Young's Modulus-210, Poisson Ratio-0.3300

• For hub, *MAT_24_Piecewise_Linear_plasticity card material is selected again and the steel properties are assigned Density-8.2000$e^{-6}$, Young's Modulus-145, Poisson Ratio-0.30,
  Yield stress-0.81000, Tangent Modulus-70.000, Failure flag-$1.000e^{21}$(Default)

Section

Section card is created individually upon the following FE models i.e., bird, blade, and engine casing with Element formulation of 16 (Fully integrated shell element modified for higher accuracy) 

• Bird-The thickness is assigned to be 15mm

• Blades-The thickness is assigned to be 2mm

• Casing-The thickness is assigned to be 1mm

• Hub-The Element formulation is assigned to be 10 (point tetrahedron)

Include

• All the FE models which are saved separately for a bird, engine case, blade, hub are included in a single .k model file 
• Using the keyword file *Include$\to$Include allows providing a new FE model

Part 

• Assign the part with respect to the material cards and section with proper Subsys.k after inserting the FE parts into a single model file

Bird

Turbine Blades

Engine casing

Turbine Hub

Contact

• Contacts are created using Part ID type, SSTYPE=3 & MSTYP=3
• Hence the blades should rotate according to the hub, *CONTACT_TIED_SURFACE_TO_SURFACE is used to tied the surfaces with the blade as a slave and hub as master 

• Contacts are created using Part ID type, SSTYPE=3 & MSTYP=3
• In order for the bird needs to hit the blade *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE is used as the bird as a slave and blade as a master

• Contacts are created using Part ID type, SSTYPE=3 & MSTYP=3
• Up-next, the blade should not suppose to penetrate the casing. Again *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE is used as the blade to be slave and casing as a master

• Contacts are created using Part ID type, SSTYPE=3 & MSTYP=3
• Finally, also the bird does not suppose to penetrate the casing. Again *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE is used as the casing to be slave and bird as a master

• Here for the single contact, we used Part ID type, SSTYPE=3 & MSTYP=0
• Eventually, once the bird hits the blades the deformed blades not supposed to penetrate with each other. Here *CONTACT_AUTOMATIC_SINGLE_SURFACE is used 

Boundary Condition

SPC

• Though the turbine engine case supposed to be fixed at the airplane wing. It has to be constrained in all the directions i.e, Translation and Rotation

Initial Velocity

• The average velocity of the bird travels from the references is observed to be approximately 20 to 80 mm/ms 
• Here the velocity of the bird is assigned to be VX=30 mm/ms along the X-axis translative direction 

• The angular velocity of the blades is roughly around an average speed of around 3000 to 20000 RPM
• Considering 5000 rpm and convert it to radiant. From which angular velocity OMEGA=0.25 rad/ms, NX=1 (x-direction cosine)

Control 

• The simulation running time is set to be 10 ms 

• For mass scaling after multiple iterations, the time-step card is given with a DT2MS -0.0010 and TSSFAC 0.9

Database

• Generally, the list of output request is specified with the Time interval between outputs DT= 0.05 ms for GLSTAT, MATSUM, NODOUT

• The frequency of the animation file is set to be Time interval between outputs DT= 0.10 ms

Renumber

Result:
Since the thickness of the bird is 15 mm which is larger than the blade with 2 mm
It can be noticed from the simulation that the blade is getting broken once after the bird impacts at 30kmph velocity

Contour plots-Von Mises stress

• The maximum strain is attained to be max=0.0730087 at an element 508531

Contour plots-Effective plastic strain

• The maximum strain is attained to be max=0.272301 at an element 508531

Energy graph:

• In this Explicit Analysis, when the bird hits the blade at a velocity of 30kmph. The Internal energy increases and kinetic energy decreases due to the impact on the blades
• There is no hourglass formation due to full integration ELFORM-16 is used the total energy of the system remains constant throughout the simulation
• Hence we can notice that the plot satisfies the law of conservation of energy 

• At the time of 1.1ms, once the bird hits the blade at the first element of 508450 the maximum stress developed is 0.0240 Gpa 

Conclusion

• All the subsystems were created and included with the Main_Include file. The simulation is assigned and performed using the mass scaling factor
• Transient-non linear bird strike application understands and done using LS-Dyna. The contour plots and global results are sketched