Bird Strike - Project - 2

BIRD STRIKE SIMULATION ON AN AERO ENGINE USING LS-DYNA
AIM
To create the simulation of bird strike on the Aero Engine from the given FE model.
Following are the information and conditions required to model the phenomenon
i.     The blades should rotate at a constant velocity but the casing should remain stationary.
ii.    The cylindrical bird model should travel along its own axis and hit the blades.
iii.   Use elastic material for the bird(2000MPa) and the casing(200GPa).
iv.   The material model for the blades is given.
v.    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.
vi.   To follow a consistent numbering approach 100000+ for nodes, 500000+ for elements, and 1000+ for the parts. Allother keywords should be numbered within 10000-19999 while following a range for each one.
vii.  The bird, casing and the blades should be in different input files as well as control cards and boundary conditions and thereshould be one main file referencing all the input files.
viii. The reference include path should be such that the folder can be copied anywhere and it is able to correctly reference allthe file and run the simulation.

Note: The unit system used is kg-mm-ms.

INTRODUCTION :
The collision of birds on an aircraft while cruising in the air is called Bird Strike event and it is one of the most dangerousaccidents that often occur. Therefore, certification of a bird strike is one of the important processes in aircraft design. Inparticular, the areas where bird strikes occur a lot are on the front leading edge of the wing, the cowling part of the engine,rotating engine fan blades, the cockpit transparency and the landing gear part. This project is a classic nonlinear transientdynamics problem similar to car crash and mobile drop. While accurate modelling of the problem requires advanced techniquessuch as SPH, this problem can be solved using generic explicit solver.

PROCEDURE :
The given FE model of Aero Engine assembly is opened in LS-PrePost and each part is assigned with section property andsaved with ‘.k’extension.

The given FE model consists of parts such as bird, blade, hub and casing.

1. Part Definition keyword files
Bird:

Fig.1 Bird part definition
The given FE model of dummy bird is retained and all other parts are deleted. The part is assigned with section shell propertieswith
ELFORM = 2 and thickness = 2.5 mm.

The keyword file is saved with suitable name using ‘.k’ extension

Blade:

Fig.2 Blade part definition
The given FE model of blade is retained and all other parts are deleted. The part is assigned with section shell properties with
ELFORM = 2
and thickness =1.2 mm.

The keyword file is saved with suitable name using ‘.k’ extension

Hub:

.

Fig.3 Hub part definition
The given FE model of hub is retained and all other parts are deleted. The part is assigned with section solid properties with
ELFORM = 4.

The keyword file is saved with suitable name using ‘.k’ extension

Casing:

Fig.4 Casing part definition
The given FE model of casing is retained and all other parts are deleted. The part is assigned with section shell properties with
ELFORM = 2 and thickness = 2 mm . The keyword file is saved with suitable name using ‘.k’ extension

Fig.5 Material property keyword file.
The MAT_ELASTIC material card with mid 11001 is assigned to part with pid 1001 i.e., bird.
The MAT_ELASTIC material card with mid 11003 is assigned to parts with pid 1003 and 1004 i.e., hub and casing.
The MAT_PIECEWISE_LINEAR_PLASTICITY material card with mid 11002 is assigned to part with pid
1002 i.e., blade
The material keyword file is saved with suitable name using ‘.k’ extension.

Fig.6 Boundary conditions keyword file.

As stated in the problem the casing has to be stationary, hence the degrees of freedom of nodes in the casing is constrained inall direction.
The velocity of the engine blades and the birds are chosen so that blade failure can be seen within a short span of time. Hence,the angular velocity of blade is assumed as 0.5 rad/ms.
The velocity of bird striking the blade is assumed as 116 mm/ms.

The cylindrical bird model is made to travel along its own axisand hit the blades.
The keyword file is saved with suitable name using ‘.k’ extension

Fig.7 Contact conditions keyword file.

In case of bird impacting the fan blades directly, it is important to assign the proper contact between,
Bird and fan blades,
Fan blades and hub,
Fan blades with other blades,
Blades with casing.

Contact between bird and fan blades was defined by using nodes to surface contact where bird is the nodes and blades aresurface. The impact of the bird with a blade induces reaction loads that counter the rotational forces of the blade thusdeforming the blade enough or fracture, portions of the same blade may even come into contact with the remaining blade andcasing.
The hub and blade are held together using tied surface to surface contact.
The contact between fan blades with other blades is defined using automatic single surface contact.
The contact between damaged fan blades with casing is defined using automatic surface to surface contact.
The keyword file is saved with suitable name using ‘.k’ extension.

4. Control and Database options keyword file

Fig.8 control functions. and Database options
The control energy card is used to calculate hourglass energy, stonewall energy, sliding interface energy and Rayleigh energy.
The termination time is set as 2 ms.

The time step value of 0.1 ms is given for the BINARY_D3PLOT and in the DATABASE_ASCII option for
GLSTAT, MATSUM,RCFORC and SLEOUT *DATABASE_EXTENT_BINARY card with STRFLG =1,
is used to compute the elastic strain in the model.
The keyword file is saved with suitable name using ‘.k’ extension.

5. Main keyword file

Fig.10 Main keyword file.

The bird, blade, hub, casing, material, boundary condition, contact and control database are in different input files. Hence toreference these input files to one main file, Open LS-PrePost, in the *INCLUDE card type the input keyword file names andinsert it and save it as main keyword file with ‘.k’ extension. The main file referencing all the input files is as shown in fig.10. Thereference include path is such that the folder can be copied anywhere and it is able to correctly reference all the files and runthe simulation.

6. Renumbering Approach

Fig.11 Renumbering.

Renumbering is done by selecting individually the keywords like nodes, elements, parts etc and entering the Start ID value andclick Set and Renumber.
A consistent numbering approach is followed i.e., 100000+ for nodes, 500000+ for elements, and 1000+ for the parts. Allother keywords are numbered within 10000-19999 while following a range for each one.
These requirements are necessary in a professional setting when dealing with large models and multiple people working withthe same model simultaneously.

RESULTS AND DISCUSSIONS

 v-m Stress contour:

Effective Plastic Strain contour:

Maximum v-m Stress in the blade:

Fig.11 Maximum v-m stress in the blade on element 508679

An element508437 of the blade at the impact zone is taken to plot the graph of v-m stress. From the graph, it is observed thatthe v-m stress induced in the element is maximum and reaches to a value of 0.00542 GPa which is more than the yield stressvalue of 0.0133 Gpa of the blade material. Hence, the blade is damaged.

Energy plot

Fig.13 Energy plot graph

From the energy plot graph, it is observed that the kinetic energy is reduced during the time of bird striking the blade of AeroEngine. The internal energy is increased due to the decrease in kinetic energy. The hourglass energy remains low during theentire time of simulation. The total energy remains constant throughout the simulation.

Mass Scaling
i. without mass scaling.

Fig.14 Without mass scaling
The estimated time required to complete the simulation was shown as 1Hr 33 mins but actually it took only 19 mins 43 sec to completethe task.

Mass scaling approach can be adopted to reduce the runtime.

ii. with mass scaling.

Trial 1:TSSFAC = 0.9 and DT2MS = -2.00E-4

Fig.15 With mass scaling trial 1.
For,
DT2MS = -2.00E-4, the estimated clock time to complete the simulation for TSSFAC= 0.9 is 16 mins . The percentageincrease in mass is 2.4908%.

Trial 2: TSSFAC = 0.9 and DT2MS = -2.20E-4

Fig.16 With mass scaling trial 2.

For,

DT2MS = -2.20E-4,the estimated clock time to complete the simulation for TSSFAC= 0.9 is 14 mins . The percentageincrease in mass is 4.3324%.
The runtime has reduced compared to trial 1. The iteration for DT2MS value is continued till thepercentage increase in mass is within the limit of 5%.

Trial 3: TSSFAC = 0.9 and DT2MS = -2.50E-4

Fig.17 With mass scaling trial 3.

For,
DT2MS = -2.50E-4, the estimated clock time to complete the simulation for TSSFAC= 0.9 is 12 mins. The percentageincrease in mass is 8.3928%.
The runtime has reduced compared to trial 2 but percentage increase in mass is above 5%. Hence,the iteration is stopped.
By considering the hard limit on mass scaling being 5%.

The optimized value of DT2MS = -2.20E-4 and TSSFAC= 0.9 withlowest possible runtime required to complete the simulation is 08 mins.

Debugging:
For the initial simulation, following parameters were assumed,
i.   Poisson’s ratio for bird material = 0.3
ii.  Angular rotational velocity of blade = 1000 RPM = 0.0167 rad/ms
iii. Initial velocity of bird = 10 mm/ms
iv. Termination time = 1 ms
v.  Contact between only bird and blade as well as blade and hub were defined.

Fig.18 Simulation of bird strike before debugging.
In the simulation, the blade failure couldn’t be seen. Hence to resolve this issue, the keyword file was debugged by running thefile for several iterations of assumed parameters with the help of literature survey.

CONCLUSION
i.  The keyword file to simulate bird strike on an Aero Engine is created by following the necessary conditions andrequirements.
ii. In the simulation, the blades at the impact zone are damaged.
iii.Mass scaling approach was adopted to reduce the runtime with optimum value of DT2MS = -2.20E-4

iv.The initial keyword files were debugged to get a realistic simulation of bird strike phenomenon.
v.Learned to create and organise as well as debug the input keyword files similar to a professional setting when dealing withlarge models and multiple people working with the same model simultaneously.