Week 12 Challenge: Bird Strike into Aero Engine Challenge

Course: LS-DYNA for Structural Mechanics/ FEA

Submitted by: Amanpreet Singh Whan

Supervisor: Tholkappiyan D

November 25, 2021

Abstract

Safety in accidental conditions is one of the important requirements an aviation jet engine must meet. It is known that one of the quite possible and very dangerous accidents is a bird strike into the engine in the flight. This case is characterized by the high-speed impact of the bird onto rotating blades of the fan, causing large dynamic deformations of the blades and other elements which may lead to the disintegration of the construction. That is why numerical investigations of the bird strike are very important and should be implemented during the design stage of a development process. The paper presents some results of numerical simulations of dynamic deformations of the fan blades loaded by the bird impact obtained using LS DYNA.

Table of Contents

Abstract.

1 Introduction.

2 Aims.

3 Methodology.

4 Results.

5 Conclusion.

1 Introduction

LS-DYNA is an advanced general-purpose multiphysics simulation software package developed by the former Livermore Software Technology Corporation (LSTC), which was acquired by Ansys in 2019. While the package continues to contain more and more possibilities for the calculation of many complex real-world problems, its origins and core competency lie in highly nonlinear transient dynamic finite element analysis (FEA) using explicit time integration. LS-DYNA is used by the automobile, aerospace, construction and civil engineering, military, manufacturing, and bioengineering industries.

Many external airframe components are susceptible to collisions with birds, particularly during the critical take-off and landing phases. In order to assure minimum safety standards in cases of bird impacts, international airworthiness standards require that airplane tests be conducted to demonstrate certain basic capabilities specified in terms of structural resistance and allowable degradation in flying qualities.

The size of the birds used for testing is set forth by the certification authorities. Different weight birds are specified for windshields, wings, empennage, and engines. The standards for the leading edge of a flying surface dictate that the bird should not cause critical structural damage to the load-carrying structural members directly interior to the leading-edge skin. The airworthiness standards for the windshields of a business jet require that the windshield transparencies and frames be capable of sustaining the impact of a 4 lb. bird at maximum cruise velocity (typically 250 knots – the maximum permitted airspeed below 10,000 ft), with no serious penetration. Jet engine certification requires the demonstration of maintaining a certain level of thrust after an encounter with a flock of small birds, or the safe shut down of an engine after ingesting a single large bird.

The purpose of the Week 12 challenge is to perform the bird strike into a jet engine simulation and compare the stress/ strain and other results for the same.

2 Aims

1. Create a simulation setup to perform a bird strike into a jet engine.
2. Analyze and compare stress/ strain and other results obtained from the simulations.
3. Use Mass Scaling to reduce the runtime for the simulation.
4. Achieve a hard limit of 8% for mass scaling by optimizing DT2MS and TSSFAC parameters.

3 Methodology

The section describes different stages that were performed to set up the simulation file using an engineering tool LS-DYNA for the completion of the Week 12 Challenge task. These stages can be broadly categorized into creating an input deck containing different cards such as contact, control, database, element, hourglass, initial (velocity), keyword, MAT, node, part, section, and title.

Since LS-DYNA does not have a default system of units, therefore, the user needs to input material properties and physical dimensions according to a consistent system of units as per their choice. Considering the challenge requirements of simulating an impact test of head form with the Hood model, kg mm ms were assumed to be the most appropriate units.

What follows hereafter is the set of images representing the process followed to create different cards and values assigned to their key parameters.

Contact:

The keyword CONTACT provides a way of treating interaction between disjoint parts. Automatic Single Surface, Automatic Surface to Surface, and Tied Surface to Surface Contacts were defined for the challenge purposes.

Control:

The keyword control cards can be used to change defaults. The control hourglass, energy, output, timestep, and control termination cards were created for the challenge purposes.

Database:

The database definitions are necessary to obtain output files containing results information. The binary D3Plot, Extent Binary, and ASCII options were selected for the challenge purposes.

Define:

The keyword DEFINE provides a way of defining boxes, coordinate systems, load curves, functions, tables, transformations, and orientation vectors. The curve option was selected to define the given stress-strain values for the blade material.

Include:

The keyword INCLUDE provides a means of reading independent input files containing model data. The file contents are placed directly at the location of the include line. For the challenge purposes, include was created.

Initial:

The keyword INITIAL provides a way of initializing velocities and detonation points. The initial velocity generation option was selected, and the assigned values of velocities are presented in the following cards.

Mat:

The keyword MAT helps in defining and assigning different types of materials to a part as per requirements. For this challenge MAT 001 Elastic and MAT 24 Piecewise Linear Plasticity were selected. The given values assigned to material card parameters are presented in the following card.

Section:

Under section, the element formulation, integration rule, nodal thicknesses, and cross-sectional properties are defined. Solid and shell sections were created as per challenge requirements and assigned values for their parameters are presented in the following picture.

Part:

Here define combine material information, section properties, hourglass type, thermal properties, and a flag for part adaptivity.

4 Results

After setting up the input deck and saving the file as. K format, the first and foremost step is to perform a model check and then run the simulation using LS-Run software. Once normal termination is achieved, the results can be viewed and analyzed using LS Pre-Post software.

Since the challenge requirements were to find an optimal value of DT2MS and TSSFAC parameters for establishing mass scaling hard limit of 8%, the trial-and-error approach was selected to find the optimal values for the discussed parameters. What follows hereafter is the set of images representing the stress-strain results, the results of percentage increase, and the addition of mass to the simulation for different values assigned to the DT2MS and TSSFAC parameters of the Control Timestep card.

Renumbering of nodes, elements, parts, and other keywords.

Mass Scaling Results.

Control Timestep Card: - DT2MS = - 3.500 E-05 & TSSFAC= 0.9

Control Timestep Card: - DT2MS = - 7.500 E-05 & TSSFAC= 0.9

Control Timestep Card: - DT2MS = - 1.00 E-04 & TSSFAC= 0.9

Control Timestep Card: - DT2MS = - 3.00 E-04 & TSSFAC= 0.9

Control Timestep Card: - DT2MS = - 3.180 E-04 & TSSFAC= 0.9

Simulation Results

From the above figure, it can be seen that when the bird hits the blades, there is a reduction in the kinetic energy of the bird while there is an increment of the internal energy of the blades and after that both energies become constant. This validates the principle of conservation of energy which states that energy can neither be created nor destroyed but can be converted from one form of energy to another. Moreover, from the plot, it can also be seen that the hourglass energy remains nearly 0 indicating a good selection of both the hourglass and element formulation for the simulation setup.

5 Conclusion

The main aim of the Week 12 challenge was to create an input deck or setup file to perform the Bird Strike into Jet Engine Simulation using LS-DYNA software. In addition, plot stress/ strain contours and perform Mass Scaling to reduce the simulation time by adjusting TSSFAC of control timestep keyword. From the GLSTAT plots, it was found that the simulation setup follows and validates the principle of conservation of energy expressing loss of kinetic energy of the bird conversely leads to the gain of internal energy of the blades.