Chapter 9: Bird Strike On Prestressed Rotating Fan Blades

9 Bird Strike on Prestressed Rotating Fan Blades

Summary 127

Introduction 128

Requested Solutions 128

Model Details 128

FEM Solution 129

Results 132

Modeling Tip 133

Input File(s) 134

127CHAPTER 9

Bird Strike on Prestressed Rotating Fan Blades

SummaryTitle Chapter 9: Bird Strike on Prestressed Rotating Fan Blades

Features Bird Strike On Prestressed Rotating Fan Blades

Geometry

Material properties Fan: Piecewise linear plastic material (MATD024)Bird: Elastic-plastic hydrodynamic material (MATD010)

Boundary conditions • Prestress analysis: One end of blade is fully fixed.1/3 and 1/2 span of rotor are fixed to x and y translational and rotational directions

• Impact analysis: One end of blade is fixed to x, y and z translational directions.1/3 and 1/2 points of rotor are fixed to x and y translational directionsDetails are explained in FEM solutions section.

Applied loads • Prestress analysis: Fan: 8000 rpm using RFORCE option (rotational static force)• Impact analysis: Fan: 8000 rpm using TIC3 option (rotational initial speed) and SPCD2 option

(enforced motion) Bird: Initial velocity of 7692 inch/s (437 m.p.h.) using TIC option.Details are explained in FEM solutions section.

Element type • Fan: 4-node shell element• Bird: 8-node hexahedral element (Impact analysis only)

FE results •Prestress analysis: Plot of final stage of implicit run nastin - ASCII result file for initial state values are included. •Impact analysis: Plot of each stage (t = 1.52 ms shown here)

44.22

6.24

44.22

D = 1.8t = 0.2

Units: inches

D = 2.36

D = 27.20.0266 < t < 0.0403

t = 1.52 ms

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IntroductionAerospace companies have performed bird strike test simulation to predict the impact-resistance properties of the aircraft structure. This is an example of a bird (made by solid elements) impacting against rotating fan blades using a sequential implicit-explicit technique. First, using the implicit solver, the initial condition (stress and displacement) on the blades and rotor was calculated followed by transient loading of bird impact, which was simulated using the explicit solver.

Requested SolutionsA numerical analysis was performed to demonstrate the pre-stressed fan blade out method. The rotational inertia effects were taken into account in implicit analysis and the resulting stress, strain and displacements were computed. Next, the results were added to the explicit analysis as initial condition.

Model DetailsMaterials

Fan: Piecewise linear plastic material (MATD024)

= 4.14e-4 lbf/inch3-s2/inch, = 0.35, E = 1.60E+7 psiy (yield stress) = 138000 psi, ET (Tangent modulus) = 100000 psiPlastic strain failure limit = 0.2

Bird: Elastic-plastic hydrodynamic material (MATD010)

= 9E-5 lbf/inch3-s2/inch, G (Shear modulus) = 145 psiy (yield stress) = 2.9 psi, ET (Tangent modulus) = 0.145 psi

Linear polynomial equation of state (EOSPOL

P a1 a22 a3

3 b0 b1 b22 b3

3+ + + 0E+ + +=

0 1–=

overall material density=

0 reference density=

E specific internal energy pur unit mass=

a1 a2 b1 b2 b3 0= = = = =

a1 4.25x106psi=

129CHAPTER 9

Bird Strike on Prestressed Rotating Fan Blades

FEM Solution

Boundary Condition and Applied Load

Prestress Analysis (Implicit)

The rotational velocity of blades and rotor is 8000 rpm which is applied using RFORCE option (rotational static force) in the prestress run. The end of the rotor is fully fixed. In addition, the bearings located at 1/3 and 1/2 of distance from the front of rotor are fixed in x, y translational as well as x, y rotational directions using SPC1 option. The applied loading and boundary conditions of prestress analysis are shown in Figure 9-1(a).

RFORCE 1 299999 -133.3330.0 0.0 1. TABLED1 321 0. 1. .001 1. ENDTSPC1 1 123456 300425 THRU 300443SPC1 1 1245 400058SPC1 1 1245 400115

Impact Analysis (Explicit)

The initial rotational velocity of 8000 rpm is applied to fan blades using the TIC3 entry as well as end of rotor using the SPC2 entry (enforced motion). The bird impact velocity of 7692 inch/s (437 m.p.h.) is applied on all the grid points of the bird model. The boundary conditions at the end of rotor is changed to constrain x, y and z translational directions and the bearing locations of rotor are constrained in x and y translational directions. The applied loading and boundary conditions of impact analysis are shown in Figure 9-1 (b).

TIC 1 1000001 3 7692....SPCD2 1 GRID 1 7 80 -1.TABLED1 80 ++ 0.0 837.758 1. 837.758 ENDT$ Displacement Constraints of Load Set : Disp1SPC1 1 3 21 THRU 31......$ Initial angular velocity for rotor +fan bladeTIC3 1 299999 1. -837.758 1 THRU 6384 300000 THRU 300018 300020 THRU

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Figure 9-1 Boundary Conditions and Applied Loads of the Fan And Bird

(a) Prestress model (implicit)

(b) Impact model (explicit)

8000 rpm

Fully fixed

Fixed (x,y direction) (x,y rotation)

8000 rpm (initial speed)

Fixed (x,y,z direction)

Fixed (x,y direction)

437 mph

8000 rpm (enforced speed)

(b) Impact model (explicit)

8000 rprr m

ully fiff xed

8000 rpprr(iniitial

(x,y,z direction)

437 mph

(a) Prestress model (implicit)

Fu

Fixed (x,y direction)(x,y rotation)

pprr ml speed)

Fixed

Fixed (x,y direction)

8000 rprr m(enfoff rced speed))

(a) Prestress model (implicit)

(b) Impact model (explicit)

8000 rpm

Fully fixed

Fixed (x,y direction) (x,y rotation)

8000 rpm (initial speed)

Fixed (x,y,z direction)

Fixed (x,y direction)

437 mph

8000 rpm (enforced speed)

131CHAPTER 9

Bird Strike on Prestressed Rotating Fan Blades

FEM Model and ContactThe rotor, hub and fan blades are modeled by shell elements while the bird is modeled by solid elements.

Prestress Analysis (implicit)

By using the PRESTRS bulk data entry, a prestress analysis is carried out. The prestress simulation requires the analysis to be run with double precision version of the implicit solver. Final deformations and stresses of elements are written to a text file named “input_file_name”.dytr.nastin to provide initial conditions for rotor and fan blades of the impact run. The definition of TSTEPNL is required in implicit run to determine the number of time steps and their increment for higher fidelity of the solution.

TSTEPNL 1 5 1.-5 1 ADAPT 2 10 PRESTRS

Impact analysis (explicit):

The end time in transient run is defined by using 100 time steps at 0.4e-4 sec. for each increment. End time is the product of these two entries. Notice here the Time Increment is only for the first step. The actual number of Time Increments and the exact value of the Time Steps are determined by MD Nastran solver during the analysis. The time step is a function of the smallest element dimension during the simulation.

The prestress results file is prestress_rotor.dytr.nastin. The name of this file was changed to rotor.dytr.nastin due to the long file name. It includes the results for grid points, elements and is used as initial condition for explicit transient run. The prestress file prestress_rotor.dytr.nastin includes all geometry information such as grids, elements and the results. Therefore, the explicit model should include only the material properties for the structure, the new boundary conditions as well as new data for the bird.

INCLUDE rotor.dytr.nastinTSTEPNL 1 100 .4e-4 1 ADAPT 2 10

The file rotor.dytr.nastin contains an entry called ISTRSSH. This entry specifies the prestress condition of the shell element as defined below (see the MD Nastran Quick Reference Guide for more details). These result values of the prestress run are to be carried over to the impact run. When other elements types other than shells are used, ISTRSBE, ISTRSTS, and ISTRSSO entries must be included in the nastin file.

ISTRSSH* 2275 1 5 5* * * * 0.000E+00 6.916E+03 7.371E+03 1.480E+02* * 7.084E+03 -3.908E+01 1.150E+02 0.000E+00* ...

In this analysis, adaptive contact is defined between the bird and the fan blades. The BCBODY and BCPROP entries are used to define a symmetric (M-S, S-M) contact bodies.

BCTABLE 1 2 SLAVE 8001 0. 0. 0.1 0. 0 0 0 0 0.1 YES++

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+ MASTERS 1001 SLAVE 1001 0. 0. 0.1 0. 0 0 0 0 0.1 YES+++ MASTERS 8001BCBODY 1001 3D DEFORM 1001 0BCPROP 1001 1 2 3 4 5 6 7...

Results

Prestress RunThe results of all increments are essentially the same which indicates that the implicit calculations are stable. The results of the last increment were written to the file prestres_rotor.dytr.nastin.

Figure 9-2 Result Increment 5: written to the .nastin file

133CHAPTER 9

Bird Strike on Prestressed Rotating Fan Blades

Impact runThe prestress result variables have been initialized at the begin of the analysis (Time = 0)

Figure 9-3 Element Mesh Applied in the MD Nastran Simulation

Modeling TipThe default values for shell integration points in implicit and explicit analyses are different. There are three integration points for implicit analysis and two integration points for explicit analysis. Therefore, the shell element type for the implicit analysis has to be modified to be consistent with that of explicit simulation.

PSHELL1 1 1 BLT GAUSS 2

t = 0 ms t = 1.00 ms

t = 2.00 ms

t = 4.00 ms

t = 1.52 ms

t = 3.00 ms

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Input File(s)

File Description

nug_9a.dat Impact analysis

nug_9b.dat Prestress model

nug_9c.dat Stresses and deformations of prestress model for input to impact analysis

nug_9d.dat Data for bird