Aerospace and Defense: Simulating Lightning...

© 2011 ANSYS, Inc. November 27, 2014

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Aerospace and Defense: Simulating Lightning Strikes


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Agenda

• Industry Trends and Lightning Strikes

• Lightning Characterization

• ANSYS Simulation Approaches

• Examples

• Direct Strike : Tank and Aircraft

• Shielding

• Composite UAV Airframe

• Cabling

• Electromagnetic Pulse on a Tank


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The increasing use of composite materials, a higher percentage of more complex electronic systems coupled with a non compromise approach to safety and survivability demands an increased focus on understanding the impact of lightning strikes

Rising Fuel Cost

Environmental Regulation

Reduce weight with composite materials Improve passenger comfort and safety Increased number of electronic systems

Competition

Understand the impact of lightning strikes


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Agenda




• Examples


• Shielding


• Cabling



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Direct Strike • Direct transmission

Indirect Strike • Coupled transmission • Conducted transmission

Lightning Strike Categories


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What is lightning… • Discharge of electricity resulting from terrestrially or atmospherically separated

charges.

Behaves like a massive current pulse.

Lightning Behavior


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Double exponential rise/fall characteristic

Lightning Characteristics

[ ] ( ) ( )( )ttm eeItI βα −− −=

Curr

ent [

A]

Time [s]

+

−+

=βα ss

II m11][L

fjs π2=Cu

rren

t [A

/Hz]

Frequency [Hz]

15kHz (-3dB)


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Multiple strikes of decreased magnitude are common • 4+ re-strikes of similar shape but smaller amplitude common. • Examples that follow look only at the first strike as it is always the most severe.

Lightning Characteristics


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Defense: 2.9/69 µs • MIL STD 464

Power Distribution: 1.2/50 µs • ANSI/IEEE C62.41

Telecommunications: 10/700 µs • TR-NWT-001089 • ITU K17-K20

Aeronautics: 6.4/69 µs • RTCA/DO-160, Section 22 • SAE/ARP5412

Lightning Waveform Standards

Decay to 50%

Rise from 10% to 90 %

Waveform time constants defined


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Waveform model parameter severity comes from averaged empirical data

Lightning Waveform Severity

>10kA >28kA >80kA >200kALightning 90% 50% 9.90% 0.10%

0%

20%

40%

60%

80%

100%

Lightning Magnitude Percentiles


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Agenda




• Examples


• Shielding


• Cabling



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3D Field Solvers • Time domain • Frequency domain

System Simulation • Parameter extraction • Circuit + 3D link

– Transient – System

ANSYS Simulation Methods

3D Field Solvers HFSS

• High Frequency FEM

Maxwell • Quasi-Static FEM

Q3D • Parameter Extraction

System Simulation Simplorer

• Multi-domain systems

Designer • Electronics, RF systems


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Simulation Methodology • Time Domain

– Direct waveform – Maximum time and time step are simulation parameters – Time steps must be solved sequentially

• Frequency Domain

– DFT waveform – Maximum frequency and frequency step are simulation

parameters – For linear problems impulse response can be obtained in highly

parallel fashion. – Alternative: Export a Reduced Order Model from HFSS Frequency

Domain with Full-Wave SPICE. Then simulate a coupled 3D + transient circuit with Designer or Simplorer

Measure current distribution and voltage levels at specific locations

ANSYS Simulation Methods

0 1 2 3 4 5

x 10-5

0

0.5

1

1.5

2x 105

Time (s)

Cur

rent

(A)

Source Current

0 1 2 3 4 5 6 7 8

x 105

0

0.5

1

1.5

2

2.5

3x 104

Frequency (Hz)

Cur

rent

(A)

Source Harmonics up to 125 of 400


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Frequency Methods (“By Hand”) • Fourier transform current input waveform • Calculate impulse response from 3D FEM

– Unit excitation as input across “all” frequencies in sweep • Convolve impulse response with input current • Inverse Fourier transform

Simulation Model

0 1 2 3 4 5

x 10-5

0

0.5

1

1.5

2

2.5x 10

5

time

curre

nt

0 5 10 15

x 104

0

0.5

1

1.5

2

2.5

3x 10

4

freq

curre

nt

X

0 5 10 15

x 104

0

0.2

0.4

0.6

0.8

freq

curre

nt

0 5 10 15

x 104

0

1

2

3

4

5

6x 10

6

freq

curre

nt

0 1 2 3 4 5

x 10-5

0

1

2

3

4

5

6x 10

4

time

curre

nt

=

F(ω) f (t)


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Lightning strike • Lightning enters through a bolt path • Lightning exits through ground contact • No material breakdown

Simulation Model

Lightning Entrance

Lightning Ground Exit


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Compare field values from time domain method to frequency domain method

Simulation Domain Comparison

0 1 2 3 4 5

x 10-5

0

0.5

1

1.5

2x 105

Time (s)

Cur

rent

(A)

Source Current

Input Current

Colored- Time Domain Gray Dashed – Freq Domain

• 2.5MHz max frequency, • 5MHz sample frequency, • 6250Hz bandwidth

Output Currents Measure current through each leg

8” thick concrete, with rebar

7.2ft

6.5ft

1 2

3 4

0.00 10.00 20.00 30.00 40.00 50.00Time [us]

0.00

10000.00

20000.00

30000.00

40000.00

50000.00

60000.00

Cur

rent

[A]

No Frequency Dependence ANSOFT

Curve InfoTransient_J1

Setup1 : TransientTransient_J2



Setup1 : Transient

More evenly distributed. Peak down 10%.


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Agenda




• Examples


• Shielding


• Cabling



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Tank: Identify surface current densities and field strength inside

Direct Strike Simulation

Lightning strike

Increased current density along edges

T=4uS

DC


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10Hz

1000Hz

Current In

Current Out


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Building with water infiltration (4S/m)


no water

1ft water

Electric current flows into the flooded space from the structure

Current animation

Current animation

0 0.2 0.4 0.6 0.8 1x 10-4

0

500

1000

1500

Time (s)C

urre

nt (A

)

Current Into Water

Without water, skin effect and low impedance rebar keeps current out of the habited area

With flooding significant current enters the habited area


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Aircraft - Direct Strike

HFSS: Aircraft direct strike


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Agenda




• Examples


• Shielding


• Cabling



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Aircraft Outer Shell • Typically made of aluminum or carbon fiber • What is the expected frequency dependent shielding provided by each for a 3mm

material thickness?

Shielding Effectiveness

Material Conductivity [S/m] Thickness Frequency below which fields penetrate

with minimal attenuation*

Aluminum 3.8e7 3mm 10Hz

Carbon Fiber 6.7e4 3mm 1KHz


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Boundary Conditions • Use empty PerfE/PerfH tube with voltage source

as reference. Expect full transmission.

Shielding Test • Can 2D boundary yield the same results vs

frequency as 3D Shell?

Shielding Test

Side Walls PerfH Top/Bottom are PerfE

Voltage Source

Polyline down center to measure E and H field values

Far end is radiation boundary


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Finite Conductivity with Thickness

Boundary Setup and Property Validation

Impedance Values


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Finite Conductivity Boundary vs 3D Solve Inside • Plotting the peak electric field along a polyline across/through 3mm of carbon

fiber. The shield is located at 0.5 meters.

PeakE vs. Distance and Frequency

Solid lines are from the 2D boundary with “thickness” option. Dotted lines are from explicitly 3D solve inside solution.

1Hz, 10Hz, 100Hz

1KHz

10KHz

100KHz

The 2D boundary yields the same attenuation as the 3D boundary for boresight energy incidence!


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Finite Conductivity Boundary vs 3D Solve Inside • Plotting the peak E-field transmission across/through 3mm of carbon fiber. The

shield is located at 0.5 meters.

Shielding Comparison vs. Incidence Angle

Solid lines are from the 2D boundary with “thickness” option. Dotted lines are from explicity 3D solve inside solution.

The 2D boundary yields the same attenuation as the 3D boundary regardless of incidence angle!*

1Hz, 10Hz, 100Hz

1KHz

10KHz

100KHz


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Summary • 2D finite conductivity boundaries with the “thickness” option provide can

effectively be used to calculate transmission through carbon fiber or metallic shells. • The 2D boundary approximation has been shown to compare well with a 3D solve

inside metal at lightning frequencies, but with much greater solution efficiency!

Shielding Test Conclusions


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Agenda




• Examples


• Shielding


• Cabling



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Predator UAV with Carbon Fiber Shell

Strike at wingtip

Exit at tail PeakH at 1Hz Penetrates the wings and fuselage


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H-field on Cut Planes Inside the Predator

H-Field vs Frequency Filled at low frequencies, and H-field repelled at higher frequencies


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Agenda




• Examples


• Shielding


• Cabling



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A320 with Cables in the Wing and Fuselage

Orange/yellow color in fuselage is at 1Hz. Blue color in fuselage is at 1MHz.


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A320 with Cables in the Wing and Fuselage

Two single strand cables inside fuselage. A four conductor cable in the wing.


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Current along A320 Cables

Current flowing on cables, here at 1MHz

Plot PeakH vs. distance along the whole cable length for each frequency.


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Multiphysics and Lighting Strikes

ANSYS HFSS Simulation of Lightning Strike ANSYS Mechanical – Transient Thermal

Mapped Heat Flux


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ANSYS Workbench


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ANSYS Mechanical - Transient Thermal

Apply HFSS Heat Flux until t1

Deactivate Heat Flux after t1

HFSS - Heat Flux

Transient Thermal - Temperature


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Agenda




• Examples


• Shielding


• Cabling



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Electromagnetic Pulse

EMP

EMP explosive device in background


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Fourier Transform

Peak 50 kV/m Rise time ~3ns

( )4 7 6 8( ) 6.5 4 e t e ty t e e e− ⋅ − ⋅= ⋅ − ( )( )2 4 7 2 6 83.64 13( )

j f e j f eeY f

π π⋅ + ⋅ +=

High Altitude EMP Signal


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103MHz

146MHz

Different scale (not shown)

Sensitive microstrip line on board inside tank

Tank Resonance Results


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p1:1p1_45:1

p2_45:1

PNUM=1

RZ=50ohmIZ=0ohm

PNUM=2

IZ=0ohm

PNUM=3RZ=50ohm

V

Name=Vout

V Nam

e=Vi

n

0

10000

20000

30000

40000

50000

60000

0.0E+00 5.0E-08 1.0E-07 1.5E-07 2.0E-07 2.5E-07

EMP Excitation

Input Pulse

What’s the EMP effect on the output signal?

Microstrip on board inside tank.

EMP and Electronics Interaction


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Microstrip line board

Effect On Board Without Tank

Effect On Board With Tank

Tank resonance is significant

EMP Effect Amplified Inside the Tank

Aerospace and Defense: Simulating Lightning...

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