1

2008 International

ANSYS Conference

Development of a 3-D Blast Overpressure

Modeling Capability Utilizing Fluent

Daniel L. Cler - U.S. Army RDECOM/ARDEC/WSEC/Benet LabsMark Doxbeck - U.S. Army RDECOM/ARDEC/WSEC/Benet Labs

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Blast Waves - Examples

Gun blasts Firearm firing

Blast wave effects on buildings

CFD animation of blast wave

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Blast Waves

• A blast wave is the pressure and flow resulting from the

deposition of a large amount of energy in a small very localized

volume.

– Flow field can be approximated as a lead shock wave, followed by a 'self-

similar' subsonic flow field

• Shock waves cause a virtually instantaneous jump in pressure at

the shock front

• The combination of the pressure jump (called the overpressure)

and the dynamic pressure causes blast damage

• It is necessary to understand blast wave to

– Estimate the damage that will result from an explosion

– To devise mechanisms for mitigating the blast

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Simulation Objective

• Accurate, effective and efficient CFD approach to simulation of

transient shock discontinuities

– Shock wave identification and tracking

– Shock front resolution

– Interaction of shocks with objects

CFD image of a blast wave Blast wave resulting from a pipe burst

in the bleed system of a jetliner

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Simulation Approach

• CFD finite-volume utilized rather than Lagrangian

– Lower computational overhead.

– Better flow prediction capabilities.

– Solution based grid adaption.

– Best in situations where blast does not cause structural deformation.

– Can be coupled to FEA solvers to determine deformations and loading of structural components

• Significant improvements to adaptation schemes are required to make CFD simulations feasible.

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Modeling Blast Waves

• FLUENT has capability of tracking and resolving traveling blast waves and shocks by means of dynamic adaption– Both refinement and coarsening of the

mesh is performed• Refinement captures traveling shocks

• Coarsening avoids excessive mesh resolution away from discontinuities

– Refinement parameters based on• Gradient of static pressure gradient:

– Static pressure has largest gradient across shock front

• Solution-based adaption criterion

• In combination, these tools make it possible to:– Model the initial combustion

– Track associated blast wave

– Determine the pressure load history on nearby objects

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Validation 1 – Small Caliber Gun

• Modeling of blast wave associated with a bullet fired

from 7.62 mm NATO G3 rifle with DM 41 round

– Modeling 1st and 2nd precursor and main propellant gas

plume without bullet

Ref: Gun Muzzle Blast and Flash, Progress in Astronautics

and Aeronautics, Vol. 139; Klingenberg, Gunter, Heimerl,

Joseph M., Seebass, A. Richard Editor-in-Chief, AIAA

CFD Analysis Process

• 2-D Axisymmetric

• Fluent 6.1

• Density-based explicit solver with explicit time stepping

• Second-order upwind scheme

• Inviscid

• Species transport of 2 non-reacting ideal gases (propellant &

air)

• Time varying pressure inlet (12.5 mm upstream of the muzzle)

– Existing experimental static pressure as a function of time

• Pressure outlets at computational domain boundaries

• Gun barrel walls are modeled

• Mesh adaption based on pressure gradient

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Validation 1 – Preprocessing and Results

1st Pre-Cursor

FLUENT: t = -350 sec t = -5 sec t = -+120 sec

2nd Pre-Cursor Main blast wave

Experiment

Quad-paved grid with coarse

spacing near outlet; structured

grid in barrel and tight spacing

near muzzle

Pressure

outlet

Pressure

outlet

Pressure-inletPressure-inlet

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BWIP Development Rationale

• Standard Gradient Adaption Limitations

– Poor Coarsening after Wave Passes

– Loss of Adaption as Blast Wave Weakens

– Over-adaption in Uncritical Areas

– Unable to Utilize Advance Register Combinations to

Improve Performance

• Blast Wave Identification Parameter (BWIP)

– Track Primary and Reflected Waves

– Ignore Other Pressure Gradients

– Fine Control of Adaption Level on Shock Fronts

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Validation 2 - 3D Tank Gun Blast

Experimental set-up of 120 mm

Advanced Technology

Demonstrator (ATD)

• Structures of blast waves associated with firing of a ballistic weapon are very complicated

• In different regions, shockwaves can have quite different strengths

• There are no universal criteria for the accurate numerical detection of shockwaves

• U.S. Army has been studying blast wave propagation numerically to propose methods of minimizing the impact of gun blasts on tank crews

• New blast wave identification parameter (BWIP) based on the flow physics capable of locating traveling shocks with disparate strengths automatically with minimal user set-up is used

Ref: Kurbatskii, K. A., Montanari, F., Cler, D. L., and

Doxbeck, M., “Numerical Blast Wave Identification and

Tracking Using Solution-Based Mesh Adaptation

Approach”, AIAA Paper 2007-4188

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Validation 2 – Mesh and Solution-based

Adaption Parameter

Initial mesh: 330,000

tetrahedral cells

Solution-based Blast Wave Identification

Parameter (BWIP)

• For any shock, the Mach vector normal to the shock

has a value of at least one just before the shock

• This normal Mach number is used as a test value for

determining the shock location

• Pressure gradient is always normal to the shock, and

it is used to find the shock orientation

• Dot product of the pressure gradient with the Mach

number vector is used to calculate a shock test value

in each cell

• Locations where the test value equals to one forms a

boundary surrounding the shock locations

• A correction is applied to the test equation to account

for the moving shock

• Utilize user defined functions.

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Validation 2 - CFD Analysis Process

• FLUENT 6.3 or later

• Density-based explicit double-precision solver

• Inviscid flow

• Species transport of two non-reacting ideal

gases (propellant and air)

• 1st-order upwind scheme

• Standard upwind flux-difference splitting of

Roe to evaluate fluxes

• Green-Gauss node-based gradient evaluation

• Explicit time stepping approach

• time step determined by the CFL condition

• 4-stage Runge-Kutta scheme with standard

coefficients for time integration

• Mesh adaption based on the BWIP function

• Outflow boundary is a pressure outlet

• Computations are completed before

propagating shocks reach outflow boundaries

120 mm ATD firing

FLUENT simulation

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Validation 2 - Results

Mesh Static pressure

Static pressure on tank chassis

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h09

0

1500 2000 2500 3000 3500 4000 4500 5000

t, sec

Ps

Fluent with adaption

Fluent without adaption

test

h10

0

1500 2000 2500 3000 3500 4000 4500 5000

t, sec

Ps

Fluent with adaption

Fluent without adaption

test

h13

0

1500 2000 2500 3000 3500 4000 4500 5000

t, sec

Ps

Fluent with adaption

Fluent without adaption

testh14

0

1500 2000 2500 3000 3500 4000 4500 5000

t, sec

Ps

h14 - Fluent with adaption

h14 - Fluent without adaption

h14 - test

Numerical and experimental time histories of pressure at four hull locations. Also shown

numerical results computed on the original mesh without adaption

Validation 2 - Results

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Advanced BWIP Development

• Utilizes to determine shock location.

• Identifies shock center based on above.

• Marks cells a prescribed distance from shock center.

• Mark cells based on mass fraction of propellant.

• Combines registers.

• Creates a buffer zone in front of shock based on dot

product of vector defined by two cell centroids and a

velocity vector

• Grid adaption frequency controlled based on time for shock

to pass to edge of buffer zone.

• U.S. Patent Application # 60/944,612 Filed on 6/18/07

minVpmax

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Validation 3 – LAEP 6

• New Muzzle Brake

• Improved Experimental

Instrumentation

– Field Probes

• Advanced BWIP

• Improved Chemical

Species Determination

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Validation 3 – Side-on Pressure

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Validation 3 – Adaption Capabilities

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Validation 3 – Maximum Overpressure

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Validation 3 – Animation 1

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Validation 3 – Animation 2

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Validation 3 – Animation 3

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Validation 4 – Fixed Mesh vs BWIP

Fixed Mesh - 976,422 cells Adaption Mesh - 94,344 cells

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Validation 4 – Performance Comparison

0 1 2 3 4 5 6 7 8 9 10 110

2

4

6

8

10

12

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Flow Time (ms)

Cu

mu

lati

ve C

PU

Tim

e (

days)

Fixed Mesh

BWIP

Advanced BWIP

0 1 2 3 4 5 6 7 8 9 10 110.5

1

1.5

2

2.5

3

3.5

4x 10

5

Flow Time (ms)

Nu

mb

er

of

Cell

s

BWIP

Advanced BWIP

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Validation 4 –

Static Pressure Probe Comparison

4 4.5 5 5.5 6 6.5 7 7.5 8

0

5

10

15

Time (ms)

Overp

ressu

re (

psi)

f02

Fixed Mesh

BWIP

Advanced BWIP

4 4.5 5 5.5 6 6.5 7 7.5 8

0

2

4

6

8

10

12

Time (ms)

Overp

ressu

re (

psi)

f06

Fixed Mesh

BWIP

Advanced BWIP

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Validation 4 – Performance Comparison

• 2-D Comparison

– Advanced BWIP is 8 times faster.

– Slight reduction in quality at farfield locations.

• 3-D Comparison

– Advanced BWIP would be 2 orders of magnitude faster than fixed mesh based on 2-D performance comparison.

– Quality should only be slightly degraded.

– Advanced BWIP makes 3-D blast simulation feasible.

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Conclusions

• High-quality 3-D blast analysis capability in

Fluent through Advanced BWIP user defined

function with very low computational overhead.

• Better solution accuracy and lower computational

cost than traditional Lagrangian blast simulation

methods.

• Validated against real world problems with good

prediction accuracy.

• Technology is licensable from RDECOM.