Shock Mitigation in Aqueous Foam

Breda Carole -PhD Student Eufoam 2014 7.07.2014

1 French-German Research Institute of Saint-Louis © ISL 2014 - All rights reserved conform to ISO 16016

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Co-financed by ISL and DGA (2012-15)

Shock Mitigation in Aqueous Foam C. Bréda 1, M.-O. Sturtzer 1, S. Kerampran 2, J.-F. Legendre 1 , Y.-M. Scolan 2

1 French-German Research Institute of Saint-Louis, ISL 2 Laboratoire Brestois de Mécanique et des Systèmes (LBMS EA 4325), ENSTA Bretagne

Co- supervised by LBMS

Contact author: [email protected]



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Steps of the study: - reproducible and stable foam generation - foam characterization - study of the interaction between foam and … -shock -blast -fragment(s) -blast and fragment(s)

Foam use for IED-disposal: - attenuation of blast effects - decrease of the velocity of metal fragments reduction of the danger zone

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huffingtonpost.fr



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Foam mitigates shock through dissipation of kinetic energy. Several reasons can account for this attenuation: - the high compressibility of the gaseous phase in bubble - the high heat capacity of the liquid phase - the low sound velocity of foam relative to air and water - the visco-elasticity of foam - the pulsation and resonance of bubbles - the reflectivity at the foam/air interface - the evaporation of the liquid films - the fragmentation of foam in smaller droplets

But the quantification of the relative effects of studied phenomena is still unclear.



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t Time

t

Pre

ssur

e

Time

Pi

Shock wave Blast wave

Pi > 7 kPa : glass breakage Pi > 5 bar : 50% of lung injuries Pi > 8 bar : lethal threshold

Pi



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• A shock wave Fast travelling wave inducing an abrupt, nearly discontinuous change in the

thermodynamical and mechanical characteristics of the medium. The properties that undergo an extremely rapid increase through a shock are : - the static pressure, temperature and density, - the internal energy, - the material velocity

• Overpressure of shock

• Mach number t v : velocity magnitude a : sound velocity in medium

Definition of a shock wave

avM =

atmospheric pressure

incident shock pressure

t Time

iP

0P

0PPP i −=∆



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Confining the charge with his double of mass in foam: 90% of pressure reduction (study for 1-100 kg/m3)

10-16kg/m3 : optimal density to attenuate shock until 0.7 bar at the minimum standoff

-Additive coal particle in foam to increase the mitigation of shock -Peak pressure higher in wet (200 kg/m3) than in dry foam at the end-wall

A network of bubbles model (Kameda theory)

2006 Hartman: (SANDIA)

2007 Britan:

2012 Del Prete: 2011 Grandjean:

1984 NEST R&D: (Boughton)

1978 Gelfand: & Borisov

1985 Vakhenko:

2010 Ghidaglia: Hydrodynamic model with phases transition (evaporated foam)

Hydrodynamic model without phases transition

History about shock/foam interaction

Foam/air interface (5,15,20,35 kg/m3) -double fronted compressive structures

Determination of drag coefficient for foam in the numerical calculation

1988 Bobin (ISL) 1992 Parmentier

Shock tube tests and blast tests with shave foam



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Gas at high pressure P0, T0, ρ0

Gas at low (ambiant) pressure

Paul Vieille, 1899 Before the diaphragm break

P1, T1, ρ1

After the diaphragm break

P0, T0, ρ0

P2 P1 P3

P0

Time

space

space

Pressure incident shock pressure

P1, T1, ρ1 P3, T3, ρ3 P2, T2, ρ2

rarefaction



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Foaming liquid

Ecopol (Bio-ex) + 60% of deionized water foaming agent used by: firemen proportions determined by: Bikermann test superficial tension: σ = 26mN/m apparent viscosity: μapp = 100 mPa.s (60t/min)

Foam generation system Flow-focusing with air

mean bubble diameter: Ф~5mm or ~100µm variation coefficient ~ 70% by ImageJ density: ρ~ 50 -80 kg/m3 by conductimetry, direct weight measurement

Characterization of foam

Ф~5mm

Ф~100µm



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3,14m

7 piezoelectric pressure sensors in low pressure section to analyze shock

LBMS shock tube in Brest

Incident shock

Reflected shock at tube wall

10cm 18cm 10 10 10 9cm

P1



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Reference test without foam (air)

Test with small bubbles foam Φ~100μm ρ ~ 80kg/m3

Test with large bubbles foam Φ~5mm ρ ~ 50kg/m3

5.15.1

==∆

MbarP

6.18.1

==∆

MbarP

time

P

time

P

time

P

pressure discontinuity less sharp

iPrP



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P2 P3 P4 P5

P7

0

1

2

3

4

5

6

7

8

time(

ms)

P2 P6

Test with foam Φ~5mm Test with foam Φ~100µm Reference test (air) 6.1

8.1==∆

MbarP

Incident wave

Reflected wave

Compression head

Compression tail



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time

P

0P

iP

0PPi

0 0,5

1 1,5

2 2,5

3 3,5

4 4,5

without foam with foam, big bubbles with foam, small bubbles

P1 P2 P3 P4 P5 P6 P1 P2 P3 P4 P5 P6 0

1

2

3

4

5

Increase of pressure could be caused by the shock transmission in a higher impedance medium. The incident pressure decreases with the foam thickness.

5.14.1

==∆

MbarP

6.18.1

==∆

MbarP

With pressure profiles, the ratio is analyzed 0P

Pi



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Assumption : - shock is plane - foam as an ideal gas - sound velocity of foam [1] Ø~5mm : ~ 200m/s - sound velocity of foam [1] with Ø~ 100µm: ~ 50m/s Relation between pressure and wave velocity given by the Rankine-Hugoniot theat the interface air/foam : The model of two ideal gas fits well with experiments: relative error < 10% The calculated pressure increases by over 125% for M=1.5 and 145% for M=1.6. [1] Pierre, Dollet, Leroy, Physical review letters 112, 148307, 2014

Increase of incident pressure

wavedtransmittewavereflected vv =

foamfoam c,ρairair c,ρ Incident wave Reflected wave Transmitted wave

time

foamP

iP

Pressure sensor just before foam



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i

i

PP

f

Attenuation performance

: Ratio of the reflected pressure in foam and reflected pressure in air by the wall of tube

: Ratio of the incident pressure in foam and shock pressure in air

Reference test

time

P

time

P

time

iPrP

frPfrP

fiPfiP

r

r

PP

f

Pressure sensors in foam with bubbles Ф~5mm Pressure sensors in foam

with bubbles Ф~100µm



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8 18 28 38 8 18 28 38 crossed foam thickness crossed foam thickness

Attenuation performance for the incident wave

0

0,5

1

1,5

2

2,5

5.14.1

==∆

MbarP

6.18.1

==∆

MbarP

i

i

PP

f

The incident pressure with a large bubble Ф~5mm (blue) is more attenuated than with small bubble Ф~100µm (red). It seems that propagating through at least 40cm of foam is necessary for the incident pressure to return to the shock pressure in air.

v

Propagation direction of the incident shock



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0 0,2 0,4 0,6 0,8

1 1,2 1,4 1,6

The reflected pressure with a small bubble Ф~100µm (red) is more attenuated than with large bubble Ф~5mm (blue). The reflected pressure is lower than pressure in air with only 10cm thick of small bubble foam.

r

r

PP

f

8 18 28 38 8 18 28 38 crossed foam thickness crossed foam thickness

5.14.1

==∆

MbarP

6.18.1

==∆

MbarP

Propagation direction of the reflected shock by tube wall

Attenuation performance for the reflected wave



ww

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If the pressure decreases with crossed foam thickness, which physical parameters increase to conserve the system energy ?

First observation: as the incident wave passes there is an increase of local foam density

80kg/m3 -> 100kg/m3

To observe the mitigation phenomenology in foam : high speed camera 15000images/s Test: large bubble (~5mm) , shock wave of

t=0ms

t=0.3ms

2cm

barP 4.1=∆



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t=0.6ms

t=0.9ms

2cm

The big bubbles at windows are not destroyed but the inner foam all are fragmented in droplets. This is the foam atomization.

t=1.2ms



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For the given experimental conditions, the transmitted wave in the foam is not a shock but rather compression zone, with a head and a tail, which is slower than an incident shock in air. Just after the wave enters the foam, an increase of pressure is observed, which is accounted for by the shock transmission in a higher impedance medium. With small bubbles (Ф~100µm), the wave is more delayed than with large bubbles (Ф~5mm). And with enough foam (thickness ~ 38cm), the reflected wave by the tube wall with small bubbles is lower than the reflected wave in air. The large bubble foam is better suited to study the incident and reflected waves, because they are clearly identified on the pressure signals. Furthermore, visualization with a high-speed camera is possible with large bubbles.



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Shock Mitigation in Aqueous Foam

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