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Optically measured explosive impulse

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Abstract

An experimental technique is investigated to optically measure the explosive impulse produced by laboratory-scale spherical charges detonated in air. Explosive impulse has historically been calculated from temporal pressure measurements obtained via piezoelectric transducers. The presented technique instead combines schlieren flow visualization and high-speed digital imaging to optically measure explosive impulse. Prior to an explosive event, schlieren system calibration is performed using known light-ray refractions and resulting digital image intensities. Explosive charges are detonated in the test section of a schlieren system and imaged by a high-speed digital camera in pseudo-streak mode. Spatiotemporal schlieren intensity maps are converted using an Abel deconvolution, Rankine-Hugoniot jump equations, ideal gas law, triangular temperature decay profile, and Schardin’s standard photometric technique to yield spatiotemporal pressure maps. Temporal integration of individual pixel pressure profiles over the positive pressure duration of the shock wave yields the explosive impulse generated for a given radial standoff. Calculated explosive impulses are shown to exhibit good agreement between optically derived values and pencil gage pressure transducers.

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Abbreviations

D ij :

Linear operator

EBW:

Exploding bridgewire

f :

Focal length

Hg–Xe:

Mercury–Xenon

I + :

Positive phase of explosive impulse

k :

Gladstone-dale coefficient

M :

Mach number

N :

Number of intervals

P :

Pressure

PETN:

Pentaerythritol tetranitrate

P 0 :

Atmospheric pressure

P s :

Peak shock wave pressure

R :

Lens radius

r :

Radial lens coordinate

r i :

Distance from center of object of interest

r 0 :

Radial lens coordinate exhibiting luminance

R air :

Air gas constant

RDX:

Cyclotrimethylene trinitramine

T :

Temperature

T max :

Peak shock wave temperature

T min :

Temperature at t a + T +

t a :

Shock wave time of arrival

t m :

Time vector

T + :

Positive pressure phase duration

w :

Graded filter width

Δr :

Data spacing interval

δ :

Normalized refractive index difference

γ :

Specific heat ratio

ε :

Refraction angle

ε min :

Minimum refraction angle

ε 0 :

Lens refraction angle constant

η :

Refractive index

η 0 :

Refractive index at ambient conditions

ρ :

Air density

ρ e :

Energetic material pressing density

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Acknowledgments

We would like to acknowledge the U.S. Army Research Laboratory’s Lethality Division Innovation Program and the National Research Council Postdoctoral Fellowship Program for funding of this research. We would like to acknowledge Mr. Roy Maulbetsch, Mr. Terry Piatt, and Mrs. Lori Pridgeon of the Ingredient, Formulation, & Processing Team for pressing of the energetic samples and Mr. Richard Benjamin, Mr. William Sickels, Mr. Ray Sparks, Mr. Gene Summers, and Mr. Ronnie Thompson of the Detonation Science Team for their assistance in conducting these experiments, and Ms. Susan Corley at DSC Laboratories for providing the linearly graded filters. Lastly, we would like to acknowledge Dr. Michael Hargather at New Mexico Tech for his thoughtful discussions and insight.

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Authors and Affiliations

  1. Lethality Division’s Energetic Technology Branch, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD, 21005-5066, USA

    Matthew M. Biss & Kevin L. McNesby

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  1. Matthew M. Biss
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  2. Kevin L. McNesby
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Correspondence to Matthew M. Biss.

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Biss, M.M., McNesby, K.L. Optically measured explosive impulse. Exp Fluids 55, 1749 (2014). https://doi.org/10.1007/s00348-014-1749-x

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  • DOI: https://doi.org/10.1007/s00348-014-1749-x

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