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Licentiate seminar

On dynamics and thermal radiation of imploding shock waves


Defendant Main Advisor Extra Advisor Date
Malte Kjellander Nicholas Apazidis Nils Tillmark 2010-04-16

Opponent
Gabi Ben-Dor, Ben-Gurion University of the Negev

Evaluation committee

Abstract

Converging cylindrical shock waves have been studied experimentally. Numerical calculations based on the Euler equations and analytical comparisons based on the approximate theory of geometrical shock dynamics have been made to complement the study. Shock waves with circular or polygonal shock front shapes have been created and focused in a shock tube. With initial Mach numbers ranging from 2 to 4, the shock fronts accelerate as they converge. The shocked gas at the centre of convergence attains temperatures high enough to emit radiation which is visible to the human eye. The strength and duration of the light pulse due to shock implosion depends on the medium. In this study, shock waves converging in air and argon have been studied. In the latter case, the implosion light pulse has a duration of roughly 10 ?s. This enables non-intrusive spectrometric measurements on the gas conditions. Circular shock waves are very sensitive to disturbances which deform the shock front, decreasing repeatability. Shocks consisting of plane sides making up a symmetrical polygon have a more stable behaviour during focusing, which provides less run-to-run variance in light strength. The radiation from the gas at the implosion centre has been studied photometrically and spectrometrically. Polygonal shocks were used to provide better repeatability. The full visible spectrum of the light pulse created by a shock wave in argon has been recorded, showing the gas behaving as a blackbody radiator with apparent temperatures up to 6000 K. This value is interpreted as a modest estimation of the temperatures actually achieved at the centre as the light has been collected from an area larger than the bright gas core. As apparent from experimental data real gas effects must be taken into consideration for calculations at the implosion focal point. Ideal gas numerical and analytical solutions show temperatures and pressures approaching infinity, which is clearly not physical. Real gas effects due to ionisation of the argon atoms have been considered in the numerical work and its effect on the temperature has been calculated. The propagation of circular and polygonal have also been experimentally studied and compared to the self-similar theory and geometrical shock dynamics, showing good agreement.
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