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On dynamics and thermal radiation of imploding shock waves
KTH, School of Engineering Sciences (SCI), Mechanics.
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Converging cylindrical shock waves have been studied experimentally. Numericalcalculations based on the Euler equations and analytical comparisons basedon the approximate theory of geometrical shock dynamics have been made tocomplement the study.Shock waves with circular or polygonal shock front shapes have been createdand focused in a shock tube. With initial Mach numbers ranging from 2 to4, the shock fronts accelerate as they converge. The shocked gas at the centreof convergence attains temperatures high enough to emit radiation which isvisible to the human eye. The strength and duration of the light pulse due toshock implosion depends on the medium. In this study, shock waves convergingin air and argon have been studied. In the latter case, the implosion lightpulse has a duration of roughly 10 μs. This enables non-intrusive spectrometricmeasurements on the gas conditions.Circular shock waves are very sensitive to disturbances which deform theshock front, decreasing repeatability. Shocks consisting of plane sides makingup a symmetrical polygon have a more stable behaviour during focusing,which provides less run-to-run variance in light strength. The radiation fromthe gas at the implosion centre has been studied photometrically and spectrometrically.Polygonal shocks were used to provide better repeatability. Thefull visible spectrum of the light pulse created by a shock wave in argon hasbeen recorded, showing the gas behaving as a blackbody radiator with apparenttemperatures up to 6000 K. This value is interpreted as a modest estimation ofthe temperatures actually achieved at the centre as the light has been collectedfrom an area larger than the bright gas core.As apparent from experimental data real gas effects must be taken intoconsideration for calculations at the implosion focal point. Ideal gas numericaland analytical solutions show temperatures and pressures approaching infinity,which is clearly not physical. Real gas effects due to ionisation of theargon atoms have been considered in the numerical work and its effect on thetemperature has been calculated.The propagation of circular and polygonal have also been experimentallystudied and compared to the self-similar theory and geometrical shock dynamics,showing good agreement.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2010. , vii, 73 p.
Series
Trita-MEK, ISSN 0348-467X ; 2010:03
Keyword [en]
converging shock waves, polygonal shock waves, temperature measurments, argon, plasma creation, ionisation, spectrometry
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12365ISBN: 978-91-7415-610-2 (print)OAI: oai:DiVA.org:kth-12365DiVA: diva2:310000
Presentation
2010-04-16, S40, KTH, Teknikringen 8D, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20110502Available from: 2010-04-12 Created: 2010-04-12 Last updated: 2011-05-02Bibliographically approved
List of papers
1. Thermal radiation from a converging shock implosion
Open this publication in new window or tab >>Thermal radiation from a converging shock implosion
2010 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 4, 046102- p.Article in journal (Refereed) Published
Abstract [en]

High energy concentration in gas is produced experimentally by focusing cylindrical shock waves in a specially constructed shock tube. The energy concentration is manifested by the formation of a hot gas core emitting light at the center of a test chamber at the instant of shock focus. Experimental and numerical investigations show that the shape of the shock wave close to the center of convergence has a large influence on the energy concentration level. Circular shocks are unstable and the resulting light emission varies greatly from run to run. Symmetry and stability of the converging shock are achieved by wing-shaped flow dividers mounted radially in the test chamber, forming the shock into a more stable polygonal shape. Photometric and spectroscopic analysis of the implosion light flash from a polygonal shock wave in argon is performed. A series of 60 ns time-resolved spectra spread over the 8 mu s light flash shows the emission variation over the flash duration. Blackbody fits of the spectroscopic data give a maximum measured gas temperature of 5800 K in the beginning of the light flash. Line emissions originating in transitions in neutral argon atoms from energy levels of up to 14.7 eV were also detected.

Keyword
aerospace components, argon, explosions, flow instability, flow visualisation, heat radiation, numerical analysis, shock tubes, shock waves
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-27927 (URN)10.1063/1.3392769 (DOI)000277242300036 ()2-s2.0-77953340892 (Scopus ID)
Note
QC 20110110Available from: 2011-01-10 Created: 2011-01-03 Last updated: 2017-12-11Bibliographically approved
2. Shock dynamics of imploding spherical and cylindrical shock waves with real gas effects
Open this publication in new window or tab >>Shock dynamics of imploding spherical and cylindrical shock waves with real gas effects
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-33293 (URN)
Note
QS 20110502Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2011-05-02Bibliographically approved
3. Regular versus Mach reflection for converging polygonal shocks
Open this publication in new window or tab >>Regular versus Mach reflection for converging polygonal shocks
2007 (English)In: Shock Waves, ISSN 0938-1287, E-ISSN 1432-2153, Vol. 17, no 1-2, 43-50 p.Article in journal (Refereed) Published
Abstract [en]

The onset of Mach reflection or regular reflection at the vertices of a converging polygonal shock wave was investigated experimentally in a horizontal annular shock tube. The converging shock waves were visualized by schlieren optics. Two different types of polygonal shock convergence patterns were observed. We compared the behavior during the focusing process for triangular and square-shaped shocks. It is shown that once a triangular shaped shock is formed, the corners in the converging shock will undergo regular reflection and consequently the shape will remain unaltered during the focusing process. A square-shaped shock suffers Mach reflections at the corners and hence a reconfiguring process takes place; the converging shock wave alternates between a square and an octagon formation during the focusing process.

Keyword
shock focusing; annular shock tube; imploding shock; shock reflection
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-7430 (URN)10.1007/s00193-007-0091-0 (DOI)000248819100004 ()2-s2.0-34547854293 (Scopus ID)
Note
QC 20100707Available from: 2007-08-31 Created: 2007-08-31 Last updated: 2012-05-21Bibliographically approved

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