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Energy concentration by converging shock waves in gases
KTH, School of Engineering Sciences (SCI), Mechanics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Converging shock waves have been studied experimentally in a shock tube, and numerically using inviscid calculations and the theory of geometrical shock dynamics. The converging shock waves were created in a shocktube with two modular test sections designed to create cylindrical respectively spherical waves. In the spherical case the shock waves take the shape of spherical cap before propagating into a cone, while the cylindrical shocks converge in a fully circular cylindrical chamber.

The dynamics and symmetry of circular and polygonal cylindrical shock waves with initial Mach numbers ranging from 2 to 4 were studied. 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, argon, nitrogen and propane have been studied. 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. 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 6,000 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. Circular shock waves attained higher temperatures but the run-to-run variation was significant. The propagation of circular and polygonal shocks was also studied using schlieren photography and compared to the self-similar theory and geometrical shock dynamics, showing good agreement.

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.

A second convergent test section was manufactured, designed to smoothly transform a plane shock wave into the shape of a spherical cap. After the convergent transformation the spherical shock propagates through a conical section, where it is aimed to retain the spherical shape and converge in the tip of the truncated cone, which has an end radius of 0.3 mm. Spherical implosion is more efficient than cylindrical and the target volume is much smaller than that in the cylindrical chamber. The new set-up does not suffer from large losses through reflections. Spectrometric and photometrical measurements of the implosion show significantly stronger radiation of longer duration. Preliminary results show measured apparent blackbody temperatures up to 27,000 K during implosion of shock waves of initial Mach number MS = 3.9.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , vii, 80 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:09
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:kth:diva-95186ISBN: 978-91-7501-368-8 (print)OAI: oai:DiVA.org:kth-95186DiVA: diva2:526904
Public defence
2012-05-25, D2, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note
QC 20120521Available from: 2012-05-21 Created: 2012-05-15 Last updated: 2012-05-21Bibliographically 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 strong imploding cylindrical and spherical shock waves with real gas effects
Open this publication in new window or tab >>Shock dynamics of strong imploding cylindrical and spherical shock waves with real gas effects
2010 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 11, 116102- p.Article in journal (Refereed) Published
Abstract [en]

Strong cylindrical and spherical shock implosion in a monatomic gas is considered A simple solution is obtained by Whitham's geometrical shock dynamics approach modified to account for the real gas effects The real gas effects are introduced by jump relations over the shock and include several levels of ionization, Coulomb interaction as well as internal energy of the excited electrons It is shown that ionization has a major effect on temperature and density behind the converging shock as well as on the shock acceleration The temperature and acceleration being substantially reduced and density substantially increased as compared to the ideal nonionizing case The ionization effect on the pressure behind the converging shock is less pronounced It is also shown that for the considered test case of initial Mach number M-0=8 the gas becomes completely ionized behind the spherical shock at approximately 1% of the initial radius from the focal point and its speed being decreased by a factor of 1 8 as compared to the ideal case

National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-30509 (URN)10.1063/1.3500684 (DOI)000285486600058 ()2-s2.0-79251560971 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110316Available from: 2011-03-16 Created: 2011-02-28 Last updated: 2017-12-11Bibliographically 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
4. Polygonal shock waves: comparison between experiments and geometrical shock dynamics
Open this publication in new window or tab >>Polygonal shock waves: comparison between experiments and geometrical shock dynamics
2012 (English)In: Proceedings of the 28th International Symposium on Shock Waves held in Manchester UK, 17 to 22 July 2011, Vol 2 / [ed] Konstantinos Kontis, Springer Berlin/Heidelberg, 2012Conference paper, Published paper (Other academic)
Place, publisher, year, edition, pages
Springer Berlin/Heidelberg, 2012
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-95183 (URN)10.1007/978-3-642-25685-1 (DOI)978-3-642-25684-1 (ISBN)978-3-642-25684-4 (ISBN)
Conference
28th International Symposium on Shock Waves
Note
QC 20120525Available from: 2012-05-15 Created: 2012-05-15 Last updated: 2012-05-21Bibliographically approved
5. Experimental determination of self-similarity constant for converging cylindrical shocks
Open this publication in new window or tab >>Experimental determination of self-similarity constant for converging cylindrical shocks
2011 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 11, 116103- p.Article in journal (Refereed) Published
Abstract [en]

Guderley's self-similarity solution r = r(0)(1 - t/t(0)) a for strong converging cylindrical shocks is investigated experimentally for three different gases with adiabatic exponents gamma = 1.13; 1.40; and 1.66 and various values of the initial Mach number. Corresponding values of the similarity exponent a which determines the strength of shock convergence are obtained for each gas thus giving the variation of alpha with gamma. Schlieren imaging with multiple exposure technique is used to track the propagation of a single shock front during convergence. The present experimental results are compared with previous experimental, numerical, and theoretical investigations.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-58822 (URN)10.1063/1.3662002 (DOI)000297939200036 ()2-s2.0-82855170856 (Scopus ID)
Note
QC 20120109Available from: 2012-01-09 Created: 2012-01-09 Last updated: 2017-12-08Bibliographically approved
6. Numerical assesment of shock tube with inner body designed to create cylindrical shock waves
Open this publication in new window or tab >>Numerical assesment of shock tube with inner body designed to create cylindrical shock waves
2012 (English)Report (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-95185 (URN)
Funder
Swedish Research Council
Available from: 2012-05-15 Created: 2012-05-15 Last updated: 2012-05-21Bibliographically approved
7. Generation of spherical converging shocks in a shock tube by wall shaping
Open this publication in new window or tab >>Generation of spherical converging shocks in a shock tube by wall shaping
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-95184 (URN)
Funder
Swedish Research Council
Available from: 2012-05-15 Created: 2012-05-15 Last updated: 2012-05-21Bibliographically approved

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