| 1. |
- Apazidis, Nicholas, et al.
(författare)
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An experimental and theoretical study of converging polygonal shock waves
- 2002
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 12:1, s. 39-58
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Tidskriftsartikel (refereegranskat)abstract
- An experimental investigation was carried out to explore the possibility of producing converging polygonal shocks in an essentially two-dimensional cavity. Previous calculations by Apazidis and Lesser (1996) suggested that such configurations could be produced by reflecting a cylindrical outgoing shock from a smoothly altered circular boundary, the alteration having n-gonal symmetry. In the experiments the outgoing shock was produced by a spark discharge which yielded shocks in the Mach number range from 1.1 to 1.7 at a radius just prior to the reflection. Polygonal shocks were observed as predicted by using a modified form of geometrical shock dynamics, derived in the above paper. In addition, the modified theory was used to calculate the results of an experiment carried out by Sturtevant and Kulkarny (1976). The results of the numerical calculations were found to be in substantial agreement with both experiments, suggesting that the modifications in geometrical shock dynamics for non-uniform flow ahead of an advancing shock are useful in the case of shock focusing. The experiment. also showed that the polygonal shapes were stable in the examined range of shock Mach numbers. a result that may be of importance for a number of practical situations in which shock focusing is present.
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| 2. |
- Apazidis, Nicholas
(författare)
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Focusing of strong shocks in an elliptic cavity
- 2003
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 13:2, s. 91-101
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Tidskriftsartikel (refereegranskat)abstract
- Focusing of strong shock waves in a gas-filled thin chamber with an elliptic boundary is investigated theoretically and numerically. The process of reflection and convergence of cylindrical shocks generated at one of the focal points of the chamber is studied by means of Whitham's theory of geometrical shock dynamics (1957, 1959). The current calculations are based on the modified version of this theory, which takes into account the non-homogeneous flow ahead of the advancing shock, Apazidis and Lesser (1996). There are two main results of the present study indicating a qualitatively different behavior of strong shocks as compared to the linear acoustic case. (1) - the form of the reflected shock is not cylindrical and (2) - the reflected, converging shock does not focus at the geometrical second focal point of the chamber. The dependence of the form of the reflected shock and the location of its focusing point on the strength of the shock and the eccentricity of the chamber is investigated.
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| 3. |
- Apazidis, Nicholas, et al.
(författare)
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High energy concentration by symmetric shock focusing
- 2013
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 23:4, s. 361-368
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Tidskriftsartikel (refereegranskat)abstract
- High-energy concentrations in gas are achieved experimentally in a specially constructed shock tube facility at KTH Mechanics. The high-energy concentration is manifested by a formation of a hot, light-emitting gas core. Experimental, numerical and theoretical investigations show that the shape of the imploding shock is of pivotal importance for the final energy concentration. Cylindrical shocks are unstable. Symmetric polygonal shocks are shown to be dynamically stable and are produced by various methods, e.g. thin wing profiles placed radially in the test section. Such symmetric polygonal shocks are able to produce extremely high energy levels at the focal point. Spectral data from 60 nanosecond short intervals of 8 microsecond light pulse give temperatures in the range of 6,000 K.
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| 4. |
- Apazidis, Nicholas
(författare)
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Numerical investigation of shock induced bubble collapse in water
- 2016
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Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 28:4
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Tidskriftsartikel (refereegranskat)abstract
- A semi-conservative, stable, interphase-capturing numerical scheme for shock propagation in heterogeneous systems is applied to the problem of shock propagation in liquid-gas systems. The scheme is based on the volume-fraction formulation of the equations of motion for liquid and gas phases with separate equations of state. The semi-conservative formulation of the governing equations ensures the absence of spurious pressure oscillations at the material interphases between liquid and gas. Interaction of a planar shock in water with a single spherical bubble as well as twin adjacent bubbles is investigated. Several stages of the interaction process are considered, including focusing of the transmitted shock within the deformed bubble, creation of a water-hammer shock as well as generation of high-speed liquid jet in the later stages of the process.
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| 5. |
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| 6. |
- Apazidis, Nicholas, et al.
(författare)
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Supersonic jet by blast wave focusing
- 2021
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:12, s. 126101-
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Tidskriftsartikel (refereegranskat)abstract
- A supersonic jet of Mach number M = 4.5 in air is produced experimentally at the apex of a miniature 150 x 50 x 5 mm converging section with a 2 x 5 mm opening by the principle of blast wave amplification through focusing. An initial plane blast wave of M = 2.4 in the convergence section is generated by the exploding wire technique. The profile of the convergence section is specially tailored to smoothly transform a plane blast wave into a perfectly cylindrical arc, imploding at the apex of the section. The cylindrical form of the imploding shock delivers maximum shock amplification in the two-dimensional test section and maximum subsequent jet flow velocity behind the shock front. Blast wave propagation in the convergence chamber as well as jet generation through a 2 mm opening at the apex into the adjacent exhaust chamber is optically captured by a high-speed camera using the shadowgraph method. Visualizing the flow provided a distinct advantage not only for obtaining detailed information on the flow characteristics but also for validating the numerical scheme which further enhanced the analysis. Experimental images together with the numerical analysis deliver detailed information on the blast wave propagation and focusing as well as subsequent jet initiation and development. One of the main advantages of the described method apart from being simple and robust is the effective focusing of low initial input energy levels of just around 500 Joules, resulting in production of supersonic jets in a small confined chamber.
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| 7. |
- Eliasson, Veronica, et al.
(författare)
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Controlling the form of strong converging shocks by means of disturbances.
- 2007
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 17:1-2, s. 29-42
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Tidskriftsartikel (refereegranskat)abstract
- The influence of artificial disturbances on the behavior of strong converging cylindrical shocks is investigated experimentally and numerically. Ring-shaped shocks, generated in an annular cross sectional shock tube are transformed to converging cylindrical shocks in a thin cylindrical test section, mounted at the rear end of the shock tube. The converging cylindrical shocks are perturbed by small cylinders placed at different locations and in various patterns in the test section. Their influence on the shock convergence and reflection process is investigated. It is found that disturbances arranged in a symmetrical pattern will produce a symmetrical deformation of the converging shockfront. For example, a square formation produces a square-like shock and an octagon formation a shock with an octagonal front. This introduces an alternative way of tailoring the form of a converging shock, instead of using a specific form of a reflector boundary. The influence of disturbances arranged in non-symmetric patterns on the shape of the shockfront is also investigated.
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| 8. |
- Eliasson, Veronica, et al.
(författare)
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Focusing of strong shocks in an annular shock tube
- 2007
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 15:3-4, s. 205-217
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Tidskriftsartikel (refereegranskat)abstract
- Focusing of strong shock waves in a gas-filled thin convergence chamber with various forms of the reflector boundary is investigated experimentally and numerically. The convergence chamber is mounted at the end of the horizontal co-axial shock tube. The construction of the convergence chamber allows the assembly of the outer chamber boundaries of various shapes. Boundaries with three different shapes have been used in the present investigation-a circle, an octagon and a smooth pentagon. The shock tube in the current study was able to produce annular shocks with the initial Mach number in the range M-s = 2.3-3.6. The influence of the shape of the boundary on the shape and properties of the converging and reflected shock waves in the chamber has then been investigated both experimentally and numerically. It was found that the form of the converging shock is initially governed by the shape of the reflector and the nonlinear interaction between the shape of the shock and velocity of shock propagation. Very close to the center of convergence the shock obtains a square-like form in case of a circular and octagonal reflector boundary. This is believed to stem from the instability of the converging shock front triggered by the disturbances in the flow field. The outgoing, reflected shocks were also observed to be influenced by the shape of the boundary through the flow ahead as created by the converging shocks.
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| 9. |
- Eliasson, Veronica, et al.
(författare)
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Light emission during shock wave focusing in air and argon
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:10, s. 106106-1-106106-17
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Tidskriftsartikel (refereegranskat)abstract
- The light emission from a converging shock wave was investigated experimentally. Results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The light emission appears as the shock wave collapses. The full width at half maximum of the light pulse is about 200 ns for all geometrical shapes. It was also found that argon as a test gas produces more light than air. Numerical simulations showed good agreement with experimental results regarding the shape of the shock and the flow field behind the shock. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there.
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| 10. |
- Eliasson, Veronica, 1978-
(författare)
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On focusing of shock waves
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Both experimental and numerical investigations of converging shock waves have been performed. In the experiments, a shock tube was used to create and study converging shock waves of various geometrical shapes. Two methods were used to create polygonally shaped shocks. In the first method, the geometry of the outer boundary of the test section of the shock tube was varied. Four different exchangeable shapes of the outer boundary were considered: a circle, a smooth pentagon, a heptagon, and an octagon. In the second method, an initially cylindrical shock wave was perturbed by metal cylinders placed in various patterns and positions inside the test section. For three or more regularly spaced cylinders, the resulting diffracted shock fronts formed polygonal shaped patterns near the point of focus. Regular reflection was observed for the case with three cylinders and Mach refection was observed for cases with four or more cylinders. When the shock wave is close to the center of convergence, light emission is observed. An experimental investigation of the light emission was conducted and results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The shock wave focusing was also studied numerically using the Euler equations for a gas obeying the ideal gas law with constant specific heats. Two problems were analyzed; an axisymmetric model of the shock tube used in the experiments and a cylindrical shock wave diffracted by cylinders in a two dimensional test section. The results showed good agreement with the experiments. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there. As a practical example of shock wave focusing, converging shocks in an electrohydraulic lithotripter were simulated. The maximum radius of a gas bubble subjected to the pressure field obtained from the lithotripter was calculated and compared for various geometrical shapes and materials of the reflector. Results showed that the shape had a large impact while the material did not influence the maximum radius of the gas bubble.
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| 11. |
- Eliasson, Veronica, 1978-
(författare)
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On focusing of strong shock waves
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Focusing of strong shock waves in a gas-filled thin test section with various forms of the reflector boundary is investigated. The test section is mounted at the end of the horizontal co-axial shock tube. Two different methods to produce shock waves of various forms are implemented. In the first method the reflector boundary of the test section is exchangeable and four different reflectors are used: a circle, a smooth pentagon, a heptagon and an octagon. It is shown that the form of the converging shock wave is influenced both by the shape of the reflector boundary and by the nonlinear dynamic interaction between the shape of the shock and the propagation velocity of the shock front. Further, the reflected outgoing shock wave is affected by the shape of the reflector through the flow ahead of the shock front. In the second method cylindrical obstacles are placed in the test section at various positions and in various patterns, to create disturbances in the flow that will shape the shock wave. It is shown that it is possible to shape the shock wave in a desired way by means of obstacles. The influence of the supports of the inner body of the co-axial shock tube on the form of the shock is also investigated. A square shaped shock wave is observed close to the center of convergence for the circular and octagonal reflector boundaries but not in any other setups. This square-like shape is believed to be caused by the supports for the inner body. The production of light, as a result of shock convergence, has been preliminary investigated. Flashes of light have been observed during the focusing and reflection process.
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| 12. |
- Eliasson, Veronica, et al.
(författare)
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Regular versus Mach reflection for converging polygonal shocks
- 2007
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 17:1-2, s. 43-50
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Eliasson, V, et al.
(författare)
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Shaping converging shock waves by means of obstacles
- 2006
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Ingår i: Journal of Visualization. - KTH Mech, SE-10044 Stockholm, Sweden. : SPRINGER. - 1343-8875 .- 1875-8975. ; 9:3, s. 240-240
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 14. |
- Essén, Hanno, et al.
(författare)
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Turning points of the spherical pendulum and the golden ratio
- 2009
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Ingår i: European journal of physics. - : IOP Publishing. - 0143-0807 .- 1361-6404. ; 30:2, s. 427-432
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Tidskriftsartikel (refereegranskat)abstract
- We study the turning point problem of a spherical pendulum. The special cases of the simple pendulum and the conical pendulum are noted. For simple initial conditions the solution to this problem involves the golden ratio, also called the golden section, or the golden number. This number often appears in mathematics where you least expect it. To put our result in perspective we briefly discuss its relevance in physics.
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| 15. |
- Kjellander, Malte
(författare)
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Energy concentration by converging shock waves in gases
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 16. |
- Kjellander, Malte, et al.
(författare)
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Energy concentration by spherical converging shocks generated in a shock tube
- 2012
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 24:12, s. 126103-
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Tidskriftsartikel (refereegranskat)abstract
- Spherical converging shock waves are produced in a conventional shock tube with a circular cross-section. Initially, plane shocks are transformed into the shape of a spherical cap by means of a smoothly convergent cross-section. The wall shape in the transformation section is designed to gradually change the form of the shock wave until it approaches a spherical shape. Thereafter, the shock enters a conical section where it converges towards the apex of the cone. Numerical calculations with the axisymmetric Euler equations show that the spherical form is only slightly dependent on the initial Mach number of the plane shock within the range 1.5 < MS < 5.5, and is preserved to a close vicinity of the focal point. The test gas is heated to very high temperatures as a result of shock convergence and emits a bright light pulse at the tip of the test section. The light radiation is collected by optical fibers mounted at the tip of the convergence chamber and investigated by photometric and spectroscopic measurements. Experiments are performed with argon and nitrogen and with different initial Mach numbers. The radiation of the shock-heated argon closely resembles blackbody radiation. Fits to the experimental data result in apparent blackbody temperatures in argon of up to ~27 000 K, some 250 ns after the focusing instant. The initial Mach number in these spectrometric runs is MS = 3.9, indicating an efficient amplification of the shock wave strength.
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| 17. |
- Kjellander, Malte, et al.
(författare)
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Experimental determination of self-similarity constant for converging cylindrical shocks
- 2011
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 23:11, s. 116103-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 18. |
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| 19. |
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| 20. |
- Kjellander, Malte
(författare)
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On dynamics and thermal radiation of imploding shock waves
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 21. |
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| 22. |
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| 23. |
- Kjellander, Melte, et al.
(författare)
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Shock dynamics of strong imploding cylindrical and spherical shock waves with real gas effects
- 2010
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 22:11, s. 116102-
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 24. |
- Kjellander, Malte, et al.
(författare)
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Thermal radiation from a converging shock implosion
- 2010
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 22:4, s. 046102-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 25. |
- Kulkarni, Rohan Ajit, et al.
(författare)
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Experimental studies of dynamic compression of cellulose pulp fibers
- 2023
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Ingår i: Sustainable Materials and Technologies. - : Elsevier BV. - 2214-9937. ; 38
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Tidskriftsartikel (refereegranskat)abstract
- The ability to control the structure of the wood-pulp fiber cell wall is an attractive means to obtain increased accessibility to the fiber interior, providing routes for functionalization of the fibers that support further processing and novel material concepts, e.g. improved degree of polymerization, nanofiltration as demonstrated in previous studies. It has been proposed that dynamic compression and decompression of the cellulose pulp fibers in the wet state make it possible to modify the cell wall significantly. We hypothesize that hydrostatic pressure exerted on fibers fully submerged in water will increase the accessibility of the fiber wall by penetrating the fiber through weak spots in the cell wall. To pursue this, we have developed an experimental facility that can subject wet cellulose pulp samples to a pressure pulse -10 ms long and with a peak pressure of -300 MPa. The experiment is thus specifically designed to elucidate the effect of a rapid high-pressure pulse passing through the cellulose sample and enables studies of changes in structural properties over different size ranges. Different characterization techniques, including Scanning electron microscopy, X-ray diffraction, and wide- and small-angle X-ray scattering, have been used to evaluate the material exposed to pulsed pressure. The mechanism of pressure build-up is estimated computationally to complement the results. Key findings from the experiments consider a decrease in crystallinity and changes in the surface morphology of the cellulose sample.
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| 26. |
- Liverts, Michael, et al.
(författare)
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Limiting Temperatures of Spherical Shock Wave Implosion
- 2016
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 116:1
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Tidskriftsartikel (refereegranskat)abstract
- Spherical shock wave implosion in argon is studied both theoretically and experimentally. It is shown that as the strength of the converging shock increases the nonideal gas effects become dominant and govern the evolution of thermal and transport gas properties limiting the shock acceleration, lowering the gas adiabatic index and the achievable energy density at the focus. Accounting for multiple-level ionization, excitation, Coulomb interaction and radiation effects, the limiting equilibrium temperatures to be achieved during the shock implosion are estimated. Focal temperatures of the order of 30 000 K are measured in experiments where converging spherical shock waves are created using a conventional gas-dynamic shock tube facility.
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| 27. |
- Liverts, Michael, et al.
(författare)
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Mitigation of exploding-wire-generated blast-waves by aqueous foam
- 2015
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 27:7
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we implement an exploding wire technique to generate small-scale cylindrical blast waves in aqueous foam. The exploding wire system offers an easy to operate and effective tool for studying blast-wave/foam interaction related phenomena in real explosion scenarios. The mitigation of blast waves as a function of the thickness of the foam barrier is discussed and quantified. A fluid mixture pseudo-gas based numerical approach with the aid of the point explosion theory is used to separate the mitigation mechanisms into the near-and the far-field related groups and to analyze the contribution of each group to the overall losses of the blast wave energy.
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| 28. |
- Sembian, Sundarapandian, et al.
(författare)
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Attenuation of strong external blast by foam barriers
- 2016
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Ingår i: Physics of fluids. - : American Institute of Physics Inc. - 1070-6631 .- 1089-7666. ; 28:9
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Tidskriftsartikel (refereegranskat)abstract
- The mitigation of externally generated strong blast waves by an aqueous foam barrier of varying configurations within fixed distance between the explosion origin and the object to be protected is investigated and quantified both experimentally and numerically. The blast waves of shock Mach number 4.8 at 190 mm from the explosion plane are generated using exploding wire technique. The initially cylindrical blast waves are transformed into a plane blast wave in a specially constructed test unit in which the experiments are performed. The shock waves emanating from the foam barrier are captured using shadowgraph technique. A simple numerical model treating the foam by a pseudo-gas approach is used in interpreting and reconstructing the experimental results. The additional contribution of the impedance mismatch factor is analysed with the aid of numerical simulation and exploited for achieving greater blast wave pressure reduction.
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| 29. |
- Sembian, Sundarapandian, et al.
(författare)
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Plane blast wave interaction with an elongated straight and inclined heat-generated inhomogeneity
- 2018
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 851, s. 245-267
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Tidskriftsartikel (refereegranskat)abstract
- The unstable evolution of an elongated elliptically shaped inhomogeneity that is embedded in ambient air and aligned both normal and at an angle to an incident plane blast wave of impact Mach number 2.15 is investigated both experimentally and numerically. The elliptic inhomogeneities and the blast waves are generated using gas heating and exploding wire technique and their interaction is captured optically using shadowgraph method. While two symmetric counter-rotating vortices due to Richtmyer-Meshkov instability are observed for the straight interaction, the formation of a train of vortices similar to Kelvin-Helmholtz instability, introducing asymmetry into the flow field, are observed for an inclined interaction. During the early phase of the interaction process in the straight case, the growth of the counter-rotating vortices (based on the sequence of images obtained from the high-speed camera) and circulation (calculated with the aid of numerical data) are found to be linear in both space and time. Moreover, the normalized circulation is independent of the inhomogeneity density and the ellipse thickness, enabling the formulation of a unique linear fit equation. Conversely, the circulation for an inclined case follows a quadratic function, with each vortex in the train estimated to move with a different velocity directly related to its size at that instant. Two factors influencing the quadratic nature are identified: the reduction in strength of the transmitted shock thereby generating vortices with reduced vorticity, along with the gradual loss of vorticity of the earlier-generated vortices.
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| 30. |
- Sembian, Sundarapandian, et al.
(författare)
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Plane blast wave propagation in air with a transverse thermal inhomogeneity
- 2018
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 67, s. 220-230
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Tidskriftsartikel (refereegranskat)abstract
- An alternate mechanism explaining the shock broadening and splitting effects observed during its propagation through an elongated region with transverse thermal inhomogeneity is described. The shock wave is generated by exploding wire technique and its propagation is captured optically using shadowgraph method. Visualizing the flow provided distinct advantage not only for obtaining detailed information on the propagation characteristics but also for validating the numerical scheme used in the analysis. Three physical features namely shock jump, precursor region and vorticity induced flow, are identified to contribute to the shock structure with the latter two being responsible for the pressure profile ‘broadening’. The physical behavior of the incident shock is also analyzed along with other factors like temperature and curvature effects.
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| 31. |
- Sembian, Sundarapandian, et al.
(författare)
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Plane shock wave interaction with a cylindrical water column
- 2016
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Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 28:5
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Tidskriftsartikel (refereegranskat)abstract
- A complex system of waves propagating inside a water column due to the impact of plane shock wave is investigated both experimentally and numerically. Flow features, such as, focusing of expansion waves generating large negative pressure, nucleation of cavitation bubbles, and a re-circulation zone are observed and discussed qualitatively and quantitatively. Experiments are conducted on a 22 mm diametrical water column hit by shock waves with Mach numbers 1.75 and 2.4 in a newly constructed exploding wire facility. A new technique to create a properly shaped, repeatable, large diameter water column with straight walls is presented. Qualitative features of the flow are captured using the shadowgraph technique. With the aid of numerical simulations the wave motions inside the column are analyzed; the spatial location of the expansion wave focusing point and the corresponding negative peak pressures is estimated.
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| 32. |
- Sundarapandian, Sembian
(författare)
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Strong blast wave interaction with multiphase media
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interaction of a blast wave propagating in air with different fluids like water column, aqueous foam and thermal/density inhomogeneity have been studied both experimentally and numerically. The blast waves were generated at atmospheric conditions in a newly constructed exploding wire facility. For fixed capacitance and wire size, the intensity of the shock front (measured typically at 200 mm from the wire explosion plane) was varied by controlling the charges stored in the capacitor and the size of the test section. Qualitative features of the interaction were captured using shadowgraph technique. Numerical simulations were performed to better analyze and understand the flow features observed in experiments. The main points across each fluid interactions are as follow:Water column: A new technique was implemented to create highly repeatable, properly shaped, large diameter water column. The impact of a blast wave with shock Mach number ranging from 1.75 to 2.4 on a 22 mm diameter water column resulted in a complex system of waves propagating inside the column. Due to the concave boundary of the downstream interface, the reflected expansion wave naturally focused at a point before travelling upstream resulting in the generation of large negative pressures leading to nucleation of cavitation bubbles. Through high speed photography, various aspects of the flow features were discussed qualitatively and quantitatively. With the aid of numerical simulation, the effect of size of water column and shock strength on the maximum attainable negative pressures in the absence of cavitation were quantified.Aqueous foam: The performance of various aqueous foam barrier configurations on the attenuation of externally generated blast wave peak pressure was examined. Here a blast wave with shock Mach number 4.8 was allowed to interact with an aqueous foam barrier of initial liquid fraction 0.1. The dominant process responsible for reduction of peak pressure was the `catching up' of the rarefaction wave with the wave front travelling in the foam barrier. Additional reduction was provided by the impedance mismatch factor at the foam-air interface which was further exploited to achieve greater reduction. A simple numerical model treating the foam by a pseudo-gas approach was used for re-constructing the experimental results.Density inhomogeneity: The unstable evolution of a 2D elongated, elliptically-shaped inhomogeneity embedded in ambient air and aligned both normal and at an angle to the incident plane blast wave of impact Mach number 2.15 was studied. The inhomogeneity was created on the basis of `Joule heating' wherein heat produced by a current carrying wire was used to heat its surrounding air. Two counter-rotating vortices primarily due to Richtmyer-Meshkov instability (RMI) and a train of vortices primarily due to Kelvin-Helmholtz instability (KHI) were observed for two different inclination angles. Similarly circulation, calculated from numerical simulation solving Navier-Stokes equation, was also found to vary from a linear to a quadratic function when the inhomogeneity was inclined.
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