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  • 1.
    Sembian, Sundarapandian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Liverts, Michael
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Attenuation of strong external blast by foam barriers2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 9, article id 096105Article in journal (Refereed)
    Abstract [en]

    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.

  • 2.
    Sembian, Sundarapandian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Liverts, Michael
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Plane blast wave interaction with an elongated straight and inclined heat-generated inhomogeneity2018In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 851, p. 245-267Article in journal (Refereed)
    Abstract [en]

    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.

  • 3.
    Sembian, Sundarapandian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Liverts, Michael
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Plane blast wave propagation in air with a transverse thermal inhomogeneity2018In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 67, p. 220-230Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Sembian, Sundarapandian
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Liverts, Michael
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Plane shock wave interaction with a cylindrical water column2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 5, article id 056102Article in journal (Refereed)
    Abstract [en]

    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.

  • 5.
    Sundarapandian, Sembian
    KTH, School of Engineering Sciences (SCI), Mechanics.
    An experimental time-based analysis and numerical parameter study on shock-water column interactionManuscript (preprint) (Other academic)
  • 6.
    Sundarapandian, Sembian
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Plane blast wave interaction with an elongated straight and inclined heat-generated inhomogeneityManuscript (preprint) (Other academic)
  • 7.
    Sundarapandian, Sembian
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Strong blast wave interaction with multiphase media2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

1 - 7 of 7
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  • fi-FI
  • nn-NO
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  • text
  • asciidoc
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