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Apazidis, Nicholas
Publications (4 of 4) Show all publications
Sembian, S., Liverts, M. & Apazidis, N. (2018). Plane blast wave interaction with an elongated straight and inclined heat-generated inhomogeneity. Journal of Fluid Mechanics, 851, 245-267
Open this publication in new window or tab >>Plane blast wave interaction with an elongated straight and inclined heat-generated inhomogeneity
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 851, p. 245-267Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
compressible flows, shock waves
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-232761 (URN)10.1017/jfm.2018.495 (DOI)000439307100008 ()2-s2.0-85050617477 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20180803

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-06Bibliographically approved
Sembian, S., Liverts, M. & Apazidis, N. (2018). Plane blast wave propagation in air with a transverse thermal inhomogeneity. European journal of mechanics. B, Fluids, 67, 220-230
Open this publication in new window or tab >>Plane blast wave propagation in air with a transverse thermal inhomogeneity
2018 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 67, p. 220-230Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Exploding wire, Shock broadening/splitting, Shock curvature, Thermal inhomogeneity, Shock waves, Wave propagation, Blast wave propagation, Exploding wire technique, Physical behaviors, Propagation characteristics, Shadowgraph method, Exploding wires
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-216806 (URN)10.1016/j.euromechflu.2017.09.011 (DOI)000418726900021 ()2-s2.0-85030309669 (Scopus ID)
Funder
Swedish Research Council, 621-2014-5678
Note

QC 20180111

Available from: 2017-12-05 Created: 2017-12-05 Last updated: 2018-05-18Bibliographically approved
Sembian, S., Liverts, M. & Apazidis, N. (2016). Attenuation of strong external blast by foam barriers. Physics of fluids, 28(9), Article ID 096105.
Open this publication in new window or tab >>Attenuation of strong external blast by foam barriers
2016 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 9, article id 096105Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Institute of Physics Inc, 2016
Keywords
Numerical models, Shock waves, A-plane, Aqueous foams, Blast waves, Exploding wire technique, Impedance mismatch factors, Shadowgraph technique, Test unit, Exploding wires
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-194875 (URN)10.1063/1.4963243 (DOI)000384878900039 ()2-s2.0-84989361083 (Scopus ID)
Note

QC 20161213

Available from: 2016-12-13 Created: 2016-11-01 Last updated: 2018-05-18Bibliographically approved
Eliasson, V., Apazidis, N. & Tillmark, N. (2006). Shaping converging shock waves by means of obstacles. Journal of Visualization, 9(3), 240-240
Open this publication in new window or tab >>Shaping converging shock waves by means of obstacles
2006 (English)In: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975, Vol. 9, no 3, p. 240-240Article in journal, Editorial material (Other academic) Published
Place, publisher, year, edition, pages
KTH Mech, SE-10044 Stockholm, Sweden.: SPRINGER, 2006
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-242436 (URN)10.1007/BF03181665 (DOI)000238663100003 ()
Note

QC 20190211

Available from: 2019-02-11 Created: 2019-02-11 Last updated: 2019-02-11Bibliographically approved
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