| 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. |
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| 5. |
- 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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| 6. |
- 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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| 7. |
- 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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| 8. |
- Mariani, Raffaello, et al.
(författare)
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Head On Collisions of Compressible Vortex Rings on a Smooth Solid Surface. : Effects of Surface Distance Variation.
- 2013
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153.
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Tidskriftsartikel (refereegranskat)abstract
- An experimental study has been conducted onthe effects of distance variation on the impingement processof compressible vortex rings on a stationary solid wall. An experimental incident Mach number of 1.61 was used. Qualitative and quantitative studies compared the impingement and interaction flow characteristics of a compressible vortex ring with a stationary, solid, smooth surface at three distances: 1.66, 3.33, and 5.00 inner diameters. The three distances corresponded to an under-developed vortex ring(1.66 inner diameters), a vortex ring at its development threshold (3.33 inner diameters), and a fully developed one (5.00 inner diameters). Qualitative schlieren results showed that the surface distance affected the shock/vortex interaction process along with the impingement process of the vortex ring and the flow structure of its trailing jet. Quantitative data were extrapolated to evaluate the propagation velocity of the vortex ring prior to impingement. The boundary layer thickness was also estimated. Particle image velocimetry studies showed the main and secondary vortices to have opposite vorticity, with the magnitude of the vorticity of the secondary vortices being approximately half of that of the main vortex. Surface pressure results reveal the symmetrical properties ofthe impinging flow, along with a direct correlation between the maximum pressure measured at the instant the vortex ring impingement and an increase in surface distance.
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| 9. |
- Ramachandra, K., et al.
(författare)
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Study of unsteadiness due to 3-D shock-boundary layer interaction in flow over a square-faced protuberance
- 2023
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Ingår i: Shock Waves. - : Springer Nature. - 0938-1287 .- 1432-2153. ; 33:7-8, s. 569-583
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of shock-induced unsteady separated flow past a three-dimensional square-faced protuberance are investigated through wind tunnel experiments. Time-resolved schlieren imaging and unsteady surface pressure measurements are the diagnostics employed. Dynamic mode decomposition (DMD) of schlieren snapshots and analysis of spectrum and correlations in pressure data are used to characterize and resolve the flow physics. The mean shock foot in the centreline is found to exhibit a Strouhal number of around 0.01, which is also the order of magnitude of the Strouhal numbers reported in the literature for two-dimensional shock-boundary layer interactions. The wall pressure spectra, in general, shift towards lower frequencies as one moves away (spanwise) from the centreline with some variation in the nature of peaks. The cross-correlation analysis depicts the strong dependence of the mean shock oscillations and the plateau pressure region, and disturbances are found to travel upstream from inside the separation bubble. Good coherence is observed between the spanwise mean shock foot locations till a Strouhal number of about 0.015 indicating that the three-dimensional shock foot largely moves to-and-fro in a coherent fashion.
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| 10. |
- Schedin, Staffan, et al.
(författare)
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Shock waves in an elliptical cavity with varying height
- 1997
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Ingår i: Shock Waves. - : Springer Science and Business Media LLC. - 0938-1287 .- 1432-2153. ; 7:6, s. 343-350
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Tidskriftsartikel (refereegranskat)abstract
- Transient shock waves in a confined elliptical chamber are experimentally investigated. Quantitative results of the pressure distribution are obtained for an air-filled cavity. Lower bounding surfaces of different geometrical shapes can be inserted making it possible to get chambers with varying height. An electrical discharge across a pair of electrodes inside the cavity gives rise to the shock waves. Double pulsed holographic interferometry is used to study the propagation and focusing process of the waves. The results are quantitatively evaluated by using the method of two-reference-beam holography. The angular pressure distribution behind the converging wave front is presented for different geometries of an air-filled cavity. The pressure distribution is non-homogeneous but symmetric along the wave front. The pressure level is higher for the geometry where the height of the chamber decreases with the radial distance from the outgoing focus and lower for increasing height of the chamber. In addition, shock waves in a water-filled cavity are studied. In this case qualitative results are obtained.
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