Filming the invisible – time-resolved visualization of compressible flows

Abstract Essentially all processes in gasdynamics are invisible to the naked eye as they occur in a transparent medium. The task to observe them is further complicated by the fact that most of these processes are also transient, often with characteristic times that are considerably below the threshold of human perception. Both difficulties can be overcome by combining visualization methods that reveal changes in the transparent medium, and high-speed photography techniques that “stop” the motion of the flow. The traditional approach is to reconstruct a transient process from a series of single images, each taken in a different experiment at a different instant. This approach, which is still widely used today, can only be expected to give reliable results when the process is reproducible. Truly time-resolved visualization, which yields a sequence of flow images in a single experiment, has been attempted for more than a century, but many of the developed camera systems were characterized by a high level of complexity and limited quality of the results. Recent advances in digital high-speed photography have changed this situation and have provided the tools to investigate, with relative ease and in sufficient detail, the true development of a transient flow with characteristic time scales down to one microsecond. This paper discusses the potential and the limitations one encounters when using density-sensitive visualization techniques in time-resolved mode. Several examples illustrate how this approach can reveal and explain a number of previously undetected phenomena in a variety of highly transient compressible flows. It is demonstrated that time-resolved visualization offers numerous advantages which normally outweigh its shortcomings, mainly the often-encountered loss in resolution. Apart from the capability to track the location and/or shape of flow features in space and time, adequate time-resolved visualization allows one to observe the development of deliberately introduced near-isentropic perturbation wavelets. This new diagnostic tool can be used to qualitatively and quantitatively determine otherwise inaccessible thermodynamic properties of a compressible flow.

[1]  August Joseph Ignaz Toepler,et al.  Beobachtungen nach einer neuen optischen Methode : ein Beitrag zur Experimentalphysik , 1906 .

[2]  Kazuyoshi Takayama,et al.  The formation of a secondary shock wave behind a shock wave diffracting at a convex corner , 1997 .

[3]  T. Etoh,et al.  Cavitation induced by low-speed underwater impact , 2009 .

[4]  Claus-Dieter Ohl,et al.  Experimental and Theoretical Bubble Dynamics , 2007 .

[5]  S. Ray.  Scientific Photography and Applied Imaging , 1999 .

[6]  Kazuyoshi Takayama,et al.  Laboratory-scale blast wave phenomena – optical diagnostics and applications , 2005 .

[7]  H. Kleine,et al.  Unsteady flow diagnostics using weak perturbations , 2009 .

[8]  H. Schardin,et al.  Kinematographie auf ruhendem Film und mit extrem hoher Bildfrequenz , 1929 .

[9]  Frank K. Lu,et al.  Optical design of Cranz-Schardin cameras , 1997 .

[10]  Jean-Michel Desse,et al.  Real-time color holographic interferometry devoted to 2D unsteady wake flows , 2004, J. Vis..

[11]  H. Schardin.  Die Schlierenverfahren und ihre Anwendungen , 1942 .

[12]  M. Brouillette,et al.  Propagation and interaction of shock-generated vortices , 1997 .

[13]  B. W. Skews,et al.  The shape of a diffracting shock wave , 1967, Journal of Fluid Mechanics.

[14]  B. W. Skews,et al.  The perturbed region behind a diffracting shock wave , 1967, Journal of Fluid Mechanics.

[15]  J. P. Baird,et al.  Supersonic vortex rings , 1987, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[16]  Kohsei Takehara,et al.  High-Speed Imaging of Drops and Bubbles , 2008 .

[17]  Gary S. Settles,et al.  Optical measurement and scaling of blasts from gram-range explosive charges , 2007 .

[18]  P. Krehl,et al.  August Toepler — The first who visualized shock waves , 1995 .

[19]  K. Takayama,et al.  On the ongoing quest to pinpoint the location of RR-MR transition in blast wave reflections , 2009 .

[20]  G. Meier,et al.  An electronic Cranz–Schardin camera , 1991 .

[21]  Eadweard Muybridge,et al.  Animals in Motion , 1957 .

[22]  G. Settles.  Schlieren and shadowgraph techniques , 2001 .

[23]  John M. Dewey,et al.  An experimental investigation of the sonic criterion for transition from regular to Mach reflection of weak shock waves , 1989 .

[24]  John M. Dewey,et al.  Chapter 13.1 - Spherical Shock Waves: 13.1 Expanding Spherical Shocks (Blast Waves) , 2001 .

[25]  Kazuyoshi Takayama,et al.  Simultaneous Shadow, Schlieren and Interferometric Visualization of Compressible Flows , 2006 .

[26]  Kazuyoshi Takayama,et al.  Studies of the TNT equivalence of silver azide charges , 2003 .

[27]  B. Schmitz,et al.  How snapping shrimp snap: through cavitating bubbles. , 2000, Science.

[28]  Wolfgang Merzkirch,et al.  3 – Optical Flow Visualization , 1987 .

[29]  N. de Haas,et al.  Experimental Study of the Formation of a Vortex Ring at the Open End of a Cylindrical Shock Tube , 1952 .

[30]  Harald Kleine.  Chapter 5.1 - Measurement Techniques and Diagnostics: 5.1 Flow Visualization , 2001 .

[31]  Kohsei Takehara,et al.  Time-resolved visualization of shock–vortex systems emitted from an open shock tube , 2010, J. Vis..

[32]  Harald Kleine,et al.  Flow features resulting from shock wave impact on a cylindrical cavity , 2007, Journal of Fluid Mechanics.

[33]  Gary S. Settles,et al.  Schlieren imaging of loud sounds and weak shock waves in air near the limit of visibility , 2010 .

[34]  H. Maruyama,et al.  High-speed time-resolved color schlieren visualization of shock wave phenomena , 2005 .