The thickness of detonation waves visualised by slight obstacles

On the Hydrodynamic Thickness of Cellular Detonations

“…Since they noted that in general the global chemical reaction length (typically a fraction of a cell width λ) is much shorter than the measured hydrodynamic thickness, the authors of [11] suggested that the appearance of the sonic plane could be linked to the equilibration of the mechanical fluctuations and dissipation of the turbulent energy into mean kinetic and internal energy. Edwards' estimates [11] for the location of the sonic surface were higher than the measurements of Vasil'ev [17] and Weber and Olivier [18], consistent with the fact the latter were only a lower bound estimate for the location of the real CJ surface. However, they agreed well with the earlier measurements of Soloukhin [8] for the onset of the Taylor expansion, whose results also reveal that the limiting characteristic is situated approximately at 4λ downstream of the leading front [8].…”

“…Since the sonic surface also represents a first-order discontinuity in the wake of a detonation, this discontinuity could perhaps be captured experimentally from mean measured profiles of pressure or density, as attempted by Edwards [11] and Soloukhin [8]. Also, the determination of the sonic plane in the laboratory frame using the bow shock technique, pioneered by Vasil'ev et al [17] and refined by Weber and Olivier [18] can also be used although it does not provide the location of the true sonic surface that serves as a limiting characteristic.…”

“…Weber and Olivier [18] reported better definition of the bow shock wave when multiple needles are used, although, as can be seen from Figure 8, the weak shock generated by the array of needles, clearly seen on frames 5 to 8, is far from being planar and hence cannot provide a very accurate measurement. The measurements for the location of the laboratory sonic plane of Vasil'ev [17] and Weber and Olivier [18] varied between 0.5 and 3 detonation cell widths λ, with Interaction of a C2H2-air detonation wave with multiple obstacles illustrating the influence of threedimensional details of the wave structure in the determination of the laboratory frame sonic surface (from [18]). the upper bound corresponding to conditions removed from marginality where the tube diameter is larger than a few characteristic cell sizes.…”

“…For large cell sizes compared to the dimension of the blunt body and for large hydrodynamic fluctuations (e.g., [16]), it may not be possible to detect a meaningful averaged one-dimensional steady flow at one location in the highly transient three-dimensional flow field. In a more recent study, however, Weber and Olivier [18] used an array of fine needles to seek an improved averaging process. Figures 7 and 8 show schlieren photographs with a single and multiple blunt bodies in the flow.…”

“…7. Interaction of a H2-O2-Ar detonation wave with an obstacle illustrating the determination of the laboratory frame sonic point at a single location (from [18]). turbulent detonation, however, tracking the local dynamics of a characteristic surface would be much more difficult.…”