Development of an ultra-high speed dynamic witness-plate particle image velocimeter for micro-detonator studies

An ultra-high speed particle image velocimetry (PIV) system is developed for measuring flow properties behind shock waves created by explosive detonators. Unique challenges are faced in extending the PIV technique to explosive output, including imaging in the presence of broadband explosive emission, nanosecond temporal resolution, and micro-scale spatial resolution. The outputs of explosive detonators are driven into dynamic witness plates, and time-resolved two-dimensional velocity-field measurements are made using the PIV system. The measurements represent the highest frame-rate PIV experiments ever performed and provide data describing the spatial profile of velocity in any direction within the experimental field-of-view.;As for the sub-microsecond properties of the flow behind detonator-induced shocks, the flow is unique in that the shock front acts as a flow boundary that moves much faster than the material flow itself, creating rapid time evolution that cannot be ignored in the PIV measurements. Further, the very small curvature of the moving shock, coupled with the change in density across it, acts like a strong, rapidly-changing lens that severely distorts the images of particles inside the shock and creates false motion of the images. These effects are analyzed and methods are developed to account for them in the experiments.;In the analysis, a Lagrangian model of the particle displacements in response to a moving shock provides the relationship between the measured displacement field and the true velocity field for the case of double-exposure PIV. It also suggests the possibility of measuring the velocity field using a single exposure and knowledge of the temporal evolution of the shock position. A dynamic mapping function between the object and image plane is developed to account for the additional light refraction at the shock-front boundary and is validated for a static case. The fact that the mapping function is dynamic leads to an interesting new equation relating the image displacements to the Eulerian velocity.;Application of the PIV system to exploding bridge wire output is demonstrated.