Observation of Shock Waves Using the Schlieren Method with the HPV-X3

Significance of the Topic

Observing shock waves with high spatial and temporal resolution is essential in aerospace, ballistics, fluid dynamics and materials testing because shock structure and transient behavior determine aerodynamic forces, fragmentation mechanisms and safety margins. Optical visualization techniques that detect small refractive-index gradients, especially the Schlieren method, provide non-intrusive access to density gradients in air and enable qualitative and semi-quantitative study of rapidly evolving shock phenomena.

Goals and Overview of the Study

This application note demonstrates use of the HyperVision HPV-X3 high-speed camera to visualize shock waves produced by pellets fired from a gas gun using a classical Schlieren arrangement. The aims were to (1) exploit the improved resolution and speed of the HPV-X3 to capture fine shock features, (2) visualize shock interaction with obstacles (rubber bands and a perforated plate) and (3) document fragmentation and subsequent shock generation from fragments.

Methodology

The experiment used a single-line Schlieren optical setup in which laser illumination was collimated between two concave mirrors, and the Schlieren image was formed using a knife-edge and a mirror arrangement to direct light into the HPV-X3 camera. Pellets were mounted in plastic holders in a gas-gun firing chamber; holders dropped away after launch leaving only the pellet to traverse the optical field. Two experimental conditions were recorded: (Imaging 1) the pellet passing through two rubber bands placed in its path, and (Imaging 2) the pellet impacting an obstacle plate with ten circular holes to provoke fragmentation and multi-source shock emission. Time spacing between captured frames in the experimental runs was on the order of microseconds (reported inter-image intervals ~15 μs for the rubber-band sequence and ~23 μs for the obstacle sequence).

Instrumentation

• High-speed camera: HyperVision HPV-X3 (noted improvement: ~3× resolution vs previous HPV-X2; maximum frame rate capability up to 20 Mfps)
• Lens: 180 mm macro lens with 2× teleconverter
• Illumination: Laser source for Schlieren illumination
• Schlieren optics: Two concave mirrors, knife-edge, and steering mirror to image into the camera
• Projectile system: Gas gun with pellet held in plastic holders; obstacles included rubber bands and a perforated plate

Main Results and Discussion

• Clear visualization of a leading shock wave ahead of the pellet was achieved. The Schlieren images revealed the shock front shape and its temporal evolution as the projectile moved through the field of view.
• Interaction with rubber bands: Upon encountering the first rubber band the incident shock split into transmitted and reflected components. Collision also produced local shocks both ahead of and behind the impact point. The second rubber band produced little observable shock reflection, interpreted as a consequence of reduced pellet speed after the first interaction.
• Interaction with perforated obstacle: Impact caused the pellet to fragment. Subsequent frames showed multiple fragments each generating their own shock waves. The experiment thus captured not only primary shock generation but also secondary shock sources arising from fragmentation.
• Temporal resolution allowed tracking of shock propagation and reflection events across successive frames (frame spacing ~15–23 μs in presented sequences). Spatial detail improved relative to earlier camera models, enabling finer observation of shock geometry and small fragmented pieces producing localized shocks.

Benefits and Practical Applications

• Enhanced spatio-temporal resolution supports more detailed qualitative analysis of shock structure, reflection, transmission and fragmentation dynamics—critical for ballistics testing, impact safety studies and aerodynamic design validation.
• Non-intrusive Schlieren imaging with a high-speed detector is well suited for experiments where placing probes is impractical or would perturb the flow.
• Data are useful for validating numerical models (CFD, shock–structure interaction simulations) and for informing materials and structural response evaluations under high-rate loading.

Limitations and Considerations

• Classical Schlieren provides line-of-sight integrated gradients of refractive index; quantitative density fields require additional calibration or alternative techniques (e.g., background-oriented Schlieren, interferometry, or tomographic approaches).
• Shock and fragment repeatability can vary shot-to-shot; statistical datasets or synchronized diagnostics (pressure transducers, high-rate photodiodes) are helpful for quantitative analysis.
• Illumination timing, camera exposure and optical alignment are critical to capture sharp images of very short-duration features; laser source stability and synchronization with the firing event must be managed carefully.

Future Trends and Applications

• Higher frame-rate and higher-resolution sensors will continue to improve capture of fine-scale shock and fragmentation dynamics, enabling more reliable quantitative extraction of shock speed and curvature.
• Multi-camera and tomographic Schlieren setups could reconstruct three-dimensional shock geometry and density fields, removing line-of-sight ambiguity.
• Combining high-speed Schlieren with complementary diagnostics—time-resolved pressure sensing, particle image velocimetry (PIV), or high-speed spectroscopy—will enable more comprehensive characterization of flow, chemical and mechanical responses.
• Advanced image processing and machine learning can enhance feature detection (shock fronts, reflections, fragment trajectories) and automate parameter extraction for large datasets.
• Applications will expand in defense testing, aerospace component validation, high-speed impact studies in material science and safety testing for protective structures.

Conclusion

Using the HPV-X3 camera with a Schlieren optical configuration, the study demonstrated improved visualization of pellet-induced shock waves, their reflection/transmission from obstacles, and shock generation from fragments following impact. The HPV-X3’s higher resolution and high frame-rate capability make it a valuable tool for detailed optical studies of high-speed compressible flows and impact phenomena, though quantitative density retrieval and full 3D characterization require complementary methods.

Reference

• K. Ohtani, High Speed Visualization Measurement of Underwater Shock Wave and Cavitation Bubble Generation, pages 17–22, 2023.