阴影成像：一种可视化超音速流动特征的技术

Schlieren Imaging is a density-based flow visualization technique that uses changes in refractive index to reveal shock waves and expansion fans in supersonic flow. A collimated light source shines through the test section, and variations in air density distort the light beam. A knife-edge at the focal plane blocks deflected light, creating high and low intensity patterns that map air density changes, enabling visualization of aerodynamic features.

At Mach numbers above 0.3, air compressibility effects become significant because air undergoes substantial density changes at high speeds. These density variations create shock waves and expansion fans around moving bodies. Compressibility effects are critical in supersonic flow analysis because they directly influence pressure, temperature, and flow properties that Schlieren Imaging can detect and visualize.

The refractive index, defined as the ratio of light velocity in vacuum to its velocity in a medium, is proportional to air density. As air density changes within shock waves and expansion fans, the refractive index changes correspondingly. This proportional relationship allows Schlieren Imaging to detect and visualize density variations by measuring how light beams are distorted passing through regions of different refractive index.

When supersonic flow exceeds Mach 1.0, an oblique shock wave forms at the nose of a cone or wedge, causing abrupt changes in pressure, temperature, and density. Expansion fans form when supersonic flow turns around a convex corner, creating an infinite series of waves where pressure, density, and temperature decrease continuously while velocity increases. Both phenomena are visible using surface dye flow visualization qualitative method observe streakline patterns in supersonic flow.

Mach number gauges flight speed relative to the speed of sound (343 m/s). At Mach 1.0, an object travels at the speed of sound. Transonic speeds range from Mach 0.8 to 1.2, while supersonic speeds exceed Mach 1.2. This classification system is essential for understanding aerodynamic behavior, as compressibility effects and shock formation depend critically on Mach number ranges.

The knife-edge is positioned at the focal plane of the transmitted light and blocks some of the deflected light beams. By selectively blocking light, the knife-edge enhances image contrast, creating distinct high and low intensity patterns on the projection screen. Adjusting the knife-edge aperture allows operators to optimize image quality and clearly visualize shock waves and expansion fans in the flow field.

Schlieren Imaging provides highly accurate shock wave measurements. In experiments with Mach 2.0 flow over a 15-degree cone, the measured oblique shock angle was 33.6 degrees compared to the theoretical value of 33.9 degrees calculated from the Taylor-Maccoll Equation, yielding less than 1 percent error. This demonstrates Schlieren's precision for validating theoretical aerodynamic predictions.