No abstract

No abstract

An analog quadrature encoder technique is coupled with a Michelson inter ferometer for capturing the refractive index in gases. When combined with a broadband source, this technique can also measure refractive index across discrete shock-wave boundaries.

The measurement of high temperature gas properties is key for characterizing high speed flows. Nearly discrete changes in density across shock waves, in particular, are difficult to resolve through traditional fringe-counting interferometric methods. Existing techniques for estimating large fringe jumps are either resolution limited or require specialized window configurations. In this Letter, we describe a unique hybrid interferometric technique that combines narrowband fringes for high resolution and broadband fringes as an absolute reference to measure changes in refractive index with a resolution of up to 7 × 10−8 across nearly discrete index changes of up to 1.5 × 10−4. By capturing fringes with an ultrahigh-speed camera, the refractive index changes across discrete shock fronts can be estimated inside a shock tube with high accuracy and time resolution. First, a novel hybrid calibration method for tracking finite fringes is discussed. Next, this technique is used to measure the post-initial-shock refractive indices for Mach 2.7 to 4.2 flows (pressures from 90.4 to 228.4 kPa). Results are then compared with theoretical values showing agreement within 2%.

Understanding the optical properties of air is essential for the validation and characterization of plasmas and hypersonic flows. Beyond 6000 K, the dissociation of nitrogen and oxygen molecules, along with other reactions, alters the equilibrium composition of air, causing a temperature and pressure dependence in the Gladstone–Dale coefficient. Due to measurement complexities, there is currently very little experimental data to validate model predictions under these conditions. In this work, a unique quadrature fringe imaging interferometer technique is applied to high temperature and pressure measurements of air in the Sandia free-piston high enthalpy shock tube. The diagnostic method combines a narrowband and broadband source to capture large, nearly-discrete changes in the index of refraction by calibrating to interference pattern changes. For the experiments, the reflected shock front is used to generate temperatures between 6000 and 7800 K at pressures up to 300 psi (20 bars). Results behind the shock front exhibit complex flow bifurcation and tail shock feature before equilibrium conditions are reached. Measurements in these flows show close agreement with theoretical predictions of the nonconstant Gladstone–Dale coefficient at high temperatures and high pressures, providing new validation data for chemical equilibrium gas models.