Digital self-aligned focusing schlieren.

In this Letter, a digital self-aligned focusing schlieren (D-SAFS) system is introduced. This system uses a digital transparent micro liquid crystal display (µLCD), in combination with a linear polarizer, to act on the linear polarization state of light transmitted in both the forward and reverse directions, essentially acting as both the source and cutoff grids. The use of the µLCD display allows for on-the-fly changes to the cutoff pattern type, spatial frequency, and orientation. This eliminates the need to physically access the source/cutoff grid in order to optimize the instrument's sensitivity, which is necessary with a conventional self-aligned focusing schlieren (SAFS) system.

Velocity measurements in a hypersonic flow using acetone molecular tagging velocimetry.

In the present work, a non-intrusive diagnostic technique known as molecular tagging velocimetry was used to collect quantitative freestream velocity measurements in the Mach 7 Ludwieg Tube Wind Tunnel located at The University of Texas at San Antonio. This laser-based diagnostic technique used a single Nd:YAG 4th harmonic laserline to excite acetone molecules seeded in the flow field. From the resulting emitted light, mean and instantaneous velocity profiles in the hypersonic freestream flow and facility boundary layer were measured. Uncertainty in the velocity measurements for individual test runs is estimated at ≤ 8% while overall 1D freestream mean velocity measurements were recorded with ±2.4% (± 21 m/s) accuracy. The effect of acetone seeding on the speed of sound was also quantified.