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.

Colinear focused laser differential interferometry and self-aligned focusing schlieren.

A colinear focused laser differential interferometer (FLDI) and self-aligned focusing schlieren (SAFS) system has been assembled and tested in the laboratory, using a turbulent jet of compressed air issuing from a small needle nozzle to provide a high frequency density object. Measurements verified that the coupling of the SAFS system onto the optical axis of the FLDI system had negligible influence on the FLDI system's data, including tests that assessed the influence of the inclusion of dichroic mirrors, dichroic mirror reflection angle, dichroic mirror positioning relative to the Wollaston prisms of the FLDI system, and SAFS light propagation direction. A qualitative comparison of the focusing ability of the two systems was made, and FLDI power spectral density estimates and SAFS spectral proper orthogonal decomposition were used for quantitative comparisons of the acoustic frequency of the jet, with good agreement between the two. The success of the system integration and resulting jet testing demonstrates the utility of this colinear, simultaneous FLDI/SAFS measurement system.

Projection background-oriented schlieren.

A background-oriented schlieren (BOS) system is developed with two benefits over traditional BOS systems. First, the dot pattern required for BOS is projected onto a retroreflective background instead of being painted/printed onto the material itself, allowing for on-the-fly updates to the size and distribution of the dot pattern. Second, a reference image is acquired for every flow image so that real-time BOS images can be displayed, and a flow-off reference image need not be acquired if the projected dot pattern is changed during a run for BOS signal optimization. The system can be made very compact, can be converted quickly to operate as a shadowgraph system, and can be integrated with polarization optics that reduce glare/reflections from wind tunnel windows.

Compact, self-aligned focusing schlieren system.

A novel, to the best of our knowledge, compact, self-aligned focusing schlieren system is presented that eliminates the need for a separate source grid and cutoff grid. A single grid element serves both to generate a projected source grid onto a retroreflective background and act as the cutoff grid for the reflected light. This is made possible by manipulating the polarization of light through the system. The use of only a single grid element eliminates the need to create a cutoff grid that is perfectly matched and scaled to the source grid, and removes the need to align the source and cutoff grids to each other. The sensitivity to density objects is adjustable with the use of a polarizing prism. Images obtained with this system show operation similar to existing focusing schlieren systems, but with much reduced complexity and setup time. Images taken with acrylic windows placed normal to the optical axis further demonstrate the system's utility for wind tunnel measurements.

Multi-point line focused laser differential interferometer for high-speed flow fluctuation measurements.

A multi-point focused laser differential interferometer (FLDI) has been developed to measure density fluctuations at 16 points along a line. A pair of cylindrical lenses on the transmitter side of a conventional single-point FLDI instrument form two closely spaced (≤200µm), orthogonally polarized, parallel laser lines at the instrument's focus. On the receiver side of the instrument, the interference of the beams on a 16-element photodiode array results in a single line of measurements. The further addition of a Nomarski prism creates two separate measurement lines, and the addition of a second photodiode array to the instrument enables simultaneous measurements of density fluctuations along the two lines separated by several millimeters. These two lines of measurement can be conveniently oriented at any azimuthal angle relative to the instrument's optical axis on the measurement plane, coinciding with the instrument's focus. Two experiments were performed to demonstrate the capabilities of the instrument. In the first experiment, a laser-induced breakdown spark generated a traveling spherical shock wave, and measurements of the resulting density disturbance and wave velocity were obtained. These results were compared to high-speed schlieren images of the shock wave acquired at 400 kHz. In the second experiment, the multi-point FLDI instrument was used to measure density disturbances in the boundary layer of a flat plate in a Mach 6 freestream flow. The measurements were made along two lines, both approximately 6 mm in length, extending from the surface of the plate through the boundary layer. High-speed schlieren images were acquired at 100 kHz during separate wind tunnel runs at matching unit Reynolds numbers to visualize the unsteady boundary layer flow and compare to the FLDI measurements.

Two-point, parallel-beam focused laser differential interferometry with a Nomarski prism.

A Nomarski polarizing prism has been used in conjunction with a focused laser differential interferometer to measure the phase velocity of a density disturbance at sampling frequencies ≥10MHz. Use of this prism enables the simultaneous measurement of density disturbances at two closely spaced points that can be arbitrarily oriented about the instrument's optical axis. The orientation is prescribed by rotating the prism about this axis. Since all four beams (one beam pair at each measurement point) propagate parallel to one another within the test volume, any bias imparted by density fluctuations away from the measurement plane on the disturbance phase velocity is minimized. A laboratory measurement of a spark-generated shock wave and a wind tunnel measurement of a second-mode instability wave on a cone model in a Mach 6 flow are presented to demonstrate the performance of the instrument. High-speed schlieren imaging is used in both cases to verify the results obtained with the instrument.