Schlieren imaging is one of the oldest flow visualization techniques to make use of optical principles. As can be seen from the setup diagram, collimated light (i.e. light with parallel rays) is passed through a region of interest (usually the test-section for wind tunnel usage) and is refocused afterwards. The Gladstone-Dale relation states that density changes result in index of refraction changes. Therefore, the collimated light rays are deviated in angle because of index of refraction changes.  This causes them to focus at slightly different points in the focal plane after the test-section. A knife edge is place at the focus and blocks some of the deflected rays, giving rise to the dark regions in the schlieren image. However, some rays are deflected in the opposite manner and are not blocked by the knife. These rays comprise the bright regions of the image.  Overall, schlieren imaging shows density gradients, which is why shockwaves show up so prominently. It is an excellent technique to visualize high-speed flow-fields.

We utilize schlieren in each of our facilities and have the capability to do 400 kHz schlieren imaging thanks to high-power pulsed-LEDS. While schlieren imaging is typically considered a qualitative diagnostic, we often utilize the advances in data analysis to extract quantitative info. This includes Fourier analysis, modal analysis techniques (e.g. SPOD, DMD, etc.), optical flow analysis, and feature tracking. We are also exploring the use of machine learning techniques for data processing.

PLIF is a technique that uses laser light to fluoresce a slice of the flow, thereby showing the structure of the flow in a plane. Beyond flow visualization, the signal can be used to determine density and temperature quantitatively depending on the setup.

The primary advantage for PLIF compared to schlieren imaging is that PLIF is not path-integrated. That is, schlieren imaging picks up disturbances that occur at all points along the collimated beam path, whereas PLIF only provides information in a slice.  The image on the left shows schlieren and PLIF images collected simultaneously. Notice that the PLIF image provides a cleaner look at the dynamics inside the jet since it is not path-integrated through the jet shear layer.

Another useful feature of PLIF is that it doesn’t require a line-of-sight setup. This means that it can be applied to complicated geometries (e.g., concave surfaces), unlike schlieren imaging.

Focused laser differential interferometry (FLDI) is a point-based measurement technique that can provide information about density fluctuations at frequencies of several MHz. Much like schlieren imaging, the technique relies on the Gladstone-Dale relation. The focusing and differential nature of the instrument are the key features. Together, they allow the instrument  to have sensitivity only near the focal region for frequencies of interest in model boundary layers. This allows the measurement to minimize the effect boundary layers on the tunnel walls. Furthermore, the high sampling rates for FLDI measurements are ideal for studying high-speed transitional and turbulent boundary layers, which usually have dynamics at the microsecond time-scale.

Our group has used FLDI extensively to study high-speed boundary layer dynamics. We also improved the technique by adding a diffractive optical element, which allowed several FLDI beam pairs to be generated for measurements at multiple points (known as Linear Array-FLDI or LA-FLDI). Spectra obtained from LA- FLDI showing a Mach 6 boundary layer transitioning from a laminar to a turbulent state are shown on the left.

Pressure sensitive paint is a technique for measuring surface pressures and can provide quantitative measurements of the global pressure distribution at rates up to 20 kHz. The paint compound is commercially available and has been used in wind tunnel measurements at NASA and AEDC. The technique relies on a surface paint layer that is comprised of a polymer layer that has luminescent particles embedded in it. Under UV light, these particles fluoresce. However, molecular oxygen can diffuse through the polymer and “quench” the excited state of a luminescent particle, causing it to transition back to a ground state without emitting the signal light. This sensitivity to quenching by oxygen gives the paint its ability to measure pressure, since higher air pressure (i.e. more oxygen) results in less paint fluorescence (i.e. more quenching occurs). Calibration and a reference image are used to convert each pixel intensity in an image into pressure.

On the left is an example of a static pressure measurement done in the UTSI Mach 4 Ludwieg tube on a hollow cylinder-flare geometry. A corresponding computational result is also shown, showing good agreement with the measurement. [Note the CFD is using the pressure coefficient, which is pressure normalized by the dynamic pressure).

Temperature sensitive paint (TSP) works in much the same way as PSP. However, the quenching is no longer associated with the diffusion of oxygen into the paint, but is instead associated with a temperature dependent transfer of internal energy within the luminescent particles. This transfer of internal energy quenches the excited state of the luminescent particles and reduces the signal emission intensity. Thus, higher temperature locations correspond to regions of the image with less signal. Again, a calibration curve and reference image are used to convert image intensity into temperature.

The image on the left is an example of TSP used in the UTSI Mach 7 Ludwieg tube to investigate the heat transfer on the flare portion of the hollow cylinder-flare geometry. The result shows the change in temperature via a colormap.

Velocity is one of the critical parameters for characterizing fluid flow, but is also one of the most difficult to measure in high-speed wind tunnels. At higher Mach numbers, PIV becomes less reliable and acts like a lowpass filter with respect to turbulent fluctuations, allowing the measurement to only resolve large scale turbulent motion. These effects are essentially due to “particle” lag associated with the seeded particles necessary for PIV. Furthermore, seeding particles into high speed flows can be troublesome, especially for impulse facilities like Ludwieg tubes.

Molecular tagging velocimetry (MTV) works by using a laser to write a fluorescent line in the flow (usually done by targeting some tracer species such as nitrogen, krypton, acetone, etc. seeded into the flow in the stagnation chamber). This line convects with the flow and its displacement is monitored with high-speed imaging. From the displacement vs time information, the velocity can be calculated along the tagged line. The technique could also be extended to 2D using an appropriate optical setup.

MTV measurements in our Mach 4 Ludwieg tube are shown on the left. The measurements were done near the tunnel flow and the turbulent boundary layer is clearly shown in the processed velocity profile. This was done with the tagged line being written at 10 kHz and the high-speed intensifier collecting images at 60 kHz. Since the camera exposure was approximately 100 µs, each frame fully captured the motion of a single written line.