Laser and Optical Diagnostics Laboratory
Laser and optical based diagnostics are used to measure quantities of interest in a variety of challenging flow environments. This lab focuses on measurements in supersonic flowfields, electrically and optically produced plasmas, and combustion environments – typically in coupled applications. These flow environments require numerous techniques to measure properties of interest for, in most cases, numerical model development or validation. Techniques in this lab include PIV for flow velocities, Raman scattering techniques for major species concentrations, fluorescence measurements for minor species concentrations, and high-speed schlieren for flow visualization. Work in the lab is supported by the Army Research Office (ARO), the Air Force Office of Scientific Research (AFOSR), and the U.S. Department of Energy (DOE).
|102 Aerodynamics Research Laboratory
Facility and Equipment Description
Optical diagnostics and capabilities
Our laser systems and detectors allow for a variety of optical diagnostics. We have two seeded Nd:YAG nanosecond lasers, another unseeded Nd:YAG, and 4 dual-cavity PIV lasers. Those alone allow for PIV, filtered Rayleigh scattering, and spontaneous Raman scattering experiments. Coupled with narrowband and broadband dye lasers, CARS and TALIF experiments are possible. Detectors include two ICCD cameras, an EMCCD, a back-illuminated CCD, 4 sCMOS cameras, 6 dual frame CCD cameras for PIV, and 2 high-speed Photron detectors.
Fluorescence and absorption based techniques
Path averaged absorption techniques are useful for the accurate determination of temperature and number density of the absorbing species. Examples are shown for rotational OH absorption lines and a model of the absorption coefficient for water vapor. For measurements of species with low number densities, more sensitive techniques are required. Probing species at a resonant frequency promotes excitation to higher energy levels, causing the molecules or atoms to fluoresce back to their respective ground states. Collection of this fluorescence provides at a minimum the location of the probed species and at best the number density. Examples in our lab include OH planar laser induce fluorescence (PLIF) and two photon laser induced fluorescence (TALIF) for the H and O radical species.
Femtosecond Laser based techniques
A new addition to our lab is a Ti:sapphire femtosecond amplifier system (800 nm, 7 mJ, 35 fs at 1 kHz) with an optical parametric amplifier and a second harmonic bandwidth compressor, advancing our diagnostic capabilities into the ultrafast regime. Increased power-per-pulse from nanosecond systems allows for single-shot CARS and TALIF measurements with combustion relevant measurements lengths. Collision-less pump-probe CARS measurements are possible with the impulsively driven Raman coherence of fs preparation pulses and time-delayed picosecond probing. This new system will allow for femtosecond laser electronically-enhanced tagging (FLEET) of nitrogen molecules for non-intrusive measurements of flow velocity. Femtosecond electric field induced second harmonic generation is also a new technique added to our lab, enabling non-intrusive pointwise electric field measurements.
Coherent anti-Stokes Raman scattering (CARS) is the standard for optical temperature measurements in combustion environments. Targeting a specific pure rotational or ro-vibrational Raman coherence of specific molecules of interest by the frequency difference of photon pairs in the pump and Stokes pulses, the generated coherence can be probed with a frequency narrow pulse to generate a molecule-specific CARS signal blue shifted off the probe beam. The resulting temperature-dependent Raman lines can be fit to a theoretical model to produce temperature information. Probing multiple molecules at once with a dual-pump CARS scheme or a broadband pure rotational scheme can also allow for measurements of relative species mole fractions.
Filtered Rayleigh Scattering
The imaging the frequency broadened Rayleigh scattering off of molecules through a molecular filter, denoted filtered Rayleigh scattering, is a technique to isolate the pressure, temperature, velocity, and species concentration dependent signal from the frequency narrow probe beam. In our lab, we take advantage of the hyperfine iodine absorption lines around the second harmonic of a seeded Nd:YAG as our filter, blocking out the Mie scattering from the laser source while still passing the Rayleigh scattering signal. To isolate all the properties of interest, the probe laser can be frequency scanned across the iodine absorption line for a steady flow application, generating multiple images for which to determine the flow variables. For unsteady applications, or combustion applications with large gradients in Rayleigh cross-sections between the fuel and air, other techniques (such as spontaneous Raman scattering) must be added in order to interpret the acquired signal. An example of this is shown for our DBD burner, where the presented raw signal referenced to room air is mostly a combination of nitrogen and hydrogen, where the Cabannes lines are drastically different from one another.
Particle Image Velocimetry
PIV is a technique to measure the flow velocity by seeding a flow with small particles, illuminating them with a laser as shown, and collecting the scattering on a detector. Accurately controlling the time between particle illumination allows for particle tracking by means of a cross-correlation algorithm through LaVision’s DaViz software. Our lab utilizes dual-cavity laser systems to produce laser sheets for 2D-PIV along a sheet with one dual frame camera or adding the out-of-plane velocity by means of stereo-PIV with two cameras. We now have the capability to illuminate a volume of particles to determine flow velocity in a volume using four cameras, denoted as tomographic PIV. Our group has performed PIV in a variety of supersonic and subsonic flow applications, a pulsed plasma jet, and in plasma-coupled diffusion flames.
Pressure Sensitive Paint (PSP)
Planar surface measurements of pressure are obtained by means of steady UniFIB PSP and a porous, fast-response PSP from ISSI, Inc. The PSP is excited by means of a UV lamp, causing a pressure-dependent fluorescence of the paint which is captured by a CCD detector and calibrated against pressure taps. PSP is also inherently dependent on the temperature of the surface, thus simultaneous measurements with an IR camera were required to correct the displayed fast-response PSP data recorded at 7 kHz of an underexpanded jet.
2D-PIV in a plasma-assisted flame
The following figure is the average velocity obtained from 1000 image pairs of our lab’s DBD burner. The contours are of velocity magnitude, with both streamlines and a water vapor emission images of the flame overlaid. The flowfield demonstrates nearly radial flow along the burner surface with a large, external toroidal vortex acting to ventilate the near flat flame. Care must be taken in determination of the actual flow velocity, as the presented values are the particle velocities – plagued with particle drag and thermophoesis. (Student: Jon Retter, Support: DOE)
Tomographic PIV of an Underexpanded Jet
Application of the tomographic PIV system on a round, underexpanded jet is shown by means of isosurfaces of the average axial flow velocity in meters per second. This visualization allows for the clear depiction of the well-known shock-cell structure of an isolated jet, demonstrating how the lowest flow velocity in the jet core is just downstream of the Mach disk in the first shock cell. (Student: Ruben Hortensius, Support: ARO)
Filtered Rayleigh Scattering (FRS)
A converging jet of air experiment was designed to showcase the power of the FRS technique. For this steady flow application, the frequency narrow probe laser was scanned through 120 points in the iodine absorption profile with 50 images recorded at each location. Therefore, for each pixel, the resulting 120-point curve is fit for the properties shown here, resulting in the measurement of pressure, density, temperature, and velocity from just this one technique. (Student: Martin Boguszko, Support: NSF)
High-speed schlieren photography
Understanding the use of a nanosecond LIB for non-intrusive ignition of combustible mixtures begins with a visualization of the actual event by means of high-speed schlieren. Doing so, as shown with the schlieren images, led to the discovery of the importance of the hydrodynamics of the LIB and how it effects the actual ignition. Clearly, the breakdown location is not always the ignition location, as the jet or tail of the LIB can also be an ignition kernel, leading to at times two separate ignition locations from a single laser shot. (Student: Jon Retter, Support: DOE)
Spatial-temporal evolution of a LIB
At very early times after the breakdown, before the initiation of the LIB hydrodynamics, the development of the LIB plasma can me measured my means of imaging the breakdown line onto a streak camera, providing 1D time-resolved emission information. A bandpass filter allowed for collection of both the 532 nm elastic scattering of the laser and the plasma emission from N+. The first row is single-mode operation (seeded), while the second row is multi-mode (unseeded). Both show the development of the plasma in both the forward and backward directions, revealing a two-lobe structure. (Postdoc: Dr. Munetake Nishihara, Support: DOE)