G. C. Herring

Nonresonant referenced laser-induced thermal acoustics thermometry in air

We report a detailed investigation of nonresonant laser-induced thermal acoustics (LITA) for the single-shot measurement of the speed of sound (v(S)) in an oven containing room air. A model for the speed of sound that includes important acoustic relaxation effects is used to convert the speed of sound into temperature. A reference LITA channel is used to reduce uncertainties in v(S). Comparing thermocouple temperatures with temperatures deduced from our v(S) measurements and model, we find the mean temperature difference from 300 to 650 K to be 1% (+/-2sigma). The advantages of using a reference LITA channel are discussed.

Simultaneous velocimetry and thermometry of air by use of nonresonant heterodyned laser-induced thermal acoustics

Nonresonant laser-induced thermal acoustics is used with heterodyne detection to measure temperature (285-295 K) and a single component of velocity (20-150 m/s) in an atmospheric pressure, subsonic, unseeded air jet. Good agreement is found with Pitot-tube measurements of velocity (0.2% at 150 m/s and 2% at 20 m/s) and the isentropic expansion model for temperature (0.3%).

Optical Measurements at the Combustor Exit of the HIFiRE 2 Ground Test Engine

The development of optical techniques capable of measuring in-stream flow properties of air breathing hypersonic engines is a goal of the Aerospace Propulsion Division at AFRL. Of particular interest are techniques such as tunable diode laser absorption spectroscopy that can be implemented in both ground and flight test efforts. An AFRL and NASA research team recently executed a diode laser-based absorption measurement campaign at the exit of the combustor of the HIFiRE 2 ground test engine during Phase II operation of the engine. Data was collected in anticipation of similar data sets to be collected during the flight experiment. The ground test optical data provides a means to evaluate signal processing algorithms particularly those associated with limited line of sight tomography. Equally important, this in-stream data was collected to compliment data acquired with surfacemounted instrumentation and the accompanying flowpath modeling efforts – both CFD and lower order modeling. This report discloses the specifics of hardware and data collection along with a preliminary look at the acquired data and approach to processing and analyzing it.

Pressure measurement in supersonic air flow by differential absorptive laser-induced thermal acoustics

Nonintrusive, off-body flow barometry in Mach 2 airflow has been demonstrated in a large-scale supersonic wind tunnel using seedless laser-induced thermal acoustics (LITA). The static pressure of the gas flow is determined with a novel differential absorption measurement of the ultrasonic sound produced by the LITA pump process. Simultaneously, the streamwise velocity and static gas temperature of the same spatially resolved sample volume were measured with this nonresonant time-averaged LITA technique. Mach number, temperature, and pressure have 0.2%, 0.4%, and 4% rms agreement, respectively, in comparison with known free-stream conditions.

Optical measurement of the speed of sound in air over the temperature range 300–650 K

Using laser-induced thermal acoustics (LITA), the speed of sound in room air (1 atm) is measured over the temperature range 300-650 K. Since the LITA apparatus maintains a fixed sound wavelength as temperature is varied, this temperature range simultaneously corresponds to a sound frequency range of 10-15 MHz. The data are compared to a published model and typically agree within 0.1%-0.4% at each of 21 temperatures.

Submicrosecond temperature measurement in liquid water with laser-induced thermal acoustics

Using laser-induced thermal acoustics, we demonstrate nonintrusive and remote sound-speed and temperature measurements in liquid water. Unsteady thermal gradients in the water sample produce fast, random laser beam misalignments, which are the primary source of uncertainty in these measurements. For water temperatures over the range 10 degrees C to 45 degrees C, the precision of a single 300-ns-duration measurement varies from +/-1 to +/-16.5 m/s for sound speed and from +/-0.3 degrees C to +/-9.5 degrees C for temperature. Averaging over 10 s (100 laser pulses) yields accuracies of +/-0.64 m/s and +/-0.45 degrees C for sound speed and temperature, respectively.

Shock-Strength Determination With Seeded and Seedless Laser Methods

Two noninvasive laser diagnostics were independently used to measure time-averaged and spatially resolved pressure change across a two-dimensional (2D) shock wave. The first method is Doppler global velocimetry (DGV) which uses water seeding and generates 2D maps of three-orthogonal components of velocity. A DGV-measured change in flow direction behind an oblique shock provides an indirect determination of pressure change across the shock, when used with the known incoming Mach number and ideal shock relations (or Prandtl–Meyer equations for an expansion fan). This approach was demonstrated at Mach 2 on 2D shock and expansion waves generated from a flat plate. This technique also works for temperature change (as well as pressure) and for normal shocks (as well as oblique). The second method, laser-induced thermal acoustics (LITA), is a seedless approach that was used to generate 1D spatial profiles of streamwise Mach number, sound speed, pressure and temperature over the same oblique waves. Excellent agreement was obtained between DGV and LITA, suggesting that either technique is viable for shock-strength measurement.

Temperature measurement by degenerate four-wave mixing with strong absorption of the excitation beams

We have made simultaneous temperature measurements by degenerate four-wave mixing (DFWM) and absorption spectroscopy of OH in a CH(4)-air, lifted-diffusion flame. After we corrected the DFWM data for laser beam absorption of as much as 60%, the DFWM-based temperatures were in good agreement with temperatures derived strictly from the absorption data, as well as a one-dimensional reacting flow simulation.

Effect of crossed beams on Stokes gain and interaction length in stimulated Raman gain spectroscopy

We have numerically calculated the change in spatial resolution and Stokes gain for stimulated Raman gain experiments that use two crossed laser beams. The laser beams are modeled as diffraction-limited, Gaussian TEM(00) beams. Results for interaction length and relative Stokes gain are presented for crossing angles of 0-15°, focusing ƒ/#s 2-250, and mismatches in the positions of the focal points of the two beams. The numerical results for spatial resolution and gain are compared with geometric approximations that have been previously published. These numerical simulations show where the approximations are valid and also extend into regions where the approximations are invalid. PACS: 42.65 Dr.

VOLUMETRIC NEAR-FIELD MICROWAVE PLASMA GENERATION

A periodic series of microwave-induced plasmoids is generated using the outgoing wave from a microwave horn and the reflected wave from a nearby on-axis concave reflector. The plasmoids are spaced at halfwavelength separations according to a standing-wave pattern. The plasmoids are enhanced by an “effective focusing” in the near field of the horn (Fresnel region) as a result of a diffractive narrowing. Optical imaging, electron density, and rotational temperature measurements characterize the near field plasma region. Volumetric microwave discharges may have application to combustion ignition in scramjet engines. A. Introduction