Roger C. Hart

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%).

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.

Spectral brightness and other improvements to the tunable ArF excimer laser

We present two oscillator designs and a new amplification design which improves many characteristics of the dual-discharge tube tunable ArF excimer laser. We demonstrate bandwidths from 0.17–11.0 cm−1 (5–330 GHz) can be selected by appropriate choice of oscillator slit width and diffraction-grating dispersion. Amplification is achieved using three consecutive passes through the discharge tubes. This amplifier design decreases divergence (9×diffraction limit) and increases output energy (33%), locking efficiency (20%), range of tunability (40%), and spectral brightness (two orders-of-magnitude) when compared to the standard unstable-resonator amplifier.

Observation of in a flame by two-colour laser-induced-grating spectroscopy

By using two-colour laser-induced-grating spectroscopy (TC-LIGS), we observed the third-overtone spectrum of the O - H stretch of water vapour at a point in a stoichiometric - air flame. We also demonstrated the extension of these point measurements to a line image in a flame. Only thermal gratings could be observed. The reasons for this and the difficulties in making a practical combustion diagnostic are discussed.

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.

Seedless laser velocimetry using heterodyne laser-induced thermal acoustics

A need exists for a seedless equivalent of laser Doppler velocimetry (LDV) for use in low-turbulence or supersonic flows or elsewhere where seeding is undesirable or impractical. A compact laser velocimeter using heterodyne non-resonant laser-induced thermal acoustics (LITA) to measure a single component of velocity is described. Neither molecular (e.g. NO/sub 2/) nor particulate seed is added to the flow. In non-resonant LITA two beams split from a short-pulse pump laser are crossed; interference produces two counterpropagating sound waves by electrostriction. A CW probe laser incident on the sound waves at the proper angle is diffracted towards a detector. Measurement of the beating between the Doppler-shifted light and a highly attenuated portion of the: probe beam allows determination of one component of flow velocity, speed of sound, and temperature. The sound waves essentially take the place of the particulate seed used in LDV. The velocimeter was used to study the flow behind a rearward-facing step in NASA Langley Research Centers Basic Aerodynamics Research Tunnel. Comparison is made with pitot-static probe data in the freestream over the range 0 m/s - 55 m/s. Comparison with LDV is made in the recirculation region behind the step and in a well-developed boundary layer in front of the step. Good agreement is found in all cases.

Application of laser-induced thermal acoustics to a high-lift configuration

Laser-Induced Thermal Acoustics (LITA) has been used to measure the flow field in the slat region of a two-dimensional, high-lift system in the NASA Langley Basic Aerodynamics Research Tunnel (BART). Unlike other point-wise, non-intrusive measurement techniques, LITA does not require the addition of molecular or particulate seed to the flow. This provides an opportunity to obtain additional insight and detailed flow-field information in complex flows where seeding may be insufficient or detection is problematic. Based on the successful use of LITA to measure the flow over a backward-facing step, the goal of this study was to further evaluate the technique by applying it to a more relevant and challenging flow field such as the slat wake on a high-lift system. Streamwise velocities were measured in the slat wake and over the main element at 11.3 degrees angle of attack and a freestream Mach Number of 0.17. The single-component LITA system is described and velocity profiles obtained using LITA are compared to profiles obtained using two-dimensional, Digital Particle Image Velocimetry (DPIV) and a steady, Reynolds-Averaged Navier-Stokes (RANS) flow solver for the same configuration. The normalized data show good agreement where the number of measurement locations had sufficient density to capture the pertinent flow phenomena.

Nonresonant referenced laser‐induced thermal acoustics (LITA) for measurement of the speed of sound in gases

Nonresonant laser‐induced thermal acoustics (LITA) is an effective tool for measuring the speed of sound in gases and is suitable for operation at elevated temperatures. Counterpropagating sound waves of known, fixed wavelength are generated electrostrictively by crossing two split laser beams which are generated from a short‐pulse pump laser. The sound waves form a Bragg grating, which is illuminated by a second, long‐pulse probe laser. A small fraction of the probe beam is diffracted to a detector, which permits accurate measurement of the sound frequency. Since the method comprises a free‐filed measurement, the corrections required of a conventional acoustic interferometer or resonator are not necessary here. The presentation will include the principle of operation of LITA, comparison with other methods of measuring the speed of sound in gases, and sample measurements at temperatures up to 650 K.