R. Jeffrey Balla

Planar Rayleigh scattering results in helium–air mixing experiments in a Mach-6 wind tunnel

Planar Rayleigh scattering measurements with an argon-fluoride excimer laser are performed to investigate helium mixing into air at supersonic speeds. The capability of the Rayleigh scattering technique for flow visualization of a turbulent environment is demonstrated in a large-scale, Mach-6 facility. The detection limit obtained with the present setup indicates that planar, quantitative measurements of density can be made over a large cross-sectional area (5 cm x 10 cm) of the flow field in the absence of clusters.

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.

Raman shifting a tunable ArF laser to wavelengths of 190–240 nm

Tunable radiation at far-ultraviolet wavelengths is produced by Raman shifting a modified (285 mJ at 1–100 Hz repetition rate) ArF excimer laser. Multiple Stokes outputs are observed in H2(20%), CH4(22%), D2(53%), N2(21%), SF6(2.1%), and CF4(0.35%). Numbers in parentheses are the first vibrational Stokes energy conversion efficiencies. We can access 70% of the frequency range 42 000–52 000 cm−1 (190–240 nm), with Stokes pulse energies that vary from 0.2 μJ to 58 mJ inside the Raman cell. One of our better results, using 110 mJ of pump energy and D2, is an energy conversion efficiency of 53% and a tunable Stokes energy that varies over the range 29–58 mJ/pulse for the wavelength range 204–206 nm.

Density measurements in air by optically exciting the Cordes bands of I2

We describe an optical method based on laser-induced fluorescence for obtaining instantaneous measurements of density along a line in low-density air seeded with I2 . The Cordes bands of I2 (D 1 u + X 1 g + ) are excited with a tunable ArF excimer laser. Air densities in the range (0.1 - 6.5) ? 1017 cm-3 are measured over 295 - 583 K using the density-dependent emission ratio of two emission bands of I2 ; the 340 nm bands and the diffuse-structured McLennan bands near 320 nm.

Thermometry in air by optically exciting the I2 Cordes bands

An optical method is described to perform instantaneous thermometry along a line in low-density air seeded with I2. Laser-induced fluorescence is generated by exciting the I2 Cordes bands (D 1Σu+ ← 1Σg+) with a tunable ArF excimer laser near 193 nm. Temperatures from 295 to 648 K are measured by fitting the I2 emission spectrum from 188 to 195 nm to a vibrational Boltzmann distribution. Uncertainties for average measurements are typically ±5% (±1σ).

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.