Instrumentation

Wall Pressure Transducer Resolution

• "Transducer resolution and the turbulent wall pressure spectrum," Journal of the Acoustical Society of America, 97:370-378, 1995. Adequate spatial resolution for microphones to resolve the smallest energetic scales of wall pressure beneath a turbulent boundary layer is a difficult problem for experimentalists. To investigate the effect of transducer spatial resolution on wall pressure measurements, I computationally applied wall pressure "transducers" to the NASA-Ames direct numerical simulation wall pressure database by first Fourier transforming the wall pressure to the wavenumber domain, then applying models of transducers of different sizes, shapes, and sensitivities as wavenumber filters, and finally transforming the wall pressure spectrum back into the spatial domain. Using this method of analysis I was able to determine that not only does spatial averaging across the face of the transducer reduce the amplitude of the spectrum, but the appearance of zeros in the spatial transducer response function can also reduce the amplitude. In fact, by using the frequency of the first zero of the response function I was able to prescribe a new criterion for the relation between the transducer size and the maximum frequency of wall pressure fluctuations that can be accurately measured. Natural Gas Fuel Composition Sensor

• "Acoustic sensor for determining combustion properties of natural gas," with S. Phillips, Measurement Science and Technology, 5:1375-1381, 1994 and "Acoustic natural gas fuel quality sensor," with S. Phillips and M. Oczkowski, SAE Paper 950529. Since natural gas is not refined, its composition and quality depend on when and where it is recovered. This results in problems with optimal performance of natural gas combustion systems. We found that the speed of sound in natural gas can be used to measure the fuel quality, since the speed of sound changes with the molecular weight of the gas. Methane, the primary constituent of natural gas, has a relatively low molecular weight which results in a high speed of sound. Contaminants such as ethane, propane, nitrogen, and carbon dioxide have a lower speed of sound. Using a bench-top prototype speed of sound sensor, we measured the speed of sound at temperatures from -30° C to 65° C for twelve representative natural gas compositions. The sound speeds measured with the prototype sensor match theoretical predictions. Using 6700 representative natural gas compositions from across the U. S. indicates that the theoretical sound speed correlates well with percent methane, percent non-methane hydrocarbons, density, and methane number. The speed of sound is more weakly correlated with the stoichiometric mass air-fuel ratio, hydrogen-to-carbon ratio, Wobbe index, and lower flammability limit. We have received patent protection for the invention.
  

Miniature Wall Pressure Measurement Sensors

• "Wall pressure and turbulent structures in a turbulent boundary layer on a cylinder in axial flow," with S. Snarski, Journal of Fluid Mechanics, 286:137-171, 1995. We pioneered the use of hearing aid microphones for measuring wall pressure, because the microphones are small enough to be enclosed in a 1 cm diameter cylinder used in our experiments. Several other researchers have followed our lead and begun using these microphones because of their small size.