Photoacoustic technique for measuring velocity, temperature and density of a gas flow

Magnetohydrodynamic (MHD) flows represent one of the most severe environments encountered by gas dynamic diagnostics. Special state-of-the-art techniques and instrumentation systems are required to monitor, control, and collect data for the MHD component systems. The technique described involves creating a brief plasma spark in a gas flow with a focused pulsed laser beam. An acoustic disturbance is produced which propagates upstream with the velocity c₀-v_f and downstream with the velocity c₀+v_f where c₀ is the sound speed and v_f is the gas flow velocity. HeNe laser beams are positioned both upstream and downstream from the spark. The change in density at the acoustic wavefront causes a change in the index of refraction of the gas, which in turn causes the beams to deflect. It is a simple matter to detect this deflection with a photodetector and an oscilloscope. The gas flow velocity and the sound speed can be determined from the arrival timer of the acoustic disturbance and the separation distances of the beams from the spark. The temperature can be determined by knowing the sound speed as a function of temperature. The density can also be determined from the perfect gas law if the composition of the gas is known and the pressure is measured independently