Velocity effects on pollutant diagnostics with tunable lasers for doppler-broadened lines

The effect of velocity distortions in high-speed gas flows must be allowed for in diagnostic studies using tunable lasers.We present the results of theoretical studies on the distortion of a Doppler-line profile produced by directed motion in a conical source flow. This type of flow is conviniently defined by the ratio of axial velocity at the observation station to the critical velocity and by the axial temperature. In addition, we present results for an idealized flow with constant flow velocity and temperature in the observation plane.Deviations from the Gaussian Doppler contour increase, under otherwise constant conditions, with increasing source-flow divergence angle, decreasing temperature, and increasing flow speed. Even under moderate condition (e.g. maximum semi-divergence angle of 30°, axis temperature T_l = 300 K, and axial flow speed divided by the speed of light = q_l/c = 1 × 10^-5), the Gaussian profile is grossly distorted because of occurrence of velocity components towards and away from the observer.Our results suggest that it will be very difficult to determine gas compositions from high-resolution spectroscopic observations (e.g. laser-absorption measurements) unless the velocity field is determined independently. Sensitivity losses in line-center and derivative spectroscopy may be estimated from our data for representative flow conditions.