Two-component seedless velocimetry utilizing laser-induced shockwaves

Abstract

A novel two-component seedless velocimetry is devised using shockwaves from laser-induced plasmas (LIP) generated by focused nanosecond laser pulses (532 nm). This velocimetry utilizes two pairs of continuous-wave (CW) laser beams traversing the region adjacent to the LIP in parallel with the horizontal path of the laser pulse. Two probe beams of a pair (633 nm) are above and below, and the two of the other pair (780 nm) are on the sides of the LIP. Each pair is from a diode laser emitting a beam traveling back and forth between a right-angle prism mirror and a hollow-roof prism retroreflector to be terminated on a fast photodiode. The spherical shockwaves propagating radially from the LIP divert the path of the probe beams when intersecting; therefore, the shockwave arrival on the beam is recorded in the photodiode signal. The shock-arrival times indicate the flow velocity near the LIP in the vertical and horizontal directions since the shockwave floats in the flow before reaching the probe beams. The four probe beams surrounding the LIP within the 5ⅹ5 mm2 square are placed in an air jet while the jet direction is varied, 0 – 90°, to test the 2D velocimetry. An empirical point-explosion blast model is employed to track the propagation of the shockwave transitioning from a strong shockwave to an acoustic wave, which is used to derive the two velocity components of the flow from the shock-arrival times. Each velocity component is measured with the uncertainty of 4–6 m/s regardless of the velocity, which exhibits better precision for faster flows. The speed accuracy of 3 m/s is verified using hot-wire.