Application of background-oriented schlieren (BOS) technique to a laser-induced underwater shock wave

摘要

We build an ultra-high-speed imaging system based on the background-orientedschlieren (BOS) technique in order to capture a laser-induced underwater shockwave. This BOS technique is able to provide two-dimensional density-gradientfield of fluid and requires a simple setup. The imaging system consists of anultra-high speed video camera, a laser stroboscope, and a patterned background.This system takes images every 0.2 $\mu$s. Furthermore, since the densitychange of water disturbed by the shock is exceedingly small, the system hashigh spatial resolution $\sim$ 10 $\mu$m/pixel. Using this BOS system, weexamine temporal position of a shock wave. The position agrees well with thatmeasured by conventional shadowgraph, which indicates that the high-speedimaging system can successfully capture the instantaneous position of theunderwater shock wave that propagates with the speed of about 1500 m/s. Thelocal density gradient can be determined up to $O$(10$^3$ kg/m$^4$), which isconfirmed by the gradient estimated from the pressure time history measured bya hydrophone.