A crossed beam laser shadow technique for spatially resolved measurements in turbulent flow

Optical methods such as the shadowgraph and schlieren techniques do not directly allow three-dimensional spatial resolution because of the inherent integration along the line of sight. For the particular case of the laser shadow technique, it is attempted to circumvent this restriction by employing a pair of crossed laser beams and cross-correlating the optical signals obtained from the beams. The information obtained is supposed to pertain only to the common volume of both beams. It is theoretically shown that this concept will work if the turbulence spectrum does not decay faster than K^-5 in the range of wave numbers corresponding to the inverse diameter of the laser beam. Experiments on a round jet confirm that local values of the mean flow velocity can be obtained.