Laser velocimeter and wall approach limitations

The SNR evolution with wall approach is theoretically investigated for three types of LV (fringe, two spots, two-dashes), in backscatter on-axis and off-axis geometries. It is pointed out that the best suited apparatus is the two-spots LV, but limited to laminar flow investigations; fringe LV works very badly in these conditions, and even in off-axis mode cannot perform measurement beyond 5 or 10 mm. A good compromise is the two-dashes LV, able to approach within 0.7 mm, which can become two-dimensional LV in the two-crosses configuration, and able to perform measurements in turbulent flows (but not when velocity sign determination is required).