Flow measurement: using a chopper together with a CW laser to realize a frame straddling technique

A simple method that uses a chopper together with a CW laser to realize a f rame straddling technique for flow measurement is introduced, in which a ch opper with two windows is used to generate double pulses for exposure. By c hanging the size and position of each window, the time and interval of expo sure can be adjusted easily. The light source is a CW semiconductor laser. The advantage of this method is in its being simple, cheap and compact. The method has been validated on a hydrodynamic flow involving a von Karman Vo rtex Street. (C) 2001 Elsevier Science Ltd. All rights reserved.