Many interesting and useful phenomena in thermal and fluid engineering occur through the changes in the flow velocity field. In Chapter 3, we saw that the flow characteristics change completely at high angle of attack for a jet aircraft (see again Figure 3.1) due to the flow separation from the aircraft wing-body surface. The resulting flow velocity field is complex and directly affects the lift and drag of the aircraft. Although flow visualization methods can provide an overall picture of the flow pattern, in many instances we would like to determine the exact magnitude and direction of the velocity vectors to better understand and control such complex flows.