In order to describe the velocity distribution of plug flow and provide the basis for plug design，stepped plug and gradually contracted plug were measured by LDV.The results showed that the time average velocity becomes uniform after 0.133 times tunnel diameter from plug inlet.The time average velocity after expansion becomes uniform after 3 times tunnel diameter from plug outlet.The reattachment points after outlet of the plug are 1~2 times diameter of the tunnel from plug outlet.In general,the radial time average velocity beyond circulation area is 10 percent of that of axial.The fluctuation velocity is 10~30 percent of time average velocity and the maximum of it lies in the area where the grad of velocity is relatively high.As far as the same section is concerned,the fluctuation velocity at axis is relatively smaller than other point.From the point of the velocity distribution,plug length should be greater than 0.133 times the pipe diameter,and the distance between plugs should be greater than 3 times the pipe diameter.
