Aberrance and rectification of a round pipe for flow field visualization

When the flow field in a circular-section pipe is visualized,there exists optical aber-ration along the radii of the pipe,which makes the effective range of flow field smaller.The aber-ration must be calibrated in order to get a bigger view.Commonly a cylindrical len with reverse foci to that of the pipe wall is used to calibrate the distortion.Therefore the design of the calibra-tion cylinder lens is the key.Firstly we calculate the foci of the half-wall of the circular-section pipe,and the other side of the pipe is the same as this side.Then the foci of the calibration cylin-der len is obtained.It is found that the calibration effect of a cylinder len with the traditional pla-no-convex cross section can not meet the requirement.So the foci of the len is recalculated in ZE-MAX software by fixing the distance between the pipe and the len,and then adjusting the two curvatures of len until the output rays are almost parallel in 80% range.The result shows that the optimal cross section takes the form of positive meniscus.After that,we manufactured two glass pipes and two pieces of calibration cylinder lens.Then the result of static state demarcation by grid imaging proves the validity of this method.Compared to the traditional method,this method is more accurate and extends the effective flow field range to more than 80%.Dynamic result is given by the high speed focusing schlieren way.This research has reference signification for flow field visualization in round pipes with a thick wall.