A schlieren motion estimation (SME) method is a physical-based schlieren image velocimetry involving both schlieren luminance characteristic equation and continuity equation. This seedless velocimetry technique is useful for flow situations where the seeding particles are difficult to supply or identify. In this study, we conducted two quantitative investigations: (1) the sensitivity in an estimation accuracy with the user-defined weight parameters in the SME method and (2) the performances of the SME method for a laminar–turbulent transition flow in which a velocity field would be difficult to be estimated by a standard schlieren image velocimetry technique driven by a block-matching algorithm. The experimental results showed that a weight parameter α between governing equation and a constraint condition is sensitive to a velocity estimation accuracy. Additionally, α is strongly related to an image contrast gradient appearing in a schlieren image, and the strong image contrast gradient leads to the better weight parameter α in a wide range. Regarding the result of another quantitative investigation, it is found that the SME method is applicable to the laminar–turbulent transition flow because the turbulent structures in different sizes can be simultaneously captured by the SME method.

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