Flow visualization is used to measure fluid flow velocity in many fields. As an example, there is a measurement of gas flow velocity in the engine influences deeply on combustion and heat transmission. So many researches have been performed to the present. The authors realized the flow visualization method using a color layer which indicates the velocity in the depth direction by color, and three-dimensional measurement can be achieved. This paper shows the problems and solutions as to applying the flow visualization by a color layer to air flow velocity. One is the making of a tracer particle being suitable for the color layer visualization. Another is the development of the new image-processing method for obtaining color information exactly when using a small tracer particle. As a result, this newly revised flow visualization method makes it possible to measure velocity of the particle within the error of 10%.

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