It was known that development of a jet from a rectangular orifice could be divided into three regions, which are 1) potential core region 2) characteristic decay region and 3) axisymmetric decay region. When a circular cylinder is placed in the rectangular jet, characteristic separation occurs around the cylinder and shapes of non-separation area on the cylinder are distinguished with the streamwise position of the cylinder. This flow geometry can be frequently seen in the industrial applications and drag and heat transfer of the cylinder would be important for the designs. In this study the rectangular jet impinging on a circular cylinder has been experimentally investigated. The relation between the jet development before impinging and separation area on the cylinder is especially focused. In the case that the cylinder is in the potential core region, the non-separation area distributes as saddle shape and the spanwise pressure distribution at the impinging line on the cylinder is almost constant around jet center and in the edge of jet impinging region it decreases with the distance. Then the pressure reaches at the minimum in the outside of the edge and recovers to the pressure of the circumference far from the edge. This pressure distribution suggests that flow just above the impinging line of cylinder blows toward the center and that the impinging flow is incredibly three-dimensional. When the cylinder is in the characteristic decay region, the non-separation area distributes as saddle shape as well as in the case of the potential core region. The spanwise impinging pressure distribution on the cylinder has maximums at the spanwise edges of impinging jet and sudden decrease out side of the jet. In this case the minima in the fringes of the jet were not observed. In the case of the axisymmetric region, the area of non-separation is elliptic and at the spanwise center the non-separated flow reaches backside of the cylinder.

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