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Flow Around the Back of a Cylinder

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Experimental procedure and explanation:

  • Place a styrofoam cylinder in the air flow generated by a fan, and then observe the conditions of air flow around the cylinder. Place a partitioning screen on one side of the cylinder to block the wind. Moreover, in order to observe the air flow, cut tissue paper into thin strips and then attach the strips to a rod.
  • At a position to the side of the cylinder, the air is flowing to the right.
  • When the rod is moved slightly to the downstream side, it is clear that the flow separates from the cylinder and flows outward in the right direction.
  • When the rod is positioned directly behind the cylinder, the air flows in the downward direction of the screen, or in other words, the air flows in the direction that is opposite to the advancement along the cylinder. At this time, the flow of air forms a vortex at the back of the cylinder rather than wrapping around the cylinder. This type of phenomenon is known as “separation.” When an object is placed in uniform flow (flow in the same direction for the entire area), this type of separation will occur in various cases.
  • Next, a dryer is used, and only a portion of the cylinder is hit with the flow of air. Flow gushing out from a dryer, hose, nozzle, hole, or the like is known as a “jet” flow. With this type of jet flow, separation is not produced so easily compared to a uniform flow. Moreover, it is clear that the flow of air wraps around and flows along the cylinder.
  • A property of the jet flow is that it curves and advances along a curved surface and that separation is more difficult to produce compared to a uniform flow. This is referred to as the “Coanda effect.” Accordingly, with a fan (uniform flow), separation occurs at an earlier position, and with a dryer (jet), separation is suppressed and the flow of air wraps around to the back side of the cylinder.
  • The trick when exposing the cylinder to the dryer is to direct the flow of air so that it is more to the inside rather than tangent to the cylinder. In other words, direct the flow of air so that a portion of the flow hits the cylinder. By forcefully causing the air to flow around the cylinder at the front half of the cylinder, the flow of air can be made to cling more easily to the cylinder from that point onward as well. It is also effective to attach a nozzle to the dryer to taper the flow and increase its speed.

[Supplementary Explanation]

  • In the case of a uniform flow, when the change in pressure along a certain streamline is measured at the upstream side, the pressure is at its atmospheric value. Then, when the side of the cylinder is passed, the pressure falls below the atmospheric value. However, at the downstream side, the pressure returns once again to the atmospheric value. At this time, the flow that became a low pressure flow at the side of the cylinder experiences an increase in pressure toward atmospheric pressure. Because the flow advances in opposition to the pressure, “separation” easily occurs. Conversely, with the jet flow, on a single streamline, the pressure becomes equal to the atmospheric pressure on the atmosphere side at any location on that line. Thus, there is no change in pressure in the forward direction in these areas, and “separation” does not occur as easily as it does in the case of a uniform flow.
  • Furthermore, at the front half of the flow, the flow is forcefully bent along the curved surface. When this occurs, a pressure difference takes place with high pressure at the outer circumferential side of the curved streamline and low pressure at the inner circumferential side thereof (streamline curvature theorem). This pressure difference causes the curvature to be contacted with the later flow and is connected with the Coanda effect. In addition, due to the fact that pressure decreases at the inner circumferential side of the curvature of a streamline, the decrease in pressure at the surface of the cylinder can be explained. Moreover, it is clear that the cylinder is drawn into the flow.
[Keywords] Coanda effect
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[Reference] Isao Imai, “Fluid Dynamics, Vol. I,” Shokabo, pp. 66–67.
Masaaki Shirakura et al. “Study of Flow (Part I) (Nagaregaku (Ue)),” Corona Publishing Co., Ltd., pp. 262.
The Japan Society of Mechanical Engineers, “The Wonders of Flow (Nagare no Fushigi),” Kodansha Blue Backs, pp. 128–133.
Ryozo Ishiwata, “Illustrated Fluid Dynamics Trivia (Zukai Zatsugaku Ryutai Rikigaku),” Natsume Publishing, pp. 214–215 and pp. 206–209.
Last Update:9.30.2016