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One Of The Mysteries In Fluid Mechanics

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1999 Annual Conference


Charlotte, North Carolina

Publication Date

June 20, 1999

Start Date

June 20, 1999

End Date

June 23, 1999



Page Count


Page Numbers

4.405.1 - 4.405.5

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Jan Lugowski

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NOTE: The first page of text has been automatically extracted and included below in lieu of an abstract

Session 3548

One of the Mysteries in Fluid Mechanics

Jan Lugowski Purdue University, West Lafayette, IN


This paper is not about known mysteries, such as what is turbulence, or how tornadoes, or twisters, work. It is about a discrepancy between an existing theory and actual measurements of flow induced forces. The flow case where the discrepancy occurs is not presented in fluid mechanics textbooks. Rather, it is presented in fluid power textbooks, when the origin of flow induced forces in hydraulic valves is discussed.

The existing theory explaining the origin of flow forces in hydraulic valves is based on Newton’s law of motion: Flow force is a reaction force, and its magnitude equals mass times acceleration. Both parameters can be accurately estimated, so the theory can be easily verified.

In the paper, simultaneous recordings of the static pressure distribution in a valve orifice are presented. The measurements show that the jet has a strong tendency to attach itself to the nearest wall. This effect is known as the Coanda effect, and is omitted by the theory. We know that the static pressure in the jet is lower than in the surrounding fluid. If such a jet were attached to a wall of a container, a force perpendicular to the wall would result. This phenomenon cannot be explained by Newton’s mechanics.


Let us consider a flow case in which a liquid jet, perpendicular to a wall, impinges upon the wall. We would apply Newton’s laws of motion to calculate the reaction force (Fig. 1) resulting on the wall. The reaction force could be simply measured with a load cell. We also could estimate the reaction force experimentally by measuring the static-pressure distribution on the wall. All forces, theoretical and experimental, should be the same. The static pressure P1 on the wall in the area of the jet impingement would be higher than outside this area, say P2 on the opposite side of the wall. What would be the pressure P3? According to Bernoulli’s equation we would expect this pressure to be lower than P2. The question arises how pressure P3 contributes to the reaction force.

Lugowski, J. (1999, June), One Of The Mysteries In Fluid Mechanics Paper presented at 1999 Annual Conference, Charlotte, North Carolina.

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