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impact force (shear stress) by waterjet impingement

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Old   May 7, 2014, 20:39
Smile impact force (shear stress) by waterjet impingement
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Hao Zhu
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Hi everyone,

I want to get help about waterjet impact force.

1. When the waterjet impacts horizontally to a flat target surface, the impact force can be calculated using the momentum equation, when neglecting the back flashing and the gravity effect, i.e.F=\rho
\upsilon^{2}A. If we assume the water flow is steady and use the Bernoulli equation to get the stagnation pressure on the stagnation zone, which is P=0.5\rho\upsilon^{2}. Then the force acting on the target surface is 0.5\rho\upsilon^{2}A, which is half of the value calculated from momentum equation. I found in most papers the force is calculated using first way, while there are still papers doing this calculation using second way. I don't know why the results are different.

2. When the waterjet impacts obliquely to the target surface, the normal impact force still can be calculated using the momentum equation in vertical direction, i.e. Fy=\rho\upsilon^{2}\A\cos{\theta}, where the \theta is the angle between waterjet axis and the vertical direction. However, in this case, the shear force cannot be calculated directly using the momentum equation since the velocity distribution and water layer thickness after impingement are unknown. Alternatively, someone just used the stagnation pressure (0.5\rho\upsilon^{2}) multiplied by the cross area of waterjet and get the impact force along the axis direction. Then this force is decomposed in the vertical and horizontal direction, as the normal and shear force. Is it reasonable?

3. According to my understanding, as the pressure comes from lot of collisions between the small water particle and the target surface. thus the force caused by pressure should be perpendicular to the target surface. Then the shear stress can only come from the friction between the still target surface and the moving water layer. Am I right?

Any suggestion will be highly appreciated. Thank you guys
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