# impact force (shear stress) by waterjet impingement

 Register Blogs Members List Search Today's Posts Mark Forums Read May 7, 2014, 20:39 impact force (shear stress) by waterjet impingement #1 New Member   Hao Zhu Join Date: May 2014 Posts: 1 Rep Power: 0 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  Tags impact force, shear stress, stagnation pressure, waterjet Thread Tools Search this Thread Show Printable Version Email this Page Search this Thread: Advanced Search Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules Similar Threads Thread Thread Starter Forum Replies Last Post be_inspired OpenFOAM Programming & Development 10 September 14, 2018 11:12 ashakn CFX 1 November 2, 2011 20:56 colopolo CFX 13 October 4, 2011 22:03 Novak Elliott CFX 0 April 6, 2003 01:45 zjm FLUENT 0 January 2, 2002 07:43

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