
[Sponsors] 
impact force (shear stress) by waterjet impingement 

LinkBack  Thread Tools  Search this Thread  Display Modes 
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 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
ActuatorDiskExplicitForce in OF2.1. Help  be_inspired  OpenFOAM Programming & Development  10  September 14, 2018 11:12 
shear stress and drag force  ashakn  CFX  1  November 2, 2011 20:56 
Force can not converge  colopolo  CFX  13  October 4, 2011 22:03 
shear stress scalar definitions?  Novak Elliott  CFX  0  April 6, 2003 01:45 
What is the detail definition of wall shear stress  zjm  FLUENT  0  January 2, 2002 07:43 