forces resulting from fluid momentum and thrust by reaction
TRANSCRIPT
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Forces resulting from fluid momentum and Thrust by reaction.
Introduction..... Symbols..... Momentum Notes..... Stationary Plates..... Moving Plates..... Vanes..... Pipe walls.....
Introduction
The study of forces resulting from the impact of fluid jets and when fluids are diverted round pipe bends involves
the application of newtons second law in the form of F = m.a. The forces are determined by calculating the change
of momentum of the flowing fluids. In nature these forces manifest themselves in the form of wind forces, and the
impact forces of the sea on the harbour walls. The operation of hydro-kinetic machines such as turbines depends
on forces developed through changing the momentum of flowing fluids.
I have created a excelcalcs spreadsheet for convenient access to all of the equations found on this page . This is
located at ExcelCalcs.com calculation Fluid Jets
Symbols
= jet angle (radian)
a = Acceleration (m/s 2)
= density (kg/m3)
= density (kg/m3)
F = Force (N)
m = mass (kg)
V = fluid velocity(m/s)
u 1 = initial velocity (m/s)
u 2 = final velocity velocity (m/s)P = Power (watts)Q = Volumetric Flow Rate(m 3/s)1 vane inlet angle angle(radian)2 vane outlet angle angle(radian)
Momentum
Newtons Second Law can be stated as: The force acting on a body in a fixed direction is equal to rate of increase
of momentum of the body in that direction. Force and momentum are vector quantities so the direction is important.
A fluid is essentially a collection of particles and the net force, in a fixed direction, on a defined quantity of fluid
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equals the total rate of momentum of that fluid quantity in that direction.
Consider a mass m which has an initial velocity u and is brought to rest. Its loss of momentum is m.u and if it
stopped in a time interval t then the rate of change of momentum is m.u /t. The force F required to stop the
moving mass is therefore F = m.u / t . Now if this is applied to a jet of fluid with a mass flow rate ( m / t ) which is
equivalent to the volumetric flow rate times the density ( Q ) the equivalent force on a flowing fluid is F =
Q u. Also in accordance with Newtons third law the resulting force of the fluid by a flowing fluid on its
surroundings is (-F). Newtons third law states that for every force there is an equal and opposite force.
The figure below illustrates this principle at two locations.
The fluid flowing into the tank is brought to rest from a velocity u to zero velocity and the force on the jet is F = Q..
u. . The reaction force on the tank contents is -F.
The fluid flowing in the pipe in the horizontal direction is forced to change direction at the bend such that its velocity
in the orginal direction is zero. Therefore the force on the flowing fluid is F = Q. u. The reaction force on the pipe
is -F in the horizontal direction as shown.
In its simplest form, with steady flow conditions, the force on a fluid flow in a set direction is equal to its mass flow
rate times by the change in velocity in the set direction. The fluid flow also exerts an equal and opposite reaction
force as a result of this change in momentum.
F = Q (u 1 - u 2). ..( F and u are vector quantities)
The resultant force on a fluid in a particular direction is equal to the rate of increase of momentum in that direction.
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Jet Forces on Stationary Plates
Jet force on a flat plate
Considering only forces in a horizontal direction u 1 = V and u 2 = 0 therefore
F = QV = AV2
Jet force on a flat plate at an angle
Considering only forces Normal to plate surface u 1 = V sin and u 2 = 0 therefore
F = QV sin = AV2
sin
when =90o
then F = AV2
as above
Jet force on an angled plate. ( < 90o)
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Considering only forces in a horizontal direction u 1 = V and u 2 = V cos therefore
F = QV (1 - cos ) = AV2(1 - cos )
Jet Force on an angled plate ( > 90o)
Considering only forces in a horizontal direction u 1 = V and u 2 = V cos therefore
F = QV (1 - cos ) = AV2(1 - cos )
Jet Force on an angled plate ( = 180o)
u 1 = V and u 2 = V cos therefore
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F = QV (1 - cos180o
) = 2QV = 2AV2
Jet Forces on Moving Plates......Fluid Machines - Pelton Wheel
Jet force on a moving flat plate
Considering only forces in a horizontal direction
u 1 = V and u 2 = Vp
and let r = V p / V therefore
F = Q( V - V p ) = AV(V -V p)
and F = AV2( 1 - r
The power (P) generated by the force on the moving plate = P = F. Vp
Jet force on an angled moving plate
Considering only forces in a horizontal direction
u 1 = V and u 2 = V p + (V - V p) cos
and let r = V p / V therefore
F = A V( V - V p ) ( 1 - cos ) = A V2
( 1 - r ) ( 1 - cos )
The power (P) generated by the force on the moving plate P = F. Vp
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Note: The moving plate with an angle = 180o
is the typical of the rotating plate of the pelton
wheel.
The ideal value for r resulting in the maximum power output is clearly 0,5
Jet Forces on Vanes......
Additional notes can be found on webpages
Fluid Machines - Francis Wheel
Steam Turbines - Impulse blades
Force on fixed Vane.
In the x Direction: u 1x = V cos 1 , u 2x = -V cos 2
F x = QV(cos 1 + cos 2 ) = AV2(cos 1 + cos 2 )
In the y Direction u 1y = V sin 1 u 2y = V sin 2
F y = QV(sin 1 - sin 2 ) = AV2(sin 1 - sin 2 )
Force on Moving Vane.
The notes below related to vanes as used in impulse turbines. These turbines derive the mechanical energy
mainly from the change in momentum as the fluid passes through the vanes. The conditions as shown when the
vectorial sum of V v + V r1 = V1 results in smooth entry with efficient transfer of energy of the fluid to the vane.
When this does not occur there will be turbulent flow over the vane with significant losses.
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In the x Direction
u 1x = V 1 cos
u 2x = V v - V r2 cos 2 .... ( V r2 = V r1 = V 1 sin /sin 1 )
F x = QV 1 (cos + [sin /sin 1 ] cos 2 - r > )
r = V v / V 1
In the y Direction u 1y = V 1 sin
u 2y = V r2 sin 2 .... ( V r2 = V r1 = V 1 sin /sin 1 )
In the y direction F y = QV 1sin (1 - sin 2 / sin 1 )
If the vane is moving in the x direction the power developed by the vane P = F x.V V
Force on Pipe Wall
The notes below related to the force on a pipe wall resulting from the changes in fluid pressure and fluid
momentum as the fluid flows round a pipe bend . Gravity and friction effects are not considered. The fluid is
assumed to be flowing under steady state conditions.
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In the x Direction
u 1 = V 1
u 2 = V 2
F x = p1.A1 - p2.A2 + (A1V12
- A2V2)2
In the x Direction
u 1x = V 1u 2x = V 2cos F x = p1.A1 - p2.A2cos + (A1V1
2 -A2V2
2 cos )
In the y Direction
u 1y = 0u 2y = V 2sin F y = - p2.A2sin - A2V2
2 sin
The resultant reaction force on the pipe = Fr = (Fx2 + Fx2 )
The angle of the resultant force to the x axis = tan-1
((Fy /(Fx)