Download Fluid statics

LUKHDHIRJI
ENGINEERING COLLAGE
Civil Engineering
B.E. - 3rd Semester
PREPARED BY:-
B.E. CIVIL SEM -3 ( B-DIVISION )
140313106002
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Properties of fluid
Fluid statics
Fluid kinemetics
Flow measuring devices
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Fluid statics: study of fluids at rest
Different from fluid dynamics in that it concerns
pressure forces perpendicular to a plane (referred to as
hydrostatic pressure)
If you pick any one point in a static fluid, that point is
going to have a specific pressure intensity associated
with it:
P = F/A where
◦ P = pressure in Pascals (Pa, lb/ft3) or Newtons (N,
kg/m3)
◦ F = normal forces acting on an area (lbs or kgs)
◦ A = area over which the force is acting (ft2 or m2)
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This equation, P = F/A, can be used to calculate
pressure on the bottom of a tank filled with a liquid
(or.. at any depth)
F = V
 = fluid specific wt
(N/m3), V = volume (m3)
P1
P = h
h = depth of water
(m or ft)
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Pressure is the same at all points at equal
height from the bottom of the tank
Point: temp doesn’t make that much difference
in pressure for most aquaculture situations
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Z1 + (P1/) + (V12/2g) = Z2 + (P2/) + (V22/2g)
Wow! Z = pressure head, V2/2g = velocity head
2g = (2)(32.2) for Eng. System
If we’re trying to figure out how quickly a tank
will drain, we use this equation in a simplified
form:
Z = V2/2g
Can you think of any applications for this?
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In actuality, fluids have losses due to friction in the
pipes and minor losses associated with tees,
elbows, valves, etc.
Also, there is usually an external power source
(pump). The equation becomes
Z1 + (P1/) + (V12/2g) + EP = Z2 + (P2/) + (V22/2g) + hm + hf
 If no pump (gravity flow), EP = 0. EP is energy from the
pump, hm and hf = minor and frictional head losses, resp.
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These are losses in pressure associated with
the fluid encountering:
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restrictions in the system (valves)
changes in direction (elbows, bends, tees, etc.)
changes in pipe size (reducers, expanders)
losses associated with fluid entering or leaving a pipe
Screens, foot valves also create minor losses
A loss coefficient, K, is associated with each
component
total minor losses, hm, = K(V2/2g)
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hf = f(L/D)(V2/2g)
Where hf = pipe friction head loss (m/ft); f =
friction factor; L = total straight length of pipe
(m/ft); D = inside pipe diameter (m/ft); V =
fluid velocity (m/s or ft/s); g = gravitational
constant (m/s2 or ft/s2)
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Pressure is the (compression) force exerted by a fluid
per unit area.
Stress vs. pressure?
Influidsgasesandliquidswespeakofpressure;insolidsthi
sisnormal
In fluids, gases and liquids, we speak of pressure; in
solids this is normal stress. For a fluid at rest, the pre
ssure at a given point is the same in all directions.
Differences or gradients in pressure drive a fluid flow,
especially in ducts and ip ipes.
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The density of a fluid is its mass per unit
volume:
Liquids are essentially incompressible.
Density is highly variable in gases nearly proportion
al to the pressure.
The specific weight of a fluid is its weight,per
unit volume.
 Density and specific weight are related by
gravity
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Specific gravity is the ratio of a fluid density to a st
andard reference fluid, typically water at 4˚C (for li
quids) and air (for gases)
Potential energy
Potential energy is the work required to move the
system of mass from the origin to a position
against a gravity field g.
 Kinetic energy
Kinetic energy is the work required to change the
speed of the mass from zero to velocity V.
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Viscosity is a measure of a fluid’s resistance to flow. It
determines the fluid strain rate that is generated by a
given applied shear stress.
Temperature has a strong and pressure has a moderate
effect on viscosity.
The viscosity of gases and most liquids increases slowly
with pressure
A liquid, being unable to expand freely, will form an in
terfacewith a second liquid or gas.
 The cohesive forces between liquid molecules are re
sponsible for the phenomenon known as surface
tension.
 Surface tension Υ(pronounced upsilon) has the
dimension of force per unit length (N/m) or of energy
per unit area (J/m2).
Υair‐water= 0.073 N/m; Υair‐mercury= 0.48 N/m
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Using a force balance, pressure increase in the int
erior of a liquid half‐cylinder droplet of length L an
d radius Ris: ypg
Contact angle θ appears when a liquid interface in
tersects with a solid fsur face.
Water is extremely wetting to a clean glass surfac
e with θ≈0. For a clean mercury‐air‐glass interfac
e, θ≈130°.
Pressure
Measurements
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Pressure is the action of one force against another
over, a surface. The pressure P of a force F
distributed over an area A is defined as:
P = F/A
System
Length
Force
Mass
Time
Pressure
MKS
Meter
Newton
Kg
Sec
N/M2 =
Pascal
CGS
CM
Dyne
Gram
Sec
D/CM2
English
Inch
Pound
Slug
Sec
PSI
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A Newton is the force necessary to accelerate a
mass of 1 kg at a rate of 1 meter per second per
second.
The acceleration of gravity is 9.8 m/sec2
The force due to gravity on a 1 kg mass is 9.8 N is
1 kg weight.
1 Newton is 0.102 kg weight.
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1 N/m2 is a very small pressure
Therefore kilopascal (kPa)
1 atmosphere (14.7 psi, 750mmHg) is
approximately 100 kPa = 1 bar
1 kPa is about 7 mmHg
1% of a gas at our altitude is about 7 mmHg
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Collision of molecule with wall
Momentum is mass x velocity
Change of momentum is double
Collision is isothermal = perfectly elastic
Sum collisions over area to get force
Static pressure is the pressure of fluids or gases that
are stationary or not in motion.
Dynamic pressure is the pressure exerted by a fluid or
gas when it impacts on a surface or an object due to its
motion or flow.
Impact pressure (total pressure) is the sum of the
static and dynamic pressures on a surface or object.
Point B in Fig. depicts the impact pressure.
Absolute pressure
The pressure is referenced to zero absolute pressure and
has units of psia. Absolute pressure can only have a
positive value.
Gauge pressure
The pressure is referenced to atmospheric pressure and
by convention is measured in the positive direction, i.e. 7
psig.
Vacuum pressure
The pressure is referenced to atmospheric pressure and
by convention is measured in the negative direction, i.e.
-50 mm Hg.
A number of measurement units are used for pressure.
They are as follows:
1.
Pounds per square foot (psf) or pounds per square
inch (psi)
2.
Atmospheres (atm)
3.
Pascals (N/m2) or kilopascal (1000Pa)*
4.
Torr = 1 mm mercury
5.
Bar (1.013 atm) = 100 kPa
6.
14.696 lbf/in2 equals 33.9 feet of H2O
7.
14.696 lbf/in2 equals 29.921 inches of of Hg
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As previously noted, pressure is force per unit area
and historically a great variety of units have been
used, depending on their suitability for the application.
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For example, blood pressure is usually measured in
mmHg because mercury manometers were used
originally.
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Atmospheric pressure is usually expressed in in mmHg
for the same reason.
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Other units used for atmospheric pressure are bar and
atm.
The following conversion factors should help in
dealing with the various units:
1 psi= 51.714 mmHg
= 2.0359 in.Hg
= 27.680 in.H2O
= 6.8946 kPa
1 bar= 14.504 psi
1 atm. = 14.696 psi
The atmospheric pressure is usually measured by
means of : Mercury barometer
 Android brometer
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Manometers are one of the oldest pressure
measuring devices.
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Change Manometer fluid.
Incline the manometer.
Combine inclined and straight sections.
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Airfoils, wings and complete airplane models
equipped with pressure ports can be used to
determine model loads and load distribution.
Pressure models provide more detail on how the
loads are generated than a force balance.
Disadvantage is pressure measurement of model
forces is more time consuming.
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Suppose you have designed a new airfoil section
and you want to determine its aerodynamic
performance and its pressure distribution, how
would make such a model?
Bourdon tube pressure gauge
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In “C” type Bourdon tube, a section of tubing that
is closed at one end is partially flattened and
coiled.
When a pressure is applied to the open end, the
tube uncoils.
This movement provides a displacement that is
proportional to the applied pressure.
The tube is mechanically linked to a pointer on a
pressure dial to give a calibrated reading.
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An opening in a tank or vessel through which the
liquid flows out is known as an orifice.
This hole or opening is called an orifice, so long as
the level of the liquid on the uptream side is above
the top of the orifice.
The orifice is used to measure discharge. An
orifice may be provided in the vertical side of a
tank or in the base.
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A mouthpices is a short length of a pipe
which is two to three time its diameter in
length fitted in a tank or vessel containing
the fluid.
orifice as well as mouthpices are used for
measuring the rate of flow of fluid.