We were discussing the basic concept of kinetic
energy correction factor and momentum correction factor, power
absorbed in viscous flow, viscous
resistance of journal bearings, viscous
resistance of foot- step bearings, viscous resistance of collar bearings
and also the difference between laminar flow and turbulent flow, in the subject
of fluid mechanics, in our recent posts.
Now we will go ahead to find out the basics of
Reynolds experiment, in the subject of fluid mechanics, with the help of this
post.
Reynolds experiment
As we are aware that for determining the
type of flow we use to calculate the Reynolds number and on the basis
of Reynolds number we use to decide the flow type. So let us see here
the basics behind the determination of type of flow based on the
Reynolds number.
Value
for Reynolds number might be calculated with the help of
following formula
Re = ρ V D /μ
Where,
V = Flow velocity of the Hydraulic fluid i.e. liquid (m/s)
D = Diameter of pipe (m)
μ = viscosity (poise)
O Reynold had explained this concept with one
experiment, which is explained here, in 1883. Reynold had concluded that
transition from laminar flow to turbulent flow in a pipe depends not only on
the velocity but also it depends on the diameter of the pipe and viscosity of
the fluid flowing through the pipe.
Reynolds experiment apparatus
Apparatus for Reynolds experiment are as mentioned
here
- A tank containing water at constant head
- A small tank containing some dye
- A glass tube with bell-mouthed entrance at one end and a regulating valve at other end
Apparatus for Reynolds experiment
Now we will allow water to pass through the glass
tube from the water tank. Regulating valve is provided here to vary the
velocity of water flowing through the glass tube.
We will introduce a liquid dye, of having same
specific weight as of water, in to the glass tube as displayed here in
following figure.
Observations made by Reynold
Observation I
When velocity of water flow is low, dye filament
will be in the form of straight line in the glass tube. It could be seen in the
glass tube that dye filament is in the form of straight line and parallel to
the wall of glass tube.
Above condition is the example of laminar fluid
flow. Therefore at lower velocity of water flow through the glass tube, the
type of water flow will be laminar.
Following figure, displayed here as figure a,
indicates the case of water flow through the glass tube at low velocity of
water flow.
Observation II
Now velocity of flow is increased with the help of
regulating valve. Dye filament will not be in the form of straight line in the glass
tube. It could be seen in the glass tube that dye filament is in the form of
wavy one now.
Above condition is the example of transition of
fluid flow. Therefore when velocity of water flow through the glass tube is
increased, the type of water flow will be transition flow. Transition flow
means the flow between laminar flow and turbulent flow.
Following figure, displayed here as figure b,
indicates the case of transition flow through the glass tube.
Observation III
Now velocity of flow is increased again with the
help of regulating valve. Wavy dye filament will be broken and finally diffused
in the water as displayed here in following figure.
It could be seen in the glass tube that particles of
dye filament liquid are moving in random and irregular fashion at this higher
velocity of water flow. Mixing of particles of water and dye filament is
intense and water flow will be random, irregular and disorderly.
Above condition is the example of turbulent fluid
flow. Therefore when velocity of water flow will be higher, the type of water
flow will be turbulent flow.
Following figure, displayed here as figure c,
indicates the case of turbulent flow through the glass tube.
In case of laminar fluid flow, loss of pressure head
will be proportional to the velocity of fluid flow.
While in case of turbulent
fluid flow, loss of pressure head will be approximately proportional to the
square of velocity of fluid flow.
Further we will go ahead to find the basics of frictional loss in pipe flow and to derive the
expression of loss of head due to friction in pipes, in the subject of fluid
mechanics, with the help of our next post.
Do you have any suggestions? Please write in comment
box.
Reference:
Fluid mechanics, By R. K. Bansal
Image courtesy: Google
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