FLUID FLOW PHENOMENA
Fluid:In physics a fluid is a substance that continuously deforms under an applied
shear force. A fluid is a substance that doesn’t permanently resist distortion. An attempt
to change the shape of mass of a fluid results in sliding of the layer of the fluid over one
Fluid Flow:Potential Flow: - The flow of incompressible fluid with no shear is
known as Potential flow
It has some important characteristics1. Neither circulation nor eddies forms within the stream.
Hence the potential flow is known as irrotational flow.
2. Friction cannot develop since there is no existence of
shear stress & hence there is no dissipation of mechanical energy into heat energy.
Boundary layer:The effect of solid boundary on the flow is confined to the layer of the
fluid immediately adjacent to the solid boundary. This layer is called Boundary Layer &
also the shear stress are confirm to this part of the fluid only.
Parts of fluid:(1)Boundary Layer (2)Remaining fluid
RHEOLOGICAL PROPERITIES OF FLUIDS
Newtonian & Non-Newtonian fluid:Newtonian fluid – Fluid flow in simple linearity are
called Newtonian fluid. In a Newtonian fluid the
shear stress is proportional to the shear rate ,
and the proportionality constant is called the
g c dy
where μ = co-efficient of
Exmp- Water , Gasses etc
Non-Newtonian fluid1. The curve starts from origin & concave
downwards represents Pseudoplastic fluid & this
type of fluid is said to be shear rate –thinning.
Exmp – Polymer solutions , starach
FIGURE : Shear stress vs shear rate 2. The curve starts from origin & concave
upwards represents Dilatant fluid & this type of
fluid is said to be shear rate –thickening.
Newtonian & Non-Newtonian fluid
Exmp – Wet beach sand , starch in water etc
3. The straight line having some intercepts in y – axis represents Bingham plastic . This type of fluid
do not flow at all until a threshold shear stress
attained & then flow linearly at shear stress
Exmp – Sludge
Reynolds stresses :- The stress is much larger in turbulent flow than the laminar
flow . Since the shear stress is higher in turbulent flow Turbulent shear stress are
called Reynolds stresses
Eddy viscosity :- By analogy , he relationship between shear stress and velocity
gradient in a turbulent stream is used to define an eddy viscosity EV .
where E v = eddy viscosity
t g c Ev
Also we know ,
μ = co-efficient of viscosity
g c dy
The above two expression is almost similar .Hence eddy viscosity is analogous to μ .
where ν=kinematic viscosity
DV DV DV
=Eddy diffusivity of momentum =
Here kinematic viscosity is analogous to eddy diffusivity.
Here the flow of fluid is parallel
to a thin plate LM . A boundary
is define as the part of a
moving fluid in which a fluid is
influence by a solid boundary .
The velocity of the fluid as
solid-liquid interface is zero.
The velocity increases with
distance from the plate as
shown in figure.
Each of the curve represents the velocity profile for definite value of
x , the distance from the leading edge of the of the plate. The curves
changes slope rapidly near the plate . Line OL represents an
imaginary surface , which separates the fluid stream into two parts ,
one in which fluid velocity is constant and the other where the
velocity varies from zero to a velocity substantially equal to that of
un disturbed fluid.
LAMINAR & TURBULANT FLOW IN BOUNDARY LAYER
Flow near the
boundary layer is
laminar flow. Since
velocity is very low as
we move further from
the solid boundary the
velocity is fairly large
and hence the floe
There are three
2. Buffer layer
3. Turbulent zone
BOUNDARY LAYER FORMATION IN STRAIGHT TUBUES
Considering a straight, thin-walled tube with fluid entering it at a
uniform velocity. As shown in the above fig. A boundary layer begins
to form at the entrance to the tube and as the fluid move to the first
part of the channel , the boundary layer thickens. During this stage the
boundary layer occupies only a part of the tube & total stream consists
of a core of fluid moves like a road like manner . But the velocity of
fluid is constant. In the boundary layer , the velocity varies from zero
to constant velocity existing in the core . As we further move down to
the tube , the boundary layer occupies an increasing portion of the
cross-section of the tube.
At this point , the velocity distribution in the tube reaches
its final point & remains unchanged for the remaining part of the fluid .
Such flow with an unchanging velocity distribution is called ‘Fully
Developed Flow' .
The length of the entrance region of the
tube necessary for the boundary layer to
reach the centre of the tube & for the fully
developed flow to be established is called
We can express it by
(Xt / D)=0.05*NRe
Xt = Transition Length
D= Diameter of the tube