A smooth pipe line of 100 mm diameter carries
2.27m3 per minute of water at 20∘C with kinematic viscosity of 0.0098 Stokes. Calculate the friction factor maximum velocity as well as shear stress at the boundary.
Solution:-
Given:-
Diameter of pipe, D=100mm=0.1m
Radius, R=0.12=0.05m
Discharge, Q=2.27m3/min=2.2760=0.0378m3/s
Kinematic viscosity, v=0.0098stokes
v=0.0098×10−4m2/s
Therefore Average velocity,
U¯¯¯¯=QA=0.0378π4(0.1)2=4.817m/s
Therefore Reynolds number is given by,
Re=U¯¯¯¯×Dv
Re=4.817×0.10.0098×10−4=4.9154×105
The flow is turbulent and Re is more than 105.
Hence for smooth pipe, the coefficient of friction ‘f’ is obtained by,
14f−−√=2.0log10(Re4f−−√)−0.8
14f−−√=2.0log10(4.9154×1054f−−√)−0.8
14f−−√=2.0[log10(4.9154×105)+log104f−−√]−0.8
14f−−√=2.0[5.6915+log104f−−√]−0.8
14f−−√=2×5.6915+2log104f−−√−0.8
14f−−√=11.3830+log10(4f−−√)2−0.8
14f−−√=11.383+log10(4f)−0.8
14f−−√−log10(4f)=11.383−0.8=10.583……………….(1)
Step 1: Friction factor
(f*)=4× coefficient of friction=4f
substitute the value in equation (1),
1f∗−−√−log10(f∗)=10.583………………(2)
The above equation is solved by hit and trial method,
Therefore, let f*=0.1, then LHS of equation (2), becomes
LHS=10.1−−−√−log10(0.1)=3.16−(−1.0)=4.16
Let f*=0.01, then LHS becomes,
LHS=10.01−−−−√−log10(0.01)=10−(−2)=12
But for exact solution, LHS should be (10.583). Hence value of f* lies between 0.1 and 0.01.
Let f*=0.013, then LHS becomes,
LHS=10.013−−−−√−log10(0.013)=8.77−(−1.886)=10.656
which is approximately (10.583).
Hence the value of (f*=0.013).
Therefore friction factor, f*=0.013.
Step 2: Maximum velocity (umax)
We know, f*=4f
∴f=f∗4
∴f=0.0134=0.00325
Now,
u∗=U¯¯¯¯f2−−√
u∗=4.817×0.003252−−−−−−−√
u∗=4.817×0.04303=0.194
For smooth,
umaxu∗=5.75log10u∗yv+5.55
The velocity will be maximum at the centre of pipe.
Where, y=R=0.05,
umaxu∗=5.75log10u∗Rv+5.55
umax0.194=5.75log100.194×0.050.0098×10−4+5.55
umax=0.194[22.974+5.55]=5.528m/s
Step 3: Shear stress at boundary (τ0)
u∗=τ0ρ−−−√
∴τ0=ρu2∗
∴τ0=1000×(0.194)2=37.63 N/m2
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