CFD -Flow over a cylinder

AIM:

• To perform CFD analysis on a cylinder in which external flow takes place and running a baseline simulation and then calculating the Reynolds number and also the simulation end time.
• To increase the Reynolds number by a factor of 20%, 40%, and 100% & then run the transient flow simulation.
• To Animations for pressure-velocity for all the three Reynolds number created.

FORMULA:

Reynolds Number ;

Reynolds number will be useful in identifying the nature of the flow of a fluid which shows either laminar, turbulent, or laminar-turbulent. The transition from laminar to turbulent flow depends on the surface roughness, flow velocity, geometry, surface temperature, and type of fluid, among others.

The Reynolds number is defined as:

Re= (rho*D*V)/ muR e = ρ u D μ = u D ν {\displaystyle \mathrm {Re} ={\frac {\rho uD}{\mu }}={\frac {uD}{\nu }}}

Where:

• Rho is the density of the fluid (SI units: kg/m³)
• V is the fluid time(m/s)
• D is the diameter of the tube (m)
• Mu is the dynamic viscosity of the fluid(Pa·s or N·s/m² or kg/(m·s))

DIMENSIONS OF THE CYLINDER:

Diameter of the cylinder = 0.04 m

Length of the cylinder = 0.2 m

BASELINE SIMULATION PARAMETERS:

Let us consider the initial velocity of the fluid as 5 m/s and taking the standard air properties since air is taken as a fluid

INPUTS	VALUES
Diameter of cylinder	0.04 m
Density of air	1.225 kg/m^3
Dynamic viscosity of air	18.14 e-06 Ns/m^2
Inlet velocity	5 m/s

Re = rho*V*D/mu

Re= (1.225*5*0.04)/18.14 e-06

Re = 13506.06

Similarly, Reynold's number at 20, 40 and 100 % is calculated,

At 20% Re = 16207.27

At 40% Re = 18908.48

At 100% Re = 27012.12

The baseline simulation was conducted based on the values we’ve known.

INPUT	VALUES
Fluid	air
Analysis type	External flow
Time dependant	Total analysis time:0.16 s
	Output time step:0.016 s
Global mesh level	5
Inlet velocity	5 m/s

CALCULATION OF SIMULATION END TIME:

Velocity = Distance / time

Time = Distance / velocity

i.e. Time = 0.40/5= 0.08 sec

The length of the domain along the X-axis is 0.40 m

Given that the end time should be 2 times the flow-through time

At Baseline For V = 5 m/s , Re = 13506.6 , Simulation end time = 0.16sec

Similarly:

At 20% For V = 6 m/s , Re = 16207.27 , Simulation end time = 0.133 sec

At 40% For V = 7m/s , Re = 18908.48 , Simulation end time = 0.114 sec

At 100% For V = 10 m/s , Re =27012.12, Simulation end time = 0.08 sec

The above relation depicts that velocity increases the time decreases.

RESULTS:

1) FOR BASELINE SIMULATION:

VELOCITY -CUT PLOT FOR 5 m/s:

PRESSURE -CUT PLOT:

ANIMATION FOR VELOCITY:

ANIMATION FOR PRESSURE:

2)CASE-1: FOR 20% INCREASE IN REYNOLDS NUMBER :

VELOCITY-CUT PLOT @6 m/s:

PRESSURE-CUT PLOT:

ANIMATION FOR VELOCITY:

ANIMATION FOR PRESSURE:

3)CASE-2: FOR 40% INCREASE IN REYNOLDS NUMBER :

VELOCITY-CUT PLOT @7 m/s:

PRESSURE-CUT PLOT:

ANIMATION FOR VELOCITY:

ANIMATION FOR PRESSURE:

4)CASE-3: FOR 100% INCREASE IN REYNOLDS NUMBER:

VELOCITY-CUT PLOT @10 m/s:

PRESSURE-CUT PLOT:

ANIMATION FOR VELOCITY:

ANIMATION FOR PRESSURE:

CONCLUSION:

From the above plots and animation, it has been clear that there is no significant change in the pressure plot and pressure is mainly acting of the frontal area of the cylinder

The Reynold's number increases as the Velocity increases which results in a decrease of time. the velocity or Renold's number increase the flow over the cylinder becomes steady and the velocity plot at 20% and 40% exhibit a slight unsteady plot whereas at 100% the flow is steady and we can draw a clear understanding of the flow when velocity is increased.