Flow over a cylinder

AIM: Flow over a cylinder.

Objectives of the project:

1. To calculate the Reynolds number for the baseline simulation and run it.
2. Run a transient flow simulation for an increasing Reynolds number by a factor of 20%, 40% and 100% of the baseline Reynold number.
3. Create an animation for pressure-velocity for all the three Reynolds number.

Introduction:

Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flow. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid (liquids and gases) with surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved, and are often required to solve the largest and most complex problems.

Geometry of the Model:

Calculations:

Assumptions made during flow simulation:

1. Roughness value is considered as zero.
2. Surface of the cylinder is smooth.
3. Effect of temperature is neglected in study.
4. Computational domain in x-direction=0.5m
5. Diameter of the cylinder=0.04m.
6. Length of the cylinder=0.1m

• For Baseline simulation:

Re=rho*U_inlet*D/mu=27071.82(laminar region)

Where, D=cylinder diameter=0.04m

rho=density=1.225 kg/m³

mu=dynamic viscosity=1.81×10^-5 kg/ms

U_inlet=inlet velocity=10 m/s

• For Re=120% of Baseline simulation=120% of 27071.82=32486.18

Re=rho*U_inlet*D/mu

U_inlet=(32486.18*1.81×10^-5)/(1.225*0.04)

U_inlet=12 m/s

Flow through time (FTT)=Length of domain/Velocity

FTT=0.5/12=0.041sec

End time=FTT*2=0.082sec

• For Re=140% of Baseline simulation=140% of 27071.82=37900.54

Re=rho*U_inlet*D/mu

U_inlet=(37900.54*1.81×10^-5)/(1.225*0.04)

U_inlet=14 m/s

Flow through time (FTT)=Length of domain/Velocity

FTT=0.5/14=0.035sec

End time=FTT*2=0.071sec

• For Re=200% of Baseline simulation=200% of 27071.82=54143.64

Re=rho*U_inlet*D/mu

U_inlet=(54143.64*1.81×10^-5)/(1.225*0.04)

U_inlet=20 m/s

Flow through time (FTT)=Length of domain/Velocity

FTT=0.5/12=0.025sec

End time=FTT*2=0.05sec

Procedure:

1. Firstly, created a cylinder of above mentioned dimensions.
2. Then in wizard, an external flow analysis id done.
3. Fluid as an air is taken into consideration.
4. Velocity in x-direction is taken as calculated U_inlet velocity.
5. Computational domain in x-direction is set to 0.5m
6. Calculated end time is taken as total analysis time.
7. Now, cut plot is made on xy plane to view the flow simulation for parameters such as pressure and velocity.
8. Run the project and finally we get the cut-plots of pressure and velocity.

Results:

• Flow over a cylinder at Baseline Velocity 10 m/s

Cut Plot of Velocity Contour:

Animation Link:https://youtu.be/aayfULrmYI8

Cut Plot of Pressure Contour:

Animation Link:https://youtu.be/v086387C7Qk

• Case1: Flow over a cylinder with increase of 20% of Reynolds Number at Velocity 12 m/s

Cut Plot of Velocity Contour:

Animation Link:https://youtu.be/Zv_s6i67wIo

Cut Plot of Pressure Contour:

Animation Link:https://youtu.be/Sp-1J7ZeAjE

• Case2: Flow over a cylinder with increase of 40% of Reynolds Number at Velocity 14 m/s

Cut Plot of Velocity Contour:

Animation Link:https://youtu.be/Ovlgna1TZkU

Cut Plot of Pressure Contour:

Animation Link:https://youtu.be/SxOFThbgfkg

• Case3: Flow over a cylinder with increase of 100% of Reynolds Number at Velocity 20 m/s

Cut Plot of Velocity Contour:

Animation Link:https://youtu.be/pdRgGERmUbc

Cut Plot of Pressure Contour:

Animation Link:https://youtu.be/lzTaKI7Czso

Obervations:

• It is observed that as Reynolds number increases, the velocity in x-direction also increases and the time required to flow decreases. Thus, increasing the flow of air over the cylinder.
• Also, with the increase in the percentage of Reynolds number, the flow shifts from laminar to turbulent flow.
• The flow over a cylinder becomes steady as the Reynolds number or velocity increases.
• The pressure variation does not vary to a large extent for Reynolds number taken.
• The pressure is mainly acting on the frontal area of the cylinder.