Flow over a cylinder

Aim: To study the flow over cylinder using Solidworks simulation feature for baseline simulation and with varying Reynolds number by a factor of 20%, 40%, 100% of the baseline simulation.

Objective:

1. Calculate the Reynolds number for the baseline simulation.
2. Run the baseline simulation.
3. Run the transient flow simulation for an increasing Reynolds number by a factor of 20%, 40%, and 100% of the baseline Reynolds number.
4. Create an animation for pressure – velocity for all the three conditions of Reynolds number.

Theory:

Prepare the cylinder for simulation of following dimensions

• Cylinder diameter = 0.04 m
• Cylinder length = 0.05 m

The characteristics of fluid  flowing inside the pipe are given below,

• Fluid: Air
• Dynamic Viscosity: 0.00001825 N.S/m²
• Density: 1.225 kg/m³

Assumption:

1. The surface of the cylinder is smooth and roughness is considered zero.
2. The effect of temperature is not considered in the study.
3. Computational domain in x-direction = 0.5 m

Problem statement:

1. Simulate the flow over a cylinder for the conditions shown in the video.
2. Calculate the baseline case Reynolds number.

Re = (ρ ×V ×D)/μ

Where,

• Re =Reynolds Number
• ρ = Density of  fuid (kg/m³)
• D = Inner Diameter of pipe (mm)
• μ = Dynamic Viscosity (N.S/m²)

Run a baseline simulation and then calculate the Reynolds number. Then increase the Reynolds number by a factor of 20%, 40%, and 100% then run the transient ow simulation.

Simulation end time should be calculated properly. Make sure that end time is set to 2 times ow through time. According to the previous example, the end time would be 10 seconds. Create animations for pressure-velocity for all the three Reynolds numbers.

Calculation:

For baseline simulation

Assume velocity over a cylinder is 10 m/s. Therefore

Re = (ρ ×V ×D)/μ

Re = (1.225 ×10 ×0.04)/0.00001825

Re = 26849.3

Similarly, Reynolds number increasing at 20, 40 and 100% is calculated,

• At 20% Re = 32219.1
• At 40% Re = 37589.02
• At 100% Re = 53698.6

For Re = 120% of 26849.3 =32219.1

Velocity = 12 m/s

Flow through time = (Domain length )/(velocity )

Flow-through time = 0.04166 sec

Simulation end time = 2 × flow through time

Simulation end time = 0.0833 sec

For Re = 140% of 26849.3 =37589.02

Velocity =14 m/s

Flow-through time = 0.0357 sec

Simulation end time = 2 × flow through time

Simulation end time = 0.0714 sec

For Re = 200% of 26849.3 =53698.6

Velocity = 20 m/s

Flow-through time = 0.025 sec

Simulation end time = 2 × flow through time

Simulation end time = 0.05 sec

Procedure:

1. The cylinder of the above-mentioned dimensions is created.
2. Set the external ow analysis in the wizard.
3. Calculated and time is taken as total analysis time.
4. Fluid as air is taken into consideration.
5. Velocity in the X –direction is taken as calculated velocity from Reynolds number.
6. Set the computational domain in X-direction as 0.5 m.
7. Run the project and get the cut plots of relative pressure and velocity.

Result:

Baseline simulation

Animation for velocity and pressure distribution over the cylinder

                

For Re = 120% of 26849.3 =32219.1

Animation for velocity and pressure distribution over the cylinder

                   

For Re = 140% of 26849.3 =37589.02

Animation for velocity and pressure distribution over the cylinder

                  

For Re = 200% of 26849.3 =53698.6

Animation for velocity and pressure distribution over the cylinder

             

Conclusion

1. Inlet velocity in x-direction increases with the increase in Reynolds number, thus increasing the Kow of air over the cylinder.
2. The relative pressure variation does not vary to a large extent for the Reynolds number.
3. It is observed that the high pressure is created on the surface facing the inlet velocity.
4. The velocity variation in the above animations has a different velocity profile, velocity is maximum at the surface of the cylinder contact with inlet velocity and it is approximately zero nearer to the surface which is not in contact with inlet velocity.