Week 2 - Flow over a Cylinder.

The definitions of Reynolds number, drag and lift coefficients, effect of Re number and Karman vortex street are explained in the written document which is attached below.

PART-1:

Reynolds number change is made by just changing the viscosity of the fluid. Now we will do the analysis (steady state and transient) for Re=100 first.

Procedure:

1.Geometry

Based on the given dimensions geometry is created in the Space Claim in ANSYS Fluent

After the geometry is ready. we will start meshing with sizing, inflation and triangle methods to accurately predict the phenomenon around cylinder.

Mesh:

Setup:

The setup for the steady state simulation and transient simulation are similar except for the type of the time (steady or transient). Once the model setup is done. Material properties are assigned based on the Reynolds number. After that boundary conditions are assigned. Then the required graphs for the results analysis are generated using definitions in solution options. All of these setup are attached below.

Now the monitor point setup is done with all the results including residuals. lets look at the results for Re=100 steady state and transient

STEADY STATE SIMULATION

Average vortex velocity:

Residuals:

Cl:

Cd:

Velocity Contour:

Pressure Contour:

Strouhal Number:

The error for Cd and cl are within 5% and the Strouhal number is 0.05. Convergence reached around 300-350 iterations.

Transient Simulation:

Average vortex velocity

Residuals

Cd

Cl

Strouhal Number

Velocity contour

Pressure contour

Re=10:

The plots are attached as follows.

Vertex average velocity, residuals, CL, Cd, Velocity contour and pressure contour.

RE=1000:

The plots are attached as follows.

Vertex average velocity, residuals, CL, Cd, Velocity contour and pressure contour.

Re=10000

The plots are attached as follows.

Vertex average velocity, residuals, CL, Cd, Velocity contour and pressure contour.

Re=100000:

The plots are attached as follows.

Vertex average velocity, residuals, CL, Cd, Velocity contour and pressure contour.

observations from Results:

In order to confirm that the solution is fully converged in the Karman vortex street simulation is difficult. So for that reason if we look at the monitor point vertex average velocity after certain number of iterations the solution oscillates between same numbers. From these observations we can tell that the solution is converged.

Effect of Re Number on Cl and Cd

Increasing the RE will increase the skin friction drag. As the Re number approaches higher there is high energy loss due to this fact the drag increases.

When it comes to Cl, the plots are oscillating plots. From the plots we can observe that with increase in Re number oscillation increases.

References:

1.https://dragonfly.tam.cornell.edu/teaching/mae5230-tritton-chap3.pdf

2.https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street

3.https://en.wikipedia.org/wiki/Strouhal_number

4.Introduction to fluid dynamics by D.J.Tritton