Transient Flow Simulation over a Cylinder

Introduction;

Usign CFD analysis an external flow over a cylinder is simulated with inital conditions. The Reynolds number calculated is then increased by a factor of 20%, 40%, and 100% & the simulation end time is calculated for the transient flow simulation. Animations for pressure-velocity for different scenarios of Reynolds number is generated to understand the variation of pressure & velocity with increase in velocity/reynolds number.

Inital Conditions for Baseline simulation & Model Geometry;

Reynold's number is calculated by;

Diameter of cylinder - 0.05m

Length of cylinder - 0.06m

Inital Velocity - 10m/sec

Fluid used for simulation - Air

Density of air - 1.224 kg/m^3

Dynamic viscosity - 1.81x10^-5 Ns/m^2

Calculating the Reynolds Number

Using the above formula the Re number is calculated as 33812.15

Similarly, the Reynold's number is increased by 20%, 40% and 100 %

At 20% Re = 40574.58

At 40% Re = 47337.01

At 100% Re = 67624.30

CALCULATiNG SMULATION END TIME

using the formula ; Velocity = Distance / time ;

here distance refers to the length of domain & time refers to flow through time

The length of the domain along the X axis is 0.3m

The Velocity is 10 m/s for Re - 33812.15 then flow through time is - 0.03 sec

Considering end time is 2 times the flow through time

At Baseline V = 10 m/sec , Re = 33812.15, Simulation end time = 0.06 sec

At 20% V = 12 m/sec , Re = 40574.58, Simulation end time = 0.05 sec

At 40% V = 14 m/sec , Re = 47337.01, Simulation end time = 0.042 sec

At 100% V = 20 m/sec , Re = 67624.30, Simulation end time = 0.03 sec

STEPS FOR SIMULATION;

1. The cyclinder is created using the above dimensions

2. Go to the flow simulation tab & select wizard option to run the simulation

3) Select external flow simulation & fluid as air.

4) We are considering the transient flow simulation and end time for simulation is calculated as above

5) Inlet velocity & end time is provided for varing cases of Re as above

6) Create two cut plot in the direction parallel to the direction of fluid flow, one cut plot for velocity profile and another cut plot for pressure profile.

7) Run the simulation & load the results.

8) Goto animation & create the animation for for both the velocity and pressure throughout the fluid flow over the cylinder.

RESULTS;

Flow over a cylinder - Baseline Velocity of 10m/sec

Velocity contour Pressure contour

Flow over a cylinder - With 20% increase in Re i.e Velocity of 12m/sec

Velocity contour Pressure contour

Flow over a cylinder - With 40% increase in Re i.e Velocity of 14m/sec

Velocity contour Pressure contour

Flow over a cylinder - With 100% increase in Re i.e Velocity of 20m/sec

Velocity contour Pressure contour

Link for Animations;

https://drive.google.com/drive/folders/1PZre6I-TButcEFL68n3ZQIKOXHG4l8wk?usp=sharing

Inference;

• With increase in the reynolds number by 20%,40% .. the Simulation end time reduces which means the working fluid (air) will be travelling over the cylinder at a faster pace & leave the domain.
• There is no significant change in the pressure variation with increase in reynolds number
• When the velocity is higher For higher values of the velocity waves are created behind the cylinder