112_Flow in Cylinder

Since its first time conducting CFD through COMSOL, I plan to follow the "Flow past a Cylinder" example found in the COMSOL application manual.

 

Fluid flow becomes unstable behind large bodies, making it challenging to compute. The wake formed behind the body generates significant resistance due to all sizes of disordered vorices. However, turbulence in the thin boundary layer beside streamlined bodies causes only weak disturbances. From an engineering perspective, predicting the frequency of oscillations at various fluid velocities and avoiding resonance between solid structure vibrations and vortex shedding is crucial.

Therefore, this example examines the unstable incompressible flow around a long cylinder. A cylinder is placed in the middle to generate vortices in the flow, Here, the Reynolds number is 100, which does not represent fully turbulent flow.

 

The viscous forces acting on the vlinder are proportional to the velocity field's gradient at the surface. While it's possible to directly differentiate the velocity gradient at the boundary for calculation, it's not accurate. Hence, a reaction force operator is used to calculate the integral of viscous forces. The coefficients are as follows.

 

 

Where ${{\text{C}}_D}$ and ${{\text{C}}_L}$ is the drag and lift forces, ${U_mean}$ is mean velocity, and $A$ is projected area.

 

In Model Wizard - 2D - Fluid FLow - LAminar Flow (spf) - add - study

in study - Time dependant - Done

 

Add parameter

name : U_mean

Expression : 1[m/s]

 

 

To specify the fluid's step, create a Step in the Functions from the Home toolber, and set the Parameter - Location to 0.1.

 

 

Create geometry

Rectangle

Width : 2.2

Height : 0.4

Circle

x : 0.2

y : 0.2

Radious : 0.05

 

Booleans and Partitions and choose Difference.

Objects to add : r1

Objects to subtract : c1

 

 

Materials

Density : 1

Dynamic viscosity : 1e-3

 

Boundary condition

Select the left edge of the cylinder as the inlet and set ${U_0}$ as ${\text{U_mean*6*s*(1 - s)*step(t[1/s])}}$.

Select the right edge of the cylinder as the outlet.

 

Mesh

Element size : Finer

 

Study - Time Dependent

Output times : range(0,0.2,3.4) range(3.5,0.02,7)

Time Stepping - Steps taken by solver : Intermediate

 

Result

Through the above example, we learned that fluid flowing around large bodies can create vortices under certain conditions. To avoid such phenomena, various methods incorporating spiral shapes, are employed.

Since it was a very simple example, it was much easier to perform than when using Fluent.