Steady vs Unsteady Flow over a Cylinder Using Ansys Fluent

Steady vs Unsteady Flow over a Cylinder

Aim:

To perform steady & transient simulations on flow over a cylinder geometry studying the characteristics of the different solvers using Ansys Fluent.

Procedures:

1. Generating the CAD Model of a channel with a cylinder using Ansys SpaceClaim plugin.
2. Mesh Processing.
3. Setting up the boundary conditions & to solve the case using the below-mentioned solver type.
   
   • Steady Type
   • Transient Type
4. Post Processing output files.

1:- Pre-Processing:

1.1 CAD Preparation:

In this case,

• The length of the channel is L = 60m.
• The Cylinder outer wall diameter is D = 2m.
• The width of the flow region is W = 20m.
• The flow is along X-axis.
• Flow medium is 'user-defined' fluid.
• Velocity at Inlet will be u=1 m/s.
• Reynold number for the flow is 100.
• The viscosity of the fluid is 0.02 kg/ms.
• The density of the fluid is 1 kg/m^3.

2:- Mesh Processing:

2.1 Meshing:

Launch the mesh generator from the project schematic window. In Mesh Quality section enable the Mesh Metric by element size to verify the mesh generated is above 10%. In order to increase the accuracy of the simulation, the element sizing is set to 0.25m.

2.2 Meshing Edge Sizing & Inflation Implementation:

In this case, the number of lines connected along the circumference of the cylinder wall will be less which results in inaccurate simulation over the cylinder wall regions. To resolve this we can implement the Edge sizing & Inflation Meshing methods. By Edge sizing the number of lines connecting along the cylinder circumference making the mesh finer at the edge region. The Inflation Method increases the number of elements along with the cylindrical shape by 6 layers. The meshing shape is set to triangles. The below image shows the final mesh generated with the edge sizing & inflation methods.

3.1- Setting up Solver:

These cases are solved using pressure-based solver as the flow is determined as non-variable with Reynold number of the fluid flow is 100. A monitor point is added for 8m away from the cylinder in order to capture the velocity at that particular point.

In setting up physical models, we use Laminar under the Viscous model section. This will provide us a stable solution as the Reynolds number is very low.

In setting up the case we use a monitor point which is located 8m away from the cylinder to measure the velocity at the region through the option Vertex average under definitions option.

4- Post-Processing:

4.1- Steady Flow Case Post-Processing:

4.1.1-Residual Plot:

This case took around 450 number of iterations to attain steady-state flow. This can be known from the above chart as the velocity & continuity curves attain a varied stable state.

4.1.2-Velocity at monitor point Plot:

The above plot shows us the velocity at the provided monitor point with respect to the number of iterations. From iteration 200-320 we can see the raising oscillating curves which helps us to find Strouhal number for this case setup.

4.1.3-Strouhal number:

The above number shows us the Strouhal numbers with respect to the velocity at the monitor point. From the above plot, the Strouhal number is found to be 0.000539243, this is calculated by the average number from the Strouhal number plot points exported from the Ansys fluent. The Strouhal number found is very low in steady flow solver. So the Strouhal number we get from the steady-state solver for the unsteady flow may not be accurate.

4.1.4-Velocity Contour:

The above figure shows us the simulation of steady flow over a cylinder. In this, we can see the vortex shedding across the cylinder.

Case1_Simulation_Clip:

4.2- Steady Flow Case Post-Processing:

4.2.1-Residual Plot:

This case took around 19000 number of iterations to attain steady-state flow. This can be known from the above chart as the velocity & continuity curves attain a varied stable state.

4.2.2-Velocity at monitor point Plot:

The above plot shows us the velocity at the provided monitor point with respect to the number of iterations. From iteration 7500-18000. we can see the raising oscillating curves which helps us to find Strouhal number for this case setup.

4.2.3-Strouhal number:

The above number shows us the Strouhal numbers with respect to the velocity at the monitor point. From the above plot, the Strouhal number is found to be 0.25, this is calculated by the average number from the Strouhal number plot points exported from the Ansys fluent.

4.2.4-Velocity Contour:

The above figure shows us the simulation of steady flow over a cylinder. In this, we can see the vortex shedding across the cylinder.

Case2_Simulation_Clip: