Week 2 - Flow over a Cylinder.

REPORT FOR CHALLENGE-2

Aim : To Simulate the flow over the cylinder and calculate the Strouhal Number for a given problem using workbench-fluent under steady and unsteady case for Re= 100. And also calculate the coefficient of drag and lift over a cylinder for Reynolds number
10,100,1000,10000 & 100000 under steady state condition. Discuss the effect of Reynolds number on the coefficient of drag.

Software Required : Ansys Workbench – Fluid Flow vR2-2022

Hardware Required : Desktop, Keyboard, Mouse, CPU.

Theory:

Under the correct flow conditions, the flow around a bluff body produces a regular pattern of alternating vortices known as a von Karman vortex street. These vortices produce a sinusoidal force acting on the body perpendicular to the flow. For a cylinder, the frequency ω (in Hz) of the shedding is related to the diameter D(m) and wind speed v (ms⁻¹) via the Strouhal Number S.

Formula: S=ωD/v.

For very small Reynolds Number, the flow is completely laminar around the cylinder, with no recirculation and no instability.

For larger Reynolds numbers, a recirculation zone appears behind the obstacle. More precisely, two eddies with opposite signs appear just behind the cylinder and remain close to it.

For larger Reynolds numbers, these eddies are unstable and successively leave the cylinder to propagate in the wake, to form the so called “Von Karman” vortex street.

Procedure :

1.Open the Ansys Workbench.

2.Double click or drag the Fluid Flow module.

3.Import the required geometry or Design the geometry with the required specifications and extract volume if need, then after mesh the geometry with the suitable element size in addition to name all the faces which model have.

• Open the Space claim and create a Cylinder with 2m.
• Create the domain with the suitable specifications for the analysis of the flow over a cylinder.
• Open the mesh block and mesh the geometry with the triangular elements.
• Create a Inflation layer to certain height corresponding to the wall boundary of the cylinder.

4.Solve the equations in the set-up and solution block for required result.

• Set 2D, Double Precision and Assign number of cores.
• Set Steady state.
• Change method to SIMPLE.
• Assign the Material Properties and Boundary conditions.
• Check and assign all the Reference values properly.
• Create definition plots for CL,CD and Avg Vortex Velocity.
• Create a monitor point at some distance behind the cylinder for to get vortex values.
• Create Contours.
• Initialize and create Animation for velocity contour and Calculate.
• Save the Animation.
• Plot the strouhal number.

5.Then do the post-processing in the CFD solver in the result block.

6.Save and close the file.

Observations :

Geometry and Mesh Details:

Inflation and Edge sizing Details:

-First Layer Thickness:5mm

-Max Layers:15

-Growth Rate:1.2

-No.of Divisions:50

Reference values:

Density	1kg/m^3
Length	2m
Depth	1m
Area	2m^2
Viscosity	0.05N-s/m^2

Boundary Conditions:

Based on the different Reynolds numbers, I did solved this problem with 5 different cases using their corresponding velocity values as the inlet boundary conditions. Those are as follows,

	Casse-1	Case-2	Case-3	Case-4	Case-5
Reynolds number	10	100	1000	10000	100000
Corresponding Velocity(m/s)	0.25	2.5	25	250	2500

Case-1:

Solution Status:

-No.of Iterations = 712

-After the required equations solved, tabulated the results that were obtained.

Plots and Contours:

Case-2:

Solution Status:

-No.of Iterations = 700

-After the required equations solved, tabulated the results that were obtained.

Plots and Contours:

Case-3:

Solution Status:

-No.of Iterations = 600

-After the required equations solved, tabulated the results that were obtained.

Plots and Contours:

Case-4:

Solution Status:

-No.of Iterations = 400

-After the required equations solved, tabulated the results that were obtained.

Plots and Contours:

Case-5:

Solution Status:

-No.of Iterations = 400

-After the required equations solved, tabulated the results that were obtained.

Plots and Contours:

Precautions :

1.Carefully solve the problem.

2.Choose the suitable methods carefully for to get the desired results.

3.Note down the values carefully which are required for theoretical calculations.

Result:

Case	Reynolds No	Corresponding Velocity	CL	CD	Vortex Avg Velocity(m/s)	(ST.no)max
1.	10	0.25	0.00227	3.3286	0.1058	0.007849
2.	10^2	2.5	-0.06842	1.3649	1.6015	0.11482
3.	10^3	25	0.3463	1.1149	18.0408	0.4529
4.	10^4	250	-0.3777	1.2019	176.492	0.6609
5.	10^5	2500	-0.8738	1.3251	2139.02	0.81101

Resulted contour images which describes the Point of seperation in the different velocity flow fields after analysis were given below,

1.Point of seperation contour for the flow when velocity=0.25m/s.

2.Point of seperation contour for the flow when velocity=2.5m/s.

3.Point of seperation contour for the flow when velocity=25m/s.

4.Point of seperation contour for the flow when velocity=250m/s.

5.Point of seperation contour for the flow when velocity=2500m/s.

Conclusion:

When the fluid is passing over a body there are many changes happens in the properties since attaching to detaching to the body. Which means properties of the fluid is not constant if there is any obstacle present in the way of fluid direction. There will be change in the Pressure, Velocity, etc,.Due to the pressure and velocity change while contact with the body, there will be some forces are generated like lift, drag, etc,.While detaching the fluid from the body, eddys or vortexes forms behind the body to certain distance. The flow around a bluff body produces a regular pattern of alternating vortices. These vortices produce a sinusoidal force acting on the body perpendicular to the flow.For very small Reynolds Number, the flow is completely laminar around the cylinder, with no recirculation and no instability.For larger Reynolds numbers, a recirculation zone appears behind the obstacle. More precisely, two eddies with opposite signs appear just behind the cylinder and remain close to it.For larger Reynolds numbers, these eddies are unstable and successively leave the cylinder to propagate in the wake. Increase in the velocity leads to increase in the Reynolds number, which leads to indrease in the vortex shedding and for every increase in the reynolds number leads to increase in the Strouhal Number.