Flow over a cylinder

1. AIM

Simulate the flow over a cylinder for different conditions

2. OBJECTIVES

1. Calculate the Reynolds number for a particular velocity. Then increase the Reynolds number by a factor of 20%, 40%, and 100% then run the transient flow simulation.
2. Create animations for pressure-velocity for all the three Reynolds number.

3. INTRODUCTION

Case 1 shows very slow flow in which we have neglected viscosity entirely. We have an ideal flow with no boundary layer along the surface, completely attached flow (no separation) and no viscous wake downstream of the cylinder. Because the flow is symmetric from upstream to downstream, there is no drag on the cylinder.

Case 2 illustrates what actually occurs for low velocity. A stable pair of vortices are formed on the downwind side of the cylinder. The flow is separated but steady and the vortices generate a high drag on the cylinder.

Case 3 illustrates more unstable pair of vortices are formed on the downwind side of the cylinder due to further increase in velocity.

Case 4 shows the flow as velocity is increased. The downstream vortices become unstable, separate from the body and are alternately shed downstream. The wake is very wide and generates a large amount of drag. The alternate shedding is called the Karman vortex street.

4.CAD GEOMETRY

5. FLOW SIMULATION

SOLIDWORKS Flow Simulation is an intuitive Computational Fluid Dynamics (CFD) solution embedded within SOLIDWORKS 3D CAD that enables you to quickly and easily simulate liquid and gas flows through and around your designs to calculate product performance and capabilities.

The major tabs in Flow simulation

PARAMETERS

1. Internal flow simulation
2. Transient state study
3. Pressure: 101325KPa
4. Temperature: 293.2K
5. Medium : Air
6. Velocity in X direction : 1m/s in case 1
7. Wall conditions: Adiabatic with 0 micrometer roughness,

STEPS

1. Model the system using solidworls sketch, boss extrude tool.
2. Activate the flow simulation plugin of solidwokrs in add-ins tab.
3. Set a domain size and calculate the flow through time required for inputing the value of time dependent study.
4. Select wizard and input respective details including medium as per parameters given above to set the project.
5. Set the computational domain to 3D and set the domain in such a way that the silumation result is not affected.
6. Redefine the gloabal mesh for more accurate results.
7. Insert cutplot of front plane for velocity and pressure.
8. Run the simulation
9. Create animation.

Domain lenght is set as 0.36m

`"Flow through Time" = "Domain length"/V`

`"End time" = 2* "Flow through Time"`

`Re = (ρVD)/μ`

	Case1 = Re1	Case2 = 1.2Re1	Case3 = 1.4Re1	Case4 = 2Re
Velocity (m/s)	1	1.2	1.4	2
Renolds number(Re)	2707.18232	3248.6188	3790.0552	5414.3646
Flow through time (s)	0.36	0.3	0.2572	0.18
End time (s)	0.72	0.6	0.5143	0.36

6. RESULTS, OBSERVATIONS AND CONCLUSIONS

Case1 = Re1

Animation of velocity plot

Animation of pressure plot

Case2 = 1.2 x Re1

Animation of velocity plot

Animation of pressure plot

Case3 = 1.4 x Re1

Animation of velocity plot

Animation of pressure plot

Case4 = 2 x Re1

Animation of velocity plot

Animation of pressure plot

1. The Reynolds number increases due to increase in velocity in each case.
2. As the Reynolds number increases we can see the increase in wake formation and unstable pair of vortices formed on the downwind side of the cylinder. The downstream vortices become unstable, separate from the body and are alternately shed downstream. The wake is very wide and generates a large amount of drag.
3. For each case the time of flow decreases due to increase in velocity.
4. Velocity of fluid is minimum at the diametrically opposite point where the air hits the cylinder first.
5. Pressure is maximum at point where air strikes the cylinder.

7. REFERENCE

https://www.grc.nasa.gov/www/k-12/airplane/dragsphere.html