Flow over a cylinder

AIM: To run a flow over cylinder simulation with an inlet Reynolds number of 100, 120 and 140.

BACKGROUND:

Flow is characterized into Laminar/Turbulent based on the Reynolds number.

Reynolds number: It is a non-dimensional number that is defined by the ratio of Inertial to viscous forces. It gives us an idea of the level of turbulence associated with the flow.

Where,

: Reynolds number

: Density of fluid

V: Velocity of fluid

: Characteristic length for the geometry

: Dynamic viscosity of fluid

PRELIMINARY CALCULATION:

For a pipe having 20mm diameter and 1000mm length with water flowing through the inner portion, the Reynolds number is calculated based on the diameter of pipe (inner).

Fluid properties: Density: 1.225 kg/m³, Dynamic viscosity: 1.78e-5 kg/m.s

Reynolds number	Velocity (m/s)
100	0.07265
1000	0.08718
10000	0.101714

SETTING UP THE PROBLEM:

• Draw a circle of diameter 20mm in a sketch
• Extrude it to 20mm along midplane
• Save the part
• Under Solidworks add-ins, open flow simulation
• Start flow wizard
  • Use correct unit systems
  • Select analysis flow type as external
  • Import fluid as air
  • Define the wall parameters
  • Define operating conditions (Inlet velocity corresponding to Re=100)
  • Define transient parameters: time step =0.1s, 10s simulation time
• Setup required results i.e. contour plots of velocity and pressure along mid plane
• Setup a parametric study for variable inlet velocities (to enforce variable Reynolds numbers)

• Setup a semi-coarse mesh
• Run the simulation
• Perform the same analysis using different meshes i.e. perform a mesh independent study

COMPUTATIONAL DOMAIN:

Domain size:

Upstream: 5* Characteristic length

Downstream: 20* Characteristic length

Top, bottom and sides: 7.5* Characteristic length

RESULTS AND DISCUSSIONS:

1. Velocity contours along midplane

Re=100

Re=120

Re=140

All three plots show similar wake region. Since the flow was simulated for 10s, it is seen that the transient behaviour of flow is absent, which confirms that the flow has become steady.

Therefore the vortex karmann street effect cannot be visualized in this case.