Flow simulation over a cylinder

Objective: To simulate a flow over a cylinder for different reynolds number

Description: The Reynolds number (Re) is an important dimensionless quantity in fluid mechanics used to help predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow (eddy current). These eddy currents begin to churn the flow, using up energy in the process, which for liquids increases the chances of cavitation. The Reynolds number has wide applications, ranging from liquid flow in a pipe to the passage of air over an aircraft wing. It is used to predict the transition from laminar to turbulent flow, and is used in the scaling of similar but different-sized flow situations, such as between an aircraft model in a wind tunnel and the full size version. The predictions of the onset of turbulence and the ability to calculate scaling effects can be used to help predict fluid behaviour on a larger scale, such as in local or global air or water movement and thereby the associated meteorological and climatological effects.

Formula used:

where:

• ρ is the density of the fluid (SI units: kg/m³)
• u is the velocity of the fluid with respect to the object (m/s)
• L is a characteristic linear dimension (m)
• μ is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/m·s)
• ν is the kinematic viscosity of the fluid (m²/s).

Data:

Radius of cylinder: 0.02 metre

Height of cylinder: 0.05 metre

Velocity of air: 10 m/s towards positive X- direction

Density of air: 1.225 kg/m^3

Dynamic viscosity: 1.81*10^-5 kg/ms

Procedure:

• Firstly the domain (cylinder) in constructed using the above data
• Using Flow wizard in flow simulation Analysis type is specified which in this case is External flow and Time dependent Simulation (Transient Simulation)
• The fluid type is also specified which in this case is air
• Inside initial and ambient conditions the velocity of air is specified which is 10 m/s in positive X-direction (Base line velocity)
• The computational Domain is then created which will bound the fluid
• The base mesh is generated using the global mesh tab from input data tree
• Then we run the solver
• Using the results generated we create a cut plane in the geometryin order to visualise the velocity contour
• Finally we create a animation in order to visualise how the velocity profile change with increase in reynolds number

Results:

Base line velovity and Reynolds number:

Velocity: 10 m/s

Reynolds number: 13535.91

With Mesh size refinement:

Animation:

https://www.youtube.com/watch?v=RQTLcKx3vUw

20% increase in reynolds number:

Velocity: 11.99 m/s

Reynolds number: 16243

With Mesh size refinement:

Animation:

https://www.youtube.com/watch?v=9nn0ImmjRu0

40% increase in reynolds number:

Velocity: 13.99 m/s

Reynolds number: 18950.21

With Mesh size refinement:

Animation:

https://www.youtube.com/watch?v=EOP4dfGO27s

100% increase in reynolds number:

Velocity: 19.99 m/s

Reynolds number: 27071.82

With Mesh size refinement:

Animation:

https://www.youtube.com/watch?v=2oWH8tRdBf0

Pressure Contour at 20% increase in reynolds number:

velocity: 11.99 m/s

Animation:

https://www.youtube.com/watch?v=kTFNcAaYROo

Pressure Contour at 40% increase in reynolds number:

velocity: 13.99 m/s

Animation:

https://www.youtube.com/watch?v=Br9Y1wCuVXA

Pressure Contour at 100% increase in reynolds number:

velocity: 19.99 m/s

Animation:

https://www.youtube.com/watch?v=G4CbtpwpLvE

Explanation:

• The velocity in X-direction increases with increase in reynolds number
• Pressure do not vary to a large extent when reynolds number is increased by a factor of 20%, 40% and 100% respectively
• With Mesh refinement we get a better result with a better velocity contour and velocity profile can be visualized more accurately