Flow over a cylinder
Simulating flow over a cylinder for different reynolds numbers
To calculate the baseline case reynolds number. we have the following formula
Re = rho*U_inlet*D/mu
- D = cylinder diameter
- mu = dynamic viscosity
- rho = density
- U_inlet=Inlet velocity
Thus for Re = 20, 40 and 100 we are runing the transient flow simulation also called as time dependent flow simulation.
Air is the working fluid in our simulation hence for inlet velocity calculation we need density and dynamic viscocity of air.
- Density of air approx 1.225 Kg/m^3[according to ISA (International Standard Atmosphere)].
- Dynamic viscosity of air is 1.81x10^(-3) Kg/ms at temperature 300 k pressure 1 atmosphere.
Thus calculating the inlet velocities by substituting the above values in the reynolds equation ,we get corresponding velocities as 0.00738 m/s ,0.0147 m/s and 0.0369 m/s. Here the inlet velocity is in the x direction.
The computational domain has a length of 300 mm in x direction, thus for the above set of velocities the flow through time would be 55 sec, 28 sec and 11 sec respectively. Since the end time is set to 2 times the flow through time, we get the end time values as 110 sec, 52 sec and 22 sec.
- In the simulation wizard the total analysis time is same as the end time .
- Output time step is taken as 1 to 2 sec for the iterations.
Cylinder dimensions in mm
- Diameter of the cylinder is 40 mm.
- Length of the cylinder is 50 mm.
- Surface roughness is taken as zero.
- Gravity is not considered.
For the above conditions the velocity and the relative pressure plot results are as follows
For Reynolds number = 20
Relative Pressure cut-plot in top view
Velocity cut-plot in front view
For Reynolds number = 40
Relative Pressure cut-plot in top view
Velocity cut-plot in front view
For Reynolds number = 100
Relative Pressure cut-plot in top view
Velocity cut-plot in front view
OBSERVATIONS
- As the reynolds number increases the inlet velocity in x direction increases ,thus increasing the flow of air over the cylinder.
- The Relative pressure variation in the above animation does not varry to a large extent for the reynolds number taken. It is observed that high pressure is created on the surface facing the inlet velocity.
- The velocity variation in the above animations has different velocity profiles ,when the velocity is larger the velocity profile is longer behind the cylinder surface and it is approximately zero nearer to the cylinder surface [The surface which is not in contact with inlet velocity].
- For higher values of the velocity waves are created behind the cylinder.