CFD analysis of External Flow over a cylinder with varying Reynold's Number.

Objective:

1. Simulate the flow over a cylinder :
2. Run a baseline simulation and then calculate the Reynolds number. Then increase the Reynolds number by a factor of 20%, 40%, and 100% then run the transient flow simulation.
3. Create animations for pressure-velocity for all the three Reynolds numbers.
4. Explain your observations

Reynolds number (Re):

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. 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 behavior on a larger scale, such as in local or global air or water movement and thereby the associated meteorological and climatological effects.

It is given by :

Cylinder Dia: 1 m

Working fluid:

• Water : Kinematic Viscosity = 1.793 x 10^-6 Kg/(m sec) )
• Density= 1000 kg/m^3

Case 01:(Leminar Flow)

• velocity of flow = 0.0035m/sec
• Re=1987

Case 02:(Transitional Region)

• Re=3910.6
• velocity of flow=0.007m/sec

Case 03:(20% increase in Re)(Terbulent Region)

• velocity of flow = 0.0085m/sec
• Re=4692.7

Observations:


1)laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion; turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices, and other flow instabilities.

2)This can be clearly seen from the velocity animations above but when it comes to pressure, there is hardly any change in the 4 animations. Thus Reynold's number is not playing a major part in the pressure distribution.