Flow over a cylinder

OBJECTIVE:

To simulate flow over a cylinder and generate pressure – velocity plots. The flow simulation is to be carried out for three different cases with Reynolds number of 120%, 140% and 200% of the initial Re taken in the base line simulation. Further, the end time needs to be precisely calculated and inputted for the different cases.

SOLUTION:

The aim of the project is to carry out a simulation for flow over cylinder and analyse the resulting pressure and velocity profiles generated. When a fluid flows over a body, the fluid layer close to the body has zero velocity and the velocity of the fluid layers increases as the distance between the layer and the body increases. This is due to the formation of boundary layers. At a specific distance the fluid velocity equals 0.99 times of free stream velocity. This distance between the fluid layer and the body wall is called as boundary layer thickness. When the flow takes place over a body, at a specific point when the velocity gradient becomes negative or equal to zero, the boundary layer detaches from the body surface. This phenomenon is called as flow separation. Flow separation created a region of high vortex called as the wake region.

The aim is to perform an analysis of flow over a cylinder with different Reynolds number. Initially a base simulation was performed using air as the fluid. The cylinder dimensions were taken as 100mm diameter and 500mm length. The velocity of the fluid was taken as 1m/s, resulting in a Reynolds number of 7044.19. Further, the Re value was increased 20%, 40% and 100% and the analysis was performed at each of these conditions.

For calculating the end time of the simulation, the length of the computational domain was finalized as 1.3m using the base simulation. The flow through time is obtained by dividing the domain length and the velocity of flow. The end time is 2 times of the flow through time.

Case 1:

Re = 1.2 x 7044.19 = 8453.04

Re = ρvd/µ

For air, ρ = 1.275 kg/m³, µ = 1.81 x 10^-5 Ns/m²

Velocity, v = 1.2 m/s

Flow through time = 1.3/1.2 = 1.083 sec

End time = 2 x 1.083 = 2.167 sec

Case 2:

Re = 1.4 x 7044.19 = 9861.878

v = 1.4 m/s

Flow through time = 1.3/1.4 = 0.928 sec

End time = 2 x 0.928 = 1.857 sec

Case 3:

Re = 2 x 7044.19 = 14088.397

v = 2 m/s

Flow through time = 1.3/2 = 0.65 sec

End time = 2 x 0.65 = 1.3 sec

After creating the cylinder model, the model was meshed as shown.

The fluid conditions were defined using the flow wizard option. The flow was defined as transient and external with each simulation having 10 time steps. The set up was run and the results were obtained.

RESULTS:

Case 0: Velocity = 1m/s, Re = 7044.19

Case 1: Velocity = 1.2 m/s, Re = 8453.04

Case 2: Velocity = 1.4 m/s, Re = 9861.878

Case 3: Velocity = 2 m/s, Re = 14088.397

OBSERVATIONS:

1. The pressure plots doesn’t exhibit much variation for the different Re values. These plots have high pressure point at the fluid entry region. This point is called the stagnation point. Stagnation point is the point at which the flow separates into two different streams.
2. It can be observed that in all of the velocity plots, there is a low velocity region at the exit side of the flow. This region consists a large number of vortex flows and is called the wake region.
3. As the Re value increases, the flow through time decreases. At low values of Re, the velocity profile is wider and there is region of high velocity around the body. As Re value increases the high velocity region becomes small and narrow.