Flow over a cylinder
Aim: To simulate the flow over a cylinder.
Description:
COMPUTATIONAL FLUID DYNAMICS: The field of study devoted to the solution of the equations of fluid flow through the use of a computer (or, more recently, several computers working in parallel). Modern engineers apply both experimental and CFD analyses and the two complement each other.
External flow: Sometimes a fluid moves over a stationary body (such as the wind blowing over a building), and other times a body moves through a quiescent fluid (such as a car moving through the air). These two seemingly different processes are equivalent to each other; what matters is the relative motion between the fluid and the body. Such motions are conveniently analyzed by fixing the coordinate system on the body and are referred to as flow over bodies or external flow.
Drag: The force a flowing fluid exerts on a body in the flow direction is called drag.
Flow Separation: A fluid climbs the uphill portion of the curved surface with no problem, but it has difficulty remaining attached to the surface on the downhill side. At sufficiently high velocities, the fluid stream detaches itself from the surface of the body. This is called flow separation.
Figure: Flow separation over a backward-facing step along a wall.
When a fluid separates from a body, it forms a separated region between the body and the fluid stream. This low-pressure region behind the body where recirculating and backflows occur is called the separated region.
Wake: The region of flow trailing the body where the effects of the body on velocity are felt is called the wake.
Figure: Flow separation during flow over a tennis ball and the wake region.
Vortex shedding:
An important consequence of flow separation is the formation and shedding of circulating fluid chunks, called vortices, in the wake region. The periodic generation of these vortices downstream is referred to as vortex shedding. This phenomenon usually occurs during normal flow over long cylinders or spheres for Re 90.
FLOW OVER CYLINDERS
The characteristic length for a circular cylinder or sphere is taken to be the external diameter D. Thus, the Reynolds number is defined as Re =VD/v where V is the uniform velocity of the fluid as it approaches the cylinder or sphere. The critical Reynolds number for flow across a circular cylinder or sphere is about Recr 2 x105. That is, the boundary layer remains laminar for about
Re 2 x105 and becomes turbulent for Re 2 x 105 .
Figure: Laminar boundary layer separation with a turbulent wake; flow over a circular cylinder at
Re = 2000.
Objective: To calculate the Baseline Reynolds number.
To obtain the plots for velocity variations and pressure variations at a cut plane.
To create the Animation videos
To perform flow simulations for the following three cases:
|
Sl. No. |
Reynolds Number increased (%) with respect to baseline Reynolds number |
|
1 |
20 |
|
2 |
40 |
|
3 |
100 |
Figure: Flow of the project.
Steps Followed:
- A cylinder is created.
- Flow simulation is opened.
- Then some initial conditions are provided such Fluid type, thermodynamic parameters, analysis type.
- The flow domain is setup.
- Boundary conditions are provided.
- The discretization is done(meshing).
- For results the plots are created at required positions are created the cut plots.
- Then flow simulation is run.
- Desired outputs are saved.
- The same process is followed in all cases.
Calculations:
ρ=1.2754 kg/m
μ=1.81*10 Pa.s
D=0.02 m
- For Baseline calculation:
V=10 m/s
Re=14033
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/10 = 0.02 s
End time = 2*Flow time = 0.04 s
- For 20% increased Reynolds number:
Re=14033*1.2 = 16840
V=12 m/s
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/12 = 0.0167 s
End time = 2*Flow time = 0.0334 s
- For 40% increased Reynolds number:
Re=14033*1.4 = 19646
V=14 m/s
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/14 = 0.0143 s
End time = 2*Flow time = 0.0286 s
- For 100% increased Reynolds number:
Re=14033*2 = 28066
V=20 m/s
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/20 = 0.01 s
End time = 2*Flow time = 0.02 s
Results:
Case 1:
Baseline
V=10 m/s
Re=14033
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/10 = 0.02 s
End time = 2*Flow time = 0.04 s
Figure: Velocity Plot
Figure: Pressure plot.
Video Animation Link: https://drive.google.com/file/d/1cy61v3RE_xalhLsX8wVe6BUDVfozy_IR/view?usp=sharing
Case 2:
For 20% increased Reynolds number:
Re=14033*1.2 = 16840
V=12 m/s
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/12 = 0.0167 s
End time = 2*Flow time = 0.0334 s
Figure: Velocity Plot
Figure: Pressure plot.
Video Animation Link: https://drive.google.com/file/d/1KSbvVpKdSTuzhdCHaIOYbh2WzetQzhT1/view?usp=sharing
Case3:
For 40% increased Reynolds number:
Re=14033*1.4 = 19646
V=14 m/s
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/14 = 0.0143 s
End time = 2*Flow time = 0.0286 s
Figure: Velocity Plot
Figure: Pressure plot.
Video Animation Link: https://drive.google.com/file/d/1aUFt8nY3fjL7u1ROPUOf8eX5JZN_LFwN/view?usp=sharing
Case 4:
For 100% increased Reynolds number:
Re=14033*2 = 28066
V=20 m/s
Domain length = 0.2 m
Flow time = domain length/velocity = 0.2/20 = 0.01 s
End time = 2*Flow time = 0.02 s
Figure: Velocity Plot
Figure: Pressure plot.
Video Animation Link: https://drive.google.com/file/d/1M-lC1UITckPNTghxCJMHyw0l-CIKFhaP/view?usp=sharing
Conclusion:
At higher velocities, the fluid still hugs the cylinder on the frontal side, but it is too fast to remain attached to the surface as it approaches the top (or bottom) of the cylinder. As a result, the boundary layer detaches from the surface, forming a separation region behind the cylinder. Flow in the wake region is characterized by periodic vortex formation and pressures much lower than the stagnation point pressure.
Link of cad files: https://drive.google.com/file/d/14Vf4tOKePoc8tRFLetK-u8iOFeO0hcVC/view?usp=sharing