Week 2 - Flow over a Cylinder.
SIMULATION OF FLOW OVER CYLINDER
AIM
To simulate the flow over a cylinder and analyse the vortex formation behind the trailing zone of the bluff body by varying the reynolds number for both steady state and transient conditions. To observe the effect of coefficient of drag by varying the reynolds number .
INTRODUCTION
What is Vortex Shedding?
It is a phenomena when the flow crosses the bluff body, vortices are shed alternately from one side to the other, & where alternating low-pressure zones are generated to the downside of the bluff body giving rise to a fluctuating force acting at right angles to the flow direction.
What is Von Kamran Vortex Street?
The vortices which are sheded alternately from either side of the bluff body & it creates the pattern of swirling vortices. These pattern of swirling vortices are repeated periodically & its called as von kamran vortex street.
Here, simulation of cylinder is done for two parts to observe the vortex shedding,
Part 1:
Simulate the bluff body with the steady state and Transient cases and calculate the Strouhal Number for Re= 100.
Part 2:
- Calculate the coefficient of drag and lift over a cylinder by setting the Reynolds number to 10,100,1000,10000 & flow over a flat plate. (Run with steady solver)
- Discuss the effect of Reynolds number on the coefficient of drag.
MODELLING APPROACH:
Formulas solved
The equation of Reynolds number is
Re=ρ⋅U⋅L /μ
where,
Re = Reynolds number
ρ = material density
U = flow velocity
L = characteristic length
μ = dynamic viscosity
The equation of Strouhals number is
St = (f • L) / U
where,
St is the Strouhal number,
f is the frequency of vortex shedding,
L is the characteristic length and
U is the flow velocity
Creating the geometry in spaceclaim
Dimensions - 60mX20m, Dia - 2m
Meshing the model
Steps followed in generating the mesh
Element length -0.250m
Element type - Tria
Inflation layers - 7
Mesh Statistics,
| Nodes | 20692 |
| Elements | 40456 |
| Element size | 0.25 m |
Setup Boundary conditions,
For Reynolds no 100, with steady state condition,
| Time | Steady |
| Type | Pressure Based |
| Viscous | Laminar |
| Materials | User defined |
| Iterations | 700 |
| Dynamic viscosity | 0.02 |
| Reynolds number | 100 |
| Freestream velocity | 1m/s |
| C.Length | 2 m |
Residual plot
Coefficient of lift plot
Coefficient of drag plot
Vortex Shedding for re.no 100
Strouhal Number plot
Flow over cylinder with Reynold's Number 100, Transient state,
| Time | Transient |
| Type | Pressure Based |
| Viscous | Laminar |
| Materials | User defined |
| Iterations | 700 |
| Dynamic viscosity | 0.02 |
| Reynolds number | 100 |
| Freestream velocity | 1 m/s |
| C.Length | 2 m |
Velocity Magnitude plot,
Coeffficeient of Lift plot,
Coefficient of Drag plot,
Vortex Shedding at re 100, transient flow
Strouhal Number plot,
coefficient of drag and lift over a cylinder
For Reynold's Number 10, & steady state conditions,
| Time | Steady |
| Type | Pressure Based |
| Viscous | Laminar |
| Materials | User defined |
| Iterations | 700 |
| Dynamic viscosity | 0.2 |
| Reynolds number | 10 |
| Freestream velocity | 1 m/s |
| C.Length | 2 m |
Coefficient of Lift Plot,
Coefficient of Drag plot,
Vortex shedding at Re no - 10
Strouhal Number plot
Coefficient of drag and lift over a cylinder
For Reynold's Number 1000, & steady state conditions,
| Time | Steady |
| Type | Pressure Based |
| Viscous | Laminar |
| Materials | User defined |
| Iterations | 700 |
| Dynamic viscosity | 0.002 |
| Reynolds number | 1000 |
| Freestream velocity | 1 m/s |
| C.Length | 2 m |
Coefficient of lift plot
Coefficient of Drag plot,
Vortex shedding at Re no - 1000
Strouhal Number plot
Coefficient of drag and lift over a cylinder
For Reynold's Number 10000, & steady state conditions,
| Time | Steady |
| Type | Pressure Based |
| Viscous | Laminar |
| Materials | User defined |
| Iterations | 700 |
| Dynamic viscosity | 0.0002 |
| Reynolds number | 10000 |
| Freestream velocity | 1 m/s |
| C.Length | 2 m |
SPECIAL CASE
Flow over a flat plate,
Mesh statistics
| Nodes | 20607 |
| Elements | 39934 |
| Element size | 0.25 m |
| Time | Steady |
| Type | Pressure Based |
| Viscous | Laminar |
| Materials | User defined |
| Iterations | 700 |
| Dynamic viscosity | 0.02 |
| Reynolds number | 100 |
| Freestream velocity | 1 m/s |
| C.Length | 2 m X 2 m |
Coefficient of Lift plot,
Coefficient of Drag plot,
Vortex shedding plot,
Strouhal Number plot,
Conclusion:
Simulation of flow over a cylinder by changing Reynold's number has been carried out and vortex shedding for different cases is observed. the first part of the challenge for simulating the flow over a cylinder with steady state and transient conditions is done and the vortex shedding for both cases is observed, where the transient flow is time consuming and requires more number iterations to converge to reach a constant state and for steady state flow it is lesss time consuming and requires less iterations to converge.For part 2 of the challenge , simulation of flow over cylinder with different reynolds numberr is perrformeed and the variations in vortex shedding and the coefficient of lift and drag is observed whereas in case of reynolds number 10 vortex shedding not occuring because the flow is laminar. A special case of flow over a flat plate is carried out and large flow separation and wake is observed over and behind the flat plate for reynolds number 100.
Link to download file:
https://drive.google.com/file/d/1EeyCKZouUg3eQ-sZVVxUfHQoCiOu4n7Z/view?usp=share_link