Week 2 - Flow over a Cylinder.

Objective:

Simulate the flow over a cylinder and explain the phenomenon of Karman vortex street.

PART-I

1. Simulate the flow with the steady and unsteady case and calculate the Strouhal Number for Re= 100.

PART-II

1. Calculate the coefficient of drag and lift over a cylinder by setting the Reynolds number to 10,100,1000,10000 & 100000. (Run with steady solver)
2. Discuss the effect of Reynolds number on the coefficient of drag. [ Results should be validated with any standard literature and error should be within 5 %]

Expected results:-

1. In both approaches, show that the flow has converged. That is, what quantities or plots need to be looked at to determine that the flow has converged?
2. Plots of the coefficient of drag and lift.
3. Plots to show vortex shedding behind the cylinder.
4. Mention the preferred material (Reference Paper or Fluid Mechanics Textbook) at the end of the challenge submission page.

We used line tool to draw with dimensions

circle - 2m, with circle origin - -20 on X and +40 on X-axis.

box - 60x20m

and then used Pull to create a surface from the geometry that looks like

And then move on to Mesh.

Here we are going to perform mesh and to achieve a very fine Triangular mesh we have to perform certain steps to make sure it is desirable and

we are to be some different methods and tools to do so. Below are the steps involved in order to do so. This is the former mesh.

and that the next steps involve naming selection and making the mesh more fine around the cylinder as well as to the whole computational mesh.

Though we are going to have undesirable physical simulation at the CFD post with this amount of MESH as we performed beforehand what elements size is suitable for that physical phenomenon.

So we further tuned it more to finer mesh with an element size of 0.25 and we have

As you can see the layers are formed as we provided the layer quantity to the wall edge and that's how we get the finer edge mesh.

PART-I

1. Simulate the flow with the steady and unsteady case and calculate the Strouhal Number for Re= 100.

Moving on to the Fluent we will simulate the above mesh with steady-state simulation and calculate the strouhal number. This part has 2 cases so first we run simulation for steady state. We will also animate the following output and to plot the contour we have

Thats how we got the contour of boh velocity and pressure here. The general information starts with pressure based solver with steady state and having Laminar Viscous model with usr defined material, which is air but we defined as custom material. This is how we find the suitable tools in fluent do perform these actions.

velocity:0.2

Here viscosity value is similar to as dynamic viscosity which is kinematic viscosity over density of the fluid. And then we ran for first 350 iteration

0.02

ART-II

1. Calculate the coefficient of drag and lift over a cylinder by setting the Reynolds number to 10,100,1000,10000 & 100000. (Run with steady solver)
2. Discuss the effect of Reynolds number on the coefficient of drag. [ Results should be validated with any standard literature and error should be within 5 %]

Solution 1:

As we move forward for calculating the coefficient of drag and lift with Reynolds number. All the steps are the same as running the case in Fluent where we have to change the viscosity value in materials with different values and run the case.

Reynolds number : 10 100 1000 10000 100000

Viscosity : 0.2 0.02 0.002 0.0002 0.00002

for Re = 100 and mu = 0.02

for Re = 1000 and mu = 0.002

for Re = 10000 and mu = 0.0002

for Re = 100000 and mu = 0.00002

he effect of Reynolds number on the coefficient of drag and Lift done by aerodynamic experimentation of an airfoil with gurney flap

Lift coefficient decreases rapidly and drag coefficient increases rapidly when Reynolds number is decreased below critical range. This occurs due to a change in flow pattern near the Gurney flap at low Reynolds numbers.

A computational investigation on the effects of Reynolds number on the aerodynamics of NACA0012 airfoil without and with Gurney flap of the height of 3% airfoil chord has been carried out. ANSYS FLUENT commercial CFD code with one-equation Spalart-Allmaras turbulence model is used for the six Reynolds numbers varying from 3.0 × 10⁵ to 3.0 × 10⁴. From this investigation, the following major conclusions are drawn.

(1)Reynolds number plays a very major role in the airfoil aerodynamics for the NACA0012 airfoil without and with Gurney flap. Lift decreases and drag increases when Reynolds number is decreased.

(2)For the airfoil with GF, Reynolds number has adverse effects on lift coefficient, while the drag coefficient of the airfoil with GF has some beneficial effects compared to the airfoil without GF.

(3)For high Reynolds number above critical range, decrease in Cl and Cd increase in are negligible.

(4)As the Reynolds number is decreased below the critical Reynolds number range, Cl decreases and Cd increases at a very steep rate and the performance degrade rapidly.

(5)For lower Reynolds numbers, the two vortices behind the Gurney flap vanish. The Gurney flap seems to increase the effective camber of the airfoil, causing a negative zero-lift angle and reduced stall angle.

Conclusion of both cases:

And In both approaches, the flow has converged and we looked at a monitored control point plot to make sure that estimation. The frequent curve frequency shows that there's the presence of a physical phenomenon of vortex shedding around the cylinder wall.