Week 2 - Flow over a Cylinder.
Solutions:
The flow over a 2D cylinder is a very simple flow simulation and has a lot of fluid properties to be monitored while running it. While the flow can be witnessed in real life, It has a specific pattern that forms the Karman Vortex Street. its an oscillation flow phenomenon that can be observed very easily. So let's understand what the Karman Vortex street pattern is all about.
To describe in an informal way the flow created by the motion of a cylinder with constant speed in a fluid at rest. For very small Reynolds numbers, the flow is completely laminar around the cylinder, with no recirculation and no instability. For larger Reynolds numbers, a recirculation zone appears behind the obstacle. More precisely, two eddies with opposite signs appear just behind the cylinder and remain close to it. For larger Reynolds numbers, these eddies are unstable and successively leave the cylinder to propagate in the wake, to form the so-called “Von Karman” vortex street.
Above are some of the examples as to how the vortex pattern forms around a cylinder or any laminar flow at the obstacle in the center. We are going to perform the same phenomenon in ANSYS-FLUENT. First, we are going to create a geometry with a cylinder-like a shape in the middle which will have the same conditions as the cylinder at the center of any fluid flow path. We are going to create this in space claim.
The following are the tools we used to create the geometry and we have the same after the attempt of creation.
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.
the following are the tools involved:
First, we try to add a method that let us convert quadrilateral mesh to triangle 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
further, for element sizing, we have a finer cylinder layer where we make it finer by increasing the number of edges by 36 having to be considered as 10 degrees between each other.
The same goes for the inflation layer.
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
- 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.
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 with boundary condition below and then to 150 more to achieve up to 500 iterations.
OUTPUT:
CASE 1 : STEADY STATE SIMULATION OF REYNOLDS NUMBER 100.
PLOT OF VORTEX SHREDDING BEHIND THE CYLINDER
DRAGCOEFFICIENT:
LIFTCOEFFICIENT:
Strouhal Number: The Strouhal number is defined as St = fstD/U, where fst is the vortex shedding frequency (or the Strouhal frequency) of a body at rest, D is the diameter of the circular cylinder and U is the velocity of the ambient flow. And the relation between Strouhal number and coefficient of drag and lift is as follows:
COEFFICIENT OF LIFT:
COEFFICIENT OF DRAG:
The contour of velocity magnitude:
And when we open the FFT data file into an excel sheet we determined the magnitude of strouhal number as 2e-2 as the highest magnitude is
7.27e-2
CASE 2: TRANSIENT STATE SIMULATION OF REYNOLDS NUMBER 100.
And when we open the FFT data file into an excel sheet we determined the magnitude of Strouhal number as 3.616e-2 as the highest magnitude is
7.98e-2
PART-II
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)
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 = 10 and mu = 0.2
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
Solution 2:
The 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.
Steady-state case
Unsteady state case