Week 2 - Flow over a Cylinder.

WEEK 2 FLOW OVER A CYLINDER

AIM: - To simulate flow over a cylinder for steady and transient state for different Reynolds numbers and its effect on co-efficient of drag.

INTRODUCTION: - Karman vortex street is a phenomenon in which the repeating pattern of swirling vortices is caused by a process known as vortex shedding, which is responsible for the unsteady separation of flow of a fluid around the bodies. It has main importance in aerodynamics as due to this parodic swirling and detachments structures gets damaged so it before creating blunt bodies it is very important study this formation.

Laminar flow: - it is a characteristics of fluid flow in which the fluid follows a smooth path in layers, without any fluctuations.

Turbulent flow: - it is a type of fluid flow in which there are fluctuations. These are caused by many reasons.

Reynolds number: - it is a dimensionless quantity that is used to determine the type of flow pattern as laminar or turbulent while flowing through a pipe. Defined by the ratio of inertial forces to that of viscous forces.

Strouhal number: - The Strouhal number is defined as St = f D/U, where f 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.

Co-efficient of drag: - The drag coefficient expresses the ratio of the drag force to the force produced by the dynamic pressure times the area.

The flow over is a basic concept helps in understanding various boundary conditions and types of fluid flow moreover, it not only just makes us understand concepts but is widely studied both computationally and physically because it shows behaviour of fluid flow over blunt surfaces.

Representation of Flow over a cylinder and separation of fluid

Steps involved in solving with Ansys fluent

1. Designing/setting up geometry
2. Meshing
3. Setting up physics and fluid flow properties
4. Setting up plots, animation and contours
5. Initializing and running calculations

Step 3,4,5 is done in Ansys fluent

1. Designing/setting up geometry: -

The cylinder either can be designed in the space claim or design modeler or else it can be also be preloaded if designed in any other software’s. While designing first draw a circle of 2-meter dia and input at a distance of 10d of 10d and output of 20d at distance of 20d from the center of the circle. After designing the outline use pull option to create surface.

2. Meshing: -

In this process first we are going to give names to the boundaries such as inlet, outlets etc., and then perform meshing in Ansys workbench to refine and get accurate results we should use triangular elements perform Edge sizing to make the wall of cylinder smoother I have given number of divisions to 72 by dividing each element to 5^o and then give inflations layers to make wall layer smoother and also to change size of meshing to 21.5 cm which will make elements up to 50,000.

3. Setting up physics and post processing in ANSYS fluent

In this step the physical parameters can be set for the design and conditions in which they will be operating in after selecting all the parameters and setting up such as velocity at inlet, properties of fluid etc, we can solve the problems after selecting required quantities to be measured such as temperature, standard deviation and velocity etc but we will be measuring vortex velocity by selecting vortex average. And also create a point in post processing section before initializing after which we can create contours of vortex velocity and then again coming back to solving where we can create animations of the contours created and proceed with calculation by giving no of iteration.

Calculation of reynolds number and velocity

Density of fluid = 1kg/m³

µ=0.05 kg/(m.s)

Re=10,100,1000,10000,100000

D=2 m

Reynolds number	Velocity
10	0.25
100	2.5
1000	25
10000	250
100000	2500

4. Simulations: -

Part 1 calculate Strouhal number for Reynold’s number 100 for steady and unsteady flow

St=f d/v

For steady flow

Where f=0.5

D=2

V=0.14

St = 7.142

Unsteady

Part 2 co eff of drag and lift over on cylinder for steady flow for Re 10,100,1000,10000,100000

For Re=10

For re 100

For re 1000

For re=10000

For re=100000

5. Conclusions: -

• Finer the mesh smoother gets the results.
• When the velocity increase vortex formations occur for the steady flow also.
• If the flow is transient vortex formation occurs for small velocity also.