Flow over a cylinder

Project title: Flow Over a Cylinder.

Objective:

1. Study the flow behaviour of air over the cylinder.

2. Do parametric study at a different velocity.

3. Study pressure distribution around the cylinder.

4. Study velocity distribution around the cylinder.

Theory:

When a fluid moves over a solid body, it exerts pressure forces normal to the surface and shear forces parallel to the surface of the body. We are usually interested in the resultant of the pressure and shear forces acting on the body rather than the details of the distributions of these forces along the entire surface of the body. The component of the resultant pressure and shear forces that act in the flow direction is called the drag force (or just drag), and the component that acts normal to the flow direction is called the lift force (or just lift).

Drag And Lift:

It is a common experience that a body meets some resistance when it is forced to move through a fluid, especially a liquid. As you may have noticed, it is very difficult to walk in water because of the much greater resistance it offers to motion compared to air. Also, you may have seen high winds knocking down trees, power lines, and even trailers and felt the strong “push” the wind exerts on your body. You experience the same feeling when you extend your arm out of the window of a moving car. A fluid may exert forces and moments on a body in and about various directions. The force a flowing fluid exerts on a body in the flow direction is called drag.

A stationary fluid exerts only normal pressure forces on the surface of a body immersed in it. A moving fluid, however, also exerts tangential shear forces on the surface because of the no-slip condition caused by viscous effects. Both of these forces, in general, have components in the direction of flow, and thus the drag force is due to the combined effects of pressure and
wall shear forces in the flow direction. The components of the pressure and wall shear forces in the direction normal to the flow tend to move the body in that direction, and their sum is called lift.

The drag and lift forces depend on the density 𝜌 of the fluid, the upstream velocity V, and the size, shape, and orientation of the body, among other things, and it is not practical to list these forces for a variety of situations. Instead, it is more convenient to work with appropriate dimensionless numbers that represent the drag and lift characteristics of the body. These
numbers are the drag coefficient CD, and the lift coefficient CL, and they are defined as

The part of drag that is due directly to wall shear stress 𝜏w is called the skin friction drag (or just friction drag FD, friction) since it is caused by frictional effects, and the part that is due directly to pressure P is called the pressure drag (also called the form drag because of its strong dependence on the form or shape of the body). The friction and pressure drag coefficients are defined as

Problem Statement

Simulate the flow over a cylinder

Simulate the flow over a cylinder for the conditions shown in the video.

• Calculate the baseline case Reynolds number.
  • Re = rho*U_inlet*D/mu
  • D = cylinder diameter
  • mu = dynamic viscosity
  • rho = density
• Run a baseline simulation and then calculate the Reynolds number. Then increase the Reynolds number by a factor of 20%, 40%, and 100% then run the transient flow simulation.
  • For example
    • If the Reynolds Number value for the baseline simulation is 100. Then it should be increased( as a factor of 20%, 40%, and 100%) to 120, 140 and 200
• Simulation end time should be calculated properly
  • If the inlet velocity is 1m/s and if the length of the domain is 5 m then the flow-through time is 5 seconds. That is, it takes 5 seconds for the flow from the inlet to go to the outlet.
  • This 5 second is called the flow-through time
  • Make sure that end time is set to 2 times flow through time. According to the previous example, the end time would be 10 seconds.
• Create animations for pressure-velocity for all the three Reynolds number.
• Explain your observations

Solution

Geometry

Cylinder

In this simulation, cylinder size is considered 0.02m.

Flow setup

Computational domain

Due to restrictions in mesh count 2D simulation is performed.

To capture the exact flow physics computational domain behind the cylinder kept large around 20D.

computational domain along the side of the cylinder is kept 10D.

computational domain in front of the cylinder is kept 5D.

Mesh

Hex mesh is used to create meshing.

Around the cylinder fine mesh is created to capture the flow around the cylinder.

Mesh count

Total mesh count is 16092 elements

Global meshing:

To capture the accurate results in globe mesh setting mesh refinement is kept at 3.

Local meshing:

In local mesh refinement tab refining cell is kept at 4.

Analysis type

Air flows around the cylinder so analysis type is selected eternal.

to capture the flow after some time interval time-dependent option is on.

Working fluid:

in this simulation working fluid air is selected.

Initial flow simulation conditions

pressure 101325 Pa

Temp 293.5 K

Inlet velocity 10 m/s

Parametric study

In parametric study, Re-value is a change in 3 stages and observe the flow behaviour behind the cylinder

In the first case, velocity is 10 m/s is considered.

In the second case Re value is increased by 20 % so that velocity is 12 m/sec.

In the third case Re value is increased by 40 % so that velocity is 14 m/sec.

Results

Case 1 with 10 m/sec velocity

Pressure distribution

In front of the cylinder, there is a stagnation point where pressure is maximum. the value of maximum pressure is 101389.27 Pa

As the airflow around the cylinder pressure starts reducing due to the converging shape of the cylinder in the first half portion of the cylinder.

the pressure is minimum at top of the cylinder. the value of minimum pressure is 101205.9 Pa

As air move ahead from top portion again pressure starts increasing due to expansion shape at second half portion of the cylinder.

velocity distribution

Pressure and velocity have a reverse correlation with each other.

Velocity is maximum where pressure is minimum and vice versa.

Due to high-velocity vertex formation take place behind the cylinder figure shows vortex formation.

maximum velocity is 15.24 m/sec

minimum velocity is 0 m/sec due to no-slip boundary condition.

Animation

An animation shows a 0.1-sec flow of air around the cylinder with 10 m/sec velocity

Case 2 with 12 m/sec velocity

Pressure distribution

In front of the cylinder, there is a stagnation point where pressure is maximum. the value of maximum pressure is 101418.85 Pa

As the airflow around the cylinder pressure starts reducing due to the converging shape of the cylinder in the first half portion of the cylinder.

the pressure is minimum at top of the cylinder. the value of minimum pressure is 101144.98 Pa

As air move ahead from top portion again pressure starts increasing due to expansion shape at second half portion of the cylinder.

velocity distribution

Pressure and velocity have a reverse correlation with each other.

Velocity is maximum where pressure is minimum and vice versa.

Due to high-velocity vertex formation take place behind the cylinder figure shows vortex formation.

maximum velocity is 19.54 m/sec

minimum velocity is 0 m/sec due to no-slip boundary condition.

Animation

An animation shows a 0.1-sec flow of air around the cylinder with 12 m/sec velocity

Case 3 with 14 m/sec velocity

Pressure distribution

In front of the cylinder, there is a stagnation point where pressure is maximum. the value of maximum pressure is 101451.60 Pa

As the airflow around the cylinder pressure starts reducing due to the converging shape of the cylinder in the first half portion of the cylinder.

the pressure is minimum at top of the cylinder. the value of minimum pressure is 101042.31 Pa

As air move ahead from top portion again pressure starts increasing due to expansion shape at second half portion of the cylinder.

velocity distribution

Pressure and velocity have a reverse correlation with each other.

Velocity is maximum where pressure is minimum and vice versa.

Due to high-velocity vertex formation take place behind the cylinder figure shows vortex formation.

maximum velocity is 23.43 m/sec

minimum velocity is 0 m/sec due to no-slip boundary condition.

Animation

An animation shows a 0.1-sec flow of air around the cylinder with 14 m/sec velocity

Conclusion:

1. Pressure and velocity have a reverse correlation with each other.

2. The pressure is maximum at a stagnation point.

3. Velocity is zero m/sec at a stagnation point.