Week 2 - Flow over a Cylinder.

Introduction:

Vortex shadding:

vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff (as opposed to

streamlined) body at certain velocities, depending on the size and shape of the body. In this flow, vortices are created at the

back of the body and detach periodically from either side of the body forming a Von Karman vortex street. The fluid flow past

the object creates alternating low-pressure vortices on the downstream side of the object. The object will tend to move

toward the low-pressure zone.

If the bluff structure is not mounted rigidly and the frequency of vortex shedding matches the resonance frequency of the

structure, then the structure can begin to resonate, vibrating with harmonic oscillations driven by the energy of the flow. This

vibration is the cause for overhead power line wires humming in the wind, and for the fluttering of automobile whip radio

antennas  at some speeds. Tall chimneys constructed of thin-walled steel tubes can be sufficiently flexible that, in air flow

with a speed in the critical range, vortex shedding can drive the chimney into violent oscillations that can damage or destroy

the chimney.

                         

Reynolds Number (Re):

Reynolds number for a flow is a measure of the ratio of inertial to viscous forces in the flow of a fluid around a body.

Re = rho*V*d/Dynamic Viscosity

The range of Re values will vary with the size and shape of the body from which the eddies are being shed, as well as with

the kinematic viscosity of the fluid. Over a large Re_d range (47<Re_d<10⁵ for circular cylinders; reference length is d:

diameter of the circular cylinder) eddies are shed continuously from each side of the circle boundary, forming rows of vortices

in its wake. The alternation leads to the core of a vortex in one row being opposite the point midway between two vortex

cores in the other row, giving rise to the distinctive pattern shown in the picture. Ultimately, the energy  of the vortices is

consumed by viscosity as they move further downstream, and the regular pattern disappears.

 

Strouhal number (St):

Strouhal number is a dimensionless number describing oscillating flow mechanisms.

The Strouhal number is often given as

St = f*d/u

Where,

f is the frequency of vortex shading, 

L is the characteristic length and 

U is the flow velocity.

 

Procedure:

Geometry:

A plate with the dimension of 20*60m is created using spaceclaim.

         

Now,we want to simulate effect of fluid flow over cylinder, select the circle and delete it.

          

Meshing:

1.select the edges of geometry and named them as inlet,outlet ,wall and symmetry.

2.Now select the body and go to mesh and click on generate mesh with mesh size 0.25 m.To ensure triangular mesh, go to

mesh > insert > select triangular mesh type.

         

3.After generating mesh,it is seen that size of cirle looks distoreted.

go to mesh > sizing > select circular edge > deffinition >type > no,of divisions >36.

        

4.To capture better results near cylinder wall, go to mesh > inflation > select the body and edge > inflation option : 1st layer

thickness >1 st layer height :5e-3 m >maximum layer :6

                           

 

Solving :

1. select laminar flow inside the viscous model.

2.As we wanted to customimize material property for simulation, go to materials > fluent database > select the material (air)

> copy it> rename the name of material.

 

Boundary Condition:

Outlet : gauge pressure = 0 Pa.

wall : no slip condition.

 

Refference values:

     

Steady state simulation:

1.Reynolds number = 10.

velocity = 1 m/s.

density = 1 kg/m3.

viscosity = 0.02 Pa-s.

 

Residual plot:

    

 

Monitor point velocity:

monitor point: A point is created at 8 m in x direction from the axis of cylinder to check velocity.

      

Coefficient of drag:

      

 

Lift Coefficient:

      

 

Velocity contour:

       

Pressure contour:

      

 

2.Reynolds number = 100.

velocity = 10 m/s.

density = 1 kg/m3.

viscosity = 0.02 Pa-s.

 

Residual plot:

    

 

Monitor point velocity:

monitor point: A point is created at 8 m in x direction from the axis of cylinder to check velocity.

      

Coefficient of drag:

      

 

Lift Coefficient:

      

 

Velocity contour:

       

Pressure contour:

      

 

3.Reynolds number = 1000.

velocity = 100 m/s.

density = 1 kg/m3.

viscosity = 0.02 Pa-s.

 

Residual plot:

    

 

Monitor point velocity:

      

 

Coefficient of drag:

      

 

Lift Coefficient:

      

 

Velocity contour:

       

Pressure contour:

      

 

4.Reynolds number = 10000.

velocity = 1000 m/s.

density = 1 kg/m3.

viscosity = 0.02 Pa-s.

 

Residual plot:

    

 

Monitor point velocity:

      

 

Coefficient of drag:

      

 

Lift Coefficient:

      

 

Velocity contour:

       

Pressure contour:

      

5.Reynolds number = 100000.

velocity = 10000 m/s.

density = 1 kg/m3.

viscosity = 0.02 Pa-s.

 

Residual plot:

    

 

Monitor point velocity:

      

 

Coefficient of drag:

      

 

Lift Coefficient:

      

 

Velocity contour:

       

Pressure contour:

      

 

Unstaedy state solver:

Reynolds number = 100.

velocity = 10 m/s.

density = 1 kg/m3.

viscosity = 0.02 Pa-s.

 

Residual plot:

    

 

Monitor point velocity:

      

 

Coefficient of drag:

      

 

Lift Coefficient:

      

 

Velocity contour:

       

 

Pressure contour:

      

Strauhall number:

Strouhal number is a dimensionless number describing oscillating flow mechanisms.

The Strouhal number is often given as

      St = f*d/u

Here,we have calculated staruhal no.using ansys fluent.

1.Go to fluent solver > results > fft analyser > load lift coefficient fiel > select plot FFT

Here, x axis denotes strauhal number and y axis denotes magnitude.

2.To get the magnitude of strauhal number,

go to results > fft analyser > click on write fft to file > click on write fft.

open the file in excel and 1st coloumn denotes the frequncy and 2nd coloumn denotes strauhal number.use the command

max and select 2nd row and find the magnitude of strauhal number.

    

 

      Strauhal Number = 0.112

 

 Result:

    Values of coefficient of drag and lift coefficient for respective reynolds number is tabulated as follows:

     

Conclusion:

1.When fluid flows over cylinder, wake region is formed next to cylinder due to decrease in pressure and recirculation of flow

occures and hence flow separation occures.

2.As flow over cylinder is being transient meaning time dependent phenomenon, we are unable to observe flow separation at

low reynolds no.(say 50).

3.In this assignment we have kept all parameters constant and have only varried velocity to change reynolds number. So as

the reynolds no. increases coefficient of drag decreases.however after reynolds no. = 4000, flow is turbulent and hence

coefficient of drag is lower.

4.Strauhal no. calculated for unsteady state with reynolds no.= 100 is 0.112.