Week 2 - Flow over a Cylinder.
AIM:
Main Aim is to Simulate the flow over a cylinder and explain the phenomenon of Karman vortex street.
Objective:
Objectives are as Follows:
1.To 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.Simulate the flow with the steady and unsteady case and calculate the Strouhal Number for Re= 100.
Introduction:
REYNOLDS NUMBER:
Reynolds number is a dimensionless quantity that is used to determine the type of flow pattern as laminar or turbulent while flowing through a pipe.
Re = rho*V*d/Dynamic Viscosity
Strouhal Number:
The Strouhal number is a dimensionless number describing oscillating flow mechanisms.
St = f*d/u
Vortex shadding Explanation:
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 he 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.
Procedure step by step:
1.Geometry:
A plate with the dimension of 20*60m is created using spaceclaim wihtin Ansys Fluent.
2. Meshing:
1.select the edges of geometry and named them as outlet ,wall, inlet 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.Mian motive is 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 problem:
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.
Steady state simulation:
by the objectives:
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:
Here,we have calculated staruhal no.using ansys fluent.
1.Go to fluent solver > results > fft analyser > load lift coefficient fiel > select plot FFT
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:
please find the values:
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.
- Done by Nakshata