Week 2 - Flow over a Cylinder.
geometry and mesh files
Unsteady vs Steady flow for a Reynolds nuber 100
It is observed that , the steady state required few number of iterations in comparison with unsteady flow, for a unsteady flow we have used the time step size of 0.1 and number of time steps are 1500 to get a converged solution, whereas for the former the solution has converged for around 400 iterations.
The criteria for convergence
Residuals were considered as a criteria for convergence, since the velocity at the point behind the cylinder will oscillate due to vortex shedding .
Once this phenomena is achieved the velocity oscillates , the frequency of this shedding can be calculated by strauhal number using an FFT plot in fluent.
Methodology
In this section we have compared the variation of velocity across the cylinder, varaition of velocity at a point 8 m away from the cylinder,drag coefficient , lift coefficient and lastly the strauhal number by keeping the reynolds number constant
Variation of the velocity contours
Steady Unsteady
Steady Unsteady
Steady Unsteady
Steady Unsteady
Steady Unsteady
Steady Unsteady
Comparison of steady state vortex shedding with varying reynolds numbers:
In this case we change the reynolds number by varying the velocity , the reynolds numbers which we consider in this case are 10,100,1000,10000 and 100000.
Re=10 Re=100 Re=1000 Re =10000
Re=100000
Re=10 Re=100 Re=1000 Re =10000
Re=100000
Re=10 Re=100 Re=1000 Re =10000
Re =100000
Re=10 Re=100 Re=1000 Re =10000
Re =100000
Re=10 Re=100 Re=1000 Re =10000
Re =100000
In this study we have observed that as we increase the velocity the reynolds number increased hence the vortex shedding also increased, we can also observe that lift and drag coefficients are also very high in the cases of reynolds number above 100, furthermore the vortex shedding is only observed in the cases of reynolds number above 100, and all this cases are converged , for validation of convergence we have used the residual plots .
From this study we can understand that , higher reynolds number gives better results for visualizing the vortex shedding.