Flow over a cylinder Simulation
Aim:
The aim of the project is to perform steady and transient state simulation of flow over a cylinder at Raynolds number and to visualize the phenomena of vortex shedding.
Introduction:
A bluff body kept in the regions of a flow field causes the occurrence of vortices in the wake due to the difference in pressure . At very low Raynolds number it has been documented that vortices formation occurs this is called as von karman vortex streets.
Geometry:
The Geometry created in this study is as shown below with a circle of diameter 2m in the flow field; The velocity of flow is given as 1 m/s; The Raynolds number is adjusted by varying the dynamic viscosity by creating a customized fluid.
Computational Mesh:
The mesh created for this project is as show below
The cellcount is 10692 and the mesh is made finer by using an element size of 0.5 ( 4 times smaller than the diameter of the bluff body)
No slip Boundary condition is used in the walls in order to have the first shear layer near the wall has a velocity equal to the wall.
An inflation layer of 9 and also an edge sizing of 36 divisions are given in order to have a smoother edge meshing
Results:
Steady state condition
As shown in the figure-1; the phenomena of vortex shedding is visualized but an exact frequency of oscillation or the strohal number cannot be calculated from this because vortex shedding is transient in nature it cannot be correlated to a particular time.
Even the vortex average velocity magnitude graph shows the same. As shown in fig.2 the solution has converged but at a particular point of time we cannot find the exact magnitude of vortex shedding; this can only be used to get an average value;. Hence we should try the simulation with a transient condition
The Strouhal Number can be important when analyzing unsteady, oscillating flow problems. The Strouhal Number represents a measure of the ratio of the inertial forces due to the unsteadiness of the flow. Hence not relevet in this condition
Transient condition
Figure below shows the formation of vortices by displaying the velocity magnitude
The figure clearly shows the vortices
The graph shows the vortex average velocity with respect to time. As shown in the figure the fluctuation has reached a steady value which means that the simulation is converged.
The magnitude of fluctuation is steady. in this case for aparticular time we can get the vortex sheddig rate.
Strohal number is used to describe the oscillating flow mechanism
Strohal number is calculated by
St=fL/U
f- frequency of vortex shedding
L- Charecteristic Diameter
U- flow velocity
By performig FFT in above graph the frequency of vortex shedding can be obtained; the length and flow velocity is known and hence strohal nuber can be calculated.
In this study the strohal nuber obtained is 0.00036