Post-processing

When the analysis is completed and the message Process '…', started on … has finished. has been displayed, we can proceed to visualise the results by pressing Postprocess.

For details on the result visualisation not explained here, please refer to the Post-processing chapter of the previous examples and to the GiD manual or GiD online help.

Below, the results corresponding to the last time step (t = 120 s) are shown (see Figure 93, Figure 94 and Figure 95).

Figure 93 Velocity module distribution

Figure 94 Pressure distribution

Figure 95 Velocity vectors (detail)

The evolution in time of any parameter can be captured and visualised by means of the Animate (accessible through the Windows menu) utility of GiD (Figure 96).

Figure 96 Animate window

Selecting Save MPEG in the Animate window will save the animation in MPEG format. In order to save disk space, it is advisable to reduce the GiD window size to the essential details, as the whole interior of the GiD main window will be saved. This can result in very large files. To prevent this, also the empty space around the area of interest should be minimised.

t=60.0 s

t=62.0 s

t=64.0 s

t=66.0 s

t=68.0 s

t=70.0 s

t=72.0 s

t=74.0 s

t=76.0 s

 t=78.0 s

Figure 97 Time evolution of velocity module

Remarks

a)      From Figure 93 - Figure 97 we can observe that the perturbances induced by the cylinder in the velocity and pressure fields reach the boundaries of the control volume. Normally this should be avoided by choosing a larger control volume, as the boundary conditions can perturb the solution. This has not been possible here because of the limits of the academic version of Tdyn (a larger domain would imply a larger mesh). Nevertheless we obtain quite accurate results.

b)      We can verify the quality of the results by comparing the calculated period of the vortex shedding with experimental and other numerical results [12, 14].

The periodical character of the vortex shedding phenomenon is described by the Strouhal number, given by

 

where f being the frequency of the vortex shedding, D the diameter of the cylinder, and v¥ the free-stream velocity.

The computed period can be evaluated in many ways: trough the evolution of a variable at a point behind the cylinder, or of the net force over it. Here we will first calculate the period using another GiD facility: the Graph menu:

Figure 98 Creating a velocity graph

We have to access the Graph point analysis utility as shown in Figure 98, and select the physical quantity we want to analyse - here the x-component of the velocity is chosen- and select the point we want to analyse using the join option. We select a lateral point in the wake of the cylinder (Figure 99). In a more central point, the x-component of the velocity would oscillate at twice the frequency, as it changes every time a vortex is shed. The lateral points, however, are only affected by vortices shed on the respective side of the cylinder. The point should also be far enough from the boundaries, as these can also affect the velocity evolution.

Figure 99 Point chosen for the velocity graph

Finally the velocity graph shown in Figure 100 is obtained:

 

Figure 100 Evolution of the X-component of the velocity at the selected point

As can be seen the resolution of the graph is not good enough since just one point is drawn every 10 time steps (every second) as fixed in the problem type window. However the period can be estimated quite accurately to be T = 6.0 s.

It is also possible to calculate the period of the phenomenon by visualising the time evolution of the forces acting on the cylinder. This can be done using the Forces Graph option of the menu View Results (see Figure 101).

Figure 101 Forces Graph menu option

In the Figure 102 the evolution of the pressure vertical force (PFy option) on the cylinder is shown. Through this window is possible to draw next components (all the values are given in standard units Kg, m, s):

Option

Component

PFx

Ox pressure force component on the boundary.

PFy

Oy pressure force component on the boundary.

PFz

Oz pressure force component on the boundary.

MFx

Ox pressure momentum component on the boundary (calculated respect to the origin).

MFy

Oy pressure momentum component on the boundary (calculated respect to the origin).

MFz

Oz pressure momentum component on the boundary (calculated respect to the origin).

VFx

Ox viscous force component on the boundary (calculated by integrating viscous stresses on surface).

VFy

Oy viscous force component on the boundary (calculated by integrating viscous stresses on surface).

VFz

Oz viscous force component on the boundary (calculated by integrating viscous stresses on surface).

MVFx

Ox viscous momentum component on the boundary (calculated respect to the origin).

MVFy

Oy viscous momentum component on the boundary (calculated respect to the origin).

MVFz

Oz viscous momentum component on the boundary (calculated respect to the origin).

Figure 102 shows how the oscillatory phenomena are completely developed after 68 seconds and that the period of the process is about 6.0 seconds, as mentioned above. This is to be compared with the experimental value of T = 5.98 s reported in [13].

The calculated period leads to a Strouhal number of Str = 0.167 which is  very close to the experimental value obtained in [13]andabout 5% below the numerical value reported in [14].

Figure 102 Forces Graph window