PRELUDE Tutorial VWT3SA, Simulating flow around a simple object

PRELUDE Tutorial VWT3SA
Simulating flow around a simple object

Author: E.O. Pankova, Moscow Baumann State Technical University

Summary

In this tutorial, you will continue learning how to use PRELUDE's Virtual-Wind-Tunnel Gateway to simulate the flow around a simple object.

The third tutorial (VWT3sa) in the VWT stand-alone series will explain how to change the object inside the Virtual-Wind-Tunnel and its initial settings.

How to start
Changing the initial settings
Making the simulation and inspecting the results
Saving the results of your work with PRELUDE
Concluding remarks

1. How to start

First create your working directory under /phoenics/d_prelud in an ordinary way.
Start this tutorial by clicking on the icon on the desktop.
To load the case which is discussed in this tutorial, click on the arrow to the right of the 'Load Gateway' box and select already familiar 'vwt', which stands for Virtual Wind Tunnel.
List of initial settings (options) will appear inviting you to introduce the necessary changes at the very beginning. Let us now replace the sphere inside the VWT with a half cylinder object for which purpose substitute the ball.dat in the test object window by halfcyl.dat (standing for a half cylinder) in the list of objects that is revealed when you click on the arrow to the right of this box.
Click on the button "Next" leaving other settings unchanged for the time being.
You will then see the following in the calculation domain.
The sphere has disappeared and instead - a half cylinder object has been placed in the same place of the VWT.
Now specify your working directory in the drop-down menu of the 'Edit' button, select 'Set Working Directory', and navigate thereafter to the directory you have just created.
Click on that folder. It will remain your working directory for PRELUDE until you change it again.

2. Changing the initial settings

First of all let us make the 'object tree' visible clicking on the icon in the toolbar.
It is good mentioning here that a substitution of a test-item can be also made at this point (not with the help of the VWT Creation Wizard as we did), but using the objects from the store cupboard.
Pay attention that no object should be selected at this moment and the sign of this is the word 'Director' in the top-right corner of the Graphics window.
Click on the 'red-tick' icon in the tool-bar and you will get the picture with the 'store cupboard' tab visible, which, when clicked upon, will reveal its contents.
You can try locating any object from the store cupboard into the VWT, but in the end restore the half cylinder object. It is another opportunity of changing the default test-item object in the VWT.
It is reasonable here to remind of the default settings introduced at the first step by the VWT Creation Wizard paying a special attention to the 'Turbulence' setting.
The word 'No' in the Turbulence box signifies that no turbulence model will be introduced into the future flow simulation, i.e. the flow is laminar. However, if you check the default Reynolds number, it will definitely be turbulent. This is inaccuracy which we introduced using default settings. Let us first reduce the inlet speed value from 2.0 m/sec to 0.5 m/sec. This can be easily done as the inlet speed being set in a parametric form ('WINDSPEED) can be edited if you click on the 'Parameters' tab.
Now type '0.5' instead of '2.0' in the corresponding box.
To be certain that this new setting has been introduced, check the inlet velocity by selecting the 'inlet' object in the object tree, then clicking on the 'Attributes' tab and from the 'Edit parameter' box choosing 'velocity'. You will get a picture like this one.
The 'X velocity' option although being still a parameter 'WINDSPEED' has now changed its value to 0.5 m/sec.
It is certainly not a sufficient reduction of the Reynolds number and in order not to change the VWT sizes, let us increase the flow viscosity. To do so, expand the 'materials' object in the object tree clicking on the sign '+' on the left, then select the 'phase1' object dealing with the heat transfer medium to be used for the simulation. You will then have a picture as follows.
To increase the flow viscosity, e.g. one thousand times, change the exponent from -5 to -2 leaving all other parameters as they are.
Note that this has been done only to get laminar flow conditions in the VWT and the new viscosity has nothing to do with this of air being used in simulation.

3. Making the simulation and inspecting the results

We are ready now to make a run. So click on 'Options' in the Menu bar, and then on 'Run Solver'. It will take some time until the simulation is finished and a user is again taken to the Prelude Graphics window.
Click on the sign '+' to the left of the vtkphi object to expand it and select the cplane (i.e. cut plane) - a plane where the calculation results will be displayed.
Now click on the object properties button to open the window as follows.
let us first display pressure (P1) contours on the cplane.
Click on the Cutplane tab, then select the Y Contour box, i.e. along the VWT to get a picture like this one.
Let us display pressure contours in Z plane, i.e. in the cross-section plane. First remove the above picture, clicking in the box 'None' in the Y-plane and then select 'Contour' in the Z-plane. You will have a picture similar to this one.

You can enlarge and rotate the images to get a better view of the results.
To display the simulation data for velocity, first make this variable a default one, clicking on the Scalar Properties tab, then on the PlotScalar button and choose VABS from the list standing for absolute velocity.
You can then repeat your previous steps to get the pictures like these.
- velocity contours in Y-plane;
- lines of constant velocity in Y-plane;
- velocity vectors in Y-plane;
  Your pictures for vectors can be different depending on the vector properties that you use. To control vector properties, click on the Vector Properties tab and experiment with vector length, radius and representation scale. The previous picture was obtained with the following vector properties.
- velocity contours in Z-plane.
The options accessed via the 'Cutplane' tab also allow display of both Lines & Vectors, Contours & Vectors as the following picture shows.
Experiment with the options to get pictures which suit more your requirements.

Pressure and velocity pictures show that there are regions of lower pressure and higher velocity close to the VWT walls and a low-pressure and low-velocity zone behind the object caused by a vortex created there.

4. Saving the results of your work

The results of the simulation will be saved in your working directory when you want to close the Prelude window and finish this rather prolonged session.

First close the present Prelude window clicking on the top right cross.
Answer 'yes' to the invitation 'Save the unsaved changes'.
To load your case, click on the tab 'Load other' and you will be asked to run any file having extension *.q1, *.q3 or *.psc from your working directory.
Open the vwtpscrun.psc file and your case will be loaded.
Then display the object tree clicking on the corresponding button in the tool bar.
Click on the VTK button and open the familiar phi.vtk file from the 'Choose VTK results file' window. The phivtk object together with its cplane will be displayed in the object tree and you can make similar steps already explained in the previous section.

5. Concluding remarks

In this tutorial you have learnt how to introduce a different object from the store cupboard into the VWT; how to change its defalt settings as well as these of the flow. You have also made simulation and got the results that confirmed the presence of the vortex zone behind the half-cylinder object.