Flow Wizard Rotating Turb Flow Mixing Tank

Tutorial 9. Rotating Turbulent Flow in a Mixing Tank

In the following example, you will use FloWizard to set up a problem, solve the problem,and view the results.

Problem Description

This example illustrates the setup and solution of turbulent flow in a mixing tank. Itis often important to predict the flow field within a mixing tank as impellers rotate toproperly design the impeller/tank assembly.

The problem to be considered is shown schematically in Figure 9.1. A tank containswater that is circulated throughout by an impeller. The tank dimensions are in meters,and the properties and boundary conditions are given in metric units. A turbulent flowmodel is necessary.

Figure 9.1: Problem Description

c© ANSYS, Inc. January 31, 2008 9-1

Rotating Turbulent Flow in a Mixing Tank

Outline of Procedure

The steps you will follow in this example are reduced to the following:

• Set the units for the problem.

• Import and describe the geometry.

• Define boundary conditions.

• Set the calculation settings.

• Calculate a solution.

• Review a summary of the results.

• Examine the results graphically.

Step 1: Welcome

Before you begin, create a local working folder where you can store files generated byFloWizard. Start FloWizard by double-clicking the icon on your desktop. Once started,FloWizard displays the Welcome page by default.

In the Welcome page, click Next to proceed to the Units page of FloWizard.

Step 2: Units

1. In the Units page, FloWizard will prompt you to set the system of default units forthe problem at hand. These are the units that you would like to use. This settingdoes not affect the units in which the CAD geometry was created (which will beset separately). For this problem, select Metric.

2. Click Next to proceed to the Geometry page of FloWizard.

9-2 c© ANSYS, Inc. January 31, 2008


Step 3: Geometry

The Geometry page of FloWizard provides an overview of the steps you will perform duringthis phase.

In the Geometry page, click Next to proceed to the File Import page of FloWizard.

1. Import your model geometry into FloWizard.

(a) In the File Import page, click Browse... to display the Select Files dialog.

(b) Locate and select the mesh file (mixingtank.cas).

(c) Once you have selected the mesh file, click Open in the Select Files dialog toimport the mesh file into FloWizard.

The table in the File Import page now contains the file name and the dimensionsof the geometry.

The page also prompts you for the appropriate unit of length for the dimen-sions of the imported geometry.

(d) Use the Length unit drop-down combo box to select meters (m) as the units inwhich the original geometry was defined.

Figure 9.2: The File Import Page



2. Change the view of the geometry and include a picture in your final report.

(a) In the graphics window, select the outer wall of the tank (wall-tank), right-click to open the context menu and select Hide Selected from the menu. Thiswill expose the interior of the mixing tank.

(b) Select the wall of the inner cylindrical region (f face.81), right-click andselect Hide Selected from the context menu again to expose the impeller blades.

(c) Select the top and bottom walls of the inner cylindrical region (f face.80,and f face.82), right-click and select Hide Selected from the context menuagain to fully expose the impeller blades.

(d) Re-orient the display in the graphics window to get a better view of the mixingtank geometry so that the imported geometry is as shown in Figure 9.3.

Figure 9.3: The Mixing Tank Geometry (With Tank Walls Hidden)

i Note that you can manipulate the view by pressing the <Shift> keyand dragging the left, middle, and right mouse buttons to rotate,translate, and zoom respectively.

(e) Include the current display of the geometry in the graphics window in yourfinal report by clicking the Include This Picture in Final Report button. Thisopens the Include Picture in Report dialog.

(f) For the Caption, enter The Mixing Tank Geometry (With Tank Walls Hidden).

(g) Click OK to close the Include Picture in Report dialog and add the text as adescriptive caption to the picture that will ultimately be included in the finalreport.



3. Examine the computational mesh (optional).

You can visually examine the computational mesh that corresponds to the geometryyou have just imported into FloWizard. In addition, you can obtain informationrelated to the types of cells that constitute the mesh.

(a) Select the Show Mesh on Boundaries button or select the Mesh on Boundariesoption in the Show sub-menu under the View menu.

View −→ Show −→Mesh on Boundaries

This will display the computational mesh on all boundaries. To get a betterview of the mesh, you may want to zoom in on portions of the model that arenot hidden.

(b) Zoom into the area around the impellers. Select the Zoom to Area icon, thenclick and drag the left mouse button in the graphics window to create a rect-angle around the impellers. Release the mouse button to expand the view sothat the impellers fill the graphics window.

(c) Select the Hidden Lines Not Shown icon to make it easier to see the computa-tional mesh on the boundary surfaces.

Figure 9.4: A Close-Up View of the Computational Mesh



(d) Obtain data about the mesh by selecting the Mesh Information... option underthe Tools menu.

Tools −→Mesh Information...

This displays the Mesh Information dialog that contains information about thenumber of computational cells and the type(s) of cells used in the mesh.

Figure 9.5: The Mesh Information Dialog

(e) In the File Import page, click Next to proceed to the Symmetry page of FloWiz-ard.

4. Indicate if symmetry conditions are applicable.

(a) Under Is your geometry symmetric?, select No since this geometry does notexhibit symmetrical properties.

i Note that in this and many other pages within FloWizard, you canclick the Guide Me button for additional information.

(b) In the Symmetry page, click Next to proceed to the Description page of FloWiz-ard.



5. Describe the imported geometry.

(a) In the Description page, select The geometry is the flow region(s) to tell FloWiz-ard that the imported geometry contains only the flow region(s).

For geometries that represent only the flow region(s), no more additional in-formation is needed about the characteristics of the flow geometry.

(b) In the Description page, click Next to proceed to the next phase of FloWizard.

6. Save your work.

Since you have not yet saved your work, FloWizard will prompt you to do so withthe Save Your Inputs dialog.

Figure 9.6: The Save Your Inputs Dialog

(a) In the Save Your Inputs dialog, click Save to save your work in the Geometryphase of FloWizard. FloWizard will display the Save File dialog (unless youhave already saved your work).

(b) Save your file in your working folder with a case file name of your choice (e.g.mixingtank).

(c) Once you click the Save button in the Save File dialog, FloWizard will saveyour work using a .flowizard file extension (i.e., mixingtank.flowizard).



Step 4: Physics

Now that your model’s geometry has been defined, you must specify the physical char-acteristics of the flow. In the Physics section of FloWizard, you define these physicalcharacteristics, such as whether or not temperature is important, whether the flow islaminar or turbulent, which fluid(s) are involved, and the conditions of the flow at theboundaries.

In the Physics page of FloWizard, click Next to proceed to the Temperatures page.

1. Define whether you want to consider temperature effects.

Temperature considerations are important when you have a compressible gas flowbecause the temperature is needed to calculate the density of the gas.

(a) In the Temperatures page, select No to ignore heat transfer for the currentproblem.

(b) Click Next to proceed to the Flow Type page of FloWizard.

2. Determine the type of the flow.

Different types of flow fields require different computational approaches. Flow fieldscan be placed into one of two categories: laminar or turbulent.

(a) In the Flow Type page, select Turbulent.

(b) Click Next to proceed to the Region Properties page of FloWizard.

i Note that the mixing tank is divided into two regions: one contain-ing the rotating impeller (fluid-impeller); and one containingthe stationary components, such as the tank wall and the baffles(fluid-tank). You will now proceed to define the properties ofeach region.



3. Define the properties of the region containing the impeller (fluid-impeller).

(a) In the Region Properties page, select fluid-impeller and click Define Properties...to display the Define Region Properties sub-wizard. Note that you can alsodouble-click the name in the list to display the sub-wizard.

(b) Keep the default settings of Fluid and the name of fluid-impeller.

(c) Click Next in the Define Region Properties sub-wizard. Here, you will specifywhether you want to consider flow characteristics for the fluid region, andwhat fluid is contained in the region.

(d) Select Yes to indicate that you want to include this fluid region.

(e) For Is the flow in this region compressible?, choose No to indicate that the flowin this region is incompressible.

(f) For Does this region contain rotating machinery, such as turbines and impellers?,select the Guide Me... button to display the Rotating Machinery Guide.



The Rotating Machinery Guide offers assistance in making the correct choiceif your geometry contains any rotating machinery. Inspect each of the op-tions, noting that the faces that are shared between fluid-impeller andfluid-tank form a cylinder with the shaft as an axis. This is a geomet-ric requirement for regions containing rotating machinery. If, for example,fluid-impeller had a cubic shape, then the problem setup would be invalidand incorrect results would be obtained. Based on this information, you canselect the last option in the Rotating Machinery Guide and click OK to closethe dialog.

Figure 9.7: The Rotating Machinery Guide

Note that, back in the Define Region Properties sub-wizard, Yes is alreadyselected for you to indicate that the flow in this region contains rotating ma-chinery.



(g) Select water-liquid from the Fluid drop down combo box.

(h) Click Next in the Define Region Properties sub-wizard to specify the motion ofthe rotating machinery.



(i) Enter 60 and select rpm from the drop-down combo box for the RotationalSpeed.

(j) Specify the location of the rotation axis.

i. Select At a specified origin point and direction vector under Where is therotation axis located?.

ii. For the Origin of the rotational axis, enter 0 for x, y, and z.

iii. For the Direction of the rotational axis, enter 0 for x, 1 for y, and 0 for z.

(k) Select Counterclockwise for the direction of the rotation around the rotationalaxis.



(l) Click the Preview Rotation Direction button and note how the graphics windowdisplay two arrows: a white arrow displaying the axis of rotation; and a bluearrow displaying the direction of rotation.

To make sure you can see the two arrows, you may want to view the geometryusing wireframe mode (using the Wireframe icon) and by zooming out a little(using the Fit to Window icon).

Figure 9.8: Previewing the Rotation of the Impellers Inside the Mixing Tank

(m) Click Finish in the Define Region Properties sub-wizard to apply the propertiesand close the sub-wizard.

Note that even though the fluid-tank region temporarily disappears, theregion still exists and reappears as soon as you leave the Region Propertiespage.



4. Define the properties of the fluid region in the main portion of the tank (fluid-tank).

(a) In the Region Properties page, select fluid-tank and click Define Properties...to display the Define Region Properties sub-wizard. Note that you can alsodouble-click the name in the list to display the sub-wizard.

(b) Keep the default settings of Fluid and the name of fluid-tank.

(c) Click Next in the Define Region Properties sub-wizard. Here, you will specifywhether you want to consider flow characteristics for the fluid region, andwhat fluid is contained in the region.

(d) Select Yes to indicate that you want to include this fluid region.

(e) For Is the flow in this region compressible?, choose No to indicate that the flowin this region is incompressible.

(f) For Does this region contain rotating machinery, such as turbines and impellers?,keep the default setting of No to indicate that the flow in this region containsno rotating machinery.

(g) Select water-liquid from the Fluid drop down combo box.

(h) Click Finish in the Define Region Properties sub-wizard to apply the propertiesand close the sub-wizard.

The Region Properties page is updated to show the name of the fluids.

(i) Click Next to proceed to the Boundary Conditions page, where you will defineboundary conditions for the simulation.

Note that by default, FloWizard sets all boundaries for this particular geometryto stationary walls. You will need to modify some of these default settings inthe following steps.



5. Define the boundary conditions for the outer wall of the mixing tank.

(a) Select wall-tank from the list of boundaries in the Boundary Conditions page.

(b) Click Define Conditions... to display the Define Boundary Conditions sub-wizard.

(c) Keep the default values for type (Wall and Stationary) and name of the wall-tankboundary.

(d) Click Finish to set the outer wall boundary conditions.

6. Define the boundary conditions for the impeller blades.

(a) Select wall-impeller from the list of boundaries in the Boundary Conditions page.


(c) Keep the default value for type of Wall, but change the sub-type to Moving,and keep the default name of the wall-impeller boundary.

(d) Click Next to define the motion of the wall. Select The rotating motion is thesame as that of the adjacent fluid region.

(e) Click Finish to set the boundary conditions for the impeller blades.

7. Define the boundary conditions for the bottom wall of the mixing tank.

(a) Select wall-bottom from the list of boundaries in the Boundary Conditions page.


(c) Keep the default values for type (Wall and Stationary) and name of the wall-bottomboundary.

(d) Click Finish to set the bottom wall boundary conditions.

8. Define the boundary conditions for the four baffles. The baffles convert a portionof the swirling motion generated by the impeller into vertical motion, assisting inthe mixing process.

(a) Select wall from the list of boundaries in the Boundary Conditions page.


(c) Keep the default values for type (Wall and Stationary).

(d) Rename the wall boundary to wall-baffles.

(e) Click Finish to set the wall boundary conditions.

i Note that the shaft inside the mixing tank is divided into two sec-tions. One section is a boundary of fluid-tank and another sec-tion that is a boundary of fluid-impeller. We will now proceedto specify boundary conditions for each of the two sections.



9. Define the boundary conditions for the top part of the shaft that is contained inthe region fluid-tank.

(a) Select top-shaft from the list of boundaries in the Boundary Conditions page.


(c) Keep the default selection of Wall for the boundary type and select Moving,keeping the name of the top-shaft boundary.

(d) Click Next to define the motion of the wall.

i. Select Rotating.

ii. Enter 60 and select rpm from the drop-down combo box for the RotationalSpeed.

iii. Select At a specified origin point and direction vector under Where is therotation axis located?.

iv. For the Origin of the rotational axis, enter 0 for x, y, and z.

v. For the Direction of the rotational axis, enter 0 for x, 1 for y, and 0 for z.

vi. Select Counterclockwise for the direction of the rotation around the rota-tional axis.

vii. Click the Preview Rotation Direction button to check your inputs.

(e) Click Finish to set the wall boundary conditions for the rotating top shaft.

10. Define the boundary conditions for the bottom part of the shaft that is containedin the region fluid-impeller.

(a) Select bottom-shaft from the list of boundaries in the Boundary Conditions page.


(c) Keep the default value for type Wall, but change the sub-type to Moving, andkeep the default name of the bottom-shaft boundary.

(d) Click Next to define the motion of the wall. Select The rotating motion is thesame as that of the adjacent fluid region.

(e) Click Finish to set the boundary conditions for the bottom shaft.



11. Define the boundary conditions for the surface of the liquid at the top of the mixingtank (liquid-surface).

(a) Select liquid-surface from the list of boundaries in the Boundary Conditionspage.


(c) Select Gas or liquid surface and keep the default name of the liquid-surface

boundary.

(d) Click Finish to set the boundary conditions for the bottom shaft.

Now all boundaries should be defined (as noted by the green checkmarks in theBoundary Conditions page) so that clicking Next will take you to the next phase ofFloWizard.

If you have not already done so, FloWizard will prompt you to save your work withthe Save Your Inputs dialog. In the Save Your Inputs dialog, click Save to save yourwork in the Physics phase of FloWizard.



Step 5: Solution

In this section of FloWizard, you can specify speed vs. accuracy settings and review inputsthat you have supplied in order to make sure they are correct before you begin yourcalculations. Finally, once you specify a maximum CPU time allotment and designatehow you want to save your work, you can start performing the CFD calculation.

Click Next in the Solution page to proceed to the Settings page.

1. Specify your solution accuracy.

On the Settings page, under Specify the desired balance between solution speed andsolution accuracy, move the slider to the Accuracy setting for good, quantitativeresults.

2. Request geometry cleanup and simplification, if necessary.

(a) On the Settings page, click No under the Does your geometry need to be cleanedup and simplified? since the imported geometry does not need to be simplified.

(b) On the Settings page, click Next to proceed to the Pre-Calculation Review page.



3. Review your inputs.

If you have skipped any inputs while entering information into the wizard pages,FloWizard will inform you of this in the Pre-Calculation Review page. It is up to youto return to the pages indicated to make the necessary corrections. If all inputshave been completed, FloWizard will display a confirmation message in the page.

(a) Click the View Summary of Inputs button to display the Summary of Inputsdialog.

The Summary of Inputs dialog contains relevant details of your inputs for thecalculation. Review the information and close the dialog when you are readyto proceed.

(b) On the Pre-Calculation Review page, click Next to proceed to the Calculationpage.

If you have not already done so, FloWizard will prompt you to save your work withthe Save Your Inputs dialog. In the Save Your Inputs dialog, click Save to save yourwork in the first part of the Solution phase of FloWizard.



4. Perform the calculation.

(a) Make sure the Calculate solution using existing mesh option is selected in theAction drop-down combo box.

(b) Make sure the On this machine option is selected in the Location drop-downcombo box.

(c) For Maximum CPU time, enter 8 hours. Note that, for this problem, thesolution will likely be complete in well under that time (approximately 2 to 3hours).

(d) Make sure the Automatically after the calculation option is selected in the Saveoption drop-down combo box.

(e) Click the Start Calculation button to start the calculations.

Figure 9.9: The Calculation Page

(f) Note the progress of the calculations in the Progress dialog that appears.

(g) Once the calculations are complete, the Interrupt button in the Progress dialogbecomes the Close button. Close the Progress dialog by clicking the Closebutton.

(h) In the Calculation page, click Next to proceed to the Results page.



Step 6: Results

Once your calculation has been completed, you can view a summary of your results,display results on the boundaries, display plane cuts and iso-surfaces, view flow pathlinesthat will show you the dynamics of the flow field, and generate a report of your CFDsimulation.

In the Results page, click Next to proceed to the Summary page.

1. Display a summary of your results.

(a) Click the View Summary of Results button to display the Summary of Resultsdialog.

The Summary of Results dialog contains relevant details of the results of thecalculation. Review the data and close the dialog when you are ready toproceed.

(b) In the Summary page, click Next to proceed to the Boundaries page, where youcan display your results on the boundaries of the geometry.

2. Display results on boundaries.

(a) In the Boundaries page, select Velocity magnitude (regions only) for the Quantityunder What do you want to display? and Distribution as the Display type.

(b) Select wall-tank, hold down the <Ctrl> key and select wall-bottom in the listunder Specify which boundaries to display the results on.

Figure 9.10: The Boundaries Page



(c) Change the color scale to use “local” values – a color scale where the minimumand maximum values are those of the currently selected object(s).

i. Select Results Display Options... from the View menu to open the ResultsDisplay Options dialog.

View −→Results Display Options...

ii. Select Local in the Color Scale tab of the Results Display Options dialog,keeping the default values.

Figure 9.11: The Results Display Options Dialog



You can view your velocity magnitude results on the tank walls in the graphicswindow. Note the change in the velocity magnitude color scale.

Figure 9.12: Velocity Magnitude (Regions Only) Distribution on Tank Walls

(d) Click Include This Picture in Final Report... to display the Include Picture inReport dialog.

(e) In the Include Picture in Report dialog, enter Velocity Magnitude (Regions

Only) Distribution On Tank Walls as a caption to be included with thepicture when it becomes part of your final report.

(f) In the Boundaries page, click Next to proceed to the Plane Cuts page, whereyou can display your results on cross-sections within your geometry.



3. Display plane cuts.

(a) In the Plane Cuts page, select XZ plane (constant y) in the list of Planes.

(b) Keep the default Distance as 0 to concentrate on the flow in the midsection ofthe mixing tank.

(c) Keep the default selection of Velocity magnitude for the Quantity under Whatdo you want to display on the selected plane(s)?

(d) Select Flow vectors (proportional to velocity) as the Display type.

Figure 9.13: The Plane Cuts Page

You can view your plane cut in the graphics window. FloWizard makes theboundaries semi-transparent so that you can easily view the plane cuts.



(e) Zoom into the area around the impeller core (using the Zoom to Area icon) toget a better view of the circulating flow.

Figure 9.14: Flow Direction Vectors Based on Velocity

(f) Click Include This Picture in Final Report... to display the Include Picture inReport dialog.

(g) In the Include Picture in Report dialog, enter Flow Direction Vectors on xz

Plane as a caption to be included with the picture when it becomes part ofyour final report.

(h) Click OK to close the Include Picture in Report dialog.

(i) Zoom back out (using the Fit to Window icon) to get an overall view of themixing tank.

(j) View an animation of the plane cut sweeping throughout the flow field byclicking the Play button.

(k) Stop the animation by clicking the Stop button.

(l) In the Plane Cuts page, click Next to proceed to the Iso-Surfaces page. Youare welcome to view your results on iso-surfaces if you wish, however, for thepurposes of this tutorial, an iso-surface analysis is not required.

(m) In the Iso-Surfaces page, click Next to proceed to the Flow Pathlines page.



4. Display flow pathlines.

(a) In the Flow Pathlines page, under What quantity do you want to use for pathlinecoloring?, select Velocity magnitude from the Quantity drop-down combo box.

FloWizard allows you to specify the location where the pathlines are released(from any boundaries or plane cut surfaces). You can change the pathlinevisualization by selecting different boundaries or plane cuts.

(b) Select XZ plane (constant y) from the list of possible Locations.

(c) Keep the default setting for the slider that controls the number of pathlines.

(d) Click the Update Pathlines button to update the pathline display in the graph-ics window.

Figure 9.15: The Flow Pathlines Page



You can view the pathlines in the graphics window. FloWizard makes theboundaries semi-transparent so that you can easily view the pathlines.

Figure 9.16: Velocity Magnitude Pathlines Throughout Flow Field

(e) Click Include This Picture in Final Report... to display the Include Picture inReport dialog.

(f) In the Include Picture in Report dialog, enter Velocity Pathlines as a captionto be included with the picture when it becomes part of your final report.

(g) View an animation of tracers moving along the pathlines by clicking the Playbutton.

(h) Stop the animation by clicking the Stop button.

(i) In the Flow Pathlines page, click Next to proceed to the Point Probes page. Youare welcome to view your results at point probes if you wish, however, for thepurposes of this tutorial, a point probe analysis is not required.

(j) In the Point Probes page, click Next to proceed to the Final Report page.



5. Generate a final report.

(a) Add a comment to the final report.

i. Select Introduction in the Report Section list.

ii. In the Your Comments text field, enter the following text:

This is an example of flow through a mixing tank. Flow rotates

throughout the tank via impellers.

Figure 9.17: The Final Report Page



(b) Click View Report to view the report.

(c) Click Save Report... to save the report.

FloWizard will display a Save File dialog where you can specify a folder anda file name for the final report. For this example, keep the default nameof mixingtank report. FloWizard will save mixingtank report.htm in thespecified folder.

i You can easily import your HTML final report into either MicrosoftWord or Excel to add or edit text and figures.



Summary

This example demonstrated how to use FloWizard to set up and solve a CFD analysisfor flow through a mixing tank where internal impeller walls are rotating and movingthe fluid around the tank. It showed how to import and describe the geometry, as wellas how to define the physics and the boundary conditions for the problem. In addition,computational results were generated and graphically analyzed in several different ways.Finally, a report was generated that contains tabulated inputs, outputs, and graphicalpictures.


Documents

Flow Wizard Rotating Turb Flow Mixing Tank