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Figure 27.8: LWC of Monodispersed Droplets Around a NACA0012

14.Go to File → Save Case as… and save this calculation in the project directory FLUENT_ICING_NACA0012. Name this simulation nac0012_rough_mvd. Do not close this Fluent Icing session.

27.3.2. Langmuir-D Distribution

There are several cloud droplet size distributions that have been published in the literature. The distributions published by Langmuir have been used by NACA to determine the MVDs currently listed in Appendix C, which is used for icing certification of aircraft. Advisory Circular No 20-37A from FAA suggests using Langmuir-D distribution for MVDs up to 50 microns. For more details on these distributions,

youcan consult the Advisory Circular, and also the book by Irving Langmuir, The Collected Works of Irving Langmuir (New York, Pergamon Press, 1960).

The most important reason for considering an analysis using a distribution is that there are droplets larger than the MVD in the distribution, which can impinge further back on the top and bottom of the airfoil, creating a thin but rough layer of ice that can have adverse effects on aerodynamics and control. In this case, solutions for each droplet size of a given distribution are calculated separately. The final solution is then created as a composite of all solutions using weights on each droplet size.

In this tutorial, you will use the set-up created in Monodispersed Calculation (p. 896) as a starting point.

1.Without closing the previous Fluent Icing session (Monodispersed Calculation (p. 896)), go to Set-up → Particles → Droplets. In the Properties window, under Particles distribution, choose Langmuir D as Droplet distribution.

Note

The current version of Fluent Icing only supports pre-defined droplet size distributions (Langmuir B to E). User defined distributions are not yet supported. Below is a representation of a Langmuir D distribution and the droplet diameters that are used to represent this distribution. Please note that this figure is taken from FENSAP-ICE native user interface and is currently unavailable in the Fluent Icing UI.

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The droplet diameters are on the horizontal axis, and the weights (the percentage of droplets of a given diameter contained in the cloud) are on the vertical axis. The individual weights are shown with the blue curve, and the overall sum, cumulative weight, is shown with the red curve. On the red curve, the data points are plotted at the mid-range of their cumulative weight intervals. For example, the 20 microns droplet, which happens to be the MVD, covers the cumulative weight range of 35% to 65% and it is therefore plotted at 50% cumulative weight on the red curve.

A Particle droplet simulation is ran for each droplet size shown in the above table.

2.Go to Solve → Particles, in its Properties window, check Save distribution solutions under Output. This will allow you to save a droplet solution for each droplet size simulated. Otherwise, only the combined solution of the distribution is saved.

3.Keep all the other settings the same.

4.Go to File → Save Case as… and save this calculation in the project directory FLUENT_ICING_NACA0012. Name this simulation naca0012_rough_LangD. This will allow you to save the droplet solution for each droplet size using a different name than naca0012_rough_mvd.

Note

When Save distribution solutions is enabled, Fluent Icing saves the droplet solution for each droplet size using the most recent case file name saved in memory (current case file).

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5.Right-click Particles under Solve, select Initialize and then choose Calculate to run the calculation. Individual runs will be executed one after the other, and the results will be combined.

6.When calculations are completed, go to the ribbon bar of your Fluent Icing window and, under View → Quick-view → Contour, choose Collection Efficiency to output the water catch of the Langmuir D droplet distribution over the NACA0012. See figure below.

Figure 27.9: Collection Efficiency of Droplets with Langmuir-D Distribution over a NACA0012

7.Repeat these steps to easily output the LWC around the NACA0012. Blue contours define the shadow zone, absence of water droplets. See figure below.

Figure 27.10: LWC of Droplets with Langmuir-D Distribution Around a NACA0012

8.Go to File → Save Case as… and save this calculation in the project directory FLUENT_ICING_NACA0012. Name this simulation naca0012_rough_LangD. This action will save the combined solution of the distribution. Do not close this Fluent Icing session if you would like to proceed to the next section.

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Droplet Impingement on the NACA0012

27.3.3. Post-Processing Using Quick-View

To complement the built-in post-processing, ANSYS distributes Viewmerical and CFD-Post with the installation package. In this tutorial, you will use Viewmerical to post-process your droplet results. In the next tutorial, you will use CFD-Post to post-process your icing results.

Viewmerical is a light-weight graphical display tool specifically designed for in-flight icing solutions and applications. Viewmerical can display solution field contours, velocity vectors, planar cuts through the volumes, 2D graphs of variables, streamlines, etc. This tutorial will demonstrate some basic features of Viewmerical while comparing the two droplet solutions obtained in the previous sections.

1.While inside your Langmuir D simulation run, go to the ribbon bar of your Fluent Icing window and under

View → Quick-view → Contour, choose View with VIEWMERICAL. The program will launch and show an isometric display of the entire grid showing the first solution field, Droplet LWC, of the combined Langmuir D solution.

2.Rename this dataset by double-clicking on the original name, data-naca0012_rough_LangD.droplet.tmp. A Rename dataset window appears, write LangD in the text box.

3.Go to the Data tab and then change the Color range to Spectrum 2 – 16.

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4.Align the view angle with the Z-symmetry plane by right-clicking on the 3D axes on the lower left, and by choosing Top (Z). Alternatively, you can left-click the Z axis itself.

5.Zoom in on the airfoil. You can use Ctrl + left-click to draw a zoom box, or scroll the mouse wheel to zoom in and middle-click to pan.

6.Change the font of your legend to bold. Click on the top left corner of the window and select Command window; then type BIGFONTS in the command line of the 3dview console and hit Enter. The legend

fonts now become bold.

7.Using the camera icon on the upper left corner, you can take a snapshot of the solution window to capture the following image.

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Droplet Impingement on the NACA0012

Figure 27.11: LWC of a Langmuir D Droplet Cloud over a NACA0012 at an AoA of 4 Degrees, Showing the Shadow Zone (Blue Region)

Examine the LWC distribution in the area close to the airfoil. The blue region is called the shadow zone, where no droplets exist. In between the shadow zone and the free stream, there are bands of high LWC concentrations which are the enrichment zones forming due to the constriction of stream tubes in the continuum domain. These features can be of special interest for downstream aircraft components.

8.Go to the Data tab and choose Collection efficiency-Droplet. Collection efficiency is only displayed on the walls of your geometry. Go to Objects tab and uncheck BC_1004 and BC_4300 to display the collection efficiency distribution only on the walls (BC_2005, BC_2006, BC_2007, and BC_2008).

Use the left mouse button to rotate, the middle mouse button to pan, and the right-mouse button to zoom in the airfoil surface to obtain the following figure.

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Figure 27.12: Collection Efficiency of a Langmuir D Droplet Cloud on the Surface of the Airfoil at an AoA of 4 Degrees

9.For a more in-depth quantitative view, it would be possible to create 2D data plots using Viewmerical. Click the Query tab and enable 2D Plot.

Change the Cutting plane to Z and the horizontal axis to Y.

On the lower right corner of Viewmerical, you can directly modify data sets and solution fields. Leave them as they are now.

10.The color and thickness of the data curve displayed in the graph can be changed by left clicking on the cube menu located on the top right and by choosing Curve Settings. Set the curve color to red and the curve widths to 2 and press OK.

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Droplet Impingement on the NACA0012

Finally, the following 2D plot is generated.

Figure 27.13: Collection Efficiency of a Langmuir D Droplet Cloud on the Surface of the Airfoil at an AoA of 4 Degrees

The maximum beta occurs at the stagnation point, just below the leading edge in this case. The points on the upper and lower surfaces where beta becomes zero are the impingement limits. In rime icing cases, all the water that impinges is frozen instantly, therefore icing limits are the same as the impingement limits. In glaze icing, water can runback and freeze past the impingement limits. Maximum beta is usually no more than 1.0, and reduces as the droplet flow becomes tangent to the surface.

11.To save data points of this collection efficiency distribution, go to the cube menu on the top right and choose Save one file. A new window pups up to browse and name the file that should contain these data points.

12.You can also open and compare several solution files using Viewmerical. Let’s display simultaneously all 7 droplet size solutions obtained in Langmuir-D Distribution (p. 900).

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13.Go to the Objects panel, uncheck LangD and click on the button located at the right corner of the panel. A window appears to load a pair of files, a grid file and its solution file.

14.Click on the folder icon of Grid file and select the naca0012_rough_LangD.grid.tmp file located inside your working directory.

15.Click on the folder icon of Solution file (optional) and select the naca0012_rough_LangD.droplet.dist.01 file located inside your working directory.

16.Press the Load button. A new data set is added to the Object panel. Rename this dataset by double-clicking on its original name and enter LangD-01 in the window Rename dataset.

17.Repeat steps 13 to 16 for the remaining droplet solutions from *.dist.02 to *.dist.07.

18.Go to the Data panel and click Shared located under Color range. Switch the Data field to Collection efficiencyDroplet.

19.Go to Query tab, enable 2D plot, and switch the Cutting plane to Z. The graph should display 8 individual beta distributions. Click on LangD, to disable the LangD curve from the 2D plot. You can change the color and thickness of the data curve displayed in the graph via the cube menu on the top right and by choosing Curve Settings. You can also draw a zoom box by Shift + left-click.

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Droplet Impingement on the NACA0012

Figure 27.14: Collection Efficiency on the Surface of the Airfoil at an AoA of 4 Degrees, Langmuir D Droplet Solutions

The curve with the lowest beta corresponds to the smallest droplet size, and the one with the largest beta corresponds to the largest droplet size. Smallest droplets are less ballistic, tend to follow the

air flow and avoid the aircraft therefore reducing their collection efficiency and impingement limits. Larger droplets are more ballistic and they do not tend to follow the airflow. Therefore, their collection efficiency and impingement are usually higher than the smallest droplets. In general, this information is crucial to properly design the IPS power requirements and coverage.

Note

The difference between beta curves of different droplet sizes become more pronounced as the aircraft surface size increases. The effect can be dramatic on large blunt surfaces like fuselage noses or radomes where the contribution from the smaller size droplets can be negligible if compared to the largest ones. As a result, the composite or combined solution of a Langmuir simulation can be very different from the solution of the MVD.

20.To compare the LangD result to that of the monodispersed (MVD), go to the Objects panel, check LangD and uncheck all the other LangD-* objects.

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21.Click on the button located at the right corner of the Object panel. A window appears to load a pair of files, a grid file and its solution file.

22.Click on the folder icon of Grid file and select the naca0012_rough_LangD.grid.tmp file located inside your working directory. Both the LangD and the MVD solutions originate from the same Fluent

.CAS file. Therefore, both solutions share the same grid.

23.Click on the folder icon of Solution file (optional) and select the naca0012_rough_mvd.droplet file located inside your working directory.

24.Press the Load button. A new data set is added to the Object panel. Rename this dataset by double-clicking on its original name and enter MVD in the window Rename dataset.

25.Go to Query tab, enable 2D plot, and switch the Cutting plane to Z. The graph should display 9 individual beta distributions. Click on LangD-01 to LangD-07 to disable these curves from the 2D plot. Change the color of the MVD to red and of the LangD to blue via the cube menu on the top right and by choosing Curve Settings. You can also draw a zoom box by Shift + left-click.

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Figure 27.15: Collection Efficiency on the Surface, Langmuir D vs. Monodisperse

The LangD solution is fairly close to that of the MVD. The impingement limits of the Langmuir D solution will always be further back due to the inclusion of larger droplets in the distribution. The maximum beta of the composite is lower than the MVD here. This is not always the case. Based on the size and shape of the impingement surface, the Langmuir D solution can have a maximum beta that is several times higher than the MVD. In this case, however, the results of the MVD and the distribution are close.

26.You will now compare the LWC of the largest and smallest droplet of a Langmuir D distribution. Go to Objects panel, uncheck LangD and MVD objects and check LangD-01 (largest droplets) and LangD-07 (smallest droplets).

27.On the lower right corner of Viewmerical, change Collection efficiency-Droplet to Droplet LWC (kg/m^3).

28.Select LangD-01 in the Objects panel and choose Horizontal-Left under Split screen menu.

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