# Download EXERCISES - FLOW-3D v11 Water Environment 3 Day Intro.pdf PDF

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E X E R C I S E S

I m p r o v i n g t h e w o r l d t h r o u g h a c c u r a t e f l o w m o d e l i n g

WATER & ENVIRONMENT TRAINING

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Guide 1: CFD Workflow Guide

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Water & Environment Training on FLOW-3D v11
Exercise 5: Analysis

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Use Solver Text Output to Diagnose Errors

1. Locate the Warnings & Errors button above the runtime plots. It means warnings are
present.

2. After the meshblock mismatch errors discussed in There is one warning, which is displayed
below.

Solver Messages:
convective flux exceeded stability limit
at t= 3.0009E-02 cycle= 1 iter= 51 delt= 3.0009E-02 mesh block 4
restarting cycle with smaller time step
maximum failure ratio = 2.26944E+00 is in x-coordinate direction
at cell ( 82, 34, 17) mesh block 4

3. The meaning is as follows:
a. At t = 0.03 seconds, a packet of fluid tried to move more than one cell at a time. The packet

probably accelerated unexpectedly, which is why the solver didn’t predict the velocity of the
packet. The stability limit is the requirement that fluid packets only move one cell per time
step. The solver takes care of the problem by restarting the time step with a smaller dt.

b. The error occurred in Mesh Block 4, the conforming block.
c. The offending cell number is i, j, k = 82, 34, 17. Remember that i,j,k values are block-specific,

while x,y,z values are global.
d. The problematic fluid packet was moving in the x-direction. In the next step you will find the

problem cell and identify why the problem occured.

4. Use the small X in the upper right corner of the dialog to close the Warnings & Errors.

5. Scroll up through the text output displayed below the plots and find the same warning. Notice
that the cell location and mesh block are not given in this summary.

6. Select Diagnostics > Solver Errors from the menu at the top of the screen: the same information

always be checked.

The only run-time error reported in this simulation is a single convective flux error which does not affect
solver accuracy. Except for the purposes of this exercise, you can safely ignore isolated errors. If
convective flux errors recur frequently, they will at best slow down the simulation, and at worst cause it
to terminate before completion. Errors should be addressed when they are repetitive.

Use 2-D Plots to Diagnose Errors

1. Go to the Analyze > 2-D tab.

2. Click the Mesh Block button at the lower right and select Mesh Block 4 only. When you click OK,
the Limits sliders will reset to show the I J K cell number in the selected block at their sides.

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Water & Environment Training on FLOW-3D v11
Exercise 5: Analysis

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3. Set plotting extents for Block 4:
a. Set Contour Variable = X-Velocity and Vector Type = Plain Velocity Vectors.
b. Select Plane = XZ.
c. Select the Mesh checkbox to plot the grid lines.
d. Select Data Source = Selected.
e. Slide the Max Time Frame slider (or use the associated arrow) so that t = 0 to 0.7 seconds.
f. Slide the X Limits sliders so that I = 78 to 86. The problem cell (I = 61) will be in the center.
g. Move the Y Limits sliders so that both Min and Max J = 34. This will display one planar slice

along the cell centers of Block 4. The slice will contain the problem cell.
h. Slide the Z Limits sliders so that K = 2 to 32.

4. Select contour plotting options:
a. Click the Contour Options button in the bottom central portion of the tab.
b. Activate the No Contour Smoothing checkbox to turn off the aesthetic blurring of cell-

centered values.
c. Select Blanking Variable = None. This clears the Blanking Variable Value entry. All cell

values will be displayed, whether they contain fluid or not.
d. Select OK to close the dialog.

5. Check that you’ve made the selections above (marked below for reference) and click Render.
The two plots closest in time to the error should be similar to the ones on the next page.

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Water & Environment Training on FLOW-3D v11
Making .STL Files from USGS Data

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STEP 12 – CHECK ACCURACY
Use CloudCompare’s Compute Cloud/Mesh Distances tool with Octree Level = 10 and compare the .stl

file to the original trimmed point cloud. Do not include the generated 2-D grid that was added to make

the upper surface of the .stl.

SIGNED ERROR ERROR MAGNITUDE

MEAN
STANDARD
DEVIATION MEAN ±

STANDARD
DEVIATION

NED (USGS 2003)
(NED vs. 13,305 geodetic controls)

-0.32 m 2.42 m
2.44 m
(RMSE)

~2.3 m
(1/2 95

th
percentile)

15-m TOPO2STL spacing
(.stl surface vs. 20.3 x 10

6
points)

-0.10 m 3.6 m 2.4 m 2.6 m

30-m TOPO2STL spacing
(.stl surface vs. 20.3 x 10

6
points)

-0.28 m 7.9 m 5.5 m 5.7 m

Table 2. Accuracy statistics for 1/3 arc-second data: original and converted to .stl

As of June 2003, the USGS NED data exhibits vertical uncertainty vs. the real world that is similar to the

uncertainty generated by converting the NED data to .stl format with 15-m spacing. For details of the

NED error estimation, see USGS (2006) “Vertical Accuracy of the National Elevation Dataset”, online as

of 04/29/2014 at http://ned.usgs.gov/documents/NED_Accuracy.pdf. The area of interest (the valleys

where the St. Francis flood occurred) has a very low error, as shown in Figure 23. The error magnitude

distribution is shown in Figure 24: it is best described as a Weibull distribution. The 15-m .stl file (Figure

25) shows the least error, and will be used in subsequent FLOW-3D simulations.

Figure 23. Original point cloud, colored by distance from 15-m resolution .stl surface

(blue = 0 m, red = ±7.6 m i.e. mean + 2 standard deviations)

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Making .STL Files from USGS Data

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Figure 24. Histogram of point-cloud to 15-m .stl facet error. Gray region is outside 2 standard deviations.

Figure 25. Close-up of the final 15-m .stl.