20.3.3 Postprocessing for the Crevice Model

A plot of cylinder mass with and without the crevice model during the motored engine simulation is shown in Figure  20.3.2. The rate of mass loss from the crevice is proportional to the pressure difference between the cylinder and the crankcase pressure defined in the text interface.

Figure 20.3.2: Cylinder Mass vs. Crank Angle

A plot of cylinder pressure with and without the crevice model for the same engine simulation is shown in Figure  20.3.3. The effect of the mass loss from the crevice is to lower the peak pressure in proportion to the total mass loss from the cylinder.

Figure 20.3.3: Cylinder Pressure vs. Crank Angle

Using the Crevice Output File

The pressure in the top ring land is defined as the cylinder pressure (i.e., the pressure in the cells defining the ring landing). Intermediate pressures are available at any point during the ANSYS FLUENT session through the (crevice-summary) command as previously shown. If the optional data file output is chosen in the crevice-model-controls, the intermediate pressures in the defined crevices are printed to the file crev.out at the start of each new time step. The format of the file is as follows:

# crank (deg)  data->press[0...1...2...3...4...5...6] total_mdot
 1.95500e+02 2.16650e+05 1.01325e+05 1.01325e+05 1.01325e+05 1.01325e+05 1.01325e+05 1.01325e+05  0.0
 1.96000e+02 2.09945e+05 1.06794e+05 1.81553e+05 1.04111e+05 1.48582e+05 1.02202e+05 1.01325e+05 -1.6
 1.96500e+02 2.17787e+05 1.13070e+05 1.88242e+05 1.07960e+05 1.53544e+05 1.03526e+05 1.01325e+05 -1.6
 1.97000e+02 2.17434e+05 1.19065e+05 1.88060e+05 1.11705e+05 1.53475e+05 1.04830e+05 1.01325e+05 -1.6
 1.97500e+02 2.17652e+05 1.24777e+05 1.88299e+05 1.15286e+05 1.53668e+05 1.06081e+05 1.01325e+05 -1.6
 1.98000e+02 2.17937e+05 1.30215e+05 1.88594e+05 1.18711e+05 1.53900e+05 1.07283e+05 1.01325e+05 -1.6

where the first column is the current flow time (or crank angle), and the next $n_{\rm cv}+2$ columns are the ring pressures (where $n_{\rm cv}$ is the number of crevice volumes, or $2n_r -1$), including the face pressure on the crevice cell, and the defined pressure at the crevice exit. The final column is the mass flow past the top ring. This file is currently formatted so that it can be read into the free Gnuplot plotting package, which is available at www.gnuplot.info.

To read the crevice output file into ANSYS FLUENT as a data file, you will need to put each column of the crevice output file in its own individual file. The first three lines of each column of the data file should be of the following form:

"Title"
"X-Label" "Y-Label"
0 0 0 0

where the title, $x$-label, and $y$-label strings are enclosed by double quotes and the third line of the file contains four zeros. The lines following the first three lines of the file are the columns you wish to plot. For example, to plot column 1 versus column 3 of the crevice model output file in ANSYS FLUENT, you would enter the following commands in a UNIX terminal:

cat > crev_col_1_3.dat
"Column 1 vs Column 3"
"Crank Angle (deg)" "Pressure behind ring 1 (Pa)"
0 0 0 0
ctrl-d

where ctrl-d is the end-of-file character made holding down the <Ctrl> key and pressing d. To append columns 1 and 3 to this file, enter the following:

tail +2 crev.out | awk '{print $1, $3}' >> crev_col_1_3.dat

The file crev_col_1_3.dat can now be read into ANSYS FLUENT using the File XY Plot dialog box. See Section  29.9.3 for details about creating $x$- $y$ plots. For Windows users, the file crev.out can be imported into Excel for plotting purposes without any modification.

A Gnuplot plot of the pressure in the ring pack crevices for the above engine simulation is shown in Figure  20.3.4. After an initial transient period where the flows in the network settle down, Figure  20.3.4 shows that the pressure in the ring crevices follows the cylinder pressure in form, though with pressure magnitudes that are controlled by the ring pack geometry.

Figure 20.3.4: Crevice Pressures