Isolated nacelle

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Isolated nacelle

An example of designing an isolated nacelle using TLNS3D-MB is shown in Figures 1 - 4. The

geometry is an isolated fan-cowl and core cowl. The grid for the fan-cowl and core-cowl comprised six

blocks containing a total of one million grid points. The core cowl was run inviscid while the fan cowl

was run viscous. The external surface of the fan cowl was redesigned.

Figure 1 shows the convergence history for the design of this isolated nacelle. The spikes indicate the

design iterations. The horizontal axis on the plot is a measure of the computational work that was

performed. It is readily seen that in order to achieve the same O(3.5) drop in residual, the design takes

roughly 420/290 ~ 1.45 times the computational work of an analysis of a fixed geometry. Similar run

comparisons were observed with the OVERFLOW runs.

Figure 2 shows that the target coefficient of pressure has been closely matched after eight design cycles.

The target stations were grid lines closest to the 45 degrees, 90 degrees, 135 degrees, 180 degrees, 225

degrees, 270 degrees and 315 degrees cuts respectively. The run was set up so that no design changes

were allowed on the upper part of the nacelle between 340 degrees and 20 degrees. One can see that the

final Cp distribution is quite different from the initial distribution.

This case is a good check-out run because the target pressure distribution was deliberately obtained from

a geometry that was known a priori. While there is no guarantee that the inverse design will produce this

identical geometry, Figures 3 and 4 show that the designed geometry is quite close to the "target"

geometry. The dotted lines for the design shape are almost completely masked by the solid lines for the

known "target" geometry. Note also that the resultant nacelle is symmetrical about the vertical plane.

This symmetry condition was not forced during the design by using the CDISC option of aliasing

corresponding left side stations to those on the right. Rather, the symmetry of the original geometry was

maintained because of the uniform inflow condition. Note also that the original and designed nacelles

are not symmetrical, about a horizontal plane passing through the nacelle centerline, because of the inlet

droop.

This example shows that a desired nacelle can be obtained from a radically different initial profile. The

technique is directly applicable to any multi-block grid containing an installed nacelle.

Installed Nacelle

The nacelle designed above was installed on a transport airplane operating at a mid-cruise Mach number

of 0.85. This flow-through nacelle was installed with a toe-in of 1.5 degrees and an upward tilt of 2

degrees. The cruise angle of attack was roughly 1.5 degrees which meant that the installed nacelle faced

an inflow stream at a much higher effective angle of attack than in the example above. Also the inflow

was non-uniform because of the presence of the airplane wing and fuselage.

This redesign was performed with OVERFLOW-CDISC. The fuselage-wing, pylon, fan-cowl,

core-cowl, bifurcator, pylon shelf and far-field are represented by seven overset grids. The grid topology

is not presented here because it is very similar to that described in reference 10 which also shows

various views of the overset grid system. During each design cycle, the fan-cowl grid was the only grid

extracted to the virtual workbench. The design was carried out over the entire circumference of the fan

cowl.

Figure 5 is a plot of the convergence history. The nine spikes indicate the design iterations. An

erroneous tenth spike is also visible. This spike may be ignored; it occurred because of a malfunction in