Method

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Thus, a tool for the inverse design of nacelles must be flexible enough for a designer to be able to

produce an initial design, then study changes required to the proposed design because of the presence of

the airplane flowfield, and finally, perform trade studies of the pros and cons of nacelle symmetry. For

example, the cruise benefit gained from achieving laminar flow over a non-symmetric nacelle might be

significant enough to outweigh other issues.

Method

The design method used in the codes is the CDISC inverse design algorithm1. CDISC is an extension of

the basic DISC method of Campbell2&3. At the very crux, DISC is a pressure based design method in

that the change in surface geometry is related to a change in pressure. The user starts with an initial

"rough" cut of the geometry. The initial geometry is gridded and then analyzed using a CFD code. The

surface pressure is then examined and a target pressure distribution is specified which is an

improvement to the original pressure distribution. The difference between the desired pressure and the

existing pressure is used to compute a change in geometry. The volume grid is automatically adjusted to

accommodate the new geometry and a fresh analysis is performed. The results from this analysis are

then compared to the target and a new geometry is computed and analyzed. The process continues until

the user is satisfied with the analysis pressure.

The nature of the relationship between geometry and pressure is different for subsonic (where change in

pressure is related to change in curvature) and supersonic (where change in pressure is related to change

in slope) flow but the basic idea is the same for both: that the resultant change in geometry depends on

the desired change in pressure. This kind of design philosophy is ideally suited for nacelle design

because nacelle flows are largely attached. Without special treatments, one would expect DISC to fail in

regions of massive separation because the streamline curvature relationship would no longer be valid on

the surface.

In CDISC, one specifies desirable characteristics of the target pressure distribution. For example, one

might specify pressure gradients in one region, shock position in another region, pressure levels in a

third and then smooth selectively certain critical regions of the design space. These regions can be both

chordwise and spanwise (circumferential) in extent. The target file is automatically generated and

updated at each step based on all these constraints. One can also specify geometry constraints such as

minimum thickness and leading edge radius.

Additionally, CDISC has some global constraints for quantities like twist distribution, lift coefficient

and pitching moment that are especially useful for the design of wings. In the current version of CDISC

an attempt is made to satisfy all constraints by iterating through them a set number of times in the order

in which they are listed.

Implementation

DISC and CDISC have been coupled to many CFD codes. Airplane winglets and installed nacelles have

been designed using DISC coupled to an Euler code4&5. DISC has been used with the CFL3D

Navier-Stokes code for the design of isolated nacelles6.

The coupling of CDISC with the multi-block codes, TLNS3D-MB (Thin-Layer Navier-Stokes

3-Dimensional Multi-Block from Vatsa7,8&9) and OVERFLOW (a 3-D Navier Stokes code for overset

Grids from Buning10-13) is presented here. These two implementations of CDISC allow for the design

of either isolated or installed nacelles with any combination of toe, inclination and roll angle for the