## Goal

** We wish to demonstrate that automatic differentiation,
when coupled with domain knowledge, can efficiently provide accurate
sensitivities (derivatives) of real computational fluid dynamics (CFD)
codes. Accurate sensitivities are a prerequisite for many efforts in
engineering computation. **
## Background and Project Description

Modern engineering design depends on sensitivity calculations. To
obtain these sensitivities, engineers have two choices: use finite
differences, or develop a program to compute the analytic
derivatives. To avoid programming, engineers have typically chosen
to use finite differences. The accuracy of finite difference
calculations, however, depends on getting a ``good'' step size. A ``good''
step size, however, depends on two competing factors. To approximate
the derivative well, the step size should be ``small.'' On the other
hand, to avoid critical cancellation, the step size should be
``moderate.'' Finding an appropriate step size to satisfy both criteria
can be difficult, especially for complex processes like CFD
calculations. Furthermore, the step size exercise may need to be
repeated for each new point in the design space.
Analytic derivatives, by definition, avoid the step size problem.
Generating an analytic derivative program by hand, however, is
tedious, time consuming, and prone to error. Fortunately, engineers
can avoid the ``by hand'' part by using an automatic differentiation
(AD) tool. An AD tool automatically applies the derivative chain rule
to all of the expressions in a program, thereby generating a program
that computes both the desired analytic derivatives, in addition to
the function value.

In this work, we describe results obtained by application of the
ADIFOR 2.0 automatic differentiation to OVERFLOW, an extensively used
CFD code from NASA Ames.