The β-assumed-p.d.f. approximation of Cook & Riley
(1994) is tested as a subgrid
model for the LES computation of non-premixed turbulent reacting flows,
in the limit
of infinitely fast chemistry, for two plane constant-density turbulent
mixing
layers with
different degrees of intermittency. Excellent results are obtained in the
computation
of plane-averaged properties, such as product mass fractions and relatively
high
powers of the temperature, and even of the p.d.f. of the conserved scalar
itself. In
all these cases the errors are small enough to be useful in practical
applications. The
analysis is extended to slightly out-of-equilibrium problems, such as
the generation of
radicals, and formulated in terms of the p.d.f. of the gradient of the
mixture fraction.
It is shown that the form of the conditional gradient distribution is universal
in a
wide range of cases, whose limits are established. Within those limits,
engineering
approximations to the radical concentration are also possible. It is argued
that the
experiments in this paper are already in the limit of high Reynolds numbers.