Conditional filtering
From CFD-Wiki
A conditional filtering operation of a variable 
is defined as
where 
is a positive defined space filter with filter width 
,
is a fine-grained probability density function,
which is taken as a Dirac delta 
.
The probability density function
is a subgrid PDF and 
is the sample space of the passive scalar
. In variable density flows, conditional density-weighted
(Favre) filtering is used,
,
