The occurrence and properties of singularities in the equation for the surface density function #sigma# ident to |#DELTA##tool for numericla simulatios of reacting flows,especially when ocmbustion instabilities are encountered.Nevertheless,a difficulty occus in developign subgrid scale models for premixed combustion,becasue the flme front is generally too thin to be resolved on the omputational grid.An approach based on the filtering of the balance quation for the progress variable c using a LES filter larger than the mesh size is propsoed and investigated.Despite its similarities with the field eescription basded on the G-equation,the advantage of this approach is that c and related quantities such as subgrid-scale flame surface density are phsiclly well defined and may be easily extracted from direct numericla simulations (DNS) or expeirmental dat ato analyze and validate models.#p.917-925#QTY##,??e is a minimum time requried for quenching.#p.711-717#QT
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