Nonlinear response of wrinkled premixed flames to time- and space-dependent forcing and stretch

Abstract

Premixed-flame wrinkling is studied via a Michelson-Sivashinsky (MS) type of evolution equation retaining the Darrieus-Landau (DL) instability, a curvature effect and a geometric nonlinearity. Here it also keeps forcing by longitudinal shearflow and wrinkle stretch by transverse flow; both imposed stimuli vary in time and space as to make the front slope comprise a given fluctuating spatial harmonics and unknown pole-decomposed pieces. A DL-free Burgers version is examined in parallel, also with Neumann conditions and symmetry. As is shown for both models, solving Ntot equations of motion for the poles in principle yields the front dynamics, the arclength increment V(t) and its time-average Vav. Yet this could be worked out analytically (or nearly so) only in high-frequency HF or low-frequency LF limits. These tackle one or two pairs of poles per cell, then a large number of pairs Ntot forming two piles viewed as continua, one per crest. Despite ample pole motions that make some commute between crests, Vav grows in a nearly parabolic way with the combined intensity of forcing and stretch. LF stimuli and DL instability can induce multiple branches and relaxation phenomena. Numerical t-averages are needed even if V(t) is analytically known. For Ntot=1,2 and short wrinkles, or Burgers fronts, Vav transitions from quadratic to sublinear as the forcing grows ; for longer wrinkles Vav keeps its MS value at moderate forcing, then bifurcates to an ultimately sublinear growth that depends on the stimulus phases. For very long wrinkles, coupled integral equations give analytical slope and pole-density profiles, but pile heights/contents need a t-dependent numerical search of up to two roots to get V(t). A summary, a discussion and hints of generalizations are provided, and open problems are evoked.