Influence of combustion on principal strain-rate transport in turbulent premixed flames

The transport of principal strain-rates (si) was experimentally investigated using high-repetition-rate (10 kHz) tomographic particle image velocimetry (T-PIV) and OH planar laser induced fluorescence (PLIF) in a Rej=13,000 turbulent premixed flame. These measurements allowed calculation of the source terms in the si transport equation associated with the strain-rate and vorticity fields. Furthermore, the Lagrangian derivatives of si could be calculated by tracking theoretical Lagrangian fluid particles through space and time using the T-PIV data. These Lagrangian derivatives and the resolved source terms allowed the combined effects of the unresolved source terms to be inferred, namely the pressure Hessian, viscous dissipation, density gradients, and viscosity gradients. Statistics conditioned on the location of the Lagrangian fluid particles relative to the flame showed slight reductions in the strain-rate and vorticity source terms in the flame, indicating that these aspects of the turbulence were attenuated by the flame. Comparing the difference between the inferred source terms in the vicinity of the flame to the non-reacting flow showed that attenuation of si arose due to the combined effects of density and pressure gradients in the flame. The effects of flame-induced dilatation were small relative to the turbulent strain-rate and no change was found in the relative alignment of vorticity and strain-rate in the flame.