  Significant progress has been made on the model development for simulating
turbulent reacting flows. As a consequence, we are currently in a position
where key-physical aspects of fairly complex combustion processes are well
understood at a qualitative and -- in many cases -- also at a quantitative
level. Examples are the prediction of temperature and major species,
statistically stationary flames, gas-phase combustion, turbulent transport, and
turbulence/flame coupling. However, current challenges lie in capturing
transient processes and stability boundaries, minor species and emissions,
multiphase flows and phase-transition, as well as multidimensional flame/flow
interactions that may involve flame curvature effects, stratification,
partially premixing, or flame-wall interaction. With this, the question arises
what steps need to be taken to elevate the current state of modeling
capabilities in order to address these deficiencies? This paper seeks to
address this question. We begin by reviewing the current state of combustion
model approaches, our quest for improving existing models, the separation of
errors arising from numerical discretization and physical models, and ideas on
model evaluations. We then proceed by examining concepts on quantitative model
evaluations, requirements on predictability, quantities of interest, and
cost/accuracy trade-offs. We close by introducing recent concepts that
assimilate time-resolved measurements into numerical simulations for state
estimation, model evaluation, and parameter determination.
