Unsteady RANS

Unsteady RANS (URANS), 11 781. See also Reynolds- averaged Navier-Stokes (RANS)... 

In those early days, when computer power was limited, often use was made of a symmetry assumption each quarter of the vessel containing one of the four baffles at the vessel wall was supposed to behave identically hence, a steady flow in the RANS approach was simulated in just a quarter vessel. Such strong simplifications are no longer in use. Precessing vortices moving around the vessel centerline contribute to flow unsteadiness and, therefore, exclude models that just assume flow steadiness or allow for domain reductions through geometrical symmetries. The most correct response to this flow unsteadiness is the concept of LES. 

The research on the flow regimes in packed tubes suggests that laminar flow CFD simulations should be reasonable for Re <100 approximately, and turbulent simulations for Re >600, also approximately. Just as RANS models provide steady solutions that are regarded as time averages of the real time-dependent turbulent flow, it may be suggested that CFD simulations in the unsteady laminar inertial range 100 time-averaged picture of the flow field. As with wall functions, comparisons with experimental data and an improved assessment of what information is really needed from the simulations will inform us as to how to proceed in these areas. 

In simulations for engineering problems involving turbulent combustion, a RANS description of the flow [1] and simplistic combustion models (e.g., [2]) are typically combined. This involves simulating only the mean flow-field features and modeling the effects of the entire range of turbulent scales. The restricted information provided by this approach, regarding the fluid dynamics, combustion, and their different interactions, precludes adequate prediction of the important phenomena required to achieve effective control of the combustion processes, such as combustion-induced flow instabilities, cycle-to-cycle variations, and combustion oscillations associated with unsteady vortex dynamics. 

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