SU2 is an open-source CFD suite that is built around a RANS-solver. The main PDE that is solved, is the following:
$$ \frac{\partial}{\partial t} \mathbf{U} + \nabla \cdot \mathbf{F^c} - \nabla \cdot \mathbf{F^v} = \mathbf{Q} \qquad \mathrm{in}\ \Omega, t > 0 $$
In this equation, $\mathbf{U}$ is the state vector and $\mathbf{F^c}$ and $\mathbf{F^v}$ represent the convective and viscous fluxes, respectively.
Palacios et al. (2013) state the following (p. 13):
Naturally, both the laminar Navier-Stokes and Euler equations are also available in the code as subsets of the RANS equations by disabling turbulence modeling and by completely removing viscosity, respectively.
This, however, is not so 'natural' to me. My thoughts/ideas so far:
Turbulent RANS to laminar NS
In the RANS-case, part of $\mathbf{F^v}$ is the term $v_j \tau_{ij} + \mu^*_{tot} C_p \frac{\partial}{\partial i} T$ (part of the energy equation). As $\mu_{tot}$ (as well as $\mu^*_{tot}$) is a combination of $\mu_{dyn}$ (dynamic viscosity) and $\mu_{turb}$ (turbulent viscosity). Given the quote on 'disabling turbulence modelling', my guess would be that that would mean setting $\mu_{turb} = 0$ (such that $\mu_{tot} = \mu_{dyn}$). Is that indeed correct?
Turbulent RANS to inviscid Euler
I get that setting $\mathbf{F^v} = 0$, the inviscid Euler equations are obtained from the main PDE above. However, taking a more theoretical approach, I would think that the process of averaging flow variables (into mean and time-fluctuating parts) that one uses to obtain RANS from NS, somehow interferes with that. Put differently: if we can go from RANS to Euler, that would imply that all viscous effects are contained in these time-fluctuating parts. On one hand that makes sense, on the other hand, I'm not so sure.
Follow-up question: does this hold for RANS formulations in general, or only the specific implementation used in SU2?