# Computational Investigation of a Novel Box-Wing Aircraft Concept
## 2 Computational Approach  
Whether it concerns the reference (CRM) or the BWA (SL) aircraft, the numerical investigation is conducted using the same computational strategy and tool; the aerodynamic performance is investigated first, using a Computational Fluid Dynamics (CFD) method relying on a steady Reynolds Averaged Navier-Stokes (RANS) approach. The structural characteristics are then explored using a Fluid-Structure Interaction (FSI) simulation, which relies on the CFD results obtained beforehand. Of note, the FSI is handled in a weak (or one-way) coupling sense, i.e., it accounts for the structural deformations induced by the aerodynamic loads while neglecting how the former are impacted by the latter. In addition, all computations are performed under the assumption of static equilibrium, i.e., all inertial effects are neglected.  
For each aircraft configuration, the RANS simulation underlying both the CFD and the FSI calculations is conducted using an unstructured mesh, which is made of polyhe  
Figure 2: “SmartLiner” triplane Box-Wing Aircraft concept (BWA/SL).  
dral cells. To save computational resources, and given the (very) high value of targeted Reynolds numbers (from 30 to 60 million), use is made of a wall function. The latter enables relaxing importantly the grid requirements at the walls (e.g., around the aircraft skin) while rendering the drag effects incurred by the boundary layers' shear stresses. Indeed, wall functions allow modelling--rather than resolving--the near-wall flow regions, whose viscous effects are of secondary importance compared to inertial ones, in high Reynolds number flow. More precisely, a wall function relies on a semi-empirical model, which allows \"bridging\" the inner (viscous sublayer and logarithmic boundary layer) to the outer (fully turbulent) regions, with no need to resolve the flow in the former one. Such a modelling is economical, robust, and reasonably accurate, thereby constituting a real advantage for industrial applications involving high Reynolds number flows, as in here. On that stage, it is worth noting that _very_ high Reynolds number flows (e.g., by aircraft, ships) exhibit a logarithmic boundary layer that extends to several thousands of dimensionless length unit (y\({}^{+}\)) away from the wall (as opposed to more moderate Reynolds number flows, for which this distance would rather be of a hundred y\({}^{+}\) units--e.g., turbine blades). Aside from a wall function, use is made of a k-\(\epsilon\) turbulence model, which is a classical, two equation transport model that is more particularly suited for resolving flows away from the wall (as opposed to k-\(\omega\) models, which rather apply to near-wall flows). Of note, use is here made of the so-called realizable k-\(\epsilon\) turbulence model, which is known to better predict the boundary layer characteristics of large pressure gradients, separated and recirculating flows [43]. Finally, the fluid properties are classically set so that they correspond to an ideal gas, with a viscosity that obeys the Sutherland law. The specific heat and thermal conductivity are kept constant (with values of 1006.43 J/kg-K and 0.0242 W/m-K, respectively).  
The computational methodology and underlying tool are first validated using the CRM reference aircraft (cf. Section 3), before they are applied to the BWA/SL aircraft concept (cf. Section 4), with additional sanity checks being performed whenever possible (e.g., grid convergence study, comparison with results obtained with a different solver).