With the mandatory requirement for more efficient aircraft due to both economic and environmental purposes, academy and industry are exploring new aircraft design opportunities including the concepts of hybrid-electric and fully electric vehicles. Within this framework, distributed electric propulsion is a key technology for future aviation, as it allows the installation of a theoretically indefinite number of small motors. The blowing effect induced by the propeller can be used to improve aerodynamic performance, hence, thanks to their reduced size, these small motors could be installed along the whole span covering the whole wing. This paper presents a study devoted to the investigation of the aerodynamic effects of distributed electric propulsion installation on a regional aircraft, computing the aerodynamic coefficients using high-fidelity CFD simulations via the RANS approach. Different propeller diameters and trust levels were analysed in climb and landing conditions, applying periodic boundary conditions on a finite span section of the wing, simulating an infinite rectangular wing. The goal of the current study is to quantify the increase of aerodynamic coefficients with reference to power off condition and report data as a function of the propeller’s characteristics. The objective is to identify and propose a simplified analytical formulation to be used in the phase of preliminary design. The implementation of such a formula in lower-fidelity tools will allow fast and reliable procedures for preliminary conceptual design.

The effect of partial slip flow on airfoil performance at high Reynolds numbers is analyzed in this paper. The link between the physical mechanism of drag reduction attained by many devices and the slip length concept has been well assessed in the literature. A slip length model is therefore here adopted in large eddy simulations to quantify the effect of slip flow on airfoil performance. The possibility to adopt a slip flow boundary condition to simulate riblets on airfoil is verified. Their effectiveness in reducing friction drag in turbulent flow has been well assessed since the end of the last century. Both theory and experiments proved that the effect of riblets only depends on the local Reynolds number. However, some experiments showed an increased effectiveness of riblets in the presence of pressure gradient. This secondary effect is still being debated and a physical explanation has not been found. This paper has the aim to provide a contribution to the understanding of this phenomenon. Large eddy simulations of flows around airfoils are proposed with an extensive analysis of riblet performance, obtained by a proper slip flow boundary condition. It is shown that riblets reduce the boundary layer displacement thickness inducing small but significant modifications to the pressure distribution, in particular in the adverse pressure gradient region. The reduced thickening of the equivalent body is the reason for the reduced form drag.

The efficiency increase that distributed propulsion could deliver for future hybrid-electric aircraft is in line with the urgent demand for higher aerodynamic performances and a lower environmental impact. Several consolidated proprietary tools (not always available) are developed worldwide for distributed propulsion simulation. Therefore, prediction and comparisons of propeller performances, with computational fluid dynamic codes featuring different implementation of solvers, numerical schemes, and turbulence models, is of interest to a wider audience of research end-users. In this framework, the paper presents a cross-comparison study among different CFD solvers, the SU2 Multiphysics Simulation and Design Software, the CIRA proprietary flow solver UZEN, and the commercial ANSYS-FLUENT code, for the simulation of a wing section with a tractor propeller at different flow attitudes. The propeller is modelled as an actuator disk according to the general momentum theory and is accounted for in the flow solvers as a boundary condition, for the momentum and energy equations. In this study, a propeller with a fixed advance ratio $J=0.63$ is considered, while propeller performances are assumed variable along with the radius. To perform the comparisons among the solvers, an in-house procedure, which provides the input thrust and torque distributions in a unified format among the three solvers, is developed. Steady RANS simulations are performed at $R{e}_{\infty }=1.7\times {10}^6$ and $M_{\infty }=0.11$, for the flowfield of an isolated propeller. Successively, a wing section with a fixed forward-mounted propeller configuration with no nacelle, is studied at $\alpha =0^{\circ },4^{\circ },$ and $8^{\circ }$ angles of attack. The comparisons in terms of the lift coefficient show a good agreement among the three flow solvers both in power-off and power-on conditions. Simulations also evidenced the strong stability preserving property of upwind schemes, applied to propeller simulation at low-Mach number. Some discrepancies in the drag coefficient are observed and related to different levels of numerical diffusion between the three codes, which affects the downstream wake. Differences in flow properties in near disk region are observed and explained considering the different hub implementations.