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A highly loaded, high-speed turn flight of Airbus Helicopters' Bluecopter demonstrator helicopter is simulated to investigate dynamic stall using a loose computational fluid dynamics/structural dynamics (CFD/CSD) coupling of the flow solver FLOWer and the rotorcraft comprehensive code CAMRAD II. The rotor aerodynamics is computed using a high-fidelity delayed detached-eddy simulation (DDES). A three-degree-of-freedom trim of an isolated rotor is performed, yielding main-rotor control angles that agree well with the flight-test measurements. The flow field in this flight condition is found to be highly unsteady and complex, featuring massively separated flow, blade–vortex interaction, multiple dynamic-stall events, and shock-induced separation. The computed pitch-link loads are compared to flight-test measurements. This shows that all CFD/CSD cases underpredict the amplitudes of the flight test and yield phase shifts. However, overall trends agree reasonably. Also, varying the computational setup reveals that the shear stress transport–DDES turbulence model performs better than Spalart–Allmaras–DDES, that the consideration of the rotor hub and fuselage improves the agreement with flight-test data, and that the elastic twist plays only a minor role in the dynamic-stall events.

Recent enhancements and applications of the flow solver FLOWer are presented. First, an Adaptive Mesh Refinement technique (AMR) is implemented and an AMR cycle is built around FLOWer. Depending on the application case, the usage of AMR significantly reduces the overall computational cost—and thus increases efficiency—of the flow solution. Exemplary, for a complete helicopter simulation a reduction in computational time of 45% is achieved without any observable loss of accuracy. Second, the implementation of a new shielding approach for detached eddy simulations is shown. This applies a sophisticated communication method to exchange flow variables in wall normal direction. As an application case, the computation of the highly resolved rotor wake in hover highlights the possibilities of high performance computing in combination with advanced numerical methods for rotorcraft flows.

This paper presents recent developments in finding alternative shielding functions in the framework of Delayed Detached-Eddy Simulation (DDES). The weaknesses of the standard shielding function are elaborated for the turbulent flow over a flat plate and an axisymmetric adverse pressure gradient flow. In both cases a small filter width compared to the boundary layer height caused a degeneration of the shielding function and led to severe model stress depletion. To overcome the strong grid dependency of the standard shielding two alternative shielding functions are proposed. The first determines the boundary layer edge by integrating the vorticity in the wall-normal direction, while separated flow is identified based on a comparative analysis of the individual vorticity components. For the second switching function the boundary layer edge is estimated by evaluating a localized formulation of the Bernoulli equation. The shielding disintegrates under resolved turbulent content by a sensor that includes the \(\sigma \)-velocity gradient operator. The novel shieldings are verified for basic canonical test cases. Compared to DDES, a superior protection of attached boundary layers could be demonstrated.

High-fidelity unsteady Reynolds–averaged Navier–Stokes (URANS) and Menter-SST delayed detached-eddy simulations (DDES) of dynamic stall on a rotor with cyclic pitch control are presented and compared to experimental surface pressures and particle-image-velocimetry (PIV) data. Before the dynamic-stall event, the DDES suffers from modeled-stress depletion (MSD) leading to grid-induced separation (GIS) due to a breakdown of the boundary-layer shielding function \(f_d\). Combined with the “grey-area” problem, this leads to severe erroneous load peaks. After dynamic stall, flow is completely separated and only DDES shows realistic small-scale, incoherent vortical structures. Two approaches are investigated to eliminate MSD/GIS: Firstly, increasing the empirical constant \(C_{d1}\) of the \(f_d\) function to 30 basically eliminates GIS. Secondly, a non-local, grid-independent vorticity-integrated algebraic DES, which replaces the \(f_d\) function, is introduced that provides robust boundary-layer shielding and enables the LES mode in case of massive flow separation.

Recent enhancements and applications of the flow solver FLOWer are presented in this paper. A locally formulated laminar-turbulent transition model is implemented and used for simulations of a steady and pitching finite wing. Corresponding experimental results show good agreement on transition points. In CFD, a separation bubble is observed that triggers laminar-turbulent transition. Furthermore, the paper focuses on CFD simulations conducted on Airbus Helicopters’ compound helicopter RACER and the underlying setup, including flexible main rotor blades, engine boundary conditions and deformable flaps. Finally, interaction phenomena and interesting flow characteristics are described for hovering and cruise conditions.

Recent optimizations and HPC-applications of the flow solver FLOWer are presented in this paper. A graph partitioning method is introduced to the MPI communication, which reduces the number of messages as well as the total message size, leading to a run time speed-up of 20%. A numerical investigation of a finite wing shows the influence of the wind tunnel wall only in the wing root area and agrees well with experimental data for attached flow. Both a URANS and a Delayed Detached-Eddy Simulation (DDES) of the massively stalled wing reveal difficulties in matching the experimental behaviour of flow separation. Finally, a simulation of a model Contra-Rotating Open Rotor (CROR) at various operating conditions exhibit interaction effects, blade loadings and noise emissions which agree well with expectations and results from literature.

The compressible block-structured flow solver FLOWer of the German Aerospace Center (DLR) has been extended towards state of the art detached-eddy simulation (DES) methods in order to conduct hybrid RANS/LES simulations of flow around rotary wings. The large-eddy simulation (LES) capabilities of the code are demonstrated for decaying isotropic turbulence. Excessive numerical dissipation is avoided by using the fifth-order WENO scheme and an appropriate low-Mach number correction. The DES implementations are validated first for the well documented test cases backward facing step and NACA0021 airfoil at \(60^\circ \) incidence, before increasing the complexity by simulating the flow around the MEXICO model wind turbine operating in stalled conditions and comparing with experimental data. From the latter, recommendations on the numerical settings are derived to successfully set up eddy resolving simulations for wind turbine or helicopter applications.

Numerical investigations of three-dimensional dynamic stall on a two-bladed Mach-scaled rotor (R = 0.65m, Ma75 = 0.21, Re75 = 3.5 × 105) with 1/rev cyclic pitch control are presented and compared to experimental surface pressure and PIV data. In addition to URANS simulations using the finite-volume flow solvers FLOWer and TAU, a delayed detached-eddy simulation (DDES) with Menter SST as underlying RANS model is carried out with FLOWer. Facing dynamic stall and flow separation, the DDES reproduces high-frequency load fluctuations, cycle-to-cycle variations and small-scale vortical structures as seen in the experiment, which is not the case with URANS. However, common hybrid RANS-LES issues – grid-induced separation and the grey area problem – play a role in this DDES and influence loads severely. FLOWer SST simulations yield load peaks of the same magnitude as individual, non-phase-averaged measurements. With TAU SST the dynamic stall event is delayed and weakened compared to FLOWer SST and experimental results. FLOWer and TAU results using the SA turbulence model are fairly comparable but in bad agreement with the experiment at the outboard station at r/R = 0.77, where they exhibit no dynamic stall at all.