Manuel Kessler

Tip Vortex Conservation on a Helicopter Main Rotor Using Vortex-Adapted Chimera Grids

This paper presents a method to improve the tip vortex conservation in a computational fluid dynamics simulation of a helicopter main rotor. Our approach uses vortex-adapted Chimera child grids to achieve a local refinement of the grid in the vicinity of the vortex and thus reduce the numerical dissipation of the vortex. The method is applied to the higher-harmonic-control aeroacoustic rotor test case to evaluate its potential with respect to the prediction of blade-vortex interaction airloads. To allow a meaningful comparison with the experimental data, the rotor is trimmed for thrust, as well as for longitudinal and lateral mast moments, using weak fluid-structure coupling with a flight mechanics code. We obtained a good overall agreement with the experimental data for both aerodynamics and blade dynamics. The effect of the improvement in tip vortex conservation is demonstrated by comparison with simulations without Chimera refinement and with the experimental results. It turned out that a very fine grid resolution in the vicinity of the vortex is necessary to capture the blade-vortex interaction airloads. The required grid resolution was provided by a refinement of the vortex-adapted grids, allowing for a very good reproduction of the blade-vortex interaction airloads, especially on the retreating blade side.

Numerical investigation of helicopter rotors in ground effect

This study intends to proof the capability of URANS computations to simulate the helicopter flight in the ground effect flow regime. Hover as well as forward flight cases are computed and compared to experimental data. For both flight conditions several parameter variations, such as ground distance, collective pitch and normalized advance ratio, were examined. Overall a good representation of the flow field structure could be obtained, as well as a satisfying approximation for the thrust coefficient. Furthermore, conclusions could be drawn regarding unsteady effects in the wake and their influence on the brown-out phenomenon. Especially the dynamic fluid movement around the Flow Separation Point is a considerable source of unsteadiness and energy introduced in the brown-out particle movement.

Aeroacoustics of a High-Fidelity CFD Calculation of a Counter-Rotating Open Rotor in Take-Off Conditions

A very high-fidelity CFD calculation has been carried out for a 9x7 counter-rotating open rotor setup at take-off conditions. The total setup consisted of roughly 170 million grid cells and was used for a grid study as well as a subsequent acoustic evaluation. The analysis contained farfield aeroacoustics obtained with the acoustic analogy of Ffowcs Williams and Hawkings as well as nearfield acoustics extracted directly from the flow field data. The grid study showed a low grid dependency for integrated aerodynamic values. The acoustic evaluation in the farfield showed that the main noise characteristics can be resolved sufficiently with all three grid resolutions. Significant errors only occur in the low intensity high frequency range. A comparison of acoustic evaluation in the nearfield with the Ffowcs Williams and Hawkings approach and direct pressure output showed no benefit of the latter justifying the higher computational costs caused by the required grid resolution for the transport of the pressure disturbances. For both approaches the noise characteristics were similar with small discrepancies in the wake of the CROR and at higher frequencies.

Trimmed Simulation of a Complete Helicopter Configuration Using Fluid-Structure Coupling

In this paper we present numerical simulation results of a complete helicopter configuration. The CFD code FLOWer (DLR) is used for the simulation of the aerodynamics. For the main rotor an aeroelastic analysis is performed using weak fluid-structure coupling between FLOWer and the flight mechanics code HOST (Eurocopter). As a reference configuration the complete helicopter configuration investigated in the EU project GOAHEAD has been chosen.

Turbulence Modeling and Detached Eddy Simulation with a High-Order Unstructured Discontinuous Galerkin Code

In the present paper a high-order Discontinuous Galerkin method is presented for the numerical simulation of the separated turbulent flow around complex geometries using unstructured grids. Bassi and Rebay extended the Discontinuous Galerkin method to solve the Navier-Stokes equations for laminar and 3D turbulent flows. Especially, an extension will be provided to calculate unsteady separated flows with a Detached Eddy Simulation, which is a hybrid method between the Unsteady Reynolds averaged Navier-Stokes approach and the Large Eddy Simulation. Some results, like flows over a flat plate and around a sphere, which could not be predicted with an Unsteady Reynolds averaged Navier-Stokes calculation, are calculated with high accuracy and compared with theory and experiments.

Aerodynamic and acoustic analysis of an extruded airfoil with a trailing edge device using Detached Eddy Simulation with a Discontinuous Galerkin method

In this paper the Discontinuous Galerkin (DG) method is applied for the simulation of an airfoil with a small trailing edge device (TED). The method discretizes the Reynoldsaveraged Navier-Stokes (RANS) equations including the Spalart-Allmaras turbulence equation, which can be extended easily to a Detached Eddy Simulation (DES) model. As the DG method is a high-order method, the turbulent length scale is adapted with the polynomial degree of the basis function to take the cell interior into account. This adaptation is validated on two examples, a backward facing step and a circular cylinder. The acoustic evaluation of the TED case is carried out in a post-processing step with a Ffowcs WilliamsHawkings (FW-H) method. A comparison to wind tunnel tests and a standard finite volume (FV) method for both the aerodynamic results and the acoustic results is presented.

Aeroelastic Simulations of Isolated Rotors Using Weak Fluid-Structure Coupling

In this paper we present a weak fluid-structure coupling method for the aeroelastic simulation of isolated helicopter main rotors. The CFD Code FLOWer (by DLR) is coupled to the flight mechanics code HOST (by Eurocopter). HOST is used to compute the blade dynamics and the rotor trim, whereas the aerodynamic loads are determined by FLOWer. The method has been applied to two different rotors: the advanced EC145 rotor in fast forward flight and the well known Bo105 rotor in slow descent flight.

Advanced Rotary Wing Aeromechanics

This paper presents recent developments and results in the aerodynamic and aeroelastic simulation of helicopter main rotors. Our current work focusses on two aspects which are of high relevance for the further improvement of helicopter rotors: The aeroelastic simulation of active rotor concepts and the correct reproduction and prediction of Blade Vortex Interaction.

Planform Design for a Five Bladed Isolated Helicopter Rotor Using Fluid-Structure Coupled CFD Simulations

The planform of a helicopter rotor blade is studied by describing the blade tip with eight parameters. The effect of those parameters on the performance of the isolated five bladed rotor in cruise speed condition is analysed using the lift over drag ratio as quality criterion. Trimmed and fluid-structure coupled Computational Fluid Dynamics computations are required to account for the deformation of the rotor blades as well as to obtain a steady flight state necessary for comparing different blade shapes with each other. The parametric domain is systematically investigated and areas with beneficial effect on the forward flight performance are identified. Improvements of up to 5% compared to a rectangularly shaped reference blade with the same thrust weighted equivalent chord are achieved.

Limited Bandwidth Aeroacoustic Schemes with Applications to Rotorcraft

Acoustic prediction for transonic aerodynamic sources is a tough problem. The highly nonlinear aerodynamics as well as the singular acoustic integrals feature enough diculties for themselves, and their coupling does not simplify the procedure. This is the case for example for helicopter rotors. The blade surface moves at high speeds, and part of the grid is even supersonic. Therefore it is necessary to handle the integrals of the acoustic analogy in a most general and robust manner, appropriate for subsonic as well as sonic and supersonic sources. The concept of limited bandwidth early in the integration process is essential to overcome numerical as well as conceptual singularities of the acoustic integrals in transonic cases. The uid physics are used as a guiding principle. The concept of ecien t evaluation is generalised in this paper to wind tunnel tests of forward igh t cases, where the uid is in motion with respect to the observer. While we are most interested in helicopter noise prediction, our scheme can be applied to any exterior acoustic computation with sucien t accurate aerodynamic data avaliable. It is especially suited for surfaces at nearly sonic speeds, while little gain is expected for stationary or only slowly moving cases.