R. Steijl

Development and Validation of a CFD Technique for the Aerodynamic Analysis of HAWT

This paper demonstrates the potential of a compressible Navier–Stokes CFD method for the analysis of horizontal axis wind turbines. The method was first validated against experimental data of the NREL/NASA-Ames Phase VI (Hand, , 2001, “Unsteady Aerodynamics Experiment Phase, VI: Wind Tunnel Test Configurations and Available Data Campaigns,” NREL, Technical Report No. TP-500-29955) wind-tunnel campaign at 7 m/s, 10 m/s, and 20 m/s freestreams for a nonyawed isolated rotor. Comparisons are shown for the surface pressure distributions at several stations along the blades as well as for the integrated thrust and torque values. In addition, a comparison between measurements and CFD results is shown for the local flow angle at several stations ahead of the wind turbine blades. For attached and moderately stalled flow conditions the thrust and torque predictions are fair, though improvements in the stalled flow regime are necessary to avoid overprediction of torque. Subsequently, the wind-tunnel wall effects on the blade aerodynamics, as well as the blade/tower interaction, were investigated. The selected case corresponded to 7 m/s up-wind wind turbine at 0 deg of yaw angle and a rotational speed of 72 rpm. The obtained results suggest that the present method can cope well with the flows encountered around wind turbines providing useful results for their aerodynamic performance and revealing flow details near and off the blades and tower.

Computational Study of the Advancing-Side Lift-Phase Problem

The prediction of airloads and the corresponding structural response in high-speed forward flight of rotors poses a significant challenge to predictive rotorcraft aeromechanics methods. One of the issues identified in the flight test data of the Puma and Black Hawk aircraft is the phase difference between the minimum lift coefficient and the minimum of the blade pitch on the advancing side of the rotor during high-speed forward flight. This is commonly referred to as the advancing-side lift-phase delay. In the present work, the unsteady three-dimensional flowfield on the advancing side of a helicopter rotor is analyzed using computational fluid dynamics in an attempt to quantify contributions to the preceding effect. Time-dependent two-dimensional computational fluid dynamics simulations of blade sections with combined pitch/freestream Mach number oscillation were carried out to isolate the contribution to the phase difference of pitch angle and Mach number variations in the absence of the complex rotor-induced flowfield, sideslip, and rotor blade dynamics. The results for the freestream Mach number oscillations show that the lift coefficient lags the Mach changes at outboard stations, but this effect is reduced for combined pitch/Mach number oscillations. Finite span and sideslip contributions to the phasing were quantified by investigating the chordwise extent of supersonic flow on the advancing side for two nonlifting rotors in high-speed flight. Finally, the UH-60A rotor in high-speed forward flight was considered. By comparing results for rigid blades with results for a prescribed blade torsional deflection, the contribution of the blade torsion to the advancing-blade lift phasing was also quantified. Furthermore, rigid-blade simulations with different flapping schedules demonstrated the sensitivity of the lift phasing to trim-state variations. It was found that Mach number effects are dominant and the lift phasing depends primarily on the encountered Mach number and pitch schedule. Further, the elastic torsional deflection of the blades effectively changes the pitch schedule of the blade sections and also plays a role in the phasing of the lift and pitching moment coefficients.

Computational Study of Helicopter Rotor-Fuselage Aerodynamic Interactions

Aerodynamic interactions between the main rotor, fuselage, and tail rotor must be considered during the design phase of a helicopter, and their effect on performance must be quantified. However, interactional helicopter aerodynamics has so far been considered by very few researchers. In this work, the Helicopter Multi-Block flow solver is used to investigate the flow around two generic rotor–fuselage cases before moving on to a more realistic fullhelicopter geometry under investigation in the European Commission Framework 6 Generation of Advanced Helicopter Experimental Aerodynamic Database project. A comparison of the computational fluid dynamics results obtained with experimental data shows that the method is capable of resolving the main interactional flow features for the generic cases. A similar comparison with experimental data for the Generation of Advanced Helicopter Experimental Aerodynamic Database test case has not yet been conducted, but the obtained results show that even for a test case of high complexity, state-of-the-art rotorcraft computational fluid dynamics methods are capable of providing realistic predictions. However, comparisons with isolated rotor cases clearly show the increased loading as the blade passes over the nose of the helicopter as the result of a fuselage-induced upwash. Similarly, the fuselage induces a reduction of the blade loading for inboard stations when the blades passes through the rear part of the rotor disk. The present results highlight and quantify the radial and azimuthal extent of the rotor–fuselage interactional effect on the rotor loading.

Numerical Study of Helicopter Rotors in a Ship Airwake

Operating helicopters in a naval environment is challenging because it imposes a pilot workload significantly higher than that during land-based operations. The aerodynamic interaction between the aircraft and the ship wake is known to play an important role in increasing the pilot workload, hence reducing the aircraft capability as a result of maintaining safety. As a further step toward numerical prediction of ship/helicopter operational limitations, computational-fluid-dynamics simulations are conducted for the Canadian Patrol Frigate. The effect of the rotor is included in the simulation, first using an actuator-disc method together with steady calculations, then using rotor blades and the unsteady Reynolds-averaged Navier–Stokes equations. Results using the actuator-disc method demonstrate the importance of coupling effects on the wake and rotor inflow when the rotor is operating close to the ship and therefore the invalidity of superposition methods. The case of a Sea King helicopter main rotor hove...

A CFD Framework for Analysis of Helicopter Rotors

A CFD method suitable for the analysis of hovering and forward-flying rotors has been developed and validated against experimental data. The Caradonna and Tung as well as the ONERA 7A/7AD1 rotors have been simulated and results were found to be in excellent agreement with wind tunnel measurements. As a second step the method was coupled with a trimming algorithm devised using rotor blade element theory. The coupled algorithm demonstrates the rapid convergence to prescribed thrust coefficient values and no deterioration of the convergence rates relative to simulations of untrimmed rotors.

Assessment of Flow Control Devices for Transonic Cavity Flows Using DES and LES

Since the implementation of internal carriage of stores on military aircraft, transonic flows in cavities were put forward as a model problem for validation of CFD methods before design studies of weapon bays can be undertaken. Depending on the free-stream Mach number and the cavity dimensions, the flow inside the cavity can become very unsteady. Below a critical length-to-depth ratio (L/D), the flow has enough energy to span across the cavity opening and a shear layer develops. When the shear layer impacts the downstream cavity corner, acoustical disturbances are generated and propagated upstream, which in turn causes further instabilities at the cavity front and a feedback loop is maintained. The acoustic environment in the cavity is so harsh in these circumstances that the noise level at the cavity rear has been found to approach 170 dB and frequencies near 1 kHz are created. The effect of this unsteady environment on the structural integrity of the contents of the cavity (e.g. stores, avionics, etc.) can be serious. Above the critical L/D ratio, the shear layer no longer has enough energy to span across the cavity and dips into it. Although this does not produce as high noise levels and frequencies as shorter cavities, the differential pressure along the cavity produces large pitching moments making store release difficult. Computational fluid dynamics analysis of cavity flows, based on the Reynolds-Averaged Navier—Stokes equations was only able to capture some of the flow physics present. On the other hand, results obtained with Large-Eddy Simulation or Detached-Eddy Simulation methods fared much better and for the cases computed, quantitative and qualitative agreement with experimental data has been obtained.

Computation of the Helicopter Fuselage Wake with the SST, SAS, DES and XLES Models

This paper presents a first industrial attempt at Eurocopter Germany to use DES-like methods to compute the wake behind a helicopter in forward flight. Three codes have been used: TAU (DLR), CFX (Ansys) and HMB (Uni. Liverpool) and four different turbulence models: standard SST and three of its unsteady extensions SAS, DES and XLES. Results for the loads, pressure distribution and skin-friction lines have been compared to each other and against wind tunnel experimental data. These more sophisticated models brought improvements over the base SST but not always markedly and consistently: the drag gets improved although the negative lift on the body departs further form the experimental value. The pressure distribution on the backdoor has been improved by one of the computation. Oil-flow visualisation results show that the mean flow in the region of the backdoor is qualitatively reasonably well captured.

CFD Analysis of Rotor-Fuselage Interactional Aerodynamics

[Abstract] The problem of rotor-fuselage interaction is central to the design and performance analysis of helicopters. However, regardless of its significance, rotor-fuselage aerodynamics has so far been addressed by very few authors. This is mainly due to the difficulties associated with both experimental and computational techniques when such complex configurations are considered. To account for the relative motion between the fuselage and the rotor blades the concept of sliding planes is introduced. A sliding plane forms a boundary between a CFD mesh around the fuselage and a rotor-fixed CFD mesh which has to be rotated to account for the motion of the rotor blades. CFD meshes adjacent to sliding plane do not necessarily have matching nodes or even the same number of cellfaces. This poses a problem of interpolation between CFD meshes and, in addition, the employed algorithms should have small CPU overhead. The sliding plane methods employed for this work are demonstrated for both simple and complex flows and emphasis is placed in the presentation of the developed algorithms.

Kinetic Models and Gas-Kinetic Schemes for Hybrid Simulation of Partially Rarefied Flows

Approaches to predict flowfields that display rarefaction effects incur a cost in computational time and memory that is considerably higher than methods commonly employed for continuum flows. For this reason, to simulate flowfields where continuum and rarefied regimes coexist, hybrid techniques have been introduced. In the present work, analytically defined gas-kinetic schemes based on the Shakhov and Rykov models, for monatomic and diatomic gas flows, respectively, are proposed and evaluated, with the aim of being used in the context of hybrid simulations. This should reduce the region where more expensive methods are needed by extending the validity of the continuum formulation. Moreover, because, for rarefied gas flows at high velocities, it is necessary to take into account the nonequilibrium among the internal degrees of freedom, the extension of the approach to employ diatomic gas models with rotational relaxation is a mandatory first step toward realistic simulations. Compared to the previous works...

CFD and aeroelastic analysis of the MEXICO wind turbine

This paper presents an aerodynamic and aeroelastic analysis of the MEXICO wind turbine, using the compressible HMB solver of Liverpool. The aeroelasticity of the blade, as well as the effect of a low-Mach scheme were studied for the zero-yaw 15m/s wind case and steady- state computations. The wake developed behind the rotor was also extracted and compared with the experimental data, using the compressible solver and a low-Mach scheme. It was found that the loads were not sensitive to the Mach number effects, although the low-Mach scheme improved the wake predictions. The sensitivity of the results to the blade structural properties was also highlighted.