George N. Barakos

Investigation of three-dimensional dynamic stall using computational fluid dynamics

Numerical simulation of three-dimensional dynamic stall has been undertaken using computational fluid dynamics. The full Navier–Stokes equations, coupled with a two-equation turbulence model, where appropriate, have been solved on multiblock strucured grids in a time-accurate fashion. Results have neen obtained for wings of square planform and of NACA 0012 section. Efforts have been devoted to the accurate modeling of the flow near the wing tips, which, for this case, were sharp without tip caps. The obtained results revealed the time evolution of the dynamic stall vortex, which, for this case, takes the shape of a capital omega+spanning the wing. The obtained results compare well against experimental data both for the surface pressure distribution on the wing and the flow topology. Of significant importance is the interaction between the three-dimensional dynamic stall vortex and the tip vortex. The present results indicate that once the two vortices are formed both appear to originate from the same region, which is located near the leading edge of the tip. During the ramping of the wing, the two vortices grow significantly in size. The dynamic stall vortex dettaches from the wing in the inboard region but remains close to the wing’s leading edge near the tip. The overall configuration of the developed vortical system takes a form. To our knowledge, this is the first detailed numerical study of three-dimensional dynamic stall appearing in the literature.

Numerical simulation of transonic buffet flows using various turbulence closures

The paper presents a numerical investigation of buffet flows using various turbulence models, including linear and non-linear low-Re eddy-viscosity models (EVM). The accuracy of the models is assessed against experimental data for transonic flows around the NACA-0012 aerofoil. The study shows that non-linear two-equation models in conjunction with functional cμ coefficient for the calculation of the eddy-viscosity (henceforth labelled NL-cμ), provide satisfactory results for transonic buffet flows. The computations also reveal that the Spalart–Allmaras one-equation model provides comparable results to the NL-cμ models, while larger inaccuracies are introduced by linear and non-linear models based on constant cμ coefficient. Moreover, the buffet onset boundaries are similarly predicted by the one-equation and NL-cμ models. The study has been performed using a second-order time accurate implicit-unfactored method which solves in a coupled fashion the Navier–Stokes and turbulence transport equations. The spatial discretisation of the equations is obtained by a Riemann solver in combination with a third-order upwind scheme.

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.

Assessment of Passive Flow Control for Transonic Cavity Flow Using Detached-Eddy Simulation

The implementation of internal store carriage on stealthy military aircraft has accelerated research into transonic cavity flows. Depending on the freestream Mach number and the cavity dimensions, flows inside cavities can become unsteady, threatening the structural integrity of the cavity and its contents (e.g., stores, avionics, etc.). Below a critical length-to-depth ratio, the shear layer formed along the cavity mouth has enough energy to span across the opening. This shear layer impacts the downstream cavity corner and the generated acoustic disturbances propagate upstream, causing further instabilities near the cavity front. Consequently, a self-sustained feedback loop is established. This extreme flow environment calls for flow control ideas aiming to pacify the cavity by breaking the feedback loop and controlling the breakdown of the shear layer. This is the objective of the present work, which aims to assess changes of the cavity geometry and their effect on the resulting flow using detached-eddy simulation. For the cases computed in this work, quantitative and qualitative agreement with experimental data has been obtained. All of the devices tested achieved similar reductions in overall sound pressure level in the rear half of the cavity; however, a slanted aft wall provided the largest noise reduction in the front half of the cavity.

Computational Fluid Dynamics Study of Three-Dimensional Dynamic Stall of Various Planform Shapes

Numerical simulation of 3-D dynamic stall has been undertaken using computational fluid dynamics. As a first step, validation calculations have been performed for cases in which experimental data were available. Although the amount and quality of the experimental data available for 3-D dynamic stall does not match what is available for 2-D cases, the computational fluid dynamics was found capable of predicting this complex 3-D flow with good accuracy. Once confidence on the computational fluid dynamics method was established, further calculations were conducted for several wing planforms. The calculations revealed the detailed structure of the 3-D dynamic stall vortex and its interaction with the tip vortex. Remarkably, strong similarities in the flow topology were identified for wings of very different planforms.

An implicit unfactored method for unsteady turbulent compressible flows with moving boundaries

An implicit, unfactored solver for the simulation of unsteady turbulent flows around moving solid bodies is presented. The method is applied to solve the turbulence transport equations in conjunction with the Navier–Stokes equations in a strong coupled fashion. A third-order accurate upwind scheme is used for discretising the inviscid fluxes and Newton-type sub-iterations are employed to obtain time-accurate solutions on a moving mesh. The flow around an oscillating aerofoil is computed as a test problem and calculations are presented both for laminar and turbulent flows. Initially, laminar flow calculations over an oscillating NACA 0012 aerofoil at various Mach numbers were carried out in order to assess the accuracy of the method in the prediction of the dynamic-stall vortex under laminar flow conditions. Furthermore, calculations were performed for quasi-steady and fully unsteady turbulent flows over the NACA 0015 aerofoil using various turbulence models at different flow conditions corresponding to attached unsteady flow as well as light- and deep-stall cases. It was found that the numerical results for unsteady, turbulent flow calculations over an aerofoil depend strongly on the turbulence model used.

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.

Aerofoil-Vortex Interaction Using the Compressible Vorticity Confinement Method

The phenomenon of blade-vortex interaction (BVI) is central to the study of rotorcraft aerodynamics and aeroacoustics. The numerical simulation of BVI is challenging because most numerical schemes tend to alter the characteristics of the vortex, which must be preserved until the interaction. The compressible vorticity confinement method (CVCM) is capable of preserving vortices and requires minimal modifications to existing computational- fluid-dynamics codes. Inviscid and viscous calculations using a range of turbulence models have been carried out for a well-known, head-on BVI case. This is the first time CVCM is employed for turbulent flow calculations with an upwind solver. The results obtained using the CVCM show a good agreement with measurements.

Investigation of Nonlinear Eddy-Viscosity Turbulence Models in Shock/Boundary-Layer Interaction

Validation of nonlinear two- and three-equation, eddy-viscosity turbulence models (NLEVM)in transonic e ows featuring shock/boundary-layer interaction and separation is presented. The accuracy of the models is assessed againstexperimentalresultsfortwotransonice owsoverbumpgeometries.Moreover,theaccuracyandefe ciencyof NLEVMsisalsoassessedincontrasttonumericalpredictionsobtainedbyavarietyofothermodelsemployedinthis study. These include two linear eddy-viscosity k‐≤ models, the k‐! shear-stress transport model, and a nonlinear version of the k‐! model. Discretization of the mean e ow and turbulence transport equations is obtained by a characteristics-based scheme (Riemann solver ) in conjunction with an implicit unfactored method. The study shows that NLEVMs improve the numerical predictions in shock/boundary-layer interaction, compared to the linear models, but they require longer computing times. Nomenclature A = Lumley’ s e atness parameter, ´ 1 i 9 (A2 i A3) A2 = second invariant of Reynolds-stress anisotropy, ´ a ija ij A3 = third invariant of Reynolds-stress anisotropy, ´ a ika kja ji c