Giorgos Papadakis

In view of accelerating CFD simulations through coupling with vortex particle approximations

In order to exploit the capabilities of Computational Fluid Dynamics in aerodynamic design, the cost should be reduced without compromising accuracy and consistency. In this direction a hybrid methodology is formulated within the context of domain decomposition. The strategy is to choose in each sub-domain the best performing method. Close to solid boundaries a grid-based Eulerian flow solver is used while in the far field the flow is described in Lagrangian coordinates using particle approximations. Aiming at consistently including compressible effects, particles carry mass, dilatation, vorticity and energy and the complete set of conservation laws is solved in Lagrangian coordinates. At software level, the URANS solver MaPFlow is coupled to the vortex code GENUVP. In the present paper the two dimensional formulation is given alongside with validation tests around airfoils in steady and inherently unsteady conditions. It is verified that: purely Eulerian and hybrid simulations are equivalent; the Eulerian domain in the hybrid solver can be effectively restricted to a layer 1.5 chord lengths wide; significant cost reduction reaching up to 1:3 ratio is achieved.

Experimental benchmark and code validation for airfoils equipped with passive vortex generators

Experimental results and complimentary computations for airfoils with vortex generators are compared in this paper, as part of an effort within the AVATAR project to develop tools for wind turbine blade control devices. Measurements from two airfoils equipped with passive vortex generators, a 30% thick DU97W300 and an 18% thick NTUA T18 have been used for benchmarking several simulation tools. These tools span low-to-high complexity, ranging from engineering-level integral boundary layer tools to fully-resolved computational fluid dynamics codes. Results indicate that with appropriate calibration, engineering-type tools can capture the effects of vortex generators and outperform more complex tools. Fully resolved CFD comes at a much higher computational cost and does not necessarily capture the increased lift due to the VGs. However, in lieu of the limited experimental data available for calibration, high fidelity tools are still required for assessing the effect of vortex generators on airfoil performance.

Assessment of the CFD capabilities to predict aerodynamic flows in presence of VG arrays

Modelling of aerodynamic flows in the presence of vortex generators constitutes a big challenge for CFD due to the different scales involved. The present paper addresses this issue in terms of accuracy and cost. In the simple case of a VG pair placed on a flat plate with no streamwise pressure gradient, the option of fully resolving the VG and that of using the jBAY model are compared with measurements and other CFD simulations. Then the case of 3D separation control on a rectangular wing is considered and comparisons to measurements are performed. Although full resolution of the VGs improves accuracy, the vorticity production is still significantly underestimated, a fact linked with the incapacity of eddy viscosity models to predict vortex flows. It is found that the simulation of one VG pair with periodic side conditions gives fair predictions as long as the VGs keep the flow attached. At angles of attack where 3D separation occurs, this cost effective modelling approach is no longer valid and simulations should include the complete array of VGs. Stereo PIV data showed that close to the VGs (up to 37.2 VG heights downstream of the VGs) turbulent transport between the vortices is strong while further downstream (up to 47.2 heights) diffusion becomes dominant. The normal Reynolds stress distributions also indicate significant vortex wandering in both the normal and spanwise directions.

Results of the AVATAR project for the validation of 2D aerodynamic models with experimental data of the DU95W180 airfoil with unsteady flap

The FP7 AdVanced Aerodynamic Tools for lArge Rotors - Avatar project aims to develop and validate advanced aerodynamic models, to be used in integral design codes for the next generation of large scale wind turbines (10-20MW). One of the approaches towards reaching rotors for 10-20MW size is the application of flow control devices, such as flaps. In Task 3.2: Development of aerodynamic codes for modelling of flow devices on aerofoils and, rotors of the Avatar project, aerodynamic codes are benchmarked and validated against the experimental data of a DU95W180 airfoil in steady and unsteady flow, for different angle of attack and flap settings, including unsteady oscillatory trailing-edge-flap motion, carried out within the framework of WP3: Models for Flow Devices and Flow Control, Task 3.1: CFD and Experimental Database. The aerodynamics codes are: AdaptFoil2D, Foil2W, FLOWer, MaPFlow, OpenFOAM, Q3UIC, ATEFlap. The codes include unsteady Eulerian CFD simulations with grid deformation, panel models and indicial engineering models. The validation cases correspond to 18 steady flow cases, and 42 unsteady flow cases, for varying angle of attack, flap deflection and reduced frequency, with free and forced transition. The validation of the models show varying degrees of agreement, varying between models and flow cases.

CFD aerodynamic analysis of non-conventional airfoil sections for very large rotor blades

The aerodynamic performance of flat-back and elliptically shaped airfoils is analyzed on the basis of CFD simulations. Incompressible and low-Mach preconditioned compressible unsteady simulations have been carried out using the k-w SST and the Spalart Allmaras turbulence models. Time averaged lift and drag coefficients are compared to wind tunnel data for the FB 3500-1750 flat back airfoil while amplitudes and frequencies are also recorded. Prior to separation averaged lift is well predicted while drag is overestimated keeping however the trend in the tests. The CFD models considered, predict separation with a 5° delay which is reflected on the load results. Similar results are provided for a modified NACA0035 with a rounded (elliptically shaped) trailing edge. Finally as regards the dynamic characteristics in the load signals, there is fair agreement in terms of Str number but significant differences in terms of lift and drag amplitudes.

CFD code comparison for 2D airfoil flows

The current paper presents the effort, in the EU AVATAR project, to establish the necessary requirements to obtain consistent lift over drag ratios among seven CFD codes. The flow around a 2D airfoil case is studied, for both transitional and fully turbulent conditions at Reynolds numbers of 3 × 106 and 15 × 106. The necessary grid resolution, domain size, and iterative convergence criteria to have consistent results are discussed, and suggestions are given for best practice. For the fully turbulent results four out of seven codes provide consistent results. For the laminar-turbulent transitional results only three out of seven provided results, and the agreement is generally lower than for the fully turbulent case.

Summary of the Blind Test Campaign to predict the High Reynolds number performance of DU00-W-210 airfoil

DU00-W-210 airfoil DTU Orbit (20/10/2019) Summary of the Blind Test Campaign to predict the High Reynolds number performance of DU00-W-210 airfoil This paper summarizes the results of a blind test campaign organized in the AVATAR project to predict the high Reynolds number performance of a wind turbine airfoil for wind turbine applications. The DU00-W-210 airfoil was tested in the DNWHDG pressurized wind tunnel in order to investigate the flow at high Reynolds number range from 3 to 15 million which is the operating condition of the future large 10MW+ offshore wind turbine rotors. The results of the experiment was used in a blind test campaign to test the prediction capability of the CFD tools used in the wind turbine rotor simulations. As a result of the blind test campaign it was found that although the codes are in general capable of predicting increased max lift and decreased minimum drag with Re number, the Re trend predictions in particular the glide ratio (lift over drag) need further improvement. In addition to that, the significant effect of the inflow turbulence on glide ratio especially at high Re numbers is found as the most important parameter where the prediction as well as the selection of the correct inflow turbulence levels is the key for correct airfoil designs for the future generation 10MW+ wind turbine blades.

Computing the flow past Vortex Generators: Comparison between RANS Simulations and Experiments

The flow around a wind turbine airfoil equipped with Vortex Generators (VGs) is examined. Predictions from three different Reynolds Averaged Navier Stokes (RANS) solvers with two different turbulence models and two different VG modelling approaches are compared between them and with experimental data. The best results are obtained with the more expensive fully resolved VG approach. The cost efficient BAY model can also provide acceptable results, if grid related numerical diffusion is minimized and only force coefficient polars are considered.

Study of Drag Reduction Devices on a Flatback Airfoil

Various trailing edge drag reduction devices, including a new Flap device, were examined experimentally on a flatback airfoil in a wind tunnel. The tests concerned a 30% thick airfoil with 10.6% thick trailing edge. Pressure, Hot Wire and Stereo PIV measurements were performed at a chord Reynolds number of Re=1.5e6. Results show that the best performing devices decrease drag, increase the vortex shedding frequency and reduce flow variation downstream of the wing trailing edge. When a characteristic height for each device is defined the corresponding Strouhal number indicates bluff body shedding at 0.20 < St < 0.25. The best performing device was a combination of the Flap with an Offset Cavity plate. It increased the lift to drag ratio of the plane airfoil by 138%, increased the shedding frequency from 217Hz to 281Hz and reduced the amplitude of the main frequency by 84%. RANS simulations predict St = 0.20 for the plane airfoil, but do not predict vortex shedding for the other cases. Further investigation is required for the optimization of the new device and in order to examine its effects on noise reduction, load mitigation and control.

Assessment of the aerodynamic characteristics of thick airfoils in high Reynolds and moderate Ma numbers using CFD modeling

The aerodynamic characteristics of thick airfoils in high Reynolds number is assessed using two different CFD RANS solvers: the compressible MaPFlow and the incompressible CRES-flowNS-2D both equipped with the k-ω SST turbulence model. Validation is carried out by comparing simulations against existing high Reynolds experimental data for the NACA 63-018 airfoil in the range of -10° to 20°. The use of two different solvers aims on one hand at increasing the credibility in the results and on the other at quantifying the compressibility effects. Convergence of steady simulations is achieved within a mean range of -10° to 14° which refers to attached or light stall conditions. Over this range the simulations from the two codes are in good agreement. As stall gets deeper, steady convergence ceases and the simulations must switch to unsteady. Lift and drag oscillations are produced which increase in amplitude as the angle of attack increases. Finally in post stall, the average CL is found to decrease up to ~24° or 32° for the FFA or the NACA 63-018 airfoils respectively, and then recover to higher values indicating a change in the unsteady features of the flow.