Panel Code Research Articles

Time domain prediction of power absorption from ocean waves with wave energy converter arrays

A three-dimensional transient numerical code ITU-WAVE based on potential theory and Neumann-Kelvin approximation is extended to take into account wave interaction in an array system using two and four truncated vertical cylinder arrays. ITU-WAVE panel code is validated against analytical array results before applied to power absorption from ocean waves for different array configurations. The effects of the separation distances between array system and heading angles on energy absorption in both sway and heave modes are studied by the support of numerical simulations which show sway mode has wider bandwidth than heave mode for energy absorption. It is also shown that wave interactions are stronger when the array systems are close and these wave interactions are reduced significantly and shifted to larger times when the separation distance is increased. The wave interaction is much stronger at the same separation distance and heading angle in heave mode than in sway mode. Numerical experience also shows that more power is absorbed in sway mode than in heave mode in both two and four array systems at any separation distances and heading angles when the bodies in array system have the same displacement in both sway and heave modes.

Design of bare and ducted axial marine current turbines

To convert the kinetic energy of marine current into electricity, the most sensible generator is a horizontal axis turbine. The know-how and the tools used for marine propulsion devices find a new range of applications in this field. An academic panel method code developed for the design of bare and ducted marine propellers was applied to design a marine current turbine. The turbine dimension and the tidal current velocity have been taken to fit the conditions in the Race of Alderney. The wing section theory and the optimum rotor theory based on the blade element momentum were used to obtain the design condition and a first geometry approaching the Betz limit for a bare rotor. The panel method was then used to verify the power coefficient obtained in the presence of the 3D effects and if the cavitation constraints are respected. Subsequently, the same panel code was used to verify if the addition of a duct could improve the power output per unit surface.

Semi-empirical modeling of fuselage–rotor interference for comprehensive codes: influence of angle of attack

The flow field around the isolated Bo105 fuselage including the tail boom and empennage is computed by an unsteady panel code. Velocities normal to the rotor rotational plane are extracted in a volume around the rotor as a data base. A simple semi-empirical analytical formulation of the fuselage-induced velocities, based on parameter estimation from the panel code data, is extended to include rotor shaft angles of attack from $$\alpha =-90^{\circ }$$ (hover, vertical climb) to +90° (vertical descent) for use in comprehensive rotor codes. This model allows the computation of fuselage–rotor interferences on the rotor blade element level in a simplified form, thus eliminating the need for costly CFD computation (of this effect). It also allows the prediction of the rotor wake geometry deformation due to the presence of the fuselage in both prescribed wake and free-wake codes. Its impact on rotor thrust, power and trim is estimated analytically using blade element momentum theory.

A refined tip correction based on decambering

AbstractA new tip correction for use in performance codes based on the blade element momentum (BEM) or the lifting‐line technique is presented. The correction modifies the circulation by taking into account the additional influence of the induction of the vortices in the wake, using the so‐called decambering effect and thin‐airfoil theory. A limitation of the standard Prandtl tip correction is that it represents the surface loading by a line distribution that does not take into account the actual shape of the rotor blade. Thus, the chord distribution does not appear as a parameter in the model, and the loading in the proximity of the tip is generally found to be overestimated. The new tip correction is implemented as an additional correction in order to represent the surface loading by a line distribution. Comparing computations using the new model with standard BEM results and computations using a 3D panel code show that the inclusion of the correction greatly improves the results. The new model also explains some of the discrepancies that earlier on have been observed when using a BEM technique based alone upon standard tip corrections. Copyright © 2015 John Wiley & Sons, Ltd.

ON THE EFFECT OF SHAPE PARAMETERIZATION ON AEROFOIL SHAPE OPTIMIZATION

The selection of suitable shape parameterization technique is one of the significant factors affecting the fidelity of the solution found during aerofoil shape optimization process. This paper investigates the effect of shape parameterization on an automated aerofoil shape optimization problem. Four well known shape parameterization techniques were considered for study; Bezier curves, Class-Shape function Transformation, Hicks-Henne “Bump” function and polynomial method. A boundary layer panel code was coupled with a surrogate-based multi-objective evolutionary algorithm and implemented within an automatic design loop. The optimization problem was formulated for NACA 0012 aerofoil at 5 degree angle of attack. The main criteria for comparison were based on the number of parameters required by each method for accurate representation of the aerofoil, the ability to find the aerofoil with the best performance within the constrained design space and also the computational cost. Preliminary results show that the optimization process was able to increase the lift-to-drag ratio of the aerofoil by 30%. ClassShape Transformation and Hicks-Henne Bump function were able to find the best aerofoil shape within the design space effectively.

Cascade Analysis of a Floating Wind Turbine Rotor

Mounting a wind turbine on a floating foundation introduces more complexity to the aerodynamic loading. The floater motion contains a wide range of frequencies. To study some of the basic dynamic load effect on the blades due to these motions, a two-dimensional cascade approach, combined with a potential vortex method, is used. This is an alternative method to study the aeroelastic behavior of wind turbines that is different from the traditional blade element momentum method. The analysis tool demands little computational power relative to a full three dimensional vortex method, and can handle unsteady flows.When using the cascade plane, a "cut" is made at a section of the wind turbine blade. The flow is viewed parallel to the blade axis at this cut. The cascade model is commonly used for analysis of turbo machineries. Due to the simplicity of the code it requires little computational resources, however it has limitations in its validity. It can only handle two-dimensional potential flow, i.e. including neither three-dimensional effects, such as the tip loss effect, nor boundary layers and stall effects are modeled. The computational tool can however be valuable in the overall analysis of floating wind turbines, and evaluation of the rotor control system.A check of the validity of the vortex panel code using an airfoil profile is performed, comparing the variation of the lift force, to the theoretically derived Wagner function. To analyse the floating wind turbine, a floating structure with hub height 90 m is chosen. An axial motion of the rotor is considered.

Design and aerodynamic analysis of a twin-engine commuter aircraft

The present paper deals with the preliminary design of a new general aviation Commuter 11 seat aircraft. The commuter aircraft market is today characterized by very few new models and the majority of aircraft in operation belonging to this category are older than 35 years. Tecnam Aircraft Industries and the Department of Industrial Engineering (DII) of the University of Naples “Federico II” have been deeply involved in the design of a new commuter aircraft that should be introduced in the market with very good opportunities of success. This paper aims to provide some guidelines on the conceptual design of this new twin-engine commuter aircraft. Aircraft configuration and cabin layout choices are shown and compared to similar solutions adopted by main competitors. The aerodynamic analyses are focused on some particular effects such as the wing–fuselage interference and the nacelle lift contribution and their effect on wing span loading. The aerodynamic analyses have been also essential to validate the preliminary estimation of aircraft stability and control derivatives (both longitudinal and lateral–directional) and to lead to a right sizing of tail surfaces. These analyses have been carried out through the use of a 3-D panel code. Finally some preliminary wind tunnel test results are presented.

Aerodynamic guidelines in the design and optimization of new regional turboprop aircraft

This paper aims to provide some guidelines in the aerodynamic design and optimization of future regional turboprop aircraft with about 90 passengers. Currently there are no configurations on the market of this type, thus a typical 70 passengers turboprop aircraft is taken as reference starting point. The most critical aircraft components in terms of aerodynamic drag contribution and possible improvement are highlighted and an automatic procedure manageable trough MATLab® is described. This interfacing procedure allows importing and modifying geometries using interpolating curves and surfaces via NURBS. Within the optimization loop, any new geometry is analyzed trough the panel code solver until optimized shapes are found. Wing–fuselage junction (also called “Karman”), undercarriage pod, fuselage nose and wing-tip device have been investigated and estimation of performance advantages has been computed. Design of the winglet is presented highlighting performance improvements during the entire mission profile. Finally two different turboprop configurations are proposed: the first with a 4-abreast fuselage arrangement and the second with 5-abreast, highlighting pros and cons of each configuration.

Superposition of Spanwise Circulation Distributions: Accuracy Assessment and Application in Wing Design

Superposition of spanwise lift distributions is a familiar concept from lifting-line theory. However, even for linear potential flows, superposition is not rigorously applicable for aerodynamic models and wing geometries in which the shape of the wake vortex system changes with angle of attack. Nonetheless, the influence of these wake effects could generally be expected to be small, insofar as downstream vortices have a weaker influence than the vortex system in the near field of the wing. In this paper, the theoretical framework for lift distribution superposition is first examined. Next, empirical studies conducted with a panel code are presented that show that superposition remains a very good approximation for a wide range of wing geometries and operating conditions. A method for correcting the small errors in induced drag predictions that result from the application of superposition is also provided. Finally, an application for wing design in which an additional and a design distribution are superimposed to produce lift distributions at other operating conditions is presented. This formulation reveals insights into the influence of the design distribution and the planform characteristics, and it supports computationally inexpensive optimization of wing twist.

A Computational Investigation of Unsteady Aerodynamics of Insect-Inspired Fixed Wing Micro Aerial Vehicle’s 2D Airfoil

A Micro air vehicle (MAV) is defined as class of unmanned air vehicle (UAV) having a linear dimension of less than 15 centimeters and a mass of less than 100 grams with flight speeds of 6 to 12 meters per second. MAVs fall within a Reynolds number (Re) range of 50,000 and 120,000, in which many causes of unsteady aerodynamic effects are not fully understood. The research field of low Reynolds number aerodynamics is currently an active one, with many defence organizations, universities, and corporations working towards a better understanding of the physical processes of this aerodynamic regime. In the present work, it is proposed to study the unsteady aerodynamic analysis of 2D airfoil using CFD software and Xfoil panel code method. The various steps involved in this work are geometric modelling using CATIA V5R17, meshing using ICEM CFD, and solution and postprocessing through FLUENT. The finite control volume analysis and Xfoil panel code method has been carried out to predict aerodynamic characteristics such as lift coefficients, drag coefficients, moment coefficients, pressure coefficients, and flow visualization. The lift and drag coefficients were compared for all the simulations with experimental results. It was observed that for the 2D airfoil, lift and drag both compared well for the midrange angle of attack from −10 to 15 degree AOA.

Airfoil family design for large offshore wind turbine blades

Wind turbine blades size has scaled-up during last years due to wind turbine platform increase especially for offshore applications.The EOLIA project 2007-2010 (Spanish Goverment funded project) was focused on the design of large offshore wind turbines for deep waters. The project was managed by ACCIONA Energia and the wind turbine technology was designed by ACCIONA Windpower. The project included the design of a wind turbine airfoil family especially conceived for large offshore wind turbine blades, in the order of 5MW machine. Large offshore wind turbines suffer high extreme loads due to their size, in addition the lack of noise restrictions allow higher tip speeds. Consequently, the airfoils presented in this work are designed for high Reynolds numbers with the main goal of reducing blade loads and mantainig power production.The new airfoil family was designed in collaboration with CENER (Spanish National Renewable Energy Centre). The airfoil family was designed using a evolutionary algorithm based optimization tool with different objectives, both aerodynamic and structural, coupled with an airfoil geometry generation tool. Force coefficients of the designed airfoil were obtained using the panel code XFOIL in which the boundary layer/inviscid flow coupling is ineracted via surface transpiration model. The desing methodology includes a novel technique to define the objective functions based on normalizing the functions using weight parameters created from data of airfoils used as reference.Four airfoils have been designed, here three of them will be presented, with relative thickness of 18%, 21%, 25%, which have been verified with the in-house CFD code, Wind Multi Block WMB, and later validated with wind tunnel experiments. Some of the objectives for the designed airfoils concern the aerodynamic behavior (high efficiency and lift, high tangential coefficient, insensitivity to rough conditions, etc.), others concern the geometry (good for structural design, compatibility for the different airfoil family members, etc.) and with the ultimate objective that the airfoils will reduce the blade loads. In this paper the whole airfoil design process and the main characteristics of the airfoil family are described. Some force coefficients for the design Reynolds number are also presented.The new designed airfoils have been studied with computational calculations (panel method code and CFD) and also in a wind tunnel experimental campaign. Some of these results will be also presented in this paper.

A New Tip Correction Based on the Decambering Approach

A limitation of the standard Blade Element Momentum (BEM) technique is that it represents the surface loading by an averaged value determined by locally computed airfoil characteristics. Thus, it does not take into account the chord wise distribution of the induction. Likewise, lifting line methods suffer from the problem that the induction from the free wake vortices is only evaluated along a line representing the center of pressure. Hence, the effect from the chord wise distribution of the induction is neglected. As a consequence, the loading in the proximity of the tip is generally found to be overestimated. To remedy this problem, a correction has been developed, which modifies the circulation by taking into account the additional influence of the induction of the free wake vortices. This is done by correcting the circulation, using the so-called decambering effect and thin-airfoil theory. The correction is implemented as an additional correction to the Prandtl tip correction. Where the Prandtl tip correction serves to correct the axisymmetric momentum theory for a finite number of blades (see Goldstein, 1929), the new model further corrects the blade element model to represent the line distribution by a surface loading. Comparing computations of the new model with results from a 'standard' BEM model and computations using a 3D panel code, show that the inclusion of the correction greatly improves the results. The new model also explains some of the discrepancies that earlier on have been observed when using a BEM technique based alone upon standard tip corrections (Shen et al., 2005).

The role of conservative forces in rotor aerodynamics

AbstractThe theory to predict the performance and loads on rotors (propellers, screws, windmills) has a history of more than a century. Apart from modern computational fluid dynamics and vortex panel models taking the true blade geometry into account, most other models proceed from an infinitely thin actuator disc or line. These models assume an externally defined force field distributed at the disc or line, representing the loads on the real rotor. Given this force field, the flow is solved by momentum balances or by the equations of motion. The use of external force fields was discussed in textbooks of the first decades of the 20th century, but has received little attention since then. This paper investigates the higher-order effect of adding thickness to the actuator disc or changing the actuator line to a blade with cross-sectional dimensions. For the generation of a Rankine vortex by a force field acting on an actuator disc with thickness, an exact solution has been found in which not only the thrust and torque determine the flow, but also a radial force. This force is conservative, in contrast to the other force components. For rotor blades, a conservative normal and radial force acting on the chordwise bound vorticity is present. This explains the experimentally observed inboard motion of the tip vortex of model wind turbine rotors before the wake induction field drives it outboard. Simulations by computational fluid mechanics and a vortex panel code reproduce the inboard motion, but an actuator line analysis, in which the chordwise vorticity is absent, does not. The conservative load is only $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}1\mbox{--}2\, \%$ of the thrust on the entire blade but ${\approx }10\, \%$ of the thrust at the tip ($r/R>0.9$). Conservative forces at the disc and rotor blade vanish for vanishing disc thickness or blade cross-section, so play no role in any of the infinitely thin actuator disc or line methods. However, if higher-order effects of non-zero dimensions are to be modelled, the conservative force field has to be included.

A Multi-Element Diffuser Augmented Wind Turbine

A new class of diffuser augmented wind turbines (DAWTs) is presented. The new diffuser concept exploits aero-dynamic principles for the creation of high-lift airfoil configurations known from the aircraft industry. Combining this with our objective of obtaining a compact power-efficient design has enabled creation of a family of DAWT designs with energy capture potentials which exceed the power efficiency based on the diffuser exit area by 50%. The paper presents the 1D momentum theory governing the DAWTs, and discusses upper limits for power extraction, similar to the Betz limit applicable for bare Horizontal-Axis Wind Turbines (HAWTs). Inviscid axisymmetric panel code calculations are then used to drive the diffuser design towards higher power coefficients. Axisymmetric actuator disk Navier-Stokes calculations reveal the types of stall that inhibit the functionality of the ideal inviscid optimum, leading the design towards the new class of DAWTs. DAWT performance has been differently measured over time, creating confusion. Proper comparison with performance of existing DAWT designs is therefore emphasized. This involves reference to established literature results, and recalculation of earlier DAWT designs in an attempt to project all results onto a common metric.

A Two-Stage Optimisation Method for Full-Scale Marine Propellers Working Behind a Ship

A procedure for the optimisation of full-scale marine propellers working behind a ship is presented. The procedure consists of two stages, the actual optimisation and a detailed investigation of selected propeller variants. In the first stage, an evolutionary algorithm in combination with the in-house panel code panMARE is used to solve the multi-objective propeller optimisation problem. Efficient methods for the evaluation of propeller variants and parameterisation of propeller shape are presented. Proceeding from the results of the optimisation, a set of selected propeller variants is investigated in the second stage taking into account all relevant propeller-hull interactions. The applied algorithm is based on coupling of panMARE with a viscous flow solver. An enhancement of this method allows investigating several propeller variants sequentially without stopping the solvers. In this work, the optimisation method is applied to improve the propeller of a container vessel.

Multi-objective optimisation of horizontal axis wind turbine structure and energy production using aerofoil and blade properties as design variables

The design of wind turbine blades is a true multi-objective engineering task. The aerodynamic effectiveness of the turbine needs to be balanced with the system loads introduced by the rotor. Moreover the problem is not dependent on a single geometric property, but besides other parameters on a combination of aerofoil family and various blade functions. The aim of this paper is therefore to present a tool which can help designers to get a deeper insight into the complexity of the design space and to find a blade design which is likely to have a low cost of energy. For the research we use a Computational Blade Optimisation and Load Deflation Tool (CoBOLDT) to investigate the three extreme point designs obtained from a multi-objective optimisation of turbine thrust, annual energy production as well as mass for a horizontal axis wind turbine blade. The optimisation algorithm utilised is based on Multi-Objective Tabu Search which constitutes the core of CoBOLDT. The methodology is capable to parametrise the spanning aerofoils with two-dimensional Free Form Deformation and blade functions with two tangentially connected cubic splines. After geometry generation we use a panel code to create aerofoil polars and a stationary Blade Element Momentum code to evaluate turbine performance. Finally, the obtained loads are fed into a structural layout module to estimate the mass and stiffness of the current blade by means of a fully stressed design. For the presented test case we chose post optimisation analysis with parallel coordinates to reveal geometrical features of the extreme point designs and to select a compromise design from the Pareto set. The research revealed that a blade with a feasible laminate layout can be obtained, that can increase the energy capture and lower steady state systems loads. The reduced aerofoil camber and an increased L/D-ratio could be identified as the main drivers. This statement could not be made with other tools of the research community before.

Aerodynamic performance benefits of utilising camber morphing wings for unmanned air vehicles

AbstractThis work considers the effects of camber morphing, both in magnitude and chord position, on the performance of a generic unmanned air vehicle (UAV). The focus is to maximise appropriate aerodynamic factors across the mission by optimising the wing camber. Specifically, the enhancement of range, endurance, and stall speed is sought by means of maximising their aerodynamic performance parameters, CL/CD, CL3/2/CD, and CLmax respectively. An analysis of the effects of camber morphing is carried out using the vortex panel code, XFOIL, utilising aerofoils from the NACA four-digit family. The results are then adjusted to account for 3D flow factors such as induced drag, offering a more realistic appraisal of their effectiveness. Flight testing is then performed on four wings of fixed aerofoil sections, optimised for each performance characteristic, to validate the trends observed in the XFOIL data onboard a 1·64m span aircraft.

Analysis of the forward speed effects on the radiation forces on a Fast Ferry

The paper presents an analysis of the radiation forces on a ship advancing with forced heave and pitch motions. This analysis is based on experimental data obtained with model tests and on numerical simulations with two codes, one based on 3D time domain Greens function method and the other is a 2D linear strip theory method. The case study consists of a hull shape from a Fast Ferry and several speeds are investigated between 0 and 40knot. The objective is to quantify the forward speed effects on the radiation forces and to verify the skill of the conventional strip theory and of a time domain panel methods in dealing with large forward speeds.The consequences of applying simplifications on the linear boundary conditions of the common seakeeping codes are investigated. It is concluded that forward speed has a large effect on the radiation forces, especially on the coupling terms, and thus it is important to consider the full linear interactions between the steady and unsteady flows in the numerical calculations as the speed of the ship increases and the time domain panel code is able to represent these effects.

CPP propeller cavitation and noise optimization at different pitches with panel code and validation by cavitation tunnel measurements

The propeller design is an activity which nowadays presents ever increasing challenges to the designer, involving not only the usual mechanical characteristics fulfillment and cavitation erosion avoidance, but also other cavitation side effects, such as radiated noise and/or pressure pulses. Moreover, in some cases propeller characteristics have to be optimized in correspondence to very different functioning points, including considerably off-design conditions, hardly captured by conventional design methods. In the present paper, a recently presented method, based on the coupling between a multiobjective optimization algorithm and a panel code, is applied to the design of a CPP propeller at different pitch settings, with the aim of reducing the cavitating phenomena and, consequently, the resultant radiated noise. Particular attention has been devoted to the slow speed (low pitch) condition, obtained at constant RPM, and characterized by considerable radiated noise and vibrations related to face cavitation. Numerical results are validated by means of an experimental campaign, testing both the original and the optimized geometry in terms of cavitation extent and radiated noise. Experimental results confirm the numerical predictions, attesting the capability of the method to assess propeller functioning characteristics, thus representing a very useful tool for the designer in correspondence of challenging problems.

Indicial lift response function: an empirical relation for finite‐thickness airfoils, and effects on aeroelastic simulations

ABSTRACTThe aeroelastic response of wind turbines is often simulated in the time domain by using indicial response techniques. Unsteady aerodynamics in attached flow are usually based on Jones's approximation of the flat plate indicial response, although the response for finite‐thickness airfoils differs from the flat plate one.The indicial lift response of finite‐thickness airfoils is simulated with a panel code, and an empirical relation is outlined connecting the airfoil indicial response to its geometric characteristics. The effects of different indicial approximations are evaluated on a 2D profile undergoing harmonic pitching motion in the attached flow region; the resulting lift forces are compared with computational fluid dynamics (CFD) simulations. The relevance for aeroelastic simulations of a wind turbine is also evaluated, and the effects are quantified in terms of variations of equivalent fatigue loads, ultimate loads, and stability limits.The agreement with CFD computations of a 2D profile in harmonic motion is improved by the indicial function accounting for the finite‐thickness of the airfoil. Concerning the full wind turbine aeroelastic behavior, the differences between simulations on the basis of Jones's and finite‐thickness indicial response functions are rather small; Jones's flat‐plate approximation results in only slightly larger fatigue and ultimate loads, and lower stability limits. Copyright © 2012 John Wiley & Sons, Ltd.