Dissertations / Theses on the topic 'Wind Turbine Airfoil'

A new method, Computational Acoustic Beamforming, is proposed in this thesis. This novel numerical sound source localization methodology combines the advantages of the Computational Fluid Dynamics (CFD) simulation and experimental acoustic beamforming, which enable this method to take directivity of sound source emission into account while maintaining a relatively low cost. This method can also aid the optimization of beamforming algorithm and microphone array design. In addition, it makes sound source prediction of large structures in the low frequency range possible. Three modules, CFD, Computational Aeroacoustics (CAA) and acoustic beamforming, are incorporated in this proposed method. This thesis adopts an open source commercial software OpenFOAM for the flow field simulation with the Improved Delayed Detached Eddy Simulation (IDDES) turbulence model. The CAA calculation is conducted by an in-house code using impermeable Ffowcs-Williams and Hawkings (FW-H) equation for static sound source. The acoustic beamforming is performed by an in-house Delay and Sum (DAS) beamformer code with several different microphone array designs. Each module has been validated with currently available experimental data and numerical results. A flow over NACA 0012 airfoil case was chosen as a demonstration case for the new method. The aerodynamics and aeroacoustics results are shown and compared with the experimental measurements. A relatively good agreement has been achieved which gives the confidence of using this newly proposed method in sound source localization applications.

A load-dependent passive camber control concept is introduced for alleviating load fluctuations on wind turbine rotor blades with the overall goal of reducing fatigue and increasing durability and turbine lifetime. The passive change of the camber line is realized through kinematically coupled leading and trailing-edge flaps. The leading-edge flap is actuated by the increased pressure forces due to the change in angle of attack. The trailing-edge flap is kinematically coupled to the rotation of the leading-edge flap. This combined motion results in an increase or decrease in airfoil camber dependent on the pressure difference along the airfoil and the restoring force applied at the leading-edge flap. This concept works fully passive, i.e. its characteristics are determined solely by the fluid-structure interaction. The quantification of these aerodynamic characteristics is the objective of the present study. The concept has been studied experimentally and numerically. The numerical simulations consider quasi-steady aerodynamics and the combined flap motion is described by one degree of freedom. The concept has been confirmed experimentally under quasi-steady conditions in the large scale low-speed wind tunnel at TU Darmstadt. The structural parameters which characterize the flap deflections are investigated systematically. The results show how the lift curve slope can be adjusted by the preload moment, the stiffness and the coupling ratio between the leading and trailing-edge flap. It is shown that it is possible to keep the lift coefficient constant due to the self-adaptive camber line. The numerical model is compared to the experimental results. The model is able to predict the effects revealed through the wind tunnel measurements. In the second part the numerical model of the airfoil section with leading and trailing-edge flaps is extended to consider also the bending and torsional degree of freedom. The results show that although the dynamic behavior of the blade changes significantly, load reduction is achieved and a flexible camber line is advantageous for the dynamic response of the rotor blade. Finally the concept is evaluated with the wind turbine simulator FAST. The underlying look-up tables are modified to incorporate the flapped airfoil characteristics. The results provide a baseline for the evaluation of the concept in conjunction with the aerodynamics encountered by wind turbines.

碩士
國立臺灣大學
工程科學及海洋工程學研究所
98
This article uses Fluent’s Computational Fluid Dynamics software to simulate the comparison of lift and drag coefficient between airfoil with microtab and airfoil prototype. Two commonly used airfoil of wind turbine blade, S809 and Naca63-415, are analyzed in this study；the variables of microtab are location and length, which respectively are simulated under three different conditions. In addition, the changes of two different Reynolds numbers also are analyzed in the simulation. After simulating the lift and drag coefficient under each condition, the formula of airfoil maximum acquired power is then applied to compare the effect on the power variation of airfoil prototype with microtab. Moreover, comparison between the efficiency of increased power is implemented to further understand under which condition microtab can effectively increase power.

The total worldwide base of installed wind energy peak capacity reached 94 GW by the end of 2007, including 1846 MW in Canada. Wind turbine systems are being installed throughout Canada and often in mountains and cold weather regions, due to their high wind energy potential. Harsh cold weather climates, involving turbulence, gusts, icing and lightning strikes in these regions, affect wind turbine performance. Ice accretion and irregular shedding during turbine operation lead to load imbalances, often causing the turbine to shut off. They create excessive turbine vibration and may change the natural frequency of blades as well as promote higher fatigue loads and increase the bending moment of blades. Icing also affects the tower structure by increasing stresses, due to increased loads from ice accretion. This can lead to structural failures, especially when coupled to strong wind loads. Icing also affects the reliability of anemometers, thereby leading to inaccurate wind speed measurements and resulting in resource estimation errors. Icing issues can directly impact personnel safety, due to falling and projected ice. It is therefore important to expand research on wind turbines operating in cold climate areas. This study presents an experimental investigation including three important fundamental aspects: 1) heat transfer characteristics of the airfoil with and without liquid water content (LWC) at varying angles of attack; 2) energy losses of wind energy while a wind turbine is operating under icing conditions; and 3) aerodynamic characteristics of an airfoil during a simulated icing event. A turbine scale model with curved 3-D blades and a DC generator is tested in a large refrigerated wind tunnel, where ice formation is simulated by spraying water droplets. A NACA 63421 airfoil is used to study the characteristics of aerodynamics and convective heat transfer. The current, voltage, rotation of the DC generator and temperature distribution along the airfoil, which are used to calculate heat transfer coefficients, are measured using a Data Acquisition (DAQ) system and recorded with LabVIEW software. The drag, lift and moment of the airfoil are measured by a force balance system to obtain the aerodynamics of an iced airfoil. This research also quantifies the power loss under various icing conditions. The data obtained can be used to valid numerical data method to predict heat transfer characteristics while wind turbine blades worked in cold climate regions.
October 2008

This dissertation purpose is to study the impact that a geometry modification of a wind turbine rotor imposes on its performance. The studied wooden rotor, with a diameter of 1.2 m, belongs to a family of small wind turbines that are built by unskilled persons using hand tools with the guidelines of Hugh Piggott. Due to its inaccuracy, the production process delivers a geometry with sharp leading edges. For the performance of an airfoil, the leading edge is one of the most important characteristics to take in mind, and so, the goal of this dissertation was to smooth the airfoils leading edge towards the lower surface in order to widen the ????- ?? curve of the rotor. To do so, numerical methods were employed to assess such modification on the performance, in a way that the technique could be later applied on the rotor using nothing but hand tools. In a previous investigation, the same rotor here approached in this dissertation, was numerically and experimentally studied for the following windspeeds: 3.0; 3.7; 4.4; 5.5; 7.2 e 7.7 m/s. In the same study, a digital scan was performed on the rotor, one for each blade, resulting in 6 different cross sections each with its chord and incidence angle. The three blades present geometric differences. Having these airfoils characteristics, the QBlade software was used for the design and analysis of the new modified airfoils based on the original Piggott airfoils. The software also allows for rotor design and uses the Blade Element Momentum Theory for the analysis of horizontal axis wind turbines. The performance of both rotors was approximated by averaging the performance of three ideal rotors, each consisting of three identical blades 1,2 and 3. The new airfoils regarding blade 1 and 3, presented better aerodynamic efficiency performance compared to the Piggott airfoils, whereas blade 2 new airfoils did not exhibited any significant improved performance compared to the Piggott airfoils.The dimensionless simulations results from QBlade, portrayed that the averaged rotor with the modified airfoils present better power coefficient (????) for high values of ?? (ratio between the tangential velocity of the blade tip and the free stream windspeed) when compared to the averaged rotor with the Piggott airfoils. For a constant rotational speed of 500 RPM, the new rotor remarkably withdraws more energy from the flow for low windspeeds. In a hypothetical approach of a optimized turbine production made up by the best modified airfoils, the optimized rotor simulations showed a significant better performance for high values of ??,as well as higher maximum ???? than the ones from the averaged rotor with the modified airfoils.
O objectivo da presente investigação é estudar a influência que uma modificação na geometria do rotor de uma turbina eólica inflige na sua eficiência. O rotor estudado, de madeira e com um diâmetro de 1,20 m, pertence a um grupo de pequenas turbinas eólicas que são construídas apenas com recurso a ferramentas manuais seguindo as indicações do Hugh Piggott. Este processo de construcção, devido à sua imprecisão, resulta numa geometria que dá origem a um bordo de ataque aguçado. Sendo que o bordo de ataque é um aspecto importante para o desempenho do mesmo, o objectivo desta dissertação passou por suavizar o bordo de ataque do perfil alar a “fugir” para o intradorso, de maneira a ampliar a curva ????- ?? do rotor. Para tal, recorreu-se a métodos numéricos para avaliar o desempenho de tal modificação, numa perspectiva que esta técnica possa ser aplicada no rotor usando ferramentas manuais, como por exemplo uma lixa. Num estudo prévio, o mesmo rotor que é estudado nesta dissertação foi alvo de um estudo numérico e experimental para as seguintes velocidades de vento: 3.0; 3.7; 4.4; 5.5; 7.2 e 7.7 m/s. No mesmo estudo o rotor foi alvo de uma digitalização na qual, cada pá do rotor foi examinada em 6 secções diferentes e que resultou em 6 perfis alares diferentes com a respectiva corda e o ângulo de incidência, sendo de realçar que as três pás apresentam diferenças geométricas entre si. Tendo estas características, usou-se o software QBlade para o desenho e análise dos novos perfis alares modificados a partir dos originais perfis alares do Piggott. O software permite o desenho de rotores e para a simulação do desempenho de turbinas eólicas de eixo horizontal, o software emprega a Blade Element Momentum Theory. O desempenho real do rotor original e do novo rotor foi estimado a partir da média de três rotores ideais, cada um constituído por três pás idênticas 1, 2 e 3. Os novos perfis alares das pás 1 e 3 revelaram melhor desempenho da sua eficiência aerodinâmica (????/????) quando comparados aos perfis oriundos da construção manual do Hugh Piggott, enquanto que os novos perfis da pá 2 não ilustraram qualquer melhoria significativa quando comparados aos perfis originais. Os resultados das simulações adimensionais do QBlade, mostraram que o rotor médio com os novos perfis apresenta melhor coeficiente de potência (????) para altos valores de ?? (razão entre a velocidade tangencial da ponta da pá e a velocidade do vento) quando comparado ao rotor médio com os perfis do Piggott. Quando submetidos a uma velocidade rotacional constante de 500 RPM, o novo rotor retira notavelmente mais energia do escoamento a baixas velocidades de vento. Numa abordagem hipotética da construcção de uma turbina optimizada composta pelos melhores perfis modificados, as simulações do rotor optimizado ilustraram um significativo melhor desempenho a altos valores de ?? como ???? máximos mais elevados do que os do rotor médio com os perfis modificados.

Experimental aeroacoustic research was conducted on a wind turbine specific airfoil at low Reynolds numbers. The goal of this thesis was to study trailing edge noise generation from the airfoil and investigate correlations between the noise and the flow field. Before experiments were performed the current wind tunnel had to be modified in order to make it more suitable for aeroacoustic tests. Sound absorbing foam was added to the inside of the tunnel to lower the background noise levels and turbulence reduction screens were added which lowered the turbulence. An S822 airfoil was chosen because it is designed for low Reynolds flows attainable in the wind tunnel which are on the order of 104. Smoke wire flow visualization was used to gain insight into the airfoil wake development and oil film flow visualization was used to qualitatively assess the boundary layer development. Laser Doppler anemometry (LDA) was used to measure two components of velocity at high data rates in the airfoil wake. Wake profiles were measured in addition to single point measurements to determine the velocity spectrum. A microphone was mounted inside the test section in order to measure the trailing edge noise. Initial plans included measuring the trailing edge noise with a microphone array capable of quantifying and locating noise sources. Although an array was built and beamforming code was written it was only used in preliminary monopole source tests. Oil film results showed the behaviour of the boundary layer to be consistent with previous low Reynolds number experiments. LDA results revealed sharp peaks in the velocity spectra at 1100 Hz from U0 = 15–24 m/s, and 3100 and 3800 Hz, from U0 = 25–35 m/s, which were inconsistent with vortex shedding results of previous researchers. Also present were a series of broad peaks in the spectra that increase from 1200–1700 Hz in the U0 = 25–35 m/s range. The shedding frequency from the smoke wire flow visualization was calculated to be 1250 Hz at U0 = 26 m/s. These sharp peaks were also present in the acoustic spectrum. It was reasoned that these peaks are due to wind tunnel resonance which is a common occurrence in hard wall wind tunnels. In particular the tone at 1100 Hz is due to a standing wave with a wavelength equal to half the tunnel width. The shedding frequency from the smoke wire flow visualization was calculated to be 1100 Hz at U0 = 20 m/s. These tones exhibited a “ladder-like” relationship with freestream velocity, another aspect indicative of wind tunnel resonance. It was reasoned that the wind tunnel resonance was forcing the shedding frequency of the airfoil in the U0 = 15–24 m/s range, and in the U0 = 25–35 m/s range, the shedding frequency corresponded to the broad peaks in the LDA spectra.

An aluminum NACA 0018 airfoil testbed was constructed with 95 static pressure taps and 25 embedded microphones to enable novel time-resolved measurements of surface pressure. The main objective of this investigation is to utilize time-resolved surface pressure measurements to estimate salient flow characteristics in the separated flow region over the upper surface of an airfoil. The flow development over the airfoil was examined using hot wire anemometry and mean surface pressure for a range of Reynolds numbers from 80x103 to 200x103 and angles of attack from 0° to 18°. For these parameters, laminar boundary layer separation takes place on the upper surface and two flow regimes occur: (i) separation is followed by flow reattachment, so that a separation bubble forms and (ii) separation occurs without subsequent reattachment. Measurements of velocity and mean surface pressure were used to characterize the separated flow region and its effect on airfoil performance using the lift coefficient. In addition, the transition process and the evolution of disturbances were examined. The lift curve characteristics were found to be linked to the rate of change of the separation, transition, and reattachment locations with the angle of attack. For both flow regimes, transition was observed in the separated shear layer. Specifically, the amplification of disturbances within a band of frequencies in the separated shear layer resulted in laminar to turbulent transition. Validation of time-resolved surface pressure measurements was performed for Rec = 100x103 at α = 8° and α = 12°, corresponding to regimes of flow separation with and without reattachment, respectively. A comparative analysis of simultaneous velocity and time-resolved surface pressure measurements showed that the characteristics and development of velocity fluctuations associated with disturbances in the separated shear layer can be extracted from time-resolved surface pressure measurements. Specifically, within the separated flow region, the amplitude of periodic oscillations in the surface pressure signal associated with disturbances in the separated shear layer grew in the streamwise direction. In addition, the frequency at the spectral peak of the amplified disturbances in the separated shear layer was identified. Based on the results of the validation analysis, time-resolved surface pressure measurement analysis techniques were applied for a Reynolds number range from 60x103 to 130x103 and angles of attack from 6° to 16°. Within the separated flow region, the streamwise growth of surface pressure fluctuations is distinctly different depending on the flow regime. Specifically, within the separation bubble, the RMS surface pressure fluctuations increase in the streamwise direction and reach a peak just upstream of the reattachment location. The observed trend is in agreement with that observed for other separating-reattaching flows on geometries such as the forward and backward facing step and splitter plate with fence. In contrast to the separation bubble formation, when the separated shear layer fails to reattach to the airfoil surface, RMS surface pressure fluctuations increase in the streamwise direction with no maximum and the amplitude is significantly lower than those observed in the separation bubble. Surface pressure signals were further examined to identify the frequency, convective velocity, and spanwise uniformity of disturbances in the separated shear layer. Specifically, for both flow regimes, the fundamental frequency and corresponding Strouhal number exhibit a power-law dependency on the Reynolds number. Based on the available data for which velocity measurements were obtained in the separated flow region, the convective velocity matched the mean velocity at the wall-normal distance corresponding to the maximum turbulence intensity. A distinct increase in the convective velocity of disturbances in the separated shear layer was found when the airfoil was stalled in comparison to that found in the separation bubble. From statistical analysis of surface pressure signals in the spanwise direction, it was found that disturbances are strongly two-dimensional in the laminar portion of the separated shear layer and become three-dimensional through the transition process.

Αντικείμενο της παρούσας διδακτορικής διατριβής είναι η πειραματική και υπολογιστική διερεύνηση αεροδυναμικής συμπεριφοράς πτερύγων σε διφασική ροή αέρα–νερού και η εφαρμογή σε πτερύγια ανεμοκινητήρων. Αρχικά, γίνεται πειραματική και υπολογιστική μελέτη μονοφασικής ροής αέρα γύρω από αεροτομές, πτέρυγες και πτερύγιο ανεμοκινητήρα και στη συνέχεια μελέτη διφασικής ροής αέρα-νερού γύρω από τα ίδια σώματα. Η σύγκριση μεταξύ των αποτελεσμάτων της μονοφασικής ροής με τα αντίστοιχα της διφασικής ροής αέρα-νερού είναι αναγκαία ώστε να μελετηθούν οι επιπτώσεις της διφασικής ροής αέρα–νερού στην αεροδυναμική απόδοση. Η πειραματική ανάλυση αφορά τη διεξαγωγή πειραμάτων για τη μελέτη της αεροδυναμικής συμπεριφοράς αεροτομών και πτερύγων σε συνθήκες μονοφασικής και διφασικής ροής. Για την προσομοίωση συνθηκών διφασικής ροής αέρα-νερού τροποποιείται η αεροσήραγγα που διαθέτει ήδη το Εργαστήριο με την προσαρμογή ειδικών ακροφυσίων ψεκασμού νερού (συνθήκες βροχής). Για τις ανάγκες των πειραμάτων χρησιμοποιούνται τα μοντέλα αεροτομών και πτερύγων NACA 0012 που συνοδεύουν την αεροσήραγγα και κατασκευάζονται αεροτομή και πτέρυγες S809. Τα πειράματα μονοφασικής και διφασικής ροής γίνονται για την ίδια ταχύτητα αέρα. Για τη διφασική ροή αέρα-νερού εξετάστηκαν τέσσερις διαφορετικές πυκνότητες περιεχόμενης βροχής. Η υπολογιστική ανάλυση γίνεται με το υπολογιστικό πακέτο ANSYS CFD-Fluent. Αρχικά, γίνονται προσομοιώσεις για μονοφασική ροή αέρα γύρω από την αεροτομή NACA 0012, για την οποία υπάρχει πλήθος δημοσιευμένων αποτελεσμάτων, με τρία διαφορετικά μοντέλα τύρβης ώστε να βρεθεί το καταλληλότερο. Ο συντελεστής άνωσης υπολογίζεται με μεγάλη ακρίβεια, σε αντίθεση με το συντελεστή αντίστασης. Το πρόβλημα αυτό οφείλεται στην αδυναμία του Fluent να υπολογίσει το σημείο μετάβασης του οριακού στρώματος από στρωτό σε τυρβώδες. Κρίνεται επομένως αναγκαίο να γίνει σύγκριση του συντελεστή αντίστασης με πειραματικά δεδομένα για πλήρως τυρβώδες οριακό στρώμα. Για ακόμα πιο ακριβή αποτελέσματα αναπτύσσεται αλγόριθμος για τον υπολογισμό του σημείου μετάβασης από στρωτό σε τυρβώδες οριακό στρώμα και γίνονται προσομοιώσεις ορίζοντας την περιοχή αριστερά από το σημείο μετάβασης ως στρωτή και δεξιά από αυτό ως τυρβώδη. Υπολογίζονται οι κατανομές πίεσης και ταχύτητας γύρω από την αεροτομή, καθώς επίσης και τα σημεία ανακοπής, μέγιστης ταχύτητας, αποκόλλησης και επανακόλλησης του οριακού στρώματος. Παρουσιάζονται επίσης οι ροϊκές γραμμές και τα διανύσματα της ταχύτητας γύρω από την αεροτομή. Αντίστοιχες προσομοιώσεις γίνονται και για την αεροτομή S809. Για τη μελέτη του τρισδιάστατου χαρακτήρα της ροής, γίνονται προσομοιώσεις γύρω από πτέρυγα S809. Υπολογίζονται οι συντελεστές άνωσης και αντίστασης, τα σημεία ανακοπής, μέγιστης ταχύτητας, αποκόλλησης και επανακόλλησης του οριακού στρώματος. Επίσης παρουσιάζονται κατανομές της έντασης της τύρβης στην άνω επιφάνεια της πτέρυγας και της συνισταμένης ταχύτητας, της ταχύτητας στη z-διεύθυνση, της έντασης της τύρβης και της επιτάχυνσης της ροής πίσω από την πτέρυγα. Για τη μελέτη της ροής γύρω από περιστρεφόμενο πτερύγιο γίνονται προσομοιώσεις γύρω από το πτερύγιο Phase IV της NREL. Γίνεται μελέτη της κατανομής της αξονικής ταχύτητας πίσω από το δρομέα, της κατανομής της στατικής πίεσης και της έντασης της τύρβης πάνω στην επιφάνεια του πτερυγίου και της κατανομής της στατικής πίεσης σε διάφορα σημεία πάνω στο πτερύγιο. Η υπολογιστική μελέτη της διφασικής ροής αέρα-νερού γίνεται αρχικά για την αεροτομή NACA 0012 με πυκνότητα περιεχόμενης βροχής LWC=30 g/m³, επειδή υπάρχουν αντίστοιχα έγκυρα πειραματικά αποτελέσματα ώστε να γίνει σύγκριση για την εγκυρότητα της διαδικασίας της προσομοίωσης. Στη συνέχεια γίνονται προσομοιώσεις για διφασική ροή αέρα-νερού γύρω από την αεροτομή S809, την πτέρυγα S809 και το περιστρεφόμενο πτερύγιο Phase IV της NREL. Προσομοιώσεις γίνονται επίσης για διαφορετικές πυκνότητες περιεχόμενης βροχής για τη ροή γύρω από τις αεροτομές σε χαμηλό αριθμό Reynolds. Τα αποτελέσματα της διφασικής ροής αέρα-νερού συγκρίνονται με τα αντίστοιχα της μονοφασικής ροής ώστε να προκύψουν συμπεράσματα για τις επιπτώσεις της βροχής στην αεροδυναμική απόδοση. Γίνεται επίσης υπολογισμός του συντελεστή ισχύος του ανεμοκινητήρα σε συνθήκες μονοφασικής ροής αέρα και διφασικής ροής αέρα-νερού. Σε συνθήκες διφασικής ροής αέρα-νερού παρατηρείται υποβάθμιση της αεροδυναμικής απόδοσης, συγκεκριμένα μείωση της άνωσης με παράλληλη αύξηση της αντίστασης. Δυο είναι οι βασικοί μηχανισμοί που επικρατούν και έχουν ως αποτέλεσμα την υποβάθμιση αυτή. Στην επιφάνεια της αεροτομής δημιουργείται ανομοιόμορφο φιλμ νερού που αυξάνει την τραχύτητα και το πάχος της αεροτομής. Τα σταγονίδια καθώς προσκρούουν πάνω στο φιλμ νερού δημιουργούν «κρατήρες» αυξάνοντας την τραχύτητα της αεροτομής. Επίσης, τα σωματίδια νερού διασπώνται κατά την πρόσκρουσή τους πάνω στην αεροτομή σε άλλα σταγονίδια μικρότερης διαμέτρου και μειωμένης ταχύτητας. Αυτό έχει ως αποτέλεσμα τα σταγονίδια αυτά, επαναεπιταχυνόμενα από τη ροή του αέρα να αποσπούν ποσό ενέργειας από το οριακό στρώμα καθιστώντας το πιο ευάλωτο σε αποκόλληση. Στόχος της μελέτης της αεροδυναμικής συμπεριφοράς των πτερυγίων σε διφασική ροή αέρα-νερού είναι η κατασκευή ανεμοκινητήρων υψηλού βαθμού απόδοσης και η παραγωγή φθηνής ενέργειας από την όσο το δυνατόν καλύτερη αξιοποίηση της αιολικής ενέργειας.
The aim of the present doctoral thesis is the experimental and computational study of the aerodynamic behavior of wings in two-phase flow and the application on wind turbine blades. First of all, experimental and computational study of one-phase flow over airfoils, wings and wind turbine blade and afterwards study of two-phase flow over the same bodies is conducted. The comparison of the results between dry and wet conditions is necessary in order to show the effects of two-phase flow at the aerodynamic performance. Wind tunnel tests were conducted to show the aerodynamic behavior of airfoils and wings in one-phase and two-phase flows. To simulate two-phase flow, the wind tunnel of the Fluid Mechanics Laboratory has to be configured with adding commercial rain simulated nozzles. For the experiments NACA 0012 airfoils and wings which come along the wind turbine are utilized and airfoil and wings S809 are constructed. The experiments of one-phase flow and two-phase flow are conducted for the same air velocity. For the two-phase flow four different Liquid Water Contents are examined. For the computational analysis the commercial CFD code ANSYS Fluent is used. In first place, simulations of one-phase flow over the NACA 0012 airfoil are done with three different turbulence models. The NACA0012 airfoil is chosen because it has been studied in depth and has a precise data base to compare the results of the simulation with. The lift coefficients are computed with accuracy in contrast to the drag coefficient. The overprediction of drag is expected since the actual airfoil has laminar flow over the forward half. The turbulence models cannot calculate the transition point from laminar to turbulent and consider that the boundary layer is turbulent throughout its length. Therefore, it is necessary to compare the computational results with experimental data of a fully turbulent boundary layer. In order to get more accurate results, the computational domain could be split into two different domains to run mixed laminar and turbulent flow. The contours of pressure and velocity over the airfoil are presented, as well as stagnation, maximum velocity, detachment and reattachment points of the boundary layer are computed. Streamlines and velocity vectors over the airfoil are also presented. Similar simulations are conducted for the S809 airfoil. In order to study the tree-dimensional effects of the flow, simulations over the S809 wing are made. Lift and drag coefficients, stagnation, maximum velocity, detachment and reattachment points of the boundary layer are computed. Moreover, contours of turbulent intensity on the upper surface of the wing and velocity, z-velocity, turbulence intensity and helicity behind the wing are presented. Simulations over the Phase IV blade of NREL are also conducted. The axial velocity behind the rotor, the static pressure and the turbulence intensity contribution on the blade’s surface and the static pressure contours at several blade cross-sections are studied. First of all, the computational study of the two-phase flow over a NACA 0012 airfoil and Liquid Water Content LWC=30 g/m3 is conducted, because there are published experimental data for comparison, in order to validate the CFD developed model. After that, simulations of two-phase flow over the S809 airfoil, S809 wing and Phase IV blade are made. In addition, computational study of the effects of different Liquid Water Content on the aerodynamic performance of NACA 0012 and S809 airfoil at low Reynolds number is made. The results from two-phase flow are compared with the corresponding results from one-phase flow in order to show the effects of two-phase flow at the aerodynamic performance. The influence of two-phase flow on the power coefficient of a wind turbine is also investigated. The results show that the aerodynamic performance degrades when encountering rain, especially lift is degreased and drag is increased. The aerodynamic degradation is caused by the water film formation on the airfoil’s surface and the cratering effects from the raindrops impact. The presence of uneven water film on the airfoil surface roughens the airfoil surface and increases the airfoil thickness. The cratering effects from the water droplets impact on the water film layer increase also the airfoil thickness. Moreover, the droplets splash-back when they impact the airfoil and as a result droplets with smaller diameter and velocity are formed. The acceleration of the splashed-back droplets by the air flowfield acts as a momentum sink, deenergizing the boundary layer and leaving it more susceptible to separation. The aim of the study of the aerodynamic behavior of blades in two-phase flow is the construction of wind turbines with greater efficiency and the production of energy from wind with low cost.

Wind turbine efficiency typically focuses on the shape, orientation, or stiffness of the turbine blades. In this thesis, the focus is instead on using static fixed airfoils in proximity to the wind turbine to control the airflow coming out of the turbine. These control devices have three beneficial effects. (1) They gather air from “higher up” where the air is moving faster on average (and therefore has more kinetic energy in it). (2) They throw the used (and slowed down air) downwards. This means that any turbines in the wind farm behind the lead turbines do not get “stale” air. (3) These control devices provide a large stabilizing lifting force for floating off-shore turbines. In this study, Reynolds-Averaged Navier-Stokes (RANS) simulations of an aligned array of two wind turbines along with various designs of these control devices is studied. The recovery in the velocity at the inlet plane of downstream turbine due to the controlled flow facilitated by these devices is measured with respect to the average streamwise wind velocity at the inlet plane of upstream turbine. A customized numerical solver was written in C++ using Opensource Field Operation And Manipulation (OpenFOAM) to model the turbines as actuator discs with axial induction and to generate an inlet velocity field similar to a turbulent atmospheric boundary layer (ABL). All the design configurations use a streamlined (airfoil shaped) structure, at an angle of attack carefully selected to prevent flow separation depending upon its location around the turbine. For strong wake displacement, the devices are placed in proximity to the upstream wind turbine so as to facilitate a substantial downwash of the faster wind from upper layers of the ABL and at the same time deflect the wake out of the way of the downstream turbine. Also, the pressure coefficient across the upstream turbine augmented with these devices can sometimes become more negative than a bare turbine, which in turn increases the mass flow rate of air passing through it, thereby also increasing the leading turbine’s efficiency slightly.

Small-scale wind turbines were not considered viable in the past due to their poor efficiencies, mainly because of their aerodynamic effects around the irfoil shape. Recently researchers have renewed interest in enhancing the aerodynamic performances of the blades’ designs inspired by the aerodynamic pattern of biological characteristics of insects and marine mammals such as locusts, dragonflies, damselflies, Humpback Whales etc. Bioinspired wing designs have advantages compared to conventional smooth irfoil blades as they can counter the bending forces that the wings experience during flapping. Bio-inspired corrugated airfoil based on dragonfly wing geometries have been reported to perform well compared to conventional airfoil at low Reynolds numbers. Corrugated airfoils reduce flow separation and enhance aerodynamic performance by trapping vortices in the corrugations thus drawing flow towards the airfoil’s surface. This results in the higher lift whilst incurring only marginally higher drag. Such airfoils also have an advantage when it comes to span-wise structural stiffness due to the corrugated cross-sections. Replacing conventional turbine blades by tubercles or corrugated blades could enhance turbine performance by reducing the pressure gradient along the leading edge; however, the aerodynamic effects at the leading edge will depend on the variations of wavelength and amplitude. In this study, two types of computational studies were investigated: Optimising a corrugated airfoil and investigating the aerodynamic effects of a sinusoidal shape at the leading edge of a blade. Previous studies used an idealized geometry based on the dragonfly wing cross-section profile but did not attempt to optimize the geometry. In the present study: a two-dimensional CFD model is constructed using ANSYS Fluent Workbench-Design Explorer to determine the optimal corrugated blade profile for four angles of attack (AOA) from 5° to 20° corresponding to typical AOA of small-scale wind turbine blades. Two modified blades with variations of wavelength and amplitude at the leading edge were studied to investigate the aerodynamic effects. Three-dimensional models were constructed using Qblade software and 3D points were exported to AutoCAD Inventor to generate the CAD model. The governing equations used are continuity and Navier-Stokes equations written in a frame reference rotating with the blade. The CFD package used is ANSYS FLUENT 19.0. The simulation was run under steady-state, using SST-k omega turbulence model. The modifications have improved the aerodynamic performance. The optimised corrugated blade produced a maximum increase of CL and L/D. Both modified blades (1 and 2) had their performances measured separately and compared to that of baseline blade SG6042 (Conventional blade). Modified blade 1 had a lower wavelength and amplitude at the leading edge of 14.3 % and 4 % respectively of the chord. It was noted that the aerodynamic performance decreased by 6%. Modified model 2, on the other hand had a higher wavelength and amplitude at the leading edge. of 40.4 % and 11.9 % respectively of the chord. It was also noted the aerodynamic performance increased by 6%. From the empirical evidence highlighted above, it can be observed that there is a direct correlation between wavelength, amplitude, and aerodynamic performance of the blade.
Electrical and Mining Engineering
M. Tech. (Engineering)

碩士
國立臺灣大學
工程科學及海洋工程學研究所
97
The purpose of the research is to investigate the noises induced by flow over the wind blades. The noise analysis is conducted by the Broadband Noise Source Model and FW-H (Ffowcs Williams and Hawkings) Formula which are based on theory of Lighthill’s acoustic analogy. How the wind velocity, angle of attack as well as the inflow turbulent intensity influence the induced aerodynamic noise is discussed. First of all, the dynamic coefficients and flow field of three airfoils NACA64(3)-618、S809 and S822 were verified, and then the accurate information of turbulence was provided as the source to evaluate the sound energy distribution. Three types of noise models that provided different characteristics of the noise distribution were adopted in this wrok. Firstly Reynolds-averaged Navier-Stokes Equation with the k-e turbulent model was used to predict the turbulent flow field. When the inflow turbulent intensity was increased to 5% and 10%, it causes great changes to the flow field and obviously it is also one of the major facts to the flow induced noise. For aerodynamic noise analysis, Proundman’s BNS model was performed to get the acoustic energy density distribution over the entire calculating domain. Further, Curle’s Formula was adopted to predict the surface acoustic power along the solid boundary. In order to understand the details of flow induced noise one step further, the Large Eddy Simulation approach for the unsteady flow combined with the FW-H equation was used to predict the unsteady sound pressure signal. Then by Fourier Transformation the spectrum of the noise can be calculated and consequently the frequency distribution and the power output are achieved. It might be useful in reducing the flow induced aerodynamic noise. However, LES requires a very fine grid resolution to capture the large scale eddy. At this stage, our current computer resources are extremely difficult to satisfy the computational efforts. Therefore, only the small wind blades were taken as the analysis object in this study. This experience may be useful in large wind blade analysis in the near future.