Design of Airfoil for downwind wind turbine Rotor

185 Pages·2009·4.27 MB·English

by  Eivind Sæta

Design of Airfoil for downwind wind
turbine Rotor
Eivind Sæta
Master of Science in Energy and Environment
Submission date: June 2009
Supervisor: Per-Åge Krogstad, EPT
Norwegian University of Science and Technology
Department of Energy and Process Engineering

Problem Description
At the institute, there has been a significant amount of work done in developing an airfoil with very
high lift-to-drag ratio. However, it has been developed for relatively low Reynolds numbers, and
has a very dramatic stall. It is desirable to develop this profile further with regards to typical
working conditions for offshore wind turbines. It is also necessary to change the profile stall
characteristics so that it is more gradual, and hopefully to achieve this without significant
reduction in efficiency.
The following questions should be considered in the project:
1 – The student is to study the work done previously at the institute.
2 - The theory of high lift-to-drag profiles is to be studied.
3 – The student will develop a new profile, either based on HOG-profile designed at the the
department or from another geometry, where the main focus should be on high lift-to-drag ratio
and stall characteristics.
4 – The properties of the developed profile should be tested with software such as Xfoil and/or
Fluent.
Assignment given: 15. January 2009
Supervisor: Per-Åge Krogstad, EPT

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Design of airfoil for downwind wind turbine rotor

Eivind Sæta

NTNU 2009

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Summary

This thesis is on the design of an airfoil for a downwind wind turbine rotor with thin
flexible wings, for offshore floating conditions. It has been suggested that such a system
would to be lighter, simpler and allow for the use of more efficient airfoils.
There has been a significant amount of work done at NTNU to develop a “high-lift”
airfoil. These are airfoils with very high lift-to-drag ratios. They operate very efficiently
at their design angle, but tend to not work well over a range of angles and conditions, and
have a sudden and dramatic stall characteristic. In this thesis, it is attempted to pick up
the work done with the high-lift profiles at NTNU in the 1980’s, and develop a new
profile which has performance in the high-lift range, but with a much smoother stall and
more stable characteristics, and to do so for the typical conditions expected for the
suggested turbine.

A fictitious 5 MW version of the suggested turbine was created and analyzed with the
blade element momentum method (BEM). This gave informative results about the
conditions the new airfoil must operate in.

The high-lift technology and the earlier reports from NTNU were studied. Based on this
knowledge and the numerical values from the BEM calculations, a serious of new airfoils
were developed. By using the simulation programs Xfoil and Fluent (CFD), it was
possible to modify and test a large number of airfoils and find the desired qualities.

It was possible to design airfoils that had performance in the high-lift range, while
maintaining stable operation and having a soft stall, and also increase the lift coefficient
to be able to design for lower angles of attack. The profiles created here appear to be
suitable for wind turbines, and provide an impressive increase in performance compared
to traditional airfoils.
Extra effort was put into making airfoils that were unaffected by roughness, air properties
and Reynolds number, as stable performance in varying conditions are necessary for
wind turbine blades. This was done by using adverse pressure gradients to control the
point of transition.

A slow stall was achieved by letting the pressure recovery distribution gradually
approach the local ideal Stratford distribution when moving back over the airfoil. This
caused the flow separate at the back first, and then the separation would grow gradually
forward with increasing angle of attack.
The inclusion of a separation ramp also worked very well together with the high-lift
design, and allowed for an increased lift coefficient and more stable operation during the
region of early stall.
AR – profile

The most successful profile created appears to be the AR profile. It combines a diverged
Stratford distribution with a separation ramp and a pressure spike at the nose to control
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transition. It has a wider range, stalls later and softer, and has a much more stable
performance with varying conditions compared to the original HOG profile from NTNU.
At the design point, the maximum performance is reduced only 5.9 % compared to the
HOG. For higher and lower angles of attack, and increased values of roughness and
turbulence, the AR has an all round higher performance than the HOG. It appears to be
usable for wind turbines, and would increase the maximum airfoil performance by up to
40 % compared to commonly used NACA profiles.

More good profiles were made, with varying thickness, stall and performance. Depending
on the exact local requirements of an application, this report offers several interesting
profiles to choose from. For instance, the D2 profile has round shape and over 16 %
thickness, it has an even softer stall than the conventional wind turbine profiles, and
would increase the maximum airfoil performance by up to ~34%. This profile would also
be usable for upwind turbines.

D2 – profile

It was found that there is a big potential for manipulating the high-lift technology to give
various shapes and performances. The usability of these profiles therefore appears to be
wider than previously assumed.

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Sammendrag

Denne hovedoppgaven omhandler design av airfoil for en nedstrøms vindturbin med
tynne, fleksible vinger. For en flytende offshore vindmølle vil et slikt design kunne
redusere vekt og gjøre systemet enklere, og samtidig tillate mer effektive vingeprofiler.
Ved NTNU er det nedlagt en betydelig mengde arbeid for å designe et ”high-lift”
vingeprofil. Dette er vinger med svært høyt løft-til-motstands-forhold. Disse fungerer
veldig bra rundt designområdet, men er vanligvis ikke veldig bra for andre forhold, og
har en svært dramatisk og plutselig steilekarakteristikk.
I denne oppgaven forsøkes det derfor å gjenoppta arbeidet som ble gjort ved NTNU i
1980-årene, og å designe et nytt profil med ytelse som et ”high-lift”-profil, men som
steiler mye langsommere og er mer stabilt i varierende forhold, og å gjøre dette for de
typiske forhold som er forventet for den foreslåtte turbinen.

En teoretisk 5 MW -versjon av den foreslåtte turbinen ble designet og testet ved hjelp av
bladelementmetoden (BEM). Dette ga informative resultater om hvilke forhold den nye
airfoilen vil møte.

”high-lift”-teorien, samt de tidligere rapportene fra NTNU, ble studert. Basert på dette og
de numeriske verdiene fra BEM-kalkulasjonene, ble flere nye profiler utviklet. Ved å
bruke simuleringsprogrammene Xfoil og Fluent (CFD), var det mulig å modifisere og
teste et stort antall vingeprofiler, for å finne de ønskede kvalitetene.

Det er mulig å designe en airfoil som har ytelse i ”high-lift”-regionen, men som har stabil
operasjon for varierende Reynoldstall og har en myk steilekarakteristikk, og det lar seg
også gjøre å øke løftekoeffisienten for å muliggjøre vingeoptimalisering ved lavere
angrepsvinkler. Profilene som ble lagd i denne oppgaven ser ut til å ha karakteristikker
som gjør at de kan brukes i vindturbiner, samtidig som de vil være en imponerende
forbedring i forhold til de profilene som tradisjonelt brukes i vindturbiner.
Det ble satt ekstra fokus på å designe vinger som i stor grad var upåvirket av varierende
Reynoldstall, ruhet og andre turbulensvariasjoner, siden vinger som skal brukes i
vindturbiner er avhengige av stabil operasjon under varierende forhold. Dette ble gjort
ved å bruke positive trykkgradienter for å kontrollere transisjonspunktet.

Myk steiling ble oppnådd ved å la fordelingen til trykkgjenvinningen gradvis nærme seg
den lokalt optimale Stratford-fordelingen når man beveger seg bakover på vingeprofilet.
Dette gjorde at det oppsto separasjon bakerst på vingen først, som så beveget seg sakte
fremmover ved økende angrepsvinkel. Det fungerte bra å inkludere en separasjonsrampe i
”high-lift”-fordelingen. Dette gjorde at løftekoeffisienten ble økt, og overgangen til
steileområdet ble forbedret.
AR – profile

Det mest vellykkede profilet som ble laget ser ut til å være AR-profilet. Det kombinerer
en divergert Stratfordfordeling med en separasjonsrampe, og en trykkoscillasjon ved
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