wind turbine aerodynamics in yaw

289 Pages·2011·14.16 MB·English

wind turbine
aerodynamics in yaw
unravelling the measured rotor wake

wind turbine
aerodynamics in yaw
unravelling the measured rotor wake
PROEFSCHRIFT
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magni(cid:12)cus Prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen
op woensdag, 21 september 2011 om 12.30 uur
door
Wouter Haans
ingenieur luchtvaart en ruimtevaart
geboren te Haaren.
Dit proefschrift is goedgekeurd door de promotoren:
Prof. dr. ir. G.A.M. van Kuik
Prof. dr. G.J.W van Bussel
Samenstelling promotiecommissie:
Rector Magnificus voorzitter
Prof. dr. ir. G.A.M. van Kuik Technische Universiteit Delft, promotor
Prof. dr. G.J.W van Bussel Technische Universiteit Delft, promotor
Prof. dr. ir. drs. H. Bijl Technische Universiteit Delft
Prof. dr. ir. H. Hoeijmakers Universiteit Twente
Prof. C.P. van Dam, B.S., M.S., D.Engr University of California, Davis
R. Mikkelsen, MSc. PhD. Technical University of Denmark
Ir. G.J. Schepers Energy Research Centre of The Nether-
lands ECN
Printed by W¨ohrmann Print Service, Zutphen, The Netherlands
Cover design by Hub`ert Hecker, Cool Pixels Media
Copyright ⃝c 2011 by W. Haans
ISBN 978-90-8570-848-3
All rights reserved. No part of the material protected by this copyright
notice may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying, recording or by any information
storage and retrieval system, without the prior permission of the author.
Typeset by the author with the LATEX Documentation System.
Authoremail: wouter [email protected]
To Irene, for her patience, support and love

Summary
Wind energy currently provides a modest contribution to the global energy
mix. Growth of the wind energy piece is driven by its Cost of Energy CoE,
relative to other means of producing energy. Fundamental research and tech-
nologydevelopmenthavebeendemonstratedtosubstantiallyreduceCoEfrom
wind. Improved technology is still anticipated to lower CoE, one of the keys
being better rotor aerodynamics modeling.
The Blade Element Momentum BEM model couples an airfoil model to
steady,one-dimensionalinviscidactuatordisctheory. Empiricalmodelsfixthe
code when conditions violate original assumptions; with the rotor misaligned
to the flow, i.e. in yaw, actuator disc theory is e.g. replaced with the Glauert
momentumequation. ItslowcomputationaldemandmakesBEMcodesbetter
suited then more advanced aerodynamic methods to integrate into aero-servo-
elastic models. Industry thus uses BEM codes to compute the loads needed
for component design.
Wind tunnel tests on model rotors illustrate the substantial modeling un-
certainty of BEM codes. Whilst even axial flow conditions are nontrivial to
model, uncertainties are particularly large in yaw. Navier-Stokes models tend
to be more accurate, but their prohibitive computational demand renders
Navier-Stokesbasedaero-servo-elastictoolsimpracticalinanindustrysetting.
BEM modeling uncertainty impacts the required structural margins of the
turbine and hence CoE. To reduce modeling uncertainty, wind tunnel tests
on model rotors study fundamental rotor aerodynamic phenomena. In light
hereof, this thesis addresses experimental rotor aerodynamics in yaw, aiming:
• to quantify the effects of experimentally observed yaw misalignment on
rotor inflow and blade loads
• to identify the aerodynamic mechanisms governing the effects of yaw
misalignment on rotor inflow and blade loads
• to assess the validity of semi-empirical submodels that are used to patch
BEM theory in yaw; unsteady airfoil models, the Glauert momentum
equation, induction non-uniformity corrections
i
AnexperimentalcampaignisruninanopenjetwindtunnelattheTUDelft
on a 1.2m diameter model rotor with two nontapered, twisted blades. A suite
of measurements is performed in yaw and the reference axial flow state;
• rotor thrust recordings with strain gauges in the blade root
• quantitative flow visualization in the wake with smoke to measure tip
vortex paths up to about 1R downstream, with R the rotor radius
t t
• flow visualization on the blades with tufts to identify unsteady flow sep-
aration across the span in yaw
• three-dimensional flow velocity measurements at three near-wake planes
with single-sensor hot-film Constant Temperature Anemometry CTA
All measurements are unsteady and are reduced to Phase Locked Average
PLA data. Furthermore, a detailed measurement uncertainty analysis is pre-
sented for each experimental technique and wind tunnel wall corrections are
methodically quantified; both analyzes yield satisfactory results.
Traditional data reduction for single-sensor CTA PLA data only solves for
the magnitudes of the three PLA flow velocity components; the experimenter
must explicitly specify the direction. Especially in yaw, this would not be a
trivial task. A novel CTA data reduction technique is therefore proposed that
implicitly derives both magnitude and direction of the three-dimensional PLA
flowvelocity. ItscapabilitiesareconfirmedinacomparisonwithNavier-Stokes
wake modeling , making it the preferred data reduction technique in yaw.
First, axial flow is studied, as this forms the reference for yaw. Measured
rotor thrust coefficient C , tip vortex paths and near-wake flow velocity gen-
T
erally comply with published experiments and theory. Recorded C matches
T
that found from the one-dimensional inviscid actuator disc model, the latter
using rotor inflow that is linearly extrapolated from the near-wake measure-
ments. This model forms the momentum part of BEM in axial flow.
In yaw, measured flow features are typical for misalignment. Tip vortex
paths demonstrate wake skewness, the near-wake flow velocity is distributed
asymmetrically across the measurement planes and C reduces with increased
T
yaw angle. To the author’s knowledge, this experiment is the first to record
PLA three-dimensional flow velocity in the skewed near-wake and the EU-
funded MEXICO project is the only alternative source.
Quantification of the experimental inflow at the rotor plane is pivotal to
achieve the objectives of this thesis. The measurements clearly demonstrate
thenonlineardownwindconvectionoftheskewedwake. Linearlyextrapolating
the measured near-wake velocity to derive the inflow at the rotor plane would
thus be inappropriate. A further challenge towards meeting the objectives is
the lack of measured sectional loads, due to model restrictions.
ii
Both inflow and loads are addressed through the development of a physics
basedmeasurementanalysistool. Thisinversevortexwakemodelreconstructs
thewakevortexgeometryfrommeasurements, usesconservationofcirculation
and the Biot-Savart law to relate measured induced velocity with unknown
bound circulation and solves the system of equations for the latter. Airfoil
tables are not needed, loads are estimated instead using the Kutta-Joukowsky
law, where required inflow and bound circulation are both derived from the
inverse vortex wake model. Earlier TUDelft studies lay the foundation of
presentmodel,wherethemostnotableimprovementistheextensionfromaxial
into yawed flow. Model verification and validation are completed successfully.
The thesis aims to base BEM model validation (objective 3) on the ex-
perimental findings of the phenomena governing yawed rotor aerodynamics
(objective 2). The analyzes with the inverse vortex wake model are thus per-
formed in the context of BEM theory.
The analyzes focus on three items; unsteady blade aerodynamics and re-
sultingloads,therelationbetweenC andinducedaxialflowattherotorplane
T
and the mechanisms driving the unsteady inflow at the blade. From the BEM
perspective, this translates to validating unsteady airfoil models, the Glauert
momentum model and induction factor non-uniformity corrections. The ax-
ial and both yawed flow cases have marginal stall, thus avoiding the added
complexity that stall would introduce to the analysis of inflow and loads.
A testimony to the unsteadiness of blade aerodynamics is the observed lift
hysteresis, even for these non-stalled conditions. Actuator line modeling with
either the Øye or Beddoes-Leishman dynamic stall model does not reproduce
the extent of the hysteresis. Differences are attributed to measurement uncer-
tainty and to boundary layer dynamics typical for the chord Reynolds number
Re of the experiments, Reϵ105, that the dynamic stall codes ignore.
Sectional c derived from experimental blade load estimates is compared
t
to that found from the Glauert momentum equation, using estimated annular
averaged induced axial velocity in the rotor plane. Reasonable agreement is
found across the blade span, for all cases including the most extreme with
◦
45 yaw angle. Based hereupon, BEM codes do not require an engineering
correction to the Glauert momentum model.
The inverse vortex wake model quantifies the nonuniformity in the inflow
at the rotor plane in yaw. Furthermore, this study is the first to decompose
the experimental induced velocity into the constituting components from tip
and root vortices and the vortex sheet. The experiments clearly demonstrate,
next to the strong effects from tip vortices, the importance of the root vortices
on the induced axial velocity locally in the inboard, downwind region.
In summary, the thesis contributes to the reduction of BEM modeling
uncertainty through the experimental analysis of yawed rotor aerodynamics.
The reduced uncertainty could lead to a reduced CoE for wind energy.
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