steadily increasing over the past decades. In the process, Horizontal Axis Wind Turbines

(HAWT) became the most widespread and largest WE harvesting machines. Nevertheless,

significant challenges still lie ahead of further expansion of HAWT, namely concerning systemrobustness

and cost-of-energy(COE) competitiveness. This dissertation studies aHAWT

design concept termed modern Active Stall Control (ASC). With this concept HAWT power

regulation is achieved using flowcontrol actuators to trim the aerodynamic loads across the

operational envelope. The underpinning idea is that as the aerodynamic loads are trimmed

by flowcontrol actuatorswithout pitching the blades, the pitch system may be mitigated. In

turn, this might lead to decreased failure-rates and down-time, and thus eventually present

a more cost-effective solution than state-of-the art HAWTs. Going specifically into ASC, if

aerodynamic load trimming is performed it is necessary to employ a flow control actuator.

From different flow control actuator types, since the aim is to reduce the maintenance and

operational costs of ASC machines, actuators with few mechanical parts become more interesting.

As such the present research also focuses on the Alternating Current Dielectric

Barrier Discharge (AC-DBD) plasma actuator, owing among other things to its absence of

moving parts, negligible mass and virtually unlimited bandwidth of actuation.

A preliminary study on the feasibility of active stall control to regulate HAWT power

production in replacement of the pitch system is conducted. By taking the National Renewable

Energy Laboratory 5MWturbine as reference, a simple blade element momentum

code is used to assess the required actuation authority. Considering half of the blade span

is equipped with actuators, the required change in the lift coefficient to regulate power is

estimated in ¢Cl Æ 0.7. Concerning actuation technologies, three flow control devices are

investigated, namely Boundary Layer Transpiration, Trailing Edge Jets and Dielectric Barrier

Discharge plasma actuators. Results indicate the authority of the actuators considered

is not sufficient to regulate power, since the change in the lift coefficient is not large enough.

Especially if a pitch-controlledmachine is used as baseline case. Active stall control of Horizontal

AxisWind Turbines appears feasible only if the rotor is re-designed from the start to

incorporate active-stall devices.

Regarding AC-DBD plasma actuators, three specific topics are investigated. The different

studies aim at DBD performance characterization, namely at the influence of external

flow on DBD plasma momentum transfer and on the frequency response of actuator flow

region characteristic of DBD pulse operation. Both these topics are important to bridge the

gap between academic-laboratory employment of DBD and large-size industrial applications.

Finally regarding DBD plasma actuator modeling, a method is developed to describe

plasma actuation effects in integral boundary layer formulation, and coupled to a viscous-inviscid panel code (similar to XFOIL), while an experimental campaign is carried to validate

the predictions. The three DBD plasma studies are further described below.

Addressing cross-talk effects between DBD plasma actuators and external flow, a study

is carried out in which an actuator is positioned in a boundary layer operated in a range of

free stream velocities from 0 to 60m/s, and tested both in counter-flow and co-flow forcing

configurations. Electrical measurements and a CCD camera are used to characterize the

DBD performance at different external flow speeds, while the actuator thrust is measured

using a sensitive load cell. Results show the power consumption is constant for different

flow velocities and actuator configurations, while the plasma light emission is constant for

co-flow forcing but increases with counter-flow forcing for increasing free stream velocities.

The measured force is constant for free stream velocities larger than 20m/s, with same

magnitude and opposite direction for the counter-flow and co-flow configurations. In quiescent

conditions the measured force is smaller due to the change in wall shear force by the

induced wall-jet. In addition to the experimental study, an analytical model is presented to

estimate the influence of external flow on the actuator force. It is based on conservation of

momentum through the ion-neutral collisional process while including the contribution of

the wall shear force. Model results compare well with experimental data at different external

flow velocities, while extrapolation to larger velocities shows variation in actuator thrust

of at least 10% for external speedU Æ 200m/s.

Concerning the response of DBD actuator region flow to pulsed operation, a methodology

is provided to derive the local frequency response of flow under actuation, in terms

of the magnitude of actuator induced velocity perturbations. The method is applied to an

AC- DBD plasma actuator but can be extended to other kinds of pulsed actuation. For the

derivation, the actuator body force termis introduced in the Navier-Stokes equations, from

which the flow is locally approximated with a linear-time-invariant (LTI) system. The proposed

semi-phenomenologicalmodel includes the effect of both viscosity and external flow

velocity, while providing a system response in the frequency domain. Experimental data is

compared with analytical results for a typical DBD plasma actuator operating in quiescent

flow and in a laminar boundary layer. Good agreement is obtained between analytical and

experimental results for cases below the model validity threshold frequency. These results

demonstrate an efficient yet simple approach towards prediction of the response of a convective

flow to pulsed actuation. Future application of the methodology might include actuation

scheduling design and optimization for different flow control scenarios.

The third study specifically addressing DBD plasma actuators presents a methodology

to model the effect of DBD plasma actuators on airfoil performance within the framework

of a viscous-inviscid airfoil analysis code. The approach is valid for incompressible, turbulent

flow applications. The effect of (plasma) body forces in the boundary layer is analyzed

with a generalized form of the von Kármán integral boundary layer equations. The additional

terms appearing in the von Kármán equations give rise to a new closure relation. The

model is implemented in a viscous-inviscid airfoil analysis code and validated by carrying

out an experimental study. PIV measurements are performed on an airfoil equipped with

DBD plasma actuators over a range of Reynolds number and angle of attack combinations.

Balance measurements are also collected to evaluate the lift and drag coefficients. Results

show the proposed model captures the magnitude of the variation in IBL parameters from

DBD actuation. Additionally the magnitude of the lift coefficient variations (¢Cl ) induced

by plasma actuation is reasonably estimated. As such, this approach enables the design of

airfoils specifically tailored for DBD plasma flow control.

Transitioning into ASCrotor design, and building on the previously presented, a methodology

is introduced for designing airfoils suitable to employ actuation in a wind energy environment.

The novel airfoil sections are baptized WAP (Wind Energy Actuated Profiles). A

genetic algorithm based multiobjective airfoil optimizer is formulated by setting two cost

functions, one for wind energy performance and the other representing actuation suitability.

The wind energy cost function considers ’reference’ wind energy airfoils while using a

probabilistic approach to include the effects of turbulence and wind shear. The actuation

suitability cost function is developed for HAWT active stall control, including two different

control strategies designated by ’enhanced’ and ’decreased’ performance. Two different

actuation types are considered, namely boundary layer transpiration and DBD plasma.

Results show that using WAP airfoils provides much higher control efficiency than adding

actuation on reference wind energy airfoils, without detrimental effects in non-actuated

operation. The WAP sections yield an actuator employment efficiency that is 2 to 4 times

larger than obtained with reference wind energy airfoils. Regarding geometry, WAP sections

for decreased performance display an upper surface concave aft-region compared to

typical wind energy ’reference’ airfoils,while retaining common sharp nose and S-tail (characteristic

aft-loading) features. Results show there is much to gain in designing airfoils from

the beginning to include actuation effects, especially compared to employing actuation on

already existing airfoils, which ultimately might pave theway for novelHAWT control strategies.

Finally addressing the complete rotor planform design, an optimization study tailors a

HAWT rotor to ASC operation, in a aero-structural-servo formulation. The study considers

the aerodynamic and structural loads are in static equilibrium, and as such no unsteady effects

are taken into account. The optimization includes planformgeometry design but also

actuation scheduling, rated rotational speed and spanwise laminate skin thickness. Results

show that, compared to variable-pitch turbines, ASC planform displays increased chord at

inboard stations with decreased twist angle towards the tip, resulting in increased AOA. Actuation

is employed to trim the (static) loads across the operational wind speed envelope

and hence reduce load overshoots and associated costs. Comparing with state-of-the-art

pitch machines, the expected COE of the ASC rotor does not indicate a significant decrease,

but appears to be at least competitive with pitch-controlled HAWTs if the pitch system is effectively

mitigated. Additionally, and though not explicitly considered in the present work,

it is foreseen ASC might become interesting if the actuation system allows for further OM

cost reduction via fatigue load-alleviation, since the actuation trimming load system is anyhow

included in an ASC machine.","Active Stall Control; DBD Plasma Actuators","en","doctoral thesis","","978-94-6299-417-1","","","","","","","","","","","","" "uuid:f2fa6bd6-4419-494c-ab14-f0c1ec020270","http://resolver.tudelft.nl/uuid:f2fa6bd6-4419-494c-ab14-f0c1ec020270","Numerical Simulations of NS-DBD Plasma Actuators For Flow Control","Popov, I. (TU Delft Aerodynamics)","Scarano, F. (promotor); Hulshoff, S.J. (copromotor); Delft University of Technology (degree granting institution)","2016","Nanosecond dielectric barrier discharge (NS-DBD) plasma actuators is relatively

new means of flow control. It has several advantages compared to more conventional

means of flow control, such as small size, low weight, fast response time

and controllability. It has been demonstrated to be able to promote transition of

boundary layers and to postpone flow separation on aerodynamic surfaces. This

makes the NS-DBD actuator a promising technology for many applications in

aerospace and wind energy industries.

This thesis presents a study of NS-DBD actuator effects by numerical simulations.

For the purposes of simulations of fluid-dynamic effects of the actuation,

complex plasma dynamic processes are modeled by their thermal effects. This

is possible due to a large separation of scales between plasmadynamic, thermodynamic

and fluid dynamic phenomena. The resulting model is embedded into

the compressible computational fluid dynamics (CFD) simulation using Navier-

Stokes equations. This model is then used in numerical simulations in two model

flows: a laminar boundary and a free shear layer. These model flows are relevant

for promotion of laminar to turbulent boundary layer transition and laminar

leading edge separation elimination.

For the laminar boundary case, the effect of a burst of discharges on a flat

plate boundary layer is studied. The shape, wavelength and propagation speed of

the disturbance introduced into the boundary layer by actuation are compared to

experimental results and found to be in agreement. This indicates that the thermal

model is adequate at predicting phenomenological effects of the actuation in this

case. POD analysis of the CFD flow fields is employed to identify the dominating

modes of the disturbance. The dominating mode is found to be the same as

the least stable mode predicted by linear stability theory. A compression wave,

however, is not found to play an important role, and the burst of pulses is found

to produce the same effects as the long pulse with the same total energy.

For the free shear layer case, the model of the actuator is placed on a centerline

in the beginning of a free shear layer. As a result of constant frequency actuation,

early formation of vortices and shear layer breakdown are observed. Each actuation event produces a convective disturbance in the flow field. Dynamics of the

disturbances are analyzed and growth rates are found to be in agreement with the

predictions of linear stability theory. A parametric study is carried out to study

scalability of the actuator effects to change of actuation frequency and energy per

pulse. A saturation effect with the increase of actuation frequency is observed.

For both studied cases, the effect of NS-DBD actuation is excitation of natural

instability modes, which then evolve according to the stability properties of the

flow.","flow control; plasma; transition; flow separation; plasma actuators; DBD; NS-DBD","en","doctoral thesis","","ISBN 978-94-6186-617-2","","","","","","","","","","","","" "uuid:e172757c-2120-437e-a9c5-3f8e35e764ce","http://resolver.tudelft.nl/uuid:e172757c-2120-437e-a9c5-3f8e35e764ce","Dielectric barrier Discharge Plasma Actuator Characterization and Application","Correale, G.","Scarano, F. (promotor)","2016","An experimental investigation about nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuator is presented in this thesis. This work aimed to answer fundamental questions on the actuation mechanism of this device. In order to do so, parametric studies in a quiescent air as well as laminar bounded of free shear layers were performed. Amplitude and location of the input with respect to the receptivity region as well as frequency of flow actuation were investigated. This work required the implementation of acquisition techniques such as Schlieren, Particle Image Velocimetry (PIV), infrared thermography, back current shunt technique and balancemeasurements. Moreover, tools of analysis were employed such as Linear Stability Theory (LST), Proper Orthogonal Decomposition (POD) and Inverse Heat Transfer Problem(IHTP). Results revealed that the effect of a ns-DBD is that of “enhancing” the development of natural hydrodynamic instabilities of the specific field of motion. Therefore, in case of a laminar boundary layer, the effect of a ns-DBD plasma actuator was to amplify Tollmien–Schlichting waves according to linear stability theory. Such results led to understand the influence of the actuator position on the achievement of a specific flow control task. A ns-DBD is capable of producing several effects: a shock wave, a small body force and a thermal gradient within the discharge volume. Thus, three were the possible causes of flow actuation. The shock wave was found to be too weak to be capable of introducing an appreciable disturbance. As the shock wave, also the momentum injection induced by the body force produced by the pulsed discharge was found to be relatively too small to justify a control authority based on momentum redistribution within the boundary layer, for cases of relatively high freestream velocity. Thus, the thermal gradient induced within the discharge volume by the energy deposition of a high voltage nanosecond discharge is the effect capable of inducing a relatively large disturbance into the field of motion. Nevertheless, a thermal gradient within a gaseous flow induces two effects, it reduces density and increases viscosity. At the moment it is still unclear which of these two effects is more relevant. Once identified the thermal gradient as the main cause of flow control mechanism, a characterization study was performed aimed to identify the properties of a ns-DBD plasma actuator (thermal, electrical and geometrical) important tomaximize the induced thermal gradient within the discharge volume. In general, a higher efficiency is achieved by a strong dielectric material concerning thermal energy deposition. A barrier of a ns-DBD plasma actuator should be as thin as possible. However, the thickness affects also the lifetime of the barrier itself. Nanosecond pulsed DBD plasma actuators have shown to have the capability to delay leading edge separation. However, in the relevant literature, an influence of the actuation frequency on the achieved results is always reported. In order to investigate this frequency effect, a parametric study on a Backward Facing Step was performed. This geometry was selected because it mimics a fixed point laminar separation, the flow sceixnario of interest. Such flow scenario is unstable at high frequencies close to the step and low frequencies downstream the step and it naturally develops a most unstable mode within it. However, when a flow is actuated, its stability changes, so do the most unstable frequencies naturally developed within it. Results showed that the effect of actuation is the redistribution of energy among modes and that the optimal frequency of actuation must be based on the new stability achieved by the flow due to the actuation itself. Moreover, results indicated that the optimal frequency of actuation is not related to the most unstable frequencies naturally present within the base non-actuated flow. A method to quantify the efficiency of ns-DBDs in depositing energy within the discharge volume is proposed. This energy is the one that eventually contributes to the formation of the thermal gradient responsible of the flow control capabilities shown by these devices. Such method is based on simultaneous implementation of infrared thermography and back-current shunt techniques. Results showed that the overall efficiency of a ns-DBD plasma actuator is inversely proportional to the thickness of the dielectric barrier. Last part of this thesis is concerned with a demonstrative application of a ns-DBD plasma actuator on a two element airfoil, at Reynolds numbers ranging between 0.2·106 and 2 ·106. Results demonstrated its capability to delay separation, increase lift and reduce drag in the post stall regime. Moreover, the plasma actuator showed the capability to eliminate both a laminar bubble separation for small angles of attack and the hysteresis behaviour of the selected airfoil. In conclusion, this work shed some light on the flow actuation mechanism of a ns- DBD plasma actuator and deepened its basic knowledge.","plasma actuator; flow control","en","doctoral thesis","","","","","","","","","Aerospace Engineering","Aerodynimics","","","","" "uuid:953d7b08-2464-4f7f-905d-06d44c65ae4e","http://resolver.tudelft.nl/uuid:953d7b08-2464-4f7f-905d-06d44c65ae4e","Experimental method to quantify the efficiency of the first two operational stages of nanosecond dielectric barrier discharge plasma actuators","Correale, G. (TU Delft Aerodynamics); Avallone, F. (TU Delft Wind Energy); Yu Starikovskiy, A. (Princeton)","","2016","A method to quantify the efficiency of the first two operational stages of a nanosecond dielectric barrier discharge (ns-DBD) plasma actuator is proposed. The method is based on the independent measurements of the energy of electrical pulses and the useful part of the energy which heats up the gas in the discharge region. Energy input is calculated via a back current shunt technique as the difference between the energy given and the energy reflected back. The ratio of the difference of the latter two quantities and the energy input gives the electrical efficiency (η E) of a ns-DBD. The extent of the energy deposited is estimated via Schlieren visualizations and infrared thermography measurements. Then, the ideal power flux obtained if all the inputted energy was converted into heat is calculated. Transient surface temperature was measured via infrared thermography and used to solve a 1D inverse heat transfer problem in a direction normal to the surface. It gives as output the actual power flux. The estimated ratio between the two power fluxes represents a quantification of the mechanical fluid efficiency (η FM) of a ns-DBD plasma actuator. Results show an inverse proportionality between η E, and η FM, and the thickness of the barrier. The efficiency of the first two operational stages of a ns-DBD is further defined as η = η E centerdot η FM.","ns-DBD; efficiency; plasma actuator","en","journal article","","","","","","","","2017-12-01","","","Aerodynamics","","","" "uuid:c8905aa0-ba4b-45ca-bc07-7dc2a93fe3a9","http://resolver.tudelft.nl/uuid:c8905aa0-ba4b-45ca-bc07-7dc2a93fe3a9","Method to quantify the electrical efficiency of a ns-DBD plasma actuator","Avallone, F.; Correale, G.","","2015","An experimental investigation was conducted on the effective efficiency of a nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuator. Back-current shunt technique and infrared thermography measurements were carried out at the same time on an upside-down flat plate in a quiescent environment. The only investigated parameter was thickness of the dielectric barrier. Voltage amplitude and frequency of discharge were kept constant at maximum values allowable by the used power generator, i.e. 10k Volt and 10k Hz respectively. The selected material for the dielectric barrier was Makrolon(r) because of its well know thermal and dielectric propriety. Energy input was calculated as difference between the pulse voltage given and the one reflected back into the system via back current shunt technique. Ideal power flux obtained if all the input energy was converted to heat is then calculated. The actual power flux was obtained by solving an IHTP (Inverse Heat Transfer Problem) once the transient temperature distribution on the surface of the dielectric barrier was measured by means of IR thermography. The ratio between these two values represents a quantification of electrical efficiency of an ns-DBD plasma actuator. Results prove the high performances of ns-DBD plasma actuator in the respect of energy deposition and that the efficiency depends on the thickness of the barrier.","electrical efficiency; ns-DBD plasma actuator; IR thermography","en","conference paper","","","","","","","","","Aerospace Engineering","Aerodynamics, Wind Energy & Propulsion","","","","" "uuid:2a3631d9-61a2-4167-b510-efa563f991b0","http://resolver.tudelft.nl/uuid:2a3631d9-61a2-4167-b510-efa563f991b0","Aeroacoustic Resonance of Slender Cavities: An experimental and numerical investigation","De Jong, A.T.","Bijl, H. (promotor)","2012","","cavity resonance; aeroacoustics; plasma actuator; trailing edge noise; flow control; Lattice Boltzmann Method; Particle Image Velocimetry","en","doctoral thesis","","","","","","","","","Aerospace Engineering","Aerodynamics","","","","" "uuid:cdaadcdc-f5d9-4459-bd5d-ad6a58d05af8","http://resolver.tudelft.nl/uuid:cdaadcdc-f5d9-4459-bd5d-ad6a58d05af8","Plasmas for Transition Delay","Kotsonis, M.; Boon, P.; Veldhuis, L.","","2009","This paper describes the experimental investigation of the properties of Dielectric Barrier Discharge (DBD) actuators aimed at transition delay techniques. A wide range of geometrical configurations are tested as well as several electrical operational conditions. For the majority of the measurements statistical data for the induced flow field are obtained and for a limited selection of actuators, high sample-rate time resolved measurements are also conducted. All measurements are made in still flow in order to eliminate free-stream effects on the induced velocities. Results show the formation of a thin near-wall jet which could be used as a flow control device.","plasma actuators; experimental study; active wave cancellation; transition delay","en","conference paper","","","","","","","","","Aerospace Engineering","Aerodynamics & Wind Energy","","","",""