Ricardo Pereira

Effect of external flow velocity on momentum transfer of dielectric barrier discharge plasma actuators

An experimental study is performed towards identifying cross-talk effects between DBD plasma actuators and external flow. An actuator is positioned in a boundary layer operated in a range of free stream velocities from 0 to 60 m/s, and tested both in counter-flow and co-flow forcing configurations. Electrical measurements are used for estimating the power consumption and the discharge formation is visualized using a CCD camera. The actuators force is measured using a sensitive load cell. Results show the power consumption is constant for different flow velocities and actuator configurations. The plasma light emission is constant for co-flow forcing but shows a trend of increasing intensity with counter-flow forcing for increasing free stream velocities. The measured force is constant for free stream velocities larger than 20 m/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. 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. Satisfactory agreement is found between the prediction of the model and experimental data at different external flow velocities.

Analysis of local frequency response of flow to actuation: Application to the dielectric barrier discharge plasma actuator

The present study provides a methodology to derive the local frequency response of flow under actuation, in terms of the magnitude of actuator induced perturbations. The method is applied to a dielectric barrier discharge (DBD) plasma actuator but can be extended to other kinds of pulsed actuation. The actuator body force term is introduced in the Navier-Stokes equations, from which the flow is locally approximated with a linear-time-invariant system. The proposed semi-phenomenological model includes the effect of both viscosity and external flow velocity, providing a system response in the frequency domain. A validity criterium is additionally devised for the estimation of the threshold frequency below which the developed approach can be applied. Analytical results are compared with experimental data 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 thresh...

How does the presence of a body affect the performance of an actuator disk

The article seeks to unify the treatment of conservative force interactions between axi-symmetric bodies and actuators in inviscid ow. Applications include the study of hub interference, di_user augmented wind turbines and boundary layer ingestion propeller con_gurations. The conservation equations are integrated over in_nitesimal streamtubes to obtain an exact momentum model contemplating the interaction between an actuator and a nearby body. No assumptions on the shape or topology of the body are made besides (axi)symmetry. Laws are derived for the thrust coe_cient, power coe_cient and propulsive e_ciency. The proposed methodology is articulated with previous e_orts and validated against the numerical predictions of a planar vorticity equation solver. Very good agreement is obtained between the analytical and numerical methods

Modeling DBD Plasma Actuators in Integral Boundary Layer Formulation for Application in Panel Methods

A methodology is presented to include the influence of DBD plasma actuators in panel methods used for airfoil design. The influence of the plasma body-force is modelled by performing an asymptotic expansion to the Navier-Stokes equations as to obtain a generalized form of the von Karman integral equations. New closure relations were thus derived, which account for boundary layer development in the presence of DBD actuators. To validate the modelling approach, an experimental study was carried out in which PIV was performed on an airfoil equipped with DBD plasma actuators. The airfoil section was tested at different operational Reynolds number and angles of attack, while balance measurements were also performed to evaluate the lift and drag coefficients. Results show the proposed model captures the magnitude of the variation in IBL parameters brought upon by the DBD actuator. This is verified for different operating conditions, while the model captures the magnitude of the lift coefficient variation (∆Cl). Ultimately this approach may enable the design of airfoils specifically tailored for flow control through DBD employment, potentially decreasing the power required for active flow control.

Active stall control for large offshore horizontal axis wind turbines: A conceptual study considering different actuation methods

The increasing size of Horizontal Axis Wind Turbines and the trend to install wind farms further offshore demand more robust design options. If the pitch system could be eliminated, the availability of Horizontal Axis Wind Turbines should increase. This research investigates the use of active stall control to regulate power production in replacement of the pitch system. A feasibility study is conducted using a blade element momentum code and taking the National Renewable Energy Laboratory 5 MW turbine as baseline case. Considering half of the blade span is equipped with actuators, the required change in the lift coefficient to regulate power was estimated in Cl = 0:7. Three actuation technologies are investigated, namely Boundary Layer Transpiration, Trailing Edge Jets and Dielectric Barrier Discharge 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. Active stall control of Horizontal Axis Wind Turbines appears feasible only if the rotor is re-designed from the start to incorporate active-stall devices.

The effect of external flow velocity on the momentum transfer of DBD plasma actuators

An experimental study is performed towards identifying possible cross-talk effects between DBD actuators and externally imposed flow. The actuator has been positioned in a boundary layer operated in a range of free stream velocities from 0 to 60 m/s, and tested both in the upstream and downstream configuration. Two experimental set-ups were tested. In the first measurement campaign the flow field and the plasma induced momentum has been measured using Particle Image Velocimetry from which the actuator momentum transfer was derived. In the second experimental campaign the DBD actuator was made stronger and the body force was measured directly with a load cell. Electrical measurements were also used for estimating the power consumption and the discharge formation has been visualized using an intensified CCD camera. Results show the measured force is practically constant over the range of considered wind speeds, with the same magnitude and opposite direction for the upstream and downstream configuration. The power consumption measured was also constant for different flow velocities and actuator configurations. This indicates that the external flow velocity does not have a significant impact on the momentum transfer of the actuator, seemingly confirming the commonly used assumption that the actuator’s body force is independent of the external flow.

Improved airfoil polar predictions with data-driven boundary-layer closure relations

The accuracy of airfoil polar predictions is limited by the usage of imperfect turbulence models. Can machine-learning improve this situation? Will airfoil polars teach the effect of turbulence on skin-friction? We try to answer these questions by refining turbulence treatment in the Rfoil code: boundary layer closure relations are learned from airfoil polar data. Two turbulent closure relations, for skin friction and energy shape factor, are parametrized with a class-shape transformation. An experimental database is then used to define code inaccuracy measures that are minimized with an interior point gradient algorithm. Results show that airfoil polars contain exploitable information about turbulent phenomena. Inferred closures agree with direct numerical simulation results of skin friction and the new code predicts drag more accurately. Maximum lift remains under-predicted but Rfoil maintains its robustness and suitability for optimization of wind energy airfoils.

Investigating the Influence of DBD Plasma Actuators on the Skin Friction in Integral Boundary Layer Formulation

Including the influence of DBD plasma actuators in rapid analysis panel methods allows this type of active flow control to be included in airfoil design from the beginning. The present study builds further on a previous modeling effort representing DBD plasma actuators in the integral boundary layer formulation. A correction to the skin friction closure relation is investigated. Order of magnitude analysis reveals the significant component of the variation in skin friction coefficient in plasma actuated flow. A more comprehensive approach is subsequently taken to investigate the influence of DBD plasma actuation on skin friction through designing and conducting an experimental campaign. The spatial distribution of the variation in skin friction is then examined and fitted with a semi-elliptical shape for various boundary layer states. Ultimately the present approach may enable the design of airfoils tailored for DBD plasma flow control, which allows to maximize the potential performance gain this technique has to offer.