Daniele Ragni

Kinetic energy entrainment in wind turbine and actuator disc wakes: an experimental analysis

The present experimental study focuses on the comparison between the wake of a two-bladed wind turbine and the one of an actuator disk. The flow field at the middle plane of the wake is measured with a stereoscopic particle image velocimetry setup, in the low-speed Open Jet Facility wind tunnel of the Delft University of Technology. The wind turbine wake is characterized by the complex dynamics of the tip vortex development and breakdown. Analysis of the flow statistics show anisotropic turbulent fluctuations in the turbine wake, with stronger components in the radial direction. The wake of the actuator disc is instead characterized by isotropic random fluctuations. The mixing process in the shear layer is further analysed in terms of flux of mean flow kinetic energy, to show the main differences between the kinetic energy entrainment in the actuator and the turbine wake. This project is intended to provide the basis for understanding the origin of the limitations of the current wake models based on the actuator disc assumption.

PIV Investigation of the Flow Past Solid and Slitted Sawtooth Serrated Trailing Edges

Turbulent boundary layer trailing edge noise (TBL-TE) is an important source of aeroacoustic noise in many aerodynamic applications. Along with conventional solid sawtooth serrated geometries, new alternative slitted sawtooth design solutions seem to further improve the reduction of TBL-TE noise. In this study, particle image velocimetry (PIV) is used to investigate the flow past solid and slitted sawtooth serrations, installed at the trailing edge of a NACA 0018 airfoil at several angles of attack. Acoustic measurements obtained from a microphone array, in combination with a conventional beamforming algorithm, are employed to investigate the noise reduction obtained from the new serration designs. At zero angle of attack, the PIV measurements show similar mean flow and turbulence statistics between serrated and slitted serrations, despite notable differences in noise reduction. At nonzero incidence, the slitted serrations show less perturbations in the mean and turbulence flow fields with respect to the solid serrations. Despite this, the noise reduction trend seen between the two serration geometries remains similar.

Flow Field Around a Serrated Trailing Edge at Incidence

The flow close to the surface and in the wake of sawtooth trailing edge serrations is investigated using particle image velocimetry and Reynolds-averaged Navier-Stokes simulations. This is done in order to establish changes it undergoes when the serrations are placed at different degrees of incidence, which is prescribed through variations in the serration flap angle, and the angle of attack of the airfoil on which the serrations are installed. What has been found is that the flow close to the surface is particularly sensitive to the flap angle and shows various degrees of deflection that develop into streamwise vortices in the wake. Because of this level of complexity in the flow, the degree of incidence should be considered an important factor when trying to predict the sound emitted by serrated trailing edges using the established analytical solution.

Drag coefficient accuracy improvement by means of particle image velocimetry for a transonic NACA0012 airfoil

A method to improve the reliability of the drag coefficient computation by means of particle image velocimetry measurements is made using experimental data acquired on a NACA0012 airfoil tested in the transonic regime, using the combination of a variable pulse separation with a new high-order Poisson spectral pressure reconstruction algorithm.

Tomographic-PIV investigation of the flow over serrated trailing-edges

The three-dimensional characterization of the flow field over the suction side of a NACA 0018 airfoil with and without trailing-edge serrations is performed by means of time-resolved tomographic particle image velocimetry. The acoustic reduction efficiency of the investigated serrations is assessed by means of beamforming measurements. The mean flow characterization shows that at the location of the straight trailing-edge the flow is weakly perturbed by the presence of the serrations. The three-dimensional flow field is characterized by counter-rotating streamwise oriented vortices developing from the root of the serrations. They generate a funneling motion that alters the characteristics of the turbulent flow approaching the edge of the serrations.

Aerodynamic and Aeroacoustic Effects of Pylon Trailing Edge Blowing on Pusher Propeller Installation

The aerodynamic and aeroacoustic effects of pylon trailing edge blowing on the propulsive performance and noise emissions of a propeller installed in a pusher configuration were studied in a wind tunnel. A propeller model and a pylon equipped with a trailing edge blowing system were installed in the large low-speed facility of the German-Dutch wind-tunnels (DNW-LLF). Particle image velocimetry measurements of the flow field downstream of the pylon confirmed a wake re-energization obtained through blowing, with a momentum deficit recovery of 80% compared to the unblown case. For the symmetric inflow conditions considered, the effect of pylon installation on the propulsive performance was found small. Increases in thrust and torque of 1% up to 6% were measured at high and low thrust settings, which was comparable to the measurement variability. Acoustic data obtained using out-of-flow microphones confirmed the strong interaction effects resulting from the installation of the upstream pylon, with an increase in noise levels due to the presence of the pylon of up to 12 dB at a medium propeller thrust setting. The application of pylon trailing edge blowing successfully eliminated the installation effects, resulting in noise levels equal to those of the isolated propeller over the entire axial directivity range. At higher thrust settings the change in blade angle of attack due to the pylon wake impingement is smaller, and the steady blade loads are larger compared to the unsteady loads experienced during the wake passage. Consequently, in this operating regime the propeller noise emissions were dominated by steady sources for all but the most upstream observer positions.

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...

Experimental comparison of a wind-turbine and of an actuator-disc near wake

The actuator disc (AD) model is commonly used to simplify the simulation of horizontal-axis wind-turbine aerodynamics. The limitations of this approach in reproducing the wake losses in wind farm simulations have been proven by a previous research. The present study is aimed at providing an experimental analysis of the near-wake turbulent flow of a wind turbine (WT) and a porous disc, emulating the actuator disc numerical model. The general purpose is to highlight the similarities and to quantify the differences of the two models in the near-wake region, characterised by the largest discrepancies. The velocity fields in the wake of a wind turbine model and a porous disc (emulation of the actuator disc numerical model) have been measured in a wind tunnel using stereo particle image velocimetry. The study has been conducted at low turbulence intensity in order to separate the problems of the flow mixing caused by the external turbulence and the one caused by the turbulence induced directly by the AD or the WT presence. The analysis, as such, showed the intrinsic differences and similarities between the flows in the two wakes, solely due to the wake-induced flow, with no influence of external flow fluctuations. The data analysis provided the time-average three-component velocity and turbulence intensity fields, pressure fields, rotor and disc loading, vorticity fields, stagnation enthalpy distribution, and mean-flow kinetic-energy fluxes in the shear layer at the border of the wake. The properties have been compared in the wakes of the two models. Even in the absence of turbulence, the results show a good match in the thrust and energy coefficient, velocity, pressure, and enthalpy fields between wind turbine and actuator disc. However, the results show a different turbulence intensity and turbulent mixing. The results suggest the possibility to extend the use of the actuator disc model in numerical simulation until the very near wake, provided that the turbulent mixing is correctly represented.

Performance Analysis of Wake and Boundary-Layer Ingestion for Aircraft Design

This paper presents conceptual studies to evaluate the performance of the propulsor and its associated vehicle in the configurations of wake ingestion and boundary-layer ingestion. A power conversion analysis uses the power balance method to elaborate the power-saving mechanism of wake ingestion, showing that the Froude’s propulsive efficiency as a figure of merit should be separated from the power conversion efficiency in these configurations. The body/propulsor interaction occurring in the boundary-layer ingestion configuration is qualitatively analyzed to clarify its influence on the performance of the integrated vehicle. The results suggest that the minimization of power consumption should be used as a design criterion for aircraft using boundary-layer ingestion.

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.