Francesco Avallone

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a positive angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relate...

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.

Computational Study Of Diffuser Augmented Wind Turbine Using Actuator Disc Force Method

In this paper, a computational approach, based on the solution of Reynolds-averaged-Navier–Stokes (RANS) equations, to describe the flow within and around a diffuser augmented wind turbine (DAWT) is reported. In order to reduce the computational cost, the turbine is modeled as an actuator disc (AD) that imposes a resistance to the passage of the flow. The effect of the AD is modeled applying two body forces, upstream and downstream of the AD, such that they impose a desired pressure jump. Comparison with experiments carried out in similar conditions shows a good agreement suggesting that the adopted methodology is able to carefully reproduce real flow features.

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.

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

Numerical analysis of noise reduction mechanisms of serrated trailing edges under zero lift condition

The relation between the far-field noise and the flow field, in presence of serrated and combed-serrated trailing edge, is studied to explain the associated noise reduction mechanisms. Both serration geometries are retrofitted to a NACA 0018 wing. Computations are carried out by solving the fully explicit, transient, compressible Lattice Boltzmann equation , while the acoustic far-field is obtained by means of the Ffowcs-Williams and Hawking integral solution. A link between the far-field noise and the relevant flow parameters that contribute to noise generation is proposed. It is confirmed that the intensity of the surface pressure fluctuations varies in the streamwise direction, and that most of the low-frequency noise is generated at the root of the serrations. It is concluded that the additional noise reduction achieved by the use of combs is due to the mitigation of the outward flow motion at the root. Furthermore, it is shown that the edge-oriented correlation length and the convection velocity of the surface pressure fluctuations are the two statistical flow parameters that influence both the intensity and the frequency range of noise reduction. It is found that a larger edge-oriented correlation length contributes to noise reduction, by generating destructive interference between the pressure waves scattered along the slanted edge.

Concave serrations on broadband trailing edge noise reduction

The far-field noise and flow field of a novel curved trailing-edge serration (i.e. iron-shaped) are investigated. Spectra of the far-field broadband noise, directivity plots and the flow-field over the iron-shaped serration are obtained from numerical computations performed using a compressible Lattice-Boltzmann solver. The new design is compared to a conventional trailing-edge serration with a triangular geometry retrofitted to a NACA 0018 airfoil at zero degree angle of attack. The iron-shaped geometry is found to reduce far-field broadband noise of approximately 2 dB more than the conventional sawtooth serration for chord-based Strouhal numbers Stc < 15. It is found that the larger noise reduction achieved by the iron-shaped trailing-edge serration is due to the mitigation of the scattered noise at the root, effect obtained by mitigating the interaction between the two sides of the serration, by delaying toward the tip both the outward and the downward flow motions present at the root.

Tractor Propeller-Pylon Interaction, Part II: Mitigation of Unsteady Pylon Loading by Application of Leading-Edge Porosity

Citation (APA) Della Corte, B., Sinnige, T., de Vries, R., Avallone, F., Ragni, D., Eitelberg, G., & Veldhuis, L. (2017). Tractor Propeller-Pylon Interaction, Part II: Mitigation of Unsteady Pylon Loading by Application of LeadingEdge Porosity. In 55th AIAA Aerospace Sciences Meeting: Grapevine, Texas [AIAA 2017-1176] American Institute of Aeronautics and Astronautics Inc. (AIAA). https://doi.org/10.2514/6.2017-1176

Experimental Study Of Flow Field Of An Aerofoil Shaped Diffuser With A Porous Screen Simulating The Rotor

This study presents an experimental investigation on a diffuser augmented wind turbine (DAWT). A screen mesh is used to simulate the energy extraction mechanisms of a wind turbine in experiment. Different screen porosities corresponding to different turbine loading coefficients are tested. Measurements of the axial force and of the velocity distribution in radial direction are reported. The general purpose is to highlight the dependency between the diffuser and the screen, and to compare the radial velocity distributions in the diffuser between unloaded and loaded conditions. It is shown that the thrust on an unshrouded screen is lower than on a shrouded screen, under the same inflow condition. Moreover, the thrust on the diffuser largely depends on the screen loading. For the present configuration, the thrust on the screen with high loading coefficient contributes for more than 70% of the total thrust on the DAWT. Smoke visualizations and radial velocity profiles reveal that the high loading screen induces flow separation on the outer surface of the diffuser, justifying the results of the thrust measurements. It is also inferred that the flow separation leads to loss of thrust and has a great effect on the total pressure drag. It should be emphasized that the experimental results indicate that the flow field around the diffuser is strongly affected by the choice of screen porosity, that is, turbine loading. And that, the thrust coefficient of the diffuser does not show a linear dependence on the thrust coefficient of the screen. The axial momentum theory, therefore, is not a solid predictor for DAWT performance with high loaded screens.

Infrared thermography of transition due to isolated roughness elements in hypersonic flows

Boundary layer transition in high-speed flows is a phenomenon that despite extensive research over the years is still extremely hard to predict. The presence of protrusions or gaps can lead to an accelerated laminar-to-turbulent transition enhancing the thermal loads and the skin friction coefficient. In the current investigation, inverse heat transfer measurements using infrared thermography are performed on the flow past different roughness geometries in the form of cylinders and diamond at free stream Mach number equal to 7.5, h/δ ranging between 0.5 and 0.9 (where h is the roughness height and δ is the boundary layer thickness), and Reθ ranging between 1305 and 2450. The roughness elements are positioned on a 5° ramp placed at zero angle of attack. The measurements indicate that the roughness geometry influences the transitional pattern while the frontal area influences both the transition location and the maximum value of the Stanton number along the centreline. Moreover, there is a strong connection...