An Experimental-Numerical Investigation of the Wake Structure of a Hovering Rotor by PIV Combined with a Γ2 Vortex Detection Criterion

The rotor wake aerodynamic characterization is a fundamental aspect for the development and optimization of future rotary-wing aircraft. The paper is aimed at experimentally and numerically characterizing the blade tip vortices of a small-scale four-bladed isolated rotor in hover conditions. The investigation of the vortex decay process during the downstream convection of the wake is addressed. Two-component PIV measurements were carried out below the rotor disk down to a distance of one rotor radius. The numerical simulations were aimed at assessing the modelling capabilities and the accuracy of a free-wake Boundary Element Methodology (BEM). The experimental and numerical results were investigated by the G2 criterion to detect the vortex location. The rotor wake mean velocity field and the instantaneous vortex characteristics were investigated. The experimental/numerical comparisons show a reasonable agreement in the estimation of the mean velocity inside the rotor wake, whereas the BEM predictions underestimate the diffusion effects. The numerical simulations provide a clear picture of the filament vortex trajectory interested in complex interactions starting at about a distance of z/R = 0.5. The time evolution of the tip vortices was investigated in terms of net circulation and swirl velocity. The PIV tip vortex characteristics show a linear mild decay up to the region interested by vortex pairing and coalescence, where a sudden decrease, characterised by a large data scattering, occurs. The numerical modelling predicts a hyperbolic decay of the swirl velocity down to z/R = 0.4 followed by an almost constant decay. Instead, the calculated net circulation shows a gradual decrease throughout the whole wake development. The comparisons show discrepancies in the region immediately downstream the rotor disk but significant similarities beyond z/R = 0.5

An assessment of vortex detection criteria for 2C-2D PIV data

The aim of the article is to propose a robust and reliable engineering method for identifying and characterizing vortical structures within a flow field measured with a classic twocomponent PIV measurement system. Some of the most popular vortex-detection criteria are briefly presented for comparison purposes. Many of these fail if spurious vectors are present within the flow field due to poor PIV image quality. The proposed method was tested both on synthetic images of ideal vortices, having different spatial resolutions and different noise levels in order to perform a parametric assessment, and on real PIV images of a four-bladed rotor wake

The Blind Test Activity of TILTAERO Project for the Numerical Aerodynamic Investigation of a Tiltrotor

The present paper illustrates an aerodynamic analysis performed by CIRA in the framework of the European research project TILTAERO (TILTrotor interactional AEROdynamics) by the application of a BEM methodology and with the aim to investigate the problem of the interactional aerodynamics of a tilt rotor configuration. Due to the lack of experimental database on tilt rotor configurations the one produced during the European research project PROPWING and referring to a propeller-nacelle-wing configuration has been chosen for the numerical-experimental comparison activity. The paper provides a synthetic description of the TILTAERO project and illustrates the main aspects of the PROPWING configuration and the relative experimental database. The main features of the numerical methodology applied is also described. The comparative study is performed by illustrating the numerical and experimental results in terms of steady, time-averaged and unsteady pressure distribution as well as steady and time-averaged normal forces acting on the wing. The results obtained indicate, in general, a satisfactory agreement of the methodology applied with the experiment

Code-to-Code Comparison for the Blind-Test Activity of the Tiltaero Project

The present paper illustrates the results obtained so far by CIRA, DLR, NTUA and ONERA during a blind-test activity performed in the framework of WP1 of the European research project TILTAERO (TILTrotor interactional AEROdynamics). The aim of the work package is to evaluate the capabilities of the prediction tools applied for the analysis of the interactional aerodynamics of a tilt rotor configuration. Two BEM methodologies, differing with respect to the wake modelling, an unsteady RANS and a quasi-steady RANS CFD methodologies have been applied by the authors for this purpose. The paper provides a synthetic description of the TILTAERO project and illustrates the main aspects of the blind-test activity. The main features of the numerical methodologies applied are also described. The analysis is performed by illustrating the numerical results obtained for three different test conditions of the TILTAERO wind tunnel mock-up in terms of quasi-steady, time-averaged and unsteady pressure distributions as well as quasi-steady, time-averaged and unsteady normal forces acting on the configuration. These results, obtained prior to the production and release of the relevant experimental data base, have provided useful indications of the complex interactional phenomenology