Eric Roosenboom

Advanced Experimental and Numerical Validation and Analysis of Propeller Slipstream Flows

A comparison of velocity(-derived) quantities is made in the slipstream of a turboprop equipped transport aircraft between phase-locked experimental Particle Image Velocimetry (PIV) data and unsteady Reynolds Averaged Navier-Stokes (uRANS) calculations. Velocity results indicate a high level of agreement between the numerical and experimental nresults. In addition to velocity vector data, also derived quantities, such as vorticity, allow a verification and validation of the modeling in CFD. Although a good agreement is nachieved, future needs for the comparison and validation of computational (CFD) and experimental (PIV) data are highlighted.

Static and Dynamic SACCON PIV Tests, Part II: Aft Flow Field

The flow above delta wing shaped UCAV (Unmanned Combat Air Vehicle) configurations is often ndominated by strong vortices, especially at moderate and high angle of attack. On delta wings with nsharp leading edges, the flow separates at the leading edges already at small angles of attack. The shear nlayer rolls up and two vortices are formed on each side of the wing starting close to the wing apex and nincreasing in size towards the wing aft. The vortices produce strong suction peaks on the upper wing nsurface, so that additional lift is generated which is of benefit by performing low speed manoeuvres like ntake-off and landing. UCAV configurations often consist of rounded leading edges to improve their nperformance under cruise conditions. In this case the flow topology becomes more complicated because nthe flow separations at the leading edges are delayed to higher angles of attack and depend strongly on the nReynolds and Mach number. In regions of attached flow around blunt leading edges, other flow neffects may produce additional vortices as observed on the VFE2 delta wing where vortices develop ninboard the leading edge vortices caused by a kind of instability within the boundary layer. The onset nof the leading edge vortices, their interactions and accompanying phenomena like vortex breakdown can ncause adverse loading effects. To avoid such non-linear aerodynamic characteristics detailed flow ninvestigations are of great importance during the design chain by improving the understanding of the naerodynamics. This all the more so if the dynamic behaviour of the vehicle has to be considered as the nflow effects become even further complicated. nWithin the scope of the Task Group of AVT-161 (Chapter 1), Stereoscopic Particle Image nVelocimetry (PIV) has been applied for flow field investigations on a SACCON (Stability And Control nCONfiguration) wind tunnel model.

Investigation of the Propeller Slipstream with Particle Image Velocimetry

The flow around an 8-bladed propeller is analyzed using particle image velocimetry in the Airbus low-speed wind tunnel facility in Bremen, Germany. The propeller is mounted on a half-model equipped with two propellers with opposing rotation directions, where only the outboard propeller is investigated. For two cases with no thrust (C T = 0) and with thrust (C T = 0.1) the propeller slipstream is visualized at several angles of attack. In particular, the development of the slipstream over the wing is investigated with particle image velocimetry derived variables such as mean velocities, fluctuations, and turbulent kinetic energy. A detailed description of the slipstream returns valuable information on the vortical structures and periodic phenomena. An identification of such events is necessary for a future validation with numerical data. It allows for the analysis of parametric cases; first, a comparison between the thrust and no-thrust cases and, second, the effect of an increasing angle of attack. It is found that at the thrust cases, for higher angles of attack, vortices are formed due to interactions in the boundary layer.

Experimental and Numerical Investigation of a Contra Rotating Open-Rotor Flowfield

Contrarotating open rotor propulsion systems have seen renewed interest as a possible economic and nenvironmentally friendly powerplant for future transport aircraft. While the potential efficiency benefits are well naccepted, concerns persist regarding the probable rotor-to-rotor interaction-driven noise penalty this type of engine nwould have in comparison to modern ducted turbofan engines. This paper presents results of a collaborative nexperimental and numerical study to quantify and study in-depth the complex flowfield of a generic contrarotating nopen rotor model at wind-tunnel scale. The model has 10 front blades and 8 aft blades, with blade design similar to nmodern propellers for high-disk loadings. The comparison of flow visualization results obtained through the use of nmodern stereoscopic particle image velocimetry and unsteady Reynolds-averaged Navier–Stokes simulations helps nto improve understanding of the interactions of front-rotor-blade wakes and tip vortices with the aft rotor, which is nan important aspect to guide the design of future efficient and quiet contrarotating open rotor engines. The generally ngood match between the experimental and numerical slipstream results gives confidence in the utility for their nanalysis capabilities in this field.

Flowfield Investigation at Propeller Thrust Reverse

The flow phenomena in the slipstream around an eight-bladed propeller at thrust reverse conditions are analyzed using double stereoscopic particle image velocimetry in the nAirbus Low-Speed Wind Tunnel Facility in Bremen, Germany. Several planes are scanned using a traversing system, as well as measurements at several phase angles. In the npresent paper only properties in the propeller symmetry plane will be compared. The fluid mechanical properties for two different propeller blade pitch angle settings are investigated and compared for their ability to deliver a negative thrust. The first setting has identical blade pitch angles for all propeller blades; the second setting has alternating blade pitch angles. It is concluded that the latter setting provides better aerodynamic flow properties.

Qualitative Investigation of a Propeller Slipstream with Background Oriented Schlieren

The Background Oriented Schlieren (BOS) method has been used to qualitatively identify the (variation of) density gradients in the helical structure of a propeller slipstream. The helical structures are identified for two sideslip angles. In contrast to standard BOS correlations between exposures and a reference image, two exposures at a given time interval were cross-correlated. This revealed a more clear description of the propeller slipstream as it determines the variation of the density gradient during this interval. It enhances the visualization of the helical structure of the propeller slipstream. Based on the visualizations of the blade tip vortex trajectories the propeller slipstream contraction can be estimated.

Comparison of PIV measurements with unsteady RANS cal- culations in a propeller slipstream

A comparison of velocity(-derived) quantities is made in the slipstream of a turboprop equipped transport aircraft between phase-locked experimental Particle Image Velocimetry (PIV) data and unsteady Reynolds Averaged Navier-Stokes (uRANS) calculations. Velocity results indicate a high level of agreement between the numerical and experimental nresults. In addition to velocity vector data, also derived quantities, such as vorticity, allow a verification and validation of the modeling in CFD. Although a good agreement is nachieved, future needs for the comparison and validation of computational (CFD) and experimental (PIV) data are highlighted.

Image Based Measurement Techniques of Increased Complexity for Industrial Propeller Flow Investigations

The image based measurement techniques Background Oriented Schlieren (BOS) and Particle Image Velocimetry (PIV) are particularly useful for the investigation of propeller flow. Dedicated measurements are performed of increasing complexity. Initial in-vestigations are performed with BOS and stereoscopic PIV on a single sting propeller. The model complexity is then increased to half-model for the investigation of propeller wing in-terference at thrust reverse and high-lift conditions (for positive and zero thrust). At each stage the techniques can be used as a validation for computational methods. BOS can readily be applied without too much costs and preparation. The utilization of stereoscopic and mono PIV set-ups requires more preparation. It is shown that BOS and PIV are indispensable tools for instantaneous propeller flow investigations. Furthermore it is stated that PIV is an invaluable tool for numerical validation purposes.

Propeller Slipstream Development

The flow around an 8-bladed propeller is analyzed using Particle Image Ve-locimetry (PIV) in the Airbus Low Speed Wind Tunnel Facility in Bremen, Germany. The propeller is mounted on a half-model equipped with 2 counter-rotating propellers, where only the outboard propeller is investigated. In particular, the development of the slipstream over the wing is investigated with PIV derived variables such as mean velocities, fluctuations and turbulent kinetic energy. A detailed description of the slipstream returns valuable in-formation on areas with separation. For two cases with no thrust (CT = 0) and with thrust (CT = 0.1) several angles of attack in several planes are measured. It allows for the analysis of parametric cases: firstly, a comparison between the thrust and no thrust cases and, secondly, the effect of an increasing angle of attack. It is found that at the thrust cases, for higher an-gles of attack, vortices are formed due to interactions in the boundary layer.