Dieter Pallek

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

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

Analysis of Flow Field Measurements Obtained in a Large Tow Tank Regarding the Decay of Wake Vortices in the Far- Field for Two- and Four-Vortex Systems

Stereo PIV measurements on a 2- and 4-vortex system have been performed in a large tow tank of SVA in Potsdam. The considered 4-vortex system consists of two counter-rotating vortex pairs generated by wings of a span width ratio of b 2 /b 1 = 0.3 and a circulation ratio of Γ 2 /Γ 1 = -0.3. As reference the corresponding 2-vortex system is also considered. A submersible Stereo PIV system attached to a vertical traversing system was employed. Cross-plane velocity fields are obtained of the descending wake vortex system to access the vortex dynamics well into the wake far field (x/b > 500, τ > 11). The spatio-temporal evolutions of the vortex systems are analyzed with respect to the vortex trajectories, circulations and core radii as well as the total kinetic energy contained in the measured flow field.

Experimental and numerical investigation of a counter rotating open rotor flow field

Stereoscopic Particle Image Velocimetry (SPIV) and unsteady Reynolds-averaged Navier Stokes (uRANS) has been applied for the flow field investigation behind a counter rotating open rotor (CROR) model. Of particular interest is the validation of numerical codes as well as a dedicated recording of the phase delays to determine the phase positions of both propellers at the experimental investigation of the flow field.

Studies of Reynolds Number Effects on Wing Tip Vortex Positions by Means of Laser Light Sheet (LLS) and Background Oriented Schlieren (BOS) Technique Under Cryogenic Conditions

The Background Oriented Schlieren Method (BOS) and the Laser Light Sheet technique (LLS) were successfully adapted to the cryogenic European Transonic Windtunnel (ETW) and applied to wing tip vortex investigations. Applying optical measurement techniques to ETW is difficult, because of the extreme wind tunnel conditions (temperature, pressure, poor optical access).