ENGINEERING AND CFD

Introduction

Blade-tip vortices are still a challenging flow phenomenon in helicopter aerodynamics since they induce drag and, thus, influence the performance of the rotor. Furthermore, they strongly affect the wake of the helicopter and result in a complex vortex form due to the rapidly varying lift with time. These vortices arise due to pressure differences between the lower and upper surfaces, driving the flow around the tip from the pressure side to the suction side. Depending on the shape of the tip, several vortices can arise.

Blade-tip vortices occur both in fixed-wing aerodynamics and helicopter aerodynamics. However, fixed-wing aircraft leave their blade-tip vortices behind, whereas helicopters re-ingest the blade-tip vortices in multiple flight conditions, resulting in blade-vortex interaction (BVI). These interactions, in turn, result in a limitation of the flight envelope of the helicopter due to dynamic stall on the highly loaded rotor.

Additionally, the vortices can strike the tail or be ingested by the tail rotor. The rapidly changing velocity in the vortex leads to high impulsive loads on the structure and, thus, an increase in vibration. Moreover, all interactions between vortices and structures generate impulsive aeroacoustic noise. This noise impact of helicopters on areas surrounding flight paths and landing platforms is relevant as it influences the social acceptance of helicopter operations.

Therefore, the understanding how blade-tip vortices are generated and propagated is of great interest.

This study analyzes the blade-tip vortex of a rotor at high cyclic pitch. In contrast with earlier investigations, the rotor test facility in Göttingen (RTG) is capable of a very fine particle image velocimetry (PIV) scan of the blade-tip vortex. This scan allows a direct comparison of experiment and numerical simulations concerning the evolution of the vortex.

Previously, this comparison could only be made at a single azimuthal point. The focus of this study is a thorough numerical investigation of the evolution and convection of the blade-tip vortex complete with a comparison to experimental results by Wolf et al. [1] and Braukmann et al. [2].

Figure 1 shows a visualization of the numerical simulations for the pitching cycle. The highest angle of incidence is reached in the upper left corner.

For simplicity, only vortices until the next blade are displayed. A variation in both the blade flow and the generated vortex structures can be seen.

Results and Methods

The DLR-TAU code was used for the numerical simulations. Each of the four blade grids has 9.5 million points. The blades are moved as rigid bodies with either prescribed static pitch or sinusoidal cyclic pitch depending on the test case.

The four identical blade grids of the four-bladed RTG rotor are embedded in a background grid which primarily uses tetrahedral cells and has a total size of 48 million points.

The computations used 1440 timesteps per period, with at least 50 inner iterations per timestep. All shown static simulations have been computed with 60 coarse periods with 36 timesteps per period, followed by 2 fine periods. The cyclic simulations have been computed with 60 coarse periods, 10 intermediate periods with 360 timesteps per period and 3 fine periods with 100 inner iterations per timestep. This setup leads to a computational requirement of one million CPU-hours per test case. 2500 cores running in parallel need roughly two weeks for a case. One revolution of the rotor with the finest setting takes four days and produces one Terabyte of data. The results have been published in a journal article [3] and a conference article [4].

Figure 2 displays the λ₂ isosurface of five different light climb (hover with axial inflow) simulations with different collective pitch angles. The creation process of the vortex starts at the leading edge of the tip, forming a slender vortex. The blade-tip vortex grows with increasing angle of incidence, staying slender except at the highest angle (Θ_r=30°) which shows a radically expanded vortex covering Δr/R≈5%, which has been shown to be associated with separated flow, which is in agreement with the expectation from the thrust polar.

On-going Research / Outlook

The investigations show a good agreement with experimental data. The combination of both methods leads to a better understanding of the blade-tip vortex of a rotor. In a further step, simulations regarding blade-tip vortex interactions can be performed.