Quieter and Greener Rotorcraft: Concurrent Aerodynamic and Acoustic Optimization

Within the DLR project VicToria an aerodynamic and aero-acoustic optimization of helicopter rotor blades is performed. During the optimization, three independent flight conditions are considered: hover, cruise and descent flight. The first two flight conditions drive the power requirements of the helicopter rotor, while the descent flight is the loudest flight condition for current helicopter generations. In order to drive down the required power and the emitted noise, a multi-objective design approach coupled with surrogate models is utilized to find a Pareto optimal set of rotors. This approach allows to identify the trade-offs to be made when laying emphasis on either goal function. The underlying CFD simulations utilize fourth order accurate spatial schemes to capture the vortex dominated flow of helicopter rotor blades. The paper presents the validation of the setups, the optimization results and the off-design analysis of a chosen set of blades from the Pareto front. The conclusion is that the utilization of the Pareto front approach is necessary to find good rotor designs, while the utilization of high order methods allows for efficient CFD setups

Successive Optimization of Airfoils, Planform and Twist for Aerodynamic Performance of Helicopter Rotor Blades

The design of new helicopter rotor blades is a challenging task. The individual blade sections undergo very different flow conditions during the various flight regimes of the helicopter. In forward flight, the advancing side operates in a transonic regime where potentially shock waves can occur, while on the retreating side little flow velocities at high angle of attacks are seen up to reverse flow. In hover, the oncoming tip vortex of the previous blade drastically influences the inflow on the rotor. This paper joins the classical blade shape design with numerical optimization techniques. Opposing to the direct solution process of directly modifying the blade shape, the rotor blade is conventionally considered as a set of airfoils, a planform and a twist distribution. First, rotor airfoils are found through numerical optimization and then placed on the reference rotor. The planform & twist of this rotor are then also numerical optimized. The obtained trade-off blade for hover and forward flight drastically improved the performance over the reference HART-II blade (11% in forward flight, 5% in hover flight). In order to arrive at industrial relevant blades, further work including more disciplines becomes necessary

A Numerical Optimization Framework for Rotor Airfoil Design

The design of new helicopter airfoils is a challenging task. The individual blade sections undergo very different flow conditions during the various flight regimes of the helicopter. In forward flight, the advancing side operates in a transonic regime where potentially shock waves can occur, while on the retreating side little flow velocities at high angle of attacks are seen up to reverse flow. In hover, the oncoming tip vortex of the previous blade drastically influences the inflow on the rotor. Therefore, after a brief review of given design techniques, a novel approach for airfoil designs is put forward. A surrogate based multi-objective approach including constraints is utilized to concurrently optimize an airfoil for hover, retreating and advancing side flow, while also enforcing a certain robustness as to not looking at single design points in these global flow regimes. Along with the estimation of design targets, this 2D flow analysis-based framework allowed to optimize the airfoil design of an existing model rotor blade. A comparison over a range of flight conditions of the rotor with and without the new airfoils proved the validity of this approach

JAXA-ONERA-DLR COOPERATION: Results from Rotor Optimization in Forward Flight

This paper presents the results of a cooperative study by JAXA, DLR, and ONERA on the optimal design of helicopter blades for high-speed forward flight. Optimizations and simulations are carried out by each agency with their own analysis codes using both blade element theory-based methods and computational fluid dynamics. These results are cross-compared and show common trends identified for optimum rotor blades obtained by each agency and the mechanism for improving forward flight performance are discussed. From the effective drag distributions, it is confirmed that, in order to improve forward flight performance, it is first important to reduce drag on the advancing side, and that a blade with a relatively small twist angle and planforms with a smaller chord length at the root and tip compared to the mid-span section is generally a suitable blade

JAXA-ONERA-DLR cooperation: results from rotor optimization in hover

A cooperation between JAXA, ONERA and DLR puts the focus on the aerodynamic optimization of helicopter rotors. This paper represents the conclusions from the first phase: optimization of a hovering rotor. The HART-II blade is first investigated with low-fidelity tools and compared against state-of-the art CFD simulations. Afterwards, the chord distribution and twist of the HART-II blade are optimized using the low-fidelity tools as well as CFD. Since the partners observed differences in the outcome of the CFD simulations for the low-fidelity optimized blades, a deeper investigation of the effects of the turbulence modelling approach, elasticity and grid topology is conducted. The findings show that the chosen flight condition is close to the thrust of the maximum Figure of Merit and that the vortex-triggered separation on the outboard sections of the blade has to be modelled correctly. In this study, the blade grids had the most noticeable effect on the results, followed by the turbulence model and elasticity. With respect to the optimization, low-fidelity methods require special care, whereas CFD optimized blades were found to lead to more robust designs, even though they have only been optimized for a single point. This is explained by the more accurate modelling of the stall phenomenon with respect to geometrical changes

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis

Investigations of a boxed rotor: The STAR II rotor in DLR's test hall

The second Smart Twisting Active Rotor (STAR II) project aims at investigating active twist on a conventional blade design in the DNW-LLF wind tunnel. Prior to the wind tunnel test, the rotor is first tested for correct operation in the rotor test hall at the German Aerospace Center (DLR) in Braunschweig. This test shall ensure that all the instrumentation, but also the active twist, works correctly along with setting up the tracking of the blades before shipping the whole test setup to the DNW-LLF wind tunnel. From previous rotor tests in this hall, it was observed that vibration levels increased drastically above certain thrusts. Since there were no PIV tests carried out within this test chamber, it was decided to carry out CFD simulations to obtain visualizations of the flow and to find the origins of these vibrations. The rotor test hall was approximated by a simple box. With the help of CFD simulations carried out by DLR and JAXA, the initial guess of potential recirculation could be confirmed. The recirculation effect and vibrations become stronger with increasing thrust and therefore limits the maximum thrust in the test hall. This paper details the results of the CFD simulation and presents them along with initial experimental data from the rotor test hall

Vortex-Induced Stall on an Actively Twisted Highly Loaded Model Rotor Blade

In preparation for a future wind tunnel test of a rotor with twist-actuated blades (STAR II), numerical predictions of this test have been carried out by the contributing partners. In this paper, the simulated results of the vortex-induced stall operating condition, synonymous with a highly loaded flight condition, are presented. The current conclusion is that a noticeable spread of the results is given when searching for the maximum attainable thrust, depending on the onset of stall once perceived by the individual simulation methodology. Some general trends among the simulations could still be identified with respect to the actuation settings that reduce vibrations and the required power. Generally, most CFD-based results allowed to capture this physical phenomenon, but carefully tuned low-fidelity aerodynamic tools also managed to capture the same trends at a fraction of the computational cost

Vortex-Induced Stall on an Actively Twisted Highly Loaded Model Rotor Blade

As preparation for a future wind tunnel test of a rotor with twist actuation blades (STAR II ), a prediction effort by the contributing partners has been executed. In this paper, the results of the planned vortex-induced stall operating condition are presented. The current conclusion is that a noticeable spread of the results is given when searching the maximum attainable thrust, depending on the onset of stall once perceived by the individual simulation methodology. Some general trends for the best suited actuation settings reduce vibrations and required power could still be identified. Generally, most CFD-based results allowed to capture this physical phenomenon, but carefully tuned low-fidelity aerodynamics also managed to capture the same trends at a fraction of the computational cost

Smart Twisting Active Rotor (STAR) - Pre-Test Predictions

A Mach-scaled model rotor with active twist capability is in preparation for a wind tunnel test in the large low-speed facility (LLF) of the German-Dutch wind tunnel (DNW) with international participation by DLR, US Ar-my, NASA, ONERA, KARI, Konkuk University, JAXA, Glasgow University and DNW. To get the maximum benefit from the test and the most valuable data within the available test time, the tentative test matrix was covered by predictions of the partners, active twist benefits were evaluated, and support was provided to the test team to focus on the key operational conditions