The Clean Sky Joint Technology Initiative (JTI) is a European Research Programme aimed at greening Air Transport through the development of more environmentally-friendly advanced technologies. It is organised into six Integrated Technology Demonstrators (ITD) covering a large variety of themes, including rotorcrafts. Airbus Helicopters is involved in the Green Rotorcraft (GRC) ITD programme. One of the GRC top objectives is a reduction of CO_2 emissions by 25 to 40%. CO_2 emissions are function of fuel consumption, and consequently of the power required to fly, which directly depends on the design of the airframe and of the non-lifting rotating components for cruise flight. A preliminary phase of the Clean Sky GRC programme consisted in a drag analysis for three different helicopter weight-classes. It was focused on discerning by CFD the H/C components from which significant gains could be obtained, depending on the specific architecture of each weight class, as comprehensively described in [1] for the medium-weight reference helicopter. The main contributors to drag have been identified for the H155 helicopter as the blade sleeves (16% of the total H/C drag) and the rotor-head/fuselage interactions (9%), whose contribution is evaluated as the increase of the fuselage drag due to the presence of the rotor head. Whereas the parasite drag of the blade sleeves may be efficiently reduced by sleeve fairings [3,4,5], mitigating the rotor-head/fuselage interaction drag is much more challenging. Early investigations dealing with wake and interactions, as described in [2,6], indicate that the hub-cap design, as well as the shape of the pylon fairing, are amongst key elements to reduce the interaction drag, because the pylon fairing is usually the place of important interactions with the wake generated by the rotor head, and because this wake is also highly affected by the downwash created by the lifting hub cap [1,2]. This is why two innovative designs of pylon fairings and rotor fairings involving two enhanced sleeve fairings and two advanced hub-cap designs have been proposed to be experimentally investigated within the framework of the Clean Sky CARD (Contribution to Analysis of Rotor-hub Drag reduction) partner project. Overviews of the shapes are proposed in Figure 1. Those innovative designs have been patented by Airbus Helicopters. The CARD consortium is composed of the Aircraft Research Association (ARA), University of Glasgow (UoG), and Vyzkumny a Zkusebni Letecky Ustav (VZLU). The Wind-Tunnel Tests (WTT) were performed in September 2014 at the VZLU Letfiany facilities (Prague), and were principally aimed at demonstrating the benefits on drag from the proposed innovative fairings for cruise flight conditions. For the tests, a 1:4 scaled H155 fuselage model without the rear parts (tail-fin, horizontal stabilizers and fenestron®) was considered. For this scale, the cruise flow condition is characterised by an incoming flow velocity of 40 m/s, hence ensuring acceptable discrepancies on the Reynolds Number with respect to full scale. The model was mounted by a 5-bladed rotor head composed of a mast, a hub, a hub cap, sleeves, blade roots, and truncated blades for which 25% of the original blade span have been retained. Independent load cell measurements for the hub cap, the rotor and the fuselage, as well as stereo PIV measurements downstream the rotor head were achieved during the CARD campaign. A comprehensive drag analysis of experimental data has been proposed at ERF2015 [7]. In this work, the innovative shapes proposed for CARD WTT are numerically investigated, at the same scale as for the experimental campaign, and under the same flow conditions. Steady-state computations have been achieved using the unstructured-mesh based DLR's TAU solver to assess numerically the benefits on drag. The high-fidelity geometries have been meshed using ICEM CFD. It consists in a non-structured mesh based on up to 220 million of elements for the most demanding configuration. 24 layers of prisms have been required to mesh properly boundary layers at the fuselage and at the rotor-head's geometrical details, hence ensuring a mean dimensionless wall distance of 1. A box of mesh refinement has been employed at the vicinity of the rotor head and of the aft upper deck, in order to well capture the wake generation and potential airframe/wake interactions. Some snapshots of both skin mesh and volume mesh are proposed in Figure 2. The simulations point out that for the baseline configuration (F0S0H0), the parasite drag is responsible for 89% of the total drag and airframe/flow interactions for 11%. The best benefits on drag are obtained at 0° angle of attack when sleeve fairings S1 are used together with the hub cap H1. This rotor-fairing combination ensures drag reduction by 9.7% of the total drag when it is mounted on the baseline H155 fuselage, as it appears in Figure 3. Figure 3 also demonstrates that the best benefits on drag come significantly from a reduction of the interactional drag (for 45%), the rest being a reduction of parasite drag (for 55%). The most complex combinations involving innovative pylon fairings in conjunction with the best rotor fairing are still under investigations (F1S1H1, F2S1H1) and results will be presented in the final paper. A comprehensive analysis of the contributors to the drag mitigation is also conducted from part-by-part drag breakdown as proposed in Figure 4 for each configuration. The parasite drag reduction comes essentially from a reduction of the pressure drag at the blade sleeves, at the blade roots and more slightly at the hub cap. In addition, Figure 4 shows that the drag generated at the cowlings, aft cowlings, pylon fairing and aft pylon fairing is significantly mitigated by interaction effects. The drag reduction is also assessed locally from the investigation of the skin distribution of the pressure drag, friction drag and interactional drag coefficient, as illustrated in Figure 5. The robustness of the benefits on drag obtained from the innovative fairings regarding a variation of the angle of attack is numerically considered. A plot of the total-drag polar with respect to the angle of attack is presented in Figure 6 for each configuration. It confirms that the drag mitigation from the most efficient combinations of fairings is well preserved for angles of attack within the range [-8˚;0˚]. The influence of the yaw angle is not investigated in the present work, but it has been assessed during the CARD WTT. Comparisons of CFD drag polar curves with WTT measurements proposed in [7] have been undertaken, and will be presented in the final paper. In addition, numerical investigations are being carried out to assess the efficiency of the innovative fairings on a complete H/C including the potential interactions with the rear parts, more realistic baseline sleeves, and additional rotor-head components such as the lead-lag dampers (Figure 7). The conclusions will be presented in the final communication.
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