Forces on Obstacles in Rotor Wake - A GARTEUR Action Group

FORCES ON OBSTACLES IN ROTOR WAKE – A GARTEUR ACTION GROUP Antonio Visingardi, [email protected], CIRA (Italy) Fabrizio De Gregorio, [email protected], CIRA (Italy) Thorsten Schwarz, [email protected], DLR (Germany) Matthias Schmid, [email protected], DLR (Germany) Richard Bakker, [email protected], NLR (The Netherlands) Spyros Voutsinas, [email protected], NTUA (Greece) Quentin Gallas, [email protected], ONERA (France) Ronan Boisard, [email protected], ONERA (France) Giuseppe Gibertini, [email protected], Politecnico di Milano (Italy) Daniele Zagaglia, [email protected], Politecnico di Milano (Italy) George Barakos, [email protected], University of Glasgow (United Kingdom) Richard Green, [email protected], University of Glasgow (United Kingdom) Giulia Chirico, [email protected], University of Glasgow (United Kingdom) Michea Giuni, [email protected], University of Glasgow (United Kingdom) Abstract The paper describes the objectives and the structure of the GARTEUR Action Group HC/AG-22 project which deals with the basic research about the forces acting on obstacles when immersed in rotor wakes. The motivation started from the observation that there was a lack of experimental databases including the evaluation of the forces on obstacles in rotor wakes; and of both numerical and experimental investigations of the rotor downwash effects at medium-to-high separation distances from the rotor, in presence or without sling load. The four research centres: CIRA (I); DLR (D); NLR (NL); ONERA (F); and three universities: NTUA (GR); Politecnico di Milano (I); University of Glasgow (UK) created a team for the promotion of activities that could contribute to fill these gaps. In particular, both numerical and experimental investigations were proposed by the team to study, primarily, the effects of the confined area geometry on a hovering helicopter rotor, and, secondarily, the downwash and its influence on the forces acting on a load, loose or slung, at low to high separation distances from the rotor disc. The following activities were planned: a) application and possible improvement of computational tools for the study of helicopter rotor wake interactions with obstacles; b) set-up and performance of four cost-effective wind tunnel test campaigns aimed at producing a valuable experimental database for the validation of the numerical methodologies applied; c) final validation of the numerical methodologies. The project started in November 2014 and has a duration of three years. hover or low-speed forward flight, in contrast to fixed-wing 1 NOMENCLATURE aircraft. These are the main reasons why helicopters are employed in missions within “confined areas”. Symbol Description Units A confined area is a region where the flight of the helicopter c Chord length m is limited in some direction by terrain or by the presence of obstacles, natural or manmade, such as steep valleys and Cp Pressure coefficient buildings. Rescue operations, emergency medical services, Ω Rotor speed RPM ship-based rotorcraft operations are just some examples of D Rotor diameter m helicopter missions within confined areas, Figure 1. R Rotor radius m 2 u, v, w Velocity components m/s x, y, z Geometrical coordinates m ∆p Pressure difference wrt p∞ Pa Θ0 Collective pitch deg µ Advance ratio σ Rotor solidity HIGE Hover In Ground Effect HOGE Hover Out of Ground Effect INTRODUCTION A helicopter is an aircraft that generates the lifting force by means of the blades, which are rotating aerodynamic surfaces. Therefore, a helicopter does not necessarily require a relative wind to fly and can efficiently operate in Figure 1: Examples of helicopter operations in confined areas The wake system generated by the helicopter rotor may interact with the airflow around the obstacles, with an intensity that increases with the proximity of the rotor to the ground and/or the obstacles. This mutual interaction generates aerodynamic forces that may result in: (a) high compensatory workload for the pilot; (b) degradation of the handling qualities and performance of the aircraft; (c) unsteady forces on the structure of the surrounding obstacles; (d) noise levels creating discomfort to the community residing in the area. loading and on the distance of the sling load from the rotor disc. The instabilities that can arise from these forces affect the rotorcraft and/or the load itself, and their avoidance is therefore crucial not only for safety reasons but also when a controlled attitude of the load is required. An example for the latter case is represented by drop test experiments, such as PHOENIX[2.], NASA X40-A[3.],[4.], ESA-IXV[5.], in which the helicopter is both the carrier and the launch station of tests articles, which represent the sling load, Figure 5. The obstacle wake can be highly unsteady, complex and challenging to predict, Figure 2. Figure 4: Helicopter with a sling load Figure 2: Complexity of the obstacle wake[1.] This flowfield is further complicated by the presence of the helicopter which induces an additional flowfield that can significantly alter the upstream airflow when the aircraft is operating close to a surface. Thus, the helicopter experiences a combination of the wake induced by the obstacle and the airflow induced by itself. The presence of vertical surfaces forces a part of the rotor wake to flow upward along the walls and to become the recirculatory inflow with respect to the rotor, Figure 3. The aerodynamic performance of the rotor changes because the upward rotor wake interferes with the flow field around the rotor. 2.1 Figure 5: NASA X40-A drop test[4.] Previous work Several publications address the problem of the helicopter ground effect in confined areas and the majority of them concern investigations of the helicopter-ship interactional problem, example papers are reported in references[6.]-[11.], whereas a numerical and experimental investigation concerning the more general problem of the rotor performance in the wake of a large structure is illustrated in Quinliven[12.]. Nevertheless, references of the evaluation of forces acting on obstacles in rotor wake are scarce. Likewise, there are few experimental databases for the validation of numerical methodologies, their accessibility is uncertain, and do not provide force measurements on obstacle surfaces. A few papers describe the characteristics of some experimental databases. They also testify the complexity of the problem and the importance to better understand the phenomenology and its implication on the flight safety. Some relevant examples are provided in the following: • a pioneering work is represented by Timm[13.] in which the author illustrates the basic requirements for the generation of an obstacle-induced flow recirculation and what the driving parameters are. Qualitative results of an experimental activity are provided and solutions for a mitigation of the problem are also proposed; • a more recent experimental investigation is illustrated in Iboshi[14.]. The authors illustrate the set-up of an experiment consisting in a rotor hovering over a ground plate delimited by one or two vertical plates. Performance evaluations are made at certain combinations of the wall height, space between walls and rotor height. An increase in the required torque Figure 3: Helicopter operations in confined areas A helicopter sling load, Figure 4, is another, yet particular, case of obstacle subjected to forces produced by its interaction with the rotor wake. Once airborne a sling load comes under the influence of aerodynamic forces and moments associated with its size, shape, mass, and transport speed. Furthermore, in the particular case of hover/low-speed forward flight conditions, where the load is fully immersed in the rotor wake system, these forces are strongly related to the intensity of the downwash effects of the wake which, in turn, are mainly dependent on the blade • • coefficient as well as in the vibratory torque coefficient is produced by the presence of the vertical walls; distances from the rotor, in presence or without sling load, is observed. the comparisons of Navier-Stokes CFD predictions of the airflow around a helicopter rotor hovering near a land-based hangar with experimental velocity data gathered during a flight test campaign by using ultrasonic anemometers are illustrated in Polsky[15]. The rotor is numerically modelled by an actuator disk and several turbulence models are tested. The importance of an atmospheric boundary layer (ABL) modelling is also considered. Although the helicopter downwash dominates the flowfield, the CFD investigation also demonstrates the importance of accurately predicting the flow over and around the hangar structure. For this purpose, experimental data were gathered by the authors around the full scale hangar and sub-scale wind tunnel data including surface pressures and oil flow but these results are not shown in the paper; An Action Group, namely HC/AG-22[22], was created in the framework of the GARTEUR organization by four research centres: CIRA (I); DLR (D); NLR (NL); ONERA (F) and three universities: NTUA (GR); Politecnico di Milano (I) - PoliMi; University of Glasgow (UK) - UoG, with the aim to promote activities which could contribute to fill these gaps. For the purpose, this team proposed to investigate, both numerically and experimentally: the occurrence of aerodynamic interference between a helicopter and obstacles of different shapes located in the vicinity of the helicopter is numerically and experimentally investigated in Lusiak[16]. For the particular case of a well-shaped obstacle, such as a typical town courtyard, numerical computations are performed by coupling the FLUENT software for the rotor aerodynamics with a panel code for the fuselage aerodynamics and compared with the measurements made in a low turbulence wind tunnel TMT in terms of tensometric measurements of forces and moments. The results indicate that the phenomenon of aerodynamic interference can seriously disturb the flow around the helicopter and change the loading of some of its elements. Substantial changes in the value of the resulting loads can make the helicopter difficult to control. The aerodynamics of helicopter slung-load systems is investigated in a few publications. A relevant example is given by Gabel[17], where a specific section is dedicated to the experimental evaluation of the aerodynamic instabilities induced by the load attitude and the separation distance of the load from the helicopter. CFD is used in Prosser[18] to resolve the unsteady Navier-Stokes equations for prediction of aerodynamic forces and moments acting on dynamic helicopter sling loads; no influence of the rotor wake is taken into account. Similarly, Theron[19] summarizes the work on the aerodynamics of a slung load cargo container without considering the effect of the rotor wake: two different CFD codes are used to study the threedimensional flow over the stationary container and the twodimensional simulation of the stationary and oscillating container. Comparisons with experimental measurements are also reported. Finally, two relevant papers concerning the measurement of the helicopter downwash velocity are represented by Leese[20],[21], where no sling load is however considered. 2.2 Objectives The analysis of the previous work highlights the lack of experimental databases including the evaluation of the forces acting on obstacles when immersed in rotor wakes. Instead, in the case of the helicopter sling-load problem, the lack of both numerical and experimental investigations of the rotor downwash effect at medium-to-high separation • primarily, the effects of the confined area geometry on a hovering helicopter rotor from the standpoints of both the phenomenological understanding of the interactional process and the evaluation of the forces acting on surrounding obstacles; • secondarily, the downwash and its influence on the forces acting on a load, loose or slung, at low to high separation distances from the rotor disc. The know-how acquired by the GARTEUR AG-17[23.],[24] about the wake modelling in the presence of ground obstacles was capitalised and set-up the basis for this new research activity. The project, started in November 2014, has a duration of three years, with conclusion planned for October 2017, during which the following activities are planned: • application and possible improvement of computational tools for the study of helicopter rotor wake interactions with obstacles; • set-up and performance of cost-effective wind tunnel test campaigns aimed at producing a valuable experimental database for the validation of the numerical methodologies applied; • final validation of the numerical methodologies. The present paper provides the details of the above indicated activities. In particular, section 3 illustrates the statement of the work. The wind tunnel experiments and the numerical activities are described, together with some example of results obtained, in sections 4 and 5, respectively. Some conclusions are finally drawn in section 6. 3 STATEMENT OF WORK In order to achieve the objectives of HC/AG-22, the project is structured in four work packages. The partners are involved in experimental activities, during which suitable databases are produced for the phenomenological understanding and the quantification of the forces arising during the rotor-ground-obstacles interactional process, as well as numerical activities aimed at both providing baseline indications for the set-up of the experiments, and enhancing and validating the employed commercial or in-house computational tools. 3.1 Project Work Package Architecture The project architecture is shown in Figure 6 and a short description of the work packages is provided in the following. Table 1 reports a summary of these experiments, while the main characteristics of the test rigs, of the wind tunnels, and of the model obstacle are illustrated in the following. Figure 6: Project work package architecture • • • • 4 WP0 – Management & Dissemination: is aimed at the fulfilment of all the obligations concerning the project management and the dissemination of the results. Through this work package the project interacts with: the Group of Responsables (GoR), by receiving inputs and providing the information required; and the scientific community, by collecting the results of the activities of the other three work packages and disseminating them; WP1 – Preliminary Computations & Code Enhancements: deals with a preparation phase during which partners are involved in literature review and computational activities aimed at providing necessary and useful inputs to the two following work packages where experimental databases are produced (WP2) and the modelling capabilities of the applied numerical tools are validated (WP3). It also provides WP0 with all the information required for management and dissemination; WP2 – Wind Tunnel Test Campaigns: concerns the performance of four wind tunnel test campaigns that have been identified by partners as particularly meaningful for the phenomenological understanding of the flow field generated by a model rotor operating in HOGE/HIGE conditions in the presence of obstacles, and the quantification of the forces acting on them. The resulting experimental databases are used in WP3 for the final validation of the numerical tools proposed by the partners. It also provides WP0 with all the information required for management and dissemination; WP3 – Final Validation of Codes: is aimed at the final validation of the numerical tools proposed by partners. The validation is performed by comparing the numerical results of the computational activity with the experimental data produced during the wind tunnel test campaigns of the project in the framework of WP2. The work package also provides WP0 with all the information required for management and dissemination. Table 1: Experimental tests 4.1 CIRA: HOGE/HIGE rotor with a loose/sling load The external loads carried by a helicopter are typically connected in the proximity to the fuselage in order to remain in its shadow, thus avoiding unsteady aerodynamic loads induced by the main rotor wake and possible instability phenomena. Instead, for some particular missions, such as rescue operations (SAR) in adverse conditions or drop tests, these loads can be located at a larger distance from the fuselage, so that the knowledge of the wake downwash and of the forces and moments induced on the loads becomes crucial for the safety of the mission. The CIRA test campaign aims at evaluating the effects of the main rotor wake on a sling load, in terms of aerodynamic loads and pressure distributions, in this latter condition. The activity is ongoing. 4.1.1 Test activity The test activity investigates firstly the characteristics of the main rotor wake several diameters downstream of the rotor disc in HOGE condition. The influence of the main rotor wake is quantified in terms of induced loads and pressure distributions on a sling load positioned at different distances form the rotor disc, while measuring the lift of the rotor. In addition, the mean load induced by the sling loads immersed in the rotor downwash wake on the rotor is measured, Figure 7. Secondly, the effect of the interaction between the rotor and the sling loads during the landing phase, when the rotor is in HIGE condition, is also evaluated, Figure 8. WIND TUNNEL EXPERIMENTAL ACTIVITIES The four partners CIRA, ONERA, PoliMi and UoG proposed four different wind tunnel test campaigns, complementary to each other, to be carried out in their own wind tunnel facilities and aiming at analysing, with a wider and deeper insight, different aspects of the phenomenology. A fully instrumented obstacle was put at disposal by DLR. Figure 7: CIRA experimental lay-out - HOGE blades with NACA0012 airfoil, diameter D = 0.71m and speed Ω = 2600 RPM. The collective pitch was set at a fixed angle θ0 = 7.5°, Figure 10. Figure 8: CIRA experimental lay-out - HIGE The tests is being conducted in free air inside the CIRA laboratory of Testing Engineering and Methodology. The experimental campaign will make use of the R/C helicopter model Blade 450 3D RTF, equipped with a four-bladed rotor, with rectangular blades and NACA 0013 airfoil, having diameter D = 0,72m and rotor speed Ω = 2400 RPM, Figure 9. The forces and moments will be measured by using the six-component balance ATI Mini40 IP65. A sling load of cylindrical shape and fineness ratio 2, having length equal to 20% of the rotor diameter, is being manufactured at CIRA via Additive Manufacturing technique. The cylinder is equipped with some pressure ports for the characterization of the pressure distributions. Figure 10: R/C helicopter Sphynx 3D The helicopter is mounted in the centre of a square-shaped courtyard as shown in Figure 11. The platform simulating a complete ground is at 1.2 m above the building floor. The walls have a parallelepiped form; they are in wood and screwed on the floor. The interior side of the walls are painted in black for the visualisations. The wall is 0.36 m high with a thickness equal to 0.30 m. Figure 11: Helicopter mounted in a square-shaped box representing a closed courtyard The tests were conducted in free air in a laboratory environment at ONERA Lille. 4.2.2 Figure 9: R/C helicopter Blade 450 3D RTF 4.2 ONERA: HIGE rotor in proximity to a wellshaped obstacle Test activity Tests were realised with and without the presence of obstacles, which represent a typical town courtyard with a squared shape, in HIGE/HOGE and quasi axisymmetric conditions. The distance between the rotor and the ground was varied as indicated in Figure 12. ONERA developed a scaled rotor bench with the objective to investigate the interactional effects between the rotor wake and its close environment, which can be infinite or finite ground effect, or obstacles of any kind. The activity is nearly completed and a number of related publications has been already produced and reported in the references[25.],[26.]. 4.2.1 Test rig and obstacle model The test rig is originally based on the commercial R/C helicopter model Sphynx 3D including a rotor head with global and cyclic control in pitch. The helicopter was strongly customised: the tail rotor and the cyclic pitch were removed; a six components balance, an external energy supply, etc. were introduced. The rotor has two rectangular Figure 12: Layout of the ONERA test cases The forces and moments on the rotor were measured via a a 6-components balance. The acquisition of the balance signals was done at 2 kHz during 15s with a high frequency filter at 1 kHz to eliminate the high frequencies folding. The pressure measurements along the floor and the wall were realised by using 9 Druck PDRCR42 of 75 mbars flush-mounted on a rod alternatively inserted in the floor and in the side wall. An example is provided in Figure 13. The pressures were characterized in static (~5 sec) with a MENSOR differential sensor with a guaranteed accuracy of 0.25 Pa on the scale ±400 Pa. Figure 16: Module of the velocity field. 4.3 Figure 13: Pressure measurements on the ground with and without the side walls The flow visualisation was made with a high speed video camera, a smoke generator and a laser light sheet aligned with the rotor head. Figure 14 shows an extracted image with the field of view focused on the blade tip, in direct negative colour. The rotor head appears on the right-up side. The vortices shed at the extremity of the blades are well visible and their core, generated by the centrifugation of the smoke, grows rapidly at their birth. Near the ground the flow expand radially with rebounds of the vortices at different height. PoliMi: HIGE rotor in proximity to an obstacle in windy conditions The wind tunnel test campaign proposed by PoliMi deals with a helicopter model hovering in close proximity to a solid obstacle, with and without natural wind, with the aim to study the mutual aerodynamic interference. A first entry for the wind-off test was carried out in a large testing environment on the side of the wind tunnel. The second test entry, comprising both wind-on (µ = 0.05) and wind-off tests, was instead carried out in the large test section of the Wind Tunnel of Politecnico di Milano (GVPM), whose dimensions allowed for an even reduced interference with the surroundings. The activity is completed and a number of related publications has been already produced and reported in the references[27.],[28.],[29.],[30.]. 4.3.1 Test rig and obstacle model The test rig consists of an in-house developed helicopter model, inspired to the MD-500, fixed to a horizontal pylon that can be moved by a system of two traversing guides so that its height from the ground and distance from the obstacle can be changed. The roll angle of the helicopter can be adjusted. The helicopter model is powered by an on-board electric motor and is fixed to the pylon by the tail (tail rotor is not present), Figure 17. The rotor model has four rectangular blades with NACA 0012 airfoil, and the following main characteristics: Figure 14: Smoke visualization of blade tip vortices The Stereo-PIV measurement were made in an area located below the rotor and on the advancing blade side of the model, Figure 15. The two PIV cameras as well as the laser were synchronized with one-per-rev signal provided by a sensor on the helicopter rotor. The acquisition frequency was set at 4.8 Hz, which is equivalent to one PIV recording for nine rotor revolutions. Figure 16 shows an example from the SPIV results of the flow field in between the helicopter and the surrounding walls. Figure 15: Configuration of the model and PIV zone • Diameter D = 0.75m; • Chord c = 0.032m; • Solidity σ = 0.11; • Fixed collective pitch θ0 = 10°; • No cyclic pitch; • Speed Ω = 2480 RPM (1st entry) - Ω = 2580 RPM (2nd entry) The obstacle was provided by DLR and the main characteristics are described in section 4.5. The following measurements were made for each test: 1) 2) 3) 4) forces and moments on the rotor by means of a 6components balance (20E12A JR3 Force Torque Sensor); steady (average values) pressures on the obstacle walls (several pressure taps linked to a PSI Pressure System); unsteady (time history) pressures on the obstacle walls (20 Kulites XCS-093). The unsteady pressure acquisition was synchronized with the acquisition of the rotor blade azimuthal position; PIV flow field survey downstream of the obstacle, Figure 20. Figure 17: Helicopter model & Test Rig 4.3.2 Wind tunnel characteristics The Wind Tunnel of Politecnico di Milano is widely used for helicopter tests thanks to the generous dimensions of its test chamber (4m x 3.84 m) and its very good flow quality. Furthermore this facility allows to use the return duct too as a quite large test chamber (13.84m x 3.84m) usually utilized for building tests. The tests of the present campaign were carried out inside this huge room as represented in Figure 18 in order to minimize the interference effect of the surroundings. Figure 19: PoliMi measuremet points Figure 20: PIV measurements An example of the measured pressures and PIV are illustrated in Figure 21 and Figure 22, respectively. Figure 18: PoliMi GVPM experimental lay-out 4.3.3 Test activity Several model settings, Figure 19, were tested during the campaign by changing the following three parameters: 1) horizontal distance from the obstacle; 2) height from the ground; 3) wind velocity. Each combination of them defined a test configuration. A matrix, containing the lists of the configurations was defined before the start of the campaign. Figure 21: Example of Cp measurements on the PoliMi obstacle in wind-off conditions - µ = 0.05 away from the rotor centre line. It was used to carry out forces and moments and LDA measurements. Figure 22: Example of PIV measurements on the PoliMi obstacle in wind-off conditions - µ = 0.05 4.4 UoG: HIGE rotor in proximity to an obstacle without wind The wind tunnel test campaign proposed by UoG was aimed at investigating the rotor flow in the vicinity of an obstacle. In particular, it consisted of a set of tests reproducing hovering flight conditions at different positions with respect to a cubic obstacle with side dimension equal to the rotor diameter. Two different rotor rigs were used and load measurements from the rotor, pressure measurements on the obstacle faces and velocity measurements of the air flow were gathered during the campaign. Figure 23: UoG large rotor test rig 1 Rotor rig 2, Figure 24, could be used in the same laboratory space, or it could be mounted in the UoG de Havilland wind tunnel, which has a 2.66m x 2.07m x 5.6m (W x H x L) working section. This rotor rig was used to perform StereoPIV and pressure measurements as well as flow visualizations. The activity is completed and a number of related publications has been already produced and reported in the references[27.],[29.]. 4.4.1 Test rig, obstacle model and equipment Two simplified rotor systems available at Glasgow were used within this project: large rotor rig 1; small rotor rig 2. The notional obstacle was a simple cuboid shape, which was mounted on a load cell. In addition it was fitted with surface mounted transducers. The main characteristics of the rigs and the obstacle are summarised in Table 2. Characteristics Rotor Rig 1 (Large) Rotor Rig 2 (Small) Obstacle size 1m 0.3 m Rotor diameter Number of blades Blade chord 1m 0.3 m 4 2 53 mm 31.7 mm Solidity 0.135 0.134 8o 8o 1200 rpm 4000 rpm Collective pitch Rotor speed Table 2: Main features of the UoG rotor rigs. Rotor rig 1, Figure 23, was placed in a large laboratory space with an even, flat ground extending to a 5m radius Figure 24: UoG small rotor test rig 2 4.4.2 Test activity The matrix of the tests conducted at UoG is reported in Figure 25 where the red circles indicate the positions of the centre of the rotor with respect to the obstacle. Some conditions on planes different from the symmetry plane were also tested. The following measurements were made for each test: 1. forces and moments generated by the rotor were measured by a 6-components AMTI MC36 model load cell mounted on the rotor rig 1. The intention to trim the rotor to zero rolling and pitching moment by varying the cyclic angle settings proved to be difficult in practice so the moments were measured for given blade settings and the changes in moment due to rotor proximity to the obstacle were recorded; 2. a 2-component LDA system mounted on a traverse system was used to measure the induced velocity on a plane 40mm above the rotor rig 1, as the distance from the ground and from the obstacle changed; 3. a ZOC valve system was used to measure the pressure on the top and side faces of the cubic obstacle. These measurements were taken by using the rotor rig 2 to provide support data for the PIV analysis in addition to the obstacle load. Some data were sampled with the rotor rig in the wind tunnel, while other cases were run in the large laboratory space; 4. The LaVision system running Davis 8 stereoscopic PIV was used to measure the flow in the region of recirculation between the obstacle and the rotor. The system was installed in the working section of the UoG de Havilland wind tunnel, and the rotor rig 2 was used for these tests. Figure 27: Smoke flow visualisation. z/D = 1.93 above the ground 4.5 DLR F20 model obstacle A wind tunnel model, property of DLR, was used as an obstacle during the activities of the project. The DLR-F20wind tunnel model[31.] resembles a container in model scale. The model was originally used to measure the airloads and pressure distributions in the context of a study for air dropping of containers from military transport aircraft (DLRMiTraPor-Project), Figure 28. Figure 25: UoG Measurement points An example of the measurements acquired during the test campaign is provided in Figure 26, referring to an LDA scan. Figure 28: DLR-F20-Model in DNW-NWB-Wind Tunnel, Braunschweig The model is a cuboid with size 0.45 m x 0.8 m x 1.0 m. The cuboid is made from an aluminium frame onto which aluminium plates are fixed, Figure 29. It is instrumented with 5 Kulites and 155 pressure taps, Figure 30. The Kulites are still installed. The PSI-module has been removed but all tubes and clutches are still installed. Figure 26: LDA scan over rotor disc. Inflow velocity Smoke flow visualizations were also made and an is shown in Figure 27. Figure 29: Internal layout of brick Figure 30: Sensor installation on brick During the HC/AG-22 experiments, the cuboid was simply laid down on the floor. 5 NUMERICAL ACTIVITIES The numerical investigations are performed by each partner applying in-house-developed or commercial computational tools. Preliminary computations were performed with the aim to test the initial modelling capabilities of the proposed tools. Code enhancements are also carried out for those cases where an improvement of the tools capabilities was required in order to better simulate the phenomenology under investigation. All partners are currently involved in the final validation activity. The simulations are performed by using computational methodologies, which span from the lower fidelity flight mechanics ones up to the high fidelity and sophisticated Navier-Stokes based ones, Figure 31. The numerical tools are summarized in Table 3 and described in the following. Figure 32: Cp distribution over PoliMi obstacle in wind-off conditions – x/R = 0; z/R = 2 Figure 31: Numerical methodologies applied Figure 33: Cp distribution over ONERA wellshaped obstacle and ground DLR applies the panel method UPM to study the interference effects between rotor and obstacle. UPM[33.] is an unsteady free wake panel method based on the Laplace equation. The ground and obstacles may be modelled by a panelised surface with sources/sinks. The rotor is trimmed allowing the analysis of the rotor trim state in the presence of an obstacle. A related publication has been already produced and reported in the reference[34.]. An example of computed flowfield velocities and streamlines around the PoliMi obstacle in wind-on conditions is shown in Figure 34. Table 3: Numerical methodologies applied CIRA computations are performed by RAMSYS[32.], which is an unsteady, inviscid and incompressible free-wake BEM solver, developed at CIRA, for multi-body configurations. It is based on Morino's boundary integral formulation for the solution of Laplace’s equation for the velocity potential φ. Ground effect problems are solved by the application of a Mirror Image Method (MIM) or by using the more sophisticated Surface Singularity Method (SSM). The forces on the obstacles are evaluated by integrating the surface pressure distributions. An example of predicted pressure distributions on the obstacle and ground surfaces is shown in Figure 32 and Figure 33. Figure 34: In-plane velocity magnitude contours and streamlines for the PoliMi obstacle in wind-on conditions at µ = 0.05 – x/R = -0.5; z/R = 2 NLR uses the commercial tool FLIGHTLAB[35.]. It contains a panel method for the modelling of ground/walls/ship deck that is used to interact with rotor wakes. The rotor wake may be modelled by means of a simple Peters/He finite-state wake or by the more complex free vortex wake methods. The vortex wake is either prescribed or free and a timeaccurate method is also available. A method for the computation of the rotorcraft wake geometry using NURBS is also proposed for further development and implementation. An example of computed Cp and streamlines for the ONERA well-obstacle is shown in Figure 35. ONERA based on the same approach than the MINT code, for the free wake model and extended with a multi-body module and the Multilevel Fast Multipole Method. To take into account for any kind of obstacle geometry, specific developments are implemented in the context of this GARTEUR. The second tool is the elsA code[43.], an unsteady Navier-Stokes code able to simulate complete helicopter configuration taking into account any kind of obstacle geometry. A related publication has been already produced and reported in the reference[26.]. An example of wake development and flow field velocity predictions by PUMA and elsA codes is shown in Figure 37 and Figure 38. Figure 35: Cp and streamlines for the ONERA wellobstacle experimental case, full-scale dimensions with BO105 NTUA simulations are performed using two codes. The first is GENUVP[36.],[37.],[38.], a panel code combined with a vortex particle approximation of the wake. The code uses a multiblock MPI-enabled Particle Mesh solver for the wake evolution and tree algorithms for the panel part. The wake is generated along prescribed lines such as trailing edges of blades and corner lines of bluff bodies. The second code is HoPFlow[39.], a fully coupled hybrid code that combines a URANS un-structured compressible solver close to solid boundaries with an overlapped Particle Mesh Lagrangian solver using compressible particle approximations. The particles carry mass, vorticity, dilatation and energy while their volume is varying in order to take into account flow compressibility. In the case of independently solid bodies (rotor blades and fuselage) separate grid are introduced for each one. Figure 37: Lifting-line + free wake modelling of the wake development for the ONERA obstacle A related publication has been already produced and reported in the reference[40.]. An example of wake development modelling for the PoliMi obstacle in wind-off conditions is shown in Figure 36. Figure 38: N-S wake modelling for the PoliMi obstacle in wind-off conditions – x/R = 1; z/R = 2 PoliMi adopted computational scheme is a coupled approach making use of the ROSITA[44.],[45.] Navier-Stokes (NS) solver and a Blade Element (BE) approximation. The coupling is made by means of an actuator disk: the loads computed with the BE approach are introduced in the NS computation through the disk actuator, then from the NS solution the velocity on the disk are obtained and used again for the load evaluation by means of BE. The iterations continue until convergence. Figure 36: Wake development for the PoliMi obstacle in wind-off conditions - x/R = -1; z/R = 2 ONERA computations are performed using two different tools. The first one is the PUMA code[41.],[42.] developed at The CFD code ROSITA numerically integrates the unsteady compressible Reynolds Averaged Navier-Stokes (RANS) equations, coupled with the one-equation turbulence model by Spalart-Allmaras. Multiple moving multi-block grids can be employed to build an overset grid system using the Chimera technique. The equations are discretised in space by means of a cell-centred finitevolume implementation of the Roe’s scheme. The Gauss theorem and a cell-centred discretisation scheme are used to compute the viscous terms of the equations. Time advancement is carried out with a dual-time formulation, employing a 2nd order backward differentiation formula to approximate thetime derivative and a fully unfactored implicit scheme in pseudo-time. a comparison between measured and computed Cp on the obstacle surface. A related publication has been already produced and reported in the reference[28.]. An example of computed flowfield velocities and streamlines is shown in Figure 39 and a comparison between measured and computed PIVs is given in Figure 40. Figure 41: N-S wake modelling and streamlines for the PoliMi obstacle in wind-off conditions - x/R = -1; z/R = 2 Figure 39: In-plane velocity magnitude contours and streamlines for the PoliMi obstacle in wind-off conditions - x/R = 0; z/R = 2 Figure 42: N-S Cp distribution for the PoliMi obstacle in wind-off conditions - x/R = -1; z/R = 2 (top) and x/r=0,z/R=2 (bottom) Figure 40: Comparison between computed (top) and measured (bottom) flow-field. PoliMi obstacle in windoff conditions - x/R = 0; y/R = 0, z/R = 2. UoG employs the Helicopter Multi-Block Method HMB2 code[47.],[48.],[49.],[50.], that has rotor-modelling capability using either resolved blades or actuator disk models. The solver uses RANS/URANS methods on overset grids and fast implicit algorithms to speed-up convergence. A related publication has been already produced and reported in the reference[46.]. An example of the N-S wake modelling and streamlines evaluation is shown in Figure 41 while Figure 42 illustrates 6 CONCLUSIONS The present paper described the objectives and the structure of the GARTEUR Action Group HC/AG-22 basic research project dealing with the evaluation of forces on obstacles in rotor wake. The project, started in November 2014 and has a duration of three years with the conclusion planned for October 2017. The activities were structured in four work packages the main ones being represented by experimental activities and numerical investigations. The experimental activities consisted of four low-budget wind tunnel test campaigns, complementary to each other, conducted in the CIRA, ONERA, PoliMi and UoG test facilities, and aimed at analysing, with a wider and deeper insight, different aspects of the phenomenology. All campaigns, with the exception of the CIRA one, were concluded. All partners were also involved in numerical investigations during which in-house developed or commercial computational tools were applied. Preliminary computations were performed with the aim to test the initial modelling capabilities of the proposed tools. Code enhancements are also carried out for those cases where an improvement of the tools capabilities was required in order to better simulate the phenomenology under investigation. All partners are currently involved in the final validation activity. [10.] [11.] [12.] Finally, a consistent dissemination activity is promoted by the project with about ten papers presented at conferences or published on relevant scientific journals. 7 [1.] [2.] [3.] [4.] [5.] [6.] [7.] [8.] [9.] REFERENCES Woo, H.G.C., Peterka, J.A., Cermak, J.E., “WindTunnel Measurements in the Wake of Structures,” Doc. 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