|Low noise aeroacoustic design for turbofan powered NATO air vehicles|
|Applied Vehicle Technology|
acoustic shielding, acoustics of noncircular jet, audible detection, Flyover noise prediction, installed jet noise, low noise design process, low noise propulsion integration
Military air vehicles cover a broad range of size and function, from large transports to small U(C)AVs to the turbojets of high-powered fighters. The typically intense exterior noise generated by all of these is of aerodynamic origin and this noise represents a problem in peace times during training operation and at wartime due to the acoustic detectability of operating combat aircraft. The state of the art of aircraft noise prediction and reduction is aligned with civil transport aircraft, while much less effort has been invested in noise reduction of military aircraft. Noise reduction concepts can only partially be adopted from the civilian side, because of the largely different aircraft configurations in the military domain. Military aircraft noise is practically entirely determined by engine noise. While turbofan engine noise reduction for civil aircraft is a very active topic of research and technology, any progress in this field is completely insufficient for military aircraft. The engine integration in combination with the aircraft configuration at military aircraft is vastly different which changes the relevant sound source mechanisms and also provides significant potential for noise reduction, not available at typical tube-and-wing civil aircraft. For instance, conceptually, certain techniques, exploited to reduce the infra-red signature of combat aircraft may be transferred to reduce sound radiation. The following STO research activities in the past have contributed to set the background to the proposed RTG:
• AVT-132 “Noise issues arising from the operation of gas turbine powered military air vehicles”
• the specialists meeting AVT-158 addressed the noise from high-powered aircraft such as fighters, both in the community and on a carrier deck
• AVT-233 “Aeroacoustics of engine installation of military air vehicles” has established a validated computational tool basis to enable the prediction of acoustic installation effects of arbitrary configurations, achieved by numerical simulation (CAA=Computational Aeroacoustics). Starting from generic geometries (airfoil) and ending with a military transport configuration and the agile NATO type UAS configuration SACCON the RTG was focused on validating these CAA codes in view of their capability to predict the acoustic shielding of engine noise. The source was deliberately kept highly generic (basically a monopole point source).
• AVT-251 has been providing the overall aircraft design of an aerodynamically advanced and realistic configuration of SACCON type, called MULDICON, which forms the focus of the proposed RTG; in that respect also AVT-161 and AVT-201 were instrumental in laying the foundations for AVT-251.
The new AVT TG will build on the work conducted under AVT-233 to now deal with realistic sources and a concurrent agile (unmanned) NATO air vehicle design (MULDICON). In this way the new group will naturally integrate the multidisciplinary design work of AVT-251 with the acoustic knowledge and tools provided by AVT-233. This combined expertise will be used not only to numerically analyze the acoustic properties of the MULDICON aircraft, but will exploit the expertise on acoustic simulation tools gained in AVT-233 to do low noise engine integration design studies on MULDICON. The simulations will be backed by a large scale complex acoustic wind tunnel test on a wind tunnel model to be built, equipped with pressurized air to simulate a highly integrated jet.
Task Group to focus on
• demonstrating (on a realistic configuration) the qualification and applicability of tools for the prediction of the effects of propulsion system installation on received sound (using acoustics characteristics of the propulsion system without the engine itself)
• additionally, the important topic of predicting noise reduction technology as an exercise in aeroacoustic design at agile NATO air vehicles will be covered, particularly at the MULDICON configuration.
• the mentioned objectives imply not only code-to-code comparisons among partners, but a dedicated validation dataset from tests in an appropriate (i.e. large enough size) acoustic wind tunnel, which is to be accomplished as well.
The task group is meant to fill an existing gap as far as i) reliable prediction of noise from military aircraft and ii) simulation assisted low noise design is concerned. After completion of the RTG this capability shall have been demonstrated at a highly relevant agile NATO air vehicle. As far as the knowledge readiness level of the activity is concerned, one has to keep in mind that aeroacoustics is relatively new to the field of aircraft design, especially providing means to include noise in the early design phase of military aircraft.
Although the proposed research task is definitely focusing on a military aircraft class, much of the new knowledge to be gained will be applicable in the civilian domain as well (e.g. sound radiation from semi-buried intakes).
The proposed RTG is naturally concerned with aeroacoustics and the disciplines, directly related to it, i.e. aerodynamics of high speed subsonic flows, including turbulence and aircraft design. Specifically:
• Focus on aero-acoustic installation because of the relevance to acoustic detectability and annoyance of military aircraft. In this respect, the design of geometries which a) maximize the exploitation of acoustic shielding, and b) minimize installation related excess source noise.
• qualification of aircraft noise prediction methods with regard to propulsion system installation effects, as far as installation sources for jet and acoustic installation properties of complex intakes and exhausts is concerned
• source noise of jet-powered agile military air vehicles as e.g. MULDICON
• source to receiver propagation as a basis for acoustic design assessment
• aero-acoustic simulation (CFD/CAA) with tools of different fidelity: RANS+perturbations, Scale resolving approaches, integral methods, as tested/validated on generic configurations in AVT-233
The proposed RTG represents the next R&T step after AVT-233 (and partially AVT-251) on the aeroacoustics of agile air vehicles of SACCON type. While in AVT-233 tools have been tested/validated on relevant but simplified aircraft geometries and with very simple sources, this group considers a much more realistic and detailed geometry including a realistic extended jet noise source. The MULDICON configuration appears highly appropriate, particularly because it has gone through a thorough MD design process in AVT-251. On the one hand, acoustic installation properties of the geometries with respect to (turbofan) noise propagating from the intake and the exhaust will be analysed. On the other hand, also installation sources will be included by considering an installed jet of rectangular cross section as featured by MULDICON. Of particular interest is a first time aero-acoustic design study. The working hypothesis here is to study the effects of e.g. some simple trailing edge extension devices on the radiated jet noise. Since an isolated jet noise test will not be affordable in the RTG as a reference to the installed case, the objective here is to use the design modification as a means to study differences in installation effects. This is not only of practical interest but as well useful in view of validating the simulation codes of the partners for installation effects.
The RTG plans to set up a sequence of respective experiments in a high quality 3m acoustic tunnel, in which aero-acoustic data of a MULDICON wt model will be acquired, characterizing the sound radiation of the installed jet and the shielding properties of intake and exhaust. For this purpose a wind tunnel model will be built and equipped with a pressure supply to generate the jet. The acoustic measurement equipment from AVT-233 will be used. If affordable, a simultaneous measurement of the surface pressure fluctuations and unsteady flow field measurements is desirable to characterize the important nearfield of the jet. The geometry features a jet exhausting over an aft deck/shielding structure and jet/trailing edge interaction will take place, which may increase the radiated noise considerably. The model will also allow for local, acoustics motivated modifications (see above).
While the test will be coordinated by one partner, all partners will contribute with various activities to the design and realization of the model, including flow simulations. Each group will do predictions of either the installed jet and/or the intake/exhaust diffraction problem with their respective aero-acoustic prediction suites. A dedicated code-to-code and codes-to-experiments study will be accomplished.
Final Report on the results of the investigation. Topics covered will include data available for evaluation of tools, tools and methods assessed, and recommendations for future work.