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ZNMF Pulsed Jet-based Active Flow Control of the UHBR-induced Flow through High Fidelity CFD

Periodic Reporting for period 3 - X-Pulse (ZNMF Pulsed Jet-based Active Flow Control of the UHBR-induced Flow through High Fidelity CFD)

Reporting period: 2020-02-01 to 2020-10-31

The X-Pulse project directly addresses the challenges inherent to the development and integration of UHBR powerplant on large passenger aircraft specifically, but more generally on any aircraft equipped with close-coupled powerplant installation. It aims at developing innovative active flow control strategies, based on synthetic pulsed jets, also referred to as ZNMF for Zero Net Mass Flux, to mitigate the flow separation induced by UHBR powerplant installation, when the aircraft is operated at a high angle of attack/low speed i.e. during take-off, initial climbing, and landing.
The proposed technological solutions stand at a TRL4 level and they will serve the improvement of the aerodynamic performances of the aircraft during these critical operating conditions while turning them off will guarantee optimal aerodynamic efficiency during cruise flight phases.

The specific objectives of X-Pulse are listed below:
• Determination of a predictive CFD methodology able to accurately estimate the flow field around a complex geometry but simplified view
of an aircraft equipped with a UHBR powerplant and operated in real flight conditions.
• Adaptation of this CFD methodology for the prediction of the impact of ZNMF pulsed jets within the Nacelle-Powerplant-Wing junction region (hereafter denoted NPW), in particular in terms of control of the flow separation.
• Determination of the optimal ZNMF-based active flow control strategy to implement on the aircraft, able to mitigate the flow separation and to improve the aerodynamics performance of the aircraft during take-off, initial climbing, and landing. This optimization relies on a multi-objective optimization formulation, by analyzing the influence of several parameters of the active flow control strategy including but not limited to actuation location, momentum coefficient, outlet geometries, actuation frequency, and amplitude modulation.
A high fidelity CFD methodology has been developed to accurately predict the flow around a complex geometry modeling UHBR powerplant-equipped aircraft, for low speed/moderate-to-high incidence flight conditions. In this context, the original geometry is the DLR F15 UHBR powerplant-equipped model. Flow incidences were varied from -4° to +24°, with a chord-based Reynolds number equal to 21.5M and a Mach number Ma=0.23.

So far, the project members have
• Identified the unsteadiness of the flow in regard to the most effective excitation frequency of the piezoelectric-based actuators, considering various flow control parameters (actuators position, momentum, amplitude modulation, frequency)
• Performed a large set of unsteady RANS computations on the DLR F15 geometry, first with/without pulsed jet-based flow forcing, (i.e. 169 ZNMF SJA) using predefined locations and actuation parameters.
• Developed an innovative body force source term-based ROM approach to model ZNMF pulsed-jet based actuators, that they have applied to 2 well-referenced test cases: the flat plate and the NASA CFDVAL2004 workshop bump cases, comparison with 3 other strategies, then to the Fraunhofer actuator implemented on the baseline. This approach has been fully validated, both with a steady and unsteady approach
• Complemented the study with a comprehensive analysis of 2 alternative geometries, referred to as variant 1 and variant 2 to secure the profitability of the project. Variant 2 reveals to be the most effective configuration in restoring a relevant flow in the region of the UHBR powerplant/pylon/wing interaction, similar to the one referenced in the few available previous studies.

Based on this study, geometry "variant 2" is now considered as the baseline geometry for the upcoming multi-parametric flow control optimization study.
Besides, control-off, unsteady simulations have revealed characteristic frequencies one order of magnitude lower than the ones required to produce significant pulsed jet velocities using the ZNMF actuators. The unsteady approach is thus not appropriate in the context of a multi-parametric optimization approach for obvious cost reasons. The proposed strategy is to apply the optimization process completed on the basis of a steady approach, while a reverse engineering approach will determine maximum pulsed velocity for a given frequency of the ZNMF Fraunhöfer SJA-based pulsed jets.

The project members have then conducted research to optimize both the location on the wing, the momentum, the blowing angles of a large series of ZNMF actuators, and finally, the blowing frequency, to mitigate the flow separation induced by the UHBR powerplant/wing integration effects.

A noticeable improvement of the flow has been achieved, with up to 87% reduction of the flow separation induced by the UHBR powerplant/wing integration effects.
This proposal aims at identifying optimal flow control parameters for application of pulsed air blowing with Zero Net Mass Flux (ZNMF) in order to control flow separation over the engine/pylon junction of civil aircraft featuring innovative turbofan engines.
To this purpose, X-Pulse will contribute to investigating the potential of a peculiar type of flow control (i.e. ZNMF pulsed blowing), leveraging previous or currently running EU projects like AFLoNext (“Active Flow- Loads & Noise control on next generation wing”). Viable and efficient systems for active flow control could play a major role in maximizing the performance of these UHBR engines.

The identification of optimal actuation parameters for separation mitigation, which is the final aim of X-Pulse, has a measurable impact at different levels:
• direct contribution to environmental objectives: reduction of fuel consumption, direct impact on CO2 and NOx emissions, perceived noise;
• direct contribution to the improvement of passenger comfort and safety;
• direct impact on the components market, since systems and equipment are required to increase their intrinsic performance in order to accomplish new aircraft requirements without penalizations in weight and volume;
• potential impact to other innovations, like for instance design, development and testing of innovative component technologies for advanced, reliable, robust and efficient flow control hardware.
• potential impact on the exploitation of enabling technologies for business jet applications thanks to the synergies of design objectives with Large Passenger Aircraft (i.e. high subsonic design Mach numbers and the targeted powerplant systems).

X-Pulse will have a direct and measurable impact on improving innovation capacity by pushing the state of the art in two challenging fields:
• the simulation and understanding of complex, highly 3-dimensional unsteady flow patterns (usually accompanied by partly separated flow regions) generated by i) the very close aerodynamic interaction occurring between components in a large-sized nacelle engine/wing combination, ii) the introduction of AFC devices in such systems.
• the development of a novel, multi-disciplinary conceptual design framework, in which engine and wing are consistently integrated since the beginning, so that the maximum benefit in terms of overall aircraft level efficiency can be achieved. This multi-disciplinary conceptual design shall embed AFC technologies in the loop as well, with a clear performance target, so these technologies are no longer add-on solutions implemented in order to mitigate a problem arising after the design closure.
vortices produced by the UHBR/wing interaction without flow control promote flow separation
control-off vs. control-on illustrates the efficiency of ZNMF actuators to reduce flow separation