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This paper demonstrates the closed-loop control of the frequency and phase of a fluidic oscillator using acoustic excitation. It is shown experimentally that the use of acoustic excitation modifies the passive feedback mechanism and slows down the jet switching process. Closed-loop control is employed to vary the oscillation frequency at a fixed flow rate and track a sinusoidal reference target without phase error, using measurements from a pitot probe in the device. The controller is demonstrated to be effective at rejecting disturbances, which is illustrated with both time and frequency domain data. The controller’s ability to track step changes to the reference signal phase is tested, and the closed-loop bandwidth is shown to be around 20% of the oscillation frequency, in close agreement with the theoretical prediction.

This paper presents the application of a multidisciplinary approach for the preliminary design and evaluation of the potential improvements in performance and environmental impact through the utilization of compressed (CGH2) and liquefied (LH2) hydrogen fuel for a civil tilt-rotor modelled after the NASA XV-15. The methodology deployed comprises models for rotorcraft flight dynamics, engine performance, flight path analysis, hydrogen tank and thermal management system sizing. Trade-offs between gravimetric efficiency, energy consumption, fuel burn, CO2 emissions, and cost are quantified and compared to the kerosene-fuelled rotorcraft. The analysis carried out suggests that for these vehicle scales, gravimetric efficiencies of the order of 13% and 30% can be attained for compressed and liquid hydrogen storage, respectively leading to reduced range capability relative to the baseline tilt-rotor by at least 40%. At mission level, it is shown that the hydrogen-fuelled configurations result in increased energy consumption by at least 12% (LH2) and 5% (CGH2) but at the same time, significantly reduced life-cycle carbon emissions compared to the kerosene counterpart. Although LH2 storage at cryogenic conditions has a higher gravimetric efficiency than CGH2 (at 700 bar), it is shown that for this class of rotorcraft, the latter is more energy efficient when the thermal management system for fuel pressurization and heating prior to combustion is accounted for.

Shear layers act as demodulators when subjected to amplitude modulated, acoustic perturbations. Recent work explained that the demodulation is a result of the hypothesized relationship between the excitation waveform and the vorticity signal of the large-scale structures, which was represented by a half-wave rectification model. In this paper, this model is explored by overmodulating the excitation signal amplitude. The rectifier model predicts several effects of overmodulation on the flow response, including a doubling of the demodulated response frequency. To validate these predictions, a free, round jet that is excited acoustically upstream of the nozzle is studied using particle image velocimetry. Analytical results from the model are confirmed using a second experimental setup where a jet that emerges from a nozzle and attaches to an adjacent, inclined wall is excited acoustically. Beyond the insight it provides into shear layer vortex dynamics, overmodulation serves as a useful excitation technique for practical applications, where the shear layer response frequency can be increased at the expense of the response amplitude.

The concept of variable rotor speed (VRS) has been recognized as an efficient means to improve rotorcraft operational performance and environmental impact, with electrification being a potential technology to further contribute to that. This paper explores the impact of optimal implementation and scheduling of VRS and power management strategy for conventional and hybrid-electric rotorcraft on energy, fuel, and emissions footprint. A multidisciplinary simulation framework for rotorcraft performance combined with models for engine performance and gaseous emissions estimation is deployed. A holistic optimization approach is developed for the derivation of globally optimal schedules for combined rotor speed and power split targeting minimum energy consumption. Application of the derived optimal schedules at mission level resulted to a 6% improvement in range capability for the VRS tilt-rotor relative to its conventional counterpart. For the hybrid-electric tilt-rotor, combined optimization of VRS and power management leads to an increase in range to 18.4% at 40% and 25% reduced payload for current (250 Wh/kg) and future (450 Wh/kg) battery technology, respectively. For representative urban air mobility (UAM) scenarios, it is demonstrated that the VRS concept resulted in up to 10% and 14% reductions in fuel burn and NOx relative to the nominal rotor speed case, respectively. The utilization of the combined optimum VRS and power split schedules can boost performance with reductions of the order of 20% and 25% in mission fuel/CO2 and NOx at a reduced payload relative to the conventional tilt-rotor.

An integrated engine cycle design methodology and mission assessment for parallel hybrid electric propulsion architectures are presented in this paper. The aircraft case study considered is inspired by Fokker 100, boosted by an electric motor on the low-pressure shaft of the gas turbine. The fuel burn benefits arising from boosting the low-pressure shaft are discussed for two different baseline engine technologies. A three-point engine cycle design method is developed to redesign the engine cycle according to the degree of hybridization. The integrated cycle design and power management optimization method is employed to identify potential fuel burn benefits from hybridization for multiple mission ranges. Genetic algorithm-based optimizer has been used to identify optimal power management strategies. The sensitivity of these mission results has also been analyzed for different assumptions on the electric powertrain. With 1 MW motor power and a battery pack of 2307 kg, a maximum of 3% fuel burn benefit can be obtained by retrofitting the gas turbine for 400 nm mission range. Optimizing the power management strategy can improve this fuel burn benefit by 0.2–0.3%. Redesigning the gas turbine and optimizing the power management strategy, finally provides a maximum fuel benefit of 4.2% on 400 nm. The results suggest that a high hybridization by power, low hybridization by energy, and ranges below 700 nm are the only cases where the redesigned hybrid electric aircraft has benefits in fuel burn and energy consumption relative to the baseline aircraft. Finally, it is found that the percentage of fuel burn benefits from the hybrid electric configuration increases with the improvement in engine technology.

The effect of acoustic excitation on the aerodynamic performance of NACA0015 aerofoil at $\alpha=11^{\circ}$ and $Re_c = 1.0 \ x 10^6$ based on $c=0.35m$ chord length is investigated numerically using Improved Delayed Detached Eddy Simulations (IDDES). Flow separation is observed over the suction surface in the uncontrolled flow that forms a separated shear layer, resulting in aerodynamic performance degradation. External acoustic excitation is used in controlling the flow separation by exciting the separated shear layer. Acoustic excitation frequency and amplitude are the control parameters considered. The flow field is excited at a frequency of the most amplified disturbances and at first harmonic and sub-harmonic frequencies with a constant excitation amplitude. Unlike the harmonics, when the separated shear layer is excited at $F+=1$, the separation characteristics of the uncontrolled flow field are significantly altered. Unsteady disturbances are locked into the excitation frequency and transition is promoted within the shear layer. A separation bubble is generated over the suction surface with a reattachment region aft of the aerofoil surface. Vortex coherence and organized wake structures are also improved. The effect of excitation amplitude is investigated at the frequency of the most amplified disturbances. Increasing the excitation amplitude has two predominant impacts; it delays the boundary layer separation and advances the reattachment location, thereby reducing the separation bubble length. The results also suggest that there is a maximum effective acoustic excitation amplitude.

A convex formulation is proposed for optimal energy management in aircraft with hybrid propulsion systems consisting of gas turbine and electric motor components. By combining a point-mass aircraft dynamical model with models of electrical and mechanical powertrain losses, the fuel consumed over a planned future flight path is minimised subject to constraints on the battery, electric motor and gas turbine. The resulting optimisation problem is used to define a predictive energy management control law that takes into account the variation in aircraft mass during flight. A simulation study based on a representative 100-seat aircraft with a prototype parallel hybrid electric propulsion system is used to investigate the properties of the controller. We show that an optimisation-based control strategy can provide significant fuel savings over heuristic energy management strategies in this context.

Fluidic oscillators show promise for use in aerodynamic flow control applications, with many studies reporting oscillation frequencies in the 1–10 kHz range. Spyropoulos, “A Sonic Oscillator,” introduced a “sonic” oscillator whose oscillation frequency depends on the inlet flow rate. The purpose of this paper is to demonstrate the existence of a second mode of operation (mode II) for such an oscillator, with a separate physical mechanism to the traditional, flow rate-dependent mode (mode I). Mode II is shown to be a back-pressure-driven oscillation that is largely independent of flow rate once instigated. This is explained by a stationary wave formed along the outlet paths, and occurs when conditions on the degree of back pressure and the weakening of the Coandă attachment strength are met. For a fixed device geometry, the conditions reduce to a minimum flow rate threshold so that the combination of high flow rate and constant oscillation frequency could make mode II an attractive flow control solution in an industrial context where minimizing device size is often critical.

The design of efficient, environmentally friendly, and quiet powerplant for rotorcraft architectures constitutes a key enabler for urban air mobility (UAM) application. This work focuses on the development and application of a generic methodology for the design, performance, and environmental impact assessment of a parallel hybrid-electric propulsion system, utilizing simple and advanced recuperated engine cycles. A simulation framework for rotorcraft analysis comprising models for rotor aerodynamics, flight dynamics, and hybrid-electric powerplant performance is deployed for the design exploration and optimization of a hybrid-electric rotorcraft, modeled after the NASA XV-15, adapted for civil applications. Optimally designed powerplants for payload-range capacity, energy efficiency, and environmental impact have been obtained. A comparative evaluation has been performed for the optimum designs. The respective tradeoffs between engine, heat exchanger weight, thermal efficiency, as well as mission fuel burn and environmental impact have been quantified. It has been demonstrated that a recuperated gas turbine-based hybrid-electric architecture may provide improvements of up to 6% in mission range capability without sacrificing useful load. At the same time, analyses performed for a representative 100 km mission suggest reductions in fuel burn and NOX emissions of up to 12.9% and 5.2%, respectively. Analyses are carried at aircraft and mission level using realistic UAM mission scenarios.

The increasing demands for air-taxi operations together with the ambitious targets for reduced environmental impact have driven significant interest in alternative rotorcraft architectures and propulsion systems. The design of hybrid-electric propulsion systems (HEPSs) for rotorcraft is seen as being able to contribute to those goals. This work aims to conduct a comprehensive design and tradeoff analysis of hybrid powerplants for rotorcraft, targeting enhanced payload-range capability and fuel economy. An integrated methodology for the design, performance assessment, and optimal implementation of HEPSs for conceptual rotorcraft has been developed. A multidisciplinary approach is devised comprising models for rotor aerodynamics, flight dynamics, HEPS performance, and weight estimation. All models are validated using experimental or flight test data. The methodology is deployed for the assessment of a hybrid-electric tilt-rotor, modeled after the NASA XV-15. This work targets to provide new insight into the preliminary design and sizing of optimally designed HEPSs for novel tilt-rotor aircraft. The paper demonstrates that at present, current battery energy densities (250 Wh/kg) severely limit the degree of hybridization if a fixed useful payload and range are to be achieved. However, it is also shown that if advancements in battery energy density to 500 Wh/kg are realized, a significant increase in the level of hybridization and hence reduction of fuel burned and carbon output relative to the conventional configuration can be attained. The methodology presented is flexible enough to be applied to alternative rotorcraft configurations and propulsion systems.

The switching mechanism and underlying flow physics of an actively controlled fluidic device are investigated using both large eddy simulation (LES) and particle imaging velocimetry (PIV). The fluidic device considered herein uses acoustic excitation of inherent flow instabilities to control the movement of the jet. Acoustic excitation at the preferred frequency is shown to yield high saturation amplitudes resulting in the formation of large vortical structures that do not undergo pairing. Basic flow features including the shear layer instabilities are further examined to explain why the excitation mode that triggers the switching process changes from a shear layer-based mode (Stθ=0.012) to a jet orifice mode (Sth=0.25) as the Reynolds number increases.

Fluidic valves based on the Coand\u{a} effect are increasingly being considered for use in aerodynamic flow control applications. A limiting factor is their variation in switching time, which often precludes their use. The purpose of this paper is to demonstrate the closed-loop control of a recently developed, novel piezo-fluidic valve that reduces response time uncertainty at the expense of operating bandwidth. Use is made of the fact that a fluidic jet responds to a piezo tone by deflecting away from its steady state position. A control signal used to vary this deflection is amplitude modulated onto the piezo tone. Using only a pressure measurement from one of the device output channels, an output-based LQG regulator was designed to follow a desired reference deflection, achieving control of a 90 m/s jet. Finally, the controller's performance in terms of disturbance rejection and response time predictability is demonstrated., Comment: 31 pages, 23 figures. Published in AIAA Journal, 4th May 2020

This paper investigates an ultrasonically driven bistable fluidic diverter at inlet nozzle Mach numbers of up to Mn = 0.3 and operating pressure ratios of up to Pr = 1.1. Part I examines the switching characteristics with respect to nondimensional parameters of excitation amplitude, frequency, required energy, switching time and inlet total pressure. It is shown that to promote switching at turbulent jet Mach numbers of up to Mn = 0.3 it is necessary to excite a jet preferred mode of St = 0.45 which differs from previously reported laminar jet operation of the similar device. For the reference case the switching time amounts to 1.2 ms suggesting oscillation frequencies of up to 500 Hz. Part II is a combined experimental and numerical study that examines the triggered instability modes in the free shear layer using large eddy simulations (LES) and visualizes the flow field using Particle Image Velocimetry (PIV).

This paper describes measurements of the thermal response times of a compressor casing from a large civil engine as a function of offtake flows. The experiments used a full-size engine compressor casing and full-sized offtake mass flow rates. The experiments were performed using the Oxford Transient Heat Transfer facility (THTF).The temperature response rates of the casing to a fast ramp change in input air temperature were measured at several metal and air locations. It is shown that the transient temperature behaviour at each location can be accurately characterised by a single time constant. The variation of time constant with offtake mass flow is also shown.Such characterised experimental data can provide valuable validated information on casing thermal response much earlier than the approach of waiting for an engine development programme thermocouple test, and thus bring forward the achievement of optimum transient casing behaviour.

The effect of acoustic excitation on a low Reynolds number jet with constant centerline velocity u0 but varying velocity profile u(y) is investigated experimentally by particle imaging velocimetry. Different initial conditions at the nozzle orifice are here used with the intent to characterize the relation between the jet preferred mode fp and the natural shear layer mode fn. The jet response to acoustic excitation is described in terms of the centerline velocity decay and the downstream increase in momentum thickness. This study intends to shed some light onto the duality of the two most fundamental flow instabilities and their controversial dependency on each other.

The dynamics of an actively controlled fluidic diverter with novel actuation method are presented. This bistable fluidic valve is based on the Coanda effect and is able to switch the main flow solely by means of acoustic excitation. The switching is explained through a combination of experiments and large eddy simulations (LES). The switching time and minimum energy required are characterized for a range of pressure ratios, acoustic excitation frequencies, and input powers. It is shown that the switching mechanism depends on the excitation of natural instabilities inside the free shear layer. An enhanced roll-up of vortices at the excitation frequency increases the rate of entrainment and results in a transverse pressure gradient sufficient to counteract the Coanda effect leading to jet detachment and switching.

This paper presents a novel approach to real-time modeling of disk temperature distribution using proper orthogonal decomposition (POD). The method combines singular value decomposition (SVD) techniques with a series of low-order transfer functions to predict the disk's thermal response over a typical flight. The model uses only typically available full authority digital electronic control (FADEC) measurements to predict temperature with accuracy of ±30 K over the whole flight cycle. A Kalman filter has also been developed based on a single temperature measurement, and the location of the measurement has been assessed in order to select the most appropriate target for instrumentation. Points all around the front and back of the disk have been assessed, and the best practice result is found to be near the center of the disk neck. This represents a compromise between matching the fast dynamic response of the rim, with the slower dynamics of the cob. The new model has been validated against an independent flight simulation.

This paper presents a computational investigation into the impact of cooling air injected through the stationary over-tip turbine casing on overall turbine efficiency. The high work axial flow turbine is representative of the high pressure turbine of a civil aviation turbofan engine. The effect of active modulation of the cooling air is assessed, as well as that of the injection locations. The influence of the through-casing coolant injection on the turbine blade over-tip leakage flow and the associated secondary flow features are examined. Transient (unsteady) sliding mesh simulations of a one turbine stage rotor-stator domain are performed using periodic boundary conditions. Cooling air configurations with a constant total pressure air supply, constant mass flow rate and actively controlled total pressure supply are assessed for a single geometric arrangement of cooling holes. The effects of both the mass flow rate of cooling air and the location of its injection relative to the turbine rotor blade are examined. The results show that all of the assessed cooling configurations provided a benefit to turbine row efficiency of between 0.2 and 0.4 percentage points. The passive and constant mass flow rate configurations reduced the over-tip leakage flow, but did so in an inefficient manner, with decreasing efficiency observed with increasing injection mass flow rate beyond 0.6% of the mainstream flow, despite the over-tip leakage mass flow rate continuing to reduce. By contrast, the active total pressure controlled injection provided a more efficient manner of controlling this leakage flow, as it permitted a redistribution of cooling air, allowing it to be applied in the regions close to the suction side of the blade tip which more directly reduced over-tip leakage flow rates and hence improved efficiency. Cooling air injected close to the pressure side of the rotor blade was less effective at controlling the leakage flow, and was associated with increased aerodynamic loss in the passage vortex.

The Transient Heat Transfer Facility (THTF) was developed at the University of Oxford to test full-scale high pressure compressor and turbine casing air systems using gas turbine engine representative secondary air system mass flow rates, total temperatures and total pressures. Transient casing response together with blade and disc responses governs achievable tip clearances in both compressors and turbines. Traditionally, casing thermal response is faster than disc response and so requires cooling. However, future engine designs may have thermal responses of the discs that are faster than the casings. In this paper we investigate the use of air impingement as a means to speed up the casing response to match such future designs. Two different impingement configurations were selected from a total of eight designs based on steady Reynolds-averaged Navier-Stokes (RANS) predictions of maximum heat transfer coefficient (HTC). The 3D thermal growth of the casing was characterised by the surface temperature rise over a given period of time to assess achievable dynamic response. The configurations were tested in the THTF as a rainbow set. The resulting casing metal temperatures were used in conjunction with an explicit solution of the conduction in the casing wall to calculate HTC due to impingement. The experimental set-up resembles a typical aircraft engine and therefore includes measurement uncertainties arising from features such as fixtures, seals, geometries and large surface areas that are then subjected to varying thermal inertias. These can lead to circumferential temperature non-uniformities, as evident from the experimental results. An uncertainty analysis was performed to quantify these effects on the overall casing thermal response. The experimental data was then compared against numerical predictions from an axisymmetric, 90° sector, conjugate heat transfer model of the facility using the two impingement plate designs. Overall, the values of HTC agreed well across the experimental and numerical results. Both approaches predicted differences in which of the two designs was more effective; however, advantages and limitations were identified in both approaches. The combined experimental and numerical study shows the significance of analysing the full annulus, at engine representative conditions and the benefit of an impingement array to potentially speed up casing response for future engines.

The combined use of the closed-loop paradigm, an augmented autonomous state space formulation, partial invariance, local affine difference inclusion, and polytopic invariance are deployed in this paper to propose an NMPC algorithm which, unlike earlier algorithms that have to tackle online a nonlinear non-convex optimization problem, requires the solution of a simple QP. The proposed algorithm is shown to outperform earlier algorithms in respect of size of region of attraction and online computational load. Conversely, for comparable computational loads, the proposed algorithm outperforms earlier algorithms in terms of optimality of dynamic performance. Copyright © 2005 John Wiley and Sons, Ltd.

Linear terminal control laws In model predictive control imply the need to compromise between performance and size of terminal region. A remedy allows the control law to vary with time, but requires very large online computation. Here, use is made of nonlinear terminal laws comprising an interpolation between predefined laws, each with its own precomputed terminal set. This reduces the online computation significantly, yet results in large terminal sets defined by the convex hull of invariant ellipsoids. The use of a different prediction class also produces significant benefits with respect to performance.

We present some initial findings of a long-term project to identify the flight dynamics of large birds of prey, during free-flight outdoors. This paper outlines our methods for dealing with the particular challenges a project of this nature provides, and some preliminary results outlining the flight characteristics of soaring flight, and providing detailed descriptions of some of the manoeuvres observed during flight.

Extending disc life through online health monitoring has been a proven method of minimising engine downtime and maintenance costs. To properly monitor the disc requires a robust model of the disc’s non-linear thermal dynamics. A model can be improved by filtering the output using a measurement of the disc in real time. The damage models can then be computed with higher statistical confidence leading to increased safe life prediction. Recently, a model of disc temperature has been developed based on the proper orthogonal decomposition of simulated data. The model produced detailed thermal gradients for use in damage calculation and life assessment. This paper presents the development and implementation of a Kalman filter to augment that model. The location of the measurement has been assessed in order to select the most appropriate target for instrumentation. Points all around the front and back of the disc have been assessed, and the best practice result is found to be near the centre of the disc neck. Matching temperatures at this point represents a compromise between the fast dynamic response of the rim, with the slower response of the cob. The new model has been validated against an independent flight simulation that had previously been excluded from any training process. The addition of the Kalman filter allows the model to match aircraft dynamics outside the regular training trajectories. The accuracy is approximately ±30K, and there is a root-mean-square error of only 2K over the whole model at any one point in time.

The continuous demand from the airlines for reduced jet engine fuel consumption results in increasingly challenging high pressure turbine nozzle guide vane (NGV) working conditions. The capability to reproduce realistic boundary conditions in a rig at the combustor-turbine interaction plane is a key feature when testing NGVs in an engine-representative environment. A large scale linear cascade rig to investigate NGV leading edge cooling systems has been designed with particular attention being paid to creating engine representative conditions at the inlet to the NGVs. The combustor simulator replicates the main features of a rich-burn design including large dilution jets and extensive endwall film cooling. A three-dimensional computational domain including the entire combustor simulator has been created and RANS CFD simulations have been run in order to match Reynolds number and mainstream-to-coolant momentum flux ratio; velocity and turbulence measurements have been acquired at the NGV inlet plane at ambient temperature. In this engine-representative environment the authors focused their attention on the flow field downstream of different endwall film cooling holes configurations: three arrangements of a double row of staggered cylindrical holes (lateral pitch-to-diameter ratio of 2–3–6) and one with intersecting holes (intersecting angle of 90°) are experimentally and numerically analyzed. Velocity, turbulence intensity and integral length scales are predicted and measured for a density ratio of 1 and coolant-to-mainstream momentum flux of 6. A hot wire sensor was mounted on a two-axis traverse mechanism able to move the probe in the spanwise and lateral directions. Three slots allowed to reposition the traverse and take measurements at three downstream locations (stream-wise distance-to-diameter ratio of 4.2–9.2–14.2). The research confirmed the strong influence of the endwall coolant on the flow field at the NGV inlet plane and the hole spacing results a key parameter in managing the film development. Closer-spaced hole configurations can assure an effective film coverage. The integral length scales are strongly connected to the hole diameter and spacing. Intersecting holes can potentially reduce the amount of required coolant at a fixed pressure ratio, but they offer worst film performance than cylindrical holes. RANS simulations proved to be able to get the main trends shown by the measurements.Copyright © 2015 by Rolls-Royce plc

This paper describes the design and development of a major new facility capable of reproducing flow conditions for large civil aero engine turbine and compressor segment cavities. The facility reproduces in 3-D, typical secondary air system cruise temperatures, pressures and mass flows. The facility allows better understanding of the circumferential heat transfer effects of air system flows on turbine and compressor casings, as these have a significant influence on both steady-state and transient blade tip clearance behavior. Different air system architectural solutions are considered and 1-D transient air system modelling is used to design architecture to ensure that they meet tight performance requirements in terms of allowable pressure and temperature ripples when subject to fast switching of flows. An explanation of the design of the double-skin pressure vessel using 2-D axisymmetric thermo-mechanical analysis to understand the life-limiting features of the design is discussed. Finally we conclude the paper with some experimental results from rig commissioning and the use of these results in the determination of engine casing transients is discussed.Copyright © 2015 by Rolls-Royce plc