bluuebunny/crossref_metadata_2025 huggingface dataset:
Infra‐slow modulation of fast beta/gamma oscillations in the mouse visual system
Authors: Orlowska‐Feuer Patrycja , Allen Annette Elisabeth , Brown Timothy Matthew , Szkudlarek Hanna Jowita , Lucas Robert James , Storchi Riccardo
Published: January 2021

Key points

Neurophysiological activity in the subcortical visual system fluctuates in both infra‐slow and fast oscillatory ranges, but the level of co‐occurrence and potential functional interaction of these rhythms is unknown.

Analysing dark‐adapted spontaneous activity in the mouse subcortical visual system, we find that these two types of oscillation interact uniquely through a population of neurons expressing both rhythms.

Genetic ablation of rod/cone signalling potentiates infra‐slow and abolishes fast beta/gamma oscillations while genetic ablation of melanopsin substantially diminishes the interaction between these two rhythms.

Our results indicate that in an intact visual system the phase of infra‐slow modulates fast beta/gamma oscillations.

Thus one possible impact of infra‐slow oscillations in vision is to guide visual processing by interacting with fast narrowband oscillations.

Infra‐slow (<0.02 Hz) and fast beta/gamma (20–100 Hz) oscillations in neurophysiological activity have been widely found in the subcortical visual system. While it is well established that fast beta/gamma oscillations are involved in visual processing, the role (if any) of infra‐slow oscillations is currently unknown. One possibility is that infra‐slow oscillations exert influence by modulating the amplitude of fast oscillations, yet the extent to which these different oscillations arise independently and interact remains unknown. We addressed these questions by recordingin vivospontaneous activity from the subcortical visual system of visually intact mice, and animals whose retinal network was disrupted by advanced rod/cone degeneration (rd/rd cl) or melanopsin loss (Opn4–/–). We found many neurons expressing only one type of oscillation, and indeed fast oscillations were absent inrd/rd cl. Conversely, neurons co‐expressing the two oscillations were also common, and were encountered more often than expected by chance in visually intact but notOpn4–/–mice. Finally, where they co‐occurred we found that beta/gamma amplitude was modulated by the infra‐slow rhythm. Our data thus reveal that: (1) infra‐slow and beta‐gamma oscillations are separable phenomena; and (2) that they actively co‐occur in a subset of neurones in which the phase of infra‐slow oscillations defines beta‐gamma oscillations amplitude. These findings suggest that infra‐slow oscillations could influence vision by modulating beta‐gamma oscillations, and raise the possibility that disruptions in these oscillatory behaviours contribute to vision dysfunction in retinal dystrophy.
Model-Based Unified Framework for Detection and Mitigation of Cyclic Torque Imbalance in a Gasoline Engine
Authors: Anjum Raheel , Yar Ahmed , Murtaza Ghulam , Ahmed Qadeer , Bhatti A. I.
Published: January 2021

Torque produced by the internal combustion (IC) engine is desired to be of similar value for consecutive combustion cycles; nevertheless, difference occurs in the cyclic torque due to uncertainties in its generation. Variations between output work of successive combustion cycles are considered as the main cause of imbalance in the cyclic torque. Such variations are displayed in engine output torque and affect the engine performance. In this paper, a novel model-based unified framework is proposed for the detection and mitigation of cyclic torque imbalance in a gasoline engine by employing the first principle-based engine model (FPEM). Engine speed dynamics in the model are transformed to develop the direct relationship between engine speed dynamics and fuel input. Fault in fuel injection subsystem is induced to generate imbalance in the cyclic torque. FPEM-based uniform second-order sliding mode (USOSM) observer is applied for estimation of the unknown input, i.e., net piston force (fn) from engine speed dynamics to detect imbalance in the cyclic torque. Estimated net piston force (f⁁n) is utilized to design the control law for certainty equivalence super twisting algorithm (CESTA) based fault tolerant control (FTC) technique to mitigate the torque imbalance. The results of numerical simulation demonstrated that the desired objective is achieved by the proposed unified framework.
Effects of Surface Irregularities and Coatings on Thermal Emittance of Selected Metals Between 373 and 1073 K
Authors: Tanda Giovanni , Misale Mario
Published: January 2021

In many areas of engineering, radiation heat transfer plays an important role and it is of great importance the knowledge of the thermal radiative properties of the surfaces involved. Radiation properties of solid materials are highly dependent on surface characteristics, e.g., surface roughness, surface damage, oxide layers, and deposited thin films, and cannot be assumed as those of pure materials, typically referred to optically smooth surfaces (OS). An experimental investigation of the thermal emittance of some metals (nickel, titanium, silver, and stainless steel) is presented. Experiments were conducted by using a radiometric apparatus able to measure the total normal emittance under different temperature and pressure conditions. The aim of this paper was to identify the separate roles of surface microgeometry alterations (surface roughness), surface damage, and surface coatings (i.e., presence of either thin films deposited onto a smooth surface or oxide layers formed on both smooth and rough surfaces) by undertaking carefully selected sets of experiments covering a relatively large temperature range.
Flexible-Joint Humanoid Balancing Augmentation via Full-State Feedback Variable Impedance Control
Authors: Spyrakos-Papastavridis Emmanouil , Dai Jian S.
Published: January 2021

This paper attempts to address the quandary of flexible-joint humanoid balancing performance augmentation, via the introduction of the Full-State Feedback Variable Impedance Control (FSFVIC), and Model-Free Compliant Floating-base VIC (MCFVIC) schemes. In comparison to rigid-joint humanoid robots, efficient balancing control of compliant bipeds, powered by Series Elastic Actuators (or harmonic drives), requires the design of more sophisticated controllers encapsulating both the motor and underactuated link dynamics. It has been demonstrated that Variable Impedance Control (VIC) can improve robotic interaction performance, albeit by introducing energy-injecting elements that may jeopardize closed-loop stability. To this end, the novel FSFVIC and MCFVIC schemes are proposed, which amalgamate both collocated and non-collocated feedback gains, with power-shaping signals that are capable of preserving the system's stability/passivity during VIC. The FSFVIC and MCFVIC stably modulate the system's collocated state gains to augment balancing performance, in addition to the non-collocated state gains that dictate the position control accuracy. Utilization of arbitrarily low-impedance gains is permitted by both the FSFVIC and MCFVIC schemes propounded herein. An array of experiments involving the COmpliant huMANoid reveals that significant balancing performance amelioration is achievable through online modulation of the full-state feedback gains (VIC), as compared to utilization of invariant impedance control.
Influence of Tip Clearance on the Hydrodynamic Damping Characteristics of a Hydrofoil
Authors: Zeng Yongshun , Yao Zhifeng , Zhang Shijie , Wang Fujun , Xiao Ruofu
Published: January 2021

Tip clearance in hydraulic machines may complicate the fluid–structure interaction (FSI) effects. In this investigation, a mode-based approach (modal work) is evaluated and employed to quantitatively predict the added mass, added stiffness, and hydrodynamic damping ratio, in relation to the first-order bending mode of a vibrating hydrofoil. The investigated relative tip clearance ranges from 0.067% to 2% of the span length. The predicted vortex shedding frequency, natural frequency, and hydrodynamic damping ratio of the hydrofoil are in good agreement with the previously published experimental results, with relative deviations within 9.92%, 6.97%, and 11.23%, respectively. Simulation results show that the added mass, added stiffness, and hydrodynamic damping ratio increase inversely as the tip clearance increases. In particular, as the relative tip clearance increases from 0.067% to 2%, the added mass in still water, the added stiffness, and hydrodynamic damping ratio at a velocity of 10 m/s decrease by 18.66%, 9.36%, and 27.99%, respectively. As the tip clearance increases, the inversely increased pressure difference between the upper and lower surfaces of the vibrating hydrofoil is the main reason for the inversely increased hydrodynamic damping ratio. This is due to the energy leakages via the tip clearance region increase as the tip clearance increases, which may cause less fluid force to resist the vibration of the hydrofoil, resulting in less negative modal work done by the fluid load on the hydrofoil.
The Effects of Specific Heat and Viscosity on Film Cooling Behavior
Authors: Wiese Connor J. , Rutledge James L.
Published: January 2021

For many years, there has been interest in evaluating the effect of density differences between the coolant and the freestream in terms of the cooling effectiveness. Numerous experiments have been conducted with different cooling gases or different temperature gases to evaluate the effect of the density ratio. With little agreement on the best way to scale the density ratio effect, it has become commonplace for some researchers to insist upon matching the density ratio for the experimental work. Unfortunately, the density is not the only property that differs between the various coolant gases used in experiments, and it is certainly not the only property difference between the coolant and the freestream in actual engines. In the present work, we isolate some of these effects through film-cooling experiments with carefully selected and conditioned coolant gases at near-identical densities but exhibiting other property differences. Most significantly, coolant specific heat varied, but subtle viscosity and thermal conductivity effects were present. Through measurements of the adiabatic effectiveness from a film-cooling hole on a leading-edge model, we are able to show that the specific heat effect is just as important as the density effect, providing more evidence that effects in prior research attributed to density differences are actually a combination of density and other property differences.
Active Flow Control Utilizing an Adaptive Blade Geometry and an Extremum Seeking Algorithm at Periodically Transient Boundary Conditions
Authors: Werder Tobias , Liebich Robert , Neuhäuser Karl , Behnsen Clara , King Rudibert
Published: January 2021

As a consequence of constant volume combustion in gas turbines, pressure waves propagating upstream the main flow into the compressor system are generated leading to incidence variations. Numerical and experimental investigations of stator vanes have shown that active flow control (AFC) by means of adaptive blade geometries is beneficial when such periodic incidence variations occur. A significant risk reduction in a compressor facing disturbances can thereby be achieved concerning stall or choke. Experimental investigations on such an AFC method with simultaneous application of a closed-loop control are missing in order to demonstrate its potential. This work investigates a linear compressor cascade that is equipped with a 3D-manufactured piezo-adaptive blade structure. The utilized actuators are piezoelectric macro-fiber-composites. A throttling device is positioned downstream the trailing edge plane to emulate an unsteady combustion process. Periodic transient throttling events with a frequency of up to 20 Hz cause incidence changes to the blade’s leading edge. Consequently, pressure fluctuations on the blade’s surface occur, having a significant impact on the pressure recovery downstream of the stator cascade. Experimental results of harmonically actuating the piezo-adaptive blade with the corresponding disturbance frequency show that the impact of disturbances can be reduced to approximately 50%. However, this is only effective if the phase shift of the harmonic actuation is adjusted correctly. Using an inadequate phase shift reverses the positive effects, causing the aforementioned stall, choke, or significant losses. In order to find the optimum phase shift, even under varying, possibly unpredictable operating conditions, an extremum seeking controller is presented. This gradient-based approach is minimizing the pressure variance over time by carefully adjusting the phase shift of the harmonic actuation of the AFC system.
Effects of Mach Number and Secondary Flows on Ultra-Low Aspect Ratio Radial Outflow Turbine Cascade Aerodynamics
Authors: Grönman Aki , Tiainen Jonna , Uusitalo Antti
Published: January 2021

Radial outflow turbines are an alternative for axial turbines for example in heat recovery applications. They are, however, also often characterized by ultra-low aspect ratios. In these designs, the secondary losses dominate the overall loss share, and under a certain aspect ratio, the secondary structures from the hub and shroud begin to interact. This interaction causes a decrease in aerodynamic performance. Previous studies have suggested that the general flow phenomena between radial outflow and axial turbines could share several similarities due to observed trends in performance prediction. The blade outlet Mach number is known to affect the spanwise positions of the secondary vortices in axial turbine blading and therefore, its effect is also tested here for an ultra-low aspect ratio radial outflow turbine cascade. In addition, there are currently no cascade level experimental data publicly available, and the suitability of axial turbine loss correlations under these conditions remains an open question. From this background, the current study presents an experimental, numerical, and loss correlation analysis of the effects of an isentropic Mach number in a radial outflow turbine cascade. An experimental campaign is used to validate the numerical model both quantitatively and qualitatively. In addition, the validity of the axial turbine loss correlation is extended to ultra-low aspect ratios by introducing a new variable called penetration length. The main findings are: 1. The flow phenomena do not differ significantly from what has been observed with axial turbines, 2. The effect of penetration length calculation method on the loss breakdown is relatively low, and 3. With ultra-low aspect ratio radial outflow turbines, the loss breakdown is markedly changed when the extended Benner’s approach is employed.
Large Eddy Simulation of a Transonic Linear Cascade With Synthetic Inlet Turbulence
Authors: Bertolini Ettore , Pieringer Paul , Sanz Wolfgang
Published: January 2021

The aim of this work is to predict the boundary layer transition and the heat transfer on a highly loaded transonic turbine cascade using Large Eddy Simulations (LESs) with prescribed inlet synthetic turbulence. The numerical simulations were performed for the flow in a linear turbine cascade tested at the von Karman Institute for Fluid Dynamic (MUR test case). For the numerical case, two operating conditions with two different levels of free-stream turbulence intensity are evaluated. For the lower turbulence level case (Tu = 0.8%, MUR132) a laminar inflow is used for the LES simulations whereas for the higher one (Tu = 6%, MUR237) the inlet turbulence is prescribed by using the Synthetic Eddy Method (SEM) of Jarrin. The first part of this work deals with the LES setup. The standard Smagorinsky model was used as closure model. A value of the Smagorinsky constant CS = 0.05 was chosen whereas the turbulent viscosity was reduced in the region closest to the wall by changing the definition of the Smagorinsky length scale. To handle the strong fluctuations in the flow field the cell fluxes are computed using the WENO-P scheme. In the second part, precursor RANS and LES simulations are used to set the optimal values of the SEM parameters and to guarantee the correct level of turbulence at the blade leading edge. The turbulence decay of the synthetic turbulence is compared with the one of the RANS κ–ωSST model. Finally, a comparison between experimental and numerical results is done and the ability of LES to predict the boundary layer transition and the heat transfer on the blade surface is evaluated for the two different inflow conditions.
Robust Aerodynamic Design of Nacelles for Future Civil Aero-Engines
Authors: Schreiner B. Deneys J. , Tejero Fernando , MacManus David G. , Sheaf Christopher
Published: January 2021

As the growth of aviation continues it is necessary to minimise the impact on the environment, through reducing NOx emissions, fuel-burn and noise. In order to achieve these goals, the next generation of Ultra-High Bypass Ratio engines are expected to increase propulsive efficiency through operating at reduced specific thrust. Consequently, there is an expected increase in fan diameter and the associated potential penalties of nacelle drag and weight. In order to ensure that these penalties do not negate the benefits obtained from the new engine cycles, it is envisaged that future civil aero-engines will be mounted in compact nacelles. While nacelle design has traditionally been tackled by multi-objective optimisation at different flight conditions within the cruise segment, it is anticipated that compact configurations will present larger sensitivity to off-design conditions. Therefore, a design method that considers the different operating conditions that are met within the full flight envelope is required for the new nacelle design challenge.

The method is employed to carry out multi-point multi-objective optimisation of axisymmetric aero-lines at different transonic and subsonic operating conditions. It considers mid-cruise conditions, end-of-cruise conditions, the sensitivity to changes in flight Mach number, windmilling conditions with a cruise engine-out case and an engine-out diversion scenario. Optimisation routines were conducted for a conventional nacelle and a future aero-engine architecture, upon which the aerodynamic trade-offs between the different flight conditions are discussed. Subsequently, the tool has been employed to identify the viable nacelle design space for future compact civil aero-engines for a range of nacelle lengths.
Combustion System Upgrades for High Operation Flexibility and Low Emission: Design, Testing and Validation of the SGT5-4000F
Authors: Blaette Lutz , Boettcher Andreas , Streb Holger
Published: January 2021

The first Siemens AG SGT5-4000F was introduced in 1996. Since then, the combustion system was improved in several evolutionary steps in order to enhance the engine performance and emission requirements. Especially emissions requirements will become more challenging in the future as the typical limits will likely drop from 25ppm NOx to 15ppm or even single digit. In addition to clean power solutions, the market simultaneously demands increased operational flexibility in terms of load gradients, fuel flexibility (special fuels, hydrogen addition) and turndown capability.

The SGT5-4000F fleet consists of more than 350 units in operation with ∼20Mio accumulated operating hours. Retrofit ability to the fleet is therefore an important aspect of the combustion system development.

The paper reports upon the development of the latest improvements of the combustion system of SGT5-4000F engines, which have been achieved by application of the latest numerical design tools, advanced statistical machine learning techniques and utilization of additive manufacturing. Results from testing and validation of the modifications at the Siemens Clean Energy Center (CEC) for stand-alone combustion tests and Berlin Test Facility (BTF) for full scale engine tests will be presented.

The system improvement includes aero-dynamic, fuel / air mixing and combustor cooling air management optimizations, which are the typical focus areas for lean premixed combustion systems in gas turbines.
Sensitivity of Multi-Stage Compressor Performance Assessment to Measurement Rake Positions
Authors: Chilla M. , Pullan G. , Thorne G.
Published: January 2021

For an accurate performance assessment of a multi-stage compressor, the circumferentially non-uniform flow at the compressor exit needs to be understood and sampled in a way that minimizes uncertainties. To quantify the effect of the measurement rake positions in the exit duct on compressor performance a combined computational and experimental approach is used on a modern 4-stage compressor. The computational analysis is based on unsteady calculations of a 180-degree sector of the test compressor and experimental verification is provided by comparing to area-traverse data downstream of the outlet guide vanes.

It is shown that the exit measurement rakes are subject to circumferential flow variations caused primarily by the combined effect of the potential field of the struts housed within the exit duct and the wakes originating from the outlet guide vanes. A circumferential camber pattern, applied to the outlet guide vanes, designed to shield the upstream compressor blade rows against the potential field of the exit struts, is found to reduce the amplitude of the circumferential variation in stagnation pressure and shift its circumferential phase. Recognizing that a smaller numerical model, consisting only of the last rotor, the outlet guide vanes and the exit struts, is sufficient to capture the relevant flow mechanisms, the circumferential variations in stagnation pressure and temperature at the rake position are quantified as a function of the exit capacity. The stagnation pressure and temperature uncertainty within a +/-2 deg circumferential range around the nominal rake position is found to be up to 2.25 times larger than the change of the nominal values over an 87.1–106.0% variation of the exit capacity.

Three options to position the rakes to reduce the uncertainty in compressor efficiency are presented — moving the rake downstream as well as leaning and verniering the rakes over the outlet guide vane pitch. Moving the rake from the leading edge to the trailing edge plane of the exit struts reduced the efficiency uncertainty by 2.6%, while leaning and verniering the rakes reduced the efficiency uncertainty by 0.2% and 0.7% respectively. The knowledge gained from the large-scale, detailed CFD predictions can used to support future measurement campaigns.
Competitive Bidding by Surrogate Modeling of Steam Parameter Influence on the Attainable Start Numbers of Turbine Casings
Authors: Seiler Marcel , Züch Vitali , Dumstorff Peter , Almstedt Henning
Published: January 2021

The continued expansion of fluctuating energy sources such as wind turbines and solar systems will increase the demand for more flexible operation modes of power plants. Especially steam turbines with all their components will have to sustain a higher amount of start-stop cycles in order to compensate for variations in wind and solar radiation. Besides the rotor, inner casings are an example for main steam turbine components which are strongly loaded by thermal cycles at each start and shut down procedure. A precise prediction of the attainable number of start-stop cycles enables a more flexible operation within the guaranteed lifetime. However, this would require time-consuming FE calculations for each power plant due to their specific steam parameters.

In this paper, a physics based surrogate model is discussed for a fast prediction of permissible start-stop cycles at plant specific steam parameters. The correlation between the physical properties from the surrogate model (wall temperature difference and the resulting stresses) and the attainable number of start-stop cycles from the FE model is determined. A validation with a different inner casing design within a usual wall temperature range confirms the high accuracy level of the surrogate model compared to uncertainties like material scatter or casting tolerances. With the provided approach typically a higher number of starts can be efficiently calculated in the bidding phase compared to assuming only one conservative value for each turbine type or size. Furthermore, the steam parameters can be optimized for increasing the number of starts to the required value without additional and time-consuming FE calculations.
Modeling of Natural Gas Composition Effect on Low NOx Burners Operation in Heavy Duty Gas Turbine
Authors: Romano S. , Meloni R. , Riccio G. , Nassini P. C. , Andreini A.
Published: January 2021

This paper addresses the impact of natural gas composition on both the operability and emissions of lean premixed gas turbine combustion system. This is an issue of growing interest due to the challenge for gas turbine manufacturers in developing fuel-flexible combustors capable of operating with variable fuel gases while producing very low emissions at the same time. Natural gas contains primarily methane (CH4) but also notable quantities of higher order hydrocarbons such as ethane (C2H6) can also be present. A deep understanding of natural gas combustion is important to obtain the highest combustion efficiency with minimal environmental impact.

For this purpose, Large Eddy Simulations of an annular combustor sector equipped with a partially-premixed burner are carried out for two different natural gas compositions with and without including the effect of flame strain rate and heat loss resulting in a more adequate description of flame shape, thermal field and extinction phenomena.

Promising results, in terms of NOx, compared against available experimental data, are obtained including these effects on the flame brush modelling, enhancing the fuel-dependency under non adiabatic condition.
A Bayesian Approach for the Identification of Cascade Loss Model Strategy
Authors: Simoni Daniele , Lengani Davide , Petronio Daniele , Bertini Francesco
Published: January 2021

A Bayesian method has been used to identify the best model strategy to describe the profile losses of low pressure turbine (LPT) cascades operating under unsteady inflow. The model has been tuned with experimental data measured in a large scale cascade facility, equipped with a moving bar system. Tests have been carried out on two different cascades, investigating three different reduced frequencies, three mass flow coefficients and several Reynolds numbers (up to eight) per condition, accounting for an overall amount of 51 different combinations of these parameters for each cascade. The predictor functions included into the model have been varied starting from a classic polynomial formulation for each influencing parameter, and then with functional relationships mimicking physical constrains and loss tendencies.

Different combinations of the predictors, also including different types and orders of the cross-terms, have been evaluated by means of a Bayesian model selection method searching for the maximum probability of the model in fitting the cloud of experimental data. In particular, the evaluation of the Model Evidence (ME) using the Bayesian Information Criterion approximation (BIC) has allowed obtaining sufficient accuracy and avoiding overfitting at the same time.

The best model here identified will be shown to be able to well reproduce the loss surface of a third different cascade that does not participate to the model selection. Realistic profile loss evolutions outside of the design space tested are provided, thus also allowing for a generalization of the structure of the model for other applications and future works.
Compressed Air Energy Storage for a Small Size Standalone Plant Powered by a Solar Power Unit and a Gas Turbine
Authors: Arnulfi Gianmario L. , Croce Giulio
Published: January 2021

Energy storage can balance heat and power demand and production over different time scales, with both technical and economic benefit. Several devices have been proposed, but only two are really utilized: hydro-pumped storage, for large size plants, and electrochemical battery energy storage, for medium and small plants. Compressed Air Energy Storage (CAES) was proposed as an alternative, and two well-known plants, Hunthorf (Germany) and Mac Intosh (United States) have been successfully working for many years. However, due to the huge capital costs, this concept never became widely popular. Here, a bit different approach is proposed: one or more reciprocating compressors and pressurized reservoirs can supply an additional amount of air to a gas turbine. During the “charging” phase, the reciprocating compressors pump air into the reservoirs; during the “discharging” phase, the turbo-expander is fed by both the turbo-compressor and the reservoirs in parallel. The turbo-compressor is partially relieved and fuel can be saved for the same power. This paper focuses on a standalone small size user, served by a solar power unit coupled with a micro gas turbine. The aim is to lay down rules for a proper storage managing.
Template-Based Hexahedral Mesh Generation for Turbine Cooling Geometries
Authors: Hao Muting , Wang Feng , Hope-Collins Joshua , Rife Max E. , di Mare Luca
Published: January 2021

This paper describes a multiblock grid generation method for turbine cooling geometries. The method is based on the observation that cooling films are essentially branches inserted on a large trunk, represented by the passage or by the cooling duct. The small size of the films compared to the overall size of turbine blades allows simplifications to be introduced with respect to general-purpose trunk and branch algorithms. The grid generation starts from an existing layout for the passage or cooling duct grid and operates on a Cartesian patch of the trunk surface. The patch is hollowed and a templated branch layout is inserted. Padding blocks are created to connect the two layouts into a single, boundary conforming layout. The resulting multiblock grid is then smoothed using a modification of Thompson’s Poisson system. The boundary mesh distribution is not prescribed. Instead, boundary orthogonality is enforced and elliptic smoothing is performed on the boundaries as well as inside the volume. The grid size control relies on a novel Newton-like update for the control functions of the Poisson system. The smoothing step is essential in achieving good grid quality throughout and determines, in part, the template for a given configuration. The algorithm is particularly suitable for large arrays of films or other cooling decoration and results show that the proposed method can produce grids of better quality than existing methods.
Meanline Calculation of Surge Margin Loss due to Inlet Flow Distortion
Authors: Menegozzo Luca , Benini Ernesto
Published: January 2021

In the present work, a methodology for the preliminary analysis of axial compressors operating under clean and distorted inflow conditions is discussed. A 1D mean-line solver has been developed, and the validation has been carried out under clean conditions considering the datasets of the subsonic Rolls-Royce HP9, as well as the transonic NASA Rotor 37 and NASA Rotor 67. Numerical results have been reported together with experimental data in terms of performance maps and spanwise distributions. The ARP1420 procedure and the parallel compressor theory have been implemented for the impact assessment of inlet flow distortion on the compressor surge line. A full-annulus CFD simulation of the NASA Rotor 37 has been carried out, in order to generate high-fidelity benchmark results. A 180° circumferential distortion has been considered as inlet boundary condition, and the surge line has been calculated using the two numerical models.
Parameter Extension Simulation of Turbulent Flows in a Compressor Cascade With a High Reynolds Number
Authors: Jin Yan
Published: January 2021

The turbulent flow in a compressor cascade is calculated by using a new simulation method, i.e., parameter extension simulation (PES). It is defined as the calculation of a turbulent flow with the help of a reference solution. A special large-eddy simulation (LES) method is developed to calculate the reference solution for PES. Then, the reference solution is extended to approximate the exact solution for the Navier-Stokes equations. The Richardson extrapolation is used to estimate the model error. The compressor cascade is made of NACA0065-009 airfoils. The Reynolds number 3.82 × 105 and the attack angles −2° to 7° are accounted for in the study. The effects of the end-walls, attack angle, and tripping bands on the flow are analyzed. The PES results are compared with the experimental data as well as the LES results using the Smagorinsky, k-equation and WALE subgrid models. The numerical results show that the PES requires a lower mesh resolution than the other LES methods. The details of the flow field including the laminar-turbulence transition can be directly captured from the PES results without introducing any additional model. These characteristics make the PES a potential method for simulating flows in turbomachinery with high Reynolds numbers.
Prediction of Ventilation Effectiveness for LM9000 Package With Machine Learning
Authors: Corsini Alessandro , Delibra Giovanni , Giovannelli Marco , Lucherini Gabriele , Minotti Stefano , Rossin Stefano , Tieghi Lorenzo
Published: January 2021

Gas turbines usually are installed inside an enclosure, which is used as protection from the external environment and to provide an acoustic insulation. A ventilation system is required to control the temperature inside the enclosed volume and to dilute any potential gas leakage that may come from faulty pipes or flanges. The system has to be properly designed to avoid any unexpected explosion which would generate an overpressure not contained by enclosure walls. The most common approach to predict the effectiveness of the ventilation system requires to perform CFD analyses, which are very expensive in computational terms. A new approach has been proposed by authors, using machine learning and artificial neural networks (ANN) to identify the poorly ventilated zones. This methodology has been further developed, optimized and applied to a real gas turbine packages of new generation. In the present paper the authors will show the application of this procedure to the LM9000 package and the comparison with the results predicted using conventional CFD techniques. The tangible improvement introduced by this methodology is that the computational time is reduced from about three weeks with the common CFD approach to few minutes. The artificial neural network is developed in a Python environment that is applied during the CFX post-process phase of a steady state CFD simulation, providing results equivalent to unsteady CFD simulation. Besides the immediate benefits of this particular application, the suggested approach looks to be a great candidate to substitute the conventional and time-consuming CFD simulations with a fast post-processing algorithm that is able to learn and self-optimize as long as it is used.
Effect of Friction on the Structural Dynamics of Built-Up Structures: An Experimental Study
Authors: Fantetti Alfredo , Schwingshackl Christoph
Published: January 2021

Frictional contacts are a major source of uncertainty in the correct prediction of the dynamic response of built-up structures. This uncertainty is partially due to a limited understanding of the effects of friction on dynamic responses. Vice versa, dynamic responses can also affect the frictional behaviour of the interfaces in contact.

In the present study, the mutual relationships between frictional behaviour and structural dynamics are investigated by means of a high frequency friction rig. The rig is characterised by a simple and localised frictional contact that is needed to accurately measure hysteresis loops. Of course, the rig also has its own dynamic response, and consequently represents an excellent test case to gain a better understanding of the correlation between hysteresis loop shapes and their effect upon the dynamics.

Impact hammer tests and shaker tests were performed on the friction rig, and lead to changes in the damping and stiffness of its dynamic response, which were linked to variations in the frictional behaviour of the contact. Furthermore, there was some indication as to how certain resonances of the system might strongly affect the frictional behaviour. In particular, it was observed that full sliding causes excitation of structural modes that in turn lead to distortions in the measured hysteresis loops.

These findings confirm the strong relationship between friction and dynamics, thus highlighting the necessity to include a detailed frictional description of contacting interfaces for more accurate modelling of the dynamics of built-up structures.
Multi-Criteria Selection of Wind Turbine Using TOPSIS Approach
Authors: Rehman Shafiqur , Khan Salman A. , Alhems Luai M.
Published: January 2021

The recent revolution in the use of renewable energy worldwide has opened many dimensions of research and development for sustainable energy. In this context, the use of wind energy has received notable attention. One critical decision in the development of a wind farm is the selection of the most appropriate turbine compatible with the characteristics of the geographical location under consideration in order to harness maximum energy. This selection process considers multiple decision criteria which are often in conflict with each other, as improving one criterion negatively affects one or more other criteria. Therefore, it is desired to find a tradeoff solution where all selection criteria are simultaneously optimized to the best possible level. This paper proposes a TOPSIS (The Technique for Order of Preference by Similarity to Ideal Solution) based approach for multi-criteria selection of wind turbine. Three decision criteria, namely, hub height, wind speed, and net capacity factor are used in the decision process. A case study is shown on real data collected from the Aljouf region located at an altitude of 753 meters above sea level in the northern part of Saudi Arabia. Seventeen turbines with rated capacities ranging from 1.5 GW to 3 GW from various manufacturers are evaluated. Results indicate that Vestas V110 turned out to be the most appropriate turbine for the underlying site.
Identification of Losses in Turbomachinery With Machine Learning
Authors: Angelini Gino , Corsini Alessandro , Delibra Giovanni , Giovannelli Marco
Published: January 2021

One of the issues of handling large CFD datasets and process them to derive important design correlations is the limitation in automating the post-processing of data. Machine learning techniques, developed to process large unlabelled dataset, can play a key role on this subject.

In this work an unsupervised approach to isolate different flow features inside a 2D cascade is proposed and validated. The approach relies on machine learning methods and in particular on Exploratory Data Analysis (EDA) and Principal Component Analysis for the pre-processing of the data and on K-means clustering for the post-processing.

The K-means algorithm was trained on a Design of Experiments (DoE) of over 140 cases of 2D linear cascade configurations to identify the boundary layer on the profiles and the wake downstream. Validation resulted in a perfect capability of identifying the regions of interest.

Then a possible exploitation of this method is presented, to compute pressure losses downstream of the cascade and train an artificial neural network to make a regression able to extend data to all the possible combinations of geometrical and operating parameters of the cascade.

The same algorithm was applied to 3D flow cascades of profiles with sinusoidal leading edges to stress its extrapolation capability in case of flow regimes not present in the training DoE.
Heat Transfer Performance of Internal Cooling Turbine Blades Using Cutted-Root Rib Design
Authors: Dinh Cong-Truong , Vu Tai-Duy , Dinh Tan-Hung , Nguyen Phi-Minh
Published: January 2021

In gas turbines, the turbine blades are always working in the highly temperature overhead the permissible metal temperatures. To safe operation, the turbine blades are needed to cool. Many researchs in turbine cooling technology can be categorized as internal and external cooling. This paper presents an investigation of cutted-root rib design, where a part of rib was truncated below to create an extra-passage in the root rib applied in the internal cooling turbine blades of jet engine using three-dimensional Reynolds-averaged Navier-Stokes with the SST model. The object of this investigation is to reduce the vortex occurring near the rib for improving the performance of heat transfer, such as the Nusselt number and thermal performance factor. To investigate the heat transfer performance and fluid flow characteristics of internal cooling turbine blades, a parametric study of the cutted-root rib was performed using various geometric parameters related to the height and shapes of the extra-passage. The cutted-root rib geometry is designed in ANSYS DesignModeler, and then meshed by using ICEM-CFD, analysed and post-processed using Ansys-CFX. The numerical results showed that all heat transfer parameter with the cutted-root rib design was greater than the original case without cutted-root rib.
Root Cause Analysis of Torsional and Lateral Vibrations on a Compressor Test Rig
Authors: Rouwenhorst Driek , Jäger Daniel , Schmieder Marcel , Hermann Jakob
Published: January 2021

In this case study an extensive root cause analysis was performed on a compressor test rig following the observation of unexpected high amplitude vibrations during the rig commissioning. Typically, the location and the frequency of an observed vibration is not enough to pinpoint the cause of the behavior in a system with multiple components. The test rig comprising of an electric motor, gearbox and compressor rotor may be subject to rotordynamics, torsional dynamics, mechanical eigenfrequencies and fluid dynamics in the compressor.

In order to understand the source of the vibrations observed at the gearbox, lateral vibrations of the casing and torsional vibrations of the rotor were measured simultaneously. In parallel, the system was modeled in MADYN 2000 in order to match the observations with theoretical torsional eigenmodes of the drivetrain.

The modeling of the system resulted in a torsional eigenfrequency lying within 10% of the observed gearbox vibration, making it a viable candidate to excite gearbox vibrations through a varying torque. To be able to measure this mode shape, a temporal measurement setup for torsional vibrations had to be installed.

The measurements of the test campaign revealed that a torsional resonance is indeed present at the frequency of the observed vibrations. However, a lack of coherence between the torsional and lateral vibrations suggests that the torsional resonance is not the root cause of the gearbox vibrations. Two separate resonances were observed as the gear mesh frequency crossed the torsional eigenfrequency and the observed high amplitude gearbox vibration. Evidence of a structural eigenfrequency of the gearbox was found through an impact test. Although the torsional resonance is ruled out as the root cause, it is presumed that the two resonances do experience a positive coupling, causing the particularly high gearbox vibrations.
Synergistic Technology Combinations for Future Commercial Aircraft Using Liquid Hydrogen
Authors: Rompokos Pavlos , Rolt Andrew , Nalianda Devaiah , Isikveren Askin T. , Senné Capucine , Grönstedt Tomas , Abedi Hamidreza
Published: January 2021

Liquid hydrogen (LH2) has long been seen as a technically feasible fuel for a fully sustainable greener aviation future. The low density of the cryogenic fuel would dictate the redesign of commercial aircraft to accommodate the large tanks, which are unlikely to be integrated within the whole internal volume of the wing.

In the ENABLEH2 project, the morphological aspects of a LH2 aircraft design are discussed and a methodology for rapid concept comparative assessment is proposed. An exercise is then carried on to down-select short-to-medium range (SMR) and long-range (LR) concepts, able to carry 200 passengers for 3000 nmi and 414 passengers for 7500 nmi respectively.

The down-selection process was split into two phases with the first considering 31 potential airframe architectures and 21 propulsion-system arrangements. The second phase made the final down-selections from a short-list of nine integrated design concepts that were ranked according to 34 criteria, relating to operating cost, revenue, noise and safety. Upon completion of the process, a tube and wing design with the tanks integrated into extended wing roots, and a blended-wing-body design were selected as the best candidates for the SMR and LR applications respectively. Both concepts feature distributed propulsion to maximise synergies from integrating the airframe and propulsion systems.
Experimental Analysis of Surge-Detection System Based on Pressure Derivatives at Part-Speed Operation
Authors: Liskiewicz Grzegorz , Kabalyk Kirill , Jaeschke Andrzej , Grapow Filip , Kulak Michał , Kryłłowicz Władysław , Ouyang Hua , Shen Xin
Published: January 2021

This paper presents tests of an anti-surge system based on pressure derivatives. The control algorithm was proven to work on different machines and with different unstable flow phenomena.

Compressors are known to be affected by unstable flow conditions appearing at low mass flow rate conditions. The best known and most dangerous phenomenon is surge, which is a global instability affecting the entire impeller and regions upstream and downstream from it. A list of identified local phenomena includes among others: impeller rotating stall, diffuser rotating stall and inlet recirculation. All have a specific pressure signature that is used for early identification.

The method presented in this paper is based on a control parameter named the Rate of Derivative Fluctuation (RDF). This approach involves a simple measure of flow instability that is universal and reacts to flow disturbances. RDF has been already confirmed to identify inlet recirculation and surge. The aim of this study is to conduct real-time tests of an anti-surge system implementing the RDF algorithm triggering the safety valve opening. The study confirmed the optimal position of the monitoring point. The results showed that the RDF is indeed sensitive to different types of flow instabilities appearing in different impellers, and that it provides efficient flow stability monitoring.