DBIAB - AERO - Journal Articles

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Robust optimization integrating aircraft trajectory and sequence under weather forecast uncertainty
(Elsevier, 2023-07-01) Kamo, Shumpei; Rosenow, Judith; Fricke, Hartmut; Soler Arnedo, Manuel Fernando
Integration of trajectory optimization into sequence optimization is required for next-generation Arrival Managers (AMANs) to support Collaborative Decision-Making (CDM) and implementation of user-preferred 4D trajectories. In addition, considering uncertainty in the optimization is also necessary for making more robust decisions. To achieve these aims, this study proposes a method to integrate the trajectory and sequence of approach aircraft in a single optimization framework and calculate optimal robust solutions against weather forecast uncertainty. This uncertainty is quantified utilizing the ensemble weather forecast and the robust optimizations for trajectory and sequence are formulated in an ensemble approach. To connect the two optimizations, we introduce the so-called performance surfaces, which represent the characteristics of the optimal trajectory. The resulting integrated Trajectory and Sequence (T&S) optimization is a combination of the robust Optimal Control (OC) and Mixed-Integer Nonlinear Programming (MINLP). The MINLP problem is relaxed to the corresponding Nonlinear Programming (NLP) problem to reduce computational costs. In the case study, the trajectory and sequence are simultaneously optimized for two different objectives: the maximum throughput at the merging point and the minimum fuel burn while maintaining the inter-aircraft separation.
Development of fatigue cracks from mechanically machined scratches on 2024-T351 aluminium alloy - part I: experimentation and fractographic analysis
(Wiley, 2017-05-01) Cini, Andrea; P.E., Irving
Clad and unclad 2024-T351 alumini um alloy sheets, weakened by mechanically machined scratches, were fatigued to investigate the effect of small surface damage, like scribe marks, on aircraft fuselage joints. The role of scratch cross section geometry on fatigue life of scribed components was analysed. Scratches between 25 and 185 mum deep, with 5, 25 and 50 mum root radii, were cut on sample surface by using diamond-tipped tools. After testing, fracture surfaces were examined using a scanning electron microscope, and crack growth rates were measured by striation counting. Scratches reduced aluminium fatigue life under tensile and bending load up to 97.8% due to multiple crack nucleation at their roots. Short cracks nucleated from sharp scratches coalesced to formunique elongated cracks growing through sample thickness. Cracks initiated fromscratches were typical short cracks, growing faster than conventional long cracks. Despite the different scribing process, fatigue data of regular diamond tool cut scribes can be used to conservatively predict life reduction owing to ploughed in-service scribe marks on fuselage joints. Finite element analyses on scribed samples and the fatigue life prediction models are described in Part II of this paper.
Effect of laser shock peening on residual stress and fatigue life of clad 2024 aluminium sheet containing scribe defects
(Elsevier, 2012-06-30) Dorman, M.; Toparli, M.B.; Smyth, N.; Cini, Andrea; Fitzpatrick, M.E.; Irving, P.E.
Laser peening at a range of power densities has been applied to 2 mm-thick sheets of 2024 T351 aluminium. The induced residual stress field was measured using incremental hole drilling and synchrotron X-ray diffraction techniques. Fatigue samples were subjected to identical laser peening treatments followed by scribing at the peen location to introduce stress concentrations, after which they were fatigue tested. The residual stresses were found to be non-biaxial: orthogonal to the peen line they were tensile at the surface, moving into the desired compression with increased depth. Regions of peen spot overlap were associated with large compression strains; the centre of the peen spot remaining tensile. Fatigue lives showed moderate improvement over the life of unpeened samples for 50 mum deep scribes, and slight improvement for samples with 150 mum scribes. Use of the residual stress intensity Kresid approach to calculate fatigue life improvement arising from peening was unsuccessful at predicting the relative effects of the different peening treatments. Possible reasons for this are explored.
A length scale for non-local multi-scale gradient interactions in isotropic turbulence
(Cambridge University Press, 2023-09-22) Perez Encinar, Miguel
Three-dimensional turbulent flows enhance velocity gradients via strong nonlinear interactions of the rate-of-strain tensor with the vorticity vector, and with itself. For statistically homogeneous flows, their total contributions to gradient production are related to each other by conservation of mass, and so are the total enstrophy and total dissipation. However, locally, they do not obey this relation and have different (often extreme) values, and for this reason both production mechanisms have been subject to numerous studies, often decomposed into multi-scale interactions. In general lines, their dynamics and contributions to the cascade processes and turbulent kinetic dissipation are different, which poses a difficulty for turbulence modelling. In this paper, we explore the consequence of the `Betchov¿ relations locally, and show that they implicitly define a length scale. This length scale is found to be approximately three times the size of the turbulent structures and their interactions. It is also found that, while the non-locality of the dissipation and enstrophy at a given scale comes mostly from larger scales that do not cancel, the non-local production of strain and vorticity comes from multi-scale interactions. An important consequence of this work is that isotropic cascade models need not distinguish between vortex stretching and strain self-amplification, but can instead consider both entities as part of a more complex transfer mechanism, provided that their detailed point value is not required and a local average of reasonable size is sufficient.
Robust 4D climate-optimal flight planning in structured airspace using parallelized simulation on GPUs: ROOST V1.0
(Copernicus Publications, 2023-07-06) Simorgh, Abolfazl; Soler Arnedo, Manuel Fernando; González Arribas, Daniel; Linke, Florian; Baumann, Sabine; Lührs, Benjamin; Meuser, Maximilian M.; Dietmüller, Simone; Matthes, Sigrun; Yamashita, Hiroshi; Yin, Feijia; Castino, Federica; Grewe, Volker; European Commission
The climate impact of non-CO2 emissions, which are responsible for two-thirds of aviation radiative forcing, highly depends on the atmospheric chemistry and weather conditions. Hence, by planning aircraft trajectories to reroute areas where the non-CO2 climate impacts are strongly enhanced, called climate-sensitive regions, there is a potential to reduce aviation-induced non-CO2 climate effects. Weather forecast is inevitably uncertain, which can lead to unreliable determination of climate-sensitive regions and aircraft dynamical behavior and, consequently, inefficient trajectories. In this study, we propose robust climate-optimal aircraft trajectory planning within the currently structured airspace considering uncertainties in standard weather forecasts. The ensemble prediction system is employed to characterize uncertainty in the weather forecast, and climate-sensitive regions are quantified using the prototype algorithmic climate change functions. As the optimization problem is constrained by the structure of airspace, it is associated with hybrid decision spaces. To account for discrete and continuous decision variables in an integrated and more efficient manner, the optimization is conducted on the space of probability distributions defined over flight plans instead of directly searching for the optimal profile. A heuristic algorithm based on the augmented random search is employed and implemented on graphics processing units to solve the proposed stochastic optimization computationally fast. An open-source Python library called ROOST (V1.0) is developed based on the aircraft trajectory optimization technique. The effectiveness of our proposed strategy to plan robust climate-optimal trajectories within the structured airspace is analyzed through two scenarios: a scenario with a large contrail climate impact and a scenario with no formation of persistent contrails. It is shown that, for a nighttime flight from Frankfurt to Kyiv, a 55 % reduction in climate impact can be achieved at the expense of a 4 % increase in the operating cost.
Linear contrails detection, tracking and matching with aircraft using geostationary satellite and air traffic data
(MDPI, 2023-07-01) Chevallier, R.; Shapiro, M.; Engberg, Z.; Soler Arnedo, Manuel Fernando; Delahaye, D.
Climate impact models of the non- (Formula presented.) emissions of aviation are still subject to significant uncertainties. Condensation trails, or contrails, are one of these non- (Formula presented.) effects. In order to validate the contrail simulation models, a dataset of observations covering the entire lifetime of the contrails will be required, as well as the characteristics of the aircraft which produced them. This study carries on the work on contrail observation from geostationary satellite by proposing a new way to track contrails and identify the flight that produced it using geostationary satellite infrared images, weather data as well as air traffic data. It solves the tracking and the identification problem as one, each process leveraging information from the other to achieve a better overall result. This study is a new step towards a consistent contrail dataset that could be used to validate contrail models.
Guidance for autonomous spacecraft repointing under attitude constraints and actuator limitations
(Elsevier Ltd, 2023-06-01) Ponche, Alison; Marcos Esteban, Andres; Ott, Thomas; Geshnizjani, Ramin; Loehr, Johannes
Current and future space observation missions need to perform many large-angle, multi-axis slew maneuvers between observations while keeping the scientific instrument's attitude in a safe region. The state-of-practice typically divides each multi-axis maneuver into a series of single-axis sub-maneuvers, each of which is computed by restricting its guidance solution to the exact spacecraft momentum capacity. This ensures that the constraints are explicitly considered and results in a simple on-board implementation of the guidance algorithm, but is time-consuming and non-optimal for the whole multi-axis maneuver. Addressing this issue, this article presents a novel analytical guidance approach that relies on the convexity of the permissible attitude zone. The proposed guidance is time-optimal for a given spacecraft design and set of admissible observation targets. Both guidance approaches are compared using a multi-body/multi-actuator benchmark spacecraft, whose complex repointing phase requires an autonomous on-board guidance computation. It is shown that the proposed approach is systematic and that the reduction in maneuver time, compared to the state-of-practice approach, is considerable.
A global-local meta-modelling technique for model updating
(Elsevier, 2024-01-01) Dessena, Gabriele; Ignatyev, Dmitry I.; Whidborne, James F.; Zanotti Fragonara, Luca
The finite element model updating procedure of large or complex structures is challenging for engineering practitioners and researchers. Iterative methods, such as genetic algorithms and response surface models, have a high computational burden for these problems. This work introduces an enhanced version of the well-known Efficient Global Optimisation technique to address this issue. The enhanced method, refined Efficient Global Optimisation or rEGO, exploits a two-step refinement and selection technique to expand the global search capability of the original method to a global–local, or hybrid, search capability. rEGO is tested and validated on four optimisation test functions against the original methods and genetic algorithms with different settings. Good results in terms of precision and computational performance are achieved, so an application for model updating is sought. A penalty function for the finite element model updating is identified in residuals of the modified total modal assurance criterion. Finally, rEGO for finite element model updating is implemented on a hybrid, numerical and experimental, case study based on a well-known experimental dataset and on a higher dimension finite element model of a wing spar. Satisfactory results in terms of precision and computational performance are achieved when compared to the original methods and genetic algorithms, needing two orders of magnitude fewer evaluations and achieving comparable results in terms of precision.
An electric model for bare-photovoltaic tethers in the passive mode
(Elsevier Ltd on behalf of IAA, 2023-11-08) Del Pino Jiménez, Ángel; Sánchez Arriaga, Gonzalo; European Commission
A bare-photovoltaic tether (BPT) is a long conductive tape in orbit that provides propellant-less propulsion and power harvesting. An electric model for a BPT operating in the passive mode and composed of a bare segment and a bare-photovoltaic (bare-pv) segment coated with thin-film solar cells on one side is presented. The pv cells, which are electrically insulated from the conductive substrate, are connected to two electric busbars that also serve as electrical contacts to the spacecraft. The power harvested by the cells can be used by the spacecraft or, as considered in this work, to boost tether performance. The electric model takes into account that, depending on the number and location of zero-bias points, four regimes of operation exist for this type of BPT. Current and voltage profiles solutions that are not possible for standard bare tethers are presented. A parametric analysis of the operation regimes varying the tether length, the length of the pv segment, and the power was carried out. Two complementary methods to find the optimum length of the bare-pv segment for a given total tether length and mission are presented. The first method is based on the optimization of the normalized average current, which controls the strength of the Lorentz force and measures tether efficiency and the second method uses as figure of merit the deorbit time in postmission disposal scenarios. For both of them it is shown that an optimum exists for the length of the pv segment. An enhancement in performance above 30% as compared with standard bare tethers was found for the considered scenario.
Fast 4D flight planning under uncertainty through parallel stochastic path simulation
(Elsevier, 2023-03-01) González Arribas, Daniel; Baneshi, Fateme; Andrés Endériz, Eduardo; Soler Arnedo, Manuel Fernando; Jardines, Aniel; García-Heras Carretero, Javier
The Air Traffic Management system is evolving to deal with efficiency, capacity, safety and environmental challenges. Progress along these fronts requires the development of trajectory planning and prediction tools that can go beyond the current deterministic planning paradigm to deal with an uncertain meteorological and operational context. In this work, we introduce a novel flight planning methodology to generate weather-optimal 4D flight plans under uncertainty. By leveraging general-purpose computing on graphics processing units and combining continuous and discrete elements in an integrated fashion, we can simulate and evaluate multiple trajectory options under multiple scenarios in parallel, allowing us to provide quick iterations to a stochastic optimization algorithm. Our computational experiments show that our proposed solutions can provide efficient solutions in seconds, as required in practical settings, while allowing for simple integration of future extensions thanks to its simulation-based nature.
Fluid-structure resonance in spanwise-flexible flapping wings
(Cambridge University Press, 2023-05-26) Martínez Muriel, Cayetano; Arranz, G.; García-Villalba Navaridas, Manuel; Flores Arias, Óscar; Ministerio de Economía y Competitividad (España)
We report direct numerical simulations of the flow around a spanwise-flexible wing in forward flight. The simulations were performed at Re = 1000 for wings of aspect ratio 2 and 4 undergoing a heaving and pitching motion at Strouhal number Stc ≈ 0.5. We have varied the effective stiffness of the wing Π1 while keeping the effective inertia constant, Π0 = 0.1. It has been found that there is an optimal aerodynamic performance of the wing linked to a damped resonance phenomenon, that occurs when the imposed frequency of oscillation approaches the first natural frequency of the structure in the fluid, ωn,f /ω ≈ 1. In that situation, the time-averaged thrust is maximum, increasing by factor 2 with respect to the rigid case with an increase in propulsive efficiency of approximately 15 %. This enhanced aerodynamic performance results from the combination of larger effective angles of attack of the outboard wing sections and a delayed development of the leading-edge vortex. With increasing flexibility beyond the resonant frequency, the aerodynamic performance drops significantly, in terms of both thrust production and propulsive efficiency. The cause of this drop lies in the increasing phase lag between the deflection of the wing and the heaving/pitching motion, which results in weaker leading-edge vortices, negative effective angles of attack in the outboard sections of the wing, and drag generation in the first half of the stroke. Our results also show that flexible wings with the same ωn,f /ω but different aspect ratio have the same aerodynamic performance, emphasizing the importance of the structural properties of the wing for its aerodynamic performance.
A sensitivity analysis on the influence of the external constraints on the dynamic behaviour of a low pollutant emissions aircraft combustor-rig
(Polish Academy of Sciences, Committee on Machine Building, 2016-06-06) Amoroso, Francesco; Fenza, Angelo de; Petrone, Giuseppe; Pecora, Rosario
The need to reduce pollutant emissions leads the engineers to design new aeronautic combustors characterized by lean burn at relatively low temperatures. This requirement can easily cause flame instability phenomena and consequent pressure pulsations which may seriously damage combustor's structure and/or compromise its fatigue life. Hence the need to study the combustor's structural dynamics and the interaction between elastic, thermal and acoustic phenomena. Finite element method represent a largely used and fairly reliable tool to address these studies; on the other hand, the idealization process may bring to results quite far from the reality whereas too simplifying assumptions are made. Constraints modelling represent a key-issue for all dynamic FE analyses; a wrong simulation of the constraints may indeed compromise entire analyses although running on very accurate and mesh-refined structural models. In this paper, a probabilistic approach to characterize the influence of external constraints on the modal behaviour of an aircraft combustor-rig is presented. The finite element model validation was performed at first by comparing numerical and experimental results for the free-free condition (no constraints). Once the model was validated, the effect of constraints elasticity on natural frequencies was investigated by means of a probabilistic design simulation (PDS); referring to a specific tool developed in the ANSYS®software, a preliminary statistical analysis was at performed via Monte-Carlo Simulation (MCS) method. The results were then correlated with the experimental ones via Response Surface Method (RSM).
Early cataloguing of fragments from break-up events
(Elsevier B.V., 2023-05-15) Pastor Rodríguez, Alejandro; Siminski, J.; Escribano Blázquez, Guillermo; Sanjurjo Rivo, Manuel; Escobar, D.; Comunidad de Madrid
Break-up events represent the dominant source of objects in space catalogues, surpassing half of the overall population. These not so uncommon events include explosions, collisions or anomalous events resulting in fragmentations and their number is estimated to be higher than 630. The early cataloguing of the fragments generated during these events poses a complex challenge for space objects catalogue build-up and maintenance processes. The provision of Space Surveillance and Tracking products and services during the few first days after a break-up event can be crucial to avoid collisions between the fragments and other space objects, particularly in highly congested regimes, such as Low Earth Orbit. In this regard, reducing the time required to accurately estimate the trajectories of the fragments may enable the execution of collision avoidance manoeuvres, in the case of operational space objects with manoeuvre capabilities, and analyse potential collision cascade events, which may endanger the space environment. This paper studies the whole cataloguing process after a break-up event, starting from a catalogue with no fragments from the fragmentation under-analysis, and until a well-established orbit is obtained for all fragments, using a ground-based sensor network. First, the observations enter a multi-sensor multi-target track-to-track association algorithm in charge of grouping observations belonging to the same objects. To resolve the ambiguity, particularly shortly after the event, hypotheses about tracks belonging to the same fragment are generated, scored, pruned, and promoted, only when there is enough confidence, leading to the initialisation of new objects in the catalogue. As soon as the catalogue is populated, a track-to-orbit correlation algorithm is responsible for the correlation of observations and already catalogued orbits. This alleviates the track-to-track association and enables the update of the orbital estimates, required for maintaining the catalogue.
A Review of Conductive Metal Nanomaterials as Conductive, Transparent, and Flexible Coatings, Thin Films, and Conductive Fillers: Different Deposition Methods and Applications
(MDPI, 2018-08-09) Naghdi, Samira; Rhee, Kyong Yop; Hui, David; Park, Soo Jin
With ever-increasing demand for lightweight, small, and portable devices, the rate of production of electronic and optoelectronic devices is constantly increasing, and alternatives to the current heavy, voluminous, fragile, conductive and transparent materials will inevitably be needed in the future. Conductive metal nanomaterials (such as silver, gold, copper, zinc oxide, aluminum, and tin) and carbon-based conductive materials (carbon nanotubes and graphene) exhibit great promise as alternatives to conventional conductive materials. Successfully incorporating conductive nanomaterials into thin films would combine their excellent electrical and optical properties with versatile mechanical characteristics superior to those of conventional conductive materials. In this review, the different conductive metal nanomaterials are introduced, and the challenges facing methods of thin film deposition and applications of thin films as conductive coatings are investigated.
On the onset of instabilities in a Bénard-Marangoni problem in an annular domain with temperature gradient
(National Library of Serbia, 2017-12) Hoyas, Sergio; Ianiro, Andrea; Pérez-Quiles, María J.; Fajardo Peña, Pablo; Ministerio de Economía y Competitividad (España)
This manuscript addresses the linear stability analysis of a thermoconvective problem in an annular domain. The flow is heated from below, with a linear decreasing horizontal temperature profile from the inner to the outer wall. The top surface of the domain is open to the atmosphere and the two lateral walls are adiabatic. The effects of several parameters in the flow are evaluated. Three different values for the ratio of the momentum dffusivity and thermal diffusivity are considered: relatively low Prandtl number (Pr = 1), intermediate Prandtl number (Pr = 5) and high Prandtl number (ideally Pr -> infinity, namely Pr = 50). The thermal boundary condition on the top surface is changed by imposing different values of the Biot number, Bi. The influence of the aspect ratio (I) is assessed for through by studying several aspect ratios, Gamma. The study has been performed for two values of the Bond number (namely Bo = 5 and 50), estimating the perturbation given by thermocapillarity effects on buoyancy effects. Different kinds of competing solutions appear on localized zones of the Gamma-Bi plane. The boundaries of these zones are made up of co-dimension two points. Co-dimension two points are found to be function of Bond number, Marangoni number and boundary conditions but to be independent on the Prandtl number.
Analyzing the departure runway capacity effects of integrating optimized continuous climb operations
(Hindawi, 2019-01-01) Villegas Diaz, Manuel; Gomez Comendador, Fernando; García-Heras Carretero, Javier; Arnaldo Valdes, Rosa Maria
Performing Continuous Climb Operation (CCO) procedures enable the reduction of the environmental footprint and the improvement of the trajectory efficiency when individually operated. However, its operation may affect negatively the overall operational efficiency at Terminal Manoeuvring Areas (TMAs). The estimation of capacity is a matter of paramount importance to all airport planning and analyzing the capacity effects of this particular operational technique on a certain scenario will definitely help on evaluating its potential applicability. In this paper, departure runway capacity at the Adolfo Suarez Madrid-Barajas airport was operationally evaluated when introducing CCOs. The considered trajectories consisted of multiobjective optimized CCOs based on the optimal control theory, using the pseudospectral direct numerical method. These scenarios allowed addressing of the incremental variations of CCOs versus conventional departures, through fast time simulation, with the objective to assess the effects on the operations.
A simple trick to improve the accuracy of PIV/PTV data
(Elsevier, 2023-02-25) Tirelli, Iacopo; Ianiro, Andrea; Discetti, Stefano; European Commission
Particle Image Velocimetry (PIV) estimates velocities through correlations of particle images within interrogation windows, leading to a spatial modulation of the velocity field. Although in principle Particle Tracking Velocimetry (PTV) estimates locally a non-modulated particle displacement, to exploit the scattered data from PTV it is necessary to interpolate these data on a structured grid, which implies a spatial modulation effect that biases the resulting velocity field. This systematic error due to finite spatial resolution inevitably depends on the interrogation window size and on the interparticle spacing. It must be observed that all these operations (cross-correlation, direct interpolation or averaging in windows) induce modulation on both the mean and the fluctuating part. We introduce a simple trick to reduce this systematic error source of PIV/PTV measurements exploiting ensemble statistics. Ensemble Particle Tracking Velocimetry (EPTV) can be leveraged to obtain the high-resolution mean flow by merging the different instantaneous realisations. The mean flow can be estimated with EPTV, and the fluctuating part can be measured from PIV/PTV. The high-resolution mean can then be superposed to the instantaneous fluctuating part to obtain velocity fields with lower systematic error. The methodology is validated against datasets with a progressively increasing level of complexity: two virtual experiments based on direct numerical simulations (DNS) of the wake of a fluidic pinball and a channel flow and the experimental data of a turbulent boundary layer. For all the cases both PTV and PIV are analysed.
Blood stasis imaging predicts cerebral microembolism during acute myocardial infarction
(Elsevier, 2020-03-03) Delgado Montero, Antonia; Martinez-Legazpi Aguilo, Pablo; Desco, M. Mar; Rodriguez Perez, Daniel; Diaz Otero, Fernando; Rossini, Lorenzo; Perez Del Villar, Candelas; Rodriguez Gonzalez, Elena; Chazo Paz, Christian; Benito, Yolanda; Flores Arias, Óscar; Antoranz, J. Carlos; Fernández-Avilés, Francisco; Álamo, Juan Carlos del; Bermejo, Javier; Ministerio de Economía y Competitividad (España); Comunidad de Madrid
Background: Cardioembolic stroke is a major source of mortality and disability worldwide. The authors hypothesized that quantitative characterization of intracardiac blood stasis may be useful to determine cardioembolic risk in order to personalize anticoagulation therapy. The aim of this study was to assess the relationship between image-based metrics of blood stasis in the left ventricle and brain microembolism, a surrogate marker of cardiac embolism, in a controlled animal experimental model of acute myocardial infarction (AMI). -- Methods: Intraventricular blood stasis maps were derived from conventional color Doppler echocardiography in 10 pigs during anterior AMI induced by sequential ligation of the mid and proximal left anterior descending coronary artery (AMI-1 and AMI-2 phases). From these maps, indices of global and local blood stasis were calculated, such as the average residence time and the size and ratio of contact with the endocardium of blood regions with long residence times. The incidence of brain microemboli (high-intensity transient signals [HITS]) was monitored using carotid Doppler ultrasound. -- Results: HITS were detected in 0%, 50%, and 90% of the animals at baseline and during AMI-1 and AMI-2 phases, respectively. The average residence time of blood in the left ventricle increased in parallel. The residence time performed well to predict microemboli (C-index = 0.89, 95% CI, 0.75–1.00) and closely correlated with the number of HITS (R = 0.87, P < .001). Multivariate and mediation analyses demonstrated that the number of HITS during AMI phases was best explained by stasis. Among conventional echocardiographic variables, only apical wall motion score weakly correlated with the number of HITS (R = 0.3, P = .04). Mural thrombosis in the left ventricle was ruled out in all animals. -- Conclusions: The degree of stasis of blood in the left ventricle caused by AMI is closely related to the incidence of brain microembolism. Therefore, stasis imaging is a promising tool for a patient-specific assessment of cardioembolic risk.
Heat transfer enhancement in turbulent boundary layers with a pulsed slot jet in crossflow
(Elsevier, 2023-01-25) Castellanos García de Blas, Rodrigo; Salih, Gianfranco; Raiola, Marco; Ianiro, Andrea; Discetti, Stefano; Agencia Estatal de Investigación (España)
The convective heat transfer enhancement in a turbulent boundary layer (TBL) employing a pulsed, slot jet in crossflow is investigated experimentally. A parametric study on actuation frequencies and duty cycles is performed. The actuator is a flush-mounted slot jet that injects fluid into a well-behaved zero-pressure-gradient TBL over a flat plate. A heated-thin-foil sensor measures the time-averaged convective heat transfer coefficient downstream of the actuator location and the flow field is characterised by means of Particle Image Velocimetry. The results show that both the jet penetration in the streamwise direction and the overall Nusselt number increase with increasing duty cycle. The frequency at which the Nusselt number is maximised is independent of the duty cycle. The flow topology is considerably altered by the jet pulsation. A wall-attached jet rises from the slot accompanied by a pair of counter-rotating vortices that promote flow entrainment and mixing. Eventually, a simplified model is proposed which decouples the effect of pulsation frequency and duty cycle in the overall heat transfer enhancement, with a good agreement with experimental data. The cost of actuation is also quantified in terms of the amount of injected fluid during the actuation, leading to conclude that the lowest duty cycle is the most efficient for heat transfer enhancement.
From snapshots to manifolds - a tale of shear flows
(Cambridge University Press, 2023-01-25) Farzamnik, E.; Ianiro, Andrea; Discetti, Stefano; Deng, N.; Oberleithner, K.; Noack, B.R.; Guerrero Lozano, Vanesa; Comunidad de Madrid; Agencia Estatal de Investigación (España)
We propose a novel nonlinear manifold learning from snapshot data and demonstrate its superiority over proper orthogonal decomposition (POD) for shedding-dominated shear flows. Key enablers are isometric feature mapping, Isomap, as encoder and, K-nearest neighbours (KNN) algorithm as decoder. The proposed technique is applied to numerical and experimental datasets including the fluidic pinball, a swirling jet and the wake behind a couple of tandem cylinders. Analysing the fluidic pinball, the manifold is able to describe the pitchfork bifurcation and the chaotic regime with only three feature coordinates. These coordinates are linked to the vortex-shedding phases and the force coefficients. The manifold coordinates of the swirling jet are comparable to the POD mode amplitudes, yet allow for a more distinct and less noise-sensitive manifold identification. A similar observation is made for the wake of two tandem cylinders. The tandem cylinders are aligned and located at a streamwise distance which corresponds to the transition between the single bluff body and the reattachment regimes of vortex shedding. Isomap unveils these two shedding regimes while the Lissajous plot of the first two POD mode amplitudes features a single circle. The reconstruction error of the manifold model is small compared with the fluctuation level, indicating that the low embedding dimensions contain the coherent structure dynamics. The proposed Isomap-KNN manifold learner is expected to be of great importance in estimation, dynamic modelling and control for a large range of configurations with dominant coherent structures.

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