Distortion Descriptors Research Articles

Building New Structural Distortion Descriptors through Pressure Engineering toward Enhanced Violet Emission in 2D Hybrid Perovskite

AbstractBuilding an explicit structure‐property relationship for two‐dimensional (2D) hybrid perovskites is critical to guide the synthesis of highly luminescent materials. However, traditional studies are limited to the deviation of individual intra‐octahedron, which fails to well reflect collective lattice distortion. Here, a set of new structural distortion descriptors associated with inter‐octahedra is introduced for 2D perovskite (C7H10N)2PbBr4, that is,(PMA)2PbBr4 through pressure engineering. An experimental conclusion is reached that adjacent Pb displacement, as the inter‐octahedron distortion, is an effective parameter in determining the structural distortion and emission behavior of (PMA)2PbBr4 under high pressure. Under high pressure, 2D perovskite (PMA)2PbBr4 exhibits two emission behaviors, free excitons (FEs) emission and self‐trapped excitons (STEs) emission. Different types of four‐Pb‐shaped quadrilateral correspond to different emissions: square‐type (only FEs emission), parallelogram‐type (coexistence of FEs emission and STEs emission), kite‐type (only STEs emission). Moreover, an enhanced violet emission at ≈431 nm is achieved under a mild pressure of 1.0 GPa, which is a crucial light source of medical sterilization. The underlying structure‐property relationship is clarified fundamentally deepens the photophysical mechanism of (PMA)2PbBr4, thus facilitating the design of high‐efficiency perovskite luminophores.

Numerical Investigation into Characteristics of WIPCC and Distortion of S-shaped Intake

Water injection pre-compressor cooling technology can efficiently increase the engine thrust and expand the flight envelope of the aircraft. Changes in the curvature of the intake and the evaporative cooling of the spray medium naturally distort the total pressure, total temperature, and characteristics of the swirl of the flow before it enters the engine. This study conducts numerical research on the aerothermal process of the WIPCC in an S-shaped intake using the Eulerian-Lagrangian method. The spatial evolution of critical variables along the flow direction and specific cross-sections were obtained by analyzing the statistical time-averaged results of multiphase flow. The transient distortion descriptors were quantified and analyzed following the SAE standard, which revealed the characteristics of various distortions and their spatial-temporal changes. Furthermore, the variables reflecting the transient flow structure were decomposed using the POD method to obtain the coherent structure of each mode and its frequency and energy characteristics. The results showed that aerodynamic breakup was the main cause of droplet size reduction and led to a decrease of 74.4% and 63.8% in Sauter droplet size under two conditions, respectively. The downstream mixing of flows in the S-duct mitigated the distortions caused by the intake's changing curvature and droplet evaporation. After the mixing of flows under specific conditions, the total pressure distortion coefficient decreased from 0.30 to 0.12, the swirl intensity decreased from 4°-6° to 2°-4°, and the high total temperature extent decreased from 0.65 to 0.35. The POD mode of the variables exhibited broadband characteristics with multiple spikes and energy dispersion. The main modes of the drop coefficients in total pressure and temperature corresponded to large- and small-scale structures, respectively, with Strouhal number ranges of 0.079-0.394 and 0.362-1.260. These results reveal the effects of WIPCC on the aerothermal processes in the S-shaped intake and the mechanism of generation and evolution of distortions. Moreover, the results provide reasonable boundary conditions for numerical investigations of wet compression under intake distortions.

Large eddy simulation investigation of S-shaped intake distortion and swirl characteristics

Curvature change of an S-duct wall naturally causes total pressure and swirl distortions in a compressor inlet. In this study, the large eddy simulation (LES) method is used to investigate the internal flow of a continuous variable curvature S-duct without a downstream compressor. A comparison between the numerical results and the experimental data verifies the effectiveness of the LES method in analyzing the internal flow mechanism of an S-duct. Based on the SAE standard, the transient distortion descriptors of the research cross-section are quantitatively analyzed, and the distortion characteristics of the total pressure and swirl in the aerodynamic interface plane (AIP), downstream outlet plane, and their spatial–temporal variation processes are revealed. Furthermore, the proper orthogonal decomposition and sparsity-promoting dynamic mode decomposition methods are used to decompose the flow structures of the AIP velocity fields, and the main coherent structures affecting the flow and their corresponding frequency/attenuation characteristics are obtained. The results indicate that the twin two swirl pattern near the wall annulus of the S-duct is dominant at high Reynolds numbers, and turbulent mixing downstream inhibits the development of distortion and swirl. In addition, some stable low-frequency mode structures exist near the Starboard, exhibiting multi-spiking frequency properties. Thus, this study reveals the generation, development, and evolution mechanisms of distortion and swirl at the upstream of a compressor inlet, and the analysis results of distortion and swirl characteristics can provide important input conditions for subsequent compressor distortion research and influence analysis of mass injection pre-compressor cooling technology.

Structural Descriptors to Correlate Pb Ion Displacement and Broadband Emission in 2D Halide Perovskites.

2D hybrid lead halide perovskites exhibit versatile photoluminescent behaviors for narrowband to broadband emissions (BBEs) and have become attractive candidates for potential applications such as solid-state lighting. Establishing the relationship between the perovskite structural distortion and BBE is key but challenging in designing and optimizing the perovskite luminophores. Conventional attention is given to analyzing the intra-octahedron distortion of the [PbX6]4- (X = halide) unit that has not yet provided a clear structure-luminescence relationship. Herein, we introduce a descriptor, Pb displacement, to describe the inter-octahedron distortion to clarify the structure-emission relationship. The displacement of adjacent Pb centers represents the lattice distortion, which determines the broadband/narrowband emission instead of the octahedron distortion itself. We find a kite-type quadrilateral rule in (001) type 2D perovskites, that is, the degree to which the four octahedral central ions deviate from a square relates to the BBE. The kite-type arrangement of the Pb ions usually corresponds to the BBEs due to the large structure distortions. In contrast, the square-type arrangement of the Pb ions corresponds to the narrowband emissions because of the small distortions. The distortion descriptor magnifies the distortion scale, making it larger than the conventional one for the intra-octahedron distortion, which matches the general concept of excitons based on the scale of the crystal lattice. Therefore, the set of structural descriptors is better to correlate the perovskite structures and emission properties.

A new distortion evaluation method based on geometric association for arbitrary cross-sectional shape of inlet

The distortion flowfields generated by convoluted inlets are major factors that compromise the stability and performance of the downstream engine. For better understanding the distortion evolution, the total pressure distortion from the arbitrary cross-sectional shape of inlets was efficiently calculated based on geometric association. First, the Poisson equation was solved to generate a grid on the original, arbitrarily shaped, non-circular plane; then, a previously simulated dataset was used as a sample to interpolate the grid, whose nodes were mapped into a circular plane according to the principle of equal areas of rings and blocks. Finally, the traditional distortion descriptors were used to evaluate the distortion of the newly generated plane. The proposed method guarantees that both the total pressure recovery coefficient of the transformed crossflow plane and the contribution of this coefficient from each grid unit remain constant. This method was also applied for the flowfield analysis of serpentine and submerged inlets; the various forms of distortion along the ducts were studied. The total pressure distribution coefficient DC(60) distributions along the internal flow direction differed completely under different exit Mach numbers and their drastic changes were located at the severe bends or expansions of the inlet profile. Such innovation in distortion evaluation can provide insights into flow distortion in various inlets and enable better integration between inlets and engines.

Flow Distortion Into the Core Engine for an Installed Variable Pitch Fan in Reverse Thrust Mode

Abstract The flow distortion at core engine entry for a variable pitch fan (VPF) in reverse thrust mode is described from a realistic flow field obtained using an integrated airframe-engine model. The model includes the VPF, core entry splitter, complete bypass nozzle flow path wrapped in a nacelle, and installed to an airframe in landing configuration through a pylon. A moving ground plane to mimic the rolling runway is included. 3D Reynolds-averaged Navier–Stokes (RANS) solutions are generated at two combinations of VPF stagger angle and rotational speed settings for the entire aircraft landing run from 140 to 20 knots. The internal reverse thrust flow field is characterized by bypass nozzle lip separation, pylon wake, and recirculation of flow turned back from the VPF. A portion of the reverse stream flow turns 180 deg with separation at the splitter leading edge to feed the core engine. The core engine feed flow exhibits circumferential and radial nonuniformities that depend on the reverse flow development at different landing speeds. The temporal dependence of the distorted flow features is also explored by an unsteady Reynolds-averaged Navier–Stokes (URANS) analysis. Total pressure and swirl angle distortion descriptors, as defined by the Society of Automotive Engineers (SAE) S-16 committee, and, total pressure loss into the core engine are described for the core feed flow at different operating conditions and landing speeds. It is observed that the radial intensity of total pressure distortion is critical to core engine operation, while the circumferential intensity is within acceptable limits. Therefore, the baseline sharp splitter edge is replaced by two larger rounded splitter edges of radii, ∼0.1x and ∼0.2x times the core duct height. This was found to reduce the radial intensity of total pressure distortion to acceptable levels. The description of the installed core feed flow distortion, as described in this study, is necessary to ascertain stable core engine operation, which powers the VPF in reverse thrust mode.

Characteristics of unsteady total pressure distortion for a complex aero-engine intake duct

Some types of aero-engine intake systems are susceptible to the generation of secondary flows with high levels of total pressure fluctuations. The resulting peak distortion events may exceed the tolerance level of a given engine, leading to handling problems or to compressor surge. Previous work used distortion descriptors for the assessment of intake-engine compatibility to characterise modestly curved intakes where most of the self-generated time-dependent distortion was typically found to be dominated by stochastic events. This work investigates the time-dependent total pressure distortion at the exit of two high off-set diffusing S-duct intakes with the aim of establishing whether this classical approach, or similar, could be applied in these instances. The assessment of joint probability maps for time dependent radial and circumferential distortion metrics demonstrated that local ring-based distortion descriptors are more appropriate to characterise peak events. Extreme Value Theory (EVT) was applied to predict the peak distortion levels that could occur for a test time beyond the experimental data set available. Systematic assessments of model sensitivities to the de-clustering frequency, the number of exceedances and sample time length were used to extend the EVT application to local distortion descriptors and to provide guidelines on its usage.

Disentangling the Effects of Inter- and Intra-octahedral Distortions on the Electronic Structure in Binary Metal Trioxides

Recently, a subclass of binary perovskite-structured metal trioxides, such as WO3 and MoO3, have been propounded for many key optoelectronic applications due to their proper band edge positions and appropriate band gap sizes. Unlike their superclass perovskites, the structure–property relationship for these binary metal trioxides is less apparent, given that they suffer from much larger structural deformities within the octahedra. In this work, by using first-principles density-functional theory calculations and atomistic scale models, we examine the internal and external distortions of WO3 and MoO3 polymorphs. We then compare our results with conventional polyhedral distortion descriptors and finally use a refined data set of different perovskite-structured oxides to establish and demonstrate how these binary metal trioxides operate with a different structure–property relationship from the conventional oxide perovskites.

Assessment methods for unsteady flow distortion in aero-engine intakes

Peak events of unsteady total pressure and swirl distortion generated within S-duct intakes can affect the engine stability, even when within acceptable mean distortion levels. Even though the swirl distortion descriptors have been evaluated in S-duct intakes, the associated flow field pattern has not been reported in detail. This is of importance since engine tolerance to distortion is usually tested with representative patterns from intake tests replicated with steady distortion generators. Despite its importance in intake/engine compatibility assessments, the spectral characteristics of the distortion descriptors and the relationship between peak unsteady swirl and both radial and circumferential total pressure distortion has not been assessed previously. The peak distortion data is typically low-pass filtered at a frequency associated with the minimum response time of the engine. However the engine design is not always known a priori in intakes investigations and a standard approach to reporting peak distortion data is needed. In addition, expensive and time-consuming tests are usually required to capture representative extreme distortion levels. This work presents a range of analyses based on Delayed Detached-Eddy Simulation and Stereo Particle Image Velocimetry data at the outlet of a representative S-duct intake to assess these aspects of the unsteady flow distortion. The distorted pattern associated with different swirl distortion metrics is identified based on a conditional averaging technique, which indicates that the most intense swirl events are associated with a single rotating structure. The main frequencies of the flow distortion descriptors are found to lie within the range in which the engine stability may be compromised. The peak total pressure and swirl distortion events are found to be not synchronous, which highlights the need to assess both types of distortion. Peak swirl and total-pressure distortion data is reported as a function of its associated time scale in a more general way that can be used in the assessment of intake unsteady flow distortion. Extreme Value Theory has been applied to predict peak distortion values beyond those measured in the available dataset, and whose measurement would otherwise require testing times two orders of magnitude longer than those typically considered.

Experimental Study of Suction Flow Control Effectiveness in a Serpentine Intake

The intakes of modern aircraft are subjected to ever-increasing demands in their performance. Particularly, they are expected to carry out diffusion with the highest isentropic efficiency while subjected to aggressive geometry requirements arising from stealth considerations. To avoid a penalty in engine performance, the flow through intake needs to be controlled using various methods of flow control. In this study, a serpentine intake is studied experimentally and its performance compared with and without boundary layer suction. The performance parameters used are nondimensional total pressure loss coefficient and standard total pressure distortion descriptors. The effect is observed on surface pressure distributions, and inferences are made regarding separation location and extent. A detailed measurement at the exit plane shows flow structures that draw attention to secondary flows within the duct. Suction is applied at three different locations, spanning different number of ports along each location, comprising of ten unique configurations. The mass flow rate of suction employed ranges from 1.1% to 6.7% of mass flow rate at the inlet of the intake. The effect is seen on exit total pressure recovery as well as circumferential and radial distortion parameters. This is examined in the context of the location of the suction ports and amount of suction mass flow, by the deviation in surface pressure distributions, as well as the separation characteristics from the baseline case. The results show that applying suction far upstream of the separation point together with a modest amount of suction downstream results in the best performance.

JET ENGINE INLET DISTORTION SCREEN AND DESCRIPTOR EVALUATION

Total pressure distortion is one of the three basic flow distortions (total pressure, total temperature and swirl distortion) that might appear at the inlet of a gas turbine engine (GTE) during operation. Different numerical parameters are used for assessing the total pressure distortion intensity and extent. These summary descriptors are based on the distribution of total pressure in the aerodynamic interface plane. There are two descriptors largely spread around the world, however, three or four others are still in use and can be found in current references. The staff at the University of Defence decided to compare the most common descriptors using basic flow distortion patterns in order to select the most appropriate descriptor for future department research. The most common descriptors were identified based on their prevalence in widely accessible publications. The construction and use of these descriptors are reviewed in the paper. Subsequently, they are applied to radial, angular, and combined distortion patterns of different intensities and with varied mass flow rates. The tests were performed on a specially designed test bench using an electrically driven standalone industrial centrifugal compressor, sucking air through the inlet of a TJ100 small turbojet engine. Distortion screens were placed into the inlet channel to create the desired total pressure distortions. Of the three basic distortions, only the total pressure distortion descriptors were evaluated. However, both total and static pressures were collected using a multi probe rotational measurement system.

Flow Distortion Measurements in Convoluted Aeroengine Intakes

The unsteady flowfields generated by convoluted aeroengine intakes are sources of instabilities that can compromise the performance of the downstream turbomachinery. Hence, there is a need for synchronous high-spatial-resolution measurements that will allow a greater understanding of the aerodynamics. In this work, stereo particle image velocimetry is used in a new application to characterize the distorted flow at the outlet of complex intakes. A suite of measurements and analyses for two S-duct configurations across a range of inlet Mach numbers is presented. The work demonstrates the feasibility of using stereo particle image velocimetry techniques for determining the flowfield at the exit of convoluted intakes with a higher spatial resolution than typical pressure measurements. Analysis of the distortion descriptors quantifies the dependency upon the S-duct configuration and highlights that the more high-offset duct generates greater levels of swirl distortion relative to the low-offset configuration. Distortion maps show that the flow is highly unsteady with a characteristic switching of the swirling flow. The results provide a new dataset that shows that the conventional assessments based on time-averaged data substantially underestimate the swirl distortion level and that the less frequent distortion events give rise to more potentially significant threats to the compression system.

NMF-Based Image Quality Assessment Using Extreme Learning Machine.

Numerous state-of-the-art perceptual image quality assessment (IQA) algorithms share a common two-stage process: distortion description followed by distortion effects pooling. As for the first stage, the distortion descriptors or measurements are expected to be effective representatives of human visual variations, while the second stage should well express the relationship among quality descriptors and the perceptual visual quality. However, most of the existing quality descriptors (e.g., luminance, contrast, and gradient) do not seem to be consistent with human perception, and the effects pooling is often done in ad-hoc ways. In this paper, we propose a novel full-reference IQA metric. It applies non-negative matrix factorization (NMF) to measure image degradations by making use of the parts-based representation of NMF. On the other hand, a new machine learning technique [extreme learning machine (ELM)] is employed to address the limitations of the existing pooling techniques. Compared with neural networks and support vector regression, ELM can achieve higher learning accuracy with faster learning speed. Extensive experimental results demonstrate that the proposed metric has better performance and lower computational complexity in comparison with the relevant state-of-the-art approaches.

Total pressure distortion levels at the aerodynamic interface plane of a military aircraft

AbstractMilitary aircrafts are often subjected to severe flight maneuvers with high Angles-of -Attack (AOA) and Angles of Sideslip (AOSS). These flight attitudes induce non-uniform in flow conditions to their gas turbine engines which may include distortion of inlet total pressure and total temperature at the Aerodynamic Interface Plane (AIP). Operation of the downstream compression system with distorted inflow typically results in reduced aerodynamic performance, reduced stall margin, and increased blade stress levels. In the present study the steady state total pressure distortion induced to the Aerodynamic Interface Plane due to the aircraft’s flight attitude have been estimated in terms of distortion descriptors. The distorted conditions at the interface between the intake and the engine have been predicted by using Computational Fluid Dynamics (CFD), where 33 different aircraft flight attitudes have been tested. Based on the obtained results the effect of Angle-of-Attack (AOA) and Angle of Side Slip (AOSS) on the distortion descriptors have been studied. The results showed that the distortion effect becomes more pronounced whenever this specific airframe configuration is exposed to incoming flow with an AOSS. Among the tested cases, the greatest total pressure defect at the AIP in terms of difference from the average value and of circumferential extent was calculated for the flight attitudes of 0·35M flight with 0° AOA and 8° AOSS and 0·35M fight with 16° AOA and 16° AOSS.

Propagation of Different Types of Inflow Distortions through a Contra Rotating Fan Stage

AbstractThis paper aims to explain the propagation of different types of inflow distortions through a contra rotating fan stage. Inflow distortions like circumferential, radial and complex distortions were artificially generated using meshes of different porosities. The overall effects of these distortions are determined in the terms of the fan performance map. It is attempted to explain the propagation of inflow distortion using distortion descriptors like circumferential intensity, circumferential extent and radial intensity at the design mass flow rate. These are described for different speed combinations of both the rotors at the inlet, the exit of rotor-1 and at the exit of rotor-2. It is important to observe here that even for purely circumferential inflow distortion, the distortion does not remain purely circumferential, rather it propagates both circumferentially and radially along the stage. For the hub-strong complex inflow distortion, the higher magnitude of circumferential intensity and circumferential extent at the exit of rotor-1 clearly indicates the strong distortion effect that leads to poor performance of the stage amongst all distortion configurations studied. With radial inflow distortion, the radial redistribution of the flow at the exit of rotor-1 plays an important role in the performance of the rotor-2 and thereby for the stage.

Ground vortex aerodynamics under crosswind conditions

An experimental study was performed to investigate ground vortex formation under crosswind conditions using stereoscopic particle image velocimetry and in-duct total pressure measurements. The effect of velocity ratio, non-dimensional height, approaching boundary layer thickness and yaw angle are assessed using vortex strength and in-duct distortion descriptors. The flow-field is characterized by a single ground vortex which increases in strength as the velocity ratio is reduced. The vortex characteristics depend on the ground clearance with a stronger vortex observed at lower intake heights. For a high ground clearance, the vortex strength reaches a local maximum before reducing towards zero as the capture streamtube no longer interacts with the ground plane. At a low ground clearance, the vortex strength is strong enough to induce a lip separation. The approaching boundary layer thickness has no notable influence on the vortex characteristics. Reducing the intake yaw angle from a crosswind to a headwind configuration leads to a monotonic decrease in vortex strength with the data following a sin3(ψ) relationship, where ψ is the yaw angle. The distortion coefficient was also monotonic with yaw angle; however, the dependence was sin6(ψ).

Improved method for estimation of the maximum instantaneous distortion values

In this article a statistical method for estimating the maximum instantaneous distortion values is developed further to estimate the maximum values of the instantaneous distortions in which the operation of peak values of dynamic pressures and correlation between them are involved. The effects of the correlation between fluctuating pressures on the estimation are discussed. Comparison between the estimated values and the experimental results shows that the developed method can quite accurately estimate the maximum value of the instantaneous distortion with operation of peak values of dynamic pressures involved, such as the distortion descriptor DC(sub theta). The accuracy estimation of maximum instantaneous distortion can generally be improved by considering the correlations between fluctuation pressures. However, when the mean correlation coefficient rho(bar) is less than 0.1, the estimation can be effected by dispensing with correlation. 4 refs.