Unsteady Flow Visualization Research Articles

Detection and localization of flow separation on wind turbines by means of unsteady thermographic flow visualization

For the first time, a thermographic detection and localization of turbulent flow separation on an operating wind turbine is presented and verified. Flow separation on wind turbine rotor blades causes power reduction, structural loads and increased noise emissions. In contrast to established methods for stall detection, the presented infrared thermographic measurement approach is non-invasive, in-process capable and provides a high spatial resolution. With respect to the state of the art for thermographic stall detection in wind tunnel experiments, the thermal surface response to unsteady inflow conditions is evaluated for measurements on an operating wind turbine, in order to achieve unambiguous thermographic features for the detection of flow separation. The evaluation of the thermodynamic response behavior shows a clear detection of flow separation by means of temperature fluctuation maxima in the regions of flow transition as well as an increasing temperature fluctuation within the separated flow region. In addition, a geometric assignment is conducted which enables a localization of the separation point with an uncertainty of 0.6% of the chord length. The detection and localization of flow separation is verified by means of tufts visualization.

Unsteady simulation of flow and heat transfer in a transonic turbine stage under non-uniform inlet conditions

In modern aero-engines, the temperature non-uniformity (hot-streak) caused by the combustors and secondary flows caused by the rotor blades have a considerable impact on the aerothermal performances of the high pressure (HP) turbine. In this study, two idealized hot-streaks (radial and rounded) from literature and one engine representative hot-streak (distorted) were evaluated within a HP turbine stage using unsteady numerical simulation. The results were analyzed and discussed and they include, the stage performance, unsteady flow visualization, unsteady hot-streak transport, secondary flows/hot-streak interaction and blades thermal loading. It was found that the rotor passage had various complex secondary flows and the boundary layer on its surface undergone several transitions. The hot-streak transport and its contact with vane and rotor blades differed from one case to another. The secondary flows had a remarkable impact on the rotor exit temperature. They induced wavy shapes and corrugations of temperature gradients near the endwalls. All hot-streaks were completely deformed and distorted across the turbine stage. The rotor surfaces suffered from hot temperatures under radial and distorted hot-streaks compared with rounded hot-streak. Very high levels of the heat transfer were generated on the rotor suction side near the endwalls due to the vortices.

Physics‐based Pathline Clustering and Exploration

AbstractMost existing unsteady flow visualization techniques concentrate on the depiction of geometric patterns in flow, assuming the geometry information provides sufficient representation of the underlying physical characteristics, which is not always the case. To address this challenge, this work proposes to analyse the time‐dependent characteristics of the physical attributes measured along pathlines which can be represented as a series of time activity curves (TAC). We demonstrate that the temporal trends of these TACs can convey the relation between pathlines and certain well‐known flow features (e.g. vortices and shearing layers), which enables us to select pathlines that can effectively represent the physical characteristics of interest and their temporal behaviour in the unsteady flow. Inspired by this observation, a new TAC‐based unsteady flow visualization and analysis framework is proposed. The centre of this framework is a new similarity measure that compares the similarity of two TACs, from which a new spatio‐temporal, hierarchical clustering that classifies pathlines based on their physical attributes, and a TAC‐based pathline exploration and selection strategy are proposed. A visual analytic system incorporating the TAC‐based pathline clustering and exploration is developed, which also provides new visualizations to support the user exploration of unsteady flow using TACs. This visual analytic system is applied to a number of unsteady flow in 2D and 3D to demonstrate its utility. The new system successfully reveals the detailed structure of vortices, the relation between shear layer and vortex formation, and vortex breakdown, which are difficult to convey with conventional methods.

A quasi three-dimensional visualization of unsteady wake flow in human undulatory swimming

Human undulatory underwater swimming (UUS) is an underwater propelling technique in competitive swimming and its propulsive mechanism is poorly understood. The purpose of this study was to visualize the three-dimensional (3D) flow field in the wake region during human UUS in a water flume. A national level male swimmer performed 41 UUS trials in a water flume. A motion capture system and stereo particle image velocimetry (PIV) equipment were used to investigate the 3D coordinates of the swimmer and 3D flow fields in the wake region. After one kick cycle was divided into eight phases, we conducted coordinate transformations and phase averaging method to construct quasi 3D flow fields. At the end of the downward kick, the lower limbs external rotations of the lower limbs were observed, and the feet approached towards each other. A strong downstream flow, i.e. a jet was observed in the wake region during the downward kick, and the paired vortex structure was accompanied by a jet. In the vortex structure, a cluster of vortices and a jet were generated in the wake during the downward kick, and the vortices were subsequently shed from the feet by the rotated leg motion. This suggested that the swimmer gained a thrust by creating vortices around the foot during the downward kick, which collided to form a jet. This paper describes, illustrates, and explains the propulsive mechanism of human UUS.

Optical oxygen sensors based on microfibers formed from fluorinated copolymers

Polymeric microfibers, particularly the fluorinated copolymers’ fibers, are promising functional materials for photoelectric devices, sensing, and energy storage due to their various surface characteristics. However, to obtain the high fluorine content copolymer-based fibers with considerable uniformity is likely to be especially challenging, which seriously debilitates their applications in sensor devices. Herein, for the first time, we presented a high fluorine content platinum porphyrin-grafted poly(isobutyl methacrylate-co-dodecafluoroheptyl methacrylate) copolymers (PtTFPP-p(IBM-co-DFHMA)) microfibrous thin-films used as optical oxygen sensors. The porous thin-film frameworks were formed of uniform microfibers, which afforded an exceptional improvement in sensitivity and exhibited 584% higher sensitivity than the solid sensing film owing to the large specific surface area, porous structures and oxygen diffusion enhancement by fluorine elements. Additionally, the remarkable emission intensity-changing characteristic of the microfiber sensing film under various air pressures facilitates convenient visualization of pressure distributions on film surface. The characteristics are particularly important for the computational fluid dynamics simulations in various sensing fields such as unsteady flow visualization and unsteady pressure measurements, etc. Owing to its attractive advantages and versatile performance, fluorine-containing copolymers fibers are expected to provide a new strategy for the rational design of high performance gas sensor devices.

Visual Analysis of Spatio-temporal Relations of Pairwise Attributes in Unsteady Flow.

Despite significant advances in the analysis and visualization of unsteady flow, the interpretation of it's behavior still remains a challenge. In this work, we focus on the linear correlation and non-linear dependency of different physical attributes of unsteady flows to aid their study from a new perspective. Specifically, we extend the existing spatial correlation quantification, i.e. the Local Correlation Coefficient (LCC), to the spatio-temporal domain to study the correlation of attribute-pairs from both the Eulerian and Lagrangian views. To study the dependency among attributes, which need not be linear, we extend and compute the mutual information (MI) among attributes over time. To help visualize and interpret the derived correlation and dependency among attributes associated with a particle, we encode the correlation and dependency values on individual pathlines. Finally, to utilize the correlation and MI computation results to identify regions with interesting flow behavior, we propose a segmentation strategy of the flow domain based on the ranking of the strength of the attributes relations. We have applied our correlation and dependency metrics to a number of 2D and 3D unsteady flows with varying spatio-temporal kernel sizes to demonstrate and assess their effectiveness.

Heat flow visualization of a chemical compound isobutane (C4H10) past a vertical cylinder in the subcritical, near critical and supercritical regions

Supercritical fluids (SCF's) found to have a large number of applications in science and engineering field particularly in separation and purification, green technologies etc. The present research paper investigates the heatline visualization of unsteady free convection supercritical fluid flow over a vertical cylinder using Bejan's heat function concept by using isobutane as a model compound. A new thermodynamic equation has been obtained to calculate the volumetric thermal expansion coefficient (β) based on the Redlich-Kwong equation of state (RK-EOS), in order to determine the free convection properties of isobutane in supercritical fluid (SCF) region. In this model the thermal expansion coefficient is characterized as a function of compressibility factor, temperature and pressure. Crank-Nicolson type of implicit finite difference method is utilized to obtain the results in terms of the streamlines (ψ), isotherms (θ) and heatlines (Π) for the different values of reduced temperature and reduced pressure in the SCF region. The numerically calculated thermal expansion coefficient values are validated with existing experimental results. Numerical simulations are performed for isobutane in three regions namely, subcritical, near critical and supercritical regions. The unsteady boundary layer flow analysis shows that the streamlines are beginning from the leading edge to the far downstream, whereas the heatlines ends at a finite distance from the surface of cylinder. The non-dimensional values of heat function are closely associated with the overall heat transfer rate. The SCF flow analysis implies that in the vicinity of the hot cylindrical surface the heatlines are observed to be denser. Further, the deviation of streamlines, isotherms and heatlines from the hot cylindrical surface increases as reduced temperature and reduced pressure increases.

Robust and magnetically recoverable dual-sensor particles: Real-time monitoring of glucose and dissolved oxygen

In this work, we presented the robust and magnetically recoverable dual sensor particles that are capable of real-time monitoring of dissolved oxygen (DO) and glucose concentrations in aqueous solutions. The dual sensor consists of glucose oxidase-functionalized polyethylenimine-coated Fe3O4 particles (Fe3O4@PEI-GOD) and sub-micro-sized Fe3O4-modified poly(Platinum porphyrin-co-Styrene) ((PtTFPPMA-PS)@Fe3O4) fluorescent spheres. The dual sensing functions originate from the cascade catalysis amplification driven by the recyclable Fe3O4@PEI-GOD particles and glucose oxidase consumption of oxygen in the process of glucose oxidation. Consequently, the (PtTFPPMA-PS)@Fe3O4 fluorescent spheres exhibited higher fluorescence intensity due to the effective alleviation of fluorescence quenching effect raised from the decrease of DO. The remarkable fluorescence intensity-changing characteristics of the solutions facilitate convenient identification of oxygen concentrations and glucose concentrations even with naked eyes. Thanks to the magnetically recoverable superiority, the recycle study proved that the multifunctional particles could be repeatedly utilized without significant catalytic activity loss after 10 cycles. Interestingly, the sensing film comprising the (PtTFPPMA-PS)@Fe3O4 fluorescent spheres can also be applied to the unsteady pressure measurement and unsteady air flow visualization due to its remarkable light intensity-changing features.

Oscillatory mode transition for supersonic open cavity flows

The transition in the primary oscillatory mode in an open cavity has been experimentally investigated and the associated characteristics in a Mach 1.71 flow has been analyzed. The length-to-depth (L/D) ratios of the rectangular cavities are varied from 1.67 to 3.33. Unsteady pressure measurement and flow visualization are employed to understand the transitional flow physics. Flow visualization revealed the change in oscillation pattern from longitudinal mode to transverse mode and is also characterized by the presence of two bow shocks at the trailing edge instead of one. The transition is found to occur between L/D 1.67 and 2, marked by a change in the feedback mechanism, resulting in a shift from the vortex circulation driven transverse feedback mode to the oscillating shear layer driven longitudinal feedback mode. Cavities oscillating in the transition mode exhibit multiple tones of comparable strength. Correlation analysis indicated the shift in the feedback mechanism. Wavelet analysis revealed the temporal behaviour of tones during transition. Tone switching is observed in deeper cavities and is attributed to the occurrence of two bow shocks as evident from the temporo-spectral characteristics of transition that affects the shear layer modal shape.

Honeycomb structured porous films from a platinum porphyrin-grafted poly(styrene-co-4-vinylpyridine) copolymer as an optical oxygen sensor

The rational design and realization of revolutionary sensing films for improving the sensitivity and photo-stability of oxygen sensors have been long-standing challenges. We presented a facile approach for the fabrication of honeycomb structured porous films for optical oxygen sensors using a platinum porphyrin-grafted poly(styrene-co-4-vinylpyridine) (PtTFPP-PSVP) copolymer. Honeycomb porous films were prepared by the “Breath figure method”, which provides an efficient and cost-effective method to produce highly ordered honeycomb patterns in PtTFPP-PSVP copolymer films at sub-micrometer dimensions. Importantly, the application of these ordered honeycomb structured porous films as oxygen sensors afford exceptional improvement in sensitivity performance and exhibited 200% higher sensitivity than the solid sensing film owing to the higher accessibility of gas molecules and larger specific surface area. Furthermore, the photo-bleaching of sensor molecules for PtTFPP grafted to PSVP copolymer films could also be effectively alleviated and the sensors’ photo-stability was significantly improved compared with the physically incorporated PtTFPP into the same PSVP copolymer. Interestingly, the remarkably light intensity-changing characteristic of the sensing film under lower O2 partial pressures facilitates convenient identification of O2 concentrations even with the naked eye. We believe that the proposed honeycomb structured porous film could be a promising candidate in various oxygen sensing fields such as nearly anoxic systems, unsteady pressure measurements and unsteady flow visualization, etc. Additionally, the “Breath figure method”, owing to its simplicity, high reproducibility and easy processability, is expected to be readily applicable to a rich variety of other high performance gas sensor devices.

Development and Integration of an In-Situ Framework for Flow Visualization of Large-Scale, Unsteady Phenomena

With large-scale simulation models on massively parallel supercomputers generating increasingly large data sets, in-situ visualization is a promising way to avoid bottlenecks. Enabling in-situ visualization in a simulation model asks for special attention to the interface between a parallel simulation model and the data analysis part of the visualization, and to presentation and interaction scenarios. Modifications to scientific workflows would potentially result in a paradigm shift, which affects compute and data intensive applications generally. We present our approach for enabling in-situ visualization within the highly parallelized climate model ICON using the DSVR visualization framework. We focus on the requirements for generalized grid and data structures, and for universal, scalable algorithms for volume and flow visualization of time series. In-situ pathline extraction as a technique for the visualization of unsteady flows has been integrated in the climate simulation model ICON and verified in first studies.

Highly enhanced sensitivity of optical oxygen sensors using microstructured PtTFPP/PDMS-pillar arrays sensing layer

In this work, we presented a facile and cost-effective approach to fabricate novel PtTFPP/PDMS micro-pillar array (MPA) sensor film with I0/I100 for gaseous oxygen and dissolved oxygen are 288 and 84, respectively. The sensitivity of the sensor film exhibited over ∼30-fold increment when compared with contemporary PtTFPP/PDMS-based oxygen sensors. The detection limit was as low as 0.10μmol/L, which means that the ultrasensitive sensor films enable reliable monitoring of dissolved oxygen (DO) within nanomolar concentration ranges that are particularly difficult to achieve in previous works. Furthermore, the remarkably light intensity-changing characteristic of the sensing film under lower O2 partial pressures facilitates convenient identification of O2 concentrations even with the naked eye. We believe that the proposed device could be a promising candidate in various oxygen sensing fields such as nearly anoxic systems, unsteady pressure measurement and unsteady flow visualization, etc.

Visualization of 2D unsteady flow using streamline-based concepts in space-time

Treating time as the third dimension of 2D time-dependent flow enables the application of a wide variety of visualization techniques for 3D stationary vector fields. In the resulting space-time representation, 3D streamlines represent 2D pathlines of the original field. In this paper, we investigate the application of different streamline-based visualization concepts to the 3D space-time representation of 2D time-dependent flow. As a consequence, we obtain from each streamline-based concept a Galilean-invariant counterpart that takes the time dependence of the original field explicitly into account. We show the advantages of the overall approach for vortex analysis and the analysis of the dynamics of material lines. In particular, we employ the concept for the extraction of vortex centers, vortex core regions, and the visualization of material line dynamics using streamsurface integration and line integral convolution in the space-time field. We exemplify the utility of our visualization approach using two 2D time-dependent datasets that exhibit vortical flow.

Image-Space Texture-Based Output-Coherent Surface Flow Visualization

Image-space line integral convolution (LIC) is a popular scheme for visualizing surface vector fields due to its simplicity and high efficiency. To avoid inconsistencies or color blur during the user interactions, existing approaches employ surface parameterization or 3D volume texture schemes. However, they often require expensive computation or memory cost, and cannot achieve consistent results in terms of both the granularity and color distribution on different scales. This paper introduces a novel image-space surface flow visualization approach that preserves the coherence during user interactions. To make the noise texture under different viewpoints coherent, we propose to precompute a sequence of mipmap noise textures in a coarse-to-fine manner for consistent transition, and map the textures onto each triangle with randomly assigned and constant texture coordinates. Further, a standard image-space LIC is performed to generate the flow texture. The proposed approach is simple and GPU-friendly, and can be easily combined with various texture-based flow visualization techniques. By leveraging viewpoint-dependent backward tracing and mipmap noise phase, our method can be incorporated with the image-based flow visualization (IBFV) technique for coherent visualization of unsteady flows. We demonstrate consistent and highly efficient flow visualization on a variety of data sets.

Mass‐Dependent Integral Curves in Unsteady Vector Fields

AbstractRecent research in flow visualization is focusing on the analysis of time‐dependent, but mass‐less particles. However, in many application scenarios, the mass of particles – and their resulting inertia – is essential in understanding fluid mechanics. This includes critical processes, such as dust particles interacting with aircraft (e.g., brown‐or white‐out effects) and particle separation based on density variation. In this paper, we contribute a generalized description of mass‐dependent particle trajectories and apply existing unsteady flow visualization methods to the mass‐dependent case. This comprises the extension of common concepts, i.e., path lines, streak lines, and time lines. Furthermore, we introduce a new class of integral curves, called mass lines that effectively visualizes mass separation and captures mass‐related features in unsteady flow fields that are inaccessible using traditional methods. We demonstrate the applicability of our method, using a number of real‐world and artificial data sets, in which mass is a crucial parameter. In particular, we focus on the analysis of brown‐out conditions, introduced by a helicopter in forward flight close to the ground.

0622 円柱と平面壁との接合部における非定常三次元流れの可視化計測(OS6-5 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

0622 円柱と平面壁との接合部における非定常三次元流れの可視化計測(OS6-5 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

Flow Radar Glyphs—Static Visualization of Unsteady Flow with Uncertainty

A new type of glyph is introduced to visualize unsteady flow with static images, allowing easier analysis of time-dependent phenomena compared to animated visualization. Adopting the visual metaphor of radar displays, this glyph represents flow directions by angles and time by radius in spherical coordinates. Dense seeding of flow radar glyphs on the flow domain naturally lends itself to multi-scale visualization: zoomed-out views show aggregated overviews, zooming-in enables detailed analysis of spatial and temporal characteristics. Uncertainty visualization is supported by extending the glyph to display possible ranges of flow directions. The paper focuses on 2D flow, but includes a discussion of 3D flow as well. Examples from CFD and the field of stochastic hydrogeology show that it is easy to discriminate regions of different spatiotemporal flow behavior and regions of different uncertainty variations in space and time. The examples also demonstrate that parameter studies can be analyzed because the glyph design facilitates comparative visualization. Finally, different variants of interactive GPU-accelerated implementations are discussed.

The State of the Art in Topology‐Based Visualization of Unsteady Flow

AbstractVector fields are a common concept for the representation of many different kinds of flow phenomena in science and engineering. Methods based on vector field topology are known for their convenience for visualizing and analysing steady flows, but a counterpart for unsteady flows is still missing. However, a lot of good and relevant work aiming at such a solution is available. We give an overview of previous research leading towards topology‐based and topology‐inspired visualization of unsteady flow, pointing out the different approaches and methodologies involved as well as their relation to each other, taking classical (i.e. steady) vector field topology as our starting point. Particularly, we focus on Lagrangian methods, space–time domain approaches, local methods and stochastic and multifield approaches. Furthermore, we illustrate our review with practical examples for the different approaches.

Real-time visualization algorithm of 2D unsteady flow field

In order to improve the rendering quality and speed of unsteady flow visualization results, the authors presented an improved unsteady flow visualization algorithm based on advection and texture blending. The temporal coherence in the flow animation was got by image advection and the spatial coherence constructed in each frame was got by particle advection, and the temporal-spatial coherence was unified by texture blending at last. In this way, the texture detail of flow animation is clear and the change in each frame is smooth. Furthermore, this algorithm can achieve a real-time frame rate. The high temporal-spatial coherence can exactly reflect unsteady flow dynamic variety.

Flow Visualization of Aerodynamic Mechanism of Insect Flapping Wings using PIV

We experimentally investigate unsteady aerodynamic mechanisms of insect flapping wings. Measurements of unsteady aerodynamic forces and flow visualization around the flapping wing using PIV are conducted with dynamically scaled mechanical models in a water tunnel, which simulates hovering and forward flapping flights. We experimentally clarify the effects of three unsteady aerodynamics on the aerodynamic characteristics of a flapping wing: delayed stall, rotational circulation, and wake capture. The contribution of delayed stall to the total lift accounts for about 90%. The feathering rotation during the flapping translation contributes to an increase of power rather than lift. The wake capture is dependent on the reduced frequency and the timing of feathering rotation. In addition, we investigate the flow interaction between fore- and hindwings like a dragonfly. The phase difference between the fore- and hindwings are effective on the aerodynamic characteristics of the tandem wings.