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We demonstrate several techniques to encourage practical uses of neural networks for fluid flow estimation. In the present paper, three perspectives which are remaining challenges for applications of machine learning to fluid dynamics are considered: 1. interpretability of machine-learned results, 2. bulking out of training data, and 3. generalizability of neural networks. For the interpretability, we first demonstrate two methods to observe the internal procedure of neural networks, i.e., visualization of hidden layers and application of gradient-weighted class activation mapping (Grad-CAM), applied to canonical fluid flow estimation problems -- $(1)$ drag coefficient estimation of a cylinder wake and $(2)$ velocity estimation from particle images. It is exemplified that both approaches can successfully tell us evidences of the great capability of machine learning-based estimations. We then utilize some techniques to bulk out training data for super-resolution analysis and temporal prediction for cylinder wake and NOAA sea surface temperature data to demonstrate that sufficient training of neural networks with limited amount of training data can be achieved for fluid flow problems. The generalizability of machine learning model is also discussed by accounting for the perspectives of inter/extrapolation of training data, considering super-resolution of wakes behind two parallel cylinders. We find that various flow patterns generated by complex interaction between two cylinders can be reconstructed well, even for the test configurations regarding the distance factor. The present paper can be a significant step toward practical uses of neural networks for both laminar and turbulent flow problems., 24 pages, 24 figures

With the use of the schlieren visualization and echo sounding method, the spatial structure of disturbances in front of a plate and behind it is investigated for the first time in the flow regime of intense internal-wave generation. In the echograms, fine structures are detected both in the upstream disturbances and in the wake within the solid-motion layer. In the schlieren graph, the fields of upstream disturbances are smooth, and thin interfaces are expressed in the confluent wake. Local peaks in the spatial disturbance spectra are expressed on scales that slightly exceed the length of internal waves, and, to a lesser extent, for lower scales. The thinnest interfaces are observed in the schlieren image of the wake immediately behind the plate.

The expansion of wind energy development has resulted in larger wind farms and closer placement of turbines to utilize the space available. Each turbine produces a wake that affects downstream turbines, which causes issues such as production loss, blade loading, and fluctuating electrical output. To minimize this impact, the wake produced by turbines must first be understood. Experimental methods were used in the exploration of the wake effects of wind turbines. Smoke visualization inside the University of Waterloo Wind Generation Facility was used to determine the helical vortex wake distribution behind a 3.3 m diameter turbine as well as the tip vortex shedding from the blade. Wind Doppler Light Detection and Ranging (LiDAR) measurement devices were modified and used to measure wind turbine wake velocities. The University of Waterloo Wind Energy Group has a ZephIR z150 LiDAR to use in this study. The LiDAR was verified under normal operation for accuracy against a cup and vane anemometer. The LiDAR was then verified for accuracy and to determine the position of measurements after modifications necessary for future wake measurement experiments.

The response of a separated boundary layer to synthetic jet flow control at the global wake instability (F+ ≈ 𝒪(1)) and the shear-layer instability (F+ ≈ 𝒪(10)) measured by particle image velocimetry are presented. The visualization shows that in each of the control cases, coherent vorticity develops and breaks down into a turbulent wake. When the jets are actuated by burst-modulation at the wake instability frequency, they induce regular formation and detachment of large-scale vorticity to form a wide turbulent wake. Excitation at the shear-layer instability frequency, on the other hand, produces a train of alternating velocity fluctuations in the boundary layer which dissipate to a narrower wake. Proper orthogonal decomposition of the velocity fields show that the physical extent of the jet-induced coherent structures is decreased with increasing addition of momentum for both excitation frequencies.

Due to wake effect, the construction of a wind farm requires a wide area of land. One of the main issues when planning a wind farm is the optimal wind farm layout design. This paper presents a new approach to the optimization of the wind farm layout, taking wake effect into consideration. The optimization problem is modelled with two objective functions, which are simultaneously optimized. The result of the optimization, considering wake effect, leads to the dependence between the wind farm Annual Energy Production (AEP) and the occupied area of the wind farm. The proposed procedure is verified using realistic measurement data of wind speed. The obtained results show that the suggested procedure can be successfully applied to the optimal wind farm layout design.

Granular flows exhibit interesting behavior as a result of complex interaction between the granular particles. This work visualizes the shock front resulting from granular flow over three cylindrical bodies having different cross-sectional shapes: circular, triangular and square, at three different flow velocities. The visualizations were done using the shadowgraph technique. The visualization data show well-defined shocks as well as some interesting instabilities in the granular media behind the shock fronts. The wave geometry resulted in estimation of the flow Mach number, the velocities of the granular particles and the speed of sound in the medium. The estimated flow Mach numbers were 6.9, 7.5 and 8.4. The estimation of the shock stand-off distance and radius of curvature of bow shock at the front stagnation point is also done for the cylindrical and square geometries. The oblique shock angle and the wake angle for the triangular model are also presented. Similarities of the wave features with continuum gas flows are discussed.

Fluid-dynamics visualization is not only intended to promote an intricate science but also to help non-scientific audiences to see that there is a beauty in this science. This can be seen in a play of colors to present the turbulent flow which can be easier to do with computational fluid dynamics (CFD) instead of experiments. The fluid visualization is now becoming an artistic medium for scientists to present our results to the general audiences.

This paper investigates the application of proper orthogonal decomposition (POD) for data obtained from visualizations. Using the POD method, the flow field behind one and two cylinders in a staggered configuration was analyzed. The data processed by this method were obtained from experimental measurements of flow fields using the particle image velocimetry (PIV) method and visualization. The dominant frequencies of the flow pattern from these data were compared using constant temperature anemometry (CTA) measurements. Attention was mainly focused on the flow at three Reynolds numbers: 500, 1200, and 2500. Velocity and vortex fields were created from PIV measurements in the wind tunnel forRe = 500, and video images of flow fields were obtained from dye visualizations in the hydrodynamic tunnel. The components of velocity, vorticity (both of PIV), and change in grayscale (from visualization) were used as input data for POD analysis. A methodology for data processing from visualizations was developed for subsequent analysis using the POD method. A new technique has been found to identify structures in the wake of the cylinders in a staggered configuration by analyzing POD based on various types of input data. The measured fields of dominant frequencies from the CTA and a thorough analysis of the POD modes and their relative energy values for each type of data made it possible to identify the structures and mechanisms that occur in the wake of cylinders. This analysis facilitated a better understanding of the importance of these structures and mechanisms, which can then be used to control the flow behind the cylinders.Graphic abstract

International audience; An experimental investigation of gas-liquid mass transfer in the wake of a confined air bubble sliding under an inclined wall in a 2D Hele-Shaw cell is reported. A colorimetric technique based on an oxygen-sensitive dye was used to visualize the oxygen transfer. Bubble velocities, shape eccentricities, interfacial areas and, for the first time, the instantaneous spatio-temporal distribution of oxygen concentration fields in the bubble wake, have been investigated for upper wall inclination angles of 10° ≤ α ≤ 60° and Archimedes numbers of 783 ≤ Ar ≤ 3221. Image processing has allowed, through a specific approach, a quantification of mass transfer. The calculation of the mass flux allowed the deduction of the liquid-side mass transfer coefficient kL. Experiments reveals that, at low angles of inclination, bubble velocities decelerates, shape eccentricities increased, and the instantaneous spatial and temporal distribution of oxygen concentration fields illustrated two distinct regions underneath the sliding bubble: a single vortex loop enclosing the near wake where oxygen is transferred, and a far wake containing oxygen in the form of a single long strip. When inclination angles and bubble sizes were increasing, velocities were increasing, the vortex elongated gradually until it disappears at high angles where total mass fluxes increased. This increase of bubble velocities has increased liquid-side mass transfer coefficient kL allowing a scaling law between the Sherwood number and the modified Archimedes number Ar.sin(α) to be proposed.

This work presents the correlation between experimental measurements and numerical simulations concerning the aerodynamics of a hovering rotor operating in ground effect above parallel and inclined surfaces. The capability of a potential-flow, free-wake aerodynamic solver based on a boundary-element-method formulation to simulate in-ground-effect problems is examined in detail. In particular, two approaches for including ground effect are outlined and compared, namely the bounded domain method and the mirror image method. Analyses of rotor aerodynamic performance and flow-field visualization reveal that the experimental data and the numerical predictions given by the solver based on the mirror-image method are in good agreement, thus proving the suitability of this developed computational tool to simulate rotor aerodynamics in the presence of close ground. Concerning the bounded domain method, it has been proven that it is suitable for the evaluation of rotor thrust and wake shape close to the rotor, but does not provide accurate simulation of the flow-field in proximity to the ground and corresponding inflow over the rotor disk.

This paper deals with a simple, fast and economical visualization method to validate two-dimensional large eddy simulations (LES) of the flow over a foil. This technique exploits the optical properties of soap film and relies on the wake patterns and the frequency at which these are shed at the trailing edge of the foil.

In [1] , the captions for Figs. 3 and 5 appeared incorrectly. The figures with their correct caption are presented here.

A flow around a three-dimensional bluff body such as an automobile sometimes exhibits a bi-stable state wherein two stable flow states exist for a single condition. Better aerodynamic characteristics can be obtained if we suppress or promote the flow state change between such bi-stable states. Hence, it is necessary to understand the trigger conditions and process of the flow state change. In this study, we investigated the transient aerodynamics of the Ahmed model with the slant angle of 32°, exceeding the critical angle of 30°, known to exhibit bi-stable state under crosswind conditions. Changing the Yaw angle by rotating the model, produced change in the flow state, accompanied by time delay. While continuously measuring fluid force, we performed PIV measurement triggered by a sudden change in fluid dynamic force corresponding to the flow state change. Using these methods, we realized the synchronous measurement of the fluid force and wake flow during the flow state change. At the beginning of the flow state change, flow velocity changed around the trailing edge of the slant surface. Subsequently, the separated flow above the slant surface increased. A gradual decrease of drag coefficient was observed before the flow state change though flow behavior associated with the drag change was not observed in the velocity field of PIV measurement.

Experiments were conducted on a 1:17 model-scale helicopter rotor hub in the Pennsylvania State University Applied Research Laboratory 12-inch diameter Water Tunnel. Qualitative methods were used to investigate the interaction between the rotor hub and a canonical engine pylon upper surface. Oil paint visualization showed flow separation on the engine pylon and a high degree of interaction between the hub and pylon flows. Fluorescent dye injection revealed significant differences in the unsteady flow character between advancing and retreating sides of the rotor hub. Stereo particle image velocimetry was used in the wake of an isolated hub configuration to quantify the spatial and phase-dependent variations in wake velocity, revealing a wake bias towards the advancing side and, unexpectedly, a phase-averaged unsteadiness bias towards the retreating side. In general, a previously unavailable broad view of the rotor hub flowfield was obtained.

The objective of this paper is to apply planar laser-induced fluorescence (PLIF) technology to flow field visualization. This experiment was carried out in a one-meter wind tunnel to study the wake flow field around a circular cylinder. This experiment studied the method of injecting tracer into the flow field; the frequency of the vortex in the wake field and the vortex speed are quantitatively analyzed. This paper gives the correspondence between the speed of the flow field and the frequency of the laser, which could be used as a rough reference standard for future wind tunnel visualization experiments. The result shows that PLIF diagnostic technology has great potential in visualization of flow field.

The present work reports incompressible, viscous flow around a rigidly suspended stationary cube placed normal to the flow. The Reynolds number range covered in this work is more than two decades from Re ∈ (500, 55,000). Measurements are conducted using particle image velocimetry and dye visualization is undertaken for better understanding of the flow. The flow is seen to be structureless at higher Reynolds number. Mean and rms velocities at different axial locations in the wake are examined along with the mean vorticity field. Two peaks are observed in the U rms profiles at different axial locations in the wake. V rms is single peaked at lower Re and has weak double peaks at higher Re. Behaviour of centerline velocity of the wake gives an insight about the recirculation length and shows its non-dependence on Re. Wake size has been determined and trajectory of the maximum vorticity in the wake is discussed. Drag coefficients are evaluated and compared with various correlations for sphere and cube. The modified wake survey method gives coefficients of drag between 0.63 and 0.89, which are close to the values for a cube given by Holzer and Sommerfeld (2008) within the experimented Reynolds number range. Using the recirculation length, the data shows a dependence on Reynolds number till Re ∼ 1200 beyond which it becomes independent of Re. Overall several similarities in the flow behaviour with respect to sphere is noted from the results. The paper is the first detailed study on a cube over such a wide range of Reynolds number.

Schlieren measurements of an air bubble sliding under a heated inclined surface reveal an intricate wake structure that is difficult to observe using alternative techniques. Image tracking is also used to extract the dynamics of the gas-liquid interface to augment these visualizations.

International audience; Visualization of fluid flows at a high-Reynoldsnumber (Re ~ 10 5 ) presents difficulties for user comprehensiondue to density and ambiguous interactions between vortices. Priorwork has used cluster-based reduced-order modelling (CROM) toanalyze the wake of a High-Speed Train (HST) with Re = 86,000.In this paper, we present a novel surface visualization to conveythe spatiotemporal changes undergone by clustered vortices in theHST wake. This visualization is accomplished throughdimensional reduction of 3D volumetric vortices into 1D ridges,and physics-based feature tracking. The result is 3D surfacesvisualizing the behavior of the vortices in the HST wake.Compared to conventional still-image representations, thesesurfaces allow the user to quickly compare and analyze the twoshedding cycles identified via CROM. The spatiotemporaldifferences of the primary vortices in these shedding cyclesprovide analytic insight to influence the aerodynamics of the HST.

At the end of 2016, the state of Oregon had an installed wind generating power capacity of over 3 Gigawatts. Wind power, while clean and renewable, requires constant load balancing due to variable weather conditions. These conditions must be met with state of the art forecasting methods in order to maximize load balancing efficiency. New techniques in Numerical Weather Prediction (NWP) have been developed to better optimize load balancing. Visualizations of these new NWP simulations show how winds behave on the leeward side of Mt. Hood. These winds generate a wake that affects Oregon wind farms; causing swings in power that require dynamic load balancing.

Low-Reynolds number testing was conducted at the 7 ft. x 10 ft. Walter H. Beech Memorial Wind Tunnel at Wichita State University to study the aerodynamic effects of ice shapes on a swept wing. A total of 17 ice shape configurations of varying geometric detail were tested. Simplified versions of an ice shape may help improve current ice accretion simulation methods and therefore aircraft design, certification, and testing. For each configuration, surface pressure, force balance, and fluorescent mini-tuft data were collected and for a selected subset of configurations oil-flow visualization and wake survey data were collected. A comparison of two ice shape geometries and two configurations with simplified geometric detail for each ice shape geometry is presented in this paper.

A modified hydrogen bubble technique is developed for simultaneous three-dimensional flow visualization and surface visualization on bluff bodies. A detailed description of the technique implementation and operating principles is presented. The technique is validated on two geometries: (1) a uniform circular cylinder for ReD = 1,050 and 2,100; and (2) a dual-step cylinder for ReD = 2,100, D/d = 2, and L/D = 0.5. The results demonstrate that flow visualizations effectively capture intricate three-dimensional wake vortex development including the formation of streamwise structures and vortex interactions in the wake of a dual-step cylinder. The surface visualizations capture boundary layer separation and its variation along the span of the models. Moreover, the results reveal complex three-dimensional surface flow patterns, shedding light on the topology of vortical structures forming near dual-step cylinder junctions.

The wake behind the cylinders is often characterized by a dominant frequency and dimensionless Strouhal number. However, the experiments performed using the CTA and visualization method and their analysis revealed that other significant frequencies were present in the wake. With the Proper Orthogonal Decomposition analysis, connections between these frequencies and their own eigenmodes were found.

A new simple optical method for visualizing flow structures generated in a rotating flow is presented. The method consists of planar PTV with sheet illumination slightly offset to the rotation axis and a geometric reconstruction of velocity vector field under the assumption of azimuthal periodicity. Two types of columnar flows intrinsically associated with rotating flows are investigated with the method. The two types of flow are a local columnar wake induced behind a moving sphere, and an asymmetrically precessing Taylor Column created below a spinning disk. By means of this new technique we obtain the first quantitative visualization of the columnar wake and the three-dimensional structure of the Taylor Column.

We report initial results of optical probing experiments (E-224) at the FACET/SLAC user facility. These experiments are being carried out in collaboration with the ongoing e-beam driven wakefield experimental campaign (E-200) at FACET. Their aim is to optically study both the short term and long term plasma structure produced by the e-beam driver. The SLAC plasma wakefield experiments have demonstrated the highest energy gain to date [1] and continue work on further optimization. Direct visualization of the plasma wake structure would aid in the understanding of the dynamics of the beam plasma interaction and acceleration and its optimization. Currently, such understanding is derived only from simulations, which are very time consuming and rely on limited knowledge of initial conditions. We describe the optical probing geometry used in this initial run, a variation of a method previously developed in our group [6], as governed by the unique experimental challenges of the FACET beam driven experiments in t...

Flow over a finite-height surface-mounted square prism of aspect ratio AR = 3 (AR = H/D where H is the height and D is the prism width) at a Reynolds number Re = 500 has been investigated using the Snapshot Proper Orthogonal Decomposition (POD) method. In particular, the instantaneous velocity fields in three horizontal spanwise planes at y/D = 1, 2 and 3 generated by Large Eddy Simulation (LES) have been studied to gain insight into the vortical structures in the prism wake. The study tracked the development of the prism wake over a single period using the most energetic POD modes. A scatter plot of the first two POD temporal coefficients successfully extracted the phase information of the flow field, and a reconstruction of the fluctuating velocity field using the first two POD modes was used to approximate the phase-averaged flow structures. Different POD modes can be associated with either anti-symmetric or symmetric vortex shedding characteristics of the wake.

A numerical investigation of three-dimensional sinusoidally oscillating flow around a circular cylinder was conducted to examine mushroom-type structures in the near wake that are manifestations of the Honji instability. The focus of this paper is to examine the flow structure through the analysis of the streaklines in the flow. Through the use of streakline visualizations and their correlation with vorticity in the flow field, the onset and development of the mushroom-type structures is followed. The parameter value range is 0.1

Flow past a circular cylinder mounted on a flat plate normal to the flow direction is numerically simulated. Three-dimensional mass and momentum conservation equations are solved to obtain the detailed spatio-temporal dynamics of wake vortices revealed through the velocity and pressure data. Particular attention is focused on the junction between the flat plate and the cylindrical cross-section, which is well known to be a rich source of three-dimensional vortical structures. The wake-boundary layer interaction region generates necklace vortices, which are popularly known as horseshoe vortices. The effect of spanwise height (H/D) of the cylinder on these vortices and their unsteady wake characteristics are investigated. The forces acting on the cylinder and the Strouhal periodicities are validated for Re=200, apart from presenting qualitative features, such as streamlines and vorticity contours. The aspect ratio of the cylinder is found to have predominant influence on the primary and secondary vortex structures. Streakline visualization is developed to study the spatio-temporal dynamics of interaction between the wake and the boundary-layer. An assessment of the two types of boundary conditions, viz., free-slip and no-slip (the latter due to bed friction) are investigated. Vortex shedding is found to be completely suppressed for smaller aspect ratios (H/D)?1.0, with a stabilized flow pattern in the aft of the cylinder, while it persists for H/D>1.0. Copyright � Taylor & Francis Group, LLC.

In this paper, an oil and dye flow visualization technique applied in two different cases of experimental testing is described, namely a single fan in crossflow and the wake of a compressor cascade. In both cases, the mixture of paint was prepared using a highly volatile light mineral or heavy machine oil together with very fine pigments of titanium dioxide (TiO2) or fluorescein sodium in various colors. After the preparation of the mixture, a homogeneous thin film was applied onto the whole investigated surface by painting with a soft brush. The air stream which flows over the surface of the plate modifies the concentration and the homogeneity of the oil film according to the flow conditions very close to the wall. The film was dried by the airflow and photographed for further consideration and field observation. A number of successful as well as unsuccessful flow visualization experiments are presented hereafter. The objective of this work is to provide further information and some general guidelines about mixture concentration at lowand high-speed incompressible flows and to discuss the applicability of this method of visualization technique in complex turbomachinery flows.

Free-stream flow and forced convection heat transfer across a rotating cylinder, dissipating uniform heat flux, are investigated numerically for Reynolds numbers of 20–160 and a Prandtl number of 0.7. The non-dimensional rotational velocity (α) is varied from 0 to 6. Finite volume based transient heatline formulation is proposed. For Re = 100, the reasons for the onset/suppression of vortex shedding at a critical rotational velocity is investigated using vorticity dynamics. At higher rotational velocity, the Nusselt number is almost independent of Reynolds number and thermal boundary conditions. Finally, a heat transfer correlation is proposed in the 2D laminar flow regime. Cylinder rotation is an efficient Nusselt number reduction or cylinder-surface temperature enhancement technique.

Forced convection heat transfer across a circular cylinder rotating with a constant non-dimensional rotation rate ( α ) varying from 0 to 6 are investigated for Reynolds numbers of 20–160 and a Prandtl number of 0.7. Flow transitions is reported here for a wider range of Reynolds number and rotation rates. Heat transfer visualization technique using heatlines is implemented here, probably for the first time, in finite volume framework for the unsteady heat transfer problem in complex domain and used for heat flow analysis. Rotation can be used as a drag reduction and heat transfer suppression technique.

Wake structure of a single “clean bubble”, rising in rectilinear, zigzagging or spiraling path, is experimentally investigated. A single nitrogen gas bubble was produced in a silicone oil pool and the wake structure development in the rear of the rising bubble was visualized by using photochromic dye. The flexibility of this visualization method enabled us to distinguish wake from drift easily. Both bubble motion and wake structure were recorded by using stereo high-speed video camera simultaneously. We present the first experimental support for the existence of the standing eddy at the rear of the clean bubble, as predicted by a previous numerical study by Ryskin and Leal [1984. Journal of Fluid Mechanics 148, 19–35], Dandy and Leal [1986. Physics of Fluids 29(5), 1360–1366] and Blanco and Magnaudet [1995. Physics of Fluids 7(6), 1265–1274]. We study motion of a pair of vortex filaments, which is called double-threaded type wake, in the case of bubble rising in an axi-asymmetric path. Visualization results of multiple formations of horse-shoe type vortices in one period of zigzag motion of rising bubble with shape oscillations, which has not been observed in previous studies are also presented.

ジグザグもしくはらせん運動を行う上昇気泡の後流について，実験的に調査した．単一の気泡をシリコンオイル中に発生させ，その後流をフォトクロミック染料を用いることで可視化した．気泡の挙動と後流は，高速度ビデオカメラを用いて同時にステレオ撮影した．その結果，気泡がらせん運動を開始する際には，ダブルスレッド型と思われる二本の渦構造が観察された．また気泡が形状振動を伴いながらジグザグ運動を行う際には，複数の馬蹄形の渦構造が観察された．

Turbulent wake of wind turbine reduces the power output of wind farm. The coherent Doppler Lidar is used to visualize the turbulent wake of wind turbine blades to analyze its scale and wind velocity deficit.

Raytheon's TACEC (Terminal Area Capacity Enhancement Concept) proposes to increase airspace capacity by using closely-spaced formations of arriving and departing aircraft to greatly increase airport capacities. However, a blunder in a tightly-spaced formation could cause a collision with another aircraft or its wake vortices. Therefore, humans need tools and visualization aids to detect and respond to blunders. This paper is the first in a series of experiments to evaluate the ability of different visualization modes to enable detection of lateral blunders. We tested four viewpoints, one warning aid, and three blunder speeds in a within-subjects design. This study had twelve participants. The main result was that changing the viewpoint made a large difference; the cockpit view was by far the worst. Blunder detection varied by speed and was difficult in the presence of noise. Future experiments should use more realistic simulators and pilots as participants.