Spatiotemporal Velocity Field Research Articles

Fluid Simulation on Neural Flow Maps

We introduce Neural Flow Maps, a novel simulation method bridging the emerging paradigm of implicit neural representations with fluid simulation based on the theory of flow maps, to achieve state-of-the-art simulation of in-viscid fluid phenomena. We devise a novel hybrid neural field representation, Spatially Sparse Neural Fields (SSNF), which fuses small neural networks with a pyramid of overlapping, multi-resolution, and spatially sparse grids, to compactly represent long-term spatiotemporal velocity fields at high accuracy. With this neural velocity buffer in hand, we compute long-term, bidirectional flow maps and their Jacobians in a mechanistically symmetric manner, to facilitate drastic accuracy improvement over existing solutions. These long-range, bidirectional flow maps enable high advection accuracy with low dissipation, which in turn facilitates high-fidelity incompressible flow simulations that manifest intricate vortical structures. We demonstrate the efficacy of our neural fluid simulation in a variety of challenging simulation scenarios, including leapfrogging vortices, colliding vortices, vortex reconnections, as well as vortex generation from moving obstacles and density differences. Our examples show increased performance over existing methods in terms of energy conservation, visual complexity, adherence to experimental observations, and preservation of detailed vortical structures.

Dynamic Mode Decomposition of merging wind turbine wakes

The design and operation of wind farms is significantly affected by the impact that upstream turbine wakes have on the power production and fatigue loading of subsequent turbines; often called the wake effect. In this work, two types of flows are considered: the wake of a single turbine with a laminar inflow and the combined wake of two turbines operating in-line where the upstream wake results in an unsteady inflow for the downstream turbine. Those two scenarios are simulated using large eddy simulation (LES) and the actuator line method (ALM). The spatio-temporal velocity fields are analyzed by means of high order dynamic mode decomposition (HODMD), a well established variant of the DMD. The results show that most of the higher frequencies characterizing the laminar case are instead dominated by the lower frequency modes in the combined wake. This suggests that structures emerging from the blade rotations in a wind turbine wake may be less significant for describing the wake dynamics when the rotor is operating in the unsteady wake of an upstream rotor.

Polynomial Chaos Expansion-Based Enhanced Gaussian Process Regression for Wind Velocity Field Estimation from Aircraft-Derived Data

This paper addresses the problem of spatiotemporal wind velocity field estimation for air traffic management applications. Using data obtained from aircraft, the eastward and northward components of the wind velocity field inside a specific air space are calculated as functions of time. Both short-term wind velocity field forecasting and wind velocity field reconstruction are performed. Wind velocity data are indirectly obtained from the states of the aircraft flying in the relevant airspace, which are broadcast by the ADS-B and Mode-S aircraft surveillance systems. The wind velocity field is estimated by combining two data-driven techniques: the polynomial chaos expansion and the Gaussian process regression. The former approximates the global behavior of the wind velocity field, whereas the latter approximates the local behavior. The eastward and northward wind components of the wind velocity field must be estimated, which causes the problem to be a multiple-output problem. This method enables the estimation of the wind velocity field at any spatiotemporal location using wind velocity observations from any spatiotemporal location, eliminating the need for spatial and temporal grids. Moreover, since the method proposed in this article allows for the probability distributions of the estimates to be computed, it causes the computation of the confidence intervals to be possible. Furthermore, since the method presented in this paper allows for data assimilation, it can be used online to continuously update the wind velocity field estimation. The method is tested on different wind scenarios and different training-test data configurations, by means of which the consistency between the results of the wind velocity field forecasting and the wind velocity field reconstruction is checked. Finally, the ERA5 meteorological reanalysis data of the European Centre for Medium-Range Weather Forecasts are used to validate the proposed technique. The results show that the method is able to reliably estimate the wind velocity field from aircraft-derived data.

Wind velocity field estimation from aircraft derived data using Gaussian process regression.

Wind velocity field knowledge is crucial for the future air traffic management paradigm and is key in many applications, such as aircraft performance studies. This paper addresses the problem of spatio-temporal windc velocity field estimation. The north and east wind components within a given air space are estimated as a function of time. Both wind velocity field reconstruction in space for a past or present time instant and short-term prediction are performed. Wind data are obtained indirectly from the states of the aircraft broadcast by the Mode-S and ADS-B aircraft surveillance systems. The Gaussian process regression method, which is a flexible and universal estimator, is employed to solve both problems. Under general conditions, the method is statistically consistent, meaning that the method converges to the ground truth when increasingly more data are available, which is especially interesting, since aircraft data availability is expected to grow in the future through the deployment of the European System-Wide Information Management. Besides estimation, the Gaussian process regression method provides the probability distribution of any particular estimate, allowing confidence intervals to be computed. Moreover, the spatial modelling is performed using raw data without relying on grids and estimation can be performed at any spatio-temporal location. Furthermore, since the training phase of the method described in this paper is fast, requiring less than 5 minutes on a standard desktop computer, it can be used online to continuously track the state of the wind velocity field, thus allowing for data assimilation. In the case study presented in this paper, the Gaussian process regression method is tested on different days with different wind intensities. The available data set is split into several training and testing data sets, which are used to check the consistency of the results of wind velocity field reconstruction and prediction. Finally, the Gaussian process regression method is validated using the European Centre for Medium-Range Weather Forecasts ERA5 meteorological reanalysis data. The obtained results show that Gaussian process regression can be used to reliably estimate the wind velocity field from aircraft derived data.

Direct 3D observation and unraveling of electroconvection phenomena during concentration polarization at ion-exchange membranes

A decade ago, two-dimensional microscopic flow visualization proved the theoretically predicted existence of electroconvection roles as well as their decisive role in destabilizing the concentration polarization layer at ion-selective fluid/membrane interfaces. Electroconvection induces chaotic flow vortices injecting volume having bulk concentration into the ion-depleted diffusion layer at the interface. Experimental quantification of these important flow patterns have so far only been carried out in 2D. Numerical direct simulations suggest 3D features, yet experimental proof is lacking. 3D simulations are also limited in covering extended spacial and temporal scales.This study presents a new comprehensive experimental method for the time-resolved recording of the 3D electroconvective velocity field near a cation-exchange membrane. For the first time, the spatio-temporal velocity field can be visualized in 3D at multiples of the overlimiting current density. In contrast to today’s simulations, these experiments cover length and time scales typical for actual electrodialytic membrane processes.We visualize coherent vortex structures and reveal the changes in the velocity field and its statistics during the transition from vortex rolls to vortex rings with increasing current density. The transition is characterized by changes in the rotational direction, mean square velocity, and temporal energy spectrum with only little influence on the spatial spectrum. These findings indicate a more significant impact of EC’s structural change on the mean square velocities and temporal spectra than on the spatial spectra. This knowledge is a prerequisite for engineering ion-selective surfaces that will enable the operation of electrically driven processes beyond the diffusion-limited Nernst regime.

Horizontal oil-water two-phase flow characterization and identification with pulse-wave ultrasonic Doppler technique

To characterize and identify the flow patterns of horizontal oil-water two-phase flow, a pulse-wave ultrasonic Doppler technique was adopted to acquire spatiotemporal flowing information non-intrusively. The echogram was firstly constructed from the echo amplitude to analyze the interrelation between the concentration distribution and the echo intensity under different flow patterns. Then the time-averaged velocity profile was calculated from the Doppler frequency shift to investigate the velocity changes at different radial positions with flow patterns. Further, the long-term and dynamic connectivity of time-varying velocity profiles were analyzed through the graph-variate dynamic (GVD) connectivity method to reveal coupling effects in the spatiotemporal velocity field. Based on the above analysis, several features were extracted objectively to characterize flow patterns from different aspects, and their evolutions with different flow conditions were analyzed. The effectiveness of these features was validated through a multi-class support vector machine model with the total identification accuracy of 93.33%.

PTV/PIV measurements of turbulent flows in interior subchannels of a 61-pin wire-wrapped hexagonal fuel bundle

This study summarizes the experimental effort to characterize the flow fields of various interior subchannels in a 61-pin wire-wrapped hexagonal fuel bundle prototypical for a sodium fast reactor. The objective was to generate high spatiotemporal velocity field data for computational fluid dynamics turbulence model validation. The experimental facility employed the matched-index-of-refraction and modern laser-based optical measurement techniques. It is the largest transparent hexagonal test fuel assembly. Measurements were performed in two planes parallel to the axial flow, Interior-1 and Center-2. The Interior-1 location captured fluid interactions in four narrower subchannels formed by the exterior row of pins near the hexagonal duct wall. The Center-2 location captured fluid interactions in two wider subchannels spanning from the center pin out to the hexagonal duct wall. All measurements have been performed at a Reynolds number of 19,000. Results include discussion about statistical convergence of the dataset, along with flow statistics such as ensemble-averaged velocity, root-mean-square fluctuating velocity, Reynolds stress, and integral length scales.

Flow control of micro-ramps on supersonic forward-facing step flow**Project supported by the National Natural Science Foundation of China (Grant Nos. 11172326 and 11502280).

The effects of the micro-ramps on supersonic turbulent flow over a forward-facing step (FFS) was experimentally investigated in a supersonic low-noise wind tunnel at Mach number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spatiotemporal resolution images and velocity fields of supersonic flow over the testing model were captured. The fine structures and their spatial evolutionary characteristics without and with the micro-ramps were revealed and compared. The large-scale structures generated by the micro-ramps can survive the downstream FFS flowfield. The micro-ramps control on the flow separation and the separation shock unsteadiness was investigated by PIV results. With the micro-ramps, the reduction in the range of the reversal flow zone in streamwise direction is 50% and the turbulence intensity is also reduced. Moreover, the reduction in the average separated region and in separation shock unsteadiness are 47% and 26%, respectively. The results indicate that the micro-ramps are effective in reducing the flow separation and the separation shock unsteadiness.

Taylor渦バイオリアクター内の超音波速度計測

Flow measurement of spatio-temporal velocity field in a Taylor-Couette vortex flow (TVF) with a short annulus was investigated with using Ultrasonic Velocity Profiler (UVP). These methods have significant advantages over conventional methods since it can obtain the instantaneous spatio-temporal information as a function of space and time even in opaque multiphase flows like bioreactors. Boundary effects with a short annulus of TVF called Ekman boundary layer are considered generating the various modes of vortex structure. In the present study, axial mean and instantaneous component of velocity profiles in the axis direction are measured with using the UVP and its flow characteristics in higher Reynolds region with the Ekman boundary is analyzed with spectrum method. The wavy vortex flow, the modulated and the chaotic flow are visualized through the spatio-temporal velocity field. These results could be useful for investigating the chemical flow characteristics and designing the industrial chemical devices or bioreactor systems.

On the turbulent energy transport related to the coherent structures in a planar jet

In this study, the turbulent energy transport related to the coherent vortex structures in the self-similarity region of a turbulent planar jet is investigated experimentally. First, two velocity components and the pressure are measured at multiple points simultaneously through the use of a “combined probe” comprising of a pressure probe and an X-type hot-wire probe. Then, proper orthogonal decomposition (POD) is applied to obtained velocity and pressure field data to extract the coherent vortex structures in the jet. A complementary technique combining POD and linear stochastic estimation is then used to reconstruct the spatiotemporal velocity and pressure field for the dominant POD modes. In the reconstructed velocity and pressure field, a coherent vortex structures which have counter-rotating vortices staggered with respect to the jet centerline are observed. This structure contained approximately 42% of the total energy. Finally, the turbulent energy transport related to the coherent vortex structure is evaluated from the reconstructed velocity and pressure field. This estimation shows that the production and pressure diffusion of the turbulent energy in the jet strongly depend on the discussed coherent vortex structures.

Feedback Guidance in Uncertain Spatiotemporal Wind Using a Vector Backstepping Algorithm

This work deals with the problem of guiding a controlled object to a given target set in a three-dimensional configuration space in the presence of wind with an uncertain spatiotemporal velocity field. The proposed guidance law uses (imperfect) measurements of the velocity of the local wind along the ensuing path of the controlled object to a given target set. No information about the velocity gradients (temporal and spatial) of the wind is assumed to be available. The development of the proposed guidance feedback law is based on the use of a backstepping algorithm that forces the controlled object to follow closely the motion of a lower-order kinematic model that is driven by a pure pursuit navigation law. In this way, the controlled object converges to its target set in finite time. Two kinematic models that describe the motion of the controlled object are considered. In the first model, the rate of change of the air velocity of the controlled object is unconstrained, whereas in the second one, it has to remain perpendicular to the air velocity at all times. Numerical simulations that are based on real wind data are presented.

Wave-shaping of pulse tube cryocooler components for improved performance

The method of wave-shaping acoustic resonators is applied to an inertance type cryogenic pulse tube refrigerator (IPTR) to improve its performance. A detailed time-dependent axisymmetric experimentally validated computational fluid dynamic (CFD) model of the PTR is used to predict its performance. The continuity, momentum and energy equations are solved for both the refrigerant gas (helium) and the porous media regions (the regenerator and the three heat-exchangers) in the PTR. An improved representation of heat transfer in the porous media is achieved by employing a thermal non-equilibrium model to couple the gas and solid (porous media) energy equations. The wave-shaped regenerator and pulse tube studied have cone geometries and the effects of different cone angles and the orientation (nozzle v/s diffuser mode) on the system performance are investigated. The resultant spatio-temporal pressure, temperature and velocity fields in the regenerator and pulse tube components are evaluated. The performance of these wave-shaped PTRs is compared to the performance of a non wave-shaped system with cylindrical components. Better cooling is predicted for the cryocooler using wave-shaped components oriented in the diffuser mode.

Imaging of supersonic flow over a double elliptic surface

The coherent structures of flow over a double elliptic surface are experimentally investigated in a supersonic low-noise wind tunnel at Mach number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spatiotemporal resolution images and velocity fields of both laminar and turbulent inflows over the test model are captured. Based on the time-correlation images, the spatial and temporal evolutionary characteristics of the coherent structures are investigated. The flow structures in the NPLS images are in good agreement with the velocity fluctuation fields by PIV. From statistically significant ensembles, spatial correlation analysis of both cases is performed to quantify the mean size and the orientation of coherent structures. The results indicate that the mean structure is elliptical in shape and the structural angles in the separated region of laminar inflow are slightly smaller than that of turbulent inflow. Moreover, the structural angles of both cases increase with their distance away from the wall.

Velocity and Flame Wrinkling Characteristics of a Transversely Forced, Bluff-Body Stabilized Flame, Part II: Flame Response Modeling and Comparison with Measurements

This article analyzes the response of bluff-body stabilized flames to transverse acoustic waves. Data were obtained for bluff-body flames at flow velocities of 50 m/s and 100 m/s with inlet air temperatures ranging from 475–750 K. Two different modes of acoustic excitation were applied, corresponding to velocity and pressure nodes/antinodes along the combustor centerline. High-speed imaging and phase-locked particle image velocimetry (PIV) were used to characterize the spatio-temporal flame front and velocity field response. The key objective of the study is to compare measurements of the fluctuating velocity and flame wrinkling using the G-equation, e.g., to compare how the ensemble averaged unsteady flame wrinkling gain/phase predicted by solving the G-equation using the measured velocity field as inputs compares to the measured values. These results show good qualitative agreement between the comparisons and measurements, and quite good quantitative accuracy in many of the cases. These comparisons also enable insight into the features controlling the unsteady flame wrinkling; for example, it enables insight into the relative contributions of acoustic and vortical disturbances on the flame wrinkling characteristics, whose different propagation velocities lead to interference patterns and oscillatory flame wrinkle amplitude characteristics.

Velocity and Flame Wrinkling Characteristics of a Transversely Forced, Bluff-Body Stabilized Flame, Part I: Experiments and Data Analysis

This article describes measurements of the response of bluff-body stabilized flames subjected to transverse acoustic waves. It is the first of a two-article series. The objective of this work was to extend prior studies of this nature to much higher Reynolds numbers and more severe environments that more closely mimic conditions encountered in applications. To this end, experiments were performed at flow velocities of 50 m/s and 100 m/s with inlet air temperatures ranging from 475–750 K. Two different modes of acoustic excitation were applied, corresponding to velocity and pressure nodes/antinodes along the combustor centerline. High-speed imaging and phase-locked particle image velocimetry (PIV) were used to characterize the spatio-temporal flame front and velocity field response. The data show that the disturbance field and the flame front response amplitude exhibit a nonmonotonic spatial distribution with interference patterns. The phase of the flame front response at the forcing frequency varies nearly linearly with downstream distance, and corresponds to a phase speed that is slightly less than the mean flow velocity. Significantly, these results show that the key features of the flame's magnitude and phase characteristics are quite similar to those observed in much lower flow velocities.

Photogrammetric Techniques for the Determination of Spatio-temporal Velocity Fields at Glaciar San Rafael, Chile

Glaciar San Rafael in the Northern Patagonia Icefield, with a length of 46 km and an ice area of 722 km2, is the lowest latitude tidewater outlet glacier in the world and one of the fastest and most productive glaciers in southern South America in terms of iceberg flux. Spatio-temporal velocity fields in the region of the glacier front were determined from monoscopic terrestrial image sequences recorded by an inter-vallometer mode high-resolution digital camera over several days. In these image sequences, a large number of glacier surface points were tracked by subpixel accuracy feature tracking techniques. Scaling and georeferencing of the trajectories obtained from image space tracking was performed using a multi-station GPS-supported photogrammetric network. The technique allows for tracking hundreds of glacier surface points at a measurement accuracy in the order of one decimeter, for typical glacier movement rates, and an almost arbitrarily high temporal resolution. The results show velocities of up to 16 meters per day.

Dynamics ofm= 0 andm= 1 modes and of streamwise vortices in a turbulent axisymmetric mixing layer

AbstractThe near field of a Reynolds number$R{e}_{0} = 2. 14\ensuremath{\times} 1{0}^{5} $and low-Mach-number cylindrical jet has been investigated by means of a high-speed stereo PIV setup that provides the spatio-temporal velocity field in a transverse plane, two diameters downstream of the jet exit. Proper orthogonal decomposition (POD) and spatio-temporal correlations are used to identify some of the main dynamical features of this flow. We show that the flow is dominated by streamwise vortices whose production and spatial organization can be related to$m= 0$and$m= 1$perturbations, and to the mean shear of the mixing layer. A dynamical scenario is proposed which describes this interaction, in accordance with our observations.

1903 アスペクト比の小さいTaylor-Couette渦流れの解析(OS19-1 超音波を利用した流動場の計測,オーガナイズドセッション)

In this study, numerical analysis and experimental measurement is carried out for Taylor-Couette vortex flow (TVF) with a small aspect ratio, which is generated between two concentric rotating cylinders with a radial ratio of 0.667. The aspect ratio defined with the gap length between the inner and outer cylinders and the height of the working fluid is 3. Successive instantaneous and mean velocity profiles were obtained with using an ultrasonic velocity profiler (UVP). The spatiotemporal velocity field was analyzed by the fast Fourier transforms. In the result, the characteristics of Wavy Vortex Flow (WVF) and Modulated Wavy Vortex Flow (MVF) transition are measured and the critical Reynolds number to the WVF is qualitatively agreed with the numerical in the case of a normal 2 Cell.

Development of a Mixing Device and Photosynthetic Bioreactor with a Taylor Vortex Flow

Flow measurement of spatio-temporal velocity field in a Taylor-Couette vortex flow (TVF) with a short annulus was investigated by using Ultrasonic Doppler velocity Profiler (UVP). These methods have significant advantages over conventional methods since it can obtain the instantaneous spatio-temporal information as a function of space and time even in the opaque liquid like solid-liquid multiphase fluids. Boundary effects with a short annulus of TVF called Ekman boundary layer are greatly related to the generation of the various modes in vortex structure. Parameters as an aspect ratio, Γ(=H/d) and radius ratio, η(=R1/R2) are important factors for flow bifurcation to these modes. Here, d is an annular gap width, H is a height of the test section, R1 and R2 are radii of the inner and outer cylinders respectively. In the present study, mean velocity profiles of the TVF with Γ=3 and η=0.667 are measured with the UVP. Instantaneous velocity profiles with oscillation in higher Reynolds region (Re=1140) are also measured and analyzed by using the UVP and a spectrum method. The characteristics of wavy vortex flow in the normal 2 and 4 cell modes are much different from each other. Particle behavior in TVF is much complicated. The limit cycle orbit, which has previously only been confirmed numerically in the multi-phased TVF were found experimentally. In the limit cycle, solid particles are gathered in a very limited orbit line. While this phenomenon is unique and does not occur in the general flow region, it could be useful for analyzing the anti-plugging characteristics in industrial rotating filter devices or bioreactor.

903 アスペクト比の小さいTaylor渦流れのUVP計測(2)(OS9-1 超音波を利用した流動場の計測(流れ場計測),OS9超音波を利用した流動場の計測,オーガナイズドセッション)

Boundary effects with a short annulus ofTaylor Vortex Flow (TVF) called Ekman boundary, are greatly related to the generation of the various modes in vortex structure, which is obtained even in the same Reynolds numbers. Some flows with these vortex structures are useful for cultivation of microorganism which is very weak in shearing flow, because it is said that a TVF has a mild flow characteristic even in higher Reynolds flows. In the present, the spatiotemporal velocity field in the higher Reynolds flow of TVF with a small aspect ratio is visualized and measured precisely by using ultrasonic velocity profiler (UVP) and discussed with the relationship between the flow structure and growth rate of the photo synthetic micro organism.