Vortical Flow Structures Research Articles

CFD Simulations of the Effects of Wave and Current on Power Performance of a Horizontal Axis Tidal Stream Turbine

To ensure the long-term reliability of tidal stream turbines, waves and currents must be considered during the design phase. In this paper, a three-bladed horizontal axis turbine with a diameter of 0.9 m was used as the baseline model. OpenFOAM, an open-source computational fluid dynamics (CFD) library platform, was used to predict the performance of a horizontal axis tidal stream turbine (HATST) under waves and currents. A mesh dependency test was carried out to select the optimal mesh to capture the flow’s features. As a validation study, the power of the turbine under only the current was predicted and was found to be consistent with the experimental results. The generated wave profile under a current was compared with the results obtained using the third-order Stokes wave theory. The performance of the HATST was predicted for various wave frequencies and heights and compared with experimental data. The effect of the wave height on the power performance was greater than the wave frequency. Vortical flow structures behind the turbine were investigated for various wave conditions. The generated tip vortices propagated upward and downward at wave crest and trough conditions, respectively.

Influence of aortic valve morphology on vortical structures and wall shear stress.

The aim of this paper is to assess the association between valve morphology and vortical structures quantitatively and to highlight the influence of valve morphology/orientation on aorta's susceptibility to shear stress, both proximal and distal. Four-dimensional phase-contrast magnetic resonance imaging (4D PCMRI) data of 6 subjects, 3 with tricuspid aortic valve (TAV) and 3 with functionally bicuspid aortic values (BAV) with right-left coronary leaflet fusion, were processed and analyzed for vorticity and wall shear stress trends. Computational fluid dynamics (CFD) has been used with moving TAV and BAV valve designs in patient-specific aortae to compare with in vivo shear stress data. Vorticity from 4D PCMRI data about the aortic centerline demonstrated that TAVs had a higher number of vortical flow structures than BAVs at peak systole. Coalescing of flow structures was shown to be possible in the arch region of all subjects. Wall shear stress (WSS) distribution from CFD results at the aortic root is predominantly symmetric for TAVs but highly asymmetric for BAVs with the region opposite the raphe (fusion location of underdeveloped leaflets) being subjected to higher WSS. Asymmetry in the size and number of leaflets in BAVs and TAVs significantly influence vortical structures and WSS in the proximal aorta for all valve types and distal aorta for certain valve orientations of BAV. Analysis of vortical structures using 4D PCMRI data (on the left side) and wall shear stress data using CFD (on the right side).

Role of Kidney Stones in Renal Pelvis Flow.

Ureteroscopy is a commonly performed medical procedure to treat stones in the kidney and ureter using a ureteroscope. Throughout the procedure, saline is irrigated through the scope to aid visibility and wash-out debris from stone fragmentation. The key challenge that this research addresses is to build a fundamental understanding of the interaction between the kidney stones/stone fragments and the flow dynamics in the renal pelvis flow. We examine the time-dependent flow dynamics inside an idealized renal pelvis in the context of a surgical procedure for kidney stone removal. Here, we examine the time-dependent evolution of these vortical flow structures in three dimensions, and incorporate the presence of rigid kidney stones. We perform direct numerical simulations, solving the transient Navier-Stokes equations in a spherical domain. Our numerical predictions for the flow dynamics in the absence of stones are validated with available experimental and numerical data, and the governing parameters and flow regimes are chosen carefully in order to satisfy several clinical constraints. The results shed light on the crucial role of flow circulation in the renal cavity and its effect on the trajectories of rigid stones. We demonstrate that stones can either be washed out of the cavity along with the fluid, or be trapped in the cavity via their interaction with vortical flow structures. Additionally, we study the effect of multiple stones in the flow field within the cavity in terms of the kinetic energy, entrapped fluid volume, and the clearance rate of a passive tracer modeled via an advection-diffusion equation. We demonstrate that the flow in the presence of stones features a higher vorticity production within the cavity compared with the stone-free cases.

Comparison and uncertainty quantification of roof pressure measurements using the NIST and TPU aerodynamic databases

The pressure coefficient of a low-rise building in the ASCE wind load design provision is usually obtained from boundary-layer wind-tunnel tests. Although these tests tend to be standardized, inconsistent results from different facilities have been acknowledged as a long-standing issue. This work compares roof pressure for a low-rise building model archived in the National Institute of Standards and Technology (NIST) aerodynamic database and the Tokyo Polytechnic University database, followed by quantifying the measurement and data reduction uncertainties of the NIST datasets. The Monte Carlo simulation propagates four elemental uncertainties to the mean Cp¯, standard deviation Cp′ and peak pressure coefficient Cp,peak. Results indicate that pronounced differences in the roof pressure from two datasets are attributed to different inflow characteristics (including the Jensen number variation) and the inherent measurement uncertainties. High measurement uncertainties of Cp¯ and Cp,peak are strongly correlated with vortical flow structures, either separated flow or conical vortices at the roof corner and windward edges. Two dominant measurement uncertainty sources are distinguished: the dynamic pressure ratio uncertainty and the surface pressure tap uncertainty. Alternative flow and pressure measurement techniques are noted to potentially reduce the two dominant uncertainty sources. This work is intended to clarify measurement uncertainty sources of obtaining pressure coefficients in wind-tunnel model tests and shed lights on why large differences exist from different tests.

LES-CHT for a Rotating Cavity With Axial Throughflow

Abstract The flow and heat transfer within rotating cavities is often discussed as a conjugate problem; the temperature distribution within the cavity disks drives the large-scale flow structure within the cavity, and the cavity aerodynamics influence the heat transfer to the disks. However, most simulations of rotating cavities only consider the fluid domain in isolation. This is particularly true for turbulence resolving approaches such as large eddy simulation (LES). The large time-scale disparity between the fluid time-steps used in LES and the characteristic solid time-scale complicates the use of LES with conjugate heat transfer (CHT). A further issue is that an under-resolved solid mesh artificially amplifies higher frequency temperature fluctuations from the fluid. This paper addresses these challenges with a new method for LES-CHT where the low frequency temperature fluctuation caused by the large-scale flow structure is accounted for using a multi-scale frequency domain approach. We investigate two cases: axially heated disks made of a low conductivity material, and disks made from a higher conductivity material with a temperature set by radial conduction from the shroud. The formation of small-scale flow structures on both the disk and shroud is dependent on the heating configuration of the cavity—indicating that high-fidelity thermal boundary conditions should be used when simulating rotating cavities. The formation of heating induced vortical flow structures near the disk is particularly interesting, as this is unexpected from the laminar Ekman layer modeling argument usually used to consider this region.

Analysis and Discussion of Two-Way Coupling Effects in Particle-Laden Turbulent Channel Flow

This paper studies the turbulence modification caused by the presence of solid particles in fully developed channel flow by means of the point particle Direct Numerical Simulations (DNS) approach. Inertial particles much smaller than the smallest vortical flow structures are considered, maintaining a volume fraction of the order , where inter-particle collisions are rare and have nearly no influence on flow development. To avoid concurrent effects that could mask the analysis of fluid turbulence interaction, gravity is not included in the study, and particle-smooth wall collisions are modelled as ideal reflections. The alteration of fluid turbulence dynamics by the particles is illustrated and discussed, providing an overview of the fluid-particle interaction phenomena occurring at both microscopic and macroscopic flow levels. Finally, the relation of such phenomena with drag-reducing effects by particles is demonstrated.

Investigation of combustion noise generated by an open lean-premixed H[formula omitted]/air low-swirl flame using the hybrid LES/APE-RF framework

An open lean-premixed hydrogen/air low-swirl (LPHALS) turbulent flame exhibiting a pronounced peak in its combustion noise spectra, is investigated numerically using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. Under this framework, the reacting flow-field of the flame is computed via Large-Eddy Simulation (LES), while the direct combustion noise it produces is captured by solving the Acoustic Perturbation Equations for Reacting Flows (APE-RF). Flame configuration and simulation conditions correspond to those of an experimental study on an open lean-premixed H2/air flame stabilized using a Low-Swirl Burner (LSB). LES results are validated against experimental data. The CAA simulation is able to predict a pronounced sharp peak in the computed combustion noise spectra, similar to one of the two characteristic peaks observed in the measured combustion noise spectra. Frequency of this spectral peak predicted by the CAA simulation is 840 Hz, which is close to that of the higher frequency secondary spectral peak at 940 Hz measured in the experiment. Upon examining the hybrid LES/APE-RF results, the noise generation mechanism at 840 Hz is found to be the intense local heat release rate fluctuations, caused by strong interaction between the flame and the periodically generated vortical flow structures in the shear layers, downstream of the LSB exit. Additionally, analysis of the spectral content and directivity of the noise generated by different acoustic source terms is performed, in order to investigate their impact on the radiated acoustic field, and hence the characteristics of direct combustion noise produced by the open LPHALS flame.

Vortical flow characteristics of a slender delta wing in ground effect

This study examined the ground effect on the vortical flow structures over a 70-deg-swept slender delta wing qualitatively and quantitatively using dye flow visualization and Particle Image Velocimetry techniques. The observations with dye visualization and PIV methods were conducted in the plan-view, side-view, and end-view planes with the angles of attack of α = 25° and 35°. The distance, h between the delta wing and the ground surface nondimensionalized with the chord, c of the delta wing, was considered as h/c = 0.1, 0.25 0.55 as well as the ground-free effect flow condition. Also, the experiments were performed at a normalized chordwise location of x/c = 0.8 over the end-view plane. The results demonstrate that magnitudes of leading-edge vortices progressively diminish when the delta wing moves from the ground-free effect region towards the wing location, h/c = 0.1. Also, the rate of inboard flow from the side leading-edge towards the central axis of the wing gets higher by the influence of the ground over the suction surface of the wing. When the wing altitude is lowered towards the ground surface, the size of the leading-edge vortex core region enlarges for both angles of attack, α = 25°, and 35°. The efficiency of the wing aerodynamics is positively affected by its closeness to the ground due to the air cushion effect that occurs as a result of the pressure increase on the suction surface of the wing due to the stagnation of air in the space between the delta wing and ground.

Bullet Time Taylor-Couette: Unwrapping The 360 Degree Field Of View For Rheoscopic Flow Visualization

The circular Taylor-Couette flow is one of the archetypical model systems for the study of flow transitions and dynamic pattern formation in experimental fluid dynamics. The emergence of the internal vortical flow structures are commonly visualized through a rheoscopic flow visualization, while their spatio-temporal dynamics can be extracted by the construction of a space-time diagram using a single camera. Although the latter is an effective method to map the various flow regimes for different inner and outer cylinder rotations, it suffers from limitations in the frame rate while the full extent of the azimuthal vortex structure along the circumference, together with its dynamic evolution through space and time, remains unclear. In this work, we perform the full 360-degree field of view panorama imaging for the rheoscopic flow visualization of the azimuthal vortex structure that wraps around the circumference. We use a set of 12 GoPro cameras that are commercially available and can be triggered remotely. We calibrate and position our cameras using methods from computer vision while we synchronize their audio channels at an inter-frame precision much greater than the frame rate. We unwrap the physical coordinates along the circumference of the outer cylinder through texture mapping its surface using a spatially weighted image interpolation and present a single representation of the azimuthal vortex structure from the rheoscopic flow visualization. We validate our methods within a submillimeter precision and showcase the application to study the steady-state and transient dynamics of a single-phase wavy vortex flow. Furthermore, we discuss the current limitations as we add neutrally buoyant PMMA particles at increasing volume fractions up to 30 %. Our methods allow us to fully decouple space and time, and study the dynamic pattern formation at bullet time accuracy.

Pressure And Velocity Fields Decomposition Around A Flapping Wing

In the present work, the velocity and pressure fields around a flapping airfoil in pitching/heaving sinusoidal motion are decomposed in modes in the wing-fixed reference frame. This approach is aimed at assessing the contribution of the fluid structures forming on the wing surface to the aerodynamic forces. The velocity field used for this work have been obtained through Particle Image Velocity experiments at a chord-based Reynolds number of 3600 and a Strouhal number equal to 0.2. The pressure field has been reconstructed from the phase-average velocity fields using a finite-differences Poisson solver. Finally, the velocity and pressure fields have been interpolated on a wing-fixed domain and decomposed using Proper Orthogonal Decomposition and Quadratic Stochastic Estimation to account for the quadratic relation between velocity and pressure. The decomposition identified vortical flow structures over the wing which model the evolution of bound vorticity, as well as leading edge and trailing edge vortices, over the wing. These modes contribute linearly to the chord-normal force by interacting with the constant free stream contained in the time average, as well as quadratically to the chord-wise force.

Experimental Investigation of Ground Effect on the Vortical Flow Structure of a 40° Swept Delta Wing

The ground effect influences the flow structure on a delta wing during landing and take-off processes. In this regard, comprehensive instantaneous velocity measurements and flow visualizations were carried out by particle image velocimetry and dye flow visualization techniques to reveal the ground effect on leading-edge vortex characteristics of a 40° swept delta wing. The flow behaviors on the delta wing under the impact of the ground were analyzed at two angles of attack, 8° and 11°, and the space between the ground and lower surface of the wing was nondimensionalized with the wing’s chord length. It was found that the presence of the ground caused premature leading-edge vortex breakdown due to the increasing adverse pressure gradient on the wing’s suction side along the chord direction. The ground effect caused an increase in peak value and distributions of turbulent kinetic energy on the wing surface that depended on the earlier leading-edge vortex bursting and complex and disorganized flow structures. The value of time-averaged vertical velocity was lower when the delta wing descended from the free-stream flow zone into the ground effect zone because of the blocking of fluid flow in the gap between the ground and pressure surface of the wing. Thus, it can be concluded that the ground effect is very influential on the change of vortical flow characteristics of nonslender delta wings.

Noise metrics of the time-varying acoustic far-field of rotors

Rotor noise comprises harmonic features, related to the blade passing frequency, as well as broadband noise. Even though an acoustic spectrum yields a frequency-distribution of energy within an acoustic pressure signal, it does not reveal phase-relations between different frequency components. The latter are of critical importance for the development of accurate prediction- and auralization-algorithms, because they may result in low-frequency modulation: temporal variations in the intensity of higher-frequency rotor noise. Baars et al. recently outlined a methodology to quantify inter-frequency modulation. The current work applies this methodology to a new and comprehensive dataset of a laboratory-scale rotor at advance ratios ranging from J = 0 to 0.6. PIV measurements of the blade-induced flow disturbances complement the acoustic data to elucidate how the vortical flow structures of one blade impact the inflow of the consecutive blade. Findings of the directivity patterns of modulation are described and interpreted using existing knowledge of the specific noise sources of the low Reynolds number (small-scale) rotor.

Fluid structure interaction of a subaqueous pendulum: Analyzing the effect of wake correction via large eddy simulations

The dynamic behavior of a subaqueous cylindrical pendulum and corresponding flow dynamics are investigated. The objectives were twofold: (i) to examine whether the two-dimensional model equations sufficiently capture the three-dimensional dynamics and (ii) to investigate the emerging three-dimensional vortical flow structures. Large eddy simulations with two-way coupling fluid structure interaction were carried out using the immersed boundary method to simulate the motion of the pendulum and its interactions with the initially stagnant water. The resulting pendulum motion is compared against measured data obtained in a series of experimental tests to validate the simulation results and the model equations with and without wake corrections. An analysis of the flow vorticity revealed the development of a vortex ring during the first swing and the formation of tip vortices. The evolution of the vortex rings emerging from the motion of the subaqueous cylindrical pendulum was visualized using Q-criteria showing a reasonable agreement with vortical structures observed in the experiment using particle imaging velocimetry. The hydrodynamic moments acting on the simulated pendulum and the moments calculated from the model equations are analyzed. Using the insights from these numerical simulations, a modification of the wake correction is proposed to enhance the accuracy of the rate of decay and period. The transient effect of coherent flow on pendulum dynamics, especially the added mass effect, is discussed.

Robust and efficient identification of optimal mixing perturbations using proxy multiscale measures.

Understanding and optimizing passive scalar mixing in a diffusive fluid flow at finite Péclet number (where and are characteristic velocity and length scales, and is the molecular diffusivisity of the scalar) is a fundamental problem of interest in many environmental and industrial flows. Particularly when , identifying initial perturbations of given energy that optimally and thoroughly mix fluids of initially different properties can be computationally challenging. To address this challenge, we consider the identification of initial perturbations in an idealized two-dimensional flow on a torus that extremize various measures over finite time horizons. We identify such ‘optimal’ initial perturbations using the ‘direct-adjoint looping’ method, thus requiring the evolving flow to satisfy the governing equations and boundary conditions at all points in space and time. We demonstrate that minimizing multiscale measures commonly known as ‘mix-norms’ over short time horizons is a computationally efficient and robust way to identify initial perturbations that thoroughly mix layered scalar distributions over relatively long time horizons, provided the magnitude of the mix-norm’s index is not too large. Minimization of such mix-norms triggers the development of coherent vortical flow structures which effectively mix, with the particular properties of these flow structures depending on and also the time horizon of interest.This article is part of the theme issue ‘Mathematical problems in physical fluid dynamics (part 1)’.

Mixing in the confined vortical flow in non-aligned T-jets reactors

In this work, flow dynamics and mixing performances in a non-aligned T-jets reactor are experimentally studied within a Reynolds number (Re) range of 50–160. Plane Laser-Induced Fluorescence (PLIF) is used to measure concentration fields and explore underlying vortical flow structures. Two vortex rings are identified in the mixing channel of the reactor at Re≥90, and the fluid pulsation induced by the vortex ring is demonstrated. The mixing in the reactor is qualified by an area-based distribution of mixing intensity, and the scale and intensity of segregation. Results show that the scale and intensity of segregation are reduced as the vortex ring appears, and the overall mixing is closely associated with vortex rings and the fluid pulsation, of which the former dominates. Further insight into the mixing is obtained by analyzing the decay time of the fluid. It is found that long decay time of the fluid in vortex rings causes the formation of well-mixed regions, and the concentration decay time in the second vortex ring is more than twice that in the first vortex ring despite the smaller size of the second vortex ring.

Lattice Boltzmann simulation of flow and sound fields around an axial flow fan

Abstract In this study, not only the flow field but the aerodynamic sound field around the axial flow fan were directly simulated using the lattice Boltzmann method (LBM). To validate the LBM results obtained in this study, they were compared with the experimental results in terms of the pressure rise performance characteristics. In order to investigate the change in the flow and sound fields due to different operating points, calculations were conducted at two operating points; one is the low flow rate point where the fan is stalled and the other is the high flow rate point where the flow field in the fan is sound. In the comparison of the performance characteristics, the calculation results showed good agreement with the experimental results, and the flow fields were well reproduced with high accuracy. The analysis in terms of the vortical flow structures and the relative velocity distributions showed that the tip vortex was generated by the pressure difference between both sides of the fan blade at the high flow rate point while at the low flow rate point the flow field near the fan tip was covered with the low velocity region and the leading edge separation occurred on the tip side of the fan blade. Therefore, it was confirmed from the flow field as well as the performance that the fan blade was stalled with the blade loading lost at the low flow rate point. Large pressure fluctuations on the fan blade surface were caused by the interaction with the tip vortex from the adjacent blade at the high flow rate point and the occurrence of leading edge separation at the low flow rate point. With regard to the comparison of the far field sound spectrum, the calculation results for both two flow rate points were in good agreement with the experimental results in a low frequency range, and the aerodynamic sounds generated from the fan were predicted well with the LBM simulation.

Numerical Study of Unsteady Cavitating Flow in an Inducer With Omega Vortex Identification

Abstract To accurately capture the behaviors of cavitation and reveal the unsteady cavitating flow mechanism, a condensate pump inducer is numerically analyzed in a separate numerical experiment with large eddy simulation (LES) at critical cavitation number σind,c under the design point. Based on the new Omega vortex identification method, the correlation between the flow structures and cavities is clearly illustrated. Besides, the pressure fluctuations around the inducer are analyzed. Special emphasis is put on the analysis of the interactions between the cavities, turbulent fluctuations, and vortical flow structures. The Omega vortex identification method could give an overall picture of the whole cavitating flow structures to present a clear correlation between the vortices and cavities. The results show that the shear cavitation dominates the cavitation characteristics under the design point. The pure rigid rotation region mainly concentrates at the edge of the cavities while the other sheet-like cavities near the casing walls are characterized by strong turbulence fluctuations. Besides, based on the analysis of the correlation between the cavities and flow structures, the rotating cavitation under the design point may mainly be attributed to the interaction between the tip leakage vortex cavitation and the next blade.

Influence of expiratory flow pulsatility on the effectiveness of a surgical mask.

Expiratory events, such as coughs, are often pulsatile in nature and result in vortical flow structures that transport expiratory particles. The World Health Organization recommends wearing face masks to reduce the airborne transmission of diseases such as SARS-CoV-2 (COVID-19). However, masks are not perfect as flow leakage occurs around the mask, and their effectiveness under realistic (multi-pulse) coughing conditions is unknown. To assess the influence of expiratory flow pulsatility on the effectiveness of a surgical face mask by quantifying and classifying the flow leakage around the mask. A custom-built pulsatile expiratory flow simulator is used to generate single- and multi-pulsed coughing events. Flow visualization and particle image velocimetry are used to assess the penetration distance and volume of leakage flow at the top and sides of a surgical mask. Leakage flow velocity profiles at the top and sides of a surgical mask take the form of a wall jet and a free-shear jet, respectively. Multi-pulsed expiratory flow events are found to generate greater leakage flow around the mask than single-pulsed events. For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough. The novelties of this study are: First, flow field measurements are used to quantify and classify the leakage flow fields around the top and sides of a surgical mask, providing a benchmark for quantitative modeling of leakage flow velocity profiles. Second, the influence of pulsatility on the effectiveness of surgical face masks is studied by quantifying the leakage volume. For the first time, the leakage volume of a surgical mask is shown to be correlated to the pulsatile nature of a cough, as multi-pulsed expiratory flow events are found to generate greater flow leakage around the mask than single-pulsed events.

Design Rule Extraction Using Multi-Fidelity Surrogate Model for Unmanned Combat Aerial Vehicles

Due to the instability of the pitching moment induced by the strong vortical flow at moderate to high angles of attack in the unmanned combat aerial vehicle (UCAV) configuration, previous design studies focused primarily on low-speed flight conditions where the longitudinal stability is a serious issue. Furthermore, because of the limitation of computational cost, univariate analyses based on low-fidelity flow solvers are performed to investigate the relationships between design variables and quantities of interest. In this study, since the high-speed condition may occupy a high proportion in the UCAV mission, it was additionally considered through multi-objective optimization. A high-fidelity flow solver is used for both low-speed and high-speed conditions to accurately capture the vortical flow structure, and corresponding computational costs are relieved by multi-fidelity surrogate modeling. Based on this surrogate model, design rules are extracted, and the effects of design variables are investigated through univariate and multivariate data mining techniques. At last, universal design rules that can be referenced regardless of the parameterization method are visualized in an airfoil shape using proper orthogonal decomposition. The time-efficient design rule extraction framework and the approach to analyzing extracted design rules presented in this study can be extended to any engineering field.

Dynamics of evaporating respiratory droplets in the vicinity of vortex dipoles

A new mathematical analysis of exhaled respiratory droplet dynamics and settling distances in the vicinity of vortical environments is presented. Recent experimental and theoretical studies suggest that vortical flow structures may enhance the settling distances of exhaled respiratory droplets beyond the two-meter distancing rule recommended by health authorities lately. We propose a mathematical framework to study the underlying physical mechanism responsible for the entrapment and subsequently delayed settling times of evaporating droplets and solid particles. A dipolar vortex is considered self-propelling through a cloud of micron-sized evaporating droplets. This configuration might be utilized to approximate an indoor environment in which similar unsteady vortical flow structures interact with exhaled respiratory droplets. We demonstrate the vortex dipole effect on droplet and solid particles settling distances, depending on the evaporation rate, the vorticity of the dipole, and the droplet’s initial diameter and location relative to the vortex core. Our theoretical analysis reveals non-intuitive interactions between the vortex dipole, droplet relaxation time, gravity, and mass transfer. The existence of optimal conditions for maximum displacement is suggested, where the droplet entrainment reaches up to an order of magnitude larger than the vortex core length scale. We present a basic model that may be applied for evaluating the spread of exhaled respiratory droplets in vortical environments. Our theoretical study suggests that exhaled respiratory droplets initially at rest can translate to significant distances, hence implying that vortical flow might enhance the transmission of airborne pathogens.