Instantaneous Flow Structures Research Articles

Multiblock High Order Large Eddy Simulation of Powered Fontan Hemodynamics: Towards Computational Surgery.

Children born with only one functional ventricle must typically undergo a series of three surgeries to obtain the so-called Fontan circulation in which the blood coming from the body passively flows from the Vena Cavae (VCs) to the Pulmonary Arteries (PAs) through the Total Cavopulmonary Connection (TCPC). The circulation is inherently inefficient due to the lack of a subpulmonary ventricle. Survivors face the risk of circulatory sequelae and eventual failure for the duration of their lives. Current efforts are focused on improving the outcomes of Fontan palliation, either passively by optimizing the TCPC, or actively by using mechanical support. We are working on a chronic implant that would be placed at the junction of the TCPC, and would provide the necessary pressure augmentation to re-establish a circulation that recapitulates a normal two-ventricle circulation. This implant is based on the Von Karman viscous pump and consists of a vaned impeller that rotates inside the TCPC. To evaluate the performance of such a device, and to study the flow features induced by the presence of the pump, Computational Fluid Dynamics (CFD) is used. CFD has become an important tool to understand hemodynamics owing to the possibility of simulating quickly a large number of designs and flow conditions without any harm for patients. The transitional and unsteady nature of the flow can make accurate simulations challenging. We developed and in-house high order Large Eddy Simulation (LES) solver coupled to a recent Immersed Boundary Method (IBM) to handle complex geometries. Multiblock capability is added to the solver to allow for efficient simulations of complex patient specific geometries. Blood simulations are performed in a complex patient specific TCPC geometry. In this study, simulations without mechanical assist are performed, as well as after virtual implantation of the temporary and chronic implants being developed. Instantaneous flow structures, hepatic factor distribution, and statistical data are presented for all three cases.

Instantaneous Flow Structures and Opportunities for Larval Settlement: Barnacle Larvae Swim to Settle.

Water flow affects settlement of marine larvae on several scales. At the smallest scale local flow regime may control the probability of adhesion to the substrate. Our aim was to mechanistically understand the transition from suspended to attached larvae in turbulent flow. Recently it was proposed that opportunities for larval settlement in turbulent boundary layers depend on time windows with suitable instantaneous flow properties. In flume flow we characterized the proportion of suitable time windows in a series of flow velocities with focus on the near-bed flow. The change in the proportion of potential settling windows with increasing free-stream velocities was compared to the proportion of temporary attachment of barnacle cypris larvae at different flow velocities. We found large instantaneous flow variations in the near-bed flow where cyprid attachment took place. The probability of temporary attachment in cyprids declined with local flow speed and this response was compatible with a settling window lasting at least 0.1 s with a maximum local flow speed of 1.9–2.4 cm s-1. Cyprids swam against the near-bed flow (negative rheotaxis) and the swimming speed (1.8 cm s-1) was close to the critical speed that permitted temporary attachment. We conclude that temporary attachment in barnacle cyprids requires upstream swimming to maintain a fixed position relative to the substrate for at least 0.1 s. This behaviour may explain the ability of barnacles to recruit to high-flow environments and give cyprids flexibility in the pre-settlement choice of substrates based on flow regime.

Experimental study of flow field on stepped airfoil at very low Reynolds number

In this article, the flow field around NACA0024 airfoil with step at lower and upper surfaces is experimentally investigated. For this purpose, particle image velocimetry technique based on the instantaneous flow structures is used to investigate the flow field around the airfoil at different times. All the experimental measurements in current study are conducted at very low Reynolds number condition based on the chord of the airfoil (Re=2000) and at angles of attack at 0° and 5° where the flow around airfoils is separated. The differences between vortical structures, mean streamlines, sizes of the wake regions, and vortex shedding of the stepped airfoils compared to unmodified airfoil are observed. The results disclose that using step in airfoil leads to a decrease in the Strouhal number. In addition, the formation of vortices in wake region and their positions at different times are discussed.

Vortex structure and breakup mechanism of gaseous jet in supersonic crossflow with laminar boundary layer

Employing nano-particle planar laser scattering and particle image velocimetry technology, underexpanded jet in supersonic crossflow with laminar boundary layer is experimental investigated in a low noise wind tunnel. Instantaneous flow structures and average velocity distribution of jet plume are captured in experimental images. Horseshoe vortex system is dominated by oscillating and coalescing regime, contributing to vortex generation of jet shear layer. The “tilting-stretching-tearing” mechanism dominating in near field raises average fractal dimension. But vortex structures generated on the windward side of jet plume scatter in jet plume and dissipate gradually, which makes the vortexes break up from outside in near field and break down into small turbulence completely in far field.

CFD simulations of the flow around a cyclist subjected to crosswinds

For the first time, an extensive numerical study of the effect of crosswinds on the flow around a cyclist on a bicycle with stationary wheels has been undertaken for crosswind (yaw) angles ranging from 0° to 90°. The flow field and the aerodynamic forces have been obtained using three numerical techniques: Reynolds Averaged Navier Stokes (RANS), Detached Eddy Simulation (DES) and Large Eddy Simulation (LES). RANS simulations have been undertaken for all the range of yaw angles to provide a general insight of the flow around a cyclist, whilst DES and LES have been undertaken at 15° yaw angle in order to investigate the time-varying flow physics in detail. The aerodynamic forces have been compared with a series of wind tunnel experiments. The RANS results showed the development of large flow separation around the bicycle with increasing yaw angles. The instantaneous flow structures and the auto-spectral densities of the time histories of the force coefficients are identified and revealed that the DES and LES turbulence models are able to predict the dominant frequencies found in the physical experiments. This work provides an improved understanding of the flow characteristics around a cyclist in crosswinds that will hopefully help to improve the safety of cyclists.

A study of the energetic turbulence structures during stall delay

This study revealed the three-dimensional instantaneous topologies of the large-scale turbulence structures in the separated flow on the suction surface of wind turbine’s blade during stall delay. These structures are the major contributors to the first two POD (proper orthogonal decomposition) modes. The two kinds of instantaneous flow structures as major contributors to the first POD mode are: (1) extended regions of downwash flow with an upstream upward flow beside it and a compact vortex pair closer to the blade’s leading edge; (2) a large-scale clockwise vortex with strong induced flows. The two kinds of flow structures contributing significantly to the second POD mode are: (1) large counter-rotating vortices inducing strong upward velocities and a series of small vortices; (2) strong downwash flow coming from the leading-edge shear layer with a large and strong vortex on the left side and small vortices upstream. The statistical impacts of these large-scale and energetic structures on the turbulence have also been studied. It was observed that when these turbulence structures were removed from the flow, the peak values of some statistics were significantly reduced.

Development of second-mode instability in a Mach 6 flat plate boundary layer with two-dimensional roughness

Particle image velocimetry, PCB pressure sensors, and planar Rayleigh scattering are combined to study the development of second-mode instability in a Mach 6 flow over a flat plate with two-dimensional roughness. To the best of the authors’ knowledge, this is the first time that the instantaneous velocity fields and flow structures of the second-mode instability waves passing through the roughness are shown experimentally. A two-dimensional transverse wall blowing is used to generate second-mode instability in the boundary layer and seeding tracer particles. The two-dimensional roughness is located upstream of the synchronization point between mode S and mode F. The experimental results showed that the amplitude of the second-mode instability will be greatly increased upstream of the roughness. Then it damps and recovers quickly in the vicinity downstream of the roughness. Further downstream, it acts as no-roughness case, which confirms Fong’s numerical results [K. D. Fong, X. W. Wang, and X. L. Zhong, “Numerical simulation of roughness effect on the stability of a hypersonic boundary layer,” Comput. Fluids 96, 350 (2014)]. It also has been observed that the strength of the amplification and damping effect depends on the height of the roughness.

Flow and Heat transfer Characteristics in a Square Cavity at High Rayleigh Number

Numerical simulations are performed to investigate the flow and heat transfer characteristics in a air-filled (Pr = 0.71), two-dimensional (2-D) cavity where the flow is induced by a buoyancy force due to differential heating of vertical walls at high Rayleigh numbers (10^9, 1.58 × 10^9, 10^(9.5), and 10^(10)). The governing equations are discretized spatially into a fourth-order-accurate compact form and integrated temporally by the second-order-accurate alternating direction implicit (ADI) method. Numerical results of instantaneous and time-averaged flow structures are presented. The time-averaged temperature and vertical velocity near the hot wall are used to generate the mean temperature and velocity profiles in the turbulent boundary layer, and to illustrate the required spatial resolution for simulations of turbulent natural convection. A universal structure is found to exist in the mean velocity and temperature turbulent boundary layer profiles for x^+ ＜ 10. This finding is also in good agreement with reported experimental data.

Optical diagnostics for high-speed flows

Since 2000 there has been a revolution in diagnostics of high-speed air flows. The foundations for this revolution were laid over the past few decades, but with the development of new short pulse and pulse burst laser technologies, higher laser powers and higher pulse energies, new high-speed cameras, better laser control and improved detection and laser delivery methodologies, many very effective new capabilities have been developed. Newly developed methods for molecular tagging velocimetry provide high fidelity visualization of transport properties and may be extended to simultaneous temperature measurements. Rapid field imaging with frequency tunable pulse burst lasers shows instantaneous flow structure and complex boundary and mixing interactions. Extending these pulse burst concepts to swept volumetric imaging is very promising for full volumetric data collection. Fast wavelength modulation spectroscopy follows real-time flow variation, and three-dimensional particle imaging extends particle imaging velocimetry to volumetric data acquisition.

Particle Transport and Deposition in a Turbulent Square Duct Flow With an Imposed Magnetic Field

In this paper, we study particle transport and deposition in a turbulent square duct flow with an imposed magnetic field using direct numerical simulations (DNS) of the continuous flow and Lagrangian tracking of particles. The magnetic field and the velocity induce a current and the interaction of this current with the magnetic field generates a Lorentz force that brakes the flow and modifies the flow structure. A second-order accurate finite volume method is used to integrate the coupled Navier–Stokes and magnetohydrodynamic (MHD) equations and the solution procedure is implemented on a graphics processing unit (GPU). Magnetically nonconducting particles of different Stokes numbers are continuously injected at random locations in the inlet cross section of the duct and their rates of deposition on the duct walls are studied with and without a magnetic field. Because of the modified instantaneous turbulent flow structures as a result of the magnetic field, the deposition rates and patterns on the walls perpendicular to the magnetic field are lower than those on the walls parallel to the magnetic field.

Extracting quantitative data from tuft flow visualizations on utility scale wind turbines

First results of a novel measurement technique that allows to extract quantitative data from tuft flow visualizations on real-world wind turbine blades are presented. The instantaneous flow structure is analyzed by tracking individual flow indicators in each of the snapshot images. The obtained per-tuft statistics are correlated with logged turbine data to provide an insight into the surface flow structure under the influence of wind speed. A histogram filter is used to identify two flow states: a separated flow state that occurs at higher wind speeds and a maximal attached flow state that mainly occurs in the lower wind speed range.

On the near-wall vortical structures at moderate Reynolds numbers

A recent database from direct numerical simulation (DNS) of a turbulent boundary layer up to Reθ=4300 (Schlatter and Örlü, 2010) is analysed to extract the dominant flow structures in the near-wall region. In particular, the question of whether hairpin vortices are significant features of near-wall turbulence is addressed. A number of different methods based on the λ2 criterion (Jeong and Hussain, 1995) is used to extract turbulent coherent structures: three-dimensional flow visualisation with quantitative estimates of hairpin population, conditional averaging and planar hairpin vortex signatures (HVS). First, visualisations show that during the initial phase of laminar–turbulent transition induced via tripping, hairpin vortices evolving from transitional Λ vortices are numerous and can be considered as the dominant structure of the immediate post-transition stage of the boundary layer. This is in agreement with previous experiments and low-Reynolds-number simulations such as Wu & Moin (2009). When the Reynolds number is increased, the fraction of hairpin vortices decreases to less than 2% for Reθ>4000. Second, conditional ensemble averages (Jeong et al., 1997) find hairpins close to the wall at low Reynolds number, while at a sufficient distance downstream from transition, the flow close to the wall is dominated by single quasi-streamwise vortices; even quantitatively, no major differences between boundary layer and channel can be detected. Moreover, three-dimensional visualisations of the neighbourhood of regions of strong swirling motion in planar cuts through the layer (the HVS) do not reveal hairpin vortices, thereby impairing statistical evidences based on HVS. The present results thus clearly confirm that transitional hairpin vortices do not persist in fully developed turbulent boundary layers, and that their dominant appearance as instantaneous flow structures in the outer boundary-layer region is very unlikely.

Effect of the instantaneous turbulent flow structures on the particle distribution near the wall of a channel

We analyzed databases obtained from direct numerical simulations of the turbulent flow in a plane channel at low Reynolds numbers. Particles with different inertia were tracked using the dilute phase approximation. The main objective of this study is the determination of the fluctuations of concentration produced by two types of instantaneous near-wall flow structures. The first type of structure produces extreme negative fluctuations of the wall shear stress (τw′≪0) and the second type of structures are associated with large positive fluctuations (τw′≫0). A conditional sampling technique was used to educe the topologies of these two types of flow structures and the corresponding conditional averaged particle concentration fluctuations. The contribution of each type of flow event to the wall shear stress history is about 10% but their contribution to the particle velocity component perpendicular to the wall, in the near-wall region, is about 50% for the flow structures associated to events with τw′≪0 and about 40% for those with τw′≫0, indicating that the role of these structures is significant in the particle fluxes from and towards the wall. The flow structures consist in two parallel counterrotating streamwise vortices that, in the case of τw′≪0, create a flow ejection from the wall between them and in the case of τw′≫0, produce a sweep towards the wall. These events produce important fluctuations of the particle concentration near the wall, with intensities up to 200% for the flow structure associated with τw′≪0 and for the largest Stokes number considered (St=25).

Large-eddy simulation of dispersed two-phase flows

Large-eddy simulation (LES) is under its rapid development. In comparison with the Reynolds-averaged Navier-Stokes modeling (RANS modeling) LES can give instantaneous flow and flame structures and in some cases may give more accurate statistical results than RANS modeling. The present author and his colleagues started to study the instantaneous flow structures of bubbly flows by LES in 2002. Subsequently, from 2005 up to now the studies on LES of gas-particle flows, the flow structure surrounding a single particle, and the flow and flame structure surrounding an evaporating and combusting droplet were carried out. In this paper, a review will be given for some recent studies by the present author and his colleagues on LES of dispersed two-phase flows, including the governing equations, sub-grid-scale models and numerical methods, main research results and their experimental validation.

Turbulence characteristics in free-surface flow over two-dimensional dunes

The turbulent structure of open-channel flows over two-dimensional dunes is investigated numerically using large-eddy simulation (LES), in order to improve our understanding of the interaction between the dune-generated turbulence and the free surface dynamics. The filtered Navier–Stokes equations in the LES model have been discretised using the finite volume method, with a dynamic sub-grid model being employed for the unresolved scales of turbulence. The partial cell treatment has been implemented in a Cartesian grid form to deal with the dune topography. Both the volume of fluid method and rigid lid approach have been employed in the numerical framework to investigate the effects of the free surface treatment on the flow characteristics. The numerical model predicted mean flow velocities, turbulence intensities and Reynolds stresses have been compared with experimental measurements published in the literature, with a detailed analysis being undertaken to assess the accuracy of the model results and the effects of the free surface treatment on the velocity and turbulence predictions. The instantaneous flow structure has been investigated, with emphasis being focused on the free surface dynamics and coherent structures.

Experimental investigation of interactions between laminar or turbulent boundary layer and shock wave

Investigation on the interactions between laminar or turbulent layer and shock wave is performed in a Mach 3.4 supersonic wind tunnel, based on nanoparticle-tracer based planar laser scattering (NPLS) system and supersonic particle image velocimetry (PIV) system. The model geometry in this experiment is composed of two flats with different positions in the test section which can provide the flat with different kinds of boundary layer, and a shock wave generator–a 12° wedge. Boundary layer separation/attachment, induced suppression waves, induced shock wave and expansion fan were clearly presented by NPLS images, velocity field and vorticity field were given by PIV results. Instantaneous flow structures and temporal evolution of two different flow conditions were analysised and compared according to both NPLS and PIV results. The experimental results show that: the separation zone in the turbulent boundary layer is long and narrow, but it’s quite different in the turbulent boundary layer where it is oval-shaped; upstream of the oblique shock wave, a serious of suppression waves can be observed outside of the laminar boundary layer and they will focus into a unsteady induced shock wave, but in contrast, only a focused shock wave can be found outside of the turbulent boundary layer and it’s quite stable; the expansion fan downstream of the oblique shock wave is small in the laminar boundary layer, leading to a sufficient acceleration of the flow, and the attachment shock wave behind the expansion fan is extremely weak, however it is a totally different condition in the turbulent boundary layer with a wide expansion fan and a strong attachment shock wave.

Large-eddy simulation of two-phase combustion

Large-eddy simulation (LES) of two-phase combustion is under its rapid development. In comparison with the Reynolds-averaged Navier-Stokes modeling (RANS modeling) LES can give instantaneous flow and flame structures and in many cases give more accurate statistical results. One of the key problems in LES is the sub-grid scale (SGS) models, in particular combustion models. Different investigators proposed or used different models. However, some modeling results are lack of detailed experimental validation, or some statistical results are not in agreement with experimental results. A second-order moment (SOM) combustion model was proposed by the present author and his colleagues, and it was successfully used in LES of gas combustion and two-phase combustion, exhibiting its superiority over other models. In this paper a brief review of our systematic studies in LES of gas combustion, spray combustion, coal combustion and droplet combustion is presented.

CFD Analysis of the Acoustic Behavior of a Centrifugal Compressor for High Performance Engine Application

The paper reports an activity aiming at the characterization of the acoustic noise of a centrifugal compressor for a currently made high performance engine. All the analyses are carried out through the use of Detached Eddy Simulation. During high-load/low- engine speed operations of the engine, the compressor exhibits noise peaks above 150 dBA at relatively low frequencies, whose origin is relatively hard to rationalize. The use of three-dimensional CFD simulation appears to be very promising to gain a better understanding of the complex flow structures at the compressor inlet as well as to promote design optimizations aiming at limiting the acoustic emissivity of the component. In view of the dependency of the acoustic phenomena on the instantaneous pressure waves and flow structures, fully transient CFD simulations are highly recommended, together with the use of sophisticated numerical techniques such as Large Eddy and Detached Eddy simulation [1], [2], which are widely recognized to be able to better capture highly unstable features than the common RANS approach [3], [4]. In order to limit the computational cost of the analyses, preliminary steady-state RANS simulations are carried out to both initialize the flow field and to evaluate the grid capability to properly match the desired frequency spectrum.

Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 2. Flow structures

AbstractWe performed large-eddy simulations of the flow over a series of three-dimensional (3D) dunes at laboratory scale. The bedform three-dimensionality was imposed by shifting a standard two-dimensional (2D) dune shape in the streamwise direction according to a sine wave. The turbulence statistics were discussed in Part 1 of this article (Omidyeganeh & Piomelli,J. Fluid Mech., vol. 721, 2013, pp. 454–483). Coherent flow structures and their statistics are discussed concentrating on two cases with the same crestline amplitudes and wavelengths but different crestline alignments: in-phase and staggered. The present paper shows that the induced large-scale mean streamwise vortices are the primary factor that alters the features of the instantaneous flow structures. Wall turbulence is insensitive to the crestline alignment; alternating high- and low-speed streaks appear in the internal boundary layer developing on the stoss side, whereas over the node plane (the plane normal to the spanwise direction at the node of the crestline), they are inclined towards the lobe plane (the plane normal to the spanwise direction at the most downstream point of the crestline) due to the mean spanwise pressure gradient. Spanwise vortices (rollers) generated by Kelvin–Helmholtz instability in the separated shear layer appear regularly over the lobe with much larger length scale than those over the saddle (the plane normal to the spanwise direction at the most upstream point of the crestline). Rollers over the lobe may extend to the saddle plane and affect the reattachment features; their shedding is more frequent than in 2D geometries. Vortices shed from the separated shear layer in the lobe plane undergo a three-dimensional instability while being advected downstream, and rise toward the free surface. They develop into a horseshoe shape (similar to the 2D case) and affect the whole channel depth, whereas those generated near the saddle are advected downstream and toward the bed. When the tip of such a horseshoe reaches the free surface, the ejection of flow at the surface causes ‘boils’ (upwelling events on the surface). Strong boil events are observed on the surface of the lobe planes of 3D dunes more frequently than in the saddle planes. They also appear more frequently than in the corresponding 2D geometry. The crestline alignment of the dune alters the dynamics of the flow structures, in that they appear in the lobe plane and are advected towards the saddle plane of the next dune, where they are dissipated. Boil events occur at a higher frequency in the staggered alignment, but with less intensity than in the in-phase alignment.

Visualization of Mach 3.0/3.8 Flow around Blunt Cone with Supersonic Film Cooling

Fine instantaneous flow structures of different scales around a blunt cone with or without supersonic film cooling were visualized via nanotracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. The Mach number of the freestream is 3.0 and 3.8 respectively and the air injection is at Mach 2.5. Lots of typical flow structures were visible clearly, such as shock wave, expansion fan, shear layer, mixing layer, K-H vortices and turbulent boundary layer. With injection, the model wall surface can be covered by a thin film layer. While no injection, the flow is similar to the supersonic flow over a backward-facing step and the structures are simpler relatively and there is a longer laminar region. Flow structures with or without film cooling at Mach 3.0 and 3.8 were compared.