Averaged Flow Field Research Articles

Experimental apparatus and flow instrumentation for the investigation of a quasi-real slug flows in vertical ducts

This paper presents the description of the experimental apparatus, instrumentation, and processing techniques developed for the study of quasi-real slug flow using Particle Image Velocimetry, High-Speed Camera and Laser Diode Photocell techniques. Most experimental studies of the flow fields around Taylor bubbles, aiming a deeper insight into slug flow pattern, analyze the situation of isolated Taylor bubbles rising in stagnant or co-current liquid flow. Furthermore, most of these studies, consider highly viscous fluids (or, in general, low values of the inverse viscosity number, Nf=ρ2gD3/μ2), to get “well behaved” Taylor bubbles, where flow fields are more easily measured. However, in most industrial situations slug flows are characterized by the presence of small dispersed bubbles in the liquid stream flowing together with large Taylor bubbles, and high values of the inverse viscosity number. On the other hand, the chaotic nature of a real slug flow would not allow an adequate characterization of the flow around single Taylor bubbles, mainly, due to the lack of the repeatability of the flow field measurement, making difficult the calculation of the averaged fields. Thus, in this work, a specific experimental apparatus and instrumentation were developed, which allows the study of the flow around Taylor bubbles with and without dispersed small bubbles in the liquid stream under controlled conditions. This allows the PIV measurements, around Taylor bubbles in flows with high Nf numbers, be done under repeatable conditions and thus, the determination of the averaged flow fields. The laser diode photocell technique is used to synchronize the PIV system with the passage of Taylor bubbles at the test section as well as to measure the terminal velocity and length of Taylor bubbles. A dynamic masking procedure was developed to mask out the Taylor bubbles noses and tails in the PIV images, since, for high values of the inverse viscosity number (Nf≈13,275) the interface fluctuations are strong and, therefore, no fixed masking can be used.

Modal analysis of the laminar boundary layer instability and tonal noise of an airfoil at Reynolds number 150,000

A direct numerical simulation of the flow field around a controlled-diffusion airfoil within an anechoic wind-tunnel at [Formula: see text] incidence and a high Reynolds number of [Formula: see text] is performed for the first time using a lattice Boltzmann method. The simulation compares favorably with experimental measurements of wall-pressure, wake statistics, and far-field sound. The simulation noticeably captures experimentally observed high-amplitude acoustic tones that rise above a broadband hump. Both noise components are related to a breathing of a recirculation bubble formed around 65–70% of the chord, and to Kelvin-Helmholtz instabilities in the separated shear layer that yield rollers that break down into turbulent vortices whose diffraction at the trailing edge produces a strong dipole acoustic field. A wavelet analysis of the wall-pressure signals combined with some flow visualization has shown that the flow statistics are dominated by intense events caused by intermittent, large and intense bursting rollers. Several modal analyses of these events are performed on both the wall-pressure fluctuations and the span averaged flow field in order to analyse the boundary layer instability which triggers the typical sharp tones over a broadband hump in airfoil noise. A suction side Kelvin-Helmholtz instability is observed to be coupled with a pressure side vortex shedding induced by the sudden transition to turbulence and the blunt trailing edge.

Evolution of the flow field in decaying active regions

Context. Fully fledged sunspots are known to be surrounded by a radial outflow called the moat flow. Aims. We investigate the evolution of the horizontal flow field around sunspots during their decay, that is, from fully fledged to naked spots, after they loose the penumbra, to the remnant region after the spot has fully dissolved. Methods. We analysed the extension and horizontal velocity of the flow field around eight sunspots using SDO/HMI Doppler maps. By assuming a radially symmetrical flow field, the applied analysis method determines the radial dependence of the azimuthally averaged flow field. For comparison, we studied the flow in supergranules using the same technique. Results. All investigated, fully fledged sunspots are surrounded by a flow field whose horizontal velocity profile decreases continuously from 881 m s−1 at 1.1 Mm off the spot boundary, down to 199 m s−1 at a mean distance of 11.9 Mm to that boundary, in agreement with other studies. Once the penumbra is fully dissolved, however, the velocity profile of the flow changes: The horizontal velocity increases with increasing distance to the spot boundary until a maximum value of about 398 m s−1 is reached. Then, the horizontal velocity decreases for farther distances to the spot boundary. In supergranules, the horizontal velocity increases with increasing distance to their centre up to a mean maximum velocity of 355 m s−1. For larger distances, the horizontal velocity decreases. We thus find that the velocity profile of naked sunspots resembles that of supergranular flows. The evolution of the flow field around individual sunspots is influenced by the way the sunspot decays and by the interplay with the surrounding flow areas. Conclusions. Observations of the flow around eight decaying sunspots suggest that as long as penumbrae are present, sunspots with their moat cell are embedded in network cells. The disappearance of the penumbra (and consequently the moat flow) and the competing surrounding supergranular cells, both have a significant role in the evolution of the flow field: The moat cell transforms into a supergranule, which hosts the remaining naked spot.

Heat transfer mechanisms of inclined jets in cross flow with different holes

Film cooling is one of the key techniques to improve the thermal performance of hot sections in gas turbines. Heat transfer coefficient is an important parameter to quantify the thermal performance, which is often analyzed based on the time averaged flow field. Nevertheless, in the case of film cooling, the mixing between the coolant and the main flow is inherently unsteady. To understand the mechanism of heat transfer, the current study uses large eddy simulation (LES) method to investigate the heat transfer coefficient of inclined jets in crossflow. The angle between the hole and the main flow is 35°. Two different geometric holes, namely a cylindrical hole and a fan-shaped hole are investigated at a blowing ratio of 0.5, which is a typical value for film cooling application. Both the time averaged and the instantaneous flow characteristics are analyzed. The mechanism of heat transfer distribution is analyzed in detail. Near the surface, the streamwise fluctuated variables u′T′‾ has a large effect on the heat transfer coefficient. Compared with the cylindrical hole, the heat transfer downstream of the fan-shaped hole is highly unsteady and streamwise fluctuated variables u′T′‾ is larger, so the heat transfer coefficient is higher.

Detached eddy simulation of flow through a circular patch of free-surface-piercing cylinders

The study carried out a numerical investigation of the open channel flow through a circular array of cylinders piercing the water surface. Unlike a lot of the previous research, this study took into account the influence of the bed friction and free water surface. A Zonal Detached-Eddy Simulation (ZDES) model was developed to simulate the turbulent free-surface flow. The non-hydrostatic model was discretized using the Finite Volume Method (FVM). Within the patch, each individual cylinder was directly modelled by specifying the no-slip boundary condition at the solid surface. The ZDES model was verified to be able to capture the small scale turbulent coherent structures. The instantaneous and average flow fields were analysed in detail with the aid of streamline visualization and vortex identifying technique (Q-criterion). The investigation shed light on the complex interaction between the free surface flow and the cylinder array arranged into a circular patch. The complexity arises from the interplay among the separated flows from individual cylinders, interference between adjacent cylinders and the bulk porous-media effect of the patch. The study highlighted the dependence of the flow characteristics on the key properties of the patch, especially the Solid Volume Fraction (SVF). The work demonstrated the encouraging capability of the established numerical technique for the investigation of vegetated flows.

Large Eddy Simulation of Bluff-Body Flame Approaching Blow-Off: A Sensitivity Study

ABSTRACTAs almost all combustion processes of practical interest take place in the presence of turbulence, the development of the increasingly refined turbulence–chemistry interaction (TCI) models has led to highly sophisticated approaches. Nearly all of the studies comparing different models focus on stable premixed/non-premixed flame configurations. In this work, the focus is on well-documented, lean premixed bluff-body stabilized flames approaching blow-off and on the blow-off sequence itself. Large Eddy Simulations (LES) have been used to capture the time-dependent, three-dimensional flow-field using Transported Probability Density Function (TPDF), Partially Stirred Reactor Model (PaSR), and Implicit LES (ILES) models. Furthermore, the influence of finite-rate chemistry and different chemical mechanisms is evaluated to determine the limitation and capability of the different TCI approaches for modeling flames just prior to and during the transient blow-off process. While the average flow-fields do not reveal any significant differences between modeling approaches, detailed analysis of the flame reveals that there are differences in the predicted flame thickness and composition. The ability of the considered TCI models to predict local as well as full-flame extinction during the blow-off is investigated as well. It is demonstrated that such a blow-off sequence is not always governed by complex chemistry.

Rheology of colloidal suspensions in confined flow: Treatment of hydrodynamic interactions in particle-based simulations inspired by dynamical density functional theory.

We investigate the microstructure and rheology of a hard-sphere suspension in a Newtonian fluid confined in a cylindrical channel and undergoing pressure-driven flow using Monte Carlo simulations. We develop a hydrodynamic framework inspired by dynamical density functional theory approaches in which the contributions due to various flow-induced hydrodynamic interactions (HI) are included in the form of thermodynamic work done by these HI-derived forces in displacing the hard spheres. Using this framework, we can self-consistently determine the effect of the local microstructure on the average flow field, and vice versa, and coevolve the inhomogeneous density distribution and the flattening velocity profile with increase in the density of suspended particles. Specifically, we explore the effect on the local microstructure due to the inclusion of forces arising from confinement-induced inertial effects, forces due to solvent-mediated interparticle interactions, and the dependence of the diffusivity on the local density. We examine the dependence of the apparent viscosity of the suspension on the volume fraction of hard spheres in the cylinder, the flow rate, and the diameter of the cylinder and investigate their effects on the local microstructure.

Large-eddy simulation of supersonic jet noise emanating from an F404 nozzle at model scale

Large-eddy simulations of supersonic jet noise emanating from an F404 nozzle at the model scale have been carried out using the JENRE flow solver. A wall-model method that was previously validated for a high subsonic flow over a flat plate is used to model the boundary layer effect inside the nozzle. The nozzle geometry is a faceted bi-conic convergent-divergent nozzle with a design Mach number equal to 1.65 and the nozzle-exit diameter equal to 5.07 inches. Both mildly and highly overexpanded conditions are tested for heated jets. The time averaged flow field, turbulence intensities, and the far-field noise are compared with available experimental data. The effects of both the boundary-layer thickness and turbulence intensity at the nozzle exit are investigated to assess their impact on jet noise generation and the noise source characteristics.

Bayesian optical flow with uncertainty quantification

Optical flow refers to the visual motion observed between two consecutive images. Since the degree of freedom is typically much larger than the constraints imposed by the image observations, the straightforward formulation of optical flow as an inverse problem is ill-posed. Standard approaches to determine optical flow rely on formulating and solving an optimization problem that contains both a data fidelity term and a regularization term, the latter effectively resolves the otherwise ill-posedness of the inverse problem. In this work, we depart from the deterministic formalism, and instead treat optical flow as a statistical inverse problem. We discuss how a classical optical flow solution can be interpreted as a point estimate in this more general framework. The statistical approach, whose ‘solution’ is a distribution of flow fields, which we refer to as Bayesian optical flow, allows not only ‘point’ estimates (e.g. the computation of average flow field), but also statistical estimates (e.g. quantification of uncertainty) that are beyond any standard method for optical flow. As application, we benchmark Bayesian optical flow together with uncertainty quantification using several types of prescribed ground-truth flow fields and images.

Numerical Study of the Circumferential Fluctuations in Swept Compressor Cascades

This paper presents comparative numerical studies to analyze the inlet flow and circumferential fluctuation(CF) source terms in compressor cascades. Forward swept cascades with different sweep angles and straight cascade were calculated numerically by a computational fluid dynamics(CFD) package to get the required three-dimensional(3D) flow parameters. A dimension reduction method was introduced and an in-house code for data reduction was conducted to obtain a circumferentially averaged flow field. The results show that different sweep angle and nominal incidence angle will change the magnitude and distribution of the CF source terms at inlet thus altering the inlet flow field. The Mach number(Ma) could also change the inlet CF source terms under the same swept angle. The trends of radial migration and the blade loading would increase with the growing of the inlet Ma.

Counter rotating vortex pair structure in a reacting jet in crossflow

This paper analyzes the time averaged flow structure of a reacting jet in cross flow (RJICF), emphasizing the structure of the counter-rotating vortex pair (CVP) by using simultaneous tomographic particle image velocimetry (TPIV) and hydroxyl radical planar laser induced fluorescence (OH-PLIF). It was performed to determine the extent to which heat release, and the associated effects of gas expansion and baroclinic vorticity production, impact the structure of the CVP. These results show the clear presence of a CVP in the time averaged flow field, whose trajectory lies below the jet centerline on either side of the centerplane. Consistent with other measurements of high momentum flux ratio JICF in nonreacting flows, there is significant asymmetry in strength of the two vortex cores. The strength and structure of the CVP was quantified with vorticity and swirling strength (λci), showing that some regions of the flow with high shear are not necessarily accompanied by large scale bulk flow rotation and vice-versa. The OH PLIF measurement allows for correlation of the flame position with the dominant vortical structures, showing that the leeward flame branch lies slightly above, as well as, in the region between the CVP cores.

Evaluating the hydrodynamics of a round jet in a vegetated crossflow through large eddy simulation

Vegetation plays an important role on the turbulence structures of the effluent spreading in an open channel, which are insufficiently studied. This paper employs a large eddy simulation model to investigate the hydrodynamic processes of a round jet in a vegetated crossflow. The time-averaged simulated results are consistent with the experimental data. Analyses of mean flow characteristics including velocity distributions in different planes are showed that the array of rigid vegetation affects the averaged flow field and the jet structure, diminishing the velocity, with a significant increase of the jet penetration height. Moreover, this simulation successfully reproduces the coherent structures of classical and well-documented types based on the mean and instantaneous flow fields, including shear-layer vortices, wake vortices, counter-rotating vortex pair and windward vortex pair. The momentum transport mechanism is quantitatively elaborated by the quadrant analysis. Spectral analysis is adopted to obtain the dominant frequencies of vortex shedding and investigated the characteristic length of the large-scale vortex at different probes in the flow field.

Numerical study of the structure and dynamics of a tornado at the sub-critical vortex breakdown stage

A tornado at the sub-critical vortex breakdown was studied in detail using large eddy simulations. The averaged flow fields and the fluctuations were examined. Correlations of the fluctuating parameters were also studied, and a special turbulence structure was found. To clarify this turbulence structure, the Reynolds averaged force balances in two directions were studied, in which the sub-terms of the turbulence forces were paid more attention. The turbulence kinetic energy budget was examined to understand the balance between turbulence production, transportation, dissipation, and advection, among others. The time histories of wind velocities at some representative points were recorded, and three types of motions were then observed. The strength of these motions was furthered examined through spectrum analysis to study the change in them at different heights. Finally, the flow field visualization was performed to reveal the mechanism underlying these motions and explain some interesting phenomena found in the present study.

Propeller wake evolution mechanisms in oblique flow conditions

In the present study the wake flow past an isolated propeller operating in oblique flow conditions is investigated experimentally. In particular, the investigation concerns a systematic topological comparison of the wake behaviour in axisymmetric and in oblique inflow conditions, for three inclination angles, and is focused on an analysis of the underlying mechanisms of wake evolution and instability. To this end, the experiment has been designed to investigate the dynamics of propeller vortical structures over a wide spatial extent covering the wake region from the propeller disk up to 4.5 diameters in the streamwise direction. Detailed flow measurements have been undertaken by particle image velocimetry (PIV), using a multicamera configuration with three cameras arranged side by side. This allowed simultaneous acquisition of a large flow extent at a spatial resolution adequate to resolve the smallest vortical structures involved in the process of propeller wake instability. The analysis has been based on both phase-locked averaged and instantaneous flow fields. The study extends the knowledge on the subject of propeller wake dynamics, highlighting the major hydrodynamic effects that non-axisymmetric propeller operating conditions exert on the mechanisms of wake evolution, instability and breakdown, such as asymmetric destabilization of the tip vortices on the leeward and windward sides of the wake, and the interference between the tip and the junction vortices, as well as the cause–effect relation between the breakdown of the blade trailing wake and the instability of the tip and hub vortices.

Effect of Prandtl number on heat transport enhancement in Rayleigh-Bénard convection under geometrical confinement

We study, using direct numerical simulations, the effect of geometrical confinement on heat transport and flow structure in Rayleigh-B\'enard convection in fluids with different Prandtl numbers. Our simulations span over two decades of Prandtl number $Pr$, $0.1 \leq Pr \leq 40$, with the Rayleigh number $Ra$ fixed at $10^8$. The width-to-height aspect ratio $\Gamma$ spans between $0.025$ and $0.25$ while the length-to-height aspect ratio is fixed at one. We first find that for $Pr \geq 0.5$, geometrical confinement can lead to a significant enhancement in heat transport as characterized by the Nusselt number $Nu$. For those cases, $Nu$ is maximal at a certain $\Gamma = \Gamma_{opt}$. It is found that $\Gamma_{opt}$ exhibits a power-law relation with $Pr$ as $\Gamma_{opt}=0.11Pr^{-0.06}$, and the maximal relative enhancement generally increases with $Pr$ over the explored parameter range. As opposed to the situation of $Pr \geq 0.5$, confinement-induced enhancement in $Nu$ is not realized for smaller values of $Pr$, such as $0.1$ and $0.2$. The $Pr$ dependence of the heat transport enhancement can be understood in its relation to the coverage area of the thermal plumes over the thermal boundary layer (BL) where larger coverage is observed for larger $Pr$ due to a smaller thermal diffusivity. We further show that $\Gamma_{opt}$ is closely related to the crossing of thermal and momentum BLs, and find that $Nu$ declines sharply when the thickness ratio of the thermal and momentum BLs exceeds a certain value of about one. In addition, through examining the temporally averaged flow fields and 2D mode decomposition, it is found that for smaller $Pr$ the large-scale circulation is robust against the geometrical confinement of the convection cell.

A New Scheme for the Simulation of Microscale Flow and Dispersion in Urban Areas by Coupling Large-Eddy Simulation with Mesoscale Models

A coupling scheme is proposed for the simulation of microscale flow and dispersion in which both the mesoscale field and small-scale turbulence are specified at the boundary of a microscale model. The small-scale turbulence is obtained individually in the inner and outer layers by the transformation of pre-computed databases, and then combined in a weighted sum. Validation of the results of a flow over a cluster of model buildings shows that the inner- and outer-layer transition height should be located in the roughness sublayer. Both the new scheme and the previous scheme are applied in the simulation of the flow over the central business district of Oklahoma City (a point source during intensive observation period 3 of the Joint Urban 2003 experimental campaign), with results showing that the wind speed is well predicted in the canopy layer. Compared with the previous scheme, the new scheme improves the prediction of the wind direction and turbulent kinetic energy (TKE) in the canopy layer. The flow field influences the scalar plume in two ways, i.e. the averaged flow field determines the advective flux and the TKE field determines the turbulent flux. Thus, the mean, root-mean-square and maximum of the concentration agree better with the observations with the new scheme. These results indicate that the new scheme is an effective means of simulating the complex flow and dispersion in urban canopies.

A POD-Based Procedure for the Split of Unsteady Losses of an LPT Cascade

A time-resolved particle image velocimetry (TR-PIV) system has been employed to investigate the unsteady propagation of upstream wakes in a low-pressure turbine cascade. Data are obtained in the steady state condition and for two passing wake reduced frequencies. The study is focused on the identification and split of the different dynamics responsible for deterministic and random oscillations, thus loss generation by means of a new procedure based on proper orthogonal decomposition (POD). The paper takes advantage of the properties of POD that reduce the data set to a low number of modes that represent the most energetic dynamics of the system. It is clearly shown that the phase averaged flow field can be represented by a few number of POD modes related to the wake passing event for the unsteady cases. Proper orthogonal decomposition is also able to capture flow features affecting the instantaneous flow field not directly related to the wake passage (i.e., the vortex shedding phenomenon induced by the intermittent separation developing between adjacent wakes), which are smeared out in the phase averaged results. A procedure exploiting the biorthogonality condition of the POD modes, and the related temporal coefficients, has been developed for the quantification of the contribution due to the different POD modes to the overall turbulence kinetic energy production, or, equivalently, the mean flow energy dissipation rate. Results into the paper clearly show that losses due to wake migration, boundary layer and vortex shedding related phenomena can be distinguished and separately quantified for the different tested conditions.

Influence of turbulent fluctuations on non-equilibrium chemical reactions in the flow

In chemically nonequilibrium flows the problem of calculation of sources (formation rates) in equations for chemical species is of utter importance. Formation rate of each component is a non-linear function of mixture density, temperature and concentration of species. Thus the suggestion that the mean rate may be determined via mean values of the flow parameters could lead to significant errors. One of the most accurate approaches here is utilization of probability density function (PDF). In this paper the method for constructing such PDFs is developed. The developed model was verified by comparison with the experimental data. On the example of supersonic combustion it was shown that while the overall effect on the averaged flow field is often negligible, the point of ignition can be considerably shifted up the flow.

Measuring solar active region inflows with local correlation tracking of granulation

Context. Local helioseismology has detected spatially extended converging surface flows into solar active regions. These play an important role in flux-transport models of the solar dynamo. Aims. We aim to validate the existence of the inflows by deriving horizontal flow velocities around active regions with local correlation tracking of granulation. Methods. We generate a six-year long-time series of full-disk maps of the horizontal velocity at the solar surface by tracking granules in continuum intensity images provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Results. On average, active regions are surrounded by inflows extending up to 10 deg from the center of the active region of magnitudes of 20-30 m/s, reaching locally up to 40 m/s, which is in agreement with results from local helioseismology. By computing an ensemble average consisting of 243 individual active regions, we show that the inflows are not azimuthally symmetric but converge predominantly towards the trailing polarity of the active region with respect to the longitudinally and temporally averaged flow field.

Numerical simulation of a Rotating Detonation with a realistic injector designed for separate supply of gaseous hydrogen and oxygen

This paper presents numerical results for a Rotating Detonation (RD) propagating in a layer of combustible mixture, created by injection of gaseous hydrogen and oxygen. 3D Large Eddy Simulations (LES) of a reacting flow have been performed in a domain of planar geometry in order to eliminate possible effects of the chamber curvature. First, the results for a 2D case with uniformly distributed premixed injection are presented to characterize the RD propagation under the most idealized conditions. Then a 3D concept is introduced for the injector composed of a series of injection elements. The RD propagation is simulated under the conditions of premixed and separate injection of the propellants at globally stoichiometric proportions. The case of separate propellant injection is the most realistic one. The computational results, represented by instantaneous and averaged flowfields, are analyzed to characterize the flowfield and the conditions of RD propagation. This analysis allows identifying the effects due to two major factors: the injection through discrete holes with respect to the distributed one and the separate propellant feeding with respect to the premixed one. Macroscopic quantities, such as the RD propagation speed, mean chamber pressure, average parameters of the mixture, and mixing efficiency are evaluated and compared in order to characterize the studied effects.