Instantaneous Flow Properties Research Articles

Investigation of Vortex Structure Modulation by Spume Droplets in the Marine Atmospheric Boundary Layer by Numerical Simulation

Direct numerical simulation (DNS) of a droplet-laden, turbulent Couette airflow over a waved water surface is performed modeling the marine atmospheric boundary (MABL) layer carrying idealized spume droplets. Both the instantaneous and mean flow properties, the characteristics of the vortex structures and the momentum exchange between air turbulence and waved water surface and droplet-mediated momentum transfer are investigated. A Eulerian–Lagrangian approach is employed in DNS where full, 3D Navier–Stokes equations for the carrier air are solved in a Eulerian frame, and the trajectories of individual droplets are simultaneously tracked in a Lagrangian frame. The impact of the droplets on the carrier air flow is modeled via a point force approximation. The droplets size is considered in the range of spume droplet sizes observed in MABL. Various water surface roughness and droplet injection scenarios are considered, and both instantaneous and phase-averaged flow fields, the Reynolds stresses and the eigenvalues of the local air velocity gradient tensor are evaluated in DNS. Numerical results show a strong dependence of the droplet-mediated airflow modification on-the-droplet injection mechanism. Droplets injected with the surrounding air velocity effectively mitigate the vortex structures by reducing their swirling strength and suppress the momentum flux from air turbulence to water surface by weakening both ejections and sweeping events, and thus accelerating the mean flow as compared to the droplet-free flow. On the other hand, droplets injected with the velocities of the Lagrangian particles of the water surface enhance both the swirling strength of the vortex structures and air-flow turbulent stresses and decelerate the mean wind. The results also show that these effects of droplet-mediated flow modification become less pronounced as the water surface wave steepness increases.

Wind tunnel measurement of three-dimensional turbulent flow structures around a building group: Impact of high-rise buildings on pedestrian wind environment

The wind velocity and surface pressure of the model of a group of urban blocks made of low-rise buildings with a high-rise building in the center were measured using a wind tunnel. Not only the time-averaged velocity field, but also the turbulent statistics, i.e., the turbulent kinetic energy, normal component of the Reynolds stresses, gust factor (GF), and probability density function (PDF), and power spectrum density (PSD) of instantaneous velocity components were investigated. Based on the results, the impact of high-rise buildings on the general flow structure and the surrounding pedestrian wind environment was discussed. The presence of high-rise buildings drastically changes the pedestrian wind environment around buildings in terms of time-averaged and instantaneous flow properties due to the complex interaction between the flows around the building and in and above the surrounding street canyons. High-rise buildings create a large GF region where the downwash flow and the street flow collide. A clear correlation between the GF and the time-averaged horizontal velocity magnitude was observed regardless of the height of the central building. The PDF of instantaneous velocity components around the high-rise building was characterized by a large skew not only near the building corners of the high-rise building but also on the surrounding streets. The high-rise building produces large PSDs of the velocity fluctuation components, particularly near the upwind corner at a high frequency and on the surrounding street at an intermediate frequency. Furthermore, the results of the time-averaged and instantaneous flow properties provide a useful database for computational fluid dynamics (CFD) validation and model development studies.

Experimental and numerical investigation of flow structure in a cylindrical corrugated channel

In this paper, details of the flow characteristics in a duct with circular corrugated channel mounted on walls were investigated both experimentally and numerically. In this research, in order to study the flow behaviors of corrugated ducts, the Particle Image Velocimetry (PIV) technique was used for obtaining extensive information. From this point of view, present experimental work, using the PIV method has been provided significant data about flow in corrugated channels. The aspect ratio of the investigated channel, a/R is selected as 1. The flow characteristics are studied as a function of four different Reynolds numbers, such as Re=2000, 3000, 4000 and 5000. Generally, the target of this comprehensive work is to investigate the flow characteristics and hydrodynamics in the circular corrugated channels in terms of instantaneous and time-averaged flow data such as velocity, streamline, turbulent kinetic energy, Reynolds shear stress and vorticity in the measurement plane. The findings have suggested that the corrugated channel have helped to improve both transporting energy and momentum transfer; hence, heat transfer rate was enhanced. The turbulence occurred in the corrugated channel with the aid of a sharp corner is the dominant factor for developing the entrainment between wake and core flows. Furthermore, in the presence of both negative pressure gradient and kinetic energy along the direction of flow, the vortices are readily mounded in cavity zones. Fluid flow can explain the phenomena of augmentations in heat transfer conveniently. At low Reynolds numbers, available cavities can cause increments in both heat transfer area and flow turbulence. On the contrary, at high Reynolds numbers, the secondary flow enables agitating hot and cold fluids in cavities besides enhanced turbulence in ribs region. In the separated shear layer zone, the entity of large coherent vortices and cyclical processes like discontinuous subtraction of the recirculation region into the core flow zone are demonstrated by instantaneous flow properties. The experimental measurements have been performed in the side view plane of a model which has the same geometry have been validated by computations. The comparative data have been showed that the distribution of velocity profile changed periodically and indicated good agreement between simulations and experimental results.

Stereoscopic TR-PIV measurements of mixed convection flow in a vertical channel with an open cavity with discrete heating

An experimental investigation of opposing mixed convection is carried out using stereoscopic time-resolved particle image velocimetry (TR-PIV) to assess the thermal effects on the flow structure over an open cubical cavity that lies at the midsection of a vertical water channel. The ratio between the channel and cavity depth is of 1, the wall facing the opening is isothermal and all other walls are adiabatic. The complex interaction between the induced incompressible forced flow at ambient temperature and buoyancy-induced flow from the heated wall of the cavity are studied experimentally for fixed Prandtl number of Pr=7, buoyancy strength or Richardson number in the range 0 ≤ Ri ≤ 20, and two Reynolds numbers defined with the channel hydraulic diameter and the mean velocity in the upstream channel of Re=1500 and 4500 that correspond to laminar and turbulent upstream boundary layers, respectively. The 3D flow features are presented in the form of mean statistics and instantaneous flow properties in planes normal to the cavity floor. It is found that the shear layer development is highly 3D due to the confinement effect induced by the lateral walls. We also show the modulation effect of opposing buoyancy on the flow distributions and the time-evolution of the shear layer. Also, the complex dynamics of the coherent structures in the region surrounding the cavity show that the entrainment rate is modified with increasing buoyancy. Root-mean-square contours of the fluctuating laminar velocity fields have been obtained, and Reynolds stress distribution and proper orthogonal decomposition analysis for the turbulent case show that an increase in the fractional energy of the characteristic POD modes takes place for increasing Ri numbers.

Supercritical fluid flow dynamics and mixing in gas-centered liquid-swirl coaxial injectors

Gas-centered, liquid-swirl coaxial injectors similar to those used in the main chambers of oxidizer-rich staged-combustion engines are investigated computationally, in terms of supercritical fluid flow dynamics and mixing. Gaseous oxygen (GOX) is axially directed through a center post at a temperature of 687.7 K. Kerosene is tangentially introduced into the outer coaxial swirler at a temperature of 492.2 K. The mean chamber pressure of 253.0 bars substantially exceeds the thermodynamic critical pressures of oxygen and kerosene. The end of the GOX post is recessed from the entrance of the taper region, which is connected downstream to an open domain. A wide range of recess lengths (and correspondingly, fuel shielding collar lengths) is considered to determine the dependence of flow characteristics on this geometric parameter. Special attention is given to the regions downstream of the GOX post end and in the taper section, where primary mixing occurs. Instantaneous and time-averaged flow properties, as well as mixing effectiveness, are examined. Results indicate that the recess length plays a critical role in determining the flow evolution and mixing behaviors. In a fully recessed injector without fuel shielding, the initial kerosene/GOX interaction resembles a swirling transverse jet into a crossflow, and flow recirculation occurs near the kerosene injection slit and the head end. In other injectors with fuel shielding, the kerosene flow is predominantly axial before it enters the mixing zone; the coflow kerosene and GOX streams expand radially and recirculate in the wake of the GOX post. Flow unsteadiness arising from the fluid injection and mixing and vorticity production in the boundary layer of the GOX stream, along the wall of the fuel passage, and at the kerosene/GOX interface cause the development of salient vortical structures in the downstream flowfield. The geometric changes at the entrance of the taper region and at the injector exit further alter the flow dynamics, inducing multiple toroidal recirculation zones and secondary vortex structures.

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.

PIV uncertainty propagation

This paper discusses the propagation of the instantaneous uncertainty of PIV measurements to statistical and instantaneous quantities of interest derived from the velocity field. The expression of the uncertainty of vorticity, velocity divergence, mean value and Reynolds stresses is derived. It is shown that the uncertainty of vorticity and velocity divergence requires the knowledge of the spatial correlation between the error of the x and y particle image displacement, which depends upon the measurement spatial resolution. The uncertainty of statistical quantities is often dominated by the random uncertainty due to the finite sample size and decreases with the square root of the effective number of independent samples. Monte Carlo simulations are conducted to assess the accuracy of the uncertainty propagation formulae. Furthermore, three experimental assessments are carried out. In the first experiment, a turntable is used to simulate a rigid rotation flow field. The estimated uncertainty of the vorticity is compared with the actual vorticity error root-mean-square, with differences between the two quantities within 5–10% for different interrogation window sizes and overlap factors. A turbulent jet flow is investigated in the second experimental assessment. The reference velocity, which is used to compute the reference value of the instantaneous flow properties of interest, is obtained with an auxiliary PIV system, which features a higher dynamic range than the measurement system. Finally, the uncertainty quantification of statistical quantities is assessed via PIV measurements in a cavity flow. The comparison between estimated uncertainty and actual error demonstrates the accuracy of the proposed uncertainty propagation methodology.

ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

Abstract. A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of polydisperse gas–particle mixtures forming volcanic plumes. Starting from the three-dimensional N-phase Eulerian transport equations for a mixture of gases and solid dispersed particles, we adopt an asymptotic expansion strategy to derive a compressible version of the first-order non-equilibrium model, valid for low-concentration regimes (particle volume fraction less than 10−3) and particle Stokes number (St – i.e., the ratio between relaxation time and flow characteristic time) not exceeding about 0.2. The new model, which is called ASHEE (ASH Equilibrium Eulerian), is significantly faster than the N-phase Eulerian model while retaining the capability to describe gas–particle non-equilibrium effects. Direct Numerical Simulation accurately reproduces the dynamics of isotropic, compressible turbulence in subsonic regimes. For gas–particle mixtures, it describes the main features of density fluctuations and the preferential concentration and clustering of particles by turbulence, thus verifying the model reliability and suitability for the numerical simulation of high-Reynolds number and high-temperature regimes in the presence of a dispersed phase. On the other hand, Large-Eddy Numerical Simulations of forced plumes are able to reproduce the averaged and instantaneous flow properties. In particular, the self-similar Gaussian radial profile and the development of large-scale coherent structures are reproduced, including the rate of turbulent mixing and entrainment of atmospheric air. Application to the Large-Eddy Simulation of the injection of the eruptive mixture in a stratified atmosphere describes some of the important features of turbulent volcanic plumes, including air entrainment, buoyancy reversal and maximum plume height. For very fine particles (St → 0, when non-equilibrium effects are negligible) the model reduces to the so-called dusty-gas model. However, coarse particles partially decouple from the gas phase within eddies (thus modifying the turbulent structure) and preferentially concentrate at the eddy periphery, eventually being lost from the plume margins due to the concurrent effect of gravity. By these mechanisms, gas–particle non-equilibrium processes are able to influence the large-scale behavior of volcanic plumes.

Grain-Scale Nonequilibrium Sediment-Transport Model for Unsteady Flow

AbstractA one-dimensional (1D) finite-volume model is developed for simulating nonequilibrium sediment transport in unsteady flow. The governing equations are the 1D mass and momentum conservation equations for sediment-laden flow and the sediment continuity equation for both bed load and suspended-load transport. The Rouse profile is modified to consider the nonequilibrium transport of suspended sediment. The spatial lag between the instantaneous flow properties (e.g., velocity, bed shear stress) and the rate of bed load transport in unsteady flow is quantified by using an adaptation length, which is derived theoretically by applying the momentum principle in the bed load layer. This new method for calculating the adaptation length is verified using data from several experiments and yields better results than other empirical formulas for a wide range of shear stress. The nonequilibrium model is applied to simulate a series of laboratory dam-break flows over erodible beds, and the simulated results agree ...

Instantaneous Plastic Flow Properties of Thin Brass Sheets Under Uniaxial and Biaxial Testing

In the prediction of formability of sheet metals, it is necessary to determine material properties which are normally evaluated on the data obtained by performing simple tests. The most important parameters affecting the value of limit strains of a sheet metal are the strain hardening exponent (n) and plastic anisotropy ratio (r). It was established that especially the value of n-parameter of brass sheets strongly depend on the specimen elongation and stress/strain state. The effect of instantaneous (differential) nt - value and rt - value on the forming limit curve was computed theoretically.

J0501-1-4 ランダムフーリエモードに基づく冷却孔モデルを用いたフィルム冷却の数値解析([J0501-1]流体機械の研究開発におけるEFD/CFD(1))

Film coolant on gas turbine's vanes and blades is subject to strong turbulent flows containing secondary flows, such as horseshoe vortex, passage vortex, etc. In general, internal flow passage should be included in the numerical domain to give accurate velocity fields at the film-hole exit, which are mandatory for reasonable prediction of the external coolant flows. But it is unrealistic to apply this technique to the LES of realistic cascade flows with multiple film cooling holes. This paper proposes a film-hole model for LES, which is based on random Fourier modes to imitate instantaneous and statistical turbulent flow properties at the film-hole exit without simultaneous analysis in the internal flow passage. The predicted time-mean film cooling effectiveness is compared with experimental data.

Turbulent Measurements in a Small Subtropical Estuary with Semidiurnal Tides

Since predictions of scalar dispersion in small estuaries can rarely be predicted accurately, new field measurements were conducted continuously at relatively high frequency for up to 50 h (per investigation) in a small subtropical estuary with semidiurnal tides. The bulk flow parameters varied in time with periods comparable to tidal cycles and other large-scale processes. The turbulence properties depended upon the instantaneous local flow properties. They were little affected by the flow history, but their structure and temporal variability were influenced by a variety of parameters including the tidal conditions and bathymetry. A striking feature of the data sets was the large fluctuations in all turbulence characteristics during the tidal cycle, and basic differences between neap and spring tide turbulence.

Flow characterization of fine powders using material characteristic parameters

Abstract Instantaneous flow properties of fine cohesive powders, i.e. limestone, glass-ballotini, alumina and talc, are evaluated using Jenike shear tester. The results are analysed in the light of elastic-plastic interparticle contact deformation theory incorporating fundamentals of fine powder adhesion and consolidation in flow behavior characterization. Thus, unconfined yield strengths (σ c) of the given samples at various consolidation levels are evaluated solely on the basis of material characteristic parameters, i.e. angle of internal friction (ø i), stationary angle of friction (ø st), isostatic tensile strength of an unconsolidated powder (σ 0) along with characteristic pre-consolidation influence (σ M,st) and subsequently predicted flow functions ( ff c) are compared with those evaluated experimentally. The correlation coefficient values for the comparison are found to be in the range of 0.96-0.99, indicating the satisfactory nature of the above parameters in explaining the flow behavior. Further, compressibility indices and elastic-plastic contact consolidation coefficient ( K) characterizing powder cohesiveness are calculated using experimental data.

Numerical analysis of dipole sound source around high speed trains.

As the maximum speed of high speed trains increases, the effect of aeroacoustic noise on the sound level on the ground becomes increasingly important. In this paper, the distribution of dipole sound sources at the bogie section of high speed trains is predicted numerically. The three-dimensional unsteady flow around a train is solved by the large eddy simulation technique. The time history of vortices shows that unstable shear layer separation at the leading edge of the bogie section sheds vortices periodically. These vortices travel downstream while growing to finally impinge upon the trailing edge of the section. The wavelength of sound produced by these vortices is large compared to the representative length of the bogie section, so that the source region can be regarded as acoustically compact. Thus a compact Green's function adapted to the shape can be used to determine the sound. By coupling the instantaneous flow properties with the compact Green's function, the distribution of dipole sources is obtained. The results reveal a strong dipole source at the trailing edge of the bogie section where the shape changes greatly and the variation of flow with time is also great. On the other hand, the bottom of the bogie section where the shape does not change, or the leading edge and boundary layer where the variation of flow with time is small, cannot generate a strong dipole source.

Numerical Analysis of Dipole Sound Source at the Bogie Section of High Speed Trains.

In this paper, the distribution of dipole sound sources at the bogie section of high speed trains are predicted numerically. The three-dimensional unsteady flow around a train is solved by the large eddy simulation technique. The time history of vortices show that the unstable shear layer separated at the leading edge of the bogie section sheds vortices periodically. These vortices travel leeward while growing to finally impinge upon the trailing edge of the section. A compact Green's function adapted to the shape is also given numerically by solving Laplace equation. By coupling the instantaneous flow properties with the compact Green's function, the distribution of dipole sources is obtained. The results show that a strong dipole source is generated at the trailing edge of the bogie section where the shape changes greatly and the variation of flow with time is also great. On the other hand, the bottom of the bogie section where the shape does not change, or leading edge and boundary layer where a variation of flow in time is small, can not generate a strong dipole source.

K-1244 高速鉄道車両下部空力音に関する数値計算(J06-6 流体関連の騒音と振動(6))(J06 流体関連の騒音と振動)

In this paper, the distribution of dipole sound sources at the bogie section of high speed trains are predicted numerically. The three-dimensional unsteady flow around a train is solved by the large eddy simulation technique. The time history of vortices show that the unstable shear layer separated at the leading edge of the bogie section releases vortices periodically. A compact Green's function adapted to the shape is also given numerically by solving Laplace equation. By coupling the instantaneous flow properties with the compact Green's function, the distribution of dipole sources is obtained. The results show that a strong dipole source is generated at the trailing edge of the bogie section where the shape changes greatly and the variation of flow over time is also great.

LABORATORY EXPERIMENTS ON TURBULENT TRANSITION IN THE BOTTOM BOUNDARY LAYER UNDER IRREGULAR WAVES

Characteristic behaviors of turbulent transition in the bottom boundary layer under irregular waves have been studied from laboratory data. It has been observed that flow transition under irregular waves is not only governed by instantaneous flow properties but also depends on the conditions in the preceding waves. While laminar-to-turbulent transition required a build-up of turbulent energy sufficient enough to produce traces of turbulence, for turbulent-to-laminar transition the turbulent wave was required to be followed by several successive smaller waves to allow total dissipation of turbulent energy. As a result waves with much higher Reynolds numbers show predominantly laminar properties, whereas, wave with Reynolds number as small as 5.0×103 shows turbulent behavior. Use of conventional wave friction factor diagram, therefore, becomes unsuited to indicate flow transition under irregular waves. Also transition takes place over a much wider range of wave Reynolds number than observed for regular waves.

Transient analysis of Rayleigh–Bénard convection with a RANS model

Abstract Rayleigh–Benard (RB) convection at high Rayleigh numbers was studied by transient Reynolds-averaged-Navier–Stokes (TRANS) approach. The aim of the study was to assess the RANS method in reproducing the coherent structure and large-scale unsteadiness in buoyancy-driven turbulent flows. The method can be regarded as a very large eddy simulation (VLES) combining the rationale of the LES and of RANS modelling. Following the experimental and DNS evidence that the RB convection is characterised by a coherent cellular motion with scales which are much larger than the scales of the rest of turbulent fluctuations, the instantaneous flow properties are decomposed into time-mean, periodic and random (triple decomposition). A conventional single-point closure (here an algebraic low-Re-number k−e− θ 2 stress/flux model), used for the unresolved motion, was found to reproduce well the near-wall turbulent heat flux and wall heat transfer. The large scale motion, believed to be the major mode of heat and momentum transfer in the bulk central region, is fully resolved by time solutions. In contrast to LES, the contribution of both modes to the turbulent fluctuations are of the same order of magnitude. In the horizontal wall boundary layers the model accounts almost fully for the turbulence statistics, with a marginal contribution of resolved scales. The approach was assessed by comparison with the available direct numerical simulations (DNS) and experimental data using several criteria: visual observation of the large structure morphology, different structure identification techniques, and long-term averaged mean flow and turbulence properties. A visible similarity with large structures in DNS was observed. The mean flow variables, second-moments and wall heat transfer show good agreement with most DNS and experimental results for different flow cases considered.

Kinematics of the quasi-coherent vortical structure in near-wall turbulence

Near-wall turbulence structures in a direct numerical simulation database of turbulent channel flow were investigated by using a three-dimensional (3-D) computer graphics software tool. Several candidate methods to detect near-wall vortical structures were tested. They were based on instantaneous flow properties such as pressure, vorticity, enstrophy, dissipation rate, and the second invariant of deformation tensor. Among them, the low-pressure regions and those of negative second invariant of the deformation tensor corresponded well to the cores of vortical fluid motion. The spatial distribution of each term in the Reynolds stress transport equations was also examined in the instantaneous field to explore the role of vortical structures in production and destruction of the Reynolds stresses. It is found that these terms are distributed highly intermittently in space; intense production occurs mostly on both sides of near-wall streamwise vortices, and high redistribution, diffusion, and destruction regions also exist near around the vortices. High helicity regions are found to be associated with the elongated near-wall streaky structures.

Navier-Stokes Computations of Cavity Aeroacoustics with Suppression Devices

Effectiveness of two devices to suppress the cavity acoustics was computationally investigated. Two dimensional, computational simulations were performed for the transonic, turbulent flows past a cavity, which was first equipped with a rear face ramp and then with a spoiler. The Reynolds-averaged, unsteady, compressible, full Navier-Stokes equations were solved time accurately by a second-order accurate, implicit, upwind, finite-volume method. The effect of turbulence was included through the Baldwin-Lomax model with modifications for the multiple-wall effects and for the highly vortical flow with a shear layer. The results included instantaneous and time-averaged flow properties, and time-series analyses of the pressure inside the cavity, which compared favorably with the available experimental data. These results were also contrasted with the computed aeroacoustics of the same cavity (length-to-depth ratio of 4.5), but without a device, to demonstrate the suppression effectiveness.