Time-averaged Flow Properties Research Articles

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.

Stereo-PIV investigation of the unsteady flow in the draft tube of a model hydro turbine

Varying the generator load of a hydro turbine results in short-term changes in the rotation frequency of the runner, leading inevitably to flow instability and strong flow swirling behind the turbine. This may lead to the formation of unsteady flow regimes featured by vortex instability of the swirling flow behind the runner, known as the precessing vortex core (PVC) Dorfler et al. (2012). This effect causes dangerous periodic pressure pulsations that propagate throughout the water column in the draft tube. The present study reports on stereo PIV measurements of the air flow field inside a transparent draft tube of a model hydro turbine for a wide range of operation conditions. The research is focused on the time-averaged flow properties (mean velocity field and the second-order moments of velocity fluctuations), pressure pulsations and coherent flow structures in the velocity field.

Benchmark PIV database for the validation of CFD simulations in a transitional cavity flow

We present an experimental benchmark database for the transitional cavity flow. The database is obtained by planar Particle Image Velocimetry measurements at the median plane of the cavity model, for Reynolds numbers between 6300 and 19,000 based on the cavity height. A detailed uncertainty analysis of the experimental results is performed via the correlation statistics method for PIV uncertainty quantification and linear error propagation.The experimental results are compared to two-dimensional Reynolds-Averaged Navier Stokes (RANS) numerical simulations with different turbulence models. It is shown that, when the standard k-ω turbulence model is employed, the discrepancy between numerical simulations and experimental results exceeds the uncertainty of the latter. Conversely, RANS simulations with the SST k-ω turbulence model agree well with the experimental data in terms of time-averaged flow properties; however, the turbulent kinetic energy results present significant discrepancies at all considered Reynolds numbers. The data presented in this paper is made available for open-access download via the 4TU.ResearchData repository with DOI: https://doi.org/10.4121/14061233.

Effect of Rear Wing on Time-Averaged Ground Vehicle Wake With Variable Slant Angle

Abstract The slant angle plays a crucial role in the flow property of hatchback ground vehicles. An optimum slant angle is obligatory for better handling the ground vehicles when fitted with a rear wing. In this regard, the variation of time-averaged flow properties around a wing-attached hatchback ground vehicle (Ahmed body) due to a variable slant angle is accessed by this paper. The design includes a scaled Ahmed body as a reference ground vehicle and a rear wing with NACA 0018 profile. The computational studies are executed with Reynolds-averaged Navier–Stokes based k-epsilon turbulence model with nonequilibrium wall function. The vehicle's model is scaled to 75% of the actual model, and analyses are conducted with Reynolds number 2.7 × 106. After the study, it is observed that a 15 deg slant angle is the critical angle for the wing attached state in which the drag coefficient is maximum. After this angle, a sudden reduction of coefficients is observed, where 25 deg is critical for without wing condition. Besides this, the two counter-rotating horseshoe vortices in the separation bubble and side edge c-pillar vortices also behave differently due to the wing's presence. The turbulent kinetic energy variation and the variation in coefficients of surface pressure are also affected by the rear wing attachment. This paper will assist in finding the optimum slant angle for hatchback ground vehicles in the presence of a rear wing. Thus the study will help in increasing stability and control for hatchback ground vehicles.

Multiscale Parallelized Computational Fluid Dynamics Modeling Toward Resolving Manufacturable Roughness

Abstract Typical turbomachinery aerothermal problems of practical interest are characterized by flow structures of wide-ranging scales, which interact with each other. Such multiscale interactions can be observed between the flow structures produced by surface roughness and by the bulk flow patterns. Moreover, additive manufacturing (AM) may sooner or later open a new chapter in the way components are designed by granting designers the ability to control the shape and patterns of surface roughness. As a result, surface finish, which so far has been treated largely as a stochastic trait, can be shifted to a set of design parameters that consist of repetitive, discrete micro-elements on a wall surface (“manufacturable roughness”). Considering this prospective capability, the question would arise regarding how surface microstructures can be incorporated in computational analyses during designing in the future. Semi-empirical methods for predicting aerothermal characteristics and the impact of manufacturable roughness could be used to minimize computational cost. However, the lack of element-to-element resolution may lead to erroneous predictions, as the interactions among the roughness micro-elements have been shown to be significant for adequate performance predictions (Kapsis and He, 2018, “Analysis of Aerothermal Characteristics of Surface Micro-Structures,” ASME J. Fluids Eng., 140(5), p. 051104). In this paper, a new multiscale approach based on the novel block spectral method (BSM) is adopted. This method aims to provide efficient resolution of the detailed local flow variation in space and time of the large-scale microstructures. This resolution is provided without resorting to modeling every single ones in detail, as a conventional large-scale computational fluid dynamics (CFD) simulation would demand, but still demonstrating similar time-accurate and time-averaged flow properties. The main emphasis of this work is to develop a parallelized solver of the method to enable tackling large problems. The work also includes a first of the kind verification and demonstration of the method for wall surfaces with a large number of microstructured elements.

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.

Estimation of Flow Turbulence Metrics With a Lateral Line Probe and Regression

The time-averaged velocity of water flow is the most commonly measured metric for both laboratory and field applications. Its employment in scientific and engineering studies often leads to an oversimplification of the underlying flow physics. In reality, complex flows are ubiquitous, and commonly arise from fluid-body interactions with man-made structures, such as bridges as well as from natural flows along rocky river beds. Studying flows outside of laboratory conditions requires more detailed information in addition to time-averaged flow properties. The choice of in situ measuring device capable of delivering turbulence metrics is determined based on site accessibility, the required measuring period, and overall flow complexity. Current devices are suitable for measuring turbulence under controlled laboratory conditions, and thus there remains a technology gap for turbulence measurement in the field. In this paper, we show how a bioinspired fish-shaped probe outfitted with an artificial lateral line can be utilized to measure turbulence metrics under challenging conditions. The device and proposed signal processing methods are experimentally validated in a scale vertical slot fishway, which represents an extreme turbulent environment, such as those commonly encountered in the field. Optimal performance is achieved after 10 s of sampling using a standard deviation feature.

Statistical Modeling of Stratified Two-Phase Flow

A new numerical model for stratified two-phase flows with wavy interface is derived in this study. Assuming an equilibrium condition between turbulent kinetic energy, potential energy, and surface energy, the turbulent length scale in the inner region of a two-layer turbulence approach can be described by a statistical model to account for the influence of the waves. The average wave number, which is an input parameter to this model, is taken from wave spectra. They can be predicted from a Boltzmann statistic of turbulent kinetic energy. The new turbulence model is compared with the two-phase k–ϵ turbulence model. Time-averaged flow properties calculated by the new approach, such as velocity, turbulence, and void profiles, are shown to be in good agreement with experimental data.

PIV measurements and Eulerian–Lagrangian simulations of the unsteady gas–liquid flow in a needle sparger rectangular bubble column

In this work, we conducted a detailed experimental as well as a computational study for the hydrodynamic characterization of the flow in a needle sparger rectangular bubble column. Particle image velocimetry technique (PIV) was used to simultaneously capture the images of both bubbles and seeding polyamide tracers of 50μm in diameter, the tracer particles were selected to provide sufficient contrast and were neutrally buoyant. A three-dimensional computational model based on an Euler–Lagrange (E–L) approach was performed to simulate the dynamic characteristics of the oscillating bubble plume. The continuous phase velocity field was obtained by solving the unsteady Reynolds averaged Navier–Stokes equations along with the k–ε turbulence model. Bubble tracking was achieved by solving the Newton equations of motion taking into account drag force, pressure, buoyancy and gravity. Two-way coupling between the liquid and gas phases are accounted for in the continuous phase momentum equation. The effect of gas flow rate on the dynamic and time-averaged flow properties were studied, the flow was found to have an unsteady structure with three vortices moving in the upward direction and the bubble plume between them oscillated. The simulated results were found to be in accordance with the PIV measurements for a gas flow rate up to 0.1l/min. The simulated turbulent kinetic energy indicates that the stronger bubble plume velocity oscillations are located near the entrance zone and are caused by the addition of a shear induced turbulence produced by the presence of an oscillating bubble plume.

Time resolved flow-field measurements of a turbulent mixing layer over a rectangular cavity

High Reynolds number, low Mach number, turbulent shear flow past a rectangular, shallow cavity has been experimentally investigated with the use of dual-camera cinematographic particle image velocimetry (CPIV). The CPIV had a 3 kHz sampling rate, which was sufficient to monitor the time evolution of large-scale vortices as they formed, evolved downstream and impinged on the downstream cavity wall. The time-averaged flow properties (velocity and vorticity fields, streamwise velocity profiles and momentum and vorticity thickness) were in agreement with previous cavity flow studies under similar operating conditions. The time-resolved results show that the separated shear layer quickly rolled-up and formed eddies immediately downstream of the separation point. The vortices convect downstream at approximately half the free-stream speed. Vorticity strength intermittency as the structures approach the downstream edge suggests an increase in the three-dimensionality of the flow. Time-resolved correlations reveal that the in-plane coherence of the vortices decays within 2–3 structure diameters, and quasi-periodic flow features are present with a vortex passage frequency of ~1 kHz. The power spectra of the vertical velocity fluctuations within the shear layer revealed a peak at a non-dimensional frequency corresponding to that predicted using linear, inviscid instability theory.

Analysis of unsteady gas–liquid flows in a rectangular tank: Comparison of Euler–Eulerian and Euler–Lagrangian simulations

We study the dynamics of gas–liquid flows experimentally and computationally in a rectangular bubble column where the gas source is introduced at the corner. The flow in this reactor is complex and inherently unsteady in nature. The two-dimensional liquid phase velocity field is calculated by an Eulerian approach solving the unsteady Reynolds Averaged Navier Stokes equations. The conservation equations are closed using a two parameter turbulence model. The two-way coupling was accounted for by adding source terms in the conservation equations of the continuous phase to take into account the interaction with the dispersed phase. Bubble tracking is achieved through a Lagrangian approach. Here the equations of motion are solved taking into account the drag, pressure, buoyancy and gravity forces. The time-averaged flows along with the variables which characterize turbulence are analyzed for a wide range of gas flow-rates using Euler–Lagrangian simulations. These simulation predictions are validated with Euler–Eulerian simulations where the gas-phase distribution is captured as a void fraction and PIV experiments. The motion of bubbles induces turbulence in the flow. The applicability of two parameter models for turbulence like the standard k– ε model on time-averaged flow properties is addressed. From the results of the time averaged velocity field, turbulence intensity, turbulent viscosity and gas hold-up profiles, it is concluded that the Euler–Lagrangian model is applicable at lower gas flow-rates. The Euler–Eulerian approach was found to be valid at lower as well as higher gas flow-rates.

Velocity Measurements of a Free-Surface Turbulent Flow Penetrating a Porous Medium Composed of Uniform-Size Spheres

We present a laboratory experimental investigation of the interaction between the turbulent flow in an open channel and the turbulent flow within its very permeable bed. The bed was composed of uniform-size spheres packed in a cubic pattern. Fluid velocities were measured by Particle Image Velocimetry (PIV), which allowed us to investigate the spatial distribution of the time-averaged flow properties in the zone where they are strongly affected by the geometry of the rough bed. We investigate the effect of bed porosity on these flow properties by comparing the results of two experimental configurations: one with an impermeable bed composed of a single layer of spheres and another with a permeable bed composed of five layers. For the latter case, PIV measurements of velocities were also carried out inside two pores adjacent to the bed surface. This data provides an insight into the mechanisms of momentum transfer between the turbulent open channel flow and the turbulent flow within its very permeable bed.

Flow structure over square bars at intermediate submergence: Large Eddy Simulation study of bar spacing effect

Turbulent open-channel flow over 2D roughness elements is investigated numerically by Large Eddy Simulation (LES). The flow over square bars for two roughness regimes (k-type roughness and transitional roughness between d-type and k-type) at a relative submergence of H/k = 6.5 is considered, where H is the maximum water depth and k is the roughness height. The selected roughness configurations are based on laboratory experiments, which are used for validating numerical simulations. Results from the LES, in turn, complement the experiments in order to investigate the time-averaged flow properties at much higher spatial resolution. The concept of the double-averaging (DA) of the governing equations is utilized to quantify roughness effects at a range of flow properties. Double-averaged velocity profiles are analysed and the applicability of the logarithmic law for rough-wall flows of intermediate submergence is evaluated. Momentum flux components are quantified and roughness effect on their vertical distribution is assessed using an integral form of the DA-equations. The relative contributions of pressure drag and viscous friction to the overall bed shear stress are also reported.

Hydrodynamics of Bubble Column Reactors at High Gas Velocity: Experiments and Computational Fluid Dynamics (CFD) Simulations

This paper focuses on the modeling of flow and mixing in a bubble column reactor operated at high gas velocities (up to 0.40 m/s). A dual-tip conductivity probe was used to measure local void properties such as local time-averaged gas holdup, chord length distribution, bubble velocity distribution, and interfacial area. Chord length distribution was converted to bubble size distribution, using the backward transformation method. Liquid-phase mixing time measurements were conducted using a conductivity probe. A computational fluid dynamics (CFD) model was developed to simulate the unsteady gas−liquid flow in a bubble column using commercial code FLUENT 6.2. The time-averaged flow properties predicted by CFD simulations were compared with the experimental data. The role of unsteady flow structures in mixing was studied. The “multiple snapshots” approach was used to simulate the mixing time using CFD. The mixing times that were predicted for all superficial gas velocities compared favorably to the measured values. This study of the hydrodynamic behavior of a bubble column at higher gas velocity provides a basis for understanding and simulating solid suspension (or solid mixing) in slurry bubble column reactors.

Large-scale turbulent structure of uniform shallow free-surface flows

The hydrodynamics of super- and sub-critical shallow uniform free-surface flows are assessed using laboratory experiments aimed at identifying and quantifying flow structure at scales larger than the flow depth. In particular, we provide information on probability distributions of horizontal velocity components, their correlation functions, velocity spectra, and structure functions for the near-water-surface flow region. The data suggest that for the high Froude number flows the structure of the near-surface layer resembles that of two-dimensional turbulence with an inverse energy cascade. In contrast, although large-scale velocity fluctuations were also present in low Froude number flow its behaviour was different, with a direct energy cascade. Based on our results and some published data we suggest a physical explanation for the observed behaviours. The experiments support Jirka’s [Jirka GH (2001) J Hydraul Res 39(6):567–573] hypothesis that secondary instabilities of the base flow may generate large-scale two-dimensional eddies, even in the absence of transverse gradients in the time-averaged flow properties.

Eulerian–Lagrangian simulations of unsteady gas–liquid flows in bubble columns

We studied the dynamics of gas–liquid flows in a rectangular bubble column using Eulerian–Lagrangian simulations. Three-dimensional, unsteady simulations were performed to simulate the dynamic characteristics of the oscillating bubble plume. The effect of superficial gas velocity and aerated liquid height-to-column width ( H/ W) ratio on the dynamic and time-averaged flow properties was studied and the simulated results were validated using wall pressure and voidage fluctuation measurements. The effect of lift force and numerical diffusion on the dynamic and time-averaged properties is discussed in detail. Further, the results obtained using the Eulerian–Lagrangian simulations were compared with the Eulerian–Eulerian simulations. The bubble scale information, which is otherwise lost in the Eulerian–Eulerian simulations, was validated using the voidage fluctuation measurements. Such experimentally validated Eulerian–Lagrangian models will be useful for the simulation of mass transfer and reactions in bubble columns.

Investigation of Stator-Rotor Interaction in a Transonic Turbine Stage Using Laser Doppler Velocimetry and Pneumatic Probes

The current paper presents steady and unsteady flow data of a transonic test turbine stage operating under flow conditions similar to modern highly loaded gas turbines. Measurements were performed between stator and rotor as well as downstream of the rotor in planes perpendicular to the rotor axis. Time-resolved axial and tangential velocities were measured by a two-component laser doppler velocimeter (LDV) to investigate unsteady phenomena, while time-averaged flow properties were measured by means of a pneumatic seven-hole probe for all three spatial directions. The time-resolved investigation done by LDV allows to present velocity fields, flow angles and turbulence data at different stator-rotor positions during one blade passing period. Averaging these results enabled comparison with the pneumatic multihole probe measurement. LDV data and stage geometry can be obtained per email request and used for computational fluid dynamics (CFD) code verification.

OSCILLATIONS OF GAS FLOW IN A TUBE WITH PERMEABLE WALLS

Characteristics of linear waves propagating in a gas flow in a long tube with permeable walls are investigated. Linear approximation of governing equations is used, and it is considered that time-averaged flow properties depend on the space co-ordinate along the tube. They are obtained from the steady flow solution. Two examples showing the comparison of theoretical and experimental results are presented.

Large eddy simulations of the turbulent flow in a stirred reactor

Large eddy simulations of a baffled reactor stirred with a single Rushton turbine were performed. The turbine was positioned at mid-height in the reactor. The unsteady spatially filtered incompressible Navier–Stokes equations have been solved numerically. These large eddy simulations (LES) provide details of the flow field that cannot be obtained with the so-called Reynolds averaged equations and corresponding models. The un-resolved scales have been modelled implicitly through the truncation errors of the discretisation. The motion of the blades was described as time-dependent momentum sources. The time-averaged flow properties, mean flow and turbulence, have been studied. Phase averaging was used to characterize the flow relative to the impeller blades. Power spectral density and correlations, both in time and space were investigated. The results have been compared with the experimental data from several sources and also with the data from other simulations. Comparing our data with measurements we see an overall good agreement both qualitatively and quantitatively. In the impeller stream we observe an acceleration of the flow in the radial direction due to the trailing vortex pair. This acceleration can also be deduced from published experimental and numerical data. There are however some discrepancies concerning velocity fluctuations in the impeller area. These discrepancies are probably caused by inaccuracy in the impeller description.

The effect of an intermittant flapping motion on the properties of merging plumes

Experiments with merging buoyant jets revealed the existence of an intermittent flapping motion in the flow trajectory. The flapping commences beyond the point where the initially axi-symmetric buoyant jets merge to form a plane plume. The size and relative positions of large-scale vortices on either boundary of the plane plume determine whether the flapping flow state occurs. The effect of the flapping on the short term timeaveraged flow properties was measured and the flapping was found to increase the spreading rate and also change the shape of the concentration fluctuation distribution in the plane plumes. The intermittent presence of flapping in plane plumes is shown to be the cause of widely accepted differences between plane and axisymmetric plumes. The new findings on the flapping behaviour were used to improve an integral model which describes merging buoyant jets.