Time-averaged Velocity Distributions Research Articles

Effect of orifice thickness-to-diameter ratio on turbulent orifice flow: An experimental and numerical investigation

This paper reports a study on the effect of orifice thickness-to-diameter ratio (τ/d) on the turbulent flow through an orifice plate. Details of the flow characteristics downstream of the orifice are investigated both experimentally and numerically for a Reynolds number of 25,000. For this purpose, seven orifice plates are manufactured with τ/d ranging between 0.27 and 1.37. Particle Image Velocimetry (PIV) is used to experimentally study the flow patterns downstream of the orifices, with particular focus on instantaneous and time-averaged velocity distributions, streamline patterns, turbulent kinetic energy, Reynolds shear stress, and out-of-plane vorticity. Results show that, with increasing τ/d, the location of the vena contracta moves closer to the orifice plate, and eventually inside for τ/d>0.55. Increasing τ/d also results in decreasing turbulent kinetic energy and Reynolds shear stress. For the selected τ/d range, experimental and numerical results are in good agreement with the thick and thin orifice classification available in the literature.

Flow field characteristics of a 127-pin rod bundle with hexagonal spacer grids

A determination of nominal flow phenomena in liquid metal fast reactor (LMFR) fuel assemblies is critical toward generation-IV reactor development. Axially positioned spacer grids are used to maintain the geometry of hexagonal rod bundles and simultaneously introduce perturbations in the flow. Three-dimensional (3D) printed asymmetric honeycomb spacer grids were installed in a prototypical 127-pin LMFR fuel assembly model to study complex fluid dynamics interactions induced by the spacer grid and rods. To characterize flow dynamics in this intricate geometry, time-resolved particle image velocimetry (TR-PIV) using the matched-index-of-refraction method was employed to obtain non-intrusive velocity measurements for three axial planes (one near-wall and two interior planes) at a Reynolds number of 6000. The statistical TR-PIV results compared sub-channel-dependent normalized time-averaged velocity, velocity fluctuations, Reynolds stress, vorticity, and turbulence kinetic energy distributions. TR-PIV line profiles characterized downstream spacer grid flow dynamics. Two-point spatial and spatial–temporal cross-correlation fields revealed local coherent structures and quantified convection velocities of traveling vortices. Spatial–temporal decomposition using dynamic mode decomposition (DMD) applied to the near-wall vorticity fields extracted turbulent structures and flow instabilities in the wake region of the spacer grid, along with their decay and frequency rates. Reduced-order velocity fields from DMD reconstructions identified the most energy-containing coherent structures persistent in the near-wall region. This research provides experimental data sets and analyses of flow behavior in rod bundles with hexagonal spacer grids. The results are critical toward LMFR design and geometry optimization, crucial for the validation of computational fluid dynamics and reduced-order flow models.

Gas Temperature Distribution and Fluctuation in a Lab-Scale Fire Whirl

Fire whirls cause an increase in fire damage. This study clarified the unsteady behavior of fire whirls, considering that instantaneous changes in the temperature and flame shape of fire whirls can affect the damage to the surrounding area. Numerical simulations of a lab-scale flame that simulates a fire whirl were performed to investigate the changes in gas temperature and velocity fields under various fuel inflow velocities. The flow field was obtained by solving a continuity equation and a three-dimensional Navier-Stokes equation, and the turbulence was resolved using a large eddy simulation. A chemical equilibrium partially premixed combustion model was used, and radiation effects were considered. The time-averaged gas temperature distribution along the burner central axis revealed that the gas temperature decreased monotonically from upstream to downstream. The time-averaged velocity distribution along the burner central axis showed that the velocity decreased as one moved downstream, but the decrease was uneven. The time variation of the gas temperature demonstrated that the higher the fuel inflow velocity, especially near the burner, the greater the gas temperature flutter. Furthermore, the larger the fuel inflow velocity, the larger the flame swell and wobble. The results showed that the fuel inflow velocity affected temperature fluctuation and flame undulating movement.

Experimental investigation on the unsteady flow fluctuation of a vertical pipe inlet/outlet of the pumped storage power station

Under the outflow condition (water from the inlet/outlet to the reservoir), the instantaneous velocity at the trash rack section of the inlet/outlet in a pumped storage power station varies greatly. Only the time-averaged velocity parameter cannot fully represent instant flow effects. Acoustic Doppler velocimetry is used to measure the velocity at the trash rack section. Under the outflow condition, the maximum instantaneous velocity is 3 times the time-averaged velocity. According to the instantaneous velocity, the trash rack section along the orifice height can be divided into a positive velocity zone, an alternating zone of positive and negative velocity, and an alternating zone of negative and positive velocity. The proportion of the zone height to the orifice height is 28 %, 52 %, and 20 %, respectively. According to the time-averaged velocity distribution, the trash rack section along the orifice height can be divided into a positive velocity zone and a negative velocity zone, with a proportion of 80 % and 20 %, respectively. Under the inflow condition (water from the reservoir to the inlet/outlet), the instantaneous velocity changes little, which is 0.9–1.1 times the time-averaged velocity. The research results will contribute to further optimizing the shape of the inlet/outlet.

Numerical Investigation of Flow Structure and Turbulence Characteristic around a Spur Dike Using Large-Eddy Simulation

Spur dikes provide significant control for flow regimes in river regulation engineering, which can help in the regeneration of stream habitats. However, the narrowing of the flow by spur dike changes the turbulence characteristics. To clarify the turbulence characteristics around the spur dike, the method of large eddy simulation (LES) was used to investigate the horizontal turbulence structure around spur dikes with different discharges in an open-channel flume. The simulations were an exact reproduction of large-scale laboratory experiments, which showed agreement with the experimental results. The distributions of time-averaged streamwise velocity, bed shear stress, and second-order turbulence statistics obtained from the LES were analyzed. An examination of the time series of velocity fluctuation as the probability density function, quadrant analysis, the power density spectra, flow instability, and the vortex separation created in the detached shear layer were estimated. The results accurately revealed the flow field under flow separation, the turbulence statistics inside the separated shear layer, and the vortex structure and emphasized the variation in the different water depths. The results demonstrated that the form of turbulence was not significantly affected by discharge. Moreover, vortex and energy transmission displayed the same periodicity, despite variances in the structural form of turbulence at different water depths. Overall, the results of the study provide an efficient basis for understanding the turbulence around spur dikes, which is crucial for their safe design.

Effect of wall slip on laminar flow past a circular cylinder

A numerical study of two-dimensional flow past a confined circular cylinder with slip wall is performed. A dimensionless number, Knudsen number ($Kn$) is used to describe the slip length of cylinder wall. The Reynolds number ($Re$) and Knudsen number ($Kn$) ranges considered are $Re = [1, 180]$ and $Kn = [0, \infty)$, respectively. Time-averaged flow separation angle ($\bar{\theta_s}$), dimensionless recirculation length ($\bar{L_s}$) and the tangential velocity ($\bar{u_{\tau}}$) distributed on the cylinder's wall, drag coefficient ($\bar{C_d}$) and drag reduction ($DR$) are investigated. The time-averaged tangential velocity distribution on the cylinder's wall fit well with the formula $\bar{u_{\tau}} = [\frac{\alpha}{1+{\beta}e^{-{\gamma}({\pi}-{\theta})}}+{\delta}]sin(\theta) $, where the coefficients ($\alpha$, $\beta$, $\gamma$, $\delta$) are related with $Re$ and $Kn$. Several scaling-laws are found, $log(\bar{u_{{\tau}max}})\sim{log(Re)}$ and $\bar{u_{{\tau}max}}\sim{Kn}$ for low $Kn$, ($\bar{u_{{\tau}max}}$ is the maximum tangential velocity on the cylinder's wall), $log(DR)\sim{log(Re)}$ ($Re\leq45$ and $Kn\leq0.1$), $log(DR)\sim{log(Kn)}$ ($Kn\leq0.05$). At low $Re$, $DR_v$ (the friction drag reduction) is the main source of $DR$. However, $DR_p$ (the differential pressure drag reduction) contributes the most to $DR$ at high $Re$ ($Re>\sim60$) and $Kn$ over a critical number. $DR_v$ is found almost independent to $Re$.

CFD Simulation of Bubbling Process in Nitric Acid Solution in a Bubble Column

Bubble column is a commonly used multiphase reactor. Owing to the complicated nature of its flow structure, computational fluid dynamics has frequently been used as a useful instrument for studying the fluid mechanics in a bubble column. However, most of the previously studied systems are air-water system, while air-nitric acid solution system is rarely reported. In the present study, the differences in bubble size distribution of pure water and 1 mol/L nitric acid in the bubble column were observed through experiments. The bubble size was found to be smaller than that in water when the liquid phase was nitric acid. According to the actual situation, the bubbling process of nitric acid solution was successfully simulated by adjusting the coalescence factor of the bubble coalescence model (deviation less than 10%). Meanwhile, it was observed by simulation that the axial time-averaged liquid velocity distribution in nitric acid solution was flatter than that in water under the same gas flow rate.

Measurements of weak and moderate oblique shock-vortex interactions in supersonic flow

Stereoscopic particle image velocimetry measurements have been performed to quantify the centerline flow field associated with oblique shock-vortex interactions. This novel data set provides a unique view into the time-averaged velocity and turbulence kinetic energy distribution of weak and moderate interactions. Traditionally visualized using qualitative techniques, these data demonstrate that even weak interactions will result in measurable alterations to the flow field.

Turbulent flow characteristics in an 84-pin rod bundle for typical and damaged spacer grids

Hexagonal rod bundles arranged in a tightly packed triangular lattice are extensively used for heat transfer and energy generation applications. Staggered spacer grids are used to maintain the structural integrity of gas-cooled fast reactor (GFR) fuel assemblies, while inducing localized turbulence in flow. Damage to these spacer grids results in a disruption of flow fields within these hexagonal fuel bundles. Experimental flow visualizations are critical to identify the differences in local flow properties that the structural damage may cause. This experimental research investigates the flow-field characteristics at a near-wall and center plane in a prototypical 84-pin GFR fuel assembly. Newly installed typical spacers and spacers subject to naturally occurring damage due to material degradation over prolonged experimentation were investigated. Velocity fields were acquired by utilizing the matched-index-of-refraction method to obtain time-resolved particle image velocimetry measurements for a Reynolds number of 12 000. Reynolds decomposition statistical results divulged differences in the time-averaged velocity, velocity fluctuations, flow anisotropy, and Reynolds stress distributions. Galilean decomposition demarcated the influence of spacer grid damage on the velocity fields. To extract turbulent structures and elucidate mechanisms of flow instabilities, proper orthogonal decomposition analysis was employed. Reduced order flow reconstructions enabled the application of vortex identification algorithms to determine the spatial and statistical characteristics of vortices generated. This research work provides unique experimental data on the spacer grid condition-dependent flow. The results offer a deeper understanding of fluid dynamics behavior to support GFR rod bundle design efforts and computational fluid dynamics model validation.

Aeroacoustic simulation of broadband sound generated from a cross-flow fan using immersed boundary-lattice Boltzmann method

Abstract In this study, we investigated the capability of the immersed boundary-lattice Boltzmann method (IB-LBM) in simulating the broadband sound generated from a cross-flow fan. The present method is based on the lattice Boltzmann equation with the D3Q27 particle velocity model, and the cascaded collision operator was used to stably simulate high-Reynolds-number flows. Furthermore, the direct-forcing approach was used to consider rotating blades in the regular lattice. To capture turbulence eddies in the rotor, we used a high-resolution grid that contained approximately 340 million nodes. The simulation successfully captured the fundamental flow features of the cross-flow fan such as the eccentric vortex, and the simulation results were consistent with the experimental results in terms of the time-averaged velocity distribution at the outlet of the fan. In addition, by resolving turbulent eddies in the impeller, the simulation successfully predicted the broadband sound spectrum in the frequency range of 2 kHz or less.

Numerical analysis of heat transfer improvement for pulsating flow in a periodic corrugated channel with discrete V-type winglets

In the present study, the effects on the hydraulic and thermal performance of pulsating flow in a periodic corrugated channel with discrete V-type winglets were numerically investigated. Three different channel flows were considered with non-winglet, solid winglet, and perforated winglet configurations. The winglets were placed in the center of the channel in a discrete V-shape downstream. Numerical analyses were carried out with the finite volume approach. The effects of pulsation amplitude (A), Strouhal number (St), and Reynolds number (Re) on the flow and heat transfer were examined. The upper and lower surfaces of the corrugated channel were protected at Tw = 350 K and thermal performance and pressure drop were acquired for a pulsating cycle across the channel. The numerical results were presented in terms of thermal performance (η), dimensionless friction factor (Γ), and thermal performance factor (TPF). To observe the effects of the pulsation parameters and Reynolds number, time-averaged velocity and temperature distributions in the channels were obtained. The findings showed that the flow and heat transfer characteristics were highly affected by the channel and winglet geometry. The pulsation parameters significantly contributed to heat transfer improvement, especially at high Reynolds numbers. Thermal performance increased considerably with increasing pulsation amplitude and frequency with a slight increase in friction factor. Also, it was observed that the perforated winglets reduced the friction factor compared to the solid winglets. The highest TPF values for non-winglet, solid winglet, and perforated winglet configurations were respectively found as 5.71, 4.68, and 5.17 at Re = 1000, A = 1.5, and St = 8.

Influence of pressure on the wake of a hydrophobic circular cylinder for a flow regime with Reynolds number of 2.2×105

Vortex flow structures in a turbulent wake behind a circular Teflon cylinder immersed in an incoming flow with a change in pressure for the Reynolds number Re = 2.2×105 have been experimentally studied using a two-dimensional image (2D-PIV) of particles in a closed-circuit water tunnel. The obtained results are presented in the form of time-averaged velocity fields, Reynolds stresses, and distributions of turbulent kinetic energy. The flow data showed that the size of the wake flow region, Reynolds stresses and turbulent kinetic energy change depending on the pressure in the flow. As a result of a 20% reduction in pressure, the size of the vortex zone in the wake increases by about 20%.

High-performance CFD for Respiratory Droplet Turbulent Dispersion in a Ventilated City Bus

This work focuses on the development of a simulation strategy able to quantify risks of airborne virus contagion in many scenarios found in enclosed domains by using high-fidelity fluid dynamics simulation to predict the trajectories and distribution of virus-loaded respiratory droplets over long times. Large-Eddy simulation is used to predict the turbulent flow fields in a city bus for different operating conditions of the Air Conditioning system. The time-averaged velocity distributions and associated turbulent kinetic energy are shown to be drastically dependent on the studied operating conditions. Lagrangian tracking of respiratory droplets is then used over long times on statically converged Eulerian flow fields to investigate their turbulent dispersion depending on the emitter position in the bus. Importance of air conditioning conditions on respiratory droplet trajectories and concentration in the configuration is illustrated indicating that air treatment devices play a crucial role in the mitigation solution of airborne virus propagation.

Experimental measurements of airflow features and velocity distribution exhaled from sneeze and speech using particle image velocimetry

Airflow exhaled from sneeze and speech is an important source of viruses and droplets in daily life and may cause imperceptible virus propagation. The velocities of sneeze and speech airflow exhaled from 10 healthy young participants repeatedly using high-frequency (2986 Hz) particle image velocimetry are measured. The parameters for describing the dynamic process of sneeze airflow, such as sneeze duration time (SDT), peak velocity time (PVT), maximum velocities, and sneeze spread angle, are analyzed. The sneeze airflow lasts 430 ms (SDT) and reaches the peak velocity in the first 20 ms (PVT). The maximum sneeze airflow velocity is approximately 15.9 m/s. The temporal variation of the sneeze velocity exhibits the gamma distribution. For speech airflow, the maximum instantaneous velocity and maximum time-averaged velocity are reported. The maximum instantaneous velocity is approximately 6.25 m/s, whereas the time-averaged value is only 0.208 m/s owing to the extremely small airflow velocity among syllables. The vertical/horizontal spread angles of the airflow are 15.1°/15.4° for sneeze and 52.9°/42.9° for speech. The difference in airflow features based on gender is generally slight for both sneeze and speech. Subsequently, an ensemble-average operation is conducted to obtain the general and representative velocity distributions. We report each component of the temporal and spatial velocity distributions of the sneeze airflow and the time-averaged velocity distribution of the speech airflow. These detailed distribution data can provide a comprehensive understanding of sneeze and speech airflow movement mechanisms as well as a detailed database for future sneeze and speech computational fluid dynamics simulations.

Numerical study of flow characteristics of the blade with different structures

In order to study the effect of blade structures on the induced excitation characteristics, two different structures are investigated, which are the blade with special trailing edge and the staggered blade. Firstly, based on the steady numerical simulation, the time-averaged velocity and pressure distribution are obtained at different Re numbers (5×103 and 1×104). Secondly, the unsteady numerical simulation method LES is applied to acquire the transient and complex flow field around the blade. It is found that both two blade structures significantly change the flow characteristics around the blade, especially in the wake flow region. The evident frequency is observed from the frequency spectrums. It is clear that the dominant frequency is originated from the vortex shedding, which is similar to the Karmen vortex street. Finally, the results will be useful for the design of blade with low noise purpose.

Experimental study of induced airflow characteristics during liquid metal pouring process through PIV, thermography, and color schlieren imaging

Contaminated airflows are frequently generated during various liquid-metal pouring processes in industrial plants. An in-depth understanding of the induced airflow characteristics is necessary to develop an energy-saving ventilation system. In this study, the airflow characteristics induced by liquid metal pouring were investigated experimentally. The results indicate that the time-averaged velocity distribution of induced airflow at different pouring heights Z/r’ essentially follow Gaussian curves and that the influence range of the dimensionless horizontal velocity of the induced airflow is 0–2.5 at a Z/r’ of 0 to −3 with the maximum pouring flow rate. The angle Ф between the mainstream movement direction and the horizontal plane is approximately 85° based on the streamline variations. In addition, the temperature attenuation of the induced airflow approximately follows an exponential decay, and the dimensionless horizontal influence range of the temperature is almost four times that of the velocity. Moreover, the air mass formation and contaminated airflow generation process are illustrated, and the temperature peak of the induced airflow at the pouring cup outlet is explained in terms of the air mass generation. By combining the air mass position S* and dimensionless air mass diameter D* variations, it is demonstrated that the polluted airflow generation function is similar to a trigonometric function during the pouring period. In summary, the obtained conclusions can assist in the design of high-efficiency ventilation systems and in reducing energy consumption in industrial plants.

Computational Characterization and Boundary Condition Models of a Jet Interaction Fluidic Oscillator

Fluidically oscillating jet actuators that generate sweeping jets when supplied with a pressurized fluid have been used to mitigate separation and reduce drag in a range of flow control applications. Their implementation in future flight vehicles would require fundamental understanding and accurate predictive techniques of the physics of their internal flow and jet formation. The present investigations focus on high-fidelity, time-accurate simulations to characterize the flow physics of the actuator in quiescent conditions. An important element of the present simulations is to demonstrate the ability of the computational fluid dynamics (CFD) solver to predict the jet characteristics and provide a basis for the development of improved boundary conditions (BC) without entirely resolving the geometrical features of the fluidic device. The CFD-predicted oscillation frequencies of the engendered jets were found to be in excellent agreement with experiments, even on two-dimensional meshes. The study revealed that three-dimensional simulations are required to capture some of the flow features of the sweeping jet such as the double peak in time-averaged velocity distributions downstream of the actuator’s orifice that were measured in experiments. Several approaches for modeling the actuator were implemented and assessed in quiescent conditions. The evaluation of a boundary condition at the device throat, based on the phase-averaged flow variables, provides the basis for devising surface-based boundary conditions. The influence and necessity of including turbulent characteristics as part of the boundary conditions have also been identified.

Hydraulic Characteristics of Emerged Rigid and Submerged Flexible Vegetations in the Riparian Zone

Flow resistance, velocity distribution, and turbulence intensity are significantly influenced by aquatic vegetations (AV) in riparian zones. Understanding the hydraulics of flow with planted floodplains is of great significance for determining the velocity distribution profile and supporting the fluvial processes management. However, the traditional flume experiment method is inefficient. Therefore, the multigroup simultaneous flume test method was carried out to describe the flow patterns affected by emerged rigid (reed and wooden stick) and submerged flexible vegetations (grass and chlorella). The Acoustic Doppler Velocimeter (ADV) was utilized to measure the velocity at one point for different experimental conditions. The results showed that hydraulic features were influenced by different types of vegetation. Furthermore, the relative depth (z/h) was a determining factor of those variations. In addition, the time-averaged velocity distributions of planted floodplains are not logarithmic. Instead, they represented “s-shape” profiles. In detail, for the vegetated floodplains, reed and wood followed an s-shape profile, but for grass and chlorella, they followed reverse s-shape profile. For all cases, turbulence is not isotropic and the change law of turbulence intensity is different in different sections. The flow resistance, turbulence intensities, and Reynold stresses influenced by different types of vegetation were also analyzed.

Experimental study on the three-dimensional wake structure of the DrivAer standard model

An automatic particle imaging velocimetry (Auto-PIV) measuring technique was developed based on the external triggering and automatic control technology. Measurements of the DrivAer model wake were performed using Auto-PIV in the spanwise and vertical direction. Three-dimensional time-averaged velocity distribution was reconstructed and the spatial distribution of the large coherent structures was described. The results indicate that regions with high velocity fluctuations mainly locate at the shear layer of the upper and lower edge of the recirculation bubble. Further proper orthogonal decomposition results show that the velocity fluctuations account for a very little proportion of kinetic energy compared with the time-averaged velocity, supporting that the DrivAer model has steady aerodynamic characteristics. The smooth outlines of the DrivAer notchback model can delay the flow separation on the top and the C-pillar of the model, which inhibits the formation of the back recirculation bubble at the rear window and reduces the size of the recirculation bubble behind the baggage compartment. These promote the pressure recovery at the rear of the DrivAer notchback model, and thus reduces the pressure drag. At the rear of the model, the airflow on both sides converges towards the center due to the pressure difference, producing the streamwise vorticity by shearing the baggage compartment, then separates at the rear of the baggage compartment and generates a pair of secondary vortices. Overall, the aerodynamic behavior of the DrivAer model is robust.

Velocity and turbulence affected by various vegetations in open channel

Water energy is a conventional source of source that is widely available in nature. It can be easily converted into high quality secondary energy-electric energy through hydroelectric power stations. Hydropower is not only a widely used conventional energy source, but also a renewable energy source. Moreover, hydroelectric power has no pollution to the environment. Therefore, water energy is an inexhaustible and high-quality energy source among many energy sources in the world. However, vegetation can change flow structure and turbulence characteristics, impacting the use of water energy. In previous researches, few studies have focused on the comparison of velocity and turbulence influenced by various vegetations. Therefore, laboratory experiments were carried out to investigate hydrodynamics affected by submerged rigid vegetation (reed and wooden stick) and merged flexible vegetation (grass and chlorella) under different conditions. The time-averaged velocity distributions of planted floodplain are not logarithmic. Instead, reed and wooden stick followed an “S-shape” profile, but for grass and chlorella, they presented reverse S-shape profile. For all cases, turbulence is not isotropic and the change law of turbulence intensity is different in different sections.