Time-averaged Distribution Research Articles

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%.

Study of the wave period effect on the time-averaged suspended sediment concentration distribution under the oscillatory sheet flow condition

ABSTRACT Existing sheet flow experimental data show the measured time-averaged suspended sediment concentration (TSSC) may increase, decrease, or even remain constant if the wave period decreases. With respect to the available sheet flow experimental data, it is confirmed that the TSSC of relatively fine sand undergoes three stages, i.e. first decreases, then increases, and finally decreases again in the case that the wave period decreases from 12 s to 2 s, whereas for the relatively coarse sand, its vertical profile remains almost unchanged. The criterion for identifying the relatively fine or coarse sand was proposed in terms of the nondimensional ratio between the settling velocity and the root-mean-square flow velocity. If this ratio is less than 0.04–0.043, it is deemed as relatively fine sand. Otherwise, it is classified as relatively coarse sand. Applying the classical gradient diffusion model, sediment diffusivity is confirmed to be insensitive to the wave period. Keeping other experimental conditions uniform, TSSCs with different wave periods are proportional to each other and only affected by the reference concentration. Subsequently, a simple quantitative expression considering the wave period effect on the TSSC under the oscillatory sheet flow regime was proposed and verified with the experimental data.

Influence of the femoral artery-graft junction patient-specific geometry on blood flow structure and wall shear stress

The paper presents a comparative numerical study of pulsatory blood flow in five patient-specific models of femoral-popliteal artery anastomosis. Three-dimensional geometric models of a proximal junction of the common femoral artery/graft were constructed on the bases of CT angiography. The influence of junction geometry on the blood flow and wall shear stress is analyzed. The ratio of the measured CFA and graft diameters and the junction angle are considered as the major geometrical parameters. Numerically calculated velocity fields are analyzed, and stagnant zones in the anastomoses flow are identified. Time-averaged distributions of wall shear stress and oscillatory shear index obtained for five patient-specific model are compared.

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.

Metallicity Dependence of the H/H2 and C+/C/CO Distributions in a Resolved Self-regulating Interstellar Medium

Abstract We study the metallicity dependence of the H/H2 and C+/C/CO distributions in a self-regulated interstellar medium (ISM) across a broad range of metallicities (0.1 < Z/Z ⊙ < 3). To this end, we conduct high-resolution (particle mass of 1 M ⊙) hydrodynamical simulations coupled with a time-dependent H2 chemistry network. The results are then post-processed with an accurate chemistry network to model the associated C+/C/CO abundances, based on the time-dependent non-steady-state (“nonequilibrium”) H2 abundances. We find that the time-averaged star formation rate and the ISM structure are insensitive to metallicity. The column densities relevant for molecular shielding appear correlated with the volume densities in gravitationally unstable gas. As metallicity decreases, H2 progressively deviates from steady state (“equilibrium”) and shows shallow abundance profiles until they sharply truncate at the photodissociation fronts. In contrast, the CO profile is sharp and controlled by photodissociation as CO quickly reaches steady state. We construct effective 1D cloud models that successfully capture the time-averaged chemical distributions in simulations. At low metallicities, the steady-state model significantly overestimates the abundance of H2 in the diffuse medium. The overestimated H2, however, has little impact on CO. Consequently, the mass fraction of CO-dark H2 gas is significantly lower than what a fully steady-state model predicts. The mass ratios of H2/C+ and H2/C both show a weaker dependence on Z ′ than H2/CO, which potentially indicates that C+ and C could be alternative tracers for H2 at low Z ′ in terms of mass budget. Our chemistry code for post-processing is publicly available.

High-speed impact of micron-sized diesel drop trains—Splashing dynamics, secondary droplet formation, and effects of pre-existing film thickness

This work investigates surface impingement of monodispersed diesel drop trains using computational simulations. The three-dimensional simulations include high-speed impact of micrometer-sized fuel drops onto initially dry and wetted stainless steel substrates. The drop size and impact velocity are representative of fuel injection conditions in internal combustion engines (ICEs). The drop trains serve as a simplified representation of fuel spray. To accurately capture the film that naturally develops on initially dry substrates, a dynamic diesel-stainless steel contact angle model was developed using experiments of single drop impact under ICE representative temperature. Simulations are presented for a highly splashing case and the effects of a pre-existing film on splashing dynamics were investigated, including the temporal evolution of splashed mass and film thickness. It was concluded that for thin films, the effect of pre-existing film thickness is less significant than in single drop impingement. Secondary droplet characterization was performed on simulation results, leading to instantaneous and time-averaged distributions of secondary droplet size, velocity magnitude, and trajectory angle. It was found that for each drop impingement, approximately 58% of the splashed mass is from the impinging drop itself, while the remainder of the splashed mass is composed of film liquid. From a high resolution case, details of secondary droplet formation are observed and three distinct phases of secondary droplet formation are identified. The detailed analysis of drop train impingement under engine-relevant conditions serves as a first step toward a robust understanding of fuel impingement and the development of cleaner and more efficient ICEs.

Experimental study on effects of plate angle on acoustic waves from supersonic impinging jets.

This study experimentally investigates the generation mechanism of a higher-angle lobe which is an acoustic field feature of a supersonic jet impinging on an inclined flat plate. The overall sound pressure level distribution and spectra, the conditional averages of near field schlieren movies, and the time-averaged wall pressure distribution were obtained for three cases of Mach 1.8 ideally expanded impinging jets with plate angles of 45°, 22.5°, and 10° to the jet downstream axis. In the 45° and 22.5° cases, a higher-angle lobe appears. The dominant acoustic waves in the higher-angle lobe are radiated from the source region that contains shock waves and are suggested to be correlated with large-scale turbulent structures. These results suggest that the higher-angle lobe is dominated by acoustic waves generated by the interaction between the shock waves and large-scale turbulent structures. This inference is supported by the fact that the near-field acoustic wave patterns are qualitatively reproduced by the interference of monopoles located near the shock waves in the 45° and 22.5° cases and that neither the shock wave nor the higher-angle lobe is observed in the 10° case.

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.

Prediction of aerothermal characteristics of a generic hypersonic inlet flow

The accurate prediction of aerothermal surface loading is of paramount importance for the design of high speed flight vehicles. In this work, we consider the numerical solution of hypersonic flow over a double-finned geometry, representative of the inlet of an air-breathing flight vehicle, characterized by three-dimensional intersecting shock-wave/turbulent boundary-layer interaction at Mach 8.3. High Reynolds numbers ($Re_L \approx 11.6 \times 10^6$ based on free-stream conditions) and the presence of cold walls ($T_w/T_o \approx 0.27$) leading to large near-wall temperature gradients necessitate the use of wall-modeled large-eddy simulation (WMLES) in order to make calculations computationally tractable. The comparison of the WMLES results with experimental measurements shows good agreement in the time-averaged surface heat flux and wall pressure distributions, and the WMLES predictions show reduced errors with respect to the experimental measurements than prior RANS calculations. The favorable comparisons are obtained using an LES wall model based on equilibrium boundary layer approximations despite the presence of numerous non-equilibrium conditions including three dimensionality, shock-boundary layer interactions, and flow separation. Lastly, it is also demonstrated that the use of semi-local eddy viscosity scaling (in lieu of the commonly used van Driest scaling) in the LES wall model is necessary to accurately predict the surface pressure loading and heat fluxes.

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.

Time-Accurate Evaluation of Film Cooling Jet Characteristics for Plenum and Crossflow Coolant Supplies

Abstract Gas turbine film cooling creates complicated and highly unsteady flow structures. This study seeks to examine the unsteady characteristics created by different cooling hole inlet geometries using a fast-response pressure-sensitive paint (PSP) technique able to capture time-accurate measurements at 2000 frames per second, resolving frequencies up to 1000 Hz. Time-accurate and time-averaged measurements are used to evaluate the performance of a plenum-style inlet and a crossflow-style inlet in varying turbulence environments over a flat plate. The results of this study are intended to begin the process of breaking down widely accepted time-averaged film effectiveness trends into the cumulative effects of freestream turbulence and momentum flux ratio, with emphasis on the cooling jet motions they cause. Jet behaviors observed in this study include a sweeping oscillation, unsteady attachment and separation from the plate, and time-accurate and time-averaged flow bias. Cooling hole inlet geometry and momentum flux ratio affect the core of the jet, and freestream turbulence affects the periphery of the jet. Crossflow-fed cooling holes show bias to the upstream side of the cooling hole with respect to the internal crossflow direction. Plenum-fed cooling holes outperform crossflow-fed cooling holes, and the difference grows with increasing momentum flux ratio. The frequency of both plenum- and crossflow-fed cooling jet motions is influenced by the freestream turbulent velocity fluctuations. The resulting mode shapes show dominant side-to-side sweeping for higher turbulence environments and a separation and reattachment motion for lower turbulence environments. At higher momentum flux ratio, the jets were seen to increasingly favor separation and reattachment motions. The results of this study are intended to better inform existing predictive tools. With a better understanding of the time-accurate behaviors responsible for creating the commonly accepted time-averaged coolant distributions, simple predictive tools may be better equipped to accurately model film cooling flows.

Skin-Friction-Based Identification of the Critical Lines in a Transonic, High Reynolds Number Flow via Temperature-Sensitive Paint.

In this contribution, three methodologies based on temperature-sensitive paint (TSP) data were further developed and applied for the optical determination of the critical locations of flow separation and reattachment in compressible, high Reynolds number flows. The methodologies rely on skin-friction extraction approaches developed for low-speed flows, which were adapted in this work to study flow separation and reattachment in the presence of shock-wave/boundary-layer interaction. In a first approach, skin-friction topological maps were obtained from time-averaged surface temperature distributions, thus enabling the identification of the critical lines as converging and diverging skin-friction lines. In the other two approaches, the critical lines were identified from the maps of the propagation celerity of temperature perturbations, which were determined from time-resolved TSP data. The experiments were conducted at a freestream Mach number of 0.72 and a chord Reynolds number of 9.7 million in the Transonic Wind Tunnel Göttingen on a VA-2 supercritical airfoil model, which was equipped with two exchangeable TSP modules specifically designed for transonic, high Reynolds number tests. The separation and reattachment lines identified via the three different TSP-based approaches were shown to be in mutual agreement, and were also found to be in agreement with reference experimental and numerical data.

Fragile many-body ergodicity from action diffusion.

Weakly nonintegrable many-body systems can restore ergodicity in distinctive ways depending on the range of the interaction network in action space. Action resonances seed chaotic dynamics into the networks. Long-range networks provide well connected resonances with ergodization controlled by the individual resonance chaos time scales. Short-range networks instead yield a dramatic slowing down of ergodization in action space, and lead to rare resonance diffusion. We use Josephson junction chains as a paradigmatic study case. We exploit finite time average distributions to characterize the thermalizing dynamics of actions. We identify an action resonance diffusion regime responsible for the slowing down. We extract the diffusion coefficient of that slow process and measure its dependence on the proximity to the integrable limit. Independent measures of correlation functions confirm our findings. The observed fragile diffusion is relying on weakly chaotic dynamics in spatially isolated action resonances. It can be suppressed, and ergodization delayed, by adding weak action noise, as a proof of concept.

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.

Numerical comparisons of the aerodynamic performances of wind-tunnel train models with different inter-carriage gap spacings under crosswind

Reasonable gap spacings between adjacent train vehicle models are of great significance for the data accuracy during a wind tunnel test. In this research, crosswind flows around 1/8th-scale high-speed train models with inter-carriage gap spacings of 0, 5, 8, 10, and 20 mm are studied using improved delayed detached eddy simulation (IDDES) with the SST (shear-stress transport) k-omega model. The numerical methodology is first validated against the wind tunnel experiment data. Then, the effects of gap spacings on aerodynamic loads and the flow field around the train are investigated. The results indicate that the side force of the tail car significantly increases with the gap spacing, in which the maximum difference occurs at 20 mm compared with the zero gap spacing, whereas the difference in the drag decreases. When the gap spacing is greater than 10 mm, the time-averaged pressure distribution, flow pattern and transient flow structures around the inter-carriage gap regions observe a significant difference from other gap spacings. Considering the overall aerodynamic effects induced by gap spacings and the practical handleability during a wind tunnel test, a gap spacing between 5 mm and 10 mm is therefore recommended for adjacent train vehicles with a 1/8th scale.

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.

Data assimilation of flow-acoustic resonance.

A data assimilation (DA) strategy was developed for accurate prediction of the flow-acoustic resonant fields within a channel-branch system. The challenges of numerical simulation of such internal aeroacoustic systems are primarily associated with determination of the transfer loss between the acoustic waves and the shear layer vortices. Thus, a data-assimilated momentum loss model that comprises a viscous loss item and an inertial loss item was established and embedded into the Navier-Stokes equations. During the DA, the acoustic pressure pulsations measured from a dynamic pressure array served as the observational data, the ensemble Kalman filter served as the optimization algorithm, and a three-dimensional transient computational fluid dynamics method comprising an explicit algebraic Reynolds stress model (EARSM) served as the predictive model system. EARSM was used because its ability to predict internal flow-acoustic resonances was superior to that of other eddy viscosity models and Reynolds stress models. The data-assimilated flow-acoustic resonant fields were then comprehensively validated in terms of their acoustic fields, time-averaged flow fields, and phase-dependent flow fields. The time-averaged flow fields were obtained from planar particle-image velocimetry (PIV) measurements, and the phase-dependent flow fields were obtained from field programmable gate array-based phase-locking PIV measurements. The results demonstrate that the use of DA afforded an optimal simulation that efficiently decreased the numerical errors in the frequencies and amplitudes of the acoustic pressure pulsations, thereby achieving better agreement between time-averaged flow distributions and fluctuations. In addition, the data-assimilated numerical simulation completely reproduced the spatiotemporal evolution of the shear layer vortices, that is, their formation, developing, transport, and collapsing regions.

On Forced-Vibroacoustic Component Analyses of an Arbitrarily Shaped Vibrating Structure

This paper presents a methodology that enables one to trace back the forced-vibroacoustic components (F-VAC) that are responsible for acoustic radiation from an arbitrarily shaped structure. This methodology relies on a modified Helmholtz equation least-square (HELS) method that takes partial normal surface velocities obtained by using a laser vibrometer and partial acoustic pressures measured using a small array of microphones as input to reconstruct the distributions of the normal surface velocity and acoustic pressure over the entire source surface. Next, the normal component of the time-averaged acoustic intensity distribution is reconstructed and linked to the field acoustic pressure. Finally, the singular value decomposition (SVD) technique is used to reveal the most critical components of the structural vibrations that are responsible for acoustic radiation. The advantages of using partial normal surface velocities and acoustic pressures as input data are that: (1) it enables one to acquire the rich near-field information embedded in the normal surface velocities; (2) it is a noncontact and noninvasive measurement approach using a laser vibrometer or scanning laser and a small microphone array at a remote location; (3) it enables one to obtain a comprehensive picture of the vibroacoustic fields that traditional near-field acoustic holography (NAH) cannot offer; (4) it allows for correlating the near-field effects to the far-field acoustic pressures and revealing the root causes of structure-borne sound radiation; (5) it provides an insight into developing the most cost-effective noise reduction measures; and (6) it is suited for engineering applications since in practice there are always areas that are inaccessible for measurements. Finally, it is emphasized that although sound may be produced by structural vibrations, not all structural vibrations may produce sound. Therefore, all one needs to do is to suppress the most critical components of structural vibrations that can emit sound. Experimental validations of using F-VAC analyses to reduce sound radiation from an arbitrarily shaped structure are demonstrated.

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.

Multiwavelength study of the quiescent states of six brightest flat-spectrum radio quasars detected by Fermi-LAT

ABSTRACT The regular monitoring of flat-spectrum radio quasars (FSRQs) in γ-rays by Fermi-LAT (Large Area Telescope) since past 12 yr indicated six sources who exhibited extreme γ-ray outbursts crossing daily flux of 10−5 photons cm−2 s−1. We obtained nearly simultaneous multiwavelength data of these sources in radio to γ-ray waveband from OVRO (Owens Valley Radio Observatory), Steward Observatory, SMARTS (Small and Moderate Aperture Research Telescope System), Swift-UVOT (UV/Optical Telescope), Swift-XRT (X-ray Telescope), and Fermi-LAT. The time-averaged broad-band spectral energy distributions (SEDs) of these sources in quiescent states were studied to get an idea about the underlying baseline radiation processes. We modelled the SEDs using one-zone leptonic synchrotron and inverse Compton emission scenario from broken power-law electron energy distribution inside a spherical plasma blob, relativistically moving down a conical jet. The model takes into account inverse Compton scattering of externally and locally originated seed photons in the jet. The big blue bumps visible in quiescent state SEDs helped to estimate the accretion disc luminosities and central black hole masses. We found a correlation between the magnetic field inside the emission region and the ratio of emission region distance to disc luminosity, which implies that the magnetic field decreases with an increase in emission region distance and decrease in disc luminosity, suggesting a disc–jet connection. The high-energy index of the electron distribution was also found to be correlated with observed γ-ray luminosity as γ-rays are produced by high-energy particles. In most cases, kinetic power carried by electrons can account for jet radiation power as jets become radiatively inefficient during quiescent states.