Low-frequency Velocity Fluctuations Research Articles

Detailed experimental and numerical analysis of hydrodynamics in the outflow measurement channel of a sewage treatment plant

Abstract The ‘Bajo Grande’ Wastewater Treatment Plant has a design capacity of 2.78 m3/s, and discharges into the Suquia River (Córdoba, Argentina). The river has an average flow rate of 10 m3/s, with lower values during the summer. Currently, the treatment plant does not have an accurate discharge-measurement system prior to the discharge into the river, which makes it difficult to evaluate the dosing of the disinfection treatment. The outflow rate is measured in a straight flume. However, at the inlet section of the flume, a 180° sharp bend induces a complex turbulent flow with instabilities and low-frequency velocity fluctuations which are not appropriate for flow quantification. In this type of flow, most of the in situ flow discharge-measurement systems have great uncertainty. Therefore, in situ flow measurements with an Acoustic Doppler Current Profiler, Large-Scale Particle-Tracking Velocimetry techniques and a prototype-scale Detached Eddy Simulation model were combined to obtain a detailed characterization of the turbulent flow. The results provide flow rates, fields of mean flow velocity, temporal evolution, and characteristic parameters of the turbulence. This allowed a better understanding of the effects of turbulence and flow instabilities. The results provide a basis to validate numerical models used in the hydraulics design of contact chambers to improve the disinfection process.

An experimental investigation on gas flow field dynamic characteristics in a reverse cyclone

The dynamic characteristics of the gas flow field in a reverse cyclone were investigated experimentally in this study by measuring instantaneous tangential velocity with hot wire anemometry (HWA). The instantaneous tangential velocity was found to continuously fluctuate by both low and high frequencies with time. Further analyses of measured data regarding time and frequency domains by standard deviation and spectral analysis revealed that the fluctuation intensities were stronger in the center region than that near the wall, and stronger still near the dust outlet. One dominant frequency (f1) prevailed in entire space of the cyclone and remained almost unchanged, while the power spectral density (PSD) of f1 decreased significantly with increased radial position. To this effect, the low-frequency velocity fluctuation exhibited transfer behavior and attenuation characteristics. Besides, another dominant frequency (f2) generated by the hopper (the hopper led to a new fluctuation) and emerged just in the local region of the dust outlet. Analyses suggested that both dominant frequencies originated from the sway of the gas swirling flow, f1 was from the cyclone, and f2 from the hopper. The dominant frequencies also increased as Re increased, and in the cyclone with a hopper, the region affected by f2 grew in size as Re increased.

Experimental study of bathymetry generated turbulence on tidal turbine behaviour

In high flow velocity areas like those suitable for tidal applications, turbulence intensity is high and flow variations may have a major impact on tidal turbine behaviour. A three-bladed horizontal axis turbine model (scale 1:20) is positioned in the wake of a square wall-mounted cylinder, representative of specific in situ bathymetric variation, to experimentally study these effects in a current flume tank. Local and global loads are acquired in synchronization with velocity measurements to study the turbine response to flow fluctuations. Velocity measurements need to be obtained close to the turbine, contrary to what is commonly considered, to properly correlate velocity and loads fluctuations. Results show that the loads phase average and their dispersion evolve according to the sheared velocity profile. We conclude that the turbine load fluctuations directly respond to the low frequency velocity fluctuations and are dominated by the turbulent structures shed from the cylinder. It is then possible to compare the effects of large coherent turbulent structures on the turbine behaviour to cases with more classical free stream turbulence commonly studied. These results provide a substantive database in high Reynolds number flows for further fatigue analysis or recommendations for turbine positioning in such flows.

Experimental study of gas swirling flow instability characteristics in a cyclone using the hot‐wire anemometry technique

AbstractThe instability characteristics of gas swirling flow in a cyclone were investigated experimentally by measuring the instantaneous tangential velocity with the hot‐wire anemometry. The results showed that the instantaneous tangential velocity fluctuated continuously with time at both low and high frequencies. Further analysis of measured data regarding time and frequency domain by probability density and spectral methods revealed that the velocity fluctuation was affected not only by the turbulence flow itself but also by gas swirling flow instability. Also, the distributions of dominant frequency and amplitude indicated that low‐frequency velocity fluctuation caused by the instability had the transfer behavior and attenuation character, which could be characterized by the dominant frequency that varied little along the radial position and decreased gradually along the axial direction, while the amplitude increased significantly with decreased radial position. Due to the gas swirling instability, the turbulence intensity and the fine particle diffusion were enhanced, which would degrade the separation efficiency of cyclone.

Effects of nozzle-exit boundary-layer profile on the initial shear-layer instability, flow field and noise of subsonic jets

The influence of the nozzle-exit boundary-layer profile on high-subsonic jets is investigated by performing compressible large-eddy simulations (LES) for three isothermal jets at a Mach number of 0.9 and a diameter-based Reynolds number of $5\times 10^{4}$, and by conducting linear stability analyses from the mean-flow fields. At the exit section of a pipe nozzle, the jets exhibit boundary layers of momentum thickness of approximately 2.8 % of the nozzle radius and a peak value of turbulence intensity of 6 %. The boundary-layer shape factors, however, vary and are equal to 2.29, 1.96 and 1.71. The LES flow and sound fields differ significantly between the first jet with a laminar mean exit velocity profile and the two others with transitional profiles. They are close to each other in these two cases, suggesting that similar results would also be obtained for a jet with a turbulent profile. For the two jets with non-laminar profiles, the instability waves in the near-nozzle region emerge at higher frequencies, the mixing layers spread more slowly and contain weaker low-frequency velocity fluctuations and the noise levels in the acoustic field are lower by 2–3 dB compared to the laminar case. These trends can be explained by the linear stability analyses. For the laminar boundary-layer profile, the initial shear-layer instability waves are most strongly amplified at a momentum-thickness-based Strouhal number $St_{\unicode[STIX]{x1D703}}=0.018$, which is very similar to the value obtained downstream in the mixing-layer velocity profiles. For the transitional profiles, on the contrary, they predominantly grow at higher Strouhal numbers, around $St_{\unicode[STIX]{x1D703}}=0.026$ and 0.032, respectively. As a consequence, the instability waves rapidly vanish during the boundary-layer/shear-layer transition in the latter cases, but continue to grow over a large distance from the nozzle in the former case, leading to persistent large-scale coherent structures in the mixing layers for the jet with a laminar exit velocity profile.

Measurements of the atmospheric turbulence in the coastal zone of the sea during weak wind from a mountainous coast

Results of measurements of the atmospheric turbulence in the layer between 1.5 and 21 m above sea level and the drag coefficient of the sea surface as the wind blows from a 4-km-long mountainous slope with a mean inclination of 11° are presented. The measurements of wind-speed profiles and its fluctuations at several levels, waves, and the main meteorological parameters were carried out in autumn 2005 and 2008 from a stationary platform located in the Black Sea at a distance of approximately 1 km from the southern coast of Crimea. It is shown that during weak synoptic wind a low-level wind jet develops at night over the sea with a maximum velocity up to 5–6 m/s at a level of approximately 6 m over the sea induced by the katabatic wind over the coastal slope. According to the approximate estimates, the horizontal scale of the low-level jet can reach a few tens of kilometers. This flow is characterized by the dissipation rate of the turbulence energy independent of height and low-frequency velocity fluctuations related to the gravity waves and advection of turbulence from the coast. It is shown that the lower part of the boundary layer (up to a height of 3 m) is adjusted to the sea-surface roughness. The dependencies of the drag coefficient on the wind speed or wave age are steadier than in the data for the open sea. However, the age of the waves is not a universal parameter at long and short fetches.

A proposed technique for determining aerodynamic pressures on residential homes

Wind loads on low-rise buildings in general and residential homes in particular can differ significantly depending upon the laboratory in which they were measured. The differences are due in large part to inadequate simulations of the low-frequency content of atmospheric velocity fluctuations in the laboratory and to the small scale of the models used for the measurements. The imperfect spatial coherence of the low frequency velocity fluctuations results in reductions of the overall wind effects with respect to the case of perfectly coherent flows. For large buildings those reductions are significant. However, for buildings with sufficiently small dimensions (e.g., residential homes) the reductions are relatively small. A technique is proposed for simulating the effect of low-frequency flow fluctuations on such buildings more effectively from the point of view of testing accuracy and repeatability than is currently the case. Experimental results are presented that validate the proposed technique. The technique eliminates a major cause of discrepancies among measurements conducted in different laboratories. In addition, the technique allows the use of considerably larger model scales than are possible in conventional testing. This makes it possible to model architectural details, and improves Reynolds number similarity. The technique is applicable to wind tunnels and large scale open jet facilities, and can help to standardize flow simulations for testing residential homes as well as significantly improving testing accuracy and repeatability. The work reported in this paper is a first step in developing the proposed technique. Additional tests are planned to further refine the technique and test the range of its applicability.

Evolution in the time series of vortex velocity fluctuations across different regimes of vortex flow

Investigations of vortex velocity fluctuation in time domain have revealed a presence of low frequency velocity fluctuations which evolve with the different driven phases of the vortex state in a single crystal of 2H-NbSe2. The observation of velocity fluctuations with a characteristic low frequency is associated with the onset of nonlinear nature of vortex flow deep in the driven elastic vortex state.

A Three-Equation Eddy-Viscosity Model for Reynolds-Averaged Navier–Stokes Simulations of Transitional Flow

An eddy-viscosity turbulence model employing three additional transport equations is presented and applied to a number of transitional flow test cases. The model is based on the k-ω framework and represents a substantial refinement to a transition-sensitive model that has been previously documented in the open literature. The third transport equation is included to predict the magnitude of low-frequency velocity fluctuations in the pretransitional boundary layer that have been identified as the precursors to transition. The closure of model terms is based on a phenomenological (i.e., physics-based) rather than a purely empirical approach and the rationale for the forms of these terms is discussed. The model has been implemented into a commercial computational fluid dynamics code and applied to a number of relevant test cases, including flat plate boundary layers with and without applied pressure gradients, as well as a variety of airfoil test cases with different geometries, Reynolds numbers, freestream turbulence conditions, and angles of attack. The test cases demonstrate the ability of the model to successfully reproduce transitional flow behavior with a reasonable degree of accuracy, particularly in comparison with commonly used models that exhibit no capability of predicting laminar-to-turbulent boundary layer development. While it is impossible to resolve all of the complex features of transitional and turbulent flows with a relatively simple Reynolds-averaged modeling approach, the results shown here demonstrate that the new model can provide a useful and practical tool for engineers addressing the simulation and prediction of transitional flow behavior in fluid systems.

Growth and breakdown of low-speed streaks leading to wall turbulence

Two-dimensional local wall suction is applied to a fully developed turbulent boundary layer such that near-wall turbulence structures are completely sucked out, but most of the turbulent vortices in the original outer layer can survive the suction and cause the resulting laminar flow to undergo re-transition. This enables us to observe and clarify the whole process by which the suction-surviving strong vortical motions give rise to near-wall low-speed streaks and eventually generate wall turbulence. Hot-wire and particle image velocimetry (PIV) measurements show that low-frequency velocity fluctuations, which are markedly suppressed near the wall by the local wall suction, soon start to grow downstream of the suction. The growth of low-frequency fluctuations is algebraic. This characterizes the streak growth caused by the suction-surviving turbulent vortices. The low-speed streaks obtain almost the same spanwise spacing as that of the original turbulent boundary layer without the suction even in the initial stage of the streak development. This indicates that the suction-surviving turbulent vortices are efficient in exciting the necessary ingredients for the wall turbulence, namely, low-speed streaks of the correct scale. After attaining near-saturation, the low-speed streaks soon undergo sinuous instability to lead to re-transition. Flow visualization shows that the streak instability and its subsequent breakdown occur at random in space and time in spite of the spanwise arrangement of streaks being almost periodic. Even under the high-intensity turbulence conditions, the sinuous instability amplifies disturbances of almost the same wavelength as predicted from the linear stability theory, though the actual growth is in the form of a wave packet with not more than two waves. It should be emphasized that the mean velocity develops the log-law profile as the streak breakdown proceeds. The transient growth and eventual breakdown of low-speed streaks are also discussed in connection with the critical condition for the wall-turbulence generation.

Large Eddy Simulations of Flow Instabilities in a Stirred Tank Generated by a Rushton Turbine

The aim of this work is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation (LES). The sliding mesh method was used for the coupling between the rotating and the stationary frame of references. The calculations were carried out on the “Shengcao-21C” supercomputer using a computational fluid dynamics (CFD) code CFX5. The flow fields predicted by the LES simulation and the simulation using standard k-ɛ model were compared to the results from particle image velocimetry (PIV) measurements. It is shown that the CFD simulations using the LES approach and the standard k-ɛ model agree well with the PIV measurements. Fluctuations of the radial and axial velocity are predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies are seen in the impeller region, while low frequencies velocity fluctuations are observed in the bulk flow. A low frequency velocity fluctuation with a nondimensional frequency of 0.027Hz is predicted by the LES simulation, which agrees with experimental investigations in the literature. Flow circulation patterns predicted by the LES simulation are asymmetric, stochastic and complex, spanning a large portion of the tanks and varying with time, while circulation patterns calculated by the simulation using the standard k-ɛ model are symmetric. The results of the present work give better understanding to the flow instabilities in the mechanically agitated tank. However, further analysis of the LES calculated velocity series by means of fast Fourier transform (FFT) and/or spectra analysis are recommended in future work in order to gain more knowledge of the complicated flow phenomena.

201 境界層外部から導入した乱れが遷移に及ぼす影響(2)(OS2-1 壁乱流の計測と制御,OS2壁乱流の計測と制御)

The effect of external disturbances on the transition of a flat plate boundary layer is studied by introducing external disturbances using a turbulence generator located behind the leading edge of the plate. The results indicate the existence of the process, where low-frequency components of the external disturbances influence the boundary layer, generating low-frequency velocity fluctuations inside the boundary layer, prior to the high-frequency components directly reaching the boundary layer.

翼型上のＳｈｏｒｔ Ｂｕｂｂｌｅ内部に生ずる低周波数速度変動に関する実験的研究

Experimental Studies have been conducted to investigate velocity disturbances observed inside the short bubble formed on NACA0012 airfoil. Measurements were done at the angle of attack 10° and at the Reynolds number of 1.3×105 based on the chord length. In the transition area in the short bubble, a dominant low frequency velocity fluctuation was observed. This frequency is much lower than that reported in the previous short bubble measurements. The phase averaging technique has been applied to analyze this low frequency velocity fluctuation. It is shown that the direction and the amount of the flow velocity vector inside the separated layer just after the separation point changes periodically. The frequency of these behaviors agrees with the observed low frequency velocity fluctuation.

Particle image velocimetry measurement of in-cylinder flow in internal combustion engines-experiment and flow structure analysis

A cross-correlation digital particle image velocimetry (PIV) system has been developed and applied to study the in-cylinder flow in a single-cylinder engine with a production-type cylinder head. The PIV system set-up and its optimization are described in the first part of the paper. Two-dimensional velocity distributions measured over 100 cycles are analysed to obtain ensemble-averaged mean and fluctuating velocities, the turbulent length scale, vorticity and strain rate distribution in the measurement plane. In particular, a spatial filtering scheme is developed in order to obtain the cycle-resolved velocity measurements. The cycle-resolved analysis shows that the low-frequency velocity fluctuation component (i.e. cyclic variation) is mainly responsible for the spatial variation in velocity and turbulent kinetic energy distributions. The integral length scale calculated from the PIV data is between 6 and 10 mm and the strain rate is estimated to be within 1000 s-1 in most areas of the measurement plane.

Modelling and simulation of the dynamic flow behaviour in a bubble column

A three-dimensional dynamic Eulerian–Eulerian two-phase model is used for the modelling of bubble column hydrodynamics in the homogeneous flow regime. The turbulence in liquid phase is considered by the standard k– ε model. Further numerical studies investigate the influence of additional turbulence production through the dispersed gas phase. The dispersed phase is represented by a mono-dispersal neglecting coalescence and break-up effects. Detailed experiments performed in a cylindrical bubble column allow for the validation of the model. The measurements focus on the local liquid-phase velocities and gas void fractions. The numerical simulations successfully predict both the time-averaged and the dynamic flow behaviour. The measured low-frequency velocity fluctuations are used for the validation of the simulated large-scale flow dynamics that is dominated by the irregular movements of the bubble plume and the vortical structures in the liquid phase. The consideration of bubble-induced turbulence shows positive as well as negative impacts on the quality of the simulation results for a coarse numerical grid. Due to the mutual influences, the turbulence model has to be discussed together with the grid resolution. Employing finer grids improves the description of the vortical flow structure in the bubble column and the agreement with the experimental data. However, the computation power increases significantly and a compromise between efficiency and quality of results has to be found.

K-1338 T字形合流管内の流れに関する研究 : 原子力プラントでの低周波流動振動に関する数値解析(S15-12 管内のはく離)(S15 噴流・はく離流れの計測と制御)

K-1338 T字形合流管内の流れに関する研究 : 原子力プラントでの低周波流動振動に関する数値解析(S15-12 管内のはく離)(S15 噴流・はく離流れの計測と制御)

Transitional Characteristics of a Two-Dimensional Jet Excited by Wake of a Two-Dimensional Cylinder Installed in Nozzle Contraction. Low-Frequency Velocity Fluctuation under Inharmonic Excitation.

Transitional characteristics of a plane jet excited by wake of a two-dimensional cylinder installed in nozzle contraction has been investigated by FFT and phase-averaging techniques. The slot width h of the nozzle was fixed to 15mm. The diameter of the cylinder d was 2mm, and the location of the cylinder inside the nozzle contraction Xc was 90 mm from the nozzle exit. The exit velocity was 10.1m/s, and Reynolds number based on the slot width was 1.0×104. In this setup condition of the cylinder, two distinctive spectral peaks (f1 and f2) centered at 1/2 the excitation frequency were observed. And also inharmonic phenomenon indicated by presence of beat frequency of the two frequencies was observed. The results show that the jet under inharmonic excitation exhibits flapping motion at frequency f1-f2, and the jet starts to rolling-up to form vortex street at x/h>8.

High Frequency Phenomena in Regurgitant Jets

Color Doppler echocardiography is accepted as a useful tool for evaluating valvular regurgitation but, so far, it does not allow for a precise quantification of the regurgitant volume since it is limited by the combination of velocity ambiguities, a limited resolution, and a low frame rate. The low frame rate is imposed by the sector angle, the packet size, the pulse repetition rate, the line density, and the sequential analysis of the ultrasound emissions. On the other hand, the images are supposed to depict pulsatile jets surrounded by high frequency pulsatile vortices. The present study was designed to search for the presence of high frequency phenomena in turbulent jets and to analyze the influence of the low frame rate on the jet images. An in vitro study using an aliasing‐free laser Doppler and a hydraulic model simulating intracardiac jets was carried out. An analysis of flow events with a high bandwidth (500 Hz) was conducted using M‐mode color displays of the velocities. It showed: (1) on the centerline, a slow propagation of the front of the jet, and low frequency velocity fluctuations (< 50 Hz); and (2) at the opposite, in the boundary layer of the jet, very rapid fluctuations with frequencies comprised between 50 and 500 Hz. On two‐dimensional color images of laser Doppler data, the size of the central laminar core appeared strongly modified by a decrease in frame rate up to the maximum one commonly used in color Doppler (30 frames / sec) as well by a decrease in line density. These data demonstrate that higher frame rate could improve the accuracy of ultrasound color Doppler flow mapping.

The unsteady structure of two-dimensional steady laminar separation

The two-dimensional unsteady incompressible Navier–Stokes equations, solved by a fractional time-step method, were used to investigate separation due to the application of an adverse pressure gradient to a low-Reynolds number boundary layer flow. The inviscid pressure distribution of Gaster [AGARD CP 4, 813 (1966)] was applied in the present computations to study the development of a laminar separation bubble. In all cases studied, periodic vortex shedding occurred from the primary separation region. The shed vortices initially lifted from the boundary layer and then returned towards the surface downstream. The shedding frequency nondimensionalized by the momentum thickness was found to be independent of Reynolds number. The value of the nondimensional Strouhal number, however, was found to differ from the results of Pauley et al. [J. Fluid Mech. 220, 397 (1990)], indicating that the shedding frequency varies with the nondimensional pressure distribution, Cp. The computational results were time averaged over several shedding cycles and the results were compared with Gaster. The numerical study accurately reproduced the major characteristics of the separation found in Gaster’s study such as the separation point, the pressure plateau within the upstream portion of the separation bubble, and the reattachment point. The similarity between the experimental results and the time-averaged two-dimensional computational results indicates that the low-frequency velocity fluctuations detected by Gaster are primarily due to the motion of large vortex structures. This suggests that large-scale two-dimensional structures control bubble reattachment and small-scale turbulence contributes a secondary role.

Deep Currents in the Gulf of Mexico

Abstract Direct current measurements using moored arrays have been made below 1000 m in the eastern, central and western Gulf of Mexico basin. The major low frequency velocity fluctuations in the lower 1000 to 2000 m of the water column in the three regions have the characteristics of topographic Rossby waves (TRWs). Spectral peaks are observed at periods of about 25 days and 40 to 100 days. Motions are highly coherent with depth. Variances increase toward the bottom despite the very weak stratification of the deep waters of the Gulf. Wave-lengths are about 150–250 km and phase propagation is offshore with energy propagation westward. A group velocity of about 9 km day−1 could be directly estimated from significantly coherent signals between eastern and western arrays. This value is consistent with estimates derived from the dispersion relation and is higher than the westward translation speed of 3 to 6 km day−1 of the large anticyclonic eddies shed from the Loop Current. It appears that a major source of...