Local Turbulent Energy Research Articles

Bubble size distribution and turbulence characterization in a bubbly flow in the presence of surfactant

This study aimed to quantify the less explored complex multiphase hydrodynamics of a bubble swarm in the presence of surfactant which is the key to flotation process widely used in the resources and environmental engineering applications.Experiments were conducted in a rectangular column (cross-section: 100 mm × 100 mm) in batch mode by varying the gas flux (0.02 to and 0.08 cm/s) in the presence of an anionic surfactant sodium dodecyl sulphate (5 to 15 ppm). High-speed imaging was then used to visualise the unsteady bubble plume dispersion behaviour and estimate the bubble size distribution (BSD) which showed a reduction in the mean bubble size with the increasing gas flux and surfactant concentration.Next, particle image velocimetry (PIV) was utilised to measure the instantaneous velocity field which was used to determine the turbulence characteristics. It was shown that the bubble plume contributes to significant anisotropy in the flow field which increased in the higher surfactant concentration and gas flux cases. The energy containing turbulence length scale was characterized by the integral length scale, which was observed to increase linearly with both the gas flux and surfactant concentration. Also, the local turbulence energy dissipation rate exhibited a strong linear correlation with the bubble surface area flux parameter. In the presence of surfactant, the turbulence energy spectrum of the system exhibited a less steep slope in the inertial subrange regime compared to the Kolmogorov −5/3 slope. The spectrum also showed a leftward shift indicating energy addition to the larger turbulence length scales which was reflected in the formation of large recirculation zones around the bubble plume.

A novel measure for long-term sediment reduction inspired by dragonfly wings.

The sediment accumulation in drainage pipes has long been recognized as a significant concern in the environmental field. This study addresses sediment accumulation in drainage pipes by introducing an innovative bioinspired approach using various shapes and angles of plates for long-term sediment reduction. Through experiments and numerical simulations, the velocity field, the turbulent kinetic energy, the head loss, and the dynamic pressure distribution in the pipeline with plates are analyzed. Results demonstrate significant increases in local velocity, dynamic pressure, and turbulence energy due to the presence of plates. The sediment reduction performance shows a positive correlation with the angle for folded plates and a non-linear relation with curvature for curved plates. Notably, the superior performance of folded plates is attributed to their exceptional ability to induce vortex formation. The head loss due to sediment reduction measures increases linearly as the angle and the curvature increase. Furthermore, the intentional induction of strong eddies and high shear flow using the undulating topography created by the locally installed folding plates in the pipeline was the main cause of sediment reduction. This novel approach holds promise for more efficient and sustainable sediment reduction in drainage systems.

Image‐based measurement of engulfment vortex to quantitatively assess micromixing efficiency in a gas–liquid stirred tank

AbstractMicromixing in reactors is typically characterized by chemical test reactions. A novel image‐based measurement system of engulfment vortex (IMEV system) is developed to assess the micromixing efficiency. Over 2000 vortices were recognized from images captured under various experimental cases, and key frames for their formation, engulfment and dissipation were identified. Inspired by the engulfment model (i.e., E model), this work quantified the micromixing time and local turbulent energy dissipation rate with the vortex lifetimes. The micromixing time obtained through the IMEV method is comparable to its value estimated by the micromixing model and segregation index of the test reaction, with a deviation of less than 20%. In turbulent gas–liquid systems, bubbles play a dual role on the local micromixing due to frequent disturbance and phase interface limitation. To summarize, this work offers an alternative approach for micromixing studies and provides valuable insights into dispersed phase effects on the micromixing efficiency.

Numerical study on the pulsating energy evolution in the cavitating flow around a mini Cascade

Cavitation is arguably a highly turbulent phenomenon in the liquid flow system. The cavitating flow around a mini cascade was carried out to investigate the turbulent characteristics and pulsation mechanisms. The results demonstrate that cavitation can significantly affect the turbulence velocity fluctuation and turbulence anisotropy, and intensively alter the local turbulent energy. To better provide an understanding of fundamental mechanisms dictating time-averaged pulsating energy, the inhomogeneity of the local concentration of pulsating energy at the vapor–liquid interface and the turbulent vortex core involves different fundamental mechanisms are expounded thoroughly through the ability of the time-averaged turbulent kinetic energy and the time-averaged pulsating entropy. The pulsating energy of cavitating flow around the mini cascade is basically obtained from the time-averaged flow, while the surrounding dissipative mechanisms are driven by the diffusion and dissipation terms. Further, the new definition of viscous diffusion term is derived based on the resolved turbulent kinetic energy, which can also clearly delineate the diffusion effect of turbulent kinetic energy produced by the molecule viscosity. Finally, the turbulent kinetic energy and pulsating enstrophy transport mechanisms inside the shedding vortex are revealed as significant characteristics of the interaction between vortex dynamics and turbulence–cavitation.

Derivation of the simplest transport equations for dissipation and vorticity from a modified equation for small-scale velocity

Transport equations for dissipation and vorticity are derived from a transport equation for small-scale velocity, using expression for turbulent viscosity through dissipation and gradients of large-scale velocity, assuming validity of local turbulent energy balance. These equations are of simple form. The equations can be used for parametrization in the various turbulence models. The approach can be generalized for magnetohydrodynamics, the flow with variable fluid density, the non-newton flow, etc.

Investigation of bubble-particle attachment, detachment and collection efficiencies in a mechanical flotation cell

The paper presents the effect of impeller speed and superficial gas velocity on bubble-particle attachment, detachment and collection efficiencies using pure quartz particles in a mechanical flotation cell. Detachment and collection efficiencies in different parts of cell were calculated by local turbulent energy dissipation rate measurements using high speed stereoscopic particle image velocimetry technique. In addition to high detachment efficiency, low attachment efficiency is also one of the reasons for low collection efficiency for coarse particles. The flotation rate constant increased with an increase in superficial gas velocity. However, the effect of superficial gas velocity on collection efficiency was negligible. This means that any effect of superficial gas velocity on flotation rate constant was due to changes in the number of bubbles and collision frequency. The flotation rate constant, collision frequency, and the number of bubbles increased with increasing impeller speed. However, collection efficiency decreased with increasing impeller speed.

Comparison of turbulent structures and energy fluxes over exposed and debris-covered glacier ice

AbstractWe present the first direct comparison of turbulence conditions measured simultaneously over exposed ice and a 0.08 m thick supraglacial debris cover on Suldenferner, a small glacier in the Italian Alps. Surface roughness, sensible heat fluxes (~20–50 W m−2), latent heat fluxes (~2–10 W m−2), topology and scale of turbulence are similar over both glacier surface types during katabatic and synoptically disturbed conditions. Exceptions are sunny days when buoyant convection becomes significant over debris-covered ice (sensible heat flux ~ −100 W m−2; latent heat flux ~ −30 W m−2) and prevailing katabatic conditions are rapidly broken down even over this thin debris cover. The similarity in turbulent properties implies that both surface types can be treated the same in terms of boundary layer similarity theory. The differences in turbulence between the two surface types on this glacier are dominated by the radiative and thermal contrasts, thus during sunny days debris cover alters both the local surface turbulent energy fluxes and the glacier component of valley circulation. These variations under different flow conditions should be accounted for when distributing temperature fields for modeling applications over partially debris-covered glaciers.

Investigation of bubble-particle interactions in a mechanical flotation cell, part 1: Collision frequencies and efficiencies

In this study the influence of impeller speed and superficial gas velocity on hydrodynamic parameters, bubble-particle collision frequency and efficiency using pure quartz particles of different sizes in an aerated Denver flotation cell was investigated. Collision frequency in different parts of cell was calculated by local turbulent energy dissipation rate measurements. Collision efficiency due to gravitational, inertial, interceptional and turbulence mechanism was determined. The overall collision efficiency (Ec) was also investigated. The results showed that the bubble-particle frequencies for different particle sizes in the turbulent zone were 10–12 times higher than in the quiescent zone. Under various hydrodynamic conditions, minimum and maximum values of Ec were observed for −38 µm and −300 + 212 µm particles, respectively. The turbulence mechanism was the main collision mechanism for fine particles. The collision efficiency of coarse particles was mainly influenced by gravitational, inertial and interceptional mechanisms.

Turbulence-Driven Ion Beams in the Magnetospheric Kelvin-Helmholtz Instability.

The description of the local turbulent energy transfer and the high-resolution ion distributions measured by the Magnetospheric Multiscale mission together provide a formidable tool to explore the cross-scale connection between the fluid-scale energy cascade and plasma processes at subion scales. When the small-scale energy transfer is dominated by Alfvénic, correlated velocity, and magnetic field fluctuations, beams of accelerated particles are more likely observed. Here, for the first time, we report observations suggesting the nonlinear wave-particle interaction as one possible mechanism for the energy dissipation in space plasmas.

Local Energy Dissipation Rate in an Agitated Vessel—a Comparison of Evaluation Methods

There are two main groups of local turbulent energy dissipation rate (e) evaluation methods, namely (i) velocity gradient methods and (ii) fitting the energy spectrum function. We calibrated our measurements and then applied these methods to evaluate e from our measurements. The experiments were carried out in the region below the impeller, wherewe assumed the existence of local isotropic turbulence, which is the main assumption for all e evaluation methods. However, the results differed from each other. Predictions obtained by the methods dealt with here are compared on the basis of our experimental program, which also brings in data obtained on relatively large vessels 0.3 and 0.4 m in inner diameter and two liquid viscosities under fully developed turbulence (Re > 50 000).

Local Turbulent Energy Dissipation Rate in an Agitated Vessel: Experimental and Turbulence Scaling

The hydrodynamics and the flow field in an agitated vessel were measured using 2-D time resolved particle image velocimetry (2-D TR PIV). The experiments were carried out in fully baffled cylindrical flat bottom vessels 300 and 400 mm in inner diameter. The 300 mm inner diameter tank was agitated by a Rushton turbine 100 mm in diameter, and the 400 mm inner diameter tank was agitated by a Rushton turbine 133 mm in diameter. Three liquids of different viscosities were used as the agitated liquid: (i) distilled water (ν = 9.35 × 10–7 m2/s), (ii) a 28 vol % aqueous solution of glycol (ν = 2 × 10–6 m2/s), and (iii) a 43 vol % aqueous solution of glycol (ν = 3 × 10–6 m2/s). The velocity fields were measured at an impeller rotation speed in the range from 300 to 850 rpm, which covers the Reynolds number range from 50000 to 189000. This means that fullydeveloped turbulent flow was reached. The experiments were performed to investigate the applicability of the following relations: e* = e/(u4/ν) = const, vK/u = const, Λ/ηK = const, τΛ/τK = const, e* = e/((Nd)4/ν) = const, Λ/d ∝ Re–1, ηK/d ∝ Re–1, vK/(Nd) = const, NτΛ ∝ R–1, NτK ∝ Re–1, and e/(Nq) ∝ Re. These formulas were theoretically derived in our previous work, using turbulence theory, in particular, using turbulence spectrum analysis. The correctness of the proposed relations is investigated by statistical hypothesis testing.

Local turbulent energy dissipation rate in an agitated vessel: New approach to dimensionless definition

Using theory of turbulence, particularly using turbulence spectrum analysis, the relations e* = e/(u 4/ν) = const., vK/u = const. and Λ/ηK = const. were derived. Assuming that u ∝ (Nd) from this it follows that the widely used dimensionless local turbulent energy dissipation rate defined as e/((N 3 d 2) is directly proportional to impeller Reynolds number, i.e. e/((N 3 d 2) ∝ Re, and length scale ratio Λ/d is indirectly proportional to impeller Reynolds number, i.e. Λ/d ∝ Re–1, in an agitated vessel at high Reynolds number. The relations obtained by turbulence spectrum analysis were used for estimation of local turbulent energy dissipation rates experimentally measured by Stahl Wernersson and Tragardh (1998, 1999) covering the range of Re = 87600–910200 and own experimental data covering the range of Re = 50000–189000. The experiments have been performed in tanks of 300 mm and 400 mm in the inner diameter for three different viscosities and for various impeller rotational speeds.

Determination of the flow field inside a Sonolator liquid whistle using PIV and CFD

PIV experiments and CFD simulations were carried out on a mimic of a pilot scale Sonolator Model A inline liquid whistle mixer (Sonic Corp. USA) for water in turbulent flow for Reynolds numbers at the orifice between 17,500 and 77,200. Three different sizes of orifice were used. The results from PIV were compared with the CFD simulations, with both global and local validations being performed. The former focusses on the pressure drop across the Sonolator and the latter was carried out by comparison of local values of velocity magnitude, turbulent kinetic energy and local specific turbulent energy dissipation rate. Velocity magnitude values were found to agree within 10% more than fifteen millimetres downstream of the orifice. A similar level of agreement was found at the orifice for lower flow rates and larger orifices. Factors which precluded this level of agreement for higher flow rates and smaller orifices were the appearance of cavitation and a minimum achievable laser pulse separation, limiting the maximum velocity measureable by the PIV. Agreement between PIV and CFD was also poorer for the turbulent parameters, although the PIV and CFD data had similar trends, the magnitudes were different. The reasons for these discrepancies include the fact that the oscillation period for the orifice jet could not be precisely identified and eliminated from the data and errors inherent in the methods used to estimate the local specific turbulent energy dissipation rate from the PIV data.

Evolutionary power spectral density analysis on the wind-induced buffeting responses of Sutong Bridge during Typhoon Haikui

Recent field measurements on long-span bridges during typhoon events have captured strong nonstationary features in the buffeting responses. In this study, the buffeting responses of Sutong Bridge during Typhoon Haikui in 2012 recorded by structural health monitoring system are analyzed to represent the nonstationary characteristics. As an accurately measured state variable, the acceleration response of the main girder is first selected to evaluate its own original stationarity in different time intervals using the run test method. The acceleration response of the main girder can be regarded as a zero-mean nonstationary random process which is in demand to extract its transient features in time–frequency domain. Hence, the evolutionary power spectral density (EPSD) of the acceleration responses, which can present the local turbulence energy distribution in both frequency and time domains, is estimated using the wavelet-based method. Also, an average wavelet spectrum is obtained by averaging the square values of wavelet coefficients along the time axis, and the comparison between the average wavelet spectrum and Fourier spectrum shows a great conformance which indirectly verifies the validity of the obtained evolutionary power spectral density. The results of this study exhibit that there are strong nonstationary characteristics existing in the buffeting responses of Sutong Bridge during Typhoon Haikui, and it is essential to incorporate the nonstationary features of winds in the analysis or design of long-span bridges from an aerodynamic viewpoint.

Measurements and analysis of non-stationary wind characteristics at Sutong Bridge in Typhoon Damrey

Recent measurements of the wind field during extreme wind events (tropical and extratropical storms) indicate that the wind speed fluctuations exhibit strong non-stationary features echoing similar observations reported at other locations. In this study, the wind data of Typhoon Damrey recorded by the Structural Health Monitoring System (SHMS) at the Sutong Bridge were analyzed to examine the non-stationary features. Following stationarity test, the wind data were analyzed by both the stationary and non-stationary wind speed models utilizing conventional time- or frequency-domain as well from the time–frequency perspective. Wind characteristics, e.g., the mean wind speed, turbulence intensity, gust factor and power spectral density including the evolutionary power spectral density (EPSD) of the wind were evaluated. The EPSD, which represents the distribution of the local turbulent energy in both time and frequency domains, was estimated using a wavelet-based scheme. The results of this study exhibit strong non-stationary characteristics in the measured wind data at the Sutong Bridge site. The findings of the data at the Sutong Bridge in the tropical storm reaffirm results from earlier studies and emphasize the need to incorporate non-stationary features of winds in the analysis and design of bridges from aerodynamic view point.

Local Turbulent Energy Dissipation Rate in a Vessel Agitated by a Rushton Turbine

Abstract The scaling of turbulence characteristics such as turbulent fluctuation velocity, turbulent kinetic energy and turbulent energy dissipation rate was investigated in a mechanically agitated vessel 300 mm in inner diameter stirred by a Rushton turbine at high Reynolds numbers in the range 50 000 < Re < 100 000. The hydrodynamics and flow field was measured using 2-D TR PIV. The convective velocity formulas proposed by Antonia et al. (1980) and Van Doorn (1981) were tested. The turbulent energy dissipation rate estimated independently in both radial and axial directions using the one-dimensional approach was not found to be the same in each direction. Using the proposed correction, the values in both directions were found to be close to each other. The relation ε/(N3·D2) ∞ const. was not conclusively confirmed.

Estimating the local turbulent energy dissipation rate using 2-D PIV measurements and a 1-D energy spectrum function

A simple method for estimating the local turbulent energy dissipation rate for a non-isotropic state is proposed. The proposed method is based on an estimation of the isotropic dissipation rate under the assumption of local isotropy, which is subsequently corrected for the actual conditions. The level of anisotropy is characterized by the fluctuation velocity components. The isotropic dissipation rates are obtained by energy spectrum function fitting. A data block averaging technique is used to smooth the spectrum. The effect of the data number within block on the calculated turbulent energy dissipation rate is taken into account. The proposed method has been tested on data obtained by a 2-D time-resolved PIV method. The effect of the spatial resolution of PIV on the estimation of the dissipation rate is also taken into account, using the correction proposed by Delafosse et al. (2011). The estimate of the local turbulent energy dissipation was found to be the same irrespective of the fluctuation velocity component that was taken for calculating the energy spectrum. This accords with the scalar character of the dissipation rate. The method also enables an estimate to be made of the integral length scale components.

Estimating the Local Turbulent Energy Dissipation Rate Using 2-D PIV Measurements and a 1-D Energy Spectrum Function

Estimating the Local Turbulent Energy Dissipation Rate Using 2-D PIV Measurements and a 1-D Energy Spectrum Function

Afforestation in China cools local land surface temperature

China has the largest afforested area in the world (∼62 million hectares in 2008), and these forests are carbon sinks. The climatic effect of these new forests depends on how radiant and turbulent energy fluxes over these plantations modify surface temperature. For instance, a lower albedo may cause warming, which negates the climatic benefits of carbon sequestration. Here, we used satellite measurements of land surface temperature (LST) from planted forests and adjacent grasslands or croplands in China to understand how afforestation affects LST. Afforestation is found to decrease daytime LST by about 1.1 ± 0.5 °C (mean ± 1 SD) and to increase nighttime LST by about 0.2 ± 0.5 °C, on average. The observed daytime cooling is a result of increased evapotranspiration. The nighttime warming is found to increase with latitude and decrease with average rainfall. Afforestation in dry regions therefore leads to net warming, as daytime cooling is offset by nighttime warming. Thus, it is necessary to carefully consider where to plant trees to realize potential climatic benefits in future afforestation projects.

CFD simulations and particle image velocimetry measurements in an industrial scale rotating disc contactor

AbstractFluid dynamics of the single‐phase and two‐phase flow in a segment of a rotating disc contactor (RDC) liquid–liquid extraction column with 450 mm inner diameter were studied by performing computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) measurements. The fluid dynamics were investigated to test the predictivity of CFD at industrial scale. Different turbulence models in conjunction with the Eulerian approach were applied in the single‐phase and two‐phase simulations. The turbulent flow characteristics were analyzed by PIV measurements to validate the CFD simulations. An iso‐optical system composed of CaCl2/water–butylacetate allows for the two‐phase PIV measurements. Local turbulent energy dissipation was derived from velocity gradients in PIV data. In this connection, the influence of the PIV spatial resolution on the measured energy dissipation was also analyzed, and different fit functions were tested to scale the measured energy dissipation. Simulated velocity fields as well as the energy dissipation were compared with the experimental PIV data. The results from the simulations and experiments are in good agreement. The work shows that CFD can predict hydrodynamic characteristics even at bigger scales but is still subject to some minor restrictions. © 2010 American Institute of Chemical Engineers AIChE J, 2011