scholarly journals Turbulent transport mechanisms in oscillating bubble plumes

2009 ◽  
Vol 633 ◽  
pp. 191-231 ◽  
Author(s):  
MARCO SIMIANO ◽  
D. LAKEHAL ◽  
M. LANCE ◽  
G. YADIGAROGLU
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Large Scale ◽  
Small Scale ◽  
Equivalent Diameter ◽  
Minor Effect ◽  
Reynolds Stresses ◽  
Bubble Plume ◽  
Low Velocity ◽  
Cross Sectional

The detailed investigation of an unstable meandering bubble plume created in a 2-m-diameter vessel with a water depth of 1.5 m is reported for void fractions up to 4% and bubble size of the order of 2.5 mm. Simultaneous particle image velocity (PIV) measurements of bubble and liquid velocities and video recordings of the projection of the plume on two vertical perpendicular planes were produced in order to characterize the state of the plume by the location of its centreline and its equivalent diameter. The data were conditionally ensemble averaged using only PIV sets corresponding to plume states in a range as narrow as possible, separating the small-scale fluctuations of the flow from the large-scale motions, namely plume meandering and instantaneous cross-sectional area fluctuations. Meandering produces an apparent spreading of the average plume velocity and void fraction profiles that were shown to remain self-similar in the instantaneous plume cross-section. Differences between the true local time-average relative velocities and the difference of the averaged phase velocities were measured; the complex variation of the relative velocity was explained by the effects of passing vortices and by the fact that the bubbles do not reach an equilibrium velocity as they migrate radially, producing momentum exchanges between high- and low-velocity regions. Local entrainment effects decrease with larger plume diameters, contradicting the classical dependence of entrainment on the time-averaged plume diameter. Small plume diameters tend to trigger ‘entrainment eddies’ that promote the inward-flow motion. The global turbulent kinetic energy was found to be dominated by the vertical stresses. Conditional averages according to the plume diameter showed that the large-scale motions did not affect the instantaneous turbulent kinetic energy distribution in the plume, suggesting that large scales and small scales are not correlated. With conditional averaging, meandering was a minor effect on the global kinetic energy and the Reynolds stresses. In contrast, plume diameter fluctuations produce a substantial effect on these quantities.

Large- and small-scale vortical motions in a shear layer perturbed by tabs

1999 ◽  
Vol 382 ◽  
pp. 307-329 ◽  
Author(s):  
JUDITH K. FOSS ◽  
K. B. M. Q. ZAMAN
Keyword(s):  
Shear Layer ◽  
Pitch Angle ◽  
Large Scale ◽  
Dimensional Space ◽  
Three Dimensional ◽  
Small Scale ◽  
Streamwise Vortices ◽  
Streamwise Vorticity ◽  
Cross Sectional ◽  
Scale Population

The large- and small-scale vortical motions produced by ‘delta tabs’ in a two-stream shear layer have been studied experimentally. An increase in mixing was observed when the base of the triangular shaped tab was affixed to the trailing edge of the splitter plate and the apex was pitched at some angle with respect to the flow axis. Such an arrangement produced a pair of counter-rotating streamwise vortices. Hot-wire measurements detailed the velocity, time-averaged vorticity (Ωx) and small-scale turbulence features in the three-dimensional space downstream of the tabs. The small-scale structures, whose scale corresponds to that of the peak in the dissipation spectrum, were identified and counted using the peak-valley-counting technique. The optimal pitch angle, θ, for a single tab and the optimal spanwise spacing, S, for a multiple tab array were identified. Since the goal was to increase mixing, the optimal tab configuration was determined from two properties of the flow field: (i) the large-scale motions with the maximum Ωx, and (ii) the largest number of small-scale motions in a given time period. The peak streamwise vorticity magnitude [mid ]Ωx−max[mid ] was found to have a unique relationship with the tab pitch angle. Furthermore, for all cases examined, the overall small-scale population was found to correlate directly with [mid ]Ωx−max[mid ]. Both quantities peaked at θ≈±45°. It is interesting to note that the peak magnitude of the corresponding circulation in the cross-sectional plane occurred for θ≈±90°. For an array of tabs, the two quantities also depended on the tab spacing. An array of contiguous tabs acted as a solid deflector producing the weakest streamwise vortices and the least small-scale population. For the measurement range covered, the optimal spacing was found to be S≈1.5 tab widths.

scholarly journals HONEYBEE PRODUCTION SYSTEM, CHALLENGES AND OPPORTUNITIES IN SELECTED DISTRICTS OF GEDEO ZONE, SOUTHERN NATION, NATIONALITIES AND PEOPLES REGIONAL STATE, ETHIOPIA

2016 ◽  
Vol 4 (4) ◽  
pp. 49-63
Author(s):  
Teklu Gebretsadik ◽  
Dinku Negash
Keyword(s):  
Production System ◽  
Large Scale ◽  
Market Access ◽  
Production Systems ◽  
Skill Training ◽  
Cross Sectional Study ◽  
Small Scale ◽  
Cross Sectional ◽  
Production Constraints ◽  
Regional State

The study was carried out in selected districts of Gedeo zones of southern Nation nationality and people’s regional state: such as Wonago, Kochere and DillaZuria/chichu/ districts. The objective of the study was to asses production systems, opportunities and constraints of apiculture farming in Gedeo zones of SNNPRs. Beekeeping is a long-standing practice in the study districts and appears as ancient history of the country as a whole. A cross sectional study, in which 90 households were purposively included and conducted in selected district to assess the current beekeeping practices, production potentials and production constraints. Most (72%) of the beekeepers in the study area have owned only traditional hives and produce honey for home consumption. The beekeeping practice was dominated by male.  Despite the area have  production constraints, the area opportunities like existence of large sized natural forest and artificial forest, due attention provision from regional, federal and local government and nongovernmental organizations, yearly flowering and variety floral availability, some small scale farmers highly experienced in apiculture farming, market access and high demand of apicultural products at nationally and international level. Therefore, Designing effective honeybee pests and predators controlling methods, Introduction of full package improved beekeeping technologies with adequate practical skill training on all bee keeping trends and queen rearing practices promoting beekeepers important indigenous knowledge, Producing areal major honeybee plants in large scale, Avoiding discarding of bee colonies after honey harvest, Availing the strategies to support farmers with beekeeping business support services, Improving pre- and post-harvest handling of bee products and Improving the utilization of stingless bee’s resources, without damaging the colony is important to make the honey production system in the area more economical, so as to enhance bee products for national and international need.

Large-scale modes of turbulent channel flow: transport and structure

2001 ◽  
Vol 448 ◽  
pp. 53-80 ◽  
Author(s):  
Z. LIU ◽  
R. J. ADRIAN ◽  
T. J. HANRATTY
Keyword(s):  
Shear Stress ◽  
Kinetic Energy ◽  
Shear Layer ◽  
Turbulent Kinetic Energy ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Reynolds Numbers ◽  
Upstream Region ◽  
Two Dimensional ◽  
Normal Plane

Turbulent flow in a rectangular channel is investigated to determine the scale and pattern of the eddies that contribute most to the total turbulent kinetic energy and the Reynolds shear stress. Instantaneous, two-dimensional particle image velocimeter measurements in the streamwise-wall-normal plane at Reynolds numbers Reh = 5378 and 29 935 are used to form two-point spatial correlation functions, from which the proper orthogonal modes are determined. Large-scale motions – having length scales of the order of the channel width and represented by a small set of low-order eigenmodes – contain a large fraction of the kinetic energy of the streamwise velocity component and a small fraction of the kinetic energy of the wall-normal velocities. Surprisingly, the set of large-scale modes that contains half of the total turbulent kinetic energy in the channel, also contains two-thirds to three-quarters of the total Reynolds shear stress in the outer region. Thus, it is the large-scale motions, rather than the main turbulent motions, that dominate turbulent transport in all parts of the channel except the buffer layer. Samples of the large-scale structures associated with the dominant eigenfunctions are found by projecting individual realizations onto the dominant modes. In the streamwise wall-normal plane their patterns often consist of an inclined region of second quadrant vectors separated from an upstream region of fourth quadrant vectors by a stagnation point/shear layer. The inclined Q4/shear layer/Q2 region of the largest motions extends beyond the centreline of the channel and lies under a region of fluid that rotates about the spanwise direction. This pattern is very similar to the signature of a hairpin vortex. Reynolds number similarity of the large structures is demonstrated, approximately, by comparing the two-dimensional correlation coefficients and the eigenvalues of the different modes at the two Reynolds numbers.

Evaluation of Turbulent Kinetic Energy Dissipation Rate in a Free Jet Flow from PIV Measurements

2012 ◽  
Vol 7 (1) ◽  
pp. 53-69
Author(s):  
Vladimir Dulin ◽  
Yuriy Kozorezov ◽  
Dmitriy Markovich
Keyword(s):  
Energy Dissipation ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Dissipation Rate ◽  
Energy Dissipation Rate ◽  
Turbulent Velocity ◽  
Small Scale ◽  
Velocity Fluctuations ◽  
Piv Measurements ◽  
Free Jet

The present paper reports PIV (Particle Image Velocimetry) measurements of turbulent velocity fluctuations statistics in development region of an axisymmetric free jet (Re = 28 000). To minimize measurement uncertainty, adaptive calibration, image processing and data post-processing algorithms were utilized. On the basis of theoretical analysis and direct measurements, the paper discusses effect of PIV spatial resolution on measured statistical characteristics of turbulent fluctuations. Underestimation of the second-order moments of velocity derivatives and of the turbulent kinetic energy dissipation rate due to a finite size of PIV interrogation area and finite thickness of laser sheet was analyzed from model spectra of turbulent velocity fluctuations. The results are in a good agreement with the measured experimental data. The paper also describes performance of possible ways to account for unresolved small-scale velocity fluctuations in PIV measurements of the dissipation rate. In particular, a turbulent viscosity model can be efficiently used to account for the unresolved pulsations in a free turbulent flow

scholarly journals Neurobehavioural symptoms and acute pesticide poisoning: a cross-sectional study among male pesticide applicators selected from three commercial farming systems in Ethiopia

2018 ◽  
Vol 75 (4) ◽  
pp. 283-289 ◽  
Author(s):  
Beyene Negatu ◽  
Roel Vermeulen ◽  
Yalemtshay Mekonnen ◽  
Hans Kromhout
Keyword(s):  
Large Scale ◽  
Pesticide Exposure ◽  
Farming Systems ◽  
Cumulative Exposure ◽  
Small Scale ◽  
Pesticide Poisoning ◽  
Cross Sectional Survey ◽  
Cross Sectional ◽  
Pesticide Applicators ◽  
Acute Pesticide Poisoning

ObjectiveTo estimate prevalence of acute pesticide poisoning (APP) and its association with neurobehavioural symptoms in Ethiopian pesticide applicators.MethodsWe performed a cross-sectional survey among 256 pesticide applicators from small-scale irrigated farms, a large-scale open farm and large-scale greenhouses. APP was ascertained using a modified WHO case definition, and neurobehavioural symptoms were collected with a standardised questionnaire (Q16). Exposure to pesticides was estimated using detailed exposure algorithms specifically developed for Ethiopian farms. Multiple logistic regression models were used to estimate risk of APP and its association with neurobehavioural symptoms.ResultsOverall APP prevalence was 16%. Working as an applicator in greenhouses was strongly associated with APP (OR 3.00, 95% CI 1.38 to 6.54). Estimated annual pesticide exposure was also associated with APP (OR 1.14, 95% CI 1.00 to 1.20). Longer duration of employment appeared to be negatively associated with APP. Having had an APP was strongly associated with reporting more neurobehavioural symptoms (OR 2.15, 95% CI 1.01 to 4.58) independent of cumulative pesticide exposure. Cumulative exposure to pesticides appeared to be associated with neurobehavioural symptoms among applicators without and with APP.ConclusionsWe showed a substantial prevalence of APP that differed between farming systems and was strongly associated with neurobehavioural symptoms. Intensity of exposure was also clearly associated with these symptoms. Reduction and control of occupational exposure to pesticides is urgently needed in Ethiopia.

scholarly journals Assessing the turbulent kinetic energy budget in an energetic tidal flow from measurements of coupled ADCPs

2020 ◽  
Vol 378 (2178) ◽  
pp. 20190496 ◽  
Author(s):  
Maxime Thiébaut ◽  
Jean-François Filipot ◽  
Christophe Maisondieu ◽  
Guillaume Damblans ◽  
Rui Duarte ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Production Rate ◽  
Tidal Flow ◽  
Tidal Energy ◽  
Reynolds Stresses ◽  
Common Assumption ◽  
Non Local ◽  
Tke Dissipation Rate ◽  
Acoustic Doppler Current Profilers

Two coupled four-beam acoustic Doppler current profilers were used to provide simultaneous and independent measurements of the turbulent kinetic energy (TKE) dissipation rate ε and the TKE production rate P over a 36 h long period at a highly energetic tidal energy site in the Alderney Race. The eight-beam arrangement enabled the evaluation of the six components of the Reynolds stress tensor which allows for an improved estimation of the TKE production rate. Depth-time series of ε, P and the Reynolds stresses are provided. The comparison between ε and P was performed by calculating individual ratios of ε corresponding to P . The depth-averaged ratio ε / P averaged over whole flood and ebb tide were found to be 2.2 and 2.8 respectively, indicating that TKE dissipation exceeds TKE production. It is shown that the term of diffusive transport of TKE is significant. As a result, non-local transport is important to the TKE budget and the common assumption of a local balance, i.e. a balance between production and dissipation, is not valid at the measurement site. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

scholarly journals Diffusional and accretional growth of water drops in a rising adiabatic parcel: effects of the turbulent collision kernel

2009 ◽  
Vol 9 (7) ◽  
pp. 2335-2353 ◽  
Author(s):  
W. W. Grabowski ◽  
L.-P. Wang
Keyword(s):  
Energy Dissipation ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Collision Kernel ◽  
Small Scale ◽  
Large Set ◽  
Initiation Time ◽  
Cloud Droplets ◽  
Dissipation Rates ◽  
Speedup Factor

Abstract. A large set of rising adiabatic parcel simulations is executed to investigate the combined diffusional and accretional growth of cloud droplets in maritime and continental conditions, and to assess the impact of enhanced droplet collisions due to small-scale cloud turbulence. The microphysical model applies the droplet number density function to represent spectral evolution of cloud and rain/drizzle drops, and various numbers of bins in the numerical implementation, ranging from 40 to 320. Simulations are performed applying two traditional gravitational collection kernels and two kernels representing collisions of cloud droplets in the turbulent environment, with turbulent kinetic energy dissipation rates of 100 and 400 cm2 s−3. The overall result is that the rain initiation time significantly depends on the number of bins used, with earlier initiation of rain when the number of bins is low. This is explained as a combination of the increase of the width of activated droplet spectrum and enhanced numerical spreading of the spectrum during diffusional and collisional growth when the number of model bins is low. Simulations applying around 300 bins seem to produce rain at times which no longer depend on the number of bins, but the activation spectra are unrealistically narrow. These results call for an improved representation of droplet activation in numerical models of the type used in this study. Despite the numerical effects that impact the rain initiation time in different simulations, the turbulent speedup factor, the ratio of the rain initiation time for the turbulent collection kernel and the corresponding time for the gravitational kernel, is approximately independent of aerosol characteristics, parcel vertical velocity, and the number of bins used in the numerical model. The turbulent speedup factor is in the range 0.75–0.85 and 0.60–0.75 for the turbulent kinetic energy dissipation rates of 100 and 400 cm2 s−3, respectively.

Turbulence Measurements from Five-Beam Acoustic Doppler Current Profilers

2017 ◽  
Vol 34 (6) ◽  
pp. 1267-1284 ◽  
Author(s):  
Maricarmen Guerra ◽  
Jim Thomson
Keyword(s):  
Kinetic Energy ◽  
Structure Function ◽  
Turbulent Kinetic Energy ◽  
Puget Sound ◽  
Inertial Subrange ◽  
Turbulence Structure ◽  
Reynolds Stresses ◽  
Frequency Spectra ◽  
Turbulence Data ◽  
Acoustic Doppler Current Profilers

AbstractTwo new five-beam acoustic Doppler current profilers—the Nortek Signature1000 AD2CP and the Teledyne RDI Sentinel V50—are demonstrated to measure turbulence at two energetic tidal channels within Puget Sound, Washington. The quality of the raw data is tested by analyzing the turbulent kinetic energy frequency spectra, the turbulence spatial structure function, the shear in the profiles, and the covariance Reynolds stresses. The five-beam configuration allows for a direct estimation of the Reynolds stresses from along-beam velocity fluctuations. The Nortek’s low Doppler noise and high sampling frequency allow for the observation of the turbulent inertial subrange in both the frequency spectra and the turbulence structure function. The turbulence parameters obtained from the five-beam acoustic Doppler current profilers are validated with turbulence data from simultaneous measurements with acoustic Doppler velocimeters. These combined results are then used to assess a turbulent kinetic energy budget in which depth profiles of the turbulent kinetic energy dissipation and production rates are compared. The associated codes are publicly available on the MATLAB File Exchange website.

Groove-induced changes of discharge in channel flows

2016 ◽  
Vol 799 ◽  
pp. 297-333 ◽  
Author(s):  
Yu Chen ◽  
J. M. Floryan ◽  
Y. T. Chew ◽  
B. C. Khoo
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Flow Regime ◽  
Turbulent Flows ◽  
Arbitrary Form ◽  
Reynolds Stresses ◽  
Bulk Fluid ◽  
Geometry Model ◽  
Smooth Channel ◽  
Channel Configuration

The changes in discharge in pressure-driven flows through channels with longitudinal grooves have been investigated in the laminar flow regime and in the turbulent flow regime with moderate Reynolds numbers ($Re_{2H}\approx 6000$) using both analytical and numerical methodologies. The results demonstrate that the long-wavelength grooves can increase discharge by 20 %–150 %, depending on the groove amplitude and the type of flow, while the short-wavelength grooves reduce the discharge. It has been shown that the reduced geometry model applies to the analysis of turbulent flows and the performance of grooves of arbitrary form is well approximated by the performance of grooves whose shape is represented by the dominant Fourier mode. The flow patterns, the turbulent kinetic energy as well as the Reynolds stresses were examined to identify the mechanisms leading to an increase in discharge. It is shown that the increase in discharge results from the rearrangement of the bulk fluid movement and not from the suppression of turbulence intensity. The turbulent kinetic energy and the Reynolds stresses are rearranged while their volume-averaged intensities remain the same as in the smooth channel. Analysis of the interaction of the groove patterns on both walls demonstrates that the converging–diverging configuration results in the greatest increase in discharge while the wavy channel configuration results in a reduction in discharge.

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