Effect Of Different Flow Regimes Research Articles

Respiration and CO2 evasion dynamics in moving streambeds as a response to flow regimes

Streams and rivers are subject to discharge fluctuations that may be natural or man-made, which influence biogeochemical processes and the release of CO2 to the atmosphere. Fluctuations in discharge are also associated with changing streamwater velocity that directly affects the movement of bedforms in sandy streambeds. However, bedform movement is rarely considered when studying effects of flow dynamics on biogeochemical processes. We used planar optodes during moving-bed flume experiments to quantify the effects of different flow regimes on aerobic respiration dynamics and the subsequent release of CO2. The flow regimes included constant flow (C), sinusoidal flow (S), and single peak flow events (F). Flow regimes substantially influence aerobic respiration in the streambed and, hence, the distribution of O2 and CO2 in the streambed. Hyporheic exchange flux (HEF), bedform celerity, and respiration rates increased non-linearly with streamwater velocity. It was found that HEF and CO2 evasion were highest under dynamic flow regimes S and F. However, respiration rates, the total O2 consumed, and the total CO2 produced were highest under flow regime C. This is due to low reaction rates once the streambed became stationary in the low discharge periods of flow regimes S and F. Thus, the distribution of celerities controls total respiration. Our results highlight that substantial aerobic respiration occurs even in slow-moving streambeds and that it is important to consider bedform movement when studying biogeochemical reactions in the hyporheic zone or predicting greenhouse gas emissions from catchments.

Study on the Flow Characteristics of Heavy Oil-Water Two-Phase Flow in the Horizontal Pipeline.

Aiming at the problem of transportation for heavy oil during the middle-later development stages of the Lvda oilfield, based on the self-developed design of a visual circulating flow experimental apparatus for heavy oil-water two-phase flow-the flow regime characteristics and corresponding drag properties of the two-phase flow of Lvda viscous oil, which is simulated by 500# industrial white oil and water in a horizontal pipeline are investigated experimentally. According to the Kelvin-Helmholtz instability theory, the flow pattern transition criteria from stratified flow to annular flow (AF) are proposed. The effects of 0.11-0.90 m/s oil superficial velocities, 0.06-1.49 m/s water superficial velocities, and 0.09-0.93 input water cuts on the drag reduction effect of different flow regimes are analyzed. The experimental results indicated that with the increase of mixing velocity and water volume fraction, stratified flow, AF, oil plug flow, and dispersed oil lump flow are successively observed in the horizontal heavy oil-water two-phase flow, in which AF is the main flow pattern. As the Froude number increases to 4.0, the input water volume fraction does not change any more and remains at about 10% of the total flow rate in the process of converting from stratified flow to AF. The four delivery approaches can archive the reduction of transportation resistance for heavy oil at different degrees, in which the transportation of heavy oil surrounded by a water ring has the best effect of drag reduction. At the optimal working conditions of 0.61 m/s oil superficial velocity, 0.07 m/s water superficial velocity, and 0.10 input water cut, the pressure drop of water annulus conveying for heavy oil is only 1/62.54 of that of separate transport for pure heavy oil under the same oil flow rate.

Sediment Transport Capacity in a Gravel-Bed River with a Sandy Tributary

Bedload transport in a river is a deeply analyzed problem, with many methodologies available in the literature. However, most of the existing methods were developed for reaches of rivers rather than for confluences and are suitable for a particular type of material, which makes them very inaccurate in cases where the sediments are comprised of a mix of different types of soil. This study considers the effect of two different bed sediment sizes, gravel and sand, in relation to bed load transport in a confluence. Five well-known and validated equations (namely Meyer-Peter and Müller, Parker + Engelund and Hansen, Ackers and White, and Yang) are applied to the case study of the Tagus–Alberche rivers confluence (in Talavera de la Reina, Spain), where main and tributary rivers transport different materials (sand and gravel). Field works in the area of the confluence were conducted, and a set of alluvial samples were collected and analyzed. The previously mentioned methods were employed to analyze the geomorphology in the confluence area and downstream of it under different flooding scenarios, concluding different trends in terms of deposition/erosion in the area under historic flooding scenarios. When the trends show erosion, all methods are very consistent in terms of numerical predictions. However, the results present high disparity in the estimated values when the predictions suggest deposition, with Parker + Engelund and Hansen yielding the highest volumes and Meyer-Peter and Müller the lowest (the latter being around 1% of the former). Yang and Ackers and White predict deposits in the same range in all cases (around 15% of Parker and Engelund Hansen). Yang’s formula was found to be suitable for the confluences of rivers with different materials, allowing for the estimation of sediment transport for different grain sizes. The effect of different flow regimes has been analyzed with the application of Yang’s formula to the Tagus-Alberche confluence.

Numerical investigation of the vapor bubble's scenarios passing through aerophobic/aerophilic porous structures using lattice Boltzmann method

Numerical investigation of bubbly flow throughout an anisotropic porous medium and the related two-phase phenomena is carried out, to provide a profound insight into the probabilities of transmitting or accumulating vapor bubbles within porous structures. The lattice Boltzmann method is used in the analysis. The effects of different flow regimes, contact angles, and porosity on a single rising bubble dynamic within a porous medium are assessed. The probability of different scenarios such as pinch-off, vapor jacket or vapor blanket at different flow regimes and conditions are considered of multiple rising bubbles. Generally, obstruction, coalescence, separation and deformation of vapor bubbles are claimed to be tightly dependent on the interaction of buoyancy, adhesion, drag and surface tension. Also, it is found that at high Eotvos (EO) number, the porous medium shows a little tendency to transmit the vapor bubbles, while at the moderate EO number, especially with aerophobic walls, the bubbles pass through the porous medium without any disturbance. Besides, the low EO number is claimed to be an inappropriate condition because of the presence of accumulated vapor bubbles within the porous medium, which is well-known as a vapor blanket phenomenon.

Numerical investigation of the flow regimes through hydraulic fractures using the X-FEM technique

In this paper, an extended finite element formulation is proposed in order to investigate the effects of different flow regimes of the fracturing fluid on the hydraulic fracturing process. To this end, the inflow and continuity equations of the fracturing fluid through the hydro-fracture are solved in conjunction with the momentum balance equation of the bulk in a sequential manner known as the staggered Newton method. The Reynolds number is applied in order to recognize the development of the laminar or turbulent flow regimes along the flow path line. The loss in energy head due to friction along the hydro-fracture faces is incorporated using the Darcy-Weisbach equation, which in turn leads to the introduction of a specific profile for the fluid pressure associated with each flow regime. At the end, through several numerical simulations the robustness of the developed framework is illustrated, and its significance in the study of hydraulic fracturing problem is demonstrated.

Effect of different flow regimes on free convection heat transfer from isothermal convex bodies over all range of Rayleigh and Prandtl numbers

In the present study, effect of different flow regimes on free convection heat transfer has been examined. In the light of this, a novel analytical method is developed to calculate free convection heat transfer from isothermal convex bodies with arbitrary shape over all range of Rayleigh number in fluids with any Prandtl number. The crux of this method is based on the concept of dynamic behaviors existing in natural convection flow. In the previous models the Body Gravity Function (BGF) and Turbulent Function (TF) have been taken as constant values. In this study, BGF accounts for the effect of body shape and orientation with respect to gravity vector in laminar free convection. Besides, TF accounts for the impact of Prandtl number, body shape and orientation with regard to gravity vector in turbulent free convection. By contrast, it is shown that these two parameters undergo a change through the variation of Rayleigh number and cannot be considered as a constant. These two parameters are modeled based upon the thermal resistance concept. Moreover, two transition criteria happening in free convection heat transfer will be obtained according to this new analytical method (conduction–laminar and laminar–turbulent transitions). Finally, three models (models 1, 2 and 3) are proposed for calculation free convection heat transfer and present results for ten isothermal convex bodies with various aspect ratios ( $$0.298 \le \frac{\sqrt A }{P} \le 2.470$$ ) have been compared with the available experimental and numerical data. Here, the results of model 2 are almost equal to those of model 3. Also, the results of model 1 are more precise than those of model 3 while the parameters computation of model 1 is more intricate in comparison with model 3. On the one hand, the model 1 has an average difference <6 % vis-a-vis numerical data in entire range of Rayleigh number (laminar and turbulent). On the other hand, the average difference of model 1 is not more than 8 % versus experimental data in complete range of Rayleigh number. Outstanding agreement exhibits that the proposed model has a great potential to predict free convection heat transfer from isothermal convex bodies in the whole range of Rayleigh numbers in fluids with any Prandtl number.

Fully-Developed Flow in a Furrowed Wavy Channel: Characterization of Unsteady Flow Regimes and Its Effect on Thermal-Hydraulic Performance

Numerical study is done at various Reynolds numbers (100–2000), for periodically fully developed flow and heat transfer in five geometrically different wavy channels. Time signal analysis is done to distinguish various unsteady flow regimes and a flow regime map is proposed; demarcating steady, two types of periodic as well as quasi-periodic, and chaotic flow regimes. Temporal variation of flow structure and Nusselt number as well as friction factor are presented – in the various unsteady regimes – and discussed for a unified cause-and-effect study. Effect of different flow regimes on the thermal-hydraulic performance of wavy as compared to plane channel is presented. Effect of Prandtl numbers (0.01–100) is also studied.

Blade–granule bed stress in a cylindrical high shear granulator: I—Online measurement and characterisation

For high shear granulators, the rotating impeller blades exert forces on the granules, which are subsequently transmitted throughout the granule bed through inter-granule collisions. The effect of these collisional stresses on the granule growth behaviour is varied; as the granules could deform, consolidate and/or break under stress. This paper focuses on the normal stress between the blade and the granule bed in a cylindrical high shear granulator, which was directly measured using a custom-built telemetric impeller pressure sensor system. The stresses, evaluated at different operating conditions (impeller speed) and granule bed attributes (bed load, granule size and granule/particle type) were also fitted to a stress model derived based on the inertial force transferred from the blade to a continuous, cohesionless granule bed block. For the granule bed velocity and height parameters required in the model, the surface velocity of the bed at the sensor region and the bed height near at the wall, measured by high speed imaging, were used. A surface velocity corrected blade–granule bed stress model to account for the use of surface velocity and the effect of different flow regimes enabled much improved prediction of the stress for all types of dry granule beds used in this study. The effects of some individual granule/particle properties such as size, shape and strength on the stress were also negligible provided that the bulk flow behaviour is similar.

The effect of different flow regimes on the growth and metabolic rates of the scleractinian coral Galaxea fascicularis

To study the effect of water flow on coral growth, four series of ten coral nubbins of Galaxea fascicularis were exposed to four different flow regimes (0, 10, 20, and 25 cm s−1, bidirectional flow) for 42 weeks. Buoyant weight, surface area, and polyp number were measured at regular intervals. Net photosynthesis and dark respiration were measured at the corresponding flow speeds, and daily amount of photosynthetic carbon left for coral growth was calculated. Finally, skeletal density and CN content, chlorophyll concentration and dry weight of coral tissue were determined for each coral. Specific growth rate (in day−1) decreased with time in each flow treatment. Absence of flow resulted in significantly lower growth rates. Average specific growth rate calculated over the entire experiment was not significantly different between 10 and 20 cm s−1, while it was significantly higher at 25 cm s−1. From 10 to 25 cm s−1, average net photosynthetic rate decreased and average dark respiration rate did not change significantly. Scope for growth based on phototrophic carbon decreased with increasing flow. Growth was not positively correlated with either photosynthesis or respiration, or scope for growth. It is suggested that higher flow rates reduce the chance of disturbance of coral growth by competing algae or cyanobacteria, allowing corals to grow more readily with the maximum specific growth rate possible under the given environmental conditions. Notably, other effects of increased flow, such as increased respiratory rates and increased (in)organic nutrient uptake, might have been equally responsible for the increased growth of the corals in 25 cm s−1.

Influence of river discharge patterns on the hydrodynamics and potential contaminant dispersion in the Douro estuary (Portugal)

Freshwater input to estuaries is a fundamental feature of these ecosystems, which may be profoundly altered by river damming as human needs for water consumption, irrigation or energy production increase. The Douro estuary is limited upstream by a dam since 1985, which reduced its length by ca. 60%. Freshwater inputs to the estuary are now irregular and greatly dependent on hydroelectric power demand; values ranging from zero to over 1000 m 3 s −1, in a matter of hours, especially in summer are common. In the present study, a three-dimensional hydrodynamic model was applied to the Douro estuary. The model was calibrated and validated against water elevation, current velocity, salinity and temperature data. Thereafter, it was used to analyse the effects of different flow regimes and magnitudes on estuarine hydrodynamics and contaminant dispersion. Results obtained suggest that the highly variable flow regimes, currently observed in the Douro, tend to reduce water column stratification and to enhance seawater intrusion, when compared with flow discharges of similar average magnitude, but lower variability. Stable flows seem to be the most effective in dispersing contaminants eventually introduced into the estuary through its small river tributaries. Overall results suggest that flow management may have important effects on estuarine hydrodynamics through non-linear interactions between flow magnitude and variability.

A Spatial Model to Assess the Effects of Hydropower Operations on Columbia River Fall Chinook Salmon Spawning Habitat

AbstractPriest Rapids Dam on the Columbia River produces large daily and hourly streamflow fluctuations throughout the Hanford Reach during the period when fall Chinook salmon Oncorhynchus tshawytscha are selecting spawning habitat, constructing redds, and actively engaged in spawning. Concern over the detrimental effects of these fluctuations prompted us to quantify the effects of variable flows on the amount and persistence of fall Chinook salmon spawning habitat in the Hanford Reach. Specifically, our goal was to develop a management tool capable of quantifying the effects of current and alternative hydrographs on predicted spawning habitat in a spatially explicit manner. Toward this goal, we modeled the water velocities and depths that fall Chinook salmon experienced during the 2004 spawning season, plus what they would probably have experienced under several alternative (i.e., synthetic) hydrographs, using both one‐ and two‐dimensional hydrodynamic models. To estimate spawning habitat under existing or alternative hydrographs, we used cell‐based modeling and logistic regression to construct and compare numerous spatial habitat models. We found that fall Chinook salmon were more likely to spawn at locations where velocities were persistently greater than 1 m/s and in areas where fluctuating water velocities were reduced. Simulations of alternative dam operations indicate that the quantity of spawning habitat is expected to increase as streamflow fluctuations are reduced during the spawning season. The spatial habitat models that we developed provide management agencies with a quantitative tool for predicting, in a spatially explicit manner, the effects of different flow regimes on fall Chinook salmon spawning habitat in the Hanford Reach. In addition to characterizing temporally varying habitat conditions, our research describes an analytical approach that could be applied in other highly variable aquatic systems.

Convective heat transfer from a cylinder in a resonant acoustic field

Experimental measurements of heat transfer rates from a cylinder in a zero-mean oscillatory flow will be presented. The cylinder is representative of a heat exchanger tube, while the oscillatory flow is typical of the strong acoustic field present in a thermoacoustic engine. A steady-state measurement technique has been used to determine the heat transfer coefficients. The effect of different flow regimes on the heat transfer behavior is considered. Correlations for the Nusselt number have been developed in terms of suitable governing dimensionless parameters for the different flow regimes. The goal is to develop a basic knowledge base of working correlations for the Nusselt number which would cover the wide operating parameter space of thermoacoustic engines. [Work supported by the Naval Postgraduate School.]

A Digital Computer Model for Predicting Reactor Coolant Pump Behavior

The development of an improved model to determine the dynamic response of the primary heat transport pump during severe transients in a Canada deuterium uranium reactor is presented. A lumped parameter model is proposed. A control volume formulation is employed for centrifugal pumps. The mathematical formulation of this model is based on mass, momentum, and energy balances as well as on Euler's Theory of Turbomachines. Several constitutive relationships are adopted in the model to describe three-dimensional effects. In addition, the proposed model includes the consequent effect of different flow regimes and the slip between the two phases. Numerical results indicated that the proposed model favorably predicted the pump response and compared well with other pump-related models (Aerojet Nuclear Company) in the literature as well as in the experimental data.