Simple Flow Regime Research Articles

Experimental study of the effect of pipe size on bubbly to beyond-bubbly flow regime transition

The two-phase flow regime and its transition is closely related to the flow channel geometry. In this study, the effect of pipe size on the bubbly-to-beyond-bubbly flow regime transition was experimentally investigated. Experiments focusing on air-water two-phase bubbly to slug flow regime transition using impedance meter and high-speed imaging were performed on 12.7 mm, 25.4 mm, and 50.8 mm inner-diameter vertical upward pipes. The flow conditions were finely selected and cover a range of superficial liquid and gas velocities of <jf> = 0.3–2.0 m/s and <jg> = 0.03–2.5 m/s. The flow regime identification was realized using a 1-D convolutional neural network-based learning algorithm, with the aid of 5 different data augmentation methods. Through the experimental results, the difference of the transition boundary between bubbly to cap-bubbly/slug flow among different pipes sizes was observed. The difference among the pipes is due to the pipe wall effect on the bubble distribution and relative motion. For a smaller diameter pipe, the transition happens in a smaller void fraction. A simple flow regime transition criterion is proposed regarding the wall effect based on Mishima-Ishii’s approach.

Fluidization in small-scale gas-solid 3D-printed fluidized beds

Additive manufacturing could be used to facilitate the rapid fabrication and testing of small-scale fluidized beds for use in screening applications, such as adsorbent screening for carbon capture. In this work, experiments were performed in order to map the different flow regimes produced in small-scale (Dh = 3–15 mm) gas-solid fluidized beds that were fabricated using additive manufacturing using the stereolithography approach. Here, the effects of bed hydraulic diameter (Dh), static particle height (Hs), and particle type/size (Dp and ρp) were considered. Pressure drop data and high speed camera images were used to develop simple flow regime maps for these printed beds showing the operating windows for packed bed, bubbling, slugging and turbulence applicable to a wide range of bed size to particle diameter ratios (Dh/Dp = 20–200) and gas velocities (Ug = 1–400 mm/s) in both ‘2D’ and ‘3D’ bed aspect ratios. Fast Fourier transforms of the pressure drop signals were also used to study the evolution of bubbling/slugging behaviour as the gas velocity was increased by creating 2D colour maps of the frequency spectra. These allowed a new quantitative method to be proposed for the identification of slugging – the point where the dominant frequency in the power spectrum becomes constant as the gas velocity increases. It is concluded in this study that the rougher surfaces generated by additive manufacturing do not influence the fluidization characteristics nor modify the wall effects of small-scale beds. Macro-scale fluidization could also be achieved at smaller Dh/Dp ratios in these 3D printed beds compared to more conventional Plexiglas beds (Dh/Dp = 75 compared to Dh/Dp = 300).

Megaripple/Flat Bed Transition in Rip Current Channels

Sherman, D.J., 2018. Megaripple/flat bed transition in rip current channels. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 246–250. Coconut Creek (Florida), ISSN 0749-0208.Megaripple and flat bed conditions are common bed configurations in energetic nearshore environments. Their existence is controlled by shear stresses associated with wave orbital motion, superimposed net flows, and water depth, representing flow regimes. This is based on the premise of a characteristic set of flow conditions associated with particular bed configurations. A simple flow regime sequence for surf zones is ripples, megaripples (or dunes), and flat bed. Each of these bed configurations may include several secondary characteristics. Despite the importance of understanding flow regime ranges and transitions, there have been few detailed studies for characteristic megaripple and flat bed conditions. This paper presents the results of a field experiment designed to acquire flow, sedimentological, and bedform data from rip current feeder channels in an inner surf zone of a barred nearshore. Waves and water depths were measured with pressure transducers, and currents were measured with bidirectional current meters. Sand samples were gathered and bed configurations were measured and photographed by divers. Analysis indicates that the mobility number, as calculated with a number of characteristic velocities, provides first-order bedform discrimination. Observations indicate that bedform transitions can occur over sub-hour time spans, although the response times are slower as combined wave and current velocities are reduced. Under the latter conditions, especially, non-equilibrium configurations may persist.

Flow environment and matrix structure interact to determine spatial competition in Pseudomonas aeruginosa biofilms.

Bacteria often live in biofilms, which are microbial communities surrounded by a secreted extracellular matrix. Here, we demonstrate that hydrodynamic flow and matrix organization interact to shape competitive dynamics in Pseudomonas aeruginosa biofilms. Irrespective of initial frequency, in competition with matrix mutants, wild-type cells always increase in relative abundance in planar microfluidic devices under simple flow regimes. By contrast, in microenvironments with complex, irregular flow profiles - which are common in natural environments - wild-type matrix-producing and isogenic non-producing strains can coexist. This result stems from local obstruction of flow by wild-type matrix producers, which generates regions of near-zero shear that allow matrix mutants to locally accumulate. Our findings connect the evolutionary stability of matrix production with the hydrodynamics and spatial structure of the surrounding environment, providing a potential explanation for the variation in biofilm matrix secretion observed among bacteria in natural environments.

Flow control design inspired by linear stability analysis

In the recent literature, a growing number of research papers have been dedicated to applying the techniques of global stability and sensitivity analysis to the design of flow controls. The controls that are designed in this way are mainly passive or open-loop controls. Among those, we consider here controls that are aimed at linearly stabilizing flow configurations which would be otherwise globally unstable. In particular, a review of the literature on flow controls designed on the basis of stability and sensitivity analysis is presented. The mentioned methods can be rigorously applied to relatively simple flow regimes, typically observed at low values of the Reynolds number. In this respect, the recent literature also demonstrates a large interest in the application of the same methods for the control of coherent large-scale flow structures in turbulent flows, as, for instance, the quasiperiodic shedding of vortices in turbulent wakes. The papers dedicated to this subject are also reviewed here. Finally, all the described methods imply the solution of eigenvalue problems which are at the state-of-the-art for computational complexity. On the one hand, there are attempts to reduce the complexity of the involved computational problems by applying local stability analysis, and some examples are illustrated. On the other hand, recent advances in numerical methods, also concisely reviewed here, allow the manipulation of large eigenvalue problems and greatly simplify the development of numerical tools for stability and sensitivity analysis of complex flow models, often built using existing fluid dynamics codes.

Influence of large size magnetic particles on the magneto-viscous properties of ferrofluid

We compare the magneto-viscous behavior in a shear flow of three different types of magnetic suspension in the presence of a magnetic field. The first suspension contains magnetite particles of average size 10 nm dispersed in transformer oil. The second one is made of large sized magnetite particles having 30 nm particle size dispersed in transformer oil. The third suspension is a mixture of the first and second fluids in different weight proportions. The size and size distribution have been confirmed by transmission microscopy and small angle neutron scattering experiments. The rheological properties of the first two suspensions were measured for varying shear and field values. The flow behavior of the nanosized dispersed ferrofluid is described with Bingham’s yield stress model and it varied from 2.2 to 5.5 Pa on increasing the field from 0 to 1 T. The large sized particle dispersed fluid exhibits magneto-viscous behavior with increasing field. The value of Bingham’s yield stress obtained is nearly 15 times higher than that of the small size dispersion. On mixing these two fluids with different weight fractions, the Bingham yield stress value increases by a factor of three compared with that of the large sized particle dispersed fluid. The Mason number provides a good scaling of data in the steady simple flow regime. The observed yielding behavior is due to the formation of a longer chain structure in the system under the field and in-field microscopy confirms the same. The present study shows that the addition of large sized magnetic particles in magnetic fluid increases the yield stress as well as the fluid stability under a field.

Environmental flows from dams: the water framework directive

The EU water framework directive (WFD) provides a template for sustainable water management across Europe. The WFD requires the development of procedures to ensure appropriate mitigation of anthropogenic impacts on river ecosystems resulting from water abstraction and impoundments. It is widely acknowledged that alterations to flow regime impact on riverine ecosystems. As a result, hydromorphology, which includes the hydrological regime, is embedded within the WFD as a supporting element to achieve good ecological status (GES). Environmental flow releases from impoundments such as reservoir dams will need to be implemented to mitigate impacts from their construction and operation. This paper outlines the process involved in the analysis of available scientific information and the development of guidance criteria for the setting of environmental flow release regimes for UK rivers. The paper describes two methods—developed by round-table expert knowledge and discussions and supported by available data—for implementation of the WFD for rivers subject to impoundments. The first is a method for preliminary assessment of a water body to determine if it is likely to fail to achieve GES because of changes to the flow regime (indexed by simple flow regime statistics) in systems where appropriate biological assessment methods are limited or currently unavailable. The second is a method for defining an environmental flow regime release based on the requirements of riverine ecological communities and indicator organisms for basic elements (building blocks) of the natural flow regime.

Transport modeling applied to the interpretation of groundwater 36Cl age

We use reactive transport modeling to consider how diffusion and hydrodynamic dispersion, cross‐formational flow, and subsurface production affect the steady state distribution in flow regimes of the radioactive isotope 36Cl, and the relationship of the isotope distribution to groundwater residence time, or “age.” The isotope forms naturally in the atmosphere, dissolves in rainwater, and then decays in the subsurface with a half‐life of ∼301,000 years; hence it is important for age dating very old groundwater. In a simple flow regime composed of an aquifer confined above and below by aquitards, isotopic age may correspond rather well with a groundwater's “piston flow” age. This correspondence is favored where the aquifer is thick, cross‐formational flow is insignificant, salinity is low, and the diffusion coefficient within the aquitards is small. The maximum dateable age, however, is somewhat smaller than expected from the isotope's half‐life. Owing to the effect of “dead” chloride, dating based on isotope abundance (the 36Cl/Cl ratio) may be less accurate than that based on 36Cl concentration. Cross‐formational flow can strongly affect the 36Cl distribution and abundance, preventing the rates and even direction of flow within an aquifer from being interpreted using the piston flow model. Where salinity is moderate or high, the isotope distribution is controlled by subsurface production, and dating on the basis of the decay of atmospheric 36Cl is not possible. In models of simple flow regimes the 36Cl method fails to predict groundwater age accurately where groundwater chlorinity exceeds ∼75–150 mg kg−1. Reactive transport models hold considerable promise for improving interpretation of the rates and patterns of groundwater flow from radioisotope distributions.

Surface-drag flow relations for zonal modeling

Zonal models, combining simple cell-to-cell viscous loss, jet momentum, and convective energy transport relations appear to model the larger features of room airflow and thermal distribution fairly well. The power-law viscous loss relations commonly used in these models, however, have little physical justification and fail, by orders of magnitude, to model the resistance offered to airflow in simple flow regimes. An alternative approach based on a surface-drag viscous loss mechanism is presented that mitigates the deficiencies of the power-law approach, provides a family of cell-to-cell flow relations that includes linear members, and appears to capture room airflow structure more faithfully near-surfaces.

Controls on the distribution and isotopic composition of helium in deep ground-water flows

The distribution and isotopic composition of helium in sedimentary basins can be used to interpret the ages of very old ground waters. The piston-flow model commonly used in such interpretation, however, does not account for several important factors and as such works well only in very simple flow regimes. In this study of helium transport in a hypothetical sedimentary basin, we develop a numerical model that accounts for the magnitude and distribution of the basal helium flux, hydrodynamic dispersion, and complexities in flow regimes such as subregional flow cells. The modeling shows that these factors exert strong controls on the helium distribution and isotopic composition. The simulations may provide a basis for more accurate interpretations of observed helium concentrations and isotopic ratios in sedimentary basins.