Wide Range Of Flows Research Articles

Quantification of the Effect of Bridge Pier Encasement on Headwater Elevation Using HEC-RAS

The deterioration of bridge substructure is a serious concern across the United States. The pier encasement is one of the most common practices for repairing and strengthening the bridge substructure. It is a rehabilitation process of existing pile piers during the repair or replacement of the bridge superstructure, which involves enclosing part of an existing pile pier with a polyethylene or PVC pipe large enough to provide at least three inches of concrete cover over the existing pier when filled. However, this process of enclosing pile piers might elevate water level due to increase in pier width, which could be hazardous in high-risk flood zones. Furthermore, it may create an adverse impact on the stability of the bridge due to scouring around the pier foundation. In order to gain knowledge on the backwater effect due to pile encasement, Hydraulic Engineering Center-River Analysis System (HEC-RAS) was used in this research to perform hydraulic simulations near the bridge sites. These simulations were carried out for various channel configurations and pier sizes with a wide range of flows, which resulted into 224 HEC-RAS models in order to investigate the effects of pile pier encasement on the headwater elevation. This study demonstrated that the water elevation measured in the upstream of the bridge showed no-rise condition, especially for wider channel sections with flatter slopes. However, the water elevation at the immediate upstream of the bridge was slightly higher, and the increasing pattern was only noticeable for a smaller channel width (20 ft), and specifically, for increased flow rate. As the area of flow was decreased resulting in increased water surface elevation due to encasement, a generic power equation in the form of Y = aXb was suggested for various channel slopes for the increased water surface elevation (Y) for each percentage decrease in channel area (X).

Evaluation of energy dissipation rate as a predictor of mechanical blood damage.

A long-standing goal in the field of biofluid mechanics has been to reliably predict hemolysis across the wide range of flows that can occur in prosthetic cardiovascular devices. A scalar representation of the complex three-dimensional fluid stresses that are exerted on cells is an attractive alternative for the simplicity that it lends to the computations. The appropriateness of the commonly used von-Mises-like scalar stress as a universal hemolysis scaling parameter was previously evaluated, finding that erythrocyte membrane tensions calculated for laminar shear and extensional flows and for three cases of turbulent flow were widely divergent for the same value of scalar stress. The same techniques are applied in this study to laminar and turbulent flows that each have the same energy dissipation rate. Results showed that agreement of membrane tension between laminar shear and turbulent shear inside an eddy was improved relative to the common scalar stress cases, but disagreement between laminar shear and laminar extension remained the same and disagreement between laminar shear and other turbulent flows increased. It is therefore concluded that energy dissipation rate alone is also likely not sufficient to universally scale blood damage across the range of different flows that can be encountered clinically.

Applicability of Preissmann box scheme for calculation of transcritical flow in pipes

AbstractThe accurate computer simulation of pipe flow is of great importance in the design of urban drainage. The Preissmann box scheme is usually used to model a wide range of subcritical and supercritical flows. However, care must be taken over the modelling of transcritical flows since, unless the correct internal boundary conditions are imposed, the scheme becomes unstable. In this paper, using the scheme in conjunction with the reduced momentum equation and applying boundary condition structure inherent to subcritical flow to all regimes, is an approach that enables efficient numerical simulation of transcritical flows in pipe networks. The approach includes three steps. First, a unified mathematical model which is based on the Preissmann slot model is derived. Second, the Preissmann box scheme is used to solve the set of equations, by analyzing and discussing the origin of the invalidity of applying the scheme, and a numerical model suitable for transcritical flow is proposed by the method of changing the convection acceleration term. Third, the numerical model is assessed by comparison with analytical, experimental and numerical results. The proposed models verified that this method can make the Preissmann box scheme applicable to the computation of transcritical flow in pipes.

Drivers of residual and tidal flow variability in the St. Lawrence fluvial estuary: Influence on tidal wave propagation

Accurately characterizing the spatiotemporal dynamics of residual and tidal flows is essential for understanding transport processes in tidal rivers and estuaries, as they directly affect salinity intrusion, water renewal capacity, sediment transport, and morphological evolution. This paper aims at characterizing the longitudinal and temporal variability of tidal discharges and currents in the St. Lawrence fluvial estuary (SLFE), under a wide range of river flow and tidal conditions. Tidal discharges and currents are reconstructed by cubature from nonstationary tidal analyses of water levels for a highly variable year, in terms of observed river flows, at 10 river cross-sections of the SLFE, distributed from the fluvial estuary entrance to the head of the tide. An analysis of the spatiotemporal variability in residual and tidal flows is hereby provided, with an emphasis on ebb-flood characteristics, residual currents, tidal constituent properties and tidal wave propagation characteristics at the intratidal, neap-spring and seasonal scales along the system. The main drivers and mechanisms responsible for this variability include (1) the system's hydrology and its associated watershed and meteorological characteristics, which dictate river flow intensity and seasonality, (2) the landward-travelling tides originating from the ocean, responsible for intratidal and tidal monthly variability in tides and currents, (3) nonlinear river-tide interactions, dissipating tidal energy, modulating tidal properties and inducing a surface level gradient, and (4) channel convergence and floodplain morphology, contributing to tidal asymmetry in horizontal and vertical tides. These factors ultimately influence tidal wave propagation and damping properties within estuaries, both longitudinally and at the neap-spring and seasonal scales.

A CFD based empirical model for assessing gas holdup in bubble columns

ABSTRACTBubble columns are widely used in many industrial applications. Gas holdup knowledge is an essential element in bubble column design and process optimization. Despite the simple structure of bubble columns, the behaviour of the bubbly flow is still not well understood owing to the complexity of gas–liquid interactions and coupling between the phases. Variation of gas density and initial liquid height are the most likely reasons for gas holdup changes in bubble columns. These parameters affect bubble breakup and coalescence as well as the rise velocity of small bubbles, which causes change to bubble size distribution that finally affect the gas holdup. In this work, through the application of computational fluid dynamics (CFD) and by investigating the effect of gas density and the initial liquid height on the bubbly flow condition, a new correlation for deducing the total gas holdup is developed. To achieve this, a wide range of bubbly flows are modelled, and gas holdup values are determined for different heights and operating pressures. Ranges of the normalized pressure (P/P0), superficial Reynolds number (Resup), and the aspect ratio of the bubble column (H0/D) were 1 ≤ P/P0 ≤ 15, 3000 ≤ Resup ≤ 57000, and1.5 ≤ H0/D ≤ 8, respectively. The proposed correlation has been found to predict the experimental as well as CFD gas holdup data fairly well. The results of this research provide a fast and accurate method for predicting gas holdup in bubble columns that work either in atmospheric or high‐pressure conditions.

Quantification of flow noise produced by an oscillating hydrofoil

After getting inspired from swimming natural species, a lot of research is being carried out in the field of unmanned underwater vehicles. During the last two decades, more emphasis on the associated hydrodynamic mechanisms, structural dynamics, control techniques and, its motion and path planning has been prominently witnessed in the literature. Considering the importance of the involved acoustic mechanisms, we focus on the quantification of flow noise produced by an oscillating hydrofoil here employed as a kinematic model for fish or its relevant appendages. In our current study, we perform numerical simulations for flow over an oscillating hydrofoil for a wide range of flow and kinematic parameters. Using the Ffowcs-Williams and Hawkings (FWH) method, we quantify the flow noise produced by a fish during its swimming for a range of kinematic and flow parameters including Reynolds number, reduced frequency, and Strouhal number. We find that the distributions of the sound pressure levels at the oscillating frequency and its first even harmonic due to the pressure fluctuations in the fluid domain are dipole-like patterns. The magnitudes of these sound pressure levels depend on the Reynolds number and Strouhal number, whereas the direction of their dipole-axes appears to be affected by the reduced frequency only. Moreover, We also correlate this emission of sound radiations with the hydrodynamic force coefficients.

An Investigation of Centrifugal Compressor Stability Enhancement Using a Novel Vaned Diffuser Recirculation Technique

The main centrifugal compressor performance criteria are pressure ratio, efficiency, and wide flow range. The relative importance of these criteria, and therefore the optimum design balance, varies between different applications. Vaned diffusers are generally used for high-performance applications as they can achieve higher efficiencies and pressure ratios, but have a reduced operating range, in comparison to vaneless diffusers. Many impeller-based casing treatments have been developed to enlarge the operating range of centrifugal compressors over the last decades but there is much less information available in open literature for diffuser focused methods, and they are not widely adopted in commercial compressor stages. The development of aerodynamic instabilities at low mass flow rate operating conditions can lead to the onset of rotating stall or surge, limiting the stable operating range of the centrifugal compressor stage. More understanding of these aerodynamic instabilities has been established in recent years. Based on this additional knowledge, new casing treatments can be developed to prevent or suppress the development of these instabilities, thus increasing the compressor stability at low mass flow rates. This paper presents a novel vaned diffuser casing treatment that successfully increased the stable operating range at low mass flow rates and high pressure ratios. Detailed experimental measurements from a high pressure ratio turbocharger compressor stage combined with complementary CFD simulations were used to examine the effect of the new diffuser casing treatment on the compressor flow field and led to the improvement in overall compressor stability. A detailed description of how the new casing treatment operates is presented within the paper.

FGCH: a fast and grid based clustering algorithm for hybrid data stream

Streaming large volumes of data has a wide range of real-world applications, e.g., video flows, internet calls, and online games etc. Thus, fast and real-time data stream processing is important. Traditionally, data clustering algorithms are efficient and effective to mine information from large data. However, they are mostly not suitable for online data stream clustering. Therefore, in this work, we propose a novel fast and grid based clustering algorithm for hybrid data stream (FGCH). Specifically, we have made the following main contributions: 1), we develop a non-uniform attenuation model to enhance the resistance to noise; 2), we propose a similarity calculation method for hybrid data, which can calculate the similarity more efficiently and accurately; and 3), we present a novel clustering center fast determination algorithm (CCFD), which can automatically determine the number, center, and radius of clusters. Our technique is compared with several state-of-art clustering algorithms. The experimental results show that our technique can achieve more than better clustering accuracy on average. Meanwhile, the running time is shorter compared with the closest algorithm.

Evaluation of Wray-Agarwal turbulence model for simulation of neutral and non-neutral atmospheric boundary layers

Wray-Agarwal (WA) turbulence model is a one-equation eddy-viscosity model derived from a blended k−ω/k−ε formulation. The model performance is better than the commonly used Spalart-Allmaras one-equation model and comparable to shear stress transport (SST) model, for a wide range of canonical flows in aerodynamics. This study proposes a Monin-Obukhov similarity theory (MOST) consistent modification of the buoyancy production/destruction term in this model to simulate the atmospheric boundary layer under different stratifications. The rough wall function is also modified to be consistent with MOST. A benchmark case for testing the streamwise homogeneity evaluated the model performance. In the tests, the wind and turbulence profiles did not decay along the stretch due to the model consistency with MOST. Adoption of the model for urban flow and wind energy simulations requires further testing.

An efficient algorithm for the multicomponent compressible Navier–Stokes equations in low- and high-Mach number regimes

The goal of this study is to develop an efficient numerical algorithm applicable to a wide range of compressible multicomponent flows, including nearly incompressible low-Mach number flows, flows with strong shocks, multicomponent flows with high density ratios and interfacial physics, inviscid and viscous flows, as well as flows featuring combinations of these phenomena and various interactions between them. Although many highly efficient algorithms have been proposed for simulating each type of the flows mentioned above, the construction of a universal solver is known to be challenging. Extreme cases, such as incompressible and highly compressible flows, or inviscid and highly viscous flows, require different numerical treatments in order to maintain the efficiency, stability, and accuracy of the method.Linearized block implicit (LBI) factored schemes (see e.g. Briley and McDonald, 1980 [1], Beam and Warming, 1978 [2]) are known to provide an efficient way of solving the compressible Navier–Stokes equations (compressible NSEs) implicitly, allowing us to avoid stability restrictions at low Mach number and high viscosity. However, the methods’ splitting error has been shown to grow and dominate physical fluxes as the Mach number approaches zero (see Choi and Merkle, 1985 [3]). In this paper, a splitting error reduction technique is proposed to solve the issue. A shock-capturing algorithm from [4] is reformulated in terms of finite differences, extended to the stiffened gas equation of state (SG EOS) and combined with the LBI factored scheme to stabilize the method around flow discontinuities at high Mach numbers. A novel stabilization term is proposed for low-Mach number applications. The resulting algorithm is shown to be efficient in both low- and high-Mach number regimes. Next, the algorithm is extended to a multicomponent case using an interface capturing strategy (see e.g. Saurel and Abgrall, 1999 [5]) with surface tension as a continuous surface force (see Perigaud and Saurel, 2005 [6]). Special care is taken to avoid spurious oscillations of pressure and generation of artificial acoustic waves in the numerical mixture layer. Numerical tests are presented to verify the performance and stability properties for a wide range of flows.

Buoyancy effects on large-scale motions in convective atmospheric boundary layers: implications for modulation of near-wall processes

A number of recent studies have demonstrated the existence of so-called large- and very-large-scale motions (LSM, VLSM) that occur in the logarithmic region of inertia-dominated wall-bounded turbulent flows. These regions exhibit significant streamwise coherence, and have been shown to modulate the amplitude and frequency of small-scale inner-layer fluctuations in smooth-wall turbulent boundary layers. In contrast, the extent to which analogous modulation occurs in inertia-dominated flows subjected to convective thermal stratification (low Richardson number) and Coriolis forcing (low Rossby number), has not been considered. And yet, these parameter values encompass a wide range of important environmental flows. In this article, we present evidence of amplitude modulation (AM) phenomena in the unstably stratified (i.e. convective) atmospheric boundary layer, and link changes in AM to changes in the topology of coherent structures with increasing instability. We perform a suite of large eddy simulations spanning weakly ($-z_{i}/L=3.1$) to highly convective ($-z_{i}/L=1082$) conditions (where $-z_{i}/L$ is the bulk stability parameter formed from the boundary-layer depth $z_{i}$ and the Obukhov length $L$) to investigate how AM is affected by buoyancy. Results demonstrate that as unstable stratification increases, the inclination angle of surface layer structures (as determined from the two-point correlation of streamwise velocity) increases from $\unicode[STIX]{x1D6FE}\approx 15^{\circ }$ for weakly convective conditions to nearly vertical for highly convective conditions. As $-z_{i}/L$ increases, LSMs in the streamwise velocity field transition from long, linear updrafts (or horizontal convective rolls) to open cellular patterns, analogous to turbulent Rayleigh–Bénard convection. These changes in the instantaneous velocity field are accompanied by a shift in the outer peak in the streamwise and vertical velocity spectra to smaller dimensionless wavelengths until the energy is concentrated at a single peak. The decoupling procedure proposed by Mathis et al. (J. Fluid Mech., vol. 628, 2009a, pp. 311–337) is used to investigate the extent to which amplitude modulation of small-scale turbulence occurs due to large-scale streamwise and vertical velocity fluctuations. As the spatial attributes of flow structures change from streamwise to vertically dominated, modulation by the large-scale streamwise velocity decreases monotonically. However, the modulating influence of the large-scale vertical velocity remains significant across the stability range considered. We report, finally, that amplitude modulation correlations are insensitive to the computational mesh resolution for flows forced by shear, buoyancy and Coriolis accelerations.

Control of viscous fingering and mixing in miscible displacements with time‐dependent rates

In miscible displacements encountered in enhanced oil recovery processes, the unfavorable viscosity contrast between injected solvent and oil usually leads to viscous fingering (VF), a hydrodynamic instability which may result in a lower sweep efficiency and oil recovery. This phenomenon can be observed in a wide range of flows in subsurface porous media. This study examined a simple cyclic time‐dependent displacement rate and its effects on the onset and longer development of VF. It is found that such varying displacement rate can either stabilize or destabilize VF, depending on the cycle period, amplitude, and displacement scenarios. The most important mechanism is that such time‐dependent rate can effectively change the competition between convection (destabilizing effect) and dispersion (stabilizing effect). This is different from the widely used constant injection rate where the flow instability is actually determined by the Peclet number and mobility contrast for a given scenario. This study therefore provided a new aspect to control VF, either enhance or reduce, with low additional costs. It is therefore both scientifically and practically important for a wide range of flows in subsurface porous media. © 2017 American Institute of Chemical Engineers AIChE J, 65: 360–371, 2019

3D transient CFD modelling of a scroll-type hydrogen pump used in FCVs

The scroll pump has a great potential to recirculate hydrogen in a fuel-cell vehicle (FCV) because of its high efficiency, low noise and vibration, reliable operation, and a wide range of adjustable flow. This paper presents three-dimensional transient computational fluid dynamics (CFD) modelling of a scroll-type hydrogen pump used in FCVs, including leakage flow through both the radial clearance (RC) and axial clearance (AC). A dynamic mesh was generated for the moving orbiting scroll, and high-quality hexahedral structured grids with sufficient grid-density were applied to the clearances to solve the multi-scale problem. The pressure and velocity fields were obtained at different rotating angles to reveal the dynamic characteristics in the compression chambers. The simulation results showed that the radial leakage through AC has more significant influence on the volumetric efficiency than the tangential leakage through RC, especially on scroll-type hydrogen pumps. The presented modelling and simulation methods were validated experimentally by operating a scroll air compressor at different speeds and pressure ratios. The volumetric efficiency of the scroll pump was 85.39% with 0.02 mm AC and 0.02 mm RC, 81.43% with 0.02 mm AC and 0.04 mm RC, and 70.17% with 0.04 mm AC and 0.02 mm RC. Further, it was found that the performance of scroll-type hydrogen pumps is more sensitive to rotating speed than air scroll pumps under the same conditions. With hydrogen, the volumetric efficiency increased by 30.68% when the rotating speed was increased from 3000 r·min−1 to 6000 r·min−1; with air, the volumetric efficiency increased by 12.81%. Therefore, it is necessary to consider both AC and RC in the CFD modelling of scroll machines, particularly in the case of hydrogen scroll pumps.

How Well Does the Mechanistic Water Quality Model CE‐QUAL‐W2 Represent Biogeochemical Responses to Climatic and Hydrologic Forcing?

AbstractMechanistic water quality models are applied globally for water quality management in rivers and lakes. However, it is unclear how well these models represent the response of lakes to changes in climate and hydrologic conditions. To address this question, we conducted a series of climate and hydrologic sensitivity analyses using a parameterized water quality model, CE‐QUAL‐W2, for the Spokane River and Lake Spokane system. Two experimental tests with climate and flow forcing showed that the model predictions were rather insensitive to climatic‐driven hydrologic changes and a wide range of hydrologic conditions (1st to 99th percentile flows) for two different wastewater treatment plant discharge scenarios. The model realistically represented some aspects of Lake Spokane's observed responses to climatic and hydrologic variability, for example, the water residence time and some water quality trends. However, the model overestimated the reservoir total phosphorus (TPR) concentration by ~37% and the chlorophyll a (Chl a) concentration by ~31% and underestimated the minimum volume‐weighted hypolimnetic dissolved oxygen (DOMIN) concentration by ~26% for a wide range of flows compared to the observed data. In contrast to the model's water quality insensitivity, the modeled temperature was more sensitive to flow than was actually observed in Lake Spokane. This study provides guidance for improving the response of one of the most important mechanistic water quality models to climatic and hydrologic forcing and should help modelers and decision‐makers to develop a rigorous assessment of climate and flow sensitivity to improve the margin of safety when setting total maximum daily load targets.

Thermal lattice Boltzmann method for multiphase flows

An alternative method to simulate heat transport in the multiphase lattice Boltzmann (LB) method is proposed. To solve the energy transport equation when phase boundaries are present, the method of a passive scalar is considerably modified. The internal energy is represented by an additional set of distribution functions, which evolve according to an LB-like equation simulating the transport of a passive scalar. Parasitic heat diffusion near boundaries with a large density gradient is suppressed by using special "pseudoforces" which prevent the spreading of energy. The compression work and heat diffusion are calculated by finite differences. A new method to take into account the latent heat of a phase transition Q(T) is realized. The latent heat is released or absorbed continuously inside a thin transition layer in a certain range of density, ρ_{1}<ρ<ρ_{2}. This allows one to avoid interface tracking. Several tests were carried out concerning all aspects of the processes. It is shown that the Galilean invariance and the scaling of the thermal conduction process hold, as well as the correct dependence of the sound speed on the heat capacity ratio. The method proposed has low scheme diffusion of the internal energy, and it can be applied to modeling a wide range of multiphase flows with heat and mass transfer even for high density ratios of liquid and vapor phases.

Delivery of sevoflurane using a neonatal ventilator.

Most anesthetic ventilators are designed to cope with a wide range of patient sizes and may lack precision at the lowest end of the minute volume scale. Neonatal intensive care ventilators on the other hand are designed specifically for this patient group, but are not able to deliver volatile anesthesia. We aimed to adapt the neonatal ventilator currently in use in our institution to deliver sevoflurane by incorporating a vaporizer and a scavenging system. We used a Diamedica draw-over vaporizer incorporated into the ventilator circuit and a custom designed open interface scavenging system. A number of safety measures are described to ensure that this equipment is correctly inserted into the circuit. Bench testing revealed that the vaporizer output is linear and stable within the circuit flow range 4-8L/min in all modes except high frequency oscillation where concentrations are not predictable. The scavenging system was found to be effective and did not affect volumes, pressures or waveforms when ventilating a test lung over a wide range of flows and pressures. This remained the case over the full range of scavenger flow adjustment. The addition of a Diamedica vaporizer to a Fabian neonatal ventilator was shown in bench testing to provide stable, linear vapor concentrations without compromise of ventilator function. The system should not be used in high frequency oscillation mode because concentrations will exceed those expected and will not maintain a linear relationship with the vaporizer setting.

One-dimensional turbulence modeling for cylindrical and spherical flows: model formulation and application

The one-dimensional turbulence (ODT) model resolves a full range of time and length scales and is computationally efficient. ODT has been applied to a wide range of complex multi-scale flows, such as turbulent combustion. Previous ODT comparisons to experimental data have focused mainly on planar flows. Applications to cylindrical flows, such as round jets, have been based on rough analogies, e.g., by exploiting the fortuitous consistency of the similarity scalings of temporally developing planar jets and spatially developing round jets. To obtain a more systematic treatment, a new formulation of the ODT model in cylindrical and spherical coordinates is presented here. The model is written in terms of a geometric factor so that planar, cylindrical, and spherical configurations are represented in the same way. Temporal and spatial versions of the model are presented. A Lagrangian finite-volume implementation is used with a dynamically adaptive mesh. The adaptive mesh facilitates the implementation of cylindrical and spherical versions of the triplet map, which is used to model turbulent advection (eddy events) in the one-dimensional flow coordinate. In cylindrical and spherical coordinates, geometric stretching of the three triplet map images occurs due to the radial dependence of volume, with the stretching being strongest near the centerline. Two triplet map variants, TMA and TMB, are presented. In TMA, the three map images have the same volume, but different radial segment lengths. In TMB, the three map images have the same radial segment lengths, but different segment volumes. Cylindrical results are presented for temporal pipe flow, a spatial nonreacting jet, and a spatial nonreacting jet flame. These results compare very well to direct numerical simulation for the pipe flow, and to experimental data for the jets. The nonreacting jet treatment overpredicts velocity fluctuations near the centerline, due to the geometric stretching of the triplet maps and its effect on the eddy event rate distribution. TMB performs better than TMA. A hybrid planar-TMB (PTMB) approach is also presented, which further improves the results. TMA, TMB, and PTMB are nearly identical in the pipe flow where the key dynamics occur near the wall away from the centerline. The jet flame illustrates effects of variable density and viscosity, including dilatational effects.

PO-421 A novel high-throughput drug screening platform using pancreatic ductal adenocarcinoma derived organoids in the organoplate®

Introduction Pancreatic cancer is one of the deadliest tumours due to the limited treatment options and late diagnosis. Here, we describe a novel high throughput drug screening platform combining the OrganoPlate, a microfluidic based 3D-culture plate, and the recently described Pancreatic Ductal AdenoCarcinoma (PDAC) derived organoids. Material and methods The OrganoPlate is a high throughput microfluidic 3D cell culture platform, supporting physiologically relevant models with a minimal requirement of cell material and enabling a wide range of flow and co-culture options (e.g. with blood vessels). Organoids were derived from human PDAC xenografts and seeded in the OrganoPlate. The low amount of tissue material required (4000 cells per chip) and the high number of replicates on one plate (n=96 on a standard microtiter format plate) renders the OrganoPlate an efficient and cost-effective platform for drug screening and toxicity assays on complex, 3D models. Results and discussions Organoids were exposed to various chemotherapeutic drugs for 72 hours. The viability of the organoids before and after drug treatment is monitored with standard viability assays and subsequently used to generate dose response curves. Conclusion In conclusion, we showed that the OrganoPlate can be used for high throughput drug screening assays and toxicity screening, and demonstrated its compatibility with human pancreatic PDAC derived organoids.

Structural characteristics of a spiral symmetry stream anaerobic bioreactor based on CFD

The structural characteristics of a spiral symmetry stream anaerobic bioreactor (SSSABR) contribute to its super-high-rate performance. This study investigated the structural characteristics of SSSABR, including distribution zone, spiral reaction zone, and three-phase separation zone, by computational fluid dynamics (CFD). In distribution zone, uniformity and hydraulic head loss increased with increasing number of orifices, and five-orifice is deemed optimal for distributor. In spiral reaction zone, a 45° cutting angle produced a tangential velocity strong enough to spin the granular sludge and maintain a moderate mixing intensity. A 30° installation angle produced a wide range of spiral flows. The 120° symmetrical arrangement of plates formed a strong–weak periodic spiral flow that strengthened material exchange. In separation zone, a value of 0.625 for the frustum characteristic parameter i (i = r1/r2, where r1 presented the bottom frustum radius of reverse cone, and r2 presented radius of spiral reaction zone of SSSABR) provided moderate recirculation. The relative error of dead zone rate between CFD and tracer experiment was 4.66%.

Turbulence Statistics in a Two-Dimensional Vortex Condensate.

Disentangling the evolution of a coherent mean-flow and turbulent fluctuations, interacting through the nonlinearity of the Navier-Stokes equations, is a central issue in fluid mechanics. It affects a wide range of flows, such as planetary atmospheres, plasmas, or wall-bounded flows, and hampers turbulence models. We consider the special case of a two-dimensional flow in a periodic box, for which the mean flow, a pair of box-size vortices called "condensate," emerges from turbulence. As was recently shown, a perturbative closure describes correctly the condensate when turbulence is excited at small scales. In this context, we obtain explicit results for the statistics of turbulence, encoded in the Reynolds stress tensor. We demonstrate that the two components of the Reynolds stress, the momentum flux and the turbulent energy, are determined by different mechanisms. It was suggested previously that the momentum flux is fixed by a balance between forcing and mean-flow advection: using unprecedently long numerical simulations, we provide the first direct evidence supporting this prediction. By contrast, combining analytical computations with numerical simulations, we show that the turbulent energy is determined only by mean-flow advection and obtain for the first time a formula describing its profile in the vortex.