Real Flow Research Articles

Flow mixing and heat transfer deterioration investigation of supercritical carbon dioxide in wire-wrapped bundles

As one selection of the fourth-generation reactors, the gas cooled reactor has received enormous attention such as high power conversion efficiency and simple cycle layout. In this paper, the flow mixing and heat transfer characteristics of supercritical carbon dioxide within a seven-pin wire-wrapped bundle were investigated. Due to the lack of more experiments data of velocity field, two experiments were selected to validate the uncertainty of different turbulence models from the view of temperature field. The analysis of mass exchange between different subchannels revealed that coolant flows consistently in a certain direction in edge and corner subchannels, whereas in center subchannels, the cross flow changes direction periodically. The mixing coefficients at three types of subchannel interfaces were fitted by a sine function with the R2 values are all greater than 0.96. For different mass flux and pressures, the root mean square error between correlations and simulations was less than 1 × 10-3, except when Re equals 15000. Additionally, the heat transfer deterioration of SCO2 was observed. While wire spacers can mitigate the extent of heat transfer deterioration, they cannot completely eliminate it. The Performance Evaluation Criterion (PEC) was introduced to assess the effectiveness of wire spacers in enhancing heat transfer. The results indicate that the PEC exceed unity, meaning that the enhanced heat transfer capability outweighs the additional resistance loss caused by wire spacers. With increasing pressure, the PEC decreases but remains greater than unity. The mixing of coolant between different subchannel induced by wire spacers can reduce the wall temperature of rods at the edge of the bundle. However, the seven-pin wire-wrapped bundle was investigated in this work, and the results may not accurately reflect the flow and heat transfer inside a real reactor rod bundle. In the future, a larger scale rod bundle should be conducted to obtain more universally applicable and realistic flow and heat transfer phenomenon for reactor assembly design.

Validation of the thermo-chemical approach to modelling of CO2 conversion in sub-atmospheric pressure microwave gas discharges

The CO 2→ CO + 12 O2 conversion experiment [D’Isa et al 2020 Plasma Sources Sci. Technol. 29 105009] has been compared with thermo-chemical calculations. The experiment is a 2.45 GHz plasma torch with a straight channel in the effluent. The 1.5D model of the CO2/CO/O2/O/C mixture without turbulent transport has been applied with plasma acting only as a prescribed heat source. The parameter range covered is the specific energy input (SEI) 0.3–5 eV/molecule at pressure p = 0.9 bar, and SEI = 0.6–2 eV/molecule at p = 0.5, 0.2 bar. The calculated conversion χ is always close to the experimental values. At the same time, the calculated temperatures T deviate significantly from the experiment, especially for p = 0.2 bar. The calculated T were also found to be sensitive with respect to the uncertain model parameters, but χ is not sensitive. The net conversion in the model is driven to a large extent by the radial diffusion of CO and O from the hot core towards the wall and steep radial temperature gradients. The main factor that reduces the energy efficiency is the re-oxidation of CO at the edge of the hot plasma region and downstream. The 1.5D approximation applied has the principle limitation that the impact of a realistic bulk flow field on the chemical process cannot be studied. Hence, the results must be considered preliminary and have to be confirmed with a more elaborate and accurate model of the vortex-stabilized flow inside the reactor.

Exploring the Aerodynamic Effect of Blade Gap Size via a Transient Simulation of a Four-Stage Turbine

With the impact of size on low-pressure turbines (LPTs) increasing, the gap between the blades has shrunk, inevitably influencing the unsteady effects inside the turbine. In this study, the aerodynamic effect of the blade gap size is investigated using a compressible unsteady Reynolds-averaged Navier–Stokes (URANS) model on the basis of a four-stage LPT. Simulations are conducted in which the gap between the third-stage stator (S3) and rotor (R3) varies from 0.2 to 0.8 times the axial chord length of the R3 blade. The multi-stage environment reflects the complexity of real low-Reynolds flow fields. Computational fluid dynamics is used to analyze the flow field in detail. The results demonstrate that in the small-gap (AG-0.2) case, the turbulence kinetic energy (TKE) level of the S3 wake close to the R3 leading edge is four-thirds of that in the large-gap (AG-0.8) case. The higher intensity of the wake impacting on the blade results in a higher inverse pressure gradient in the rear part of the R3 suction surface, which increases the profile loss. However, the AG-0.2 case leads to fewer losses caused by the passage vortex in the hub area under the influence of the higher intensity of the wake.

Role of streak secondary instabilities on free-stream turbulence-induced transition

We study the stability of a zero-pressure gradient boundary layer subjected to free-stream disturbances by means of local stability analysis. The dataset under study corresponds to a direct numerical simulation (DNS) of a flat plate with a sharp leading edge in realistic wind tunnel conditions, with a turbulence level of 3.45 % at the leading edge. We present a method to track the convective evolution of the secondary instabilities of streaks by performing sequential stability calculations following the wave packet, connecting successive unstable eigenfunctions. A scattered nature, in time and space, of secondary instabilities is seen in the stability calculations. These instabilities can be detected before they reach finite amplitude in the DNS, preceding the nucleation of turbulent spots, and whose appearance is well correlated to the transition onset. This represents further evidence regarding the relevance of secondary instabilities of streaks in the bypass transition in realistic flow conditions. Consistent with the spatio-temporal nature of this problem, our approach allows us to integrate directly the local growth rates to obtain the spatial amplification ratio of the individual instabilities, where it is shown that instabilities reaching an $N$ -factor in the range [2.5,4] can be directly correlated to more than 65 % of the nucleation events. Interestingly, it is found that high amplification is not only attained by modes with high growth rates, but also by instabilities with sustained low growth rates for a long time.

Water content for clot composition prediction in acute ischemic stroke.

Mechanical thrombectomy (MT) has become the gold standard care for treating acute ischemic stroke (AIS) due to large vessel occlusion. Emerging evidence suggests that understanding the composition of clots prior to intervention could be useful for the selection of neuroendovascular techniques, potentially improving the efficacy of treatments. However, current imaging modalities lack the ability to distinguish clot composition accurately and reliably. Since water content can influence signal intensity on CT and MRI scans, its assessment may provide indirect clues about clot composition. This study aimed to elucidate the correlation between water content and clot composition using human clots retrieved from stroke patients and experimentally generated ovine clots. This study involved an analysis of ten clots retrieved from patients with AIS undergoing MT. Additionally, we created ten red blood cells (RBC)-rich and ten fibrin-rich ovine blood clots, which were placed in a human intracranial vascular model under realistic flow conditions. The water content and compositions of these clots were evaluated, and linear regression analyses were performed to determine the relationship between clot composition and water content. The regression analysis in human stroke clots revealed a significant negative association between RBC concentration and water content. We also observed a positive correlation between water content and both fibrin and platelets in ovine blood clots. Conclusion.

Effect of the hole configurations on effusion cooling effectiveness under swirl impact in gas turbine combustor

Effusion cooling characteristics of the cylindrical and fan-shaped hole configurations are studied under realistic swirl flows at blowing ratios ranging from 1.2 to 6.0. RANS computations with the k-ω SST model are used to evaluate the interaction between swirl mainstream and cooling air. The results show that the cooling effectiveness distribution for the cylindrical and fan-shaped hole configurations are similarly controlled by swirl impact. Two high-temperature regions emerge near the impact location of the swirl main flow on the liner wall. The fan-shaped hole configuration has higher cooling effectiveness, and the difference is relative to location. Quantitatively analyzing, the fan-shaped holes are 19.7 %–53.2 % higher than the cylindrical holes in impact zones. In the corner recirculation zone, the difference ranges from 39.1 % to 84.2 %. The computations reflect the interaction between swirl flows and cooling jets is stronger for fan-shaped holes due to lower outlet velocity. Therefore the cooling air is easier to be suppressed by swirl impact under low BR, while the increasing blowing ratio can enhance the resistance of cooling air against swirl flows.

A Study on the Heterogeneity and Anisotropy of the Porous Grout Body Created in the Stabilization of a Methane Hydrate Reservoir through Grouting

To solve the sand problem during the depressurization of methane hydrate (MH), we proposed a method to build a porous grout body with sufficient permeability and strength around the wellbore through inhibitor pre-injection and grouting, and verified its effectiveness and potential in our previous research using artificial cores created with silica sand and alternative hydrates such as TBAB- hydrate and iso-butane hydrate. However, all of the artificial cores mentioned above were created with high homogeneity, injected, cured, and had their physical properties measured in the vertical direction, which differs from real reservoir conditions. To investigate the effects of grouting in a more realistic fluid flow, we conducted further experiments using horizontal 1D cores, 1D cubic models, and a 2D cross-sectional model mimicking the near wellbore. These experiments revealed that (1) the generated gas somewhat suppressed the effects of grouting as in the case of previous experiments, and (2) grouted reservoirs would be heterogenous and anisotropic due to the fluid densities and the distribution of grout particles and turbidite sediments, but sufficient permeability and satisfactory strength could still be attained. The above series of experiments demonstrated that our method has the potential to effectively produce actual MH, preventing sand problems even in heterogeneous and anisotropic grouted reservoirs.

The viscous force and torque on a closed irrotational surface

The force and torque on a solid body in a viscous potential flow are often taken to be independent of viscosity. Joseph et al. (Eur. J. Mech. B/Fluids, vol. 12, 1993, pp. 97–106; J. Fluid Mech., vol. 265, 1994, pp. 1–23) proved that this holds for (i) the force (not the torque) in any two-dimensional flow, and (ii) the drag force experienced by a purely translating three-dimensional body. The remaining components of the force and torque, along with general three-dimensional flows, were not considered. Importantly, the flow was assumed to be unbounded and irrotational everywhere. We eliminate this rarely satisfied assumption and consider the viscous force and torque experienced by any closed surface where the flow is irrotational locally; this can include a body's surface. Any vorticity distribution is permitted away from the closed irrotational surface. In so doing, we complete the analysis of Joseph et al. for all components of the viscous force and torque in two and three dimensions and enable application to real flows that inevitably contain regions of vorticity.

The Relationship Between the German Current Account and Financial Account: Evidence from the Toda-Yamamoto Causality Approach

Abstract The paper examines the causality between the German current account and financial account. It contrasts with past research which assumes the current account and financial account to be jointly determined by a saving-investment imbalance. Our analysis decomposes the current account into exports and imports (real resource flows) and the financial account into domestic capital outflows and foreign capital inflows (gross capital flows). Evidence from the Toda-Yamamoto causality test shows that for Germany from Q1.1980 to Q2.2023, the causality runs from the financial account to the current account. It is not real resource flows but gross capital flows which exert significant impacts on the German real exchange rate. The finding implies that over the long run, strong German capital outflows and weak foreign capital inflows contributed to weak wage growth and stagnant investment in Germany, sustaining the persistent German current account surpluses. A reduction of the German current account surpluses requires a policy mix of fiscal expansion and monetary tightening which would expand the absorption of German and foreign capital in the German economy.

Small-scale turbulent characteristics in transcritical wall-bounded flows

Considerable efforts have been devoted to the understanding of the small-scale characteristics in turbulent flows. While the universality of small-scale quantities has been established for incompressible flows, their extension to high-pressure transcritical flows remains an open area of research. To address this question, we investigate the real-fluid thermodynamic effects on small-scale velocity statistics of high-pressure transcritical wall-bounded turbulence. We show that in the locally isotropic region for transcritical flows, low-order moments of small-scale statistics collapse for all cases and Kolmogorov's (1941) theory holds. However, real-fluid thermodynamic effects introduce deviations in the tails of the probability density function for the velocity derivative and, consequently, high-order moments of velocity gradients and dissipation rate in transcritical flows cannot collapse in the locally isotropic region. Analysis of the intermittency shows that the low-order structure functions in transcritical flows follow extended self-similarity, while the dependence of the intermittency factor on real-fluid effects is observed for high-order structure functions. The real-fluid effects on intermittency are explained by turbulent structures related to rare events. Additionally, the dissipation rate moments for transcritical flows follow a universal scaling with Reynolds number, and the scaling exponents are different from those of incompressible flows. These results extend the small-scale universality in incompressible flows (Schumacher et al., Proc. Natl Acad. Sci. USA, vol. 111, 2014, pp. 10961–10965) to realistic flows with significant changes in thermodynamic properties, and provide a physical underpinning of the scaling laws of small-scale statistics at transcritical conditions.

Traffic Flow Prediction by an Ensemble Framework with Data Denoising and Functional Principal Components Analysis

Accurate traffic flow data are important for congestion identification and real-time traffic control. Raw traffic flow data may be disturbed by different noises during the acquisition process, which leads to the degradation of model prediction performance. To address this problems, this paper proposes a data denoising method based on the improved complete ensemble empirical mode decomposition with adaptive noise (ICEEMDAN) combined with wavelet threshold denoising (WTD) to suppress potential outliers in the data, and then introduces functional principal component analysis to accomplish the traffic forecasting task. We use the proposed framework for real traffic flow prediction and validate the prediction performance of the proposed framework using root mean square error and mean absolute error. In the prediction performance comparison, five denoising methods are considered: empirical mode decomposition (EMD), ensemble empirical mode decomposition, complete ensemble empirical mode decomposition with adaptive noise, ICEEMDAN, and WTD. The prediction results show that the model combined with the denoising method outperforms the model without the denoising method. ICEEMDAN combined with WTD’s denoising method improves prediction performance more than other methods. Furthermore, the WTD method with dmey type achieves higher accuracy than other types.

Quantitative analysis of hemodynamic changes induced by the discrepancy between the sizes of the flow diverter and parent artery

The efficacy of flow diverters is influenced by the strut configuration changes resulting from size discrepancies between the stent and the parent artery. This study aimed to quantitatively analyze the impact of size discrepancies between flow diverters and parent arteries on the flow diversion effects, using computational fluid dynamics. Four silicone models with varying parent artery sizes were developed. Real flow diverters were deployed in these models to assess stent configurations at the aneurysm neck. Virtual stents were generated based on these configurations for computational fluid dynamics analysis. The changes in the reduction rate of the hemodynamic parameters were quantified to evaluate the flow diversion effect. Implanting 4.0 mm flow diverters in aneurysm models with parent artery diameters of 3.0–4.5 mm, in 0.5 mm increments, revealed that a shift from oversized to undersized flow diverters led to an increase in the reduction rates of hemodynamic parameter, accompanied by enhanced metal coverage rate and pore density. However, the flow diversion effect observed transitioning from oversizing to matching was less pronounced when moving from matching to undersizing. This emphasizes the importance of proper sizing of flow diverters, considering the benefits of undersizing and not to exceed the threshold of advantages.

Numerical investigation of fluid flow behavior in steel cord with lattice Boltzmann methodology: The impacts of microstructure and loading force.

Steel cord materials were found to have internal porous microstructures and complex fluid flow properties. However, current studies have rarely reported the transport behavior of steel cord materials from a microscopic viewpoint. The computed tomography (CT) scanning technology and lattice Boltzmann method (LBM) were used in this study to reconstruct and compare the real three-dimensional (3D) pore structures and fluid flow in the original and tensile (by loading 800 N force) steel cord samples. The pore-scale LBM results showed that fluid velocities increased as displacement differential pressure increased in both the original and tensile steel cord samples, but with two different critical values of 3.3273 Pa and 2.6122 Pa, respectively. The original steel cord sample had higher maximal and average seepage velocities at the 1/2 sections of 3D construction images than the tensile steel cord sample. These phenomena should be attributed to the fact that when the original steel cord sample was stretched, its porosity decreased, pore radius increased, flow channel connectivity improved, and thus flow velocity increased. Moreover, when the internal porosity of tensile steel cord sample was increased by 1 time, lead the maximum velocity to increase by 1.52 times, and the average velocity was increased by 1.66 times. Furthermore, when the density range was determined to be 0-38, the pore phase showed the best consistency with the segmentation area. Depending on the Zou-He Boundary and Regularized Boundary, the relative error of simulated average velocities was only 0.2602 percent.

On the Secondary Flow System of an Aggressive Inter Compressor Duct

Abstract In contrast to the highly optimized rotating components, the inter compressor duct (ICD) is offering a high potential for further improvements. Due to a more aggressive design, the length of the whole engine can be reduced. Shorter and thus lighter engines are leading to further fuel savings for the next engine generation. The design of current ICDs is very conservative because of high uncertainties in design space. An extensive test campaign on two highly aggressive ICDs has been conducted at an annular cascade at German Aerospace Center (DLR) Cologne to explore these limitations. The test section consists of low-pressure compressor (LPC)-OGVs, struts, and high-pressure compressor (HPC)-inlet guide vanes (IGVs). To simulate the influence of the last rotor of the LPC and to vary the incidence of the OGV, a moveable swirler is placed in front of the OGV. This simplification results in differences compared to a real rotor outlet flow. Due to geometric constraints, the ability to move the swirler can only be achieved by adding partial gaps in the rear part of the swirler at hub and shroud. These gaps have a huge impact on the secondary flow structure of the swirler outflow and therefore on the inflow of the ICD. In this article, the secondary flow system of an aggressive ICD is analyzed in detail by the means of experimentally validated computational fluid dynamics (CFD) simulations. Special attention is given to the influence of the nonrotating swirler on the secondary flow system. Furthermore, a recommendation for future experimental setups with respect to the described effects is concluded.

Investigation the Stochastic behaviour of the Traffic Flow: A Case Study of a Section of a Road

The stochastic behavior is one of the key for the current state of vehicles flow for the real time traffic behavior. This paper describe the study to investigate the stochastic behavior of real time traffic flow for a section of road using probability distribution fit over the section of road, the traffic data was collected for a week from 7:00 to 19:00 at the location Nawabshah Pakistan. The different distribution such as Normal, Lognormal, Weibull, Gamma, Exponential distribution was fit using MATLAB distribution fit by probability plot of traffic flow data. The same distribution was used for the goodness-of-fit tests by considering Kolmogorov-Smirnov, Kolmogorov-Smirnov modified, Anderson-Darling were used with p-values at 95% of confidence level and justification to accept the hypothesis test are accepted or rejects. The hypothesis accept for Normal, Weibull and Gamma distribution which accept the all hypothesis test and among these three accepted fit distribution the Normal probability distribution fit is most fitted distribution using the rank by p-value of the hypothesis tests. Keywords: Traffic flow, Goodness-of-fit, Probability Distributions, Nawabshah

Modelling urban canopy with object-based porosity model

We present a novel approach to modelling urban canopies based on the porosity model. The impact of objects in the canopy layer is implicitly accounted for by applying a high-value drag force in the parts of the canopy layer that objects would occupy. This effectively halts fluid flow in the defined space of the objects, mimicking the impact of solid objects in the canopy layer. The model addresses two common issues in urban canopy modelling: the definition of the drag coefficient and the loss of canopy morphology, which significantly affect flow and turbulence fields. The model is assessed for three cases: two idealised urban-like canopies with realistic urban flow conditions and one benchmark case based on the measurements of Hilderman and Chong (2007). The benchmark results, where the explicitly modelled objects are reduced from 109 to 9, show that the new model exhibits a level of accuracy similar to explicit modelling. We also computed the commonly used zone-based porosity and roughness models for the two idealised urban-like canopies. The new model produced results comparable to the results of explicit modelling and superior to those of the alternative approaches. All cases were computed in unsteady mode using a hybrid RANS/LES model based on elliptic-relaxation eddy-viscosity RANS and the dynamic Smagorinsky model. The simulations were performed using the open-source computational fluid dynamics code T-Flows, available on Github.

Gas state equation and flow mechanism of gas–liquid two-phase flow in airlift pump system

Airlift pumps (ALPs) have a simple structure and significant application potential. However, previous studies on ALPs have generally assumed that the gas density remains constant. In this paper, the gas state equation (GSE) for ALP is established based on the van der Waals formula, which explicitly considers the density of gas. An electrical resistance tomography system is used to collect the gas void fractions at different heights under different gas flow rates, and an empirical formula for the gas void fraction is established. To verify the effectiveness of the proposed model, high-precision pressure sensors and a high-speed camera are used alongside an electrical resistance tomography system to determine the realistic ALP flow parameters. The results of 409 sets of experiments show that: (1) the gas in ALP cannot be regarded as ideal gas, because the ideal GSE cannot distinguish between different gas flow rates; (2) the state change of gas in ALP is a quasi-equilibrium process, whereby the GSE of ALP can be obtained from the pressures under several locations; (3) the axial pressures predicted by the proposed GSE for ALP are in good agreement with experimental data; and (4) a single parameter of the GSE uniquely determines the state process. The proposed model and the experimental data provide a new methodology and comprehensive references for studying the working mechanism and efficiency of ALPs.

Low-Carbon Mixed Traffic Route Recommendation for Community Residents Based on Multilayer Complex Traffic Network

With the proposed “carbon peaking” and “carbon neutral” goals in China, the transportation sector, as the second largest consumer of oil and a major producer of greenhouse gases, is a critical area for energy efficiency and emission reduction actions. However, few studies have focused on the effective combination of solving residents' commuting challenges and low-carbon travel. In this paper, by extracting real traffic flow data from taxi and bike-sharing trajectory data, a multilayer complex traffic network is formed to realize an interactive visual exploration of urban traffic patterns. Based on this network a low-carbon travel route recommendation is implemented using a modified genetic algorithm to reduce personal carbon emission and travel costs. Meanwhile, the trip chain level carbon emission estimation method is defined for city streets and recommended routes. With the integration of the above algorithms, a visual analytics system is designed and implemented to support the joint exploration of urban traffic patterns and the street carbon emission distribution, low-carbon mixed traffic route recommendations for inter-community commuting, and optimization of low-carbon recommended routes by adjusting bike stations. Take the taxi and bike-sharing trajectory data in Xiamen, China as an example, an evaluation analysis of the system shows that the method is effective in reducing commuting costs for community residents while reducing personal travel carbon emission.

On the extreme wall shear stress events in a turbulent pipe flow

This study investigates the extreme wall shear stress events in a turbulent pipe flow by direct numerical simulation at a frictional Reynolds number Reτ≈500. A two-step conditional averaging scheme is implemented to identify the locations of extreme events and construct their spatial structures. Combined with the joint probability density functions of shear stresses, further evidence is provided for the argument that extreme positive events occur below an intense sweep event (Q4), and the formation of the backflow events is predominantly aided by an identifiable oblique vortex. Moreover, the conditional probability distribution of shear stress for varying thresholds used to define extreme events reveals that, when the threshold is above or below the mean, the probability distributions of the extreme positive events or the backflow events generally follow an exponential relationship, suggesting the extreme wall shear stress events are a threshold-independent process. Finally, the conditional space–time proper orthogonal decomposition is performed to extract the dominant modes and characterize the evolution of the extreme events from inception to dissipation, which exhibits morphological features of real flow structures. It is found that the observation of uθ modes can provide a basic representation of the entire variation process and the extreme values return to normal levels in a very short time.

Numerical and experimental investigation of cryogenic fluid flow characteristics on corrugated packing surfaces

Cryogenic distillation is a significant method for producing pure oxygen and nitrogen gas. In the past, due to the high cost and difficulty of conducting cryogenic experiments, the structured packings used in cryogenic distillation are mainly developed and designed by referring to the testing results of room-temperature fluids. The real flow characteristics of cryogenic fluids on structured packings, such as flow pattern and wettability which significantly determines the distillation performance, have seldom been revealed. In this study, with the help of a three-dimensional CFD model and a visual experimental setup, the liquid nitrogen (LN2) flow characteristics within the geometric unit of a 45° inclined packing element were revealed detailedly under different liquid loads and crimping angles. Comparison studies were also conducted to show the different film flow characteristics between water and LN2. Additionally, optimization of the crimping angles based on 250Y structured packing was explored. Both simulation and experiment show that the flow pattern and wetting mechanism of the hydraulic test on water are different from those of LN2 in the actual cryogenic distillation. The LN2 has much better film flow performance than water due to its different physical properties. In addition, it is found that the crimping angle has a significant effect on the wetting area ratio. For LN2, the crimping angle of 90° can basically result in a high enough wetting area ratio, while for water, the crimping angle as large as 120° is needed to achieve a high wetting area ratio. It shows that testing and designing the structured packing with water as the main working medium will bring apparent errors in real cryogenic distillation process.