Turbulence Eddy Dissipation Research Articles

Prediction of turbulence eddy dissipation of water flow in a heated metal foam tube

The insertion of porous metal media inside the pipes and channels has already shown a significant heat transfer enhancement by experimental and numerical studies. Porous media could make a mixing flow and small-scale eddies. Therefore, the turbulence parameters are attractive in such cases. The computational fluid dynamics (CFD) approach can predict the turbulence parameters using the turbulence models. However, the CFD is unable to find the relation of the turbulence parameters to the boundary conditions. The artificial intelligence (AI) has shown potential in combination with the CFD to build high-performance predictive models. This study is aimed to establish a new AI algorithm to capture the patterns of the CFD results by changing the system’s boundary conditions. The ant colony optimization-based fuzzy inference system (ACOFIS) method is used for the first time to reduce time and computational effort needed in the CFD simulation. This investigation is done on turbulent forced convection of water through an aluminum metal foam tube under constant wall heat flux. The ANSYS-FLUENT CFD software is used for the simulations. The x and y of the fluid nodal locations, inlet temperature, velocity, and turbulent kinetic energy (TKE) are the inputs of the ACOFIS to predict turbulence eddy dissipation (TED) as the output. The results revealed that for the best intelligence of the ACOFIS, the number of inputs, the number of ants, the number of membership functions (MFs) and the rule are 5, 10, 93 and 93, respectively. Further comparison is made with the adaptive network-based fuzzy inference system (ANFIS). The coefficient of determination for both methods was close to 1. The ANFIS showed more learning and prediction times (785 s and 10 s, respectively) than the ACOFIS (556 s and 3 s, respectively). Finding the member function versus the inputs, the value of TED is calculated without the CFD modeling. So, solving the complicated equations by the CFD is replaced with a simple correlation.

Transient characteristics of internal flow fields of mixed-flow pump with different tip clearances under stall condition

This paper investigates the influence of different tip clearances on the transient characteristics of mixed-flow pump under stall condition. The instantaneous internal flow fields of mixed-flow pump with four tip clearances (0.2 mm, 0.5 mm, 0.8 mm and 1.1 mm) are explored by conducting unsteady time accurate simulations. Reynolds-averaged Navier-Stokes (RANS) equations are employed in the simulations and the results of computations are compared with experimental data. The results show that the pump head decreases by 22.1% and the pump efficiency drops by 13.9% at design flow condition when the impeller tip clearance increases from 0.2 mm to 1.1 mm. The swirling flow occurs in the inlet pipe of the mixed-flow pump with different tip clearances under stall condition, and the initial starting point of the swirling flow gets further away from the impeller inlet with increase in tip clearance because of increase in circumferential velocity and change in momentum of the tip leakage flow (TLF). The high turbulent eddy dissipation (TED) regions in the flow are attributed to the TLF, swirling flow, back flow and stall vortex, and their intensity are affected by the change in tip clearance. The oscillating trend of time domain distribution of TED enhances first and then decreases with increase in tip clearance and it exhibits a propagation feature under the effect of stall vortex, while most of the energy in the frequency domain remains concentrated in the low frequency part under stall condition.

Erozyon Korozyonunda Tasarım Etkisinin Sonlu Elemanlar Analizi İle Araştırılması

The aim of this study is to minimize the corrosion effect of moving fluids in a pipe by design variable. The pipes used for fluid transport in the petrochemical industry have different designs. SolidWorks program is used for 3D studies considering two different pipe designs. Intra-pipe flow analyses are performed based on the finite volume method by using computational fluids dynamics program-Ansys Fluent. 90 ° sharp elbow pipe and rounded elbow pipe are used for analyses. Flow velocity, pressure, the turbulence kinetic energy, turbulence eddy dissipation, eddy viscosity, and erosion rate for each model have been obtained in the results of the analysis for two different pipes. The geometrical structure of the rounded elbow pipe model allows turbulence and hence eddy formation less than the 90 ° sharp elbow pipe.

Study on critical speed of vortex rupture and gas-liquid two-phase flow in rotating soybean milk machine

Abstract The gas-liquid two-phase flow field inside a soybean milk machine is simulated with the Navier–Stokes equations, Renormalization group (RNG) k-ɛ turbulence model and the particle two-phase flow model. The critical speed of vortex breakdown in the container and the influence of the characteristics of the two-phase flow on the soybean crushing effect at different speeds are investigated. The results show that there is a critical value of rotating speed of 1500 rpm for the vortex breakdown in the studied machine. At this rotational speed, the turbulent eddy dissipation (TED) reaches the maximum, and the position where the vortex first rupture is near the central axis and with a distance of 0.01 m from the central axis. It is also found that with the increase of rotating speed, the pressure difference around the blade varies significantly. The research in this study has important implication for the design of the soybean milk machine.

Liquid Seepage in Coal Granular-Type Porous Medium.

To investigate liquid seepage process in a coal granular-type porous medium, a new sampling device was designed to obtain coal samples with required porosity. Meanwhile, an approach combining ultra-deep-field microscopy with advanced digital image processing technologies was proposed to rebuild granular-type porous medium models. The liquid seepage process was simulated with CFD, and the effects of head pressure, liquid viscosity, and pore size were studied. The results show that only liquids with head pressures above a critical value can penetrate into coal stacks and the hydraulic conductivity and permeability are positively correlated to the driving head pressure. Liquid viscosity enhances flow deformation, causing more eddy current energy dissipation; the turbulent eddy dissipation caused by acetone, methanol, and ethanol was 700, 1200, and 4700 m2/s3, respectively. Larger pores can strengthen the additional pressure at the front end of the flow, reducing the flow resistance and thus increasing the fluid kinetic energy and seepage velocity.

Computational fluid dynamics of rectangular external loop airlift reactor

AbstractThe novel design of a rectangular external loop airlift reactor is at present the most used large-scale reactor for microalgae culture. It has a unique future for a large surface to volume ratio for exposure of light radiation for photosynthesis reaction. The 3D simulations have been performed in rectangular EL-ALR. The Eulerian–Eulerian approach has been used with a dispersed gas phase for different turbulent models. The performance and applicability of different turbulent model’s i.e., K-epsilon standard, K-epsilon realizable, K-omega, and Reynolds stress model are used and compared with experimental results. All drag forces and non-drag forces (turbulent dispersion, virtual mass, and lift coefficient) are included in the model. The experimental values of overall gas hold-up and average liquid circulation velocity have been compared with simulation and literature results. It is seemed to give good agreements. For the different elevations in the downcomer section, liquid axial velocity, turbulent kinetic energy, and turbulent eddy dissipation experimental have been compared with different turbulent models. The K-epsilon Realizable model gives better prediction with experimental results.

Enhanced Bauxite Recovery Using a Flotation Column Packed with Multilayers of Medium

An innovative self-designed medium was packed in a bench-scale flotation column to study its influence on the flotation recovery of bauxite. Computational fluid dynamics (CFD) simulation was conducted to reveal the impact of the packing medium on the turbulent characteristics of collection zone in the column. Simulation results show that multilayer packing of the medium divides the collection zone into small units having different turbulent intensities, which is more suitable for flotation separation. The packing medium decreases the turbulence kinetic energy (from 1.08 × 10−2 m2/s2 to 2.1 × 10−3 m2/s2), turbulence eddy dissipation (from 3.71 × 10−2 m2/s3 to 9.8 × 10−3 m2/s3) and axial fluid velocity of fluid in the column. With three layers of packing, the recovery of Al2O3 increased by 2.11% and the aluminum to silicon content ratio of the concentrate improved from 5.16 to 9.72.

Soot Formation Model Performance in Turbulent Non-Premix Ethylene Flame: A Comparison Study

This paper presents results obtained from the application of a computational fluid dynamics (CFD) approach to modelling of non-premixed turbulent ethylene sooting flame. The study focuses on comparing the two soot models available in the Fluent in predicting the soot level in the turbulent non-premixed ethylene flame. A standard k-ε model and Eddy Dissipation model are utilized for the representation of flow field and combustion of the flame being investigated. For performance comparison study, a single step soot model of Khan and Greeves and two-step soot model proposed by Tesner are tested. The results of calculations are compared with experimental data for a turbulent sooting flame taken from literature. The results of the study show that a combination of the standard k-ε turbulence model and eddy dissipation model is capable of producing reasonable predictions of temperature both in axial and radial profiles; although further downstream of the flame over-predicted temperatures are evidence. With regard to soot model performance study, it shows that the two-step model clearly performed far better than the single-step model in predicting the soot level in ethylene flame at both axial and radial profiles.

The Correlation of Mesoscale Humidity Anomalies With Mesoscale Organization of Marine Stratocumulus From Observations Over the ARM Eastern North Atlantic Site

AbstractThe structure of mesoscale cellular organization in marine cloud‐topped boundary layers is found to be well characterized by using a novel compositing approach based on mesoscale variations in the column water vapor path (WVP). The approach is applied to ground‐based observations from the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic site. Based on a set of satellite and ground‐based observational criteria, 381 hr of closed‐cell and 227 hr of open‐cell cases were selected from late 2015 to early 2018. Strong correlations are found between mesoscale‐filtered cloud properties and WVP for both closed‐cell and open‐cell regimes. In the moist atmospheric columns, the clouds are thicker with higher tops and lower bases, and stronger precipitation compared to that in the dry columns. Overall, cloud properties of open and closed cells covary similarly with mesoscale moisture perturbations, except that the correlation of liquid water path with WVP is much weaker for open cells. The relations of subcloud properties with WVP are also examined. In the moist columns, surface equivalent potential temperature and relative humidity are higher than in the dry columns. A marginally negative correlation between mesoscale‐filtered subcloud turbulence eddy dissipation rate and WVP is found for closed cells. These results are consistent with the conceptual model of closed cellular mesoscale circulations of Zhou and Bretherton (2019).

The effects of porous wall as a novel flame stabilization method on flame characteristics in a premixed burner for CH4/air mixture by numerical simulation

In the present investigation, the effects of inner diameter and length of porous wall on flame stability limits, temperature distribution, methane conversion, and pressure drop in a premixed burner are numerically analyzed. The governing equations are solved by the control volume method, considering Re-Normalization Group k-ɛ for turbulence modeling and eddy dissipation concept for turbulence–chemistry interaction modeling. The simulations are performed for various porous walls with the inner diameter of 30, 40, and 50 mm and the length of 22, 44, and 66 mm. The results demonstrate that the increase in inner diameter of porous wall causes an increase in the lower flame stability limit and a decrease in the upper flame stability limit, gas and solid temperatures, pressure drop, and methane conversion. Also, the maximum solid and gas temperatures in the porous wall and methane conversion are related to the porous wall with 44 mm length. Furthermore, the methane conversion and pressure drop increase with the rise in the equivalence ratio. Finally, it can be said that the change in inner diameter of porous wall is more important than the length of porous wall in the studied phenomena.

Numerical and experimental evaluation of droplet breakage of O/W emulsions in rotor-stator mixers

Emulsification of a high viscosity oil in water mixture was studied in a rotor-stator mixer. Experimental characterization of drop sizes and high-speed visualizations of drop breakup phenomena were performed, combined with computational fluid dynamics analysis of the flow field and population balance modeling. Single-phase flow simulations were carried out with a sliding mesh CFD model and combined with analysis of streamlines, turbulent eddy dissipation fields and high-speed visualizations of drop breakage. From frames taken over few seconds, with the oil-water system initially fully segregated, three drop breakage mechanisms were identified, supported by the CFD analysis: (i) a combination of the large scale circular motion in the tank and the fluid jet emanating from the stator holes; (ii) droplets that are pulled into the rotor – stator system from the bottom and flow through the gap between the stator and rotor, undergoing extremely high shear stresses, being expelled through the stator slots in several fragments, and iii) droplets that remain caught at the recirculation zones in the stator holes, increasing exposure time to high shear during the rotor blade passage. Finally, an optimization algorithm was coupled to a population balance model and the measured drop size distributions were reproduced in good agreement with experimental data, including the bimodal behavior attributed to the generation of satellite droplets.

Numerical investigation of jet agitation in a nuclear liquid waste storage tank

Radioactive liquid waste is often stored in a large capacity (150 m3) horizontal cylindrical tanks. It is necessary to keep the contents of the tank agitated to prevent the settling of fine solids in the tank. In extreme cases, solids settled at the tank base may invoke the system malfunction. This paper deals with the Computational Fluid Dynamic (CFD) modeling of the agitation mechanism considering the turbulent dispersion effects. The simulations are conducted on a standard tank geometry to arrive the optimum jet velocity required to suspend the settled solid particles thoroughly. The distribution of volume phase fraction, velocity magnitude, turbulence kinetic energy (TKE) and eddy dissipation of each phase for different inlet jet velocities (v = 10–25 m/s) have been presented. The spatial variation of these quantities are measured at three different planes in the vessel mainly in the vicinity of the nozzle exit. The results indicate that the jet velocity is a significant parameter that influences the particle suspension. Parametric studies have also been carried out for four different particle sizes, viz. dp = 5, 10, 50 and 100 μm. The present study revealed that, for the range of parameters covered, the smallest (dp = 5 μm) and the largest (dp = 100 μm) particle sizes has least effect in terms of solids suspension.

Computational fluid dynamic on the temperature simulation of air preheat effect combustion in propane turbulent flame

this paper presents results obtained from the application of a computational fluid dynamics (CFD) code Fluent 6.3 to modelling of temperature in propane flames with and without air preheat. The study focuses to investigate the effect of air preheat temperature on the temperature of the flame. A standard k-ε model and Eddy Dissipation model are utilized to represent the flow field and combustion of the flame being investigated, respectively. The results of calculations are compared with experimental data of propane flame taken from literature. The results of the study show that a combination of the standard k-ε turbulence model and eddy dissipation model is capable of producing reasonable predictions of temperature, particularly in axial profile of all three flames. Both experimental works and numerical simulation showed that increasing the temperature of the combustion air significantly increases the flame temperature.

Effect of Free-Stream Turbulence Intensity on Transonic Airfoil with Shock Wave

Airplanes regularly operate switching between various flight modes such as take-off, climb, cruise, descend and landing. During these flight conditions the free-stream approaching the wings undergo fundamental changes. In transonic flow conditions, typically in the military or aerospace applications, existence of nonlinear and unsteady effects of the airflow stream significantly alters the performance of an airfoil. This paper presents the influence of free-stream turbulence intensity on transonic flow over an airfoil in the presence of a weak shock wave. In particular, NACA 0012 airfoil performance at Ma∞ = 0.7 is considered in terms of drag, lift, turbulence kinetic energy, and turbulence eddy dissipation parameters under the influence of varying angle of attacks and free-stream turbulence. The finite volume method in a commercial CFD package ANSYS-CFX is used to perform the numerical analysis of the flow. Mesh refinement using a mesh-adaption technique based on velocity gradient is presented for more accurate prediction of shocks and boundary layers. A Shear Stress Transport (SST) turbulence model is validated against experimental data available in the literature. Numerical simulations were performed, with free stream turbulence intensity ranging from low (1%), medium (5%) to high (10%) levels. Results revealed that drag and lift coefficients are approximately the same at every aforementioned value of turbulence intensity. However, turbulence kinetic energy and eddy dissipation contours vary as turbulence intensity changes, but their changes are disproportionally small, compared with values adopted for free-stream turbulence.

Flow Field Computational Analysis in a Solid Fuel Scramjet Combustor

According to the domestic and foreign institutions of solid fuel scramjet experimental data, adopt the numerical simulation method, design to construct the numerical model of the combustion chamber, using RNG k- turbulence model and eddy dissipation combustion model, simulation combustor working condition, analysis of the combustion chamber flow field data, concluded that the combustion phenomenon occurs mainly in the center of the flow field area. At the entrance of the combustion chamber can produce normal shock. The cavity can reduce the air velocity and it can guarantee the stability of the flame burning, and that to calculate the specific impulse is 126N·s/kg.

Directional moisture transfer through a wild silkworm cocoon wall.

A silkworm cocoon is a porous biological structure with multiple protective functions. In the current work, the authors have used both experimental and numerical methods to reveal the unique moisture transfer characteristics through a wild Antheraea pernyi silkworm cocoon wall, in comparison with the long-domesticated Bombyx mori silkworm cocoon walls. The water vapor transmission and water vapor permeability (WVP) properties show that the A. pernyi cocoons exhibit directional moisture transfer behavior, with easier moisture transfer from inside out than outside in [e.g., the average WVP is 0.057 g/(h m bar) from inside out and is 0.034 g/(h m bar) from outside in]. Numerical analysis shows that the cubic mineral crystals in the outer section of the A. pernyi cocoon wall create a rough surface that facilitates air turbulence and promotes disturbance amplitude of the flow field, leading to lengthened water vapor transfer path and increased tortuosity of the moist air. It also indicates the vortex of water vapor can be generated in the outer section of cocoon wall, which increases the diffusion distance of water vapor and enhances the turbulence kinetic energy and turbulence eddy dissipation, signifying higher moisture resistance in the outer section. The difference in moisture resistance of the multiple A. pernyi cocoon layers is largely responsible for the unique directional moisture transfer behavior of this wild silkworm cocoon. These findings may inspire a biomimicry approach to develop novel lightweight moisture management materials and structures.

ウィンドプロファイラとゾンデから求めた乱流エネルギー消散率の比較

ウィンドプロファイラとゾンデから求めた乱流エネルギー消散率の比較

Comparison of horizontal winds and turbulent eddy dissipation rates obtained by two different multi-receiver techniques – Spaced Antenna and Postset Beam Steering

The horizontal winds and eddy dissipation rates (ε) obtained by two multi-receiver techniques, Spaced Antenna (SA) and Postset Beam Steering (PBS), are compared and contrasted, using Middle and Upper atmospheric radar measurements. Comparison of mean horizontal winds obtained by both methods indicates a good agreement in zonal and meridional winds. On the other hand, the agreement in ε derived by SA and PBS is good in the lower troposphere (below ~9km), but the profiles of ε show some discrepancy in the upper troposphere. While the discrepancy is relatively more in layers of depleted turbulence, it is small in layers of enhanced turbulence. Comparison of signal-to-noise ratio and turbulence variations reveal that these variations are not due to specular echoes, rather due to anisotropic turbulence. The study suggests that these techniques and correction formulae for spectral width work well in the regions of enhanced turbulence but yield relatively less accurate values in regions of depleted turbulence.

Quantification of turbulence in cryogenic liquid using high speed flow visualization

A high speed flow visualization experiment was conducted to characterize the boiling induced turbulence when a cryogenic liquid is released on water. The advective transport of turbulent structures traversing through the liquid was captured and reconstructed using image processing software to obtain information on velocity components. The numerical results obtained from image processing were used to determine turbulence parameters like turbulent intensity, turbulent kinetic energy and eddy dissipation rate. An interesting aspect of the study was the formation of wavy structures called ‘thermals’ which were characteristic of turbulent convection. The thermals were found to act as a catalyst in increasing heat transfer and turbulence between water and cryogenic pool. The turbulent intensity was influenced by the turbulent velocity and had direct effects on the vaporization flux. Among the turbulence parameters, increase in turbulent kinetic energy resulted in faster vaporization of cryogenic liquid through enhanced mixing, whereas variations in the eddy dissipation rate had weak dependence on vaporization. Additionally, the initial height of cryogenic liquid was also found to strongly affect the vaporization mass flux.

Southern Annular Mode and westerly‐wind‐driven changes in Indian‐Atlantic exchange mechanisms

AbstractThe dynamical link between the Indian Ocean and Atlantic Meridional Overturning Circulation (AMOC) remains poorly understood. This partly arises from the complex Agulhas leakage, which occurs via rings, cyclones, and non‐eddy flux. Hindcast simulations suggest that leakage has recently increased but have not decomposed this signal into its constituent mechanisms. Here these are isolated in a realistic ocean model. Increases in simulated leakage are attributed to stronger eddy and non‐eddy‐driven transports, and a strong warming and salinification, especially within Agulhas rings. Variability in both regimes is associated with strengthening Indian Ocean westerly winds, reflecting an increasingly positive Southern Annular Mode. While eddy and non‐eddy flux signals are tied through turbulent eddy dissipation, the ratio between the two varies decadally. Consequently, while altimetry suggests a recent increase in retroflection turbulence and implied leakage, non‐eddy flux may also play a significant role in modulating the leakage AMOC connection.