CFD Techniques Research Articles

A study of energy transfer during water entry of solids using incompressible SPH simulations

Cavity formation during water entry of a solid corresponds to the deceleration experienced by the solid. Several experimental studies in the past have facilitated qualitative understanding of the relation between flow and impact properties and the type of cavity formed. The types of cavities formed are classified primarily based on the nature of the seal, such as (a) surface seal, (b) deep seal, (c) shallow seal and (d) quasi-static seal. The flow mechanism behind these features and their effects on the speed of the impacting solid require further quantitative understanding. A study of such phenomenon is difficult using the existing CFD techniques owing to the fact that the high density ratios between the two phases, namely water and air, bring in issues with respect to the convergence of the linear system used to solve for the pressure field for a divergence-free velocity field. Based on a free surface modeling method, we present Incompressible Smoothed Particle Hydrodynamics (ISPH) simulations of water entry of two-dimensional solids of different shapes, densities and initial angular momenta. From the velocity field of the fluid and shape of the cavity, we relate the transfer of kinetic energy from the solid to the fluid through different phases of the cavity formation. Finally, we present a three-dimensional simulation of water entry to assert the utility of the method for analysis of real life water entry scenarios.

전산해석을 활용한 두께비 18%익형(Case1)의 공력특성 분석

두께비 18% 익형(KARI-11-180)에 대한 공력특성 분석이 전산해석기법을 활용해 수행되었다. 익형주위의 격자는 벽면에서 수직으로 투영하여 경계층 격자를 형성하였고 익형 후방에는 정밀한 후류 예측을 위해 조밀한 격자를 위치하였다. 원방경계까지의 거리는 익형 코드의 100배로 정하였고 익형시험결과와의 비교를 위해 자유류 속도, 익형 코드 및 Reynolds수를 풍동시험과 동일하게 정하였다. 또한 난류 모델은 천이지점 예측이 우수한 transition SST 및 DES 모델을 사용하였다. 3차원 전산해석은 세장비가 2와 5인 익형모델에 대해 수행되었는데 양력은 풍동시험결과보다 높은 값을 항력은 낮은 값을 예측하였다. Aerodynamic analysis for the airfoil, KARI-11-180 having 18% thickness ratio, was performed with CFD techniques. The boundary layer grid was generated by projecting the wall grid normally and fine grid was placed behind the trailing edge to capture the wake accurately. The distance to the far boundary is 100 chords and the flow condition is same as the wind tunnel test condition. Transition SST and DES turbulence models were utilized for accurate prediction of the transiton point. The predicted lift is higher but the drag is predicted lower than the wind tunnel test. 3-dimensional results with airfoil models of which aspect ratio were 2 and 5 were compared with 2-dimensional results.

Estimation of Leakage Mass Flow Rate through Labyrinth Seal Using CFD Techniques

Seal design is an essential part for turbo machinery. Seal consisting of fins is placed in a gap between stationary and rotating component to minimize the leakage flow. Seal leakage flow has been considered as an inevitable loss factor that highly affects the efficiency of any machine. During operation of the equipment, thermal expansion/contraction of components take place, which causes variation of the gap between stationary and rotating component. Importance of the study is to understand the flow behavior due to variation of the gap. The variation of gap leads to change of radial clearance between fin to metal component and subsequent change of flow pattern.
 The main focus of the paper is to estimate the leakage flow through a labyrinth seal placed between rotor and casing of a typical steam turbine. Numerical techniques using 3D CFD tool are used for this purpose. Three different seal configurations are proposed in the study. The variables of the three seal configurations are radial clearance, number of fins in the flow passage and pressure drop across the seal passages.
 As an alternative methodology, an empirical correlation is formulated based on numerical simulation results for one set of radial clearance to estimate mass flow rate through the seal. In order to validate the formulated correlation, mass flow rate is determined for another set of radial clearance and compared with numerical simulation results. It is observed that flow rate estimated from 3D CFD study is around 20% lower compared to empirical correlation.

Characterization of Modular Deposits for Urban Drainage Networks Using CFD Techniques

Storm tanks are an increasingly solution used to control floods caused by the increase of runoff flows generated by rain. To expedite the construction of these infrastructures and reduce downtime of urban supplies plastic modular elements as basic building structures are used. This paper presents the characterization of this sort of modular elements determining its energy dissipation to the passage of the water and comparing the hydraulic behavior of modular deposits to conventional ones.This paper justifies the use of computational fluid dynamic techniques (CFD) for characterizing hydraulic behavior of these modular elements based on three-dimensional models. These models have been validated by an experimental prototype built in laboratory. This one consists of a channel along which various completely monitored modular blocks are disposed in order to analyze both, circulating flows and water levels reached at different points of the system. The final result is a validated and calibrated model that allows to represent the behavior of this kind of infrastructures against strong intensity storms.A one-dimensional model of such structures is proposed as a way to join it with a specific computational model of drainage networks analysis as is the case of SWMM. Finally, a comparison between the results obtained by CFD techniques and those obtained after adjustment in SWMM is shown.

Non-stationary CFD Simulation of a Gear Pump

The use of modern CAD and CFD techniques in the conception and simulation of industrial products has had huge applications in the mechanical, automotive and aerospace industries. This paper includes all the steps to follow, from the treatment of CAD geometry up to the analysis of the simulation results. In particular we treat CAD simplification, the meshing of the geometry, the numerical strategy and, finally, analysis of the simulation results.This paper presents important aspects of a practical methodology applied to design a gear pump using the ANSYS Gambit software parametric abilities followed by a FVM analysis of its assembly to identify the most stressed components. A model for the prediction of the dynamic behaviour of an external gear pump is presented. We took into account the most important phenomena involved in the operation of this kind of machines. Fluid pressure distribution on gears, which is time-varying, is instantaneously computed and included as a resultant external force and torque acting on the gears.

Computation of Aerodynamic Noise Radiated from Ducted Tail Rotor Using Boundary Element Method

A detailed aerodynamic performance of a ducted tail rotor in hover has been numerically studied using CFD technique. The general governing equations of turbulent flow around ducted tail rotor are given and directly solved by using finite volume discretization and Runge-Kutta time integration. The calculations of the lift characteristics of the ducted tail rotor can be obtained. In order to predict the aerodynamic noise, a hybrid method combining computational aeroacoustic with boundary element method (BEM) has been proposed. The computational steps include the following: firstly, the unsteady flow around rotor is calculated using the CFD method to get the noise source information; secondly, the radiate sound pressure is calculated using the acoustic analogy Curle equation in the frequency domain; lastly, the scattering effect of the duct wall on the propagation of the sound wave is presented using an acoustic thin-body BEM. The aerodynamic results and the calculated sound pressure levels are compared with the known technique for validation. The sound pressure directivity and scattering effect are shown to demonstrate the validity and applicability of the method.

Investigation and Assessment of the CFD for Horizontal Flow in the VHTR Core

A nuclear power station using gas as a cooling medium has attracted so much attention because it offers high efficiency and greater safety. For a nuclear station that operates at a very high temperature, a gas-cooled reactor is fueled by uranium, moderated by graphite, and customarily cooled by helium. Nevertheless, throughout the operation, the bypass flow might be a result of a change in graphite shape that is caused by neutron damage. Core bypass and cross flows are significant elements to consider since the cross gap set hurdles to the flow field that are capable of diverting sufficient amount of coolant from reactor core location and initiating a possible fuel overheating. However, there is a great need to sufficiently validate this method by carrying out a thorough evaluation based on working environment analysis. Comparing the computed results with the existing data from Groehn’s NHDA PMR-200 study was the only way to validate data. A model simulation was performed on a two-prismatic fuel block with a cross gap to examine the gaping size effect. Finally, the prediction methods for horizontal flow phenomena using a CFD technique and the field investigation results from the VHTR core were verified, and the identification of the horizontal flow behavior played a vital role in investigating the coolant velocity and pressure distribution in the horizontal gap.

Computational fluid dynamics modeling patterns and force characteristics of flow over in-line four square cylinders

The flow over four square cylinders in an in-line, square arrangement was numerically investigated by using the finite volume method with CFD techniques. The working fluid is an incompressible ideal gas. The length of the sides of the array, L, is equal. The analysis is carried out for a Reynolds number of 300, with center-to-center distance ratios, L/D, ranging from 1.5 to 8.0. To fully understand the flow mechanism, details in terms of lift and drag coefficients and Strouhal numbers of the unsteady wake frequencies are analyzed, and the vortex shedding patterns around the four square cylinders are described. It is concluded that L/D has important effects on the drag and lift coefficients, vortex shedding frequencies, and flow field characteristics.

CFD Simulation of Hydraulic Tank

The aim of this paper is the examination of fluid dynamics in a tank. The use of modern CAD and CFD techniques in the conception and simulation of industrial products has huge applications in the mechanical, automotive and aerospace industries. This paper includes all the steps from treatment of CAD geometry up to the analysis of simulation results. The presented approach involved CAD simplification, meshing of the geometry, CFD simulation and analysis of the simulation results.A case study of a hydraulic tank partially filled with hydraulic oil was simulated in this paper using Volume of Fluid (VOF) multiphase model. Simulations compared the amplitude of sloshing in tank. In this paper, a part of the project which aims to develop a computer aided methodology for developing/designing of the fuel tanks based on static and dynamic analysis is presented.

Investigation of the Flow Characteristics of Titanium - Oxide - Water Nanofluid in Microchannel with Circular Cross Section

The presented paper reports the analysis of the flows characteristic of TiO2-water nanofluid flowing inside a horizontal microchannel with circular cross section area. The flow is investigated by CFD techniques using a finite volume method. A recently introduced viscosity correlation was used to model the effective viscosity of the nanofluid. A range of Re number is tested in the present study. Various temperature ranges were used as constant temperature boundary conditions. The increase of the nanoparticle volume fraction was found to increase the heat transfer rate; water showed less enhancement in heat transfer compared to the nanofluid. The increase in Re number promoted Nu number. The effect of the temperature on the effective viscosity in the channel was also reported. The change of the velocity in the entrance region was studied and discussed. The velocity gradient in the microchannel is calculated, and the results are shown and discussed.

Study on numerical calculation method for hydrodynamic parameters of WEC

For the effect of hydrodynamic parameters on the dynamic performance of wave energy devices is very significant, these parameters must be considered carefully when adjusting dynamic characteristics of devices. On the other hand calculating hydrodynamic parameter of devices accurately can guarantee rational dynamic property parameter adjustment. By using CFD technique and considering the definition of hydrodynamic parameters, the phase relationship between added mass and damp as well as the equation of forces, one new calculation method of hydrodynamic parameter was presented. Finally one example demonstrated the effectiveness of the new analysis method presented in this paper.

Effect of the Configuration of the Dual Impeller Agitated Vessel on the Flow Pattern and Rate of Mass Transfer at the Wall

Agitated vessels are now being extensively used in many applications regarding water and wastewater treatment where reaction and physical treatment processes can occur at the wall. Thus, the aim of the present work is to study the flow pattern of double impeller agitated vessel and its effect on the rate of mass transfer at the wall. Three different configurations were studied: double Rushton turbine, and a combination of pitched blade turbine and Rushton turbine to form pumping up and pumping down configurations. The rotational speed and the distance between the impellers were varied. Computational Fluid Dynamic was used to evaluate the flow pattern in the vessel and the velocity distribution near the wall of the tank, while the limiting current technique was used to determine the value of mass transfer coefficient at different heights of the wall. The mass transfer coefficient was found to depend strongly on the position of the impellers and the speed of rotation. Moreover, the configuration of impellers has a noticeable effect. The results show the potential usefulness of the CFD technique as a tool for designing stirred reactors for different applications.

CFD Technique for Solving Low Water Level Problem of Axial Flow Pumps

In this paper, water levels variation in the sump intake is studied in El- SHABAB Pumping Station. ANSYS Ver.17.1 flow simulation software is used to simulate the flow conditions to overcome decreasing water level in the low demands period, winter closure. Seven scenarios are studied to obtain the best one to apply it at that condition. Case 1 represents the optimum design when the submergence water level is 5m above the sump floor and the length of submersed part in the sump is 3.5m. Case 2 represents the winter closure period where the submergence water level decreased by 1.5m to become 3.5m in the period winter closure and the length of submersed part in the sump is 2m. As a result, velocity in the suction pipe decreased 26%, a swirl angle 10.79° is attained, and vibration level increased 4 times representing hydraulic and dynamic problems. Case 3 is done by adding a joint with length 0.5m to the submersed part of pipe. Case 4 is done by increasing a joint length by 1m to the submersed part of pipe. Case 5 is done by adding a cone under the bell mouse with height 1m and with the same pipe diameter. Case 6 is done by adding a cone under the bell mouse with height 1m and 0.5m joint leading to increase the submersed part of pipe to become 2.5m. Case 7 is done by repeating case 6 with increasing water submerging level after finishing the period of winter closure where the water level in the sump is 5m. The results indicate that, during the period of winter closure, Case 6 is the best condition to use where it prevents vortex and has the best swirling angle 5.12° with no turbulence is observed at the entrance of the pump improving the dynamic and hydraulic performance of the pump. Also, the results indicate that Case 7 after finishing the period of winter closure and water level returned to become 5m (optimum design) is the best condition to use during all seasons where it prevents vortex and has the best swirling angle 4.66°.

CFD Simulation of Ejector in Steam Jet Refrigeration

In this study CFD technique was employed to investigate the effect of divergent angle of primary nozzle, NXP (NXP = Distance between the nozzle exit to mixing chamber inlet) and throat of the ejector on the performance of ejector using the steam jet refrigeration cycle. In all these cases, only one fixed mixing chamber with different divergent primary nozzle, NXP and throat of an ejector was investigated numerically using the commercial CFD package, the effect of primary fluid pressure mass flow rate and mach numbers were observed and analyzed. The velocity contour lines were used to explain the happening of process inside the ejector it was found that the throat diameter, NXP and diverging angle of primary nozzle plays a crucial role in the steam jet refrigeration.

CFD modeling of a fixed-bed biofilm reactor coupling hydrodynamics and biokinetics

Rigorous modeling of transport phenomena is essential to reproduce accurately biofiltration systems performance. In this sense, the aim of this study was to investigate the effect of integrating fluid flow dynamics in the development of these bioreactor models, mimicking their hydrodynamics and behavior in a fixed biofilm reactor. 2D bioreactor models were developed using three different well-established tools for modeling bioreactors (AQUASIM, MATLAB, and Computational Fluid Dynamics – CFD), considering from ideal flow patterns to more complex fluid dynamics. A detailed comparison was performed among the results, taking into account the simulation of dissolved oxygen profiles in the liquid phase, inside the biofilm and in the boundary layer along a bioreactor. These models were validated by comparing the simulations with direct measurements obtained by means of dissolved oxygen microsensors of high spatial resolution. In all cases, deviations were below 6%, nevertheless CFD predictions obtained the lowest deviations below 3.5%. Thus, these results underline that CFD techniques are appropriate to model more accurately the performance of fixed-bed biofilm reactors, allowing the study in detail of all the hydrodynamics variables involved in the process. In addition, a 3D CFD model, combining hydrodynamics and biological reactions, was developed and solved to simulate local transient flow and dynamic behaviors of oxygen consumption in the bioreactor. The results of CFD simulations were evaluated in order to know the effect of mass transport phenomena (advection and diffusion) by characterizing hydrodynamics and, finally, to predict the oxygen degradation along the bioreactor.

Conjugate Heat Transfer Study of Combined Radiation and Forced Convection Turbulent Separated Flow

Abstract In the current study, a numerical investigation of two-dimensional combined convection-radiation heat transfer of turbulent gas flow over a backward-facing step (BFS) in a horizontal rectangular duct is presented. The computational domain contains two different parts including gas flow and solid element that makes the problem as a conjugate one. The gas phase is considered to be a radiating media that can absorb, emit and scatter thermal radiation, where in solid phase, heat transfer takes place by conduction. The set of governing equations for gas flow is solved numerically using the CFD technique and the k − ε $$k - \varepsilon $$ model is employed for computation of turbulence fluctuations. To evaluate the radiative term in the gas energy equation, the radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM). Inside the solid phase, the conduction equation is solved to obtain the temperature distribution. The effects of conduction ratio, optical thickness, radiation-conduction parameter and albedo coefficient on heat transfer behavior of the system are carried out.

Analyzing the fluidization of a gas-sand-biomass mixture using CFD techniques

Fluidization taking into account the presence of the material to be gasified is a differential addressed in this study. Accordingly the solid phase was composed of a binary mixture of sand and biomass. This work deals with the numerical simulation using CFD of a gasifier bubbling fluidized bed for the system composed of gas - biomass – sand. In order to determine the best fluidization conditions, a factorial design 23 was carried out varying the biomass particle density and diameter and the biomass percentage in the solid phase. To perform the simulations, ANSYS CFX 15.0 was used, adopting an Eulerian approach coupled to the Kinetic Theory of Granular Flow. The k-ε turbulence model was adopted. Seventeen simulations were performed setting the gas superficial velocity to 0.38ms−1. Based on the results of the factorial design, it was possible to qualitatively identify the tests to which the system reached a bubbling fluidization. Segregation of the particulate medium occurred for assays where the ratio between the mass of each biomass particle and the mass of each sand particle was >1.0 coupled to a ratio between biomass and sand volume fractions in the bed ≤0.5. The variable with the highest significance in the model equation was the diameter of the biomass particle. Volumetric fraction profiles of gas, sand and biomass were obtained to the 17 factorial design conditions as well as a model that predicts the bed expansion. The assay that reached greater bed height (0.50m), staying on bubbling regime, was the one with 15% biomass volume fraction with 375μm diameter and 45% sand, indicating those are good conditions for fluidization.

Optimum design for new gas lift valve seat

The Testing Gas Lift Valve (GLV) Beveled Seat showed excellent performance improvement as compared to the Sharp-Edged Seat design as reported by Elldakli et al. (2014). To optimize the initial Beveled Seat design requires manufacturing and testing numerous cases which is a very costly and time-consuming process. Use of CFD technique provides an excellent alternative solution. In this case the testing of the beveled seat experiments were matched with a CFD software. The validated model is used to optimize the design.The goal of the Gas Lift Valve seat design optimization is to find the best possible design out of a set of alternatives. The numerical model is used for further GLV improvement by investigating the effect of various parameters on the performance of the valve. The effect of GLV seat geometry such as Port Bottom Height (PBH), Port Top Height (PTH), Port Top Diameter (PTD) and Ball Size were studied. Fourteen cases representing three different basic scenarios were simulated.The optimum design for each port was compared within the initial modified design (Beveled seat) and sharp-edged seat. The results show significant improvement with the optimized design over the original design.

Study and Optimization of a CO2 Sparger for Carbonated Beverages and Beer by Means of CFD Modeling

Abstract In the present work, two different geometries of spargers for beverage carbonation were modeled by means of CFD technique, taking into consideration three different flow rates. The first geometry presented a radial inlet of liquid food while the second one a tangential one. Calculation allowed to study the effect of fluid velocities on mixing and to identify the best solution; mathematical results were then confirmed from a qualitative point of view by experimental tests with both water and apple juice. CFD resulted a very useful technique for in-silico designing, not only for technically-simple parts of plants but also for very complicated ones such as carbon dioxide spargers in which gas and liquid are mixed together.

Actuator disc methods for open propellers: assessments of numerical methods

ABSTRACTThe paper describes the assessment of two different actuator disc models as applied to the flow around open propellers. The first method is based on a semi-analytical approach returning the solution for the nonlinear differential equation governing the axisymmetric, steady, inviscid and incompressible flow around an actuator disc. Despite its low computational cost, the method does not require simplifying assumptions regarding the shape of the slipstream, e.g. the wake contraction is not disregarded or prescribed in advance. Moreover, the presence of a tangential velocity in the wake as well as the spanwise variation of the load are taken into account. The second one is a commonly used procedure based on CFD techniques in which the effects of the propeller are synthetically described through a set of body forces distributed over the disc surface. Both methods avoid the difficulties and the computational costs associated with the resolution of the propeller blades geometrical details. The comparison is based on an in-depth error analysis of the two procedures which results in a set of reference data with controlled accuracy. An excellent agreement has been documented between the two methods while the computational complexity is obviously very different. Among other things the comparison is also aimed at verifying the accuracy of the semi-analytical approach at each point of the computational domain and at quantifying the effect of the errors embodied in the two methods on the quality of the solution, both in terms of global and local performance parameters. Furthermore, the paper provides a set of reference solutions with controlled accuracy that could be used for the verification of new and existing computational methods. Finally, the computational cost of the semi-analytical model is quantified, thus providing a valuable information to designers who need to select a cost effective and reliable analysis tool.