Computational Fluid Dynamics Tools Research Articles

Automatic Design of Microfluidic Gradient Generators

Concentration gradient generators are common components of microfluidic devices when specified mixing ratios have to be acquired. However, depending on their complexity, the creation of a suitable design is an elaborate task. Although approaches exist that derive such a design, they suffer from low flexibility and usability, which is why most concentration gradient generators are still mainly designed manually thus far—a time-consuming and error-prone task. To tackle this problem, we propose a method for the automatic design of tree-shaped concentration gradient generators and introduce a publicly available online tool that automatically generates a suitable design by adjusting the hydrodynamic resistances within the network. Moreover, the proposed tool is not only able to generate designs complying with user-defined requirements, such as mixing ratios, flow rates, and channel widths but also works for any number of outlets. Simulations using a Computational Fluid Dynamics (CFD) tool as well as evaluations on fabricated designs confirm the quality of the results obtained by the proposed tool.

Development of efficient compressor wheel for turbocharger by varying number of blades

The present work is done to find out the change in the efficiency of the compressor wheel by varying the number of blades and the velocity and pressure variation at a particular speed. Efficiency values are observed oncalculated pressure ratio versus corrected air flow graph for varying number of blades on standard compressor map of GT-15 Turbocharger. GT–15 Turbocharger is considered for the analysis, which is used in sports vehicles. Modelling of compressor wheel is done by 3D modelling software PTC Creo parametric 2.0 and mesh generation is done by using Computational Fluid Dynamics (CFD) tools. The model is solved with a different number of blades i.e., 10, 12, and 15. An approach to increase the efficiency of the compressor wheel by increasing the number of blades is studied.

Nanosatellite case study: Issue of heat dissipation across passive thermal analysis

One of the major reasons, why nanosatellites are still in a state of vulnerability, is the increase of their thermal temperature, which in many cases leads to the failure of the missions they were originally designed for. In order to maintain the quality of the thermal management system, while keeping the electronic equipment in reliable working conditions. This paper presents a research on the thermal environment and the passive thermal control systems (PTC) of nanosatellites, based on a CFD computational fluid dynamics tool. The idea of this work is to provide a PTC to nanosatellites at a specific altitude in low earth orbit in order to determine their temperature values. From the results collected, the temperatures of the nanosatellites change drastically depending on the presence or absence of heat dissipation. This also depends on the type of the considered coating. It was also noticed that the temperature inclined with the choice of surface coating which was stipulated by the α/ε ratio. Therefore, it would be wise to choose the right surface coating at the very beginning of the design.

Design and analysis of hybrid electric multi-lobed airship for cargo transportation

Though a lot of curiosity has been developed towards a sustainable mode of cargo transportation, multi-lobed hybrid airships are touted to be a viable alternative to the conventional aircraft. The advantages of these Lighter Than Air (LTA) vehicles can further be boosted by incorporating a renewable source of energy. The requirements of high power in cargo airships and the current technological barriers call for the development of a hybrid propulsion system that can couple the best of both conventional and electric propulsion. The electric propulsion system is established by integrating solar arrays with fuel cells, which is then combined with conventional engines in a parallel power train architecture ensuring minimum transfer of energy and compact size. A solar irradiance model is adopted, to evaluate the amount of energy generated by the solar arrays installed over the airship envelope. A multi-lobed hybrid airship model is presented whose aerodynamic characteristics are obtained using computational fluid dynamic tools, which are then utilized to evaluate the power requirements through flight performance analysis. The hybrid propulsion system is then modified in accord with the power needs of the hybrid airship. Finally, detailed weight distribution between hybrid-powered and conventional hybrid-airship is presented displaying significant reduction in fuel weight of ∼350 kg due to the hybrid propulsion system.

On predicting particle capture rates in aquatic ecosystems.

Recent advances in understanding the capture of moving suspended particles in aquatic ecosystems have opened up new possibilities for predicting rates of suspension feeding, larval settlement, seagrass pollination and sediment removal. Drawing on results from both highly-resolved computational fluid dynamics (CFD) simulations and existing experimental data, we quantify the controlling influence of flow velocity, particle size and collector size on rates of contact between suspended particles and biological collectors over the parameter space characterising a diverse range of aquatic ecosystems. As distinct from assumptions in previous modeling studies, the functional relationships describing capture are highly variable. Contact rates can vary in opposing directions in response to changes in collector size, an organism’s size, the size of particles being intercepted (related to diet in the case of suspension feeders), and the flow strength. Contact rates shift from decreasing to increasing with collector diameter when particles become relatively large and there is vortex shedding in the collector wake. And in some ranges of the ecologically relevant parameter space, contact rates do not increase strongly with velocity or particle size. The understanding of these complex dependencies allows us to reformulate some hypotheses of selection pressure on the physiology and ecology of aquatic organisms. We discuss the benefits and limitations of CFD tools in predicting rates of particle capture in aquatic ecosystems. Finally, across the complete parameter space relevant to real aquatic ecosystems, all quantitative estimates of particle capture from our model are provided here.

Real-fluid thermophysicalModels: An OpenFOAM-based library for reacting flow simulations at high pressure

Although OpenFOAM is a widely-used open source computational fluid dynamics (CFD) tool, it is limited to numerical simulations of multi-dimensional reacting/nonreacting flows at relatively-low pressures. This is not only because real-fluid models that can evaluate thermophysical properties at high pressures are not available in the thermophysicalModels library of OpenFOAM, but also because the existing mixing model cannot handle various mixing rules of real-fluid models. In the present study, we develop a novel algorithm applicable for a mixture model incorporating various mixing rules in OpenFOAM. Based on the new algorithm, we update the thermophysicalModels library of OpenFOAM 6.0 by implementing a set of real-fluid models such as the Soave-Redlich-Kwong/Peng-Robinson equation of state, Chung's model for dynamic viscosity and thermal conductivity, mixture averaged model for mass diffusivity using Takahashi's correction for binary diffusion coefficients at high pressure. The new library is validated against experimental data and is further assessed for compressible reacting flows by performing two-dimensional numerical simulations of axisymmetric laminar non-premixed counterflow flames and one-dimensional numerical simulations of premixed CH4/air flames at high pressures. The developed library can be used for any reacting flow solvers in OpenFOAM 6.0 that adopt a set of implemented real-fluid models. Program summaryProgram title: Real-fluid thermophysicalModelsCPC Library link to program files:https://doi.org/10.17632/n8zb2wpjp6.1Developer's repository link:https://github.com/danhnam11/realFluidThermophysicalModels-6Licensing provisions: GPLv3Programming language: C++Nature of problem: The mixing rules required for evaluating the real-fluid based thermophysical properties of mixtures are relatively complicated and different from each other, while the existing thermophysicalModels library offers only a mixing rule based on the mass fraction weighted average of each species, for which overloading operator functions (“*” and “+=”) are utilized to update parameters for a mixture. However, this approach is not applicable for complicated mixing rules in the real-fluid models.Solution method: To build a mixture class containing different mixing rules in different models such as the equation of states and thermodynamics/transport properties in OpenFOAM, we adopt void-type functions (parameter updating functions) to update parameters of a mixture instead of the original overloading operator functions in OpenFOAM. The parameters in the mixing rules that depend on pressure and temperature are handled by decomposition such that they can be updated in the mixture class.

Optimum Operating Parameters and Blade Setting of a High-Speed Propeller

Traditionally, the design of propellers is accomplished on the bases of engineering experience and the availability of well-established experimental data. However, the emergence of computational fluid dynamics (CFD) tools coupled with advanced optimizers has made this task easier. This paper presents the essentials of an optimization-based CFD chain aimed at finding the optimal operating parameters and blade settings of a high-speed propeller. This fully automated procedure combines an improved hypercube sampling (IHS) for the initial design of experiment and a radial basis function (RBF) as the meta-model, hence leading to lesser CFD simulations. The coupling between IHS and RBF is judiciously implemented, and the sampling strategy and updates are thoroughly examined. The optimization procedure uses the multi-objective evolutionary algorithm nondominated sorting genetic algorithm to determine the operating parameters and blade setting that maximize the propeller efficiency and thrust. At the optimum point the blade operates with mild pressure gradients, whereas the transonic region is restricted to the upper corner of the blade tip.

NUMERICAL PREDICTION OF VERTICAL SHIP MOTIONS AND ADDED RESISTANCE

Along with the development of computer technology, the capability of Computational Fluid Dynamics (CFD) to conduct ‘virtual computer experiments’ has increased. CFD tools have become the most important tools for researchers to deal with several complex problems. In this study, the viscous approach called URANS (Unsteady Reynolds Averaged Navier-Stokes) which has a fully non-linear base has been used to solve the vertical ship motions and added resistance problems in head waves. In the solution strategy, the FVM (Finite Volume Method) is used that enables numerical discretization. The ship model DTMB 5512 has been chosen for a series of computational studies at Fn=0.41 representing a high speed case. Firstly, by using CFD tools the TF (Transfer Function) graphs for the coupled heave- pitch motions in deep water have been generated and then comparisons have been made with IIHR (Iowa Institute of Hydraulic Research) experimental results and ordinary strip theory outputs. In the latter step, TF graphs of added resistance for deep water have been generated by using CFD and comparisons have been made only with strip theory.

Quantification of Uncertainty in CFD Simulation of Accidental Gas Release for O & G Quantitative Risk Assessment

Quantitative Risk Assessment (QRA) of Oil & Gas installations implies modeling accidents’ evolution. Computational Fluid Dynamics (CFD) is one way to do this, and off-the-shelf tools are available, such as FLACS developed by Gexcon US and KFX developed by DNV-GL. A recent model based on ANSYS Fluent, named SBAM (Source Box Accident Model) was proposed by the SEADOG lab at Politecnico di Torino. In this work, we address one major concern related to the use of CFD tools for accident simulation, which is the relevant computational demand that limits the number of simulations that can be performed. This brings with it the challenge of quantifying the uncertainty of the results obtained, which requires performing a large number of simulations. Here we propose a procedure for the Uncertainty Quantification (UQ) of FLACX, KFX and SBAM, and show its performance considering an accidental high-pressure methane release scenario in a realistic offshore Oil & Gas (O & G) platform deck. The novelty of the work is that the UQ of the CFD models, which is performed relying on well-consolidated approaches such as the Grid Convergence Index (GCI) method and a generalization of Richardson’s extrapolation, is originally propagated to a set of risk measures that can be used to support the decision-making process to prevent/mitigate accidental scenarios.

Quadrature moment simulation of silica nanoparticles aggregation and breakage in chemical mechanical polishing

Chemical mechanical polishing (CMP), a wafer surface planarization method, is critical in the semiconductor industry because uniform and scratch-free processing is required for a highly integrated circuit on the wafer surface. Thus, the prediction of aggregated particle size during CMP helps achieve stable and precise processing during experimental circumstances. Herein, silica nanoparticle aggregation simulation during CMP process-based Quadrature Method of Moments (QMOM) was first described using ANSYS Fluent as a computational fluid dynamics (CFD) tool. Experiments were also implemented in a 10,000 class cleanroom using a CMP polisher actually used in the semiconductor industry. Moreover, a comparison of experimental particle size measured through zeta-potential and particle size analyzer and simulated Sauter mean diameter (D32) showed that the experimental data agreed well with the simulation results.

On the flash fire of stratified cloud of liquefied natural gas

This work was focused on the characterization of flash fire scenarios resulting from the accidental release of liquefied natural gas (LNG) by keeping into account vapour stratification and temperature gradient, which deeply modify the combustion phenomena for the standard homogeneous cloud. An open-source computational fluid dynamic (CFD) tool was used to this aim, provided premixed and diffusive/convective modification for the combustion model.The effects of the source model for the evaporation and the representative boundary conditions (wind velocity, heat flux from the substrate, relative humidity of the atmosphere, liquid pool diameter) were investigated. The heat flux from the substrate was found the most impacting parameter since it considerably helps vertical spreading, thus reducing the stand-off distances. The comparison with standardized procedures for the consequence assessment of flash fire highlights that the latter approach provides non-conservative results under certain circumstances. Hence, the adoption of the presented procedure is suggested.Considering the nature of the investigated scenario, the effects of thermal and mass stratifications on combustion efficiency were analysed too. As a result, the safety distances calculated by CFD for stratified mixtures were found almost double than the corresponding values estimated for homogeneous cloud and by integral models, commonly adopted in risk analyses.

Mixing Enhancement of Non-Newtonian Shear-Thinning Fluid for a Kenics Micromixer.

In this work, a numerical investigation was analyzed to exhibit the mixing behaviors of non-Newtonian shear-thinning fluids in Kenics micromixers. The numerical analysis was performed using the computational fluid dynamic (CFD) tool to solve 3D Navier-Stokes equations with the species transport equations. The efficiency of mixing is estimated by the calculation of the mixing index for different cases of Reynolds number. The geometry of micro Kenics collected with a series of six helical elements twisted 180° and arranged alternately to achieve the higher level of chaotic mixing, inside a pipe with a Y-inlet. Under a wide range of Reynolds numbers between 0.1 to 500 and the carboxymethyl cellulose (CMC) solutions with power-law indices among 1 to 0.49, the micro-Kenics proves high mixing Performances at low and high Reynolds number. Moreover the pressure losses of the shear-thinning fluids for different Reynolds numbers was validated and represented.

Effects of Selection of Inlet Perturbations, Multiphase and Turbulence Equations on Slug Flow Characteristics Using Altair® AcuSolve™

Selection of inlet perturbations, multiphase equations, and the turbulence equation may affect the development of slug flow using computational fluid dynamic simulation tools. The inlet perturbation, such as sinusoidal and random perturbations, play an essential role in inducing slug formation. Multiphase equations such as volume of fluid and level set methods are used to track and capture the gas-liquid immiscible interface. Similarly, turbulence equations such as Spalart Allmaras (SA), Detached Eddy Simulations (DES), k-omega, and k-epsilon can be used to predict the evolution of turbulence within the flow. At present, no direct comparison is available in the literature on the selection of (i) types of inlet perturbations, (ii) the choice of multiphase equations, and (iii) the turbulence equation on the development of slug flow using the Altair computational package. This article aims to compare the effects of the selection of inlet perturbations, multiphase models and turbulence equations on slug flow characteristics using Altair® AcuSolve™. The findings by Altair® simulation were compared to published experimental data and simulation works using ANSYS and STAR-CCM+. The slug flow characteristics of interest include slug morphology, a body length-to-diameter ratio, velocity, frequency, and pressure gradient. It was found that the slug flow could be developed for all combinations of settings. Although level set approach in Altair® can track fluid motion successfully, it has a limitation in modelling the convective transport of the multiphase mixture well, unlike ANSYS and STAR-CCM+. Compared to the standard level set method, the coupling of back-and-forth error compensation and correction with the level set function helps to capture the internal boundary more accurately by reducing errors caused by numerical diffusion in the transport of the level set. It was revealed that the Spalart Allmaras turbulence equation could mimic published experimental result better than DES as it produced the closest slug translational velocity. Since the frequency of the slugs for the developed models showed a good agreement with the published data, the models could be sufficient for the investigation of fluid-structure interaction.

Numerical Simulation of Low-Pressure Drop Static Mixers for Mixing Enhancement

Static mixers (SM), also generally known as inline mixers, form a newly developing industry trend. They have no moving parts, hence have lower energy consumption, lower installation cost, and very low maintenance cost, and are thus an attractive alternative to the conventional agitators. Modifications were made in the design to reduce pressure drop and increase the mixing intensity across the mixer and increase the application of inline mixers in the industries. Three hybrid geometries (different combinations of Kenics and LPD) of static mixers were constructed and simulated using computational fluid dynamics (CFD) tools. Kenics is an excellent radial mixing device, and low-pressure drop (LPD) an excellent axial mixing device. The key design parameter to modify LPD was the slope angle of elliptical plates which affects the mixer performance. Different slope angles from 90o to 120o were simulated. Kenics was modified for different aspect ratios, and the edge of Kenics was curved. Pressure drop, thermal, and Discrete Phase Model (DPM) analysis were performed on these three different classifications of hybrid geometries. The most promising geometry to emerge based on the low-pressure drop and good mixing efficiency both was the curved edge Kenics. Keen-sighted these results, further analysis was performed on curved edge Kenics after the modification of blend radius. It was concluded that for a lower Reynold number, the curved edge with a higher blend radius dominates all other mixers. Result validation was done by comparing the trends and sensitivity of process variables with the established results and standards.

Computational fluid dynamic simulations to improve heat transfer in shell tube heat exchangers

Abstract Improving the thermal efficiency of shell-tube heat exchangers is essential in industries related to these heat exchangers. Installing heat transfer boosters on the side of the converter tube is one of the most appropriate ways to enhance heat transfer and increase the efficiency of this equipment. In this article, spring turbulence is studied using the computational fluid dynamics tool. The displacement heat transfer coefficient and the friction coefficient were selected as the primary target parameters, and the effect of using spring tabulators on them was investigated. The ratio of torsion step length to turbulence pipe length, wire diameter to pipe diameter ratio, and flow regime was studied as the main simulation variables, and the simulation results were compared with a simple pipe. The effect of water-acting fluid, R22, and copper Nanofluid on tubes containing turbidity was compared and investigated. This study showed that due to the pressure drop, the pipe with a torsional pitch to pipe length ratio of 0.17, a turbulent diameter to pipe diameter ratio of 0.15, and a Reynolds number of 50,000 with fluid R22 has the best performance for heat transfer.

Predicting indoor particle dispersion under dynamic ventilation modes with high-order Markov chain model.

Mechanical and natural ventilations are effective measures to remove indoor airborne contaminants, thereby creating improved indoor air quality (IAQ). Among various simulation techniques, Markov chain model is a relatively new and efficient method in predicting indoor airborne pollutants. The existing Markov chain model (for indoor airborne pollutants) is basically assumed as first-order, which however is difficult to deal with airborne particles with non-negligible inertial. In this study, a novel weight-factor-based high-order (second-order and third-order) Markov chain model is developed to simulate particle dispersion and deposition indoors under fixed and dynamic ventilation modes. Flow fields under various ventilation modes are solved by computational fluid dynamics (CFD) tools in advance, and then the basic first-order Markov chain model is implemented and validated by both simulation results and experimental data from literature. Furthermore, different groups of weight factors are tested to estimate appropriate weight factors for both second-order and third-order Markov chain models. Finally, the calculation process is properly designed and controlled, so that the proposed high-order (second-order) Markov chain model can be used to perform particle-phase simulation under consecutively changed ventilation modes. Results indicate that the proposed second-order model does well in predicting particle dispersion and deposition under fixed ventilation mode as well as consecutively changed ventilation modes. Compared with traditional first-order Markov chain model, the proposed high-order model performs with more reasonable accuracy but without significant computing cost increment. The most suitable weight factors of the simulation case in this study are found to be (λ1 = 0.7, λ2 = 0.3, λ3 = 0) for second-order Markov chain model, and (λ1 = 0.8, λ2 = 0.1, λ3 = 0.1) for third-order Markov chain model in terms of reducing errors in particle deposition and escape prediction. With the improvements of the efficiency of state transfer matrix construction and flow field data acquisition/processing, the proposed high-order Markov chain model is expected to provide an alternative choice for fast prediction of indoor airborne particulate (as well as gaseous) pollutants under transient flows.

A Review of Numerical Simulation as a Precedence Method for Prediction and Evaluation of Building Ventilation Performance

Natural ventilation has been used widely in buildings to deliver a healthy and comfortable indoor environment for occupants. It also reduces the consumption of energy in the built environment and dilutes the concentration of carbon dioxide. Various methods and techniques have been used to evaluate and predict indoor airspeed and patterns in buildings. However, few studies have been implemented to investigate the relevant methods and tools for the evaluation of ventilation performance in indoor and outdoor spaces. The current study aims to review available methods, identifying reliable ones to apply in future research. This study investigates scientific databases and compares the advantages and drawbacks of methods including analytical models, empirical models, zonal models, and CFD models. The findings indicated the computational fluid dynamics (CFD) model is the most relevant method because of cost-effectiveness, informative technique, and proficiency to predict air velocity patterns and ratios in buildings. Finally, widely used CFD codes and tools are compared considering previous studies. It is concluded the application of codes for research is subject to the complexity and characteristics of a studied model, the area and field of study, the desired turbulence model, and the user interface.

Numerical modelling of wave transformation and runup over rough fringing reefs using VARANS equations

Computational Fluid Dynamics (CFD) models are gaining popularity recently in efforts to understand the complex dynamics of wave interaction with coral reef profiles. To provide an alternative approach to the current CFD models to account for the reef surface with large bottom roughness, a 3D numerical wave tank based on the CFD tool OpenFOAM® is developed in this study. The VARANS equations are solved for two-phase incompressible flow with the k-ω SST model for the turbulence closure and VOF method for tracking the free surface. The reef surface with high friction is modeled by using a porous media model in the VARANS equations. The numerical model is first validated with laboratory experiments. The model is then applied to evaluate the impacts of forcing condition (wave and water level), reef morphology (reef-flat width, fore-reef slope, and back-reef slope), and reef roughness on wave transformation and wave runup. Moreover, an empirical formula is derived from the numerical results to predict wave runup. Subsequently, the simulated cross-reef variations of energy dissipation, wave spectrum, wave shape parameters (skewness and asymmetry) and nonlinearity parameter (Ursell number) are discussed for both the smooth and rough reefs. Finally, the cross-reef variations of the TKE and its dissipation rate are also examined for both reefs via the numerical simulations.

Blade Roughness Effects on Compressor and Engine Performance—A CFD and Thermodynamic Study

Degradation of compressors is a common concern for operators of gas turbine engines (GTEs). Surface roughness, due to erosion or fouling, is considered one of the major factors of the degradation phenomenon in compressors that can negatively affect the designed pressure rise, efficiency, and, therefore, the engine aero/thermodynamic performance. The understanding of the aerodynamic implications of varying the blade surface roughness plays a significant role in establishing the magnitude of performance degradation. The present work investigates the implications due to the degradation of the compressor caused by the operation in eroding environments on the gas turbine cycle performance linking, thereby, the compressor aerodynamics with a thermodynamic cycle. At the core of the present study is the numerical assessment of the effect of surface roughness on compressor performance employing the Computational Fluid Dynamics (CFD) tools. The research engine test case employed in the study comprised a fan, bypass, and two stages of the low pressure compressor (booster). Three operating conditions on the 100% speed-line, including the design point, were investigated. Five roughness cases, in addition to the smooth case, with equivalent sand-grain roughness (ks) of 15, 30, 45, 60, and 150 µm were simulated. Turbomatch the Cranfield in-house gas turbine performance simulation software, was employed to model the degraded engine performance. The study showed that the increase in the uniform roughness is associated with sizable drops in efficiency, booster pressure ratio (PR), non-dimensional mass flow (NDMF), and overall engine pressure ratio (EPR) together with rises in turbine entry temperature (TET) and specific fuel consumption (SFC). The performance degradation evaluation employed variables such as isentropic efficiency (ηis), low pressure compressor (LPC) PR, NDMF, TET, SFC, andEPR. The variation in these quantities showed, for the maximum blade surface degradation case, drops of 7.68%, 2.62% and 3.53%, rises of 1.14% and 0.69%, and a drop of 0.86%, respectively.

Fluid flow characteristics of two types rotary engines

Hydrogen fueled rotary engines are potential alternatives to be green engines in vehicles. In this study, a numerical approach is used to investigate and compare the internal flow field characteristics of two types of rotary engines: triangular rotary engine (TRE) and liquid piston engine (LPE) also known as elliptical rotary engine. First, the three-dimensional fluid analysis models are developed for the two engines based on geometric designs of the rotors. The fluid analysis is carried out using the computational fluid dynamics (CFD) tool by assuming two working fluids: air and H2, and the dynamic mesh and turbulent flow models are integrated and simulated with ANSYS Fluent solver. For simulation convenience, combustion and thermal effects are neglected. The results show that the flow rate fluctuation coefficient and gas moment fluctuation coefficient are higher in LPE as compared to TRE, indicating LPE has relatively lower stability than TRE. The vortex number is found to be higher in LPE, signifying higher combustion efficiency of LPE as compared to TRE. However, LPE may have more energy dissipations, reduced exhaust and suction efficiency and high risk of leakages. The TRE has more stable flow, larger exhaust, lesser vortex, lower leakages, and simpler flow path as compared to LPE. Comparison of the two fluids i.e. air and H₂ shows that in TRE, air has high magnitude of average gas moment, high mass flow rate, high pressure and low flow velocity as compared to H₂, implying that air is more stable than H₂ in TRE. Conversely, H₂ is more stable than air in the case of LPE. The vortex number is relatively lower in both LPE and TRE using air indicating that air has low leakages than H₂ in both rotary engine types. This study can be used as a reference for a better rotary engine design in the future.