Commercial Computational Fluid Dynamics Solver Research Articles

Roughness constant selection for atmospheric boundary layer simulations using a k-ω SST turbulence model within a commercial CFD solver

Transit of an atmospheric boundary layer (ABL) profile within computational fluid dynamics (CFD) simulations is often hindered through unintended streamwise gradients in the flow resulting in ABL inhomogeneity. Within Reynolds-Averaged Navier–Stokes (RANS) turbulence modeling in Ansys Fluent, user-defined wall functions are not available for the k–ω class of RANS models as a remedy to this problem. Instead, the practitioner is required to use the sand–grain roughness property for ground surface roughness calibration, and specify a roughness height and roughness constant, Cs, accordingly. To-date, no clear guidance on their selection is available that can accommodate both high and low roughness terrains. To overcome this, a straightforward and practical calculation is presented for the roughness constant based on the standard wall function implementation. When this calculation is synthesized with other best-practice guidelines, it is possible to reliably model transiting ABL profiles based on wind-tunnel data whilst minimizing effects due to ABL inhomogeneity.

CFD-based model of adsorption columns: Validation

The computational research on adsorption is of significant interest for process development and optimization. Despite the existence of unidimensional models that reproduce simple vertical columns, a further level of detail is required to understand the intricacy of transport phenomena within industrial-grade columns across time and space. With that in mind, this research work focuses on developing and validating a higher-dimensional model to reproduce the flow, heat, and mass transfer phenomena inside adsorption columns using Computational Fluid Dynamics (CFD). The mathematics of adsorption are coded into the commercial CFD solver, ANSYS Fluent v19.2, using User-Defined Functions (UDF). The model is implemented using a mathematical adaptation of Fluent's standard transport equations, so that the material and thermal effects of adsorption can be accounted for. The resulting model is validated against published experimental results for the single-component and competitive adsorption of N2 and CO2 on zeolite 13X on a small-scale pilot unit. Good agreement between our modelling results and experimental data is demonstrated. Afterwards, a modified column geometry with an inlet jet is used to explore the impact of gas injection on the unsteady spatial distribution of matter and energy. Radial gradients and 2D contour plots are illustrated in all cases to showcase the enhanced insight that a higher-dimensional model provides. Finally, this model can be used to predict comprehensive characteristics of adsorption columns of any design and size, taking into account different designs of inflow gas distributors in industrial scale columns. It includes a new sort of adsorption devices using monolith instead of packed beds. All model UDFs are open-source and can be found in supplementary materials.

Hemodynamic analysis of coil filled patient-specific middle cerebral artery aneurysm using porous medium approach

Cerebral aneurysms are bulges of an artery, which could be life-threatening when ruptured. Depending on their size, shape, and location, they need to be managed either through clipping or an endovascular coiling intervention. When coiled, reduced hemodynamic activity enables the coil to get thrombosed and achieve flow stasis. However, some coils delivered into the aneurysm tend to prolapse into the parent vessel and cause stroke due to obstruction and embolization. The recurrence of an aneurysm after endovascular coiling is of concern in the treatment of wide necked aneurysms. The initial packing density or improper coiling of the aneurysm and its relation to recurrence remains uncertain. This study investigates the influence of reduction in coil fill volume and packing density on the aneurysm recurrence using hemodynamic parameters by analyzing its flow features. Finite element method based commercial computational fluid dynamics solver is employed for performing patient-specific simulations for the coil filled aneurysm. The present approach uses porous medium based formulation. The numerical simulations show that any reduction below the optimal coil fill volume and packing density inside the aneurysm increases the velocity magnitude, wall shear stress, time-averaged wall shear stress, and spatial gradient of wall shear stress and reduces the relative residence time. The hemodynamic parameters and flow features suggest that a reduction in the coil packing density inside the aneurysm increases the chances of aneurysm recurrence. Hence, an assessment on how to achieve optimal coil fill volume and packing density is critical in reducing the risk of aneurysm recurrence.

CFD Validation of Wall Laws in K-ω SST Model for Adverse Pressure Gradient-Free Downward Flow

This study introduces a validation investigation employing Computational Fluid Dynamics (CFD) to assess the utilization of the Consistent Wall Law (CWL) in tandem with the K-Shear Stress Transport (SST) turbulence model, specifically in scenarios characterized by downward flow without adverse pressure gradients. The precise prediction of turbulent flows near walls is of paramount significance in the realm of engineering applications. The CWL is meticulously designed to enhance the precision of near-wall turbulence modeling by ensuring a harmonious alignment between wall functions and the flow field. To gauge the performance of CWL in the challenging context of downward flows marked by adverse pressure gradients, we conducted numerical simulations using a commercial CFD solver. The results reveal a substantial improvement in agreement with experimental data, particularly in critical near-wall regions. CWL effectively showcases its prowess in managing adverse pressure gradients and offering precise predictions for wall-related parameters. Moreover, we executed a sensitivity analysis to explore the durability of CWL across various parameter variations, confirming its effectiveness and trustworthiness. This research significantly contributes to a more profound comprehension of turbulence modeling, thereby facilitating the generation of more accurate predictions in intricate flow scenarios that are pertinent to engineering applications.

Performance improvement of a Wells turbine through an automated optimization technique

• A new and efficient Wells turbine blade geometry is proposed. • Computational Fluid Dynamics is coupled with multi-objective optimization tool. • Influence of ring-type endplate is quantified. • Improvement of peak torque coefficient by up to 120% & efficiency by up to 9% The present article reports optimization of the performance of a Wells turbine, which is an axial turbine utilized for wave energy conversion, through a computational fluid dynamics (CFD) based automated optimization technique. The in-house optimization library OPAL++ was coupled to a commercial CFD solver to get the Pareto front defining the relationships between two objectives. Four different geometric parameters of the turbine with ring-type endplate were used, and the objectives were to maximize the torque coefficient and simultaneously minimize the pressure drop coefficient. From the Pareto front, two designs (G1 and G2) were chosen for further analysis. G1 improved the peak torque-coefficient by more than 120% and delayed the stall point from φ = 0.225 to φ = 0.3, while the peak efficiency dropped. Whereas, G2 improved the peak efficiency by 9.1%, but the peak torque coefficient was reduced by about 50%. The main contribution of the study is to develop an optimum Wells turbine geometry through the coupling of CFD and automated optimization algorithm - first of its kind applied to a Wells turbine with endplate. A detailed flow analysis, the influence of the endplate, and a comparison of the optimized geometries are presented.

Numerical investigation of thermal failure of a fuel subassembly foot wall during molten material relocation under various flow blockage scenarios in sodium-cooled fast reactor

The innovative Fuel Assembly Inner DUct Structure (FAIDUS) design inherently offers a quick relocation path for the molten core material to descend downwards via the inner duct and arrive at the foot of the Subassembly (SA) following the local core meltdown during Total coolant Flow Blockage (TFB) accident. With its high thermal energy and continual heat generation, the amassed molten mass targets the surrounding foot structure, causing the foot wall to fail. The thermal damage progression and discrete failure of the foot wall govern the later phase of the melt relocation that signals the accident state. To understand the thermal damage progression of SA foot, transient numerical simulations are performed using a commercial computational fluid dynamics solver, ANSYS Fluent. An enthalpy-porosity approach based solidification/melting model is employed to predict the wall failure mechanism and associated event sequences. The computational model is validated against the wall failure test results available in the literature. Subsequently, the foot wall failure analysis is undertaken using a 2-D axisymmetric physical model for various plausible blockage occasions that initiate the TFB accident near the SA foot. The numerical solution endeavors to explicate the transient behavior of molten pool, convection-diffusion controlled phase interface development, thermal transport mechanism, liquid front propagation across the foot cross-section, and associated thermal hydraulics. The present findings indicate that the standard SA foot design perpetuates the accident scenario by releasing the molten mass through the ruptured wall segment after failure. To address this, an improved slotted discriminator-based foot design is proposed in the present study, and the effectiveness of the modified foot design in realizing a positive melt relocation to the core catcher is examined. The simulation results suggest that the modified foot design permits the intended melt release to the core catcher, confirming the Controlled Material Relocation (CMR) strategy and restraining the accident propagation to neighboring SA.

COUPLED RANSE AND LIFTING LINE THEORY FOR ESTIMATION OF SHIP PROPULSION FACTORS

Advances in Computational Fluid Dynamics (CFD) techniques through the development of the Reynolds-Averaged Navier-Stokes Equations (RANSE) have assisted in estimation of resistance and propulsion characteristics of ships to a reasonable level of accuracy. The aim of this paper is to test and demonstrate the capabilities of the coupled RANSE and Lifting Line theory for undertaking ship resistance, propeller open-water and self-propulsion simulations. Further, parametric studies for generation of numerical propeller design sheets and optimisation of propulsive efficiency using the coupled simulation approach has been discussed. Commercial CFD solver “M/s Flowtech - Shipflow” has been used for the study. Initially, some benchmark experimental/numerical model results are validated with the results of the CFD simulations and then, further parametric analyses have been undertaken with the KRISO Container Ship and the KP505 Propeller. The numerical propeller series and the preliminary study methodology for optimization of location of propeller disc behind the ship’s hull are being proposed as an effective concept/feasibility design stage tool for estimation of ship propulsion characteristics.

Development and Verification of Nonequilibrium Reacting Airflow Modeling in ANSYS Fluent

High-speed flow problems of practical interest require a solution of nonequilibrium aerothermochemistry to accurately predict important flow phenomena, including surface heat transfer and stresses. The complex coupling of nonequilibrium effects on the aerothermal load remains a challenging aspect of hypersonic flow modeling. As a majority of these flow problems are in the continuum regime, computational fluid dynamics (CFD) is a useful tool for modeling nonequilibrium flows. However, there are a limited number of CFD solvers with nonequilibrium flow modeling capabilities that are highly scalable on parallel architectures and can handle complex geometries. This work demonstrates that a commercial general purpose CFD solver can be effectively modified to include high-fidelity nonequilibrium aerothermochemistry and transport models. The paper presents modifications of a general purpose CFD solver, ANSYS Fluent, using the user defined functions to model salient aspects of nonequilibrium flow in air with a five-species reacting mixture: , , NO, N, and O. The modifications to the solver include Gupta–Yos high temperature mixing laws and Park’s two-temperature nonequilibrium chemistry models. The developed solver was verified for several benchmark nonequilibrium flow problems and compared with the available experimental data and other nonequilibrium flow simulations. The work establishes future possibilities for coupled aerothermal and electromagnetic simulations in thermochemical nonequilibrium.

Bubble dynamics and heat transfer performance on micro-pillars structured surfaces with various pillars heights

Micro-pillars structured surfaces have been found to enhance nucleate boiling significantly, as compared with smooth surfaces. In this study, a VOF-based (volume of fluid) numerical model is developed to investigate the single-bubble dynamics on micro-pillars structured surfaces in nucleate boiling, aiming to understand the effect of pillar height (h) on boiling enhancement. The model is solving by the commercial CFD (computational fluid dynamics) solver Fluent. The results show that increasing h modifies remarkably the bubble morphology. An interesting phenomenon is observed for larger h that the bubble bottom undergoes expansion, shrinkage, and re-expansion in gaps between micro-pillars in the growth stage, so that a mushroom-like bubble is generated, with a small root and a large head. The modified bubble morphology promotes the bubble departure, leading to a shorter departure time. Moreover, increasing h also enhances both evaporation rate in the microlayer and evaporation rate on the liquid-vapor interface, and thereby increasing bubble departure diameter. Thus, the shorter departure time and larger departure diameter are responsible for the enhanced boiling heat transfer on micro-pillar structured surfaces with larger h.

Computational investigation of multi-phase flow effects on the performance of the steam turbine exhaust hood

In this research work, a computational modeling of multi-phase flow through an asymmetric exhaust hood is presented. The three-dimensional Navier–Stokes equations along with the standard [Formula: see text] turbulent model and the Eulerian–Eulerian multi-phase equations were solved. The coupling of the last stage turbine blades and the exhaust hood has been carried out using the actuator disc model, which is less computationally demanding. The finite volume-based commercial computational fluid dynamics solver, ANSYS FLUENT, is used for the present numerical simulations. The effects of wetness on the flow structure and the pressure recovery capacity of a steam turbine exhaust hood have been investigated. One of the salient findings is that the pressure recovery capacity of a steam turbine exhaust hood enhances due to wetness effects. Wetness-induced turbulence damping is noted to be playing a crucial role in the enhancement of pressure recovery capacity of an exhaust hood.

Two Closely Spaced Aneurysms of the Supraclinoid Internal Carotid Artery: How Does One Influence the Other?

The objective of this study was to use image-based computational fluid dynamics (CFD) techniques to analyze the impact that multiple closely spaced intracranial aneurysm (IAs) of the supra-clinoid segment of the internal carotid artery (ICA) have on each other's hemodynamic characteristics. The vascular geometry of fifteen (15) subjects with 2 IAs was gathered using a 3D digital subtraction angiography clinical system. Two groups of computer models were created for each subject's vascular geometry: both IAs present (model A) and after removal of one IA (model B). Models were separated into two groups based on IA separation: tandem (one proximal and one distal) and adjacent (aneurysms directly opposite on a vessel). Simulations using a pulsatile velocity waveform were solved by a commercial CFD solver. Proximal IAs altered flow into distal IAs (5 of 7), increasing flow energy and spatial-temporally averaged wall shear stress (STA-WSS: 3-50% comparing models A to B) while decreasing flow stability within distal IAs. Thus, proximal IAs may "protect" a distal aneurysm from destructive remodeling due to flow stagnation. Among adjacent IAs, the presence of both IAs decreased each other's flow characteristics, lowering WSS (models A to B) and increasing flow stability: all changes statistically significant (p < 0.05). A negative relationship exists between the mean percent change in flow stability in relation to adjacent IA volume and ostium area. Closely spaced IAs impact hemodynamic alterations onto each other concerning flow energy, stressors, and stability. Understanding these alterations (especially after surgical repair of one IA) may help uncover risk factor(s) pertaining to the growth of (remaining) IAs.

Surgical Aortic Valve Replacement: Are We Able to Improve Hemodynamic Outcome?

Aortic valve replacement (AVR) does not usually restore physiological flow profiles. Complex flow profiles are associated with aorta dilatation, ventricle remodeling, aneurysms, and development of atherosclerosis. All these affect long-term morbidity and often require reoperations. In this pilot study, we aim to investigate an ability to optimize the real surgical AVR procedure toward flow profile associated with healthy persons. Four cases of surgical AVR (two with biological and two with mechanical valve prosthesis) with available post-treatment cardiac magnetic resonance imaging (MRI), including four-dimensional flow MRI and showing abnormal complex post-treatment hemodynamics, were investigated. All cases feature complex hemodynamic outcomes associated with valve-jet eccentricity and strong secondary flow characterized by helical flow and recirculation regions. A commercial computational fluid dynamics solver was used to simulate peak systolic hemodynamics of the real post-treatment outcome using patient-specific MRI measured boundary conditions. Then, an attempt to optimize hemodynamic outcome by modifying valve size and orientation as well as ascending aorta size reduction was made. Pressure drop, wall shear stress, secondary flow degree, helicity, maximal velocity, and turbulent kinetic energy were evaluated to characterize the AVR hemodynamic outcome. The proposed optimization strategy was successful in three of four cases investigated. Although no single parameter was identified as the sole predictor for a successful flow optimization, downsizing of the ascending aorta in combination with the valve orientation was the most effective optimization approach. Simulations promise to become an effective tool to predict hemodynamic outcome. The translation of these tools requires, however, studies with a larger cohort of patients followed by a prospective clinical validation study.

Performance Prediction and Validation of a Small-Capacity Twisted Savonius Wind Turbine

In this study, an off-grid–type small wind turbine for street lighting was designed and analyzed. Its performance was predicted using a computational fluid dynamics model. The proposed wind turbine has two blades with a radius of 0.29 m and a height of 1.30 m. Ansys Fluent, a commercial computational fluid dynamics solver, was used to predict the performance, and the k-omega SST model was used as the turbulence model. The simulation result revealed a tip-speed ratio of 0.54 with a maximum power coefficient, or an aerodynamic rotor efficiency of 0.17. A wind turbine was installed at a measurement site to validate the simulation, and a performance test was used to measure the power production. To compare the simulation results obtained from the CFD simulation with the measured electrical power performance, the efficiencies of the generator and the controller were measured using a motor-generator testbed. Also, the control strategy of the controller was found from the field test and applied to the simulation results. Comparing the results of the numerical simulation with the experiment, the maximum power-production error at the same wind speed was found to be 4.32%.

Numerical Validation of NACA 0009 Airfoil in Ultra-Low Reynolds Number Flows

Validation of the sectional airfoil performance is necessary to test a propeller. This study presents a numerical validation of the computational fluid dynamics (CFD) method for testing a symmetric NACA airfoil called NACA 0009. An ultra-low Reynolds number of 20000 is assumed and investigated. The implementation is performed using a commercial CFD solver ANSYS Fluent. The obtained results are compared with experimental data obtained from literature. A coarse grid is adopted as meshing technique and a realizable k-ε turbulence model, which has provided the best results, is assumed. Results show favourable prediction for most of the considered angles of attack. Thus, an overall reliable model has been developed.

Effects of Upstream Step Geometry on Axisymmetric Converging Vane Endwall Secondary Flow and Heat Transfer at Transonic Conditions

This paper presents a detailed experimental and numerical study on the effects of upstream step geometry on the endwall secondary flow and heat transfer in a transonic linear turbine vane passage with axisymmetric converging endwalls. The upstream step geometry represents the misalignment between the combustor exit and the nozzle guide vane endwall. The experimental measurements were performed in a blowdown wind tunnel with an exit Mach number of 0.85 and an exit Re of 1.5×106. A high freestream turbulence level of 16% was set at the inlet, which represents the typical turbulence conditions in a gas turbine engine. Two upstream step geometries were tested for the same vane profile: a baseline configuration with a gap located 0.88Cx (43.8 mm) upstream of the vane leading edge (upstream step height = 0 mm) and a misaligned configuration with a backward-facing step located just before the gap at 0.88Cx (43.8 mm) upstream of the vane leading edge (step height = 4.45% span). The endwall temperature history was measured using transient infrared thermography, from which the endwall thermal load distribution, namely, Nusselt number, was derived. This paper also presents a comparison with computational fluid dynamics (CFD) predictions performed by solving the steady-state Reynolds-averaged Navier–Stokes with Reynolds stress model using the commercial CFD solver ansysfluent v.15. The CFD simulations were conducted at a range of different upstream step geometries: three forward-facing (upstream step geometries with step heights from −5.25% to 0% span), and five backward-facing, upstream step geometries (step heights from 0% to 6.56% span). These CFD results were used to highlight the link between heat transfer patterns and the secondary flow structures and explain the effects of upstream step geometry. Experimental and numerical results indicate that the backward-facing upstream step geometry will significantly enlarge the high thermal load region and result in an obvious increase (up to 140%) in the heat transfer coefficient (HTC) level, especially for arched regions around the vane leading edge. However, the forward-facing upstream geometry will modestly shrink the high thermal load region and reduce the HTC (by ∼10% to 40% decrease), especially for the suction side regions near the vane leading edge. The aerodynamic loss appears to have a slight increase (0.3–1.3%) because of the forward-facing upstream step geometry but is slightly reduced (by 0.1–0.3%) by the presence of the backward upstream step geometry.

Coupled Ranse and Lifting Line Theory for Estimation of Ship Propulsion Factors

Advances in Computational Fluid Dynamics (CFD) techniques through the development of the Reynolds-Averaged Navier-Stokes Equations (RANSE) have assisted in estimation of resistance and propulsion characteristics of ships to a reasonable level of accuracy. The aim of this paper is to test and demonstrate the capabilities of the coupled RANSE and Lifting Line theory for undertaking ship resistance, propeller open-water and self-propulsion simulations. Further, parametric studies for generation of numerical propeller design sheets and optimisation of propulsive efficiency using the coupled simulation approach has been discussed. Commercial CFD solver “M/s Flowtech - Shipflow” has been used for the study. Initially, some benchmark experimental/numerical model results are validated with the results of the CFD simulations and then, further parametric analyses have been undertaken with the KRISO Container Ship and the KP505 Propeller. The numerical propeller series and the preliminary study methodology for optimization of location of propeller disc behind the ship’s hull are being proposed as an effective concept/feasibility design stage tool for estimation of ship propulsion characteristics.

Influence of Contact Angle Boundary Condition on CFD Simulation of T-Junction

In this work, we study the influence of the contact angle boundary condition on 3D CFD simulations of the bubble generation process occurring in a capillary T-junction. Numerical simulations have been performed with the commercial Computational Fluid Dynamics solver ANSYS Fluent v15.0.7. Experimental results serve as a reference to validate numerical results for four independent parameters: the bubble generation frequency, volume, velocity and length. CFD simulations accurately reproduce experimental results both from qualitative and quantitative points of view. Numerical results are very sensitive to the gas-liquid-wall contact angle boundary conditions, confirming that this is a fundamental parameter to obtain accurate CFD results for simulations of this kind of problems.

Unconfined laminar nanofluid flow and heat transfer around a rotating circular cylinder in the steady regime

AbstractIn this work, steady flow-field and heat transfer through a copper- water nanofluid around a rotating circular cylinder with a constant nondimensional rotation rate α varying from 0 to 5 was investigated for Reynolds numbers of 5–40. Furthermore, the range of nanoparticle volume fractions considered is 0–5%. The effect of volume fraction of nanoparticles on the fluid flow and heat transfer characteristics are carried out by using a finite-volume method based commercial computational fluid dynamics solver. The variation of the local and the average Nusselt numbers with Reynolds number, volume fractions, and rotation rate are presented for the range of conditions. The average Nusselt number is found to decrease with increasing value of the rotation rate for the fixed value of the Reynolds number and volume fraction of nanoparticles. In addition, rotation can be used as a drag reduction technique.

Improved combustion model of boron particles for ducted rocket combustion chambers

A combustion model of boron particles for detailed Computational Fluid Dynamics (CFD) based simulations of ducted rocket combustion chambers is studied. It is aimed to construct a model for combustion of boron containing gas mixtures ejected from a solid propellant gas generator. This model includes all main physical processes required to define an accurate particle combustion simulation. The reaction rate modeling in similar, previous studies are improved for ramjet combustion chambers and this model provides better predictions for all particle sizes. The reactions in the ignition stage are reformulated as competing reactions for consumption of (BO)n polymers. Large discrepancies between the experimental and calculated ignition times for the 3 μm diameter particles in similar studies are eliminated. The developed model is added to a commercial CFD solver and can be used along with gas phase detailed turbulent combustion simulations of ducted rocket combustion chambers. Our simulation approach has provided us an effective tool, which allows us to forecast the effects of the changes on the performance and efficiency. This detailed combustion model is validated with existing experimental results available in open literature. The model is also compared with the results of similar previous studies.

Secondary flow and heat transfer coefficient distributions in the developing flow region of ribbed turbine blade cooling passages

This paper reports an experimental and numerical study of the development and coupling of aerodynamic flows and heat transfer within a model ribbed internal cooling passage to provide insight into the development of secondary flows. Static instrumentation was installed at the end of a long smooth passage and used to measure local flow features in a series of experiments where ribs were incrementally added upstream. This improves test turnaround time and allows higher-resolution heat transfer coefficient distributions to be captured, using a hybrid transient liquid crystal technique. A composite heat transfer coefficient distribution for a 12-rib-pitch passage is reported: notably the behaviour is dominated by the development of the secondary flow in the passage throughout. Both the aerodynamic and heat transfer test data were compared to numerical simulations developed using a commercial computational fluid dynamics solver. By conducting a number of simulations it was possible to interrogate the validity of the underlying assumptions of the experimental strategy; their validity is discussed. The results capture the developing size and strength of the vortical structures in secondary flow. The local flow field was shown to be strongly coupled to the enhancement of heat transfer coefficient. Comparison of the experimental and numerical data generally shows excellent agreement in the level of heat transfer coefficient predicted, though the numerical simulations fail to capture some local enhancement on both the ribbed and smooth surfaces. Where this was the case, the coupled flow and heat transfer measurements were able to identify missing velocity field characteristics.