Capability Of Computational Fluid Dynamics Research Articles

Computational flow noise

This work focuses on combining the capabilities of computational fluid dynamics with computational structural acoustics to enable the calculation of flow noise primarily for undersea vehicles. The work is limited to the non-coupled problem, where the flow calculations are made over a non-deforming boundary with the goal of calculating wall pressure fluctuations and using them as loads on a finite element structural acoustics model. The ultimate goal of this work is to develop the capability to calculate flow noise for three-dimensional undersea structures for which analytical approaches are not possible. Results will be presented that make use of wall pressure fluctuations calculated using Large Eddy Simulations (LES) and variants of Improved Delayed Detached Eddy Simulations (IDDES).

A validation of computational fluid dynamics temperature distribution prediction in a pulverized coal boiler with acoustic temperature measurement

The main objective of this work was to examine the capability of CFD (Computational Fluid Dynamics) on properly predicting temperature distribution in the combustion chamber. Numerous approaches were employed to verify CFD models of large-scale utility boilers. Furnace Exit Gas Temperature is one of the key values used for verification studies. Harsh environment and large dimensions inside the furnace make temperature measurement a complex task. Traditionally used suction pyrometry provides only local information. With this technique, while extremely accurate, it is practically impossible to obtain a representative temperature distribution at the furnace exit as measurements in different locations are not taken at the same time. Acoustic Pyrometry technique is the most appropriate for comprehensive CFD flame shape prediction verification. Not only average temperature value in a certain boiler cross-section can be continuously measured but also its complete two-dimensional distribution. CFD code was used to simulate the OP-650 front-fired boiler operation. The boiler is equipped with Acoustic Gas Temperature Measuring system located in a horizontal plane approximately 4 m under the furnace exit. Comparison of simulation results with measurements proves good accuracy of CFD results.

Adaptive neuro-fuzzy system (ANFIS) based appraisal of accumulated heat from hydrogen-fueled engine

Hydrogen as an ecological fuel has called a great deal of attention and has been a dynamic field of studying interest for engine designers. The work presented herein is an investigation of the accumulated heat created in hydrogen fueled engine as affected by crank angle, equivalence ratio, penetration and H2 mass fraction. The capability of computational fluid dynamics (CFD) approach was applied to simulate the combustion process while the potential of a universal approximator, ANFIS (adaptive neuro-fuzzy inference system), was utilized to model the problem. Two statistical performance criteria of root mean square error (RMSE) and coefficient of determination (R2) were considered to further assess the modeling performance. The findings revealed that due to high diffusivity characteristics of hydrogen, the optimum level of equivalence ratio is affected by the addition of hydrogen. It was observed that hydrogen addition yields higher heat release since hydrogen and air can mix well at any standard scale. Higher engine speed than that of 3500 rpm resulted in a higher spray penetration at any specified crank-angle position. It was also concluded that an ANFIS structure with Trimf membership algorithm each at three membership functions yielded RMSE of 0.011527 and R2 equal to 0.999.

Low-Reynolds-Number One-Equation Turbulence Model Based on Closure

Accurate turbulence modeling remains a critical problem in the prediction capability of computational fluid dynamics. In this paper, a new one-equation eddy-viscosity model is derived from closure. The equation used in this derivation includes a cross-diffusion term that allows the new model to switch between the model properties exhibited by the two-equation or turbulence models. In addition, the damping function used in conjunction with the proposed one-equation model has not been studied before in the literature. The new model is used to simulate several benchmark canonical flows involving both the free shear layer and wall-bounded turbulent flows with small separation regions. The open-source software OpenFOAM is used for the flowfield calculations. It is shown that the new model improves the accuracy of flow simulations compared to the widely used one-equation Spalart–Allmaras model and is competitive with the shear-stress-transport model.

Understanding Wind-Turbine Wake Breakdown Using Computational Fluid Dynamics

This work explores the breakdown of the wake downstream of the Model Experiments in Controlled Conditions Project (known as the MEXICO project) wind-turbine rotor and assesses the capability of computational fluid dynamics in predicting its correct physical mechanism. The wake is resolved on a fine mesh able to capture the vortices up to eight rotor radii downstream of the blades. At a wind speed of , the main frequency present in the computational fluid dynamics signals for up to four radii was the blade-passing frequency (21.4 Hz), where the vortex cores fall on a perfect spiral. Between four and five radii downstream, higher-frequency content was present, which indicated the onset of instabilities and results in vortex pairing. The effect of modeling a 120 deg azimuthally periodic domain and a 360 deg three-bladed rotor domain was studied, showing similar predictions for the location of the onset of instabilities. An increased frequency content was captured in the latter case. Empirical and wake models were also explored, they were compared with computational fluid dynamics, and a combination of kinematic and field models was proposed. The obtained results are encouraging and suggest that the wake instability of wind turbines can be predicted with computational fluid dynamics methods, provided adequate mesh resolution is used.

In view of accelerating CFD simulations through coupling with vortex particle approximations

In order to exploit the capabilities of Computational Fluid Dynamics in aerodynamic design, the cost should be reduced without compromising accuracy and consistency. In this direction a hybrid methodology is formulated within the context of domain decomposition. The strategy is to choose in each sub-domain the best performing method. Close to solid boundaries a grid-based Eulerian flow solver is used while in the far field the flow is described in Lagrangian coordinates using particle approximations. Aiming at consistently including compressible effects, particles carry mass, dilatation, vorticity and energy and the complete set of conservation laws is solved in Lagrangian coordinates. At software level, the URANS solver MaPFlow is coupled to the vortex code GENUVP. In the present paper the two dimensional formulation is given alongside with validation tests around airfoils in steady and inherently unsteady conditions. It is verified that: purely Eulerian and hybrid simulations are equivalent; the Eulerian domain in the hybrid solver can be effectively restricted to a layer 1.5 chord lengths wide; significant cost reduction reaching up to 1:3 ratio is achieved.

Wind tunnel and CFD study of the natural ventilation performance of a commercial multi-directional wind tower

Scaled wind tunnel testing and Computational Fluid Dynamics (CFD) analysis were conducted to investigate the natural ventilation performance of a commercial multi-directional wind tower. The 1:10 scaled model of the wind tower was connected to the test room to investigate the velocity and pressure patterns inside the micro-climate. The tests were conducted at various wind speeds in the range of 0.5–5 m/s and various incidence angles, ranging from 0° to 90°. Extensive smoke visualisation experiments were conducted to further analyse the detailed airflow structure within the wind tower and also inside the test room. An accurate geometrical representation of the wind tunnel test set-up was recreated in the numerical modelling. Care was taken to generate a high-quality grid, specify consistent boundary conditions and compare the simulation results with detailed wind tunnel measurements. The results indicated that the wind tower was capable of providing the recommended supply rates at external wind speeds as low as 2 m/s for the considered test configuration. In order to examine the performance quantitatively, the indoor airflow rate, supply and extract rates, external airflow and pressure coefficients were also measured. The CFD simulations were generally in good agreement (0–20%) with the wind tunnel measurements. Moreover, the smoke visualisation test showed the capability of CFD in replicating the air flow distribution inside the wind tower and also the test room.

Static Computational Fluid Dynamics simulations around a specialised delta wing

The primary aim of this paper is to determine a suitable and reliable model for the full static angle of attack range in Computational Fluid Dynamics (CFD) applications while determining associated model dependencies. This would allow CFD to be utilised as a more reliable tool in the development of aircraft, reducing dependency on wind tunnel investigations, with a consequent reduction in development costs. The model used in this study is based on a specialised delta wing configuration. The study has been undertaken by incorporating simulation parameters such as mesh resolution, discretisation schemes, turbulence and transition models, time step sizes and the order of the time integration operator. The modelling has been carried out using specialised meshing software, the flow simulation software (TAU) developed by the German Aerospace Agency (DLR), and the graphical interface Tecplot. Findings indicate that current CFD capabilities to model the flight envelope of a configuration are near-sufficient. The findings also show the difficulties in utilising one CFD model to represent the entire angle of attack range and the effect of model dependencies.

Numerical simulation and analysis of power consumption and Metzner-Otto constant for impeller of 6PBT

Majority of non-Newtonian fluids are pseudoplastic with shear-thinning property, which means that the viscosity will be different in different parts of the stirred tank. In such mixing process, it is difficult to predict accurately the power consumption and mean shear rate for designing novel impeller. Metzner-Otto method is a widely accepted method to solve these questions in mixing non-Newtonian fluids. As a result, Metzner-Otto constant will become a key factor to achieve an optimum way of economical mixing. In this paper, taking glycerine and xanthan gum solutions as research system, the power consumption, stirred by the impeller composed of perturbed six-bent-bladed turbine (6PBT) with differently geometrical characteristics in a cylindrical vessel, is studied by means of computational fluid dynamics (CFD). The flow is modeled as laminar and a multiple reference frame (MRF) approach is used to solve the discretized equations of motion. In order to determine the capability of CFD to forecast the flow process, the torque test experiment is used to measure the glycerine solution power consumption. The rheological properties of the xanthan gum solutions are determined by a Brookfield rheometer. It is observed that the power consumption predicted by numerical simulation agrees well with those measured using torque experiment method in stirring glycerine solution, which validate the numerical model. Metzner-Otto constant is almost not correlated with the flow behavior index of pseudoplastic fluids. This paper establishes the complete correlations of power constant and Metzner-Otto constant with impeller geometrical characteristics through linear regression analysis, which provides the valuable instructions and references for accurately predicting the power consumption and mean shear rate of pseudoplastic fluids in laminar flow, comparatively.

CFD simulation of a pharmaceutical bubbling bed drying process at three different scales

Drying is one of the major unit operations in the manufacturing of solid pharmaceuticals. In many pharmaceutical processes, optimum design of the drying process will significantly enhance the rate and reliability of the production and, in turn, decrease the cost of the pharmaceutical products. In this study, gas–solid flow patterns, mixing, and drying of pharmaceutical particles for three different scale bubbling fluidized bed dryers were simulated using ANSYS/FLUENT Computational Fluid Dynamics (CFD) code. The simulations are based on the two-fluid granular model (Eulerian–Eulerian approach). The mathematical model that we used in this study is composed of the continuity, momentum, energy, and species transfer equations to simulate the flow pattern and heat and mass transfer processes in the pharmaceutical drying processes based on the bubbling fluidized beds.To extend FLUENT code capability to account for moisture transport and to simulate the drying process, needed user-defined scalar transport equations (UDS) were added to the code. The results of the CFD simulation were compared with the experimental data obtained at the laboratory scale (Duquesne University experiments) to validate and refine our CFD model. Then, the modified model was used to simulate the drying of the same material in Abbott Laboratories kilo- and 10-kilo-scale units. Our simulation results for solid particles drying as a function of dimensionless time showed that our CFD model along with the same dimensionless group can be used as a tool to scale up the drying process from experimental-scale to both kilo-scale and 10-kilo-scale fluidized bed dryers. In addition, the numerical simulation results compared well with the experimental data (performed by Duquesne University and Abbott Laboratories) on moisture removal rate and outlet gas temperature. This also could provide a very good demonstration of the CFD capability as a tool to be used in the design and scale-up of the drying processes.

A computational approach to flow noise

The ability to calculate the noise internal to a structure due to external flow is a necessity for the optimal design of a low noise structure. Whether the structure is an automobile, an airplane or an acoustical array, the goal is the same: to in some way minimize the acoustic pressure/particle velocity resulting from flow excitation at a design location. The excitation is usually that due to wall pressure fluctuations resulting from turbulent boundary layer (TBL). There is a long history of modeling plate excitation due to TBL loads. In most cases, the existing work makes use of statistical, empirically based models for the TBL excitation. This work focuses on combining the capabilities of computational fluid dynamics with computational structural acoustics to enable the calculation of flow noise primarily for undersea vehicles. The work is limited to the non-coupled problem, where the flow calculations are made over a non-deforming boundary with the goal of calculating wall pressure fluctuations and using them as loads on a finite element structural acoustics model. The ultimate goal of this work is to develop the capability to calculate flow noise for three dimensional undersea structures for which analytical approaches are not possible.

Investigation of Effect of Diaphragms on the Efficiency of Hybrid Rockets

This paper describes computational fluid dynamics applied to the analysis of a hybrid rocket motor with a diaphragm in the combustion chamber to enhance rocket performance. This work follows the last author’s experiments: An engine was tested with nitrous oxide and paraffin wax as propellants. Several of the tests have been used as references for numerical simulations. The following approximations have been made: steady-state conditions, eddy dissipation model with one-step reaction, gaseous injection of fuel and oxidizer, and no droplets entrainment (typical of a paraffin grain). First of all, a single geometry without a diaphragm has been analyzed with different turbulence models (, shear stress transport, , renormalization group). It has been shown that the model predicts a lower flame temperature and chamber pressure than the model. Then, five geometries have been studied to compare two different types of diaphragms (one hole and four holes) in two positions (24 and 33% of the total length) to a configuration without any mixer. The effect of the diaphragm is to increase the mixing of the chemical species participating in the combustion process. The use of the diaphragm causes a performance enhancement, as shown in the experimental study. There is a good agreement between computational fluid dynamics results and experimental data: The efficiency is overestimated by less than 5.5%. This work proves the capability of computational fluid dynamics to predict global hybrid rocket performance: It can be considered as a useful tool in the study of mixing devices.

The Effect of Inlet Guide Vanes on Inlet Flow Distortion Transfer and Transonic Fan Stability

An investigation was carried out into the effects of variable inlet guide vanes (VIGVs) on the performance and stability margin of a transonic fan in the presence of inlet flow distortion. The study was carried out using computational fluid dynamics (CFD) and validated with experimental data. The capability of CFD to predict the changes in performance with or without VIGVs in the presence of an inlet flow distortion is assessed. Results show that the VIGVs improve the performance and stability margin and do so by reducing the amount of swirl at inlet to the rotor component of the fan.

Application of a New One-Equation Turbulence Model to Computation of Separated Flows

Accurate turbulence modeling remains a critical problem in the prediction capability of computational fluid dynamics. One particular flow regime lacking accurate simulation is instances of separated flow. In this paper the Rahman-Agarwal-Siikonen (RAS) model is used to simulate the flow of several canonical separated flow cases. The commercially available software ANSYS FLUENT and the open source software OpenFOAM are used for the flow calculations. It is shown that the RAS model significantly increases the accuracy of flow simulations compared to the commonly used Spalart-Allmaras (SA) and Shear-Stress-Transport (SST) k-ω models.

CFD simulations of dense solid–liquid suspensions in baffled stirred tanks: Prediction of solid particle distribution

Industrial tanks devoted to the mixing of solid particles into liquids are often operated at an impeller speed N less than Njs (defined as the lowest speed allowing the suspension of all particles): under such conditions the distribution of solid-particles is very far from being homogeneous and very significant concentration gradients exist. The present work is devoted to assessing the capability of Computational Fluid Dynamics (CFD) in predicting the particle distribution throughout the tank.The CFD model proposed by Tamburini et al. [58] and successfully applied to the prediction of the sediment amount and shape was adopted here to simulate the particle distribution under partial-to-complete suspension conditions. Both transient (via the Sliding Grid approach) and steady state (via the Multiple Reference Frame approach) RANS simulations were carried out for the case of a flat bottomed baffled tank stirred by a Rushton turbine.Results show that the model can reliably predict the experimental particle distribution at all investigated impeller speeds. Transient simulations were found to predict slightly better the experimental data with respect to steady state simulations. Radial gradients of solids concentration, usually neglected in the literature, where found to be significant in the presence of unsuspended solid particles settled on the vessel bottom (i.e. incomplete suspension conditions).

Numerical study on human model shape and grid dependency for indoor thermal comfort evaluation

Various computer-simulated person (CSP) models have been used to represent occupants in indoor airflow simulations using computational fluid dynamics (CFD). Despite the capability of CFD to predict temperature and velocity fields in an automotive cabin or a room in a building, it is more difficult to evaluate the degree of thermal comfort considered by the CSP models. Up to now, the shapes of CSP models and their grid characteristics have not been studied for the evaluation of indoor thermal comfort. In this paper, the effects of the human model’s shape and the physical characteristics of the grids are studied. The FLUENT code is used for analysis, and the predicted mean vote (PMV), predicted percentage dissatisfied (PPD), and equivalent homogeneous temperature (EHT) values are used for the evaluation and comparison of thermal comfort. The computational results show that the CSP shape and grid features do not affect the global flow fields or the evaluations of PMV and PPD. However, more precise results are obtained from the evaluation of thermal comfort by EHT when detailed human models with a prism grid are used.

Computational Fluid Dynamics: NASA Glenn Research Center’s Legacy and Contributions

Development and contributions to computational fluid dynamics (CFD) at the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) during the period from 1947 to the present are reviewed in five categories: numerical methods, physical modeling, CFD codes development, CFD validation and engineering applications, and multidisciplinary design optimization. Some representative results in applications to aero and propulsion systems are included to illustrate the developed capabilities. GRC has a long history of investing resources to develop these key subject matters, with an interest in a wide range of applications, primarily focusing on propulsion-related technologies and concepts. The evolved CFD capabilities have enabled simulations of complex three-dimensional flow fields for engine components and integrated configurations, as illustrated in this article. This article is intended to give a useful, albeit noncomplete, overview into GRC’s work in CFD.

CFD simulation of DEBORA boiling experiments

CFD simulation of DEBORA boiling experimentsIn this work we investigate the present capabilities of computational fluid dynamics for wall boiling. The computational model used combines the Euler/Euler two-phase flow description with heat flux partitioning. This kind of modeling was previously applied to boiling water under high pressure conditions relevant to nuclear power systems. Similar conditions in terms of the relevant non-dimensional numbers have been realized in the DEBORA tests using dichlorodifluoromethane (R12) as the working fluid. This facilitated measurements of radial profiles for gas volume fraction, gas velocity, bubble size and liquid temperature as well as axial profiles of wall temperature. After reviewing the theoretical and experimental basis of correlations used in the ANSYS CFX model used for the calculations, we give a careful assessment of the necessary recalibrations to describe the DEBORA tests. The basic CFX model is validated by a detailed comparison to the experimental data for two selected test cases. Simulations with a single set of calibrated parameters are found to give reasonable quantitative agreement with the data for several tests within a certain range of conditions and reproduce the observed tendencies correctly. Several model refinements are then presented each of which is designed to improve one of the remaining deviations between simulation and measurements. Specifically we consider a homogeneous MUSIG model for the bubble size, modified bubble forces, a wall function for turbulent boiling flow and a partial slip boundary condition for the liquid phase. Finally, needs for further model developments are identified and promising directions discussed.

Cavern Development of Pseudoplastic Fluids Stirred by Impeller of Perturbed Six-bent-bladed Turbine

The prediction of the cavern size is important in the mixing of shear-thinning fluids with yield stress.The cavern development of xanthan gum solutions,a pseudoplastic fluid with yield stress,stirred by a impeller of perturbed six-bent-bladed turbine was numerical researched using computational fluid dynamics(CFD).The cavern boundary velocity is properly determined and the cavern dimensions of shape,diameter and height to diameter ratio are obtained versus apparent Reynolds number,Re*.In order to determine the capability of CFD to forecast the flow process,the agitational experiment is used to measure the power consumption.The results show that the power data predicted by the simulation are in good agreement with those measured in the experiment,which validate CFD model developed in laminar flow.The cylindrical cavern development is found as Re* increased,which shows a good agreement with the representation of the cylindrical model(Elson’s model).The variations of dimensionless cavern diameter(Dc/D) verse dimensionless NpRey,as well as cavern height to diameter ratio,have nothing to do with the rheological index of fluids all together.CFD simulation predicts a linear slope of 0.35 from a log-log plot of Dc/D verse NpReyat DcT,which agrees well with the theoretical value,0.33,of Elson’model.The height to diameter ratio is 0.68 until the cavern reaches the tank wall,while the rate constant of axial fluid expansion describing cavern height once the cavern touched the vessel wall,β is 0.79.The research results promote the understanding of the mixing performance of the impeller of perturbed six-bent-bladed turbine and provide valuable instructions and references for the design,application and development of new type impeller in pseudoplastic fluids.

Non-uniform rational B-splines-based aerodynamic shape design optimization with the DLR TAU code

With the improving capabilities of computational fluid dynamics (CFD) for the prediction of aerodynamic performance, CFD tools are now being increasingly used for aerodynamic design optimization in the aerospace industry. In this article, an automated optimization framework is presented to address inviscid aerodynamic design problems. Key aspects of this framework include the use of the continuous adjoint methodology to make the computational requirements independent of the number of design variables, and computer-aided design-based shape parameterization, which uses the flexibility of non-uniform rational B-splines to handle complex configurations.