Computational Fluid Simulation Research Articles

A Numerical Study: Transitional Hydrodynamic Behaviour of a Moored Barge in Different Ultra-shallow Water Depths of Malaysia

Background: Malaysia has most of its oil reservoirs in the South China sea. The water depth ranges from 50 m to 200 m. The effects of ultra-shallow water are of prime importance in the exploration of marginal oil fields. Hence, there is an increasing demand for understanding the hydrodynamic behavior of FPSO in ultra-shallow water depths. Objective: A simulation study in both frequency-domain and time-domain analyses has been performed to understand the dynamic responses of a moored barge in varying shallow water depths. The objective of this study was to observe the transitional hydrodynamic behavior of the moored barge under varying shallow water depths. Methods: The moored barge was administered under regular and irregular waves. Operating conditions for irregular waves in terms of significant wave height and peak time period were incorporated from PETRONAS Technical Standards (PTS). The wave-body interactions and mooring effects have been numerically modelled using a commercial Computational Fluid Dynamics (CFD) and simulation software (ANSYS AQWA) successfully. In order to gain confidence in the simulation software, additional experimental validation was performed for a FPSO model. Results: Though the barge was primarily free to rotate in all Degrees Of Freedom (DOF), however, only three DOFs were considered for our study; viz, heave, roll and yaw respectively. The force spectral density, cable RAO’s in addition to the time series of cable forces, along with the effect of significant motions on the mooring cables behavior have been discussed. Conclusion: In irregular beam sea state, the significant motions in ultra-shallow water were greater than that for deep waters, this was primarily the main reason for higher cable responses in ultra-shallow water.

STUDI NUMERIK PERBANDINGAN KARAKTERISTIK ALIRAN BACKWARD FACING STEP DENGAN BODI PENGGANGGU (BUMP) TIPE RECTANGULAR DENGAN BILANGAN REYNOLDS TINGGI

This research uses a backward facing step geometry with additional rectangular bump on the inlet. The rectangular bump with variation mixed length (l) model developed in this study is very important for predicting fluid flow among boundary layer especially in reattachment length, the prediction obtained by applying standard wall function and non-equilibrium wall function with turbulence model RKE Realizable. Furthermore, the results are compared with previous experimental research data Kim dkk. 2005. This research is investigated using 2D simulation Computational Fluid Dynamics (CFD) with Parameter to be used are: step height (h): 1 cm; variation of mixing length (l): 5; 4; 3; and 2 cm; rectangular side (d): 0.5 cm. All investigations are running with Reynolds Number 38,000, based on the step height and the mean stream. From this study we will obtain the best combination of turbulent models and the near-wall treatment method. It also give prediction show the best reattachment length prediction in position l/h 5 from the comparison of the results of the distance variation of the body model.

Tonal Fan-Noise Radiation From Aero-Engine Bypass With Serrated End Treatments

Abstract Chevrons, which are also known as serrations, are initially developed to suppress jet noise radiating from aero-engine nozzles. The associated fluid mechanics are already well known. Compared with jet noise, turbomachinery fan noise has become relatively more important along with the ever-increasing bypass ratio. However, it is still unclear whether the trailing-edge chevrons on the bypass duct would attenuate fan noise and, if the answer is yes, what is the associated mechanism. In this work, we first use a theoretical model based on the Wiener–Hopf method to rapidly conduct parametric studies across a number of different setups. The results from such a theoretical model suggest that the chevrons are also effective in the reduction of fan noise scattering. Next, we perform high-fidelity computational fluid and acoustic simulations for a realistic aero-engine with some representative setups, and the results further confirm the effectiveness of chevrons. Both analytical and numerical results show the associated noise control mechanism, that is, chevrons would induce acoustic mode conversion (especially from low modes to high modes), which shall further result in evanescent waves in the radial direction and the final noise reduction at various radiation angles. The findings may find applications in the next-generation low-noise aero-engine design.

Modeling and Simulation of a Piston-Type Work Exchanger for Mechanical Energy Recovery

Heat and work are two common forms of energy. Over past decades, thermal energy recovery via heat integration has been very successful in the process industry. Recovery of mechanical energy, however, has not drawn sufficient attention until recently. It is shown that mechanical energy can be recovered systematically through integrating work exchange networks in plants. It is conceived that the pressure-driven work exchange device can be used as a main type of unit in work exchanger networks. In this paper, we introduce a piston-type device to perform work exchange for mechanical energy recovery among pairs of process streams in the gas phase. Computational fluid dynamics-based modeling and simulation is conducted to study the design of the device and its operational behavior under different operating conditions. A case study is illustrated to demonstrate its application potential for mechanical energy recovery.

A Downhole Hydrocyclone for the Recovery of Natural Gas Hydrates and Desanding: The CFD Simulation of the Flow Field and Separation Performance

The application of a hydrocyclone to recycle NGH and desand during NGH exploitation is a novel idea. The flow field and performance of this hydrocyclone is in the frontier of the research in this field and is unclear so far. This research aimed to reveal the flow field characteristics and performance of NGH downhole hydrocyclones. In this paper, flow field, solid phase particle volume distribution and separation efficiency were investigated according to the two objectives of NGH recovery efficiency and sand removal efficiency with different inlet velocities by computational fluid simulations (CFD)-FLUENT software. The results show that the short circuit flow contributed to the recovery of NGH. Axial velocity is a decisive factor in balancing the two objectives of NGH recovery efficiency and sand removal efficiency. In addition, the same as those in traditional hydrocyclones, the static pressure, tangential velocity and turbulence intensity play key roles in separation performance, hydrocyclone performance can be improved by increasing the inlet velocity. On the other hand, most separation efficiencies were greater than 80%, when the particle size was larger than 15 µm, and the differential pressure was less than 0.6 MPa. Therefore, all the above results confirm that hydrocyclone has good performance in NGH exploitation, and the basis of its structural design and optimization are provided.

Parametric Study on Determining Optimum Shading Techniques for Urban High-Rise Dwellings

Shading techniques constitute one of the most passive, beneficial strategies for reducing energy consumption in urban dwellings. Shading affects many factors, for example, the solar gains and radiations falling on the façade, which are considered the most significant in increasing the cooling energy demand in hot climates. This paper conducts a parametric study on external and internal shading devices and establishes their impact on energy consumption, daylight levels, and ventilation. The work was conducted using Integrated Environmental Simulation Virtual Environment (IES-VE) and Computational Fluid Dynamics (CFD) numerical methods. The results revealed that optimised shading can influence savings in terms of energy and cooling, in addition to the enhancement of daylighting and reduction of glare. After studying all these factors associated with the different shading techniques investigated, the findings revealed that all shades affect the energy, daylight and ventilation parameters positively. However, despite all external and internal shadings showing improvements, the egg crate shade was determined as that which provided the optimum energy saving, while enhancing daylight and improving natural ventilation for a sustainable building design.

Simulation Analysis of Flow Field in Engine Compartment of a Rear-Mounted Exhaust

The numerical simulation of the flow field in the engine compartment of a car is the basis for the evaluation and research of the air flow characteristics in the engine compartment. In this paper, the computational fluid simulation software STAR-CCM+ was applied to simplify the vehicle model of a rear-mounted exhaust. The surface wrapper and surface remesher were used to obtain the whole vehicle surface mesh. Combining with the vehicle wind tunnel model was generated the volume mesh based on Trim mesh sub-regional method. Then, the finite element model of the car engine compartment was established. Three-dimensional flow field in engine compartment was analyzed under reasonable boundary conditions, and found the reflow at climbing is more apparent as compared with that at high constant speed. Then the flow rate experiment of the whole vehicle was obtained wind speed value of the windward surface of the radiator. The error between the simulated value and the experimental value is within 10%, indicating that the simulation model established in this paper is reliable and possesses high simulating accuracy.

Active Mixing of Reactive Materials for 3D Printing

The mixing of materials during additive manufacturing is a major benefit which allows one to compositionally and spatially tailor material properties, for example to locally control the reactivity in fuel–oxidizer systems known as thermites. This work characterizes an active mixing printhead used in conjunction with a 3D printing process known as Direct Ink Writing. Besides compositional control, a major benefit of this approach is that it offers a safe method for working with these materials, which can otherwise be hazardous once mixed. Custom fuel and oxidizer inks are fed at fixed volumetric rates into an active mixing printhead, and both the rotational speed of the mixing impeller as well as the fuel–oxidizer ratio are varied. Upon ignition, the propagation speed increases with the rotational speed of the mixer and plateaus above a critical value of ≈750 rpm. The critical mixing speed is corroborated by computational fluid simulations and an analytical expression that considers the inks’ complex fluid behavior. Additionally, varying the composition results in a wide range of propagation speeds with peak reactivity corresponding to a fuel‐rich formulation (ϕ = 1.5). A test article incorporating a fast‐ and slow‐burning region demonstrates how spatial composition can manipulate the reactivity.

Rainfall erosivity in interrill areas: Insights about the choice of an erosive factor

Defining a suitable erosive factor for interrill erosion has been a long-going discussion that was not resolved by experimental results. In this paper, by using computational fluid simulations of individual raindrop impacts, the relevance of four erosive factors was assessed for a range of soil resistance, rainfall type and water layer depth. Computation results show that the erosivity exponent associated to the erosive factors had a very low sensitivity to soil resistance. This confirms the common practice of separating rainfall erosivity from soil erodibility. The erosivity exponent was found to be very sensitive to the rainfall type. Therefore, an universal exponent could not be found, its value depending on the rainfall type. As a result of the balance between the shear stress development and the protective effect of the water layer, the soil detachment rate was maximum for a specific water depth. This supports to better account for the water layer depth in erosion models. Overall, the computational approach shows that the choice of an erosive factor may not be of practical importance. It also encourages for new experimental designs, able to evaluate soil detachment by raindrops — and not only the amount of splashed soil.

Determining building-specific wind pressure coefficients to account for the microclimate in the calculation of air infiltration in buildings

ABSTRACT Wind pressure coefficients (Cp ) are important for the correct calculation of the air infiltration of a building. Cp depends on wind direction, position on the building façade and site exposure, and is therefore influenced by the microclimate. The external coupling between building energy simulation (BES) software and computational fluid dynamics (CFD) pre-processing allows calculating building-specific wind pressure coefficients that can account for the microclimate. BESs have been performed in order to calculate the infiltration rate of a reference building using surface-averaged Cp values from two different sources; a standard database and CFD simulations from OpenFOAM. Tracer gas measurements were performed in the reference building in order to validate the simulation results. The results show the coupled CFD/BES method gives building-specific Cp values that represent adequately the microclimatic conditions, leading up to 45% more accurate air infiltration rates compared to conventional methods.

Aeroelastic behaviour of a parameterised circulation-controlled wing

The aeroelastic behaviour of the wing of a short take-off and landing aircraft using the Coandă effect depends on its properties and shape. An existing reduced-order model is parameterised for detailed investigations. On the one hand, varying mass due to tank level and varying overall stiffness is implemented in the reduced-order model. The influence of the mass change on the aeroelastic behaviour is reflected in the stability maps. On the other hand, two-dimensional steady and unsteady aerodynamics of different nose shapes are investigated with detailed computational fluid simulations and included in the reduced-order model. The dependence on the profile shape and the frequency is described. Their influence on the aeroelastic behaviour is reflected by the stability maps as well.

Asymptotic Approximation of the Eigenvalues and the Eigenfunctions for the Orr-Sommerfeld Equation on Infinite Intervals

Asymptotic eigenvalues and eigenfunctions for the Orr-Sommerfeld equation in two and three dimensional incompressible flows on an infinite domain and on a semi-infinite domain are obtained. Two configurations are considered, one in which a short-wave limit approximation is used, and another in which a long-wave limit approximation is used. In the short-wave limit, WKB methods are utilized to estimate the eigenvalues, and the eigenfunctions are approximated in terms Green's functions. The procedure consists of transforming the Orr-Sommerfeld equation into a system of two second order ordinary differential equations for which eigenvalues and eigenfunctions can be approximated. The approximated eigenvalues can, for instance, be used as a starting point in predicting transitions in boundary layers with computer simulations (computational fluid dynamics). In the long-wave limit approximation, solutions are expressed in terms of generalized hypergeometric functions.

Lattice Boltzmann method-based large-eddy simulation of indoor isothermal airflow

As a relatively new computational fluid simulation method, the Lattice Boltzmann method (LBM) has recently garnered widespread attention in various engineering fields. The LBM-based large-eddy simulation (LBM-LES) model is commonly used to predict flows with high Reynolds numbers, and is considered to yield a prediction accuracy comparable to that of the traditional finite volume method (FVM-LES). Nonetheless, in the indoor environment, a detailed benchmark for the LBM-LES accuracy is currently underdeveloped, and its computational time has not been sufficiently compared with that of FVM-LES.In this study, simulations of an indoor isothermal-forced convection benchmark case were carried out in the forms of LBM-LES and FVM-LES, using different grid resolutions, relaxation time schemes, and discrete velocity schemes of the LBM, with the aim of verifying the prediction accuracy of LBM-LES and investigating its consistency with FVM-LES in indoor turbulent flow. Their computational speeds and parallel computational efficiencies were also compared. The results demonstrate that LBM-LES can achieve the same level of indoor-turbulent-flow prediction accuracy as FVM-LES; however, it requires a more refined grid system (in this study, its grid width was half of that of FVM-LES). Furthermore, the relaxation time and discrete velocity schemes of LBM barely influenced the accuracy of the indoor flow. For the same level of accuracy, their computational speeds approached the same level, even though the LBM-LES required an eight times larger mesh quantity than the FVM-LES; however, the LBM-LES computational speed can surpass that of the FVM-LES when more cores are utilized because of its outstanding parallel efficiency.

A refined method for digitally modeling small and complex plant structures in 3D: An example from the grasses (Poaceae).

Premise of the StudyA refined procedure is described for modeling small, intricate plant structures using computer‐aided design software. The procedure facilitates the study of wind pollination in the family Poaceae and provides virtual biological illustrations for public outreach.Methods and ResultsSpikelets were fixed in gFAA, dehydrated using ethanol and xylene, embedded in paraffin wax, and then sectioned with a rotary microtome. Images of serial sections were used as a reference for modeling the shape of bracts with splines in a computer‐aided design program. Virtual models produced by this method have many potential uses; examples include geometric morphometric analyses and simulations of computational fluid dynamics.ConclusionsThis protocol is a synthesis of modern biological illustration and engineering technology. Virtual models facilitate quantitative experiments that may address questions about reproductive biology, conditions shaping the form of anatomical support, or the morphological evolution of structures of biomechanical interest.

Optimizing the heat transfer performance of the recovery boiler superheaters using simulated annealing, surrogate modeling, and computational fluid dynamics

The energy efficiency of recovery boiler power plants is largely influenced by the heat transfer to the superheaters. In the design process of such very large-scale applications, one of the key challenges is the a priori geometry optimization by robust numerical approaches. The main objectives of this work are to demonstrate a numerical optimization framework and to optimize the geometry of the superheater region to enhance the heat transfer. The framework is implemented as a surrogate-based optimization method, which combines simulated annealing, local polynomial regression, and computational fluid dynamics. The novelty of this work consists of the following: 1) The optimization framework is designed and introduced. 2) The connection between the geometry and heat transfer is quantified by formulating the optimal design curve. 3) The optimal design for a typical, existing recovery boiler is identified. The results indicate that the uniformity of the flow field is improved, and the heat transfer rate is increased by 5%. 4) The results indicate the importance of minimizing the separation vortices through the geometrical design with a strong linkage to the overall heat transfer rate. 5) The potential of optimization methods in this very large-scale energy production application is demonstrated for the first time.

Effect of a speaking valve on biomechanical properties of swallowing and the upper airway flow characteristics for tracheotomized patients after acquired brain damage

Alterations in swallowing and the upper airway flow characteristics can occur after tracheotomy and can result in varying degrees of aspiration. We investigated the effect of a Passy-Muir speaking valve (PMV) on biomechanical properties of swallowing and the upper airway flow characteristics, explored swallowing-breathing interactions preliminarily for tracheotomized patients after acquired brain damage. Six tracheotomized patients who were able to tolerate the PMV and eight healthy volunteers were recruited. High-resolution manometry (HRM) and videofluoroscopic swallowing study (VFSS) were performed to evaluate the swallowing function of the healthy and patients without and with PMV. Iohexol injection was used. Three-dimensional models of the upper airway were constructed based on the CT-scan data. Pressure in pharynx, which was measured by HRM was used as boundary conditions. Computational fluid simulations were performed to acquire upper airway flow characteristics. The patient group showed significantly lower upper esophageal sphincter (UES) resting pressure, shorter UES opening times and less hyoid anterior movement (HAM), but significantly higher UES residual pressure, as well as higher airflow velocity and pressure during inspiratory, expiratory, especially swallowing phase. With the PMV, higher UES resting pressure, larger hyoid superior movement (HSM) and UES opening diameter, lower airflow velocity and pressure were observed, the airflow velocity dropped to almost normal level during swallowing phase. However, there was no significant difference of the maximum pharyngeal pressure, UES residual pressure, laryngeal closure times, UES opening times, HAM, or Penetration-Aspiration Scale score before and instantly after wearing the PMV. The PMV demonstrated significant effects on UES resting pressure, HSM, UES opening diameter, upper airway airflow velocity and pressure in tracheotomized patients after acquired brain damage.

Computational investigation of noise interaction for a nano counter-rotating rotor in a static condition

The interaction between the upper and lower rotors greatly influences the aeroacoustic characteristics of a counter-rotating nano-coaxial rotor. To study this influence, a numerical investigation was carried out. The unsteady flow field of a single upper rotor was first studied with a large-eddy simulation computational fluid dynamics method coupled with a sliding-mesh technique. The Ffowcs Williams–Hawking equation method was used to investigate the aeroacoustic characteristics of the upper rotor based on the flow field. An experimental setup was established to validate the computational approach. The experimental results matched well with the computational results. Additionally, results show that the peak value of the total sound pressure level appeared near the blade tip, which verified that the tip vortex was one of the most important sources of rotor noise. Then the aeroacoustic noise of the nano-coaxial rotor was studied numerically. It was found that the total sound pressure level of the nano-coaxial rotor was greater than that of the upper rotor. Flow field analysis showed that the shedding vortices of the upper rotor interacted with the lower rotor, resulting in a blade–vortex interaction. It was evident that the aeroacoustic noise was enhanced by the interference between the upper and lower rotors.

Optimization of a solar collector with evacuated tubes using the simulated annealing and computational fluid dynamics

In this work, the optimization of a low temperature, water-in-glass, evacuated tubes solar collector is presented. The process of optimization combined the simulated annealing method and a computational fluid dynamics model. The numerical study was carried out in three dimensions, steady-state and laminar regime. A design of experiments study via computational fluid dynamics was carried out with two levels and five parameters, 25, the parameters with significance in the performance of the collector were found from a commercial collector. This collector was used as base case in the process of optimization. In the optimization process, the absorber area was analyzed under three different cases because of the combination of geometrical parameters: length, diameter and number of tubes. Thus, 259 different collector geometries were constructed and modeled. Results from the design of experiments showed that the significant parameters on the thermal performance of the solar collector are: the diameter of the tubes, the absorber area, and the mass flow rate. Results of the optimization process showed that the minimum absorber area for an optimal geometry is 2.49 m2, which is 19.4% lower than the commercial geometry considering the same outlet temperature. The diameter of the tubes increased around 30%, the length of the tubes decreased 40%, the cost of the optimal geometry and the number of evacuated tubes decreased 38.9% and the thermal efficiency increased 26.3%, compared to the commercial geometry. The results of this work can be helpful in further specific applications where the maximum performance and the minimum costs are important, such as: the design of low temperature, water-in-glass, evacuated tubes solar collector networks for heating water in swimming pools, buildings, hospitals and industries.

A novel hydro-kite like energy converter for harnessing both ocean wave and current energy

A novel hydro-kite like ocean energy converter is proposed in this paper to harness both ocean current and wave energy. The proposed ocean energy converter is typically designed to consist of a hydro-kite like kite floating or submerged in the ocean and an energy conversion unit set on a ship. The ocean wave or current energies can be generated by pulling a pre-tensioned flexible tether that connects the hydro-kite-like kite and the energy conversion unit. The general dynamics of the novel ocean energy converter is basically described and modelled based on the dynamics of key components under both ocean wave and current conditions. The feasibility and effectiveness of the proposed ocean energy converter are evaluated through computational fluid simulations under both fully submerged ocean current and semi-submerged ocean wave conditions. The simulation results verify that the hydro-kite can be used to generate power from both ocean current and wave conditions and the design parameters of the hydro-kite such as the angle of attack, draft should be optimally controlled or designed to maximize the ocean energy generations under different ocean current speeds, wave periods and heights.

A mixed computational and experimental approach to improved biogas burner flame port design

Anaerobic digestion is a well-known and potentially beneficial process for rural communities in emerging markets, providing the opportunity to generate usable gaseous fuel from waste resources. With recent developments in low-cost digestion technology, communities across the world are gaining affordable access to the benefits of anaerobic digestion derived biogas. For example, biogas provides a more efficient and cleaner burning alternative to biomass (wood, charcoal, dung), effectively reducing harmful emissions and fuel consumption.This study sought to develop and test a design approach for optimizing flame port geometry for household biogas-fired burners. The approach consists of a multi-component simulation that incorporates three-dimensional CAD designs with simulated chemical kinetics and computational fluid dynamics. The simulated flame port designs included an array of circular and rectangular geometries using a widely available biogas burner. The three highest performing designs identified were manufactured and tested experimentally to validate model outputs and to compare against a baseline port geometry. In the experiment, each of the three designs suggested improved thermal efficiency relative to the baseline. A configuration of four millimeter circular ports resulted in a 7.17% improvement, raising an average thermal efficiency of 53.0% to 56.8%. The results indicated that hydraulic diameter, velocity and mixture density are relevant factors in port geometry design to improve the thermal efficiency of a biogas burner. Conversely, the emissions predictions made by the model were found to be unreliable and incongruent with laboratory experiments.