Computational Fluid Dynamics Numerical Method Research Articles

Evolution of CFD numerical methods and physical models towards a full discrete approach

The physical models and numerical methodologies of Computational Fluid Dynamics (CFD) are historically linked to the concept of continuous medium and to analysis where continuity, derivation and integration are defined as limits at a point. The first consequence is the need to extend these notions in a multidimensional space by establishing a global inertial frame of reference in order to project the variables there. In recent decades, the emergence of methodologies based on differential geometry or exterior calculus has changed the point of view by starting with the creation of entangled polygonal and polyhedral structures where the variables are located. Mimetic methods and Discrete Exterior Calculus, notably, have intrinsic conservation properties which make them very efficient for solving fluid dynamics equations. The natural extension of this discrete vision relates to the derivation of the equations of mechanics by abandoning the notion of continuous medium. The Galilean frame of reference is replaced by a local frame of reference composed of an oriented segment where the acceleration of the material medium or of a particle is defined. The extension to a higher dimensional space is carried from cause to effect, from one local structure to another. The conservation of acceleration over a segment and the Helmholtz–Hodge decomposition are two essential principles adopted for the derivation of a discrete law of motion. As the fields covered by CFD are increasingly broad, it is natural to return to the deeper meaning of physical phenomena to try a new research or new path which would preserve the properties of current formulations.

Numerical study on the sensitivity of photovoltaic panels to wind load on array layout

Offshore floating photovoltaics began to gradually enter people's vision due to the limitation of inland land resources. However, the offshore environment is harsh, and wind load is an essential factor in floating photovoltaic design. At the same time, the photovoltaic panel will be subjected to a large wind load in strong typhoon weather, which may cause a drag-driven or lift-driven instability for offshore floating photovoltaics. In this work, the effects of wind loads on six PV array structure configurations installed on offshore floating PV platforms at high Reynolds numbers are investigated by using the computational fluid dynamics numerical method. The differences in wind load on photovoltaic panels under different layout structures are analyzed and explained, including analysis of velocity and pressure distribution, turbulence field, and lift and drag coefficient of photovoltaic panels. The results showed array b with symmetrically staggered has less sensitivity to wind direction, less lifting torque, and relatively small pressure on the surface of the photovoltaic panel, which is a potential arrangement.

Numerical Simulation Research Considering the Influence of Bridge Deck Cross Slope on Wind Performance of π-Type Composite Beam Cable-Stayed Bridge with a Symmetric Elevation Arrangement

Based on a symmetrical cable-stayed bridge, the wind resistance performance of a proposed π-type composite beam is analyzed, and the influence of changing the bridge deck cross slope ratio on the static and dynamic wind characteristics is emphatically studied. The purpose is to provide a design reference for the preliminary wind resistance study of the cable-stayed bridge. The CFD (Computational Fluid Dynamics) numerical calculation method is used to solve the vibration problem of wind-bridge coupling with the help of the software Fluent. It is found that variation of four deck cross slope rates 0%, 1.5%, 2%, and 2.5% has large effects on the static wind coefficient and flutter critical wind speed of a π-type combination beam. In the range of a −3–3° wind attack angle, the static wind drag coefficient will be decreased as the beam deck cross slope rate increases, and in which the drag coefficient at 0% slope rate is the largest. Within the discounted wind speed 13, changing the bridge deck cross slope has little effect on the aerodynamic derivatives of a π-type composite beam. However, the beam deck slope increases the critical wind speed of the bridge, and the critical wind speed at 2.5% is the largest of the four deck slope rates. In addition, it is found that the beam leeward surface keeps generating up and down vortices when the incoming flow wind speed is in the range of 2.5–4.0 m/s, which led the beam to be under vertical vortex vibration, and further research is needed on vibration suppression measures.

3D Printing of a Biocompatible Nanoink Derived from Waste Animal Bones.

Direct ink writing (DIW) additive manufacturing is a versatile 3D printing technique for a broad range of materials. DIW can print a variety of materials provided that the ink is well-engineered with appropriate rheological properties. DIW could be an ideal technique in tissue engineering to repair and regenerate deformed or missing organs or tissues, for example, bone and tooth fracture that is a common problem that needs surgeon attention. A critical criterion in tissue engineering is that inserts must be compatible with their surrounding environment. Chemically produced calcium-rich materials are dominant in this application, especially for bone-related applications. These materials may be toxic leading to a rejection by the body that may need secondary surgery to repair. On the other hand, there is an abundance of biowaste building blocks that can be used for grafting with little adverse effect on the body. In this work, we report a bioderived ink made entirely of calcium derived from waste animal bones using a benign process. Calcium nanoparticles are extracted from the bones and the ink prepared by mixing with different biocompatible binders. The ink is used to print scaffolds with controlled porosity that allows better growth of cells. DIW printed parts show better mechanical properties and biocompatibility that are important for the grafting application. Degradation tests and a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay study were done to examine the biocompatibility of the extracted materials. In addition, discrete element modeling and computational fluid dynamics numerical methods are used in Rocky and Ansys software programs. This work shows that biowaste materials if well-engineered can be a never-ending source of raw materials for advanced application in orthopedic grafting.

Performance analysis of the hypersonic vehicle with dorsal and ventral intake

The integrated design of the airframe and inlet is the favorite choice for the aerodynamic configuration design of the air-breathing hypersonic vehicle. To better understand the integrated configurations with a dorsal intake, this paper utilizes the computational fluid dynamics (CFD) method to comprehensively compare the performance of integrated configurations with the dorsal and traditionally used ventral intake. The performance indexes include aerodynamic performance, such as lift-to-drag ratio, the inlet performance, and thrust-to-drag balance performance. First, under the same geometric constraints, two integrated aerodynamic configurations with dorsal and ventral intake are designed based on a truncated Busemann inlet. Second, the CFD numerical simulation method based on multi-GPU parallel computing is developed and verified. Then, the flow-field characteristics, the aerodynamic, inlet, and thrust-to-drag balance performance of the two types of configurations in a wide range of Mach numbers are comprehensively compared using the GPU-based efficient numerical simulation method. The advantages and disadvantages of the two types of configurations are then analyzed to provide a meaningful reference for the aerodynamic configuration design of air-breathing hypersonic vehicles.

Effects of varied bend turning vanes on flow control and heat transfer performance in a smooth two-pass channel

Turbine blades are always withstanding high temperature and thermal stress from hot gas. Serpentine passages with 180-deg turns are widely used within the turbine blade so as to guide cooling flow and enhance heat transfer. However, there is always a large pressure drop when cooling airflow passes the 180-deg bend. Turning vanes are usually arranged in the bend of two-pass channel to reduce the flow separation and pressure loss, and improve mainstream uniformity in the second passage. In this study, the heat transfer performance and flow field features of six two-pass channels with / without varied turning vanes are investigated by using Computational Fluid Dynamics numerical methods. The studies on effects of shape lines and curvature of the turning vane are conducted by commercial software Fluent based on realizable k - ε turbulent model. The influences of Reynolds number are also included. Numerical results indicate that the friction loss factors of channels with turning vane are decreased by 10.5%–31.4% compared to smooth channel without turning vane. However, the Nusselt number in the second passage is reduced greatly by 12.6%–22.1%. Considering the heat transfer and pressure loss, semicircular turning vane yields the highest thermal performance factor with an improvement by 5.2%. Moreover, turning vane with big curvature shows the best ability to suppress the flow separation and recirculation around the divider in the second passage. Among all the turning vanes studied, the novel airfoil-shaped turning vane produces the highest transfer augmentation at the region downstream the bend.

Моделирование траектории судна на волнах на базе STAR-CCM+

The reliable ship motion trajectory signal provides a basis for six-degree-of-freedom test bench to simulate ship motion in waves. The characteristics of ship motion in waves were analyzed. Aiming at the problem of complex hull shape and large displacement at sea, the dynamic overlapping grid technique was used. Based on STAR-CCM+, the free surface was solved by CFD (Computational Fluid Dynamics) numerical simulation method. The numerical model of ship motion in waves was established, and the DFBI (Dynamic Fluid Body Interaction) method was used to simulate the ship motion in the waves. The ship’s motion trajectory was obtained in the regular and irregular heading waves, which provides important support for the six-degree-of-freedom test bench wave simulation system.

Study the influence of concavity shapes on augmentation of heat‐transfer performance, pressure field, and fluid pattern in three‐dimensional pipe

AbstractIn this current study, the heat‐transfer augmentation mechanism and pressure drop with flow field structures over different dimple arrays in turbulent flow of three‐dimensional circular tube are investigated based on computational fluid dynamics numerical methods using the standard k − e turbulent model. The water is the working fluid over the range of Reynolds numbers from 1500 to 24,000, with three different tubes dimpling arrangements are analyzed. The numerical simulation results are validated through available experimental data, and they show good agreement results between them. The numerical analysis results showed that the flow fields are symmetric within the middle cross‐sectional direction in the pipe with and without dimples. Also, the flow near or close to the dimples is chaotic and including small vortexes and eddies. The results found that the temperature difference in the dimple tube with 2 mm diameter at low flow range 0.56 L/min was higher than that of smooth pipe, dimple tube 0.75 and 1.5 mm by 26.8%, 10.57%, and 3.68%, respectively. It can be concluded that using dimples in heat‐exchanger tubes can provide rates of heat transfer that is higher than that without dimpled tubes at the same operating conditions. Hence, this is an important enhancement in process industries for the energy conversion applications.

Aerodynamic analysis and modeling of coaxial ducted fan aircraft with the ceiling effect

In this study, a novel coaxial ducted aircraft structure is considered, based on which the flow field with the ceiling effect under different vertical velocities and heights is obtained by exploiting the CFD (Computational Fluid Dynamics) numerical calculation methods. Taking into account the ceiling effect, a total lift mechanism model of the coaxial ducted aircraft structure is established based on the momentum theory and the blade element theory. In the proposed mechanism model, the relationship between the vertical climbing rate and the induced speed of the rotors and the induced velocity interaction between the coaxial rotors and the effect of the wake of the upper rotor on the lower rotor are considered. To verify the effectiveness and feasibility of the mechanism model, the numerical calculation and the bench test are conducted. The results indicated that, under the hovering condition, the error between the mechanism model and the bench test results falls into 15%; under the vertical climbing condition, the error between the mechanism model and the CFD numerical calculation results falls into 10%. It is concluded that the proposed mechanism model exhibits high adaptability and accuracy.

A numerical study of aerodynamic characteristics of a high-speed train with different rail models under crosswind

The CFD (Computational Fluid Dynamics) numerical simulation method with the DES (detached eddy simulation) approach was adopted in this paper to investigate and compare the aerodynamic performance, pressure distributions of the train surface, and flow fields near the train model placed above the subgrade with non-rail, realistic rail, and simplified rail models under crosswind. The numerical methods were verified with the wind tunnel tests. Significant differences in aerodynamic performances of the train body and bogie were found in the cases with and without a rail model as the presence of the rail model had significant impacts on the flow field underneath the vehicle. A larger yaw angle can result in a more significant difference in aerodynamic coefficients. The deviations of the train aerodynamic forces and the pressure distribution on the train body with the realistic and simplified rail models were not significant. It was concluded that a rail model is necessary to get more realistic results, especially for large yaw angle conditions. Moreover, a simplified rectangular rail model is suggested to be employed instead of the realistic rail and is capable to get accurate results.

Numerical research of the effects of fouling on the performance of marine propellers

As it is known, propeller has been most widely-used ship thruster, propeller blade surfaces are of polished metal and have no antifouling provision, which makes them vulnerable to fouling. Under the effects of fouling, the propulsion efficiency of propellers decreases and fuel consumption of ships increases, thus greenhouse gas emissions increase, although the effects are rarely studied. The present work aims at quantifying the effects and providing a deep insight into the physical mechanism by means of Computational Fluid Dynamics (CFD) and the surface panel method. The simulation employs the SST k-ω turbulence model and is carried on NSRDC-NACA-66-mod blade section which is widely applied to marine propeller. Barnacles are selected as the study subject from the fouling community and are directly modeled at geometry level. With Propeller 4383 being studied, the induced velocity field under several advance coefficients through the surface panel method, hence the incoming velocity and effective attack angle of the blade sections at each radius are obtained. Then the lift and drag force of each blade section under fouling conditions are got through the CFD numerical method, and then the propeller open water performance parameters under fouling conditions are obtained through blade elements theory, such as the thrust coefficient TK, torque coefficient QK and open water efficiency η0. The results shows that the fouling has a serious adverse effect on the propeller performance parameters, and the serious calcareous fouling leads TK goes down by 36.1%, QK increases by 25.7% and η0 reduces by 49.2%.

Synthetic aperture radar image simulation of the internal waves excited by a submerged object in a stratified ocean

ABSTRACTA submarine moving in a stratified fluid with sharp interfaces is considered. The velocity field and the waves created by a moving submarine on a sea surface are simulated with the computational fluid dynamics numerical method. The wakes in various patterns with different parameters are discussed and illustrated in both geometries and synthetic aperture radar (SAR) images. The hydrodynamic fields are utilized to obtain the electromagnetic (EM) scattering signatures of the internal waves. In order to simulate SAR images of the generated wakes, the spatial distributed EM field is necessary. For this purpose, the semi-deterministic facet scattering model is employed. The SAR images in different look directions demonstrate that the micro-internal waves on sea surface can be sensed from its Bragg component moving in the radar look direction by SAR.

Predicting urban heat island circulation using CFD

Most existing microscale computational fluid dynamics (CFD) models and mesoscale meteorological models cannot consider multi-scale urban wind flows, as neither can completely take into account the mesoscale and microscale physics, and their interactions. Here we suggest a CFD model which has good potential for development of the meso-micro scale models for predicting and designing multi-scale urban airflows. The principal idea is to use CFD numerical methods to solve a set of governing equations with appropriate boundary conditions that can govern both major mesoscale and microscale flow characteristics. Based on the coordinate transformation method proposed by Kristóf, Rácz and Balogh (2009), we derived a similar set of governing equations with some improvements and used an existing porous turbulence model for modeling the urban canopy layer. The approach was then successfully implemented using a commercial CFD package (Fluent) for studying urban heat island circulation (UHIC), which is considered to be one of the most difficult problems in CFD. Our predicted mean quantities agree well with existing data in the literature obtained from large eddy simulations, mesoscale models, and laboratory experiments. Our predicted results also reveal the effects of the different heat fluxes and urban height of a city on UHIC characteristics.

Numerical Comparison of Rotordynamic Characteristics for a Fully Partitioned Pocket Damper Seal and a Labyrinth Seal With High Positive and Negative Inlet Preswirl

Pocket damper seals (PDSs) are used as replacements for labyrinth seals in high-pressure centrifugal compressors at the balance-piston location or center seal location to enhance rotordynamic stability. A concern exists that this enhanced stability will be lost at high positive inlet preswirl. Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates were presented and compared for a fully partitioned PDS (FPDS) and a labyrinth seal at high positive and negative inlet preswirl, using a proposed transient computational fluid dynamics (CFD) method based on the multifrequency elliptical orbit whirling model. The negative preswirl indicates a fluid swirl in a direction opposite to rotor rotation at seal inlet. Both seals have identical diameter and sealing clearance. The full 3D concentric CFD model and mesh were built for the labyrinth seal and FPDS, respectively. The accuracy and availability of the present transient CFD numerical method were demonstrated with the experiment data of frequency-dependent rotordynamic coefficients of the labyrinth seal and FPDS at zero and high positive preswirl conditions. The numerical boundary conditions include two high positive preswirl, two high negative preswirl, and a zero preswirl. Numerical results show that the effect of inlet preswirl on the direct force coefficients is weak, but the effect on the cross-coupling stiffness and effective damping is dramatic. Both seals possess negative effective damping at lower excitation frequencies due to positive preswirl, and the crossover frequency of effective damping term increases with increasing positive preswirl. Negative preswirl produces negative cross-coupling stiffness and positive effective damping over the whole excitation frequency range. Increasing negative preswirl is a stabilizing factor for annular gas seals, which results in a significant increase in the effective damping and a decrease in the crossover frequency. It is desirable to reduce the inlet preswirl to zero or even negative through applications of negative-swirl brakes and negative injection devices.

Analysis of turbulent flow through an orifice meter using experimental and computational fluid dynamics simulation approach—A case study

This paper describes the use of computational fluid dynamics in teaching graduate students who were in a four‐year B. Tech program. Many of these students did not have a good background in mathematics, fluid dynamics, heat transfer, and programming; however, most of them were good at computer‐aided design in ProE and were very interested in learning computational fluid dynamics as a design tool in industries. Solidworks flow simulator was chosen as the computational fluid dynamics software to teach students the entire computational fluid dynamics process in a single integrated software environment. Based on projects, computational fluid dynamics numerical methods and fundamentals of heat transfer and fluid flow were introduced to help students understand the computational fluid dynamics process, interpret, and validate simulation results. The computational fluid dynamics simulation of an orifice meter is given as the basic example for the students. Orifice meters are the most common equipment used for measuring fluid flow because of their simple mechanical structure, versatility, and low cost. In this paper, computational fluid dynamics simulation has been used to predict the orifice flow with better accuracy. Computational fluid dynamics simulations have been performed using solidworks flow simulator and validated with the data available in published literature. A new system has been proposed to accurately measure the flow using orifice metering systems.

Numerical Investigations on the Leakage and Rotordynamic Characteristics of Pocket Damper Seals—Part I: Effects of Pressure Ratio, Rotational Speed, and Inlet Preswirl

Effects of pressure ratio, rotational speed and inlet preswirl on the leakage and rotordynamic characteristics of a eight-bladed fully partitioned pocket damper seal (FPDS) were numerically investigated using proposed three-dimensional (3D) transient computational fluid dynamics (CFD) methods based on the multifrequency elliptical whirling orbit model. The accuracy and availability of the multifrequency elliptical whirling orbit model and the transient CFD numerical methods were demonstrated with the experimental data of frequency-dependent rotordynamic coefficients of the FPDS at two rotational speeds with high preswirl conditions. The frequency-dependent rotordynamic coefficients of the FPDS at three pressure ratios (three inlet pressures and three outlet pressures), three rotational speeds, three inlet preswirls were computed. The numerical results show that changes in outlet pressure have only weak effects on most rotordynamic coefficients. The direct damping and effective damping slightly increase in magnitude with decreasing outlet pressure at the frequency range of 20–200 Hz. The effect of inlet pressure is most prominent, and increasing inlet pressure for the FPDS results in a significant increase in the magnitudes of all rotordynamic coefficients. The magnitudes of the seal response force and effective damping are proportional to pressure drop through the seal. Increasing rotational speed and increasing inlet preswirl velocity both result in a significant decrease in the effective damping term due to the obvious increase in the magnitude of the destabilizing cross-coupling stiffness with increasing rotational speed or increasing preswirl velocity. The crossover frequency of effective damping significantly increases and the peak magnitude of effective damping decreases with increasing rotational speed or increasing preswirl velocity. The destabilizing cross-coupling stiffness is mainly caused by the circumferential swirl velocity generating from high rotational speed and inlet preswirl. Reducing swirl velocity (such as swirl brake) can greatly enhance the stabilizing capacity of the FPDS.

Numerical Investigation of Static and Dynamic Aerodynamic Characteristics for a Guided Projectile with Fin Slot

The flow field of a cruciform finned guided projectile with and without fin slot was simulated by the mean of CFD(computational fluid dynamic) numerical method. The characteristics of pressure field distribution without and different widths slots were calculated and compared. The influence of fin slot to the aerodynamic characteristics such as drag, lift, roll moment, pitch moment static aerodynamic characteristics as well as pitch, roll damping moment dynamic aerodynamic characteristics were emphasized in different angle of attack. The numerical results were certified by the good agreement between the wind tunnel test results. Important reference was provided to theoretical research and engineering for guided projectile shape design.

Two-Phase Flow Model Applied in the Oxidation Ditch

The unique hydraulic characteristics in oxidation ditch have a close relation with the quality of treated water, the design and optimization of the oxidation ditch. The experimentally validated numerical tools, based on computational fluid dynamics(CFD), were proposed. Sewage- sludge two-phase model and liquid-gas two-phase model of an oxidation ditch were built through CFD numerical method. Meanwhile, a velocity field measurement was enforced on the ditch by Acoustic Doppler Velocimeter(ADV). The simulated results and experimental data were in good correspondence, which verify the simulation methods are reasonable and the simulation results are acceptable. The combination of simulation and experimrntal measurement has profound influence on the hydraulic optimization of oxidation ditch. 3D simulation could be a good supplement for improving the hydrodynamic performance in oxidation ditch designs.

Simulations of the kettle reboiler shell side thermal-hydraulics with different two-phase flow models

A computational fluid dynamics approach is presented for the simulation and analyses of the kettle reboiler shell side thermal-hydraulics with two different models of two-phase flow – the mixture and two fluid model. The mixture model is based on solving one momentum equation for two-phase mixture flow and a closure law for the calculation of the slip between gas and liquid phase velocities. In the two fluid modeling approach the momentum balance is formed for each phase, while the gas-liquid interaction due to momentum exchange at the interface surface is predicted with an empirical correlation for the interface friction coefficient. In both approaches the two-phase flow is observed as two inter-penetrating continua. The models are solved for the two-dimensional geometry of the kettle reboiler shell side vertical cross section. The computational fluid dynamics numerical method based on the SIMPLE type algorithm is applied. The results of both liquid and vapor velocity fields and void fraction are presented for each modeling approach. The calculated void fraction distributions are compared with available experimental data. The differences in the modeling approaches and obtained results are discussed. The main finding is that the void fraction distribution and two-phase flow field strongly depends on the modeling of the slip between liquid and gas phase velocity in mixture model or on the interface friction model in two fluid model. The better agreement of the numerically predicted void fraction with the experimental data is obtained with the two fluid model and an interfacial friction model developed for the conditions of two-phase flows in large volumes of kettle reboilers or different designs of steam generators.

Numerical black holes: A moving grid approach.

Spherically symmetric (1D) black-hole spacetimes are considered as a test for numerical relativity. A finite difference code, based in the hyperbolic structure of Einstein's equations with the harmonic slicing condition is presented. Significant errors in the mass function are shown to arise from the steep gradient zone behind the black hole horizon, which challenge the Computational Fluid Dynamics numerical methods used in the code. The formalism is extended to moving numerical grids, which are adapted to follow horizon motion. The black hole exterior region can then be modeled with higher accuracy.