Incompressible Flow Field Research Articles

Modeling of magnetohydrodynamic free convection in an enclosure having elliptical sinks using lattice Boltzmann method

AbstractThis paper adopted the Lattice Boltzmann Method (LBM) to compute a two‐dimensional incompressible convective flow field with two isothermal elliptical sinks attached vertically to the horizontal bottom north side of the enclosure, which is subjected to a horizontal magnetic field. The attained results show that LBM can effectively capture vortex shedding structures and other features inside such complex flow. Also, the Hartmann numbers with a panoply of heated sinks location were studied, and the results showed that fluid flow and heat transfer rate depend on the Hartmann number and are greatly influenced by the heated sinks positions. The effect of the Prandtl number on the average Nusselt number is also highlighted.

Pressure characteristics of a nonspherical underwater explosion bubble in a compressible fluid

The pressures produced by underwater explosions present serious threats to ships, submarines, and other marine structures. A significant part of underwater explosion pressure comes from the explosion bubble. Most computational studies on bubble pressure have considered the case of an incompressible fluid or have confined calculations to the time before the formation of a toroidal bubble, because of the complexity and strong nonlinearity of a compressible flow field with a doubly connected geometry. The few compressible models that are capable of calculating the pressure after jet impact suffer from computational difficulties. In this paper, we calculate the bubble pressure by constructing a new form for solving an auxiliary function based on a boundary integral method that takes account of the compressibility of the flow field. We verify out numerical algorithm by comparison with a classical theoretical model and a set of experimental results. We then compare the bubble pressure in a compressible flow field in both the first and second oscillation periods with that in an incompressible flow field. The results of this comparison confirm that it is necessary to consider the compressibility of the fluid and the multiperiod pulsations of a bubble in pressure calculations. We present a comprehensive discussion of the pressure characteristics in the central axial and circumferential directions induced by a nonspherical bubble in a free field. Finally, we obtain a critical bubble–wall distance rw > 1.8 for which the bubble is prevented from splitting after the first jet impact, and we investigate the pressure characteristics of a bubble near a rigid wall for both cases of rw < 1.8 and rw > 1.8.

Procena modela turbulencije za nestišljiv tok u venturijevoj cevi

Turbulence models are semi-empirical transport equations that model flow behavior. They are compared on a recurring basis to know which of them presents the best fit with experimental data for different laboratory equipment. In the present work, the incompressible flow field (water) is simulated with the computational fluid dynamics (CFD) tool and RANS model for the geometry of a Venturi tube in 2D computational domains, with the objective of evaluating six turbulence models: standard k-e, RNG k-e, standard k-o, SST k-o, RSM and SA. The numerical results of the trajectories of the pressure pattern curves at the walls and axial symmetry are close to each other. Pressure drops occur at the throat. The percentage errors of the turbulence models increase as the magnitude of the pressure ratio increases, for r p =1.0932, r p =1.1118, r p =1.1377, and r p =1.1531. It is concluded that the SA turbulence model of Spalart-Allmaras (1992) best fits the experimental pressure data, with percentage errors of less than 10%.

Differentiable Curl-Noise

We present Differentiable Curl-Noise, a C1 procedural method to animate strictly incompressible fluid flows in two dimensions. While both the original Curl-Noise method of Bridson et al. [2007] and a recent modification by Chang et al. [2022] have been used to design incompressible flow fields, they often suffer from non-smoothness in their handling of obstacles, owing in part to properties of the underlying Euclidean distance function or closest point function. We therefore propose a differentiable scheme that modulates the background potential in a manner that respects arbitrary solid simple polygonal objects placed at any location, without introducing discontinuities. We demonstrate that our new method yields improved flow fields in a set of two dimensional examples, including when obstacles are in close proximity or possess concavities.

Fast aerodynamics prediction of laminar airfoils based on deep attention network

The traditional method for obtaining aerodynamic parameters of airfoils by solving Navier–Stokes equations is a time-consuming computing task. In this article, a novel data-driven deep attention network (DAN) is proposed for reconstruction of incompressible steady flow fields around airfoils. To extract the geometric representation of the input airfoils, the grayscale image of the airfoil is divided into a set of patches, and these are input into the transformer encoder by embedding. The geometric parameters extracted from the transformer encoder, together with the Reynolds number, angle of attack, flow field coordinates, and distance field, are input into a multilayer perceptron to predict the flow field of the airfoil. Through analysis of a large number of qualitative and quantitative experimental results, it is concluded that the proposed DAN can improve the interpretability of the model while obtaining good prediction accuracy and generalization capability for different airfoils and flow-field states.

SCR형 탈질 시스템의 암모니아 분사 노즐의 유량 제어에 대한 CFD기법 적용에 관한 연구

In the present study, computational analysis tools have been applied to improving the performance of the denitrification process by flow control of the ammonia injection nozzles adjusting to the flow distribution of the nitrogen oxides in the inlet flue gas of the selective catalytic reduction system. The uniformity of NH3/NO molar ratio is selected as the performance parameter. Design Xplorer was used to minimize the root mean square of NH₃/NO. Four types of inlet flow pattern were chosen as the simulated parameters, i. e. uniform, parabolic, upper-skewed and random. The flow rate of the sixteen nozzles installed in the ammonia injection grid was adjusted to the inlet conditions. In order to solve the three-dimensional steady, incompressible and viscous flow fields, the commercial software named as ANSYS-Fluent were used with the turbulence model. From the present computational analysis, the denitrification performance was improved between 1.47% and 88.6% according to flow pattern of the inlet flue gas.

Field synergy principle for compressible laminar flow and the application for drag reduction in microchannel

Considering the density variation of compressible fluid, the field synergy principle for compressible laminar flow is presented based on the incompressible flow field synergy principle. The flow resistance is related to the synergy of velocity field and density logarithmic gradient field. Based on the principle of minimum mechanical energy dissipation, the compressible laminar flow field synergy equation is derived. The field synergy principle is verified by an example of the microchannel with expansion cross section, and the structure is optimized. The results indicate that the viscous dissipation value in the entire flow domain of the optimized structure can be reduced by 13.5%, and the angle between the velocity vector and the gradient field of logarithm of density increases. In fluid flows, it will reduce the fluid flow drag to decrease the synergy angle between the velocity vector and the gradient field of logarithm of density.

Fast sparse flow field prediction around airfoils via multi-head perceptron based deep learning architecture

In order to obtain the information about flow field, traditional computational fluid dynamics methods need to solve the Navier-Stokes equations on the mesh with boundary conditions, which is a time-consuming task. In this work, a data-driven method based on convolutional neural network and multi-head perceptron is used to predict the incompressible laminar steady sparse flow field around the airfoils. Firstly, we use convolutional neural network to extract the geometry parameters of the airfoil from the input gray scale image. Secondly, the extracted geometric parameters together with Reynolds number, angle of attack and flow field coordinates are used as the input of the multi-layer perceptron and the multi-head perceptron. The proposed multi-head neural network architecture can predict the aerodynamic coefficients of the airfoil in seconds. Furthermore, the experimental results show that for sparse flow field, multi-head perceptron can achieve better prediction results than multi-layer perceptron.

Influence Analysis of Simulation Parameters on Numerical Prediction of Compressible External Flow Field Based on NACA0012 Airfoil under Hypersonic Speed

Currently, the influence of numerical parameters on the prediction accuracy of the compressible external flow field characterized by a NACA0012 airfoil under hypersonic speed, especially the influence analyses of the trailing edge shape, the modeling methods, and the adopted data points on it are relatively sparse. In this paper, using three modeling approaches and two data point sources, six NACA0012 airfoils are designed including two types of trailing edge shapes. Unlike under the incompressible external flow field, the comparative analysis shows that though the optimal accuracy is obtained by the sharp trailing edge, the improper sharp trailing edge design could result in a greater error ratio than that of the blunt trailing edge. Similarly, the definition formula could provide the best performance, while in other cases, NACA4 is preferred and the selection between them depends on the adopted data point source. Furthermore, the increase in the number of the adopted two types of data points would lead to a decrease in prediction accuracy. According to the comparison among all the optimal total error ratios, the suggested configuration is the sharp trailing edge based on the definition formula adopting 200 points sourced from NACA4 + 16 m far-field distance + SA turbulence model + ROE flux type and the best result is 2.05%.

An improved hydrodynamic/acoustic splitting method for fluid–structure interaction feedback with elastic boundaries

To simulate the noise caused by acoustic radiation, scattering, and flow, standard hydrodynamic/acoustic splitting methods have been extensively researched and developed. These methods decompose compressible Navier–Stokes equations into incompressible Navier–Stokes equations and acoustic perturbed equations but generally do not consider the influence of the fluid–structure interaction with elastic boundaries. In response to this defect, a wavelet finite element method is introduced to construct the elastic boundary static model, and a wavelet immersed boundary method is used to simulate the fluid–structure coupling incompressible flow field. In addition, a perfectly matched layer technique is applied to form non-reflective boundary conditions in the acoustic perturbed equations. Based on these approaches, an improved hydrodynamic/acoustic splitting method is presented for fluid–structure interaction feedback with elastic boundaries. Finally, several numerical examples were calculated, including acoustic radiations and scatterings, as well as flow-induced noise, to verify that the new method is a simple and efficient method for fluid–structure–sound interactions.

About 3D Incompressible Flow Reconstruction from 2D Flow Field Measurements.

In this paper, an assessment of the uncertainty affecting a hybrid procedure (experimental/numerical) is carried out to validate it for industrial applications, at the least. The procedure in question serves to depict 3D incompressible flow fields by using 2D measurements of it and computing the third velocity component by means of the continuity equation. A quasi-3D test case of an incompressible flow has been inspected in the wake of a NACA 0012 airfoil immersed in a forced flow of water running in a rectangular open channel. Specifically, starting from a 2D measurement data in planes orthogonal to the stream-wise direction, the computational approach can predict the third flow velocity component. A 3D ADV instrument has been utilized to measure the flow field, but only two velocity components have been considered as measured quantities, while the third one has been considered as reference with which to compare the computed component from the continuity equation to check the accuracy and validity of the hybrid procedure. At this aim, the uncertainties of the quantities have been evaluated, according to the GUM, to assess the agreement between experiments and predictions, in addition to other metrics. This aspect of uncertainty is not a technical sophistication but a substantial way to bring to the use of a 1D and 2D measurement system in lieu of a 3D one, which is costly in terms of maintenance, calibration, and economic issues. Moreover, the magnitude of the most relevant flow indicators by means of experimental data and predictions have been estimated and compared, for further confirmation by means of a supervised learning classification. Further, the sensed data have been processed, by means of a machine learning algorithm, to express them in a 3D way along with accuracy and epoch metrics. Two additional metrics have been included in the effort to show paramount interest, which are a geostatistical estimator and Sobol sensitivity. The statements of this paper can be used to design and test several devices for industrial purposes more easily.

Integration over discrete closed surfaces using the Method of Fundamental Solutions

The Method of Fundamental Solutions (MFS) is an established technique for solving linear partial differential equations. In this paper it is used for a new purpose: the approximation of integrals over closed surfaces from a finite set of known points and values. The MFS is used to fit an implicit surface through the surface points, where the implicit equation is chosen such that a surface integral is provided by summing the weights of the fit. From the divergence theorem, these surface integrals can be related to specific integrals over the enclosed volume. As a demonstration, we calculate the surface area, volume, centroid and radius of gyration, for three solid geometries: a sphere, a torus, and an ellipsoid. Very quick convergence to analytical results is shown. Local surface properties, such as the components of curvature, can also be obtained accurately. The drawbacks and advantages of the method are discussed, and the potential to calculate properties of constant-density rigid bodies (e.g. the moment of inertia tensor) and averages of incompressible flow fields (e.g. average flow velocity and strain rate) is highlighted.

Improved Stall Delay Model for HAWT Performance Predictions using 3D Navier-Stokes Solver and Actuator Disk Method

The Actuator Disk Method (ADM), in its analytical formulation or combined with Navier-Stokes equations, is widely used for design and/or for aerodynamic analysis of Horizontal Axis Wind Turbines (HAWT). This method has demonstrated its capabilities for performance predictions of HAWT rotors for limited range of wind speeds with lower angles of attack values, i.e. attached flow conditions. However, for typical wind speeds that rotor encounters, under higher angles of attack i.e. stall conditions, such a method cannot describe accurately the flow characteristics around rotor-blades due to severe flow separations coupled with the effects of blades rotation as well as the radial flow over the blades. In this paper, original correction approaches have been proposed for the existing stall delay models to take into account both the blade rotation and the radial flow effects over the rotor blades. For this purpose, the ADM combined with 3D- NavierStokes equations formulation using Large Eddy Simulations (LES) model has been considered to describe the incompressible turbulent flow field around HAWT rotor blades. The resulting mathematical model has been solved using a 3D in-house subroutine developed with OpenFOAM code. The proposed numerical method has been validated against the well recognized reference measurements obtained using the New MEXICO and the NREL Phase VI experimental wind turbines. In comparison with existing stall delay models, the proposed correctionapproaches, especially the radial flow approach, have shown noticeable enhancements on performance predictions of HAWT rotors compared to the experimental measurements. It has been found very low discrepancies to the experimental torque and thrust values, up to 1% and 10% have been recorded respectively.

Aeroacoustic noise prediction of a vertical axis wind turbine using large eddy simulation

This study investigates the numerical prediction for the aerodynamic noise of the vertical axis wind turbine using large eddy simulation and the acoustic analogy. Low noise designs are required especially in residential areas, and sound level generated by the wind turbine is therefore important to estimate. In this paper, the incompressible flow field around the 12 kW straight-bladed vertical axis wind turbine with the rotor diameter of 6.5 m is solved, and the sound propagation is calculated based on the Ffowcs Williams and Hawkings acoustic analogy. The sound pressure for the turbine operating at high tip speed ratio is predicted, and it is validated by comparing with measurement. The measured spectra of the sound pressure observed at several azimuth angles show the broadband characteristics, and the prediction is able to reproduce the shape of these spectra. While previous works studying small-scaled vertical axis wind turbines found that the thickness noise is the dominant sound source, the loading noise can be considered to be a main contribution to the total sound for this turbine. The simulation also indicates that the received noise level is higher when the blade moves in the downwind than in the upwind side.

Application of clustering and the Hungarian algorithm to the problem of consistent vortex tracking in incompressible flowfields

The ability to track vortices spatially and temporally is of great interest for the study of complex and turbulent flows. A methodology to solve the problem of vortex tracking by the application of machine learning approaches is investigated. First a well-known vortex detection algorithm is applied to identify coherent structures. Hierarchical clustering is then conducted followed by a unique application of the Hungarian assignment algorithm. Application to a synthetic flowfield of merging Batchelor vortices results in robust vortex labelling even in a vortex merging event. A robotic PIV experimental dataset of a canonical Ahmed body is used to demonstrate the applicability of the method to three-dimensional flows.Graphic abstract

CFD Simulation for Estimating Efficiency of PBCF Installed on a 176K Bulk Carrier under Both POW and Self-Propulsion Conditions

In this paper, the efficiency of Propeller Boss Cap Fins (PBCF) installed at the bulk carrier was estimated under both Propeller Open Water (POW) and self-propulsion conditions. For this estimation, virtual model-basin tests (resistance, POW, and self-propulsion tests) were conducted through Computational Fluid Dynamics (CFDs) simulation. In the resistance test, the total resistance and the wake distribution according to ship speed were investigated. In the POW test, changes of thrust, torque coefficient, and open water efficiency on the propeller according to PBCF installation were investigated. Finally, the International Towing Tank Conference (ITTC) 1978 method was used to predict the effect of PBCF installation on self-propulsive coefficient and brake horsepower. For analyzing incompressible viscous flow field, the Reynolds-Averaged Navier–Stokes (RANS) equation with SST k-ω turbulence model was calculated using Star-CCM+ 11.06.010-R8. All simulation results were validated by comparing the results of model tests conducted at the Korea Research Institute of Ships and Ocean Engineering (KRISO). Consequently, for the self-propulsion test with the PBCF, a 1.5% reduction of brake horsepower was estimated in the simulation and a 0.5% reduction of the brake horsepower was estimated in the experiment.

등가누출직경을 적용한 도시가스 감지기의 설치 위치에 대한 수치 해석적 연구

In this study, the location of gas detectors installed in urban gas-using facilities was investigated by using Computational Fluid Dynamics(CFD). Three-dimensional incompressible flow fields coupled with a κ-ε turbulence model were solved by using the equivalent leakage diameter to efficiently analyze the sonic jet leakage. Numerical simulation consisting of twenty methane gas leakage scenarios with various leakage hole sizes and gas pressures provided quantitative data of a detectable height that depends on an alarm set concentration. Lowering the alarm set concentration from 25% of Lower Explosive Level(LEL) to 20% of LEL was found to increase the detectable heights by 23∼31%. Although the maximum width of 20% contour of LEL was increased by 16%~30% compared to that of 25%, it was found that the width of the 25% contour of LEL predicted by CFD was as quite a small as about 30 ㎝.

Aeroacoustic simulation of flexible structures in low Mach number turbulent flows

This work deals with the numerical simulation of aeroacoustic sound generated by flexible structures in low Mach number turbulent flows. A partitioned computation scheme is investigated to solve the coupled fluid-solid problem. A hybrid method based on the acoustic/viscous splitting technique is applied for aeroacoustic computation using large eddy simulations (LES) to resolve the incompressible turbulent flow field. Combining the acoustic/viscous splitting approach with a Kirchhoff wave extrapolation method enables an efficient evaluation of the far-field noise. The arbitrary Lagrangian-Eulerian (ALE) technique is applied for both the incompressible flow field and the acoustic field to take moving boundaries into account. Validation of the acoustic computation process is presented for stationary and moving surfaces. Finally, the approach described in this work is applied to study the aeroacoustic characteristics of a membrane airfoil.

Inferring incompressible two-phase flow fields from the interface motion using physics-informed neural networks

In this work, physics-informed neural networks are applied to incompressible two-phase flow problems. We investigate the forward problem, where the governing equations are solved from initial and boundary conditions, as well as the inverse problem, where continuous velocity and pressure fields are inferred from scattered-time data on the interface position. We employ a volume of fluid approach, i.e. the auxiliary variable here is the volume fraction of the fluids within each phase. For the forward problem, we solve the two-phase Couette and Poiseuille flow. For the inverse problem, three classical test cases for two-phase modeling are investigated: (i) drop in a shear flow, (ii) oscillating drop and (iii) rising bubble. Data of the interface position over time is generated by numerical simulation. An effective way to distribute spatial training points to fit the interface, i.e. the volume fraction field, and the residual points is proposed. Furthermore, we show that appropriate weighting of losses associated with the residual of the partial differential equations is crucial for successful training. The benefit of using adaptive activation functions is evaluated for both the forward and inverse problem.

Petrov–Galerkin flux upwinding for mixed mimetic spectral elements, and its application to geophysical flow problems

Upwinded mass fluxes are described and analysed for advection operators discretised using mixed mimetic spectral elements. This involves a Petrov–Galerkin formulation by which the mass flux test functions are evaluated at downstream locations along velocity characteristics. As for the original mixed mimetic spectral element advection operator, the upwinded mass flux advection operator is conservative, however unlike the original advection operator, which is purely hyperbolic, the upwinded advection operator adds dissipation which is biased towards high wave numbers. The upwinded advection operator also removes the spectral gaps present in the dispersion relation for the original advection operator. As for the original advection operator, a material form advection operator may be constructed by similarly downwinding the trial functions of the tracer gradients. Both methods allow for the recovery of exact energy conservation for an incompressible flow field via skew-symmetric formulations. However these skew-symmetric formulations are once again purely hyperbolic operators which do not suppress oscillations. The scheme is implemented within a shallow water code on the sphere in order to diagnose and interpolate the potential vorticity. In the absence of other dissipation terms, it is shown to yield more coherent results for a standard test case of barotropic instability.