Boundary Layer Grid Research Articles

Fine scale simulation of sea salt aerosol under strong wind: WRF sensitivity test on PBL schemes and LES under Typhoon Hagibis

Identifying which factors among the planetary boundary layer (PBL) scheme, grid resolution, and source or sink terms dominantly influence the sea salt aerosol (SSA) spatial distribution is necessary. The dependence of SSA transport on PBL processes was investigated using a numerical weather prediction (NWP) model with a grid scale of ∼143 m. Size-segregated SSA transport during Typhoon Hagibis, which made landfall in Japan in 2019, was simulated using the Weather Research and Forecasting (WRF) model. The PBL schemes of Yonsei University (YSU), scale-aware YSU (Shin and Hong, 2015's scheme, SH) and Mellor-Yamada Nakanishi-Niino (MYNN), and large-eddy simulation (LES) subjected to turbulence transition enhancement via a cell perturbation method were tested to evaluate the reproducibility of observed values of wind speed, surface pressure, relative humidity, and temperature at meteorology masts. The results showed that wind speed was most influenced by the selection of the PBL scheme and LES. The distance from the coastline, as determined by the wind direction during the most intense typhoon stage, was correlated with a decrease in the time averaged SSA concentration when the storm moved inland. Despite the simulated SSA emission scheme's high sensitivity to wind speed (nearly 3rd power), the near-surface average concentration was less sensitive to the PBL schemes and LES selection than wind speed. However, the maximum arrival of the SSA differed by ∼20 % during the most vigorous stage of the typhoon. Budget analysis of the SSA transport equation highlighted that the advection term and gravity settling were most dependent on the PBL schemes and LES. The impacts of both factors on SSA transport were not limited to near-surface tendencies but were in a broader range over the atmosphere, up to 1–2 km. These results indicated that the vertical structure of the SSA field over the boundary layer depended on the PBL schemes and LES, thereby modulating the inland arrival of the SSA.

Использование приповерхностных сеток для численного моделирования вязкостных явлений в задачах гидродинамики судна

Использование приповерхностных сеток для численного моделирования вязкостных явлений в задачах гидродинамики судна

CFD Analysis of Over-Expanded Flow in Convergent Divergent Nozzles

The classical one-dimensional inviscid theory does not reveal the complex flow features in an over-expanded nozzle accurately. The code Fluent has been used to simulate the viscous fluid (air) flow passing through three different 2-D Convergent-Divergent (C-D) nozzles for nozzle pressure ratios (NPR) corresponding to over-expanded flow. The steady state RANS equation with five different turbulence models has been simulated for turbulence model study. Two different types of boundary layer grids: grid1 (y<sup>+</sup>=1) and grid 2 (y<sup>+</sup=50) have been simulated. Shear Stress Transport k-w (SSTKW) turbulence model with grid 1 has been chosen for turbulence modeling. Both inviscid and viscous flows have been simulated. The converged solutions captured asymmetric lambda shock in all the three nozzles at higher NPRs for viscous flows. It also predicted aftershock and flow separation depending upon NPR. The shock wave angle of the incident and reflected shock generally reduces with increase in NPR for NPR£1.92. The thrust co-efficient of all the theoretical, predicted inviscid and predicted viscous solutions reduce with increase in NPR upto a certain value of NPR and then starts increasing. The predicted viscous results (shock structure, shock location, size of normal shock, aftershock and asymmetric lambda shocks) are close to the experiments in most of the cases. The aftershock phenomenon provides dual mode flow at the exit of the nozzle.

Enhancement of the Robustness on Advancing Layer Method with Trimmed Hexahedral Volume Mesh for the Generation of the Boundary Layer Grids

When performing simulations using computational fluid dynamics, the grid systems in the viscous boundary layer regions are important because the velocity and pressure change very rapidly in these regions. Especially for the turbulent flows, thin grids should be arranged densely in the direction perpendicular to the wall. In this study, the advancing layer method, which has been applied mostly to tetrahedral meshes, is applied to trimmed hexahedral meshes. To generate boundary layer meshes with non-intersecting grid lines near the wall boundaries having concave corners and narrow gaps, the directional vectors of grid lines and faces are smoothed, and the displacement vector fields calculated using the Laplace equation were utilized. Firstly, the details on the newly developed methods are introduced showing simple two-dimensional cases as examples. After applying the methods for a complex three-dimensional geometry to check its applicability and investigating the generated grid systems, the numerical simulations of propeller open water test for INSEAN E779A marine propeller were carried out by simpleFoam, one of the standard solvers of OpenFOAM. The computational results showed good agreement with the experimental results. Therefore, in conclusion, the developed advancing layer method is an appropriate method for generating boundary layer grids of a trimmed hexahedral mesh.

Research on grid‐dependence of neural network turbulence model

AbstractTurbulence machine learning based on deep neural network has become a research hotspot in turbulence modeling. Although most turbulence models have strict requirements on grid dependence, there are few analyses on grid dependence on the coupling of models and equations. In this article, a neural network turbulence model is constructed for the flow around airfoil with high Reynolds number, and the effects of wall‐normal grid spacing on the calculation accuracy are studied. The results show that compared with the traditional differential equation turbulence model, the neural network turbulence model can break through the limitation of y+ < 1 and relax the requirement of the normal density of the boundary layer grid while ensuring the accuracy.

Numerical Investigation of Boundary Grid Effect on Heat Transfer Computation of RP-3 at Supercritical Temperature of Helical Tube Wall

To get reliable computational results, the RNG k-e turbulence model with enhanced wall treatment was validated to solve the heat transfer of supercritical RP-3 in a helically coiled tube, and models of the thermo-physical properties of RP-3 were optimally chosen. Most significantly, the grid independence was validated by two-step procedure, and the effect of boundary grids of the supercritical-temperature wall on the computational accuracy was well studied. Through adjusting boundary-layer girds’ size, four regions (increased, pseudo-convergence, decreased and convergence) of the outlet temperature Tout were obtained and analyzed. The results showed that the maximum computation errors of Tout and the pressure differential between the inlet and outlet ΔP reached 20.65% and 98.15%, respectively, indicating that boundary grids have a significant influence on computation of flow and heat transfer. Based on this, a dimensionless distance from the wall-adjacent cell to the wall y+ = Prw−1/1.78 (Prw denotes Wall Prandtl number) was recommended as a convergence point. The variation laws of viscous length scale y were discussed under different structural parameters, operation parameters, and helical lengths. An explicit model of y* was proposed to calculate the height (y) of the first boundary layer grids and refine boundary grids efficiently. A modified model for coefficient of friction factor Cf was proposed based on Rogers’s, and Nusselt number Nu was proposed based on an analogy of momentum and heat transfer. The above models about y*, Cf and Nu could apply to both the entrance region and the whole tube length, and showed good performance when Reynolds number was extended to above 70 000, or whenever the outlet temperature is below or above the critical point.

Sub-kilometer dispersion simulation of a CO tracer for an inter-Andean urban valley

Air quality planning and management can benefit from the ability of numerical models to produce skillful forecasts. However, obtaining these forecasts is a continuing challenge, particularly in complex terrain. Here we examine the dispersion of traffic emissions in the Aburrá valley (located in the Colombian Andes) during an episode of severe air pollution, using the WRF-Chem and a Lagrangian model at sub-kilometer resolution. We focus on the identification of areas with higher CO concentrations (the pollutant analyzed), and examine the role of local and regional airflows in the distribution of CO inside the valley. The meteorological model performance, considering different planetary boundary layer parameterizations and grid sizes, is evaluated using available observations. Overall, the meteorological model performance is within recommended benchmarks for complex terrain. Model performance improves with increasing grid resolution for surface temperature and wind direction, but not for wind speed. Both dispersion models reproduce important features of the spatial and diurnal variability of CO in the valley, but underestimate CO concentrations throughout the simulation period. The south and southeast areas of the valley present the largest CO concentrations, associated with a prevalent northerly transport along the valley axis and reduced transport on the eastern slope. The representation of CO improves when the model adequately reproduces observed rainfall, due to effects of rainfall on boundary layer height and stability, which condition dispersion. Better simulation of transport processes is crucial for informing decisions on air quality management in critical areas such as urban valleys.

전산해석을 활용한 두께비 18%익형(Case1)의 공력특성 분석

두께비 18% 익형(KARI-11-180)에 대한 공력특성 분석이 전산해석기법을 활용해 수행되었다. 익형주위의 격자는 벽면에서 수직으로 투영하여 경계층 격자를 형성하였고 익형 후방에는 정밀한 후류 예측을 위해 조밀한 격자를 위치하였다. 원방경계까지의 거리는 익형 코드의 100배로 정하였고 익형시험결과와의 비교를 위해 자유류 속도, 익형 코드 및 Reynolds수를 풍동시험과 동일하게 정하였다. 또한 난류 모델은 천이지점 예측이 우수한 transition SST 및 DES 모델을 사용하였다. 3차원 전산해석은 세장비가 2와 5인 익형모델에 대해 수행되었는데 양력은 풍동시험결과보다 높은 값을 항력은 낮은 값을 예측하였다. Aerodynamic analysis for the airfoil, KARI-11-180 having 18% thickness ratio, was performed with CFD techniques. The boundary layer grid was generated by projecting the wall grid normally and fine grid was placed behind the trailing edge to capture the wake accurately. The distance to the far boundary is 100 chords and the flow condition is same as the wind tunnel test condition. Transition SST and DES turbulence models were utilized for accurate prediction of the transiton point. The predicted lift is higher but the drag is predicted lower than the wind tunnel test. 3-dimensional results with airfoil models of which aspect ratio were 2 and 5 were compared with 2-dimensional results.

WRF Hub-Height Wind Forecast Sensitivity to PBL Scheme, Grid Length, and Initial Condition Choice in Complex Terrain

Abstract This study evaluates the sensitivity of wind turbine hub-height wind speed forecasts to the planetary boundary layer (PBL) scheme, grid length, and initial condition selection in the Weather Research and Forecasting (WRF) Model over complex terrain. Eight PBL schemes available for the WRF-ARW dynamical core were tested with initial conditions sources from the North American Mesoscale (NAM) model and Global Forecast System (GFS) to produce short-term wind speed forecasts. The largest improvements in forecast accuracy primarily depended on the grid length or PBL scheme choice, although the most important factor varied by location, season, time of day, and bias-correction application. Aggregated over all locations, the Asymmetric Convective Model, version 2 (ACM2) PBL scheme provided the best forecast accuracy, particularly for the 12-km grid length. Other PBL schemes and grid lengths, however, did perform better than the ACM2 scheme for individual seasons or locations.

Refined simulation of temperature distribution in molds during autoclave process

Since the production of advanced composites, they have gained popularity in many industrial applications because of their unique performances. The advantages of these materials are very important, especially in the area of aviation. Among all the molding methods adopted in their production, the autoclave stands out as distinct method. Considering the temperature distribution in molds, it is one of the crucial aspects to impart quality to composite components during the process of autoclave molding, and it is necessary to do research and analysis on temperature distribution in molds, especially those of large frame-type molds. In previous studies, many authors improved the simulation accuracy simply by changing the grids or boundary conditions. A few of them took boundary layer grids into consideration for further precision. With the aid of computer softwares, this paper conducts simulation on large frame type of molds. Given that the boundary layer grids will largely determine the simulation precision, this study attempts to give emphasis on the study of boundary layer grids to make the simulation results more accurate, and finally make a comparison with the experimental data. The error between simulation and experiment results was within 5 %. This indicated that the boundary layer grids had great influence on the accuracy of simulation of the paper conducted, and should be given more attention. Therefore, the method of simulation in this study can be further used to accurately simulate the temperature distribution in molds.

Effects of boundary layer and liquid viscosity and compressible air on sloshing characteristics

ABSTRACT In this paper, numerical investigations for tank sloshing, based on commercial CFD package FLUENT, are performed to study effects of boundary layer grid, liquid viscosity and compressible air on sloshing pressure, wave height and rising time of impact pressure. Also, sloshing experiments for liquids of different viscosity are carried out to validate the numerical results. Through comparison of numerical and experimental results, a computational model in-cluding boundary layer grid can predict the sloshing pressure more accurately. Energy dissipation due to viscous friction leads to reduction of sloshing pressure and wave elevation. Sloshing pressure is also reduced because of cushion effect of compressible air. Due to high viscosity damping effect and compressible air effect, the rising time of impact pressure becomes longer. It is also found that liquid viscosity and compressible air influence distribution of dynamic pressure along the vertical tank wall.

A Computational Framework for Helicopter Fuselage Drag Reduction Using Vortex Generators

A computational framework is described for the computational fluid dynamics (CFD) simulation of a bluff helicopter fuselage equipped with vortex generators (VGs). The VGs are explicitly discretized in the CFD mesh, using an overset grid method. A computational setup was developed allowing parametric investigations for VG size, pitch angle, position, number, arrangement, and thickness effects. High-density meshes were generated with successive grid overlap between body-fittedmeshes with high grid resolution in the boundary layer and Cartesian grids for the near and far field. The methodology was applied to the model-scale GOAHEAD model with sponsons. This model permits drag reduction by passive flow control on the model back ramp where the pronounced upsweep is responsible for a large separated flow at the backdoor/tail-boom junction. A test matrix was completed, and a VG layout proved its effectiveness for an eight-pair array of counterrotatingzero-thickness vane-type VGs, with the device height defined according to the local boundary layer thickness. At cruise conditions, some VG configurations tested achieve up to 5% drag reduction by cumulated effects of flow reattachment, limited device drag, and static pressure recovery.

Fluid–structure interaction for an elastic structure interacting with free surface in a rolling tank

A fully coupled approach to large deformation fluid–structure interaction (FSI) using a nonlinear finite element (FEM) solver and a URANS/DES overset solver is presented. Since the relationship between strains and displacement cannot be treated as linear at large deformation problems, a nonlinear FEM treating geometric nonlinearity is applied instead of a linear modal analysis for a structure solver. For large deformation the reinitialization of the overset hole-cutting, which is carried out by default only at the beginning of the computation, is performed automatically at each time step. The approach uses the gluing method to transfer the forces and displacements on non-matching grids for fluid and structure domains. A linear FEM solver is applied to deform the outer boundary of the boundary layer grids which wrap around the deformable geometries. The deformation of interior points in the boundary layer grid is obtained using linear interpolation. Three cases of rolling tanks partially filled with fluid with an elastic bar clamped to bottom or top are simulated and compared with experiments and other numerical simulation results. The simulation results of the presented method show good agreement with the experiments for bar deformation and free surface elevation.

Improving the accuracy of discontinuous Galerkin schemes at boundary layers

SUMMARYThe resolution of boundary layers typically requires fine grids in the wall normal direction, which leads to anisotropic elements being refined towards the wall. Best practice guidelines for the mesh generation of stretched boundary layer grids exist for the finite volume or finite difference discretizations. A similar resolution of boundary layers with DG schemes can be achieved with a coarser grid because of the subgrid resolution of the DG scheme. High order schemes incorporate the possibility of high order element mappings, resulting in different resolution properties inside the element. In this paper, we show that the use of an internal element mapping in combination with a stretched grid can be used to reduce the error of the boundary layer approximation by an order of magnitude in comparison with the classical linear internal element mapping. The boundary layer is modeled by a one‐dimensional singular perturbation problem. In addition, we discuss the construction of the element mappings by interpolation and investigate the limits of the stretching function such that the resulting element Jacobian remains positive. A parameter study shows the influence of the element mapping for different polynomial degrees on the solution. Copyright © 2014 John Wiley & Sons, Ltd.

Computational fluid dynamics assessment of damaging wind loads on the One Indiana Square tower

In this paper, we simulated damaging wind loads on the One Indiana Square tower in Indianapolis due to the storm of April 2nd 2006. We followed recommended practice guidelines for this urban wind modeling. First, a test case, Aerodynamics of Commonwealth Advisory Aeronautical Council (CAARC) building were modeled and simulated to compare with a publicly available experiment and other computational studies. Based on the modeling parameters in the CAARC study, then, as a clean building configuration, we modeled the One Indiana tower alone without surrounding buildings. Finally, the flow field around the tower including nearby downtown buildings were simulated. We used the Fluent flow analysis software tools. The domain was meshed using unstructured grids, the first boundary layer grid element being 10 cm (4 in.) and 15 cm (6 in.) in height from the tower and the ground for the CAARC building and the One Indiana tower, respectively. Two different wind directions of 260 $$^\circ $$ and 280 $$^\circ $$ at 137 km/h (85 mph) speed were considered to estimate wind loads on the One Indiana tower facades. Simulated pressure distributions on the tower and flow patterns over the downtown buildings were discussed to draw conclusions about the mechanism of extreme wind load that caused the damage. The simulations revealed that suction forces are almost twice higher hence more damaging at the corners of the west facades than straight wind. It was also seen in the simulation results that upstream building topology, specifically Chase, One America, and some low-rise towers, augmented the actual wind load unfavorably on the One Indiana Square tower. Although this study presents a specific case, the applicability of its findings are of more general interest. Similar wind events are common especially during storm seasons both in urban and suburban areas. In similar incidents, one can follow the same procedure to analyze their problems as certain modeling guidelines were followed in this study.

Scaling Evaluations on the Drag of a Hub System

Parasite drag on rotorcraft can become a crucial factor in forward flight, especially during high-speed flight. Prior evaluations of the ability of computational methods to predict hub drag have focused on the ability of these solvers to match model-scale experimental data, but the results have not typically been examined for full-scale conditions. Using an unstructured computational method, the sources of hub drag on a moderately complex model are examined at different Reynolds number scales. Correlations with a 1/3.5-scale wind tunnel test and empirical data are provided to confirm the accuracy of the computations. Unlike prior efforts, grid adaptation that crosses overset mesh boundaries permits grid refinement where needed while minimizing the computational cost. For the moderately complex hub evaluated, utilization of the same grid is permissible, provided the boundary layer grid is tailored for the highest Reynolds number studied and that grid adaptation is applied. Drag evaluation illustrates that the drag of each component should be estimated at the component-level Reynolds number before consideration of the interference effects. Estimation of the interference drag for rotating hubs should additionally account for the Magnus effect, which influences the nonlinearities observed in scaling the drag and the wake.

Hybrid grid generation for two-dimensional high-Reynolds flows

An hybrid mesh generation algorithm for two-dimensional viscous flows at high-Reynolds number is presented. An advancing-front method is used close to solid surfaces and in the wake region(s). The boundary layer and wake grids possibly contain both highly stretched quadrilateral and triangular elements. The latter are inserted locally to improve the quality of the grid, thus circumventing some drawbacks of standard structured grid advancing-front methods. An advancing-front/Delaunay algorithm triangulates the remaining portion of the computational domain. An overview of both algorithms is given and results for viscous laminar and turbulent compressible flows around single and multi-element airfoils are shown to support the present approach.

Budget analysis of the boundary layer grid flights during FIFE 1987

An aircraft grid pattern was flown by the Canadian Twin Otter to map the low‐level fluxes and structure over the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) research area in 1987. The time dependence and horizontal advection of heat and moisture were extracted from these flights, combined with surface flux measurements and boundary layer top measurements from radiosondes, to analyze the boundary layer budget using a mixed layer model. The results confirm the suggestion of an earlier study that the boundary layer top entrainment (when parameterized using the buoyancy flux) is nearly double the value used by many modeling studies. Both surface and aircraft data have been revised, and it now appears that the direct measurements of the sensible and latent heat fluxes by the aircraft underestimated these fluxes by about 20%, because of filtering and undersampling of long wavelength contributions.