Development and Evaluation of a Reynolds-Averaged Navier–Stokes Solver in WindNinja for Operational Wildland Fire Applications

An open source computational fluid dynamics (CFD) solver has been incorporated into the WindNinja modeling framework. WindNinja is widely used by wildland fire managers, as well as researchers and practitioners in other fields, such as wind energy, wind erosion, and search and rescue. Here, we describe the CFD solver and evaluate its performance against the WindNinja conservation of mass (COM) solver, and previously published large-eddy simulations (LES), for three field campaigns with varying terrain complexity: Askervein Hill, Bolund Hill, and Big Southern Butte. We also compare the effects of two model settings in the CFD solver, namely the discretization scheme used for the advection term of the momentum equation and the turbulence model, and provide guidance on model sensitivity to these settings. Additionally, we investigate the computational mesh and difficulties regarding terrain representation. Two important findings from this work are: (1) CFD solver predictions are significantly better than COM solver predictions at windward and lee side observation locations, but no difference was found in predicted speed-up at ridgetop locations between the two solvers, and (2) the choice of discretization scheme for advection has a significantly larger effect on the simulated winds than the choice of turbulence model.

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Development and Evaluation of a Reynolds-Averaged Navier-Stokes Solver in WindNinja for Operational Wildland Fire Applications

10.20944/preprints201909.0315.v1 ◽

2019 ◽

Author(s):

Natalie S. Wagenbrenner ◽

Jason M. Forthofer ◽

Wesley G. Page ◽

Bret W. Butler

Keyword(s):

Wildland Fire ◽

Navier Stokes ◽

Modeling Framework ◽

Computational Mesh ◽

Large Eddy ◽

Speed Up ◽

Computational Fluid Dynamics Cfd ◽

Terrain Representation ◽

Bolund Hill ◽

Askervein Hill

An open source computational fluid dynamics (CFD) solver has been incorporated into the WindNinja modeling framework widely used by wildland fire managers as well as researchers and practitioners in other fields, such as wind energy, wind erosion, and search and rescue. Here we describe incorporation of the CFD solver and evaluate its performance compared to the conservation of mass (COM) solver in WindNinja and previously published large-eddy simulations (LES) for three field campaigns conducted over isolated terrain obstacles of varying terrain complexity: Askervein Hill, Bolund Hill, and Big Southern Butte. We also compare the effects of two important model settings in the CFD solver and provide guidance on model sensitivity to these settings. Additionally, we investigate the computational mesh and difficulties regarding terrain representation. Two important findings from this work are: (1) the choice of discretization scheme for advection has a significantly larger effect on the simulated winds than the choice of turbulence model and (2) CFD solver predictions are significantly better than the COM solver predictions at windward and lee side observation locations, but no difference was found in predicted speed-up at ridgetop locations between the two solvers.

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Effect of RANS Method on Stall Inception Eigenvalue Approach

Volume 2B: Turbomachinery ◽

10.1115/gt2017-64708 ◽

2017 ◽

Author(s):

Zhe Xie ◽

Yangwei Liu ◽

Xiaohua Liu ◽

Lipeng Lu ◽

Xiaofeng Sun

Keyword(s):

Turbulence Model ◽

Wake Flow ◽

Navier Stokes ◽

Flow Structures ◽

Discretization Scheme ◽

Spatial Discretization ◽

Eigenvalue Approach ◽

Stall Inception ◽

Flow Mechanisms ◽

Mesh Density

The effect of mesh, turbulence model and discretization scheme in Reynolds-averaged Navier-Stokes method (RANS) on a stall inception eigenvalue approach is investigated in a transonic compressor rotor. The most influencing flow structures on the result of eigenvalue approach are also identified. The compressor stall point is calculated by a recently developed eigenvalue model. Based on the 3D Navier-Stokes equations, the body-force term and small disturbance were used to transform the original equations into the eigenvalue approach. Because the eigenvalue mainly relies on the results from RANS, the sensitivity of the eigenvalue to the mesh density, turbulence model, and numerical scheme needs to be clearly identified before it is applied to engineering. The effect of mesh density is firstly specified. Several grids with different densities and distributions are employed in RANS. The eigenvalue results indicate that the solution converges at the same grid density as RANS does. Besides, the eigenvalue approach has the ability to predict a more accurate stall point compare to RANS with a coarse computational grid. The investigation of the detailed flow field indicates that the flow structures in the vicinity of blade tip region change significantly with three different grid densities, the eigenvalue is also influenced. Two important flow mechanisms are found to be the decisive factors for the eigenvalue, namely the blockage generated by the shock-vortex interaction, the separated flow and the wake near the trailing edge. These flow patterns are consistent with the flow mechanisms of the compressor stall inception. Further investigations are conducted with four different turbulence models combined with three different spatial discretization schemes. Calculated eigenvalue proves that the turbulence model changes the eigenvalue with an over-prediction of stall point at about 1%. The spatial discretization scheme has small effect on stall point prediction using k-ε and SA models, whereas it has large effect when using SST model. The scheme shows great influence in the simulations with specific turbulence model by changing the predicted stall point at least 1.7%. The existence of blockage, the separation and the wake flow are identified as the major and secondary factor which contributes to an unstable prediction of eigenvalue approach, respectively.

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Simulation of the Transitional Flow in a Low Pressure Gas Turbine Cascade With a High-Order Discontinuous Galerkin Method

Journal of Fluids Engineering ◽

10.1115/1.4024107 ◽

2013 ◽

Vol 135 (7) ◽

Cited By ~ 15

Author(s):

A. Ghidoni ◽

A. Colombo ◽

S. Rebay ◽

F. Bassi

Keyword(s):

Discontinuous Galerkin ◽

Turbulence Model ◽

Stokes Equations ◽

Time Integration ◽

High Order ◽

Transitional Flow ◽

Navier Stokes ◽

Implicit Time ◽

Turbine Cascade ◽

High Reynolds

In the last decade, discontinuous Galerkin (DG) methods have been the subject of extensive research efforts because of their excellent performance in the high-order accurate discretization of advection-diffusion problems on general unstructured grids, and are nowadays finding use in several different applications. In this paper, the potential offered by a high-order accurate DG space discretization method with implicit time integration for the solution of the Reynolds-averaged Navier–Stokes equations coupled with the k-ω turbulence model is investigated in the numerical simulation of the turbulent flow through the well-known T106A turbine cascade. The numerical results demonstrate that, by exploiting high order accurate DG schemes, it is possible to compute accurate simulations of this flow on very coarse grids, with both the high-Reynolds and low-Reynolds number versions of the k-ω turbulence model.

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Simulation of Vortex Instability in a Pipe Trifurcation

Volume 1: Fora, Parts A, B, C, and D ◽

10.1115/fedsm2003-45438 ◽

2003 ◽

Cited By ~ 1

Author(s):

Albert Ruprecht ◽

Ralf Neubauer ◽

Thomas Helmrich

Keyword(s):

Large Eddy Simulation ◽

Turbulence Model ◽

Model Test ◽

Reasonable Agreement ◽

Navier Stokes ◽

Extended Version ◽

Vortex Instability ◽

Flow Phenomenon ◽

Eddy Simulation ◽

Large Eddy

The vortex instability in a spherical pipe trifurcation is investigated by applying a Very Large Eddy Simulation (VLES). For this approach an new adaptive turbulence model based on an extended version of the k-ε model is used. Applying a classical Reynolds-averaged Navier-Stokes-Simulation with the standard k-ε model is not able to forecast the vortex instability. However the prescribed VLES method is capable to predict this flow phenomenon. The obtained results show a reasonable agreement with measurements in a model test.

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Development of a Numerical Investigation Framework for Ground Vehicle Platooning

Fluids ◽

10.3390/fluids6110404 ◽

2021 ◽

Vol 6 (11) ◽

pp. 404

Author(s):

Charles Patrick Bounds ◽

Sudhan Rajasekar ◽

Mesbah Uddin

Keyword(s):

Turbulence Model ◽

Turbulence Models ◽

Navier Stokes ◽

Ground Vehicle ◽

Simulation Methodology ◽

Vehicle Platooning ◽

Length Separation ◽

Dynamics Simulations ◽

The Impact ◽

Far Wake

This paper presents a study on the flow dynamics involving vehicle interactions. In order to do so, this study first explores aerodynamic prediction capabilities of popular turbulence models used in computational fluid dynamics simulations involving tandem objects and thus, ultimately presents a framework for CFD simulations of ground vehicle platooning using a realistic vehicle model, DrivAer. Considering the availability of experimental data, the simulation methodology is first developed using a tandem arrangement of surface-mounted cubes which requires an understanding on the role of turbulence models and the impacts of the associated turbulence model closure coefficients on the prediction veracity. It was observed that the prediction accuracy of the SST k−ω turbulence model can be significantly improved through the use of a combination of modified values for the closure coefficients. Additionally, the initial validation studies reveal the inability of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach to resolve the far wake, and its frailty in simulating tandem body interactions. The Improved Delayed Detached Eddy Simulations (IDDES) approach can resolve the wakes with a reasonable accuracy. The validated simulation methodology is then applied to the fastback DrivAer model at different longitudinal spacing. The results show that, as the longitudinal spacing is reduced, the trailing car’s drag is increased while the leading car’s drag is decreased which supports prior explanations of vortex impingement as the reason for drag changes. Additionally, unlike the case of platooning involving Ahmed bodies, the trailing model drag does not return to an isolated state value at a two car-length separation. However, the impact of the resolution of the far wake of a detailed DrivAer model, and its implication on the CFD characterization of vehicle interaction aerodynamics need further investigations.

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Nonlinear amplification in hydrodynamic turbulence

Journal of Fluid Mechanics ◽

10.1017/jfm.2021.914 ◽

2021 ◽

Vol 930 ◽

Author(s):

Kartik P. Iyer ◽

Katepalli R. Sreenivasan ◽

P.K. Yeung

Keyword(s):

Reynolds Number ◽

Isotropic Turbulence ◽

Stokes Equations ◽

Three Dimensional ◽

Reynolds Numbers ◽

Navier Stokes ◽

Navier Stokes Equations ◽

Developed Turbulence ◽

Advection Term ◽

Nonlinear Amplification

Using direct numerical simulations performed on periodic cubes of various sizes, the largest being $8192^3$ , we examine the nonlinear advection term in the Navier–Stokes equations generating fully developed turbulence. We find significant dissipation even in flow regions where nonlinearity is locally absent. With increasing Reynolds number, the Navier–Stokes dynamics amplifies the nonlinearity in a global sense. This nonlinear amplification with increasing Reynolds number renders the vortex stretching mechanism more intermittent, with the global suppression of nonlinearity, reported previously, restricted to low Reynolds numbers. In regions where vortex stretching is absent, the angle and the ratio between the convective vorticity and solenoidal advection in three-dimensional isotropic turbulence are statistically similar to those in the two-dimensional case, despite the fundamental differences between them.

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The Simulation of Natural Ventilation of Buildings With Different Location of Windows/Openings

Volume 6B: Energy ◽

10.1115/imece2015-51168 ◽

2015 ◽

Author(s):

A. Idris ◽

B. P. Huynh ◽

Z. Abdullah

Keyword(s):

Air Quality ◽

Flow Pattern ◽

Turbulence Model ◽

Natural Ventilation ◽

Air Flow ◽

Building Design ◽

Building Envelope ◽

Navier Stokes ◽

Computational Domain ◽

Numerical Predictions

Ventilation is a process of changing air in an enclosed space. Air should continuously be withdrawn and replaced by fresh air from a clean external source to maintain internal good air quality, which may referred to air quality within and around the building structures. In natural ventilation the air flow is due through cracks in the building envelope or purposely installed openings. Its can save significant amount of fossil fuel based energy by reducing the needs for mechanical ventilation and air conditioning. Numerical predictions of air velocities and the flow patterns inside the building are determined. To achieve optimum efficiency of natural ventilation, the building design should start from the climatic conditions and orography of the construction to ensure the building permeability to the outside airflow to absorb heat from indoors to reduce temperatures. Effective ventilation in a building will affects the occupant health and productivity. In this work, computational simulation is performed on a real-sized box-room with dimensions 5 m × 5 m × 5 m. Single-sided ventilation is considered whereby openings are located only on the same wall. Two opening of the total area 4 m2 are differently arranged, resulting in 16 configurations to be investigated. A logarithmic wind profile upwind of the building is employed. A commercial Computational Fluid Dynamics (CFD) software package CFD-ACE of ESI group is used. A Reynolds Average Navier Stokes (RANS) turbulence model & LES turbulence model are used to predict the air’s flow rate and air flow pattern. The governing equations for large eddy motion were obtained by filtering the Navier-Stokes and continuity equations. The computational domain was constructed had a height of 4H, width of 9H and length of 13H (H=5m), sufficiently large to avoid disturbance of air flow around the building. From the overall results, the lowest and the highest ventilation rates were obtained with windward opening and leeward opening respectively. The location and arrangement of opening affects ventilation and air flow pattern.

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Large-eddy simulations of tip leakage and secondary flows in an axial compressor cascade using a near-wall turbulence model

Proceedings of the Institution of Mechanical Engineers Part A Journal of Power and Energy ◽

10.1243/09576509jpe825 ◽

2009 ◽

Vol 223 (6) ◽

pp. 645-655 ◽

Cited By ~ 12

Author(s):

D Borello ◽

G Delibra ◽

K Hanjalić ◽

F Rispoli

Keyword(s):

Secondary Flows ◽

Tip Clearance ◽

Navier Stokes ◽

Wall Region ◽

Compressor Cascade ◽

Tip Leakage ◽

Computational Mesh ◽

Large Eddy ◽

Reynolds Averaged Navier Stokes ◽

Near Wall

This paper reports on the application of unsteady Reynolds averaged Navier—Stokes (U-RANS) and hybrid large-eddy simulation (LES)/Reynolds averaged Navier—Stokes (RANS) methods to predict flows in compressor cascades using an affordable computational mesh. Both approaches use the ζ— f elliptic relaxation eddy-viscosity model, which for U-RANS prevails throughout the flow, whereas for the hybrid the U-RANS is active only in the near-wall region, coupled with the dynamic LES in the rest of the flow. In this ‘seamless’ coupling the dissipation rate in the k-equation is multiplied by a grid-detection function in terms of the ratio of the RANS and LES length scales. The potential of both approaches was tested in several benchmark flows showing satisfactory agreement with the available experimental results. The flow pattern through the tip clearance in a low-speed linear cascade shows close similarity with experimental evidence, indicating that both approaches can reproduce qualitatively the tip leakage and tip separation vortices with a relatively coarse computational mesh. The hybrid method, however, showed to be superior in capturing the evolution of vortical structures and related unsteadiness in the hub and wake regions.

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ACAT1 Benchmark of RANS-Informed Analytical Methods for Fan Broadband Noise Prediction—Part I—Influence of the RANS Simulation

Acoustics ◽

10.3390/acoustics2030029 ◽

2020 ◽

Vol 2 (3) ◽

pp. 539-578

Author(s):

Carolin Kissner ◽

Sébastien Guérin ◽

Pascal Seeler ◽

Mattias Billson ◽

Paruchuri Chaitanya ◽

...

Keyword(s):

Turbulence Model ◽

Analytical Methods ◽

Companion Paper ◽

Broadband Noise ◽

Operating Conditions ◽

Navier Stokes ◽

Data Set ◽

The Past ◽

Flow Turbulence ◽

Diverse Data

A benchmark of Reynolds-Averaged Navier-Stokes (RANS)-informed analytical methods, which are attractive for predicting fan broadband noise, was conducted within the framework of the European project TurboNoiseBB. This paper discusses the first part of the benchmark, which investigates the influence of the RANS inputs. Its companion paper focuses on the influence of the applied acoustic models on predicted fan broadband noise levels. While similar benchmarking activities were conducted in the past, this benchmark is unique due to its large and diverse data set involving members from more than ten institutions. In this work, the authors analyze RANS solutions performed at approach conditions for the ACAT1 fan. The RANS solutions were obtained using different CFD codes, mesh resolutions, and computational settings. The flow, turbulence, and resulting fan broadband noise predictions are analyzed to pinpoint critical influencing parameters related to the RANS inputs. Experimental data are used for comparison. It is shown that when turbomachinery experts perform RANS simulations using the same geometry and the same operating conditions, the most crucial choices in terms of predicted fan broadband noise are the type of turbulence model and applied turbulence model extensions. Chosen mesh resolutions, CFD solvers, and other computational settings are less critical.

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