Verification of Length Scale Effects on Solution Accuracy of Hybrid RANS-LES Methods

2016 ◽  
Author(s):  
Gaurav Kumar ◽  
Harish Gopalan ◽  
Dominic Chandar ◽  
Vinh-Tan Nguyen ◽  
Ashoke De
Keyword(s):  
Bluff Body ◽  
Scale Effects ◽  
Computational Cost ◽  
Separated Flow ◽  
Reynolds Numbers ◽  
Test Problems ◽  
Length Scale ◽  
High Reynolds ◽  
Solution Accuracy ◽  
Bluff Body Flows

Hybrid RANS-LES methods are gaining popularity for the simulation of the complex bluff body flows at high Reynolds numbers due to their reduced computational cost and good accuracy. A number of such methods have been proposed in the literature. Each of these methods have enjoyed varying degree of success for different applications. One of the most important parameter which determines the switching between near-wall RANS region and off-body LES region is the length scale parameter. This parameter can be grid based or physics based and numerous choices exist for defining this parameter. This study proposes to investigate the effect of this parameter on the size of the RANS and LES regions and also on the solution accuracy. Four test problems are chosen covering attached, mildly separated and massively separated flow regimes. Results will help us to identify length scale definitions to be used for different flow scenarios.

A Vortex Immersed Boundary Method for Bluff Body Flows

2010 ◽  
Author(s):  
Georges-Henri Cottet ◽  
Federico Gallizio ◽  
Adrien Magni ◽  
Iraj Mortazavi
Keyword(s):  
Bluff Body ◽  
Vortex Sheet ◽  
Reynolds Numbers ◽  
Immersed Boundary ◽  
Vortex Methods ◽  
Slip Boundary ◽  
Wide Range ◽  
Penalization Methods ◽  
High Reynolds ◽  
Bluff Body Flows

The aim of this work is to couple vortex methods with the penalization methods in order to take advantage from both of them. This immersed boundary approach maintains the efficiency of vortex methods for high Reynolds numbers focusing the computational task on the rotational zones and avoids their lack on the no-slip boundary conditions replacing the vortex sheet method by the penalization of obstacles. This method that is very appropriate for bluff-body flows is validated for the flow around a circular cylinder on a wide range of Reynolds numbers.

scholarly journals MESHING STRATEGY EVALUATION FOR A SQUARE SHAPED BLUFF BODY UNDER HIGH REYNOLDS NUMBER CROSS FLOW

2018 ◽  
Vol 3 (2) ◽  
Author(s):  
Filipe Branco Teixeira ◽  
Andrei Luís Garcia Santos ◽  
Luiz Alberto Oliveira Rocha ◽  
Liércio André Isoldi ◽  
Elizaldo Domingues Dos Santos
Keyword(s):  
Nusselt Number ◽  
Bluff Body ◽  
Transient Flow ◽  
Computational Cost ◽  
Cross Flow ◽  
Reynolds Numbers ◽  
Strategy Evaluation ◽  
The Mean ◽  
Prandtl Numbers ◽  
High Reynolds

This work consists in the numerical evaluation of meshes employed in a two-dimensional, incompressible, and transient flow, with high Reynolds numbers and forced convection that passes through a square shaped bluff body. The objective of the study is to evaluate different strategies of mesh construction to solve this type of problem, seeking to reduce the computational cost and improve the solution. The simulations were performed for Reynolds and Prandtl numbers of ReD = 22,000 and Pr = 0.71. The turbulence behavior is solved with the RANS SST–k-w model. The evaluation of the solutions is performed analyzing the drag coefficient (CD), Nusselt number (NuD) and Strouhal number (St). The use of an unstructured mesh with fully structured local refinement was able to validate the fluid dynamics of the case with a relative deviation of 14.95% for the velocity field, compared to the literature. The deviations for CD and St were as low as 0.43% and 0.79% respectively. For thermal verification, the deviation of the mean local Nusselt number along the bluff body (NuD) was 5.93%, and the deviation for the global Nusselt number (Nug) was as low as 0.56% compared to the literature

Heat Transfer from a Plate Elevated above a Host Surface and Washed by a Separated Flow Induced by the Elevation Step

1981 ◽  
Vol 103 (3) ◽  
pp. 441-447 ◽  
Author(s):  
E. M. Sparrow ◽  
F. Samie ◽  
S. C. Lau
Keyword(s):  
Heat Transfer ◽  
Flow Separation ◽  
Angle Of Attack ◽  
Separated Flow ◽  
Active Surface ◽  
Reynolds Numbers ◽  
Transfer Coefficients ◽  
Reynolds Number Flow ◽  
Heat Transfer Coefficients ◽  
High Reynolds

Wind tunnel experiments were performed to determine heat transfer coefficients and fluid flow patterns for a thermally active surface elevated above a parallel host surface. The step-like blockage associated with the elevation causes flow separation and recirculation on the forward portion of the thermally active surface. Four parameters were varied during the course of the experiments, including the angle of attack of the oncoming airflow relative to the surface, the step height, the extent of the host surface which frames the active surface (i.e., the skirt width), and the Reynolds number. Flow visualization studies, performed with the oil-lampblack technique, showed that the streamwise extent of the separation zone increases with decreasing angle of attack, with larger step heights and skirt widths, and at higher Reynolds numbers. At larger angles of attack, separation does not occur. The experimentally determined heat transfer coefficients were found to increase markedly due to the flow separation, and separation-related enhancements as large as a factor of two were encountered. The enhancement was accentuated at small angles of attack, at large step heights and skirt widths, and at high Reynolds numbers. A main finding of the study is that the separation-affected heat transfer coefficients are generally greater than those for no separation, so that the use of the latter may underestimate the heat transfer rates. For an application such as a retrofit solar collector, such an underestimation of the wind-related heat loss would yield an optimistic prediction of the collector efficiency.

Vortex-Induced Motion of a Square Cylinder at Moderate Reynolds Numbers

2016 ◽  
Author(s):  
Chih-Hua Wu ◽  
Shengwei Ma ◽  
Chang-Wei Kang ◽  
Teck-Bin Arthur Lim ◽  
Rajeev Kumar Jaiman ◽  
...  
Keyword(s):  
Vortex Shedding ◽  
Bluff Body ◽  
Computational Cost ◽  
Turbulence Models ◽  
Reynolds Numbers ◽  
Spanwise Direction ◽  
Detached Eddy Simulation ◽  
Eddy Simulation ◽  
Cylinder Length ◽  
Induced Motion

Bluff body structures exposed to ocean current can undergo vortex-induced motion (VIM) for certain geometric and physical conditions. Recently, the study of VIM has been gaining attention for many engineering applications, in particular offshore structures such as buoys, FPSOs, semi-submersibles, Spars and TLPs. The present work is a part of a systematic effort to investigate the VIM response of multi-columns floating platform. In real sea condition, floating platforms are in high Reynolds numbers region and flow patterns around structures are turbulent in nature. For the purpose of investigating and simulating accurately the nonlinear dynamic processes of vortex shedding, transport and wake interactions with the bluff body, the fundamental study of VIM around a square column at moderate Reynolds numbers (1500 ≤ Re ≤ 14000) is firstly investigated. In the present work, the transient flow pattern around a free vibrating square cylinder at moderate Reynolds numbers is numerically investigated by an open source CFD toolbox, OpenFOAM. Good consistency and agreement are found between the present numerical findings and that of experiments. The cylinder, with a blockage area of 4.2%, is mounted on an elastic support for free vibration in the transverse direction. Hybrid RANS-LES turbulence models are considered here for accurate prediction of massively separated turbulent wake flow while maintaining the reasonable computational cost. Three hybrid turbulence models, the DDES (Delayed Detached Eddy Simulation, the k-ω SST-DES (Detached Eddy Simulation), and the k–ω SST-SAS (Scale Adaptive Simulation), are studied and their results are compared with the reported experimental measurements. It is shown that the result of simulation with the k–ω SST-SAS model is closer to the reported literature than the other two and therefore, subsequently adopted for all the simulations of our study in this paper. The scaling effect of cylinder length in the spanwise direction is also studied with the objective to reduce the computational cost. From the comparison with the recent experimental measurements, the discrepancy between the present simulations of reducing cylinder length and the experiment increases only when Re ≥ 4000. This might stem from the increase in wavelength of some vortex shedding modes and turbulence intensity variation in the spanwise direction near the cylinder as Re ≥ 4000. The detailed flow patterns, 3D vortex structures (based on Q-criterion) and vortex-shedding modes are presented in this work as well.

scholarly journals A fast direct numerical simulation method for characterising hydraulic roughness

2015 ◽  
Vol 773 ◽  
pp. 418-431 ◽  
Author(s):  
D. Chung ◽  
L. Chan ◽  
M. MacDonald ◽  
N. Hutchins ◽  
A. Ooi
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Computational Cost ◽  
Simulation Method ◽  
Reynolds Numbers ◽  
Proof Of Concept ◽  
Hydraulic Roughness ◽  
Velocity Shift ◽  
High Reynolds ◽  
Reynolds Number Flows

We describe a fast direct numerical simulation (DNS) method that promises to directly characterise the hydraulic roughness of any given rough surface, from the hydraulically smooth to the fully rough regime. The method circumvents the unfavourable computational cost associated with simulating high-Reynolds-number flows by employing minimal-span channels (Jiménez & Moin, J. Fluid Mech., vol. 225, 1991, pp. 213–240). Proof-of-concept simulations demonstrate that flows in minimal-span channels are sufficient for capturing the downward velocity shift, that is, the Hama roughness function, predicted by flows in full-span channels. We consider two sets of simulations, first with modelled roughness imposed by body forces, and second with explicit roughness described by roughness-conforming grids. Owing to the minimal cost, we are able to conduct direct numerical simulations with increasing roughness Reynolds numbers while maintaining a fixed blockage ratio, as is typical in full-scale applications. The present method promises a practical, fast and accurate tool for characterising hydraulic resistance directly from profilometry data of rough surfaces.

The effect of base bleed on the steady separated flow past bluff objects

1969 ◽  
Vol 39 (4) ◽  
pp. 735-752 ◽  
Author(s):  
L. G. Leal ◽  
A. Acrivos
Keyword(s):  
Bluff Body ◽  
Separated Flow ◽  
Pressure Profile ◽  
Reynolds Numbers ◽  
Two Dimensional ◽  
Wake Region ◽  
Near Wake ◽  
Flow Past ◽  
Modifying Effect ◽  
Base Bleed

The modifying effect of base bleed on the steady separated flow past a two-dimensional bluff body is considered. Detailed experimental results are presented for Reynolds numbers R between 50 and 250 and for bleed coefficients b in the range 0 to 0·15. The streamline pattern near the object is found to be strongly affected by small changes in the rate of bleed, with the recirculating closed wake disappearing altogether for b > 0·15. Nevertheless, the qualitative dependence on R of the physical dimensions of the near-wake region and the associated streamwise pressure profile appear to be unaffected by base bleed.

Multiscale Calculations Using an Adaptive Wavelet Algorithm

2002 ◽  
Author(s):  
Yevgenii A. Rastigejev ◽  
Samuel Paolucci
Keyword(s):  
Degrees Of Freedom ◽  
Computational Cost ◽  
Reynolds Numbers ◽  
Adaptive Wavelet ◽  
Great Ability ◽  
Driven Cavity ◽  
Multiresolution Representation ◽  
Order Of Magnitude ◽  
Comparable Accuracy ◽  
Solution Accuracy

We present a new wavelet-based adaptive multiresolution representation (WAMR) algorithm for the numerical solution of multiscale evolution problems. Key features of the algorithm are fast procedures for grid rearrangement, computation of derivatives, as well as the ability to minimize the degrees of freedom for a prescribed solution accuracy. To demonstrate the efficiency and accuracy of the algorithm, we use it to solve the two-dimensional benchmark problem of incompressible fluid-flow in a lid-driven cavity at large Reynolds numbers. The numerical experiments demonstrate the great ability of the algorithm to adapt to different scales at different locations and at different times so as to produce accurate solutions at low computational cost. Specifically, we show that solutions of comparable accuracy as the benchmarks are obtained with more than an order of magnitude reduction in degrees of freedom.

Numerical Investigation of Flow Interference Effects in Fixed and Vibrating Tandem Square Columns Using Hybrid RANS-LES Models

2015 ◽  
Author(s):  
Harish Gopalan ◽  
Dominic Denver John Chandar ◽  
Narasimha Rao Pillamarri ◽  
Guan Mengzhao ◽  
Rajeev K. Jaiman ◽  
...  
Keyword(s):  
Solid Wall ◽  
Computational Cost ◽  
Reynolds Numbers ◽  
Navier Stokes ◽  
Arbitrary Lagrangian Eulerian ◽  
High Reynolds Numbers ◽  
Flow Past ◽  
Ale Methods ◽  
High Reynolds ◽  
Flow Interference

Investigation of flow past tandem and side-by-side circular and square columns is of interest in offshore engineering. Flow past fixed and vibrating circular columns has received a lot of focus in the literature. However, the studies focused on square columns, especially at high Reynolds numbers are very limited due to the computational cost of large eddy simulation (LES). Unsteady Reynolds-averaged Navier-Stokes (URANS) methods are limited by their accuracy, especially for tandem columns in the wake interference regime (spacing to diameter ratio: L=D ∼ 3:0). Hybrid URANS-LES models (URANS near the solid-wall and LES away from the wall) can overcome the drawbacks of the traditional URANS methods and can provide a reasonable prediction of the flow physics at a fraction of the cost of LES without significantly sacrificing numerical accuracy. Arbitrary Lagrangian-Eulerian (ALE) methods fails when vibrating tandem bodies are in close proximity to each other or vibrate at high reduced velocities. Remeshing the domain can be expensive, especially at high Reynolds numbers (Re). Alternate strategies are necessary to efficiently simulation this problems. This study proposes the use of a non-linear URANS-LES model, coupled with an overset mesh method (for vibrating columns), for studying flow past tandem square columns. Simulations are performed at sub-critical Re to match the experimental Re. The initial results are encouraging for further investigation of fixed and vibrating tandem square column flow interference at high Reynolds numbers.

Bluff-Body Simulation by SPH Method With Relatively High Reynolds Number in Laminar Flow Regime

2010 ◽  
Author(s):  
Mostafa Safdari Shadloo ◽  
Amir Zainali ◽  
Mehmet Yildiz
Keyword(s):  
Bluff Body ◽  
Separated Flow ◽  
Fracture Repair ◽  
Solid Boundary ◽  
Bluff Bodies ◽  
Element Mesh ◽  
Particle Hydrodynamics ◽  
Repair Procedure ◽  
High Reynolds ◽  
Wcsph Method

In this work, we present solutions for flow over an airfoil and square obstacle using Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) method. For the solution of these two problems, we present an improved WCSPH algorithm that can handle complex geometries with the usage of multiple tangent solid boundary method, and eliminate particle clustering induced instabilities with the implementation of particle fracture repair procedure as well as the corrected SPH discretization scheme. We have shown that the improved WCSPH method can be effectively used for flow simulations over bluff-bodies with Reynolds numbers as high as 1400, which is not achievable with standard WCSPH formulations. Our simulation results are validated with a Finite Element mesh-dependent Method (FEM), and excellent agreements among the results were observed. We illustrated that the improved WCSPH method is able to capture the complex physics of bluff-body flows naturally such as flow separation, detachment of separated flow, wake formation at the trailing edge, and vortex shedding without any extra effort to increase the particle resolution in some specific areas of interest.

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