scispace - formally typeset
Search or ask a question
Journal ArticleDOI

3D CFD computations of transitional flows using DES and a correlation based transition model

01 Jan 2011-Wind Energy (John Wiley & Sons, Ltd)-Vol. 14, Iss: 1, pp 77-90
TL;DR: In this article, the correlation based transition model of Menter et al. in combination with the Detached Eddy Simulation (DES) methodology is applied to two cases with large degree of flow separation typically considered difficult to compute.
Abstract: The present article describes the application of the correlation based transition model of Menter et al. in combination with the Detached Eddy Simulation (DES) methodology to two cases with large degree of flow separation typically considered difficult to compute. Firstly, the flow is computed over a circular cylinder from Re = 10 to 1 × 106 reproducing the cylinder drag crisis. The computations show good quantitative and qualitative agreement with the behaviour seen in experiments. This case shows that the methodology performs smoothly from the laminar cases at low Re to the turbulent cases at high Re. Secondly, the flow is computed over a thick airfoil at high angle of attack, in this case the DU-96-W351 is considered. These computations show that a transition model is needed to obtain correct drag predictions at low angle of attack, and that the combination of transition and the DES method improve agreement in the deep stall region. Copyright © 2010 John Wiley & Sons, Ltd.

Summary (3 min read)

2 Introduction

  • During the last years, Computational Fluid Dynamics has found wide spread use within the wind energy community, and has been shown to perform well in many cases.
  • Attempting to shed some light on some of the fundamental problems connected to the deep stall physics of wind turbine blades, often equipped with thick airfoil, the present work firstly addresses the classical problem of predicting the drag crisis of a circular cylinder, secondly applying the same methodology to the flow over a thick airfoil.
  • The cylinder case is mainly chosen because a large body of high quality experimental data exists, which for many other flows can be problematic to obtain for deep stall cases.
  • It is well known that the movement of the separation point on the circular cylinder is highly influenced by the laminar to turbulent transition process.
  • Additionally it is well know that typical RANS are not sufficiently accurate in massively separated flows, and to help alleviate this problem, the DES technique is applied.

3 Code description

  • The in-house flow solver EllipSys3D is used in all computations presented in this paper.
  • Both steady state and unsteady computations can be performed.
  • When a convergent solution is obtained, the variables are updated, and the authors continue with the next time step.
  • The problem of the LES region contaminating the RANS layer can happen when computing flows with high values of inflow turbulence typically used with the correlation based transition model.

3.1 Transition Model

  • The γ− R̃eθ correlation based transition model of Menter [1], is a framework for implementing empirical correlations based transition criterions in general purpose flow solvers, that can be used together with structured, unstructured and parallelized solvers.
  • The backbone of the model is two transport equations one for intermittency γ and one for the local transition onset momentum thickness Reynolds number R̃eθt .
  • Based on a series of zero pressure gradient flat plate boundary layers, expressions for the two missing correlation functions relating Reθc and Flength to R̃eθt have been determined by Sørensen [15].
  • (3) Comparing the present correlation for Reθc with the two correlations proposed by Toyoda et al. [16] and Pettersson et al. [17] good agreement is observed at low Reθt , see Figure 1. the expression for the Flength parameter has dimension of length, and not a dimensionless quantity as it should be.
  • It is well known that the turbulence will decay from the inlet value, in the case of zero shear where there is no production in the farfield.

4.1 Computational grids

  • The meshes for the computations in the present work are generated with the 2D enhanced hyperbolic grid generation program HypGrid2D [19] as a 2D slice, and the 3D grid is then generated by sweeping the grid in the span-wise direction, see Figure 2.
  • The grid holds in total 8.4 million cells.
  • The surface of the cylinder is modeled as a no-slip surface, and periodic conditions are used in the spanwise direction.
  • The outer domain boundary is specified as inlet, except for the area downstream of the airfoil covering around +/- 45 degrees in azimuth direction, where outlet condition is used specifying fully developed flow.

4.2 Flow Over a Circular Cylinder

  • The challenging case of flow over the cylinder covering the full range from laminar flow at low Reynolds numbers to transitional flow at high Reynolds numbers is investigated.
  • Each machine has 2GB of memory, the interconnect is based on Gigabit Ethernet network.
  • Comparing the computed drag ( FD0.5ρU2 ) as function of Reynolds Number with measured values [20], an improved agreement around the drag crisis is observed, see Figure 5, where the transitional computations predict the increase of the drag in the region of Reynolds number from 1× 104 to 2× 105 observed in the measurements.
  • Observing the development of the limiting streamlines at the surface of the cylinder and the movement of the laminar to turbulent transition, insight about the actual physical process can be obtained.

4.3 Discussion of Cylinder Results

  • The use of the combined DES/transitional methodology improves the prediction of the drag crisis on the cylinder compared to pure DES simulation.
  • Additionally, the DES/transitional methodology predicts the correct flow phenomena, with laminar separation, turbulent reattachment followed Risø-R-1692(EN) 9 by turbulent separation along with backward shift of the separation point and the narrowing of the cylinder wake with increasing Reynolds number.

4.4 Flow over a thick airfoil

  • Next, the method is applied to predicting the flow over the DU-96-W-351 airfoil, that has a thickness of 35% chord.
  • Similar to the cylinder case, the separation of the flow over an airfoil is controlled by the momentum in the boundary layer flow and the skin friction distribution.
  • This was identified as grid induced separation or MSD, and the application of the 10 Risø-R-1692(EN) DDES methodology seems to alleviate this problem.
  • Right around the abrupt stall at 15 degrees, the computed results exhibit a large dependency on the inflow turbulence, showing a variation of nearly 20%, see Figure 12 and 13.
  • Similar to the cylinder analysis, the power spectrum of the tangential force for the DU-96W-351 airfoil is analyzed to determine the Strouhal frequency of most energetic variations, performed for the case of 45 degrees angle of attack.

4.5 Discussion of Airfoil Results

  • In the present work, computations of the flow over the DU-96-W-351, 35% thick airfoil in the region of stalled flow have been compared to measurements from the LM tunnel.
  • In the computations the airfoil section is modeled with an extent of two chords in the spanwise direction and the use of periodic conditions.
  • This is done to limit the number of cells needed to model the actual configuration, where the airfoil section is enclosed by walls.
  • The problem of the wind tunnel corrections, that do not account for the interaction of the separation and the wall effects would additionally require further studies.
  • The results show an improved agreement with the measured data, compared to 2D computations.

5 Conclusion

  • The combination of Detached Eddy Simulations with a laminar to turbulent transition model has been demonstrated.
  • The methodology has proven to be numerically very robust, and similar to the transitional computations using Reynolds Averaged Navier-Stokes approach, capable of predicting both laminar separation, turbulent reattachment, and turbulent separation.
  • For the prediction of the cylinder drag crisis, a distinct improvement of the drag in the critical region is observed.
  • For the airfoil the results are more inconclusive, but indicates that a very high dependency of the inflow turbulence intensity may exist for thick airfoils.

Did you find this useful? Give us your feedback

Figures (17)

Content maybe subject to copyright    Report

General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright
owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
You may not further distribute the material or use it for any profit-making activity or commercial gain
You may freely distribute the URL identifying the publication in the public portal
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from orbit.dtu.dk on: Aug 10, 2022
3D CFD computations of transitional flows using DES and a correlation based
transition model
Sørensen, Niels N.
Publication date:
2009
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Sørensen, N. N. (2009). 3D CFD computations of transitional flows using DES and a correlation based transition
model. Danmarks Tekniske Universitet, Risø Nationallaboratoriet for Bæredygtig Energi. Denmark.
Forskningscenter Risoe. Risoe-R No. 1692(EN)

Risø-R-Report
3D CFD computations of transitional flows
using DES and a correlation based transition
model
Niels N. Sørensen
Risø-R-1692(EN)
July 2009
Re = 1.e6

Author: Niels N. Srensen Risø-R-1692(EN)
Title: 3D CFD computations of transitional flows using DES and a correlation based
transition model
Department: Aeroelastic Design Wind Energy Division
Abstract:
ISSN
The report describes the application of the correlation based tran-
sition model of of Menter et. al. [1, 2] to the cylinder drag crisis
and the stalled flow over an DU-96-W-351 airfoil using the DES
methodology. When predicting the flow over airfoils and rotors, the
laminar-turbulent transition process can be important for the aero-
dynamic performance. Today, the most widespread approach is to
use fully turbulent computations, where the transitional process is
ignored and the entire boundary layer on the wings or airfoils is
handled by the turbulence model. The correlation based transition
model has lately shown promising results, and the present paper de-
scribes the application of the model to predict the drag and shed-
ding frequency for flow around a cylinder from sub to super-critical
Reynolds numbers. Additionally, the model is applied to the flow
around the DU-96 airfoil, at high angles of attack.
ISBN 978-87-550-3749-6
Contract no.:
ENS-33033-0055
Group’s own reg. no.:
Sponsorship:
Danish Energy Agency
Cover:
Separation behind cylinder
Pages: 18
Tables: 17
References: 24
Information Service Department
Risø National Laboratory for Sustain-
able Energy
Technical University of Denmark
P.O.Box 49
DK-4000 Roskilde
Denmark
Telephone +45 4677 4004
bibl@risoe.dk
Fax +45 4677 4013
www.risoe.dk

Contents
1 Abstract 3
2 Introduction 3
3 Code description 3
3.1 Transition Model 4
4 Results 5
4.1 Computational grids 5
4.2 Flow Over a Circular Cylinder 6
4.3 Discussion of Cylinder Results 9
4.4 Flow over a thick airfoil 10
4.5 Discussion of Airfoil Results 15
5 Conclusion 15
6 Acknowledgement 15
2 Risø-R-1692(EN)

1 Abstract
The present report describes the application of the correlation based transition model of of
Menter et al. [1, 2] to the flow over a cylinder and a thick airfoils using the Detached Eddy
Simulation (DES) methodology. When predicting the flow over airfoils and rotors, the laminar-
turbulent transition process can be important for the aerodynamic performance. Today, the most
widespread approach is to use fully turbulent computations, where the transitional process is
ignored and the entire boundary layer on the wings or airfoils is handled by the turbulence
model. In the present work the possibility of combining the DES technique with a transition
model is tested for two flows featuring large separated areas.
2 Introduction
During the last years, Computational Fluid Dynamics has found wide spread use within the
wind energy community, and has been shown to perform well in many cases. Even though
much success has been achieved, important problems still exist where the standard fully tur-
bulent Reynold Averaged Navier-Stokes (RANS) approach fails to give sufficiently accurate
answers. The most obvious problem is the failure to predict the power production and load for
stall controlled turbines at high wind, corresponding to high angle of attack along the blade
span. Attempting to shed some light on some of the fundamental problems connected to the
deep stall physics of wind turbine blades, often equipped with thick airfoil, the present work
firstly addresses the classical problem of predicting the drag crisis of a circular cylinder, sec-
ondly applying the same methodology to the flow over a thick airfoil. The cylinder case is
mainly chosen because a large body of high quality experimental data exists, which for many
other flows can be problematic to obtain for deep stall cases. Additionally, cylindrical or nearly
cylindrical sections exist at the inboard part of most modern wind turbine rotors. To accomplish
the flow simulations, the new correlation based γ Re
θ
model by Menter et al. [1] and the DES
version of the k ω SST model by Strelets [3] is applied. It is well known that the movement
of the separation point on the circular cylinder is highly influenced by the laminar to turbulent
transition process. Additionally it is well know that typical RANS are not sufficiently accurate
in massively separated flows, and to help alleviate this problem, the DES technique is applied.
3 Code description
The in-house flow solver EllipSys3D is used in all computations presented in this paper. The
code is developed in co-operation between the Department of Mechanical Engineering at the
Technical University of Denmark and The Department of Wind Energy at Risø National Lab-
oratory, see [4, 5] and [6]. The EllipSys3D code is a multiblock finite volume discretization of
the incompressible Reynolds Averaged Navier-Stokes (RANS) equations in general curvilin-
ear coordinates. The code uses a collocated variable arrangement, and Rhie/Chow interpolation
[7] is used to avoid odd/even pressure decoupling. As the code solves the incompressible flow
equations, no equation of state exists for the pressure, and in the present work the Semi-Implicit
Method for Pressure-Linked Equations (SIMPLE) algorithm of Patankar and Spalding [8, 9] or
the Pressure Implicit with Splitting of Operators (PISO) algorithm of Issa [10, 11] is used to
enforce the pressure/velocity coupling, for steady state and transient computations respectively.
The EllipSys3D code is parallelized with the Message-Passing Interface (MPI) for executions
on distributed memory machines, using a non-overlapping domain decomposition technique.
Both steady state and unsteady computations can be performed. For the unsteady computa-
Risø-R-1692(EN) 3

Citations
More filters
Journal ArticleDOI
TL;DR: In this article, the authors describe the development and description of the aerodynamic models used to estimate aerodynamic loads on wind turbine constructions, including a status of the capabilities of computation fluid dynamics and the need for reliable airfoil data for the simpler engineering models.
Abstract: The paper describes the development and description of the aerodynamic models used to estimate the aerodynamic loads on wind turbine constructions. This includes a status of the capabilities of computation fluid dynamics and the need for reliable airfoil data for the simpler engineering models. Also a discussion of the use of passive and active aerodynamic devices is included such as, e.g., Vortex Generators and distributed active flaps. Finally the problem of wakes in wind farms is addressed and a section of the likely future development of aerodynamic models for wind turbines is included.

83 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present an aerodynamic shape optimization framework consisting of a Reynolds-averaged Navier Stokes solver coupled with a numerical optimization algorithm, a geometry modeler, and a mesh perturbation algorithm.
Abstract: Computational fluid dynamics (CFD) is increasingly used to analyze wind turbines, and the next logical step is to develop CFD-based optimization to enable further gains in performance and reduce model uncertainties. We present an aerodynamic shape optimization framework consisting of a Reynolds-averaged Navier Stokes solver coupled to a numerical optimization algorithm, a geometry modeler, and a mesh perturbation algorithm. To efficiently handle the large number of design variables, we use a gradient-based optimization technique together with an adjoint method for computing the gradients of the torque coefficient with respect to the design variables. To demonstrate the effectiveness of the proposed approach, we maximize the torque of the NREL VI wind turbine blade with respect to pitch, twist, and airfoil shape design variables while constraining the blade thickness. We present a series of optimization cases with increasing number of variables, both for a single wind speed and for multiple wind speeds. For the optimization at a single wind speed performed with respect to all the design variables (1 pitch, 11 twist, and 240 airfoil shape variables), the torque coefficient increased by 22.4% relative to the NREL VI design. For the multiple-speed optimization, the torque increased by an average of 22.1%. Depending on the CFD mesh size and number of design variables, the optimization time ranges from 2 to 24h when using 256 cores, which means that wind turbine designers can use this process routinely. Copyright © 2016 John Wiley & Sons, Ltd.

78 citations


Cites background from "3D CFD computations of transitional..."

  • ...These studies used various levels of fidelity in the models, ranging from low-fidelity (blade element momentumbased fidelity) [14, 15, 16, 17, 18, 19, 20] to higher-fidelity (free and prescribed vortex-based fidelity) [21, 22, 23, 24, 25, 26, 27]....

    [...]

Journal ArticleDOI
TL;DR: In this paper, a variable blade pitch automatic optimization platform (VBPAOP) composed of genetic algorithm and computational fluid dynamics (CFD) simulation modules is built to search for optimal blade pitches that can maximize turbine power.

65 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the flow behavior and its features in the blade's root region of a horizontal axis wind turbine by using stereoscopic particle image velocimetry (PIV) technique.
Abstract: This research investigates the flow behavior and its features in the blade's root region of a horizontal axis wind turbine by using stereoscopic particle image velocimetry (PIV) technique. Wind tunnel tests are conducted to measure the velocity field, phase-locked with the blade motion, at different azimuth angles and at different spanwise positions. The pressure distribution is obtained from PIV velocity field by solving the Navier–Stokes momentum equations. In this paper, we aim to answer two questions: (i) How is the flow behavior in the root region? (ii) How is the evolution of the root vortex? The analysis of the velocity fields shows an outboard radial flow motion in the root region and a vorticity driven inboard motion at the bladeSs maximum chord position. As a result of this vorticity driven flow, an increase in the axial velocity close to nacelle is measured. Wake sheets are observed and further discussed in the measured velocity and vorticity distributions. The formation and evolution of the root vortices conveyed downstream by the axial velocity are analyzed through vorticity and pressure distributions. Although the azimuthal vorticity in 3D representation is showing the trailing vorticity, the tilting of the root vortex tube is observed in the axial vorticity distribution. Moreover, the radial vorticity and azimuthal velocity from chordwise measurements show separation on the suction surface of the blade. This research concluded that the flow in the blade wake is driven by the root vortex; hence, the local effects of the root vortex cannot be ignored. Copyright © 2013 John Wiley & Sons, Ltd.

32 citations

References
More filters
Journal ArticleDOI
TL;DR: Langtry et al. as discussed by the authors developed a new correlation-based transition model based strictly on local variables, which is compatible with modern computational fluid dynamics (CFD) approaches, such as unstructured grids and massive parallel execution.
Abstract: A new correlation-based transition model has been developed, which is based strictly on local variables. As a result, the transition model is compatible with modern computational fluid dynamics (CFD) approaches, such as unstructured grids and massive parallel execution. The model is based on two transport equations, one for intermittency and one for the transition onset criteria in terms of momentum thickness Reynolds number. The proposed transport equations do not attempt to model the physics of the transition process (unlike, e.g., turbulence models) but form a framework for the implementation of correlation-based models into general-purpose CFD methods. Part I (this part) of this paper gives a detailed description of the mathematical formulation of the model and some of the basic test cases used for model validation, including a two-dimensional turbine blade. Part II (Langtry, R. B., Menter, F. R., Likki, S. R., Suzen, Y. B., Huang, P. G., and Volker, S., 2006, ASME J. Turbomach., 128(3), pp. 423–434) of the paper details a significant number of test cases that have been used to validate the transition model for turbomachinery and aerodynamic applications. The authors believe that the current formulation is a significant step forward in engineering transition modeling, as it allows the combination of correlation-based transition models with general purpose CFD codes.

1,131 citations

Proceedings ArticleDOI
08 Jan 2001

1,065 citations

Journal ArticleDOI
TL;DR: In this paper, the authors measured the local pressure and skin friction distribution around a cylinder and calculated the total drag, the pressure drag and the friction drag, which can be used to define three states of the flow: the subcritical flow, where the boundary layer separates laminarly; the critical flow, in which a separation bubble, followed by a turbulent reattachment, occurs; and the supercritical flow where an immediate transition from the laminars to the turbulent boundary layer is observed at a critical distance from the stagnation point.
Abstract: In a large range of Reynolds numbers, 6 × 104 < Re < 5 × 106, the flow around single cylinders with smooth surfaces has been investigated. The high values of the Reynolds numbers were obtained in a test channel which could be pressurized up to 40 bar of static pressure. New experiments were performed to measure the local pressure and skin friction distribution around the cylinder. From these results the total drag, the pressure drag and the friction drag were calculated. By means of the skin friction distribution the position of the separation points, separation bubbles or transition points can be localized. These data allow one to define three states of the flow: the subcritical flow, where the boundary layer separates laminarly; the critical flow, in which a separation bubble, followed by a turbulent reattachment, occurs; and the supercritical flow, where an immediate transition from the laminar to the turbulent boundary layer is observed at a critical distance from the stagnation point. According to the total drag coefficient the values found in this paper connect the subcritical region represented by the measurements of Wieselsberger (1923) and Fage & Warsap (1930) with the supercritical range in which Roshko (1961) carried out his experiments.

670 citations


"3D CFD computations of transitional..." refers background or methods in this paper

  • ...Even though the overall behavior is the same for different experiments, there is a wide spread in the details of the flow, as shown by Achenbach [24] for the drag variation....

    [...]

  • ...Computed averaged azimuth position of the wake separation point measured from the stagnation point, compare with data from [24] by turbulent separation along with backward shift of the separation point and the narrowing of the cylinder wake with increasing Reynolds number....

    [...]

Journal ArticleDOI
TL;DR: In this article, a modification to Kirchhoff's free streamline introduces the parameter k = √(1-C_ps), which allows arbitrary base pressure and which must depend on the dynamics of the wake.
Abstract: A modification to Kirchhoff's free streamline introduces the parameter k = √(1- C_ps), which allows arbitrary base pressure and which must depend on the dynamics of the wake. For a cylinder of given cross-sectional shape, the drag, C_D, and the wake width, d', are functions of k only. These functions are used to relate C_D and the dimensionless shedding frequency , S = nd/U_ ∞ to another number, S* = nd' / U_s, which is based on wake parameters. It is found that S* = 0.16 for all cylinders. In another approach, k is evaluated by using Karman's solution for the vortex street.

646 citations


"3D CFD computations of transitional..." refers background or result in this paper

  • ...to Re = 1× 106 the computations predict an increase of the Strouhal frequency, not normally discussed, but in fair agreement with several references [21, 22, 23]....

    [...]

  • ...As already addressed by Roshko [23], typical wall corrections used in wind tunnel experiments do not account for possible interference effects connected to flow separation which may be problematic for cases with a high degree of separation....

    [...]

Frequently Asked Questions (2)
Q1. What are the contributions mentioned in the paper "3d cfd computations of transitional flows using des and a correlation based transition model" ?

The report describes the application of the correlation based transition model of of Menter et. al. [ 1, 2 ] to the cylinder drag crisis and the stalled flow over an DU-96-W-351 airfoil using the DES methodology. The correlation based transition model has lately shown promising results, and the present paper describes the application of the model to predict the drag and shedding frequency for flow around a cylinder from sub to super-critical Reynolds numbers. 

This could be the focus of further work, along with efforts to actually resolve the wind tunnel walls. The problem of the wind tunnel corrections, that do not account for the interaction of the separation and the wall effects would additionally require further studies.