Reports of Research Institute for Applied Mechanics, Kyushu University No.150 (47–59) March 2016

Reproducibility of Complex Turbulent Flow
Using Commercially-Available CFD Software―Report 1: For the Case of a Three-Dimensional Isolated-Hill With Steep Slopes―
Takanori UCHIDA*1
E-mail of corresponding author: takanori@riam.kyushu-u.ac.jp(Received January 29, 2016)
Abstract
Selecting a high-quality site for wind turbine installation is frequently challenging in Japan because the majorityof the topography of Japan is steep complex terrain and the spatial distribution of the wind speed is quite complex
over such terrain. To address this issue, an unsteady CFD model called RIAM-COMPACT® has been developed.
RIAM-COMPACT® is based on an LES turbulence model. In the present paper, to assess the accuracy of
RIAM-COMPACT®, we performed a numerical simulation of uniform, non-stratified airflow past athree-dimensional isolated-hill. Attention is focused on airflow characteristics in the wake region. The results of
RIAM-COMPACT® using the LES model were in good agreement with those of another commercially-available
CFD software (STAR-CCM+) using LES models.
Key words: Commercially-available CFD software, STAR-CCM+, RIAM-COMPACT®, LES, RANS, Isolated-hill

and also with GPGPU (General Purpose computing on GPU),

1．Introduction
The author’s research group has developed the numericalwindsynopsis

COMPACT®1,

prediction
2)

.

The

technique

core

named

technology

of

RIAMRIAM-

COMPACT® is under continuous development at the
Research Institute for Applied Mechanics, Kyushu University.
An exclusive license of the core technology has been granted
by Kyushu TLO Co., Ltd. (Kyushu University TLO) to
RIAM-COMPACT Co., Ltd. (http://www.riam-compact.
com/), a venture corporation which was founded by theauthor and originated at Kyushu University in 2006. A
trademark, RIAM-COMPACT®, and a utility model patent
were granted to RIAM-COMPACT Co., Ltd. in the same

which has drastically reduced the computation time, leaving
no appreciable problems in terms of the practical use of the
RIAM-COMPACT® software.
In the present study, simulation results of airflow over an
isolated-hill with steep slopes obtained from the RIAMCOMPACT® software are compared to those from anothercommercially-available

CFD(Computational

Fluid

Dynamics) software. The results from this comparison arereported.

2. Summary of Commercially-Available
CFD Software

year. In the meantime, a software package has been

Commercially-available CFD software has developed

developed based on the above-mentioned technique and ismainly as an engineering tool primarily in the automobile andnamed the RIAM-COMPACT® natural terrain version

aviation industries until the present time. The following is a

software. Efforts have been made to promote this software aslist of the major CFD software packages available on thea standard model in the wind power industry.

market:Some of the major corporations, which have adopted the
RIAM-COMPACT® software, are Eurus Energy Holdings
Corporation,

ElectricPower

Development

Co.,

Ltd.

(J-POWER), Japan Wind Development Co., Ltd., and
EcoPower Co., Ltd. etc.
Computation time had been an issue of concern for the

General-purpose CFD thermal fluid analysis software■STAR-CCM+
http://www.cd-adapco.co.jp/products/star_ccm_plus/index.
html

RIAM-COMPACT® software, which focuses on unsteady■ANSYS (CFD, Fluent, CFX)

turbulence simulations (LES, Large-Eddy Simulation). The

http://ansys.jp/solutions/analysis/fluid/index.html

present fluid simulation solver is compatible with multi-core■SCRYU/Tetra

CPUs (Central Processing Units) such as the Intel Core i7

http://www.cradle.co.jp/products/scryutetra/*1 Research Institute for Applied Mechanics, Kyushu University

48

Uchida : Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software, Report 1■STREAM■METEODYN

http://www.cradle.co.jp/products/stream/index.html

http://meteodyn.com/■CFD2000In the present study, the simulation results from the
RIAM-COMPACT® software are compared to those from
STAR-CCM+, one of the leading commercially-available
CFD software packages. The results of the comparison are
discussed.

http://www.cae-sc.jp/docs/cfd2000/index.htm■PHOENICS
http://www.phoenics.co.jp/■Autodesk Simulation CFD
http://www.cfdesign.com/■CFD++

3. Summary of STAR-CCM+ Software
In this section, a summary of STAR-CCM+, a general-

http://bakuhatsu.jp/software/cfd/

purpose thermal fluid analysis software package distributed■CFD-ACE+

by CD-adapco will be provided. The version of the software

http://www.wavefront.co.jp/CAE/cfd-ace-plus/

package used in the present study is 6.02.007 (for 64-bit■AcuSolve
http://acusolve.jsol.co.jp/index.html

Windows).
STAR-CCM+ uses a single GUI (Graphical User
Interface) for computational mesh generation, execution of■FLOW-3D

fluid analyses, and data post-processing. In the present study,

http://www.terrabyte.co.jp/FLOW-3D/flow3d.htm

a three-dimensional model of an isolated-hill (IGES format)■FloEFD

is created using CATIA, a three-dimensional CAD software

http://www.sbd.jp/product/netsu/floefd3cad.shtml

package developed by Dassault Systèmes. After reading in■Flow Designer

the created three-dimensional CAD data on STAR-CCM+,

http://www.akl.co.jp/■PowerFLOW
http://www.exajapan.jp/pages/products/pflow_main.html■KeyFlow
http://www.kagiken.co.jp/product/keyflow/index.shtml

the mesh type, the mesh spacing, the turbulence model(details to be described later), the time-step interval, theboundary

conditions,

Subsequently,

and

other

pre-processing,variables

fluid

are

set.

analyses,andpost-processing are performed.
The mesh generation method in STAR-CCM+ is■OpenFOAM

distinctive. In STAR-CCM+, both a polyhedral mesh and a

http://www.cae-sc.jp/docs/FOAM/

prism layer mesh can be used (see Fig.1). The polyhedral■FrontFlow
http://www.advancesoft.jp/product/advance_frontflow_red/
The wind power industry has on its own developed anddistributed CFD software designed for selecting sites
appropriate for the installation of wind power generators (seethe listing below). Recently, some of the above-mentioned
general-purpose thermal fluid analysis software has started

mesh is a new type of mesh offered by CD-adapco andconsists of polyhedral cells which possess 10 to 15 faces on
average. The use of this cell type makes it possible todramatically reduce the number of mesh cells required toobtain analysis results equivalent to those that can beobtained using a conventional tetrahedral mesh and 2) the
memory required by the solver. With the use of this cell type,the computational stability improves significantly, and thebeing adopted in the wind power industry.

time required to obtain convergent solutions also decreases.

CFD software designed for the wind power industry (windthe behavior of the boundary layer developed over an object.

farm design tools)

In this type of mesh, layers of thin mesh cells are distributed■RIAM-COMPACT®
http://www.riam-compact.com/

The prism layer mesh is a refined mesh designed to capture

regularly over the object. Since the thickness and the numberof layers in the normal direction with respect to the object
surface can be freely adjusted, the behavior of the boundary■MASCOT

layer in the vicinity of a wall can be captured with highhttp://aquanet21.ddo.jp/mascot/

accuracy. However, when the number of the prism layer■WindSimmesh cells is very large, the computation time increases

http://www.windsim.com/

significantly.

Reports of Research Institute for Applied Mechanics, Kyushu University No.150 March 2016
Numerical simulations are based on FVM (Finite-

49

modeled on the physical considerations of the SGS stress.

Volume Method), and the Navier-Stokes equation is used as

For the governing equations of the flow, a spatially-filtered

the governing equation. Iterative calculations are performed

continuity equation for incompressible fluid (Eq.(1)) and afor the velocity and pressure fields using an AMG (Algebraic

spatially filtered Navier-Stokes equation (Eq.(2)) are used:

Multi-Grid) linear solver. For the time marching method, afirst-order implicit method is used. For the convective term, asecond-order upwind scheme is adopted for the case of
RANS (Reynolds Averaged Navier-Stokes Simulation)models. For the case of LES models, a BCD (Bounded
Central Differencing) scheme is employed.

ui
0
x i

-(1)

ij
ui
ui
p
1 2 ui
 uj



t
x j
x i Re x jx j x j-(2)

Supporting equations are given in Eqs.(3) - (8):Region filled with fluid

Prism layer

SGS   Cs fs   S2

S  2SijSij
Object surface

ObjectFig.1 Polyhedral mesh and prism layer mesh

1 ' 'u k u k ij  2SGS Sij
3

ij  u 'i u 'j 

Sij 

1/2



fs  1  exp  z  / 25



-(4)



1  u i u j 2  x j x i   hx h yhz



1/3

-(3)

-(5)
-(6)



-(7)
-(8)

Because mean wind speeds of 6.0 m/s or higher are
considered in the RIAM-COMPACT® software , the effect

4. Summary of RIAM-COMPACT® Software
In this section, a summary of the RIAM-COMPACT®
natural terrain version software is provided. The RIAMCOMPACT® natural terrain version software uses collocatedgrids in a general curvilinear coordinate system in order tonumerically predict local airflows over complex terrain withhigh accuracy while avoiding numerical instability. In thesecollocated grids, the velocity components and pressure aredefined at the mesh cell centers, and variables which result
from the contravariant velocity components multiplied by the
Jacobian are defined at the cell faces. As for the simulation
technique, the FDM (Finite-Difference Method) is adopted,
and an LES model is used for the turbulence model. In LES, aspatial filter is applied to the flow field to separate eddies ofvarious scales into GS (Grid Scale) components, which arelarger than the computational grid cells, and SGS (Sub-Grid
Scale) components, which are smaller than the computational
grid cells. Large-scale eddies, i.e., the GS components of
turbulence eddies, are directly numerically simulated without
relying on the use of a physically simplified model. On theother hand, the main effect of small-scale eddies, i.e., the SGS
components, is to dissipate energy, and this dissipation isof vertical thermal stratification (density stratification), whichis generally present in the atmosphere, is neglected.
Furthermore, the effect of surface roughness is included byreconstructing the irregularities of the terrain surface in highresolution.
The computational algorithm and the time marching
method are based on a FS (Fractional Step) method and theEuler explicit method, respectively. The Poisson’s equation
for pressure is solved by the SOR (Successive OverRelaxation) method. For discretization of all the spatial terms
in Eq.(2) except for the convective term, a second-order
central difference scheme is applied. For the convective term,a third-order upwind difference scheme is applied. Aninterpolation technique based on four-point differencing andfour-point interpolation by Kajishima is used for the
fourth-order central differencing that appears in thediscretized form of the convective term. For the weighting of
the numerical diffusion term in the convective term
discretized by third-order upwind differencing, α = 3.0 iscommonly applied in the Kawamura-Kuwahara Scheme.
However, α = 0.5 is used in the present study to minimize theinfluence of numerical diffusion. For LES sub-grid scalemodeling, the commonly used Smagorinsky model is adopted.

50

Uchida : Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software, Report 1

A wall-damping function is used with a model coefficient of
0.1.

5. Flow Field and Simulation Set-up
In this section, the flow field investigated and thecoordinate system and simulation set-up adopted in thehα  40a=2hpresent study are described. The dimensions of thecomputational domain are 13h (x) × 9h (y) × 10h (z), where x,

Fig.3 Structured computational mesh used in the vicinity of they, and z are the streamwise, spanwise, and vertical directions,

isolated-hill in the RIAM-COMPACT® simulation.

respectively, and h is the height of the isolated-hill (see Fig.2).

The cross-section shown is for the central plane (y = 0).

The geometry of the isolated-hill is represented by thefollowing function:z (r) = 0.5h × ｛1 + cos (πr / a)｝，r = (x2 + y2)1/2，a = 2h-(9)where a is the radius of the isolated-hill at its base.
Regardingtheboundary

conditions,

uniform

inflowconditions are applied at the inflow boundary, slip conditions
are applied at the side and upper boundaries, and convective
outflow conditions are applied at the outflow boundary. At the
ground surface, non-slip boundary conditions are imposed. The
Reynolds number is set to Re (= Uh / ν) = 104, where h is theisolated-hill height and U is the wind speed at height h at the
inflow boundary. The time steps are set to Δt = 2 × 10-3 (h / U)
and Δt = 1 × 10-2 (h / U) in the simulations with RIAM-(a) Side view (y = 0)

COMPACT® and STAR-CCM+, respectively.
Inflow boundary:uniform inflow conditions
Outflow boundary:
convective outflow
conditions

9h
Upper boundary &Side boundaries:
slip condition

Flow

U

3h

(b) Top view (z = 0.5h)
Three-dimensional
isolated-hill model(height h)

10h

Ground surface:non-slip conditions

Fig.4 Side and top cross-sectional views of the computationalmesh (unstructured mesh) distributed over the entire
computational domain for the STAR-CCM+ simulations.
Fig.3 shows the computational mesh (structured mesh)which is used in RIAM-COMPACT® in the vicinity of theisolated-hill for the present simulation. The numbers of

10hmesh points in the x-, y-, and z-directions are 326×226×67
points, respectively (approximately five million points in

Fig.2 Computational domain, coordinate system,
and boundary conditions

total). The mesh points in the x- and y-directions are spaced
at an even interval of 0.04h while the mesh points in theReports of Research Institute for Applied Mechanics, Kyushu University No.150 March 2016
z-direction are spaced unevenly (from 0.003h to 0.6h).

51

Flow

Fig.4 shows the computational mesh (unstructured mesh)used in the present STAR-CCM+ simulations. The total
number of mesh points is approximately one million (i.e.,
approximately 1/5 of that used in the RIAM-COMPACT®
simulation). In the STAR-CCM+ simulations, the mesh(a) Wind tunnel experiment, smoke-wire techniqueresolution in the vicinity of the isolated-hill is set to benearly identical to that in the RIAM-COMPACT®
simulation.
Tables 1 and 2 summarize all of the turbulence models(RANS, LES) adopted in the present comparative study. For
convenience, simulations performed with the use of each of(b) Numerical simulation (RIAM-COMPACT®),the models are referred to as Cases 1 to 5. In Case 4, i.e., thepassive particle tracking method

WALE model, the eddy viscosity coefficient becomes zeronear the wall without the use of a wall damping function.

Fig.5 Visualization of the flow field

Furthermore, this model has been designed in such a way

in the vicinity of an isolated-hill; instantaneous fieldthat the eddy viscosity coefficient is not calculated forlaminar shear layers.

Figs.6 to 12 illustrate comparisons of simulation results. In
these figures, the time-averaged flow fields and turbulence
Spalart-Allmaras single equation

statistics were evaluated from the fully-developed flow field,

eddy-viscosity turbulence model,

specifically from the time interval of t = 100 – 200 (h / U).

RANS

steady RANS

Examinations of these figures (Figs.6 to 10) reveal that themodelsSST k-ω two-equation eddy

trends of the simulation results from STAR-CCM+ with the-viscosity model, unsteady RANS

use of the RANS models (steady and unsteady) are similar to(URANS)

each other. In addition, the three sets of simulation results

Case 1Case 2LES

Case 3

Smagorinsky model, LES

from STAR-CCM+ and RIAM-COMPACT® with the use ofmodelsCase 4WALE model, LES

the LES models (i.e., STAR-CCM+ with the standard

Table 1 Turbulence models used in the present

Smagorinsky model and the WALE model and RIAM-

STAR-CCM+ simulations

COMPACT® with the standard Smagorinsky model)
resemble one another very closely with no distinction

Smagorinsky model, LES

between the use of STAR-CCM+ and the use of RIAM-

Table 2 Turbulence model used in the present

COMPACT®. It should be noted that a clear vortex center is

RIAM-COMPACT® simulation

simulated downstream of the isolated-hill in the simulations

LES model

Case 5

with the LES models, however, not in the simulations with

6. Simulation Results and Discussions
First, the flow patterns which form in the vicinity of theisolated-hill are examined (see Fig.5). The qualitative
behaviors of the flows numerically simulated by RIAMCOMPACT® and observed in a wind tunnel experiment
resemble each other quite closely. More specifically, a shear
layer separates in the vicinity of the top of the isolated-hilland rolls up into isolated-eddies, forming large eddies.
Subsequently, these eddies are periodically shed downstreamof the isolated-hill. Refer to Uchida et al.1) for a detailed
comparison between the simulation results from RIAMCOMPACT® and those from the wind tunnel experiment.

the RANS models.
Subsequently, the vertical profiles of turbulence statistics(the mean and standard deviation of the streamwise (x)velocity component), shown in Figs.11 and 12, are examined.
The turbulence statistics were evaluated at a total of four
points, specifically, at x / h = -2, 0, +2, and +6 as shown in
Fig.10. In the mean velocity profiles, no significant
differences are found at any of the points given by x / h = -2,0, and +2 (Fig.11). At the top of the isolated-hill, which islocated at x / h = 0, locally occurring increased wind velocity
is evident. Immediately downstream of the isolated-hill, thatis at x / h = +2, a region of reversed flow is present. At x/ h
= +6, the trends of the wind velocity profiles differ between

52

Uchida : Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software, Report 1

Flow(a) Spalart-Allmaras single equation eddy-viscosity turbulence model, steady RANS(b) SST k-ω two-equation eddy-viscosity model, unsteady RANS (URANS), time-averaged field
Fig.6 Distribution of the streamwise (x) velocity component; simulation results from STAR-CCM+ with the use of RANS
turbulence models. The cross-sections shown are for the central plane (y = 0) aligned with the streamwise direction (x).

Reports of Research Institute for Applied Mechanics, Kyushu University No.150 March 2016

53

Flow

(a) Standard Smagorinsky model, LES(b) WALE model, LES
Fig.7 Distribution of the time-averaged streamwise (x) velocity component; simulation results from STAR-CCM+ with the use of
LES turbulence models. The cross-sections shown are for the central plane (y = 0) aligned with the streamwise direction (x).

54

Uchida : Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software, Report 1

Flow

The vortex region is not simulated clearly.

(a) Spalart-Allmaras single equation eddy-viscosity turbulence model, steady RANS

The vortex region is not simulated clearly.

(b) SST k-ω two-equation eddy-viscosity model, unsteady RANS (URANS), time-averaged field
Fig.8 Distribution of velocity vectors; simulation results from STAR-CCM+ with the use of RANS turbulence models.
The cross-sections shown are for the central plane (y = 0) aligned with the streamwise direction (x).

Reports of Research Institute for Applied Mechanics, Kyushu University No.150 March 2016

55

Flow

Vortex center(a) Standard Smagorinsky model, LES

Vortex center(b) WALE model, LES
Fig.9 Distribution of time-averaged velocity vectors; simulation results from STAR-CCM+ with the use of LES turbulencemodels. The cross-sections shown are for the central plane (y = 0) aligned with the streamwise direction (x).

56

Uchida : Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software, Report 1

Flow

-2h

0h

+2h

+6h

height h(a) Distribution of the streamwise (x) velocity component

-2h

0h

+2h

+6h

Vortex center

height h(b) Distribution of velocity vectors
Fig.10 Time-averaged results from RIAM-COMPACT® with the use of the standard Smagorinsky LES model.
The cross-sections shown are for the central plane (y = 0) aligned with the streamwise direction (x).

Reports of Research Institute for Applied Mechanics, Kyushu University No.150 March 2016

STAR-CCM+
RIAM-COMPACT®

x=0h

x=-2h
No significant differencesamong the values

x=+2hNo significant differencesamong the values

Locally occurring
increased velocity

x=+6h
RANS results↓
The trend isdifferent from
LES results.

Fig.11 Comparisons of profiles of the mean streamwise (x) velocity component

57

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Uchida : Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software, Report 1

STAR-CCM+
RIAM-COMPACT®

x=0h

x=-2h

Only values close tozero were output insimulations with RANS
models.

Similar trends are foundamong values output by
LES models.

Similar trends are foundamong values output by
LES models.

x=+2h
Only values close tozero were output insimulations with RANS
models.

Similar trends are foundamong values output by
LES models.

x=+6h
Only values close tozero were output insimulations withRANS models.

Fig.12 Comparisons of profiles of the standard deviation of the streamwise (x) wind velocity componentReports of Research Institute for Applied Mechanics, Kyushu University No.150 March 201659

the simulation results from RANS models and those fromwere nearly the same. In addition, the center of the vortex

LES models.

(region of reversed flow) downstream of the isolated-hill

From the point of view of predicting the airflow field overwhich was evident in the LES simulations, was notcomplex terrain, these errors in the simulation results from

adequately simulated by STAR-CCM+ using the RANS

the RANS models would significantly affect assessments of

turbulence models.
In the present study, the vertical profiles of turbulence

the power to be generated and other variables.
In the vertical profiles of the standard deviation of thestatistics (the mean and standard deviation of the streamwise

streamwise (x) wind velocity component predicted by the(x) wind velocity component) were evaluated and compared

RANS models, no significant differences are found at any ofat four points, x / h = -2, 0, +2, and +6. The comparisonsthe four examined points (Fig.12). Similarly, it can be said

revealed the following. No significant difference was foundthat the trends of the profiles from simulations which usedamong the mean velocity profiles obtained from all thethe LES models, i.e., simulations by STAR-CCM+ with both

turbulence models at x / h = -2, 0, and +2. At the top of thethe standard Smagorinsky model and the WALE model andisolated-hill, which was located at x/h = 0, locally occurring

the simulation by RIAM-COMPACT®, are nearly identical.

increased velocity was identified. At x / h = +2, which was

In the simulations performed in the present study, theimmediately downstream of the isolated-hill, an area of

number of mesh points used in RIAM-COMPACT®

negative velocity was found, indicating the presence of a(structured mesh, approx. five million mesh points) isregion of reversed flow. At x / h = +6, the trends of theapproximately five times that used in STAR-CCM+

results differed between the simulations which used the(unstructured mesh, approx. one million mesh points).

RANS models and those which used the LES models.

Nonetheless, it has been confirmed that the simulation by

From the perspective of predicting the airflow field over

RIAM-COMPACT® completes approximately ten times

complex terrain, the prediction errors in the mean windfaster than those by STAR-CCM+.

velocity in the RANS model simulations would greatly affectthe assessments of the power to be generated and other7. Conclusions
In the present paper, the airflow field in the vicinity of an
isolated-hill with steep slopes was simulated by one of theleading, commercially-available CFD packages, STARCCM+ as well as by RIAM-COMPACT® (turbulence model:
LES based on the standard Smagorinsky model). The
simulation results were compared to examine the prediction
accuracy of RIAM-COMPACT®. In STAR-CCM+, twotypes of RANS turbulence models were selected: theSpalart-Allmaras single equation eddy-viscosity turbulencemodel (steady RANS) and the SST k-ω two-equation

variables.
In the simulations which used the RANS turbulencemodels, the values of the standard deviation were notsignificantly different from zero throughout the profiles at all
the examined points. In the simulations which used the LES
models, i.e., STAR-CCM+ with the standard Smagorinsky
model and the WALE model and RIAM-COMPACT® withthe standard Smagorinsky model, the trends of the profiles ofthe mean and standard deviation of the wind velocity
resembled one another quite closely as was the case for thetrends of the visualized flow field.

eddy-viscosity model (unsteady RANS). In addition, for LES

References

turbulence models (SGS models), the standard Smagorinsky
model and the WALE model were selected.
From the comparisons of the simulation results, the1)

technique for wind turbine generators by using

following have been found. Images visualizing the flow fields

large-eddy simulation, Journal of Wind Engineering &time-averaged over the interval of t = 100 – 200 (h / U)suggested that the trends of the simulation results from
STAR-CCM+ with the RANS turbulence models (steady andunsteady) were quite similar to each other. In addition, thetrends of the flow patterns simulated by RIAM-COMPACT®
with the LES turbulence model (the standard Smagorinsky
model) and STAR-CCM+ with the LES turbulence models(the standard Smagorinsky model and the WALE model)

Takanori UCHIDA and Yuji OHYA, Micro-siting

Industrial Aerodynamics, Vol.96, pp.2121-2138, 2008
2)

Takanori

UCHIDA,

Validation

TestingofthePrediction Accuracy of the Numerical Wind Synopsis
Prediction Technique RIAM-COMPACT for the Caseof the Bolund Experiment –Comparison against aWind-Tunnel Experiment–, Reports of RIAM, Kyushu
University, No.147, pp.7-14, 2014