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Overview of the TurbSim 

Stochastic Inflow Turbulence 

Simulator 

Version 1.21 

(Revised February 1, 2007) 

N.D. Kelley and B.J. Jonkman 

Technical Report 

NREL/TP-500-41137 

April 2007 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Overview of the TurbSim 

Stochastic Inflow Turbulence 

Simulator 

Version 1.21 

(Revised February 1, 2007) 

N.D. Kelley and B.J. Jonkman 

Prepared under Task No. WER7.2501 

Technical Report 

NREL/TP-500-41137 

April 2007 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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OVERVIEW OF THE TURBSIM STOCHASTIC INFLOW 

TURBULENCE SIMULATOR (Version 1.21) 

(Revised February 1, 2007) 

Background 

The TurbSim stochastic inflow turbulence code was developed to provide a numerical 

simulation of a full-field flow that contains bursts of coherent turbulence (organized 

turbulent structures in the flow that have a well-defined spatial relationship) that reflect 

the proper spatiotemporal turbulent velocity field relationships seen in instabilities 

associated with nocturnal boundary layer flows. Such features are not simulated well by 

the International Electrotechnical Commission (IEC) Normal Turbulence Models 

(NTMs). Its purpose is to provide the wind turbine designer with the ability to drive 

design code (FAST, MSC.ADAMS®, or YawDyn) simulations of advanced turbine 

designs with simulated inflow turbulence environments that incorporate many of the 

important fluid dynamic features known to adversely affect turbine aeroelastic response 

and loading. The purpose of this document is to provide the user with a generalized 

overview of how the code has been developed and some of the theory behind that 

development. The details for the use of the TurbSim code are provided in the TurbSim 

User’s Guide [1]. 

The TurbSim Code 

The TurbSim code supports all of the features found in the previous SNLWIND-3D [2] 

and SNwind inflow turbulence simulator codes that had their roots in Paul Veers’ original 

stochastic wind simulator, SNLWIND [3]. All of the spectral models available in 

SNLWIND-3D [SMOOTH (flows over flat, homogeneous terrain), WF_UPW, WF_07D, 

and WF_14D (flows upwind of a multi-row wind farm and with 7 and 14 rotor diameter 

row-to-row spacing within the farm)] as well as the IEC Kaimal and von Karman NTMs 

that are provided in SNwind are included. The wind farm spectral models (WF_xxx) were 

originally developed to mimic conditions seen upwind and at two row-to-row separations 

within a large, multi-row wind farm at San Gorgonio Pass in California. They have been 

validated up to a height of 50 m above ground level. 

NWTCUP Spectral Model 

TurbSim Version 1.01 and later versions include the NWTCUP (National Wind 

Technology Center UPwind) spectral model. This model generates a turbulent inflow 

that is characteristic of the highly turbulent conditions experienced when testing turbines 

at National Renewable Energy Laboratory’s National Wind Technology Center (NWTC); 

i.e., downwind of a major mountain range. The NWTCUP model was validated using 

measurements from the upwind planar sonic anemometer array and response of the 

NWTC Advanced Research Turbine (ART) employed as part of the NWTC component 

of the Long-term Inflow and Structural Testing (LIST) Experiment [6]. It represents a 

test environment corresponding to those found in complex terrain situations that often 

1 
present challenges for turbine operations. The NWTCUP model incorporates additional 

scaling features to better reflect conditions seen at the NWTC (high turbulence intensities 

and intense coherent turbulent structures encountered during stable, nighttime flows). In 

earlier versions of the code, the NWTCUP model utilized the low-frequency (longperiod) turbulent spectral model derived from measurements in San Gorgonio Pass, 

California [2] that did not fully reflect conditions seen at the NWTC. However, with the 

release of version 1.20, TurbSim incorporates NWTC spectral scaling derived from 

measurements taken from a planar sonic anemometer array that was part of the NWTC 

LIST experiment. 

GP_LLJ Spectral Model 

With the release of Version 1.10, a new model GP_LLJ (Great Plains Low-Level Jet) is 

included that simulates flow conditions associated with the large vertical shears and 

coherent turbulence encountered within and beneath Great Plains low-level jet streams. 

A low-level jet stream (LLJ) is a current of higher speed air that usually forms at night 

near the ground and is characterized by strong vertical wind shear beneath a wind speed 

maximum that typically occurs at heights between 50 and 500 m above ground level 

(AGL). Figure 1 shows an example of a LLJ as a series of vertical wind profiles 

observed 10-minutes apart between 02:40 and 04:00 LST in Southeast Colorado. The 

lower pair of horizontal dashed lines outline the height range of a 1.5 MW turbine rotor 

while the upper two lines depict the expected rotor depth of a future onshore 10 MW 

turbine. 

10-minute mean wind speed (m/s)

6 8 10 12 14 16 18 20 22

Height AGL (m)

50

100

150

200

250

300

350

400

450

500

02:40

02:50

03:00

03:10

03:20

03:30

03:40

03:50

04:00

Local Std Time

Figure 1. LLJ example. 

The GP_LLJ model reflects conditions seen over the Great Plains with and without the 

presence of a low-level jet stream that occurs at a height between 70 and 490 m AGL. 

2 
Local turbulence scaling and low-level jet vertical wind profiles used with the GP_LLJ 

model have been derived from measurements collected on a 120-m tower and with a 

medium range Doppler acoustic wind profiler (SODAR) located in the southwestern 

Great Plains near Lamar, Colorado [4]. The GP_LLJ model allows the user to specify the 

jet height and peak wind speed (intensity) or to let the code randomly choose a jet height 

and/or wind speed based on the other specified boundary conditions. 

The GP_LLJ model produces both wind speed and direction profiles that can be extended 

down to a height of 3 m AGL when the JET profile option is chosen. It is not possible to 

specify a shear exponent to describe a power law variation in wind speed across the 

specified rotor disk when using the JET option as the shape of the jet profiles are 

determined by the boundary conditions provided. The summary file generated for each 

simulation does contain an equivalent power law exponent across the rotor disk however. 

Requesting the LOG profile option when using the GP_LLJ model generates a simulation 

with a logarithmically varying vertical wind speed profile over a user-specified height 

range which can begin as low as 3 m AGL.

Coherent Turbulent Structures 

The TurbSim code provides the ability to efficiently generate randomized coherent 

turbulent structures that are superimposed on the more random background turbulent 

field as produced by one of the diabatic (non-neutral) spectral models; i.e., SMOOTH, 

WF_UPW, WF_07D, WF_14D, NWTCUP, or GP_LLJ. Typically the size of the 

structures have a higher probability of being scaled to the dimensions of the requested 

rotor diameter but the user may request the placement of the coherent disturbance in 

either the upper or lower half of the specified rotor disk for rotor diameters of 30 m or 

larger. The presence of superimposed coherent structures can induce transient loading 

events on simulated turbine rotors and supporting structures [4,5]. The simulation of 

such structures is not adequately provided for in the inflow simulators up to now because 

of basic limitations in the mathematical procedures used. This combination of 

techniques, i.e., Fourier inversion plus time-domain Poisson and lognormal modeled 

events, produces the highly non-Gaussian flow behavior associated with the presence of 

coherent structures in actual flows. 

The impact of coherent turbulent structures on wind turbine operations is discussed in [5]. 

The process of generating coherent turbulent structures involves first generating a 

background turbulent field using one of the diabatic spectral models listed above. The 

SMOOTH spectral model is generally recommended for conditions seen in homogeneous 

terrain when no low-level jets are expected. It can be considered a “reference” to which 

load simulations using the other spectral models can be compared. It can also be adjusted 

to meet measured scaling information which is sometimes useful. The NWTCUP 

spectral model is suggested for complex terrain applications, and the GP_LLJ spectral 

model is recommended for use in the Great Plains where low-level jets are frequent. As 

previously discussed, the GP_LLJ model has been based on observations in the 

southwestern Great Plains. The code also allows the user to use available observed 

turbulence information to improve the agreement in specific locations. 

3 
The coherent turbulent structures exhibit the correct temporal and spatial phase 

relationships associated with the flow elements of the breakdown of a Kelvin-Helmholtz 

(KH) wave or billow that have been derived from both Large-Eddy and Direct Numerical 

Simulations (LES and DNS). KH instability (KHI) is known to be more or less 

ubiquitous in the stable, nocturnal boundary layer. A more complete discussion of KHI 

and its impact on wind turbines is available in Sections 1-3 of [4] and in [5]. The 

coherent structures generated are then superimposed on the more random background 

similar to what occurs in natural flows. 

Modeled as a combination of non-homogenous Poisson and lognormal stochastic 

processes, the randomized scaling of the coherent structures in this version of TurbSim is 

based on measurements taken from the anemometer array upwind of the NWTC ART 

Turbine as part of the LIST program [4,6]. A flow chart of the TurbSim code is shown in 

Figure 2. When the GP_LLJ model is specified, the scaling is based on measurements 

derived from the Lamar 120-m tower used as part of the Lamar Low-Level Jet Project 

(LLLJP) and any systematic differences based on that location and height above the 

ground are accounted for. 

Every effort has been made to randomize the occurrence and scaling of coherent event 

structures that occur in natural, nocturnal boundary layer flows. If a flow containing 

coherent turbulent elements is desired, it is highly recommended that an ensemble of 

simulations using the same boundary conditions with a varying random seed be generated 

and the resulting statistics evaluated. Ideally, each of these individual flow realizations 

should be employed to drive the target wind turbine simulation, and the resulting 

aeroelastic response statistics should be compared in order to obtain some feel of the 

expected variability. A PERL-script to compute an ensemble of inflow realizations for a 

given set of boundary conditions is included in the distribution. 

If coherent turbulence is requested and the specified gradient Richardson number 

(vertical dynamic stability) is greater than -0.05, a series of coherent disturbances are 

generated with random properties. This option should always be requested when using 

the NWTCUP model. Observations of low-level jets in the high plains of Southeast 

Colorado have revealed that coherent structures may or may not be present so the user 

may choose not to request them to simulate such situations when using the GP_LLJ 

model. The properties of the coherent structures include the number of disturbances, 

time of occurrence in the simulated record, and intensity. They are empirically scaled by 

the hub-height mean wind speed (or height of the center of the disturbance requested in 

the upper or lower half of the disk), the gradient Richardson number stability parameter, 

and the hub-height standard deviation of the vertical wind component (σw) derived from 

the simulated background turbulence field. 

The incorporation of the discrete coherent events requires the storage of about 270 MB in 

addition to the TurbSim executable and associated support files. The coherent events are 

separate from the normal TurbSim archive. The use of the coherent events does require a 

bit more execution time in the AeroDyn program (where they are superimposed on the 

background turbulence) with the exact amount of time dependent on the number of 

events and their lengths called for in each realization that are, in turn, a function of the 

4 
specified boundary conditions. The Cholesky factorization algorithm in TurbSim has 

been updated, and other parts of the code have been rewritten to be more efficient in 

terms of both CPU and memory usage. 

The KHTEST Function 

The ability to test the effects of intense coherent turbulent structures has been 

incorporated within TurbSim. Specifying “KHTEST” in the turbulence characteristic 

field generates a single intense disturbance in the center of a time series, without the 

random properties otherwise employed when coherent turbulence is requested. This test 

option overrides the Richardson number stability criterion and wind shear of the 

background turbulence field. It is available with only the NWTCUP spectral model. 

One use is to compare turbine simulations that have been excited with one of the IEC 

NTM models in order to assess the effects of intense coherent turbulence. 

Random Number Generators (RNGs) 

As opposed to the previous simulators in which only one random number generator 

(RNG) was provided, TurbSim allows the user a choice of one of three RNGs. A 

specification of “RNSNLW” in the input file invokes the original Sandia RNG that is 

used with SNLWIND and SNLWIND-3D. Adding a second seed value in place of the 

“RNSNLW” character string invokes an intrinsic Multiple Linear Congruential Generator 

(MLCG and used with SNwind), or substituting the character string “RANLUX” invokes 

level 3 of the Lüscher “Luxury Pseudorandom Numbers” RNG (see the TurbSim User’s 

Guide [1] for additional information). After an extensive testing of these three 

generators, we recommend the use of RANLUX. 

TurbSim Applicability 

With a limited comparison of simulated results with measurements between 50 and 120 

m from the NWTC LIST and Lamar 120-m towers, we believe the NWTCUP model is 

applicable up to heights of 100-120 m. We have extrapolated the vertical scaling 

measured on the Lamar 120-m tower and believe the GP_LLJ model is applicable up to a 

height of 230 m AGL based on limited SODAR observations. In the future, we plan to 

further evaluate the model simulation in the 120-230 m height range using data available 

from a wind profiling Doppler LIDAR. The wind farm related models, WF_xxx, are 

generally assumed to be applicable up to a height of 50 m. The SMOOTH model is 

considered to be applicable in the lower portion of the atmospheric boundary layer (the 

surface layer), in which the vertical flux of momentum can be considered constant with 

height. The upper limit of this layer is generally considered to be the order of 100 m 

when no low-level jets are expected. 

TurbSim Applications 

TurbSim is intended to generate a series of inflow simulations based on a given set of 

initial boundary conditions that are employed with the design codes (FAST, 

MSC.ADAMS®, or YawDyn) to produce response (load) solutions that can be analyzed 

5 
using ensemble statistics. For example, at least nine stochastic degrees of freedom have 

been incorporated into the NWTCUP model and twelve in the GP_LLJ in order to 

provide a measure of the range of variation seen in actual flows. A minimum of 31 

realizations is recommended for a specific set of boundary conditions to allow the use of 

large sample statistics to be employed properly. 

If the objective is to evaluate the impacts of coherent turbulence on the structural 

response of a particular wind turbine design, there are several considerations that must be 

addressed. Our research has shown in [5] that coherent turbulence primarily drives the 

higher order modal responses as transport or flux of turbulent kinetic energy into the 

turbine structure where it must be dissipated. In order to assess such an impact, the 

turbine design code being used must provide sufficient fidelity to encompass a range of 

modal frequencies that fully describe the turbulence-induced harmonic vibrations. Thus 

the design code being employed must include a sufficient number of degrees of freedom 

to properly model this flux of energy. 

In order for the FAST Code to accomplish simulations in a minimum amount of time, it 

supports only a relatively few degrees of freedom (currently 24). For a somewhat small 

and stiff turbine structure such as the NWTC ART we have found them to be sufficient 

(assuming the model has been “tuned” by actual field measurements). However for 

larger and more flexible designs such as the fictitious WindPACT 1.5 MW reference 

turbine or even larger turbines, many more degrees of freedom are required to fully 

simulate and evaluate the response to coherent turbulence excitation. Thus if the 

objective is to evaluate the impact of operating a large and flexible turbine design in 

coherent turbulence, the use of a multi-body design code which supports a large number 

of degrees of freedom (300-400 or more) such as MSC.ADAMS® is generally required. 

This capability is necessary to fully appreciate the differences in response between the 

IEC NTM and KHTEST excitations. 

References 

1. Jonkman, B.J.; Buhl, M.L., Jr. (March 2007). “TurbSim User’s Guide.” (Version 

1.21) NREL/TP-500-41136. Golden, CO: National Renewable Energy Laboratory. 

2. Kelley, N.D. (1992). “Full Vector (3-D) Inflow Simulation in Natural and Wind Farm 

Environments Using an Expanded Version of the SNLWIND (Veers) Turbulence 

Code.” NREL/TP-442-5225. Golden, CO: National Renewable Energy Laboratory. 

3. Veers, P.S. (March 1988). Three-Dimensional Wind Simulation, SAND88-0152. 

Albuquerque, NM: Sandia National Laboratories. 

4. Kelley, N.; Shirazi, M.; Jager, D., Wilde, S.; Adams, J.; Buhl, M.; Sullivan, P., 

Patton, E. (January 2004). Lamar Low-Level Jet Project – Interim Report, NREL/TP500-34593. Golden, CO: National Renewable Energy Laboratory. 

5. Kelley, N. D., Jonkman, B.J., Scott, G.N. (August 2005) “The Impact of Coherent 

Turbulence on Wind Turbine Aeroelastic Response and Its Simulation.” NREL/CP500-38074. Golden, CO: National Renewable Energy Laboratory. 

6 
6. Kelley, N.; Hand, M.; Larwood, S.; McKenna, E. (January 2002). “The NREL LargeScale Turbine Inflow and Response Experiment – Preliminary Results.” NREL/CP500-30917. Golden, CO: National Renewable Energy Laboratory.

7 
8 

List of Abbreviations 

AGL above ground level 

ART Advanced Research Turbine 

CS coherent structure 

DNS Direct Numerical Simulation 

GP_LLJ Great Plains Low-Level Jet spectral model 

IEC International Electrotechnical Commission 

KH Kelvin-Helmholtz 

KHI Kelvin-Helmholtz Instability 

KHTEST Kelvin-Helmholtz Billow Test Function 

LIST Long-Term Inflow and Structural Testing 

LLJ low-level jet stream 

LLLJP Lamar Low-Level Jet Project 

LES Large-Eddy Simulation 

LIDAR wind profiler utilizing LIght Detection And Ranging 

LST local standard time 

MLCG multiple linear congruential random number generator 

NTM Normal Turbulence Model 

NWTC National Wind Technology Center 

NWTCUP NWTC UPwind spectral model 

RNG random number generator 

SODAR vertical wind profiler utilizing SOund Detection And Ranging 

WindPACT fictitious 1.5 MW reference turbine design 
9 

Figure 2. TurbSim Flowchart 

TurbSim 

Input File 

Compute full field 

 NWTCUP model

 values for u* and crossc oefficients, based on LIST 

data, GP_LLJ model uses

values for jet wind speed and hei 

cross-correlation coefficients, based on 

Lamar data.

background turbulence

Uses random phase from a uniform 

distribution; uses 

randomized

orrelation c

randomized

ght, and

Generate 

coherent 

structures 

(CS)? 

Gene

backg

wind 

No

Choose LES or 

DNS event types 

Equal probability

Calculate billow size 

and location

 it covers rotor disk; 

(each) it covers 

lf of rotor disk 

75% probability

12.5% probability 

upper or lower ha

Compute the time 

scale factor

Factor is a function of billow size 

and wind shear, and scaled to 

achieve better agreement with 

LIST data; if billow is smaller than 

the rotor, randomly (from a 

) increase the s

to 100% 

uniform 

distribution hear up 

Yes

Calculate the times 

when CS will be 

generated

 Modeled as a 

 with rate f 

d and Richardson 

number or height above 

ground 

Poisson 

process a function o

wind spee

Calculate the expected 

length of the CS 

Length is a random variate from a 

 distribution, a function of 

height 

lognormal

Gene

cohe

turbu

wind 

Insert events at 

previously 

calculated times 

until expected 

length is met

Choose events randomly 

from a uniform distribution of 

available events 

Calculate ve

factor ba

maximum 

Maximum CTKE is a 

on Lamar data for GP 

models; otherwise 

Richardson numbe

billow center, σw o

turbulence, and u* a

residual is added fo

WF_07D 
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4. TITLE AND SUBTITLE

Overview of the TurbSim Stochastic Inflow Turbulence Simulator: 

Version 1.21 (Revised February 1, 2001) 

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NREL/TP-500-41137 

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N.D. Kelley and B.J. Jonkman 

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14. ABSTRACT (Maximum 200 Words)

The TurbSim stochastic inflow turbulence code was developed to provide a numerical simulation of a full-field flow 

that contains coherent turbulence structures that reflect the proper spatiotemporal turbulent velocity field relationships 

seen in instabilities associated with nocturnal boundary layer flows. This report provides the user with an overview of 

how the TurbSim code has been developed and some of the theory behind that development. 

15. SUBJECT TERMS

Turbulence simulation; TurbSim stochastic inflow turbulence code; design codes; wind turbine design 

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