1. Using CFD to gain insight into the turbulence inflow conditions for a small wind turbine on the rooftop of a building Amir Bashirzadeh Tabrizi, Jonathan Whale, Thomas Lyons, Tania Urmee Murdoch University

2. Aim To gain insight into the turbulent inflow conditions for a SWT in the highly turbulent setting of the rooftop of a large building based on the key parameters of turbulence intensity and turbulence kinetic energy in order to provide guidance in the micro-siting of wind turbines

3. Objectives To assess the combination of the CFD package CFX and TurbSim stochastic simulator as a tool to investigate the turbulence inflow conditions for a small rooftop wind turbine. In particular an assessment is carried out to compare the agreement with measured values between using TKE inlet profiles to the CFX domain and using a prescribed percentage TI in CFX. To use the combination of CFX and TurbSim to investigate the effects of the complex urban topography on turbulence on the rooftop via identifying high and low turbulence zones. Identification of zones of high turbulence is important in terms of protection of the SWT from excessive loading from coherent turbulence structures by avoiding installation in such area.

4. Methodology CFX assumes neutral atmospheric stability WAsP generates regional wind atlas up to 200 m a.g.l.

5. The Reference Site

6. The Target Site

7. 3D ultrasonic 10Hz data from August 2011 to January 2012 Data filtered by Monin- Obukhov length to isolate neutrally stable data

8. The CFD Model Shear Stress Transport (SST) scheme used to model turbulence CFD used to model all buildings within radius of 200 m of target site WAsP used to estimate wind shear profile in each of 8 wind sectors TurbSim used to estimate TKE profile for each wind sector using IEC Kaimal model CFX-TurbSim approach validated using test case from CEDVAL wind tunnel data set from the University of Hamburg

9. Results Wind Sector Measurement Neutrally Stable Condition(m 2 /s 2 ) CFD (TurbSim Approach) (m 2 /s 2 ) CFD (TI =10%) (m 2 /s 2 ) N1.8021.6020.013 NE1.0610.6820.065 E1.7492.2550.457 SE1.3701.8190.036 S1.4732.2520.030 SW2.7112.4740.013 W2.0722.1850.003 NW2.1291.8240.026 Average discrepancy between simulations and measurements using TKE profile is reduced compared to selecting a TI value in the CFD code Average gain in accuracy of around 70%

10. TI contours along front façade of building Prevailing wind direction TI contours along middle of building Prevailing wind direction

11. Conclusions and Recommendations CFX-TurbSim provided reasonable accuracy for predicting TKE levels on roof, particularly in neutrally stable conditions and for wind sectors where there are fewer obstacles CFX-TurbSim was less accurate at simulating the TKE after the flow passes over the building before encountering the measurement mast  Inability of CFX to model flow redevelopment after separation?  Limitation of Kaimal model in TurbSim? Significant gain in accuracy in using TurbSim to generate TKE profiles for the inlet of the CFD domain rather than using a preset value of turbulence intensity in the CFD code. Installers should erect turbines in middle of roof, avoiding edges of roof as well as areas close to windward and leeward walls of building in prevailing wind direction

12. Australian Research Council (ARC) Discovery Project Grant Jonathan Whale, Tom Lyons and Tania Urmee Murdoch University

13. Background Over period 2009 – 2013 IEA Task 27 worked closely with IEC TC88 MT 2 team in producing edition 3 of small wind design standard, IEC61400-2 One key issue was the suitability of the current turbulence models in the standard to the ranges of sites and wind conditions where SWTs are being installed Some analysis of existing data (mostly from urban sites) showed wind conditions that are outside the Standard Wind Conditions (SWC) that are used for the design load cases in the standard The MT2 team described these as “Other Wind Conditions” (OWC) and made the designer aware of them by writing a new informative Annex (Annex M in ed. 3)

14. Research Questions Annex M states “(for OWC sites)…the standard wind conditions model is no longer valid for use by the designer without modification” What are the required modifications to the SWC model that guide the designer to design a turbine for installation in OWC? What changes are required to existing wind models in standard to simulate normal wind conditions at sites with OWC? What changes are required to existing wind models in standard simulate extreme wind conditions at sites with OWC? How important is it that we incorporate these changes into the standard? (Perhaps siting in OWC is covered by the designer through design safety factors?)

15. Grant Application Grant: ARC Discovery Project Grant Working Title: Adaption and improvement of the wind models in the international wind turbine design standards to cater for an extended range of inflow conditions Chief Investigators: Murdoch University, Australia Partner Investigators: ForWind Centre of Wind Energy Research, University of Oldenburg, Germany Timeline: 2016 – 2019 inclusive Due date of application: March 19, 2015

16. Aim The Project aim is to improve the current turbulence models in the small wind turbine design standard to take into account the actual inflow conditions where the turbine is intended to be installed, by extending the range of inflow conditions to include OWC sites as well as SWC sites.

17. Methodology Turbulence models will be improved by first looking at corrections to existing model parameters that would make the models suitable for an extended range of inflow conditions e.g. that would make them suitable for OWC. In some cases changes to modelling approaches may be needed. It must be kept in mind that changes to approaches must be “usable” for designers Any changes should also be made out of necessity. Changes in modelling approach as well as adaptions/corrections will be ranked in order of importance based on their impact on turbine performance and safety.

18. Overview of Activities Activity 1. Insight into turbulence characteristics of OWC sites using existing data Activity 2. Measurements of loads on turbines in OWC Activity 3. Impact of changes to modelling normal wind conditions in the standard Activity 4. Impact of changes to modelling extreme wind conditions in the standard Activity 5. Synthesis of research Activity 6. Recommendations to IEC TC88 MT2 team in time for 4 th edition of IEC61400-2

19. Synergy with IEA Task 27 Synergy between Activity 1 and Working Package 2 of IEA Task 27 extension We would welcome any data from IEA Task 27 members for the OWC sites database Some synergies between Activity 2 and Working Package 3 of IEA Task 27 extension Activity 6: Results of research grant can be shared and discussed with IEA Task 27 throughout the research so that recommendations can be refined and passed on to IEC TC88 MT2 team in 2019-2020 for revision of standard.

20. Expected Outcomes Suggestions for corrections/adaptions to existing wind models in standards for OWC sites  List parameters that have the most impact to loads on turbine Suggestions for new wind modelling approaches  Trade-off between added complexity and importance for designers Characterisation of OWC sites into Design Classes Final recommendations for changes to existing wind models in IEC61400-2 for OWC sites

21. Letters of Support We welcome letters of support from IEA Task 27 members! Trudy and Ignacio have the letter template – please ask them for it Please mention in the letter any data and/or information you can share with us through your participation in the IEA Task 27 Letter can be from SWAT member or IEC CAC SWT committee member if that is more appropriate for you than IEA Understand that permission may need to be gained from your organisation Grant application due March 19, 2015 but need your letters well before then – please send before end of year!