1. Fluid Dynamic Analysis of Wind Turbine Wakes
P M V Subbarao
Professor
Mechanical Engineering Department
Comprehensive Diagnosis of WT Wakes

2. Measurement of Wind Turbine Wakes
Field measurements of wake flows produced by real wind turbines are also necessary in order to avoid issues connected to uncertainties associated with turbulence and turbine simulations.
Very large measurement volumes involved is a great challenge for field measurements.
The non-stationary and nonhomogeneous nature of atmospheric boundary layer flows created another challenge.
However, field measurements are extremely important for an accurate prediction of wind turbine power production.

3. Remote Sensing Instruments for Wind Filed Measurement
Remote sensing systems measure the wind conditions from the ground up to a height of 200 meters.
There are two types of remote sensing systems on the market: SoDAR (Sonic Detection and Ranging) and LiDAR (Light Detection and Ranging).
SoDAR instruments measure the wind conditions by means of sound.
LiDAR instruments use light to measure the wind characteristics.
LiDAR systems can also be installed in top of a wind turbine to measure wind conditions in front and behind the turbine.

4. LiDAR Measurements & Validation of Jensen MKodel
V = 7m/s
Measured Profile
Jensen Model

5. Need for An Accurate Wake Model
V = 11m/s
V = 17m/s

6. Need for Wake Models : to Minimize Power Loss
It is very important to determine the optimum spacing between neighbouring turbines, when developing wind farms.
A wake model must predict average wind velocity more accurately select this optimal spacing.
An under prediction of wake velocity leads higher spacing and lost of capacity.

7. Need for Wake Models : Safety of WTs
Fatigue is in particular a problem for the turbine in other than first row.
The downstream turbine potentially facing risks, being hit by the tip vortices from the turbines located at the front of the wind farm.
It is important to determine the lifetime of the tip vortices and the parameters that govern their breakdown into small-scale turbulence.
This location is called as Fully Turbulent wake zone and no danger of high fatigue.

8. Computations of Signatures left by Wind Turbine on Downstream Wind & Recovery
The flow field in immediate near wake is approximated by solving the filtered three-dimensional incompressible Navier–Stokes equations.
Define filtered velocity as:
Continuity equation
Navier–Stokes equations
Two body forces (fWT and fturb) are explicitly introduced in the simulations to model the effect of the wind turbine and atmospheric turbulence.

9. Computational Study of Nearwake
Distribution of axial velocity
Evolution of Vorticity in wake.

10. Computational Study of Mid & Far Wakes

11. Computational Study of Wakes of Different WTs
CT=0.85
CT=0.9
x=8D
x=6D
x=2D

12. Vorticity Generation & Evolution in the Wake of A WT
A WT introduces helical tip vortices into the wake.
The amplitude of Voriticity introduced by rotor is identified as A0

13. Evolution of Tip Vortices & Instability
Evolution analysis shows that the amplitude of helical vortices increases in the streamwise direction.
is the dimensional growth rate of an instability R

14. Instability of Wake
A growing vorticity is a measure of infection in laminar wake.
A growing infection leads to instability, transition and turbulence.
Stability Analysis shows that helical tip vortices are inherently unstable and that they will break down at some distance behind the wind turbine.
The breakdown position of tip vortices is assumed to take place where simple logarithmic amplification reaches its maximum. R

15. Evolution of Turbulence Level

16. Breakdown Position
The breakdown position of tip vortices is assumed to take place where simple logarithmic amplification reaches its maximum.
The constants
This expression gives a measure of the position where the helical tip vortices break down.
This position is a function of the intensity of the ambient turbulence level and of parameters depending uniquely on the turbine’s operational characteristics.