INVESTIGATION ON AERODYNAMICS CHARACTERISTIC OF WIND TURBINE

BLADE S809 AIRFOIL UNDER VARIOUS WIND SPEED AND PITCH ANGLE

SITUATIONS USING CFD TECHNIQUE

Agung Bayu Kusumo, NIP 7907003-Z

NOMENCLATURE

C chord length of the airfoil

p pressure r radius of the airfoil

INTRODUCTION

Understanding aerodynamic characteristic of a specific wind turbine blade under various wind

speed and pitch angle situations are considerably important to evaluate the performance of the

wind power system. It is also needed to determine the best option of parameter of wind blade,

such as the type of the airfoil and pitch angle setting, to get better energy output of the system.

This report is purposed to investigate the aerodynamic behaviour of a particular wind turbine

blade (S809 airfoil) under various wind speed and pitch angle situations using CFD technique.

The flow field around the airfoil is modelled in 2D with the axis of airfoil perpendicular to the

direction of flow. Velocity at the inlet of the flow domain and pitch angle of the blade are

specified by the user. The aims of simulating this model are:

1. To learn the process of creating and exporting a mesh for quality CFD modelling.

2. To learn how to set boundary conditions and process a numerical model. 3. To explore the post-processing abilities of the CFD code to analyse numerical results.

4. To practice writing concise and well developed professional reports.

METHODOLOGY

• Geometry of the Airfoil and Flow Conditions

The simulation will be conducted in three different flow conditions over a two dimensional

airfoil in a channel of an infinite length. A flow domain is created surrounding the airfoil

(S809). The chord length of the airfoil (C) is set to be 0.728m. The upstream length is around

5 times the chord length of the airfoil, and downstream length is 10 times the chord length of

the airfoil. The width of the flow domain is 20 times the chord length of the airfoil. The 2-D

geometry of the simulation boundary can be seen in figure-1. The radius of the airfoil (r) is

1.343 m.

Figure-1. 2-D geometry of the square channel with a cylinder

C 20C

5C 10C

inlet

face

outlet

face airfoil

face

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Figure-2. The airfoil shape and parameter

The picture of the airfoil model in Ansys CFD Software can be seen in Figure-3 below.

Figure-3. The airfoil model in Ansys CFD software

As mentioned, aerodynamics characteristics under three different flow situations are investigated using CFD technique. Details of the three flow conditions are tabulated as follow:

Wind Speed

(m/s)

Rotational

Speed (rpm)

Pitch Angle

(degree)

Case 1 7 72 30

Case 2 7 72 18

Case 3 10 72 18

Table-1: Flow conditions case

Wind direction

Relative wind

Relative blade speed

Rotation Plane

Pitch Angle Pitch Angle Angle

of Attack

Radius of the arifoil: r=1.343m

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• Creating the mess and the boundary specifications

The next step is creating the mesh based on the geometry have been built in previous step. The mesh configuration can be observed in Table-2 below. From the table, it can be observed

that the mesh around the airfoil region is created higher in density compared to the other regions because the simulation is mainly intended to analyse the airflow behaviour in those

region (see figure-4 for detailed picture of the mesh).

Inflation

Inflation Option 1st Layer Thickness

First Layer Height 0.0017 m

Maximum Layers 12

Growth Rate 1.2 m

Element size 0.01 m

Table-2: The mesh configuration

Figure-4: The mesh of the airfoil

After creating the mesh, we need to determine the boundaries of the domain and their settings.

The airflow materials is air at 250C which flows in with a specific value of velocity and an

angle of attack to the aerofoil. The properties of the air is defined as below:

- Density (ρ) � 1.185��/3

- Temperature (T) = 250C

- Dynamic viscosity (µ) � 1.831 � 10�� kg/ms

For the fluid models turbulence, we choose k-epsilon with no heat transfer.

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As mentioned before, we will analyse 3 cases of flow conditions (see table-1) which resulting

different value of inlet air velocity. Table-3 shows the calculation for velocity in 3 dimensional (U,V,W) of the inlet air flow, with U component is parallel to the chord of the

aerofoil and the V component is normal to the chord. The turbulence option for inlet and global initialisation is medium (intensity=5%)

Case Wind

velocity (m/s)

Blade angular speed (rpm)

Blade speed (m/s)

Pitch angle

Relative Wind

Velocity (m/s)

Pitch angle +

Attack of angle

Attack of

Angle

Inlet Velocity

U V W

(m/s) (m/s) (m/s)

case 1 7 72 10.1260 30 12.310 34.656 4.656 12.269 0.999 0.000

case 2 7 72 10.1260 18 12.310 34.656 16.656 11.793 3.528 0.000

case 3 10 72 10.1260 18 14.231 44.641 26.641 12.721 6.381 0.000

Table-3: The calculation for inlet velocity in 3 dimensional direction

• Solving the Airflow Cases and Creating the Result To solve the simulation, we will use the CFX Solver module to calculate the velocity,

pressure profile, lift and drag force coefficient, pressure and friction coefficient profile. The result from 3 cases will be analysed to understand the aerodynamic behaviour of air flow

passing the airfoil with various velocity and angle of attack.

RESULTS

Flow Separation for all cases

Case-1 (no Flow Separation)

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Case-2

Case-3

Figure-5: Flow separation in 3 cases

In case 1, it could be seen that there is no flow separation, the air is always attached to the aerofoil. For case 2, on the lower surface of the airfoil, flow separation is happened near the tip

point, however after that point, the airflow is attached to the surface. On the upper surface, the flow is separated in 2 points: near the tip point and in the end of the airfoil (roughly at 0.7 m

from the tip point). For case 3, the separation point of the airflow is nearly the same position with

case 2, both on the upper and lower surface. However, if we see it in more detail, it can be seen

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that the 2nd separation point in upper surface (in the end of the airfoil) of case 3 is started earlier

than in case 2, and in lower surface it is started later than case 2.

Flow Streamlines Plot

Case 1

Case 2

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Case 3

Figure-6: Flow streamlines plot in 3 cases

Figure-6 shows streamlines plot result for all 3 cases, with 2000 streamlines max points setting.

As discussed in previous chapter of this report, in case 1, there is no flow separation and the

flows in the upper surface have higher velocity than in the lower surface. This is because the

upper surface is curved more than the lower surface, which means the flow in upper surface take longer distance than in lower surface for the same time of flow and produce the faster flow. The

airfoil is also hit by the wind with an angle of attack, which increase the velocity on the upper surface. The difference in the flow pattern between the two surfaces will result a lift force to

move the airfoil or the wind turbine blade.

In case 2, the flow starts to separate near the tip point of the upper surface and reduce to free-stream velocity. After that point, the flows on the upper surface are only turbulences. There is

still difference in the velocity profile between the upper and the lower surface and hence there is still pressure difference and lift force. However, the turbulence after the separation point causes a

pressure drag on the airfoil and reduce the lift force on the wind blade significantly. In other words, the force of the wind in case 2 is much less compared to case 1 though the wind speed is

the same in both case, or even stalled.

Similar to the case 2, the streamlines in case 3 once hit the airfoil immediately separate from the

upper surface. There are only turbulences and eddy flows near the upper surface of the airfoil

completely. Since the velocity on the upper surface is zero or almost zero, the pressure on the

upper surface is very high and probably there is no lift force on the airfoil anymore in case 3 –

the wind turbine is stalled in this case and likely will not produce energy although the wind

speed now is higher than in case 2 and case 1.

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Pressure Coefficient Profiles for all cases

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Figure-7: Pressure coefficient profiles for 3 cases

The figure-7 above shows the pressure coefficient along the poly-line of the airfoil surface in all

cases. In all cases, the upper line is the value of the lower surface and the lower line is for the upper surface.

In case 1, when the wind hits the tip point, it stops produce the highest pressure. After that, the

velocity increases and the pressure drops. When the air passes the shoulder of the airfoil, the speed decreases towards to trailing edge and at the same time the pressure increases again. Those

phenomena is happened in both surface, upper and lower, and creating a relatively constant pressure coefficient difference between those 2 surfaces after the shoulder of the airfoil.

In case 2 and 3, after the 1st separation point (near the tip point), as turbulences are happened

here, the pressure and velocity in upper surface are relatively constant, and results a static pressure coefficient in those regions (pressure coefficient = pressure/dynamic force). However,

the difference pressure between upper and lower surface near the tip point of airfoil (early stage

of airflow) in case 3 are higher than in case 2, due to higher wind speed which produces more

pressure drop in upper surface in those regions.

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Friction Coefficient (wall shear stress) Profiles all cases

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Figure-8: Friction coefficient profiles for 3 cases

Figure-8 above shows the friction coefficient along the poly-line of the airfoil surface in all cases. In case 1, the graph never hit the horizontal axis which means that the friction coefficient

never has 0 value along the surface and there is no flow separation. However, in case 2, the graph line of the upper surface reaches 0 value at early stage (around x = 0.02 m) which means

that there is flow separation after this point. The separation is also happened near the end of the upper surface of the airfoil (x = 0.7 m). The graph also confirms the observation on the vector

and streamline plot figures of case 2 whereas flow in upper surface starts to separate in 2 points: early stage and the end of the airfoil the upper surface.

Regarding the figure of case 3, the friction coefficient of the upper surface is nearly zero all the

time and this means that the air velocity around the upper surface is very low. Similar to the vector and the streamline result, the flow separation starts very early near the tip point and on the

upper surface, there is only turbulence and eddy flows.

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CONCLUSION

Using the Ansys CFD software, we can build a model of wind turbine airfoil, creating the mesh,

setting the boundary paramater, and conducting a simulation on the airflow, which are very

helpful in evaluating the aerodynamic characteristic of a specific wind turbine blade under

various wind speed and pitch angle situations.

From the simulation, it can be concluded generally that the airflow behaviour around the airfoil

depends mainly on the angle of attack applied on the airfoil and also the wind velocity. In the

simulation using airfoil S809 with different attack of angle, when the angle of attack is higher

than the optimum value (case 2 and case 3 in this report), the flow will likely be separated and cause the pressure drag and resulting on the reduction of useful force on the airfoil, or even

stalled. According to Jain (2010), each airfoil design has an optimum angle of attack where the lift force is highest and the drag force is lowest.

The simulation also shows that there are correlation between the rotating speed of the blade and

the wind velocity attach to the blade, that should be considered when design a wind power system. There is an optimum ratio of the speed of rotating blade tip to the speed of the free

stream wind (Kidwind Project, 2011).

REFERENCES

Jain, P 2010, Wind Energy Engineering: a Practical Guide to Wind Energy Engineering and

Design, McGraw-Hill, New York

Kinwind Project, 2011, Wind Energy Technology, New York, US, viewed 25 May 2012, <http://learn.kidwind.org/sites/default/files/windturbinetechnology.ppt>

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Computer Guide for ANSYS CFD Simulation

Learning Objectives:

1. Learn to create 3D model geometry in ANSYS DesignModeler

2. Learn to generate an inflated boundary layer mesh around a 3D wall in ANSYS Meshing

3. Learn to generate a 3D grid/mesh with an acceptable quality using ANSYS Meshing

4. Learn to perform the CFD analysis for external turbulent flows using ANSYS CFX,

including assigning model property parameters, selecting the appropriate CFD solver, setting up the boundary and initial conditions et cetera.

5. Learn to write expressions in ANSYS CFX Post to calculate the required CFD data, for example the lift and drag coefficients

6. Learn to utilise the various visualisation tools in ANSYS CFX Post to present the

turbulent flow characteristics around a typical blade of a wind turbine

Procedure

Start ANSYS Workbench:

(1) From Windows Start menu, select ANSYS 13.0 > Workbench

(2) When Workbench opens, select File > Save, then save the project as Assign2 into the

folder under the D:\ drive on the computer, e.g. D:\S3000000\AssignB

(3) In the Analysis Systems, drag and drop a Fluid Flow (CFX) system into the window of

Project Schematic.

Stage 1 3D Model Geometry Creation

� Double click on the Geometry cell to launch DesignModeler. Select meter as the unit of

measure for the session when prompted.

� Under Tree Outline, click on XY Plane, then click on New Sketch icon on the top menu. This is to insert Sketch1 in XY Plane.

� Click on Look At icon to orient the sketch plane in the normal direction.

� Click on the Sketching tab to switch to the sketch mode.

We first create the computational fluid domain:

� In Sketching: click on Draw > Rectangle, then draw a 2D rectangle box as shown in Figure 1. Make sure that the bottom line of the box overlaps with the x-axis.

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� In Sketching: Click on Dimensions, then set up the dimension parameters as shown in

Figure 1 (note: your dimension numbers may be different).

Figure 1 The 2D rectangle box of the computational domain with dimension parameters

� In the Details for Dimension window, modify the values of the dimension parameters as

shown in Table 1.

Table 1 The 2D rectangle dimension parameter values

H1 H2 V3 and V4

5 10 10

� Click on Extrude icon on the top menu bar above the model viewer window to bring up the Details View for the Extrude 3D operation (the view will consequently switch

to the Modeling view).

� Edit the Details of Extrude 1 page: (1) select Add Material for Operation; (2) select

Reversed for Direction; (3) set FD1 Depth to 0.1. Click on icon to complete the Extrude operation.

We then create a plane parallel with the XY plane:

� Ensure the XY Plane is active, and then click on the New Plane icon to insert a plane that parallel with the XY Plane.

� Click on icon on the top menu bar to complete the operation.

We finally create the car body (refer to Figure 3 below)

� Highlight the newly created Plane 4 by clicking on Plane 4 under the Tree Outline

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� Click on icon on the Top bar menu

� In the Details of Point 1,

(1) set Definition to be “From Coordinates File”;

(2) At Coordinate File, click on to browse;

(3) Browse necessary to locate the file “upper-profile”, and Open it;

(4) Click on icon to complete the operation.

(5) Zoom in to get a detailed view of the points, as shown in Figure 3.

Figure 3 Import the airfoil upper profile data points into

DesignModerler

� Repeat above steps to import the lower profile

� Click on Sketching tab > Draw > Spline

� In the Graphics window, click on the upper points sequentially from left to the right,

then RMB click to bring up a menu, and select Open End to complete the spline as the top profile of the airfoil.

� Repeat the same procedure to create a spline though the lower points along the

bottom for the base profile of the airfoil.

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Figure 4 Sketch the car profile using Spline and Line operations

� Click on the icon on the top menu bar to bring up the Details View for

the Extrude 3D operation.

� In the Details of Exdrude2, as

shown in Figure 5,

(1) select Cut Material for

Operation;

(2) select Reverse for Direction,

and

(3) type 0.1 for FD 1 Depth (>0).

� Click on icon on the top menu bar to complete the Extrude

operation.

Figure 5 Details of Extrude2

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Figure 7 The completed 3D airfoil model geometry, with the names of boundary

faces that enclose the fluid domain.

� Create Named Selections for the boundaries of the flow domain:

(1) Ensure that the Face Selection Filter is active, click on the inlet face to highlight the face;

(2) RMB > Create Named Selection ;

(3) In the Details View panel, next to the Geometry cell, click on Apply;

(4) Click on the Named Selection cell, and change the name to inlet;

(5) Then click on the icon on the top menu bar to complete the

creation.

� Repeat the same procedure to create the Named Selections for outlet, symmplane,

sky, ground and airfoil.

� Save the Project and exit DesignModeler to return to the Workbench and ready for

Meshing.

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Stage 2 Create the Mesh

� In the Workbench, RMB on the Geometry cell.

(1) In the pop up menu select Properties.

(2) Ensure that the Named Selection cell is ticked ON.

(3) Delete the value: NS for the Named Selection Key.

This is to ensure that the Named Selections are transferred to the next operation.

� Double click on Mesh cell of the Fluid Flow (CFX) system block to bring up ANSYS

Mesh.

� Click on Mesh under the Outline view to bring up the Details of Mesh panel, set the Physics Preference to CFD, Solver Reference: CFX.

� Expand Sizing and set the Relevance Centre to Fine.

Insert an Inflation to create the boundary layers on the faces of the car body

� RMB on Mesh under the Outline view, select Insert > Inflation (see Figure 8(a)).

� Note that a panel named Details of “Inflation” is now appeared on the left bottom corner

of the screen.

� Note that the cell next to the Geometry cell under Scope in the Details of “Inflation” is

highlighted. This means that the Geometry requires definition.

� Click on the highlighted cell next to Geometry to bring up Apply and Cancel buttons, and

then move the curser to the Graphics window to select the fluid domain by clicking on it, then click on the Apply button.

� Under Definition, click on the highlighted cell next to Boundary to bring up Apply and Cancel buttons, and then move the curser to the Graphics window to select the2 faces of

the airfoil by Ctrl_clicking on them, then click on the Apply button.

� Set Inflation Options to First Layer Thickness.

� Type 0.0017 m for First Layer Height

Note: Refer to Appendix 1 for the calculation of the 1st layer height.

� Type 12 for Maximum Layers.

� Keep the default value1.2 for Growth Rate.

Insert a sizing function

� Ensure that Face selection on the top menu bar is active.

� RMB on Mesh under the Outline view, select Insert > Sizing (see Figure 8(b))

� Note that a panel named Details of “Face Sizing” is now appeared on the left bottom corner of the screen

� In the Details of “Face Sizing” panel, select Geometry Selection for Scoping Method.

� Select the 2 faces for the airfoil.

� Enter 0.01 for Element Size.

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(a) (b)

Figure 8 Mesh settings for (a) Inflation; (b) Sizing

Preview the inflated boundary layer mesh

� RMB on Mesh under Outline view, select Preview > Inflation.

� Wait for a couple of seconds for the boundary layer mesh to appear (see Figure 9).

Figure 9 Preview inflation

Generate the body mesh

� In Outline view, RMB click on Mesh, select Generate Mesh.

� Wait for the body mesh to be completed (see Figure 10).

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(a) (b)

Figure 10 3D body mesh (a) over the computational domain; and (b) around the airfoil.

� File > Save Project

� File > Close Meshing to exit ANSYS Mesh and return to the Workbench Project page.

� RMB click on the Mesh cell > Update. Wait until a green tick appears for the Mesh cell.

Stage 3 Specify boundary types, materials, fluid flow models and initial values

Note: Though the flow phenomenon in this assignment is, by nature, transient, we are seeking steady state solutions. This is because the main objective of the assignment is

to obtain the drag coefficient, for which the steady-state turbulence modelling in most CFD software can provide good solutions with much lower computational cost.

� In Workbench, double click on Setup cell to open CFX Pre

� Change the default domain name to a meaningful name. RMB on Default Domain in

the Outline tree and select Rename, then change Default Domain to Airflow.

� Under Flow Analysis 1, double click on Airflow to open the panel named Details of

Airflow in Flow Analysis 1, which contains three tabs named Basic Settings, Fluid

Models and Initialisation. Go through each of them as follows:

- In the Basic Settings, note the default material is Air at 25 C, which is the material we are using in this analysis. Leave other fields to the default settings.

- Click on the Fluid Models tab, select None for Heat transfer; select K-Epsilon for Turbulence. Leave other fields to the default settings. Accept all the settings

by clicking on Apply.

- Click on Initialisation tab, tick the Domain Initialisation to view the default

settings. Accept these settings by clicking on Apply, then OK to exit the Details

panel.

� Under Materials, double click on Air at 25 C to open the panel named Details of Air

at 25 C. Click on Material Properties tab, find the values of Density under Equation

of State, and Dynamic Viscosity under Transport Properties, as shown below. These values will be used to calculate: (1) Inlet velocity; (2) inlet turbulent kinetic

energy k; (3) inlet turbulent dissipation rate ε; (4) lift and drag coefficients.

Density: ρ = 1.185 [kg m^-3]; Dynamic Viscosity: µ = 1.831E-05 [kg m^-1 s^-

1].

� Click on Close to exit the panel.

� Specify the boundary conditions of the flow domain

(1) RMB on Airflow > Insert > Boundary, type: inlet in the pup up window, then

OK

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This brings up Details of inlet in Airflow in Flow Analysis 1

(a) Select Inlet for Boundary Type; select inlet for Location.

(b) Click on Boundary Details tab, see Figure 11 below, enter 10.0 for

Normal Speed

(c) Under Turbulence, select Medium (Intensity = 5%) for Option.

(d) Click Apply to finalise the setting, then OK to exit the Details of

Inlet panel.

Figure 11 The inlet condition settings

(2) RMB on Airflow > Insert > Boundary, type: farfield in the pup up window, then OK

This brings up Details of farfield in Airflow in Flow Analysis 1

(a) Select Opening for Boundary Type; select sky, ground and outlet for

Location, refer to Figure 13(a).

(b) Click on Boundary Details tab. In Mass and Momentum cell, select Open Pres. and Dirn for Option, and enter 0 for Relative pressure. In

Turbulence cell select Zero Gradient for Option. Keep the default Subsonic for Option in the Flow Regime cell (see Figure 13(b)).

(3) Click Apply to finalise the settings, then OK to exit the Details panel.

(4) Repeat the same procedure to define the remaining boundaries of the fluid

domain named Airflow. Refer to the following Table 1 for the corresponding

settings.

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(a) (b)

Figure 13 The sky and backplane condition settings

Table 1: Some boundary settings of the Airflow domain

Name Boundary Type Location Boundary Details

outlet Outlet Sky, outlet and

Ground

Average Static Pressure

= 0 Pa

symm Symmetry symmplane

Airfoil Wall Airfoil No Slip Wall

� Under Flow Analysis 1, double click on Solver Control to open the panel named:

Details of Solver Control in Flow Analysis 1.

Accept the default Basic Settings by clicking on Apply (as shown in Figure 14), then OK

to exit the Details of Solver Control panel.

� Under Flow Analysis 1, double click on Output Control to open the panel named:

Details of Output Control in Flow Analysis 1.

Click on the Monitor tab, then refer to Figure 15, (1) expand Monitor Points and

Expressions; (2) add a monitor point: DragCoef; (3) select Expression for Option; (4) type the following drag coefficient expression for the Expression Value:

force_x()@Airfoil *2/1.185[kg m^-3]/27.78[m s^-1]^2/0.1[m]/0.1525[m]

� Click on Apply, to save the settings, then OK to exit the Details of Output panel.

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Figure 14 Solver Control settings Figure 15 output Control settings

� Click on the global Initialization icon on the top menu bar to open the panel

named Details of Global Initialization in Flow Analysis 1. Refer to Figure 16:

(a) Tick the Velocity Scale to open the cell, and then

(b) Enter 10.0 for Value. Leave other settings as the default.

(c) Click on Apply to save the settings, then OK to exit the panel.

AU

FC

d

d⋅

=

∞

2

2

1 ρ

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Figure 16 Global Initialization settings

� Exit CFX Pre and return to the Workbench Project page.

Stage 4 Solving for a solution

� Save the Workbench Project

� In Workbench double-click Solution cell to launch the CFX Solver Manager

� The CFX Solver Manager will start with the simulation ready to run.

� Click on Start Run to begin the solution process.

� The solution process will stop either the residual target is met or the Max. Iteration

number is reached, whichever comes first. Steadily decreased residual curves indicate a convergent solution. It will take approximately 8 mins to obtain the

solution)

� Exit CFX Solver Manager to return to Workbench Project page.

� File > Save.

� Click File > Exit … to quit ANSYS Workbench.

Appendix 1

In ANSYS CFX, hybrid methods are provided for turbulence models (automatic near-wall

treatment) involving boundary layer simulation. The value of y+ < 300 is recommended.

Furthermore, a minimum 10 grid nodes inside the boundary layers are also required.

To save time in meshing, it is useful to estimate the distance between the wall and the first grid

node, y1. ANSYS CFX recommends using the following formula to estimate the y1 value based

on the desired y+ value:

14/13

1 Re74−+

⋅= LyLy

where L = 1 is the length scale (i.e. length of the car); and

LRe - is the Reynolds number based on the length scale.

Based on the recommendations of ANSYS CFX for the max y+ value, in this guide I took a third

of the maximum y+ value, i.e. y

+ =100, to estimate the 1

st inflation layer height y1 as:

y1 = 0.0017

for the inflation settings.

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Stage 5: Post-processing the results in ANSYS CFD-Post

� In the Workbench make sure that the cells 1 – 5 have a green tick.

� In Workbench Project Schematic page, double-click Results cell to launch

CFX Post.

� Create the y+ contour plot around the wind turbine (refer to Figure 1):

- Click on the Contour icon from the top menu bar.

- In the pup up window, enter a name, e.g. YplusBlade, then OK.

- In the Details of YplusBlade, set the Locations to Airfoil

- Set the Variable to Yplus (click on icon to open the Variable Selector and

scroll down to find the Yplus, then click OK).

- Set the Range to Local.

- Click Apply to generate the y+ contour plot.

- Zoom in to observe y+ variations around the wind turbine surface (see Figure

2). Note that the maximum y+ value is less than 155, which satisfies the basic

requirement for CFD turbulence models.

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Figure 1 Create the y+ contour graph on the wind turbine

Figure 2 The y+ contour plot on the wind turbine surfaces

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� Create an expression that calculates the drag force on the wind turbine:

- Click on the Expressions tab to open the Expression Tree (refer to Figure 3a).

- RMB on Expressions, select New. In the pup up window, enter a name, e.g.

DragForce, click on OK to open the panel named Details of DragForce

- In the Details of DragForce panel (refer to Figure 3b):

(1) RMB > Functions > CFD Post > force_x

(2) RMB >Locations > Airfoil

(3) You will see that the following expression has been entered for the Definition of DragForce (see Figure 3c)

force_x()@Airfoil

- Click Apply to evaluate the expression, and note an answer of 0.072611 [N] is

given for the Value of the drag force (see Figure 3c).

(a)

(b) (c)

Figure 3 Create an expression: (a) inset a new expression, (b) define the expression

(c) calculate the value of the expression.

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� Repeat the same procedure to insert and define the following expressions:

DynamicForce, LiftForce, FrontalArea, HorizontalArea, DragCoef and LiftCoef,

PressCoef and FrictioCoef, as shown in Figure 4 below:

Figure 4 List of the user defined expressions (within the red rectangle)

� Create a pressure contour plot on the symmetry plane:

- Click on the Contour icon from the top menu bar.

- In the pup up window, enter a name, e.g. PressOnSymm, then OK.

- In the Details of PressOnSymm, set the Locations to symm

- Set the Variable to Pressure,

- Click Apply to generate the pressure contour plot

- Re-orientate the model view and zoom in to observe pressure variations around

the wind turbine (see Figure 5).

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Figure 5 Pressure contour on the symmetry plane

� Create a velocity vector plot on the symmetry plane:

- Click on the Vector icon from the top menu bar.

- In the pup up window, enter a name, e.g. VectorVelSymm, then OK.

- In Details of VectorVelSymm, set the Locations to symm

- Set the Variable to Velocity.

- Click on Symbol tab and set the Symbol Size to be 0.1.

- Click Apply to generate the velocity vector plot on the symmetry plane.

- Zoom in to observe the flow phenomenon surrounding the wind turbine (see

Figure 6).

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Figure 6 Velocity vectors at the vicinity of the wind

turbine

� Create a 3D streamline plot:

- Click on the Streamline icon from the top menu bar.

- In the pup up window, click OK to keep the default name: Streamline 1.

- In Details of Streamline 1, set the Locations to Airflow, Airfoil and inlet

- Set the Maximum Points to Reduction.

- Enter 2000 for Max Points

- Click Apply to generate the Streamline plot.

- Zoom in to the wind turbine to observe the 3D flow phenomenon around the wind turbine surfaces (see Figure 7 for an example)

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Figure 7 The 3D Streamline plot

� Create a turbulence kinetic energy contour plot on the symmetry plane:

- Click on the Contour icon from the top menu bar.

- In the pup up window, enter a name, e.g. TKEonSymm, then OK.

- In the Details of TKEonSymm, set the Locations to symm

- Set the Variable to Turbulence Kinetic Energy,

- Set the Range to Local

- Increase the # of Contours to 30.

- Click Apply to generate the TKE contour plot

- Re-orientate the model view and zoom in to observe TKE variations

particularly in the wake (see Figure 8).

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Figure 8 Turbulence kinetic energy contour on the symmetry plane

� Create a series turbulence kinetic energy contour plots in the wake of the wind

turbine:

- Click on the Location icon from the top menu bar to expand the

list, then select Plane.

- In the pup up window, enter a name, e.g. WakePlane 1, then OK (see Figure 9(a)

- In the Details of WakePlane 1, set the Method to YZ Plane (see Figure 9(b)).

- Set the X to 1.10 [m], then click OK to create the plane

- Click on the Contour icon from the top menu bar.

- In the pup up window, enter a name, e.g. TKEonWakePlane1, then OK.

- In the Details of TKEonWakePlane1, set the Locations to WakePlane 1

- Set the Variable to Turbulence Kinetic Energy, and the Range to User

Specified, then entre a fixed value for the maximum e.g. 25 (see Figure 10)

- Click Apply to generate the TKE contour plot on the WakePlane 1.

- Repeat the procedure to create a series TKE plots in the wake. Plot them together as shown in Figure 10

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(a)

(b)

Figure 9 Create a YZ plane in the wake of the wind turbine.

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Figure 10 Turbulence kinetic energy on the wind turbine and in the wake

� Create chart showing the pressure coefficient over the whole wind turbine surface

- Click on the Location icon from the top menu bar to expand the

list, then select Plane.

- In the pup up window, enter a name, e.g. MidPlane 1, then OK

- In the Details of MidPlane 1, set the Method to XY Plane.

- Set the Z to -0.05 [m], then click OK to create the plane.

- Click on the Location icon from the top menu bar to expand the

list, then select Polyline.

- In the pup up window, enter a name, e.g. Polyline 1, then OK

- In the Details of Polyline 1, set the Method to Boundary Intersection

- Set the Boundary List to Airfoil, and Intersect with MidPlane then click OK to

create the Polyline

Figure 11 Create a Polyline by boundary intersection method

- Click on the Variables tab to open the Variables Tree (refer to Figure 12).

- RMB on Variables, select New. In the pup up window, enter a name, e.g.

PressCoef, set the Method to Expression, and set the Expression to

PressureCoef click on Apply to create a user variable.

- Repeat the procedure to create a series TKE plots in the wake

Figure 11 Create a user variables

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- Click on the Contour icon from the top menu bar.

- In the pup up window, enter a name, e.g. PressCoef Chart, then OK.

- In the Details of PressCoef Chart, under the Data Series Tab, set the Locations

to Polyline 1

- Under the X axis Tab, set the Variable to x

- Under the Y axis Tab, set the Variable to PressCoef

- Click Apply to generate the Chart for Pressure Coefficient.

- Repeat the procedure to create another Chart for Friction Coefficient

� Save the project and exit CFD Post to return to Workbench Project page.

Create other flow conditions

� In Workbench, duplicate the system block for the first case. Modify the name to a

meaningful name, e.g. Case 2

� Double click on the Geometry cell of the copied system block to launch the

DesignModeler.

� Double click on the Set Up cell to launch CFX Pre

� Revise the inlet boundary condition and specify the velocity according the flow condition (Be careful in calculating the relative wind velocity and angle of attack!!).

� Return to Workbench. Solve for the solution

� In CFX Post, observe the phenomenon and calculate Cd.