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Table of Contents

Introduction

At each blade station (MissingReference Bladed geometry), the aerodynamic characteristics of the blade section must be defined so that lift and drag coefficients can be calculated. Pitch moment coefficients may also be calculated if desired.

Aerodynamic characteristics are defined empirically, by means of lookup tables of lift, drag and pitch moment coefficients tabulated against angle of attack. One such lookup table is referred to here as an aerofoil dataset (see aerofoil data set MissingReference).

A number of aerofoil datasets may be combined to define one aerofoil section referred to as either Normal Aerofoil Sections or in cases where Ailerons are used Aileron Sections.

Aerofoil Datasets

An aerofoil dataset is a lookup table of lift coefficient, drag coefficient and pitching moment coefficient (optionally) as a function of angle of attack. Such tables are usually obtained empirically for particular aerofoil sections at particular Reynolds numbers, for example in wind tunnel tests.

As well as the table of aerodynamic coefficients, each aerofoil dataset is characterised by a set of general data giving the characteristics of the aerofoil section to which it applies, namely:

  • Name: this is an identifier for referencing the dataset within Bladed.

  • Comments: to identify the source of the data and the full name of the aerofoil section.

  • Thickness to chord ratio for the aerofoil section.

  • Reynolds number for which the dataset applies.

  • Pitching moment centre: the point about which the aerodynamic pitching moment is defined, as a percentage of the chord backwards from the leading edge.

  • Deployment angle: This is used in the case of Aileron Sections.

  • Include pitching moment: set to No if the pitching moment data is not available. If any dataset used on the turbine does not have pitching moment data defined, then no pitching moments will be calculated for that turbine.

During simulation Bladed uses linear interpolation to lookup the lift, drag and pitching moment coefficient based on angle of attack. Nearest neighbour values in cases where the angle of attack is exceeded. The user should supply data for angles of attack ranging from -180° up to 180° as the full range will be needed for most calculations.

Importing a Dataset

Bladed maintains a database of aerofoil datasets. Click the aerofoil icon on the toolbar for access to the database. Aerofoil datasets may be imported from an ASCII file of aerofoil data by clicking the Import button. A standard file selection screen appears. Select the drive, directory and file where the data is stored. If the identifier specified in the file duplicates one which exists in the database, you will be prompted for a different identifier to be used.

Adding a New Dataset Manually or via the Clipboard

Data may be provided in a separate file format or a spreadsheet. Bladed allows a user to copy and paste the data into Bladed as follows. Click the New button, fill in the General data to characterise the aerofoil dataset, add comments as required, and give the dataset an appropriate identifier in the Name box.

The aerofoil data itself may be pasted in from the clipboard by clicking the Paste button. The data should be copied from a spreadsheet or tab-delimited file, and should consist of three, or four columns: angle of attack in degrees, Lift coefficient, Drag coefficient, and Pitching moment coefficients.

Alternatively, type the data for each angle of attack in the coefficients box, clicking Add after each one.

Then press Save. New datasets may also be created by editing existing ones, as described below.

Editing an Existing Dataset

Click the Load button to list the names of all currently available datasets. Clicking one of these names displays the main characteristics of that dataset, which can then be selected by double-clicking or pressing OK. The data for that aerofoil may then be edited. To add a new angle of attack, enter it in the coefficients window and press Add. To modify a datapoint, click on it and press Edit to make it appear in the coefficients window where it can be edited. To remove a datapoint, click on it and press Delete. When editing is finished, click the Save button. To save it as a new aerofoil dataset, modify the General data as appropriate, change or insert any comments in the Comments box, change the Name as required, and then click Save.

The Copy button may also be used to transfer aerofoil data via the clipboard to another application.

Removing a Dataset

Click the Delete dataset button.

Viewing Aerofoil Data Graphically

Select the view required, change the range of angles of attack if desired, and click View Data. The graph may be printed or saved as a Metafile or copied to the clipboard for subsequent inclusion in a report.

Normal Aerofoil Sections

A normal aerofoil section should be defined for every blade station which is either:

  • Fixed (see Blade geometry MissingReference) or

  • Moving (see Blade geometry MissingReference) and pitchable.

Up to 6 separate aerofoil datasets may be provided to define a normal aerofoil section. Amongst the 6 dataset up to three different Reynolds numbers can be input. Interpolation between these datasets will occur depending on the Reynolds number during the calculation. For each of the fixed Reynolds numbers, aerofoil datasets may be provided for two different thickness/chord ratios, one either side of the actual thickness/chord ratio for the blade station in question. In this case, a further interpolation will be carried out to give aerodynamic coefficients appropriate to the actual thickness/chord ratio. Linear interpolation is used and nearest neighbour is used

A number of different normal aerofoil sections may be defined, for use at different blade stations.

Normal aerofoil section editing

On the Blade properties window, click on the foil section to be defined. A drop-down list allows an already-defined section to be selected, or select Define\... to open the Define Aerofoil Sections window. This allows the characteristics of already-defined sections to be viewed or edited, or new ones created by clicking New.

To view or edit an existing aerofoil section go to the Define aerofoil Sections window, select the section number required.

To set up a new aerofoil section go to the Define aerofoil Sections window, press New to start a new foil section.

To edit an aerofoil section:

Up to six aerofoil Dataset names may be entered in the boxes provided. Click on any white or red box to open the Aerofoil Dataset Selection window. This presents a list of all suitable datasets from the database (they must be in ascending order of Reynolds number and thickness). If desired, the selection criteria may be modified to further restrict the list displayed. Select a dataset from the list and click OK.

If fewer than six boxes are required, the boxes used must form a rectangular pattern so that the interpolation scheme is fully defined. The top left hand box must always be used.

To remove an aerofoil section:

There is no need to delete an aerofoil section if it is no longer needed at any blade station. The sections which are still in use will be renumbered when the Blade Properties window is closed, starting from 1. The aerofoil datasets themselves of course remain in the database.

Last updated 05-12-2024