Wind Speed, v _{T}m/s Newton Pulling Force, F |
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*) The Persian windmill has been reinvented many times, and several inventors have patented many, rather useless variations on the theme with parachutes, yawing wind shields etc. Drag based windmills are very inefficient, and really only of interest for educational or hobby purposes. (With all due respect, think before you act - there are thousands of worthless wind turbine patents around. Contrary to popular belief patenting guarantees neither efficiency, usefulness nor workability).
You can easily change the values in the input boxes to the left up or down by one unit by clicking the arrows, or you may enter a number. If you enter a number, press the tab key or click outside the box to tell the programme to calculate the result. If you are a non-English speaker, remember to use decimal points, and not commas when entering decimal fractions in the boxes. The calculator will not accept unreasonable quantities. You can read the input and output data for each experiment in the Test Results to the right. Each variable is explained in the section How does the Calculator Work? at the bottom of this page.You can sort the test results by any column by clicking on the variable name at the top of the column. You can delete a row by clicking on the row number (in the first column). You can plot any two variables against one another by selecting the variables from the popup menus at the top of the list and then clicking the "Plot" button. Remember to sort your data by the variable on the horizontal (x-) axis before you plot. Finally, you can delete all of the test results by clicking the "Delete" button.
How does the Calculator Work?We need the same variables which we used for our drag machine: v Now, let us try a more realistic assumption than in the simplified analysis, i.e. that we subtract the drag on the sails behind the wall from the output of the machine. In order to find the propulsion speed of the Persian windmill, we solve this equation for v 0 = c The second half of the expression to the right of the equal sign (shown in red) is the undesirable drag from the rotor sails. The formula is simply the definition of the drag force explained previously. We subtract the propulsion speed from the wind speed (v The equation for the Anemometer, which we can solve for v 0 = c The second half of the expression to the right (shown in dark red) is the undesirable drag force from the anemometer cups moving upwind. The drag coefficients c *) = The figure is from Gash (1996). Frank M. White (1999) has 1.18-1.2, see the bibliography. |
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