The knowledge of the mean wind speed at a certain altitude (and its statistical properties) is essential to calculate the aerodynamic forces acting on the aerostat and in particular to determine the forces along the mooring cable.
The wind speed data described in this section were provided by the Natural Environment Research Council (NERC), from the Mesosphere-Stratosphere-Troposphere (MST) Radar station located at Capel Dewi (52.42°N, 4.01°W), near Aberystwyth in west Wales (UK). This facility can provide vertical and horizontal wind speed data, covering an altitude range from 2 to 20 km, with 300 m resolution. However for this study only the data up to 10km were processed. The particular set of data described here covers the period January – December 2007 and measurements were acquired every day continuously. The radar is located near the coast, where the wind speed is expected to be slightly higher than inland and therefore the estimate should be conservative.
Fig. 4. Wind speed variation with altitude, year mean and 3 sigma values shown
Fig. 4 shows the mean wind speed and the 3 sigma value, and it is possible to notice that at 6km altitude these values are 20m/s and 55m/s respectively.
Another factor to be considered is that, in the time domain the wind speed is quite variable with relatively rapid transients (i. e. gusts). As discussed in Aglietti, 2009 the tethered aerostat is a non-linear system, with considerable damping, and therefore rapid transients of the input in reality produce a response that is significantly lower than what is calculated simply using the maximum wind speed in a static analysis.