The surface wind gust regime and aircraft operations at Sydney Airport

This paper reviews a series of conditional sampling techniques developed at Oxford that allow pseudo-instantaneous velocity field pictures to be synthesised from a succession of single-point measurements made with a two-component laser Doppler anemometer (LDA). The first attempt to apply this technique was by Marwood in his study of roof-edge vortices. This pressure conditional velocity method involved extracting appropriate velocity values after examination of pre-recorded simultaneous pressure and velocity time series. In order to obtain the required data from a reasonably short experiment, Marwood accepted velocities within a small range of pressure trigger levels and calculated the flow field mean values of the samples obtained. Subsequently, bending-moment-conditional velocity sampling has been applied to the detection of identifiable coherent flow structures over a forest canopy. In this application, a significant improvement in trigger detection is under investigation. Instead of setting a trigger based upon a single instantaneous bending moment, the bending moment time series is being examined using the relatively new technique of wavelet analysis. In this case, a wavelet function is chosen to detect the short-term fluctuations in bending moment which might be expected to follow the impact of a discrete canopy-penetrating gust. Within a bending moment event the wavelet analysis will give a precise marker on the velocity time series, and once again, repeated experiments with velocity components measured by the LDA at a sequence of locations provide the necessary information to build a pseudo-instantaneous picture of the gust velocity field above and within the forest canopy. This paper reports that preliminary experiments using this method have been unsuccessful because of the inability of the wavelet analysis to specifically detect coherent gusts in a bending moment time series. However, when an inverted analysis is carried out (using velocity-conditional bending-moment wavelet analysis) clear bending moment events can be recognised to coincide with gust fluctuations of similar wavelength as the tree natural frequency. This suggests that the method is viable, but not set up appropriately. More interestingly it leads to the question of whether coherent gusts produced by trees in a forest can lead to resonance damage to the trees themselves. © 1999 Elsevier Science Ltd. All rights reserved.