Generation and Mitigation of Large Amplitude Gusts

Abstract

Gusts and turbulence affect a range of sectors. For example, in transport, they are a leading cause of accidents, and in energy generation, the load variations they generate on wind and tidal turbines reduce their fatigue life. It is vital for designers to be able to assess unsteady loads from gusts quickly and accurately when designing devices. Linear theories have therefore been derived to study the load response of a lifting surface to a gust, and gust generators have been developed to investigate the limits of these linear theories in a controlled environment. Load alleviation devices such as minitabs have also been developed to attempt to reduce the load variations in order to prevent unwanted vehicle motion and increase component fatigue life.

This thesis considers the generation and mitigation of unsteady gusts in a wind tunnel environment. First, a multi-vane gust generator is characterised. The gust produced is a harmonic, sinusoidal gust for which the load response of an aerofoil is predictable by the Sears theory. Normalised velocity root-mean-square and two-point cross-correlation show that the gust is two-dimensional and spatially uniform, in both the streamwise and transverse directions. Proper Orthogonal Decomposition reveals one dominant, uniform mode in the transverse plane, and two dominant modes in the streamwise plane, which displays the spatial uniformity and the convective nature of the gust through the measurement window. In the streamwise plane, mode 1 for vorticity exhibits small wavelength vorticity shedding from the vanes. The reduced frequency of this vorticity shedding is calculated as ten times higher than that of the gust generator, and so there is little risk of interaction between the gust produced and the vorticity generated by the vanes of the gust generator. In the transverse plane, the dominant vorticity mode displays only noise, confirming there is negligible risk of interaction from the vane wakes. The load response of two aerofoils, NACA 0012 (a canonical aerofoil) and NACA~63-421 (a thick and cambered aerofoil representative of a tidal turbine geometry) are recorded, and it is demonstrated that at an angle of attack of zero, the normalised lift amplitude and the phase of both aerofoils follow the Sears prediction well. This agreement gives further evidence that the gust generator produces a two-dimensional, spatially uniform gust for which the load response can be predicted using linear theory.

The same gust generator had been shown in previous work to produce an Atassi-type gust, where a streamwise component is also present and the resulting aerofoil load deviates from the Sears function. In order to investigate possible reasons for this phenomenon, the vane motion was modified such that the vanes oscillate from zero to -2 gust amplitudes (instead of oscillating symmetrically about zero degrees). This was repeated for two different vane motion amplitudes, such that four different gust configurations were investigated (large centred gust, large off-centre gust, small centred gust, small off-centre gust). The uniformity of each gust configuration is assessed, and all cases are found to produce two-dimensional and spatially uniform gusts. The streamwise reduced frequency and gust strength are calculated using a linear regression, and the gust amplitude and gust strength are found to be approximatively equal for both centred and off-centre gust. The streamwise reduced frequency is found to be of the order of 10^-3 for the off-centre gusts, and 10^-7 for the centred gusts, and therefore negligible in both cases. The load response of NACA 0012 and NACA 63-421 aerofoils are studied, and the normalised lift amplitude is found to follow the Sears prediction at zero angle of incidence for both aerofoils, while the response deviates from the Sears function for all other angles of attack tested. In this case, since the streamwise reduced frequency k₂ is so small, the Sears-normalised Atassi function is equal to the Sears function over a wide range of transverse reduced frequencies k=k₁, only deviating at very low frequencies (k₁<0.02). The reasons behind a Sears or Atassi response are discussed, and are attributed to small non-uniformities and three-dimensional features in the gust produced, so while a classical Atassi-type gust is not generated, the response deviates from the Sears solution. Work on similar gust generators by other authors has also found a coincidental agreement with the Atassi function that was caused by a spatially non-uniform flowfield. The fact that the same gust generator produced an Atassi-type gust in a different wind tunnel is attributed to the length of the plenum in the original tests, which allowed the growth of shear layers on the tunnel walls. Multi-vane gust generators therefore do not inherently produce an Atassi-type gust, however the inflow needs to be very uniform as only a small variation can cause non-uniformities and therefore an Atassi-type gust.

Finally, the mitigation of the large amplitude centred gust was investigated by using a minitab device placed near the leading edge (x/c=0.04) of a NACA 63-421 aerofoil (representative of a tidal turbine blade). A plain, solid tab was trialled first. In static conditions, the tab is only effective at reducing lift at angles of attack above 6.62°, due to a favourable pressure gradient on the aerofoil upper surface reattaching the flow past the deployed tab. At higher angles of attack, the minitab separates the boundary layer over the remainder of the aerofoil surface. The lift reduction occurs past a critical tab height dependent on angle of attack, before which no lift reduction was observed. In dynamic conditions, the tab is found to generate a somewhat coherent vortex at 6.62° and k=0.29, and this tab vortex grows in coherency as the angle of attack and reduced frequency are increased. Proper Orthogonal Decomposition reveals the shift of energy from the flow separation mode to the vortex shedding mode as reduced frequency and/or angle of attack is increased. Different tab geometries are tested to attempt to reduce or inhibit the creation of the tab vortex: two base-vented tabs with different slot sizes, and a serrated tab. In static conditions, the slotted tabs present similar responses to the solid tab with a critical tab height above which lift reduction occurs, whereas the serrated tab instead gives a progressive reduction in lift with tab height. In dynamic conditions, both the slotted 1 tab (smaller slot) and slotted 2 tab (larger slot) create a coherent tab vortex, and the serrated tab shows signs of a vortex being generated. To assess the effectiveness of the tab in gusty conditions, the aerofoil is tested with the gust generator using a large amplitude, centred gust. The results reveal that the tabs behaved differently at the two frequencies of oscillation tested, but are most likely to reduce the lift and drag with a phase lead with respect to the gust reaching the leading edge. The slotted 1 tab is found to be the most effective, reducing lift and drag with a phase lead of 90°. This could be achievable in real conditions with an appropriate controller.

The work presented in this thesis has given insight into the design requirements for generating a Sears-type gust with a multi-vane gust generator and paves the way for further experimental studies of load alleviation and control.

Date of Award	25 Mar 2026
Original language	English
Awarding Institution	University of Bath
Supervisor	Anna Young (Supervisor), Samuel Bull (Supervisor) & Ismet Gursul (Supervisor)