Wind tunnel tests on a 1/16th scale laser model

An analysis of full-scale wind structure relevant to sailing yachts is carried out in order to develop a target model for wind-tunnel simulations. Unavoidable differences between real onset yacht flows and idealised wind-tunnel simulations are pointed out. Detailed measurements of the actual wind-tunnel flows developed for the present tests are discussed in depth, and are compared to idealised cases, concluding that although the simulations are not as good as physically possible, they are perfectly adequate for the present test programme.Measurements of the forces and moments acting on a 1/16th scale model of a laser yacht for apparent wind directions of 30 and 60 degrees, for heel angles of 0, 10, 20, and 30 degrees, and for smooth and turbulent sheared flow are presented. The results are discussed in terms of sail aerodynamics, and a transformation procedure is developed which leads to an excellent collapse of the measured results on the basis of calculated sail lift and drag coefficients

Tidal turbine blade load experiments for oscillatory motion

This paper presents blade root bending moment measurements of a horizontal-axis tidal turbine for planar oscillatory motion, conducted in a stationary water towing tank. By comparing the measurements with quasi-steady reconstructions for both single and multiple frequency oscillatory motion, the bending moment was shown to be sensitive to both frequency and amplitude, as well as to the mean tip-speed ratio. The unsteady loads associated with the separation of the flow and dynamic stall are shown to be of considerably greater importance than those which are already present for attached flow, such as added mass and dynamic inflow. A linear model fit to the unsteady bending moment also indicates that the inertia contribution is relatively small. For cases where attached flow exists over the majority of the load cycle, these reconstruction methods are likely to be sufficient to obtain a reasonable prediction of the root out-of-plane bending moment. However, turbines whose blades are likely to operate near stall are likely to require more complex models for accurate load predictions to mitigate the risk of fatigue failure

The characterisation of the hydrodynamic loads on tidal turbines due to turbulence

An improved characterisation of the hydrodynamic blade loads due to onset turbulence is essential in order to mitigate premature failures, reduce excessive levels of conservativeness and ultimately ensure the commercial viability of tidal turbines. The literature focussing on the turbulence in fast flowing tidal streams and of the unsteady loads that are subsequently imparted to rotors has previously been very limited. However, increased activity in the tidal energy community has led to new investigations and insights which are reported in this paper. It has been found that through the use of acoustic Doppler-based sensors, the streamwise turbulence intensities generally tend to a value of approximately 6–8% at the mid-depth of proposed tidal energy sites. Evidence that the anisotropic structure and scales of the turbulence are more consistent with open-channel-based models than atmospheric-based correlations has also been found. Rapid distortion theory has been applied to estimate that the standard deviation of the streamwise turbulent velocity fluctuations in the onset free-stream flow may be amplified significantly by 15% due to the presence of a turbine. The turbulent fluctuations have also been predicted to remain well correlated over the outer span of the blades at the rotational frequency of the rotor. Recent model-scale experiments have enabled the unsteady hydrodynamic loading to be isolated from the steady-flow loading. For cases where the boundary layer remains primarily attached across the blades, this has enabled linear transfer functions to be developed and applied to model the response to a multi-frequency forcing. It has also been found that phenomena consistent with delayed separation and dynamic stall can result in a blade root bending moment that exceeds the steady value by 25%, and this needs to be taken into account in design to reduce the probability of failure

Blade loads on tidal turbines in planar oscillatory flow

Characterisation of the unsteady hydrodynamic loads is essential for accurate predictions of the fatigue life and ultimate loads of tidal turbine blades. This paper analyses a set of experimental tests of the hydrodynamic blade root out-of-plane bending moment response to planar oscillatory motion, chosen as an idealised representation of the unsteadiness imparted by waves and turbulence. Phenomena associated with dynamic stall are observed which are sensitive to the oscillatory frequency and velocity amplitude. Flow separation is shown to result in loads significantly greater in magnitude than that for steady flow. Following flow reattachment, the load cycles compare relatively well with Theodorsen’s theory for a two-dimensional foil oscillating in heave, suggesting that circulation due to the shed wake dominates the unsteadiness in phase with acceleration, over added mass effects. For attached flow, the effect of unsteadiness is comparatively much smaller. At low frequencies a phase lead over the velocity is observed, compared to a lag at higher frequencies. Multiple frequency oscillations are also briefly considered. Reconstruction of the multi-frequency response using both the steady flow measurements, and the single frequency measured response, is shown to offer a relatively good fit when the flow is attached, for lower frequency combination

Blade loading on tidal turbines for uniform unsteady flow

An improved characterisation of the unsteady hydrodynamic loads on tidal turbine blades is necessary to enable more reliable predictions of their fatigue life and to avoid premature failures. To this end, this paper presents a set of blade-root bending moment responses for a scale-model tidal turbine subjected to an unsteady planar forcing in a towing tank. In cases where the boundary layer was believed to be attached to the outer sections of the blade, the out-of-plane bending moment amplitude for unsteady flow was up to 15% greater than the corresponding load measured in steady flow and exhibited a phase-lead of up to 4.5°. Both these observations are qualitatively consistent with the effects of dynamic inflow and non-circulatory forcing. The bending moment responses for a forcing time history that comprised multiple frequencies, as well as for a discrete half-sinusoidal perturbation, were able to be reconstructed reasonably well using the responses obtained from single-frequency oscillatory flows. This suggests that blade designers could utilise relatively low fidelity techniques and conduct potentially fewer experimental tests to acquire the fatigue load spectrum

A Wind Tunnel Investigation of the Aerodynamics of Sailing Dhows

This paper presents the results of experimental tests conducted to study the aerodynamic performance of sailing dhows. The investigation emerged from the interest shown by designers and sail-makers to understand how these sails perform, since they have never been studied before. The 43ft (13.1m) and 60ft (18.m) classes have been tested in a wind tunnel where the effects of varying several parameters were investigated. These parameters were: heel angle, apparent wind angle, bending and stiffness of the yard, optimization of the trimming and the influence of the mizzen on the mainsail. This is the first investigation where the aerodynamic performance of the lateen sails on sailing dhows has been investigated in a wind tunnel study