CFD analysis of an exceptional cyclist sprint position

A few riders have adopted a rather exceptional and more aerodynamic sprint position where the torso is held low and nearly horizontal and close to the handle bar to reduce the frontal area. The question arises how much aerodynamic benefit can be gained by such a position. This paper presents an aerodynamic analysis of both the regular and the low sprint position in comparison to three more common cycling positions. Computational fluid dynamics simulations are performed with the 3D RANS simulations and the transition SST k–ω model, validated with wind-tunnel measurements. The results are analyzed in terms of frontal area, drag coefficient, drag area, air speed and static pressure distribution, and static pressure coefficient and skin friction coefficient on the cyclist surfaces. It is shown that the drag area for the low sprint position is 24% lower than for the regular position, which renders the former 15% faster than the latter. This 24% improvement is not only the result of the 19% reduction in frontal area, but also caused by a reduction of 7% in drag coefficient due to the changed body position and the related changes in pressure distribution. Evidently, specific training is required to exert large power in the low sprint position.</p

Generalized lift force model under vortex shedding

peer reviewe

Prévention des blessures chez le cycliste : développement d’un système d’évaluation de la position

editorial reviewe

Parameter identification of wake-oscillator from wind tunnel data

peer reviewedThis paper proposes a procedure for the parameter identification of Tamura's wake-oscillator model. A multiple timescale analysis of the dimensionless model shows that the response is governed by two dimensionless groups D0 and D1, highlighting the importance of the forcing terms in the two governing equations, the total (aerodynamic and structural) damping and the coefficient ɛ of the fluid Van der Pol oscillator. In particular, this approach provides a simple closed form expression for the steady state amplitude of the structural displacement, which is usually measured in wind tunnel experiments. The proposed method of identification consists in fitting the parameters of the model by adjusting the closed-form expression of the VIV curve on experimental points. It is developed into two variants: a least-square fitting and a fitting based on some simple geometrical indicators (height, width, asymmetry). The model is sufficiently versatile to estimate the maximum amplitude and lock-in range. Applications of VIV in air for different geometries and Scruton numbers show that the two variants give equivalent results thanks to the robustness of the method. The paper is first intended for experimenters looking for a simple robust procedure to identify the parameters of the wake-oscillator, which can then be used in a prediction phase. The derivation of the slow phase version of Tamura's model might also be appealing to better understand the main features of this model

A Cross-Validation Approach to Approximate Basis Function Selection of the Stall Flutter Response of a Rectangular Wing in a Wind Tunnel

The stall flutter response of a rectangular wing in a low speed wind tunnel is modelled using a nonlinear difference equation description. Static and dynamic tests are used to select a suitable model structure and basis function. Bifurcation criteria such as the Hopf condition and vibration amplitude variation with airspeed were used to ensure the model was representative of experimentally measured stall flutter phenomena. Dynamic test data were used to estimate model parameters and estimate an approximate basis function

Aeroelastic measurements on a vertical axis wind turbine

peer reviewedAerodynamic pressure and acceleration are measured synchronously on a blade of a full scale vertical axis wind turbine during operation using a novel type of aeroelastic measurement device: the WAMB (Wireless Aeroelastic Measurement Belt). The objective is to investigate the aeroelastic behaviour of the blade during the rotation, where the flow might change from attached to separated from the surface of the blade, depending on the azimutal position. These important variations of the aerodynamic loading might lead to dynamics stall. When coupled to the flexibility of the blades, important vibrations of the turbine are expected. At this stage, convincing preliminary tests have been performed to demonstrate the capability of the WAMB to measure pressure and vibrations on a rotating blade. A future test campaign is planned to perform similar in situ measurements on the wind turbine for different operating conditions, including low tip speed ratios, for which dynamic stall is expected to take place