References of "Rigo, François"
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See detailMixture model in high-order statistics for peak factor estimation on low-rise building
Rigo, François ULiege; Andrianne, Thomas ULiege; Denoël, Vincent ULiege

in Proceedings of the XV Conference of the Italian Association for Wind Engineering (2018, December 10)

To design reliable structures, extreme pressures and peak factors are required. In many applications of Wind Engineering, their statistical analysis has to be performed taking into account the non ... [more ▼]

To design reliable structures, extreme pressures and peak factors are required. In many applications of Wind Engineering, their statistical analysis has to be performed taking into account the non-Gaussianity of the wind pressures. With the increasing precision and sampling frequency of pressure sensors, large short and local peak events are more usually captured. Their relevance is naturally questioned in the context of a structural design. Furthermore, the increasing computational power allows for accumulation and analysis of larger data sets revealing the detailed nature of wind flows around bluff bodies. In particular, in the shear layers and where local vortices form, it is commonly admitted that the Probability Density Function (PDF) of measured pressures might exhibit two or more significant components. These mixed flows can be modelled with mixture models [Cook (2016)]. Whenever several processes coexist, and when one of them is leading in the tail of the statistical distribution, as will be seen next in the context of corner vortices over a flat roof, it is natural to construct the extreme value model with this leading process and not with the mixed observed pressures. It is therefore important to separate the different processes that can be observed in the pressure histories. Once this is done, specific analytical formulations of non-Gaussian peak factors can be used to evaluate the statistics of extreme values [Kareem and Zhao (1994), Chen (2009)]. The separation of mixed processes is usually done by means of the PDF of the signals [Cook (2016)]. This information is of course essential to perform an accurate decomposition but it might be facilitated by considering higher rank information like auto-correlations and higher correlations like the triple or quadruple correlation. Indeed, the two phenomena that need to be separated and identified might be characterized by significantly different timescales, which are not reflected in the PDF. In this paper, the large negative pressures measured on a flat roof are analyzed and decomposed into two elementary processes, namely, the flapping corner vortex and the turbulent flow detaching from the sharp upstream edge. The full paper will finally show that an accurate decomposition of the recorded pressures into their underlying modes provides a more meaningful evaluation of the extreme pressures. [less ▲]

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See detailVIV response of a suspended sphere nearby the critical Reynolds number
Andrianne, Thomas ULiege; Rigo, François ULiege; Denoël, Vincent ULiege

Conference (2018, September 10)

The Vortex-Induced Vibration of a sphere connected to a flexible beam is investigated in air for Reynolds numbers including the critical value. For this value (ReC =3e5 ), wake disorganisation is expected ... [more ▼]

The Vortex-Induced Vibration of a sphere connected to a flexible beam is investigated in air for Reynolds numbers including the critical value. For this value (ReC =3e5 ), wake disorganisation is expected in a static configuration. This fundamental fluid-structure interaction characterised by an axisymmetric configuration leads to vibrations in the lateral and longitudinal directions. Similarly to the well-known circular cylinder, a lock-in phenomenon will take place due to the effect of the motion of the body on the shedding process. An experimental aeroelastic model is designed, instrumented and tested in the wind tunnel of University of Liège. The objective is twofold: (i) solve a practical wind-engineering problem (a spherical suspended streetlight) and (ii) propose a fundamental investigation of the VIV of a smooth sphere for sub-critical and critical Reynolds numbers. [less ▲]

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See detailVortex induced vibrations of rectangular cylinders arranged on a grid
Rigo, François ULiege; Andrianne, Thomas ULiege

Scientific conference (2018, September 10)

A grid arrangement made of one hundred rectangular cylinders fixed to the facade of a house generates strong and disturbing mono-harmonic noise. The cross-flow vibration of the rectangular cylinders is ... [more ▼]

A grid arrangement made of one hundred rectangular cylinders fixed to the facade of a house generates strong and disturbing mono-harmonic noise. The cross-flow vibration of the rectangular cylinders is identified as the origin of the noise. The present article proposes a complete investigation of the Vortex Induced Vibration (VIV) combined with a grid effect. It is based on in situ measurements, numerical (finite elements and Computational Fluid Dynamics (CFD)) and extensive wind tunnel (WT) modelling. A comparison between unsteady pressure tap measurements and CFD results allows to understand the vortex shedding process and synchronisation type depending on the wind incidence and spacing of the cylinders. On the basis of this multi-approach parametric investigation, a deep understanding of the VIV-grid phenomenon enables to propose two mitigation techniques. These techniques are tested and their effectiveness is reported in terms of vibration amplitude and acoustic intensity. [less ▲]

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See detailVortex induced vibrations of rectangular cylinders arranged on a grid
Rigo, François ULiege; Denoël, Vincent ULiege; Andrianne, Thomas ULiege

in Journal of Wind Engineering and Industrial Aerodynamics (2018)

A grid arrangement of hundred rectangular cylinders fixed to the facade of a house generates strong and disturbing mono-harmonic noise. The cross-flow vibration of the rectangular cylinders is identified ... [more ▼]

A grid arrangement of hundred rectangular cylinders fixed to the facade of a house generates strong and disturbing mono-harmonic noise. The cross-flow vibration of the rectangular cylinders is identified as the origin of the noise. The present article proposes a complete investigation of the Vortex Induced Vibration (VIV) combined with a grid effect. It is based on in situ measurements, numerical (finite elements and Computational Fluid Dynamics (CFD)) and extensive wind tunnel modelling. A comparison between unsteady pressure tap measurements and CFD results allow to understand the vortex shedding process and synchronization type depending on the wind incidence and spacing of the cylinders. On the basis of this multi-approaches parametric investigation, a deep understanding of the VIV-grid phenomenon enables to propose two mitigation techniques. These techniques are tested and their effectiveness is reported in terms of vibration amplitude and acoustic intensity. [less ▲]

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See detailUnsteady pressure measurement around aerodynamic bodies: Development of a calibration apparatus and wind tunnel testing
Rigo, François ULiege

Master's dissertation (2017)

Separated flows are complex but interesting to study because they are variable and unsteady. They are present for every bluff bodies and stalled streamlined bodies (at high angle of attack). Experimental ... [more ▼]

Separated flows are complex but interesting to study because they are variable and unsteady. They are present for every bluff bodies and stalled streamlined bodies (at high angle of attack). Experimental aerodynamics is able to study these types of flow, using pressure sensors. Due to sensor size, pressure tubes are used to connect the pressure scanner to the tap (where the pressure is effectively measured). Statically, nothing is changed, but when an unsteady flow is studied, the signal measured by the sensor is perturbed by the tube. The Transfer Function of the tube has to be computed, to correct for the pressure measure using an inverse Fourier Transform and to obtain the pressure effectively present at the tap. The correction is made on the fluctuation amplitudes (around the mean) and the phase of the signal. The synchronization is important when vortex shedding is studied. This Transfer Function is computed by comparing the pressure measured at the begin and at the end of the tube. For that purpose, pressure with a frequency content has been applied on the tube entry (periodic for KTH calibrator and aperiodic for ULg calibrator). The ratio between these pressures gave the desired correction, showing resonance peaks for some frequencies. When a simple tube is used, theoretical models from fluid equations give very similar results to experimental ones. A parallel with electricity has also been made, replacing the pressure tube by an RLC circuit or a transmission line. The longer and the narrower the tube, the higher the signal distortion. 3D printed models are nowadays commonly used in experimental aerodynamics, allowing not only to build complex shaped models easily, but also pressure taps directly on the model and pressure channels into the structure. These more complex measurement systems have also to be experimentally calibrated. Indeed, diameter restriction on tap or shrinks in tube channels highly distort the signal. We used this calibration to correct the pressure on a stalled wind turbine wing (at high incidence). The stall is linked to viscous effects, the flow becoming separated and turbulent. The fluctuations and phase of pressure taps signal have been studied to understand the Reynolds effect on a stalled wind turbine wing. Experiments were compared with CFD and theoretical models to validate the results. Another application of unsteady pressure that we have studied is the vortex shedding process, occurring around bluff bodies (in particular for rectangular cylinders). The synchronization and amplitude fluctuations of these vortices have been corrected using the dynamic calibration device. Fluid-structure interaction (vortex induced vibration) has then been studied: when vortices were ejected at the resonance frequency of the cylinder, the structure entered auto-excitation and vibrated a lot. When the cylinders were closely spaced in the flow (assembled into a grid), they interfered with each other and the vortex shedding process was changed compared to a single cylinder. To understand deeply this grid, theoretical and numerical models have been used (FEM and CFD) in parallel with experimental sensors: accelerometers (for vibration), pressure sensors connected on taps by tubes, Cobra Probe (velocity in the wake of cylinders), Hot Wire (free stream velocity). The study of this process in function of the incidence and the cylinder spacing allowed us to predict airspeed that induces instability. This is crucial in order to find parameters that minimize vibrations occurring on a real grid, with undesirable noise. In conclusion, this work can be used to take into account unsteady effects when pressure is measured around streamlined and bluff bodies. [less ▲]

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