Anas Sakout

On the use of a diffusion model for acoustically coupled rooms

A numerical model is proposed to predict the reverberant sound field in a system of two coupled volumes that are connected through an open aperture. The model is based on the numerical implementation of a diffusion model that has already been applied to predict the sound-energy distribution and the sound decay in single rooms. In comparison with the statistical theory, the proposed approach permits the prediction of the sound field by taking into account the sound source location and the receiver locations as well as the transition from one room to the other at the coupling aperture. Moreover, the diffusion model results match satisfactorily the experimental data in terms of sound-pressure level and reverberation times, both in the room containing the source and in the receiving room. Simulations with a ray-based model are also carried out, leading to results similar to those of the diffusion model, but at a cost of larger computation times.

Simulations of the Fluid Flow around a Rotating Vertical Axis Wind Turbine

Numerical simulations of the fluid flow around a small rotating vertical axis wind turbine are performed using a time accurate Reynolds Averaged Navier Stokes (RANS) solver. A moving mesh technique is applied for the simulation of the rotating wind turbine. The RANS equations are then formulated in ALE. All computations are performed assuming 2D incompressible fully turbulent flow. Turbulence is modelled by the SST k/ω model of Menter. Blade forces and torque are obtained from the solution of the RANS equations by integrating the pressure and shear stress over the blade surface. The expected wind turbine output is determined at given rotational speeds. The resulting power coefficients are compared to that obtained by applying a multiple stream tube theory.

Modeling the sound transmission between rooms coupled through partition walls by using a diffusion model

In this paper, a modification of the diffusion model for room acoustics is proposed to account for sound transmission between two rooms, a source room and an adjacent room, which are coupled through a partition wall. A system of two diffusion equations, one for each room, together with a set of two boundary conditions, one for the partition wall and one for the other walls of a room, is obtained and numerically solved. The modified diffusion model is validated by numerical comparisons with the statistical theory for several coupled-room configurations by varying the coupling area surface, the absorption coefficient of each room, and the volume of the adjacent room. An experimental comparison is also carried out for two coupled classrooms. The modified diffusion model results agree very well with both the statistical theory and the experimental data. The diffusion model can then be used as an alternative to the statistical theory, especially when the statistical theory is not applicable, that is, when the reverberant sound field is not diffuse. Moreover, the diffusion model allows the prediction of the spatial distribution of sound energy within each coupled room, while the statistical theory gives only one sound level for each room.

Introducing atmospheric attenuation within a diffusion model for room-acoustic predictions

This paper presents an extension of a diffusion model for room acoustics to handle the atmospheric attenuation. This phenomenon is critical at high frequencies and in large rooms to obtain correct acoustic predictions. An additional term is introduced in the diffusion equation as well as in the diffusion constant, in order to take the atmospheric attenuation into account. The modified diffusion model is then compared with the statistical theory and a cone-tracing software. Three typical room-acoustic configurations are investigated: a proportionate room, a long room and a flat room. The modified diffusion model agrees well with the statistical theory (when applicable, as in proportionate rooms) and with the cone-tracing software, both in terms of sound pressure levels and reverberation times.

Experimental investigation of the wall shear stress in a circular impinging jet

The influence of the large-scale vortical structures on the wall shear stress in a circular impinging jet is investigated experimentally for a Reynolds number of 1260. Time-resolved particle image velocimetry and polarographic measurements are performed simultaneously. It is found that the instantaneous wall shear stress is strongly dependent on the vortex dynamics, particularly for different parts of the transverse vortex. The influence of the vortex ring, the secondary and tertiary vortices on the ejection/sweep process near the wall is the main mechanism involved in the wall shear stress variation. In the region of the boundary layer separation, the wall shear stress amplitude increases just upstream of the separation and dramatically decreases in the recirculation zone downstream from the separation. The interaction between primary and secondary structures and their pairing process with the tertiary structure affects the sweep/ejection process near the wall and subsequently the wall shear stress variation. A comparison between the Finite Time Lyapunov Exponent (FTLE) method and the phase average technique is performed. It is shown that both methods describe the flow dynamics in the impinging region of the vortex ring. However, the FTLE method is more suitable for describing the unsteady separation of the boundary layer.

An Empirical Diffusion Model for Acoustic Prediction in Rooms with Mixed Diffuse and Specular Reflections

In this paper, a modification to the room-acoustic diffusion model is proposed to take different amounts of wall scattering into account. An extensive set of numerical simulations using a cone-tracing software has first been carried out, in order to highlight the impact of the scattering coefficient on the diffusion process in rooms, in terms of sound pressure levels. An iterative method is then proposed to identify, for a given value of the walls scattering coefficient, the diffusion constant that allows the stationary sound field to be governed by a diffusion process, regardless of the rooms geometry. Using this method, an empirical law can be proposed between the diffusion constant and the scattering coefficient. The empirical diffusion model is then compared to scale model experiments, as well as to other models from the literature, with a satisfactory agreement for the sound pressure level. However, the empirical diffusion model fails to predict the sound decay for rooms with perfectly specularly reflecting surfaces, due to the inherent concept of a diffusion process.

Numerical Simulation of the Fluid Flow Around a Roof Mounted Wind Turbine

The aerodynamic performances of a roof mounted small vertical axis wind turbine are evaluated using a Computational Fluid Dynamics (CFD) code and a computer program based on the Blade Element Momentum theory. The CFD code solves the incompressible Reynolds Averaged Navier-Stokes (RANS) equations for the simulation of the flow around a building with a roof mounted wind turbine. These computations are performed with a 3D grid, assuming steady and fully turbulent flow. The CFD results are used to derive the wind velocity vector approaching the wind turbine. Then the aerodynamic performances of the roof mounted wind turbine are determined using a Multiple Stream tube Theory. Beforehand, to check the reliability of the CFD computations, the simulations of the flow around reference buildings are performed and the results are compared with reference data.

Fluid Structure Interaction of Wind Turbine Airfoils

Flow induced vibrations of two airfoils used in wind turbine blades are investigated by a strong coupled fluid structure interaction approach. The method is based on a general Computational Fluid Dynamics (CFD) code that solves the Navier-Stokes equations defined in Arbitrary Lagrangian Eulerian (ALE) coordinates by a finite volume method. A straightforward technique is implemented in a user subroutine for the coupling of the CFD code to a structural dynamics program to determine the airfoil displacements due to the aerodynamics forces and for updating the grid at each time step. Simulations are carried out for a free pitch oscillating airfoil and for a combined pitch and vertical oscillating airfoil. Beforehand, the problem of the flow around a forced pitch oscillating airfoil is considered to check the reliability of the moving mesh technique and the CFD computations. All computations are performed in 2D, incompressible and low Reynolds number flows.

Amplification of Single Mechanical Fault Signatures Using Full Adaptive PMSM Observer

This paper presents a localized mechanical fault detection method using a full adaptive permanent-magnet synchronous machine (PMSM) observer in a d/q reference frame. It appears that the fault sensitivity of PMSM stator currents and rotor speed measurements is very low with regard to single mechanical faults. That is why an amplification of single mechanical fault signatures in frequency domain is proposed by adjusting the adaptive PMSM observer gains. To study the influence of the adaptive PMSM observer gains on fault signatures, the observer model is first of all linearized at an operating point. Then, the static gain, the resonance, and the quality factor Q of different transfer functions are analyzed. A dedicated experimental setup based on an original mechanical fault emulator and a 7.8 kW PMSM drive is designed to validate the simulation results. The simulation and the experimental results show that the proposed method is effective to amplify single mechanical fault signatures and the calculated fault indicator.

Bearing Fault diagnosis using a pre-filtering and a spectral identification algorithm

The Time Synchronous Averaging (TSA) is a well known method used for early detection of defects in rolling elements. This paper applies and evaluates the effectiveness of this method, spefifies its limitations, and improves it using a pre-filtering of the envelop signal associated with an identification method based on Marquardt algorithm. The proposed procedure is evaluated and applied theoretically and practically on simulated and real signals.