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Register a new userRay tracing and finite element modeling of sound propagation in a compartment fire
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A compartment fire (a fire in a room or building) creates temperature gradients and inhomogeneous time-varying temperature, density, and flow fields. This work compared experimental measurements of the room acoustic impulse/frequency response in a room with a fire to numerically modeled responses. The fire is modeled using Fire Dynamics Simulator (FDS). Acoustic modeling was performed using the temperature field computed by FDS. COMSOL Multiphysics was used for finite element acoustic modeling and Bellhop for ray-trace acoustics modeling. The results show that the fire causes wave-fronts to arrive earlier (due to the higher sound speed) and with more variation in the delay times (due to the sound speed perturbations). The frequency response shows that the modes are shifted up in frequency and high frequency (>2500 Hz) modes are significantly attenuated. Model results are compared with data and show good agreement in observed trends.
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The Personal Alert Safety System (PASS) is an alarm signal device carried by firefighters to help rescuers locate and extricate downed firefighters. A fire creates temperature gradients and inhomogeneous time-varying temperature, density, and flow fields that modify the acoustic properties of a room. To understand the effect of the fire on an alarm signal, experimental measurements of head-related transfer functions (HRTF) in a room with fire are presented in time and frequency domains. The results show that low frequency (<1000 Hz) modes in the HRTF increase in frequency and higher frequency modal structure weakens and becomes unstable in time. In the time domain, the time difference of arrival between the ears changes and becomes unstable over time. Both these effects could impact alarm signal detection and localization. Received level of narrowband tones is presented that shows the fire makes the received level of a source vary by >10 dB. All these effects could impact the detection and localization of the PASS alarm, and life safety consequences.
In this work, the finite elements method (FEM) is used to analyse the growth of fretting cracks. FEM can be favourably used to extract the stress intensity factors in mixed mode, a typical situation for cracks growing in the vicinity of a fretting contact. The present study is limited to straight cracks which is a simple system chosen to develop and validate the FEM analysis. The FEM model is tested and validated against popular weight functions for straight cracks perpendicular to the surface. The model is then used to study fretting crack growth and understand the effect of key parameters such as the crack angle and the friction between crack faces. Predictions achieved by this analysis match the essential features of former experimental fretting results, in particular the average crack arrest length can be predicted accurately.
We study the propagation of a density wave in a magnetically trapped Bose-Einstein condensate at finite temperatures. The thermal cloud is in the hydrodynamic regime and the system is therefore described by the two-fluid model. A phase-contrast imaging technique is used to image the cloud of atoms and allows us to observe small density excitations. The propagation of the density wave in the condensate is used to determine the speed of sound as a function of the temperature. We find the speed of sound to be in good agreement with calculations based on the Landau two-fluid model.
This study presents a finite element analysis approach to non-linear and linearized tensegrity dynamics based on the Lagrangian method with nodal coordinate vectors as the generalized coordinates. In this paper, nonlinear tensegrity dynamics with and without constraints are first derived. The equilibrium equations in three standard forms (in terms of nodal coordinate, force density, and force vectors) and the compatibility equation are also given. Then, we present the linearized dynamics and modal analysis equations with and without constraints. The developed approach is capable of conducting the following comprehensive dynamics studies for any tensegrity structures accurately: 1. Performing rigid body dynamics with acceptable errors, which is achieved by setting relatively high stiffness for bars in the simulation. 2. Simulating FEM dynamics accurately, where bars and strings can have elastic or plastic deformations. 3. Dealing with various kinds of boundary conditions, for example, fixing or applying static/dynamic loads at any nodes in any direction (i.e., gravitational force, some specified forces, or arbitrary seismic vibrations). 4. Conducting accurate modal analysis, including natural frequency and corresponding modes. Three examples, a double pendulum, a cantilever truss with external force, and a double prism tensegrity tower, are carefully selected and studied. The results are compared with rigid body dynamics and FEM software ANSYS. This study provides a deep insight into structures, materials, performances, as well as an interface towards integrating control theories.
This paper proposes a noise insulation cavity composed of two parallel plates and a micro-perforated plate insertion parallel to the plates, which divides the cavity between the plates into two parts. A theoretical model was established that takes into account of all the couplings among the major parts of the structure, namely the two solid plates, the perforated plate, and the air cavity, together with the simply support boundary conditions. Numerical calculations were performed with different parameters of the micro-perforated plate including its position, perforation ratio, plate thickness, and hole diameters. The calculations indicated that the proposed double-panel structure with a micro-perforated plate insertion exhibited significant improvements in the sound transmission loss (STL) in certain frequency range as compared to a double- or triple-panel structure without a micro-perforated plate. Below 200 Hz the improvement in STL is mainly due to the weakening of the resonances by the energy dissipation of the perforated plate, while in the medium to high frequency range the STL enhancement is mostly due to the dissipation by the perforated plate in the broad frequency band. The theoretical results are in good agreement with the experimental results.
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