Computational aeroacoustics models of phonationAbstract: The main objectives are to assess the influence of turbulence and separated flows and to investigate possible coupling mechanisms between the jet and the sound field by using a computational tool for understanding the detailed aerodynamic mechanisms of human voice production.
Direct numerical simulations (DNS) of the flow and sound radiation in an idealized model of the human vocal tract are conducted by accurate numerical integration of the compressible Navier-Stokes equations. The above movie shows vorticity contours during a typical cycle at 250Hz. The far-field sound is recorded from the simulations and compared to the sound predicted using the Ffowcs Williams-Hawking acoustic analogy. Excellent agreement between the directly simulated sound and the acoustic analogy prediction allows acoustic sources to be identified in the flow. Monopole, dipole and quadrupole sources have been detected with the dipole source due to the unsteady forces exerted by the glottal jet on the walls be the dominant source. These are two-dimensional simulations and hence turbulence effects are not included. Extension of this to 3D Large Eddy Simulations (LES) is the focus of our current efforts. The movie below is an animation of streamwise velocity from an LES of separated flow in a diffuser for which detailed experimental data exists. This simulations was performed using the FLUENT commercial code and will be combined with our acoustic analogy code to predict diffuser noise. Extensions of our DNS code for LES of phonation are also underway.
Finally, only forced vocal fold oscillations were studied above, and so we are currently working on fluid-structure interactions issues into the model to capture flow-induced oscillations and phonation onset/offset.
The work is funded by NIH.
People
involved: Former students: Wei Zhao, Cheng Zhang
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Experimental aeroacoustics of phonation and fluid-structure interactionAbstract: The aerodynamics and acoustics of voice production are being investigated using self-oscillating laryngeal models, with particular focus on the vocal folds and the immediate supraglottal region. Both physical and numerical models are being employed. Physical models have been developed using latex tubes and exhibit motion similar to the bulk motion of the vocal folds, with frequency and onset pressure similar to human phonation. Flow visualization, hot-wire anemometry, and microphone signals are used for flow and acoustic measurements. Numerical models using FIDAP, a commercial code capable of incorporating fluid-structure interaction and developed by Fluent, Inc., are being investigated; the present studies are focused on verification of the code using the previously published results of a membrane in a 2-D channel by Luo and Pedley (1995 and 1996).
Self oscillating physical model of the vocal folds. Frequency of oscillation is approximately 160 Hz and transglottal pressure is 4 cmH2O. Strobe light is used for illumination.
Vortex shedding from an oscillating (driven) mechanical model of the vocal folds. The
orifice shape is convergent. Transglottal pressure is 6 cmH2O,
frequency of oscillation is 60Hz. FIDAP simulation of the unsteady deformation of a flexible membrane occupying a portion of one wall of a 2-D channel.
People involved: |
DNS and LES of blood flow through stenotic vesselsAbstract: Artherosclerosis is the main cause of heart attacks and stroke and leads to more deaths every year than cancer. The main cause of the disease is somehow related to the biochemical response of blood vessels to local occlusions (blockages), called stenoses, within blood vessels due to a build up of plaque deposits. Due to the pulsatile nature of blood flow, this flow restriction creates the possibility for transitional or turbulent vortical flows downstream of this blockage. The fluid dynamics of pulsatile flow in stenotic blood vessels are being studied using DNS techniques based on the spectral-element approach. The NEKTAR code, written and provided by Prof. Spencer Sherwin of Imperial College, UK, is being used here. The effect of stenosis morphology on vortical flow structure is the focus. The movie below shows some preliminary results for a 60% asymmetric stenosis. The blue and red circles represent vorticity marker particles at 3/4 and 1/4 diameters of the proximal vessel and their depicted evolution indicates the presence of large-scale vortical structures and significant crosstream mixing. Our results show this mixing is highly influence by the morphology of the stenosis. The images/movies below this, depict a typical grid for a circular (symmetric) stenosis and axial velocity contours at various cross sections.
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