The spiral shape of the cochlea
BORIS DOI
Subtitle
transverse flow visualizations and emerging phenomena in idealized models
Abstract
The cochlea, our auditory organ, has a unique spiral shape and is fluid-filled. The physiological role of its shape remains, to a great extent, elusive and is the subject of ongoing research and debate. The curvature and torsion of the cochlear duct are most pronounced toward the apex, therefore low frequencies, which are processed there, are particularly interesting. Flow in the radial direction in the cochlea, i.e., transverse flow, is of relevance because of the microanatomy of the cochlea. This thesis aims to investigate the interplay between geometry and fluid flow, especially transverse flows, using numerical and experimental methods.
In the first part, computational fluid dynamics (CFD) simulations are performed in helical square ducts with curvature and torsion, similar to the ones observed in human cochleae. Transverse flows are examined under a harmonically oscillating axial flow for frequencies covering infrasound and low-frequency hearing, with a mean inlet velocity amplitude representing sound pressure levels from normal conversation up to the threshold of pain. Torsion significantly increases transverse flow, especially when the influence of curvature drops to negligible amounts. Interestingly, the combined effect of curvature and torsion is larger than expected from a superposition of the two.
The cochlea’s thin membranes, some as little as two cell layers thick, make the study of shear stresses, exerted by the fluid on the walls, interesting. Subsequently, the effect of geometry on wall shear stresses, and pressure fluctuations is examined numerically. The helical shape experiences the most pronounced transverse wall shear stresses at the inner wall of the curve, and its maximal wall shear stresses are higher than the sum of the maximal ones in toroidal and twisted ducts.
The second part addresses the visualization of transverse flows in millimetric ducts using an in-house developed scanning particle image velocimetry (PIV) system to capture the full flow field in a volume. Our system is adapted to small-scale oscillatory flows and allows for precise measurements of both time-resolved and net velocities. The experimental outcomes align well with the CFD simulations. The developed PIV system offers means for studying multiscale periodic flows in micro- to millimeter-scale models across a variety of applications, including biological systems like the inner ear.
The observed transverse flow phenomena offer new insights into cochlear fluid dynamics, with potential implications for metabolite transport. Further research is needed to elucidate their impact on cochlear mechanics.
In the first part, computational fluid dynamics (CFD) simulations are performed in helical square ducts with curvature and torsion, similar to the ones observed in human cochleae. Transverse flows are examined under a harmonically oscillating axial flow for frequencies covering infrasound and low-frequency hearing, with a mean inlet velocity amplitude representing sound pressure levels from normal conversation up to the threshold of pain. Torsion significantly increases transverse flow, especially when the influence of curvature drops to negligible amounts. Interestingly, the combined effect of curvature and torsion is larger than expected from a superposition of the two.
The cochlea’s thin membranes, some as little as two cell layers thick, make the study of shear stresses, exerted by the fluid on the walls, interesting. Subsequently, the effect of geometry on wall shear stresses, and pressure fluctuations is examined numerically. The helical shape experiences the most pronounced transverse wall shear stresses at the inner wall of the curve, and its maximal wall shear stresses are higher than the sum of the maximal ones in toroidal and twisted ducts.
The second part addresses the visualization of transverse flows in millimetric ducts using an in-house developed scanning particle image velocimetry (PIV) system to capture the full flow field in a volume. Our system is adapted to small-scale oscillatory flows and allows for precise measurements of both time-resolved and net velocities. The experimental outcomes align well with the CFD simulations. The developed PIV system offers means for studying multiscale periodic flows in micro- to millimeter-scale models across a variety of applications, including biological systems like the inner ear.
The observed transverse flow phenomena offer new insights into cochlear fluid dynamics, with potential implications for metabolite transport. Further research is needed to elucidate their impact on cochlear mechanics.
Date of Publication
2023
Year of graduation
2023
Theses Type
dissertation
Subject(s)
Language(s)
en
Author(s)
Harte, Noëlle Claudia |
Faculty/Graduate School
Access(Rights)
open.access
Primary OA Publication
true