Rolf Henniger

The effect of rocking stapes motions on the cochlear fluid flow and on the basilar membrane motion

The basilar membrane (BM) and perilymph motion in the cochlea due to rocking stapes motion (RSM) and piston-like stapes motion (PSM) is modeled by numerical simulations. The full Navier-Stokes equations are solved in a two-dimensional box geometry. The BM motion is modeled by independent oscillators using an immersed boundary technique. The traveling waves generated by both stimulation modes are studied. A comparison of the peak amplitudes of the BM motion is presented and their dependence on the frequency and on the model geometry (stapes position and cochlear channel height) is investigated. It is found that the peak amplitudes for the RSM are lower and decrease as frequency decreases whereas those for the PSM increase as frequency decreases. This scaling behavior can be explained by the different mechanisms that excite the membrane oscillation. Stimulation with both modes at the same time leads to either a slight increase or a slight decrease of the peak amplitudes compared to the pure PSM, depending on the phase shift between the two modes. While the BM motion is dominated by the PSM mode under normal conditions, the RSM may lead to hearing if no PSM is present or possible, e.g., due to round window atresia.

Direct numerical simulations of particle transport in a model estuary

We investigate numerically the mixing of freshwater with ambient saltwater in a model estuary along with associated particle settling processes. We first discuss and specify two numerical setups that consider several relevant features to study the particle settling. The first configuration is a large rectangular basin with a small inlet for the (particle-laden) freshwater; the second is geometrically identical to the first except that the flow is laterally confined to the narrow inlet width. The two flows are computed until a statistically stationary solution is reached. We perform highly resolved direct numerical simulations using a high-order finite difference approach to yield reliable and accurate results. Accordingly, all relevant turbulent scales are resolved and turbulence modeling is not needed. The main target of this study is to describe and illustrate the fluid dynamics and the particle settling processes under the influence of turbulence arising in the freshwater/saltwater-stratified mixing la...

Temporal evolution, morphology, and settling of the sediment plume in a model estuary

We examine the transport and settling of particles in a laboratory-scale model estuary using direct numerical simulation. The configuration is a shallow saltwater-filled basin with large horizontal dimensions. The particle-laden freshwater enters the basin over a relatively small inlet. Turbulence is generated by the collapse of Kelvin–Helmholtz vortices triggered in the freshwater/saltwater stratified mixing layer. The flow is computed until a statistically stationary solution is attained. The results demonstrate a significant increase of the particle settling speed compared to Stokes particle settling. For the onset of the simulation, this increase is attributed to sheet and finger convection. At later stages, we find that the increased settling speed is a result of the turbulent mixing of the particle suspension with clear ambient fluid, the density differences between the different phases, and the Stokes settling. The increase of the settling velocity is most pronounced for large Reynolds numbers, Ric...

Simulation of gravity-driven flows using an iterative high-order accurate Navier--Stokes solver

We investigate by DNS the spatial growth of disturbances imposed at the inflow in 2D and 3D stratified mixing layer configurations. At the present moderate Reynolds, Schmidt and Richardson numbers the disturbances trigger Kelvin–Helmholtz waves which break down to small-scale fluctuations. The results are found to be strongly influenced by the choice of the inflow setup, especially the disturbance type and magnitude.

Reynolds Number Influence on the Particle Transport in a Model Estuary

The details of the transport of riverine sediments to the ocean are not fully understood (Geyer et al., 2004; McCool and Parsons, 2004). Normally the freshwater-particle mixture is lighter than estuarine saltwater such that most river plumes are positively buoyant. Thus, the particles can be transported over relatively large distances with the freshwater current until their settling dominates over the horizontal transport. Generally, two different settling modes are known to increase the average particle settling speed significantly (McCool and Parsons, 2004): flocculation of individual particles forming larger effective aggregates with larger Stokes settling speeds (Geyer et al., 2004) and the settling enhancement due to turbulence (McCool and Parsons, 2004).

Large-eddy simulation of particle-driven gravity currents using the Relaxation-Term model

We present Large-Eddy Simulations (LES) of two laboratory-scale lock-exchange flows at Grashof numbers of up to 108 and a Schmidt number of unity. The unresolved subgrid scales (SGS) are modeled with the Relaxation-Term (RT) approach. We compare the LES results with those of fully resolved Direct Numerical Simulations (DNS) to rate the quality of the approach. We find that reductions of the grid resolution by factors of about four in each spatial and in the temporal direction are feasible compared to marginally resolved DNS. This corresponds to a reduction of computer time by about two orders of magnitude.