The present study covers both a numerical and experimental investigation of the processes
in the oscillatory boundary layer. In the first part a direct numerical simulation
(DNS) is conducted to study the vertical pressure gradient, and its role in relation to
laminar to turbulent transition and its role in the fully turbulent boundary layer. The
pressure in the flow is obtained from the flow fields of the oscillatory boundary layer.
What differs, the vertical pressure gradient, from other turbulent quantities, like e.g.
velocity fluctuations is that it can detect newly generated turbulence. This is in contrast
to velocity fluctuations that are diffusive, so they can also contain residual turbulence
from the previous half cycle until they are dissipated. Furthermore, the magnitude of
the mean value of conditionally averaged vertical pressure gradient (for −∂p∗/∂x∗
2 > 0)
is compared to the submerged weight of sediment. This revels that the upward directed
vertical pressure gradient on average has a magnitude that yields in a contribution to
the force needed to overcome the submerged weight of the water-sediment mixture.
Secondly particle motion in the oscillatory boundary layer is investigated. The
experiment is conducted in a oscillating water tunnel, for both smooth bed and rough
bed. The particle motion is determined by utilizing particle tracking base on a video
recording of the particle motion in the flow. In the oscillatory flow, in contrast to
steady current, the particle motion is a function of phase. Therefore the particle will
settle towards the end of each half period, and after flow reversal, when the turbulent
intensity becomes large enough it can be suspended. If the particle is light enough it
can be maintained in suspension, otherwise it will settle before it is resuspended. The
governing parameter for the particle motions after after it is brought into suspension, the
Rouse parameter (β = ωs/(κUf ), ωs the settling velocity, κ = 0.4 Karmans constant).
For large values of the Rouse parameter, the particle tend to stay near the bed while
for smaller values the particle spends more time away from the bed.