The influence of bed roughness on the dynamics of gravity currents

To date the influence of bed roughness onl the propagation and dynamics of gravity currents
has been largely neglected. A new physical modelling dataset has been compiled,
which details the fundamental affects of several bed roughnesses on lock-release gravity
currents. Five bed configurations were chosen encompassing 'grain' and 'form' type elements
at a range of spacings. 1%, 5% and 10% initial density excesses were studied and
the effect of removing the buoyant ambient fluid between the elements examined. Observations
due to changing the current depth relative to the element height were also made.
Ultrasonic Doppler velocimetry profiling (UDVP) and video capture techniques were
used to analyse stream wise and vertical velocity structures and the affects on the front
speed and distance travelled by the current. A 10 depth-averaged model solves modified
2-layer shallow water equations using the method of characteristics to obtain temporal
velocity and depth evolution for a current under the influence of a general roughness
quantity. 2D and 3D depth-resolved CFD simulations use the commercial software FLUENT
to solve the RANS equations and transport of a scalar for the dense current with
the RNG k - € turbulence model. The CFD predictions were well validated by the new
experimental dataset and provide supplementary predictions of concentration, lateral
motion and activity in the vicinity of the roughness elements. Comparison of 20 and 30
models resulted in the conclusion that the 3D model is vital for accurate simulation of
internal dynamics of gravity current propagation over beam type bed roughness. In general
general,
the distance that the front travels decreases with any bed roughness present.This
reduction increases with element spacing. The stream wise mean velocity profiles show
a reduced velocity maximum further from the bed. Decreased entrainment results from
breakdown of larger billows. Also observed is a thicker current, a rounder profile and a
shorter, diluted head. Areas of increased vertical motion within the current. associated
with decreased horizontal motion are observed, indicative of ejections of ambient fluid
from between the elements. The presence of this fluid is found to contribute to ~ 50% of
the current retardation. There are also similarities with the effects of bed roughness in
open channel and pipe flows, most notably there is a critical element spacing (11'/ kr ~ 7)
where the effects of roughness are greatest (where w is element spacing and kr is element
height). The experimental and numerical results demonstrate that the application
of existing models that rely on experimental validation with smooth beds to situations
where a rough boundary is present may lead to significant errors.