Showing papers by "Rex Britter published in 1980"

The motion of the front of a gravity current travelling down an incline

Abstract: The motion of the head of a gravity current travelling down a slope of angle θ to the horizontal is investigated in the laboratory. The head is produced by suddenly initiating a buoyancy flux from a line source at the top of the slope. It is found that for very small slopes (θ [les ] 0.5°) the head decelerates with distance from the source, but at greater slopes the buoyancy force is large enough to overcome frictional effects and a steady head velocity results. Over a wide range of slope angles the front velocity Uf, non-dimensionalized by the cube root of the buoyancy flux (g′0Q)1/3, is almost independent of the slope angle and Uf/(g’0Q)1/3 = 1.5 ± 0.2 for 5° [les ] θ [les ] 90°. This result is shown to follow from some simple analysis which relates the velocity of the front to the following flow. For a Boussinesq plume the front velocity is found to be approximately 60% of the mean velocity of the following flow. This means that the head increases in size as it travels down the slope, both by direct entrainment into the head itself and by addition of fluid from the following flow. We find that direct entrainment increases with increasing slope and accounts for one-tenth of the growth of the head at 10° and about two-thirds at 90°.

A laboratory model of an atmospheric mesofront

Abstract: Consideration is given to the leading edge of a well-defined flow of dense air without significant condensation, such as occurs at a thunderstorm outflow or a sea-breeze front. Explanation of the front is proposed in terms of a gravity (density) current formed by the denser air. Observations of such flows are related both to laboratory gravity current heads in which head, tail and calm wind conditions are simulated and to a simple, theoretical gravity current model based primarily on a momentum balance. Good agreement is found between the atmosphere, laboratory and analytical results. The mean flow in the gravity current behind the head is greater than the velocity of the head, a result of mixing between the two fluids. This mixing nearly all occurs at the leading edge of the current. The excess height of the head of the gravity current is interpreted as the mixing region at the leading edge, and this mixed fluid lays down a stable layer above the following gravity current. In the laboratory the effect of a headwind is to flatten the profile of the raised head at the front and to reduce its rate of advance by about three-fifths of the value of the opposing flow.

A fluid modeling study of the flow structure and plume impingement on a three-dimensional hill in stably stratified flow

Abstract: The flow structure around simple three-dimensional obstacles in stably stratified flow is experimentally studied using a large towing tank. Under certain conditions, plumes may impinge directly on an obstacle giving far higher concentrations in comparison with plumes diffusing to the surface. The conditions for plume impingement and resulting concentrations are quantified and interpreted. Requirements for a strong downslope wind in the lee of the obstacle are noted. The results are relevant to winds around hills in a stable atmosphere and local transport and dispersion of pollutants.