Isolated Roughness Flow Research Articles

Strategic guidelines for street canyon geometry to achieve sustainable street air quality

This paper is concerned with the motion of air within the urban street canyon and is directed towards a deeper understanding of pollutant dispersion with respect to various simple canyon geometries and source positions. Taking into account the present days typical urban configurations, three principal flow regimes “isolated roughness flow”, “skimming flow” and “wake interference flow” (Boundary Layer Climates, 2nd edition, Methuen, London) and their corresponding pollutant dispersion characteristics are studied for various canopies aspect ratios, namely relative height ( h 2/ h 1), canyon height to width ratio ( h/ w) and canyon length to height ratio ( l/ h). A field-size canyon has been analyzed through numerical simulations using the standard k- ε turbulence closure model. It is found that the pollutant transport and diffusion is strongly dependent upon the type of flow regime inside the canyon and exchange between canyon and the above roof air. Some rules of thumbs have been established to get urban canyon geometries for efficient dispersion of pollutants.

A Laboratory Model of Urban Street-Canyon Flows

Abstract A circulating water channel is constructed to examine urban street-canyon flow. In the cases of an even-notch street canyon in which model buildings on both sides of the street have equal heights, one vortex is observed in model canyons with aspect ratios of 1 and 1.5, and two counterrotating vortices are observed in canyons with aspect ratios of 2, 2.4, and 3. In all of the even-notch cases, the center of the vortex (or the upper vortex) is located slightly downstream of the canyon center, and the downward motion downstream is stronger than the upward motion upstream. The magnitudes of the maximum updraft and downdraft are almost independent of the aspect ratio. In the case of a stepup notch, one vortex is observed in the canyon. In the case of a stepdown notch, two counterrotating vortices are observed. The upper vortex resembles to some extent an isolated roughness flow, and the lower vortex is characterized by a skimming flow. It is shown that the results of the water-channel experiments are ...

Influences of Woody Debris on Flow Patterns and Channel Morphology in a Low Energy, Sand-Bed Stream Reach

In lowland areas, such as the glacial landscapes of eastern Germany, sand-bed streams are the most common stream type. They have low gradients and their hydrological regime is often subdued due to the frequent interruption by lakes. Very few is known about the influence of woody debris in these streams, since nearly all previous studies are from high-gradient conditions, where streams have coarse bed sediments and harsh hydrological regimes. The research objectives of this study were first to assess the quasi-natural quantity and quality of wood in a lowland sand-bed stream and second to understand the influence of wood on the channel morphology and the flow patterns at base-flow. The three-dimensional stream bed relief was surveyed by electronic distance measurement. The position and the size of large woody debris was assessed by close-up photography. An acoustic Doppler velocimeter was used to record the patterns of flow velocity and turbulence. Overlay and analysis of the spatial data was done using a Geographic Information System. The standing stock of wood was 1.9 m3 and 39 woody elements per 100 m2 of stream bed. The flow pattern was clearly controlled by the wood. Woody elements elevated above the stream bed deflected flow and locally caused strong secondary current, high turbulence, and scour of the stream bed at baseflow. Wood resting directly on the stream bed, which contributed the majority of the wood inside the bank-full channel, determined the roughness of the stream bed. Near-bed flow patterns observed were isolated roughness flow and wake interference flow, which was registered inside the accumulations of wood. 68% of the stream bed had shear stress above critical. Hence, the secondary morphological structures of the sand-bed were controlled at base-flow by the flow which was determined by the woody debris distribution.

Influences of Woody Debris on Flow Patterns and Channel Morphology in a Low Energy, Sand-Bed Stream Reach

In lowland areas, such as the glacial landscapes of eastern Germany, sand-bed streams are the most common stream type. They have low gradients and their hydrological regime is often subdued due to the frequent interruption by lakes. Very few is known about the influence of woody debris in these streams, since nearly all previous studies are from high-gradient conditions, where streams have coarse bed sediments and harsh hydrological regimes. The research objectives of this study were first to assess the quasi-natural quantity and quality of wood in a lowland sand-bed stream and second to understand the influence of wood on the channel morphology and the flow patterns at base-flow. The three-dimensional stream bed relief was surveyed by electronic distance measurement. The position and the size of large woody debris was assessed by close-up photography. An acoustic Doppler velocimeter was used to record the patterns of flow velocity and turbulence. Overlay and analysis of the spatial data was done using a Geographic Information System. The standing stock of wood was 1.9 m3 and 39 woody elements per 100 m2 of stream bed. The flow pattern was clearly controlled by the wood. Woody elements elevated above the stream bed deflected flow and locally caused strong secondary current, high turbulence, and scour of the stream bed at baseflow. Wood resting directly on the stream bed, which contributed the majority of the wood inside the bank-full channel, determined the roughness of the stream bed. Near-bed flow patterns observed were isolated roughness flow and wake interference flow, which was registered inside the accumulations of wood. 68% of the stream bed had shear stress above critical. Hence, the secondary morphological structures of the sand-bed were controlled at base-flow by the flow which was determined by the woody debris distribution.

An ecologically useful classification of mean and near‐bed flows in streams and rivers

SUMMARY. 1. Mean motion and near‐bed flows in streams and rivers can be described using a classification derived from fairly simple field measurements. Our proposed classification is ecologically useful because it incorporates the combined effects of velocity, depth and substrate roughness to provide a means of quantifying the flow regimes occurring within the microhabitats of stream benthos.2. Mean motion is characterized by the Reynolds number and the Froude number. Both are easily calculated, and because they are dimensionless they provide a means of comparing flows at different sites.3. Five categories of near‐bed flows (i.e. the flow microenvironments of stream benthos) are recognized. Flow may be hydraulically smooth or hydraulically rough and the latter category is subdivided further into: chaotic flow, wake interference flow, isolated roughness flow and skimming flow. Hydraulically smooth flows occur in sections of a river bed with fine sediments (e.g. sands, muds and clays). over flat sheets of bedrock, or in association with the flat blades of submerged macrophytes. Hydraulically rough flows occur where the substrate elements are larger (e. g. pebbles, cobbles and boulders) and are a function of substrate roughness and the depth of flow relative to the height of the roughness elements. Chaotic flows and wake interference flows predominate in riffles whilst isolated roughness flows and skimming flows are more likely to be a feature of runs.4. Conventional stream sampling methods (e.g. the Surber and box or cylinder samplers) may collect across several different flow microhabitats. Our classification should enable different flow microenvironments to be recognized and so sampled more appropriately which, in turn, may reduce apparent clumping and the wide confidence intervals of benthic population estimates. Because our classification identifies ‘patches’ within the flow regime associated with the stream bed it enables stream ecologists to generate testable hypotheses regarding the distribution and abundance of benthic species in response to flow.5. Our classification identifies spatial patterns in the flow regimes associated with the stream bed. Temporal patterns have not been identified: however, predictable changes in spatial patterns will resuh from temporal changes in stream discharge.