Skimming Flow Research Articles

Spatially-averaged turbulent flow over cubical roughness in wave-current co-existing environment

The paper describes an experimental study carried out in a laboratory flume to investigate the interaction of surface-wave with unidirectional current over cube mounted rough-bed (3D flow). The cubes were made of wood and were positioned at the channel-bed with different spacing. The spacing was chosen to represent different roughness types under different spacing conditions as: isolated roughness flow, wake interference flow and skimming flow. The three-dimensional velocity field was measured by an acoustic Doppler velocimetre (ADV). The study particularly focused on the changes induced in the spatially-averaged velocity profiles, turbulence intensities and Reynolds shear stress due to the superposition of waves of different frequencies on current-induced flow. Spatially-averaged velocity profiles show two distinct distributions: linear or exponential distribution below the roughness crest and logarithmic distribution above the roughness crest. It is found that the shift of the velocity profile depends strongly on the spacing of roughness elements. The profile of stream-wise turbulence intensity changes significantly below the roughness crest due to presence of surface wave, whereas bottom-normal spatially-averaged turbulence intensity does not change much with increase in wave frequency. Within the roughness crest near the bottom, the form-induced shear stress changes significantly from negative to positive value with change in roughness spacing, but is not affected much due to the presence of surface-wave.

Hydraulics of the Developing Flow Region of Stepped Spillways. II: Pressure and Velocity Fields

AbstractIn skimming flow on a stepped spillway, the upstream flow motion is nonaerated and a turbulent boundary layer develops until the outer edge of the boundary layer interacts with the free surface; that is, at the inception point of air entrainment. Herein, new experiments were performed in the developing flow region on a large 1 V∶1 H stepped spillway model with step height h=0.10 m. The flow properties in the developing flow region were carefully documented. In the developing boundary layer, the velocity distributions followed a 1/4.5th power law at the step edges. Detailed velocity and pressure measurements showed some rapid flow redistribution between step edges and above step cavities. The application of the momentum integral equation indicated an average friction factor of 0.18, which is close to the observed air-water-flow friction factor of 0.23, suggesting that the spatially averaged dimensionless shear stress was comparable in the developing flow and fully aerated flow regions.

Hydraulics of the Developing Flow Region of Stepped Spillways. I: Physical Modeling and Boundary Layer Development

AbstractOn a stepped spillway, the steps act as macroroughness elements, contributing to enhanced energy dissipation and significant aeration. In a skimming flow, the upstream flow motion is nonaerated, and the free surface appears smooth and glossy up to the inception point of free-surface aeration. In this developing flow region, a turbulent boundary layer grows until the outer edge of the boundary layer interacts with the free surface and air entrainment takes place. The flow properties in the developing flow region were documented carefully in a large stepped spillway model (1V:1H; h=0.10 m). The upstream flow was controlled by a broad-crested weir and critical flow conditions were observed along most of the weir crest, although the pressure distributions were not hydrostatic at the upstream and downstream ends. Downstream of the broad crest and upstream of the inception point, the free surface was smooth, although some significant free-surface curvature was observed for all discharges. The boundary ...

Improvement of a stochastic backscatter model and application to large‐eddy simulation of street canyon flow

A stochastic backscatter (SB) subgrid‐scale (SGS) model is applied, for the first time, to large‐eddy simulation (LES) of street canyon flow. We model a ‘skimming flow’ regime under a neutrally stratified atmosphere, in which the approaching wind is perpendicular to the along‐street axis of a street canyon of unity aspect ratio. Previous LESs of this type have shown an under‐prediction of the intensity of the primary eddy (PE) that forms within the street canyon, indicating a lack of momentum transfer across the roof‐level shear layer. The SB model, however, acts to increase this momentum transfer, bringing the simulated PE intensity significantly closer towards that observed in a corresponding wind‐tunnel experiment. A metric for the PE intensity, ωPE, based on the two‐dimensional vorticity field, is increased from around 70% of the wind‐tunnel ωPE value (with the Smagorinsky SGS model) to as much as 90% (with the SB model). Calculation of the air exchange rate at roof‐level confirms that the rate of entrainment into the street canyon is increased with the inclusion of backscatter.We also outline an improvement to the SB model prior to its application. In its previous version, a constraint on the magnitude of the backscatter acceleration variances ensured a theoretically appropriate level of additional grid‐scale (backscattered) energy. Here, a further constraint on the magnitude of the main covariance term also facilitates a better representation of grid‐scale vertical momentum flux. This new constraint alone can help to increase the simulated ωPE value by as much as 10% of the wind‐tunnel ωPE value, and requires almost no additional computational effort. The effect of varying the magnitude and length‐scale of the imposed backscatter (via the backscatter coefficient and length of the filter used to generate the backscatter acceleration fields, respectively) is also investigated.

Revisiting the flow regimes for urban street canyons using the numerical Green’s function

Vortex interactions within a two-dimensional street canyon are analysed using the numerical Green’s function. On account of the inhomogeneity of the domain, vortex interactions are asymmetric: the influence of a street-level vorticity source on the roof-level shear layer differs from that of the latter on the street level. Consequently the magnitudes of the induced vertical velocities are maximised at different aspect ratios. It is argued that the transition from isolated roughness to wake interference is related to the onset of strong long-range interactions while the transition from wake interference to skimming flow is related to the weakening of these interactions. The Green’s function analysis is verified using three-dimensional large-eddy simulations.

Large eddy simulation of natural ventilation for idealized terrace houses due to the effect of setback distance

Similar to most tropical countries, Malaysia have low wind speed and airflow characteristics to provide an effective natural ventilation system for comfortable living especially in terrace houses. Even so, by designing them with suitable threshold height/width, H/W, ratio may help reduce heat sink, or even the accumulation of contaminants within the setback distance. Through this study, the downstream building of these terrace houses will be investigated due to the effects from an upstream building. With the use of Large-Eddy Simulation (LES) method, the formation of the vortex within the threshold H/W ratio will be clearly simulated and allow the observation of flow regimes developed by each model. With increasing threshold H/W ratios the models will exhibit some wake interference flow and skimming flow which will determine the negative or positive effect of ventilation from the upstream building towards the downstream building. The airflow characteristics of the downstream house will also be analysed and the most effective layout in providing a better air circulation may be determined. Improving the natural ventilation of such houses could significantly reduce these negative effects such as the accumulation of dust, smoke or bacteria. In turn, with the alarming rate of depletion in natural resources and its effects to the environment, this study can significantly reduce energy usage for ventilation and space cooling.

Flow pattern and pollutant dispersion over three dimensional building arrays

The flow pattern and pollutant dispersion in urban canopies is investigated by large eddy simulation of flow over an array of cubes. It had been found that the pattern of flow over an isolated cubical obstacle can be characterized by an external wake of horseshoe vortex around the lower part of the windward face and an internal wake of recirculation cavity leeward. The width of the external wake Wex and the size of the internal wake Lin in the isolated roughness flow are used as key parameters to determine the wake effects on the flow and dispersion in the canopy of the same roughness height h in the isolated roughness flow. Flow patterns are categorized into five types based on the packing density as a result. The five types of urban canopy flow are introduced as (1) Isolated roughness flow when the lateral building interval WL is much greater than Wex and the streamwise building interval WS is much greater than Lin; (2) External wake interference flow when WL is less than Wex while WS is greater than Lin; (3) Internal wake interference flow when WS is in the same order of the size of Lin; (4) Skimming flow when WL is less than Wex and WS is less than Lin (5) Street network flow when WL and WS are much less than the Wex and Lin respectively. Results of time-averaging velocity field and pollutant concentration contours are demonstrated for each type of flow patterns. It is concluded that the behavior of flow pattern and pollutant dispersion is governed by the packing density from a very low packing density case, approximated as the flow around an isolated roughness element, to a high packing density case, resembled as the network flow.

Modelling atmospheric dry deposition in urban areas using an urban canopy approach

Abstract. Atmospheric dry deposition is typically modelled using an average roughness length, which depends on land use. This classical roughness-length approach cannot account for the spatial variability of dry deposition in complex settings such as urban areas. Urban canopy models have been developed to parametrise momentum and heat transfer. We extend this approach here to mass transfer, and a new dry deposition model based on the urban canyon concept is presented. It uses a local mixing-length parametrisation of turbulence within the canopy, and a description of the urban canopy via key parameters to provide spatially distributed dry deposition fluxes. Three different flow regimes are distinguished in the urban canyon depending on the height-to-width ratio of built areas: isolated roughness flow, wake interference flow and skimming flow. Differences between the classical roughness-length model and the model developed here are investigated. Sensitivity to key parameters are discussed. This approach provides spatially distributed dry deposition fluxes that depend on surfaces (streets, walls, roofs) and flow regimes (recirculation and ventilation) within the urban area.

A GLOBAL RELATION OF FIRE SMOKE RE-CIRCULATION BEHAVIOUR IN URBAN STREET CANYONS

Street canyon, formed by buildings along two sides of a road, is an important and ubiquitous architectural element in the metropolises. When a fire occurs in an urban street canyon, a critical serious phenomenon is found to happen where the uprising fire smoke is re-circulated back into the base of the street canyon by the wind flow beyond a critical velocity. Theoretical analysis is derived based on Froude number (Fr), by balancing the inertial force of the wind flow to the buoyancy strength of the fire smoke. It is found that the critical re-circulation wind velocity is proportional to an integrated global parameter . Large Eddy Simulations (LES) are carried out by Fire Dynamics Simulator (FDS). The critical Froude number (Fr) is found to be about 0.7~0.8. The simulation results are well collapsed by the generalized theoretical relation. It is further revealed that the non-dimensional critical re-circulation wind velocity . against aspect ratio of street canyons in the skimming flow pattern (W/H < 1.43) falls into two behavioural regimes, where it firstly increases then remains constant with the increase in street canyon aspect ratio (W/H) with a turning point at W/H = 1. A global non-dimensional relation is finally achieved for the critical re-circulation wind velocity (u), fire heat release rate (Q) and its height (H) to the top of the street canyon as well as the street canyon aspect ratio (W/H).

Oxygen Transfer through Air Entrainment over Stepped Weir

Oxygen transfer through the air entrainment over the stepped weir by the flow types was presented through the laboratory experiments. In the nappe flow, dominant flow features included an enclosed air pocket, a free-falling nappe impact and subsequent hydraulic jump on the downstream step. Most air was entrained through a free-falling nappe impact and a hydraulic jump. In the skimming flow, air entrainment occurs from the step edges. Downstream of the inception point, the flow is highly aerated at each and every step with very significant splashing. The average values of the oxygen transfer efficiency in the region of the nappe flow and of the skimming flow are about 0.40 and 0.28, respectively. The stepped type of the weir was found to be efficient for water treatment associated with substantial air entrainment.

Water-Channel Study of Flow and Turbulence Past a Two-Dimensional Array of Obstacles

A neutral boundary layer was generated in the laboratory to analyze the mean velocity field and the turbulence field within and above an array of two-dimensional obstacles simulating an urban canopy. Different geometrical configurations were considered in order to investigate the main characteristics of the flow as a function of the aspect ratio (AR) of the canopy. To this end, a summary of the two-dimensional fields of the fundamental turbulence parameters is given for AR ranging from 1 to 2. The results show that the flow field depends strongly on AR only within the canyon, while the outer flow seems to be less sensitive to this parameter. This is not true for the vertical momentum flux, which is one of the parameters most affected by AR, both within and outside the canyon. The experiments also indicate that, when (i.e. the skimming flow regime), the roughness sub-layer extends up to a height equal to 1.25 times the height of the obstacles (H), surmounted by an inertial sub-layer that extends up to 2.7 H. In contrast, for (i.e. the wake-interference regime) the inertial sub-layer is not present. This has significant implications when using similarity laws for deriving wind and turbulence profiles in canopy flows. Furthermore, two estimations of the viscous dissipation rate of turbulent kinetic energy of the flow are given. The first one is based on the fluctuating strain rate tensor, while the second is related to the mean strain rate tensor. It is shown that the two expressions give similar results, but the former is more complicated, suggesting that the latter might be used in numerical models with a certain degree of reliability. Finally, the data presented can also be used as a dataset for the validation of numerical models.

Oscillating Water Surface Measurement of Free Overfall with a Plunge Pool

This study presents a laser-sheet imaging technique to measure the water surface of free overfalls with plunge pool. Varying the plunge pool length of a constant approach discharge created skimming, periodic oscillatory, and nappe flows. This study analyzes the resulting oscillating water surfaces and temporal variations of water stages at particular positions in the periodic oscillatory flow condition. The oscillation period and amplitude of temporal water stage variation were determined by directly measuring the time interval and water stage differences in water stage variations, respectively. The plunge pool length and air pocket characteristics seriously affected the periodic oscillatory flow. The oscillation period increased as the plunge pool length increased. On the other hand, the water stage amplitude decreased sharply when the plunge pool exceeded a specific length. The absence of an air pocket beneath the falling nappe significantly increased the oscillation period and decreased the water stage amplitude. This study investigates water surface oscillation in the plunge pool and the upstream side of the drop. However, the mean end depth of a periodic oscillatory flow with an air pocket is also applicable to discharge estimation using the end depth method.

Empirical Modeling of Oxygen Uptake of Flow Over Stepped Chutes

The present investigation evaluates the influence of three different step chute geometry when skimming flow was allowed over them with the aim of determining the aerated flow length which is a significant factor when developing empirical equations for estimating aeration efficiency of flow. Overall, forty experiments were conducted in the Hydraulics Laboratory of the Department of Water Resources and Environmental Engineering, Ahmadu Bello University Zaria-Nigeria to determined the aeration efficiency of flow over the three different chute geometry at the water temperature T and at the standard temperature of 20 0 C. Data of the aeration parameters were analysed using a regression model to develop an empirical equation used for the prediction of aeration efficiency of flow for the three chute geometry under consideration. The results revealed that the aeration efficiency increases with an increased in the length of aeration but decreases with an increased in both the critical depth and roughness Froude number. The results also indicated that percentage of the length of aeration range from 51.6-71.4% over the stepped chute length for the range of unit discharge of 43.4-26.1 L/s.m. The aerated length in all the three chute geometry showed that they could contribute significantly to the overall aeration efficiency, especially the inclined chute type. The inclined chute configuration could be employed, optimally, as a water treatment structure. http://dx.doi.org/10.4314/njt.v34i1.6

Determination of aerodynamic parameters of urban surfaces: methods and results revisited

Estimates of aerodynamic parameters, in particular roughness length z0 and displacement height d, are important for the analysis of the roughness of an urban surface, which affects processes that occur within the urban boundary layer such as pollutant dispersion and urban ventilation. Findings regarding the aerodynamic effects of various configurations of urban arrays were compiled from various studies. Several experimental, numerical and semi-empirical studies to estimate z0 and d were reviewed and compared with each other. The results can be summarized as follows: (1) the influence of the frontal area index (λf) on z0 is significant and their relationship has been confirmed by both experimental and numerical data; (2) compared to one-parameter and two-parameter fitting methods, the three-parameter fitting method is the least accurate; (3) the physical meaning of d remains vague because its definition as the height where surface drag acts may not be accurate for sharp-edged roughness blocks and (4) the peak values of z0 for uniform and heterogeneous block heights indicate presence of skimming or wake-interference flow effects, which may influence surface roughness. Finally, the semi-empirical models were found to be limited to cases derived from available experimental data, which normally involve uniform arrays of cubes.

Discharge coefficient and energy dissipation over stepped spillway under skimming flow regime

Stepped spillways have a wide range of applicability as a hydraulic structure in large dams to dissipate energy of high-velocity flows at a downstream area of dams as well as release overflows into the downstream. In this study, the numerical simulation of the flow over the stepped spillway was investigated by using Flow3D software as an analytic flow field. The RNG k-ε model was applied as the turbulence model, and Volume of Fluid (VOF) model was used to determine the free surface flow profiles. At the first stage, the model was verified by reliable experimental data. Then, in order to investigate the various features of skimming flow regime, 112 numerical spillway models was designed, which 96 models were stepped spillway models and 16 models were smooth spillway models (i.e., WES profile). In these numerical experiment, two step sizes, six configuration, four passing discharge and four profile slopes (15, 30, 45 and 60 degrees) with various relative discharges were considered to investigate the energy dissipation and discharge coefficient rates. The results indicated that discharge coefficient rate and energy dissipation rate have inverse relationship. Also it was observed that when relative discharges increased, energy dissipation rate decreased and discharge coefficient rate increased.

Phase-detection probe measurements in high-velocity free-surface flows including a discussion of key sampling parameters

Air–water high-velocity flows are characterised by strong interactions of air bubbles and water droplets. The void fraction ranges from a few percent in bubbly flows to up to 100% at the free-surface and a reliable measurement instrumentation is the phase-detection intrusive probe. Herein new experiments were conducted on a stepped spillway (θ=26.6°) in transition and skimming flow sub-regimes yielding new insights into the turbulent air–water flow properties including the turbulence intensities and integral turbulent time and length scales. The integral turbulent scales showed self-similarity independently of the flow regime. A sensitivity analysis was conducted on the phase-detection probe signals to investigate the optimum sampling duration and frequency as well as the data analysis parameters threshold, sub-sampling duration, histogram bin sizes and cut-off effects. The results provide recommendations in terms of optimum sampling and processing parameters for high-velocity air–water flows.

Numerical investigation of flow in pool-and-weir fishways using a meshless particle method

ABSTRACTA Lagrangian meshless particle method (moving particle semi-implicit method; MPS) has been widely applied to simulate engineering problems. In this study, a new solid boundary treatment using imaginary particles instead of traditionally predefined ghost particles is developed to accommodate sharp edges in addressing practical problems. The proposed solid boundary treatment in the weakly compressible MPS (WC-MPS) model is verified for a sharp-crested weir flow, and then applied to simulate flow in pool-and-weir fishways with thin walls, which can be considered as a flow system with a series of weirs. It was found that flow characteristics in pool-and-weir fishways can be determined for given design parameters such as pool length, height of weir, fishway bed slope, and water discharge. The plunging flow and skimming flow observed in experiments are reproduced successfully by the WC-MPS model. Simulated results demonstrate satisfactory agreement with experimental data.

Effect of upstream flow regime on street canyon flow mean turbulence statistics

The effect on the flow over a street canyon (lateral length/height, L/h $$=$$ 30) of using either 3D (cube) or 2D (rectangular block) upstream roughness arrays, of the same height as the canyon, has been studied for two streamwise canyon width to height aspect ratios (AR $$=$$ W/h) of 1 and 3, in a wind tunnel using Particle Image Velocimetry. The mean streamwise velocity, shear stress, turbulent intensities and length scales, together with shear layer boundaries and mass fluxes across the canyon opening are presented for different combinations of skimming and wake-interference regimes using different upstream roughness and canyon configurations. These results show significant trends with canyon aspect ratio and roughness array plan area packing density $$(\uplambda _{\mathrm{p}})$$ with respect to 2D and 3D configurations. The mean streamwise velocity for configurations of equal $$\uplambda _{\mathrm{p}}$$ is higher in 3D than 2D configurations, while the spatially averaged shear stress is shown to be lower in 3D than 2D configurations. The relative contribution to the total turbulent kinetic energy (TKE) demonstrates that staggered and aligned arrays or 2D and 3D arrays do not produce similar profiles of TKE. Finally, the integral length scale is larger in 2D cases than 3D cases of equal $$\uplambda _{\mathrm{p}}$$ . Urban air quality is a significant concern for human health. By investigating the influence of upstream roughness on canyon flow one can determine which cases or flow regimes in both the upstream roughness and canyon will result in decreased ventilation and negatively effect the air quality of urban areas. From the present work decreased ventilation occurs in the skimming flow regime and is lowest in the case of upstream 2D bar roughness with $$\uplambda _{\mathrm{p}} = 50~\%$$ and canyon AR $$=$$ 1.

Discussion of “Skimming, Nonaerated Flow on Stepped Spillways over Roller Compacted Concrete Dams” by Inês Meireles, Floriana Renna, Jorge Matos, and Fabián Bombardelli

The nonaerated region may occupy a large portion of the skimming flow in steep, stepped spillways, particularly for relatively high unit flow rates. In spite of the numerous contributions on the hydraulic properties at both the inception point of air entrainment and the aerated region, much less is known regarding the flow in the nonaerated region. In this paper, new empirical evidence, based on an extensive data set obtained during several years in a large-scale facility, sheds light on the features of the nonaerated-flow region. Diverse ways to locate and estimate the main hydraulic properties at the inception point are first discussed and compared. Then, expressions capable of characterizing the main flow variables along the nonaerated region are presented, namely, the boundary-layer development, the velocity distribution, the equivalent clear-water depth, the characteristic depth taking into account the free-surface unsteadiness due to turbulence, and the energy dissipation. The energy dissipation is observed to be larger than that for smooth spillways, although much smaller than values typically reported for aerated flows at the toe of stepped spillways.

Initial Investigation into the Protection of Manufactured-Home Parks from Hurricanes Using Wind-Deflection Barriers

AbstractParks containing manufactured homes and trailers exist in the hurricane-prone areas of the southern United States. Homes in these parks may suffer heavy damage from high-speed winds. This paper’s objective is to present a methodology and to provide suggestions to the engineering community and protection agencies such as FEMA and the U.S. Army Corps of Engineers (USACE) about a method to lessen damage to groups of manufactured homes. To support the development of this methodology with analysis, an unsteady, Reynolds-averaged, Navier-Stokes computational tool was employed to model a manufactured-home park to evaluate how well a wind barrier installed along its upwind periphery could establish skimming flow over all the buildings in the park. A manufactured-home park that had existed in Punta Gorda, Florida, was studied because satellite pictures before and after it was heavily damaged by Hurricane Charley on August 13, 2004, were available. Computations were performed using a computer program and th...