Spatial Flow Heterogeneity Research Articles

Patch‐scale representation of vegetation within hydraulic models

AbstractSubmerged aquatic vegetation affects flow, sediment and ecological processes within rivers. Quantifying these effects is key to effective river management. Despite a wealth of research into vegetated flows, the detailed flow characteristics around real plants in natural channels are still poorly understood. Here we present a new methodology for representing vegetation patches within computational fluid dynamics (CFD) models of vegetated channels. Vegetation is represented using a Mass Flux Scaling Algorithm (MFSA) and drag term within the Reynolds‐averaged Navier–Stokes Equations, which account for the mass and momentum effects of the vegetation, respectively. The model is applied using three different grid resolutions (0.2, 0.1 and 0.05 m) using time‐averaged solution methods and compared to field data. The results show that the model reproduces the complex spatial flow heterogeneity within the channel and that increasing the resolution leads to enhanced model accuracy. Future applications of the model to the prediction of channel roughness, sedimentation and key eco‐hydraulic variables are presented, likely to be valuable for informing effective river management. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Acoustic Doppler Current Profiler measurements near a weir with fish pass: assessing solutions to compass errors, spatial data referencing and spatial flow heterogeneity

There has been an increasing interest in the use of Acoustic Doppler Current Profilers (ADCPs) to characterise the hydraulic conditions near river engineering structures such as dams, fish passes and groins, as part of ecological and hydromorphological assessments. However, such ADCP applications can be limited by compass errors, obstructed view to navigation satellites, frequent loss of bottom tracking and spatially heterogeneous flow leading to erroneous water velocity measurements. This study addresses these limitations by (i) developing a heading sensor integration algorithm that corrects compass errors from magnetic interference, (ii) testing a Total Station based technique for spatial ADCP data referencing and (iii) evaluating a recently proposed data processing technique that reduces bias from spatial flow heterogeneity. The integration of these techniques on a radio control ADCP platform is illustrated downstream of a weir with fish pass on the River Severn, UK. The results show that each of the techniques can have a statistically significant effect on the estimated total water velocities and can strongly affect measures of vorticity. The obtained three-dimensional flow maps are suitable to describe the magnitude and orientation of the fish pass attraction flow in relation to competing flows and to highlight areas of increased vorticity.

Physical heterogeneity increases biofilm resource use and its molecular diversity in stream mesocosms.

BackgroundEvidence increasingly shows that stream ecosystems greatly contribute to global carbon fluxes. This involves a tight coupling between biofilms, the dominant form of microbial life in streams, and dissolved organic carbon (DOC), a very significant pool of organic carbon on Earth. Yet, the interactions between microbial biodiversity and the molecular diversity of resource use are poorly understood.Methodology/Principal FindingsUsing six 40-m-long streamside flumes, we created a gradient of streambed landscapes with increasing spatial flow heterogeneity to assess how physical heterogeneity, inherent to streams, affects biofilm diversity and DOC use. We determined bacterial biodiversity in all six landscapes using 16S-rRNA fingerprinting and measured carbon uptake from glucose and DOC experimentally injected to all six flumes. The diversity of DOC molecules removed from the water was determined from ultrahigh-resolution Fourier Transform Ion Cyclotron Resonance mass spectrometry (FTICR-MS). Bacterial beta diversity, glucose and DOC uptake, and the molecular diversity of DOC use all increased with increasing flow heterogeneity. Causal modeling and path analyses of the experimental data revealed that the uptake of glucose was largely driven by physical processes related to flow heterogeneity, whereas biodiversity effects, such as complementarity, most likely contributed to the enhanced uptake of putatively recalcitrant DOC compounds in the streambeds with higher flow heterogeneity.Conclusions/SignificanceOur results suggest biophysical mechanisms, including hydrodynamics and microbial complementarity effects, through which physical heterogeneity induces changes of resource use and carbon fluxes in streams. These findings highlight the importance of fine-scale streambed heterogeneity for microbial biodiversity and ecosystem functioning in streams, where homogenization and loss of habitats increasingly reduce biodiversity.

Bacterial Community Composition of Stream Biofilms in Spatially Variable-Flow Environments

Streams are highly heterogeneous ecosystems, in terms of both geomorphology and hydrodynamics. While flow is recognized to shape the physical architecture of benthic biofilms, we do not yet understand what drives community assembly and biodiversity of benthic biofilms in the heterogeneous flow landscapes of streams. Within a metacommunity ecology framework, we experimented with streambed landscapes constructed from bedforms in large-scale flumes to illuminate the role of spatial flow heterogeneity in biofilm community composition and biodiversity in streams. Our results show that the spatial variation of hydrodynamics explained a remarkable percentage (up to 47%) of the variation in community composition along bedforms. This suggests species sorting as a model of metacommunity dynamics in stream biofilms, though natural biofilm communities will clearly not conform to a single model offered by metacommunity ecology. The spatial variation induced by the hydrodynamics along the bedforms resulted in a gradient of bacterial beta diversity, measured by a range of diversity and similarity indices, that increased with bedform height and hence with spatial flow heterogeneity at the flume level. Our results underscore the necessity to maintain small-scale physical heterogeneity for community composition and biodiversity of biofilms in stream ecosystems.

Form induced stresses over rough gravel-beds

This paper presents an analysis of spatial flow heterogeneity over rough gravel beds for shallow flows in terms of form induced stresses. Data from experiments specifically designed with the intention to analyze the flow data with the double-averaging methodology are used to investigate the nature of form induced stresses. It is shown that spatial flow heterogeneity is small at greater distances to the roughness tops (z100), increases slightly towards z100, and increases significantly below z100. Form induced stresses determined over the same bed and with the same slope are found to be independent of discharge. The influence of the number of measuring verticals on the magnitude of form induced stresses is discussed. The distributions of form induced stresses − are used to define the geodetic level of the roughness crest for rough, irregular beds from hydraulic data.

Role of turbulent shear rate distribution in aggregation and breakage processes

AbstractThe effect of spatial flow heterogeneity on shear‐induced aggregation and breakage of fine particles in turbulent flow conditions is investigated using computational fluid dynamics (CFD) and population balance modeling. The quadrature method of moments (QMOM), particularly suitable for implementation in commercial CFD codes, has been used to solve the corresponding population balance equation. QMOM is first tested and compared with alternative numerical methods (sectional/fixed‐pivot methods) for a specific set of realistic operating conditions. Then QMOM is implemented in a CFD code and the effect of spatial heterogeneities on the cluster mass distribution in a Taylor–Couette vessel is investigated. Simplified models have been derived based on the separation of the timescales of mixing on one hand and of aggregation and breakage on the other and compared with the full CFD model. Guidelines for use and limitations of such models and the identification of the underlying aggregation and breakage kernels are discussed. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Heterogeneous Microcirculation in the Rat Small Intestine during Hemorrhagic Shock: Quantification of the Effects of Hypertonic-Hyperoncotic Resuscitation

Purpose: In the event of a spatial or temporal microvascular perfusion heterogeneity conventional methods are often inadequate to describe the microcirculatory changes. Our aim was to use a new formula to characterize and compare the microcirculatory reactions in the mucosa and longitudinal muscle of the rat small intestine in response to hypertonic/hyperoncotic and normotonic resuscitation strategies. Methods: Intravital videomicroscopy with an orthogonal polarization spectral (OPS) imaging technique was utilized. Microcirculatory variables were recorded during hemorrhagic shock (HS; 50 mm Hg mean arterial pressure for 60 min) and fluid replacement with 0.9% saline or with 7.2% saline containing 10% hydroxyethylstarch 200/0.5 (Osmohes; 4 ml/kg). Due to the temporal perfusion variability, microcirculatory changes were described using the calculation of the average red blood cell velocity (A-RBCV), while the spatial changes were calculated as a function of the size of the perfused capillary network. Results: During HS and the late phase of resuscitation, perfusion was characterized by capillary flow motion (i.e. variability in time) in the villi, and by spatial flow heterogeneity in the longitudinal muscle layer. The approximately 40% decrease in the calculated villus A-RBCV during HS was only partially affected by 0.9% saline, whereas Osmohes completely restored A-RBCV by increasing both the red blood cell velocity and the duration of high-flow periods at the onset of resuscitation in the villi. The approximately 60% reduction in A-RBCV in the muscle layer during HS was not followed by an appreciable recovery in either group, but Osmohes significantly increased A-RBCV in the late resuscitation phase. Conclusions: The hypertonic/hyperoncotic solution induces a considerable microcirculatory improvement in two distinct layers of the small intestine after HS. This positive effect is related to the amelioration of the intestinal microcirculatory heterogeneity.

Muscle blood flow and flow heterogeneity during exercise studied with positron emission tomography in humans.

Blood flow is the main regulator of skeletal muscle's oxygen supply, and several studies have shown heterogeneous blood flow among and within muscles. However, it remains unclear whether exercise changes the heterogeneity of flow in exercising human skeletal muscle. Muscle blood flow and spatial flow heterogeneity were measured simultaneously in exercising and in the contralateral resting quadriceps femoris (QF) muscle in eight healthy men using H2(15)O and positron emission tomography. The relative dispersion (standard deviation/mean) of blood flow was calculated as an index of spatial flow heterogeneity. Average muscle blood flow in QF was 29 (10) ml x (kg muscle)(-1) x min(-1) at rest and 146 (54) ml x (kg muscle)(-1) x min(-1) during exercise (P = 0.008 for the difference). Blood flow was significantly (P < 0.001) higher in the vastus medialis and the vastus intermedius than in the vastus lateralis and the rectus femoris, both in the resting and the exercising legs. Flow was more homogeneous in the exercising vastus medialis and more heterogeneous (P < 0.001) in the exercising vastus lateralis (P = 0.01) than in the resting contralateral muscle. Flow was more homogeneous (P < 0.001) in those exercising muscles in which flow was highest (vastus intermedius and vastus medialis) as compared to muscles with the lowest flow (vastus lateralis and the rectus femoris). These data demonstrate that muscle blood flow varies among different muscles in humans both at rest and during exercise. Muscle perfusion is spatially heterogeneous at rest and during exercise, but responses to exercise are different depending on the muscle.

Blood flow heterogeneity in the heart.

Local deposition density of microspheres is heterogeneous in histologically homogeneous myocardium under physiological conditions. The underlying biological heterogeneity must be distinguished from a methodological heterogeneity which depends preferentially on the number of microspheres injected, blood flow to a particular myocardial region and sample mass. As the variables space (spat), time (temp), and method (meth) are independent of each other, the observed (obs) variability may be approximated using the coefficients of variation (CV) of the individual variables: CVobs = (CV2spat+CV2temp+CV2meth)0.5. Studies in which these different variables have been quantified indicate that the largest fraction of the observed variability of microsphere deposition density is contributed by spatial flow heterogeneity which exists independent of the myocardial layer. Spatial flow heterogeneity increases with decreasing sample mass and decreasing mean flow. Fractal and autocorrelation analyses have shown that adjacent myocardial flows are spatially correlated and nonrandom. Local blood flow was shown to correlate with various metabolic and transport rates, while no differences were found between low and high flow regions with respect to several metabolic markers of tissue hypoxia. In conclusion, the evidence available to date indicates that 1) in histologically homogeneous myocardium there exists a spatial blood flow heterogeneity which 2) is temporally stable, 3) resolution dependent, 4) largely layer-independent, 5) nonrandom, and 6) related to local aerobic metabolism.

Temporal and Spatial Heterogeneity of Blood Supply to the Heart: Visualization and Interpretation

Blood flow of the heart muscle (the coronary circulation) exhibits both temporal and spatial heterogeneity. In this paper, the unique blood velocity waveform during a cardiac cycle, the flow fluctuation of a longer period and the within-layer spatial flow heterogeneity of the coronary circulation are described with a reference to the pathogenesis of angina pectoris and myocardial infarction.

Autoradiographic assessment of blood flow heterogeneity in the hamster heart.

Provide regional flow measurement in the hearts of small mammals using a new, higher-resolution technique based on the deposition of a molecular marker. We determined the instantaneous extraction and retention of the "molecular microsphere" radiolabeled desmethylimipramine in retrogradely perfused hamster hearts. In a separate series of experiments, autoradiography was used to measure regional myocardial deposition densities in hamster hearts of about 0.5 g with spatial area resolution of 16 x 16 microns. Radiolabeled desmethylimipramine is almost 100% extracted during a single transcapillary passage and is retained in the tissue for many minutes. Autoradiographic images demonstrated a spatial flow heterogeneity with standard deviations of 31 +/- 4% of the mean flow (N = 5) in 16 x 16 x 20-micronm3 voxels. This is equivalent to the projections made using fractal relationships from cruder observations obtained with microspheres in the hearts of baboons, sheep, and rabbits. Autoradiography using a molecular deposition marker provides quantitative information on myocardial flow heterogeneities with resolution at the size of cardiac myocytes. Because the regions resolved are smaller than the volume of regions supplied by single arterioles, the results must slightly exaggerate the true heterogeneity of regional flows.

Heterogeneity of unsaturated, gravitational flow of water through beds of large particles

Water was applied uniformly as a fine spray onto the surface of beds of nonabsorbing particles. The outflow at the base of the bed was collected separately from 74 equal, small collecting areas. The spatial heterogeneity of the outflow was determined as a function of bed height, particle diameter, and the size of the collection areas. The results show that spatial heterogeneity develops with increasing bed depth and decreases with increasing area of the collectors. Particle diameter has only a small effect on flow heterogeneity. It is hypothesized that spatial heterogeneity develops because individual flow pathways, or rivulets, take a “random walk” downward through the particles in the bed. When two flows meet by chance, they combine and stay combined. In this way, the number of rivulets per unit cross‐sectional area of bed decreases with increasing depth, and the spatial flow heterogeneity increases. It is proposed that this process may occur in the tilled layer of soil.