Drag reduction in wave-swept macroalgae: Alternative strategies and new predictions

? Premise of the study: Intertidal macroalgae must resist extreme hydrodynamic forces imposed by crashing waves. How does frond flexibility mitigate drag, and how does flexibility affect predictions of drag and dislodgement in the field? ? Methods: We characterized flexible reconfiguration of six seaweed species in a recirculating water flume, documenting both shape change and area reduction as fronds reorient. We then used a high-speed gravity-accelerated water flume to test our ability to predict drag under waves based on extrapolations of drag recorded at slower speeds. We compared dislodgement forces to drag forces predicted from slow- and high-speed data to generate new predictions of survivorship and maximum sustainable frond size along wave-swept shores. ? Key results: Bladed algae were generally "shape changers", limiting drag by reducing drag coefficients, whereas the branched alga Calliarthron was an "area reducer", limiting drag by reducing projected area in flow. Drag predictions often underestimated actual drag measurements at high speeds, suggesting that slow-speed data may not reflect the performance of flexible seaweeds under breaking waves. Several seaweeds were predicted to dislodge at similar combinations of velocity and frond size, suggesting common scaling factors of dislodgement strength and drag. ? Conclusions: Changing shape and reducing projected area in flow are two distinct strategies employed by flexible seaweeds to resist drag. Flexible reconfiguration contributes to the uncertainty of drag extrapolation, and researchers should use caution when predicting drag and dislodgement of seaweeds in the field.     

Drag forces on common plant species in temperate streams: consequences of morphology, velocity and biomass

Swift flow in streams may physically influence the morphology and distribution of plants. I quantified drag as a function of velocity, biomass and their interaction on the trailing canopy of seven European stream species in an experimental flume and evaluated its importance for species distribution. Drag increased at a power of 1.3–1.9 with velocity and 0.59–0.77 with biomass in 75% of the measurements. Velocity and biomass interacted because higher velocity causes reconfiguration and greater internal shelter to unimpeded flow and higher biomass enhances shelter among neighbouring shoots. Increase of drag with velocity did not differ systematically among inherently streamlined or non-streamlined species while increase of drag with biomass was smallest among non-streamlined shoots which provide greater mutual shelter. At low shoot density, inherently streamlined species usually experienced the lowest drag conducive to colonisation and growth in swift flow. At high shoot density, no systematic differences in drag existed between the two morphologies. No clear relationship existed between drag forces, morphology and field distribution of species as a function of current velocity probably because a variety of environmental conditions and plant traits influences distribution. Drag on the trailing canopy usually increased 15- to 35-fold for a 100-fold increase of biomass suggesting that an even distribution of plants at low density across the stream bed offers greater resistance to downstream flow than an uneven distribution with the same biomass confined to dense patches surrounded by open flow channels. Thus, management strategies to ensure a patchy plants distribution should be suitable for combining agricultural drainage and ecological stream quality. Handling editor: S. M. Thomaz     

Velocity gradients and turbulence around macrophyte stands in streams

1. Submerged macrophytes strongly modify water flow in small lowland streams. The present study investigated turbulence and vertical velocity gradients using small hot-wire anemometers in the vicinity and within the canopies of four macrophyte species with the objective of evaluating: (a) how plant canopies influence velocity gradients and shear force on the surfaces of the plants and the stream bed; and (b) how the presence and morphology of plants influence the intensity of turbulence. 2. Water velocity was often relatively constant with water depth both outside and inside the plant canopies, but the velocity declined steeply immediately above the unvegetated stream bed. Steep vertical velocity profiles were also observed in the transition to the surface of the macrophyte canopy of three of the plant species forming a dense shielding structure of high biomass. Less steep vertical profiles were observed at the open canopy surface of the fourth plant species, growing from a basal meristem and having the biomass more homogeneously distributed with depth. The complex distribution of hydraulic roughness between the stream bed, the banks and the plants resulted in velocity profiles which often fitted better to a linear than to a logarithmic function of distance above the sediment and canopy surfaces. 3. Turbulence increased in proportion to the mean flow velocity, but the slope of the relationships differed in a predictable manner among positions outside and inside the canopies of the different species, suggesting that their morphology and movements influenced the intensity of turbulence. Turbulence was maintained in the attenuated flow inside the plant canopies, despite estimates of low Reynolds numbers, demonstrating that reliable evaluation of flow patterns requires direct measurements. The mean velocity inside plant canopies mostly exceeded 2 cm s^??1 and turbulence intensity remained above 0.2 cm s^??1, which should be sufficient to prevent carbon limitation of photosynthesis in CO₂-rich streams, while plant growth may benefit from the reduced physical disturbance and the retention of nutrient-rich sediment particles. 4. Flow patterns were highly reproducible within canopies of the individual species despite differences in stand size and location among streams. We propose that individual plant stands are suitable functional units for analysing the influence of submerged macrophytes on flow patterns, retention of particles and biological communities in lowland streams.     

The drag and reconfiguration experienced by five macrophytes from a lowland river

Drag and flexibility of five macrophytes (fresh mass 3 g) collected from the same river were measured at velocities from 0 to 0.5 m s⁻¹ in a flume. Drag increased with increasing velocity for all five species examined. Sparganium emersum Rehmann, which has simple strap-like leaves experienced significantly less drag than the other, bushier species whilst there was no significant difference between the drag on Callitriche stagnalis Scop., Ranunculus penicillatus pseudofluitans (Syme) S.D. Webster, and Myriophyllum spicatum L. above 0.4 m s⁻¹. Potamogeton x zizii W.D.J. Koch ex Roth, which has large flat leaves, experienced significantly higher drag than all the other species. All the plants were very flexible but flexibility (as angle of bend) did not explain the drag experienced by the plants, e.g. S. emersum was the least flexible. The plants also changed shape and compressed (reconfigured) under increasing water velocity which reduced the rate at which drag increased. Reconfiguration capacity was assessed as E-values. There were no significant differences in E-values between species indicating that all the samples examined had a similar capacity to reconfigure. It is concluded that measurement of the drag experienced by plants is useful and may prove helpful in explaining the distribution of macrophytes in rivers.     

EFFECT OF CURRENT VELOCITY ON THE DETACHMENT OF THALLI OF ULVA LACTUCA (CHLOROPHYTA) IN A NEW ZEALAND ESTUARY

Experiments were undertaken in a recirculating flume to determine the relationships among water velocity, thallus area, drag, and the probability of thallus breakage or detachment in the foliose green alga Ulva lactuca L. In all specimens tested to breaking point, thalli detached from their bivalve substrates as a result of stipe breakage rather than in midthallus or by holdfast detachment. There was no relationship between thallus size and drag at which detachment occurred. Rather, the probability of detachment was normally distributed about a mean drag of 0. 70 N (95% confidence limits 0.55–0.85 N). Average breaking stress of stipes was 345 kN.m^-2 (95% cl 250–485 kN.m^-2). Similar results were obtained in field experiments where the horizontal force required to detach thalli was measured directly as 0.93 N (95% cl 0.69–1.15 N). Drag coefficients of plants were not constant with water velocity but increased up to 0.4 m.s^-1, declining exponentially at velocities above this. Empirical relationships were established between coefficient of drag and Reynold's number and, hence, among drag, thallus area and water velocity. These relationships permitted estimation of mean water velocity at which plants of a given area would detach .     

Flow-induced forces on free-floating macrophytes

Free-floating macrophytes have buoyant petioles and unanchored roots; certain species are highly invasive, owing to characteristics such as high growth rates and the formation of dense floating mats that drift on wind and water currents. Water hyacinth (Eichhornia crassipes) is one example; its invasion of tropical and subtropical freshwater systems worldwide harms native ecosystems and impedes human activities. This research examines flow-induced forces and biomechanical properties of E. crassipes to better understand flow interactions and transport mechanisms. Drag forces were measured in a flume and a wind tunnel for varying approach velocities and raft configurations; from this data, drag coefficients in water (C _D^w) and air (C _D^a) were developed. Over similar Reynolds number (Re _b ) regimes, C _D^w decrease as Re _b increases while C _D ^a are invariant. For the same raft tested in air and water, water drag exceeds air drag and the value of C _D^w approaches C _D^a at high Re _b . Force–velocity relationships indicate root canopies reconfigure by streamlining in higher flow velocities while leaf canopies do not. Root canopy streamlining is further explained through biomechanical testing: we found the major vegetative structures of E. crassipes (roots, stolons, and petioles) have similar moduli of elasticity but second moments of area are three orders of magnitude smaller in roots compared to stolons or petioles, leading to significantly lower flexural rigidity in roots than in stolons or petioles. Flow interactions with the root canopy differ for an individual plant compared to a raft assemblage. Laboratory results suggest that water currents are the dominant mechanism for E. crassipes dispersal.     

The debate about drag and reconfiguration of freshwater macrophytes: comparing results obtained by three recently discussed approaches

1. The paper by Sand‐Jensen (2003 , Freshwater Biology, 48 , 271–283) on drag and reconfiguration of freshwater macrophytes stimulated comments and a reply about the use of variables in assessments of the drag coefficient (C_d) or the Reynolds number (Re) of such plants. Although the physical argument in this debate starts from the same equations, it diverges into approaches that address differently the dynamic behaviour of flexible plants in the boundary layer flow (the typical condition experienced by lotic macrophytes). 2. We compared three (potentially among many more) such different approaches using some preliminary experiments with Egeria densa by measuring drag (and other physical variables of interest) on (1) a single shoot exposed to varying flume flows; (2) a single shoot exposed to a constant flume flow, from which we subsequently pruned off pieces from the distal end; and (3) multiple shoots exposed to a constant flume flow after they had experienced replicated flow disturbances (causing shoot reconfiguration). 3. These experiments illustrated that the three approaches can produce opposite trends in the relationship between C_d and Re and that, for a given plant and flow, the C_d values obtained by these approaches can differ by about two orders of magnitude. Thus, conventions about the use of variables are required for experiments on drag and reconfiguration of freshwater macrophytes, otherwise the field will be plagued by a multitude of incomparable results.     

Reconfiguration as a Prerequisite for Survival in Highly Unstable Flow-Dominated Habitats

Unstable and mechanically demanding habitats like wind-exposed open fields or the wave-swept intertidal require rapid adaptive processes to ensure survival. The mechanism of passive reconfiguration was analyzed in two plant models exposed to irregular flow of water or air, two species of the brown seaweed Durvillaea and the giant reed Arundo donax. Irrespective of the surrounding media and the subsequent Reynolds numbers (Re ~ 10⁵ - 10⁷), reconfiguration seems to be the key strategy for streamlining to avoid overcritical drag-induced loads. This passive mechanism is also discussed in the context of the requirement of a maximized surface area for light interception, so that morphological adaptations to rapid reconfiguration represent at least a bifactorial optimization. Both tested plant models exhibited the same principles in streamlining. At a specific threshold value, the proportionality between drag forces and flow velocity can be reduced from the second power close to an almost linear relation. This empirically derived relation could be characterized by a figure of merit or Vogel number (B). A value close to B = -1, resulting in a linear increase of drag force with velocity, was found at higher velocities for both the seaweeds and the giant reed, as well as for a variety of plants described in the literature. It is therefore concluded that the ability to reduce velocity-dependent drag force to a linear relation is a potentially important adaptation for plants to survive in unstable flow-dominated habitats.     

A three‐dimensional numerical model investigation of the impact of submerged macrophytes on flow dynamics in a large fluvial lake

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In situ measurements of hydrodynamic forces imposed on Chondrus crispus Stackhouse

Among the hydrodynamic forces experienced by intertidal organisms, drag and the impingement force are thought to have the greatest effect on macroalgae. These forces are modified by biotic factors such as algal morphology, reconfiguration, and the presence of a canopy. However, much of what is known about the hydrodynamics of macroalgae has been garnered from low-velocity laboratory flume studies. Few field studies have measured drag and none have directly measured the effects of the canopy on force. To examine in situ hydrodynamic forces imposed on the turf forming macroalga Chondrus crispus, compact digital force sensors were developed that measure and record the 3-dimensional force imposed on a macroalga without disturbing the surrounding canopy. Sensors were positioned within natural Chondrus beds and the effects of the canopy, algal morphology, and sea state on in situ hydrodynamic force were examined. Additionally, the predictions of a new model for drag on flexible macroalgae were tested by simultaneously measuring force and water velocity. Digital force recordings indicated that Chondrus only experience drag; lift and impingement force were negligible in all combinations of factors. Canopies significantly reduced drag by 15-65%. Morphology and size also influenced drag, such that lower forces were imposed on small planar algae than large arborescent individuals. Further, planar algae experienced low drag in all combinations of sea and canopy state, indicating that these individuals may not be as susceptible to wave disturbance as arborescent individuals. Overall, these data indicate that the ability for Chondrus to grow large, arborescent individuals is dependent on the drag reducing properties of the canopy, while more hydrodynamically harsh habitats may be accessible to planar morphologies. Additionally, these data suggest that drag models for canopy forming macroalgae must incorporate the effects of the canopy to predict drag accurately in situ.     

Effects of water velocity on photosynthesis and dark respiration in submerged stream macrophytes

The effects of flow velocities on dark respiration and net photosynthesis of eight submerged stream macrophytes were examined in a laboratory oxygen chamber. The shoots/leaves were exposed to saturating free-CO₂ concentrations and were attached basally so that they could move in the flowing water. Net photosynthesis declined by 34–61% as flow velocity increased from 1 to 8.6cm s^?1, while dark respiration increased 2.4-fold over the same range. The increase in dark respiration could only account for between 19 and 67% of the decrease in net photosynthesis. The relationship between flow velocity (U) and net photosynthesis (P) was described by: P=b×U^a. The exponent, a, varied from -0.20 to –0.48 and showed a negative correlation to the surface: volume (SA: V) ratio of the plants, i.e. species with high SA: V ratio were more sensitive to flow. In contrast, net photosynthesis of plants firmly attached to a supporting frame was not significantly affected by increasing flow velocity. This result indicates that the physical stress imposed on the plants by agitation or stretching in the flowing water is a key factor for the observed response.     

The behavior in flow of the morphologically variable seaweed Hedophyllum sessile (C. Ag.) Setchell

Drag force was measured on individual specimens of Hedophyllum sessile in a variable-speed flow tank. Those from sheltered localities, which are broad, bullate blades, experience greater drag at a given water velocity than ones from localities more exposed to the action of waves, which have smooth, deeply dissected blades. All specimens rearranged their blades as water velocity increased, resulting in a decrease in effective drag at higher water speeds, but individuals with smooth, dissected blades assumed a more compact shape at high current speeds and thus reduced their effective drag over that of broad-bladed individuals at the same speed. In habitats chronically exposed to strong wave action, drag reduction may be an important survival mechanism; in calm habitats the turbulence induced by lack of such streamlining may enhance mixing of the water in the immediate vicinity of a plant.     

Streamlining of plant patches in streams

1. Plants in shallow streams often grow in well‐defined monospecific patches experiencing a predictable unidirectional flow, though of temporally variable velocity. During maximum patch development in summer we studied: (i) the shape and streamlining of 59 patches of Callitriche cophocarpa, (ii) allometric relationships between canopy size and sediment area colonized by roots (root area) and (iii) fine‐scale flow gradients for a representative patch exposed to a range of velocities to evaluate relationships between patch shape and physical impact. 2. Canopy and root area viewed from above were elongated and streamlined in the flow direction, while uniform vegetative growth in all directions from a single colonizing shoot would have generated a circular form. Canopies were slightly wider in the upstream part than in the gradually tapering part downstream and the maximum height to length ratio averaged 0.25. The canopy and root area of the patches were more elongate and slender in sites with shallow water, where currents accelerate alongside patches and restrict lateral expansion, compared to deeper sites where currents can pass above the canopy. Similarly, the frontal area relative to planform area or root area was significantly lower in shallow water . Canopy shape and indices of streamlining did not change significantly with approach velocity (0.02–0.40 m s^?1), either because canopy shape is not sensitive to approach velocity or summer velocities were too low to induce such changes. 3. Sediment elevation within patches (avg. 4.1 cm) increased significantly with patch length, but did not differ between unstable sand or more stable coarse sediment for the same patch length. Shape of canopy and root area did not change significantly with sediment type. 4. Pressure drag on the canopy as a whole is probably reduced by its rounded front, restricted height and overall slender form with a low frontal area, while the downstream overhanging canopy increases drag compared to an ideal streamlined object. Across a 100‐fold range of root areas from 0.01 to 1 m², the frontal area of the canopy increased 29 times, planform area increased 38 times and volume increased 76 times, suggesting a trade‐off between physical impact of flow, light interception and anchoring strength. 5. The canopy was compressed at high approach velocities, with low current velocity within the canopy while steep velocity gradients developed across the exposed outer surfaces as the diverted flow accelerated. Because drag processes are additive, and exist at different spatial scales and Reynolds numbers on the surface and inside of plant canopies, direct measurements on entire canopies under controlled conditions are needed to test the functional importance of their shape, size and porosity to flow.     

Predicting the hydraulic forces on submerged macrophytes from current velocity, biomass and morphology

Aquatic macrophytes are important in stabilising moderately eutrophic, shallow freshwater lakes in the clear-water state. The failure of macrophyte recovery in lakes with very soft, highly organic sediments that have been restored to clear water by biomanipulation (e.g. in the Norfolk Broads, UK) has suggested that the physical stability of the sediment may limit plant establishment. Hydraulic forces from water currents may be sufficient to break or remove plants. Our aim was to develop a simple model that could predict these forces from plant biomass, current velocity and plant form. We used an experimental flume to measure the hydraulic forces acting on shoots of 18 species of aquatic macrophyte of varying size and morphology. The hydraulic drag on the shoots was regressed on a theoretically derived predictor (shoot biomass × current velocity^1.5). Such linear regressions proved to be highly significant for most species. The slopes of these lines represent species-specific, hydraulic roughness factors that are analogous to classical drag coefficients. Shoot architecture parameters describing leaf and shoot shape had significant effects on the hydraulic roughness factor. Leaf width and shoot stiffness individually did not have a significant influence, but in combination with shoot shape they were significant. This hydraulic model was validated for a subset of species using measurements from an independent set of shoots. When measured and predicted hydraulic forces were compared, the fit was generally very good, except for two species with morphological variations. This simple model, together with the plant-specific factors, provides a basis for predicting the hydraulic forces acting on the root systems of macrophytes under field conditions. This information should allow prediction of the physical stability of individual plants, as an aid to shallow-lake management. Received: 11 March 1999 / Accepted: 18 January 2000     

Morphological adaptations of emergent plants to water flow: a case study with Typha angustifolia, Zizania latifolia and Phragmites australis

1. Water velocity plays an important role in shaping plant community structure in flowing waters although few authors have yet attempted to explain the adaptation of plants to flow. 2. We aimed to test two hypotheses, that: (i) some emergent macrophytes reconfigure their shoot distribution in fast currents and form clumps, and (ii) the shape and morphology of such clumps minimises drag caused by the current. The study focuses on three emergent macrophytes that co‐occur along a gradient of water velocity. 3. The species showed a clear zonation in response to water depth and current velocity. Phragmites australis occupied shallower and more slowly flowing water than Typha angustifolia and Zizania latifolia, which had similar preferences. 4. Both T. angustifolia and Z. latifolia shoots were more clumped at high velocity, whereas they were more randomly distributed at low flow or in stagnant water. Because of the low shoot density, water flowed more easily through T. angustifolia clumps, whereas Z. latifolia clumps had a high shoot density and large amounts of trapped litter, causing stagnant water in the centre of the clump. The clumps of Z. latifolia with a high density of shoots were longer and narrower than T. angustifolia clumps. Phragmites australis was less tolerant of flow than the other two species and large amounts of litter trapped in the clumps impaired flow. 5. The shoot distribution of both T. angustifolia and Z. latifolia is reconfigured at high flow and this minimises drag on the clumps.     

Strömungsanpassung des Pinguins beim Schwimmen unter Wasser

Summary Penguins (Pygoscelis papua) swimming parallel to the glass screen of a large aquarium in Zoologischer Garten, Frankfurt, were filmed during deceleration phases during which their extremities were not moved relative to the trunk. Drag coefficients were determined using a newly developed method of analysis (the reciprocal value of velocity plotted as a function of time gives a linear curve during undisturbed deceleration). The frontal drag coefficient is 7·10^–2, surface drag coefficient 4.4·10^–3 and volume drag coefficient 3.1·10^–2. The ratio of length to diameter is 4.2. The length from head to broadest part of trunk relative to body length is 0.48. These results are compared with technical hydro- and aerodynamic measurements found in literature. The body form of penguins appears to be well adapted to transporting unit mass with least resistance. Their frontal and volume drag coefficients cannot be further improved. The surface drag coefficient shows that, at Reynolds numbers of deceleration swimming (Re10⁶), unusual effects need not be taken into account. This indicates that the boundary layer is partly laminar, partly turbulent as found in optimally stream-lined technical objects. We suppose that the smooth feathered surface of a penguin dampens boundary layer oscillations especially during fast swimming at Reynolds numbers of 10⁷ and guarantees laminar flow over larger regions of the trunk.     

Variation in velocity of cytoplasmic streaming and gravity effect in characean internodal cells measured by laser-Doppler-velocimetry

Summary Velocities of cytoplasmic streaming were measured in internodal cells ofNitella flexilis L. andChara corallina Klein ex Willd. by laser-Doppler-velocimetry to investigate the possibility of non-statolith-based perception of gravity. This was recently proposed, based on a report of gravity-dependent polarity of cytoplasmic streaming. Our measurements revealed large spatial and temporal variation in streaming velocity within a cell, independent of the position of the cell with respect to the direction of gravity. In 58% of the horizontally positioned cells the velocities of acropetal and basipetal streaming, measured at opposite locations in the cell, differed significantly. In 45% of these, basipetal streaming was faster than acropetal streaming. In 60% of the vertically positioned cells however the difference was significant, downward streaming was faster in only 61% of these. When cell positions were changed from vertical to horizontal and vice versa the cells reacted variably. A significant difference between velocities in one direction, before and after the change, was observed in approx. 70% of the measurements, but the velocity was faster in the downward direction, as the second position, in only 70% of the significantly different. The ratio of basipetal to acropetal streaming velocities at opposite locations of a cell was quite variable within groups of cells with a particular orientation (horizontal, normal vertical, inverted vertical). On average, however, the ratio was close to 1.00 in the horizontal position and approx. 1.03 in the normal vertical position (basipetal streaming directed downwards), which indicates a small direct effect of gravity on streaming velocity. Individual cells, however, showed an increased, as well as a decreased, ratio when moved from the horizontal to the vertical position. No discernible effect of media (either Ca^{2 +}-buffered medium or 1.2% agar in distilled water) on the streaming velocities was observed. The above mentioned phenomenon of graviperception is not supported by our data.Abbreviations g gravitational acceleration (9.81 m/s²) - LDV laser-Doppler-velocimetry - VR velocity ratio Dedicated to Professor Peter Sitte on the occasion of his 65th birthday     

Drag and Flexibility in Sessile Organisms

Most large, sessile organisms when exposed to rapid flows ofair or water are markedly deformed as a consequence of theirstructural flexibility. Responses to air and water movementare similar, although both extreme and typical forces generatedby water flows are greater, and erect organisms are commonlyshorter in water than in air. A useful way of viewing data onthe scaling of drag with flow speed is with a graph of speed-specificdrag (drag divided by the square of speed) against speed. Sincean ordinary solid body usually gives a horizontal line on sucha plot, deviations from the ordinary are immediately evident.The slopes of the double logarithmic version of these graphsprovide useful numerical comparisons. All of the cases consideredhere—trees, macroalgae, sea pens, etc.—give negativeslopes at high flow rates, indicating that speed-specific dragdrops with increasing flow. Such results may be taken as evidencethat the flexible response commonly constitutes an adaptivelyuseful reconfiguration as opposed to a mere incidental consequenceof the material economy afforded by flexibility.     

Explosive lower limb extension mechanics: An on-land vs. in-water exploratory comparison

During a horizontal underwater push-off, performance is strongly limited by the presence of water, inducing resistances due to its dense and viscous nature. At the same time, aquatic environments offer a support to the swimmer with the hydrostatic buoyancy counteracting the effects of gravity. Squat jump is a vertical terrestrial push-off with a maximal lower limb extension limited by the gravity force, which attracts the body to the ground. Following this observation, we characterized the effects of environment (water vs. air) on the mechanical characteristics of the leg push-off. Underwater horizontal wall push-off and vertical on-land squat jumps of two local swimmers were evaluated with force plates, synchronized with a lateral camera. To better understand the resistances of the aquatic movement, a quasi-steady Computational Fluid Dynamics (CFD) analysis was performed. The force-, velocity- and power-time curves presented similarities in both environments corresponding to a proximo-distal joints organization. In water, swimmers developed a three-step explosive rise of force, which the first one mainly related to the initiation of body movement. Drag increase, which was observed from the beginning to the end of the push-off, related to the continuous increase of body velocity with high values of drag coefficient (C_D) and frontal areas before take-off. Specifically, with velocity, frontal area was the main drag component to explain inter-individual differences, suggesting that the streamlined position of the lower limbs is decisive to perform an efficient push-off. This study motivates future CFD simulations under more ecological, unsteady conditions.     

Reduced root anchorage of freshwater plants in sandy sediments enriched with fine organic matter

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