Fluctuating Flow Conditions Research Articles

Fluid flow drives phenotypic heterogeneity in bacterial growth and adhesion on surfaces

Bacteria often thrive in surface-attached communities, where they can form biofilms affording them multiple advantages. In this sessile form, fluid flow is a key component of their environments, renewing nutrients and transporting metabolic products and signaling molecules. It also controls colonization patterns and growth rates on surfaces, through bacteria transport, attachment and detachment. However, the current understanding of bacterial growth on surfaces neglects the possibility that bacteria may modulate their division behavior as a response to flow. Here, we employed single-cell imaging in microfluidic experiments to demonstrate that attached Escherichia coli cells can enter a growth arrest state while simultaneously enhancing their adhesion underflow. Despite utilizing clonal populations, we observed a non-uniform response characterized by bistable dynamics, with co-existing subpopulations of non-dividing and actively dividing bacteria. As the proportion of non-dividing bacteria increased with the applied flow rate, it resulted in a reduction in the average growth rate of bacterial populations on flow-exposed surfaces. Dividing bacteria exhibited asymmetric attachment, whereas non-dividing counterparts adhered to the surface via both cell poles. Hence, this phenotypic diversity allows bacterial colonies to combine enhanced attachment with sustained growth, although at a reduced rate, which may be a significant advantage in fluctuating flow conditions.

AI-based shape optimization of galloping micro-power generators: exploring the benefits of curved surfaces

The advent of flow micro-power generation has resparked the interest in researching the galloping instability with the objective of determining the shape of the bluff body that is most prone to galloping. Such shape, which is sought to maximize the efficacy of galloping micro-power generators (GMPGs), must possess a very low cut-in flow speed while achieving large-amplitude steady-state oscillations beyond it. Additionally, since GMPGs can operate in environments with fluctuating flow conditions, the optimal shape must also have a very short rise time to its steady-state amplitude. In this work, we utilize computational fluid dynamics in conjunction with machine learning to optimize the shape of the bluff body of GMPGs for both steady-state and transient performance. We investigate a continuum shape description which encapsulates most of the cases studied earlier in the literature. The continuum has a straight frontal and dorsal faces with varying lengths, and side faces described by surfaces of different curvatures. The optimization study reveals that a curved-trapezoidal bluff body with the highest side surface curvature and frontal-to-dorsal ratio is the perfect shape for steady flow conditions. On the other hand, a square profile with the highest side surface curvature is the ideal choice for highly-fluctuating flow conditions because of its shortest rise time. The theoretical findings are replicated experimentally using wind tunnel tests.

Microfluidic investigation of the impacts of flow fluctuations on the development of Pseudomonas putida biofilms

Biofilms play critical roles in wastewater treatment, bioremediation, and medical-device-related infections. Understanding the dynamics of biofilm formation and growth is essential for controlling and exploiting their properties. However, the majority of current studies have focused on the impact of steady flows on biofilm growth, while flow fluctuations are common in natural and engineered systems such as water pipes and blood vessels. Here, we reveal the effects of flow fluctuations on the development of Pseudomonas putida biofilms through systematic microfluidic experiments and the development of a theoretical model. Our experimental results showed that biofilm growth under fluctuating flow conditions followed three phases: lag, exponential, and fluctuation phases. In contrast, biofilm growth under steady-flow conditions followed four phases: lag, exponential, stationary, and decline phases. Furthermore, we demonstrated that low-frequency flow fluctuations promoted biofilm growth, while high-frequency fluctuations inhibited its development. We attributed the contradictory impacts of flow fluctuations on biofilm growth to the adjustment time (T0) needed for biofilm to grow after the shear stress changed from high to low. Furthermore, we developed a theoretical model that explains the observed biofilm growth under fluctuating flow conditions. Our insights into the mechanisms underlying biofilm development under fluctuating flows can inform the design of strategies to control biofilm formation in diverse natural and engineered systems.

Stall-induced fatigue damage in nonlinear aeroelastic systems under stochastic inflow: Numerical and experimental analyses

This study focuses on characterizing the fatigue damage accumulated in nonlinear aeroelastic systems subjected to stochastic inflows through both numerical simulations and wind tunnel experiments. In the mathematical model, nonlinearities are assumed to exist either in the structure (via a cubic hardening nonlinearity in the pitch stiffness), or in the flow (via dynamic stall condition), or simultaneously in both the structural and aerodynamic counterparts. The aerodynamic loads in the attached flow and dynamic stall conditions are estimated using Wagner’s formulation and semi-empirical Leishman–Beddoes model, respectively. To augment the findings to in-field flow conditions, the oncoming wind flow is considered to be randomly time-varying in nature. The stochastic input flow fluctuations are modeled using a Karhunen–Loeve Expansion formulation. The response dynamics and the associated fatigue damage of the aeroelastic system, possessing different sources of nonlinearities, are systematically investigated under isolated cases of deterministic and stochastic input flows. Specifically, the pertinent role of stochasticity in the input flow is brought out by presenting the response dynamics and the associated fatigue damage accumulation for different values of noise intensity and time scale of the input flow fluctuation. It is demonstrated that under fluctuating flow conditions, the dynamics intermittently switch between attached flow and the dynamic stall regimes even at low mean flow speeds. The intermittent nature of the response varies as the time scale and intensity of the oncoming flow are varied. The role of torsional stresses as the predominant component dictating the fatigue damage accumulation irrespective of the source of nonlinearity is illustrated. Using the rainflow counting method and Miner’s linear damage accumulation theory, it is shown that the accumulated fatigue damage is substantially higher under stochastic flow conditions as compared to deterministic input flows. Importantly, it is observed that different time scales and intensities of the oncoming flow fluctuation play a pivotal role in dictating the fatigue damage in aeroelastic systems. Finally, fatigue damage is observed to be significantly higher for torsionally dominant oscillations in the dynamical stall regime compared to the oscillations at the attached flow regime. The numerical findings are strengthened by drawing comparisons with the preliminary results obtained from wind tunnel experiments performed on a NACA 0012 airfoil undergoing dynamic stall. To the best of our knowledge, this is the first study that systematically bridges the dichotomy between the stall-induced dynamical signatures in stochastic aeroelastic systems and maps the same to the corresponding structural damage.

Effects of syringe pump fluctuations on cell-free layer in hydrodynamic separation microfluidic devices

Syringe pumps are widely used biomedical equipment, which offer low-cost solutions to drive and control flow through microfluidic chips. However, they have been shown to transmit mechanical oscillations resulting from their stepper motors into the flow, perturbing device performance. These detrimental effects have mostly been reported on microdroplet production, but have never been reported on hydrodynamic two-phase separation, such as in microdevices making use of cell-free layer phenomena. While various mechanisms can be used to circumvent syringe pump oscillations, it is of interest to study the oscillation effects in naïve systems, which are common in research settings. Previous fluctuation studies focused on relatively low flow rates, typically below 5 ml/h, and showed a linear decay of the relative pressure fluctuations as a function of the flow rate. In this work, we have uncovered that the relative pressure fluctuations reach a plateau at higher flow rates, typically above 5 ml/h. Using a novel low-cost coded compressive rotating mirror camera, we investigated the effect of fluctuations in a hydrodynamic microfluidic separation device based on a cell-free layer concept. We demonstrated that cell-free zone width fluctuations have the same frequency and amplitude than the syringe pump-induced pressure oscillations and illustrated the subsequent degradation of particle separation. This work provides an insight into the effect of syringe pump fluctuations on microfluidic separation, which will inform the design of microfluidic systems and improve their resilience to pulsating or fluctuating flow conditions without the use of ancillary equipment.

Thermal storage analysis of a foam-filled PCM heat exchanger subjected to fluctuating flow conditions

A dynamic heat transfer model is developed to investigate the transient thermal storage characteristics of a heat exchanger with foam-filled phase change material (PCM) under the fluctuating flow conditions. The thermal energy storage (TES) configuration of double pipe is considered, while using the highly porous metal foam to intensify the heat transfer in both the PCM and heat transfer fluid (HTF) regions. The paraffin RT50 is used as the PCM and water is selected as the HTF. The energy transport between the inner and outer pipes is thoroughly taken into account. The Darcy-Forchheimer equation is adopted to model the fluid flow through metal foam, and the enthalpy-porosity method is employed to simulate the phase change process within the PCM/foam composite under the local thermal non-equilibrium (LTNE) condition. The performance of heat exchanger during the charging process is firstly predicted under steady HTF flow, and then the effects of fluctuating inlet temperature and velocity are examined respectively. In the case of steady flow, compared with the pure PCM case, inserting metal foam in both the PCM and HTF regions shortens the melting time by 93.6% and augments the heat storage rate by 9 times. Fluctuation in the inlet temperature leads to a visibly oscillating variation in the PCM temperature and heat storage, which are 15 K, 65 kJ at a fluctuation amplitude of 30 K and 17 K, 105 kJ at a periodicity of 400 s. However, fluctuation in the velocity has no considerable influence. The heat exchanger with lower porosity foam presents more sensitive to the fluctuating inlet temperature.

Ecological Risk Dynamics of Pharmaceuticals in Micro-Estuary Environments.

Micro-estuarine ecosystems have a surface area <1 km2 and are abundant in Mediterranean regions. As a result of their small size, these systems are particularly vulnerable to the effects of chemical pollution. Due to the fluctuating flow conditions of base flow dominated by treated wastewater effluents and flood events transporting rural and urban non-point-source pollution, micro-estuaries are under a dynamic risk regime, consequently struggling to provide ecological services. This 2 year study explored the occurrence and risks of pharmaceutical contamination in the Alexander micro-estuary in Israel. Pharmaceuticals were detected in all samples (n = 280) at as high as 18 μg L-1 in flood events and 14 μg L-1 in base flow. The pharmaceutical mixture composition was affected by flow conditions with carbamazepine dominating the base flow and caffeine dominating flood events. The median annual risk quotients for fish, crustaceans, and algae were 19.6, 5.2, and 4.5, respectively, indicating that pharmaceuticals pose a high risk to the ecosystem. Ibuprofen, carbamazepine, and caffeine contributed most to the risk quotients. The current work highlights that micro-estuary ecosystems, like the Alexander estuary, are continuously exposed to pharmaceuticals and most likely to other pollutants, placing these ecologically important systems under an elevated risk in comparison to the more frequently studied large estuarine systems.

Performance evaluation of a down-flow hanging sponge (DHS) reactor as a decentralized domestic wastewater treatment system in tropical regions

In this study, a pilot-scale down-flow hanging sponge (DHS) reactor was operated in the community plant of Bangkok for the treatment of domestic wastewater (COD 285 mg/L, BOD 105 mg/L) collected by separate sewer to evaluate the reactor’s feasibility as a decentralized treatment system. The DHS reactor was operated for 600 days at ambient temperatures of 25–30 °C, both with constant flow conditions and with fluctuating flow conditions that simulated wastewater discharge patterns of the community. The results indicate that under constant flow at an HRT of 5 h, the volumetric loading rates of 0.36 kgBOD/m3-sponge/day and 0.16 kgN/m3-sponge/day were the optimum operational conditions of the DHS reactor in order to satisfy the effluent discharge standards. The DHS achieved removal rates of 89, 95, 91 and 90% for COD, BOD, TSS and NH4-N. Under the fluctuating flow condition, improvement of denitrification was confirmed at volumetric loading rates of 0.50 kgBOD/m3-sponge/day and 0.18 kgN/m3-sponge/day. The fluctuating flow of wastewater positively affects retained sludge activities in terms of homogenizing sludge concentration and stimulating oxygen uptake rates. These results suggest that the DHS reactor can be applied as a decentralized treatment system for domestic wastewater with fluctuating flow rates in tropical regions.

Performance Evaluation of Baffled and Floating Hybrid Constructed Wetland Operated Under Field Conditions

Baffled and Floating Hybrid Constructed Wetland (BFHCW) consisting of constructed wetland with brickbat medium and Floating Treatment Wetland (FTW) was developed and operated under actual field flow conditions. BFHCW was vegetated with dual-species (Typha angustifolia L. and Canna indica). The performance of continuously operated system was evaluated for varied Hydraulic Loading Rate (HLR) and Organic Loading Rate (OLR) for a longer period. The system was also assessed for controlled flow rate resulting more uniform OLR and HLR. The results showed that COD removal efficiency was enhanced by 5 to 15% with controlled flow than uncontrolled highly fluctuating flow conditions. HLR has more impact on COD removal than OLR. BFHCW is effective to an extent of 25 to 40% for COD removal. BFHCW is found to be cost effective and reasonably efficient treatment for pretreated domestic wastewater.

Flow visualization and transient behavior analysis of luminescent mini-tufts after a backward-facing step

Luminescent mini-tufts method has been used for surface flow visualization for a long time. One major challenging point of this method is quantitative analysis of transient flows and the dynamic structures. This study is focused on the application of luminescent mini-tufts method in transient flows. A backward-facing step (BFS) is used in this analysis, which is one classic model that consists both flow separation and re-attachment processes. In this study, the instantaneous mini-tufts recognition, image averaging and tuft inclination angle/tuft angle estimation processes are introduced for the analysis of luminescent mini-tufts for the first time on backward-facing step flow (Rem = 2.0 × 105–7.9 × 105 and Reh = 1.3 × 104–5.3 × 104). Detailed transient features and characterization process for the backward-facing step model are explained in this study. The combination of optical-oil flow and hot-wire anemometry methods with luminescent mini-tufts are also shown useful to give comprehensive flow field information, including the surface flow behaviors, boundary layer, re-attachment position identification, etc. In addition, the decomposition of the luminescent mini-tufts visualization data is also conducted to give the power spectral density (PSD) and characteristic frequencies for the mini-tufts behaviors under transient fluctuating flow conditions.

Controls on denitrification potential in nitrate-rich waterways and riparian zones of an irrigated agricultural setting.

Denitrification, the microbial conversion of NO3- to N gases, is an important process contributing to whether lotic and riparian ecosystems act as sinks for excess NO3- from agricultural activities. Though agricultural waterways and riparian zones have been a focus of denitrification research for decades, almost none of this research has occurred in the irrigated agricultural settings of arid and semiarid climates. In this study, we conducted a broad survey of denitrification potential in riparian soils and channel sediments from 79 waterway reaches in the irrigated agricultural landscape of California's Central Valley. With this approach, we sought to capture the wide range of variation that arose from diverse waterway management and fluctuating flow conditions, and use this variation to identify promising management interventions. We explored associations of denitrification potentials with surface water NO3- -N, organic matter, flow conditions, vegetation cover, near-channel riparian bank slope, and channel geomorphic features using generalized linear mixed models. We found strong associations of sediment denitrification potentials with reach flow conditions, which we hypothesize was the result of variation in microbial communities' tolerance to dry-wet cycles. Denitrification potentials in riparian soils, in contrast, did not appear affected by flow conditions, but instead were associated with organic matter, vegetation cover, and bank slope in the riparian zone. These results suggest a strong need for further work on how denitrification responds to varying flow conditions and dry-wet cycles in non-perennial lotic ecosystems. Our findings also demonstrate that denitrifier communities respond to key features of waterway management, which can therefore be leveraged to control denitrification through a variety of management actions.

Effects of Heterogeneity, Connectivity, and Density Variations on Mixing and Chemical Reactions Under Temporally Fluctuating Flow Conditions and the Formation of Reaction Patterns

AbstractSolute mixing, spreading, and fast chemical reactions in aquifers are strongly influenced by spatial variability of the hydraulic properties, temporal flow fluctuations, and fluid density differences. We study the coupling of heterogeneity, transient forcing, and density‐driven flow on mixing and chemical reactions between two fluids of different density under a stable stratification. We consider the reaction of the fast dissolution of calcite. We find that temporal fluctuations and heterogeneity cause strong local enhancement of the mixing and reaction rates and this impact increases with the degree of connectivity of hydraulic conductivity. The global mixing and reactivity, however, are on the order of or smaller than their homogeneous counterparts due to heterogeneity‐induced fluid segregation. The local maxima of the mixing and reaction rates are found to be located around strongly stretched regions corresponding to high velocity zones where dispersive mass transfer mechanisms are increased by dispersion. We also find that density variations compress the interface, which in turn emphasizes local maxima in mixing and reaction rates. Numerical results provide evidence that the stretching of the interface induced by spatial heterogeneity and transient effects coupled with density variations lead to the formation of complex patterns of reactive hotspots, zones of enhanced reaction efficiency, and that its distribution is directly linked to the deformation properties and topology of the flow field. These results provide new insights into the role of spatial and temporal variability on the mixing and reaction efficiency as well as the formation of reactive geochemical patterns in actual environmental systems.

Offshore Gas Well Flow and Orifice Metering System: An Overview

This research presents a concise account of offshore surface well test from objective, organization, to practical approach and in relation to orifice metering system of natural gas; against the perspective of regulatory standards. With reference to reliability, availability, affordability and including control measures governing the design, the orifice meter sometimes called a head loss flow meter is chosen most often because of its reputation in the oil and gas industry. Alternative metering system of natural gas, including robust and cost effective innovations within the industry which addressed some key limitations of orifice meter was examined. The advanced flow computer with transducers suited for orifice measurement installations is a cost effective electronic flow real time measurement system. It has telemetry features and improved accuracy under fluctuating flow conditions based on functional differential pressure root mean squared volume calculation principles. These new developments and their capabilities have reduced the market share of the mechanical orifice chart meters. Except that Electronic flow meters has environmental limitations; thus, the proven mechanical orifice metering systems are still an effective solution for many flow measurement applications.

Physostomous channel catfish, Ictalurus punctatus, modify swimming mode and buoyancy based on flow conditions.

The employment of gliding in aquatic animals as a means of conserving energy has been theoretically predicted and discussed for decades. Several studies have shown that some species glide, whereas others do not. Freshwater fish species that widely inhabit both lentic and lotic environments are thought to be able to adapt to fluctuating flow conditions in terms of locomotion. In adapting to the different functional demands of lentic and lotic environments on fish energetics, physostomous (open swim bladder) fish may optimise their locomotion and activity by controlling their net buoyancy; however, few buoyancy studies have been conducted on physostomous fish in the wild. We deployed accelerometers on free-ranging channel catfish, Ictalurus punctatus, in both lentic and lotic environments to quantify their swimming activity, and to determine their buoyancy condition preferences and whether gliding conserves energy. Individual comparisons of swimming efforts between ascent and descent phases revealed that all fish in the lentic environment had negative buoyancy. However, all individuals showed many descents without gliding phases, which was contrary to the behaviour predicted to minimise the cost of transport. The fact that significantly fewer gliding phases were observed in the lotic environment, together with the existence of neutrally buoyant fish, indicated that channel catfish seem to optimise their locomotion through buoyancy control based on flow conditions. The buoyancy optimisation of channel catfish relative to the flow conditions that they inhabit not only reflects differences in swimming behaviour but also provides new insights into the adaptation of physostome fish species to various freshwater environments.

Urban wind: Characterisation of useful gust and energy capture

Small-scale wind turbine operations within the urban environment are exposed to high levels of gusts and turbulence compared to flows over less rough surfaces. There is therefore a need for such systems to not only cope with, but to thrive under such fluctuating flow conditions. This paper addresses the potential importance of gust tracking technologies within the urban environment via the analysis of the additional energy present in the gusty wind resource using high resolution measurements at two urban roof-top locations. Results demonstrate significant additional energy present in the gusty wind resource at high temporal resolution. This energy is usually under-represented by the use of mean wind speeds in quantifying the power in the wind over longer averaging times. The results support the promise of capturing a portion of this extra energy through gust tracking solutions. The sensitivity of this “additional” wind energy to averaging time interval is also explored, providing useful information for the design of gust tracking or dynamic control algorithms for small-scale turbines. Relationships between turbulence intensity and excess energy available are drawn. Thus, an analytical model is proposed which may prove useful in predicting the excess energy available across wide areas from, for example, boundary layer turbulence models.

Heat and mass transfer in automotive catalysts—The influence of turbulent velocity fluctuations

Monolithic reactors employed in automotive exhaust gas aftertreatment typically operate under fluctuating flow conditions. These fluctuations in the volumetric flow rate through the catalyst are caused by engine pulsations and the upstream exhaust gas turbulence.In the current work, the effects of the turbulent-to-laminar transition on the conversion of pollutants under mass-transfer limited operation of a monolithic reactor are investigated using numerical simulations. The most notable effect of the turbulence is an introduction of temporal fluctuations to the observed conversion at the outlet. These fluctuations are attributed to variations in the axial velocity (and thus the retention time) in the channel.A simplistic model for a single monolith channel in a tanks-in-series framework is suggested that captures this effect of the upstream turbulence on the dimensionless heat and mass transfer numbers in automotive catalysts. The model can be used to resolve the transient variations of the heat and mass transfer when the upstream turbulent characteristics are known. The effects of such variations on the catalytic conversion are likely to be most pronounced for reactions with negative-order kinetics when operating close to ignition or extinction.

Dynamic lift measurements on a FX79W151A airfoil via pressure distribution on the wind tunnel walls

We report on an experimental setup for measurements of dynamic stall for airfoils via the pressure distribution over wind tunnel walls. This measuring technique, hitherto used for lift measurements under static conditions, is also an adequate method for dynamic conditions until stall occurs. A step motor is used, allowing for sinusoidal as well as non-sinusoidal and stochastic pitching to simulate fast fluctuating flow conditions. Measurements with sinusoidal pitching and constant angular velocities were done and show dynamic stall characteristics. Under dynamic stall conditions, maximum lift coefficients were up to 80% higher than the maximum for static lift.

Effective dispersion in a chemically heterogeneous medium under temporally fluctuating flow conditions

We investigate effective solute transport in a chemically heterogeneous medium subject to temporal fluctuations of the flow conditions. Focusing on spatial variations in the equilibrium adsorption properties, the corresponding fluctuating retardation factor is modeled as a stationary random space function. The temporal variability of the flow is represented by a stationary temporal random process. Solute spreading is quantified by effective dispersion coefficients, which are derived from the ensemble average of the second centered moments of the normalized solute distribution in a single disorder realization. Using first-order expansions in the variances of the respective random fields, we derive explicit compact expressions for the time behavior of the disorder induced contributions to the effective dispersion coefficients. Focusing on the contributions due to chemical heterogeneity and temporal fluctuations, we find enhanced transverse spreading characterized by a transverse effective dispersion coefficient that, in contrast to transport in steady flow fields, evolves to a disorder-induced macroscopic value (i.e., independent of local dispersion). At the same time, the asymptotic longitudinal dispersion coefficient can decrease. Under certain conditions the contribution to the longitudinal effective dispersion coefficient shows superdiffusive behavior, similar to that observed for transport in s stratified porous medium, before it decreases to its asymptotic value. The presented compact and easy to use expressions for the longitudinal and transverse effective dispersion coefficients can be used for the quantification of effective spreading and mixing in the context of the groundwater remediation based on hydraulic manipulation and for the effective modeling of reactive transport in heterogeneous media in general.

Computed effect of guide vane shape on performance of impulse turbine for wave energy conversion

This paper deals with the computational fluid dynamics (CFD) analysis on effect of guide vane shape on performance of impulse turbine for wave energy conversion. Initially, experiments have been conducted on the impulse turbine to validate the present CFD method and to analyse the aerodynamics in rotor and guide vanes, which demonstrates the necessity to improve the guide vanes shape. The results showed that the downstream guide vanes make considerable total pressure drop leads low performance of the turbine and hence three-dimensional (3-D) inlet and downstream guide vanes have been designed based on well-known vortex theory to improve the efficiency of the turbine. In order to prove the improvement in efficiency due to 3-D guide vanes, CFD analysis has been made on impulse turbine with 2-D and 3-D guide vanes for various flow coefficients. As a result, it is seen that the present CFD model can predict the experimental values with reasonable accuracy. Also, it is showed from the numerical results that the efficiency of the turbine can be improved by average of 4.5 percentage points by incorporating 3-D guide vanes instead of 2-D guide vanes. The physical reason for improvement in efficiency of the turbine due to 3-D guide vanes has been explained with the CFD flow insight pictures. As the turbine operates in fluctuating flow conditions, the performance of the turbine with 2-D and 3-D guide vanes have been calculated numerically using quasi-steady analysis. Furthermore, the performance of the turbine has been predicted for one year based on Irish wave climate to show the feasibility of using 3-D guide vanes in actual sea wave conditions.

Net‐Spinning byAmphipsyche senegalensisLarvae (Trichoptera: Hydropsychidae) in Lake Kariba (South Central Africa)

Net‐spinning larvae of the hydropsychid Amphipsyche senegalensis Brauer are commonly found on submerged trees in exposed regions of Lake Kariba as well as in nearby rivers. Final instar larvae were removed from the lake, placed into a laboratory stream channel and the nets compared with those spun by larvae in Kariba. No significant differences were found in mean mesh size between nets from the channel (44 × 34 μm) and those from the lake (48 × 40 μm). The width of the silk strands used in net building varied from 1.5–3.2 μm, but strand width differences between nets from the two habitats were not statistically significant. However, nets spun by A. senegalensis larvae in nearby rivers were in marked contrast to those built by larvae from Kariba. A final instar net had a mean mesh size of 77 × 67 μm and a mean silk strand diameter of 7.1 μm, while a net from a younger instar measured 54 × 47 μm with a mean silk strand width of 3.6 μm, indicating that flow regime may play a part in influencing net construction. Data for all A. senegalensis nets examined showed a positive correlation between mesh size and silk strand diameter (P < 0.001). It is suggested that the apparent flexibility in net‐spinning behaviour may assist in adapting larvae to the fluctuating flow conditions often associated with lotic habitats.