Fixed Flow Conditions Research Articles

Learning Mappings from Iced Airfoils to Aerodynamic Coefficients Using a Deep Operator Network

This study abstracted the prediction of the aerodynamic coefficients of an iced airfoil as a mapping from the iced airfoil space to the aerodynamic coefficient space. Thus, a deep network called Airfoils2AeroNet that can learn this mapping was established based on Deep Operator Network (DeepONet). The deep network consists of a branch network for encoding the iced airfoil images and a trunk network that learns a nonlinear mapping from the one-dimensional aerodynamic coefficient function input to p-dimensional outputs. The branch network consists of deep convolutional neural networks (CNNs), and the trunk network consists of fully connected neural networks (FNNs). Then the network was trained and tested on iced airfoils based on NACA 0012 airfoils. Comparing the prediction results of Airfoils2AeroNet with those of the conventional direct CNN network, the network proposed in this paper has a strong advantage for generalization. Unlike the traditional CNN, which can only predict the aerodynamic coefficients at fixed flow conditions consistent with the training data, the network can flexibly predict the aerodynamic coefficients at different flow conditions. Finally, the influence of the structure of the branch network and trunk network on the prediction results was analyzed.

The assessment of an acoustic Doppler velocimetry profiler from a user’s perspective

Acoustic Doppler velocimetry profilers (ADVPs) are widely used in both experimental and field studies because of their robustness in velocity measurements. The acquired measurements do not only offer estimates of the local and instantaneous flow velocity at the interrogated measurement volume, but can also be further processed for the estimation of the bed surface shear stresses, thus they are finding a wide range of applications ranging from water engineering to geomorphology and eco-hydraulics. This study aims to evaluate the performance of an ADVP in obtaining hydrodynamics measurements under fixed flow conditions, with various probe configurations. To this goal, a robust search is conducted where ADVP probe settings are sequentially altered. A number of assessment criteria are used including qualitative observations, such as checking the shape of the velocity profile, as well as quantitative error metrics, including signal-to-noise ratio, correlations and number of spikes. Further, estimation of the bed shear stresses computed by means of using the log Law of the Wall and turbulent kinetic energy, allow obtaining a better understanding of the uncertainties involved and the importance of making a better informed choice with respect to the probe configuration settings. Thus, the methodology and performance metrics provided herein, although presented for a given flow, can generally be applied from practitioners and researchers alike.

Energy harvesting from inline vibration of an elastically mounted circular cylinder in oscillatory flow

Harvesting energy from ocean waves has attracted increasing attention due to the depletion of fossil fuels. Water motion under ocean waves can be modelled as oscillatory flow when its impact on small-scale structures are studied. In this paper, energy harvesting from inline vibration of an elastically mounted circular cylinder in oscillatory flow is studied numerically at a low Reynolds number of 150, two Keulegan–Carpenter (KC) numbers of 5 and 10 and three mass ratios of 1, 2 and 3. The energy-harvesting device is an electromagnetic device comprising of electric coils and a magnet. For specific KC number, mass ratio and electromagnetic damping ratio, the maximum power occurs when the natural frequency of the cylinder measured in water is the same as the oscillatory flow frequency and is defined as lock-in power. For a fixed flow condition, the lock-in power is affected by the electromagnetic damping ratio and there exists a best electromagnetic damping ratio, where the lock-in power reaches its maximum value, which is defined as the best power. The best power is found to be independent of the mass ratio and decreases with the increase of KC number. The best electromagnetic damping ratio decreases with the increase of mass ratio and increase with the increase of KC number. The irregular vibration of the cylinder at KC = 10 causes significant reduction in the power. The power is dominated by the first harmonic vibration and is closely correlated to the phase between the force and vibration velocity.

Net-exergetic, hydraulic and thermal optimization of coaxial heat exchangers using fixed flow conditions instead of fixed flow rates

Previous studies optimized the dimensions of coaxial heat exchangers using constant mass flow rates as a boundary condition. They show a thermal optimal circular ring width of nearly zero. Hydraulically optimal is an inner to outer pipe radius ratio of 0.65 for turbulent and 0.68 for laminar flow types. In contrast, in this study, flow conditions in the circular ring are kept constant (a set of fixed Reynolds numbers) during optimization. This approach ensures fixed flow conditions and prevents inappropriately high or low mass flow rates. The optimization is carried out for three objectives: Maximum energy gain, minimum hydraulic effort and eventually optimum net-exergy balance. The optimization changes the inner pipe radius and mass flow rate but not the Reynolds number of the circular ring. The thermal calculations base on Hellström’s borehole resistance and the hydraulic optimization on individually calculated linear loss of head coefficients. Increasing the inner pipe radius results in decreased hydraulic losses in the inner pipe but increased losses in the circular ring. The net-exergy difference is a key performance indicator and combines thermal and hydraulic calculations. It is the difference between thermal exergy flux and hydraulic effort. The Reynolds number in the circular ring is instead of the mass flow rate constant during all optimizations. The result from a thermal perspective is an optimal width of the circular ring of nearly zero. The hydraulically optimal inner pipe radius is 54% of the outer pipe radius for laminar flow and 60% for turbulent flow scenarios. Net-exergetic optimization shows a predominant influence of hydraulic losses, especially for small temperature gains. The exact result depends on the earth’s thermal properties and the flow type. Conclusively, coaxial geothermal probes’ design should focus on the hydraulic optimum and take the thermal optimum as a secondary criterion due to the dominating hydraulics.

Droplet-based mixing characteristics in bumpy serpentine microchannel

In the present study, a three-dimensional unsteady model is developed via coupling volume-of-fluid (VOF) method with component transport model to investigate the droplet-based mixing in the bumpy serpentine microchannel with different bumps structures. The hydrodynamics underlying the droplet-based mixing in bumpy serpentine microchannel is revealed and the influences of layout and number of bumps on comprehensive mixing performance inside the droplets with different size are clarified. The results indicate that, centrifugal component immersion and component entrainment by the nonuniform vortices in droplet are two main factors for promoting droplet-based mixing in serpentine microchannel. The centrifugal component immersion can be enhanced by the outside bumps of bumpy serpentine microchannel but suppressed by the inside bumps. Owing to lower viscous friction, the pressure drop in the serpentine microchannel with outside bumps is less than that with inside bumps, causing higher mixing efficiency. There is an optimal bump number for the best absolute mixing performance for a fixed flow condition in the serpentine microchannel with outside bumps, when the interval between the adjacent bumps is nearly as same as the squeezed droplet length. The absolute mixing performance of the larger droplet is smaller and less sensitive to the variation of the bump number.

High-Throughput Microfluidic Characterization of Erythrocyte Shapes and Mechanical Variability

The motion of red blood cells (RBCs) in microchannels is important for microvascular blood flow and biomedical applications such as blood analysis in microfluidics. The current understanding of the complexity of RBC shapes and dynamics in microchannels is mainly based on several simulation studies, but there are a few systematic experimental investigations. Here, we present a combined study that systematically characterizes RBC behavior for a wide range of flow rates and channel sizes. Even though simulations and experiments generally show good agreement, experimental observations demonstrate that there is no single well-defined RBC state for fixed flow conditions but rather a broad distribution of states. This result can be attributed to the inherent variability in RBC mechanical properties, which is confirmed by a model that takes the variation in RBC shear elasticity into account. This represents a significant step toward a quantitative connection between RBC behavior in microfluidic devices and their mechanical properties, which is essential for a high-throughput characterization of diseased cells.

A microfluidic binary logic device using inertia-elastic particle focusing

While a few attempts have been made on microfluidic logic devices, e.g., digital microfluidics and a fluid transistor, complete integration of digitalization into microfluidics is still a challenge. In this study, we investigate a microfluidic logic device that is built based on the particle dynamics in viscoelastic fluid. A logic gate system employing a Boolean function is implemented by utilizing multiple line particle focusing behavior in the microfluidic channel. The device is designed and fabricated to hydrodynamically control the logic operations of XOR, OR, AND, Buffer, and NOT under the fixed flow condition (e.g., flow rate, particle size, and fluid elasticity∼1.483). In addition, numerical simulation is carried out to understand the fundamental physics of the particle and fluid behavior in viscoelastic flow. Clear multiple particle focusing lines with high separation resolution (Rij∼6.19) were observed and the particle extraction at the outlets were analyzed by image-processing. This study is expected to open a new route to the incorporation of microfluidics with electronics by demonstrating logic gates based on microfluidics.

Impact of periodic flow reversal of heat transfer fluid on the melting and solidification processes in a latent heat shell and tube storage system

A numerical study has been conducted on a shell and tube latent heat storage system whereby the inlet heat transfer fluid direction is periodically reversed during charging and discharging. The impact of varying the boundary condition at the interface of the tubes carrying the heat transfer fluid and phase change material (PCM) on the evolution of the phase change front, heat transfer area and heat transfer rate have been evaluated during the charging and discharging processes. Results for the charging processes show a higher heat transfer area develops during the early stages and amplification of natural convection after 40% melt fraction, leading to a higher heat transfer rate. In comparison to the fixed flow condition, periodic flow reversal for the discharge cases results in an increased heat transfer area for a longer period of time, leading to a higher heat transfer rate particularly after 75% solidification. This effect is more important for discharging cases in the absence of convection heat transfer. Periodically reversing the direction of heat transfer fluid, which produced a periodic boundary condition at the tube-PCM interface, also resulted in a lower temperature gradient in space and time and consequently higher exergy recovery, and about a 6% increase in the time-average heat transfer rate in the charging and discharging cases. The novel reversal flow method provides a means to implement a periodic boundary condition without changing the heat source/sink, enhancing the thermal performance and cost effectiveness of latent heat storage systems. Phase change storage systems incorporating periodic flow reversal provide higher energy delivery rates, greater power density and more exergy recovery. This method can support fast heat release to respond to a peak load in a CSP plant or fast heat storage to protect a tubular receiver from high thermal stresses.

Particle dispersion in morphologically-inspired computational models of alveolar capillary networks

With growing evidence of the ability of inhaled nanoparticles (NP) to translocate from the lungs into the pulmonary circulation, increased concerns regarding the fate of such particles reaching body organs have risen. Until present, there is still a limited understanding on the transport dynamics of NPs following translocation into the pulmonary microcirculation. To gain initial insight into such processes, simulations of the transport and dispersion of various particle sizes are conducted in anatomically-inspired alveolar capillary networks (ACN). Our models, based on the seminal 'sheet flow' model, investigate quantitatively the influence of network porosity on particle dynamics. For fixed flow conditions, we find that the effective dispersion coefficient is sensibly enhanced with decreasing porosity levels and decreasing particle sizes. Furthermore, particle size significantly influences the characteristics of particle spreading and tortuosity. Overall, our findings represent a tangible first step in further understanding inhaled NP transport in networks representative of the alveolar capillaries.

CCD Series No-14: SWRO-CCD under fixed-pressure and variable flow compared with fixed-flow and variable pressure conditions

A theoretical model on the basis of the conceptual principles of closed circuit desalination (CCD) is explored in the present study for the performance assessment of the single-element seawater desalination reverse osmosis (SWRO) unit ME (E = SWC6-MAX) of near absolute energy efficiency without an energy recovery device (ERD) under fixed-pressure (FP) of variable flow compared with fixed-flow (FF) of variable pressure conditions. Performance simulations of same feed source (3.2% NaCl), sequence duration (9.01 min), recovery (60.3%), average flux (14.9 lmh), concentrate recycling flow (4.0 m3/h), number of CCD cycles (10), and cycle period (0.90 min/cycle) under FP (65 bar) compared with FF (37.1–71.3 bar) conditions revealed the respective specific energy values (SE) of 2.177 and 1.792 kWh/m3 with average permeate Total Dissolved Salts (TDS) of 574 and 464 ppm. The average SE sequential progression under FF conditions (1.234–1.792 kWh/m3) compared with that under FP conditions (2.140–2.177 kWh/m3) reveals the declined percent energy-saving mode of the former as function of increased recovery (R) in the order: 32.2% (R = 40%); 30.1% (R = 45%); 26.4% (R = 50%), 21.9% (R = 55%); and 17.7% (R = 60,3%). Since conventional SWRO desalination of Ocean seawater (~35,000 ppm TDS equivalent to ~3.2 NaCl) is normally being carried out with ~45% recovery, average flux near 15 lmh and an applied pressure around 65 bar using ERD, the findings of the current study suggest RO energy saving of 30.1% before accounting for ERD energy losses if the desalination is performed with SWRO-CCD under FF conditions. This conclusion is supported by the reported SE (2.46 kWh/m3) for the conventional SWRO desalination plant in Australia compared with the extrapolated SE value for Ocean water (~1.70 kWh/m3) from normalized experimental SWRO-CCD data with Mediterranean seawater. The average TDS of permeate projections during the batch sequence progression under FP (110–574 ppm) compared with FF (291–454 ppm) conditions reveals higher quality permeates by the former up to 54% recovery and thereafter, the preference of the latter mode. The observed inversion takes place due to rapidly declined flux under the FP conditions towards the end of the batch sequence which creates a fast rise in TDS per cycle manifested also by the average TDS.

Adjoint-based control of loud events in a turbulent jet

AbstractEfforts to reduce the noise from turbulent jets at fixed flow conditions, with aircraft noise as the principal technological motivation, have generally involved some degree of parametric empiricism often based upon a series of trial-and-error testing. As a result, it is unclear if the modest reductions found, in rare cases that do not greatly affect the flow field or incur prohibitive losses, are near the limit of what can be accomplished or if there are undiscovered opportunities for more substantive reductions with better designs or active control. We assess this using an adjoint-based optimization procedure in conjunction with an experimentally validated large-eddy simulation of a Mach 1.3 turbulent jet. The adjoint solution provides a definitive direction in which to adjust a model control actuation in order to reduce noise, providing guidance that seems lacking by any other current means. It is found that three conjugate-gradient iterations in the control space provide ∼3.5 dB of reduction, comparable to other reductions found empirically. The control seems to work by disrupting the coherence of acoustically efficient axisymmetric flow structures. The control and noise-reduction mechanisms are informative, but also suggest that any significantly quieter state would not be a simple perturbation from the uncontrolled jet. Additional iterations might reduce noise more significantly, but there might be only modest opportunities to reduce the sound from simple round turbulent jets without radical changes or relatively sophisticated controls. Though it is difficult to prove any behaviour in a global space of actuations, there does not seem to be a direct route based upon a local sensitivity gradient to substantially quieting a jet, even with an unrealistically flexible actuation. More complex jets or other noisy flows may be more amenable to control, in which case the adjoint-based optimization procedure demonstrated here could provide invaluable engineering guidance.

Second law analysis of compressible flow through a diffuser subjected to constant heat flux at wall

Entropy generation is equivalent to destruction of available work (exergy). The useful energy is destroyed due to the intrinsic irreversibility associated within thermodynamic systems. Exergy analysis can be used as an effective basis for the development and improvement of systems design not only in the overall perspective but also in the individual component level. Second law analysis provides a useful tool to identify the irreversibility in any thermal system. This study presents the investigation of local and total entropy generation in compressible flow through a diffuser. Air is used as the fluid. Uniform heat flux boundary condition is applied at the wall. Two dimensional solution of velocity and temperature fields are obtained using the CFD code FLUENT. Distribution of entropy generation rate is investigated throughout the volume of the fluid as it flows through the diffuser. Regions of high entropy generation in the diffuser have been predicted. The angle of expansion of diffuser is varied and its desire value corresponding to minimum entropy generation is determined at fixed flow conditions.

Soot-field structure in laminar soot-emitting microgravity nonpremixed flames

Due to the intrinsic interest in laminar flame regimes that cannot be attained in normal gravity (1 g), the microgravity environment (0 g) offers an attractive and promising perspective to enhance our understanding of soot formation mechanisms in diffusion flames. An experimental investigation conducted at the 2.2-s drop tower of the NASA Lewis Research Center is presented to define the soot-field structure within 0-g laminar jet nonpremixed flames which operate above their smoke point. Parallel work on earthgravity flames is also presented to facilitate comparisons and define the effect of gravity on the soot fields. The experiment considers jet diffusion flames of nitrogen-diluted acetylene burning in quiescent air at atmospheric pressure. A full-field laser extinction technique is utilized to determine the transient soot spatial distributions in axisymmetric flames with fixed flow conditions corresponding to Rc=O(100). Results are presented for a nonsmoking normal-gravity flame and its nonbuoyant counterpart which releases soot from its blunt tip. Quantitative measurements on the transient postignition character of the soot field in 0 g are presented for the first time and indicate that millimeter-diameter burners result in brief soot-field transients in microgravity. At a fuel flow rate which is nearly twice that at the smoke point in 0 g, the soot field after the initial transient period sustains its annular structure throughout the luminous nonbuoyant-flamezone. The maximum soot volume fraction measured at 0 g is nearly double that at 1 g for the same flow rate, thus attesting to the higher sooting tendency of nonbuoyant flames. The results indicate that the prolonged residence times under microgravity promote soot growth more than oxidation, thus producing more soot in 0 g. Side-by-side comparisons of the soot distributions in 0-g and 1-g flames demonstrate the increased spatial resolution afforded in microgravity flames.

Gas Transport Processes In Human Airways

Numerous mathematical models describe the behavior of inhaled gases of health effects concerns. Such models are often developed for particular applications and have commensurate limitations (e.g., applicability to water-soluble or -insoluble gases) based upon the assumptions used in their respective derivations. The various models have certain restrictions in common; for instance, they assume airways of the lung to be smooth-walled tubes with fixed flow conditions. But, as documented herein, airways of the lung contain pronounced surface irregularities, including natural structural features like cartilaginous rings in the upper bronchi, and various airway lengths insufficient for flow development. Therefore, we have focused upon the behavior of inhaled gases within human conducting airways while accounting for effects of localized features of morphology and developing flow conditions. The role of respiratory intensity (i.e., ventilatory parameters) was also examined. The new model has been developed to quantitatively describe convective gas diffusion in a lung environment that is realistic from perspectives of biology and physics. The results include the following observations. (1) The Schmidt number is important for the characterization of inhaled gases. (2) Human activity levels have considerable effects on gas diffusion efficiencies within the lung; for example, the difference between diffusion efficiencies during sedentary and light activity conditions is about 100% for a gas Schmidt number of 1. (3) Airway surface irregularities have prominent effects on gas transport. For instance, diffusion efficiencies can be enhanced by up to 35% (relative to smooth-walled tubes) due to the presence of cartilaginous rings in the large airways of the tracheobronchial tree.

The effects of Damköhler number in a turbulent shear layer

A chemical reaction for which the reaction rate can be varied in studied in a fully developed, two-dimensional, turbulent mixing layer. The layer is formed between two nitrogen streams, one carrying low concentrations of fluorine and the other hydrogen and nitric oxide. For fixed concentrations of fluorine and hydrogen and for nitric oxide concentrations that are small fractions of the fluorine concentration, the heat release is fixed but the overall reaction rate is controlled by the nitric oxide concentration. Therefore, for fixed flow conditions, the nitric oxide concentration determines the ratio of the reaction rate to the mixing rate and hence a Damköhler number. For large values of this ratio, the amount of product, at a given downstream location, measured by the mean temperature rise, is independent of the reaction rate, i.e., the reaction is mixing limited. As the reaction rate is reduced and the amount of product declines, other effects are: (1) the mean temperature profile, which is initially somewhat unsymmetrical because the hydrogenfluorine freestream concentration ratio is set at a large value, becomes symmetrical, and (2) the ramplike instantaneous temperature traces within the large structure gradually become more like square waves. Based upon two choices for the mixing rate, numerical values of the Damköhler number are proposed to quantify fast and slow chemistry regimes.

Mono- and disilicon radicals in silane and silane-argon dc discharges

Measurements of monosilicon (SiHn) and disilicon (Si2Hn) radicals at the cathode surface of dc discharges in silane and silane-argon mixtures are reported. Silyl radical density per decomposed silane was constant for fixed flow conditions over a range of powers and silane-argon ratios. The relative densities for other monosilicon radicals SiHn/SiH3 decreased with increased fraction of silane in silane-argon mixtures. The density of disilicon radicals was observed to be comparable to some of the monosilicon radicals, with Si2H2 and Si2H4 the dominant Si2Hn species. Formation and destruction reactions are discussed for these radicals, disilane, and the deposited film. We deduce that disilane is formed primarily on surfaces and that sputtering is a significant source for radicals near the cathode.

Graphical methods used in the numerical solution of Jeffery-Hamel flow at fixed flow rates

A basic problem in the area of Fluid Dynamics is the solution of the Navier-Stokes equations describing the radial flow of viscous incompressible fluid between non-parallel plane walls. Since the known solutions involve transcendental equations containing elliptic functions, there are no explicit equations or methods available for determining the shape of the velocity profile for specified boundary conditions that do not require an iterative or graphical technique. The purpose of this paper is to describe the computer generated graphical solutions which allow for an easy determination of the velocity profile for fixed flow conditions. This paper will describe the step by step process of extending the solution of the problem by working back and forth between the graphical representation of the equations and the numerical solution.