Laminar Region Research Articles

Flow sharing and turbulence phenomena in proton exchange membrane fuel cell stack headers

The distribution of the gas flow in a PEMFC stack is of paramount importance to the stack's performance and lifetime. Uneven flow distribution influences the flow rate through each cell, which in turn causes uneven distribution of the current flow of the entire cell stack and ultimately reduces the performance of the fuel cell stack. In this work, different simulation methods are compared, and large eddy simulations are selected to investigate the flow characteristics in a model stack and study the effects of operating conditions on flow sharing. The simulation results indicate different flow patterns in the inlet header and outlet header; the former features a turbulent entrance region that progressively transits to a laminar region, whereas the latter exhibits a complex flow with jets mixing downstream. Moreover, the flow patterns and distributions for different inlet/outlet configurations, i.e., U-type and Z-type, are investigated. The distribution of the flow through the unit cells for both configurations is different. The Z-type arrangement offers a more uniform flow distribution and has a smaller number of fluctuations than the U-type. The effects of different inlet flow velocity and jet inflow pattern are also studied. The findings from this work can provide guidelines to improve header design.

Correlation of Power Consumption of Double Impeller Based on Impeller Spacing in Laminar Region

Power consumption is an important parameter for the design of mixing equipment. The aim of this study is to develop a new correlation of the power consumption of a double impeller. The effect of impeller spacing on the double-impeller flow pattern and power consumption was investigated in the laminar region. As a result, the effect of impeller spacing on the flow pattern was described based on the ratio of impeller spacing to the impeller blade height. Moreover, the power consumption of a double impeller could be correlated with the same ratio.

Experimental analyses on heat transfer performance of TiO2–water nanofluid in double-pipe counter-flow heat exchanger for various flow regimes

Nanofluids are widely used in heat transfer applications. This article presents the effect of heat transfer and pressure drop of the TiO2–water nanofluids flowing in a double-tube counter-flow heat exchanger with various flow patterns. In this experimental work, performance of TiO2–water nanofluid on heat transfer in three different cases such as laminar, transition and turbulent flow region were analyzed. TiO2 nanoparticles with average diameters of 20 nm dispersed in water with three volume concentrations of 0.1, 0.3 and 0.5 vol% were used as the test fluid. The results show that the heat transfer of nanofluids is higher than that of the base liquid (water) and increased with the increase in Reynolds number and particle concentrations. The heat transfer rate of nanofluid with 0.5 vol% was 25% greater than that of base liquid, and the results also show that the heat transfer coefficient of the nanofluids at a volume concentration of 0.5 vol% was 15% higher than that of base fluid at given conditions. Pressure drop of nanofluid was increased with increase in volume concentration, and it is slightly higher than that of the base fluid.

EFFECT OF ARTIFICIAL CAVITIES ON HEAT TRANSFER AND FLOW CHARACTERISTICS MICROCHANNEL

An experimental investigation was conducted to study single-phase fluid flow and heat transfer in a copper micro channel. To investigate the effect of artificial cavities on fluid flow and single phase heat transfer in micro channel heat sink, two model of straight micro channel recognized as two models (model -1and model -2) were designed and manufactured ,where model-1 have smooth bottom surface while Model-2 have 47 artificial cavities distributed uniformly at the bottom surface along the micro channel length. The two models having the same nominal dimension of 300?m height and 300?m depth while the real dimension value are 367 ?m for width and 296 ?m for depth .De-ionized water was used as the working fluids. Experimental test was conducted at 30?C inlet temperature with Reynolds numbers range from 700 to 2200 covering laminar flow conditions. The experiments were conducted with horizontal micro channel under both adiabatic (for friction factor calculation) and diabatic (for Nusselt number calculation) conditions. The results indicated that the experimental Darcy friction factor can be predicted well with conventional scale fanning friction factor correlations for developing flow in laminar region by shah and London (1978) correlation for two models. Also, the experimental Nusselt number Agree well with each correlation of shah and London (1978) and Mirmanto correlation in laminar region.

Fuzzy Logic-Based Control for a Morphing Wing Tip Actuation System: Design, Numerical Simulation, and Wind Tunnel Experimental Testing.

The paper presents the design, numerical simulation, and wind tunnel experimental testing of a fuzzy logic-based control system for a new morphing wing actuation system equipped with Brushless DC (BLDC) motors, under the framework of an international project between Canada and Italy. Morphing wing is a prime concern of the aviation industry and, due to the promising results, it can improve fuel optimization. In this idea, a major international morphing wing project has been carried out by our university team from Canada, in collaboration with industrial, research, and university entities from our country, but also from Italy, by using a full-scaled portion of a real aircraft wing equipped with an aileron. The target was to conceive, manufacture, and test an experimental wing model able to be morphed in a controlled manner and to provide in this way an extension of the laminar airflow region over its upper surface, producing a drag reduction with direct impact on the fuel consumption economy. The work presented in the paper aims to describe how the experimental model has been developed, controlled, and tested, to prove the feasibility of the morphing wing technology for the next generation of aircraft.

A study on thermohydraulic characteristics of fluid flow through microchannels

At present, miniaturization of the devices in order to make them for effective performance, reliability and ease at cost is the primary need of any nation. In this direction, the role of microchannels can play an important role in the further development of industrial growth. The increased demand of the microchips in the industrial areas has increased the number of transistors for the improved functionality which leads to the emission of the higher heat flux, which is already a big challenge in the electronic sector. In the present paper, a comprehensive review on microchannels has been done regarding single-phase and multi-phase studies. In the present paper, an intensive review of the thermal and hydraulic characteristics of fluid flow in microchannels at different hydraulic diameters and their effects on performance has been done. The effects of the various parameters such as the Reynolds number (Re), Nusselt number (Nu), friction factor (f), pressure drop (P), working fluid and cross-sectional geometry of duct, as well as the hydrodynamic and thermal aspects, has been also studied. It was concluded through the literature study that transition from laminar to turbulent is very much affected by channel cross-sectional geometry, aspect ratio, channel wall roughness and compressibility effects. Also, researchers only explored the laminar region for its pressure drop and heat transfer characteristics, while the turbulent flow regime is yet to be explored. It was observed that most of the correlation over-estimated the value of pressure drop in multi-phase flow.

A big data analytical framework for analyzing solar energy receptors using evolutionary computing approach

Data science has been empowered with the emerging concept of big data enabling data scalability in many ways. Effective prediction systems for complex analytical problems dealing with big data can be created using evolutionary computing, associate feature selection and reduction techniques. In the current work, we put forward a big data analytical scheme to analyze solar energy receptors based on a set of features. Correct estimation of pressure loss coefficients (PLC) greatly improves the design of a solar collector. Evaluation of PLC is a time and resource consuming process as the flow rate and Reynolds number changes at every junction. Moreover, a suitable and appropriate algebraic expression is not yet defined in the laminar region of flow for approximation of the complex relationship among different geometrical features and flow variables. The overall heat gain of the solar receptor is dependent upon flow rates and flow distribution in risers. Also, the local disturbances during the flow division and combining process from manifold to risers affects the performance of the solar collector. Owing to these reasons, mostly they are calculated using experiments, primarily due to the complexity involved. The proposed big data framework involves acquiring huge feature sets at each point along the flow of thermal fluid. The data is experimentally acquired in a set of around forty features for large number of Reynolds number and discharge ratio variations. Reynolds number varies from 200 to 15,000 while discharge ratio variation is in the range of 0–1. Feature reduction in the big data set is done by calculating the relevancy score using ReliefF algorithm that extracts the most relevant features. Later, the framework employs a suitably selected optimal ANN architecture of layers, neurons and activation functions. The selected topology is trained using reduced features sets using Levenberg–Marquardt backpropagation algorithm. Test and validation results bespeaks the efficacy of the proposed strategy and indicate that future PLC values can be forecasted close to experimental data. The relative percent error is around 10% of the experimental data set and is found better than computational fluid dynamics based approaches in terms of memory and processing time.

Characterizing Influence of Transition to Turbulence on the Propulsive Performance of Underwater Gliders

Two models of underwater gliders were tested in a wind tunnel: one corresponding to a legacy shape commonly used in contemporary vehicles and the other a scaled down version of a new design. Performance of the two vehicles was characterized over a range of speeds and angles of attack. Particular attention was paid to the effect of sharp features along the hulls of the two vehicles and how they affect the observed flow regime. It has been shown that the new design, which uses a bow shape designed to encourage natural laminar flow, benefits from a 10% reduction of parasitic drag and 13% increase in lift-to-drag when the hull surface is smooth. The legacy glider, made up of a faired bow and a cylindrical hull, suffers from laminar separation and up to 100% increase in induced drag if the flow over its bow is prevented from transitioning to a turbulent state before encountering adverse pressure gradient at lower Reynolds numbers. This results in lowering of attainable speed at shallow glide path angles, whereas the associated parasitic drag reduction is demonstrated to increase the maximum velocity of the glider when moving at glide slopes greater than approximately 30°. 1. Introduction Underwater gliders are autonomous underwater vehicles (AUVs) that rely on using a buoyancy engine to ascend or descend through the water column, and by adjusting their pitch, they can use this vertical motion to develop forward thrust from their hydrofoils. This propulsion method allows them to undertake long-endurance missions, often several months long (Eriksen 2003; Rudnick et al. 2004; Graver 2005). Hydrodynamic performance of an underwater glider is primarily governed by its lift-to-drag (L/D) ratio, which dictates the minimum glide path angle the vehicle may adopt, and drag coefficient, which affects the maximum forward speed the glider may achieve for a fixed amount of vertical force developed (Graver 2005). It is thus important to minimize the drag of the AUV to allow it to perform longer deployments and gather more science data without increasing the size of the engine. Because of the typical Reynolds numbers on the vehicle hulls being less than 106 and of the order of 104–5 on the appendages, laminar and transitional flow regions may occur. Correctly identifying these is critical to achieve a robust performance prediction.

Impact of single nucleotide polymorphisms in the VEGFR2 gene on endothelial cell activation under non‑uniform shear stress.

Single nucleotide polymorphisms (SNPs) in vascular endothelial growth factor receptor 2 (VEGFR2) are associated with coronary artery disease, hypertension and myocardial infarction. However, their association with atherosclerosis remains to be fully elucidated. The purpose of the present study was to determine whether SNPs are involved in athero-genesis, by analyzing their impact on human umbilical vein endothelial cells (HUVECs) under laminar and non-uniform shear stress in a well-established in vitro model that simulates shear stress-induced proatherogenic processes at vessel bifurcations. All experiments were performed using freshly isolated HUVECs. Three SNPs in the VEGFR2 gene (rs1870377 T>A, rs2071559 A>G and rs2305948 C>T) were genotyped and the expression levels of VEGFR2 were semi-quantitatively determined using western blotting. Subsequently, the HUVECs were seeded in bifurcating flow-through cell culture slides and flow (9.6 ml/min) was applied for 19 h, including tumor necrosis factor-α stimulation during the final 2 h of flow. The protein expression levels of VCAM-1, E-selectin and VEGFR2 and the adhesion of THP-1 cells were analyzed in laminar and non-uniform shear stress regions. Data were analyzed for associations with the respective SNPs. The total expression of VEGFR2 was significantly lower under non-uniform shear stress than under laminar shear stress conditions, independent of the genotype. The expression of VEGFR2 between the different shear stress patterns was not significantly altered by the different SNPs. The expression levels of VCAM-1 and E-selectin were lower in the A/A genotype compared with those in other genotypes in rs1870377 T>A and rs2071559 A>G. In conclusion, the results suggested that SNPs within the VEGFR2 gene have a significant impact on shear stress-related endothelial activation.

Exact invariant solution reveals the origin of self-organized oblique turbulent-laminar stripes

Wall-bounded shear flows transitioning to turbulence may self-organize into alternating turbulent and laminar regions forming a stripe pattern with non-trivial oblique orientation. Different experiments and flow simulations identify oblique stripe patterns as the preferred solution of the well-known Navier-Stokes equations, but the origin of stripes and their oblique orientation remains unexplained. In concluding his lectures, Feynman highlights the unexplained stripe pattern hidden in the solution space of the Navier-Stokes equations as an example demonstrating the need for improved theoretical tools to analyze the fluid flow equations. Here we exploit dynamical systems methods and demonstrate the existence of an exact equilibrium solution of the fully nonlinear 3D Navier-Stokes equations that resembles oblique stripe patterns in plane Couette flow. The stripe equilibrium emerges from the well-studied Nagata equilibrium and exists only for a limited range of pattern angles. This suggests a mechanism selecting the non-trivial oblique orientation angle of turbulent-laminar stripes.

High-Reynolds number transitional flow simulation via parabolized stability equations with an adaptive RANS solver

The accurate prediction of transition is relevant for aerodynamic analysis and design applications. Extending the laminar flow region over airframes is a potential way to reduce the skin friction drag, which in turn reduces fuel burn and greenhouse gas emissions. This paper introduces a numerical framework that includes the modeling of transition effects for high Reynolds number flows in a high-fidelity, Reynolds–averaged Navier–Stokes (RANS) aerodynamic design framework. The CFD solver uses a discontinuous Galerkin (DG) finite element approach and includes goal-oriented adaptation. The Spalart–Allmaras (SA) turbulence model is used for the closure of the governing equations. In the flow stability analysis, the nonlocal, nonparallel effects that characterize boundary layers are accounted for by using the parabolized stability equations (PSE). Transition onset is obtained through an eN method based on the PSE computations, while a smooth intermittency function includes the transition region length. Numerical results for the NLF(1)-0416 airfoil present good agreement with experimental data, improving the computations when compared to fully-turbulent ones.

A General Sediment Carrying Capacity Formula and Its Analysis

With the extension of the energy equation of turbid density flow to the whole water depth of sediment-laden flow, the formula of equilibrium sediment concentration under the condition of steady state is obtained. On this basis, with the theory of minimum rate of flow energy dissipation, a formula of sediment carrying capacity is derived. In addition, the coefficient of the formula is proportional to the coefficient of hydraulic resistance. In different flow regions, including laminar region, turbulent smooth region, turbulent transition region, turbulent rough region, the coefficient of hydraulic resistance changes with the exponential change of the Reynolds number. In different turbulent flow, the general formula can be evolved into the sediment carrying capacity formula forms of Han Qiwei, Dou Guoren, Liu Jiaju and Zhang Ruijin. The relationship between this formula and other sediment carrying capacity formula which have been done is revealed. And this formula can be used as a general formula is confirmed. The formula is compared with the Liu Jiaju's formula and the Zhang Ruijin's which have been used for many years by the measured data. It is shown that this formula has certain advantages.

Thermal mixing enhancement of liquid metal film-flow by various obstacles under vertical magnetic field

Heat removal techniques of various liquid divertor concepts have been widely studied. In this study, thermal mixing characteristics of the liquid metal film-flow with locally heated on the surface under the vertical magnetic field were experimentally investigated by using various types of obstacles as a vortex generator. Especially, the efficiency of heat transport was investigated by the thermocouples installed on the bottom-wall. The velocity distributions were measured using the electric potential probe rake on the downstream of the channel. A delta-wing, a hemisphere or a cubic obstacle was used as a vortex generator installed at the center of the bottom-wall. The experiments were conducted for laminar flow region where Re = 487–1583 and in the range of N (=Ha2/Re) = 0–149 in the vertical magnetic field in the acrylic rectangular duct. GaInSn alloy was used as a working fluid. According to the comparison of heat flux distributions on the bottom-wall, the entire distributions moved towards the upstream with increasing of the strength of the vertical magnetic field for all cases. This tendency means the heat removal from the free-surface of liquid metal film-flow to the bottom-wall can be shortly achieved in case of the relatively high vertical magnetic field.

On the time scales and structure of Lagrangian intermittency in homogeneous isotropic turbulence

We present a study of Lagrangian intermittency and its characteristic time scales. Using the concepts of flying and diving residence times above and below a given threshold in the magnitude of turbulence quantities, we infer the time spectra of the Lagrangian temporal fluctuations of dissipation, acceleration and enstrophy by means of a direct numerical simulation in homogeneous and isotropic turbulence. We then relate these time scales, first, to the presence of extreme events in turbulence and, second, to the local flow characteristics. Analyses confirm the existence in turbulent quantities of holes mirroring bursts, both of which are at the core of what constitutes Lagrangian intermittency. It is shown that holes are associated with quiescent laminar regions of the flow. Moreover, Lagrangian holes occur over few Kolmogorov time scales while Lagrangian bursts happen over longer periods scaling with the global decorrelation time scale, hence showing that loss of the history of the turbulence quantities along particle trajectories in turbulence is not continuous. Such a characteristic partially explains why current Lagrangian stochastic models fail at reproducing our results. More generally, the Lagrangian dataset of residence times shown here represents another manner for qualifying the accuracy of models. We also deliver a theoretical approximation of mean residence times, which highlights the importance of the correlation between turbulence quantities and their time derivatives in setting temporal statistics. Finally, whether in a hole or a burst, the straining structure along particle trajectories always evolves self-similarly (in a statistical sense) from shearless two-dimensional to shear bi-axial configurations. We speculate that this latter configuration represents the optimum manner to dissipate locally the available energy.

Heat-flux measurements on a hypersonic inlet ramp using fast-response temperature-sensitive paint

Global heat-flux measurements are performed using a newly developed temperature-sensitive paint (TSP) on an inclined ramp with sidewalls in a hypersonic shock tunnel. The paint response and image acquisition rate are sufficiently fast to allow flow phenomena on timescales of around $$100\,\upmu \mathrm{{s}}$$ to be resolved. Although a priori calibration of the new TSP proves inaccurate, in situ calibration allows the recovery of heat fluxes that agree well with embedded thermocouple measurements on both short and long timescales. At low unit Reynolds numbers, the flow on the main ramp surface is entirely laminar, but transition occurs in the corner-flow regions, causing a turbulent region to spread inwards from each sidewall and producing weak, unsteady features in the heat-flux distribution of the main laminar region. Within this laminar region, roughly steady streamwise streaks with a period of approximately ten times the boundary-layer thickness are also observed. At higher unit Reynolds numbers, the boundary layer on the main ramp surface transitions to turbulence. The fast-response TSP allows tracking of the time-resolved transition front: significant unsteadiness is observed, which appears to be only weakly correlated to unsteadiness in the freestream flow conditions. Based on the heat-flux signature in the transition region, the breakdown mechanism seems to be quite different from that observed in earlier measurements on a slender cone at similar conditions.

Numerical and experimental analysis of the flow and transition of free synthetic jets

In this paper, flow characteristics and the transition of synthetic jets are investigated numerically and experimentally. The investigated synthetic jets have a dimensionsless stroke length of 146 and 167, an U0 of 7.3 and and a jet Reynolds number of 471 and 539. Novelty of this paper is the extensive analysis and explanation of why synthetic jets with result to transition and turbulent flow: it is shown that the jets exhibit swirl which originates from the cavity. The rotation results in more rapid divergence and deceleration of the free jets. Both processes initially cause disruptions of the primary vortex and the secondary vortices, also vortex breakdown, and ultimately result in jet turbulence. Extension of the laminar region spatially and temporally is possible by increasing the height of the cavity.

Aerodynamic Parameter Prediction via Artificial Hair Sensors with Signal Power in Turbulent Flow

Spatiotemporal surface flow information obtained from distributed arrays of bioinspired hair sensors are capable of predicting the real-time aerodynamic parameters (i.e., lift, moment, angle of attack, and freestream velocity) on a representative wing section. When combined with an appropriate model of the system, these sensors can provide vital information about gust disturbance as well as state estimation of an aeroelastic system, both of which are essential for effective vibration suppression control system design. In a typical aeroelastic system undergoing a periodic change in bending and torsion motion, a number of hair sensors can be in turbulent flow regions at any instant, resulting in random vibration response. This paper specifically investigates the effect in aerodynamic parameter prediction when sensor measurements from both laminar and turbulent flow regions are combined. It also investigates the idea of incorporating the sensor signal power along with sensor information to increase the prediction accuracy in such a scenario. The experimental results show that incorporating sensor response from both laminar and turbulent flow regions improves the prediction for the whole operating region containing both positive and negative angle of attack. Moreover, incorporating the signal power further improves the prediction accuracy and precision.

An innovative study on low surface energy micro-nano coatings with multilevel structures for laminar flow design

Laminar flow design is one of the most effective ways to reduce the drag of a commercial aircraft by expanding the laminar flow region on the surface of the aircraft. As material science develops, the emergence of new materials such as low surface energy materials has offered new choices for laminar flow design of commercial aircraft. Different types of low surface energy micro-nano coatings are prepared to verify the effects on the boundary layer transition position and the drag of the airfoil through wind tunnel tests. The infrared thermal imaging technology is adopted for measuring the boundary layer transition, while the momentum integral approach is employed to measure the drag coefficient through a wake rake. Infrared thermal imaging results indicate that the coatings are capable of moving backward the boundary layer transition position at both a low velocity of Mach number 0.15 and a high velocity of Mach number 0.785. Results of the momentum integral approach demonstrate that the drag coefficients are reduced obviously within the cruising angle of attack range from 1° and 5° by introducing the low surface energy micro-nano coating technology.

Development of dissimilar heat transfer promoter by genetic topology optimization

From the effective use of energy, it is desired to develop a vortex generator that realizes heat transfer enhancement while suppressing flow resistance, that is, the dissimilar heat transfer promoter. In recent years, various thermal-fluid engineering devices such as heat sinks have been developed in laminar region by topology optimization using adjoint analysis. However, in a strong non-linear flow field, second or higher-order variation cannot be ignored if integration is performed for a long time, in other words, the variational principle breaks down. Hence this optimization method cannot be applied to the development of dissimilar heat transfer promoters. Therefore, in this study, we develop a dissimilar heat transfer promoter by genetic topology optimization called the NG-net method, which is the method without using gradient information of objective functional. The purpose of this study is to find out what shape vortex generators should be installed in order to maximize heat transfer in plane channel with temperature differences on the upper and lower walls. The Reynolds number is set in a range where vortices appear time-dependently. In the poster presentation, the optimum shape, the Reynolds-number and time-integration-dependence of this shape will be shown, and the reason why the obtained shape realizes greater heat transfer will be discussed.

Modeling Frictional Characteristics of Water Flowing Through Microchannel

The present study aims at modeling the real random rough surface of a microchannel with structured rough channel of known geometric parameters. The surface of the microchannel is created by sinusoidal function using MATLAB code and 2D simulation of the model is carried out with commercial software ANSYS Fluent. The height of the channel is varied from 100 to 250 µm and length of the channel is 12.5 mm. The range of Reynolds number selected for analysis is 100 to 500 with water as the fluid medium. The roughness height is selected within the range of actual manufacturing roughness level of microchannels. The results show that channel geometry has significant influence on flow characteristics. A new non-dimensional roughness parameter β, is proposed to represent the dependence of friction factor on geometric parameters in the laminar region. A correlation for flow friction is developed as a function of roughness parameter and Reynolds number.