Net Drag Research Articles

Effects of clamping force on the water transport and performance of a PEM (proton electrolyte membrane) fuel cell with relative humidity and current density

The clamping force should be applied to a proton electrolyte membrane (PEM) fuel cell due to its structural characteristics. The clamping force affects the ohmic and mass transport resistances in the PEM fuel cell. In this study, the effects of the clamping force on the water transport and performance characteristics of a PEM fuel cell are experimentally investigated with variations in the relative humidity and current density. The water transport characteristics were analyzed by calculating the net drag coefficient. The ohmic resistance decreased with the increase in the clamping force due to the reduced contact resistance and more even membrane hydration. However, the mass transport resistance increased with the increase in the clamping force due to the gas diffusion layer compression. The net drag coefficient decreased with the increase in the clamping force due to high water back-diffusion. Additionally, the relationship between the total resistance and the net drag coefficient was investigated.

The coupled solute drag effect during ferrite growth in Fe–C–Mn–Si alloys using controlled decarburization

The kinetics of ferrite growth in three Fe–C–Mn–Si quaternary alloys has been measured using controlled decarburization tests at four temperatures between 755°C and 825°C, and a critical, quantitative test of the existence of a coupled solute drag effect (CSDE) is performed. The Mn and Si binding energies and trans-interface diffusivities (critical inputs in solute drag modelling) are taken directly from analogous experiments recently reported for ternary Fe–C–Mn and Fe–C–Si systems. The net solute drag effect experimentally observed in the quaternary system is found to be significantly less than that predicted by modelling using binding energies and trans-interface diffusivities calibrated on the respective ternary systems, i.e. an anti-CSDE is observed. This surprising observation is discussed in the context of solute interactions within the migrating interface and a number of future directions requiring investigation are highlighted.

Hydrodynamics study of a BCF mode bioinspired robotic-fish underwater vehicle using Lighthill’s slender body model

This paper investigates the hydrodynamic characteristics of the rectilinear motion of a robotic fish underwater vehicle. This 2-joint, 3-link multibody vehicle model is biologically inspired by a body caudal fin carangiform fish propulsion mechanism. Navier–Stokes equations are used to compute the unsteady flow fields generated due to the interaction between the vehicle and the surrounding incompressible and Newtonian fluid (water) environment. The NACA 0014 airfoil aerodynamic profile has been designed to boost the swimming efficiency by reducing drag as the vehicle undergoes an undulatory/oscillatory motion. Using the Lighthill slender body model, a traveling wave mathematical function is defined to undulate the robotic fish posterior (caudal) region while the motion tracking is carried out by dynamic meshing technique. The results obtained show that though the net lift force approaches to zero, the net thrust or negative drag coefficient maintains a finite value dependent on kinematic parameters like tail beat frequency (TBF) and amplitude span (AS) at a given propulsive wavelength and the forward velocity of the vehicle. The results reveal the effects of TBF and AS on the coefficient of drag friction and the thrust force. Drag coefficients obtained from the simulations are compared and validated with the experimental results. The hydrodynamic results are found to be similar to the kinematic study results and suggest that TBF and AS play the most effective roles in the bioinspired propulsion technique. Relation of these parameters with propelling thrust force and forward velocity is also in conjunction over a given range of TBF and AS values.

Drag of Clean and Fouled Net Panels--Measurements and Parameterization of Fouling.

Biofouling is a serious problem in marine aquaculture and it has a number of negative impacts including increased forces on aquaculture structures and reduced water exchange across nets. This in turn affects the behavior of fish cages in waves and currents and has an impact on the water volume and quality inside net pens. Even though these negative effects are acknowledged by the research community and governmental institutions, there is limited knowledge about fouling related effects on the flow past nets, and more detailed investigations distinguishing between different fouling types have been called for. This study evaluates the effect of hydroids, an important fouling organism in Norwegian aquaculture, on the forces acting on net panels. Drag forces on clean and fouled nets were measured in a flume tank, and net solidity including effect of fouling were determined using image analysis. The relationship between net solidity and drag was assessed, and it was found that a solidity increase due to hydroids caused less additional drag than a similar increase caused by change in clean net parameters. For solidities tested in this study, the difference in drag force increase could be as high as 43% between fouled and clean nets with same solidity. The relationship between solidity and drag force is well described by exponential functions for clean as well as for fouled nets. A method is proposed to parameterize the effect of fouling in terms of an increase in net solidity. This allows existing numerical methods developed for clean nets to be used to model the effects of biofouling on nets. Measurements with other types of fouling can be added to build a database on effects of the accumulation of different fouling organisms on aquaculture nets.

Drag reduction on a rectangular bluff body with base flaps and fluidic oscillators

The present paper investigates drag reduction on a rectangular bluff body by employing base flaps and controlling flow separation with fluidic oscillators. Wind tunnel experiments are conducted to assess the influence of various parameters. The flap length has to be sufficiently long to shift the wake structures far enough downstream away from the base plate. Any additional increase in flap length does not yield any further benefits. The flap angle has to be large enough to provide a sufficient inward deflection of the outer flow. If the angle is too large, actuation becomes inefficient due to the pressure gradient imposed by the opposite side of the base perimeter. Furthermore, the flaps at high deflection angles provide additional area for low pressure to act in the streamwise direction and therefore negate the positive effects of actuation. The required actuation intensity is best governed by the ratio between jet and freestream velocity for varying oscillator spacing. For a flap angle of 20°, the smallest net drag is obtained at a velocity ratio of 4.5. Furthermore, the optimal velocity ratio for the most efficient drag reduction changes linearly with flap angle. Smaller flap deflections require a smaller velocity ratio for optimal control at different oscillator spacing. A net drag reduction of about 13 % is measured at a flap angle of 20° when the drag is corrected by the momentum input. Even if the measured drag is conservatively corrected by the energy coefficient, a net improvement of 7 % is achieved. For the current setup, the most efficient drag reduction is still obtained at smaller flap angles with a lower momentum input. However, the presented results support the general feasibility of this drag reduction approach with significant room left for optimization.

The Resolution of d'Alembert's Paradox and Thermodynamic Admissibility

The d'Alembert paradox, annunciated in 1752, was established after it was shown that the result of a net zero drag, obtained by applying potential theory to the incompressible flow past a sphere, was in contradiction with experimental results which showed a positive drag. Interpreting the result as a flaw in the theory, resulted in the declaration of the paradox. Following d'Alembert, we assume a potential motion, and proceed to analyze the consequences of this assumption using the global principles of continuum mechanics. We show that if the fluid is inviscid, the potential motion is thermodynamically admissible, the drag is zero, and the motion can persist indefinitely. Although no conventional fluid is available to either falsify (or validate), this result experimentally, in principle, the theory could be tested by using a superfluid, such as liquid Helium. If the fluid is viscous, we show that the potential irrotational motion is thermodynamically inadmissible, it is in violation of the second law of thermodynamics, and hence such a motion cannot persist. Indeed, observations show that when a rigid body is impulsively set into motion, an irrotational motion may exist initially but does not persist. Any flow property which is derived from a thermodynamically inadmissible motion cannot be expected to be in agreement with experimental data. As an illustration we show that the drag, calculated from the viscous potential solution of the flow past a sphere, is zero. We submit that since the theory of continuum mechanics predicts that no agreement between results obtained from viscous potential theory and experimental data can be expected, there is no room for a paradox once a contradiction is in fact observed. In nature, or under experimental conditions, the nonpersistence of the thermodynamically inadmissible motion proceeds in a breakup of the irrotational motion which transforms into a rotational and obviously admissible motion. We show that by selecting boundary conditions, required in the solution of the differential equations of motion, such that they satisfy the Clausius–Duhem jump conditions inequality, the thermodynamic admissibility of the solution is a priori assured. We also show the vorticity distribution at the wall associated with the particular choice of boundary condition.

Effects of Hydrogen Fuel Injection in a Mach 12 Scramjet Inlet

To investigate the potential benefits of hydrogen fuel injection in a three-dimensional scramjet inlet, flow through a Mach 12 rectangular-to-elliptical shape transitioning scramjet inlet was simulated with and without hydrogen fuel injection along its body-side compression surface. The observed flowfields showed that, at an equivalence ratio of 0.33, the fuel cannot escape the body-side boundary layer, having little effect on macroscopic inlet flow structures. Interaction with the surrounding boundary-layer turbulence causes robust mixing of the fuel in the inlet. Fuel radicals are produced immediately following injection, particularly where the fuel plumes interact with thin, hot hypersonic boundary layers sweeping inward from the inlet side walls. Combustion does not proceed until the inlet compression process is nearly complete due to low static pressures in the inlet. Once the well-mixed fuel is processed by the final inlet compression shock, ignition and combustion occurs rapidly in the nearly constant cross-section region just upstream of the throat. The net drag increase observed was less than 5%. This modest increase was primarily due to the fuel-injection process, with almost no additional drag due to fuel combustion. Similar flow structures in other inlets suggest they would benefit from inlet fuel injection.

Turbulent Taylor–Couette flow over riblets: drag reduction and the effect of bulk fluid rotation

A Taylor–Couette facility was used to measure the drag reduction of a riblet surface on the inner cylinder. The drag on the surfaces of the inner and outer cylinders is determined from the measured torque when the cylinders are in exact counter-rotation. The three velocity components in the instantaneous flow field were obtained by tomographic PIV and indicate that the friction coefficients are strongly influenced by the flow regimes and structures. The riblet surface changes the friction at the inner-cylinder wall, which generates an average bulk fluid rotation. A simple model is proposed to distinguish drag changes due to the rotation effect and the riblet effect, as a function of the measured drag change ??w/?w,0 and shear Reynolds number Res . An uncorrected maximum drag reduction of 5.3 % was found at Res=4.7×104 that corresponds to riblet spacing Reynolds number s+=14 . For these conditions, the model predicts an azimuthal bulk velocity shift of 1.4 %, which is confirmed by PIV measurements. This shift indicates a drag change due to a rotation effect of ?1.9 %, resulting in a net maximum drag reduction of 3.4 %. The results correspond well with earlier reported results and demonstrate that the Taylor–Couette facility is a suitable and accurate measurement tool to characterize the drag performance of surfaces.

Computational Fluid Dynamics Simulation of Tip-to-Tail for Hypersonic Test Vehicle

Computational fluid dynamics simulations are carried out for a complete hypersonic vehicle integrating both external (nonreacting) and internal (reacting) flow together to calculate the scramjet combustor performance and vehicle net thrust minus drag. Simulations are carried out for a flight Mach number of about six. Three-dimensional Navier–Stokes equations are solved along with the shear stress transport turbulence model and single-step chemical reaction based on fast chemistry. The Lagrangian particle tracking method for droplets is used for combustion of kerosene fuel. Flow is largely nonuniform at the inlet of the combustor. Mass flow of ingested air increases with increase in angle of attack. Because of more combustion of fuel, wall surface pressure is higher for compared with . Combustion efficiency and thrust achieved are found to increase with the increase in angle of attack. Considerable amount of thrust is obtained from a single expansion ram nozzle and achievement of positive thrust–drag for the whole vehicle is demonstrated. The computational analysis of the whole vehicle provides net forces and moments of the whole vehicle, which is very useful for the mission analysis of the vehicle.

Power-saving device for air bubble generation using a hydrofoil to reduce ship drag: Theory, experiments, and application to ships

We have developed a new power-saving device to reduce the drag of a ship’s hull using small bubbles. The device reduces the energy required for bubble generation. The device, which consists of angled hydrofoils with air introducers, uses the low-pressure region produced above the hydrofoil as the ship moves forward to drive atmospheric air into the water. We describe the device principles obtained from simple fluid dynamic theory, and, through experiments performed in a small towing tank, show the fundamental air entrainment and bubble generation processes for the flow behavior around a hydrofoil beneath a free surface. We also present a semi-empirical scaling process for practical application to full-size ships to estimate the net drag reduction achieved by this device. Finally, the results of a series of full-scale tests are reported and show that, with correct operation, our device can produce a net power saving of 5–15% for ships.

某热电厂循环水系统静特性分析

利用一维数学模型对某热电厂循环冷却水系统进行了计算分析。考虑循环水管路及各主要设备阀门的阻力特性，计算得到了整个冷却水管网阻力特性曲线。并结合泵特性曲线，分析了循环水泵组与管网匹配后的系统特性及流量特性。为循环水泵系统实际运行提供理论基础。 The cooling water system of a certain thermal power plant has been simulated using one-dimensional method. The drag characteristic of circulating water pipe and main value has been considered, so the whole pipe net drag characteristic curve was calculated, with combinations of axial pump operating curve, the static operating and flow characteristics of the cooling water system have been analysed. This paper could provide some references for the cooling water system real operation.

Two dimensional modelling of water uptake in proton exchange membrane fuel cell

With the focus on water uptake by proton exchange membrane, a two-phase, non isothermal, transient and two-dimensional model of fuel cell is developed. Further, in order to obtain the equilibrium concentration of water in the membrane, two different approaches of water-uptake by the membrane are considered; though each takes into account the Schroeder's paradox as well as individual contributions of water vapour and liquid water. Furthermore, in both the approaches, rate of water uptake is proportional to the difference between equilibrium concentration and actual concentration of water in membrane. Model results show good agreement with the experimental results. A comparative analysis of the two approaches has been presented for various results, such as liquid saturation, net drag coefficient, temperature, water content in membrane, etc. Obtained results revealed significant difference between predicted current densities, water content of membrane and temperatures for the two approaches. These differences may be reflecting the need to correctly understand water uptake by membrane and its importance for accurate modelling of fuel cell. Response in transient state of fuel cell is also studied when a step change to cell voltage is applied. Likewise, studies on rate of sorption and desorption of water by membrane explain the increase or decrease of the water content of membrane.

(Invited) Engineering Net Water Balance in Polymer Electrolyte Fuel Cells

The net water drag in a polymer electrolyte fuel cell is a result of a variety of transport processes. There is often a need to tailor the water balance to achieve higher performance under dry operating conditions. The motivation of this collected work is to understand the methods by which we can control the water transport in a PEFC via engineering of the components and architecture. Experimental and numerical models have been developed to measure and predict this, and various pathways for control of water storage and transport will be discussed. In particular, control of the micro and macroporous layer thermal and mass transport resistance, cell architecture, and microporous layer|catalyst layer interface will be shown to be capable of manipulating the net water storage and drag coefficient.

Invited: Engineering Net Water Balance in Polymer Electrolyte Fuel Cells

The motivation of this work is to understand and control the water balance of a polymer electrolyte fuel cell to reduce anode water buildup and degradation. A custom 4.8 cm2 fuel cell with precision liquid coolant control was used to acquire the water balance data. The precision liquid coolant ensures stable, high accuracy thermal boundary conditions for the analysis. The net water drag coefficient was measured by placing high accuracy dew point temperature sensors at the inlets and outlets of the fuel cell to measure water balance. Gradients in pressure, inlet relative humidity, and temperature were used to isolate hydraulic, diffusion, and temperature driven flow through the membrane and porous media.This presentation will focus on the effects of anode land width, channel shape, and diffusion media tortuosity on net water drag. Using neutron radiography data that has been previously presented (1, 2) coupled with net water drag data, conclusions will be presented relating performance and water transport and storage. Data from this work will build upon an online database of benchmark data for the modeling community made available through this Department of Energy EERE supported program. Acknowledgements This work is funded by the United States Department of Energy Efficiency and Renewable Energy (EREE) Program through General Motors under Award Number DE-EE0000470.1. J. M. LaManna, S. Chakraborty, J. J. Gagliardo and M. M. Mench, International Journal of Hydrogen Energy, 39, 3387 (2014).2. J. LaManna and M. Mench, Meeting s, MA2014-01, 646 (2014).

The influence of the breathing action on net drag force production in front crawl swimming.

20 elite swimmers completed a total of 6 randomized net drag force trials in 2 conditions (i) 3 breathing and (ii) 3 non-breathing. Net drag force was measured using an assisted motorized dynamometer device mounted upon a Kistler force-platform. The male participants demonstrated no statistical differences in stroke rates between breathing and non-breathing trials. Female participants, however, demonstrated a statistical difference stroke rate. The male participants demonstrated that the breathing action caused a greater (26%) net drag force compared to the females (16%). To further understand the influence of breathing on swimming technique, each stroke was analyzed and comparisons were made between the breathing and non-breathing conditions. The male participants demonstrated a similar minimum net drag force when comparing the breathing and non-breathing conditions. Analysis showed that minimum net drag force and maximum net drag force for the males changed when integrating the breathing action, while female participants demonstrated similar swimming technique, regardless of condition or stroke.

Consecutive turbulence transition delay with reinforced passive control.

Miniature vortex generators (MVGs) are able to delay the transition to turbulence in a flat plate boundary layer if properly designed. Unfortunately, the natural recovery of the modulated laminar base flow in the streamwise direction is of exponential space scale and hence the passive laminar control fades away fairly rapidly. Here we show that by placing a second array of MVGs downstream of the first one it is possible to nourish the counter-rotating streamwise vortices responsible for the modulation, which results in a prolonged streamwise extent of the control. With this control strategy it is possible to delay the transition to turbulence, consecutively, by reinforcing the control effect and with the ultimate implication of obtaining a net skin-friction drag reduction of at least 65%.

Quantifying stroke coordination during the breathing action in front-crawl swimming using an instantaneous net drag force profile

This study used both an instantaneous net drag force profile and a symmetry timing to evaluate the effect of the breathing action on stroke coordination. Twenty elite swimmers completed a total of six randomised front-crawl towing trials: (i) three breathing trials and (ii) three non-breathing trials. The net drag force was measured using an assisted towing device mounted upon a Kistler force platform, and this equipment towed the swimmer at a constant speed. The net drag force profile was used to create a stroke symmetry index for each swimming trial. Analysis using the symmetry indices identified that the majority of participants demonstrated an asymmetrical instantaneous net drag force stroke profile in both the breathing and non-breathing conditions, despite no significant differences in the time from finger-tip entry to finger-tip exit. Within the breathing condition, the faster swimmers compared to the slower swimmers demonstrated a lesser percentage of overlap between stroke phases on their breathing stroke side. During the non-breathing condition, the faster participants compared to the slower swimmers recorded a reduction in the percentage of overlap between stroke phases and less duration in the underwater stroke on their breathing stroke side. This study identified that the majority of participants demonstrated an asymmetrical net drag force profile within both conditions; however, asymmetry was less prevalent when examining with only the timing symmetry index.

Comparison of Water Transport Mechanisms Between Commercial and Experimental Virgin and Aged Diffusion Media Using High Resolution Neutron Radiography

The motivation of this work is to characterize water transport for a new high tortuosity anode diffusion media (DM) and to provide the modeling community with valuable benchmark validation data. The high resolution neutron images in this work were taken at the Neutron Imaging Facility at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Tests were conducted with well-defined boundary conditions and publically available materials. Gradients in pressure, inlet relative humidity, and temperature were used to isolate hydraulic, diffusion, and temperature driven flow through the membrane and porous media.A custom designed 4.8 cm2fuel cell with liquid coolant for precise thermal boundary conditions was used to acquire the neutron images. Test conditions were selected to produce extreme gradients within the cell to push model prediction capabilities. The high resolution neutron images allow for the determination of through-plane water profiles and water transfer rates as a function of current density as shown in Figure 1. Additionally, real-time water balance and net water drag measurements were made with dew point sensors.This talk will discuss the differences in performance and water content of the experimental DM and baseline commercial DM presented previously, and attempt to draw conclusions relating the structural properties of the porous media and resulting performance and water transport and distribution. Additionally, partially degraded materials (e.g. electrodes used for hundreds of hours) will be compared to virgin materials to understand the impact of age and lifetime on water transport and performance. Data from this work will build upon an online database of benchmark data for the modeling community made available through this Department of Energy EREE supported program. Acknowledgements This work is funded by the United States Department of Energy Efficiency and Renewable Energy (EREE) Program through General Motors under Award Number DE-EE0000470. The authors would like to thank Dr. Dan Hussy and Dr. David Jacobson of NIST for their guidance.Figure 1: Liquid water profiles for all 1.5 A/cm2 test conditions for baseline DM (upper) and high tortuosity DM (lower). The high tortuosity DM significantly reduces anode water content for identical operating conditions to baseline.

Large-scale structure alternation in rotating plane Poiseuille flow at transitional Reynolds number

We performed a series of direct numerical simulations of transitional rotating Poiseuille flow, employing a large computational domain and discussed alternations of large-scale structures, such as the turbulent stripe and roll cell. The structure of the turbulent stripe is a laminar–turbulent banded pattern and the roll cell has an array of longitudinal vortexes aligning regularly in the spanwise direction. We observed the occurrence of turbulent stripes only under a low-rotation condition and the roll cells under high rotation. In the intermediate region, we found a transitional state from a static to a rotating system, which involved the coexistence of turbulent stripes and roll cells. In this study, we examined the process of disappearance of turbulent stripes and of the development of turbulent diffusion. A small net drag reduction can be achieved when relaminalization occurs and the turbulent stripe disappears on the suction side, although the drag increases monotonically once the roll cell dominantly affects the flow on the pressure side.

Laser Modified Fuel Cell Diffusion Media: Engineering Enhanced Performance via Localized Water Redistribution

The present work demonstrates an innovative concept of obtaining enhanced performance via laser treatment of the cathode-side diffusion medium (DM) while mitigating identified degradation modes. A diffusion medium was modified such that hydrophilic heat affected zones (HAZ) were introduced, which led to localized water redistribution. However, no perforation was created, thus mitigating accelerated degradation of the catalyst layer and diffusion medium. This material was compared to a diffusion medium with 100-μm diameter perforations that contained heat affected zones surrounding the perforations. In-situ net water drag experiments indicate that at low humidity and low-to-moderate current densities, a non-perforated microporous layer (MPL) forces more water to back diffuse from the cathode to the anode. However, when more water is produced at higher currents or the inlet streams are close to saturation, the non-perforated MPL acts as a barrier to prevent liquid water in the cathode DM from moving toward the anode. Furthermore, a computational model showed that the thermal gradients introduced as a result of the perforations can significantly change the water transport, particularly due to phase-change induced flow. This work adds understanding to the role of the MPL and the laser-induced heat affected zones in polymer electrolyte fuel cell performance.