Optimal Flow Field Research Articles

Fluid flow field synergy principle and its application to drag reduction

The concept of field synergy for fluid flow is introduced, which refers to the synergy of the velocity field and the velocity gradient field in an entire flow domain. Analyses show that the flow drag depends not only on the velocity and the velocity gradient fields but also on their synergy. The principle of minimum dissipation of mechanical energy is developed, which may be stated as follows: the worse the synergy between the velocity and velocity gradient fields is, the smaller the resistance becomes. Furthermore, based on the principle of minimum dissipation of mechanical energy together with conservation equations, a field synergy equation with a set of specified constraints has been established for optimizing flow processes. The optimal flow field can be obtained by solving the field synergy equation, which leads to the minimum resistance to fluid flow in the fixed flow domain. Finally, as an example, the field synergy analysis for duct flow with two parallel branches is presented. The optimized velocity distributor nearby the fork, which was designed based on the principle of minimum dissipation of mechanical energy, may reduce the drag of duct flow with two parallel branches.

Analysis of 3D rolling forming with generalized rigid-plastic boundaries approach

A new formulation of a generalized rigid-plastic boundaries approach is proposed to analyze the three-dimensional forming by rolling. In this approach, the dual-stream function that satisfies the velocity boundary conditions at the surfaces of rigid-plastic boundaries is derived. The arbitrary shapes of rigid-plastic boundaries with chosen optimization parameters are used to minimize the power required for rolling. The optimal flow field is found and used to calculate the velocity field, forming load and the width spread of the metal strip. Experiments were carried out to compare with the theoretical results of different dimensions. It is found that the predicted load, flow patterns and width spread are in good agreement with the experimental observations and results from literatures.

Field synergy equation for turbulent heat transfer and its application

A field synergy equation with a set of specified constraints for turbulent heat transfer developed based on the extremum entransy dissipation principle can be used to increase the field synergy between the time-averaged velocity and time-averaged temperature gradient fields over the entire fluid flow domain to optimize the heat transfer in turbulent flow. The solution of the field synergy equation gives the optimal flow field having the best field synergy for a given decrement of the mean kinetic energy, which maximizes the heat transfer. As an example, the field synergy analysis for turbulent heat transfer between parallel plates is presented. The analysis shows that a velocity field with small eddies near the boundary effectively enhances the heat transfer in turbulent flow especially when the eddy height which are perpendicular to the primary flow direction, are about half of the turbulent flow transition layer thickness. With the guide of this optimal velocity field, appropriate internal fins can be attached to the parallel plates to produce a velocity field close to the optimal one, so as to increase the field synergy and optimize the turbulent heat transfer.

Field synergy analysis and optimization of the convective mass transfer in photocatalytic oxidation reactors

A convective mass transfer field synergy equation with a specific boundary condition for photocatalytic oxidation reactors developed based on the extremum principle of mass transfer potential capacity dissipation can be used to increase the field synergy between the velocity and contaminant concentration gradient fields over the entire fluid flow domain to enhance the convective mass transfer and increase the contaminant removal effectiveness of photocatalytic oxidation reactors. The solution of the field synergy equation gives the optimal flow field, having the best field synergy for a given viscous dissipation, which maximize the contaminant removal effectiveness. As an illustrative example, the field synergy analysis for laminar mass transfer in plate type reactors is presented. The analysis shows that generating multiple longitudinal vortex flow in the plate type reactor effectively enhances the laminar mass transfer. With the guide of the optimal velocity pattern, the discrete double-inclined ribs can be introduced in actual applications to generate the desired multi-longitudinal vortex flow, so as to enhance the laminar mass transfer, and consequently, improve the contaminant removal performance. The experimental result shows that the contaminant removal effectiveness for the discrete double-inclined ribs plate reactor is increased by 22% compared to the smooth plate reactor.

Flow field control for larviculture of the seven-band grouper Epinephelus septemfasciatus

This paper reviews recent findings on establishing flow field control for larval rearing by hydrodynamics approach from case studies of the seven-band grouper ( Epinephelus septemfasciatus). We chose this species because grouper larvae are known to be highly sensitive to physical stress, although it is expected for the new target species of aquaculture and stock enhancement in Japan. We quantified and visualized the flow field by aeration in the rearing tank with hydrodynamics approach. A series of systematic experiments was conducted to evaluate the optimal flow for the survival of larvae in the 1 m 3 rearing tanks. Aeration at 200 mL/min produced the highest survival and growth for grouper larvae and the flow field was identified and then a computing method for estimating the flow field was established. We expanded this approach to the mass-culture scale (100 m 3) and controlled the flow field similar to the optimal flow field in the 1 m 3 rearing tank. It was quite successful resulting in 3 times higher survival than the former rearing methods. The results from these studies may be very useful for estimating the stationary flow in a rearing tank and for designing suitable tanks for rearing larvae. We also discussed the possibility of larviculture without aeration.

Gas Flow Characteristics in the Model Channel of a Disk Shape Planar-Type SOFC

A radial flow between two parallel disk shape plates, which simulates a flow in the disk shape planar-type solid oxide fuel cell (SOFC), was experimentally investigated. A temperature distribution in the planar-type SOFC is strongly affected by the gas flow behavior, and hence we tried to clarify flow pattern at various flowrates by using particle image velocimetry (PIV). It was found that optimum flow condition in the fuel channel was attained in the case of the Reynolds number of about 5. For this operating condition, a flow field near the anode surface shows uniform velocity distribution. On the other hand, in the upper air channel, non-uniform flow distribution was enlarged and large-scale vortex appears with increase of flow rate. In the lower air channel, local high velocity region exists at a high flow rate. For the present disk channel the optimum flow field in the air channel was obtained around the Reynolds number of 30. These results suggest the necessity of improvement of the channel shape and injection method of air.

Analog Integrated 2-D Optical Flow Sensor

I present a new focal-plane analog very-large-scale-integrated (aVLSI) sensor that estimates optical flow in two visual dimensions. Its computational architecture consists of a two-layer network of locally connected motion units that collectively estimate the optimal optical flow field. The applied gradient-based optical flow model assumes visual motion to be translational and smooth, and is formulated as a convex optimization problem. The model also guarantees that the estimation problem is well-posed regardless of the visual input by imposing a bias towards a preferred motion under ambiguous or noisy visual conditions. Model parameters can be globally adjusted, leading to a rich output behavior. Varying the smoothness strength, for example, can provide a continuous spectrum of motion estimates, ranging from normal to global optical flow. The non-linear network conductances improve the resulting optical flow estimate because they reduce spatial smoothing across large velocity differences and minimize the bias for reliable stimuli. Extended characterization and recorded optical flow fields from a 30 × 30 array prototype sensor demonstrate the validity of the optical flow model and the robustness and functionality of the computational architecture and its implementation.

Development of a stack having an optimized flow field structure with low cross transport effects

PEM fuel cells when operated on hydrogen from renewable sources are viewed as one of the most environmentally friendly energy conversion systems due to their high electrical efficiency. However, this advantage is depending on the overall system design, which is largely determined by the allowable operating conditions of the fuel cell stack itself. Besides the active materials, design and shape of the gas distribution zone have a significant influence on stack operation. In order to optimize overall system performance, a fuel cell stack with improved flow field design and performance was developed. An investigation on channel geometries led to a serpentine flow field with a moderate degree of parallelization and ribs with variable width to reduce cross transport effects. The resulting flow field subsequently has been modified slightly to allow a high volume production process. Summarizing, power as well as the degrees of H 2 and air utilization could be enhanced leading to a power density enhancement. Furthermore, weight reduction of end plates nearly by half using an improved end plate design led to an overall improved stack design.

Reduced Two-Phase Model for Analysis of the Anode of a DMFC

An isothermal two-phase ternary mixture model that takes into account conservation of momentum, mass, and species in the anode of a direct methanol fuel cell (DMFC) is presented and analyzed. The slenderness of the anode allows a considerable reduction of the mathematical formulation, without sacrificing the essential physics. The reduced model is then verified and validated against data obtained from an experimental DMFC outfitted with a transparent end plate. Good agreement is achieved. The effect of mass-transfer resistances in the flow field and porous backing are highlighted. The presence of a gas phase is shown to improve the mass transfer of methanol at higher temperatures (>30°C). It is also found that at a temperature of around 30°C, a one-phase model predicts the same current density distribution as a more sophisticated two-phase model. Analysis of the results from the two-phase model, in combination with the experiments, results in a suggestion for an optimal flow field for the liquid-fed DMFC. © 2004 The Electrochemical Society. All rights reserved.

Control of stretching rate in time-periodic chaotic flows

In this study we consider a method for enhancement of mixing in time-periodic laminar flows. The stretching rate along unstable manifold of the chaotic flow is considered as a quantitative criterion of mixing quality. Optimal control theory is applied for improving the stretching rate in a prototypic cavity flow. It is demonstrated that the optimal flow field tends to stretch the infinitesimally small vectors that are distributed along the material interface. The higher is the length of a vector at the final moment of time the more effective is the stretching along this vector. It is shown that a high stretching rate does not imply uniformity of mixing. In the periodic flows with the relatively short periods the chaotic region does not span over the entire flow, and the increase of a stretching rate in the chaotic region leads to expansion of regular islands. Thus, the proposed method for enhancement of mixing is applicable for the flows without regular unmixed islands.

Research on Inverse, Hybrid and Optimization Problems in Engineering Sciences with Emphasis on Turbomachine Aerodynamics: A Review of Chinese Advances

Advances in inverse design and optimization theory in engineering fields in China are presented. Two original approaches, the image-space approach and the variational approach, are discussed in terms of turbomachine aerodynamic inverse design. Other areas of research in turbomachine aerodynamic inverse design include the improved mean-streamline (stream surface) method and optimization theory based on optimal control. Among the additional engineering fields discussed are the following: the inverse problem of heat conduction, free-surface flow, variational cogeneration of optimal grid and flow field, and optimal meshing theory of gears.

'Vector white noise': a technique for mapping the motion receptive fields of direction-selective visual neurons.

A technique is described and tested for mapping the sensitivities and preferred directions of motion at different locations within the receptive fields of direction-selective motion-detecting visual neurons. The procedure is to record the responses to a number of visual stimuli, each stimulus presentation consisting of a set of short, randomly-oriented, moving bars arranged in a square grid. Each bar moves perpendicularly to its long axis. The vector describing the sensitivity and preferred direction of motion at each grid location is obtained as a sum of the unit vectors defining the directions of motion of the bars in each of the stimuli at that location, weighted by the strengths of the corresponding responses. The resulting vector field specifies the optimum flow field for the neuron. The advantage of this technique over the conventional approach of probing the receptive field sequentially at each grid location is that the parallel nature of the stimulus is sensitive to nonlinear interactions (such as shunting inhibition for mutual facilitation) between different regions of the visual field. The technique is used to determine accurately the motion receptive fields of direction-selective motion detecting neurons in the optic lobes of insects. It is potentially applicable to motion-sensitive neurons with highly structured receptive fields, such as those in the optic tectum of the pigeon or in area MST of the monkey.