CFD assisted modeling for control system design: A case study

This paper presents a novel modeling approach of coupling transient computational fluid dynamics (CFD) simulation with system identification for control system involving fluid flow and heat transfer. In order to illuminate the feasibility of this method, a fluid flow and heat transfer related process, i.e. a three dimension (3-D) spatio-temporal air temperature distribution and input (inlet air temperature) dependent process in the desert climate chamber, is considered. The distributed parameter models of the chamber temperature are identified using transient CFD simulation results and are then validated against the results obtained from the CFD simulations with high RT² (more than 0.97) and negative Young’s information criterion (YIC, less than ?11.8). The PI controllers embedded in CFD simulation are then developed based on the models. The performance of the closed-loop systems is also evaluated within the full-scale CFD model. The results show that CFD-based system identification is feasible to model fluid flow and heat transfer related processes.     

Shewhart-type control charts for variation in phase I data analysis

Control charts for variation play a key role in the overall statistical process control (SPC) regime. We study the popular Shewhart-type S², S and R control charts when the mean and the variance of a normally distributed process are both unknown and are estimated from m independent samples (subgroups) each of size n. This is the Phase I setting. Current uses of these charts do not recognize that in this setting the signalling events are statistically dependent and that m comparisons are made with the same control limits simultaneously. These are important issues because they affect the design and the performance of the control charts. The proposed methodology addresses these issues (which leads to working with the joint distribution of a set of dependent random variables) by calculating the correct control limits, so that the false alarm probability (FAP), defined as the probability of at least one false alarm, is at most equal to some given nominal value FAP₀. To aid practical implementation, tables are provided for the charting constants for each Phase I chart, for an FAP₀ of 0.01 and 0.05, respectively. An illustrative example is given.     

Evolution of mixing in a microfluidic reverse-staggered herringbone micromixer

Microfluidic platforms offer a variety of advantages including improved heat transfer, low working volumes, ease of scale-up, and stronger user control on operating parameters. However, flow within microfluidic channels occurs at low Reynolds number (Re), which makes mixing difficult to accomplish. Adding V-shaped ridges to channel walls, a pattern called the staggered herringbone design (SHB), alleviates this problem by introducing transverse flow patterns that enable enhanced mixing. Building on our prior work, we here developed a microfluidic mixer utilizing the SHB geometry and characterized using CFD simulations and complimentary experiments. Specifically, we investigated the performance of this type of mixer for unequal species diffusivities and inlet flows. A channel design with SHB ridges was simulated in COMSOL Multiphysics® software under a variety of operating conditions to evaluate its mixing capabilities. The device was fabricated using soft-lithography techniques to experimentally visualize the mixing process. Mixing within the device was enabled by injecting fluorescent dyes through the device and imaging using a confocal microscope. The device was found to efficiently mix fluids rapidly, based on both simulations and experiments. Varying Re or species diffusion coefficients had a weak effect on the mixing profile, due to the laminar flow regime and insufficient residence time, respectively. Mixing effectiveness increased as the species flow rate ratio increased. Fluid flow patterns visualized in confocal microscope images for selective cases were strikingly similar to CFD results, suggesting that the simulations serve as good predictors of device performance. This SHB mixer design would be a good candidate for further implementation as a microfluidic reactor.     

Numerical simulation of heat transfer enhancement by elastic turbulence in a curvy channel

Although many investigations on elastic turbulence have been conducted in recent years, two major research topics still call for in-depth mechanistic investigations. Specifically, one is heat transfer performance affected by elastic turbulence; the other is so-called high Weissenberg number problem (HWNP) in numerical simulation of viscoelastic fluid flow. Taking these two topics into account simultaneously, the coupled problem becomes heat transfer characteristic of viscoelastic fluid in elastic turbulence at high Weissenberg number (Wi) and very low Reynolds number (Re). In this work, we implement numerical simulations by embedding log-conformation reformulation algorithm into the open-source software OpenFOAM. The heat transfer process of viscoelastic fluid flow in a three-dimensional (3D) curvy channel is simulated over a wide range of Wi. For the first time, significant heat transfer enhancement induced by elastic turbulence in a curvy channel at high Wi was identified numerically. When Wi is above the critical value of O(1), the heat transfer performance is found to be dramatically improved by elastic turbulence and then approaches a saturation. From the transient analysis of flow motions in the axial and cross sections, it can be seen that the flow twists and wiggles in the curvy channel and the field synergy effect of viscoelastic fluid flow becomes more intensive than that of Newtonian fluid flow. These effects give rise to the extremely irregular flow motions in the cross section and consequently lead to heat transfer enhancement.     

Numerical investigations on dynamic stall of low Reynolds number flow around oscillating airfoils

This paper presents a 2D computational investigation on the dynamic stall phenomenon associated with unsteady flow around the NACA0012 airfoil at low Reynolds number (Re_c ≈ 10⁵). Two sets of oscillating patterns with different frequencies, mean oscillating angles and amplitudes are numerically simulated using Computational Fluid Dynamics (CFD), and the results obtained are validated against the corresponding published experimental data. It is concluded that the CFD prediction captures well the vortex-shedding predominated flow structure which is experimentally obtained and the results quantitatively agree well with the experimental data, except when the blade is at a very high angle of attack.     

Modelling heat,water and carbon fluxes in mown grassland under multi-objective and multi-criteria constraints

A Monte Carlo-based calibration and uncertainty assessment was performed for heat, water and carbon (C) fluxes, simulated by a soil-plant-atmosphere system model (CoupModel), in mown grassland. Impact of different multi-objective and multi-criteria constraints was investigated on model performance and parameter behaviour. Good agreements between hourly modelled and measurement data were obtained for latent and sensible heat fluxes (R² = 0.61, ME = 0.48 MJ m⁻² day⁻¹), soil water contents (R² = 0.68, ME = 0.34%) and carbon-dioxide flux (R² = 0.60, ME = −0.18 g C m⁻² day⁻¹). Multi-objective and multi-criteria constraints were efficient in parameter conditioning, reducing simulation uncertainty and identifying critical parameters. Enforcing multi-constraints separately on heat, water and C processes resulted in the highest model improvement for that specific process, including some improvement too for other processes. Imposing multi-constraints on all groups of variables, associated with heat, water and C fluxes together, resulted in general effective parameters conditioning and model improvement.     

Process control of time-varying systems using parameter-less self-organizing maps

Traditional control charts, such as Hotelling’s T², are effective in detecting abnormal patterns. However, most control charts do not take into account a time-varying property in a process. In the present study, we propose a parameter-less self-organizing map-based control chart that can handle a situation in which changes occur in the distribution or parameter of the target observations. The control limits of the proposed chart are determined by estimating the empirical level of significance on the percentile using the bootstrap method. Experimental results obtained by using simulated data and actual process data from the manufacturing process for a thin-film transistor-liquid crystal display demonstrate the effectiveness and usefulness of the proposed algorithm.     

An investigation of turbulent open channel flow with heat transfer by large eddy simulation

Large eddy simulation of fully developed turbulent open channel flow with heat transfer is performed. The three-dimensional filtered Navier-Stokes and energy equations are numerically solved using a fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stress and heat flux are employed to close the governing equations. Two typical temperature boundary conditions, i.e., constant temperature and constant heat flux being maintained at the free surface, respectively, are used. The objective of this study is to explore the behavior of heat transfer in the turbulent open channel flow for different temperature boundary conditions and to examine the reliability of the LES technique for predicting turbulent heat transfer at the free surface, in particular, for high Prandtl number. Calculated parameters are chosen as the Prandtl number (Pr) from 1 up to 100, the Reynolds number (Re_τ) 180 based on the wall friction velocity and the channel depth. Some typical quantities, including the mean velocity, temperature and their fluctuations, heat transfer coefficients, turbulent heat fluxes, and flow structures based on the velocity, vorticity and temperature fluctuations, are analyzed.     

Vortex instability of MHD natural convection flow over a horizontal plate in a porous medium

The present work investigates the vortex instability of a horizontal MHD natural convection boundary layer flow in a saturated porous medium including the radiation effect. The numerical results are solves by Keller-Box method incorporated with linear stability theory. The velocity and temperature profiles, local Nusselt number, as well as instability parameters for magnetic parameter M ranging from 0 to 2 and radiation parameter R ranging from 0 to 0.03 are presented. Numerical results showed that, as magnetic parameter M increases or radiation parameter R decreases, the heat transfer rate decrease. In addition, the magnetic effect destabilizes the flow to vortex mode of disturbance, while the radiation effect stabilizes it.     

Towards simulating urban canyon circulations with a 2D lattice Boltzmann model

The Lattice Boltzmann (LB) method is a novel fluid modelling technique developed from cellular automata. Instead of numerically solving the continuum Navier–Stokes equations, it simulates the interactions of mesoscopic particle populations p_α using discrete speeds and positions to obtain the macroscopic velocity, density and temperature fields. Localised at neighbouring grid nodes, the method handles complex geometries and multiple fluids more easily than traditional continuum CFD methods.Rothman and Zaleski (Lattice-Gas Cellular Automata: Simple Models of Complex Hydrodynamics (1997) Cambridge University Press, Cambridge) discuss LB method theory and development in more detail.To demonstrate the power of the technique, a 2D LB model is first used to perform urban canyon configuration studies at Reynolds number Re=100 for Height to Width (H/W) ratios from 0.125 to 2. Then, thermal lid driven cavity simulations for Re=100 and Rayleigh number Ra=2000 are performed for different locations of a relatively hot wall. The simulated flow fields appear qualitatively consistent with physical flows observed in wind tunnel and field studies, and indicate that LB methods generate results comparable to traditional CFD methods for the selected flow situations.     

Improved parameterisation for the numerical modelling of air pollution within an urban street canyon

Numerical modelling for application to wind flow and dispersion in urban environments has noticeably progressed in recent years, to currently represent a widely used tool for simulating mechanical processes governing air pollution in complex geometries. In particular, Computational Fluid Dynamic (CFD) techniques based on RANS (Reynolds-Averaged Navier–Stokes equations) models, are extensively used to produce detailed simulations of the wind flow and turbulence in the urban canopy. However, several studies have indicated that RANS models, and in particular the widely used standard k–? turbulence model, are sensitive to the particular form of inlet profiles for turbulence and velocity. In the present study, simulations of the wind flow and dispersion within an idealised street canyon were carried out using the standard k–? turbulence model provided by the commercial software FLUENT. The aim of this study was to improve the standard k–? model performance by modifying the model parameters according to the chosen form of inlet profiles for velocity and turbulence. Capability of the model to reproduce real wind flow fields, turbulence and concentration patterns was evaluated by comparing the model results against recently published wind tunnel data. Results for turbulent kinetic energy and concentration showed that the redefinition of the default dispersive parameters can significantly enhance the model performance. The newly proposed parameterisations of the standard k–? turbulence model can be readily implemented within commercial CFD software packages, offering a reliable modelling tool for application to urban air pollution and other environmental studies.     

Arbitrary high order PNPM schemes on unstructured meshes for the compressible Navier-Stokes equations

In this paper, we propose a new unified family of arbitrary high order accurate explicit one-step finite volume and discontinuous Galerkin schemes on unstructured triangular and tetrahedral meshes for the solution of the compressible Navier-Stokes equations. This new family of numerical methods has first been proposed in [16] for purely hyperbolic systems and has been called P_NP_M schemes, where N indicates the polynomial degree of the test functions and M is the degree of the polynomials used for flux and source computation. A particular feature of the general P_NP_M schemes is that they contain classical high order accurate finite volume schemes (N=0) as well as standard discontinuous Galerkin methods (M=N) just as special cases, which therefore allows for a direct efficiency comparison.In the application section of this paper we first show numerical convergence results on unstructured meshes obtained for the compressible Navier-Stokes equations with Sutherland’s viscosity law, comparing all third to sixth order accurate P_NP_M schemes with each other. In order to validate the method also in practice we show several classical steady and unsteady CFD applications, such as the laminar boundary layer flow over a flat plate at high Reynolds numbers, flow past a NACA0012 airfoil, the unsteady flows past a circular cylinder and a sphere, the unsteady flows of a compressible mixing layer in two space dimensions and finally we also show applications to supersonic flows with shock Mach numbers up to M_s=10.     

Monitoring the software development process using a short-run control chart

Techniques for statistical process control (SPC), such as using a control chart, have recently garnered considerable attention in the software industry. These techniques are applied to manage a project quantitatively and meet established quality and process-performance objectives. Although many studies have demonstrated the benefits of using a control chart to monitor software development processes (SDPs), some controversy exists regarding the suitability of employing conventional control charts to monitor SDPs. One major problem is that conventional control charts require a large amount of data from a homogeneous source of variation when constructing valid control limits. However, a large dataset is typically unavailable for SDPs. Aggregating data from projects with similar attributes to acquire the required number of observations may lead to wide control limits due to mixed multiple common causes when applying a conventional control chart. To overcome these problems, this study utilizes a Q chart for short-run manufacturing processes as an alternative technique for monitoring SDPs. The Q chart, which has early detection capability, real-time charting, and fixed control limits, allows software practitioners to monitor process performance using a small amount of data in early SDP stages. To assess the performance of the Q chart for monitoring SDPs, three examples are utilized to demonstrate Q chart effectiveness. Some recommendations for practical use of Q charts for SDPs are provided.     

Watershed model parameter estimation and uncertainty in data-limited environments

Parameter uncertainty and sensitivity for a watershed-scale simulation model in Portugal were explored to identify the most critical model parameters in terms of model calibration and prediction. The research is intended to help provide guidance regarding allocation of limited data collection and model parameterization resources for modelers working in any data and resource limited environment. The watershed-scale hydrology and water quality simulation model, Hydrologic Simulation Program – FORTRAN (HSPF), was used to predict the hydrology of Lis River basin in Portugal. The model was calibrated for a 5-year period 1985–1989 and validated for a 4-year period 2003–2006. Agreement between simulated and observed streamflow data was satisfactory considering the performance measures such as Nash–Sutcliffe efficiency (E), deviation runoff (Dv) and coefficient of determination (R²). The Generalized Likelihood Uncertainty Estimation (GLUE) method was used to establish uncertainty bounds for the simulated flow using the Nash–Sutcliffe coefficient as a performance likelihood measure. Sensitivity analysis results indicate that runoff estimations are most sensitive to parameters related to climate conditions, soil and land use. These results state that even though climate conditions are generally most significant in water balance modeling, attention should also focus on land use characteristics as well. Specifically with respect to HSPF, the two most sensitive parameters, INFILT and LZSN, are both directly dependent on soil and land use characteristics.     

Estimating evapotranspiration of European forests from NOAA-imagery at satellite overpass time: Towards an operational processing chain for integrated optical and thermal sensor data products

Evapotranspiration (ET) using the Integral NOAA-imagery processing Chain (iNOAA-Chain) is quantified by implementing visible and thermal satellite information on a regional scale. ET is calculated based on the energy balance closure principle. The combination of evaporative fraction (EF), soil heat flux and instantaneous net radiation, results in an instantaneous spatial distribution of ET values. Surface broadband albedo and land surface temperature (LST) serve to determine EF. EF is derived using four methods based on NOAA/AVHRR satellite imagery. Instantaneous evapotranspiration, i.e. at time of satellite overpass, on European continental scale with emphasis on forest stands is estimated using the iNOAA-Chain. Finally, the estimated net radiation (R_n), soil heat fluxes (G₀) and evaporative fraction and evapotranspiration at time of satellite overpass are validated against EUROFLUX site data for the growing season of 1997 (March-October). The regression line for the pooled R_n (iNOAA-Chain versus EUROFLUX) has a slope, intercept, Pearson product moment correlation coefficient (R²) and relative root mean square error (RRMSE) of respectively 0.943, 17.120, 0.926 and 5.5%. The soil heat fluxes, calculated with two approaches are not-well modelled with slopes smaller than − 3.000 and a R² in the order of zero. We observe a slight underestimation of the iNOAA Chain estimated EF. The regression line for pooled EF data for the best performing method (SPLIT-method) has a slope of 0.935, an intercept of 0.041 and the R² is 0.847. A pooled RRMSE EF value of 12.3% is found. The pooled slope, intercept, R² and RRMSE for EF derived with SORT-method 1 are respectively 0.449, 0.251, 0.043 and 65.1%, with SORT-method 2, 0.567, 0.203, 0.174 and 39.1%, and with SORT-method 3, 0.568, 0.254, 0.288, and 32.8%. Also instantaneous evapotranspiration is underestimated with a pooled RRMSE on ET of 23.4%. The regression curve of pooled ET data for the best performing method has a slope of 0.889 an intercept of 15.880 and a Pearson product moment correlation coefficient of 0.771. The other method gives a slope of 0.781, an intercept of 17.541 and a R² of 0.776. Error propagation analysis reveals that the relative error on evapotranspiration at satellite overpass time is at least 27%.     

基于iSIGHT平台的倾斜冲击射流传热特性优化

为了研究和优化半封闭倾斜射流中击移动平板的传热性能,采用iSIGHT与CFD软件的联合仿真,根据多岛遗传算法动态调整射流角度和平板移动速度,进行狭缝湍流冲击射流的数值模拟.结果表明,平板表面的传热效果主要是由板速决定的,低速时高角度下的平板表面的平均努塞尔数较高;初始设计时流场左侧的回流区和二次回流区消失,优化后的流场结构得到改善,移动平板表面的平均努塞尔数比初始设计结果提高7.62％,热传导更加均匀.     

New algorithms for solving fuzzy relation equations

A new algorithm is proposed to solve the fuzzy relation equation P∘Q=R with max–min composition and max–product composition. The algorithm operates systematically and graphically on a matrix pattern to get all the solutions of P. An example is given to illustrate its effectiveness.     

Evaluation of complex and dynamic safety tasks in human learning using the ACT-R and SOAR skill acquisition theories

This paper provides a human-centered analytical approach to learning dynamic and complex tasks using the Adaptive Control of Thought-Rational (ACT-R) and the State, Operator And Result (SOAR) models by comparing the task times of the model and the subjects. Twenty-one full time assembly line workers at a local computer company (14 men and 7 women) from ages 18-32 (Mean = 19.86 years, SD = 0.96 years) were randomly selected for this analysis. The task involved the placement of printed circuit board (PCB) components on the flow line of the desktop computer mother board manufacturing process. The overall timed performance of the subjects indicated that the match between the model and the subjects was good, resulting in an R² - value of 0.94. At the unit task level performance, and R² - value of 0.96 for placing the PCBs on the flow line. For tasks involving picking and searching of PCBs, the obtained R² - value was 0.76 and R² of 0.68 at the keystroke level. Findings revealed that the model already started out with a complete strategy of performing the task, whereas the human participants had to acquire additional learning information during the trials. Efforts will be made in the future to determine how the performance of the human subjects could be enhanced to meet or the same level as the model performance.     

Large-eddy simulations of film cooling flows

Large-eddy simulations (LES) of a jet in a cross-flow (JICF) problem are carried out to investigate the turbulent flow structure and the vortex dynamics in gas turbine blade film cooling. A turbulent flat plate boundary layer at a Reynolds number of Re_∞ = 400,000 interacts with a jet issued from a pipe. To study the effect of the jet inclination angle α on the flow field, two angles are chosen, the perpendicular injection at 90° and the streamwise inclined injection at 30°. For the normal injection case a small blowing ratio of the jet velocity to the cross-stream velocity R = 0.1 is examined. For the streamwise inclined injection case two blowing ratios R = 0.1 and R = 0.48 are investigated to check the impact of the jet velocity on the cooling performance. The time-dependent turbulent inflow information for the cross-flow is provided by a simultaneously performed LES of a spatially developing turbulent boundary layer. Whereas in the perpendicular injection case a rather large separation region is found at the leading edge of the jet hole, in the streamwise inclined injection cases no separation is observed. Compared with the normal injection case at the same blowing ratio, the streamwise inclination weakens the jet-cross-flow interaction significantly. Thus, the first appearance of the counter-rotating vortex pair (CVP) is shifted downstream and its strength is reduced. The increase of the blowing ratio leads to a stronger penetration of the jet into the cross-flow, resulting in a more upstream located and more pronounced CVP. Downstream of the jet exit the streamwise vortices are so large that besides the jet fluid also the cross-stream is partially entrained into this zone, which yields the worst cooling performance.     

Performance evaluation of directory protocols on an optical broadcast-based distributed shared memory multiprocessor

Recent advances in the development of optical technologies suggest the possible emergence of broadcast-based optical interconnects within cache-coherent distributed shared memory (DSM) multiprocessor architectures. It is well known that the cache-coherence protocol is a critical issue in designing such architectures because it directly affects memory latencies. In this paper, we evaluate via simulation the performance of three directory-based cache-coherence protocols; strict request-response, intervention forwarding and reply forwarding on the Simultaneous Optical Multiprocessor Exchange Bus (SOME-Bus), which is a low-latency and high-bandwidth broadcast-based fiber-optic interconnection network supporting DSM. The simulated system contains 64 nodes, each of which has a processor, a cache controller, a directory controller and an output channel. Simulations have been conducted for each protocol to measure average processor utilization, average network latency and average number of packets transferred over the network for varying values of the important DSM parameters such as the ratio of the mean channel service time to mean thread run time (T/R), probability of a cache block being in modified state {P(M)}, the fraction of write misses {P(W)} and home node contention rate. The results reveal that for all cases, except for low values of P(M), intervention forwarding gives the worst performance (lowest processor utilization and highest latency). The performance of strict request-response and reply forwarding is comparable for several values of the DSM parameters and contention rate. For a contention rate of 0%, the increase of P(M) makes reply forwarding perform better than strict request-response. The performance of all protocols decreases with the increase of P(W) and contention rate. However, the performance of strict request-response is the least affected among other protocols due to the negative impact of the increase of P(W) and contention rate. Therefore, for the full contention case (i.e. contention rate of 100%); for low values of P(M), or for mid values of P(M) and high values of P(W), strict request-response performs better than reply forwarding. These results are significant in the sense that they provide an insight to multiprocessor architecture designers for comparing the performance of different directory-based cache-coherence protocols on a broadcast-based interconnection network for different values of the DSM parameters and varying rates of contention.