Steady Flow Method Research Articles

Durability of concrete assessed via different unsaturated and steady-flow methods

Recently, there is a growing interest in how to decrease the environmental impact of concrete and to increase its durability. As concrete is mostly unsaturated during its service life, measuring unsaturated flow (through imbibition or diffusion) can provide information to assess its durability. Additionally, permeability values are often used to describe concrete’s ability to withstand the ingress of aggressive agents. However, it is not known whether the measured performance is representative of service-life behavior. In this research, the results obtained via two unsaturated flow methods (imbibition and diffusion) and two permeability tests (CEMBUREAU and Torrent method) were compared. In order to do this, 6 concrete mixes with 3 different cement types (CEM I, CEM II and CEM III) and two water to cement (w/c) ratios (0.4 and 0.7), were tested after 28 days of curing. Capillary imbibition was tested on specimens with a diameter of 100 mm and a height of 100 mm for a duration of 1 month. Furthermore, water vapour diffusion through the concrete was measured using the cup method, which consists of creating a moisture gradient by using a hygroscopic salt, with a concrete slice as a barrier. The results showed that for concrete with a w/c of 0.4 all test methods indicated that CEM III performed worse in comparison to CEM I and CEM II, which had a similar performance. However, for a w/c of 0.7, CEM III performed superior in comparison to CEM I and CEM II in the capillary imbibition test and the water vapour diffusion. This can possibly be explained by an increased tortuosity in the CEM III mix. In the gas permeability tests, CEM III performed worse compared to CEM I and CEM II.

Comparison of Soil Hydraulic Properties Estimated by Steady- and Unsteady-Flow Methods in the Laboratory

In this study, soil hydraulic conductivity (K) and soil sorptivity (S) values estimated by applying various steady- and unsteady-flow methods using cumulative infiltration data of three disturbed soils (sandy loam, loam, clay) obtained from a disc infiltrometer in the laboratory at various negative pressure heads were compared. The steady-flow methods used were those of Ankeny et al. and Reynolds and Elrick as well as Logsdon and Jaynes, while the unsteady-flow methods were those of Haverkamp et al. (two-term (2T) and three-term (3T) infiltration equations) and Zhang. The method of White et al., which is a steady-flow method but also uses unsteady-flow infiltration data, was also examined. The results showed that the three steady-flow methods, as well as the Zhang equation, for values of the van Genuchten coefficient n > 1.35, tend to give similar values of K. The 2T infiltration equation with β = 0.6 provided hydraulic conductivity values greater than those estimated by the steady-state methods but gave negative K values in some cases. The values of the coefficients C1 and C2 of the 2T equation were affected by the infiltration time. The coefficient C1 increased while C2 decreased with increasing time when the cumulative linearization method (CL) was applied, but the change in C1 tended to be smaller than that in C2. The inverse solution of the 3T equation using the Excel Solver application for β = 0.75 and β = 1.6, when positive values of K were obtained, approached better the K values estimated by the steady-flow methods compared with those estimated using β = 0.6. Regarding the estimation of S from the unsteady-flow equations (2T, 3T, Zhang), comparable S values were obtained by all equations. The differences between the S values of the various methods are smaller compared to those of K, and S is less affected than K in terms of time. The problem of negative estimates of K might be attributed to the fact that the soils used in this study are classified as soils situated in the domain of lateral capillarity or are not completely homogeneous or soil compaction is observed at some depth. In the case where the soils are not completely homogeneous, the Sequential Infiltration Analysis (SIA) method with β = 0.75 corresponding to the soil types studied was proved to be effective in estimating K values.

A least-squares finite element method for steady flows across an unconfined square cylinder placed symmetrically in a plane channel

This paper develops a least-squares finite element method (LSFEM) for non-Newtonian flows of power-law fluid across an unconfined square cylinder at two inclination angles, placed symmetrically in a two-dimensional channel. We used Newton's method to linearize the equation of motion and employed the least-squares approach with systematic mesh refinements to flow past the square cylinder. The LSFEM offers a direct approximation of the extra stress tensor components, a symmetric positive definite system, and the openness of choosing finite element spaces. We proved that the least-squares approximation converges to linearized solutions of non-Newtonian fluid flows. We demonstrated that the numerical results using low-order basis functions agree with the theoretical estimates and presented the effects of different physical parameters on the physical attributes of the power-law model at a low Reynolds number, such as drag coefficients, viscosity, and velocity. These results agree with others published in the literature. Hence, the LSFEM in non-Newtonian fluid simulations can accomplish the identification of shear-thinning and shear-thickening characteristics.

Nonoverlapping domain decomposition method with preconditioner from asymptotic analysis of steady flow in high contrast media

ABSTRACT We present a nonoverlapping domain decomposition method for steady flow in high contrast heterogeneous media modelled by an elliptic equation with coefficients that have very large amplitude variations on a small spatial scale. The linear system of equations resulting from matching the solution trace and the fluxes through the boundary of the subdomains is ill-conditioned, especially for fine meshes needed to capture the rapid variations of the solution. Our main contribution is to show with analysis and numerical simulations how to use an asymptotic approximation of the Dirichlet to Neumann map of the sub-domain problems to obtain a preconditioner for an efficient domain decomposition algorithm.

A Review on Numerical Development of Tidal Stream Turbine Performance and Wake Prediction

Recently, the output of Computational Fluid Dynamic (CFD) prediction on tidal stream turbine systems has been receiving great attention owing to the vast and untapped tidal stream potential. For several years, many publications have documented the accuracy of CFD methods for steady flows, but not for unsteady flows. A challenging area persisting in the computational field is its dependence on large computing resources, and the unsteady nature of usual tidal streams. To overcome this problem, researchers have proposed modelling simpler, representative devices or combined CFD with alternative predictive methods. Nevertheless, the turbulent modelling has been a critical issue in the CFD methods and great effort has been devoted to the study of turbulence and wave effect on the wake and functioning of the turbine. The present paper is a review of CFD application on tidal stream turbine performance in both steady and unsteady flows. The performance of the turbine predicted by actuator and blade resolved methods, has been in accordance with laboratory observations. Findings of the wake have been both consistent and inconsistent with measurements, arising from interpretation of the blade force, fluid-solving method and onset flow model, and downstream range distance. With regards to arrays, preliminary work shows turbine arrangement can have profound effects in the onset flow, and in consequence, the performance of adjacent turbine rows. The results reported appear to support the wind similarity assumption, such as wake Gaussian velocity distributions and fluctuating turbine output in unsteady flows. Under the influence of surface waves, the wake recovers faster than steady flow condition due to the flow's convective acceleration, but the mean performance stays almost equal. The findings in this paper give a critical review and insight of important implications for future turbine research, such as experimental validation of the turbine prediction output in small and large arrays.

A fractional-step method for steady-state flow

A fractional step method for solving the steady-state, incompressible, Navier-Stokes equations is presented. The proposed iterative method uses an estimate for the optimal under-relaxation coefficient at each iteration. This estimate uses prior information that is already available in the iterative method so the calculation has negligible cost. Numerical tests show that the convergence rate of the fractional step method using this optimal relaxation parameter is nearly independent of the mesh size, resulting in significant cost savings for problems with fine meshes. The ideas demonstrated in this paper are general and can be used to improve many existing iterative methods for steady flows, as well as methods for the unsteady Navier-Stokes equations and methods for other saddle-point problems.

定常流量法による排水負荷算定に関する研究 （第２報）節水型大便器を対象とした排水負荷算定に関する検討

定常流量法による排水負荷算定に関する研究 （第２報）節水型大便器を対象とした排水負荷算定に関する検討

A novel method to measure thermal conductivity of nanofluids

To measure the thermal conductivity of nanofluids under flow condition accurately, a novel method defined as the steady flow method (SFM) was employed and improved based on the heat transfer of laminar flow theory under the uniform heat flux condition. Considering the buoyance is existed in the actual pipe flow, Nusselt number would increase rather than a constant value of 4.364 in the fully developed region. The evaluation of Nu was optimized by experimental validation, and the mixed convection equation was used to improve Nu in the thermal conductivity measurement coupled with SFM. Then the thermal conductivities of water based nanofluids with 0.2 wt%, 0.5 wt% and 1 wt% Al2O3 were measured respectively by using the improved SFM (ISFM). Compared to the reference thermal conductivity obtained from REFPROP 9.1, the experimental results show that the maximum relative deviation on measured values of deionized water is reduced from 10.42% to 3.35% by ISFM. The measured thermal conductivities of water based nanofluids with 0.2 wt%, 0.5 wt% and 1 wt% Al2O3 are increased by 10.5%, 16.7%, 22.8% compared with the base fluid. Moreover, the thermal conductivity of nanofluids can be enhanced with the fluid temperature, which corresponds with other literatures. The ISFM reduces the influence of natural convection in the processes of measurement, and then leads to improving the measuring accuracy of the thermal conductivity significantly.

Multi-stage water permeability measurements on claystone by steady and transient flow methods

The present paper deals with the comparison of permeability measurements between steady flow and transient flow tests (pulse decay test). We submitted one sample to a series of four sequences of one pulse test followed by one steady flow test. This whole multi-stage test lasted approximately 120 days. The pulse tests were interpreted with an inverse method proposed by Giot et al. (2011) allowing assessment of mechanical and poromechanical parameters in addition to intrinsic permeability. Overall, the pulse test measurements showed that the claystone progressively turned into mud, exhibiting constant decreases in permeability and Young's modulus and a constant increase in the Biot coefficient. In contrast, the steady flow tests exhibited constant permeability corresponding to the final value assessed by the pulse test. These results are explicated by the evolution of the claystone fabric with wetting and call into question the representativeness of permeability measurements by the steady flow method on these kinds of materials.

Aerodynamic Instabilities of Swept Airfoil Design in Transonic Axial-Flow Compressors

This paper investigates the effect of a swept airfoil design on the aerodynamic instabilities of transonic axial-flow compressors based on global stability analysis. An approach aimed at predicting the stall inception point is developed by integrating an improved blade force model and a new mathematical treatment method for steady flow. This approach takes both the concrete blade geometry and complicated base flow into consideration, which promises to provide a highly efficient assessment of the stall margin of a compressor in the design phase. Specifically, the relevant validation is conducted through a typical transonic axial-flow compressor, which shows that the predicted stall inception point agrees well with the experimental result. Furthermore, the compressor is also adopted as a prototype, from which a series of forward- and backward-swept rotors are modeled. Aerodynamic instabilities in the form of eigenvalues for each swept rotor, along with the throttling process, are predicted by solving the developed stall inception model. The comparison of the predicted results for these rotors shows that the backward sweep deteriorates the stall margin; the larger the backward-swept degree, the poorer the compressor stability becomes. However, the forward sweep does not exhibit remarkable and regular influence on the stall margin of the compressor in the present research. A discussion of the predicted results is made from the perspectives of blade loading and passage shock location.

Preconditioned lattice Boltzmann method for steady flows: A noncascaded central-moments-based approach.

We present a concise yet effective central-moments-based lattice Boltzmann method with an accelerated convergence to the steady state through preconditioning. It is demonstrated that the proposed scheme reduces to a slight modification of the unaccelerated one, as the preconditioning affects only the equilibrium state. Different from previous efforts carried out within the lattice Boltzmann community, the present scheme is built on an original model. In fact, the corresponding collision operator loses the pyramidal orchestrated nature that is typical of the cascaded scheme, hence we coin the name "noncascaded." Our model is very general, characterized by highly intelligible formulations, simple to implement, and it can be derived for any lattice velocity space.

Thermo-stability and aging performance of modified asphalt with crumb rubber activated by microwave and TOR

More effective approaches need to be developed for solving the aging and thermal stability problems of crumb rubber modified asphalt (CRMA). This paper focused on microwave and trans-polyoctenamer (TOR). Therefore, various CRMA were prepared by crumb rubber (CR), microwave irradiated CR, TOR and base asphalt. The samples were subjected to thin film oven test (TFOT), pressure aging vessel (PAV) and high temperature storage tests. The effects of microwave and TOR were evaluated by rheological, FTIR and SEM tests. The results showed that microwave activated CR increases the risk of fatigue cracking caused by hardening effect of aging but endows the aged residue with stronger resistance to thermal cracking. These results attribute to the degradation and de-vulcanization effect of microwave on CR (verified by FTIR). TOR activated CRMA displayed a more obvious hardening effect and the improved low temperature creep behavior after aging. TOR improves the compatibility and storage stability of CRMA. Double bonds in TOR participate in chemical reaction and benefit to stability, which are verified by the decreased intensity of peak at 966cm−1. However, microwave treatment on CR has negative effect on stability of CRMA. Thermal stability was further verified by steady flow method.

A matrix-free, implicit finite volume lattice Boltzmann method for steady flows

Abstract In the present paper, a matrix-free, implicit finite volume lattice Boltzmann method for steady flow on unstructured mesh is proposed. The approximate linear system arising from the implicit finite volume discretization of lattice Boltzmann equation (LBE) is solved by a novel algorithm, which combines the lower-upper symmetric Gauss–Seidel (LU-SGS) and the Jacobi iteration schemes. A remarkable feature of the present implicit method is that the storage of the Jacobian matrixes of the convection and collision terms can be completely eliminated by approximating the Jacobian matrix-solution incremental vector product with appropriate numerical flux incremental vector and the numerical increment of collision term, resulting a matrix-free implicit scheme. The present method is validated by several two-dimensional testing cases.

Analysis heat transfer in engineering barriers used FEM and experimental method

Purpose of the calculations is to determine the temperature field at various points in the area in question. Estimate the losses of heat given off by the analysed structures to the environment. Numerical algorithm based on the Finite Element Method for steady flow of heat and areas of heterogeneous. To analyse was used windows with double-glazed unit made PCV with different option gasses and glasses. Verification the simulation which used numerical procedures by experimental method.

Domain decomposition based coupling between the lattice Boltzmann method and traditional CFD methods—Part I: Formulation and application to the 2-D Burgers’ equation

The lattice Boltzmann method is being increasingly employed in the field of computational fluid dynamics due to its computational efficiency. Floating-point operations in the lattice Boltzmann method involve local data and therefore allow easy cache optimization and parallelization. Due to this, the cache-optimized lattice Boltzmann method has superior computational performance over traditional finite difference methods for solving unsteady flow problems. When solving steady flow problems, the explicit nature of the lattice Boltzmann discretization limits the time step size and therefore the efficiency of the lattice Boltzmann method for steady flows. To quantify the computational performance of the lattice Boltzmann method for steady flows, a comparison study between the lattice Boltzmann method (LBM) and the alternating direction implicit (ADI) method was performed using the 2-D steady Burgers’ equation. The comparison study showed that the LBM performs comparatively poor on high-resolution meshes due to smaller time step sizes, while on coarser meshes where the time step size is similar for both methods, the cache-optimized LBM performance is superior. Because flow domains can be discretized with multiblock grids consisting of coarse and fine grid blocks, the cache-optimized LBM can be applied on the coarse grid block while the traditional implicit methods are applied on the fine grid blocks. This paper finds the coupled cache-optimized lattice Boltzmann-ADI method to be faster by a factor of 4.5 over the traditional methods while maintaining similar accuracy.

The Experimental Study of Temperature Dependence of Binary Diffusion Coefficients of Gases at Different Pressures

An experimental study of the temperature dependence of the binary diffusion coefficients (BDCs) was conducted for five binary mixtures of gases: \(\mathrm{H}_{2}{-}\mathrm{N}_{2}, \mathrm{H}_{2}{-}\mathrm{CO}, \mathrm{H}_{2}{-}\mathrm{CH}_{4}, \mathrm{H}_{2}{-}\mathrm{C}_{2}\mathrm{H}_{6}\), and \(\mathrm{H}_{2}{-}\mathrm{C}_{3}\mathrm{H}_{8}\). Measurements were carried out with the use of a steady-flow method in the temperature range from 250 K to 900 K and the pressure range from 0.1 MPa to 15 MPa. The determination of the BDCs is based on analysis of the volume fraction of the diffusing gas in the gas flow. The experimental data were compared with the results of calculations by the proposed formula evaluated within the framework of the elementary kinetic theory. The obtained results exhibit considerably good agreement with the experimental data within the experimental error. The results of investigations of the temperature dependence of the BDCs show that this dependence can be fitted with a power law only at atmospheric pressure.

Asymptotic numerical method for steady flow of power-law fluids

This work concerns numerical simulations of Power-law fluids. This non-linear problem is solved by using the Asymptotic-Numerical Method (ANM). As this problem is strongly non-linear, we show how the ANM can be used (introduction of new variables, regularization parameter). A numerical method to compute critical Reynolds numbers, bifurcation points, is also proposed. This method makes it possible to determine accurate critical Reynolds without increasing the computational times. Several numerical examples help to demonstrate the efficiency and the reliability of the proposed methods.

A calibration setting with uncertainty measurements for passive/active radon monitors using flow-through source type

Based upon the steady flow method, a setting has been introduced for fast calibration of almost all types of active and passive radon detectors. In this method, a traceable radon reference flow-through sources is connected to a portable rectangular cubic transparent chamber (volume of 50L) which is completely sealed and has 6 valves on its sides. The initializing calibration times are less than 150min by this chamber. The radon concentration in the chamber is in consistent with the source activity. The Best Measurement Capability (BMC) of the method has been calculated less than 5% at 68% confidence level. As well by using grab-sampling method, the difference of radon concentration of different points in the chamber was measured less than 1.2%. So the flow-through sources can be developed for a fast calibration of almost all types of the radon monitors with an acceptable uncertainty using this calibration setting.

Optimal preconditioning of lattice Boltzmann methods

A preconditioning technique to accelerate the simulation of steady-state problems using the single-relaxation-time (SRT) lattice Boltzmann (LB) method was first proposed by Guo et al. [Z. Guo, T. Zhao, Y. Shi, Preconditioned lattice-Boltzmann method for steady flows, Phys. Rev. E 70 (2004) 066706-1]. The key idea in this preconditioner is to modify the equilibrium distribution function in such a way that, by means of a Chapman–Enskog expansion, a time-derivative preconditioner of the Navier–Stokes (NS) equations is obtained. In the present contribution, the optimal values for the free parameter γ of this preconditioner are searched both numerically and theoretically; the later with the aid of linear-stability analysis and with the condition number of the system of NS equations. The influence of the collision operator, single- versus multiple-relaxation-times (MRT), is also studied. Three steady-state laminar test cases are used for validation, namely: the two-dimensional lid-driven cavity, a two-dimensional microchannel and the three-dimensional backward-facing step. Finally, guidelines are suggested for an a priori definition of optimal preconditioning parameters as a function of the Reynolds and Mach numbers. The new optimally preconditioned MRT method derived is shown to improve, simultaneously, the rate of convergence, the stability and the accuracy of the lattice Boltzmann simulations, when compared to the non-preconditioned methods and to the optimally preconditioned SRT one. Additionally, direct time-derivative preconditioning of the LB equation is also studied.

Label-Free Detection of Heparin, Streptavidin, and Other Probes by Pulsed Streaming Potentials in Plastic Microfluidic Channels

In this paper, pulsed streaming potentials generated in plastic microfluidic channels are used for the label-free detection of some model analytes. The microchannels are fabricated with the commodity plastic cyclic olefin copolymer (COC), and the detection signal arises from a change in the surface charge upon analyte adsorption on the modified microchannel surface. The role of the surface modification is to confer the microchannel with a predetermined charge and a particular specificity toward the adsorption of the target analyte. In this work, several target probes displaying different levels of specificity were investigated. Heparin and streptavidin were detected by adsorption on microchannel surfaces modified with protamine and biotin, respectively, whereas bovine serum albumin (BSA) and methylene blue (MB) showed nonspecific adsorption on almost any modified or unmodified COC microchannel surface. The magnitude of the streaming potential was found to be proportional to the liquid pressure and the surface charge of the microchannel in accord with the Smoluchowski equation. Because the relative polarity of the streaming potential is determined by the surface charge, the most straightforward detection with this method occurs when the charge is reversed upon analyte adsorption. This strategy was used for the species described in this work, and the lowest concentrations detected were approximately 0.01 units/mL for heparin (below clinical relevance), approximately 10 (-9) M for BSA, and approximately 10 (-6) M for MB. Unlike the conventional method of steady flow, in this work, the streaming potentials were measured under pulsed conditions of flow and using nonreference electrodes. This approach removes the need of special electrolytes as it is usually required when using reference electrodes, and at the same time, it mitigates the interference of electrochemical drift from the electrodes. Relative standard deviations of approximately 1-2% and measuring times of approximately 10 s are readily attained with this experimental setup. The on-channel modification of the surface was carried out by UV-photografting methods given the significant UV transparency of COC.