Quasi-steady State Simulations Research Articles

Importance of Non-Newtonian Computational Fluid Modeling on Severely Calcified Aortic Valve Geometries-Insights From Quasi-Steady State Simulations.

The Newtonian model has commonly been used to represent the viscosity of blood in the aorta, despite blood itself being a non-Newtonian fluid. This is justified where shear rates tend to be large. However, we hypothesized that using the Newtonian model to predict the hemodynamics on the aortic valve, particularly in those with severe calcifications, is inaccurate owing to valve leaflet geometry irregularities inducing multiple regions of low shear rates, <100 s-1, where a Newtonian model is invalid. We investigated the utility of three fluid viscosity models via quasi-static simulations: Newtonian, Carreau, and Quemada on a severely calcified aortic heart valve and compared their ability to capture important hemodynamic parameters of wall shear stress (WSS) and the oscillatory shear index (OSI). Our findings indicate that when the shear rates were large enough, >100 s-1, the use of a Newtonian model was justified. However, in spatial regions of relatively low shear rates, <100 s-1, specifically on the inner cusps of the fibrosa side of the valve, WSS calculations under a Newtonian model were found to be noticeably different when compared with their non-Newtonian, Carreau and Quemada counterparts. We hereby conclude that to facilitate more accurate computational flow simulations in severe aortic heart valve calcification, which is subjected to relatively large spatial regions of low shear (<100 s-1), a non-Newtonian model should be applied.

Super Residual Circulation: a new perspective on ocean vertical heat transport

AbstractOcean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities to diagnose circulation. Using quasi-steady state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature-depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of Super-Residual Transport (SRT), which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies, but excludes small-scale three dimensional turbulent mixing effect, to construct a new overturning circulation – the ‘Super Residual Circulation’ (SRC).We find that in the coarse resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11 Sv circulation which transports heat upward. The SRC’s upward heat transport is ~2 times larger in a finer horizontal resolution (0.1°) version of ACCESS, suggesting a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into super-residual processes, as the SRC elucidates the pathways in temperature and depth space along which watermass transformation occurs.

Coupling electrodialysis desalination with photovoltaic and wind energy systems for energy storage: Dynamic simulations and control strategy

The presence of desalination systems in polygeneration facilities is usually limited by important difficulties in operating under non-stationary regimes typical of renewable energy sources. Reverse osmosis, namely the most common desalination technology, is characterised by slow dynamics that rarely adapts to the power fluctuations of renewables. Therefore, the possibility of using electrodialysis coupled with a hybrid photovoltaic/wind energy source was investigated in this work. In particular, the combination of photovoltaic and wind energy is very attractive in order to achieve a more stable energy production, while electrodialysis is claimed to be a more flexible process compared to reverse osmosis. For this reason, the aim of this work was to analyse the technical advantages of using electrodialysis in the aforementioned scenarios, Suitable transitory simulation models are implemented for modelling electrodialysis units, photovoltaic panels and wind turbines. Dynamic scenarios were analysed, looking at two different time scales. Quasi steady-state simulations were used to study the yearly operation of 4 electrodialysis units operating in parallel, demonstrating process flexibility over a wide range of produced flowrates (from 920 to 230 m3/d) and power inputs (5–45 kW) when producing drinking water at a constant NaCl outlet concentration of 0.25 g/l. Dynamic simulations were adopted to study the daily time scale, where the desalination unit control system, purposely designed and tuned, was able to maintain a relatively stable target value in presence of disturbances in power availability, i.e. with a fluctuation of the outlet concentration lower than ± 10%, in between 0,23 and 0,27 g/l . Simulation results show how the electrodialysis process is particularly suitable for the integration within polygeneration systems as energy-buffer.

A posteriori error analysis of fluid–structure interactions: Time dependent error

A posteriori error analysis is a technique to quantify the error in particular simulations of a numerical approximation method. In this article, we use such an approach to analyze how various error components propagate in certain moving boundary problems. We study quasi-steady state simulations where slowly moving boundaries remain in mechanical equilibrium with a surrounding fluid. Such problems can be numerically approximated with the Method of Regularized Stokelets (MRS), a popular method used for studying viscous fluid–structure interactions, especially in biological applications. Our approach to monitoring the regularization error of the MRS is novel, along with the derivation of linearized adjoint equations to the governing equations of the MRS with a elastic elements. Our main numerical results provide a clear illustration of how the error evolves over time in several MRS simulations.

Simulation of Organic Rankine Cycle – Quasi-steady state vs dynamic approach for optimal economic performance

Computer-based simulations of Organic Rankine Cycles (ORC) have been extensively used in the last two decades to predict the behaviour of existing plants or already in the design phase. For time-varying heat sources, researchers typically rely on either quasi-steady state or dynamic simulations. In this work, the two approaches are compared and the trade-off between them is analysed, taking as benchmark waste heat recovery with ORC from a billet reheating furnace. The system is firstly optimized in MATLAB® using a quasi-steady state approach. The results are then compared with a corresponding dynamic simulation in Dymola. In the case of waste heat from billet reheat furnace, the quasi-steady state approach can successfully capture the fluctuations in waste heat. For heat source ramps from 110% to 40% the nominal value in 30 s, dynamic effects lead to 1.1% discrepancies in ORC net power. The results highlight the validity of the quasi-steady state approach for techno-economic optimization of ORC for industrial waste heat and provide a valuable guideline for developers, companies and researchers when choosing the most suitable tool for their analysis, helping them save time and costs to find the most appropriate approach.

Selection of Energy Conservation Measures for Building Energy Retrofit: a Comparison between Quasi-steady State and Dynamic Simulations in the Hands of Users

Purpose: Quasi-steady state simulations have played a pivoting role to expand the user group of simulation to design engineers and architects in Korea. Initially they are introduced in the market as a building energy performance rating tool. In domestic practice, however, quasi-steady state simulations seem to be regarded as a de facto simulation only available for energy retrofit. Selection of ECMs and economic feasibility analysis are being decided through these tools, which implies that running these tools has become a norm step of the Investment-grade Audit. Method: This study aims at identifying issues and problems with the current practice via test cases, analyzing the reasons and opportunities, and then eventually suggesting proper uses of quasi-steady state and dynamic simulations. Result: The functionality of quasi-steady state simulations is more optimized to the rating. If they are to used for energy retrofits, their off-the-shelf functions also need to be expanded for customization and detailed reports. Yet their roles may be limited only to the go/no go decision; because their algorithms are still weak at precisely estimating energy and load savings that are required for making investment decisions compared to detailed simulations.

High fidelity quasi steady-state aerodynamic model effects on race vehicle performance predictions using multi-body simulation

ABSTRACTWe described in this paper the development of a high fidelity vehicle aerodynamic model to fit wind tunnel test data over a wide range of vehicle orientations. We also present a comparison between the effects of this proposed model and a conventional quasi steady-state aerodynamic model on race vehicle simulation results. This is done by implementing both of these models independently in multi-body quasi steady-state simulations to determine the effects of the high fidelity aerodynamic model on race vehicle performance metrics. The quasi steady state vehicle simulation is developed with a multi-body NASCAR Truck vehicle model, and simulations are conducted for three different types of NASCAR race tracks, a short track, a one and a half mile intermediate track, and a higher speed, two mile intermediate race track. For each track simulation, the effects of the aerodynamic model on handling, maximum corner speed, and drive force metrics are analysed. The accuracy of the high-fidelity model is shown to reduce the aerodynamic model error relative to the conventional aerodynamic model, and the increased accuracy of the high fidelity aerodynamic model is found to have realisable effects on the performance metric predictions on the intermediate tracks resulting from the quasi steady-state simulation.

Experimental and numerical investigation of a gearless one-motor contra-rotating fan

A gearless one-motor concept for contra-rotating fans is presented in this article. The rotors are mounted to an electric motor using only one shaft. The coupling between both rotors is realised by utilising the conservation of angular momentum. The contra-rotating fans has a diameter of 200 mm at a design speed of 2100 min−1 for the first stage and 1200 min−1 for the second stage. It has been designed and investigated through a series of experiments by the Institute of Air Handling and Refrigeration in Dresden. The performance map and 2D particle image velocimetry measurements have been conducted. Numerical models for 3D quasi-steady state and transient simulations have been implemented and carried out by the Institute of Mechanics and Fluid Dynamics. The results show a good agreement between the quasi-steady, the transient simulations and the experiment. However, when close to stall, the time-resolved simulations show a superior performance compared with steady-state computations.

An MHD simulation of the inner heliosphere during Carrington rotations 2060 and 2068: Comparison with MESSENGER and ACE spacecraft observations

We present results from a new magnetohydrodynamic (MHD) model of the inner heliosphere. The model is adapted from the well-established Lyon–Fedder–Mobarry (LFM) MHD simulation code, which until recently mostly applied to studies of the terrestrial magnetosphere. We perform quasi steady-state simulations of two Carrington rotations: 2060 and 2068. During both of these periods, the heliosphere remained quiet and undisturbed by transient phenomena, making them well-suited for simulation studies of Corotating Interaction Regions (CIRs). The MHD model of the solar wind is driven at the inner boundary by the Wang–Sheeley–Arge (WSA) model of the corona augmented with empirical relations to infer the solar wind velocity, density, and temperature. Here we report on a validation exercise whereby LFM-helio simulation results are compared with in situ data from the Advanced Composition Explorer (ACE) and MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft. We find that the model successfully reproduces the large-scale configuration of the inner heliosphere, namely timing and duration of high-speed streams and heliospheric current sheet crossings, as reflected in ACE and MESSENGER observations. Discrepancies between in situ measurements and simulations, such as 1–2 day errors in the time of arrival of a CIR or the strength of the simulated magnetic field at the spacecraft, are attributed to the uncertainty in the specification of the coronal conditions, rather than a poor performance of the solar wind model. More comparisons between different inner heliosphere models driven with identical coronal conditions are suggested as a way to explore their comparative strengths and weaknesses.

Transient and quasi‐stationary simulation of heat and mass transfer in Czochralski silicon crystal growth

AbstractThe formation of grown‐in defects in silicon crystals is controlled by the concentration of intrinsic point defects. Under steady state conditions the type of the prevailing point defect species is linked to the ratio of pull rate and temperature gradient in the crystal at the solidification front. It has been shown that this ratio as well as computed point defect distributions are in good agreement with experimental data. In this paper we compare a coupled transient heat transfer and transient point defect transport model with quasi steady state simulations at various time steps. Both simulations show the same qualitative results, quantitative differences in temperature are less than 1 %. But already for constant pull rates the defect distributions show qualitative differences between transient and quasi steady state simulations. Therefore, for a detailed understanding how defects are related to growth conditions, the thermal history should not be neglected.