Full Numerical Model Research Articles

Laser-cooled polyatomic molecules for improved electron electric dipole moment searches

Doppler and Sisyphus cooling of 174YbOH are achieved and studied. This polyatomic molecule has high sensitivity to physics beyond the Standard Model and represents a new class of species for future high-precision probes of new T-violating physics. The transverse temperature of the YbOH beam is reduced by nearly two orders of magnitude to < 600 μK and the phase-space density is increased by a factor of > 6 via Sisyphus cooling. We develop a full numerical model of the laser cooling of YbOH and find excellent agreement with the data. We project that laser cooling and magneto-optical trapping of long-lived samples of YbOH molecules are within reach and these will allow a high sensitivity probe of the electric dipole moment of the electron. The approach demonstrated here is easily generalized to other isotopologues of YbOH that have enhanced sensitivity to other symmetry-violating electromagnetic moments.

Heat and mass transfer evaluation of air-gap diffusion distillation by ε-NTU method

The effectiveness-number of transfer units (ε-NTU) method was used to develop the heat and mass transfer model of air-gap diffusion distillation (AGDD). In this model, the latent heat transfer of vapor in air-gap was regarded as an equivalent convective heat transfer to make a relationship with NTU. Compared with the full numerical model developed in our previous work, the new model was simple and performing with dramatic high computational efficiency. The accuracy of the model was as well as the full numerical model. The influences of structure and operation parameters on the performances of AGDD system were discussed furtherly by employing the model. Results shown that the air-gap height, the inlet temperature and feed flow rate of hot stream could be regarded as the strong parameters effected significantly on the performances of the system in comparison with the air-gap thickness and inlet temperature and salt concentration of cold stream. Therefore, these strong parameters should be considered firstly for improving the heat and mass transfer of AGDD.

Mixed convection and heat flow characteristics in a lid-driven enclosure with porous fins: Full numerical modeling and parametric investigations

The effects of porous fins on mixed convection inside lid-driven square enclosures were reported in the present work. The porous medium with varying permeability, instead of the solid one, could widely change the baffling performance on the fluid flow, where this characteristic could be beneficial for controlling fluid flow and energy transport, typically electronic cooling process. The top lid could have the two-way movement. The working fluid in the cavity was governed by the N-S equations while that within porous medium was determined by the non-Darcy model called the Darcy-Forchheimer model based on representative element-averaging method. Relevant governing parameters, including Richardson number, Darcy number, quantities, length and distribution ratio of porous fins, are sensitively varied to identify their effects and roles on mixed convection heat transfer. Numerical results illustrate that the mechanism of main circulation development is modified by the additional fins. The adding porous fins with excellent permeability enhance heat transfer dramatically, while these fins with tiny Darcy number deteriorate heat transfer rate. The rate of heat transfer enhancement due to the increasing of the length and quantities of fins decreases. Furthermore, the distribution ratio shows the significant effects on the fluid flow and heat transfer. Present investigations could benefit the microelectronic cooling through the installation of porous-alike materials or modules.

Mode instabilities in Yb:YAG crystalline fiber amplifiers.

Mode instabilities (MI) threshold in the Yb:YAG crystalline fiber amplifier is simulated by a full numerical model. The propagation of signal fields is simulated by the finite-difference beam-propagation method combined with the rate equations, and the time-dependent heat equation is solved by the alternating-direction-implicit method. Considering the strong temperature-dependent laser performance of Yb:YAG, an iterative method is applied to reach the steady state of Yb:YAG, the crystalline fiber amplifier, before the simulation of MI behavior. The simulated MI thresholds in Yb:YAG crystalline fiber amplifiers are found to be at least 28 times of those in Yb-doped silica-glass fiber amplifiers, up to tens of kilowatts. Simulation results show that, in addition to the expected higher thermal conductivity and lower thermo-optic coefficient, strong gain saturation also plays an important role in the high MI threshold of the Yb:YAG crystalline fiber.

A fundamental rule: Determining the importance of flow prior to polymer crystallization

A continuum-level model for nonisothermal polymer crystallization following a complex flow is presented, along with a fundamental rule that may be employed to determine if the flow will influence the ensuing crystallization dynamics. This rule is based on two dimensionless parameters: the (Rouse) Weissenberg number and an inverse Deborah number defined by the ratio between the time taken to cool and the melting point vs the stretch relaxation time, which determines the time available for flow-enhanced crystallization. Moreover, we show how the time to reach the melting point can be derived semianalytically and expressed in terms of the processing conditions in the case of pipe flow—ubiquitous in polymer processing. While the full numerical model is required to quantitatively predict induction times and spherulite-size distributions, the proposed fundamental rule may be used practically to ensure, or eliminate, flow-enhanced structures by controlling the processing conditions or material properties. We discuss how flow-enhanced structures may be revealed only after postprocessing annealing and finally examine previous works that have successfully applied the model to extrusion-based three-dimensional printing.

Fixed Grid Numerical Models for Solidification and Melting of Phase Change Materials (PCMs)

Phase change materials (PCMs) are classified according to their phase change process, temperature, and composition. The utilization of PCMs lies mainly in the field of solar energy and building applications as well as in industrial processes. The main advantage of such materials is the use of latent heat, which allows the storage of a large amount of thermal energy with small temperature variation, improving the energy efficiency of the system. The study of PCMs using computational fluid dynamics (CFD) is widespread and has been documented in several papers, following the tendency that CFD nowadays tends to become increasingly widespread. Numerical studies of solidification and melting processes use a combination of formulations to describe the physical phenomena related to such processes, these being mainly the latent heat and the velocity transition between the liquid and the solid phases. The methods used to describe the latent heat are divided into three main groups: source term methods (E-STM), enthalpy methods (E-EM), and temperature-transforming models (E-TTM). The description of the velocity transition is, in turn, divided into three main groups: switch-off methods (SOM), source term methods (STM), and variable viscosity methods (VVM). Since a full numerical model uses a combination of at least one of the methods for each phenomenon, several combinations are possible. The main objective of the present paper was to review the numerical approaches used to describe solidification and melting processes in fixed grid models. In the first part of the present review, we focus on the PCM classification and applications, as well as analyze the main features of solidification and melting processes in different container shapes and boundary conditions. Regarding numerical models adopted in phase-change processes, the review is focused on the fixed grid methods used to describe both latent heat and velocity transition between the phases. Additionally, we discuss the most common simplifications and boundary conditions used when studying solidification and melting processes, as well as the impact of such simplifications on computational cost. Afterwards, we compare the combinations of formulations used in numerical studies of solidification and melting processes, concluding that “enthalpy–porosity” is the most widespread numerical model used in PCM studies. Moreover, several combinations of formulations are barely explored. Regarding the simulation performance, we also show a new basic method that can be employed to evaluate the computing performance in transient numerical simulations.

Acoustics of a piezo inkjet channel with an entrained air bubble

Piezo-acoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. However, the stability of piezo-DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. The acoustically driven air bubble modifies the ink channel acoustics, and conversely, the modified ink channel acoustics influences the bubble dynamics. Here, we measure the acoustic eigenfrequency of a MEMS based silicon ink channel as a function of the bubble size. The eigenfrequency was measured using a pulse-echo system and the bubble size using a short-wave infrared imaging setup. We show that the measured eigenfrequency increases when an air bubble is entrained. Surprisingly, the ink channel resonance frequency plateaus at total bubble volumes larger than 10 pl. Moreover, it was found that at a constant total bubble volume, the resonance frequency increases with the number of entrained bubbles. We show that both experimental observations can be quantitatively explained from a simple lumped element model (LEM) comprising the ink channel Helmholtz resonator coupled to the bubble mass-spring system. The results of the LEM model were validated using a full numerical model of the coupled ink channel—bubble system.

Numerical Simulation of Corneal Fibril Reorientation in Response to External Loading.

Purpose: To simulate numerically the collagen fibril reorientation observed experimentally in the cornea. Methods: Fibril distribution in corneal strip specimens was monitored using X-ray scattering while under gradually increasing axial loading. The data were analysed at each strain level in order to quantify the changes in the angular distribution of fibrils with strain growth. The resulting relationship between stain and fibril reorientation was adopted in a constitutive model to control the mechanical anisotropy of the tissue material. The outcome of the model was validated against the experimental measurements before using the model in simplified representations of two surgical procedures. Results: The numerical model was able to reproduce the experimental measurements of specimen deformation and fibril reorientation under uniaxial loading with errors below 8.0%. With tissue removal simulated in a full eye numerical model, fibril reorientation could be predicted around the affected area, and this change both increased with larger tissue removal and reduced gradually away from that area. Conclusion: The presented method can successfully simulate fibril reorientation with changes in the strain regime affecting cornea tissue. Analyses based on this method showed that fibrils tend to align parallel to the tissue cut following keratoplasty operations. With the ability to simulate fibril reorientation, numerical modelling can have a greater potential in modelling the behaviour following surgery and injury to the cornea.

Optimal design and thermal modelling for liquid-cooled heat sink based on multi-objective topology optimization: An experimental and numerical study

This work suggests a design method of liquid-cooled heat sink based on topology optimization. First, a multi-objective optimization problem is developed in order to manage the tradeoff between the minimization of fluid power dissipation and the maximization of heat exchange. The novel cooling channels with clear topology information are generated under both single-uniform and the multiple-non uniform heat source conditions. The effects of initial guess, weighting factor, and Reynolds number on the Pareto optimal solutions are discussed. Next, a full scale 3D conjugate heat transfer numerical model is built to test the flow and thermal performances of the heat sinks. The results show that the optimized cooling channel can achieve a lower thermal resistance and a higher Nusselts number in comparison to the conventional parallel channel, which means the optimal channel can remove more heat energy while the pumping power supply is minimum. Finally, both traditional parallel and topology optimized channel are manufactured by CNC machine and the high performance of the optimally designed heat sink is validated by conducting experimental tests.

Unsteady three-dimensional modeling of the Fluid–Structure Interaction in the check valves of diaphragm volumetric pumps

A full 3D unsteady numerical model with dynamic meshes is developed to simulate the fluid–structure interaction in the non-returning valves of air-operated diaphragm volumetric pumps. This new three-dimensional CFD model provides a more accurate description of the flow patterns and a superior evaluation than a previous 2D model, published by the authors, regarding the dynamic response of the valves motion, which are responsible for internal volumetric losses that penalize the overall pump efficiency. Both piston-like and deformable geometries for the prescribed sinusoidal displacement of the membrane have been checked and compared in the modeling, resulting in similar behavior concerning the basic performance of the pump. Standard operation and free-delivery conditions are exhaustively analyzed, confirming more instabilities in the check valves in case of low air-supplied pressures. In particular, the exhausting valve is found to experience severe tapping with repetitive partial closures during the forward stroke due to an intense Fluid–Structure Interaction. On the contrary, the aspirating valve presents much better sealing characteristics with a partial reopening only at the initial moments of the backward stroke. All these numerical evidences have provided useful information for the pump manufacturers concerning the design, selection of materials and maintenance routines, which have been employed for the development of a series of new prototypes.

Development of a full 3D numerical model to investigate the hydraulic fracture propagation under the impact of orthogonal natural fractures

Although hydraulic fracturing has been massively studied and applied as a key technique to enhance the gas production from tight formations, some problems and uncertainties exist to accurately predict and analyze the fracture behavior in complex reservoirs, especially in the naturally fractured reservoirs like shale reservoirs. This paper presents a full 3D numerical model (FLAC3D) to study hydraulic fracturing behavior under the impact of preexisting orthogonal natural fractures. In this numerical model, the hydraulic fracture propagation direction is assumed perpendicular to the minimum principal stress and activated only by tensile failure, whereas the preexisting natural fractures can be activated by tensile or shear failure or a combination of them, and only tensile failure can open the natural fracture as well. The newly developed model was used to study the impact of preexisting orthogonal natural fractures on hydraulic fracturing behavior, based on a multistage hydraulic fracturing operation in a naturally fractured reservoir from the Barnett Shale formation, northwest of Texas in USA. In this multistage operation, two more representative stages, i.e., stage 1 with a relatively large horizontal stress anisotropy of 3.3 MPa and stage 4 with a comparatively small one of 1.3 MPa, were selected to conduct the simulation. Based on the numerical results, one can observe that the interaction between hydraulic and natural fracture is driven mainly by induced stress around fracture tip. Besides, the horizontal stress anisotropy plays a key role in opening the natural fracture. Thus, no significant opened fracture is activated on natural fracture in stage 1, while in stage 4 an opened fracture invades to about 90 m into the first natural fracture. Conversely, the hydraulic fracture length in stage 1 is much longer than in stage 4, as some fluid volume is stored in the opened natural fracture in stage 4. In this work, the shear failure on natural fractures is treated as the main factor for inducing the seismic events. And the simulated seismic events, i.e., shear failure on natural fractures, are very comparable with the measured seismic events.

Full scale experimental tests and numerical model validation of reinforced concrete slab subjected to direct contact explosion

In this work, a numerical model for analyzing the mechanical behavior of a reinforced concrete slab subjected to a direct contact explosion was developed, using the explicit finite element code LS-DYNA and facing the following issues: generation and propagation of the blast wave, interaction with the solid structure, and mechanical behavior of the slab. The different elements that constitutes the slab were modeled as deformable bodies and the constitutive model coefficients for each material, when not directly measured, were calibrated by comparison with experimental measures. To this purpose, a reinforced concrete slab used for civil buildings was loaded with three different charge of EXEM 100: 2.1, 6.3, and 10.5 kg. For each test, the blast wave pressure-time profile was measured at two different locations and the damage extension in concrete and reinforcing elements was estimated. Using the same sets of material dependent parameters, a good agreement between experimental and numerical results was found for all tested configurations. The validated numerical model provided insight into the role of different structural elements on the failure mechanisms into the slab and is a useful tool for investigating alternative loading configurations and designing potential reinforcement solutions.

Maximum Efficiency Despite Lowest Specific Speed—Simulation and Optimisation of a Side Channel Pump

Side channel pumps provide high pressure at relatively low flow rates. This comes along with a quite low specific speed and thus with the known disadvantage of a quite poor maximum efficiency. This paper describes the detailed analysis and optimisation of a typical 1-stage side channel pump with an additional radial suction impeller by means of computational fluid dynamics (CFD) simulations. In a first step, the model was successively generated and it was obvious that it has to contain all details including suction impeller and main stage (both 360° models) as well as the pressure housing and all narrow gaps to provide useful simulation results. Numerical simulations were carried out in a stationary and transient way with scale resolving turbulence models to analyse the components in detail. Finally the CFD-simulations were validated with model tests. For the optimisation process it was necessary to generate a reduced numerical model to analyse the effects of more than 300 geometry variations. The findings were then combined to establish the desired objectives. Finally the best combinations were validated again with the full numerical model. Those simulations predict a relative efficiency increase at best efficiency point (BEP) and part load &gt;30% with respect to all given limitations like identical head curve, suction behavior, and dimensions.

An Experimental Test for Application of Analytical Model of Surge Flow under Drought and Wet Conditions in a Semi-Arid Region

In this paper, an analytical solution is investigated in the zero-inertia problem for surge flow phenomena in non-prismatic channels considering the impact of infiltration (introduced by Technische Univ. Dresden) in dry-channels under drought and wet conditions in a semi-arid climate in Iran. The hydrodynamic analytical model introduced by Dresden University is free of numerical troubles, easy to run, computationally efficient, and fully satisfies the law of volume conservation. In a first test series, the hydrodynamic analytical ZI model can be compared favorably with a full hydrodynamic numerical model (HEC-RAS) in a permeable non-prismatic channel. In the next step, a sensitivity and error analysis was conducted and found that the Manning-Strickler coefficient, hydraulic radius, and inflow hydrograph have important effects on the velocity of the surge, suggesting the importance of an adequate description of the topography. The new approach is finally applied to simulate and compare 10 scenarios to manage artificial groundwater recharge in three channels in the downstream of Garin Dam under wet (more than 25%) and drought (less than 5%) conditions in the west of Iran. Based on tested scenarios, infiltration rate of bad and suitable scenarios varies from 3 to 85% and 1 to 76% of the total volume of released water in Garin in wet and drought phases, respectively.

Multistage canned motor pump – getting deep insights with CFD – simulation, optimization and validation

This paper describes the detailed analysis of a multi-stage canned motor pump by means of CFD (computational fluid dynamics) simulations to find the main losses and show optimization potential of the hydraulic parts. In a first step, the numerical model was successively generated including all details like impellers, guide vanes (GV), return vanes (RV), gaps and pressure relief holes as well as the hydraulically coupled rotor of the drive. Simulations were carried out with the commercial CFD package ANSYS CFX 17.1 in stationary and transient way where mainly structured grids were used with a final model of the existing pump consisting of more than 35 mio. nodes. The behavior of the components was analyzed in detail and additionally the CFD-simulations were validated with model tests. Based on the loss analysis, a 2-stage optimization procedure was performed by combining manual engineering with automated optimization to establish the desired objectives and validated with the full numerical model. The simulations predict a huge efficiency increase from part load up to the best efficiency point (BEP) with respect to all given limitations (identical head curve, suction behavior and dimensions) and were then validated on the test rig where they have proven their high accuracy and reliability.

Real-Time Formation Evaluation and Contamination Prediction Through Inversion of Downhole Fluid-Sampling Measurements

Summary Acquisition of fluid samples using wireline-formation testers (WFTs) is an integral part of reservoir evaluation and fluid characterization. Recent developments in formation-tester hardware have enabled wireline-based fluid sampling in a wide range of downhole conditions. However, accurate quantification of oil-based-mud (OBM) filtrate contamination using data from downhole-fluid-analysis (DFA) sensors alone remains challenging, especially in difficult sampling environments and for advanced sampling tools that have complex inflow geometries and active guarding of filtrate flows. Such tools and conditions lead to contamination behaviors that do not follow simple power-law models that are commonly assumed in OBM-contamination-monitoring (OCM) algorithms. In this paper, we introduce a new OCM algorithm derived from an inversion of DFA data using a full 3D numerical flow model of the contamination-cleanup process. Using formation and fluid properties and operational tool settings, the model predicts the evolution of filtrate contamination as a function of time and pumped volume, and can thus be used to forward model the DFA sensor responses. Sensor data are then inverted in real time to provide contamination predictions. Real-time computation is enabled through fast, high-fidelity proxy models for the cleanup process. The proxy models are trained on and thoroughly vetted against a large number of full-scale numerical simulations. Compared with existing algorithms, the new OCM method is now applicable for all types of sampling hardware and a wider set of operating conditions. By directly relying on a model of the cleanup process, the physical properties of the formation and fluids (such as porosity, permeability, viscosity, and depth of filtrate invasion) are estimated during the inversion, thus providing additional valuable information for formation evaluation. The new method is demonstrated by practical application in both synthetic and field examples of oil sampling in OBM. The synthetic examples demonstrate the robustness of the algorithm and show that the true formation and fluid properties can be recovered from noise-corrupted sensor data. The field example presented demonstrates that contamination predictions are in good agreement with results from laboratory analysis, and the inverted formation properties are consistent with estimates derived from openhole logs and pressure measurements.

Dynamic viscosity of colloidal silica suspensions at low and high volume fractions

A comprehensive study was carried out on the dynamic viscosity of X30 silica dispersions at both high and low volume fractions of colloidal silica particles at various electrolyte ionic strength and pH values. Booth and Ruiz-Reina and Carrique theoretical models (R-R&C) were compared in predicting the primary electroviscous effect (PEE) for viscosity at low volume fractions. To this respect the colloidal dispersion was well characterised with regards to electrolyte properties such as the Debye length, κ−1, calculated from the ionic strength, and zeta potential, ζ, calculated from the electrophoretic mobility using the full numerical model by O’Brien and White (O’B&W). R-R&C hard sphere model (which is a modified version of Simha hard sphere model that includes a boundary condition by Happel on the outer radius of the cell) and the semi-empirical Krieger-Dougherty (K-D) models were fitted to the experimental data at high volume fractions. At both low and high volume fractions the viscosity increased with pH and decreased with ionic strength. At low volume fractions both theoretical models significantly underestimated the experimental dynamic viscosities obtained in this work. This could be attributed to the fuzzy structures for silica particles in aqueous conditions reported previously in the literature, where a significantly larger electroviscous parameter, p, was obtained experimentally for silica particles. The experimental electroviscous parameter, pexp, in this work was found to be roughly an order of magnitude up to 26 times larger than that predicted by Booth and around 5 ± 1 times larger than the predicted p by R-R&C model allowing the introduction of a correction factor in the PEE coefficient obtained from R-R&C model enabling good prediction for X30 silica dispersions by the latter model. The significant improvement of the electroviscous effect predictions by R-R&C model compared to Booth model may be attributed to the limitations invalidating the Booth model at the electrolyte conditions in this study. At high volume fractions, the R-R&C hard sphere cell model, gave a much better fit to the experimental data compared to the K-D model, which also had the advantage of being only dependent on a coefficient that linearly relates an effective volume fraction postulated for the fuzzy silica particles to the experimental. The K-D model however, depends on the intrinsic viscosity, [η], which requires the calculation of the experimental slope of the dynamic viscosity against volume fraction in the dilute limit, and also on a maximum packing fraction as a fitting parameter. Due to the effect of the fuzzy structures on the viscosity, the latter effective volume fraction ϕeff was calculated using two approaches: (i) as a fitting parameter by fitting the R-R&C hard sphere model to the experimental viscosity data over the entire volume fraction range, and (ii) by fitting only the linear part of the experimental viscosity at low volume fractions, It is concluded that the R-R&C hard sphere model with the effective volume fraction accounting for the fuzzy structures fits reasonably well the full range of experimental results at low and high volume fractions. When the model was used with the Adamczyk effective volume fraction (i.e., considering only the dilute region in the fitting procedure), the predictions became worse at high volume fractions where significant deviations from the experimental results were found upon the increase of pH.

Effects of tank sloshing on submerged oil leakage from damaged tankers

In order to gain advanced understanding of the mechanism of the submerged oil leakage from damaged tankers subjected to wave-excited motions, a three-dimensional (3D) full scale numerical model is developed in this paper. The model is based on the three-phase Navier-Stokes equation and the continuity equation, which are solved by using the finite volume method (FVM). The volume of fluid (VOF) method is implemented to identify the interfaces between different phases and the k-ε turbulence model is employed to approximate the turbulence effects. The prototype of the oil tanker is taken as the side tank of VLCC, which is subjected to a periodically forced motions yielding a liquid sloshing inside the tank. After being validated by comparing its prediction with experimental data, the present model is utilized to a systematic investigation with wide range of applications including different motion amplitudes and periods. The dynamic characteristics of both the macroscopic parameters, e.g. the volume of the oil/water, and the microscopic parameters, e.g. the velocity distributions, are analyzed. The results fill the gap in the existing numerical and experimental work, in which the tank is assumed to be stationary, and produce a more reliable prediction on the dynamic process of the oil leakage and the stability of the damaged oil tankers subjected to wave actions.

Protection of Historical Buildings in Metro Line Construction: A Case Study in Shanghai, China

This research presents a holistic approach to the protection of historical building group adjacent to metro line construction, in which health investigation, protection criterion determining, numerical simulation analyses, corresponding protection measures and field monitoring analyses are incorporated as main procedures. A case relating to the conservation of a seven-storey brick-wood pagoda, Longhua Pagoda, adjacent to the construction of Shanghai Metro Lines 11 and 12 in China, is presented. A reasonable protection criterion is determined rationally by means of numerical simulation based on the health investigation results combined with related standards, which can help to achieve a fine balance between system safety and cost constraints in protection measures determining. A full numerical model is developed to analyze the effect of dewatering, excavation, tunneling, and shield departure/reception, and corresponding measures are adopted to ensure the safety of the pagoda. Especially a brand new method, steel jacket method, is introduced in detail, which has a good performance on decreasing the amount of dewatering and maintaining a stable working face. Results demonstrate the application potential of the proposed approach, as well as its feasibility. The proposed approach can be used in the industry by practitioners to provide positive guidelines on the conservation of historical building group against metro line constructions. The reliable and detailed data in this case can also provide verification for subsequent research.

Efficient Circuit and an EM Model of an Electrostatic Discharge Generator

In this work, a new technique for an efficient, simple, and fast equivalent circuit and full wave numerical modeling of the electrostatic discharge (ESD) generator is presented. A novel circuit model of the NoiseKen ESD simulator is proposed based on the frequency domain measurement of the standard waveform calibration setup. The simple full wave electromagnetic model of the same generator, which requires much less computation resources, is also proposed using commercial CST Microwave Studio software. The reliability of the proposed models as an authentic ESD excitation source is validated through an example discharge application. The good agreement between the ESD reference waveform obtained through measurement, circuit model, and 3-D model of the generator is observed.