In this study, a new hybrid large-eddy simulation / Reynolds-averaged Navier-Stokes (LES / RANS) turbulence model has been presented. Assessment and tests of the new model have been conducted on different categories of cases. The reason for a hybrid LES/RANS model is to reduce the near- wall computational cost of LES, while resolving unsteady turbulence structures elsewhere. The goal of the new model development is to come up with a hybrid LES / RANS model that reverts to a viable SGS model far away from surfaces, preserves the composite structure of a turbulent boundary layer, and responds to changes in turbulence length scales while using only local and instantaneous information. To determine the transition point between RANS and LES regions, an estimated outer-layer length scale information is needed. While other models obtain the outer- length scale in LES region through case-specific pre-calibration, ensemble averaging, or other non-local or non-instantaneous means, the new model utilizes a one-equation RANS-type eddy viscosity transport (EVT) model based on the unsteady flow field to estimate the outer-length scale. The use of this transported ’EVT’ quantity is solely for the determination of the hybrid blending function, thus there is no risk of over-constraining the LES field with a RANS component. Moreover, the amplified production of EVT eddy viscosity due to high fluctuating strain rates is compensated by a modification of the destruction term based on an estimated von Kármán length scale.
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However, it is apparent from the two eddy examples shown above that eddy evolutions are not always characterized by monotonic decay with time. The am- plitude of the eddy might increase from time to time (Figure 1). Thus, the use of the amplitude–lifetime relations might lead to an underestimation of the ampli- tude decaying rate a, which is not the case for the values of decaying rate a from this case study. For example, a = 0.009 cm/day as inferred from a long-lived eddy (Souza et al ., 2011 ), which represents only approximately 1/40 to 1/20 of those reported in the present study. Thus, eddy viscosity reported in this study might serve as a lower bound of the true eddy viscosity in the ocean. For in- stance, the viscosity in ACC is approximately 173 m 2 / s as inferred from the Li
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It is well known that a major drawback of the EVMs is their incapacity to capture the effects resulting from the rotating system. More specifically, if the flow exhibits a swirl motion, EVMs fail to represent the near wall region ”free-loss vortex” part of the Rankine vortex. This is attributed to the use of the Boussinesq hypothesis that assumes that the eddy viscosity is an isotropic scalar which is untrue for more complex flows such as cyclones. In order to handle this weakness, many modifications aimed at the sensitization of EVMs to rotation and curvature have been suggested (see Refs. [5, 6, 7]). These attempts are limited and not universal, especially when dealing with 3D flows. Moreover, these corrections are not Galilean-invariant. In 1997, Spalart and Shur  proposed an empirical alteration to EVMs to account for the system of rotation and streamline curvature, which in a sense is close to an idea of Knight and Saffman . The former is more efficient as it measures the extra influence of the invariant contributor to the turbulence. It is also relatively easy to apply to 3D flows and unifies the description of the curvature and rotation effects in the mathematical model. Also in 1997, Hellsten  proposed some improvement to the well known k − ω SST turbulence model. The modification included the sensitization for the effects of system rotation and streamline curvature. Among several different definitions of Richardson number (Ri), Hellsten realisation replaces the turbulent time scale appearing in the Khodak and Hirch  definition by the mean-flow time scale 1/S ij . This results in a
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temperature boundary condition for the application of the direct-contact condensation. The van Driest-type eddy viscos- ity model is used in the near-wall region and Ueda’s eddy diffusivity model near the free surface of open channel flow is used for the near-interface region. Based on the eddy viscosity model, the velocity profile in the condensate film and the condensate film thickness are obtained. Axial conduction is ignored and the interface temperature is assumed to be constant in the energy conservation. The local steam flow rates and the local heat transfer coefficients are computed. It is found that the condensing rates of the steam flow is under-estimated from that of the experimental data.
The capability of the hybrid approach is tested and evaluated on two diﬀerent conﬁgurations. The ﬁrst one is the ramped-cavity ﬂow ﬁeld experimentally mapped by Settles, et. al.  The results indicate that while the predictions in the shear layer are not that satisfactory, the hybrid approach performs well in capturing the structure of the recovering boundary layer downstream of reattachment. This is a major advantage over purely RANS models, as these generally fail to generate the required levels of turbulent ﬂuctuation ampliﬁcation in order to produce a rapid recovery. The MILES approach, which sets the eddy viscosity to zero, produces similar results, but it responds better to grid reﬁnement than the hybrid model does. This implies that with suﬃcient grid resolution, only very minimal levels of eddy viscosity are necessary away from solid surfaces.
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The aim of this study is to evaluate a three-equation turbulence model applied to pipe flow. Un- certainty is approximated by comparing with published direct numerical simulation results for fully-developed average pipe flow. The model is based on the Reynolds averaged Navier-Stokes equations. Boussinesq hypothesis is invoked for determining the Reynolds stresses. Three local length scales are solved, based on which the eddy viscosity is calculated. There are two parame- ters in the model; one accounts for surface roughness and the other is possibly attributed to the fluid. Error in the mean axial velocity and Reynolds stress is found to be negligible.
In order to investigate the performance of the turbulence models: the widely used k-ε eddy viscosity and the second order closure RSM to predict reasonably the flow behavior in a 180 deg U-Bend curved pipe at a pipe Reynolds number of 4.45× 10 4 and a radius ratio of and a radius ratio of 3.375 has been carried. The numerical study reveals that the secondary flow pattern in a curved pipe is very complicated. The results support the notion of an additional (symmetrical) pair of counter-rotating vortical structures embedded in the core of the flow within the curved pipe.
In this study computational fluid dynamics (CFD) approach was used to study mixing in an Indus- trial gold leaching tank. The objective was to analyze the extent of mixing in the tank by producing visual images of the various mixing zones in the tank domain. Eddy viscosity plots that charac- terise the extent of mixing in the tank were generated in the flow field obtained by an Eulerian- Eulerian approach. The extent of mixing was found to be greatest in the circulation loops of the impeller discharge region and least at the top and bottom portions of the tank. Trailing vortices that contribute to some level of mixing were identified in between the impeller blades. This ap- proach could be used to enhance optimum design of mixing vessels and to eliminate the need for pilot plants.
It is clearer in Fig. 11 which illustrates the turbulent kinetic energies in the region near the broken holes obtained by using different turbulence modelling. From Fig.11, a significantly higher level of the turbulent kinetic energy is observed in the main body of the oil jet given by the standard, low- Re and realizable k-ε models. A similar conclusion has also been made in the comparative studies associated with the orifice flow, suggesting that the standard and realizable k-ε models may yield undesirable results for the cases with Reynolds numbers similar to the Case G1, i.e. Re<2000 (e.g., [53, 54]). The low-Re k-ε model performs better than the realizable k-ε models, partially attributing to the empirical treatment of the flow near the wall with local low turbulent Reynolds number effects and the wall damping effects [55,56]. Compared to the standard, low-Re and realizable k-ε models, the RNG k-ε model shows a dramatic improvement, perhaps attributing to its special concern on smaller scales of the fluid motion, making it more feasible to deal with the turbulence associated with the interface between different phases and triggered by convective shearing layers . Our conclusion on the poor performance of the k-ε models also conforms to the comments by , i.e. without special treatment of a turbulence damping, the differential eddy viscosity models, such as the k-ε models, generate levels of turbulence that are too high throughout the interface of the multi-phase flow. It is also found from Fig. 10(a) that the k-ω SST model leads to a better estimation of the velocity head (and turbulent energy loss) compared to the k- ε models, conforming
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In seeking ways in which we could further improve the damping performance for the quadruple suspensions other materials or schemes were considered. The effectiveness of eddy-current damping depends on the resistance of the cur- rent paths in the conductive material. The lower the resis- tance ( or the higher the conductivity ) the greater the damp- ing. In order to improve the damping, therefore, one should select the highest conductivity material. At room temperature only silver is a better conductor than copper and then only by ⬃ 5%. 10 Therefore, assuming a linear relationship between conductivity and damping performance the improvement in selecting silver over copper is marginal. Beryllium is a better conductor than either of these materials but only below 180 K. At these low temperatures one can also consider su- perconductors. The highest temperature superconductor cur- rently known is a mercury-based cuprate with a transition temperature of around 130 K at atmospheric pressure. 11 Peltier devices would not provide sufficient cooling and al- though miniature electromechanical refrigeration units ( cryo- coolers ) exist for these low temperatures they are inherently noisy. Pulse tube cryocoolers 12 have typically two orders of magnitude lower vibration amplitudes than standard types; however, their vibration levels are still higher than the noise requirement for Advanced LIGO.
et al., 2010). A shallow undersea ridge known as the Alboran Ridge extends northeastward from Cape Three Forks towards the center of the Eastern Alboran Gyre (EAG); the 0.7 km 2 Alboran Island is found on the ridge at 35.9 ◦ N, − 3.0 ◦ W. The Alboran Trough is a deep water channel along the north- ern base of the ridge that connects the western and eastern basins. Mass exchange with the North Atlantic takes place at the open Strait of Gibraltar to the west and to the east with the wider western Mediterranean. In the transition region be- tween the Alboran Sea and the Algerian sub-basin, intense eddies and fronts are also generated, although they are less frequent than in the Alboran Sea (Pascual et al., 2017). The presence of large eddies in the Algerian basin has been sys- tematically documented (e.g., Ruiz et al., 2001; Puillat et al., 2002; Escudier et al., 2016; Pessini et al., 2018). These Al- gerian eddies typically form as a result of instabilities in the cool and fresh Algerian coastal current (e.g., Millot, 1999; Testor et al., 2005; Capó et al., 2019). Recent studies in this basin using high-resolution observations (Cotroneo et al., 2016; Aulicino et al., 2018) have demonstrated the presence of fine-scale features associated with the large eddies. Re- garding the Balearic Sea, the spatial–temporal variability of the surface circulation was investigated by Mason and Pas- cual (2013), revealing intense mesoscale eddy activity in this northern sub-basin.
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Abstract The wet gets wetter, dry gets drier paradigm explains the expected moistening of the extratropics and drying of the subtropics as the atmospheric moisture content increases with global warming. Here we show, using precipitation minus evaporation (P − E) data from climate models, that it cannot be extended to apply regionally to deviations from the zonal mean. Wet and dry zones shift substantially in response to shifts in the stationary-eddy circulations that cause them. Additionally, atmospheric circulation changes lead to a smaller increase in the zonal variance of P − E than would be expected from atmospheric moistening alone. The P − E variance change can be split into dynamic and thermodynamic components through an analysis of the atmospheric moisture budget. This reveals that a weakening of stationary-eddy circulations and changes in the zonal variation of transient-eddy moisture ﬂuxes moderate the strengthening of the zonally anomalous hydrological cycle with global warming.
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That change must happen for a business to survive is well known. Those that rely on the status quo are disappearing quickly. The business landscape continues to change at an accelerating and alarming rate. Change discussions are not about evolutionary growth but growth hacks that launch and disrupt. Business units are temporary, built around short product lifecycles. The ability to harvest from these lifecycles is fraught with risk. Value is temporary and talent is temporary. A company may not bother with gaining intellectual property protection, because the lifecycle is over before the protection is granted. Companies do not need to eliminate waste, or get lean, they need to be born lean and born global, or face extinction. The question, therefor, for businesses is about whether or not they are accelerating forward at a suitable rate in comparison to their competitors. The idea of velocity may be viewed as a spot check at a time, however, each check needs to be assessed as velocity is replaced by the concept of acceleration. Threatening competitors are horizontally oriented in the market. They are vertically oriented in an accelerated mergers and acquisitions (M&A) environment. And, they are in emerging technologies that change the landscape overnight. While the competition is fierce, little is known about the counterforce to the required acceleration. What is holding organizations back? With this understanding business entities can be liberated from their uncompetitive position and launch into higher profits. Again, the author calls resistance to rapid and fluid progress, Change Viscosity. First, an understanding of the analogy.
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Second experiment of printing trial was done on PDMS printing plate by using CNT water base ink. The printing trial had been done successfully on paper substrate. The multiple fine solid lines pattern image printed result liked showing in Figure 9(a-d) depended on the viscosity setting. The ink viscosity was setting from low to high which was ratio between water and CNT ink. Figure 9(a) showed the lowest viscosity setting with unclear fine solid lines image. When the viscosity was increased, fine solid line pattern image will be more cleared as shown in Figure 9(d).
19 over a circular cylinder, etc.) and have not been used in any practical cases or in engineering applications (Lee and Moser 2015). For simple flow cases, well-developed DNS reference data are either available or can be generated without much difficulty. Therefore, it was straightforward to choose the grids and/or filter width (explicit scheme) for LES, based on the available/required DNS grid resolutions. Furthermore, there are no guidelines available to select the filter width for other complex fluid flow cases where no reference resolutions (DNS) are available. DNS cannot be easily performed to determine the filter width for explicit LES for practical cases. An important question in the absence of DNS resolution is how to select the filter width which is a purely model parameter in explicitly filtered LES. Can it be based on a percentage of a physical parameter such as the boundary layer thickness (BLT), which is considered the same size as the largest eddy (Tennekes and Lumley 1972)? It then raises the second question: how can we select the percentage of a physical parameter and select an FGR consistent with the principles of LES and grid convergence? The main objective of this current work is to address the above issues and to develop an appropriate scheme to obtain a simulation result consistent with the principles of LES which are (a) the filter width lies within inertial range and (b) all energy containing eddies are adequately captured and grid convergence is obtained in explicit schemes (Pope 2004).
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Abstract Pulsed eddy current (PEC) non-destructive test- ing and evaluation (NDT&E) has been around for some time and it is still attracting extensive attention from researchers around the globe, which can be witnessed through the reports reviewed in this paper. Thanks to its richness of spectral components, various applications of this technique have been proposed and reported in the lit- erature covering both structural integrity inspection and material characterization in various industrial sectors. To support its development and for better understanding of the phenomena around the transient induced eddy currents, attempts for its modelling both analytically and numeri- cally have been made by researchers around the world. This review is an attempt to capture the state-of-the-art development and applications of PEC, especially in the last 15 years and it is not intended to be exhaustive. Future challenges and opportunities for PEC NDT&E are also presented.
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Our first focus in this paper is to obtain a relationship be- tween the growth (or decay) of such eddies, and the charac- teristics of the scalar potential vorticity field. Would it be possible, for example, to view the field, identify a particu- lar eddy, and predict its chances of survival based on simple geometric properties of the scalar field? In response to this, we are able to develop a collection of (diffusivity-driven) ge- ometric conditions for eddy growth, outlined in Sect. 5. It would be instructive to test our criteria upon available data sets with sufficient resolution. Moreover, in Sect. 7, we also obtain a qualitative condition on (small) wind forcing, which also contributes to eddy growth. ‘Growth,’ as specified in both these cases, will be defined through the enlargement of the eddy boundary; a shrinking boundary will correspond to a ‘draining’ eddy. Growing eddies have the potential of be- ing more visible, and, therefore, are expected to be the longer lasting eddies in the ocean. Draining (shrinking) eddies, on the other hand, will eventually lose their constituent water to the ambient flow, and disappear. Therefore, in a sense, our eddy growth criteria reflect a form of eddy stability in the presence of (small) eddy diffusivity and wind forcing. It must be re-emphasised that this ‘stability’ is not in the tra- ditional sense of linear stability, in which the growth rate of various modes of imposed perturbations is analysed, as in Flierl (1988); Helfrich and Send (1988); Paldor (1999); De- war and Killworth (1995); Dewar et al. (1999).
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In this paper, we introduce the GEM to describe mesoscale eddies in a tracking process with a total number of time steps T . The GEM allows the eddy to have multiple eddies as its parents or as its children in a multi-branch model. It also solves the missing eddy problem by using a new look-ahead method similar to the MHA. Compared with the computer time O(M N+1 T ) of MHA, the new method is much faster and has much less computer time O(LM(N + 1)T ), where L denotes the number of pixels of a target region. Besides, if the GEM was implemented with the computer codes prop- erly, the output data of the GEM also record the dynamic evolution of the eddy in detail and will potentially be useful for other research fields, e.g., the dynamics of cyclones in meteorology. As an example, the GEM is applied to eddies in the North Pacific Ocean (NPO) only, and we assume the eddies do not cross the Equator.
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Falkowski and Wirick (1981) claimed that turbulence had only a small effect on the vertical integrated primary productivity. The difference between their study and the present one is their use of constant eddy viscosities and their assumption of an even vertical distribution of phytoplankton in the water column. However, the turbulence model used in the present work is based on the k e model; this leads to an uneven vertical distribution of the plankton, which is indicated by a varying mean depth of all plankton over the growth season. Furthermore, an empirical formula has been used to describe the bottom water turbulence, which of course also affects the vertical distribution of plankton. For an ensemble of diatoms, the net production over 24 hours was integrated and that for each cell was tracked individually. This enabled growth conditions for the entire population to be studied statistically, which should give a more reliable description than tracking individual plankton. One interesting result is that the integrated net production was larger in the cold and weakly-stratified summer of 1999 than in the warm and more stratified summer of 2002; in the summer of 1999, the turbulence intensity was greater and so more phytoplankton were brought from deeper layers into the euphotic zone.
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Two hours later (T2 in Fig. 6), the whole water column becomes more or less well-mixed across the deep channel and eastern shoal, while in the western shoal a sharp salinity stratification develops with bottom-top salinity difference ~3 in 2 m water column (Fig. 9g). The distribution of salinity, and thus density, in the cross-channel direction is such that lateral baroclinic pressure gradients are directed from the central part of the deep channel towards the shoals. This is similar to the situation pointed out in Nunes and Simpson . Based on their theory, such pressure gradient force will induce a lateral circulation with convergence at the surface and divergence near the bottom. This indeed occurs in our numerical results. Surface lateral flow at the west half of the channel changes from ~0.1 m/s westward to ~0.2 m/s eastward between T1 and T2. A pair of counter-rotating circulations can be clearly seen at T2 with strong convergence 2 m below the surface. Contrast to the idealized case in Lerczak and Geyer , the two circulation cells are not closed at this time. The maximum along- channel tidal velocity reaches ~ 0.8 m/s, confined at the mid-depth of the central channel (Fig. 9e). The vertical shear in along-channel velocity is relatively weak, while the horizontal shear is great, which is, over the western slope, ~ 0.6 m/s within 200 m distance. It follows that Nunes and Simpson’s argument is also applicable here in that differential advection of along-channel current is at least one of the mechanisms to generate the lateral salinity gradient. Strong vertical mixing (maximum eddy viscosity ~0.05 m 2 /s, Fig. 9h) occurs at the mid-depth and bottom boundary layer, where either tidal
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