Momentum Mixing Research Articles

New Nusselt Number Correlation and Turbulent Prandtl Number Model for Turbulent Convection with Liquid Metal Based on Quasi-DNS Results

Liquid metal is widely used as the primary coolant in many advanced nuclear energy systems. Prandtl number of liquid metal is much lower than that of the conventional coolant of water or gas. Based on the Reynolds analogy, the turbulent Prandtl number is assumed to be a constant around unity. For the turbulent convection of liquid metal, dissipations of half the temperature variance are larger than those of turbulent kinetic energies. The dissimilarity between the thermal and momentum fields increases as Pr decreases. The turbulent Prandtl number is larger than one for the liquid metal. In the current investigation, the turbulent convection of liquid metal in the channel is quasi-directly simulated with OpenFOAM-7. The turbulent statistics of the momentum and the thermal field are compared with the existing database to validate the numerical model. The power law for dimensionless temperature distribution with different Prandtl numbers is obtained by regression analysis of numerical results. A new Nusselt number correlation is derived based on the power law. The new Nusselt number correlation agrees well with the DNS results in the literature. The momentum mixing process between different layers in the cross section is compared with the thermal mixing process. The effects of the Prandtl number on the difference between the turbulence time scale and scalar time scale are analyzed. A new turbulent Prandtl number model with local parameters is obtained for turbulent convection with liquid metal. Combined with the k−ω model, the temperature distributions with the new turbulent Prandtl number model agree well with the DNS results in the literature. The new turbulent Prandtl number model can be used for turbulent convection with different Prandtl and different Reynolds numbers.

Flow field generated by a plasma actuator at different elevations in quiescent air

Motivated by the need for improving the lift of unmanned aerial vehicle and replacing the trailing-edge flap under high-altitude environment, the electrical and the flow characteristics of an asymmetrical plasma actuator for a range of elevations [0.2 km ≤ E (elevations) ≤ 4 km] in quiescent air were studied in the present work. In general, compared with the plasma actuator under normal environment, the disturbance capability of the plasma actuator can be enhanced in high-elevation environments and more beneficial to flow control. Initially, the average total power consumption and the maximum amplitude of the induced current increase monotonously with the elevation. Meanwhile, the maximum velocity, Vmax, of the wall jet created by the plasma actuator increases with increasing the elevation and Vmax (velocity) ∼ E0.36. The velocity profile becomes irregular at E ≥ 2 km. Moreover, the relationship between the electrical and the flow characteristics of the plasma actuator is revealed based on the body force and the power consumption. In addition, compared with the case of normal condition, the starting vortex travels further from the upper electrode and the traveling angle is lower at higher elevation, which is benefit for momentum transfer and mixing. It is of great importance that a series of coherent structures in the vicinity of the wall surface were first observed at higher elevation, which is quite different from the case with normal condition.

Direct calculation of the eddy viscosity operator in turbulent channel flow at Re τ = 180

This study aims to quantify how turbulence in a channel flow mixes momentum in the mean sense. We applied the macroscopic forcing method (Mani & Park, Phys. Rev. Fluids, 2021, 054607) to direct numerical simulation (DNS) of a turbulent channel flow at $Re_\tau =180$ using two different forcing strategies that are designed to separately assess the anisotropy and non-locality of momentum mixing. In the first strategy, the leading term of the Kramers–Moyal expansion of the eddy viscosity is quantified, revealing all 81 tensorial coefficients that essentially characterise the local-limit eddy viscosity. The results indicate the following: (1) the eddy viscosity has significant anisotropy, (2) Reynolds stresses are generated by both the mean strain rate and mean rotation rate tensors associated with the momentum field and (3) the local-limit eddy viscosity generates asymmetric Reynolds stress tensors. In the second strategy, the eddy viscosity is quantified as an integration kernel revealing the non-local influence of the mean momentum gradient at each wall-normal coordinate on all nine components of the Reynolds stresses over the channel width. Our results indicate that while the shear component of the Reynolds stress is reasonably reproduced by the local mean gradients, other components of the Reynolds stress are highly non-local. These results provide an understanding of anisotropy and non-locality requirements for closure modelling of momentum transport in attached wall-bounded turbulent flows.

Estimation of Turbulent Mixing Factor and Study of Turbulent Flow Structures in Pressurized Water Reactor Sub Channel by Direct Numerical Simulation

Abstract Subchannel analysis codes are presently a requirement for design and safety analysis of nuclear reactors. Among the crucial inputs for these codes, the turbulent mixing factor holds particular significance. However, acquiring this factor through experimental means proves to be a challenging endeavor, primarily due to the necessity for precise pressure equilibrium between subchannels. Consequently, this requirement leads to the undertaking of expensive and intricate experiments for each new reactor or in cases where there are modifications in fuel bundle design. The need for direct numerical simulation (DNS) stems from the challenges and costs involved in experimental techniques, and the uncertainties due to empiricism in computational fluid dynamics (CFD) models. In this study, DNS has been conducted across six Reynolds numbers, ranging from 17,640 to 1.176 × 105, in the geometry of a pressurized water reactor (PWR) subchannel. The resulting turbulent flow structures have been computed and their dynamics are examined. Furthermore, this paper presents a methodology for directly calculating the turbulent mixing factor from the fluctuating velocity field obtained from DNS data. The turbulent mixing process has been scrutinized in-depth, and a correlation for the turbulent mixing factor is developed. It is noted that most of the mixing occurs in the near-wall region. The study suggests different mixing factors for mass and momentum mixing. This paper aims to provide a comprehensive insight into the turbulent mixing phenomenon.

Magneto-Stokes flow in a shallow free-surface annulus

In this study, we analyse ‘magneto-Stokes’ flow, a fundamental magnetohydrodynamic (MHD) flow that shares the cylindrical-annular geometry of the Taylor–Couette cell but uses applied electromagnetic forces to circulate a free-surface layer of electrolyte at low Reynolds numbers. The first complete, analytical solution for time-dependent magneto-Stokes flow is presented and validated with coupled laboratory and numerical experiments. Three regimes are distinguished (shallow-layer, transitional and deep-layer flow regimes), and their influence on the efficiency of microscale mixing is clarified. The solution in the shallow-layer limit belongs to a newly identified class of MHD potential flows, and thus induces mixing without the aid of axial vorticity. We show that these shallow-layer magneto-Stokes flows can still augment mixing in distinct Taylor dispersion and advection-dominated mixing regimes. The existence of enhanced mixing across all three distinguished flow regimes is predicted by asymptotic scaling laws and supported by three-dimensional numerical simulations. Mixing enhancement is initiated with the least electromagnetic forcing in channels with order-unity depth-to-gap-width ratios. If the strength of the electromagnetic forcing is not a constraint, then shallow-layer flows can still yield the shortest mixing times in the advection-dominated limit. Our robust description of momentum evolution and mixing of passive tracers makes the annular magneto-Stokes system fit for use as an MHD reference flow.

Preferential enhancement of convective heat transfer over drag via near-wall turbulence manipulation using spanwise wall oscillations

This study investigates the manipulation of convective heat transfer through spanwise wall oscillations in a turbulent channel flow. Direct numerical simulations are performed at Reτ=180 and Pr=1.The primary focus of this work is to explore the heat transfer response to oscillation parameters that promote drag increase, a regime that has received limited attention. By adopting an extended oscillation period (T+=500) and amplitude (W+=30), which have been reported to enhance drag, a remarkable dissimilarity between momentum and heat transport emerges. Under these conditions, the convective heat transfer undergoes a substantial 15% intensification, while the drag increases by a comparatively moderate 7.7%, effectively breaking the Reynolds analogy. To elucidate the physical mechanisms responsible for this dissimilar behaviour, a comprehensive statistical analysis is conducted. The control effect on the near-wall streaks and the associated mixing of momentum and heat is investigated by examining the energy distribution across scales and wall-normal locations. This analysis provides valuable insights into the control’s impact on the turbulent structures. Furthermore, the correlation between wall-normal velocity fluctuations and both streamwise velocity and temperature fluctuations is scrutinized to understand the modification of sweep and ejection events, which drive the transport of momentum and heat. The Fukagata–Iwamoto–Kasagi (FIK) identity is employed to identify the contributing factors to the changes in drag and heat transfer. The analysis highlights the importance of the pressure term in the streamwise velocity equation and the linearity of the temperature equation. Further investigation is necessary to fully unravel the complex mechanisms governing the decoupling of heat and momentum transport. The results of this study underscore the potential of using unconventional spanwise wall oscillations parameters to preferentially enhance convective heat transfer while minimizing the associated drag penalty.

Experimental investigation on heat transfer enhancement of supercritical pressure aviation kerosene in tubular laminar flow by vibration

In advanced aero-engine thermal management systems, aviation kerosene serving as a coolant unavoidably works in a vibration environment. In this article, the laminar heat transfer performance of Chinese aviation kerosene RP-3 flowing through a horizontal micro-tube under various vibration conditions at supercritical pressures was investigated experimentally. The effects of several impact factors such as system pressure, heat flux, mass flux, inlet temperature, vibration acceleration, and vibration frequency on the heat transfer enhancement were explored in a systematic manner. Experimental results indicate that: (i) the vibration could lead to intense thermal and momentum mixing among different boundary layers of tubular laminar flow and significantly strengthens the heat transfer, and the higher Re can lead to a stronger enhancement effect. The maximum observed HTER across all experimental data is 2.8, occurring at x/d = 224.1 with the inlet temperature of 373 K; (ii) HTER hardly changes with system pressures, exhibiting a maximum relative deviation of 3.9 % at different pressures. Heat transfer enhancement has a strong dependency on heat flux, as the heat flux increases from 36 kW/m2 to 108 kW/m2, the average HTC increased by up to 36.4 %; (iii) the HTC and HTER monotonically rise with increasing vibration acceleration. Peak values in HTC and HTER are observed at vibration frequencies of 625 Hz, 191 Hz, and 242 Hz; (iv) vibration has little impact on the thermal acceleration but noticeably weakens the buoyancy close to the outlet area at high heat flux. Two well-predicted correlations for the Nu in tubular laminar flow, one with vibration and one without, are proposed.

Global climate modeling of the Jupiter troposphere and effect of dry and moist convection on jets

Aims. The atmosphere of Jupiter is characterized by banded jets, including an equatorial super-rotating jet, by an intense moist con-vective activity, and by perturbations exerted by vortices, waves, and turbulence. Even after space exploration missions to Jupiter and detailed numerical modeling of Jupiter, questions remain about the mechanisms underlying the banded jets and the role played by dry and moist convection in maintaining these jets. Methods. We report three-dimensional simulations of the Jupiter weather layer using a global climate model (GCM) called Jupiter-DYNAMICO, which couples hydrodynamical integrations on an icosahedral grid with detailed radiative transfer computations. We added a thermal plume model for Jupiter that emulates the effect of mixing of heat, momentum, and tracers by dry and moist convec-tive plumes that are left unresolved in the GCM mesh spacing with a physics-based approach. Results. Our Jupiter-DYNAMICO global climate simulations show that the large-scale Jovian flow, in particular the jet structure, could be highly sensitive to the water abundance in the troposphere and that an abundance threshold exists at which equatorial super-rotation develops. In contrast to our dry (or weakly moist) simulations, simulations that include the observed amount of tropospheric water exhibit a clear-cut super-rotating eastward jet at the equator and a dozen eastward mid-latitude jets that do not migrate poleward. The magnitudes agree with the observations. The convective activity simulated by our thermal plume model is weaker in the equatorial regions than in mid to high latitudes, as indicated by lightning observations. Regardless of whether they are dry or moist, our simulations exhibit the observed inverse energy cascade from small (eddies) to large scales (jets) in a zonostrophic regime.

Experimental study on three-dimensional blade designs in an aggressive compressor transition duct

The increasing performance requirements for aero-engines necessitate the integrated design of compressor transition ducts with upstream components to reduce the axial length of the engine. This paper presents an experimental investigation of three-dimensional (3D) blade designs within an aggressive compressor transition duct integrated with the upstream stator. The aim is to reduce the total pressure loss and decrease the radial distortion of the flow field at the outlet of the transition duct, thus providing better inflow conditions for the high-pressure compressor. Detailed measurements of the internal flow field were conducted under both near stall (NS) and design (DE) conditions for three 3D blades. The results showed that in the downward path, adopting the 3D blades with positive dihedral and forward sweep contributed to mitigating the stator hub corner stall under the NS condition. Under the same operating conditions, enhanced kinetic energy at the stator root improved the spanwise uniformity of the transition duct outlet profile, while also increasing the anti-separation capability of the downstream hub boundary layer. The corner vortex at the stator root could enhance momentum mixing between the downstream hub boundary layer and the mainstream, thereby providing stronger separation resistance for the hub boundary layer under the NS condition. Compared to the baseline stators, the optimized 3D stators reduced system total pressure loss by 24% under the NS condition, while maintaining essentially unchanged total pressure loss under the DE condition.

Axial friction coefficient of turbulent spiral Poiseuille flows

Direct numerical simulations of spiral Poiseuille flows in a narrow gap geometry are performed with the aim of identifying the mechanisms governing the dynamics of the axial friction coefficient. The investigation has explored a small portion of the Reynolds number–Taylor number phase space ( $600 \leq Re \leq 5766$ and $1500 \leq Ta \leq 5000$ ), for which reference experimental results are available. The study is focused on the mechanism leading to the enhancement of the axial friction coefficient with the Taylor number when the Reynolds number is kept constant. The analysis of the spatial distribution of the Reynolds stress tensor and of the turbulent energy budget has evidenced the key role of the pressure–strain correlation in the energy transfer from the azimuthal to the axial component. The latter eventually determines the increase of the axial friction coefficient through the enhanced radial mixing of axial momentum. Data have also shown that the flow dynamics is heavily dependent on the $Ta/Re$ ratio, and different regimes develop (ranging from laminar to turbulent), each with peculiar behaviours.

Atmospheric Fronts Shaping the (Sub)Mesoscale SST‐Wind Coupling Over the Southern Ocean: Observational Case

AbstractSurface wind divergence is largely modulated by the sea surface temperature (SST) gradient through vertical momentum mixing and pressure adjustment. Here, the two mechanisms affecting the coupling strength between SST gradient and surface wind divergence are examined during an atmospheric front passage in the Southern Ocean. This event is also recorded by an uncrewed surface vehicle (USV). The reanalysis product (ERA5) revealed that downward momentum mixing is the dominant mechanism on the daily time scale. The coupling strength during the day when the atmospheric front passed over declined by 75%, compared to the adjacent days. This implies that the atmospheric front can partially attenuate the SST gradient effect on the surface wind divergence. Furthermore, a decade‐long statistic also showed a decreasing trend of SST‐wind coupling when the atmospheric fronts occur more. Additionally, after removing the mesoscale weather variation, the USV observations showed a remarkable SST imprint on the atmospheric boundary layer in the oceanic submesoscale regime, which denotes the scale below the deformation radius (∼16 km). The submesoscale air‐sea interaction processes also displayed decreased air‐sea coupling strength during atmospheric front passage. This is possible as the vertical velocity induced by the atmospheric front can compensate for the daily averaged uprising vertical velocity due to surface wind convergence. This analysis indicates that the atmospheric front can diminish the coupling between the SST gradient and surface wind divergence, which contrasts the existing statistical results showing that atmospheric fronts tend to enhance such coupling.

Why does tropical cyclone intensify faster under weaker parameterized horizontal diffusion in three-dimensional numerical simulations?

The mechanisms by which the parameterized horizontal diffusion impacts the tropical cyclone (TC) intensification are investigated via a series of numerical simulations of Supertyphoon Lekima (2019) with different horizontal mixing lengths (Lh). Consistent with previous studies, the TC intensifies faster and attains a higher peak intensity as Lh decreases, which is attributed to a more compact wind structure and the stronger eyewall convective heating. The azimuthal-mean tangential wind and vertical momentum budgets are conducted to explain the differences in TC size and convective strength between the simulations. It is found that when horizontal diffusion weakens, the eddy mixing of tangential momentum becomes larger inside the radius of maximum wind, therefore promoting size contraction. The stronger eddy mixing is attributed to the more active eyewall mesovortices whose growth is suppressed when horizontal diffusion strengthens. The vertical momentum budget results indicate that the air parcels in the simulation with smaller Lh experience stronger upward buoyancy force in the lower-to-mid troposphere (3–7 km) than those in the larger-Lh simulation, leading to more vigorous eyewall convection. It is further revealed that the reduced horizontal diffusion helps maintain the strength of virtual potential temperature perturbations and therefore the buoyancy in the eyewall updrafts. The findings from this study imply that the horizontal diffusion plays an important role in regulating the mesoscale and convective-scale processes in the inner-core region and improvements in its parameterization are urged for improving TC intensity forecast.

Oceanic Frontal Turbulence

Abstract Upper-ocean turbulence results from a complex set of interactions between submesoscale turbulence and local boundary layer processes. The interaction between larger-scale currents and turbulent fluctuations is two-way: large-scale shearing motions generate turbulence, and the resulting coherent turbulent fluxes of momentum and buoyancy feed back onto the larger flow. Here we examine the evolution and role of turbulence in the intensification, instability, arrest, and decay (i.e., the life cycle) of a dense filament undergoing frontogenesis in the upper-ocean boundary layer, i.e., cold filament frontogenesis (CFF). This phenomenon is examined in large-eddy simulations (LES) with resolved turbulent motions in large horizontal domains using 109 grid points. The boundary layer turbulence is generated by surface buoyancy loss (cooling flux) and is allowed to freely interact with an initially imposed cold filament, and the evolution is followed through the frontal life cycle. Two control parameters are explored: the initial frontal strength M2 = ∂xb and the surface flux . The former is more consequent: initially weaker fronts sharpen more slowly and become arrested at a later time with a larger width. This reflects a competition between the frontogenetic rate induced by the secondary circulation associated with vertical momentum mixing by the turbulence and the instability rate for the along-filament shear flow. The frontal turbulence is energized by the shear production of the latter, is nonlocally transported away from the primary production zone at the filament centerline, and cascades to dissipation in a broad region surrounding the filament. The turbulent momentum fluxes arresting the frontogenesis are supported across a wide range of horizontal scales.

On the Role of Wind–Evaporation–SST Feedbacks in the Subseasonal Variability of the East Pacific ITCZ

Abstract The intertropical convergence zone (ITCZ) is a zonally elongated band of near-surface convergence and precipitation near the equator. During boreal spring, the eastern Pacific ITCZ migrates latitudinally on daily to subseasonal time scales, and climate models exhibit the greatest ITCZ biases during this time of the year. In this work, we investigate the air–sea interactions associated with the variability in the eastern Pacific ITCZ’s latitudinal location for consecutive days when the ITCZ is only located north of the equator (nITCZ events) compared to when the ITCZ is on both sides of the equator or south of the equator (dsITCZ events) during February–April. The distribution of sea surface temperature (SST) anomalies and surface latent heat flux (SLHF) anomalies during the nITCZ and dsITCZ events follow the classic wind–evaporation–SST (WES) positive feedback mechanism. However, an alternative mechanism, embracing the effect of SST anomalies on vertical stratification and momentum mixing, gives rise to a negative WES feedback. Our results show that in the surface layer, there is a general progression of positive WES feedbacks happening in the weeks leading to the events followed by negative WES feedbacks occurring after the ITCZ events, with an alternate mechanism involving air–sea humidity differences limiting evaporation occurring in between. Additionally, the spatial structures of the components of the feedbacks are nearly mirror images for these opposite ITCZ events over the east Pacific during boreal spring. In closing, we find that understanding the air–sea interactions during daily to weekly varying ITCZ events (nITCZ and dsITCZ) helps to pinpoint how fundamental processes differ for ITCZs in different hemispheres.

Numerical study on the periodic control of supersonic compression corner flow using a nanosecond pulsed plasma actuator

This study investigates the effectiveness of a pulsed nanosecond dielectric barrier discharge (NSDBD) plasma actuator for flow control over a supersonic compression corner through numerical simulations. The effects of varying applied voltages, repetitive frequencies, and activated locations of the plasma actuator are examined under large-scale laminar flow separation conditions around a compression corner. The unit Reynolds number and Mach number are 7.8 × 106 m−1 and 4, respectively. The results indicate that the discharge induces a pressure rise and leads to misalignment between the pressure gradient and density gradient in the residual heat region. Because of the interaction between the supersonic freestream and the actuation-induced shock/compression flow, convection, compressibility of the fluid element, and baroclinicity of the residual heat region collectively lead to the formation of an induced spanwise vortex, which in turn enables momentum migration. The induced vortex disrupts the initial flow structures and entrains high-energy fluid from the main flow into the boundary layer, promoting momentum mixing between the main flow and the separated flow, which increases the energy of the boundary layer to resist the adverse pressure gradient. The time-averaged flow structures imply that it is possible to totally eliminate the flow separation near the supersonic compression corner. For aerodynamics on the surface, the normal force produces a pitching moment that can be potentially utilized to control the body's orientation and trajectory. Additionally, total drag on the surface can be reduced by 5 %. This suggests that choosing the most appropriate position based on its local fluid characteristics can strongly increase the control effectiveness.

Two-dimensional turbulent thermal classical far wake

The two-dimensional turbulent thermal classical far wake is investigated. The turbulence is described by the Boussinesq hypothesis for the Reynolds stresses with Prandtl’s mixing length model for the eddy viscosity and eddy thermal conductivity. Two conservation laws are derived for the thermal boundary layer equations for the far wake using the multiplier method and two conserved quantities are obtained from the conserved vectors and boundary conditions. The Lie point symmetry associated with the momentum and thermal conserved vectors is derived. It is found that the momentum and thermal mixing lengths are proportional. In obtaining the invariant solution the cases ν≠0, κ≠0 and ν=0, κ=0 needed to be treated separately, where ν is the kinematic viscosity and κ is the thermal conductivity of the fluid. When ν≠0, κ≠0 the associated Lie point symmetry is determined in full but an analytical solution of the reduced ordinary differential equations could not be derived. The invariant solution is obtained numerically using a shooting method with the two conserved quantities as targets. The width of the wake is infinite. When ν=0, κ=0 the width of the wake is finite. In order to obtain the associated Lie point symmetry in full, Prandtl’s hypothesis that the mixing length is proportional to the width of the wake was imposed. The invariant solution is obtained analytically. The lines of constant mean temperature difference and the streamlines for the mean velocity deficit are plotted. The numerical and analytical solutions agree in the limit ν→0, κ→0.

Hydrodynamic roughness induced by a multiscale topography

Turbulent flows above a solid surface are characterised by a hydrodynamic roughness that represents, for the far velocity field, the typical length scale at which momentum mixing occurs close to the surface. Here, we are theoretically interested in the hydrodynamic roughness induced by a two-dimensional modulated surface, the elevation profile of which is decomposed in Fourier modes. We describe the flow for a sinusoidal mode of given wavelength and amplitude with Reynolds-averaged Navier–Stokes equations closed by means of a mixing-length approach that takes into account a possible surface geometrical roughness as well as the presence of a viscous sublayer. It also incorporates spatial transient effects at the laminar–turbulent transition. Performing a weekly nonlinear expansion in the bedform aspect ratio, we predict the effective hydrodynamic roughness when the surface wavelength is varied and we show that it presents a non-monotonic behaviour at the laminar–turbulent transition when the surface is hydrodynamically smooth. Further, with a self-consistent looped calculation, we are able to recover the smooth–rough transition of a flat surface, for which the hydrodynamic roughness changes from a regime where it is dominated by the viscous length to another one where it scales with the surface corrugation. We finally apply the results to natural patterns resulting from hydrodynamic instabilities such as those associated with dissolution or sediment transport. We discuss in particular the aspect ratio selection of dissolution bedforms and roughness hierarchy in superimposed ripples and dunes.

Improving the Intensity Forecast of Tropical Cyclones in the Hurricane Analysis and Forecast System

Abstract A modification to the mixing length formulation in a planetary boundary layer (PBL) scheme is introduced to improve the intensity forecast of tropical cyclones (TCs) in a basin-scale Hurricane Analysis and Forecast System (HAFS) for the real-time experiment in 2021. The 2020 basin-scale HAFS with the physics suite of the NCEP operational Global Forecast System performs well in terms of the reduced root-mean-square (RMS) errors in track and intensity except for the mean intensity bias, compared with NCEP operational hurricane models. To address the large intensity bias issue, the vertical mixing length near the surface used in the PBL scheme is increased to follow the similarity theory, consistent with that used in the surface layer scheme. Test results show that the RMS error and bias in intensity are further reduced without the degradation of the track forecast. An idealized one-dimensional TC PBL model is used to understand the model response to the modification, indicating that the radial wind is strengthened to dynamically balance the enhanced downward momentum mixing. This is also exhibited in the case study of a three-dimensional HAFS simulation, with the improved vertical distribution of the simulated wind speed in the eyewall area. Given the improvement, the modification has been implemented in one of the configurations of the first version of the operational HAFS at NCEP. Finally, the adjustment of the parameterization of diffusion and mixing in TC simulations is discussed. Significance Statement A modification to the mixing length formulation in a PBL scheme is described, which improves the intensity forecast of tropical cyclones simulated in the Hurricane Analysis and Forecast System (HAFS). Retrospective tests indicate that the modification can reduce the root-mean-square error and bias of the simulated TC intensity by 5%–10% and 50%, respectively. This modification has been implemented in one of the operational configurations of HAFS, version 1, at NCEP, improving the hurricane model guidance.

On the role of low frequency modes in the thermal and momentum mixing in parallel plane jets

Parallel plane jet mixing has been the subject of extensive study and its physics are characterized well in literature. However, the physics of parallel plane jet mixing in relatively small confined regions is not well known. In this study we investigate the effects of re-circulatory flow on thermal and momentum mixing in confined parallel plane jets. Studying confined jet interaction is important for a variety of engineering applications. One of these applications is sodium cooled fast reactors (SFR). It is common in SFRs to have coolant stream mixing in a confined domain, often the upper plenum. The Reactor Cavity Cooling System (RCCS) facility at the University of Michigan models this problem in a simplified enclosure with isothermal parallel plane jets as the incoming coolant streams. A Large Eddy Simulation (LES) of this simplified plenum was developed using optimal boundary conditions, proven mesh convergence, and consistent resolution. This model was then validated against experimental results from the RCCS facility. After model validation, flow structures in the jet interaction were discerned by means of Proper Orthogonal Decomposition (POD). Comparison of 3D POD to a clipped 2D POD region reveals that re-circulatory flow can lead to dominant oscillations within the jet mixing region. Subsequently, Power Spectrum Density (PSD) and Continuous Wavelet Transforms (CWT) were used to ascertain frequency-based phenomena that might occur in this mixing region. Frequency analysis uncovered a dominant low-frequency oscillation in both velocity and temperature PSDs. These frequencies fall within the known ranges of thermal striping in sodium. The data generated in this study can be used for the benchmarking of less computationally intensive approaches. This data will be made available upon request to ebm5351@psu.edu.

Two-way Eulerian-Lagrangian coupling approach in GASFLOW code for containment spray modelling

Spray droplets are considered to have significant effects on the gas mixing and depressurization in the containment of nuclear power plants during hypothetical accident scenarios. The Eulerian-Lagrangian approach, which enables the two-way mass, momentum, and energy coupling of the continuous gas and the dispersed spray droplets, is promising to model the behaviours of spray droplets in 3D simulations of the full-scale containment. Two-way heat and mass transfer models have been developed in the 3D CFD code GASFLOW. The particle group method is applied in the Lagrangian approach to track the droplets. The ordinary differential equations which couple the local heat and mass exchanges between each droplet group and the surrounding gas mixtures have been solved using the Runge-Kutta method. The GASFLOW coupling this new approach is validated by TOSQAN water spray experiments, which demonstrate the good feasibility of implementing the approach. The analysis of the impact of the spray on containment atmosphere indicates that: 1) the droplet swarm entrains and mixes the surrounding gas, breaking up the gas stratification, with turbulent diffusion dominating momentum mixing in dead zones; 2) the thermal and mass exchanges between the spray droplet swarm and gas jointly determine the atmosphere depressurization, with the evaporation of the spray-formed film/sump on heat structures of great importance for the containment pressure development; and 3) these spray thermodynamic and dynamic effects highly depend on the droplet size distribution and the spray shape.