Production Of Turbulent Kinetic Energy Research Articles

Turbulent Wake-Flow Characteristics in the Near Wake of Freely Flying Raptors: A Comparative Analysis Between an Owl and a Hawk

Owl flight has been studied over multiple decades associated with bio-inspiration for silent flight. However, their aerodynamics has been less researched. The aerodynamic noise generated during flight depends on the turbulent state of the flow. In order to document the turbulent characteristics of the owl during flapping flight, we measured the wake flow behind a freely flying great horned owl (Bubo virginianus). For comparison purposes, we chose to fly a similar-sized raptor a Harris's hawk (Parabuteo unicinctus): one is nocturnal and the other is a diurnal bird of prey. Here, we focus on the wake turbulent aspects and their impact on the birds' flight performances. The birds were trained to fly inside a large-scale wind tunnel in a perch-to-perch flight mode. The near wake of the freely flying birds was characterized using a long duration time-resolved particle image velocimetry system. The velocity fields in the near wake were acquired simultaneously with the birds' motion during flight which was sampled using multiple high-speed cameras. The turbulent momentum fluxes, turbulent kinetic energy production, and dissipation profiles are examined in the wake and compared. The near wake of the owl exhibited significantly higher turbulent activity than the hawk in all cases, though both birds are similar in size and followed similar flight behavior. It is suggested that owls modulate the turbulence activity of the near wake in the vicinity of the wing, resulting in rapid decay before radiating into the far-field; thus, suppressing the aerodynamic noise at the far wake.

Large Eddy Simulation of Microbubble Drag Reduction in Fully Developed Turbulent Boundary Layers

Microbubble drag reduction has good application prospects. It operates by injecting a large number of bubbles with tiny diameters into a turbulent boundary layer. However, its mechanism is not yet fully understood. In this paper, the mechanisms of microbubble drag reduction in a fully developed turbulent boundary layer over a flat-plate is investigated using a two-way coupled Euler-Lagrange approach based on large eddy simulation. The results show good agreement with theoretical values in the velocity distribution and the distribution of fluctuation intensities. As the results show, the presence of bubbles reduces the frequency of bursts associated with the sweep events from 637.8 Hz to 611.2 Hz, indicating that the sweep events, namely the impacting of high-speed fluids on the wall surface, are suppressed and the streamwise velocity near the wall is decreased, hence reducing the velocity gradient at the wall and consequently lessening the skin friction. The suppression on burst frequency also, with the fluid fluctuation reduced in degree, decreases the intensity of vortices near the wall, leading to reduced production of turbulent kinetic energy.

Numerical investigation of spatial-developing turbulent heat transfer in forced convections at different supercritical pressures

Direct numerical simulations have been adopted to study the turbulent heat transfer in forced convections of supercritical water at two different supercritical pressures P=23 MPa and P=25 MPa in a heated pipe with constant wall heat flux and a bulk Reynolds number of Re0=5400. The present study aims to reveal the mechanisms of turbulent heat transfer of superciritical fluids at different pressures in a spatial-developing flow. The results show that at the smaller pressure ratio Pr=P/Pc, where Pc is the critical pressure, the property variations become more drastic, and both the skin friction coefficient and Nusselt number become smaller. The decompositions of skin friction and Nusselt number show that it is mainly due to the large turbulence reduction along the streamwise direction. The analyses of turbulent kinetic energy (TKE), the turbulent shear stress, and the production of TKE confirm this point. Moreover, it was found that the thermophysical property fluctuations are very large and significantly influence the turbulent statistics in the supercritical fluid flows. Due to the large property fluctuations, it was found that the density-fluctuation-related terms are significant and their values are actually comparable to the mean-density-related terms. Due to their negative contributions to turbulent shear stress and turbulent heat flux, the turbulence and heat transfer are severely attenuated by the large thermophysical property fluctuations. For near-wall scaling in spatial-developing flows at supercritical pressures, the semi-local velocity transformation with a semi-local coordinate shows a better agreement in the logarithmic region. However, a clear deviation still exists, especially for mean temperature because all the transformations only incorporate the local mean property variations and cannot consider their fluctuations.

On sustaining turbulence production in interacting supersonic streamwise vortices

Interacting supersonic streamwise vortices have been investigated as a means to enhance mixing at supersonic velocities. However, the turbulence dynamics associated with this class of flow has been left largely unexplored despite turbulence playing a major role in the achievement of molecular mixing. For this reason, the process of turbulent kinetic energy production in interacting supersonic streamwise vortices is investigated in this work. Specifically, the link between the mean flow motion and turbulence, represented by the production term of the turbulent transport equation, is analyzed. An approach is proposed in which the mean flow strain rates can be cast such that their intensity and morphology properly couple with the requirements imposed by the Reynolds stresses that, in the framework of this work, are shown to be intimately linked with the resulting plume morphology. The analysis is leveraged for the design of a mode of vortex interaction targeted to maintain a positive production of turbulent kinetic energy in the downstream evolution. The resulting configuration was experimentally investigated in a Mach 2.5 flow by stereoscopic particle image velocimetry. The results from these experiments show that the turbulence production remained positive and sustained turbulence levels, a unique result in the available literature. The role of turbulence anisotropy and the principal direction of negative planar strain rates are highlighted in detail in this work.

Comparison of the Atmospheric and Oceanic Boundary Layers During Convection and their Latitudinal Dependence

AbstractThe boundary layers of the atmosphere and the ocean are compared during convection, and their latitudinal dependence is investigated. The results are applied to examine the parameterization of the boundary layer depth in the K‐profile parameterization (KPP) model. The bulk Richardson number Rib varies excessively with time in the high‐latitude (HL) oceanic boundary layer (OBL) without unresolved shear , as a result of inertial oscillation. Inclusion of is also necessary in the atmospheric boundary layer (ABL) to mitigate the large variation of Rib with wind stress. Stratification and velocity shear near the boundary layer height/depth are stronger and thicker at low latitudes (LLs), where the Ekman length scale is larger and the inertial time scale is longer. This enhanced shear makes the entrainment buoyancy flux larger at LL. Analysis of the turbulent kinetic energy (TKE) budget in the entrainment zone is carried out to explain the variation of Rib depending on the boundary layer and the latitude. This analysis shows that the TKE production in the entrainment zone is dominated by shear production at LL, but by the TKE flux at HL, and that represents the contribution from the TKE flux to the entrainment zone. The enhancement of vertical TKE by Langmuir circulation (LC) does not depend on the latitude, but it decreases with depth faster at LL. The result suggests that the parameterization of the boundary layer depth in the KPP model should be different depending on whether it is the ABL or the OBL and depending on the latitude.

A model to predict surface gas transfer rate in streams based on turbulence production by aquatic vegetation

Turbulence generated by aquatic vegetation in lakes, estuaries, and rivers can significantly alter the flow structure throughout the entire water column, affecting gas transfer mechanisms at the air-water interface, thus modifying indicators of water quality. A series of laboratory experiments with rigid, acrylic cylinder arrays to mimic vegetation was conducted in a recirculating Odell-Kovasznay type race-track flume. Particle Image Velocimetry was used to characterize mean and turbulent flow statistics, to investigate the effect of emergent and submerged vegetation on gas transfer rate in terms of turbulent kinetic energy (TKE), Reynolds stresses, and TKE production. Surface gas transfer rates were determined by measuring dissolved oxygen concentration during re-aeration using an optical sensor. The results provided new insights on how stem- and canopy-scale turbulence affect the surface gas transfer rate at different submergence ratios and array densities. The relation between mean flow velocity and TKE production in each scenario is discussed, and a modified surface renewal model using TKE production as an indicator of gas transfer efficiency is developed to more accurately predict surface gas transfer rates in vegetated streams.

Modelling of Stably Stratified Atmospheric Boundary Layers with Varying Stratifications

A recently developed explicit algebraic Reynolds-stress (EARS) model is validated for an idealized representation of the night-time high-latitude stably stratified atmospheric boundary layer. The simulations are made with four surface cooling rates that result in weakly to moderately stratified stable boundary layers. The predictions of the EARS model are compared to high-resolution large-eddy simulations (LES) of Sullivan et al. (J Atmos Sci 73(4):1815–1840, 2016). First- and second-order statistics are shown to be well predicted by the EARS model. The EARS model also predicts the horizontal turbulent fluxes and turbulence anisotropy and these compare well with the LES results. The sensitivity to the model coefficients is studied by comparing the EARS model results with LES results. Finally, we propose a new scaling for the production of turbulence kinetic energy and show that the EARS model captures the essential trends of the LES results for different cooling rates.

Transitional and turbulent flows in rectangular ducts: budgets and projection in principal mean strain axes

We carry out Direct Numerical Simulation (DNS) of flows in closed rectangular ducts with several aspect ratios. The Navier–Stokes equations are discretised through a second-order finite difference scheme, with non-uniform grids in two directions. The duct cross-sectional area is maintained constant as well as the flow rate, which allows to investigate which is the appropriate length scale in the Reynolds number for a good scaling in the laminar and in the fully turbulent regimes. We find that the Reynolds number based on the half length of the short side leads to a critical Reynolds number which is independent on the aspect ratio (), for ducts with . The mean and rms wall-normal velocity profiles are found to scale with the local value of the friction velocity. At high friction Reynolds numbers, the Reynolds number dependence is similar to that in turbulent plane channels, hence flows in rectangular ducts allow to investigate the Reynolds number dependency through a reduced number of simulations. At low Re, the profiles of the statistics differ from those in the two-dimensional channel due to the interaction of flow structures of different size. The projection of the velocity vector and of the Reynolds stress tensor along the eigenvectors of the strain-rate tensor yields reduced Reynolds stress anisotropy and simple turbulence kinetic energy budgets. We further show that the isotropic rate of dissipation is more difficult to model than the full dissipation rate, whose distribution does not largely differ from that of turbulence kinetic energy production. We expect that this information may be exploited for the development of advanced RANS models for complex flows.

Turbulence statistics in a negatively buoyant multiphase plume

We investigate the turbulence statistics in a {multiphase plume made of heavy particles (particle Reynolds number at terminal velocity is 450)}. Using refractive-index-matched stereoscopic particle image velocimetry, we measure the locations of particles {whose buoyancy drives the formation of a multiphase plume,} {together with the local velocity of the induced flow in the ambient salt-water}. {Measurements in the plume centerplane exhibit self-similarity in mean flow characteristics consistent with classic integral plume theories.} The turbulence characteristics resemble those measured in a bubble plume, {including strong anisotropy in the normal Reynolds stresses. However, we observe structural differences between the two multiphase plumes. First, the skewness of the probability density function (PDF) of the axial velocity fluctuations is not that which would be predicted by simply reversing the direction of a bubble plume. Second, in contrast to a bubble plume, the particle plume has a non-negligible fluid-shear production term in the turbulent kinetic energy (TKE) budget. Third, the radial decay of all measured terms in the TKE budget is slower than those in a bubble plume.} Despite these dissimilarities, a bigger picture emerges that applies to both flows. The TKE production by particles (or bubbles) roughly balances the viscous dissipation, except near the plume centerline. The one-dimensional power-spectra of the velocity fluctuations show a -3 power-law that puts both the particle and bubble plume in a category different from single-phase shear-flow turbulence.

Assessment of Regional Climate Model Simulations of the Katabatic Boundary Layer Structure over Greenland

The parameterization of the boundary layer is a challenge for regional climate models of the Arctic. In particular, the stable boundary layer (SBL) over Greenland, being the main driver for substantial katabatic winds over the slopes, is simulated differently by different regional climate models or using different parameterizations of the same model. However, verification data sets with high-resolution profiles of the katabatic wind are rare. In the present paper, detailed aircraft measurements of profiles in the katabatic wind and automatic weather station data during the experiment KABEG (Katabatic wind and boundary-layer front experiment around Greenland) in April and May 1997 are used for the verification of the regional climate model COSMO-CLM (CCLM) nested in ERA-Interim reanalyses. CCLM is used in a forecast mode for the whole Arctic with 15 km resolution and is run in the standard configuration of SBL parameterization and with modified SBL parameterization. In the modified version, turbulent kinetic energy (TKE) production and the transfer coefficients for turbulent fluxes in the SBL are reduced, leading to higher stability of the SBL. This leads to a more realistic representation of the daily temperature cycle and of the SBL structure in terms of temperature and wind profiles for the lowest 200 m.

Effects of aspect ratio on higher-order moments, conditional statistics, TKE budget and anisotropy in narrow open channel flow

The purpose of the present study is to investigate the influence of aspect ratio on the higher- order statistics of velocity fluctuations, conditional statistics, turbulent kinetic energy (TKE) fluxes, TKE budget, and Reynolds stress anisotropy in hydraulically rough narrow open channel flow (OCF). In the experiments, the aspect ratios were maintained low and varied between 2.5 and 4. The velocities were measured with an acoustic Doppler velocimeter (ADV). The higher-order moments, quadrant analysis, and TKE budget from the present experiments were compared with available literature data in wide and narrow OCFs to elucidate the effect of the aspect ratio. In this analysis, the third-order moments of velocity fluctuations were found to be sensitive to the aspect ratio in the outer region. The fractional contributions of all quadrant events are approximately equal in magnitude in lower aspect ratio flows, whereas ejections and sweeps are the dominant events as the aspect ratio increases. The upward transfer of TKE flux increases in the outer layer with increase in aspect ratio. In the inner layer, the TKE production and rate of dissipation are found to be increasing with decreasing aspect ratio. Therefore, the production and dissipation of TKE are dependent on the aspect ratio of the flow. The analysis of Reynolds stress AIM reveals that for low aspect ratio flows turbulence tends to attain rod-like axisymmetric turbulence only in the intermediate layer whereas for higher aspect ratio, turbulence attains rod-like axisymmetric turbulence throughout the depth.

Mitigation of heat transfer deterioration in a circular tube with supercritical CO2 using a novel small-scale multiple vortex generator

Abstracts This paper explores the performance effects of a novel small-scale multiple vortex generators (MVG) on heat transfer deterioration (HTD) mitigation and pressure drop using supercritical CO2 (sCO2) as the working fluid. The HTD mitigation and pressure drop are examined using an overall performance factor, R = ΔHTD/(Δp/Δp0). Numerical calculations are first conducted to investigate the effects of single conventional vortex generator's (VG) configurations (rectangular, triangular and trapezoidal). The results indicate that under the same aspect ratio, rectangular VG has the largest area facing the fluid flow and the largest angle of attack which induces the strongest longitudinal vortices leading to its highest value of R. Then, the morphological properties (density and height) of five pairs of evenly distributed rectangular VGs in an MVG are varied while their uniformity is maintained. With the increase in MVG's height, the channel's heat transfer coefficient (HTC) profiles increase significantly at the locations where MVG is installed while the opposite trend is found for R. The density and nonuniformity of the MVG have the highest performance effects on HTD mitigation, with R of more than 18% and 11% for the density of 2.5 and non-uniform MVG respectively. Analysis reveals that as the density of MVG increases, the fluid recirculation zone behind each VG diminishes correspondingly, causing a greater vortex intensity mixing by swirling flows which enhances the downstream production of turbulence kinetic energy (TKE), thus weakening the buoyancy forces and leading to HTD mitigations. In addition, by further increasing the MVG density beyond 2.5, it was found that the HTC decreases while the pressure drop increases, with the wall temperature peaks stabilizing at 178 °C, which demonstrates the breakdown of Reynold's analogy. Increasing the number of VGs in an MVG increases its delay and broadening effects on the wall temperature peaks by further delaying the boundary layer recovery caused by interaction of longitudinal vortices generated by each VG, but has relatively little effects on R. Also, analysis further reveals the presence of buoyancy-induced flow oscillations near the channel walls, which are attenuated by the VG and MVG effects. Overall, MVG offers superior performance than convectional single VGs in terms of HTD mitigation, and the results presented here can be employed as a reference guide in the design of highly efficient, safe and reliable supercritical heat exchanger systems.

A Review on Hydrodynamics of Free Surface Flows in Emergent Vegetated Channels

This review paper addresses the structure of the mean flow and key turbulence quantities in free-surface flows with emergent vegetation. Emergent vegetation in open channel flow affects turbulence, flow patterns, flow resistance, sediment transport, and morphological changes. The last 15 years have witnessed significant advances in field, laboratory, and numerical investigations of turbulent flows within reaches of different types of emergent vegetation, such as rigid stems, flexible stems, with foliage or without foliage, and combinations of these. The influence of stem diameter, volume fraction, frontal area of stems, staggered and non-staggered arrangements of stems, and arrangement of stems in patches on mean flow and turbulence has been quantified in different research contexts using different instrumentation and numerical strategies. In this paper, a summary of key findings on emergent vegetation flows is offered, with particular emphasis on: (1) vertical structure of flow field, (2) velocity distribution, 2nd order moments, and distribution of turbulent kinetic energy (TKE) in horizontal plane, (3) horizontal structures which includes wake and shear flows and, (4) drag effect of emergent vegetation on the flow. It can be concluded that the drag coefficient of an emergent vegetation patch is proportional to the solid volume fraction and average drag of an individual vegetation stem is a linear function of the stem Reynolds number. The distribution of TKE in a horizontal plane demonstrates that the production of TKE is mostly associated with vortex shedding from individual stems. Production and dissipation of TKE are not in equilibrium, resulting in strong fluxes of TKE directed outward the near wake of each stem. In addition to Kelvin–Helmholtz and von Kármán vortices, the ejections and sweeps have profound influence on sediment dynamics in the emergent vegetated flows.

Novel free surface boundary conditions for spilling breaking waves

The breaking of gravity water waves induces both strong turbulence near the free surface and air–water mixing, which are not captured in currently-available single-phase, non-hydrostatic Reynolds Averaged Navier Stokes (RANS) models. In order to account for such dynamics, the boundary conditions proposed by Brocchini (2002) have been implemented in a two-dimensional vertical (2DV) non-hydrostatic RANS numerical model. For the theoretical boundary conditions of Brocchini (2002) to be applicable in the numerical model, appropriate closures have been adopted and mathematical equations have been proposed to calculate the coefficients introduced in the mentioned boundary conditions. Navier Stokes equations along with different turbulence closure models have been solved using finite volume method and pressure correction technique. According to the new boundary conditions, the normal-to-mean surface gradient of the turbulent kinetic energy (TKE) differs from zero and is computed as a balance between production and dissipation of TKE within the air–water mixing layer. Also, the kinematic and dynamic boundary conditions have been modified accordingly, to account for the effects of the thin two-phase layer formed at the free surface. The modified kinematic boundary condition allows for the mass exchange between the two-phase layer and the main body of the water. For the first time it is demonstrated that the proposed simple analytical model of Brocchini (2002) leads to improvements in the prediction of incipient breaking and the wave characteristics in the surf zone. This opens the way to a new, accurate but numerically low cost, approach for the computation of the air–water mixing that characterizes breaking waves. All the many benchmarking tests run to verify the ability of the new model show that significant improvements are achieved. Improvements are specifically observed in the prediction of: (I) breaking point and the breaking wave height, (II) the consequent dissipation of the wave energy observed in the form of the crest level distribution throughout the surf zone, (III) the magnitude of the horizontal velocity near the free surface, and (IV) the TKE distribution in depth. Notably, the novel numerical model does not use any parametric criteria for detecting the inception of breaking, thus it does not need calibration for different flow circumstances. Also, since no special treatments, such as hydrostatic pressure assumptions at the front face of the breaking waves are considered, the new model well captures the breaking-induced dissipation while giving a more accurate estimation of the dynamic pressure.

Instability analysis for a centrifugal pump with straight inlet pipe using partially averaged Navier–Stokes model

The current study numerically investigates the flow instability under several part-load conditions in a centrifugal pump with a straight inlet pipe to explore the underlying relationship between a positive slope phenomenon and internal flow using a partially averaged Navier–Stokes model. The model was validated by comparing the hydraulic performance and averaged flow in the impeller between the numerical results and experimental data of a tested pump. The internal flows in pumps have been intensively investigated based on Batchelor vortex family, Rayleigh–Taylor criterion, entropy generation rate, and energy equation to analyze the flow instability from different aspects. The simulation results using partially averaged Navier–Stokes model are acceptable due to the good agreement with the experimental data for the tested pump. No matter the geometry of the inlet pipe, the pre-swirling flows in the inlet pipe are in the convective instability region. Under the part-load condition of φ = 0.5 φbep, the axial vorticity coefficient is affected by the geometry of the inlet pipe. However, under the part-load condition with rotating stall, e.g. φ = 0.78 φbep, the flow in the inlet pipe is affected by the unstable flow in the pump impeller. For the pump with a straight inlet pipe, the vortex inside the blade-to-blade passage is in a stable state according to Rayleigh–Taylor criterion under the condition of φ = 0.5 φbep. However, the vortex in the blade-to-blade passage is in an unstable state due to centrifugal instability under those operation conditions with rotating stall cells in the impeller, and the dominant oscillations are dependent on the propagation of rotating stall cells. Finally, head loss analysis based on energy equations elucidates that turbulent kinetic energy production term is predominant in the head loss in pump impeller. The present results are helpful for better understanding of the unstable flows and positive slope phenomenon for centrifugal pumps.

Turbulence Adjustment and Scaling in an Offshore Convective Internal Boundary Layer: A CASPER Case Study

AbstractThe characteristics of a convective internal boundary layer (CIBL) documented offshore during the East Coast phase of the Coupled Air–Sea Processes and Electromagnetic Ducting Research (CASPER-EAST) field campaign has been examined using field observations, a coupled mesoscale model (i.e., Navy’s COAMPS) simulation, and a couple of surface-layer-resolving large-eddy simulations (LESs). The Lagrangian modeling approach has been adopted with the LES domain being advected from a cool and rough land surface to a warmer and smoother sea surface by the mean offshore winds in the CIBL. The surface fluxes from the LES control run are in reasonable agreement with field observations, and the general CIBL characteristics are consistent with previous studies. According to the LESs, in the nearshore adjustment zone (i.e., fetch < 8 km), the low-level winds and surface friction velocity increase rapidly, and the mean wind profile and vertical velocity skewness in the surface layer deviate substantially from the Monin–Obukhov similarity theory (MOST) scaling. Farther offshore, the nondimensional vertical wind shear and scalar gradients and higher-order moments are consistent with the MOST scaling. An elevated turbulent layer is present immediately below the CIBL top, associated with the vertical wind shear across the CIBL top inversion. Episodic shear instability events occur with a time scale between 10 and 30 min, leading to the formation of elevated maxima in turbulence kinetic energy and momentum fluxes. During these events, the turbulence kinetic energy production exceeds the dissipation, suggesting that the CIBL remains in nonequilibrium.

Experiments and Stress-Blended Eddy Simulations of turbulent flow over edge-rounded dimples

Experiments and numerical simulations have been conducted to reveal the detailed turbulent flow and heat transfer characteristics over the surfaces with spherical dimples with different edge schemes, including the sharp edge, the fully rounded edge and the upstream rounded edge. The dimples have the same depth-to-diameter ratio of hd/d = 0.2. Steady-state heat transfer experiments were carried out for the turbulent flow over the three dimpled surfaces within the Reynolds number range of 10,000 ≤ ReDh ≤ 60,000, and the globally averaged heat transfer enhancements and friction factor ratios were obtained. Furthermore, transient liquid crystal thermography technique was used to obtain the local heat transfer characteristics on the dimpled surfaces. In addition, numerical simulations were carried out with a newly developed hybrid RANS-LES method of Stress-Blended Eddy Simulation (SBES) and the local heat transfer enhancement, detailed flow structure and turbulence statistics data were obtained and analyzed. The experimental results show that the best thermal performance is achieved by the dimples with upstream rounded edge, indicating an increase of about 11.4% in total heat transfer enhancement and an increase of about 5.2% in pressure loss when compared to the conventional dimples with sharp edge. The numerical results show that the flow over the dimpled surfaces can be divided into five zones: the mainstream zone, the recirculation zone, the Kelvin–Helmholtz vortex zone, the upwash zone and the rear separation zone, and each zone has its unique flow features. It is also found that the lowest pressure is achieved by the dimple with fully rounded edge, which is due to the absence of flow separation behind the rear edge of the dimple. The upstream-edge-rounded dimples show the reduced recirculation flow in the recirculation zone, the enhanced impingement and upwash flow in the upwash zone near the rear edge. These cause the stronger production of turbulent kinetic energy behind the rear edge, which are responsible for the higher thermal performance.

Oceanic Mixing over the Northern Arabian Sea in a Warming Scenario: Tug of War between Wind and Buoyancy Forces

AbstractA consensus of the twenty-first-century climate change in the ocean is surface warming, stratification due to extreme freshening and subsequent weakening of mixing, overturning circulation, and biological production. Counterintuitively, certain parts of the tropical ocean may develop a resistance to changes in mixing, where the climate change impacts of atmosphere and ocean are complementary to each other. Under the poleward shift of monsoon low-level jet (LLJ) in the twenty-first century, a part of the northern Arabian Sea has a tendency to maintain the mixed layer depth intact. The process is studied using a set of high-resolution regional ocean model downscaling experiments for the present and future climate. It is found that the wind intensification caused by the shift in LLJ tends to counteract the stratification gained by surface ocean warming and maintains the mixing process in a warming scenario. The mixing energetics shed light on the way in which this is achieved. Intensified winds promote shear production and surface ocean warming demotes buoyancy production of turbulent kinetic energy (TKE), with a net effect of an increase in TKE. However, TKE appears to be dissipating quickly because of the presence of a larger number of small-scale eddies. This causes the mixing length and mixed layer depth to remain intact. Therefore, the interpretations of impacts of future climate change in ocean mixing should be viewed with caution, at least regionally, by focusing on the detailed changes of the governing mechanisms.

Effects of Rear Angle on the Turbulent Wake Flow between Two in-Line Ahmed Bodies

Understanding the wake characteristics between two in-line vehicles is essential for improving and developing new strategies for reducing in-cabin air pollution. In this study, Ahmed bodies are used to investigate the effects of the rear slant angle of a leading vehicle on the mean flow and turbulent statistics between two vehicles. The experiments were conducted with a particle image velocimetry at a fixed Reynolds number, R e H = 1.7 × 10 4 , and inter-vehicle spacing distance of 0.75 L , where H and L are the height and length of the model. The rear slant angles investigated were a reference square back, high-drag angle ( α = 25 ° ) and low-drag angle ( α = 35 ° ). The mean velocities, Reynolds stresses, production of turbulent kinetic energy and instantaneous swirling strength are used to provide physical insight into the wake dynamics between the two bodies. The results indicate that the recirculation region behind the square back Ahmed body increases while those behind the slant rear-end bodies decreases in the presence of a follower. For the square back models, the dominant motion in the wake region is a strong upwash of jet-like flow away from the road but increasing the rear slant angle induces a stronger downwash flow that suppresses the upwash and dominates the wake region.

Modified Formulation of Laminar Kinetic Energy Transition Models by Means of Elastic-Net of a Big Experimental Database of Separated Flows

New variants of the terms involved in the transport equations of Laminar Kinetic Energy (LKE) transition schemes are proposed. The model here proposed was tuned by means of a big experimental data set describing transitional separated flows. Data were acquired by means of time-resolved Particle Image Velocimetry (TR-PIV) measurements above a flat plate centered in a variable area opening channel. Tests were carried out for different free-stream turbulence intensities, Reynolds numbers and adverse pressure gradients. The large amount of test conditions provides the statistical response of the separated flow transition process to the parameter variation. For each condition, Proper Orthogonal Decomposition (POD) was applied to the ensemble of snapshots. Analysis of POD modes and spectra of related temporal coefficients allows us to define a scale separation criterion, and thus construct reduced order models of laminar and turbulent terms appearing in LKE schemes. Combinations of the POD modes of the two velocity components, their gradients and mixed terms involving velocity scales and mean flow strain rates are used to compute the targeting function vectors for the learning of the models. The role played by normal and shear stress-strain mechanisms in the different parts of the separated flow region was addressed for a fine tuning of both the laminar and the turbulent kinetic energy production terms. Additionally, a new definition of the energy transfer rate between the laminar and the turbulent scales is educated, using ingredients representing the work done by the finer scale on the larger one, inspired by the exact definition of the transfer rate appearing in the transport equations of coherent and incoherent fluctuations. An elastic-net technique was used to identify the best predictors, thus the model learning. The ability of the proposed model is verified by using it to predict results obtained in a different experimental database that did not participate to the education of the model.