Mean Temperature Gradient Research Articles

Flavonoid Synthesis Pathway Response to Low-Temperature Stress in a Desert Medicinal Plant, Agriophyllum Squarrosum (Sandrice).

Background/Objectives:Agriophyllum squarrosum (L.) Moq. (A. squarrosum), also known as sandrice, is an important medicinal plant widely distributed in dunes across all the deserts of China. Common garden trials have shown content variations in flavonoids among the ecotypes of sandrice, which correlated with temperature heterogeneity in situ. However, there have not been any environmental control experiments to further elucidate whether the accumulation of flavonoids was triggered by cold stress; Methods: This study conducted a four-day ambient 4 °C low-temperature treatment on three ecotypes along with an in situ annual mean temperature gradient (Dulan (DL), Aerxiang (AEX), and Dengkou (DK)); Results: Target metabolomics showed that 12 out of 14 flavonoids in sandrice were driven by cold stress. Among them, several flavonoids were significantly up-regulated, such as naringenin and naringenin chalcone in all three ecotypes; isorhamnetin, quercetin, dihydroquercetin, and kaempferol in DL and AEX; and astragalin in DK. They were accompanied by 19 structural genes of flavonoid synthesis and 33 transcription factors were markedly triggered by cold stress in sandrice. The upstream genes, AsqAEX006535-CHS, AsqAEX016074-C4H, and AsqAEX004011-4CL, were highly correlated with the enrichment of naringenin, which could be fine-tuned by AsqAEX015868-bHLH62, AsqAEX001711-MYB12, and AsqAEX002220-MYB1R1; Conclusions: This study sheds light on how desert plants like sandrice adapt to cold stress by relying on a unique flavonoid biosynthesis mechanism that regulating the accumulation of naringenin. It also supports the precise development of sandrice for the medicinal industry. Specifically, quercetin and isorhamnetin should be targeted for development in DL and AEX, while astragalin should be precisely developed in DK.

Assessment of climate change on river streamflow under different representative concentration pathways

Climate change and excessive greenhouse gas emissions profoundly impact hydrological cycles, particularly in arid and semi-arid regions, necessitating assessments of their effects on water resource management, agriculture, soil fertility, nutrient transport, hydropower generation, and flood risk. This study investigates climate change repercussions on streamflow in the Zarrineh River Basin, Iran, across three decadal intervals (2020–2029, 2055–2064, and 2090–2099) aiming to develop effective adaptation and mitigation strategies. Four General Circulation Models (GCMs), chosen based on distinct Representative Concentration Pathways (RCPs) determined by the annual mean temperature gradient, are employed. These models generate daily maximum (Tmax) and minimum (Tmin) temperatures along with precipitation data. Subsequently, these variables are integrated into the Soil and Water Assessment Tool (SWAT) model to analyze river flow alterations for each decadal timeframe. Comparison between future projections and observed climate data reveals a gradual decline in precipitation and Tmax, coupled with a substantial increase in Tmin. The average precipitation diminishes from 0.77 mm in the period 1985–1994 to a range of 0.42–0.28 mm in 2090–2099. The simulated flow at the basin outlet highlights that the GCM with the highest annual mean temperature gradient yields the lowest streamflow, while conversely, the model with the lowest gradient generates the highest. Consequently, streamflow experiences a decline from 52 m3/s in 1985–1994 to a range of 41–20 m3/s in 2090–2099.

The Impact of CO2 and Climate State on Whether Dansgaard–Oeschger Type Oscillations Occur in Climate Models

AbstractGreenland ice core records feature Dansgaard–Oeschger (D‐O) events, which are abrupt warming episodes followed by gradual cooling during ice age climate. The three climate models used in this study (CCSM4, MPI‐ESM, and HadCM3) show spontaneous self‐sustained D‐O‐like oscillations (albeit with differences in amplitude, duration, and shape) in a remarkably similar, narrow window of carbon dioxide (CO2) concentration, roughly 185–230 ppm. This range matches atmospheric CO2 during Marine Isotopic Stage 3 (MIS 3: between 27.8 and 59.4 thousand of years BP, hereafter ka), a period when D‐O events were most frequent. Insights from the three climate models point to North Atlantic (NA) sea‐ice coverage as a key ingredient behind D‐O type oscillations, which acts as a “tipping element.” Other climate state properties such as Mean Atlantic Meridional Overturning Circulation strength, global mean temperature and salinity gradient in the Atlantic Ocean do not determine whether D‐O type behavior can occur in all three models.

Ocean Heat Convergence and North Atlantic multidecadal heat content variability

Abstract We construct an upper ocean (0-1000m) North Atlantic heat budget (26°-67°N) for the period 1950-2020 using multiple observational datasets and an eddy-permitting global ocean model. On multidecadal timescales ocean heat transport convergence controls ocean heat content (OHC) tendency in most regions of the North Atlantic with little role for diffusive processes. In the subpolar North Atlantic (45°N-67°N) heat transport convergence is explained by geostrophic currents whereas ageostrophic currents make a significant contribution in the subtropics (26°N-45°N). The geostrophic contribution in all regions is dominated by anomalous advection across the time-mean temperature gradient although other processes make a significant contribution particularly in the subtropics. The timescale and spatial distribution of the anomalous geostrophic currents are consistent with a simple model of basin scale thermal Rossby waves propagating westwards/northwestwards in the subpolar gyre and multidecadal variations in regional OHC are explained by geostrophic currents periodically coming into alignment with the mean temperature gradient as the Rossby wave passes through. The global ocean model simulation shows that multidecadal variations in the Atlantic Meridional Overturning Circulation are synchronized with the ocean heat transport convergence consistent with modulation of the west-east pressure gradient by the propagating Rossby wave.

Температурные изменения в мясных изделиях при жарке в пароконвекционных печах

Combination steam ovens, or combi steamers, have entered all spheres of food production, including the meat industry. Their rational use requires a scientific and practical foundation. This research featured the changes in mean volume temperature and temperature gradient that occur in meat products (one-dimensional bodies with different nutrient compositions) during heat treatment in dry air and a steam-air mix.
 The research involved two samples of meat products with different moisture and fat contents. The chicken fillet sample had a moisture content of 74.5% and a fat content of 1.9% while the pork shoulder sample had a moisture content of 55.1% and a fat content of 29.4%. Shaped as a one-dimensional cylinder and a plate, the samples were subjected to heating at the temperature range of 160–240°C in a Unox-203G steam-convection oven (Italy). Dry air and a steam-air mix with a humidity of 80–85% served as a heating medium. The temperature was measured using thermocouples attached to a Sosna-004 meter.
 The research revealed some patterns in the mean volume temperature and temperature gradient. The temperature gradient involved three stages during processing in a steam-air mix and four stages when treated with dry air. The change in the mean volume temperature for the steam-air mix could be described by a power law equation; the dry air treatment was described using a linear equation. When heated in dry air, the rate of change in the temperature gradient was constant at first but started to decrease at a certain stage. The change rate in the mean volume temperature remained low for 5 min and started to increase onwards, maintaining its value until the end of the process. When heated in a steam-air mix, the change rate in the temperature gradient dropped to its minimum in 4–5 min and started to grow. The mean volume temperature demonstrated a high change rate during the first 5 min and went down. The chicken fillet with its low fat content warmed up faster by 13–26% when processed in a steam-air mix and by 9–23% when treated in dry air. The plate-shaped products needed longer heat treatment. The composition and form had no significant effect on the nature of the change in the temperature gradient and mean volume temperature.
 The obtained dependencies made it possible to select the optimal temperature and humidity conditions for convective frying of meat products.

Characteristics of planar buoyant jets and plumes in a turbulent channel crossflow from direct numerical simulations

AbstractThis paper is motivated by an interest in understanding the characteristics of buoyant fluids discharged from the bottom wall of channels, such as encountered during tunnel fires or in river effluent discharge. Direct numerical simulation is used to model the upward release of a planar buoyant jet or plume from the bottom wall of a channel into an incoming turbulent crossflow. The well-studied jet-in-crossflow with only a momentum source is simulated first, and subsequently, fixing the incoming Reynolds number, buoyancy source as heat flux is added alongside varying momentum source, with two cases where only a buoyancy source is present. Appropriate five non-dimensional parameters relevant for this flow are defined, of which three are fixed and two—source to channel momentum ratio and Richardson number—are varied. The changes in turbulence characteristics as the buoyant jet or plume evolves downstream are presented. In all cases with buoyancy, except for the pure jet case, the plume is initially confined to the lower half of the channel before it suddenly lifts to the top half, an effect that occurs at an increasingly smaller downstream distance with increasing buoyancy, and dividing the flow into a near and far field. The distributions of mean and Reynolds stresses in the near and far field of the source are reported, and it is found that the channel flow becomes more turbulent downstream of the source, and further, the turbulent vertical temperature flux switches sign from near to far field owing the a change in the mean temperature gradient sign. From the input parameters and using the integrated temperature equation a reasonable estimate of the far field mean channel temperature can be obtained by a reference temperature based on the heat conservation that includes the convective and diffusive source heat flux. A monotonic behaviour of the back-layering distance is also observed a function of this reference temperature, which was difficult to obtain with the two specified non-dimensional parameters.

Outlet boundary condition and mean temperature gradient effects on the minimum acoustics disturbances energy in triggering nonlinear thermoacoustic instability

In this study, we theoretically investigate the impact of outlet boundary conditions and mean temperature gradients on the maximum transient growth rate of acoustical energy and the critical energy required for triggering. Our analysis encompasses open–open and open–closed thermoacoustic systems. The theoretical models developed focus on horizontal ducts with a mean temperature jump over the heat source, employing the modified King's law. By linearizing the unsteady heat release, the nonlinear thermoacoustic equations transform into linearized-delay ones. This approach enables us to predict optimal initial perturbations for linearized-delay and nonlinear systems, corresponding to maximum transient growth rates of acoustic energy over short and long periods, respectively, thus providing insights into critical energy for triggering. We find that a closed outlet leads to higher transient energy growth and a lower critical energy for triggering compared to an open outlet. The increased mean temperature gradient has a “destructive” impact on triggering in open–open systems but a “constructive” effect in open–closed systems. Raising the mean temperature ratio generally increases the critical energy for triggering in the open–open system, whereas it decreases the critical energy in the open–closed system. The critical energy for nonlinear optimal initial perturbations is notably affected by the minimum energy of critical unstable periodic solutions, while the critical energy for linearized-delay optimal initial perturbations is closely tied to the energy level of stable periodic solutions. Due to the transient energy growth rate, the critical energy for nonlinear optimal initial perturbations is significantly lower than that for linearized-delay optimal initial perturbations.

Turbulent heat flux and wall heat transfer in hypersonic turbulent boundary layers with wall disturbances

Hypersonic turbulence over disturbed walls can be encountered when the fuselage of the vehicles is ablated or installed with transpiration-cooling porous media, resulting in the variation of the wall heat transfer that complicates its accurate predictions. In this paper, we study the turbulent heat flux and wall heat transfer in hypersonic turbulent boundary layers at the free-stream Mach number of 6.0 subject to wall disturbances with different wall temperatures, with our focus on the compressibility effects. We found that the temperature fluctuation intensity and the turbulent heat flux are enhanced in the outer region, which is attributed to the higher mean temperature gradient induced by the wall disturbances. The solenoidal and dilatational components of the turbulent heat flux, obtained by the Helmholtz decomposition that splits the velocity fluctuations into the solenoidal and dilatational components, are of opposite signs, with the latter smaller in magnitude but showing the tendency of mitigating the former. The integration formula that decomposes the wall heat transfer into various physical processes reveals that the viscous dissipation generates the wall heat transfer, while the turbulent heat flux, work of pressure on dilatation and the mean flow convection transport it towards the free stream. The contribution of the dilatational motions to the viscous dissipation and turbulent heat flux is negligible compared with that of the solenoidal components.

On the wall-bounded model of fingering double diffusive convection

The present work aims at clarifying the effects of a solid boundary on the salt fingers in the wall-bounded double diffusive convection turbulence driven by the salinity and temperature differences between the top and bottom plates. The fluid properties are the same as the seawater, and two-dimensional direct numerical simulations are conducted over a wide range of the thermal and salinity Rayleigh numbers which measure the strength of driving salinity difference and stabilising temperature difference. We find that the bulk density ratio $\varLambda _b$ , defined by the mean temperature and salinity gradients at the bulk, controls the flow morphology. As $\varLambda _b$ exceeds unity, the bulk flow shifts from wide convection rolls to slender salt fingers. Two different regimes are further identified for the cases of salt-finger type. One is the confined salt-finger regime where the characteristic height of salt fingers is comparable to the bulk height and the influences of the solid boundary are noticeable. The other is the free salt-finger regime where the salt fingers are much shorter than the bulk height. In this latter regime, the transport properties versus $\varLambda _b$ are in quantitative agreement with those obtained in the fully periodic domain (e.g. Traxler et al., J. Fluid Mech., vol. 677, 2011, pp. 530–553). For a limited range of density ratio at the highest salinity Rayleigh number considered here, multiple states can be obtained from different initial conditions. The large-scale secondary instability and spontaneous formation of staircase from finger layers are not observed in the current study.

Meridional Variation of the Mascarene High and Atmospheric Transient Eddy Dynamical Forcing over the Southern Indian Ocean in Austral Winter

Abstract This study reveals a close relation between the Mascarene high (MH), atmospheric transient eddies (hereafter transient eddies or eddies for short), and the sea surface temperature (SST) front over the southern Indian Ocean in austral winter. Climatologically, the subpolar westerly jet couples well with transient eddies via eddy–mean flow interaction and the anticyclonic vorticity to its north helps with anchoring the MH. On the interannual time scale, the MH exhibits a dominant meridional variation accompanied by intensity variability. When the MH moves poleward and intensifies, positive quasi-barotropic geopotential height anomalies associated with a warm temperature feature the southern flank of MH. As a result of the modified mean temperature gradient, the subpolar jet and transient eddies’ activity are enhanced near the jet exit; in contrast, the subtropical westerly jet and eddies are weakened over the jet entrance, mainly via the baroclinic energy conversion. As feedback, the anomalous transient eddies can trigger the poleward shift of MH by diverging the extended Eliassen–Palm (E-P) flux from subpolar to subtropical region and thus the intensification of subpolar jet and weakness of subtropical jet. Such positive feedback between the meridional variation of MH and transient eddies could be attributed to the underlying SST anomalies. Early SST warming appears over the southwestern Indian Ocean 3 months prior and shifts the Agulhas SST front poleward. The poleward Agulhas SST front could further induce a southward displacement of the activity of transient eddies by changing the low-level atmospheric baroclinicity. Hence, the SST anomalies over the southern Indian Ocean may trigger the meridional variation of MH via the positive eddies–mean flow feedback.

Predicting thermoacoustic stability characteristics of longitudinal combustors using different endpoint conditions with a low Mach number flow

Combustion instability frequently occurs in propulsion and power generation systems. It is characterized by large-amplitude acoustic oscillations leading to undesirable consequences. Designing a stable combustor by predicting its stability characteristics is therefore essential. This study centers upon modeling a straight one-dimensional combustor with an acoustically compact heat source, low Mach numbers, and different end point conditions. To predict the stability characteristics, we examine six combustor configurations (open–closed, closed–closed, open–choked, closed–choked, open–open, and closed–open). A Galerkin expansion technique is implemented to capture the acoustic disturbances. The unsteady heat release is modeled using an N−τ formulation. The results show that steepening of the mean temperature gradient causes the eigenfrequency associated with an open outlet to increase more rapidly than that of a choked nozzle. Compared to a choked boundary, an open outlet generates higher eigenfrequencies and lower sound energy when coupled with an open inlet. Conversely, it triggers lower eigenfrequencies and higher sound energy using a closed inlet. The maximum possible growth of sound energy, Gmax, remains positively correlated with the inlet temperature, interaction index N, and inlet Mach number, but inversely proportional to the temperature gradient. The heat source extrema leading to the most and least amplified system energy seem to shift upstream, when the mean temperature gradient is successively increased. Their coordinates are similar in half-open tubes and exhibit a converse relation between the open–open and closed–choked tubes. At sufficiently low Mach numbers, the choked and closed outlets show equivalence in acoustic frequencies, transient energy evolution, and optimal heat source locations.

Semi-empirical model of the spatiotemporal surface temperature distribution on the plain part of Ukraine

The spatial variation of temperature is found to depend linearly on climate continentality, morphology of the relief, the position of the site with respect to seas, in addition to the usual elevation, latitude and longitude predictors. There are other factors that can have an additional significant influence: big bodies of water, terrain attributes relief, atmospheric factors (local circulation), configuration and aspect of coasts and vegetation. Therefore, these multifactorial influences form the climatic field of temperature.
 In this study, the regional semi—empirical model of the spatiotemporal distribution of the average annual and monthly temperature for the plain part of Ukraine on the basis of the methodology for assessing the influence of height above sea level and geographic coordinates is proposed. Based on the method for determining the altitudinal, latitudinal, and longitudinal gradients of meteorological parameters, we calculated these gradients for annual and monthly air surface temperature for the periods 1961—1990 and 1991—2020.
 Thus, on the plain part of Ukraine, the annual surface air temperature decreases by an average on 0.60—0.63 °C with a shift of 100 m height above sea level, on 0.51—0.55 °C with a shift of one latitude degree to the north, on 0.067—0.071 °C with a shift of one longitude degree to the east. Also, the variations of these annual mean temperature gradients from year to year over the period 1991—2020 are characteristic.
 The seasonal variation of gradients has a pronounced non—monotonic character: highest values of altitudinal gradientare typical for July—August (from –0.63 to –0.73 °C per 100 m), and the lowest values — for April—May (from –0.45 to –0.55 °C per 100 m); highest values of latitudinal gradient are typical for August—September (from –0.60 to –0.70 °С per 1 °N), and the lowest values — for April—May (from –0.20 to –0.35 °С per 1° N); the longitudinal gradients have positive values in June—August (0.074—0.128 °C per 1° E), and negative values in November—March (from –0.228 to –0.154 °C per 1° E).
 We found that the altitudinal and latitudinal gradients of temperature have the most spatiotemporal variability and the longitudinal gradient has the smallest one. Greatest variabilities of temperature gradient values are typical for February—March and July—September, and the least variability — for April—May.
 The analysis of the dynamics of gradient changes in the period 1991—2020 compared to the period 1961—1991 showed the following: the altitudinal gradientvalues increased by 8—13 %. in January and March—May; the latitudinal gradient values increased by ~30 % in December—February and decreased by ~20 % in May—August.
 The proposed semi—empirical model contains a coefficient that takes into account influence of additional effects associated with pronounced orographic and other terrain features. This study presents the numerical values of this coefficient for some specific microclimate regions of the plain part of Ukraine.
 The model estimates of thirty—year monthly mean temperature in Ukraine for the periods 1961—1990 and 1991—2020was calculated. A comparison of the model estimates of of the average annual and monthly temperature for 72 meteostations in Ukraine with their actual values showed a statistically significant correlation (the reliability of the linear approximation is 0.89—0.98). Thus, the presented design of the semi-empirical model makes it possible to quite well restore the annual and monthly temperature on the territory of Ukraine

Numerical study of rotating convection with bi-directional temperature gradients

In this work, we report results from three-dimensional numerical simulations of convection in a rotating cylindrical annulus with mean temperature gradients present along the radial and vertical directions. The simulations have been performed in OpenFOAM (Open-source Field Operation And Manipulation). The bi-directional temperature gradient is achieved by imposing uniform heat flux in a thin circular strip at the outer periphery of the bottom surface, and a uniform temperature at the entire inner cylindrical surface. The study has been carried out for a particular Rayleigh number, Ra = 4.76×108, and a range of Taylor numbers, Ta = 6.5×108–2.7×109. The Ra value is chosen so that some qualitative comparison could be done with available experimental data. A laminar flow solver has been used to perform the computations in a rotating frame of reference. Complex empirical orthogonal function analysis has been carried out for the velocity and temperature fields. The contour maps of the radial velocity field in the r−θ plane clearly show the existence of mode 4 and 4 AV (amplitude vacillation) baroclinic waves at low values of Ta, which break down into eddies as Ta increases. The mean temperature field in the r–z plane supports the existence of Convective Columnar Plumes (CCP) on the outer edge, which interacts with the baroclinic waves, aiding in the transport of heat in the system. The fluctuations in CCP disintegrate into smaller-scale structures at the highest Ta. Quantification of turbulent fluxes shows the effectiveness of the heat transport at Ta∼O(108), as compared to Ta∼O(109). Our results clearly demonstrate the role of waves, plumes, and eddies on heat transport in the presence of bi-directional temperature gradients.

Some Further Aspects of Stable Boundary-Layer Simulation in a Stratified-Flow Wind Tunnel

It is demonstrated that the vertical profile of gradient Richardson number, Ri, can be shaped by control of the working-section inlet temperature profile. In previous work (Hancock and Hayden in Boundary-Layer Meteorol 168:20–57, 2018; 175:93–112, 2020; 180:5–26, 2021) the inlet temperature profile had been specified but without control of the profile of Ri in the developed-flow region of the working section. Control of the inlet temperature profile is provided by 15 inlet heaters (spread uniformly across the height of the working section), allowing control of the temperature gradient over the bulk of the boundary layer, and the overall temperature level above that of the surface. The bulk Richardson number for the 11 cases covers the range 0.01–0.17 (there is no overlying inversion). In the upper approx 2/3 of the boundary layer the Reynolds stresses and turbulent heat flux are controlled by the gradient in mean temperature, while in the lower approx 1/3 they are controlled both by this gradient and by the level above the surface temperature. In three examples, Ri is approximately constant at 0.07, 0.10 and 0.13 across the bulk of the layer. The previous observation of horizontally homogenous behaviour in the temperature profiles in the top approx 2/3 of the boundary layer but not in the lower approx 1/3 is repeated here, except when, tentatively, Ri does not exceed 0.05 over the bulk of the boundary layer. Favourable validation comparisons are made against two sets of local scaling systems and field data over the full depth of the boundary layer, over the range 0.006 le Ri le 0.3, or, in terms of height and local Obukhov length, 0.005 le z/L le 1.

Coupled nonlinear dynamics of acoustic modes in a quasi 1-D duct with inhomogeneous mean properties and mean flow

This study investigates the coupled nonlinear evolution of acoustic modes in a quasi 1-D duct with axially inhomogeneous mean velocity and mean thermodynamic properties. A novel modification of the Krylov–Bogoliubov method of averaging (KBMA) is developed to analytically solve the modal-amplitude equations that are linearly and nonlinearly coupled with both quadratic and cubic nonlinearities. Approximate analytical solutions based on the method of multiple scales (MMS) are also derived for the coupled modal-amplitude equations with cubic nonlinearities. The KBMA and MMS solutions are presented for the cases with and without internal resonance among the acoustic modes. Furthermore, novel initial conditions are derived for the KBMA and MMS modal amplitude and phase equations that are essential to validate the analytical solutions using numerical solutions. While both the analytical solutions are seen to be in good to excellent agreement with the numerical solutions, only KBMA captures the low-frequency oscillations in the peak amplitude of the limit cycle oscillations. The role of the order of nonlinear terms in the evolution to limiting oscillations is elucidated by successively considering coupled modal equations with both quadratic and cubic nonlinearities, only cubic nonlinearities and only quadratic nonlinearities. It is seen that while the cubic nonlinearities determine the asymptotic temporal behavior, the quadratic nonlinearities give rise to complex oscillation patterns characterized by a range of non-integral frequencies. For coupled oscillators with only quadratic nonlinearities, it is observed that for mean temperature gradients below a threshold, the pressure fluctuations are characterized by intermittent bursts of high-amplitude fluctuations, as well as by a frequency spectrum that is essentially continuous. The effects of linear coupling of the modal-amplitude equations on the limit-cycle behavior of the solutions are also illustrated.

Uniform momentum and temperature zones in unstably stratified turbulent flows

Wall-bounded turbulent flows exhibit a zonal arrangement, in which streamwise velocity organizes into uniform momentum zones (UMZs), separated by thin layers of elevated interfacial shear. While significant research efforts have focused on these structural features in neutrally stratified flows, the effects of unstable thermal stratification on UMZs and on analogous uniform temperature zones (UTZs) have not been considered previously. In this article, statistical properties of UMZs and UTZs are investigated using a suite of large eddy simulations of unstably stratified turbulent channel flow spanning weakly to highly convective conditions. When normalized by the friction velocity and stability-dependent mixing length, the mean velocity gradient based on UMZ interfacial velocity jumps and the vorticity thickness exhibits good collapse for all stabilities, establishing a link between UMZ properties and scaling predictions from Monin–Obukhov similarity theory. A similar relationship is found between UTZ properties and surface-layer scaling of the mean temperature gradient. In the mixed layer, mean UMZ depth is quasi-constant with wall-normal distance, while the deepest UTZs are found in the centre of the boundary layer. These instantaneous structures are found to be linked to the well-mixed velocity and temperature profiles in the convective mixed layer. Conditional averaging indicates that both UMZ and UTZ interfaces are associated with ejections of momentum and warm updrafts below the interface and sweeps of momentum and cool downdrafts above the interface. These results demonstrate a tangible connection between instantaneous structural features, mean properties and scaling laws in unstably stratified flows.

The effect of inlet velocity profile on entropy generation and hydrothermal performance of a pin-fin heatsink with biologically prepared silver/water nanofluid coolant: Two-phase mixture model

The prediction of heat transfer characteristics numerically depends on several factors such as the numerical method, the boundary and operating conditions, etc. The present paper investigated the influence of using the uniform and non-uniform inlet velocity profiles on the heat transfer coefficient (h), pumping power (W˙p), temperature uniformity (θ), and thermal and frictional entropy generation rates (S˙th and S˙fr) in a pin-fin heatsink with silver/water nanofluid (NF) coolant. The numerical study is conducted utilizing the two-phase mixture scheme. The results proved that h is almost the same for two inlet profiles with the maximum deviation of 0.2%. θ and W˙p, however, for uniform profile is nearly 2.52–1.86% and 15–26% higher than those for the other case at Reynolds numbers of 500–2000, respectively, indicating a lower temperature uniformity and higher power consumption for uniform velocity profile. The figure of merit was obtained as 1.19–1.36 at Re numbers of 500–2000; indicating the better hydrothermal performance of the non-uniform inlet velocity profile over the other case. In addition, S˙th was almost the same for both cases due to same mean temperature and temperature gradients. However, S˙fr for non-uniform profiles was 4.41–8.84% lower than that for other profile at Reynolds numbers of 500–2000.

The phosphorus limitation in the post-fire forest soils increases soil CO2 emission via declining cellular carbon use efficiency and increasing extracellular phosphatase

Wildfires cause severe disruptions to forest soils and modify the carbon (C) cycle in the post-fire soils. Yet, the contribution of abiotic and biotic drivers to CO2 emission in the post-fire forest soils is not well understood. This study investigated soil CO2 emission from the burned and corresponding unburned forest soils across a large latitudinal gradient of mean average precipitation (from 422 to 1640 mm) and temperature (from 0.20 to 25.8 °C). We compared the soil chemical properties (e.g., dissolved organic carbon, nutrients) and microbial properties (biomass, metabolic quotient (qCO2), and community diversity and composition) in the burned and unburned soils. Microbial metabolism was also addressed through extracellular enzyme activity and intracellular C use efficiency (CUE) using the stoichiometric modeling approach. Here, we showed that the wildfires: i) increased phosphorous (P) pressure to soil microbes (indicated by increased qCO2), ii) decreased microbial biomass, community diversity and abundance of some microbial groups (e.g., Phylum Actinobacteria), and iii) resulted in larger basal respiration due to lower cellular CUE and higher extracellular alkaline phosphatase (AP) ratio to other enzymes like β-glucosidase, Cellobiohydrolase and N-Acetyl-glucosaminidase. The latter indicated the wildfires-induced P:N imbalance (P-limitation) caused significant C loss via microbial exploitation of P from soil organic matter. The study highlights the ecological significance of intracellular and extracellular metabolisms in soil respiration and consequent C sequestration in post-fire forest soils.

Turbulence Structure and Mixing in Strongly Stable Boundary-Layer Flows over Thermally Heterogeneous Surfaces

Direct numerical simulations (DNS) at bulk Reynolds number Re =10^4 and bulk Richardson number Ri =0.25 of plane Couette flow are performed with the results used to analyze the structure and mixing intensity in strongly stable boundary-layer flows. The Couette flow set-up is used as a proxy for a real-world stable boundary layer flow with surface thermal heterogeneity. Along the upper and lower walls, the temperature is either homogeneous or varies sinusoidally, but the horizontal-mean surface temperature is the same in all cases. Over homogeneous surfaces, the strong stratification always quenches turbulence resulting in linear velocity and temperature profiles. However, over a heterogeneous surface turbulence survives. Molecular diffusion and turbulence contribute to down-gradient momentum transfer. The total (diffusive plus turbulent) heat flux is directed downward, but its turbulent contribution is positive, i.e., up the mean temperature gradient. Analysis of covariances of velocity and temperature, their skewness, and the flow structure suggests that counter-gradient heat transport is due to quasi-organized cell-like vortical motions generated by surface thermal heterogeneity. These motions transfer heat upwards similar to their counterparts in highly convective boundary layers. Thus, the flow over heterogeneous surface features local convective instabilities and upward eddy heat transport, although the overall stratification remains stable with downward mean heat transfer. The DNS results are compared to the results from large-eddy simulations of weakly stable boundary layers (Mironov and Sullivan in J Atmos Sci 73:449–464, 2016). The DNS findings corroborate the key role of temperature variance in setting the structure and transport properties of stably stratified flow over heterogeneous surfaces, and the importance of third-order transport of the temperature variance.

Scaling transition of thermal dissipation in turbulent convection

Direct numerical simulation (DNS) of non-slip two dimensional (2D) Rayleigh–Benard convection (RBC) is conducted for a wide range of Rayleigh number (Ra up to 1013) at Prandtl number Pr = 0.7 and aspect ratio Γ = 1. The thermal dissipation rate is shown to display an evident scaling transition through the compensated plot, i.e., ⟨εθ⟩∝Raγ with γ≈−0.17 for 106≤Ra≤109, while γ≈−0.19 for Ra=109≤Ra≤1013. To track the transition, separate contributions from the thermal boundary layer (BL) and the bulk flow region are examined, incorporated also with the mean and fluctuation decomposition. It turns out that the mean temperature gradient in the BL is the dominant contribution, and together with other parts (i.e., fluctuations in the BL and bulk, and the mean gradient in the bulk), they all exhibit an obvious transition at Ra≈109. We have further checked the Nusselt number (Nu), which also shows the transition at Ra≈109. Interestingly, Nu∝Ra0.33 is observed for small Ra, while Nu∝Ra2/7 is absent in 2D RBC cases. To understand the physical origin of transition, spatial distributions and probability density functions of thermal dissipation rate are finally discussed, with notable statistical features changed at Ra≈109.