Modelling plant transpiration and leaf climate using CFD

Research into vertical farms or plant factories is steadily increasing over the years, as the demand for sustainable food production and a shift to more environmental friendly food production is occurring. Modelling plant climate in these confined spaces is therefore essential to guarantee optimal growing conditions. Modelling of plant climate has already been done in greenhouses, but at length scales much bigger than individual leaves. In this study, one single plant will be modelled, using computational fluid dynamics and by incorporating additional source terms in the relevant transport equations. Plants are modelled using the big leaf approach, where a plant is modelled as one artificial leaf. Water vapour flux in plants is controlled by two resistances in series, the aerodynamic resistance, which is a function of the boundary layer around the leaves and the stomatal resistance, which is the resistance against water vapour transport in leaves. Two different plants are studied, impatiens pot plant and basil plants. Values of stomatal resistance for these crops are obtained from literature or were measured. Evapotranspiration was compared with the Penman-Monteith equation.

Determination of Evaporative Fluxes Using a Bench-Scale Atmosphere Simulator

An accurate determination of evaporative fluxes is critical for efficient water management in semi-arid climates such as in the Canadian Prairies. The main achievements of this research are the design and operation of a bench-scale atmosphere simulator, performance evaluation using selected weather scenarios pertaining to regional atmospheric conditions, validation using established empirical correlations, and estimation of evaporation rates and the amount for a typical local water body. Results indicate that the measured data achieved the target values for the various parameters and the data were found to be stable during the 3-h test duration. The vapour flux was found to have large variation during summer (0.120 g∙s−1∙m−2 during the day and 0.047 g∙s−1∙m−2 at night), low variation during spring (0.116 g∙s−1∙m−2 during the day and 0.062 g∙s−1∙m−2 at night), and negligible change during fall (0.100 g∙s−1∙m−2 during the day and 0.076 g∙s−1∙m−2 at night). The measured vapour flux was generally within one standard deviation of the equality line when compared with that predicted by both the mass-transfer equations and the combination equations. The average evaporation ranged from 4 mm∙d−1 to 8 mm∙d−1 during the day and decreased to 1 mm∙d−1 to 3 mm∙d−1 at night. The 24-h evaporation was found to be 8 ± 1 mm∙d−1 from late April through late October. Likewise, the cumulative annual evaporation was found to be 1781 mm, of which 82% occurs during the day and 18% at night.

Moisture transport during large snowfall events in the New Zealand Southern Alps: The role of atmospheric rivers

Synoptic-scale moisture transport during large snowfall events in the New Zealand Southern Alps is largely unknown due to a lack of long-term snow observations. In this study, records from three recently developed automatic weather stations (Mahanga, Mueller Hut and Mt Larkins) near the Main Divide of the Southern Alps were used to identify large snowfall events between 2010 and 2018. The large snowfall events are defined as those events with daily snow depth increase by greater than the 90th percentile at each site. ERA-Interim reanalysis data were used to characterize the hydrometeorological features of the selected events. Our findings show that large snowfall events in the Southern Alps generally coincide with strong fields of integrated vapour transport (IVT) within a north-westerly airflow and concomitant deepening low pressure systems. Considering the frequency of large snowfall events, approximately 61% of such events at Mahanga were associated with narrow corridors of strong water vapour flux, known as atmospheric rivers (ARs). The contributions of ARs to the large snowfall events at Mueller Hut and Mt Larkins were 70% and 71%, respectively. Analysis of the vertical profiles of moisture transport dynamics during the passage of a landfalling AR during 11-12th October 2016 revealed the key characteristics of a snow-generating AR in the Southern Alps. An enhanced presence of low and mid-level moisture between 700-850 hPa and pronounced increases of wind velocities (more than 30 m s-1) with high values of the meridional component between 750-850 hPa were identified over the Southern Alps during the event.

Warmer–wetter climate drives shift in δD–δ18O composition of precipitation across the Queen Elizabeth Islands, Arctic Canada

We examine how recent increases in air temperature and precipitation, together with reductions in sea ice extent, may have affected the regional δD–δ18O composition of precipitation. In spring 2014, 80 snow samples were collected from six glaciers and ice caps across the Queen Elizabeth Islands, and in 2009 and 2014, two shallow ice cores were collected from Agassiz Ice Cap and White Glacier, respectively. The snow samples showed average δ18O values from 2013 to 2014 to be approximately 2‰–3‰ higher than those recorded in 1973–1974 in nearby locations, with the ice cores showing similar trends in δ18O values. A zonal average water isotope model was used to help understand the causes of the increased δ18O values, using inputs calibrated for observed changes in temperature, vapour flux, and sea ice extent. Model results indicate that atmospheric temperature changes account for <1‰ of the observed change in δ18O values, and that changes in local water input and precipitation driven by changes in sea ice extent only have an effect in coastal regions. Enhanced meridional vapour flux to the Queen Elizabeth Islands is, therefore, also required to explain the observed increases in δ18O values, with fluxes ∼7% higher today than in the 1970s, consistent with the change in precipitation.

How comets work: nucleus erosion versus dehydration

ABSTRACT We develop an activity model based on ice sublimation and gas diffusion inside cm-sized pebbles making-up a cometary nucleus. Our model explains cometary activity assuming no free parameters and fixing the nucleus surface temperature Ts, its gradient below the nucleus surface at thermal equilibrium, the pressure inside the porous pebbles, and the gas flux from them. We find that (i) the nucleus erosion rate and water vapour flux are independent of the nucleus refractory-to-ice ratio, which affects the dehydration rate only; (ii) water-driven dust ejection occurs in thermal quasi-equilibrium at Ts &gt; 205 K; (iii) the smallest and largest ejected dust sizes depend on the nucleus surface temperature and its gradient at depths of few cm; and (iv) the water-driven nucleus erosion rate is independent of the water vapour flux. Regarding comet 67P/Churyumov–Gerasimenko, we find that (i) during the northern and southern polar summers, the nucleus active areas are ≈5 km2; (ii) &gt;95 per cent of the southern pristine nucleus has a refractory-to-water-ice mass ratio &gt;5; and (iii) the different temperature dependences of the dehydration and erosion rates explain the seasonal cycle: at perihelion, dm-sized chunks ejected by the sublimation of CO2 ices are rapidly enveloped by an insulating crust, preserving most water ice up to their fallout on the northern dust deposits; the inbound water-driven activity at low temperatures triggers a complete erosion of the fallout if its water-ice mass fraction is &gt;0.1 per cent.

Influence of the lateral source/building separation on vapour intrusion: A numerical study

Various vapour intrusion (VI) models have been proposed in order to predict indoor concentration of Volatile Organic Compounds (VOCs) in buildings. However, these models tend to be conservative, and overestimate or underestimate vapour flux emissions due to several assumptions. Particularly, most of these VI models only consider an infinite uniform contaminated groundwater as the principal source of VOCs in the soil, and lateral pollution source in the vadose zone are disregarded. It has been shown that ignoring the lateral source position may lead to uncertainties on the estimations. In this paper, a numerical model is developed in order to better understand the relationship between the lateral source position in the soil, including both a source in the vadose zone and a source located at the groundwater level, and the resulting indoor air concentration. Results show that source position plays a significant role on vapour intrusion attenuation. In fact, indoor concentration of VOCs decreases with increasing lateral separation. Finally, it is shown that considering the source position can significantly improve the quality of VI predictions.

A novel and flexible test setup to measure the vapour diffusion resistance of building materials and wall components

A novel test setup and procedure to measure the vapour diffusion resistance of building materials and components are presented. In this test setup, a vapour flux across the test sample is induced by cooling down one of the sample’s surfaces by a cooling plate. The cooling plate also acts as a vapour tight plane and hence condensation is created. The vapour diffusion resistance is, via a Glaser-based calculation, inferred from the mass of condensation. Benefits of the novel procedure are its applicability to building components such as masonry, CLT, etc., and its larger flexibility in respect to the boundary conditions. The non-isothermal approach allows the induction of a large (and thus measurable) vapour flux while a quasi-constant relative humidity across the sample can be imposed. In the paper, the novel method is validated based on a bituminous impregnated fibreboard with known diffusion resistance. Thereafter, the method is applied to a masonry wall, showing the importance of diffusion measurements on the component level.

The crackling sound of Leidenfrost stars

Liquid drops deposited on a hot plate can experience a boiling crisis, when the vapour flux is strong enough to ensure the levitation of the drop and the relative insulation of the liquid from the solid. It is often denoted Leidenfrost effect, after the German Johann Gottlob Leidenfrost, who first reported it in 1756. While many studies have encompassed various applied issues associated with this phenomenon, aiming to control and prevent its appearance, Ma & Burton (J. Fluid Mech., vol. 846, 2018, pp. 263–291) focused on the spontaneous appearance of a standing wave at the free surface, together with temporal oscillations, making the drop adopt the shape of a star. Their far-reaching study presents exhaustive results using six different liquids with a range of different volumes and temperatures, in which they systematically extracted the drop dynamics together with the pressure fluctuations in the vapour cushion below.

Issues of conventional model of transfer of latent heat in soil

Upper limit of experimental coefficient between the measured transfer of latent heat and the estimated vapour flux in the frame of the conventional model of latent heat transfer in soil was examined by analysing the measured latent heat transfer and temperature gradient in soil under steady-state temperature gradient. To exclude the temperature gradient as an uncertainty factor from the experimental coefficient, the temperature gradients of overall soil and soil pore were included into the vapour fluxes in the atmosphere. The estimated experimental coefficient did not exceed unity, which indicated that both the latent heat transfer and the vapour fluxes in the soil were smaller than those in the atmosphere. The gap that appeared between the experimental coefficient and the product of the tortuosity factor and air-filled porosity implied the existence of an unidentified parameter relevant to characteristic of the circulation of water in soil which is the main mechanism of latent heat transfer in soil. By quantifying this characteristic with simultaneous measurements of the latent heat transfer, distributions of temperature, water content and solute content in various soils under the steady-state condition, the conventional model would be modified, or an alternative model being independent of the conventional model would be developed.