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A viscous sublayer was introduced into a PBL model in order to specify the lower boundary condition for temperature. The simulated results have been compared against available observations. However, for such a comparison, some of the variables and parameters that are necessary are not known but can be deduced from observed data. In this way, surface temperature and thermal diffusivity of the soil, representative for the four-day period studied here, have been estimated from measured data. An optimized relation for the thickness of the viscous sublayer δ$\delta$ was found that includes the diurnal variation of the properties of the air flow. Including this approach in the model, simulated temperatures in the ground at different levels as well as temperature in the atmosphere agree very well with the observations. The applicability for a wider range of wind speeds was demonstrated by calculating daily maximum temperatures Tmax$T_{\text{max}}$. An analysis of long-term observations for the summer season at different operational weather stations consistently show a distinct maximum of Tmax$T_{\text{max}}$ for a 10‑m wind between 2–3 m/s, which can be explained by the interaction between the molecular transport of heat within the viscous sublayer of thickness δ$\delta$ and the turbulent heat flux.

A one-dimensional boundary layer model was combined with a routine to calculate surface temperatures on road bridges. Forced by reanalysis data for the year 2016, simulations were performed for two bridges 80 and 130 m in height, respectively. The results for each hour of the year were compared with on-site observations. In addition to the annual variation in general, temperature extremes captured by the model were quite reasonable. Although hourly wind speeds above the bridge were calculated less perfectly, the few cases of simulated and observed maximum mean winds were in good agreement.

A one-dimensional boundary layer model was used to study the effects of uncertain input data on 2‑m temperature and 10‑m wind. Based on a very large number of numerical results, it can be demonstrated, that even a small degree of ambiguity can have significant implications, especially for temperature. In 50 % of all 6,000 simulations for flat terrain and randomly chosen sets of input data within narrow limits, temperature uncertainty was more than 2 K, and in 14 % of all cases more than 4 K, with a maximum of 9 K. Effect on wind speed is much smaller and depends mostly on surface roughness length. For a forest scenario, the results for temperature of 18,000 simulations are in the same order, with 25 % of the ensemble show temperature uncertainties of more than 2 K, and 6 % of more than 4 K, while wind speed above a forest is much more affected than for the bare soil case. In addition, the contribution of uncertainties of individual input data was estimated.

Width effects on concurrent forced flow flame spread over thermally-thick PMMA slabs were studied. Different flame spread behaviors were observed depending on the sample width. For wider slabs (width ≥ 10 cm), the flame spread was width-independent. The total burning rate decreased with the streamwise location to the −0.45 power, which was very close to that predicted by the laminar boundary layer theory. For narrower slabs (2.5 and 3 cm), the flame length and flame spread rate decreased with the width. The total burning rate was nearly constant along the streamwise direction, suggesting that turbulence set in at some point. It was found that width effects changed from heat and fuel loss-dominated to air entrainment-dominated as the flame grew. Width-dependent B -numbers were calculated from the measured standoff distances. The B -number was width-independent for wider slabs but decreased with the decreasing sample width for narrower slabs as a result of lateral fuel diffusion and heat loss. The lateral fuel diffusion rate decreased with the increase of sample width, streamwise location and flow velocity. The flow velocities in this study had a slight effect on the B -number. The good agreement between predicted and experimental data suggested that classical flame spread theories under the infinite width assumption can apply to the finite width case with a modified B -number. [ABSTRACT FROM AUTHOR]

It is assumed that the description of the exchange processes between heterogeneous natural surfaces and the atmosphere within turbulence closure models is mainly limited by a realistic three-dimensional (3D) representation of the vegetation architecture. Within this contribution we present a method to record the 3D vegetation structure and to use this information to derive model parameters that are suitable for numerical flow models. A mixed conifer forest stand around a clearing was scanned and represented by a dense 3D point cloud applying a terrestrial laser scanner. Thus, the plant area density (PAD) with a resolution of one cubic meter was provided for analysis and for numerical simulations. Multi-level high-frequency wind velocity measurements were recorded simultaneously by 27 ultrasonic anemometers on 4 towers for a period of one year. The relationship between wind speed, Reynolds stress and PAD was investigated and a parametrization of the drag coefficient CD$C_D$ by the PAD is suggested. The derived 3D vegetation model and a simpler model (based on classical forest assessments of the site) were applied in a boundary layer model (BLM) and in large-eddy simulations (LES). The spatial development of the turbulent flow over the clearing is further demonstrated by the results of a wind tunnel experiment. The project showed, that the simulation results were improved significantly by the usage of realistic vegetation models. 3D simulations are necessary to depict the influence of heterogeneous canopies on the turbulent flow. Whereas we found limits for the mapping of the vegetation structure within the wind tunnel, there is a considerable potential for numerical simulations. The field measurements and the LES gave new insight into the turbulent flow in the vicinity and across the clearing. The results show that the zones of intensive turbulence development can not be restricted to the locations found in previous studies with more idealized canopies.

A non steady-state one-dimensional boundary layer model is presented which contains additional terms to describe the mechanical and thermal influences of vegetation elements on the air flow. Two model versions are used to simulate the diurnal variations of temperature, wind speed and direction and other parameters as well as sensible and latent heat fluxes in forests and above them. Some characteristical features of the temperature and wind speed fields in forests are reproduced by the simulation experiments in general agreement with observations.

Abstract: An experimental study and a boundary layer analysis were performed for the steam condensation in a vertical tube bundle passive condenser operating in a through flow mode. Four condenser tubes were submerged in a water pool and the heat from the condenser tube was removed through boiling. Experimental data were obtained for various system pressures (100–170kPa), inlet steam flow rates (15–47g/s) and non-condensable gas concentration (0–15%). The experimental results showed substantial deterioration in condensation when non-condensable gas was present. With increase in steam flow rate and system pressure the condensate rate increased. The boundary layer thickness and non-condensable gas concentration increased along the condenser tube length. [Copyright &y& Elsevier]

A quasi three–dimensional, intermediate planetary boundary layer (PBL) model is developed by including inertial acceleration with the Ekman momentum approximation, but a nonlinear eddy viscosity based on Blackadar’s scheme was included to improve the theoretical model proposed by Tan and Wu (1993). The model could keep the same complexity as the classical Ekman model in numerical, but extends the conventional Ekman model to include the horizontal accelerated flow with the Ekman momentum approximation. A comparison between this modified Ekman model and other simplified accelerating PBL models is made. Results show that the Ekman model overestimates (underestimates) the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow due to the neglect of the acceleration flow, however, the semi–geostrophic Ekman model overestimates the acceleration effects resulting from the underestimating (overestimating) of the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow. The Ekman momentum approximation boundary layer model could be applied to the baroclinic atmosphere. The baroclinic Ekman momentum approximation boundary layer solution has both features of classical baroclinic Ekman layer and the Ekman momentum approximate boundary lager. [ABSTRACT FROM AUTHOR]