The effect of surface roughness in laminar flow has been the focus of recent research related to drag reduction. However, although particle transport is governed by laminar flow in most applications, the effect of surface texture on the drag of a sphere has mostly been addressed in the transitional and turbulent regimes. The aim of the present study is to explore the drag behavior of rough spherical micro-particles in laminar flow. The spheres’ roughness has been structured based on a 3D complex Weaire–Phelan model, as well as on a simpler orthogonal lattice one, and quantified as per various definitions. The emerging surface roughness comprises irregular elements in terms of shape and size. The investigation has been performed at Reynolds numbers ranging from 2 to 8. The drag coefficient is found to drop quadratically with increasing roughness. Relative roughness can reduce the total drag on the particle by over 21%. The key physical mechanism is explained by the particles’ surface cavities, which contain recirculating, nearly stagnant fluid, thus creating a self-lubricating effect that reduces skin friction, as the main flow skims over the top without entering the cavities. A reduction in total drag arises when skin friction drag reduction is larger than the increase in form drag. Understanding the drag behavior of spherical particles with irregular surface texture provides new and useful insight into low Reynolds number transport phenomena related to a variety of engineering applications.

Greening is considered a sustainable and environmentally friendly policy for urban development but despite the progress made so far, the aerodynamic implications of building embedded greening have yet to be determined. Greening may have a variety of implications on wind loads, natural ventilation and air exchange, all related to the pressure distribution on buildings' external surfaces. In the present study, a wind tunnel investigation deals with the qualitative and quantitative effects of building greening on the external surface pressure distribution of a model building. Greening was considered to fully cover the external windward fa & ccedil;ade and the roof under various permeabilities and thicknesses. Openings on the side walls allow for indoor-outdoor air exchange. The position and the thickness of greening were found to be decisive parameters for the overall pressure distribution but attention was also directed towards the fluctuating pressures near the openings, which are expected to be the driving mechanism for air exchange in this configuration. Effects of rooftop greening were confined mainly to the roof and the wake but for windward fa & ccedil;ade greening, mean and fluctuating pressure differences of up to 35%, compared to the bare building, were observed in the vicinity of the openings. The major effect was a damping of the lower frequencies of pressure fluctuations and it had a stronger dependence on the thickness rather than the material properties of the windward fa & ccedil;ade greening.

This study presents the preliminary results of the ongoing research project “Re.Nature Cities”, in which the ability of street trees to act as an effective measure against increased urban air temperatures is evaluated via experimental and simulation means. In the existing literature, numerous studies highlight that the addition of street trees inside the canyons of urban areas may result in a significant reduction of the peak ambient summer Tair, having also a prominent effect on outdoor thermal comfort regulation. Yet, street trees also impact urban ventilation as they act as barriers, disturbing the wind flow and affecting buildings’ energy needs and thermal comfort; the positive effect of wind sheltering during the cold winter period, can be thus significantly counterbalanced during the warmer periods of the year. The existing evidence reveals that the green elements’ implementation in the built environment without holistically accounting for all the vegetation-air-buildings interactions, can even exacerbate human discomfort and deteriorate indoor natural ventilation.Based on the above, this study evaluates the mitigation potential of a tree type that is commonly encountered in Greek cities – the citrus- since it has low irrigation needs and high drought tolerance. An integrated experimental campaign, employing wind tunnel measurements, albedo and Leaf Area Index/Leaf Area Density (LAI/LAD) measurements is conducted so as to define of the aerodynamic, thermal and foliage characteristics of real trees. Wind tunnel measurements of total drag are carried out in a wind tunnel section of 3.5m width and 2.5m height, while LAI measurements are conducted using a plant canopy analyzer, with the LAD of each layer (1 m/layer) then calculated from LAI by empirical equations. The obtained values are then used as input parameters in the vegetation model of the ENVI-met microclimate model, which is employed for the evaluation of the thermal environment of typical building blocks in Greece, considering different planting patterns and vegetation coverage scenarios.The experimental database of foliage, thermal and aerodynamic characteristics of common urban tree species, along with the detailed microclimatic simulations of typical urban districts provide a valuable tool for decision-making regarding the optimal vegetation coverage and the planting pattern for urban areas.

Wind turbine wake and modelling is crucial to optimizing future wind farm layouts and hence reducing the cost of energy. This paper presents the first phase of a blind test on modelling controlled and uncontrolled wind turbine wakes. The blind test is based on wind tunnel experiments of two model scale wind turbines (D = 1.1 m) one downstream of the other. The exercise is split into two phases and the first one is presented here, where participants are invited to simulate the baseline case, in which both turbines are aligned with the flow and there is no control on the either turbine. The objective of this phase is to ensure all participants can benchmark their numerical approach against a baseline open data set, where no wake control is applied. Experimental measurements include inflow velocity, turbine power and loads for a range of tip speed ratios. In the second phase, not presented here, the wake of the upstream turbine will be controlled and the performance of the downstream one will be recorded. This will be a blind test with the data not released prior to submissions. The present paper gives an overview of the initial, open benchmark case, including its objectives, methodology and experimental results.

The application of Data Assimilation (DA) methods in Computational Fluid Dynamics (CFD) problems is a concept actively being explored to couple CFD with Experimental Fluid Dynamics data. Here, Particle Image Velocimetry (PIV) data are assimilated in an OpenFOAM based CFD solver to calculate the velocity and pressure fields of the turbulent flow past a surface mounted cube inside an atmospheric boundary layer for three planes belonging to the symmetry plane of the flow. At first, the SIMPLE algorithm is used to correct both pressure and velocity fields, with the PIV data used to formulate the initial and boundary conditions. The Reynolds stresses are calculated directly from the PIV data instead of using a turbulence model. Next, we use two implementations of the nudging method and two formulations of the Kalman Filter in order to assimilate the PIV data into the iterative SIMPLE procedure. A grid independence study is performed, and the performance of the different methods is assessed. The CFD predicted pressure field is in good agreement with pressure measurements on the cube surface. The results also show that the SIMPLE based correction step already leads to a significant reduction of both the mean and the variance of the continuity errors as well as the difference between the original PIV data and the resulting velocity fields. The application of the DA methods, particularly the KF, leads to minor further improvement of the results but does improve convergence of the CFD solver.

Surface pressure measurement via pressure taps is an integral part of wind tunnel testing. Commonly, the pressure signal is transferred from the taps to a pressure measurement device through an appropriate system of tubing and possibly other components. Depending on its characteristics, the system distorts the dynamics of the signal and, when these dynamics are of interest, appropriate correction/calibration is necessary, taking into account its frequency response. In this context, a novel approach of tubing dynamic calibration is proposed here, using a single pressure measurement device instead of two or more, as is commonly done. The approach accounts for both the amplitude distortion and the phase shift of a selectable range of computer-generated dynamic signals, produced through a speaker. Apart from the innovative use of a single pressure sensor, e.g. in situations where a suitable multi-port pressure scanner is unavailable, the principal merits of the proposed procedure include its straightforward implementation whenever a component of the tubing system is altered (e.g. tube length, different pressure scanner) and the elimination of uncertainty stemming from differences between pressure sensors due to malfunction, inappropriate calibration and inattentive maintenance. The procedure is successfully validated by applying it to two different types of input signal.

Windblown dust plays a crucial role in the Earth system, impacting climate, ecosystems, human activities, and health. The spatiotemporal evolution of dust plumes during transport is determined by wind, the primary driver of dust emission. In this study, we utilize outputs from the ECMWF-IFS, assimilating quality-assured Aeolus wind profiles, to initialize dust simulations with the WRF-Chem model. The aim is to assess the impact of Aeolus wind observations on modeling the desert dust cycle. Focusing on the ASKOS/JATAC campaign in September 2021 near Cabo Verde, we qualitatively and quantitatively evaluate the simulated dust-related outputs, revealing that even small differences in wind significantly affect the simulated dust emission rates and dust optical depth.

Modeling homogeneous Atmospheric Boundary Layers (ABLs) in Computational Wind Engineering (CWE) requires that all of the conservation equations are in equilibrium. A standard approach is to apply appropriate boundary conditions in order to maintain the inlet prescribed ABL profiles. Recently, the investigation has turned to the implementation of suitable source terms in the momentum equation. In the present work, the ABL is numerically simulated for neutral, stable and unstable atmosphere using nudging, a type of Data Assimilation. Simulations were performed with an in-house code that solves the Reynolds-averaged Navier-Stokes equations and the standard k-epsilon turbulence model in steady state. The prescribed inlet flow profiles correspond to the reference data, which is used to define the nudging source terms that are added to the equations. The source terms play a role resembling a body-force and a source-sink mechanism in the momentum and TKE equations, respectively. This novel approach is shown to be reliable in ensuring streamwise homogeneity, under both neutral and non-neutral conditions, for the equilibrium ABL.

Ventilation through openings has long been considered an effective means of improving indoor air quality in living spaces. Although green roofs and facade greening are wide spread for urban buildings, their effect on ventilation is not yet fully understood. This paper presents a fundamental experimental investigation of the relevant aerodynamic effect on the air exchange of an isolated building exposed to wind. The study was performed in an atmospheric boundary layer wind tunnel using a reduced scale (M = 1:300) cubic model building exposed to 0 degrees wind with openings on its two side faces. Vegetation of different permeabilities, heights and thicknesses were placed on the building, and tracer gas measurements were performed. These properties may correspond to different vegetation species e.g ivy or shrubby plants, commonly used for facade or rooftop greening. Experimental results show that, regardless of species, facade greening on the windward wall attenuates air exchange rate while rooftop greening enhances it, especially at the upper building levels. When applying facade or rooftop greening of the same properties, the building's air exchange rate is reduced by up to 16% but increases by only up to 8%, respectively. The effect of vegetation thickness is more evident for the thicker layers of facade greening. It is notable however, that a layer of low-permeability facade greening has a minor influence on air exchange rate.

One of the deficiencies of atmospheric dust models is that they struggle to accurately reproduce the transport of coarse and giant dust particles, according to observational evidence. Among the reasons behind that model incapacity that have been proposed in the literature, is the issue of numerical diffusion inside the advection codes of the models. In this study, we examine the importance of that issue in the WRF-L model. To do so, we update the default numerical scheme (UPWIND) which is used for the vertical advection of dust due to gravitational settling. The diffusive UPWIND scheme is replaced with a non-diffusive one, named UNO3 (third-order Upstream Non-Oscillatory scheme). To test the code performance, we perform simulations reproducing the 2D transport of a dust plume which is released at 4 km height above Cabo Verde towards Barbados. The model is initialized on 13/06/2014 at 12 UTC (which coincides with the day of the SALTRACE flight above Cabo Verde) using meteorological conditions of radiosonde from Tenerife airport and wind profile based on ECMWF model climatology. The results suggest that, in the UNO3 simulation, dust particles with a diameter 26 μm can be transported more than 500 km longer than in the BASE simulation and the dust in the atmosphere can be 10% more in the UNO3 simulation compared to the BASE simulation. In future studies, the UNO3 scheme will be tested in other aerosol types also (e.g. volcanic ash, smoke from fires).Acknowledgements: Authors acknowledge support by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “2nd Call for H.F.R.I. Research Projects to support Post-Doctoral Researchers” (Project Acronym: StratoFIRE, Project number: 3995) and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Project Acronym: D-TECT, Grant Agreement: 725698).

Dust particles larger than 20 µm in diameter have been regularly observed to remain airborne during long-range transport. In this work, we modify the parameterization of the mineral dust cycle in the GOCART-AFWA dust scheme of WRFV4.2.1 to also include such coarse and giant particles, and we further discuss the underlying misrepresented physical mechanisms which hamper the model in reproducing adequately the transport of the coarse and giant mineral particles. The initial particle size distribution is constrained by observations over desert dust sources. Furthermore, the Stokes drag coefficient has been updated to account for realistic dust particle sizes (Re < 105). The new code was applied to simulate dust transport over Cabo Verde in August 2015 (AER-D campaign). Model results are evaluated against airborne dust measurements and the CALIPSO-LIVAS pure dust product. The results show that the modeled lifetimes of the coarser particles are shorter than those observed. Several sensitivity runs are performed by reducing artificially the particles' settling velocities in order to compensate underrepresented mechanisms, such as the non-spherical aerodynamics, in the relevant parameterization schemes. Our simulations reveal that particles with diameters of 5.5–17 and 40–100 µm are better represented under the assumption of an 80 % reduction in the settling velocity (UR80), while particles with sizes ranging between 17 and 40 µm are better represented in a 60 % reduction in settling velocity (UR60) scenario. The overall statistical analysis indicates that the best agreement with airborne in situ measurements downwind (Cabo Verde) is achieved with a 40 % reduction in settling velocity (UR40). Moreover, the UR80 experiment improves the representation of the vertical structure of the dust layers as those are captured by the CALIPSO-LIVAS vertically resolved pure dust observations. The current study highlights the necessity of upgrading the existing model parameterization schemes of the dust life-cycle components towards improving the assessment of the dust-related impacts within the Earth–atmosphere system.

The flow past a square cylinder under the influence of a one dimensional gust was investigated using computational fluid dynamics (CFD). The effect of upstream wind gusts of the same amplitude but different duration was investigated with respect to their effect on the flow, the vortex-shedding, and the pressure distribution around the square cylinder. For the computations, a very large eddy simulation (VLES) model was implemented in an in-house code and validated against numerical and experimental results from the literature. The gusts of different duration were found to have a distinctly different effect. The short-duration gust causes a lock-on behavior with cessation of the alternating vortex shedding, and a symmetric pair-vortex was created above and below the square cylinder. It was observed that the pressure distribution on the lateral sides of the cylinder has the same magnitude and phase, which resulted in a zero total lift coefficient. In terms of a free-standing structures, such as a building, this would lead to zero instantaneous forces and pressure difference in the lateral direction with obvious implications for dynamic response and cross ventilation.

The calculation of the pressure field on and around solid bodies exposed to external flow is of paramount importance to a number of engineering applications. However, conventional pressure measurement techniques are inherently linked to problems principally caused by their point-wise and/or intrusive nature. In the present paper, we attempt to calculate a time-averaged two-dimensional pressure field by integrating PIV (particle image velocimetry) velocity measurements into a CFD code and modifying them by the respective correction step of the SIMPLE algorithm. Boundary conditions are applied from the PIV data as a three-layer area of constant velocities adjacent to the boundaries. A novel characteristic of the approach is the straightforward inclusion of the Reynolds stresses into the source terms of the momentum equations, calculated directly from the PIV statistics. The methodology is applied to three regions of the symmetry plane parallel to the main boundary layer flow past a surface-mounted cube. In spite of findings of deviations from the planar 2D flow assumption, the derived pressure fields and the adjusted velocity fields are found to be reliable, while the intrinsic turbulent nature of the flow is considered without modelling the Reynolds stresses.

Mesoscale numerical weather prediction models usually provide information regarding environmental parameters near urban areas at a spatial resolution of the order of thousands or hundreds of meters, at best. If detailed information is required at the building scale, an urban-scale model is necessary. Proper definition of the boundary conditions for the urban-scale simulation is very demanding in terms of its compatibility with environmental conditions and numerical modeling. Here, steady-state computational fluid dynamics (CFD) microscale simulations of the wind and thermal environment are performed over an urban area of Kozani, Greece, using both the k-ε and k-ω SST turbulence models. For the boundary conditions, instead of interpolating vertical profiles from the mesoscale solution, which is obtained with the atmospheric pollution model (TAPM), a novel approach is proposed, relying on previously developed analytic expressions, based on the Monin Obuhkov similarity theory, and one-way coupling with minimal information from mesoscale indices (Vy = 10 m, Ty = 100 m, L*). The extra computational cost is negligible compared to direct interpolation from mesoscale data, and the methodology provides design phase flexibility, allowing for the representation of discrete urban-scale atmospheric conditions, as defined by the mesoscale indices. The results compared favorably with the common interpolation practice and with the following measurements obtained for the current study: SODAR for vertical profiles of wind speed and a meteorological temperature profiler for temperature. The significance of including the effects of diverse atmospheric conditions is manifested in the microscale simulations, through significant variations (~30%) in the critical building-related design parameters, such as the surface pressure distributions and local wind patterns.

Heat absorbers critically affect efficiency in a wide range of applications, such as solar power generation and industrial heat utilization. The present study experimentally compares the thermal performance of two conceptually different flat-plate heat absorbers, based on: a) a plate with straight rectangular, parallel channels and b) a serpentine tube-on-plate configuration with a series of unconventional, composite U-bends. This is a novel study as the devices and operating conditions are primarily intended for use in the receiver of a linear-focus, concentrating solar collector with Fresnel reflectors. Experiments were performed on an electrically heated laboratory-scale setup with a thermal oil as the heat transfer fluid. Measurements were taken under a wide range of operating conditions, representative of a medium-temperature concentrating solar thermal system. Steady-state efficiencies, heat removal factors and heat transfer coefficients were determined. For the absorber with rectangular channels, the steady-state thermal efficiency was found to be up to 18% higher than the serpentine-tube configuration, except for the upper range of operating temperatures, where the latter configuration exhibited a higher overall efficiency by up to 9%. The thermal-hydraulic behavior of the two absorbers was analyzed in terms of several physical effects such as the flow regime (which differed between the two configurations), non-uniform heating and buoyancy. For the serpentine-tube configuration, a new correlation was proposed, accounting for the transition and buoyancy effects present in the operating range considered. Given that the majority of flat receivers for solar thermal systems investigated so far have been characterized by low geometric (length-to-width) aspect ratios, the study of the present elongated geometries, indeed including an unconventional composite U-bend concept, aims to provide insight into the thermal-hydraulic behavior and to contribute to the efficient designs of receivers for linear-focus, concentrating solar collectors.

Air pollution episodes are common in the Greater Athens Area (GAA) because of its pollutant source concentration, complex topography, and regional meteorology. For the GAA, over 90% of the existing building stock is classified below energy class B, so the effects of upgrades on energy savings are obvious and have been widely discussed. The present study focuses on the potential effects that a realistic level of implementation of the EPBD will have on the air quality over the GAA. Three renovation scenarios were examined with an implementation rate of 20%. Numerical simulation of primary pollutants’ dispersion over the GAA was performed using the CALPUFF modelling system. Ground level concentrations of SO2, NOx, CO and PM10 were calculated, taking into account pollutant releases from major roads and highways, passenger ports and cargo transport, residential heating installations and major industrial installations. The required source data was taken from National and European statistics, demographics and local topography while the Weather Research and Forecasting (WRF) model was implemented for the required high resolution meteorological field data. The modelling system was first successfully validated by comparison with available measurements of pollutant concentrations and then applied to case studies of EPBD implementation. Reductions up to ∼6% were found in the ground level concentrations of major pollutants.

The high concentration of pollutant sources, complex topography, and regional meteorology are all factors that may contribute to air episodes in dense urban areas. Energy use in buildings is a significant source of pollution in the Greater Athens Area (GAA), Greece, where over 90% of the existing building stock has been classified below energy class B. The present study focuses on the potential effects that a realistic level of building energy efficiency upgrades will have on the air quality over the GAA. Results are expected to be relevant to similar urban areas. Furthermore, the study of primary pollutants’ dispersion is applied at a 1.2 × 1.2 km spatial resolution, providing significant local (neighborhood) level information. Numerical simulations were performed using EPA’s CALPUFF modeling system with wind field input from an independent numerical weather prediction using NCAR’s Weather Research and Forecasting (WRF) model. In order to calculate emission rates from major roads, highways, shipping ports, residential heating installations, and major industrial facilities, data were taken from National and European statistics, demographics, and local topography. After validation, the modeling system was used to examine three building energy efficiency upgrade scenarios, implemented on 20% of the buildings. Ground level concentrations of SO2, NOx, CO, and PM10 were calculated and reductions of up to 9% were found for GAA maximum values but up to 18% for local values that were also close to or above the European safety thresholds.

This paper provides the performance evaluation of the meteorological component of The Air Pollution Model (TAPM), a nestable prognostic model, in predicting meteorological variables in urban areas, for both its surface layer and atmospheric boundary layer (ABL) turbulence parameterizations. The model was modified by incorporating four urban land surface types, replacing the existing single urban surface. Control runs were carried out over the wider area of Kozani, an urban area in NW Greece. The model was evaluated for both surface and ABL meteorological variables by using measurements of near-surface and vertical profiles of wind and temperature. The data were collected by using monitoring surface stations in selected sites as well as an acoustic sounder (SOnic Detection And Ranging (SODAR), up to 300 m above ground) and a radiometer profiler (up to 600 m above ground). The results showed the model demonstrated good performance in predicting the near-surface meteorology in the Kozani region for both a winter and a summer month. In the ABL, the comparison showed that the model’s forecasts generally performed well with respect to the thermal structure (temperature profiles and ABL height) but overestimated wind speed at the heights of comparison (mostly below 200 m) up to 3–4 ms−1.

Electricity generation on site is a design challenge aiming at supporting the concept of energy-autonomous building. Many projects worldwide have promoted the installation of photovoltaic panels on urban buildings, aiming at utilizing a large area to produce electricity. In most cases, photovoltaics are considered strictly as electricity generators, neglecting their effect to the efficiency and to the thermal behaviour of the building envelope. The integrated performance of photovoltaic ventilated fa?ades, where the photovoltaics are regarded as part of a complicated envelope system, provides design challenges and problems that cannot be overlooked within the framework of the Nearly Zero Energy Building concept. In this study, a finite volume model for photovoltaic ventilated fa?ades is developed, experimentally validated and found to have a significant convergence to measured data.