University of Architecture, Civil Engineering and Geodesy
Recent publications
Solving engineering design tasks requires the use of analytical formulas and dependencies. The direct inclusion of mathematical expressions in the artificial neural network (ANN) is not possible. This research studies the possibility of applying the neural networks method for designing of single or double-side reinforced concrete sections. A Visual Basic for Applications (VBA) macro was developed in the MS Excel environment to solve the task of determining the required area of the reinforcement by given geometric dimensions and bending moment and applying classical analytical formulas for reinforced concrete sections design. The training of the pre-configured neural network is performed by approximately 34000 sets of matching input parameters. The presented results from the trained ANN are compared and analysed against the exact analytical solutions. The study presents an approach to the application of structural design calculations. The results suggest that the approach is applicable to more complicated structural design problems.
The present research focuses on the application of peanut and walnut shells, in their natural state, for the removal of methylene blue (MB) from water solutions in batch and dynamic (flow-through) conditions. Under batch conditions, at a 100 mg/L MB concentration of aqueous solution, the optimum dose of the studied biosorbents was determined to be 2.5 g/L, reaching about 95–97% efficiency of MB removal for both materials. Langmuir and Freundlich isotherms were used to model and evaluate the experimental data under different initial concentrations of MB (25 to 100 mg/L). The determined maximum adsorption capacities are 41.50 mg/g for walnut shells and 46.80 mg/g for peanut shells. Under flow-through conditions, breakthrough curves are determined for three sizes of fractions (<0.25 mm, 0.315–0.5 mm, and 0.8–1.6 mm). For peanut shells, the smaller the particle size, the higher the adsorption capacity and the column breakthrough time. For walnut shells, however, the particle size seems to have a more complex influence on the adsorption process parameters, and this phenomenon deserves future investigation. The adsorption capacity for one and the same fraction size of 0.315–0.5 mm and initial MB concentration of 50 mg/L is higher under dynamic flow conditions, i.e., 51 mg/g compared to 20 mg/g for walnut shells and 46 mg/g compared to 17.5 mg/g for peanut shells.
Algae-based wastewater treatment is a promising technology with various applications for excess biomass such as biofertilizer production or valuable elements extraction. The benefits of the technology have been discussed for larger wastewater treatment plants (WWTPs), but the use of microalgae in decentralized wastewater treatment has been barely reported. The current study screens the possible resource recovery potential of onsite technology, which adds algae-based post-treatment to the conventional biological treatment of domestic wastewater. The effluent from the onsite sequencing batch reactor (SBR) of a household was further processed in laboratory conditions using an SBR technology with two local monocultures of algae-Klebsormidium nitens (Kützing) Lokhorst and Tetradesmus obliquus (Turpin) M. J. Wynne. The decant and the generated algal biomass were analyzed in terms of their element content. The post-treated effluent has a slightly better quality for irrigation purposes than the effluent of the onsite treatment facility-up to 1.6 times increased concentration for macro-elements and up to 1.9 times for microelements. However, the generated algal biomass shows promising potential for re-use as a fertilizing agent since it contains valuable macro-and micro-elements and the heavy (hazardous) metal content is considerably lower than the limiting values in the current European and national legislations. The K. nitens strain may attract interest since it accumulates valuable metals such as chromium (36 mg/kgDS), nickel (83 mg/kgDS), and silver (0.7 mg/kgDS) that can be derived from the biomass and turn the technology to a circular one.
In the present paper we propose a microscopic model to study the multiferroic properties of Bi\(_2\)Fe\(_4\)O\(_9\) nanoparticles. The spontaneous magnetization \(M_s\) increases with decreasing nanoparticle size. \(M_s\) is shape dependent. It is larger for cylindrical than for spherical nanoparticles. \(M_s\) increases with increasing Co and Ho concentration, whereas by Mn doping it decreases. These tunable magnetic properties can be widely applied in spintronics. The polarization \(P_s\) increases also with decreasing nanoparticle size. Mn ion doping leads to increase of \(P_s\), the phase transition temperature \(T_C\) and the dielectric constant and so to enhanced electric and dielectric properties of Bi\(_2\)Fe\(_4\)O\(_9\) nanoparticles. Applying an external magnetic field \(P_s\) is enhanced, which is indirect evidence for a strong magnetoelectric coupling. The specific heat \(C_p\) shows an anomaly at the Neel temperature \(T_N\) which vanishes by applying an external magnetic field. The band gap energy \(E_g\) decreases with increasing Ti, Co and Ho dopants whereas by Mn doping \(E_g\) increases.
In this work, a sensitive coating based on Langmuir-Blodgett (LB) films containing monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) with an immobilized glucose oxidase (GOx) enzyme was created. The immobilization of the enzyme in the LB film occurred during the formation of the monolayer. The effect of the immobilization of GOx enzyme molecules on the surface properties of a Langmuir DPPE monolayer was investigated. The sensory properties of the resulting LB DPPE film with an immobilized GOx enzyme in a glucose solution of various concentrations were studied. It has shown that the immobilization of GOx enzyme molecules into the LB DPPE film leads to a rising LB film conductivity with an increasing glucose concentration. Such an effect made it possible to conclude that acoustic methods can be used to determine the concentration of glucose molecules in an aqueous solution. It was found that for an aqueous glucose solution in the concentration range from 0 to 0.8 mg/mL the phase response of the acoustic mode at a frequency of 42.7 MHz has a linear form, and its maximum change is 55 •. The maximum change in the insertion loss for this mode was 18 dB for a glucose concentration in the working solution of 0.4 mg/mL. The range of glucose concentrations measured using this method, from 0 to 0.9 mg/mL, corresponds to the corresponding range in the blood. The possibility of changing the conductivity range of a glucose solution depending on the concentration of the GOx enzyme in the LB film will make it possible to develop glucose sensors for higher concentrations. Such technological sensors would be in demand in the food and pharmaceutical industries. The developed technology can become the basis for creating a new generation of acoustoelectronic biosensors in the case of using other enzymatic reactions.
The magnetic, electric, and optical properties in Tb-doped BiFeO3 nanoparticles as functions of size and doping concentrations were investigated using a microscopic model, taking into account both linear and quadratic magnetoelectric (ME) coupling. We observed improved multiferroic properties and band-gap tuning. The magnetization and polarization increased with the decreased nanoparticle size and increased Tb-doping substitution x. The Neel temperature remained nearly unchanged whereas the Curie temperature was reduced with the increased x. There was doping-induced ME coupling. The dielectric constant is discussed as a function of the size, doping, and the magnetic field. The band gap decreased with the decreased size or increased Tb dopants due to competing effects of the compressive strain, oxygen defects on the surface, and Coulomb interactions. Increasing the Tb dopants and decreasing the nanoparticle size improved the ME effect.
The magnetic properties of pure and rare earth ion-doped Fe 3 O 4 nanoparticles are investigated using a microscopic model and the Green's function theory. The magnetization M s and Curie temperature T C are calculated depending on size, temperature and Gd doping concentration. M s and T C decrease with decreasing nanoparticle size and with increasing the doping concentration. The band gap energy increases with decreasing size and Gd dopants. The obtained results are in good agreement with the experimental data.
The band gap energy E g of orthorhombic β ‐ and hexagonal α ‐NaFeO 2 ‐bulk and nanoparticles is investigated for the first time based on a microscopic model combined with Green's function theory. E g increases with decreasing nanoparticles size and is shape‐dependent. The competition between Coulomb and electron–phonon interactions is demonstrated. Moreover, E g decreases with increasing Ge and Si doping concentration or increases by Mn, Cr, and Ni ion doping due to different radii of the doping and the host Fe ions, leading to varying strains. The substitution for Na ion by K or Li(Cu) ions can cause both enhancing or reducing of E g . Thus, the band gap energy can be regulated by nanoparticles size, ion doping, electron–phonon, and Coulomb interactions.
The water quality assessment of the surface water bodies (SWBs) is one of the major tasks of environmental authorities dealing with water management. The present study proposes a water quality assessment scheme for the investigation of the surface waters' physicochemical status changes and the identification of significant anthropogenic pressures. It is designed to extract valuable knowledge from the Water Frame Directive (WFD) mandatory monitoring datasets. The water quality assessment scheme is based on the Canadian Council of Ministers of the Environment water quality index (CCME-WQI), trend analysis of estimated WQI values, and Principal Component Analysis (PCA) using calculated excursions during the determination of WQI values. The combination of the abovementioned techniques preserves their benefits and additionally provides important information for water management by revealing the latent factors controlling water quality, taking into account the type of the SWB. The results enable the identification of the anthropogenic impact on SWBs and the type of the corresponding anthropogenic pressure, prioritization and monitoring restoration measures, and optimization of conducted monitoring programs to reflect significant anthropogenic pressures. The proposed simple and reliable assessment scheme is flexible to introducing additional water quality indicators (hydrological, biological, specific pollutants, etc.), which could lead to a more comprehensive surface water quality assessment.
Background Little is still known of how multiple urban exposures interact as health determinants. This study investigated various ways in which greenspace, traffic-related air pollution, and noise could operate together, influencing general health status. Methods In 2022, a cross-sectional population-based survey was conducted in Sofia, Bulgaria. Included were 917 long-term adult residents who completed questionnaires on poor self-rated health (PSRH), total time spent in physical activity (PA), home garden presence, time spent in urban greenspace and nature, and sociodemographics. Residential greenspace was operationalized using the normalized difference vegetation index (NDVI), tree cover density, number of trees, and access to local greenspace and parks. Nitrogen dioxide (NO2) was modeled for the study area. Road traffic, railway, and aircraft day-evening-night sound levels (Lden) were extracted from EU noise maps. Area-level income and urbanicity were considered. Analyses included multivariate ordinal regressions, interactions, and structural equation modeling (SEM). Results Associations with PSRH were per 0.10 NDVI 300 m: OR = 0.65 (0.42–1.01), home garden: OR = 0.72 (0.49–1.07), per 5 μg/m3 NO2: OR = 1.57 (1.00–2.48), per 5 dB(A) Lden road traffic: OR = 1.06 (0.91–1.23), railway: OR = 1.11 (1.03–1.20), and aircraft: OR = 1.22 (1.11–1.34). Spending >30 min/week in nature related to better health. In multi-exposure models, only associations with aircraft and railway Lden persisted. People with lower education and financial difficulties or living in poorer districts experienced some exposures stronger. In SEM, time spent in nature and PA mediated the effect of greenspace. Conclusions Greenspace was associated with better general health, with time spent in nature and PA emerging as intermediate pathways. NO2, railway, and aircraft noise were associated with poorer general health. These results could inform decision-makers, urban planners, and civil society organizations facing urban development problems. Mitigation and abatement policies and measures should target socioeconomically disadvantaged citizens.
Using a microscopic model, the magnetic and electric properties of pure and ion‐doped (on the Lu or Fe sites) hexagonal LuFeO 3 (LuFO) are studied and compared with the properties of ion‐doped LaFeO 3 (LaFO). The magnetization increases by Sr and Sc doping which is caused by the strain and the changes of the exchange interaction constants at the doped states. It is observed that by Sm doping of LuFO the polarization decreases, whereas by Sm doping of LaFO, the polarization increases with increasing Sm concentration. By Ir or Co ion doping on the Fe site, the magnetization and the phonon energy increase whereas the bandgap energy decreases. By doping on the Fe sites with the same ion, for example, Sc, there are no differences between LuFO and LaFO. A good agreement with the experimental data is observed. The doping effects can be used for different applications.
Traffic models have gained much popularity in recent years, in the context of smart cities and urban planning, as well as environmental and health research. With the development of Machine Learning (ML) and Artificial Intelligence (AI) some limitations imposed by the traditional analytical, numerical and statistical methods have been overcome. The present paper shows a case study of traffic modeling with scarce reliable data. The approach we propose resorts on the advantages of ensemble learning using a large number of related features such as road and street categories, population density, functional analysis, space syntax, previous traffic measurements and models, etc. We use advanced regression models such as Random Forest, XGBoost, CatBoost etc., ranked according to the chosen evaluation metrics and stacked in a weighted ensemble for optimal fitting. After a series of consecutive data imputations we estimate the annual average daily traffic distribution in the street and road network of Sofia city and the metropolitan municipality for 2018 and 2022, and the NO2 levels for 2021 with accuracy resp. 78%, 74% and 92%, using AutoGluon and Scikit-Learn.
The influence of size and doping effects on the magnetization M, phonon ω and band gap energy E g of MgO nanoparticles is studied using a microscopic model. The room-temperature ferromagnetism is due to surface or/and doping effects in MgO nanoparticles (NPs). The influence of the spin-phonon interaction is discussed. M increases with decreasing NP size. M and E g can increase or decrease by different ion doping (Co, Al, La, Fe) due to the different strain that appears. It changes the lattice parameters and the exchange interaction constants. We found that MgO NP with size of 20 nm and Fe-or Co-doping concentration x = 0.1 and x = 0.2, respectively, have a Curie temperature T C = 315 K, i.e., they are appropriate for application in magnetic hyperthermia, they satisfy the conditions for that. The energy of the phonon mode ω = 448 cm −1 increases with decreasing NP size. It increases with increasing Co and Fe, or decreases with Sr ion doping.
We have studied the magnetic properties of ion doped MnFe 2 O 4 nanoparticles with the help of a modified Heisenberg model and Green's function theory taking into account all correlation functions. The magnetization M s and the Curie temperature T C increase with decreasing particle size. This is the opposite behavior than that observed in CoFe 2 O 4 and CoCr 2 O 4 nanoparticles. By Co, Mg or Ni doping, M s and T C increase with enhancing the dopant concentration, whereas, by La or Gd doping, the opposite effect is obtained due to the different doping and host ionic radii which change the exchange interaction constants. The band gap energy E g is calculated from the s-d model. It can decrease or increase by different ion doping. The peak observed in the temperature dependence of the specific heat at T C is field dependent.
Sustainable development goals (SDGs) in the United Nations 2030 Agenda call for action by all nations to promote economic prosperity while protecting the planet. Projection of future land-use change under SDG scenarios is a new attempt to scientifically achieve the SDGs. Herein, we proposed four scenario assumptions based on the SDGs, including the sustainable economy (ECO), sustainable grain (GRA), sustainable environment (ENV), and reference (REF) scenarios. We forecasted land-use change along the Silk Road (resolution: 300 m) and compared the impacts of urban expansion and forest conversion on terrestrial carbon pools. There were significant differences in future land use change and carbon stocks, under the four SDG scenarios, by 2030. In the ENV scenario, the trend of decreasing forest land was mitigated, and forest carbon stocks in China increased by approximately 0.60% compared to 2020. In the GRA scenario, the decreasing rate of cultivated land area has slowed down. Cultivated land area in South and Southeast Asia only shows an increasing trend in the GRA scenario, while it shows a decreasing trend in other SDG scenarios. The ECO scenario showed highest carbon losses associated with increased urban expansion. The study enhances our understanding of how SDGs can contribute to mitigate future environmental degradation via accurate simulations that can be applied on a global scale.
Rainwater harvesting attracts rising interest in solving the new challenges associated with climate change and socio-economic development. Rainwater harvesting is addressed in various policies, but standards related to the harvested rainwater quality (HRWQ) are currently set mostly for reuse for agricultural purposes. This paper discusses the necessity for the introduction of specific legislative requirements for the HRWQ tailored to the reuse purpose, which would reduce the health and environmental risks. Based on a literature review of research outcomes regarding HRWQ parameters and existing legislation, the paper discusses the complexity of the factors influencing HRWQ and provides some thoughts for possible actions forward that could be undertaken toward the development of specific legislation. The actions include the application of a risk-based approach, the development of a database and guidance with technological solutions.
The finite elastic anisotropic solid with multiple elastic isotropic/anisotropic nanoinclusions of arbitrary shape, size, number and geometrical configuration subjected to time‐harmonic or transient loads is examined under plane strain conditions. The proposed mechanical model is based on a hybrid usage of elastodynamic theory for the bulk elastic anisotropic solid under non‐classical boundary conditions (BC), supplemented with a localized constitutive equation for the solid‐nanoinclusion interface in the framework of the Gurtin‐Murdoch theory of surface elasticity. The developed and verified numerical scheme for solution of the transient problem is based on the hybrid usage of direct and inverse Fourier transform (FFT), boundary element method (BEM) in conjunction with closed form frequency dependent fundamental solution of elastodynamic equation for anisotropic materials. The proposed model is flexible, numerically efficient and has virtually no limitations regarding the type of material anisotropy, nanoinclusions’ shape, size, number and geometrical configuration. The numerical results show a marked dependence of the wave fields on the surface elasticity effects, on the size, number and position of the nanoinclusions, on the type of material anisotropy, on the type and properties of the dynamic load, on the mutual interactions between nanoinclusions and between them and the external solid's boundary.
The multiferroic properties of ion-doped hexagonal and orthorhombic YFeO 3 (YFO) nanoparticles (NPs) are studied theoretically. The magnetization [Formula: see text] in h-YFO NPs increases, whereas for o-YFO NPs it decreases with decreasing NP size. In the dielectric constant (DC) both h- and o-YFO have a peak around [Formula: see text] and 460 K, respectively, but only in h-YFO an anomaly appears at [Formula: see text] K in the DC and the polarization which could be connected with a possible [Formula: see text]–[Formula: see text] phase transition. The polarization in pure and Bi-doped o-YFO NPs increases with increasing magnetic field. [Formula: see text] is studied by doping of a o-YFO NP with Ti[Formula: see text] ions at the octahedral Fe[Formula: see text] sites. [Formula: see text] in undoped YFO shows a small kink at [Formula: see text] K, whereas in the doped YFO it shows at [Formula: see text]480 K. By different ion doping on the Y or Fe sites in YFO there is a transformation from the h- to the o-phase or vice versa. In Mn-doped o-YFO a spin-reorientation transition appears. The bandgap of h-YFO is smaller compared to that of o-YFO.
Properties of the Langmuir-Blodgett (LB) films of arachidic and stearic acids, versus the amount of the films’ monolayers were studied and applied for chloroform vapor detection with acoustoelectric high-frequency SAW sensors, based on an AT quartz two-port Rayleigh type SAW resonator (414 MHz) and ST-X quartz SAW delay line (157.5 MHz). Using both devices, it was confirmed that the film with 17 monolayers of stearic acid deposited on the surface of the SAW delay line at a surface pressure of 30 mN/m in the solid phase has the best sensitivity towards chloroform vapors, compared with the same films with other numbers of monolayers. For the SAW resonator sensing using slightly longer arachidic acid molecules, the optimum performance was reached with 17 LB film layers due to a sharper decrease in the Q-factor with mass loading. To understand the background of the result, Atomic Force Microscopy (AFM) in intermittent contact mode was used to study the morphology of the films, depending on the number of monolayers. The presence of the advanced morphology of the film surface with a maximal average roughness (9.3 nm) and surface area (29.7 µm2) was found only for 17-monolayer film. The effects of the chloroform vapors on the amplitude and the phase of the acoustic signal for both SAW devices at 20 °C were measured and compared with those for toluene and ethanol vapors; the largest responses were detected for chloroform vapor. For the film with an optimal number of monolayers, the largest amplitude response was measured for the resonator-based device. Conversely, the largest change in the acoustic phase produced by chloroform adsorption was measured for delay-line configuration. Finally, it was established that the gas responses for both devices coated with the LB films are completely restored 60 s after chamber cleaning with dry air.
Magnetoelectroporation is an effective method of opening nanopores in cell membranes using magnetoelectric nanoparticles (MENPs) for the purpose of delivery of in vivo and in vitro of drug substances to cancer cells. We propose a microscopic approach as theoretical basis for that phenomenon. The underlying Hamiltonian includes the magnetic and ferroelectric subsystems characterized by two order parameters. The related magnetoelectric coefficient αHE characterizes the relationship between the applied magnetic field and a generated local electric field. Whereas the spontaneous polarization P → S of the magnetoelectric NP is due to the arrangement of electric dipoles there appears an additional spin‐assisted polarization Δ P → owing to the magnetic phase transition. We show the main contribution to the local field comes from P → S . Moreover the local electric field depends on the orientation of the easy‐axis magnetization of the MENPs with respect to applied external magnetic field. The magnetoelectric coefficient exhibits a non‐linear dependence of the external magnetic field. The results are based on an analytical Green’s function method. The numerical calculations are performed for spherical, structurally heterogeneous nanoparticles composed on a core and a shell, where the non‐interacting nanoparticles have the same diameter of 25 nm. The results are in good qualitative agreement with experimental observations This article is protected by copyright. All rights reserved.
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509 members
Lachezar Hrischev
  • Department of Construction Technology and Mechanisation
Milena Tasheva - Petrova
  • Department of Urban Planning
Lyubomir Zdravkov
  • Department of Steel, Timber and Plastic Structures
Konstantin Velinov
  • Department of Construction Management and Economics
Stoyanka M. Ivanova
  • Department of Computer-Aided Engineering
Hr. Smirnenski 1, 1046, Sofia, Bulgaria
Head of institution
Prof. Dr. Eng. Ivan Markov
+359 2 963 5245