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ENVI-met Model Architecture
Overview in a Nutshell
ENVI-met is a holistic three-dimensional non-hydrostatic model for the simulation of surface-plant-air interactions not only limited to, but very often used to simulate urban environments and to asses the effects of green architecture visions. It is designed for microscale with a typical horizontal resolution from 0.5 to 10 m and a typical time frame of 24 to 48 hours with a time step of 1 to 5 seconds. This resolution allows to analyze small-scale interactions between individual buildings, surfaces and plants.
The model calculation includes:
- Shortwave and longwave radiation fluxes with respect to shading, reflection and re-radiation from building systems and the vegetation
- Transpiration, Evaporation and sensible heat flux from the vegetation into the air including full simulation of all plant physical parameters (e.g. photosynthesis rate)
- Dynamic surface temperature and wall temperature calculation for each facade and roof element supporting up to 3 layers of materials and 7 calculation points in the wall/ roof.
- Water- and heat exchange inside the soil system including plant water uptake
- 3D representation of vegetation including dynamic water balance modelling of the individual species
- Dispersion of gases and particles. The model supports particles (including sedimentation and deposition at leafs and surfaces), inert gases and reactive gases of the NO-NO2-Ozone reaction cycle.
- Calculation of biometeorological indices such as Mean Radiant Temperature, PMV/PPD, PET or UTCI via BioMet
The LITE/TRIAL version of ENVI-met is free and comes along as a complete model including the basic simulation core for non-commercial applications, but is restricted in domian and calculation features. In addition to the free version (“ENVI-met LITE”), there are the BUSINESS Version and SCIENCE Version which include advanced modelling features and the newest simulation techniques. While the LITE version can be downloaded and used freely based on the Creative Commons License BY-NC-SA, the BUSINESS Version and SCIENCE Version need to be licensed and are not free.
Wind field ENVI-met includes a full 3D Computational Fluid Dynamics (CFD) modell. It solves the Reynolds-averaged non-hydrostatic Navier-Stokes equations for each grid in space and for each time step. The effects of vegetation are included as drag forces in the wind field. For detailed building physics simulation, the wind flow close to each facade and roof segment is calculated. With the new Single Wall feature, wind patterns inside complex or semi-open structures can be simulated as well.
The wind flow is updated at given time intervals. ENVI-met also supports a real-time flow calculation which means that the flow field is treated as a normal prognostic variable and calculated each step. Due to the very small time steps needed here, this way of calculation need very powerful computers.
Air temperature and humidity Air temperature and specific humidity of the air are determined by the different sources and sinks of sensible heat and vapour inside the model domain. Based on the calculated three-dimensional wind field, advection and diffusion in the air is simulated. The ground surface and vegetation leafs act as sources or sinks for both temperature and humidity in the atmosphere model. Building walls and roofs mainly act as surfaces interchanging heat with the atmosphere, but can also act as humidity sources if facade or rooftop greening is applied.
Turbulence Turbulence is calculated using the E-epsilon 1.5order closure (“E-epsilon” or “k-epsilon” model). Two prognostic equations for turbulent energy production (E) and its dissipation (epsilon) are used to simulate the distribution of turbulent energy. Exchange coefficients (K) in the air are calculated using the Prandtl-Kolmogorov relation. For low wind situations, the 1st order mixing length model can be used instead of the E-epsilon model (which often fails in this situations).
Radiative fluxes ENVI-met contains newly developed analysis modules to model the fluxes of shortwave and longwave radiation inside of complex environments. The scheme takes into account shading by complex geometries, reflections by different surface and building materials and the effect of vegetation on all radiative fluxes. The Expert Version introduces the new IVS method, in which each urban element is considered using its actual state (sun reflection, thermal radiation) instead of averaged fluxes.
Pollutant dispersion The pollutant dispersion model of ENVI-met allows the synchronous release, dispersion and deposition of up to 6 different pollutants including particles, passive gases and reactive gases. Sedimentation and deposition at surfaces and vegetation is taken into account as well as the photochemical reaction between NO, NO2 and Ozone (O3). The Expert version of ENVI-met also allows to simulate the effects of photocatalytic activated surfaces on air quality.
Surface and soil temperature The surface temperature and the distribution of soil temperature is calculated for natural soils and for artificial seal materials down to a depth of -4m. For each vertical grid layer a different soil or sealing material can be chosen in order to simulate different soils structures. The heat conductivity of natural soils is calculated with respect to the actual soil water content.
Soil water content Simulating the water balance of the surface and the soil is a crucial aspect in urban microclimatology. While humid soils can act as cooling devices, dry soils are often hotter than asphalt. In addition, the cooling effect, and -on a longer time perspective- the vitality of vegetation depends on available soil water. ENVI-met dynamically solves the soil hydraulic state of the soil based on Darcy's law taking into account evaporation, water exchange inside the soil and water uptake by plant roots. The consulting version of ENVI-met can also handle complex types of irrigation methods or subsurface water sources.
Vegetation water supply Plants are living organisms and will only contribute in positive way to the local microclimate, if enough water is available in the soil within the root zone. Together with the simulation of the soil water content and the new sophisticated 3D root model, the dynamic water supply of the plant and the resulting water extraction from the soil can be calculated.
Water bodies and pounds Water bodies are represented as a special type of soil. The calculated processes inside the water include the transmission and absorption of shortwave radiation inside the water. No second energy balance is used for the ground surface of the water pool, so that heating of shallow systems is lower than under real conditions where the main source of energy is the convection from the water ground surface rather than the absorption of radiation. In addition, no turbulent mixing is included in the model so that the use is restricted to still waters (e.g. lakes).
3D Plant Geometry ENVI-met supports simple vertical plants such as grass or corn, but also allows complex 3D vegetation geometries like large trees. All plants are treated as individual species with an integrated water balance control and heat and water stress reaction concept.
Foliage temperature The temperature of the leafs is calculated by solving the energy balance of the leaf surface with respect to the actual meteorological and plant physiological conditions for each grid box of the plant canopy. The health status of the plant and the water supply by the soil regulate, beside other factors, the plants transpiration rate and thereby the leaf temperature. ENVI-met uses a sophisticated model to simulate the stomata behaviour of the vegetation in response to microclimate, CO2 availability and water stress level.
Exchange processes with the environment Vegetation interacts in various ways with the environment: Heat and vapour are exchanged between the plants leafs and the atmosphere. Transpired water will be -if possible- extracted out of the soils hydraulic model using the plant root distribution. A complex raytracing algorithm is used to analyse the plants impact of solar radiation (shadow casting) and on longwave radiation exchange (thermal shielding).
Vegetation health assessment/ TreePass/ Wind Risk Assessment The impact of vegetation on the microclimate is only one side of the story. To grow and live, plants also need adequate local climate conditions fitting to their individual demand profile. Too much or too little sun, to heavy wind loads on the trees or stagnating air: Many aspects of the microclimate can interfere with the plants requirements and hinder a healthy and robust growth. Using our TreePass technology, the local growing conditions and the plants profiles can be matched to provide an optimal placement. Our wind risk assessment simulates the tree mechanics in order to assess the risk of wind and storm damage depending on the crown structure and the tree location. (Consulting only).
Built environment & Building system
Full 3D building geometry & Single walls Complex buildings and other structures can be constructed in full 3D with no limitations in complexity as far as the cubic base structure allows. This allows the simulation of semi-open spaces such as the soccer stadium shown in the icon and in this example. Moreover, ENVI-met Expert allows the usage of single thin walls that can be applied to any grid which can be used to represent spaces wich are enclosed by walls but do not behave like a building e.g. bus stop shelters, shading structures…
Detailed building materials In Detailed Design Mode, ENVI-met allows to assign individual wall types to each wall and roof surface. The wall types can be composed out of 3 layers of different materials with individual physical properties such as solar radiation transmission, heat capacity or heat conductivity. The different wall and roof materials can be designed graphically using the Database Manager.
High resolution building physics Each wall and roof segment in ENVI-met is represented by its own thermodynamical model consisting of 7 prognostic calculation nodes. The temperature of the outside node is updated continuously with respect to the meteorological variables at the facade and the thermal state of the buildings and other objects within the view range of the facade/roof element considered. The thermal state of the inner wall nodes is calculated from the physical properties assigned to the wall/roof based on Fourier's law of heat conduction. →More...
Building energy performance ENVI-met dynamically calculates the development of the building indoor temperature as a result of the incoming and outgoing fluxes through the associated wall and roof segments. This building energy simulation is executed parallel to the outdoor microclimate simulation for each building in the model domain so that a constant feedback between the outdoor and indoor climate conditions and of the interactions between buildings is provided. The recent version allows an initial zoning model to define building sections and thermal zones.
Green Wall and Roof Systems ENVI-met Expert allows a detailed simulation of the energy and vapour exchange processes take place at green walls and green roof tops. The Green Wall System (GWS) integrates seamlessly into the dynamic calculation of the building energy performance and the facade/ wall temperature and supports a wide range of different systems from simple climbing plants up to living wall systems.
Numerical discretisation scheme ENVI-met uses an orthogonal Arakawa C-grid to represent its environment. Topography is included by marking cells as being filled with soil. As a consequence of this scheme, ENVI-met does only allow straight and rectangular structures. For the ground surface, the exact exposition and inclination is taken into account for the energy balance calculations. For building walls and roofs, inclined or curvy surfaces must be approximated by grid points.
Numerical methods ENVI-met uses the Finite Difference Method to solve the multitude of partial differential equations (PDE) and other aspects in the model. The scheme is partly implicit, partly explicit depending on the sub system analysed. The atmospheric advection and diffusion equations are implemented in a fully implicit scheme, which allows ENVI-met to use relatively large time steps by still remaining numerically stable. This, in the final effect, reduces computing costs and allows the ENVI-met model to run on any normal computer available at your local hardware store.
Programming language 20 years ago, the choice of the programming language was a holy war. In these days, most models have been coded in FORTRAN, some of them in C. Today, the program languages are pretty much the same and their capabilities mainly depend on the libraries they load. ENVI-met is coded in Object Pascal, for WINDOWS using DELPHI. There is also a complete version in C++, but it showed no benefits so we returned to Object Pascal to minimize language confusions. Switching the core code from one language to another is about 3 days of work, which is less the work you have once a new FORTRAN compiler was released… So, for now we stick with Object Pascal until we are convinced to obtain benefits from changing.