Introduction

ENVI_MET V4 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 5 metres 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.

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Wind flow patterns and drag forces on urban trees

Wind flow and turbulence at the German Foreign Office, Berlin

Software Features

Software suite

The software suite consists out of several applications supporting the user in all different steps of the Design-Simulate-Analyse process. The spatial resolution of the model is individually adjustable with typical grid sizes between 0.5 to 5 m. The simulation results can be analyzed from a multitude of perspectives with the software tool LEONARDO, which comes with plenty of illustration facilities. The ENVI_MET database provides a wide variety of different vegetation and materials for walls, roofs and surfaces and can easily be extend by the user to fit the individual demands.

Simulating plant-atmosphere interaction

One particular strength of the software is the high-resolution simulation of complex plant-atmosphere interactions. Plants in ENVI_MET are not just static objects but instead represented as dynamical organisms that react to stimuli of the imminent local microclimate. Conversely, plants can also be placed in the center of the observation and so for example wind loads, water consumption and plant vitality at different locations can be examined. However ENVI_MET is not just a tool to prevent damage to vegetation, but also a scientifically recognized model to evaluate the effectiveness of urban green in order to reduce heat stress in cities.

Solar Access Analysis:

ENVI_MET contains newly developed analysis modules to simulate the fluxes of shortwave and longwave radiation inside complex environments. The model takes into account shading by complex geometries, reflections by different surface and building materials and the effect of vegetation on all radiative fluxes. In the new IVS method each urban element is considered using its actual state (sun reflection, thermal radiation) instead of averaged fluxes. The solar access analysis provides both hourly and annually information of solar irradiation and of façade and ground surfaces.

Pollutant emission

Urban areas are exposed to high levels of air pollution, resulting from high traffic density, fuel combustion and industries. The pollutant dispersion and chemistry 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). Using our consulting services ENVI_MET can also simulate the effects of photocatalytic activated surfaces on air quality and the emission of VOC from plants.

Dynamic building model

With ENVI_MET’s multiple node model even complex building structures can be represented with high accuracy. The state of the art model enables the construction of walls and roofs out of a multitude of different materials. By placing nodes within each material, prognostic calculations can be obtained that determine the heat fluxes between the indoor and outdoor environment. All nodes are updated continuously taking into account the interactions of the meteorological variables at the façade as well as the thermal condition inside the building.

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 in 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.

ENVI_MET Model Architecture

Wind field

ENVI_MET includes a full 3D Computational Fluid Dynamics (CFD) model. It solves the Reynolds-averaged non-hydrostatic Navier-Stokes equations for each grid in space and for each time step.

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Turbulence

ENVI_MET uses a sophisticated 2-equation turbulence model to simulate the generation, transport and dissipation of turbulence energy.

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Atmospheric model

Radiative fluxes

In urban areas, a great variety of different materials combined with sealed and unsealed surfaces create complex patterns of radiation fluxes, consisting of short wave direct, diffuse and reflected solar radiation.

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Pollutant dispersion

Urban areas are threatened by air pollution, resulting from high traffic density, fuel combustion, biomass burning and industries.

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Air temperature and humidity

Urban areas differ from their non-urban surroundings by an increase of air temperature, due to the urban heat island effect.

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3D simulation of surface and soil temperature

The surface temperature and the distribution of soil temperature is calculated for natural soils and for artificial components down to a depth of 5m.

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Water bodies and pounds

Water bodies are represented as a special type of soil.

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Soil model

Coupling with vegetation model

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.

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Simulation of soil water flux

Simulating the water balance of the surface and the soil is a crucial aspect in urban microclimatology.

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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.

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Green Wall and Roof Systems

Within our consulting services ENVI_MET allows the detailed simulation of the energy and vapour exchange processes which take place at green walls and green roof tops.

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Built environment & Building system

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.

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Building energy performance

ENVI_MET dynamically calculates the building indoor temperature as a result of the incoming and outgoing fluxes through the associated wall and roof segments.

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Detailed building materials

In Detailed Design Mode, individual wall types can be assigned to each wall and roof surface.

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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 containing a heat and water stress reaction concept.

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Vegetation health assessment & 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.

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Vegetation Model

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.

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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.

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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.

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Computing Infrastructure

As a 64 Bit application, ENVI_MET can access the full computer memory for microclimate simulations so that the domain sizes are only limited by the available RAM.

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Internals

Numerical methods

ENVI_MET uses the Finite Difference Method to solve the multitude of partial differential equations (PDE) and other aspects in the model.

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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.

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