Ingenieurbüro Gratzl used cloud-based CFD from SimScale as a cost-effective alternative for building optimization.

The Challenge

Evaluating Thermal Comfort and External Wind Conditions for Althan Quarter

Markus Gratzl’s project consisted of evaluating the thermal comfort of an immense entrance hall of a former university building. This project, as part of a larger developmental project called the Althan Quarter on Alsergrund in Vienna, aims to bring a sustainable mix of accommodation, offices, hotels, local suppliers, and additional infrastructures to the area, and is expected to be completed in 2023.

Markus’ simulation dealt with the four-storey entrance hall of the building, including the entrance to the railway station’s platforms. The goal of the analysis was to clarify whether or not the entrance hall could be thermally conditioned by passive ventilation measures during hot summer periods. If not, they would need to develop measures to improve thermal comfort during hot periods with as little energy usage as possible.

How they solved it with SimScale

Investigating Thermal Comfort

Markus turned to computational fluid dynamics (CFD) using SimScale to fulfill the needs of the project. Through CFD simulation, he was able to address two main goals:

•  Evaluating thermal comfort via internal flow simulation: Thermal comfort and conditioning requirements in multi-storey halls are largely determined by surface temperatures of the components and the temperature stratification in the room. In this case, surface temperatures could best be evaluated through transient building simulations, but their influence on thermal comfort were strongly geometry-dependent. Therefore, the results in the one-dimensional node models of a building simulation could only be meaningful to a limited extent, and could only be fully assessed in three-dimensional building models that also take into account airflow and temperature stratification provided by internal flow simulation.

•  Determining pressure coefficients on building envelope via external flow simulation: To evaluate the effectiveness of natural cross-ventilation as a measure for passive cooling, it was necessary to consider the wind pressure acting on the facades. Due to the structure of buildings in an urban environment, it is difficult to use default values for the wind pressure coefficients, especially for larger buildings. Thus, it was necessary to carry out external flow simulations, which resulted in wind pressure coefficients of all facades for different flow directions.

With the completion of these goals leading the way for the project’s success, it was determined that such investigations could only be performed with flow simulation software. When Markus discovered SimScale’s cloud-based CFD software, it was a perfect fit for his small and specialized business. With the need for expensive hardware and high investment costs eliminated, Ingenieurbüro Gratzl got to work using SimScale.

Simulation Setup

Internal and External Flow Simulations

Ingenieurbüro Gratzl ran the two flow simulations crucial to the project: internal and external airflow simulations. The setup for each consisted of the following:

• Internal flow simulation setup:In order to determine and evaluate the internal thermal comfort of the building, a 3D flow region was generated using CAD software, then imported as geometry into SimScale. Next, a mesh with 3.2M cells was created (hex-dominant parametric, with several refinements at inlets and outlets, etc.). Details such as people and devices as defined loads were also modeled in the geometry, and included within the mesh. The internal flow simulation used convective heat transfer analysis with radiation and set the boundary conditions (wall temperatures, initial conditions, additional radiative sources, etc.) for a worst-case scenario transient building simulation.The results for both simulations, including temperature stratification, flow velocities, and ultimately thermal comfort, were directly transferred to a results document. Additionally, the temperature stratification was integrated as a refinement into the model of the transient building simulation as an input variable.
• Pressure coefficients on building envelope via external flow simulation: Similar to the internal flow simulation setup, a 3D building model of the environment (about 1.00 km x 1.00 km around the investigated object) was created from the 2D cadastral plan and imported as geometry into SimScale. The direct transfer of available 3D city models was not possible due to non-waterproof buildings. Based on the geometry model, a mesh was created (hex dominant, external, automatic sizing “coarse”) containing 3.9M cells.Boundary conditions for the external flow simulation (incompressible, steady-state, k-omega SST) were defined. Result values for the relevant facade surfaces (surface data, area average) were to be used as boundary conditions in the building simulation.

The Simulations

Internal and External Flow Simulations

Markus used a step-by-step approach to investigate the airflow and achieve their specified goals. The steps can be visualized below:

• Step 1: External flow simulation
• Step 2: Shading study
• Step 3: Transient building and system simulation
• Step 4: Internal flow simulation entrance hall

The external flow simulation took around 30 minutes, and used an automatic number of cores (14 core hours). The internal flow simulation was longer with a three-hour runtime, using 300 core hours on 96 cores.

Due to the requirements of the project, it took some time to calibrate the correct settings for both the external and internal flow simulations. Additionally, integrating radiation and thermal comfort for the internal flow simulation needed some prep time. After a few test runs and help from the SimScale support team, Markus was able to get the desired results.

The Results

Design Confirmation

The internal flow simulation found that a pronounced temperature stratification developed in the entrance hall, which is around 16 m high. The temperature difference between the ground floor and the ceiling on the second floor was approximately 10 K. Additionally, the simulation found that in the priority area investigated (seating steps), the temperature values were in the range of 26 to 27°C, while in the upper area adjacent to the seating steps, the temperatures at head height were 28 to 29°C. When investigating air velocity, the results found that air velocities of over 0.1 m/s are generated in parts of the entrance hall, therefore local discomfort due to draughts could not be excluded. Therefore the position of air inlets was changed to satisfy thermal comfort demands of the users.

The external flow simulation was carried out to determine wind pressure coefficients as input values for the building and system simulation. Simulations were carried out for wind flowing from the main directions: N, NO, O, SO, S, SW, W, NW. A wind speed of 5 m/s at the reference height of 10 m was chosen. The resulting dynamic wind pressure values were normalized over the wind speed, and converted to wind pressure coefficients. These were transferred to the building and plant simulation.

The results confirmed the assumptions of the construction plans, and that the building design will fulfill the requirements on thermal comfort.

Next Steps

Continued Collaboration

Now that Ingenieurbüro Grazl has incorporated SimScale into their workflow, they are able to handle more cases than ever before. With more and more clients requesting CFD and thermal comfort analysis, SimScale makes it easy to run simulations and obtain results in a cost-effective manner.

The ongoing project will move from the planning phase to the construction phase before the end of 2020, and will be implemented taking into account the simulation results obtained through SimScale.

More on Ingenieurbüro Gratzl

Markus Gratzl has specialized his engineering office in the field of Green Building Simulation, and offers customized solutions for simulation tasks in the field of energy-efficient buildings and plant engineering. As a professor for “Simulation Methods in Building Physics,” he integrates the results of current research projects into his daily work in the optimization of buildings.