Using Autodesk CFD for Occupant Thermal Comfort in a Large Auditorium Hall

DR. MUNIRAJULU: Greetings and welcome, everyone. Thank you for joining this session about Autodesk CFD simulation for evaluating thermal comfort in a large auditorium hall. We will go through how Autodesk CFD simulation is implemented in the design of air conditioning system.

Let me introduce myself. My name is Dr. Munirajulu. Currently, I am responsible for CFD analysis in MEP/AEC design for commercial buildings and airports in L&T Construction, Larsen & Toubro Limited, India. L&T Construction is India's largest construction organization and ranked among the world's top 30 contractors. The company's capabilities span the entire gamut of construction-- civil, mechanical, electrical, and instrumentation engineering.

I am passionate about performance-based design and use of simulation tools in design of buildings. I have more than two decades of experience in using CFD technology for design, analysis, validation, and optimization. I use CFD simulation extensively in the design of data center, fire and smoke simulation, thermal comfort in occupied spaces in buildings such as auditoriums, convention center, and airports. I have been a regular speaker at Autodesk University since 2017, both in the US and India events. And it has been a great learning experience being part of AU learning community. I am sure you will have important takeaways from this session.

Before we jump into the session, let us talk a little bit about CFD simulation. CFD stands for Computational Fluid Dynamics. CFD is a design analysis tool. It uses a computer software based on physics of fluid flow and heat transfer. It requires a high-end workstation for solving large-sized problems such as thermal comfort in auditorium hall. It is used in AEC industry and MEP design. CFD is a useful tool to predict and visualize design parameters-- engineering design parameters related to fluid flow and heat transfer throughout the building space.

In combination with BIM, Building Information Modeling, CFD simulation finds its relevance in virtual design, meaning design before going for construction. CFD simulation can be used to address performance concerns, optimize the design through simulation of what-if scenarios, and predict how effectively the proposed design would function. Simulation helps in evaluating and analyzing the performance of the design at concept as well as detailed design stage.

Autodesk CFD is capable of simulating airflow, leveraging mechanical ventilation to remove heat loads in the AEC applications. It can also predict temperature distribution due to heat transfer throughout the building space of interest. Outcome of CFD simulation and analysis are helpful in evaluating design performance for proper airflow and temperature control within the occupied building space.

The purpose of mechanical ventilation is to create air movement driven by fans and provide air changes within the occupied space, replacing stale air with fresh air as well as removal of heat. This results in human comfort and good indoor air quality. In Autodesk CFD, applications of mechanical ventilation include thermal comfort of occupants within a building, data center cooling analysis, and smoke extraction analysis for life safety.

In this session, we will look at simulation capabilities in Autodesk CFD and how those capabilities can be leveraged towards design of thermal comfort. Autodesk CFD simulation provides solution to fluid flow and heat transfer problems, which are part of AEC/MEP design. We will cover four learning objectives as we proceed, namely, identifying specific workflows in Autodesk CFD, implementing those workflows, assessing the CFD results, and finally, validating the design for thermal comfort.

Hopefully, at the end of this session, you will have certain useful takeaways as to how simulation CFD helps in the design process. The goal of CFD simulation is to evaluate if the design meets the performance criteria as evidenced by the simulation results. We will go through these topics one by one in this session.

Building considered for CFD analysis is a large auditorium hall, which is part of a large convention center called India International Convention Center located in Delhi. Auditorium Hall is a multi-use events area for plenary convention sessions, corporate banquets, corporate exhibitions, training, conferences, et cetera. It has a seating capacity of 6,000-plus people in one volume over several levels as we see in the next slides.

Convention Center comprises of Main Auditorium Hall, Convention Rooms, and Grand Ballroom with a total surface of 12,830 square meters. Main Auditorium Hall has a net surface of 5,047 square meters. Out of the 5,047 square meter of surface area, 3,048 square meters are occupied by the seating area to accommodate 6,000 attendees. Due to this large occupancy, large amount of heat is generated in the auditorium hall, which, if not removed, will lead to occupant discomfort.

So the goal of air conditioning system design must provide occupant thermal comfort during events held in the auditorium hall. Ensuring thermal comfort to occupants is critical for hosting events that will attract contributed income to keep the venue full and active with financial success year after year.

Main Auditorium Hall is a large building. It is 74 meters in length, 23 meters in height at the stage, and 14-meter height at the Balcony level. Auditorium has a width of 14 meters to accommodate seating rows both in the Main Hall as well as Balcony as shown in the figure in this slide. Air conditioning system is a vital part for use of auditorium.

Air conditioning system, in this case, uses displacement ventilation. Cool air is introduced at low level and, as such, ventilates the occupied zone of the volume. Air is supplied at low level via outlets incorporated into the seating with a low temperature differential. Air is supplied at low velocity and rises as it is warmed by heat gains within the space. Extract is from high level to ensure the ventilation strategy complements thermal buoyancy rather than fighting against it.

Also, displacement ventilation gives a better air quality as indoor air is pushed out of the occupied zone. The displacement ventilation system returns stale air at high level to the air handling units, where it is passed through heat recovery. Return air is taken from a higher stratified level outside the occupied zone. The heat recovery system within the air handling units operates by extracting the cold from the return air, which, although warm by occupants and other heat gains, is cooler than the outside fresh air would be and, thereby, pre-cooling the supply airstream.

First and foremost, a key feature for a successful CFD simulation is how we characterize or idealize building components and other geometrical features for CFD modeling. Characterization is a strategy to capture design intent with a minimum amount of necessary complexity to optimize simulation performance in terms of ability to mesh or reduce solver time and provide valid results. While a fully detailed building model is necessary for construction, most of the geometry complexity may not be required when leveraging simulation for obtaining reliable performance insight quickly and effectively.

From the CAD geometry of the auditorium hall, the model suitable for CFD simulation is prepared. Input details for CFD simulation include building geometry from CAD data, including seating, air conditioning scheme details, such as supply diffusers and outlets for the return air.

Here, in this image, you can see a section view of the CFD model indicating various zones in the auditorium hall. The lower level is the Main Auditorium Hall, and the upper area is the Balcony. And the supply diffusers are installed at the bottom of the seating.

Actual supply diffuser units will have a number of details, such as swirl discharge elements, swirl discharge slots, and diagonal discharge slots, et cetera. It would be practically cumbersome to model individual components with all the details because it would increase mesh count and compute time. However, for system-level performance, which is our goal in this project case study, actual diffuser geometry is represented in the Autodesk CFD model as a simple geometrical shape. We will see that in the next slide.

The diffuser is modeled as a cylindrical body with a circular opening representing the shape of the diffuser. The outlet face is provided with a resistance value of 50%, meaning 0.5 or 50% free area ratio, to depict the diffuser opening.

The occupant seating and the occupants, they are modeled as block of air volume due to the fact that supply diffusers are integrated to the seating, and it would be computationally efficient to model the occupants as air volumes.

It is important to assign heat gains to the occupant volume in order to evaluate air temperature. Heat gains include people load, equipment and lighting load, and external loads from the walls. Seating height is considered here as 1.1 meters from the floor for evaluating the temperatures.

Strategies for modeling airflow and heat transfer are different for different CFD applications. We will look at strategies for simulation and analysis for thermal comfort in an auditorium in this case.

The Setup Tasks for simulation, for Materials, Boundary Conditions, and Meshing, depend on and apply to the input geometry. Each volume or surface created in CAD is selectable in simulation CFD. Selectable items are used to define inputs and extract results.

The CAD model for CFD is created by modifying existing CAD data or create a new geometry for simulation by referencing the existing geometry. And it is a great way to develop a clean simulation model that captures the design intent. This strategy is often less time consuming than modifying and troubleshooting the existing data. So modified CAD data is loaded into the Autodesk CFD, and the model will look like this.

Materials-- materials define the properties of parts, which impact the physics of simulation. Simulation CFD provides a comprehensive library of materials along with the ability to define custom materials as needed. For AEC applications, air is by far the most common fluid material. For auditorium CFD simulation, fluid domain is assigned Air as material with fixed properties. That is, density does not vary with temperature. Other materials include solids for building enclosure, and these solids are suppressed from the simulation.

The default resistance is provided to give resistance to the flow with a free area ratio of 0.5 in the supply inlet diffusers. The structural elements in the building are assigned solid with Concrete material, and humans are assigned as air volume with heat generation assigned in the boundary condition workflow.

We have now come to the boundary condition workflow. We specify boundary conditions after assigning materials in the CFD model. Boundary conditions, BCs in short, are necessary inputs to produce a valid flow and thermal solution. Defining these quantities is critical with a converged solution in simulation CFD. The simulation environment interacts with the simulation model based on the boundary conditions. Pressure boundary condition is applied on the surface of the air volume extension on the outlet. Inlets are extended by at least a hydraulic diameter to a wide flow re-circulation.

Air is mechanically moved out of the building space, so inlets are assigned the volume flow rate on each supply or inlet diffuser. And air flow direction indicates the flow in the right direction from the supply inlet diffusers. That is, direction is into the auditorium.

For auditorium hall, thermal simulation is driven by thermal boundary conditions and deals with heat generation components such as people, equipment, and lighting. And that is taken care by defining the total heat generation boundary condition for the occupied volume.

After assigning materials and boundary conditions, we go for meshing. And automatic mesh sizing is used to define mesh distribution in the CFD model domain. CFD simulation uses finite element method to calculate the fluid flow and thermal results. The finite element mesh is the backbone of CFD simulation calculation and has a direct impact on solution accuracy.

In the meshing task, the model geometry is divided into smaller regions called elements, where each corner of the element is a node at which flow and thermal variables will be calculated. For simulation, a certain number of elements will be required to adequately capture flow and thermal characteristics. As element count increases, so does the solution time and hardware requirements.

Question is, how many elements would be required? Well, we just use enough elements for a mesh-independent solution. Here, for our auditorium simulation, we have gone up to 16 million elements. For better accuracy, first, we do automatic mesh sizing for the entire domain, and then we do refinement on volumes as necessary.

Local mesh refinement with uniform mesh is used for mesh on supply diffusers and occupant volumes to capture the flow through them. So we click on this Uniform tab here, and that will enable the uniform mesh on the selected item. This is for the occupants, uniform meshing on the occupants.

Having gone through the setup workflow, we will now see how we have used the CFD simulation and the Results Visualization Tasks in Autodesk CFD. The Solve dialog in the Simulation task is the command center for simulation. It controls the physics of the simulation as well as what data is output. Results dialog in the simulation CFD controls various methods for visualizing the results, results using Global, Plane, Traces, Iso Surfaces, and Iso Volumes option.

Solver settings are based not only on the physics of the application but also user preferences such as how much data is to be output. The Solve dialog is comprised of three tabs-- Control, Physics, and Adaptation. Each of these tabs have a variety of input fields and buttons, which will be used in setting up the simulation.

Simulation CFD has the capability of solving a wide variety of fluid flow and heat transfer applications. The first step is to know which settings are required, and that is determined by the physics of the problem that is being solved. So here, you can see that we have set up Flow as Steady State and Incompressible. Since heat transfer needs to be included in the analysis, we have enabled the Heat Transfer. For thermal comfort study, we need to calculate temperature, so we have to solve flow as well as heat transfer. Buoyancy effects due to gravity need not be considered. So heat transfer is by Forced Convection only, so we have not enabled the Gravity here.

After the simulation is set up, run, and converged, we can look at results using the Results Visualization tools here. Autodesk CFD simulation helps the designer to validate or optimize the design by providing visualization of performance characteristics, such as temperature, velocity, that are difficult to capture in the real world at all locations. Anemometers, manometers, thermocouples, and infrared cameras can be used to capture performance characteristics at specific locations and time. So physical testing would give only limited depiction of the performance characteristics.

On the other hand, simulation CFD extends the limitation of the physical testing by providing needed values everywhere in the building. So CFD analysis results are useful to evaluate performance, identify opportunities to improve, and effect of design modifications on performance.

Flow and thermal performance of auditorium can be evaluated by looking at the results using Global option. Global results for temperature provide visualization of thermal gradient on the model surfaces, showing the minimum and the maximum temperatures in the analysis.

Airflow velocity and the temperature values are the key results from CFD analysis to evaluate thermal comfort. Results Plane option in this image provides temperature as well as velocity distribution in the occupied space.

Results Plane in this slide provides velocity distribution in the occupied space at 1.7 meters from the finished floor level. These are the values. This helps us to look at local values as well as variation from point to point. So that is advantage of CFD analysis compared to testing. Testing will give only limited values; whereas CFD analysis will provide the values anywhere that we want to look at.

Here, the Results Plane in this slide provides temperature distribution in the occupied space at 1.7 meters from the finished floor level. And so we can see here the local variation of thermal comfort condition in terms of temperature. When the temperatures are plotted in the range of 22 to 30 degrees Celsius, we can see that most of the auditorium space is below 22 degrees Celsius, which is acceptable, except a few locations which are showing slightly higher than 22 degrees Celsius.

And the results have to be plotted at 0.7 meter level also because in auditorium there could be children. The head height of children is 0.7 meters. So here, you can see that from these temperature plots, results plotted in the range of 22 to 30 degrees Celsius indicate that the temperatures are within 22 degrees Celsius or 0.7 meters from the floor in most of the auditorium space. Similarly, at 1.1 meters from the floor level, 1.1 meters is the head height when an adult is seated. So the temperature contour at this height is very important to evaluate the thermal comfort.

Here, in this slide, we can see how the air flow happens in the Balcony at 0.7 meters from the floor, 1.1 meters from the floor, and 1.7 meters from the floor. So you can see the local variations, the velocity values ranging from very low values to almost 0.25 meters per second.

Here, in this slide, we can see temperatures plotted in the range of 22 to-- 16 to 22 degrees Celsius for 0.7 meters and 1.1 meters and 1.7 meters. This kind of output from Autodesk CFD helps us to understand temperature variation and distribution, and we will be able to identify any hot spots, where the temperature is beyond the acceptable limit.

And when plotted in the range of 22 to 30 degrees Celsius, it gives a clear picture about-- if the occupied space is having temperature that would provide thermal comfort or not. So here, you can see temperature contour at 0.7 meters, 1.1 meters, and 1.7 meters from the floor. All of them are indicating temperatures are below 22 degrees Celsius. So based on the CFD results of temperature distribution and values at the occupied level, we can evaluate if air conditioning system design is adequate or not for thermal comfort.

We have seen that from relevancy of the results, temperature profiles are within acceptable limits for occupant thermal comfort, thereby indicating confidence in the adequacy of the air conditioning system designed. The velocity contour over the Main Auditorium Hall shows a uniform distribution more or less of the airflow, and the temperature distribution again shows most of the occupied area is within the recommended value of 22 degrees Celsius. And in the Balcony also, the temperature distribution shows that most of the occupied area has a temperature in the range of 19 to 21.5 degrees Celsius, which is quite acceptable.

So based on the output from Autodesk CFD simulation, it is possible to evaluate and conclude if the proposed air conditioning system is adequate or not.

So to wrap up, we have covered in this session the following and hope you have benefited from these key learnings, which are identifying specific workflows in Autodesk CFD, implementing those workflows, assessing the CFD results, validating the design for thermal comfort in a large auditorium hall.

Thank you for your attention, and happy learning at AU 2021.