Room air motion due to the typical ventilation system is best described as unstable. The Reynolds number for ventilation grille slots is typically 3500. This transitional region in flow dynamics has been difficult to model using standard turbulence models adapted to the Navier-Stokes equations. Turbulence is generated at the grille where the fluctuating component of velocity is only a fraction of the mean velocity. As the air travels further into the room, the mean velocity drops off quickly with the normal momentum exchange in jet entrainment. However, the fluctuating component does not drop off as quickly, so that levels as high as 200% of the mean velocity occur within the occupied space. Turbulence in ventilated spaces is an important parameter for thermal comfort. However, our understanding is very limited on turbulence in ventilated spaces.

The problem to model the airflow is persistent. Commercial CFD codes are intended for a wide range of flow regimes and perform well especially when predicting mean velocities. However, the fluctuating velocity component is still under-predicted. If this portion of the flow field can be predicted accurately, it will aid in the understanding of such diverse applications as occupant comfort, combustion and fire propagation, aerodynamic loads and airborne contaminant analysis.

The research project will benefit ASHRAE and the general public as follows:

Understand why turbulent length and time scales are different in various ventilated spaces.

Produce evidence why conventional eddy-viscosity models fail to predict accurately turbulence in ventilated spaces.

Provide general information to design an HVAC system with lower turbulent intensity.

Produce high-quality data for validation of CFD results in various designs.

Explore how to use high precision velocimetries for indoor environment measurement and design

Generate at least a reliable and accurate CFD model to study indoor environment

Instruct computational-fluid-dynamics (CFD) users how to predict turbulent flow in ventilated spaces.

Phase-I: Propose a feasible method to measure turbulent length and time scale for typical flow elements in ventilated spaces, such as free jet and plume and wall jet and plume, and for typical room air flows in ventilated spaces, such as forced, natural, and mixed convection. The level of quality should be attained through instrumentation methods such as Laser Doppler Velocimetry or Particle Image Velocimetry (PIV) that can provide dynamic flow characteristics and a resolution equivalent to the CFD model. The investigation will carry out measurements to provide a full set of benchmark data to validate modeling results by CFD.

Phase-II: Provide a means to accurately predict the large turbulence levels that occur within transitional flows, such as to modify existing low-Reynolds turbulence models to a limited Reynolds number range or to test the Large Eddy Simulation (LES) technique. The means should accurately capture the features of the mean flow and the fluctuating components observed in the ventilated spaces

Many occupied environments contain large thermal heat loads. For instance, industrial facilities, power plants and computer rooms are all spaces in which the focus of the activities within them may impose large thermal loads on the room airflow. Other scenarios with large heat loads include fires within tunnels and buildings, as well as uncontrolled industrial processes. These are examples of circumstances in which the thermal load is not part of the standard operating conditions, but for which a designer might have to prepare.

Modeling of spaces with heat sources is not straight forward. There are a number of technical challenges:

For instance, it is possible to introduce heat as a surface distribution by specifying either the heat transfer rate, heat transfer coefficient or the temperature of the surface. It is also possible to treat the heat source as a volumetric source. Unfortunately, in some cases, incorrect specification of the boundary condition leads to unrealistic temperatures at points close to the source. An additional complication is if the thermal source also has a mass and momentum component to it as well.

Another consequence of having a large heat source in the space is that a significant portion of the flow is buoyancy driven. In many cases, these flows are not stable and may require attention to their transient nature.

Other issues include grid resolution at or near the heat source, and the choice of turbulence model – including whether to enable buoyancy models.

There are a number of benefits for ASHRAE members and the public arising from this project:

Some environments have dangerous thermal sources, this work will contribute to understanding their effect on the occupied zone.

This work will lead to better prediction and control of hot contaminant sources.

It will provide a means to eliminate another uncertainty in the prediction of thermal comfort.

This project involves the following stages:

Survey the modeling techniques employed in other disciplines for large thermal loads (e.g. inside computer CPU cases.)

Perform a literature survey of the existing data for room air flows in which a thermal source plays the dominant (but not necessarily exclusive) role.

Take a series of data measurements from an example environment.

Use new and existing data to test various boundary condition specification methods to introduce the heat into the environment and the effects of other modeling parameters.

The last 10 years has seen significant shift in the development and integration of environmental, ecological and energy issues into the architectural design of buildings. The impact of this has been seen principally in Europe where the generation of leading architects has turned its attention to a more sustainable form of practice both in the form of building system technologies and building typologies. The issue of resources and the environment is at the heart of making intelligent and crafted architecture. Natural ventilation has been used to reduce energy use, improve indoor air quality and comfort while allowing more efficient use of interior space. When properly designed and operated, natural ventilation can produce more healthy, comfortable interiors while reducing the overall energy consumption of the building. Natural ventilation is facilitated by proper overall building design and the use of advanced facade components.

The U.S. has a great potential to use natural ventilation (Lechner 1992). In many regions, no air-conditioning system is needed for residential buildings during the summer with proper natural ventilation. Even if it is not possible to eliminate the air-conditioning system completely, a smaller air conditioning system together with natural ventilation can provide a satisfactory indoor environment. A smaller air-conditioning system means lower first costs and operating costs.

It is not easy to design and control natural ventilation. Natural ventilation is related to wind speed and direction, building shape and density, surrounding landscape and buildings, thermal conditions in and around buildings, window size and location, and building internal spatial arrangement. There are no reliable tools available to determine natural ventilation in buildings. Empirical data and equations for airflow around buildings are not suitable for practical design where building conditions are generally not the same as those to obtain the data and equations. A wind tunnel or water channel with scaled building models can be used to study airflow around building. Such an experimental facility is expensive and detailed measurements can be time consuming. Neglecting buoyancy can not be justified for the study of natural convection. Full-scale testing of airflow around buildings is not generally useful for design because of the time and expense required to obtain meaningful information. It is difficult to use a small-scale similitude model to study indoor airflow with natural ventilation because the heat transfer and airflow lead to a contradicting scaling factor with normal room air. A full-scale environmental chamber for indoor airflow study is again expensive and time consuming. The tools for predicting natural ventilation, taking into account the characteristics of the thermal and geometrical conditions in building, need to be developed.

Due to the fast development in the computational-fluid-dynamics (CFD) technique, encouraging results have been obtained with the CFD technique for study and design of indoor environment. The CFD technique can be a very useful tool to design natural ventilation. There are three types of CFD: modeling with Reynolds averaged Navier-Stokes equations (RANS), large eddy simulations (LES), and direct numerical simulations (DNS). RANS and LES seem most appropriate as a design tool for natural ventilation. However, there are many different types of RANS and LES models.

The objective of the project is to identify a suitable CFD model to study natural ventilation and to develop a computer program for ASHRAE designers to design natural ventilation.