Virtual mannequins make comfort a reality

CFD has long been an important tool in the prediction of thermal comfort in occupied spaces. However, despite a proven track record in accurately predicing physical quantities such as temperature and velocity, relating primative flow variables to the degree of thermal comfort actually experiened by an occupant is much more difficult. Although the level of thermal comfort experienced obviously depends on the local flow environment and the exposure of the occupant to thermal radiation, individual human beings respond in a subjective way to the local heat transfer in different parts of their bodies.

Fig. 1: Three climate evaluation methods: human subjective ratings, manikin measurements and computer modeling	Fig. 2: Flow tracks around MANIKIN3 inside the virtual calibration chamber " dressed" in the same clothing as the physical MANIKIN2	Fig. 3: MANIKIN3 and computer inside the virtual office equipped with mixing ventilation

Thermal sensation ratings provided by panels of subjects are probably still the most objective way to gather information about the effects of different thermal climates. However, the work with panels is expensive and time consuming, as well as difficult to standardize and use for predictions. As a response to this, artificial human shaped mannequins are increasingly used in-place of actual human subjects. Measurements in various parts of the mannequin’s body in a specially controlled thermal environment can be correlated to the responses from panels of real human beings in an identical situation.

This article describes work performed to construct a numerical thermal mannequin known as ”MANIKIN3” using STAR-CD, from data collected by an existing physical thermal mannequin “MANIKIN2” as well as more than 500 experiments with subjects. (Fig.1)

The first problem to overcome when constructing such a model is how to define a simple, yet effective measure of thermal comfort. One of the best measures is the equivalent temperature (teq), which is a measure of the effects of non-evaporative heat loss from the human body. The major advantage of this measure is that it expresses the combined effects of thermal influences in a single figure that is easy to interpret and explain. It is consequently particularly useful for local assessment of complex climatic conditions.

The relationship between changes in different thermal factors and the change in equivalent temperature

Single measurements of thermal factors do not account for all effects on a human body. Human shaped thermal manikins measure equivalent temperature over the whole surface, in all directions simultaneously. Consequently new, extended definitions of the concept of equivalent temperature have also been developed, along with theories describing equivalent temperature as a vector-valued function. These new theories are used to make more efficient computer codes and increase the understanding of measurement differences.

Fig. 4: Comfort zone diagram for 16 segments of the body for the case with mixing ventilation (Notice the measured and simulated seat zones)	Fig. 5: MANIKIN2 inside the vehicle simulator exposed to the artificial sun at the Swedish Institute for Agricultural and Environmental Engineering

The virtual MANIKIN3 has been formulated to be identical in size, area and number of zones to the real MANIKIN2. Calibration is carried out by locating the computational MANIKIN3 in a fictitious chamber. This arrangement gives an ideal environment for the determination of the heat transfer coefficients needed for the equivalent temperature calculations (Fig. 2).

In order to investigate how well CFD calculations can predict the effects of different climate situations, a number of reference cases have been carried out.

Fig. 6a & b: Temperature fields with reflective (adove) and clear glass (below) (Note the heating effects from the manikin and the untreated windows)

The office room case
This first investigation was a part of the Swedish project called "The Healthy Building" that investigated how the use of modern calculation tools can improve the thermal climate in buildings. MANIKIN3 had no problem simulating the increased insulation of the seat zones supplied by the virtual office chair. The office CFD simulations produced slightly higher equivalent temperatures due to a lower simulated air temperature (Fig. 3 & 4).

Fig. 7: The first picture of MANIKIN4 made with methods for manikin shape reconstruction from point cloud data (Scanned with a CYRA Cyclone (Cyra Technologies Inc.) at the University of Gävle)

The vehicle simulator case
This second study was a part of a research project investigating how the use of "special glazing" can improve the thermal climate inside a vehicle.

In this study a small climatic test cabin, a vehicle simulator, was positioned in a large climatic chamber (Fig. 5). The intensity of the transmitted solar radiation varied with the glazing tested. The supplied air was distributed through defroster and panel inlets in the front and at the doors. The two outlets were positioned at the back wall (Fig. 6a & b). The calculated results from this study show good agreement with the measurements made with MANIKIN2. Some deviations were found at the hands and the zones at the middle of the mannequin. MANIKIN3 simulates the reduced insulation of the ventilated seat well.

Conclusions
The objective of this work is to use and develop computer-based methods to visualize, analyze, and evaluate thermal climate in order to improve the environmental conditions. The methods are used to find useful system solutions that provide improved thermal climate in many different situations, e.g. homes, schools, offices, cars, trucks, and trains. The cubical MANIKIN3 is sufficient for most engineering calculations, but it is interesting from a research point of view to further develop the similarities between reality and the CFD model.

For further information, contact: hakann@kth.se