nuLUCA Studio

Biomimicry research and consulting, digital modeling, simulations, and visualizations

What can we learn from mammalian vessel networks?

This research was inspired by Anuj Jain at bioSEA. The idea revolves around the efficiency of vessel networks in mammals—especially those in the ears of elephants. Thermal windows are dense collections of blood vessels in elephants (and many other mammals, like jackrabbits). Thermal windows are near the surface of the skin where heat can be efficiently lost to the environment, cooling the animal. One mechanism involved in this process is the dilation of these vessels when more cooling is needs, bringing more blood to the area and releasing more heat. Beyond the efficiency of liquid transport for thermoregulation, there is the efficiency (low resistance) of the fluid movement within the vessels due to their branching patterns. The branching of mammalian vessels networks follow Murray’s Law (which describes the ratios of parent vessel diameters to daughter vessel diameters at each branching event). We combined this information with some of the basic ideas in Michael Hermann’s FracTherm® algorithm (which describes a near-uniform coverage by a branching vessel network given a specific two-dimensional surface, similar to what might be observed in nature). The vessel animation shows a result of this—a vessel network that adapts to different surfaces. To capture some of the efficiency exhibited by mammalian vessels, the branching ratios follow those laid out by Murray’s Law and can adapt to either laminar or turbulent flow.

We are currently measuring the resistance to fluid flow and the efficiency of heat transfer in these branching systems compared to other systems used for radiant heating and cooling in buildings today. Thanks to the Ansys Startup Program for providing the software we use to carry out these simulations.

How can surface geometry help us capture water from the air?

This work is featured in bioSEA’s Biomimicry Toolkit, a grant funded project out of Singapore.

In the harsh climate of the Namibian Desert, water is a precious resource. This darkling beetle’s elytra (the hard casings that cover its wings) is covered with tiny bumps. These bumps help the beetle condense water from fog by increasing the rate of droplet formation. Material properties do contribute to condensation. In this project, though, we focused primarily on geometry. The tiny bumps on the elytra surface give the surface curvature at the scale of tiny droplets, making droplets more likely to form more quickly. Droplet formation is important since, once in droplet form, water will roll, leaving the surface area bare and ready to condense more water. This means more water can be condensed in less time and more water can be captured overall. The diameter of the bumps, and the spacing between them, vary—usually between 0.5 -1.5 mm.

We built digital models of bumpy tiles, with different bump sizes, shapes, and spacings, to see if geometry is actually playing as much of a role as we hypothesized; and to see if we could determine which combination of size, shape, and spacing captures the most water. We used Ansys Fluent for CFD simulations.

Different bump shapes and sizes we tested

Research by Park et al. (2016) published in the journal Nature combined features from other organisms that capture water from the air, including the directionality of cactus spines, to arrive at a slightly different bump shape. We looked at shapes similar to theirs to see if this directionality, and the capillary action associated with the canyon-like character to the bumps, would increase the rate of condensation. Simulations involved sending a slight breeze of humid air across the surfaces (to match the conditions the beetle is in as it assumes a “fog basking” position). Water condenses on the surface and rolls off into a cistern. We rank the tiles by both the total volume of water condensed and by the total volume divided by their surface areas (to understand the influence purely from geometry—without a bias toward surfaces with larger surface areas).

Our preliminary results suggest that bumps can increase the rate of condensation, even when we correct for the increase surface area they represent. But this is not always the case. The specific size and spacing matters a great deal. Our results also suggest that asymmetric bumps perform better (relative to rate of condensation) when compared to spherical bumps—as hypothesized and demonstrated by Park et al. (2016).

This is a work in progress. There’s so much more we’d like to explore and understand related to evolved geometries that specialize in water capture.

How can the patterns on elephant skin help us keep buildings cool?

Close up of an elephant looking at us, showing the different kinds of wrinkles on the skin

This project was also born from bioSEA’s Biomimicry Toolkit (a grant funded project out of Singapore) and we are continuing work on it today.

Elephants have a unique way of staying cool. New skin cells develop faster than dead cells shed. As the skin accumulates, it cracks. This bumpy, cracked geometry combines several strategies for cooling into an extremely effective thermoregulation system. Again, nuLUCA’s role in this project was to analyze the geometry and translate it to an architectural component with the functional characteristics intact.

We followed research by Dr. Lidia Badarnah (and we continue to work with her) that looked at the specific mechanisms for cooling and how they might translate to building facades. There are three main mechanisms hypothesized, that are part of this geometric pattern: self-shading, evaporative cooling (assisted by self-shading and capillary action through the networked crevices), and convective cooling (driven by hot and cool spots creating currents very near the skin surface).

A key function of the elephant skin that contributes to evaporative cooling is its capacity to pull water across the surface and hold that water on the surface. Water movement across the surface is driven by capillary action. Capillary action is the pull of a fluid through a narrow space. This pull is due to an attraction between fluid molecules and molecules in the material of the tube or crevice. Because there is a slight attraction between the water molecules and the surface molecules, water moves to cover more of the surface. The distance water will travel depends on the material and is inversely related to the diameter of the tube or crevice.

Future research will look at widths and depths of crevices, the networked character of crevice patterns, along with materials that work with the desired capillary action effects. Current research is examining whether or not the self-shading aspects of the bump pattern, alone, can play a significant role in cooling.

We brought radiation data into Ansys, along with temperature data for different days (for both Singapore and Phoenix), to do more sophisticated thermal analyses. Our goal is to identify specific pattern characteristics that relate to particularly good thermal performance, to see how pattern performances vary in different locations and at different orientation (e.g., if some patterns perform well in some scenarios and poorly in others), and to see if we could monotonically relate our constructed fitness values to thermal performance measures in Ansys.

We measured both the temperature and heat flux on the back of our tiles in Ansys. Our results show that bumps, similar in shape and scale to those on elephant skin, do help with cooling. Our models suggest that this is due to the increase surface area, much of which is shaded. Because of the added surface area, the heat that is generated by radiation (now in smaller, more concentrated areas), has more opportunities to escape to the ambient environment instead of continuing through to the other side of the tile. Some of our results are shown in the graphs below.

Heat gradients on the backs of different tiles

Temperature on the back surface of various tiles over the course of one day as simulated by Ansys

Heat flux on the back surface of various tiles over the course of one day as simulated by Ansys