Imagine wind blows through a smooth surface obstacle. In some specific condition, the air behind the obstacle would be trapped for a while instead of being blew out. To observe this phenomenon, I built a small wind tunnel and tried to analyze the interaction between the flow and the cylindrical obstacle. In the experimental setup, the visible smoke taken away by the fan is generated by adding hot water into the liquid nitrogen. After passing through the grids on the corrugated board, the flow is striaghten and become ordered laminar flow. By using this visible flow, the flow change near the obstacle can be observed.

The experimental results are shown in the above figure (a). When the flow passes from right to left through a cylindrical obstacle, two symmetrical eddies (black regions) behind the obstacle can be observed. This is because the low pressure zone formed behind the obstacle causes the gas to return and stay in this region for a period of time. To further prove this concept, another experiment was done and shown in figure (b). However, instead of using visible flow generated by the liquid nitrogen, invisible pure gas is used this time. Moreover, I also placed a smoking incense (not shown in figure) behind the low pressure zone to determine whether the gas would return to this low pressure zone. The result is shown in figure (b). A visible smoke originating from the incense can be observed, thus proving this concept.

Another interesting question is what happens when the flow velocity changes? Will the flow behavior change as well? To answer this question, we can introduce the Reynolds number which is an important dimensionless value in analyzing any type of flow. The Reynolds number is defined as follows.

Generally, at low Reynolds numbers (low fluid velocity), the viscosity term (μ) dominates, thus causing an ordered laminar flow. As the Reynolds number increases (high fluid velocity), the inertia term (ρＶＤ) becomes dominant, causing chaotic and unpredictable turbulence. In addition, there is a transitional behavior between laminar and turbulent flow, which is a mixed behavior of these two flows. As the previous experimental result, the eddies (turbulence) behind the obstacle is surrounded by nearby laminar flow. In addition, to observe the change in flow behavior corresponding to its velocity, the following experiment was also performed. From the result above, it can be found that as the velocity increases, the flow behavior evolves from ordered laminar to transient and eventually becomes chaotic turbulent.