Where we stand now is we have our “tunnel” and our measurement/test stand. We need to complete construction of our tunnel. Let’s now review the few basic remaining parts of a wind tunnel. Please refer to my highly technical drawing below which lays out our plan. The main drawing is in blue, the rest are a few dimensions I was working out.
There are five main parts to this design. For your reference the air moves from the left to right. First, on the far left is the bell mouth, which is like a nozzle. It starts larger and funnels the air into the “tunnel” of our wind tunnel. Second is the “tunnel” which is straight and holds our specimens. From now on, we will be a little more technical and call this the test section. Third, the next part on the right is the diffuser, which provides the transition from the test section to the fan. Fourth, on the far right, but not drawn is the fan which will suck air into our wind tunnel from the left and blow out to the right. The last part is the measurement/test stand, which we just built. It goes beneath the test section, but here is unfortunately drawn backwards…oops. The whole system is on top a staggered bench top, because that is what I had.
Let me explain the purpose of the bell mouth and diffuser. One goal for a wind tunnel design is to have as smooth of air as possible in the test section. This is to have consistent results. If there was no bell mouth, the air flow would come into the test section all whirly twirly, therefore it needs a little help. Whenever we humans need help, we ask for a guide. That is what the air needs too, a guide. The bell mouth starts very large so the air is slow, and guides the air to the shape of the test section as the bell mouth narrows and the air speed increases. Tada, much smoother air flow. In addition to being a guide, it is also a nozzle, which research has found kills turbulence (any extra whirly twirly in the flow). Also, just to make sure, we will add what is called honeycomb at the connection between the bell mouth and test section. This is like what the name suggests, but more information to come during that part of construction.
The reason for the diffuser is then to provide a guide for the air from the test section to the fan. When we get a fan, we want to get one with as much airflow as possible (and we can afford) which usually means as large a diameter as possible. Like before, we would like as gentle of a transition as possible between the test section and fan, so that means a long gradual slope. If there is a dramatic slope and transition, the airflow will separate from the walls and create lots of efficiency killing turbulence, see below. On this end, because we are past the test section, we don’t care about turbulence for consistency of measurements but for efficiency of the system. Greater efficiency means greater air speed because we only have so much power from the fan. There are a few other details in this construction, but more information to come during that part of construction.
Not drawn in the first picture is a few legs we need to add to the test section so its wont fall over. Our next installment of Basement Science will be their design and construction. Until then, live long and explore!