The Wind Tunnel (Page 1)

What is a wind tunnel?

Wind tunnels date back to the 1870's. Scientists realized it didn't matter if an object was stationary (not moving) and air was blown over the object or if the object was moving through the air. The resultant forces over the object would be the same. The idea of blowing air over an object and determining the forces led to the invention of the wind tunnel.

A wind tunnel is generally sort of a duct or pipe shape and air is either blown or pulled out of the tunnel. Typically in about the middle of the tunnel is what is called the "test section". This is where the model object to be tested is placed. In aerodynamics this is typically an airplane wing or a scale model of an airplane - for our project it will be an 11" tennis ball. Wind tunnels can be small devices that fit on a table or be as big as several city blocks like one of the tunnels at NASA Ames. The tunnel we will use is not that large but does fill a large room.

Our wind tunnel expert from NASA Ames, Dr. Rabindra Mehta (Rabi), has been interested in the aerodynamics of sports balls (how air flows around objects and the forces created by this air flow) for a long time. Rabi has studied the aerodynamics of cricket balls, baseballs and golf balls - and now tennis balls! Rabi's branch chief (boss), Dr. Sanford Davis, agreed that we could use one of the wind tunnels in the Fluid Mechanics Laboratory where Rabi works.

The team had discussed how we would perform the wind tunnel test and what type of results we want to see for a while. Based on the answers to those questions the type and specific wind tunnel you use will change. So we'll review what are goals were here.

First, the team wanted to look for some specific flow patterns that would help us with our computer simulations of the tennis ball. Our computer simulations are called "CFD" simulations or Computational Fluid Dynamics. In particular we wanted to see where the air "flow separates" and when "transition" from laminar (smooth air flow) to turbulence (rough air flow) occurs on the tennis ball.

Additionally, we want to be able to "show you something" when we hold our next videoconference so you can see what we did. Typically, you do not really see that much while you are watching a wind tunnel test, but you collect a lot of information. So the team decided that we would add smoke into the wind tunnel - if you've never seen this before its very cool and you'll be able to see the flow deflect and separate and the air change from smooth to rough (or transition from laminar to turbulent as the aerodynamicists would say).