Wind tunnel testing has long been an important component common to many introductory fluid mechanics and aerodynamics courses. Demonstrations of the basic physical mechanisms of viscous and pressure drag associated with the formation of drag forces on various aerodynamic shapes are readily conducted using standard electronic or mechanical balance hardware. Experimental measurements of lift, drag, pitching moment, and pressure distribution on small-scale models likewise play a significant role in supporting basic fluid mechanics theory in such introductory courses. Understanding these physical characteristics is very important to automotive aerodynamic design, for maximizing fuel economy, and in the teaching of basic principles of aerodynamic design as applied to aircraft. In addition to the more common use of the wind tunnel as a tool for investigation of the aerodynamics of sting-mounted test models, however, the wind tunnel as a whole provides the means to demonstrate several significant principles of fluid mechanics and the application of these principles to engineering design. One such recent application of the wind tunnel involved instrumenting the entire wind tunnel for pressure distribution measurements, to demonstrate ideal inviscid fluid flow behavior, as well to illustrate the relative importance of various sources of mechanical energy losses to wind tunnel design. This paper presents the authors experience with modifying an Aerolab educational wind tunnel test facility for experimental work associated with an Undergraduate Campus Internship (CSI) mentoring program project. The purpose of this laboratory activity was to demonstrate characteristics of variable area duct flow and diffuser boundary layer separation using flow visualization by smoke injection. A simple modification to the test section region of the wind tunnel was made to conform to a converging and/or diverging (diffuser) duct flow configuration. This setup was used in conjunction with a special-purpose smoke rake injection system of our own design, and a relatively inexpensive low-power laser-based lighting system, for visualization of the associated air flow. The paper describes various ways of constructing and testing simple duct flows for the wind tunnel test section region using inexpensive materials, as well as the requirements for obtaining good quality flow visualization using smoke injection. The smoke visualization tests can potentially reveal the details of inviscid streamline flow for converging ducts or channels and the onset of boundary layer separation, along with various flow instabilities associated with discharge from a low-speed diverging (diffuser) duct outlet. The connection between diffuser and nozzle head loss behavior and boundary layer separation phenomena are also brought out in these experiments, and they should be adaptable to most educational wind tunnel laboratory test facilities.
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