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The Wind Tunnel as a Practical Tool for the Demonstration of Engineering Fluid Mechanics and Principles of Aerodynamic Design

Abstract

Laboratories which make use of wind tunnel experimentation play an important role in many undergraduate courses in the fluid and thermal sciences area, including those associated with aerodynamics and fluid mechanics courses. Measurements of the lift, drag and pitching moment behavior of airfoils as a function of angle of attack are a common occurrence in such labs. The importance of reinforcing theoretical aerodynamic concepts is clear, but the challenge is to provide meaningful experiments that demonstrate the desired effects without either introducing numerous extraneous phenomena, or overly complicating the experimental procedure.

This paper presents the authors experience with an Aerolab educational wind tunnel test facility as part of course-related work in both junior-level Fluid Mechanics (ME571) and senior-level Aerodynamics (ME628) courses. The simple addition of several special-purpose pressure taps, to complement existing built-in pressure taps normally associated with the standard test section region, provide the means to map the axial pressure distribution within the entire wind tunnel. This allows direct identification of the location(s) of significant mechanical energy losses, through comparison with ideal inviscid stream tube analysis associated with fluid mechanics principles. In particular, the losses associated with the diffuser section become very apparent, in contrast with the inlet convergent section. Pressure recovery in the diffuser section is modeled in a very simple manner and compared directly with wind tunnel measurements. Fan power requirements associated with wind tunnel design are also included as part of the experimentation. The connection between diffuser loss behavior and boundary layer separation phenomena associated with flow over a wing is also brought out in these experiments.

Introduction

Wind tunnel testing is a common component to an introductory engineering aerodynamics course. Experimental measurements of lift, drag, pitching moment, and pressure distribution (or pressure coefficient) have long been a significant part of such introductory courses, and the analysis of these physical characteristics is a very important step toward the introduction of principles of aerodynamic design—in particular airplane design. 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 itself provides a means to demonstrate significant principles of fluid mechanics and the application of these principles to aerodynamic design. Properly instrumented, it can provide an excellent demonstration of both ideal inviscid fluid flow behavior, as well as the affect of mechanical energy losses on wind tunnel design.

Typically commercially-available wind tunnels come equipped with standard pressure taps for sensing the test section pressure level. This test section pressure, relative to the ambient atmospheric pressure (i.e., the test section gage pressure), is also commonly used to determine the test section airspeed, which can then be directly displayed on the instrument panel associated with the wind tunnel. For more accurate local airspeed measurements, a small Pitot probe can be

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Beck, B. T., & Anderson, B. (2007, June), The Wind Tunnel As A Practical Tool For The Demonstration Of Engineering Fluid Mechanics And Principles Of Aerodynamic Design Paper presented at 2007 Annual Conference & Exposition, Honolulu, Hawaii. 10.18260/1-2--2968