Aerodynamics and Acoustics of the Virginia Tech Stability Tunnel Anechoic System

The acoustic treatment and calibration of a new anechoic system for the Virginia Tech Stability Wind Tunnel has been performed. This novel design utilizes Kevlar cloth to provide a stable flow boundary, which eliminates the need for a free jet and jet catcher. To test this concept a series of measurements were performed both to validate the reduction in overall test section noise levels and to ascertain the effect of these modifications on the test section aerodynamics.

An extensive program of experiments has been conducted to examine the performance of this new hardware under a range of conditions. These include baseline experiments that reveal the aerodynamic and aeroacoustic performance of the tunnel in its original configuration, treatment of the tunnel circuit with validation of in-flow noise reduction, wind tunnel tests to examine the effect of the test section acoustic treatment, and measurements of the aerodynamic and aeroacoustic characteristics of a NACA 0012 airfoil model over a range of angles of attack and Reynolds numbers.

These measurements show that acoustically treating the walls of the circuit both upstream and downstream of the test section, as well as the fan, result in an overall reduction of 5 dB depending on frequency, of the in-flow noise level. These measurements also show that the complete system provides a reduction of between 15 to 20dB depending on frequency, in the in-flow background noise level. Measurements taken both within the test section and in the adjacent chambers also show that large Kevlar windows can be used to quietly and stably contain the flow, eliminating the need for an open-jet and jet catcher system, as well as overall noise levels competitive with many other facilities. Measurements on several airfoils at various angles of attack and Reynolds number show that the interference correction for the fully anechoic configuration is approximately -22% for model with a chord length equal to half the test section height.

Aerodynamic measurements with the NACA 0012 airfoil show its lift, drag and boundary layer characteristics at high Reynolds numbers are consistent with theoretical expectations. Measurements of the window deflection as well as examination of flow transpiration through the Kevlar windows were accomplished, both with and without the NACA 0012 model. These measurements, along with the interference correction data, confirm that the Kevlar windows are a stable flow boundary.