Design and qualification of an unsteady low-speed wind tunnel with an upstream louver system

The design and qualification of a longitudinal gust generator in a low-speed, blow-down, open-return wind tunnel was conducted. Flow-impeding louvers were used to rapidly change the blockage ratio at the inlet, effectively reducing test section flow speeds up to $$60\%$$ and $$52\%$$ for the closed and open test section configurations of the wind tunnel, respectively. The wind tunnel responds more quickly for higher fan speeds, shorter tunnel lengths, and during louver closing motions (deceleration). A mathematical model developed to predict the wind tunnel’s response shows these same trends and agrees with the experimental data to within 0.03 in amplitude ratio and $$4.3^\circ$$ in phase lag, for all cases studied. Comparison of the closed to open test section configurations reveals that flow disturbances propagate nearly instantaneously, or in a globally unsteady manner, within the closed configuration, while they convect at speeds similar to that of the mean flow when testing in the open configuration.