The use of random jet arrays, already employed in water tank facilities to generate zero-mean-flow homogeneous turbulence, is extended to air as a working fluid. A novel facility is introduced that uses two facing arrays of individually controlled jets (256 in total) to force steady homogeneous turbulence with negligible mean flow, shear, and strain. Quasi-synthetic jet pumps are created by expanding pressurized air through small straight nozzles and are actuated by fast-response low-voltage solenoid valves. Velocity fields, two-point correlations, energy spectra, and second-order structure functions are obtained from 2D PIV and are used to characterize the turbulence from the integral-to-the Kolmogorov scales. Several metrics are defined to quantify how well zero-mean-flow homogeneous turbulence is approximated for a wide range of forcing and geometric parameters. With increasing jet firing time duration, both the velocity fluctuations and the integral length scales are augmented and therefore the Reynolds number is increased. We reach a Taylor-microscale Reynolds number of 470, a large-scale Reynolds number of 74,000, and an integral-to-Kolmogorov length scale ratio of 680. The volume of the present homogeneous turbulence, the largest reported to date in a zero-mean-flow facility, is much larger than the integral length scale, allowing for the natural development of the energy cascade. The turbulence is found to be anisotropic irrespective of the distance between the jet arrays. Fine grids placed in front of the jets are effective at modulating the turbulence, reducing both velocity fluctuations and integral scales. Varying the jet-to-jet spacing within each array has no effect on the integral length scale, suggesting that this is dictated by the length scale of the jets.
Bibliographical noteFunding Information:
Alec Petersen is supported by a National Science Foundation Graduate Research Fellowship. Funding from the 3 M Company (Maplewood, MN) through a Non-Tenured Faculty Award is gratefully acknowledged. Lucia Baker, Andras Nemes, Jeff Marr, Jim Mullin, and Chris Ellis (St. Anthony Falls Laboratory, University of Minnesota) provided precious help in the design and construction of the experimental apparatus. We are especially grateful to Kurtis Rakhola (Posi-Flate, Saint Paul, MN) for his contribution to all aspects of the design and construction. We are also indebted to Evan Variano (University of California, Berkeley) and John Eaton (Stanford University) for many helpful discussions and useful suggestions during the conception of the facility.
© 2016, Springer-Verlag Berlin Heidelberg.