Evaluation of a New Concept for Reducing Freestream Turbulence in Wind Tunnels

A 45 deg honeycomb flow manipulator, mounted parallel to the corner turning vanes, was investigated for improving the flow quality in wind tunnels with little or no settling chamber length. This manipulator permits increased turbulence decay distance in comparison to a conventional honeycomb arrangement. The resulting turbulence levels in a wind tunnel using the 45 deg honeycomb are comparable to those obtained using a con- ventional honeycomb, but only when a 45 deg screen is mounted immediately downstream of the honeycomb, and when some separation distance between the turning vanes and the 45 deg honeycomb is used to permit adequate decay of the turning vane wakes (at least twice the spacing between the turning vanes). As the tur- bulence scales are different at the start of the contraction for each of these honeycomb types, the best choice will depend on the relative effect of the contraction on reducing the different scales of turbulence. OST experiments conducted in wind tunnels require a low turbulence intensity in the test section, with good spatial uniformity of both the mean velocity and the tur- bulence intensity. Recent proposed experiments, such as laminar flow control (LFC), impose a severe restriction on the maximum allowable turbulence level in the test section. According to Dr. W. Pfenninger of NASA LaRC, turbulence intensities of less than 0.1 % will be required. Due to recent work in the area of turbulence control and management, ^ as well as on control of secondary flows in ducts4'6 this level is not unattainable under certain conditions. Specifically, efficient compact combinations of honeycombs, grids, screens, and other flow manipulators have been developed which lead to substantial reductions in the freestream turbulence levels with a minimum of power loss due to manipulator pressure drop. The optimum arrangements depend on maximization of the manipulator turbulence suppression functions and minimization of their generation functions.!'3 In particular, the important role of the shear layer instabilities and the proper balance between the different scales of the manipulator and the turbulence are considered in the design of efficient manipulator com- binations.2'3 In most cases, available power limitations of the facility make the use of these combinations desirable, if not mandatory, to achieve the required turbulence level. However, another extremely important feature of using these flow manipulator combinations is the provision for adequate decay distance between manipulators, as well as downstream of the final manipulator ahead of the con- traction. Unfortunately, most wind tunnels have little, if any, settling chamber between the turning vanes and the start of the contraction. This lack of any appreciable settling chamber has a severe restrictive effect on the type and number of manipulators that can be used for improving the turbulence level, with subsequent limits on the amount of turbulence reduction that can be accomplished. While new wind tunnel facilities are, or should be, designed with a settling chamber long enough for proper turbulence management, and some older facilities can be modified to provide an adequate settling