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A-1Â Â Wind-Tunnel Experimental Facility and Instrumentation A P P E N D I X A FACILITY AND INSTRUMENTATION For this research project, Dr. James Buchholz and his group at the University of Iowa conducted experiments in the recirculating wind-tunnel facility of the university, illustrated in Figure A1. The wind tunnel is designed for precision aerodynamic and boundary layer studies. The test section has cross- sectional dimensions of 35.5 inches by 29.5 inches and is 14 feet long, with an adjustable ceiling for control of the streamwise pressure gradient. The walls of the test section are glass, and the ceiling is clear acrylic, providing extensive optical access. In addition, the aluminum floor is highly modular, permitting custom model installations or additional optical access. The 60-hp axial fan and flow conditioningâconsisting of honeycomb, screens, and an 8-to-1 contractionâoffer a low-turbulence flow in the test section with free- stream speed as high as 164 ft/s (50 m/s). The instrument carriage can support pitot probes and thermal anemometry probes and is capable of precise automated vertical positioning via a stepper-motor-driven traverse and manual adjustment in the streamwise and transverse directions. A pitot probe mounted to the wind-tunnel ceiling and a resistance temperature detector measure wind speed. A curtained enclosure surrounding the test section enables safe operation of lasers for optical measurements, including four high-speed cameras and a particle tracking velocimetry system capable of acquiring time-resolved volumetric flow velocity measurements. The instrument carriage and traverse, equipped with a pitot probe and thermal anemometry probe, were used to characterize flow conditions. In addition, precision miniature force balances were constructed and installed to accurately measure the drag force on the model traffic signs.
A-2 Wind Drag Coefficients for Highway Signs and Support Structures Figure A2: Model force balance, with wind flow from left to rightandsign model at left of figure A1a: Panoramic (deformed) view of test section and wind-tunnel circuit A1b: Laser enclosure over test section A1c: Instrumentation traverse inside test section Figure A1: Recirculating wind-tunnel facility
Wind-Tunnel Experimental Facility and Instrumentation A-3 Figure A3: Final design for force measurement apparatus and base plane configuration, showing a plan view, side view, and lateral view Notes: The base plane is modular to facilitate reconfiguration of signs on the middle section and to accommodate instrumentation traverse for inflow velocity characterization. Dimensions are in inches and millimeters. Miniature road sign models were constructed from flat stainless steel shim stock. Precision miniature force balances employing miniature ball-bearing slides were constructed to support the signs and measure the drag forces, as shown in Figure A2. Drag force was measured with an anticipated uncertainty typically below 1% of the measured value with Interface SMT-type single-axis load cells integrated into the force balances. The road sign and force balance assemblies were mounted on an elevated base plane (Figure A3), representing the ground surface. Use of the base plane ensured precise control of experimental conditions by keeping the models out of the wind- tunnel floor boundary layer. The construction enabled the central âmodel section,â which supports the sign models, to be removed and replaced with a replica section that allowed access for the vertical traverse and for performance of inflow velocity profile measurements. The signs were positioned by using machined setup jigs to ensure accurate spacing between the two signs and between signs and the base plane.
A-4 Wind Drag Coefficients for Highway Signs and Support Structures Figure A4: Detail view of the sign force balances with the load cells A National Instruments LabViewâbased data acquisition system was used to acquire force and flow velocity measurements in the wind tunnel. Mean drag forces were measured on the sign models over sampling durations sufficient to achieve a statistically converged measurement of the drag force. The force measurement system was calibrated and tested. The force measurement system consists of streamwise forks to support the sign models (Figure A4). Each force balance consists of a two-prong fork in which the signs are precisely attached to the ends of the prongs with magnets and alignment bushings, allowing signs to be rapidly interchanged to investigate the planned configurations. The forks are installed on precision ball slides that constrain the forks from lateral motions while transmitting the aerodynamic loads to Interface SMT1-5N load cells. Transverse rails in the base plane allow lateral adjustment of the sign position to vary spacing between signs for two-sign configurations. Precision setup blocks were constructed to accurately position the signs relative to each other. The load cell and slide are enclosed in a fairing that shields the components from wind loading. Figure A5 shows the base plane mounted to the wind-tunnel floor with a single force balance assembly.
Wind-Tunnel Experimental Facility and Instrumentation A-5 Note: The instrumentation traverse with pitot probe installed is visible at top left. Figure A5: Acrylic base plane installed in the wind tunnel with a single force balance assembly and a model traffic sign installed Inflow conditions were characterized by several vertical profiles of wind velocity over the base plane at a location upstream of the sign installation positions. The profiles were measured using single-velocity- component hotwire measurements, which allow accurate resolution of the boundary layer profile over the base plane. Flow measurements were conducted (1) at 7.87 in. (200 mm) from the leading edge of the base plane to provide inflow boundary conditions for numerical simulations and (2) at the streamwise location of the signs. These measurements consist of vertical profiles extending approximately 3hg from the base plane where hg is 1.97 inches (50 mm). Profiles were acquired with a pitot probe and a single-wire hotwire probe. The hotwire probe was used to measure the mean boundary layer profile on the base plane as well as the root-mean-square (RMS) values of the streamwise velocity component throughout the 3hg measurement profile. The data indicate a thin turbulent boundary layer developing on the base plane at both streamwise locations and also streamwise RMS velocity fluctuations below 1% of the free-stream velocity. Neither the wind tunnel nor the building housing the wind tunnel is temperature controlled. Measurements were corrected for differences in air density resulting from the temperature variations. A protocol was adopted for sign drag measurement in which the wind tunnel was run until it reached a nearly steady-state temperature, and then a pitot-probe velocity measurement was performed before installing the sign and force balance and acquiring the sign drag force. The wind-tunnel temperature slowly drops while it is not running but then increases when restarted. This design permits temperatures and air properties to be matched between the local free-stream velocity measurement and the sign drag measurement.
