Session F20: Experimental Techniques: Aerodynamics/Wind Tunnel

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One of the parameters that define the quality of a wind tunnel is the uniformity of the wind profile in the test section. While improving this parameter in a regular wind tunnel is a challenging process, doing so in a fan array wind facility is facilitated by the high degree of control over the generated wind. In this study, an iterative process for optimizing the uniformity of the wind profile in an array of 162 fans is elaborated. For each iteration, the local mean flow velocity is measured over the whole cross-section using a motor-actuated multi-hole pressure probe. The measured profile then serves to modify the fan input for the next step of the iteration process. The process is repeated until a desired degree of uniformity is reached.   

Characterization of fluid flow inside a multi-fan-array wind tunnel for insects

How flying insects, like fruit flies, navigate turbulent environments in their day to day lives continues to be an active area of interest. To study their behavior in a controlled environment, we have developed a multi-fan-array wind tunnel with a 90x45x45cm^3 working test section. The fan array consists of 36 80mm fans configured in a 6x6 grid, with each fan being independently controlled to provide wind speeds between 0 and 100 cm/s. The fans can be operated in several different configurations designed to produce laminar, shear, and turbulent flow. To characterize the fluid flow in each of these conditions, we used a hot wire anemometer to measure wind speed and turbulence intensity at the front middle and back sections. With the incorporation of flow manipulators our wind tunnel can achieve laminar flow with a turbulence intensity of just 1.1%, approximately .1% larger than an industrially built wind tunnel at the same wind speed. Utilizing a splitter plate we can generate a significant distinction between a laminar region and turbulent region inside the wind tunnel. Finally, we can induce turbulence in the entirety of the wind tunnel with either a static fan array pattern or a dynamically shifting fan pattern. Through analyzing the power spectral density of the time trace for each turbulence set up, we found that the static configuration, in which the fans are set to be on and off in a checkered pattern, has a -5/3 slope indicating turbulence in the frequency range of 10 to 100 hz while the dynamic setup has a turbulence range around 1 to 100 hz. Our fan-array design provides a convenient test-bed for studying how insects navigate both laminar and turbulent environments, with a particular future interest in how turbulence impacts odor plume tracking behavior.   

Turbulent flows generated by fan array wind tunnel

The low-level atmosphere with unsteady wind and gust is a common working condition for general aviation and Micro Aerial Vehicle (MAV) flights. To better understand the aircraft flight performance within such environment, an open wind tunnel, which not only can offer space for aircraft free-flight experiments, but also can generate turbulent flow with desired characteristics, is an ideal tool to generate such environment. A Fan Array Wind Tunnel (FAWT) can easily fulfill these two requirements. In this research, an FAWT is utilized in a large indoor environment to generate certain turbulent flow profile and the flight control performance of the MAV is recorded by the motion capture system. The spectral and spatial characteristics of different turbulent income flows are obtained by a pressure transducer and an array of hotwire probes. The coherent structures of the generated turbulent flow profiles are also studied in this research.   

The New Penn State University Compressed Air Tunnel for Investigating High Reynolds Number Unsteady Aerodynamics

Achieving large Reynolds numbers is a continual challenge for the experimentalist, who is often limited by space and resources to relatively small wind tunnel facilities of moderate power. Periodicity in the flow, due to unsteady motion or rotation, creates further complications by producing a nondimensional frequency which must also be matched, typically at the expense of the Reynolds number. However, by using compressed air as the working fluid, a dynamically similar flow is possible in a small facility with modest drive power, enabling high Reynolds numbers without high cost. This is the approach taken by the new Penn State University Compressed Air Tunnel (PSU-CAT) where static pressures in the facility can be increased to 34 bar (500 psi). This results in kinematic viscosities which are 34 times lower than atmospheric air and Reynolds numbers of 29 million/meter in the 1 meter diameter test section, enabling the PSU-CAT to act as a much larger wind tunnel . The design and fabrication of the facility will be discussed as well as measurement capabilities including stereoscopic particle image velocimetry (sPIV) and hot wire anemometry (HWA) on the wakes of static and rotating models.   

Novel method to control the floating motion of a sphere in vertical wind

The feasibility of controlling a levitating sphere over a vertical fan array wind tunnel (FAWT) is investigated. Based on the Coanda effect, and using a velocity gradient to exert a horizontal force on the sphere in levitation, a control algorithm is developed, allowing translation in a horizontal plane and precise positioning of the levitating sphere across the vertical test section. By tuning the fans’ speeds as well as various parameters of the controller, the sphere was able to translate in aerodynamic levitation and in a stable manner.

In order to gain more stability, several stabilisation methods were investigated. The stability of the sphere was demonstrated to be proportional to the size of the sphere.

To further understand the phenomena at play, 3D flow measurements with a five-hole pressure probe allowed to visualise the air flow generated by the FAWT in two different configurations: a stationary levitation pattern, and a translation one. A motion tracking camera system was used to capture the motion of the sphere in the test volume.   

Development of a large-array hot-film sensor for detecting flow separation on a large airfoil.

Hot-film sensors have been used for a variety of applications such as measuring wind speed, temperature, and wall shear stress. This study aims to develop a low-cost, constant-current hot-film circuit in an array configuration to detect the boundary layer condition on large, complex lifting surfaces under the influence of unsteady inflow. A specific case of interest is the three-dimensional flow separation which occurs on wind turbine blades caused by atmospheric turbulence. The hot-film array will be tested on both a flat plate and a cambered airfoil actuated by a plunging-pitching rig to evaluate stall detection and frequency response capabilities of the array in both static and unsteady conditions. Dimensional analysis will be used to extrapolate the results to full-scale. The detection capabilities of the array and driving circuitry package will be compared against validation data taken with particle image velocimetry and flow visualization techniques such as tufts.