Session U16: Experimental Techniques: Wind Tunnel

Tomo-PIV Measurements in a Refractive Index Matched (RIM) Flow

Refractive index matched (RIM) flow facilities are used to mitigate optical distortions and enable PIV measurements about complex geometries. A comprehensive assessment of planar-PIV performance when viewing through transparent acrylic and urethane models in a sodium iodide (NaI) RIM facility at Notre Dame was provided in Blois et al. 2020. In the current work, tomo-PIV performance in the RIM facility is assessed. Due to the number of cameras required for a tomo-PIV measurement, the direct, instantaneous measurement comparisons that were performed in the prior work are not practical. Therefore, unique data sets obtained with and without an acrylic model in the optical path are compared. Performance metrics include calibration disparity, reconstructed volume characteristics, and ensemble flow statistics.   

The Development of a Photoelastic Floating Element Force Balance

We present a new floating element drag balance design that uses an optical measurement of the force using photoelastic stress analysis. The force sensing element consists of pins embedded in photoelastic polyurethane pads, which generate internal stress when the floating element is loaded. A series of known loads and their corresponding fringe patterns allow a calibration matrix to be derived using a 5th-order polynomial model solved by a least square regression. FEA is carried out to validate the proposed photoelastic method. The balance then measured the lift curve of the NACA0015 wing at 5m/s and 7m/s. A comparison of the photoelastic balance and an ATI Mini 40 load cell showed differences of 6%. This optical approach enables accurate measurements with inexpensive and simple components inside the sensor. The balance design minimizes the element's misalignments relative to the surface and can be sealed to avoid horizontal buoyancy forces from adverse pressure gradients, crucial for wall shear stress measurements. It can also be tailored for different load cases and scaled to fit complex setups across various magnitudes. A photoelastic balance is a simple, effective, and versatile means of force measurement, able to measure forces in a time-averaged manner and instantaneously.   

Fabrication and characterization of a turbulent channel flow facility for studying super-hydrophobic surface stability

A turbulent channel flow facility was designed, fabricated, and tested at the University of Massachusetts Dartmouth. This facility aims to study the stability of super-hydrophobic surfaces (SHS) subjected to turbulent flows. The water flow is driven by one 3-horsepower centrifugal pump. The channel has optical access from all sides and has removable windows for the installation of SHS. The channel's internal dimensions are 1016 mm × 50 mm × 6.4 mm (length × width × height). Due to the 8:1 aspect ratio of the cross-section, the flow in the channel is nearly two-dimensional. The mean flow speed in the test section varies from 0.5 to 10 m/s. The Reynolds number based on channel height and mean flow speed varies from 3200 to 64,000. In this presentation, we will describe major components of the channel flow facility, including the storage tank, settling chamber, test section, diffuser, etc. We will also discuss the skin-friction coefficient measured by pressure drop, and the profiles of mean velocity and Reynolds stresses measured by particle image velocimetry (PIV). Furthermore, we will describe the fabrication technologies of various SHSs, and two optical technologies for examining the gas layer on the SHS based on total internal reflection and reflection interference.    

Calibration and verification techniques for a multi-axis force balance to measure small changes in viscous drag on riblet surfaces

At laboratory scales, direct measurement of viscous drag reduction by riblet surfaces is a difficult task to perform accurately because of the milli-Newton scale force measurements required. This is particularly the case when comparing the performance characteristics of different riblet geometries and evaluating the effects of slight damage that could be realistically expected in real-world use. To address this challenge, a multi-axis force balance has been developed that incorporates the unique combination of an easily deformable elastomeric structure and precision Hall Effect sensors to measure structure deformation. To interpret the data, an automated calibration stand has been developed to tilt the balance to create force components in the stream-wise and span-wise directions. A multi-variable regression analysis of calibration data allows subsequent balance measurements from wind tunnel experiments to be analyzed in terms of viscous drag. These measurements have been verified against Oil Film Interferometry taken on a flat plate.   

Designing a stable droplet levitation wind tunnel

The efficient study of liquid droplets ranging from micrometers to a few centimeters by levitation is usually hindered by conventional design limitations. This is due to continuous droplet deformation in the test section. This research discusses the development of a robust design methodology for large droplet-stabilization (d > Ca) vertical wind tunnels. A modeling and simulation environment has been developed which involves component sizing and integration on analysis software followed by design optimization. Extensive research on guide vanes design to minimize the viscous losses and wind tunnel size using numerical software for cascade analyses has been successfully accomplished. Sensitivity analysis has been performed on the geometric parameters to identify the design variables for optimization. A statistical modeling of the multivariable, non-linear and discontinuous design problem has been performed to investigate the optimum design space using the stochastic optimization technique. The design of honeycomb and wire screens and their optimization for a given design has been performed using the Navier-Stokes equations. A non-conventional design with varying test area cross-section has been introduced to investigate the droplet stability in the test section. The research highlights a systematic design methodology and an alternate configuration for liquid droplet wind tunnels while focusing on stable droplet levitation.   

Design and Fabrication of Compressed Air Wind Tunnel

The compressed air wind tunnel (CAWT) is a specialized tool at Penn State that investigates the scaling of full-size aerodynamics of rotorcraft and wind turbines. This new facility is being constructed entirely inside a high-pressure vessel (34 bar) to enable large Reynolds numbers via the high air density. Ongoing work is focused on the design and fabrication of the internal, modular flow conditioning for the CAWT pressure vessel, to mitigate the losses within the circuit. The current focus is to fabricate a wind tunnel with a balance between the aerodynamic qualities in the test section and the performance of the circuit. Two areas of focus within the circuit are the flow-conditioning screens and wide-angle diffuser. The screens have been carefully selected to facilitate the desired velocity and flow uniformity within the test section. Another concurrent goal is achieving the desired turbulence levels within the test section with the use of flow conditioning. In addition, the layout of the pressure vessel necessitates the use of a rapid expansion preceding the flow conditioning. The final presentation will consist of the results from the design and construction of this compressed air wind tunnel.   

Fan Array Wind Tunnels: Spatially Resolved Characterization and Data-Driven Modeling

Fan array wind tunnels are an emerging technology to design bespoke wind fields through the independent control of a grid of individually addressable fans. They are especially well suited for the turbulent, dynamic, spatially varying flight conditions found close to the ground, and they have been used successfully in recent applications from entomology to flight on Mars. However, the physics of fan arrays are still not fully characterized, making it challenging to design a custom wind field without heuristic trial and error optimization. We present the results of an experimental campaign of spatially resolved measurements of fan array wind tunnels under various non-uniform input profiles to begin addressing this modeling challenge. Specifically, we characterize the velocity field at various downstream locations and sensor configurations. We then test different data-driven modeling techniques on this input-output data, with the goal of enabling advanced model-based inverse design, optimization, and control.   

Turbulent flows and UAV flight test with fan array wind tunnel

The low-level Atmospheric Boundary Layer (ABL) with unsteady wind and gust is a common, but challenging 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. A Fan Array Wind Tunnel (FAWT) can easily fulfill these two requirements. By changing the RPMs of the fans in certain locations of FAWT, one can obtain accustomed turbulent profiles with different characteristics for MAV flights. In this research, an FAWT is utilized in an indoor autonomous flight lab to generate certain turbulent flow profile and the flight control performances of the MAV are recorded by the motion capture system. The spectral and spatial characteristics of different turbulent profiles are obtained by a pressure transducer and a hotwire probe. The relaionship between turbulence characteristics and corresponding MAV flight control performances are investigated.   

Characterization of a surface mount hot-film sensor using an open-jet unsteady flow wind tunnel

Hot-film sensors are used for a variety of applications such as measuring wind speed, temperature, and wall shear stress. The characterization of these sensors and their driving circuits is vital for determining their viability in the field, specifically in applications which may contain higher frequency flow events such as gusts as well as with separation detection in turbulent flow. An open-jet, unsteady-flow wind tunnel is used to evaluate the time response and dynamic range of a hot film sensor, where the tunnel can simulate both a velocity impulse as well as periodic input at varying wind speeds. The hot-film is located downstream of the wind tunnel outlet and is surface mounted to a flat plate, which is able to be pitched. Testing at field relevant conditions will be achieved by matching a range of reduced frequencies as expected in the field. Hot-film sensor response will be validated using two methods: pressure taps and particle image velocimetry.

    

Characterization of periodic changes in an inductively coupled plasma using emission spectroscopy

Inductively coupled plasma (ICP) torches are used in material processing, propulsion applications, and the testing of thermal protection systems (TPS). In most cases, stable and steady operation of the plasma stream is desirable. However, properties of the power circuit, swirl stabilization of the plasma core, and the interaction between fluid mechanics and plasma kinetics can lead to instabilities and time-periodic behavior. Our ICP torch operates with argon and air plasmas at atmospheric pressure, at flow rates of a few grams per second and input powers in the range of 30 to 60 kW. Using optical emission spectroscopy, circuit measurements, and analytic estimates of plasma plume properties, we investigate the origin and impact of several periodic changes in the plasma plume on temperature and excited species. One dominant periodic mode is caused by the imperfect rectification of the AC power supply. It occurs at 180 Hz and, at atmospheric pressure, seems to cause negligible changes in the plasma temperature. Another much slower variation appears at <1 Hz, causing significant changes in the plasma temperature and warranting further characterization. Finally, we discuss the possible impact of the periodic behavior on ICP applications.

    

A high Hele-Shaw cell based on Tank-in-Tank configuration

A high vertical "Hele-Shaw" apparatus for the quasi-2D fluid experiments has been proposed to address large deformation of high channel due to static fluid pressure. The apparatus is designed with a "Tank in Tank" configuration and consists of an outer tank and an inner tank. The outer tank is cuboid-shaped and used to load fluid medium, while the inner tank consisting of two pieces of parallelly installed glass serves as the experimental channel. The pressures acting on the channel are balanced so that a high-level uniformity is maintained over the whole channel. Both heavy/light experimental objects can fall/rise freely along the channel to achieve a 2D motion style. The channel area is around 2800×1500 mm, while the gap distance of the channel can be adjusted from 0 to 120 mm. A case experiment utilizing a circular disk freely falling from rest is implemented. Four distinct falling styles have been found and visualized. The effects of the channel gap distance and disk diameter on the falling motion are discussed. As measured, a flow with Reynolds number from 400 to 63000 can be realized in this study. A theoretical model is established to characterize basic features of the apparatus. Some potential research fields and improvements of the apparatus are suggested finally.