Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session X21: Experimental Techniques: General II |
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Chair: Jiarong Hong, University of Minnesota Room: 147A |
Tuesday, November 21, 2023 8:00AM - 8:13AM |
X21.00001: On the development and implementation of MEMS sensors for instantaneous wall-shear stress measurements in wind tunnel applications Joseph Barrow, Michael Wilkes, Tao Liu, Barry Gallacher, Richard D Whalley In this work a new optical Moiré fringe sensor was designed, developed, and characterised for direct measurements of instantaneous wall-shear stress in turbulent air boundary layers. The sensors in this study consist of a small sensing pad of order 100 microns by 100 microns suspended by micro-springs as part of a 5mm square die achieving a hydraulically smooth sensor due to in-plane microfabrication and the absence of any topside connections. The instantaneous displacement and fluctuations of this device are measured through the scattering of light from a pair of gratings illuminated from the backside. The illumination is achieved using an array of LEDs focused onto the backside of the sensors and rippled at high frequency with the reflections measured using a photodiode. The Moiré fringe pattern created by this pair of closely matched pitched gratings amplifying the sensors motion by at least 50 times, improving the overall sensitivity of the device. The devices first have their sensitivities outlined using an analytical framework prior to characterization and calibration in a laminar flow channel before experimental measurements being made in a zero-pressure-gradient turbulent boundary layer. To validate the outcomes of the boundary layer experiments, laser Doppler velocimetry (LDV) will be run simultaneously to the MEMS sensor being positioned above the MEMS sensor. These two independent techniques can then be compared, as well as the moments of the statistics. |
Tuesday, November 21, 2023 8:13AM - 8:26AM |
X21.00002: Water Tunnel in a Box: A Novel Small Footprint Flow Facility Sean P Devey, Shana L Hartwick, Morteza Gharib Fluid dynamics experiments often require flow-producing facilities, which can involve large investments of time, money, and physical space. A traditional water tunnel involves one or several pumps which drive water around a circuit composed of a series of physically large and geometrically complicated guiding features including a diffuser, contraction, flow manipulators, and connecting pipes. An alternate design presented here leverages an array of electric thrusters to drive flow through an enclosed test section within a rectangular aquarium tank. In this arrangement, the test section is about 35% of the total fluid volume, with a test section footprint on the order of 45% the total tunnel footprint area. A combination of dye visualization and PIV results will be presented concerning the spatial and temporal uniformity of the flow field within the test section. Opportunities and challenges with this type of water tunnel will be discussed, including the potential for studying unsteady and impulsive flows, maintenance and operation considerations, and software tunable flow conditions. |
Tuesday, November 21, 2023 8:26AM - 8:39AM |
X21.00003: Effect of Ground Boundary Condition on Aerodynamics of a Non-Slender Delta Wing in Static Ground Effect GOKTUG KOCAK, Mehmet M Yavuz Aerodynamics of a non-slender delta wing in static ground effect was studied in a low-speed wind tunnel. Ground boundary conditions in static ground effect at three different fidelity levels including a fixed flat plate, tunnel floor (belt off), and dynamic ground condition with a moving belt mechanism (belt on) were characterized using surface pressure measurements and force measurements. The findings suggest that the aerodynamic performance and longitudinal static stability exhibit significant differences in static and dynamic boundary conditions. In dynamic ground condition, the lift-to-drag ratio versus angle of attack curve shows a larger area underneath compared to out of ground cases, rather than increased peak values observed in static ground conditions. Considering the belt on and off cases, the slopes of the aerodynamic coefficients are reduced compared to the static ground condition (a fixed flat plate), which in turn, result in different positions of the aerodynamic center along the longitudinal axis. It is possible that changes in dynamic pressure loss and flow angularity caused by the ground boundary condition are the main factors contributing to these altered stability characteristics. |
Tuesday, November 21, 2023 8:39AM - 8:52AM |
X21.00004: On the generation of single and multiple plane compound shear layers with multi-fan wind facility Malicia Leipold, Matteo Di Luca, Flavio Noca Although multi-fan wind facilities can generate complex spatial flows, due to their large number of degrees of freedom, finding the control laws has been elusive so far. This study demonstrates that plane compound shear flows can be generated using a simple control law. An existing solution to the two-dimensional incompressible Navier-Stokes equations in the absence of pressure gradients, i.e. Goertler, predicts the normalized time averaged axial velocity profile inside a compound shear layer and adding an empirically determined parameter allows predicting the dimensional velocity profiles. The model combines previous literature experiments with additional PIV measurements and shows that when the downstream distance is normalized with a characteristic fan size, i.e. its height, the velocity ratio between adjacent fans is the main additional variable that determines the downstream velocity field. The boundaries of applicability of the simple control law, predicted by the model, were validated experimentally through PIV measurements on a multi-fan wind facility. The applicability of the simple control law to multiple adjacent compound shear layers was also demonstrated experimentally. Turbulence profiles for different shear ratios were measured through hot wire anemometry to characterize the time scales of the produced shear flows, with quasi-steady applications in mind. Although a small-scale wind facility was used for model validation, the results are applicable to the control of multi-fan facilities of any size. |
Tuesday, November 21, 2023 8:52AM - 9:05AM |
X21.00005: Design and fabrication of a low-speed wind tunnel for evaluation of drag reduction surfaces Frank A Mier, Connor Wilkinson, Maria Allen, Peyton Chandler, Kyler Pritchard, Jonathan W Naughton Laboratory scale evaluation of various drag reduction surface morphologies can provide valuable insight on their capabilities for both design optimization and as-built performance verification. Specifically, the primary drag reduction technique evaluated here is the implementation of riblet surfaces which refers to small scale, streamwise structures which tailor the turbulent boundary layer to reduce viscous drag. Ongoing work between multiple collaborating parties at the University of Wyoming has established the abilities to both produce and measure the performance of these riblet surfaces. Here, the design and construction of a new wind low-speed tunnel specifically intended for these experiments is discussed and preliminary characterization of the tunnel is shown. Care has been taken to design a tunnel optimized for producing appropriately scaled boundary layers to test riblet surfaces manufactured by the same methods intended for in-field use on aircraft. |
Tuesday, November 21, 2023 9:05AM - 9:18AM |
X21.00006: Design and Flow Characterization of the Penn State Compressed Air Wind Tunnel Zarif Rahman, Mark A Miller The Penn State compressed air wind tunnel (CAWT) is a closed-loop wind tunnel that can be pressurized to 34 bar (500 psi) to enable access to high Reynolds number flows. The tunnel flow circuit is contained inside a large, toroidal pressure vessel which requires the installation of an internal liner that consists of four main components, including a rapid expansion, a turbulence management section, a contraction, and a working section. The main purpose of the liner is to provide uniform, low-turbulence flow at the test section. This talk will describe the design and implementation of the CAWT turbulence management sections and the challenges associated with constructing a wind tunnel inside a pressure vessel with two access points. In addition, flow quality will be assessed inside the test section using a hot-wire anemometer to characterize the background turbulence level and a series of Pitot-static probe measurements will be used to determine flow uniformity. |
Tuesday, November 21, 2023 9:18AM - 9:31AM |
X21.00007: OpenFlume: An accessible and reproducible benchtop flume for research and education Eli Silver, Robert Hunt, Daniel M Harris Open-channel flumes are an important tool in fluid mechanics research and education. However, the few commercially available small-scale flumes are generally expensive and lack complete characterization. In this work, we present a custom, low-cost, modular benchtop laboratory flume that is designed to be accessible and reproducible. The flume is assembled from commonly available materials and hardware and fabricated using a computer numerically controlled (CNC) router. Sections of the flume are mated using a standardized interface with a compression seal, allowing the flume components to be fabricated as discrete modules, rearranged into different configurations, and easily modified. Our design has a flow cross-section of 5 x 5 cm, adjustable flow velocity, and a modest footprint. The flow in the test section is characterized using particle image velocimetry (PIV) and shown to be of high uniformity and low turbulent intensity. Disturbances on the free surface are evaluated using an optical refraction-based method. A first application of the flume to study drag on partially immersed spheres will be briefly discussed. |
Tuesday, November 21, 2023 9:31AM - 9:44AM |
X21.00008: Advanced flow profile generation with Windshaper fan-array using real-time feedback loop control Aurélien Walpen, Nicolas Bosson, Tony Govoni, Guillaume Catry, Flavio Noca Windshapers, or fan-array wind generators, enable users to generate complex flow profiles through the independent control of thousands of small fans. However, configuring, measuring, and validating flow profiles for aerodynamic tests can be time-consuming and difficult. |
Tuesday, November 21, 2023 9:44AM - 9:57AM |
X21.00009: Quantifying Numerical Uncertainty in Background-Oriented Schlieren Pranjal Anand, Jiacheng Zhang, Lalit K Rajendran, Sally P Bane, Pavlos P Vlachos Our study presents and evaluates a method for computing the numerical uncertainty in Background Oriented Schlieren (BOS). We use Richardson extrapolation to assess the uncertainty of numerical integration of density gradients, based on residuals of density results across two grid levels. By integrating this numerical uncertainty with the existing random uncertainty, we obtain the final uncertainty of the density field. We assess the method's effectiveness using synthetic fields (hyperbolic) with artificial noise. Our error analysis shows that the sharpness of the density gradient significantly affects bias error and the prediction of numerical uncertainty. The prediction of numerical uncertainty corresponds to variations in bias error, particularly when the noise level and wavelength of the flow field are altered. By accounting for the numerical uncertainty, our method achieves up to 91% accuracy in predicting total uncertainty, as measured against the root-mean-square of the total error. We further demonstrate the utility of our methodology by applying it to experimental BOS images. Our proposed approach offers a more accurate understanding of uncertainty estimation in the BOS technique, with implications for future experiments. |
Tuesday, November 21, 2023 9:57AM - 10:10AM |
X21.00010: Continuous Stochastic Data Acquisition for Improved Calibration of Multi-Hole Pressure Probes in Flow Measurements Tony Govoni, Aurélien Walpen, Nicolas Bosson, Guillaume Catry, Flavio Noca A new data acquisition method for calibrating multi-hole pressure probes is being investigated to improve the overall measurement quality of these instruments, when operating in quasi-transitory mode. In particular, when these probes are used to perform sweeping measurements of a flow field by continuously changing their orientation or position. A continuous stochastic acquisition of calibration data has been used to account for transient effects and eliminate bias from a predefined acquisition pattern or mitigate hysteresis effects. Different rate of unsteadiness in the positioning of the probe were investigated in order to understand the limit of this calibration methods as well as the range of unsteadiness at which the measuring instrument could operate. The accuracy of measurements obtained using this calibration method was compared with that of the traditional time-averaged calibration method. The new calibration method was found to be more accurate when multi-hole pressure probes are used by a human operator, or for continuous scanning measurements. |
Tuesday, November 21, 2023 10:10AM - 10:23AM |
X21.00011: Passive makes perfect - reconstructing mean velocity and passive scalar fields from limited observations Sean P Symon, Uttam Cadambi Padmanaban, Joshua Rawden, Christina M Vanderwel Flows with passive scalars are common in many engineering and industrial applications. One current challenge, which involves pollutants as the passive scalar, is modelling and predicting urban air quality. Simulating these flows requires vast computational resources and modelling assumptions whereas experimental measurements are sparse and incomplete. To improve predictions of mean (time-averaged) velocity and passive scalar concentration, a data assimilation (DA) framework is developed which minimizes the discrepancy between a RANS simulation and partial observations of mean velocity and passive scalar fields. In doing so, the DA infers unknown forcing terms in the momentum and advection diffusion equations corresponding to Reynolds stress gradients and scalar fluxes, respectively. The algorithm is demonstrated on the laminar flow past a heated cylinder, i.e. temperature is the passive scalar. DA successfully reconstructs the mean velocity and temperature fields even with limited input data. It is also shown that with just temperature data as the input, DA improves predictions of the mean velocity fields. |
Tuesday, November 21, 2023 10:23AM - 10:36AM |
X21.00012: Effect of Refractive Distortions on Volumetric Reconstructions of PIV, PTV and LIF Light Fields Fernando Zigunov, John J Charonko In this work, we analyze the effect of refractive distortions produced by a turbulent flow containing random index of refraction fluctuations on the accuracy of tomographic reconstruction utilizing standard algorithms for light field reconstruction (Tomo-PIV, Tomo-LIF) and particle tracking (Shake-the-box PTV). In order to produce images from a known emission field, an in-house implementation of a raytracer using CUDA/C++ was developed. The raytracer generates synthetic images of particle fields or continuous emission fields with refractive distortions from a known turbulent flow field for different refractive index strengths. The synthetic images are then fed to the volumetric reconstruction algorithm.
It was found that the reconstruction quality quickly starts degrading for ray displacements greater than 3 pixels as seen by the cameras, for both Tomo-PIV/LIF and Shake-the-box PTV. In particle applications, the number of ghost particles greatly increases when the ray displacement is greater than 3 pixels. The trends observed in the synthetic data will be evaluated experimentally using a water jet with temperature gradients. A theoretical foundation based on stochastic ray displacement (“random walk”) is also presented to establish the limits of the cited tomographic reconstruction techniques for arbitrary flows with refractive distortions by relating the ray distortion to the turbulent length scales of the flow.
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