Session E20: Experimental Techniques: General

Rapid Extraction of Propeller Geometry Using Photogrammetry

As small Uninhabited Aerial Vehicles (sUAS) increase in popularity, computational analysis is increasingly being used to model and improve their performance. However, most manufacturers of propellers for this size of vehicle do not publish geometric information, making simulation of propeller aerodynamics difficult. While techniques exist to accurately extract the geometry of a propeller, these methods are often expensive or time-consuming. The method of choice among sUAS researchers is to cast the propeller in resin, slice the resulting piece into thin sections, and scan each section to extract the airfoil at that station. This technique is destructive and labor intensive as a human must perform each step. 

This paper describes a method to produce point clouds using readily available photo equipment and software, and subsequently fit typical airfoil and propeller blade parameters with minimal human input to produce a 3D model suitable for use in techniques such as Blade Element Momentum Theory. The propeller geometry generated is compared to one extracted using established methods and good agreement is shown.   

High Speed Transient Laminar Flow Meter

The ongoing development of new monopropellant spacecraft thrusters requires accurate characterization of propellant flows which contain flow transients outside the range of current commercial flowmeters. This study suggests a novel flowmeter design, which uses Womersley's series solution to determine the transient flowrate within a laminar flow passage through differential pressure measurements. A series of bench-top experiments was conducted for transient flows with a Womersley number range of α = 0.8-10, Reynolds numbers Re < 400 and a comparatively large tube inner diameter (5.45 mm) so flows remained within the range of commercially available flowmeters. Direct comparison between the proposed method and commercial flowmeter shows agreement across the full target range. In a real application, where flows are non-periodic, the Womersley solution's requirement for fully developed flow and periodic solutions means that the initial flow in the system is unresolvable. However, the Womersley solution rapidly converges to the true flowrate and experimental data was used to confirm theoretical estimates for this convergence time.   

Modal decomposition application to study large-scale flow structures in an aneurysm

Modal decomposition methods such as  Proper  Orthogonal  Decomposition  (POD)  and dynamic mode decomposition (DMD) have been used to study complex flows and to gain physical insight into the flow phenomena in detail.  The study's objective is to use the modal decomposition methods to extract physically important features and understand their impact on the flow in an ideal saccular aneurysms model.  An in-house experimental setup was fabricated where a pump system controls the non-dimensional inflow conditions Womersly number (α) and Reynolds number (Re).  Velocity measurements in the aneurysm are conducted on the mid-vertical plane using Particle Image Velocimetry (PIV). The modal decomposition results each identified the coherent structures and their spatial and temporal behavior.  These modal decompositions reveal complex interactions of the flow structures on the critical fluid flow parameters like impinging locations and wall shear stresses.     

AI-driven digital inline holography for real-time in situ particle analysis

Real-time in situ analysis of particles (e.g., size, concentration, and shape) is crucial for environmental monitoring, medical examination, advanced manufacturing, and many other areas. The digital inline holography (DIH) has emerged as a low-cost imaging-based solution for high-fidelity in situ measurements of particle shape and types (in addition to size and concentration) which are not available from conventional particle sensors based on light scattering or aerodynamic characteristics. However, the existing DIH method is computationally expensive, conducted offline and lacks robustness to deal with changing image quality (due to fluctuation in background intensity and noise, etc.) present in in situ applications. To address these challenges, we introduce a machine learning framework for simultaneous detection, classification, and other analyses (e.g., viability) of particles of various forms in real-time. The framework involves a combination of different convolutional neuron networks and generative adversarial networks for synthesis, autonomous labeling, and processing of DIH data. Integrated with various hardware setups, the performance of this AI-driven DIH approach has been demonstrated in monitoring of indoor air quality, harmful algal blooms and cell sorting.   

Direct measurement of vorticity using tracer particles with internal markers

We demonstrate an optical imaging technique to obtain a direct measurement of 3D vorticity in a flow field, based on the measurement of the instantaneous rotational rate of microscale tracer particles. The tracer particles of ~50 µm with internal markers (~2 µm) are fabricated using a flow-focusing microfluidic device. Digital inline holography (DIH), which consists of a collimated coherent light beam and a digital camera that capture the diffraction signals (holograms) from the objects within the beam path, is employed to image several tracer particles within a field of view of centimeter scale. The holograms are then processed using an inverse reconstruction approach to obtain the 3D positions of each internal marker within a tracer particle. The translation and rotation of the particles are then derived from the time-resolved 3D positions of internal markers. The proposed approach is calibrated using a solid-body-rotational flow system and will be applied to probe the small-scale vorticity dynamics in different turbulent flows.      

Towards quantum-enhanced flow sensing using nitrogen-vacancy (NV) centers in diamond

Nitrogen-vacancy (NV) centers have garnered significant interest in the past decade as a versatile platform for quantum-based sensing. These vacancies are point defects within a diamond lattice whose electron spin-states are highly sensitive to external perturbations such as temperature, strain, magnetic fields, and electric fields. NV center-based sensors measure the changes in the spin-states in either an individual or ensemble of NV centers to deduce information about the parameter of interest. Changes in the spin-states are most often observed using the photoluminescence generated by electronic state transitions in the form of optically detected magnetic resonance (ODMR). Because the spin-state can be manipulated and read at room temperature, NV center-based diagnostics are becoming an increasingly accessible option for nanoscale sensing. Despite the rapid development and proliferation of NV center-based sensing protocols, these measurement capabilities have yet to be fully adopted by the fluid mechanics community. Here, we present our recent progress towards implementing a nanoscale magnetometry and thermometry system using a continuous-wave ODMR and discuss the outlook for applying this form of sensing to problems in fluid mechanics.   

Negative meniscus lens for enhancing depth-of focus in schlieren imaging systems

Non-ideal optical systems inherently suffer from image quality degradation due to geometric aberration. The degradation is often corrected by the use of "meniscus correctors" or negative meniscus lenses. Negative meniscus lenses were introduced into a z-type focused shadowgraph imaging system to enhance the depth-of-focus. Focus was quantified using image processing techniques to determine focus range in the imaging system. High zoom camera lenses were used to image a focusing target within a 4-meter-long test section. Fixed focal length and variable focal length camera lenses were compared to quantify the ability to enhance focus when a meniscus corrector is introduced. Although most schlieren and focused shadowgraph systems do not need these corrections, systems with long distances between the collimating optics almost always require these meniscus lenses.     

Nanoscale hot-wire probe for supersonic flow measurments

The heat transfer response of a nanoscale thermal anemometry probe (NSTAP) was investigated in supersonic conditions. The fabrication of the platinum based NSTAP hot-wire was conducted using standard semiconductor manufacturing techniques. Measurements of the sensor’s thermal response using a constant current anemometer shows a response time an order of magnitude smaller than classical cylindrical hot-wires. The critical role of the supporting structure of the sensing element was attested by modifying the metal composition of the studs to reduce the heat loss from Joule effect outside the sensing element, modifying the temperature profile along the sensing element. The addition of a gold layer on top of the platinum layer enabled a considerable reduction of the response time leading to 33 ms. Increasing the cutoff frequency by concentration of the heat transfer in the nano-ribbon element provides the possibility to explore the relationship between geometrical characteristics of the sensor and Nusselt dependence on Mach number in rarefied gas.   

Estimating wind speed and direction with optimal sparse sensor placement on a cylinder

For a wide variety of applications, specifically those where automation is an integral part of the operation, there is a need to determine the magnitude and direction of an oncoming flow. Recently, there has been a growing body of work that has looked at flow field estimation, under the implicit assumption that one already knows the uniform flow field direction. In this work, we consider the simplified scenario of estimating the wind speed and direction of uniform flow over a 2D cylinder. Specifically, we are interested in determining the minimum number and location of sensors on the obstacle with the aim of developing a framework for other geometries, such that they can be used for subsequent flow field estimation methodologies. We consider Reynolds numbers in the range of 25,000 < Re < 125,000, where Re = UD/ν, with U being the oncoming flow velocity, D the cylinder diameter, and ν the kinematic viscosity, and use surface pressure readings to determine the wind speed and direction. Our results are compared to other established sparse sensing techniques, such as POD with QR-pivoting, with the results suggesting that the same level of accuracy can be obtained using fewer sensors for the proposed technique.   

Refractive-index matched polymer to water

We report a polymer refractive index-matched with water at room temperature and that can be cast in thick specimens.  We cast it to thicknesses up to 3 mm and by using appropriate primer glued it to fused silica and acrylic viewports.  Specifics of the mold fabrication and casting process will be presented. We successfully demonstrated its use with three common laser-based diagnostics: laser-induced fluorescence (LIF), particle image velocimetry (PIV), and molecular tagging velocimetry (MTV).