Session T07: Experimental Techniques: General (8:00am - 8:45am CST)

Design and Testing of an Absolute Pressure Sensor Mote to Measure Full-Scale Wind Pressure Loads on Buildings

Several studies comparing wind loads measured at the full- and model-scale on low-rise buildings have consistently found peak pressures to be underestimated at the wind tunnel scale. However, there is a lack of data comparing full- and model-scale wind loads over larger buildings. This is primarily due to experimental challenges: the distribution of differential sensors around a large building is too complicated, and non-intrusive absolute pressure sensors have traditionally lacked the resolution required to measure pressure coefficients. Our objective is to leverage a new generation of absolute pressure sensors, such as the Bosch Sensortec BMP388, to measure wind loads on large buildings. First, we determined that the sensor has low enough noise (1.7 Pa RMS) to make meaningful $C_p$ measurements, and we demonstrated that it is capable of measuring fluctuations on the time scales associated with turbulence in a wind tunnel experiment. Subsequently, we have designed a low-cost, compact, wireless mote that features the BMP388. Extensive testing of several motes has shown it to be a robust data-acquisition system, attainable at a fraction of the cost of commercially-available dataloggers with similar capabilities, and we have begun deploying motes over a high-rise building.   

Assessment of three-dimensional density measurements from tomographic background-oriented schlieren (BOS)

Tomographic background-oriented schlieren (TBOS) is used to measure 3D instantaneous density fields in turbulent flows. TBOS uses the Gladstone-Dale relation between fluid density and refractive index (RI) to obtain the 3D density field based on images looking through a flow, which capture information on path-integrated RI gradients from background image displacements. We examine four error sources: i) defocus blurring; ii) spatial averaging in the solution; iii) limited-view tomographic reconstruction; and, iv) displacement field noise. Synthetic BOS displacements are generated by raytracing through the refractive index field of a heated jet DNS at Re = 5000 and jet exit centreline-to-ambient density ratio of 0.83. The virtual BOS setup uses 15 cameras placed circumferentially around the jet. We show defocus blurring has the greatest impact on accuracy, especially near the nozzle. Sources ii) and iv) have marginal impact. Three reconstruction methods are tested: i) filtered back-projection (FBP); ii) an algebraic reconstruction technique (ART); and, iii) sequential FBP-ART. ART and FBP-ART are similarly accurate, with under half the error of FBP, which suffers from significant reconstruction artefacts. Reconstruction error is modest compared to the impact of defocus blurring.   

Investigation on the thermal performance and flow characteristics of oscillating heat pipe under different cooling conditions.

The performance of the oscillating heat pipe is reliable on the oscillatory and pulsatory motions of fluid inside the capillaries. Heat load, working fluid, filling ratio and convection sources are the essential factors which influence the internal flow dynamics and heat transfer performance of oscillating heat pipe. In this study, first a two-turn closed loop oscillating heat pipe was fabricated from a Pyrex glass and then its thermal performance and internal fluid flow characteristics was investigated under active and passive cooling conditions. A 50{\%} fill ratio methanol was used as a working fluid and heat load was provided step-wise. Flow visualization technique was used to observe the flow dynamics inside the capillaries for both active and passive cooling. The experimental result shows that internal flow dynamics and thermal performance are strongly dependent upon the cooling conditions. The velocity, acceleration and displacement of bubbles inside the capillaries are also influenced by the cooling conditions. Slug/plug flow are dominant at low heat load for both active and passive cooling while the elongated plug, annular and semi annular oscillatory flow are dominant at higher heat load. This study also reveals about the start-up performance and dry-out conditions and can be helpful to design a oscillating heat pipe based on its thermal performance and flow characteristics when different cooling conditions are applied.   

Optical Characterization of Underwater Contact Mechanics

For survival in extreme environments, organisms have evolved adhesive mechanisms and materials that out-compete those of human technology. While chemistry of underwater bio-adhesion is a source of valuable insight, the mechanics by which surfaces expel water and come in contact underlies critical understanding of natural solutions and evaluating biomimetic analogs. Hydrophobicity of an adhesive surface has been shown to be crucial in removing water from a hydrophilic substrate, but the resulting contact is typically heterogeneous, with patches of unevacuated water. Thermodynamically, the hydrophobic part of any adhesive or surface drives the water out in presence of any other akin moiety due to long-range hydrophobic forces. In this work, we present a simple, FTIR-based imaging technique to spatially resolve and quantify thickness of nanoscopic puddles formed between two solids in contact under water. The technique is validated by comparing measured air gap thickness of a glass lens in contact with glass prism with Hertzian contact theory, and then applied to characterize the drainage and formation of patches of water as a soft PDMS lens approaches a functionalized and a pristine glass surface at varying speeds. The work paves the way for better characterization of interfaces in contact under water and can find application in adhesive development, biological study and tribology.   

Framework for unsteady flow analysis in a circular cavity using Dynamic Mode Decomposition

Flows in a circular cavity display complex fluid behavior which contain large-scale flow structures and are impacted by different unsteady inflow conditions. Therefore, the objective of this study is to develop a framework for extraction of fluid dynamics in a circular cavity to identify large-scale structures, highlight their temporal features, and quantify flow behavior for a given flow scenario. To achieve this objective, velocity field measurements using Particle Image Velocimetry (PIV) were performed near the center plane of an idealized rigid closed-cavity model. The velocity field data were then sequenced and phase-averaged to a single flow cycle. Dynamic Mode Decomposition (DMD) was then used to the sequenced and phase-averaged data to obtain dynamic modes, DMD energies, and associated frequencies and decay rates which are related to the large-scale structures and temporal behavior in the circular cavity flow. A velocity field approximation which contained significant dynamic modes was used to highlight important temporal flow features and obtain information related to the flow evolution in the cavity. The combination of phase-averaging and DMD for this given flow scenario was able to capture the large-scale flow dynamics and their associated temporal behavior.   

Development and Testing of a 360° Differential Pressure Gust Sensor for Extreme Weather Environments

Measuring rapidly fluctuating high wind speeds and direction in extreme weather environments such as hurricanes and severe storms proves difficult with current sensors. For example, mechanical anemometers do not provide sufficient response time to detect micro-gusts and ultrasonic anemometers are quickly obscured by rain and other particles, and all are susceptible to damage from hail and debris. This presentation details the development and testing of a 360° gust probe using differential pressure sensors. Similar in concept to a multi-hole pitot or boundary layer probes, this circular probe is surrounded by multiple inlets connected to pressure sensors whose differential readings provide high-speed wind magnitude and vector results. The probe is equipped with a purge system to vent water and debris from the inlets and its small size and rigid construction make it damage resistant, creating a system uniquely suited to capture data in an environment that was previously unavailable. Results of wind tunnel and field tests of the system are presented.   

Three-Dimensional Density Reconstruction of Supersonic Twin Jets by Background Oriented Schlieren Technique.

Strong acoustic waves emitted from a rocket plume possibly damage to payloads of a rocket because the acoustic waves vibrate the payloads such as a satellite. Recent rockets employ multiple engines configuration and the acoustic waves from multiple jets interaction are becoming important. Therefore, it is important to three-dimensionally visualize multiple jets flow structure and to accurately understand multiple jets interaction. The present study applies three-dimensional background oriented schlieren (3D-BOS) technique to density fields of supersonic twin jets. Here, 3D-BOS is a technique that enables quantitatively to measure the three-dimensional density field. The operating condition of the twin jets is an ideally expanded condition with a Mach number of 2.0. The three-dimensional density field of the twin jets was simply reconstructed by using a matrix in 3D-BOS. The interaction of the twin jets according to the nozzle spacing was compared. The results show that the interaction of each jet in the twin jets configuration is occurring and moves the downstream side with increasing the nozzle spacing. In particular, the density distribution of small nozzle spacing elliptically spreads towards the downstream side due to the strong interaction.   

Simultaneous Temperature and Velocity Field Measurements of Liquid Hydrocarbons by Dual-luminescent Imaging and Particle Tracking Velocimetry in a Side Heated/Cooled Cavity.

Arctic oil spills are detrimental as they could cause extensive ice melting in addition to the overall environmental pollutions. Floating oil slicks among ice floes absorb ambient energy and transfer that energy to the ice to aggravate melting in thaw season. However, no studies have revealed how oil-ice interaction impacts ice melting. This research investigates the heat transfer pathways from oil slicks to the ice. Dual-luminescent imaging and particle tracking velocimetry (PTV) in a side heated/cooled cavity is performed for temperature and velocity measurements of liquid hydrocarbons, respectively. Dual-luminescent images and images of the seeding particles in PTV captured spatiotemporal temperature distribution and velocity field of oil in the cavity, respectively. The results show the convective field is directly coupled with the temperature field induced by temperature difference in the liquid. Successful implementation of the two measuring techniques together is a step toward analyzing heat transfer pathways in a liquid fuel adjacent to an ice body.   

2D and 3D Temperature Measurements with a Multi-band Plenoptic Camera

Re-entry vehicles experience very harsh environments that can compromise the integrity of materials. At these extreme temperatures, materials can have wildly varying properties. Emissivity, an important property for optical pyrometry, is a function of wavelength and temperature. Traditional optical pyrometers have difficulty with varying spectral emissivity. The multi-band plenoptic camera is a combination of a camera, microlens array, and a wavelength filter array placed at the aperture plane. The microlens array placed at a specific location samples the aperture plane, which traditionally captures spatial and angular information of light rays. Additionally, the inclusion of a filter array provides spectral content. With the spectral filter placed forward of the camera, the multi-band plenoptic camera provides flexibility in filter and wavelength design. The multi-band camera captured surface temperatures of a copper (Cu) melt pool during solidification to show efficacy in capturing temperatures when a material has large spectral emissivity and reflectivity variation. With the availability of angular and spectral information, the information can be used to produce 3D scalar field reconstructions. Finally, some preliminary 3D reconstructions of simulated flames will be presented.   

Separating Particulate Scattering from Molecular Absorption in Radiative Transmission Measurements of a High-Speed, Two-Phase, Reacting Flow

Radiative transmission through a two-phase, reacting fluid is extinguished by a combination of absorption and scattering effects due to the reaction product molecules and the bulk particles. If the particle scattering coefficient is slowly varying with wavelength, the scattering effect can be removed from the transmission spectra. To verify this, we have studied the near UV spectral extinction due to scattering in a two-phase, high speed jet with various mass loadings of Alumina particulate (Al$_{2}$O$_{3}$). Then, in the high-speed, reacting flow of a lab scale rocket exhaust plume, we have measured the transmission spectra with and without the alumina particulate added. The particle scattering effect in the transmission spectra is removed so that the difference between the flows with and without particles can be observed in the absorption spectra of Hydroxyl (OH).   

Sparsity-Based Classification for Remotely Sensed Subsurface Bubble Saturated Turbulent Flow

Subsurface bubble saturated turbulence is a complex phenomenon possessing imagery signatures which can delineate uniquely important turbulence generation processes. Naval geo-intelligence agencies possess sub-surface turbulence imaging technology supporting a serious need for classification algorithms for turbulent flow state imagery observations. A fluid dynamical and machine learning experiment explored the ability to robustly classify imagery data taken from six different bubble saturated turbulence scenarios using sparsity-based classification. Overcomplete image dictionaries, formed from image sequences acquired using an off-the-shelf underwater high-speed camera, were used to decompose and reconstruct test images to be classified. Groups of image dictionaries for each turbulence scenario were used to estimate the minimum image Euclidean reconstruction error repetitively for single test images. Global minimum reconstruction error provided a test image vote for each turbulence scenario and was used to assign turbulence scenario membership. Preliminary results show robust sparsity-based classification of test images for five of the six scenarios with classification error due to photonic similarity existing in two scenarios.   

Inverse problems in magnetic resonance velocimetry: shape, velocity and boundary condition inference

Magnetic Resonance Velocimetry (MRV) is a non-intrusive experimental technique used for example in medical imaging and porous rock sampling. It provides 3D velocity data in a 2D or 3D box, usually with low signal to noise ratio. Given noisy MRV data, we wish to infer the most likely boundaries of a flow, the boundary conditions, and the velocity of the flow. We do this by assimilating the data into a qualitatively-accurate flow model, such as the Navier-Stokes equations with unknown uniform viscosity, thus rendering the model quantitatively accurate. The revised model is used to infer hidden quantities of the fluid or the flow, which cannot be measured directly (e.g. wall-shear stress). At the same time, we obtain a denoised and higher-resolution version of the original MRV signal. The above methodologies combine fluid mechanics with optimal control and Bayesian inference and can be further applied to other experimental techniques such as PIV.   

Educing properties of wave-like structures from the spatial wavenumber spectrum

Abstract: In this work we present a method for estimating the properties of wave-like coherent structures in turbulent flows. There is increasing recognition that many flows are characterized by multiple mutually coherent wave-like structures at the same frequency, as recently identified in compressible subsonic jets (Towne et al., 2017) and supersonic jets undergoing resonance (Edgington-Mitchell, 2019). Existing techniques such as SPOD (Towne et al., 2018) can separate components with either differing phase or frequency, but an additional step is needed to educe the properties of multiple coherent structures at the same frequency. Here we present an empirical method to educe wave properties via peak fitting in the spatial wavenumber domain. Various functional forms capable of describing waves that are growing and decaying within the domain are developed. The analytical Fourier transforms of these functions are used to fit the spectral peaks by taking a spatial Fourier transform of the flow data. The performance of the technique is demonstrated on both synthetic and real data, and limitations discussed. Acknowledgements: This work was supported by the Australian Research Council through the Discovery Project scheme.