Session A21: Experimental Techniques: Aerodynamics/High-Speed Flow

Dynamic temperature measurements in the exhaust plume of an aluminized ammonium perchlorate composite propellant

Non-intrusive temperature diagnostic techniques were developed and implemented for measuring temperatures in rocket exhaust plumes during static firing. Ammonium perchlorate composite propellant (APCP) grains were cast and burn tested in a static rocket motor test facility. Temperature measurements in the exhaust plume were taken using two optical methods: Four-color pyrometry and ultraviolet-visible range (UV-VIS) spectroscopy. Using four bandpass optical filters at varying wavelengths, the four-color pyrometer measurements were used to infer temperature with reference to Planck's Law. A microHR spectrometer with a 300 groove/mm ruled grating with a range from 400 - 1400 nm was used to verify there were no instances that the bandpass filters chosen would interfere with any major emission features of the rocket plume. For the purposes of initial testing, the bandpass filters chosen were 550, 650, 700, and 800 nm based on published spectra. Temporally-resolved UV-VIS emission measurements were used to obtain the temperature history during the rocket firing events. The measured temperatures were compared to predicted combustion temperatures calculated using NASA chemical equilibrium and applications (CEA) software.   

MRV Challenge 3—Overview and Initial Findings

The third installment of a global measurement challenge for pushing the state-of-the-art capability in magnetic resonance imaging (MRI)–based measurement technology in turbulent flows is presented. A fully turbulent water flow with Reynolds number of 15,000 based on the channel hydraulic diameter passes over building-like roughness elements in a square cross section water channel. The full elements align in the channel centerplane, with partial elements along both side walls at two alternating heights. The flow is steady and includes a second inlet along the floor in the wake of the third building element, which merges with the flow with an initial velocity that matches the bulk free stream. MRI-based measurement techniques include a comparison of three components of time-averaged velocities as well as a scalar—either concentration or temperature for all challenge participants. In this case, concentration measurements are presented from a combined team of researchers from Stanford University, the US Military Academy at West Point, and Oak Ridge National Laboratory. The results depict the neutrally buoyant isothermal flow as it mixes turbulently with the free stream and impacts the local velocity field. In addition, the concentration of a dilute copper sulfate solution from an injection port is tracked downstream as it interacts with the clean free stream flow and is entrained in wakes of the roughness elements, depicting a highly 3D flow field. Because the flow is nonreacting, it represents a passive scalar that can be matched with temperature-based measurements in flows with modest differences in temperature, forming the basis of an intriguing comparative activity. The outcome of the measurement campaign has utility in turbulent mixing and represents an additional capability of MRI-based measurement systems in turbulent channel flows.   

Autonomous drone swarm system for in situ characterization of atmospheric particle dispersion

Understanding aerosol dispersion from wildfires is crucial for enhancing air quality and radiative forcing models. However, due to measurement challenges, field data on aerosol dispersion, which is strongly influenced by properties such as concentration, morphology, and composition, is scarce. To bridge this gap, we've introduced an autonomous drone swarm system. This system, comprising four drones equipped with a digital holographic sensor, uses machine vision for autonomous flight guidance, enabling precise tracking and measurement of smoke plumes. A significant development has been a fully simulated environment that integrates fluid dynamics, drone flight and control, and machine vision. This environment has been instrumental in refining drone control systems and testing swarm control strategies, particularly under simulated smoke flow conditions. The system has been successfully deployed in Cedar Creek prescribed burn experiments, providing valuable data on aerosol properties and dispersion patterns. These advancements revolutionize in situ characterization of wildfire smoke aerosols, providing real-time data for air quality and climate science, while also offering a versatile tool for studying other atmospheric particle transport phenomena like dust and pollen dispersion.   

Experimental Reproduction of Hemodynamic Index to Assess Severity of Arterial Stenoses: Implications for Revascularization Recommendations

Arterial stenosis, a prevalent medical condition characterized by the narrowing of an arterial lumen, hinders blood flow and can lead to life-threatening consequences like myocardial ischemia and ischemic stroke. The trans-stenotic pressure gradient (TSPG) is considered a key indicator of the hemodynamic significance of noncoronary arterial stenoses. However, its clinical application is limited due to challenges in noninvasively measuring distal and proximal pressure and the absence of standardized guidelines for assessing stenosis severity. To overcome these limitations, we recently introduced a new, noninvasive, and patient-specific hemodynamic index using image-based computational hemodynamics techniques. This index effectively evaluates the severity of noncoronary stenoses and has been successfully applied to four renal stenosis cases, producing computational results consistent with medical practice. The hemodynamic index is based on the correlation between TSPG and arterial lumen volume reduction, utilizing standard imaging data and parametric analysis to determine the degree of hemodynamic severity and offer revascularization recommendations. In this study, we utilize a newly developed mock circulation loop to experimentally derive the hemodynamic index for both renal and iliac stenosis cases. This unique experimental approach validates and reinforces our computational research, significantly enhancing the reliability and clinical applicability of our findings in real-world medical settings. By synergistically combining computational and experimental techniques, our work aims to provide valuable insights to improve the assessment and management of arterial stenosis, ultimately benefiting patient care and enhancing their overall well-being.   

Three-Dimensional Acoustic Travel-Time Tomography for Wind Energy

Understanding turbulent flow around wind turbines is crucial for their design, control, and numerical model validation. Existing remote sensing methods, relying on light or sound wave backscatter from airborne particulate matter, limit the resolution of spatial and temporal observations near solid objects and reflective surfaces.

Acoustic tomography (AT) presents a novel approach to remote sensing by utilizing atmospheric thermodynamic properties and the time-dependent stochastic inversion (TDSI) algorithm. This enables the reconstruction of velocity and temperature turbulence fields within an array of acoustic transducers, surpassing the limitations of existing techniques.

Despite its success in homogeneous flow fields, AT's potential application in wind energy research remains unexplored due to the nonhomogeneous nature of turbulent flow in wind farms. This study aims to employ a nonhomogeneous model and implement TDSI in a three-dimensional (3D) configuration to reconstruct turbulent flow in the wake of a wind turbine.

The Flatirons Campus (FC) at the National Renewable Energy Laboratory (NREL) hosts a 3D acoustic tomography array designed for renewable energy and atmospheric science research. The array consists of eight 10-m towers, each equipped with three layers of speakers and microphones, as well as one tower supporting three sonic anemometers and Vaisala pressure/temperature/humidity probes for reference atmospheric data. A sub-scale 1.2kW SkyStream wind turbine is located within the AT array. To date, 3D acoustic tomography has been limited to masts no taller than 6-m, making the NREL AT array the first of its kind.

Data is recorded in 0.5s frames, allowing precise travel time calculations for TDSI implementation. Preliminary results, comparing reconstructed flow velocities in the 2D configuration with homogenous background flow to measurements obtained from a sonic anemometer, show promising normalized-mean square error (NMSE) and standard deviations (STD) values of approximately 0.5 and 0.03, respectively. This study will present results of the turbulent field reconstruction in the 3D configuration, including the subscale wind turbine wake.   

Integral Imaging Microscope for measurements in near wall fluid flow

Plenoptic cameras are a cost-effective and fast method to capture a 3D scene. Multiple views of a scene can be captured at different angles using a number of positioned cameras or with the use of a microlens array. This later technique is called Fourier Integral Microscopy (FIMic) or Integral Microscopy (IMic). In both IMic and FIMic, a micro lens array (MLA) is placed in front of an image sensor to capture several elemental images (EI) that provide different perspectives of a 3D scene. This integral image contains spatial and angular information that can be used to reconstruct the image at various depths. IMic is simply a variation of the very compact FIMic, where the lenslets are placed at the Fourier plane of the objective, whereas in IMic, the lenslets are placed at the image plane of the objective.

The system’s performance is characterized in terms of lateral resolution, FOV, Depth of field (DOF) and 3D reconstructions of the images. The axial and lateral resolution of the system at focus is determined by the size of the diffraction-limited spot beneath a single microlens. The computation of the PSF (Point Spread Function) and MTF (Modulation Transfer Function) allows us to determine the most advantageous imager in term of resolution and compactness. By playing both with the distances and the optical elements (focal length, quantity of elements, …), the change in the MTF, is immediate and this can be used to improve our imager. A Zemax model of the microscope, a powerful and versatile optical software that offers ray optics visualization as well as wave optics computation, is depicted along with its calculated performance and comparison to the final design.

This instrument allows for compact stereoscopic measurements for different laboratory scenarios, mainly for biological samples. Because of its compactness and flexibility, we want to adapt this imaging technique for measurements in near wall fluid flow.   

Probing dynamics of elliptical vortex rings via direct vorticity measurements with digital inline holography

Characterizing the dynamics of vortex structures is critical for understanding the fundamental mechanisms of the behavior of vortices in fluid flows across different scales, including high vorticity filaments in turbulence. Elliptical vortex rings, as one of the fundamental elements of vortical structures, exhibit intriguing temporal vorticity variation due to the compression and stretching of the vortex core through a process known as axis-switching. This phenomenon, while observed in numerical simulations, has remained experimentally elusive due to the constraints of spatial resolution in traditional measurement techniques. In this study, we employ the innovative method of digital inline holography (DIH) for direct vorticity measurement, enabling us to characterize the temporal evolution of vorticity in elliptical vortex rings with unprecedented detail. These rings are generated using a piston arrangement and elliptical nozzles with varying aspect ratios. We specifically quantify the temporal evolution of rings generated from a nozzle with an aspect ratio of 3 over two complete axis-switching cycles. Our DIH-based vorticimetry successfully resolves the differences in vorticity dynamics of elliptical vortex rings with varying aspect ratios. This study demonstrates the potential of our DIH-based vorticimetry for characterizing the intricate dynamics of small-scale vortex structures in turbulent flows.   

Standalone Software for Background Oriented Schlieren (BOS) Processing with Wavelet-based Optical Flow Analysis (wOFA)

A software for performing background oriented schlieren (BOS) processing is presented and demonstrated. The software uses wavelet-based optical flow analysis to perform the processing, which has been shown to produce more accurate results with higher spatial resolution than commercially available iterative least squares- or correlation-based algorithms. The new BOS software is freely available from the PI's research website, https://case.edu/engineering/labs/fpi/downloads, and is distributed as a Microsoft OS executable file for easy installation. It features a graphical user interface and is accompanied by a full user guide. The software has an additional tool for generating publication-quality schlieren images from the BOS data, including an option for displaying the images with line integral convolution (LIC). This talk will include explanation of the features of the software and demonstrate its performance on a few example BOS images.