Validation of CFD Density Field Validation in Supersonic Axisymmetric Flows Using BOS and Differential Interferometry

Background-Oriented Schlieren (BOS) is a non-intrusive and non-destructive technique for measuring density gradient in transparent media. Given the dependency of the refractive index and density of a medium, its static density field can be determined as a secondary measurand from BOS measurements. For an ideal gas, if the pressure variation is negligible, the static temperature field can be determined as a tertiary measurand. While the BOS technique for far-field measurements has been extensively researched, the implementation of this technique for near-field measurements still needs to be investigated. Near-field BOS is especially important for the measurement and visualization of internal flows, such as turbomachine flows, in which access to flow passage is limited and might be feasible using endoscopes (borescopes). This paper presents the features and challenges of the endoscopic BOS technique and presents methods and solutions for the application of this method in near-field measurements. The features of endoscopic BOS records as well as the comparison of endoscopic BOS and BOS measurements with camera are presented.

The Background Oriented Schlieren (BOS) technique is one of the novel measurement techniques and its application range is very wide. The principle of BOS is similar to that of the conventional schlieren technique, it exploits the bending of light ray caused by a refractive-index change corresponding to the density change in the medium. The BOS technique allows the quantitative measurement of density with very simple experimental setup and proper image analysis. Only a background and a digital camera are required for the experiment, so that even the real scale experiments can be realized. In recent years, the development of the high-speed camera is remarkable and so many high-speed phenomena can now be captured. To realize the precise measurement with BOS technique using high-speed camera, higher resolution (larger number of pixels) is desirable. In this paper, with a technical support from Nobby Tech Ltd., a 4K high-speed camera (4096 × 2160 pixels) is applied to the BOS measurement of the lateral jet/cross flow interaction filed in the supersonic wind tunnel test as a trial of the quantitative density measurement with higher resolution. The measurement system consists of a 4K high-speed camera and a pulsed laser for background illumination. A telecentric optical system is also employed to improve the spatial resolution of the measurement. The measurement results of BOS technique up to 1000 fps with higher resolution are discussed.

The refractive index fields of several media, say, transparent solids, transparent liquids, and air are often crucial when using these media as optical elements, probing optical properties, and deriving other physical information related to the refractive index. For example, in an optical system using a high-power laser, the thermal lens effect of optical elements is one of great issues because the laser focal position will be changed by the effect [1]. Thus, a method to measure a gradient-index of the optical element should be useful to take a step to reduce the thermal lens effect. Basically, light rays passing through a medium with an inhomogeneous refractive index field will be deflected toward the area of higher refractive index. This way, it should be possible to obtain information about the refractive index field from measurement of such deflection. Consequently, there have been several methods for visualizing gradient-index fields, including schlieren photography [2] and shadowgraphy [3]. Although these visualization methods are powerful tools for qualitatively analyzing the gradient-index field, quantitative analysis requires additional methods. Rainbow schlieren photography is one candidate. In this method, the strength of deflection of light rays is obtained by using a rainbow aperture [4-6]. The quantitative phase-contrast imaging has also developed with incoherent-light [7, 8], coherent-light [9, 10], and asymmetric illumination [11]. There is also background-oriented schlieren (BOS) technique for measuring gradient-index field quantitatively [12-20]. In the BOS technique, displacements of dot-patterns that are printed on a background are caused by deflection of light rays passing through a gradient-index field. The optical flow algorithm can be used to obtain the displacements of the background-dot-patterns. However, obtaining depth information of the gradient-index field with the BOS technique is often difficult because the dot-pattern displacement results from the light ray deflected all along its path, requiring integration over the path. To address this, a method of reconstructing the depth information of a gradient-index field is proposed here on the basis of the Lagrangian optics with the BOS technique.

This study investigates Reynolds number effects on rotor wake vortex development using a hyperbaric rotor facility capable of pressurizing air up to 100 bar. Background-oriented schlieren (BOS) and hot-wire anemometry (HWA) were applied to characterize vortex trajectories, core growth, and circumferential velocity distribution. BOS measurements revealed consistent blade-to-blade trajectory deviations and vortex pairing across all operating conditions, despite that the investigated three-bladed rotor was milled from a single piece of aluminum, ensuring precise manufacturing and a highly symmetric geometry. A statistical scheme was developed to analyze the radial structure of fluctuating tip vortices, which traverse the pointwise fiber-film sensor in a fixed position. With increasing vortex Reynolds number, the tip vortices are more compact with a reduction in core growth. The circulation in the vortices grows with the vortex radial coordinate, and converges at a radial position basically independent of the vortex core size. Observed asymmetries in young vortices at low Reynolds numbers indicate enhanced roll-up dynamics. The results demonstrate the facility’s ability to isolate Reynolds number effects in rotor wake dynamics.

Thermal energy storages (TES) are increasingly important for storing energy from renewable energy sources. TES that work with liquid storage materials are used in their most efficient way by stratifying the storage fluid by its thermal density gradient. Mixing of the stratification layers during stand-by periods decreases the thermal efficiency of the TES. Tank sidewalls, unlike the often poorly heat-conducting storage fluids, promote a heat flux from the hot to the cold layer and lead to thermal convection. In this experimental study planar particle image velocimetry (PIV) measurements and background-oriented schlieren (BOS) temperature measurements are performed in a model experiment of a TES to characterise the influence of the thermal convection on the stratification and thus the storage efficiency. The PIV results show two vertical, counter-directed wall jets that approach in the thermocline between the stratification layers. The wall jet in the hot part of the thermal stratification shows compared to the wall jet in the cold region strong fluctuations in the vertical velocity, that promote mixing of the two layers. The BOS measurements have proven that the technique is capable of measuring temperature fields in thermally stratified storage tanks. The density gradient field as an intermediate result during the evaluation of the temperature field can be used to indicate convective structures that are in good agreement to the measured velocity fields.

Numerical comparisons of temperature measurement through background-oriented schlieren (BOS) and fringe deflection (FD) are presented. Both techniques are based on ray deflection and on the comparison of two different states of a region of observation. A background image displayed on a screen is used in both techniques: for BOS, randomly located spots, and for FD, sinusoidal straight fringes. When a phase object is incorporated into the layout, these spatial structures undergo displacements that are proportional to the gradient of the change of index of refraction. These displacement fields are calculated through digital correlation in BOS and by means of the Fourier phase extraction method in FD. Numerical simulations that model a flame issued by a gas nozzle are presented. The results show that FD presents a slightly larger accuracy for images that either contain relatively high temperature gradients or show low contrast.

An experimental study was performed to evaluate the spatial resolution, sensitivity and dynamic range of schlieren and background-oriented schlieren (BOS) applied to hypersonic flows with multiple density gradients. For a single Mach number, shock profile extracts for three values of freestream density are compared. Three processing metrics are proposed for BOS to optimise the sharpness of captured shocks and minimise processing blur are presented. The BOS results have a lower spatial resolution and sharpness than the schlieren images, but captured the model flow field features that were resolved by schlieren. A higher dynamic range was afforded for BOS due to the user-determined synthetic knife edge cut-off.

The effective acquisition of a distorted wavefront is the foundation of studying how to suppress and correct aero-optical effects. The background-oriented schlieren (BOS) technique, as a commonly quantitative measurement method of flow field, has great application potential in aero-optical effect measurement. With consideration of the thickness change of the refractive index field, introduction of near-field correction and construction of the double telecentric configuration, wavefront measurement accuracy was improved. A standard plano-convex lens was utilized to quantitatively evaluate the improved results, and the effectiveness of the improvement was verified. Determination methods of spatial resolution, sensitivity, and dynamic measurement range of wavefront measurement based on BOS were studied. The influences of cross-correlation interrogation window size and step length on the wavefront reconstruction accuracy were investigated. The step length should be less than the size of the interrogation window to make the reconstructed results have a satisfactory accuracy. Wavefront results of supersonic film at different positions and cases were obtained. The fitting results show that there is a good linear relationship between the root mean square of the optical path difference (OPDrms) and ρ1/ρ∞ at different locations. Under different experimental cases, the aero-optical distortion tends to become stronger first and then weaker along the flow direction, and the aero-optical distortion around Position 3 is relatively strong. The large aperture approximation can be used to predict Strehl ratio values effectively in a considerable range by OPDrms.

The background oriented schlieren (BOS) technique provides quantitative measurements of integrated light deflection caused by gradients of refractive index in an optical medium. For an axisymmetric medium and parallel rays orthogonal to the axis of symmetry, Abel inversion enables fast, direct, and non-iterative reconstruction of the refractive index distribution. In this work, we relax the parallel ray hypothesis, demonstrating that Abel inversion can be effectively applied to observations obtained by central projection with a standard entocentric lens, with a simple correction for measured deflections. This correction derives from an original second-order approximation of ray deflection under the paraxial approximation. The accuracy of the method is demonstrated using a synthetic low-density jet and a cone of vision aperture of approximately 40°. Its practical relevance is further illustrated by reconstructing the temporal mean density fields of two experimental cases: a heated high-subsonic dual-stream jet and an under-expanded supersonic jet characterized by significant shock-induced discontinuities.

Rock glaciers represent a typical landform characterizing high mountain periglacial terrains. They are composed by a mixture of frozen debris and interstitial ice and when are affected by downslope displacement related to the permafrost deformation or melting, they are called active rock glaciers. The observation of their changes over time is a critical indicator of the state of water resources and permafrost distribution and is therefore of great importance for risk scenario definition and related natural hazards management, as well as can be related to the climate change.  Synthetic Aperture Radar (SAR) data have proven to be a useful tool to estimate surface displacement phenomena in mountainous areas that can be difficult to access. In particular, Differential Interferometry can measure displacement in the line of site direction with high accuracy. However, sometimes the characteristics of the areas in terms of displacement rate and number of available images lead to some limitations, which can be overcome by using other complementary tools such as SAR offset-tracking (OT) or by integrating both techniques together. In the OT case, the estimation of the deformation in azimuth and range directions is obtained maximizing the normalized cross-correlation between pairs of images and the minimum detectable displacement depends on the spatial resolution of SAR images, in the sense that a finer spatial resolution allows estimating a lower minimum detectable displacement. Consequently, these techniques can be profitably used for medium- or long-term displacement measurements [1]. In this work, OT is used to monitor the active alpine Lazaun rock glacier, located in the Ötztal in South Tyrol (Italy). Lazaun has a snow cover free period limited typically to 3 months per year, covers an area of about 0.12 km², and its altitude is between 2480 and 2700 m a.s.l.. Previous measurements performed by using GPS have detected displacements reaching ca. 1.5 m per year [2]. Different kind of data (TerraSAR-X, Cosmo-Skymed First and Second generation, Sentinel-1 and SAOCOM) characterized by different wavelengths, exposure and resolutions have been tested to estimate the rock glacier displacements and the results have been compared with GPS measurements executed in 2016-2018 and 2022. The results show that considering the limited size of the area of interest and the displacement rate, the spatial resolution of the data is of fundamental importance and only using spotlight SAR data is possible to estimate the displacement on both seasonal and inter-annual temporal scale. This research is part of the 2021-2023 project ‘CRIOSAR: Applicazioni SAR multifrequenza alla criosfera’, funded by ASI under grant agreement n. 2018-12-U.0. TerraSAR-X data were provided by the European Space Agency, Project Proposal id 34722, © DLR, distribution Airbus DS Geo GmbH, all rights reserved.   [1] Strozzi, Tazio, et al. "Glacier motion estimation using SAR offset-tracking procedures." IEEE Transactions on Geoscience and Remote Sensing 40.11 (2002): 2384-2391.   [2] Krainer, Karl, et al. "A 10,300-year-old permafrost core from the active rock glacier Lazaun, southern Ötztal Alps (South Tyrol, northern Italy)." Quaternary Research 83.2 (2015): 324-335.

<p>In recent decades, snowfalls, snow cover and duration over Central Italy have decreased and there have been some extreme snowfall events followed by extreme avalanche activities. In this regard, the Calderone Glacier (hereinafter Calderone) represents a geographical and geomorphological element of great interest and is defined as a sentinel of climate change in central Italy, as it is going through a strong phase of reduction, it is the only glacier in the Apennines,  and the southernmost in Europe, and for its position on the summit of the Italian Gran Sasso (2912 m asl), a mountain group located in the center of the Apennine belt in the Mediterranean area.</p><p>The Italian Glaciological Committee (Comitato Glaciologico Italiano (CGI) )  every year with ad hoc in-situ inspections in late spring and early autumn monitor the Calderone mass balance. The mass balance of a glacier depends on the interplay between the mass gains and losses associated with climate and those associated with the inherent flux, its monitoring is essential because it can contribute to the knowledge of the current ongoing evolution of glaciers. </p><p>Continuation of the traditional type of monitoring, like the one performed by CGI, based on direct measurements of accumulation and ablation by means of a network of stakes, appears to be an unlikely prospect, because in-situ data gathering usually implies expensive field campaigns and with difficult access to the sites, resulting in limited spatial and temporal resolution.  In contrast, techniques based on remotely sensed data, among several techniques, those relying on Synthetic Aperture Radar (SAR) demonstrated to be very effective due to the instrument's capability of operating day and night independently of the weather conditions.</p><p>Differential interferometry or DInSAR is a tool for accurate displacement measurements, and it is useful in identifying footprints of progressing movement. DInSAR is interferometry itself, the only difference is that topographical effects are compensated by using a Digital Elevation Model (DEM) of the area of interest, creating what is referred to as a differential interferogram.</p><p>In this work we propose the mass balance for the Calderone through the DInSAR results and its comparison with CGI in-situ measurements for the winter period 2018-2020. The data used in this study consist of COSMO-SkyMed satellite X-band single-look complex images in slant geometry (SCS, level 1A product),  Stripmap Himage mode (HH polarization) at 3 m per pixel of spatial resolution, and acquisition geometry Right Descending. The processing of this satellite data was applied over the entire area covered by the images and then refined to Calderone area, it includes a pre-processing first step that include: coregistration, interferogram formation, filtering and speckle; and a second part focused on obtaining the average values, active area and total area for the calculation of the mass balance.</p>

<p>The large availability of Synthetic Aperture Radar (SAR) data collected over the last decade by several satellite missions, such as COSMO-SkyMed, TerraSAR-X and Sentinel-1 constellations, has been pushing toward the present Earth Observation (EO) scenario into a “golden age”, which is rapidly moving towards a real Big Data scenario. The widespread use of satellite SAR data have fostered the development of several SAR applications, one of those referred to as Differential SAR Interferometry (DInSAR) technology, which has deeply demonstrated to profitably detect the surface deformations over a wide spatial extent in both natural and anthropic hazard scenarios, through the generation of spatially dense deformation maps with millimetric accuracies. In particular, the advanced DInSAR algorithm referred to as Parallel Small BAseline Subset (P-SBAS) approach allows retrieving the temporal and spatial characteristics of the detected displacements at two spatial resolution scales, referred to as regional (spatial resolution in the 30-90 m range) and local (spatial resolution in the 3-10 m range) scales, suitable for a large variety of application fields, from natural hazards (volcano eruptions, seismic events, landslides) to anthropic contexts (urban areas, archaeological sites, oil-gas extraction, structures and transport infrastructures).</p><p>However, the interferometric processing performed at local scale needs to deal with hundreds of SAR acquisitions at full spatial resolution, i.e. to manage several hundreds of million points; consequently, the processing of such a data amount is particularly heavy from a computational point of view and can not be carried out in reasonable time frames through the traditional (sequential) implementation of the full resolution DInSAR processing chains.</p><p>Accordingly, to profitably benefit from the current SAR scenario, it is crucial to develop innovative solutions to automatically and efficiently handle large DInSAR data stacks. These solutions are principally based on the exploitation of advanced distributed computing environments, to achieve high efficiency in terms of scalability performances, as well as on the development of much more advanced DInSAR methodologies (and codes) able to effectively maximize the information related to these huge amount of DInSAR data.</p><p>This work is aimed at describing an innovative DInSAR solution, based on the exploitation of distributed HPC and Cloud Computing environments, which benefits from parallel programming techniques (multi-node, multi-threads, GPUs) implemented within an automatic full resolution P-SBAS processing pipeline. Starting from large SAR datasets acquired by the COSMO-SkyMed constellation, the developed parallel full resolution P-SBAS processing chain allows retrieving in short time frame (less than 24 hours) displacement time series and deformation maps, at the single buildings/infrastructure level, relevant to extended urban areas. The presented experimental results and the related performance analyses are achieved by applying the developed parallel P-SBAS pipeline to a number of large full resolution COSMO-SkyMed datasets acquired over some important Italian cities (e.g. Rome and Naples urban areas).</p>

*The flow field analysis over various reentry configurations is studied numerically by solving time -dep endent compressible three -dimensional Euler equations. The governing fluid flow equations are discretized in spatial coordinates employing a finite volume approach, which reduces the equations to semi -discretized ordinary differential equations. Temporal i ntegration is performed using multi -stage Runge -Kutta time stepping scheme. A local time step is used to achieve steady state solution. The numerical simulation is carried out on structured grid without employing multi -block mesh arrangement. The numerical computation is carried out for freestream Mach numbers range of 1.2 - 10.0 and angle of attack up to 10 degree. The thermodynamic and transport properties are considered as a function of temperature using Hansen’s table. The flow features around the reent ry body are characterized by a bow shock, expansion fan and base flow region. The numerical scheme captures all the flow field features well such as bow shock, expansion fan and downstream recompression shock. Comparisons of the flow field and surface pres sure distribution results are made between different configurations of the reentry capsules such as ARD (ESA’s Atmospheric Reentry Demonstrator), Apollo II, OREX (Orbital Reentry Experiments) with and without shoulder radius, MUSES -C and spherically blunte d cone with flare angle of 25, 30 and 35 degree. A comparison between present numerical results and with the Newtonian flow assumptions for the forebody drag of the reentry capsules are made for the freestream Mach number 10 and the comparison shows a goo d agreement between them.

This paper outlines an experimental investigation conducted at the INCAS trisonic wind tunnel, focusing on the determination of pitch damping coefficient. The model used for this investigation is the Basic Finner Model, a standard model for dynamic tests which consists in a cone-cylinder body with four rectangular fins. The study aims to evaluate the influence of various parameters—including the Mach number, angle of attack, reduced frequency, center of rotation, and roll angle—on pitch damping coefficient. The employed method for determining these coefficients is the free oscillation method which consists in measuring the model oscillation in free stream after an initial perturbation. In order to perform these dynamic tests in the wind tunnel, a dedicated rig was developed to initiate the model’s oscillation using a linear servo-actuator and to record its oscillation using a strain gauge. The results obtained from the experiments illustrate how each parameter impacts the pitch damping coefficient, highlighting the precision of the measurements. The paper’s conclusion presents that the developed rig and the method used provide accurate results, and the variation in different parameters can change the damping coefficient.

Quality assurance (QA) of the image quality for image‐guided localization systems is crucial to ensure accurate visualization and localization of target volumes. In this study, a methodology was developed to assess and evaluate the constancy of the high‐contrast spatial resolution, dose, energy, contrast, and geometrical accuracy of the BrainLAB ExacTrac system. An in‐house fixation device was constructed to hold the QCkV‐1 phantom firmly and reproducibly against the face of the flat panel detectors. Two image sets per detector were acquired using ExacTrac preset console settings over a period of three months. The image sets were analyzed in PIPSpro and the following metrics were recorded: high‐contrast spatial resolution (f30,f40,f50 (lp/mm)), noise, and contrast‐to‐noise ratio. Geometrical image accuracy was evaluated by assessing the length between to predetermined points of the QCkV‐1 phantom. Dose and kVp were recorded using the Unfors RaySafe Xi R/F Detector. The kVp and dose were evaluated for the following: Cranial Standard (CS) (80 kV,80 mA,80 ms), Thorax Standard (TS) (120 kV,160 mA,160 ms), Abdomen Standard (AS) (120 kV,160 mA,130 ms), and Pelvis Standard (PS) (120 kV,160 mA,160 ms). With regard to high‐contrast spatial resolution, the mean values of the f30 (lp/mm), f40 (lp/mm) and f50 (lp/mm) for the left detector were 1.39±0.04,1.24±0.05, and 1.09±0.04, respectively, while for the right detector they were 1.38±0.04,1.22±0.05, and 1.09±0.05, respectively. Mean CNRs for the left and right detectors were 148±3 and 143±4, respectively. For geometrical accuracy, both detectors had a measured image length of the QCkV‐1 of 57.9±0.5mm. The left detector showed dose measurements of 20.4±0.2μGy(CS), 191.8±0.7μGy(TS), 154.2±0.7μGy(AS), and 192.2±0.6μGy(PS), while the right detector showed 20.3±0.3μGy(CS), 189.7±0.8μGy(TS), 151.0±0.7μGy(AS), and 189.7±0.8μGy(PS), respectively. For X‐ray energy, the left detector (right X‐ray tube) had mean kVp readings of 81.6±0.5(CS), 122.5±0.5(TS), 122.0±0.8(AS), and 122.1±0.7(PS), and the right detector (left X‐ray tube) had 81.6±0.5(CS), 120.8±0.5(TS), 120.9±0.6(AS), and 121.3±0.7(PS). Run charts were created so that each parameter could be tracked over time and the constancy of the system could be monitored. A methodology was developed to assess the basic image quality parameters recommended by TG‐142 for the ExacTrac system. The ExacTrac system shows a consistent dose, kVp, high‐contrast spatial resolution, CNR, and geometrical accuracy for each detector over the evaluated timeframe.PACS number: 87.10.‐e