Background-oriented Schlieren Research Articles

Development of a Background-Oriented Schlieren (BOS) System for Thermal Characterization of Flow Induced by Plasma Actuators

Cold climate regions have great potential for wind power generation. The available wind energy in these regions is about 10% higher than in other regions due to higher wind speeds and increased air density. However, these regions usually have favorable icing conditions that lead to ice accumulation on the wind turbine blades, which in turn increases the weight of the blades and disrupts local airflow, resulting in a reduction in wind turbine performance. Considering this problem, plasma actuators have been proposed as devices for simultaneous flow control and deicing. These devices transfer momentum to the local airflow, improving the aerodynamic performances of the turbine blades while producing significant thermal effects that can be used to prevent ice formation. Considering the potential application of plasma actuators for simultaneous flow control and deicing, it is very important to investigate the thermal effects induced by these devices. However, due to the significant electromagnetic interference generated by the operation of these devices, there is a lack of experimental techniques that can be used to analyze them. In the current work, a background-oriented Schlieren system was developed and is presented as a new experimental technique for the thermal characterization of the plasma-induced flow. For the first time, the induced flow temperatures are characterized for plasma actuators with different dielectric materials and different dielectric thicknesses. The results demonstrate that, due to the plasma discharge, the temperature of the plasma-induced flow increases with the increase of the applied voltage and may achieve temperatures five times higher than the room temperature, which proves the potential of plasma actuators for deicing applications. The results are presented and discussed with respect to the potential application of plasma actuators for simultaneous flow control and deicing of wind turbine blades.

Estimating density, velocity, and pressure fields in supersonic flows using physics-informed BOS

We report a new workflow for background-oriented schlieren (BOS), termed “physics-informed BOS,” to extract density, velocity, and pressure fields from a pair of reference and distorted images. Our method uses a physics-informed neural network (PINN) to produce flow fields that simultaneously satisfy the measurement data and governing equations. For the high-speed, approximately inviscid flows of interest in this work, we specify a physics loss based on the Euler and irrotationality equations. BOS is a quantitative fluid visualization technique that is commonly used to characterize compressible flow. Images of a background pattern, positioned behind the measurement volume, are processed with computer vision and tomography algorithms to determine the density field. Crucially, BOS features a series of ill-posed inverse problems that require supplemental information (i.e., in addition to the images) to accurately reconstruct the flow. Current methods for BOS rely upon interpolation of the images or a penalty term to promote a globally- or piecewise-smooth solution. However, these algorithms are invariably incompatible with the flow physics, leading to errors in the density field. Physics-informed BOS directly reconstructs all the flow fields using a PINN that includes the BOS measurement model and governing equations. This procedure improves the accuracy of density estimates and also yields velocity and pressure data, which were not previously available. We demonstrate our approach by reconstructing synthetic data that corresponds to analytical and numerical phantoms as well as a single pair of experimental measurements. Our physics-informed reconstructions are significantly more accurate than conventional BOS estimates. Furthermore, to the best of our knowledge, this work represents the first use of a PINN to reconstruct a supersonic flow from experimental data of any kind.

Assessment and application of optical flow in background-oriented schlieren for compressible flows

Optical flow provides an opportunity to elevate the resolution and sensitivity of deflection sensing in background-oriented schlieren (BOS). Despite extensive relevant literature within the field of computer vision, there is a lack of proper quantification of its abilities and limitations with regard to the state-of-the-art BOS experiments. Thus, this study performs an assessment of accuracy and resolution limits in different flow field scenarios utilizing background patterns generated with random dot and wavelet noise distributions. Accordingly, a synthetic assessment over a theoretically generated Prandtl–Meyer expansion fan is conducted with variations introduced in the background patterns and operational parameters of optical flow. A clear superiority of accuracy and resolvable range of density gradient amplitudes over cross-correlation is demonstrated. Moreover, an experimental assessment of supersonic flow features over multiple wind tunnel models is performed. The influence of experimental constraints, limitations and uncertainties related to the application of optical flow in BOS and its comparative performance against the block-matching counterpart is characterized.Graphical abstract

Boundary layer and near wake measurements of a two-dimensional airfoil with background-oriented schlieren method

Boundary layer and near wake measurements of a two-dimensional airfoil with background-oriented schlieren method

Flexibility and versatility enhancements due to a portable optical system for the Background-Oriented Schlieren techniques

Flexibility and versatility enhancements due to a portable optical system for the Background-Oriented Schlieren techniques

Three-dimensional density measurements of a heated jet using laser-speckle tomographic background-oriented schlieren

Tomographic background-oriented schlieren (TBOS) is a promising optical technique for 3D density field measurement in turbulent flows. Path-integrated information on refractive index gradients is captured by cameras looking through the flow at textured background patterns in the form of apparent displacements of the patterns. A tomographic reconstruction of the three-dimensional refractive index gradients is performed, from which the instantaneous three-dimensional density field is obtained. Most TBOS measurements to date have been limited by large temporal blurring and/or large defocus blurring. Our 15-camera experimental set-up uses a pulsed laser for both illumination and creation of a laser-speckle background pattern. The high-power laser ensures adequate exposures in nanoseconds, so temporal integration is minimised in each measurement. The beam is spread to illuminate a surface that is observed by fifteen cameras that are evenly spaced circumferentially around the flow. Laser speckle patterns are observed by the cameras due to the diffuse reflection and interference of coherent light from roughness on the surface. We develop a methodology for selecting the optimal focal length, focus distance, and lens aperture by considering the compromise between displacement sensitivity, defocus blurring and speckle size. Low temporal- and spatial-blurring 3D temperature field measurements with excellent resolution of turbulent flow features are obtained and good agreement with thermocouple. Classical behaviours of the potential core instability such as vortex rollup and radial ejections are identified. The behaviour of the potential core is examined further, which reveals that the potential core undergoes considerable deformation and fragmentation during the transition to turbulence.

Simultaneous background-oriented schlieren and retroreflective shadowgraph visualization

This paper describes a large-scale visualization system that can simultaneously produce background-oriented schlieren (BOS) images and retroreflective shadowgrams (RS) of a compressible flow. The simultaneous measurements with BOS and RS in two optically equal systems allowed a direct comparison of the performance of the two methods and to quantify the relative differences, primarily with respect to identifying the broad range of length scales in a turbulent compressible flow. Further, direct comparisons are made between the schlieren image obtained from the BOS measurement, the displacement map obtained by subtracting a no-flow image from the unprocessed BOS record, the divergence of the BOS displacement field (the latter two are forms of a BOS-generated shadowgram), and the simultaneously obtained direct shadowgram from the RS setup. A qualitative comparison of the results indicated that all the three methods (BOS schlieren, direct difference, and divergence of the BOS-schlieren record) can capture the large-scale flow structures but that the smaller scales in the plume can only be resolved to a much lesser extent. These structures were, however, easily resolved in the RS images. The results show that, in the described system, the resolution of the BOS images is lower than that of the shadowgrams by a factor of about five. Further tests indicate that these differences in the spatial resolution of the BOS technique cannot be removed using a different design of the random-dot background pattern. These observations apply to the used large-scale system in which a minimum distance between the observed object and each of the two screens (background pattern/reflective) has to be maintained. This constraint also meant that neither system could be optimized with respect to spatial resolution.

Comparison between Density and Velocity Fields in Burst Modulation of a Dielectric-Barrier-Discharge Plasma Actuator

The flow field produced by a dielectric-barrier-discharge plasma actuator using burst modulation was experimentally investigated in quiescent air from two viewpoints: density and vorticity fields. A wide range of burst signal parameters were evaluated using particle-image velocimetry and background-oriented schlieren measurements. Four types of flow-field patterns were found: Type 1 was a wall jet, similar to continuous operation; Type 2 was a periodical, independent vortex moving along the wall surface; Types 3 and 4 demonstrated a feature wherein the periodic shedding of the vortex pair (primary and secondary vortices) occurred while moving over the surface. While Types 3 and 4 demonstrated a shared feature, they had different density and vorticity structures. The change of the flow-field pattern from Type 1 to Type 4 was triggered by a lower burst frequency and ratio, as well as a higher base frequency. In addition, the vorticity strength and density were strongly negatively correlated and depended on the rate of power consumption to generate one vortex.

Background-oriented schlieren technique with vector tomography for measurement of axisymmetric pressure fields of laser-induced underwater shock waves

This study aims to overcome the problems that existing background-oriented schlieren (BOS) techniques based on computed tomography (CT-BOS) face when measuring pressure fields of laser-induced underwater shock waves. To do this, it proposes a novel BOS technique based on vector tomography (VT-BOS) of an axisymmetric target. The remarkable feature of the proposed technique is the reconstruction of an axisymmetric vector field with nonzero divergence, such as the field of a laser-induced underwater shock wave. This approach is based on an approximate relation between the projection of the axisymmetric vector field and the reconstructed vector field. For comparison, the pressure fields of underwater shock waves are measured with VT-BOS, CT-BOS, and a needle hydrophone. It is found that VT-BOS is significantly better than CT-BOS in terms of better convergence, less dependence on the spatial resolution of the acquired images, and lower computational cost. The proposed technique can be applied not only to fluid dynamical fields, but also to other axisymmetric targets in other areas, such as electromagnetics and thermodynamics.Graphical abstract

Effect of Mach Number and Pitching Eigenfrequency on Transonic Buffet Onset

The transonic flow around an elastically suspended supercritical airfoil was experimentally investigated in order to enhance knowledge of buffet boundary dependencies and the corresponding self-excited fluid–structure interaction (buffeting). For that purpose, an experimental setup was designed, manufactured, and integrated in the Trisonic Wind Tunnel Munich. The design consisted of a rigid, two-dimensional, supercritical airfoil (OAT15A) with optional pitching degree of freedom and variable torsional spring stiffness. High-speed background-oriented schlieren measurements were used to observe the shock with its dynamics, while a high-speed stereo camera setup for correlation-based deformation measurements was implemented to track the dynamics of the structural motion. Pre-buffet and buffet flows were analyzed by a continuous increase of the angle of attack. The detailed observation of the shock position with increasing angle of attack exhibited a clear effect of Mach number and pitch eigenfrequency on the buffet phenomenon. The corresponding onset boundary clearly shifted into the “pre-buffet” regime given specific structural settings. Furthermore, the resulting fluid–structure interaction of shock and coupled pitch–heave motion showed characteristics of structural frequency lock-in for the pitch-to-buffet frequency ratios of 1.2 and the mode veering region for ratios of 1 and 0.9, indicating the transition region from fluid mode flutter to structural mode flutter.

Tomographic imaging using multi-simultaneous measurements (TIMes) of emission and refractive index 3D fields in turbulent flames

The present work combines background-oriented schlieren tomography (BOST) with computed tomography of chemiluminescence (CTC) using the TIMes technique to provide simultaneous three-dimensional fields of refractive index (i.e., density) and CH* chemiluminescence (i.e., flame luminescence) on a single-shot basis. The feasibility of the proposed non-intrusive technique is assessed based on simulations and experiments. Its accuracy with respect to the number of cameras is evaluated using phantoms of a realistic simulated turbulent flame. The technique is experimentally demonstrated to investigate turbulent flames issuing through a double-annular bluff-body burner (Cambridge-Sandia stratified swirl burner) under a variety of operating conditions. The simultaneous measurements are performed using a cost-effective setup consisting of 29 cameras split between BOST and CTC. The reconstructed instantaneous and averaged results reveal the three-dimensional fields of the refractive index (related to the temperature, pressure and species concentration) and CH* chemiluminescence (indicator of the flame front), showing their structure interaction.

Obtaining velocity and pressure distributions in natural convection flows using experimental temperature fields

New hybrid simulation technique is proposed for natural convection flows, which combines experimental measurements of the instantaneous temperature fields with numerical integration of the fluid dynamics equations. Experimental temperature distributions are used to determine the buoyancy term, which enables one to obtain the velocity and pressure fields without extra measurements. Solution of the energy equation is replaced with experimental data, hence the technique can be used even if the energy equation contains unknown source terms or if the boundary conditions for temperature are unknown. The approach is demonstrated for Background Oriented Schlieren (BOS) measurements of three natural convection flows: natural convection near a heated vertical plate, convective plume from a heated horizontal wire and horizontal convection driven by nonuniform radiative heating of liquid surface. The velocity fields, obtained by reconstruction in steady and unsteady flows, exhibit good agreement with numerical simulations if the input temperature data are accurate. Moreover, the hybrid simulation takes into account the flow asymmetries in a particular experimental run. However, spatially limited measurement region or underestimation of the peak temperature, typical for BOS measurements of the thin thermal layers in liquids, lead to underprediction of the velocity and pressure disturbances in hybrid simulation. • Velocity and pressure fields are determined from the measured temperature fields. • The proposed approach is validated for three different natural convection flows. • Instantaneous temperature fields are measured using Background Oriented Schlieren. • The proposed approach can be used if the problem contains unknown heat sources. • Influence of temperature uncertainty and limited measurement region is analyzed.

Finite-Wing and Sweep Effects on Transonic Buffet Behavior

This study experimentally investigates the effects of the sweep angle and finite wing on transonic buffet, studying two-dimensional (2-D) and three-dimensional wing configurations. Background-oriented schlieren and stereographic particle image velocimetry (PIV) have been used as measurement techniques, performing experiments on an OAT15A airfoil (clamped to both the side windows of the wind tunnel), an unswept wing, and two swept wings with sweep angles of 15 and 30 deg, respectively. The three wings are also based on the OAT15A airfoil and are clamped at the wind tunnel only at their root (free wingtip). All wings have been tested at a constant normal Mach number () with respect to the leading edge. The results show that the buffet oscillations are much stronger for the airfoil than for the three finite-span wings. A large difference in the buffet behavior can be noticed between the airfoil and the unswept wing, as is also seen in oil flow visualizations. This difference is particularly evident in correspondence of the more outboard spanwise locations, suggesting that for the unswept wing, an important role could be played by finite-wing effects: notably, the tip vortex. A spectral analysis has shown that for the swept wings, the classical 2-D buffet peak (occurring at for the present conditions) is substantially attenuated, whereas additional contributions in the range of 450–850 Hz appear. The PIV results showed, for the 30 deg sweep angle wing, a periodical occurrence of a secondary supersonic area downstream of the main shock wave structure, which is absent for the other wing models. The stereographic PIV configuration allowed the reconstruction of the spanwise-oriented velocity component, obtaining, in proximity of the trailing-edge, values of the spanwise velocity component () which are in agreement with the spanwise convection of buffet cells observed in the literature in this region.

Fractal analysis of hydrogen jet mixing on the basis of BOS method

Structures formed as a result of mixing of hydrogen with air are analyzed by optical methods using detection of density non-uniformities. Methods for determination of fractal parameters for a random distribution of these non-uniformities are described, and information about the structure of gas mixing revealed is analyzed. The BOS (Background Oriented Schlieren) method is used to obtain the optical image of the formed structures and then treat by the correlation method that allows to obtain the quantitative information on the mixing. As a result, the possibility to link the characteristics of the injected gas source and the fractal parameters were demonstrated. The method can be used in the development of the non-intrusive technique for the evaluation of the gaseous system parameters based on the optical diagnostics and, potentially, for the obtaining more detailed information of the turbulence in gases.

Reconstruction of surface pressures on flat plates impacted by blast waves using the Virtual Fields Method

An accurate description of surface pressure loads imposed by blast waves is crucial for the design of the next generation of blast-resistant structures. However, experimental techniques for non-intrusive, full-field surface pressure measurements are not readily available. To address this challenge, a new application of the Virtual Fields Method (VFM) has been explored to reconstruct the surface pressures acting on thin steel plates using full-field deformation measurements of the plate dynamics. A shock tube facility was used to generate a blast-like loading in controlled, laboratory environments, where the plate dynamics were measured using the deflectometry technique. Different blockages at the shock tube exit allowed for varying the spatial distribution and temporal history of the blast loading. The surface pressures were reconstructed from experimentally measured kinematic fields using the VFM. A nearly non-deformable plate equipped with point-wise pressure sensors was also used to obtain a reference, allowing to assess the performance and reliability of the proposed methodology in capturing the surface pressure distributions. Moreover, visualizations of the different blast wave impacts on the plate were obtained using a background-oriented schlieren setup. Finally, the influence of potential error sources was investigated by means of a virtual laboratory using a finite element analysis to generate synthetic input to the load reconstruction analysis. The proposed methodology provided robust, precise predictions using noise-free input from virtual experiments. The presence of both systematic and random errors during the experimental campaign resulted in a reduced pressure reconstruction accuracy, where the peak pressure amplitudes were approximately 10–20% lower compared to the pointwise transducer data. Pressure reconstructions from experimental data still showed qualitatively good estimates of the pressure distributions that were extrapolated from transducer data. Hence, this work highlights the capabilities of a promising methodology to obtain more insight into the effective action of the loading during blast–structure interaction of plated structures.

Temperature reconstruction of an axisymmetric enclosed reactive flow using simultaneous background oriented schlieren and infrared thermography

The temperature distribution of a premixed methane air flame running at a Reynolds number of 1300 on a circular burner, 12.7 diameter, enclosed in a fused silica cylindrical liner has been experimentally reconstructed using a non-invasive approach combining background oriented schlieren (BOS) and infrared (IR) thermography. BOS is used to characterize both the air ambient to the system, using an existing technique called 3D ray tracing, and the reactive flow inside the enclosure, with a novel modified version of 3D ray tracing. IR thermography is used to characterize the thermal/optical characteristics of the quartz glass enclosure itself, since the information is required as BOS is a line of sight imaging technique. Out of necessity, an approximated species independent relationship is used to calculate flow temperature from refractive index. A simulation is used to show this error is in the range of 5.8%–7%. Additionally, it is found that drastically simplifying the approach by removing the IR thermography system entirely and using the near outer wall air temperature from BOS/3D ray tracing to characterize the internal temperature of the quartz liner itself only causes a 1.5%–3.8% degradation in the accuracy of the reconstructed temperature field. The technique as presented is a relatively inexpensive, experimentally simple approach capable of determining the steady state temperature characteristics of optically accessible axisymmetric reactive flows.

On the interfacial dynamics and capillary waves during impingement of a drop on liquid pool: A background-oriented schlieren study at low Weber numbers

Understanding the dynamics of a droplet impinging on a liquid pool and the associated phenomena have been of interest due to its prevalence in nature as well as in technical applications. This paper aims toward studying the characteristics of the capillary waves generated due to the low Weber number droplet interactions with the liquid pool. In this direction, experiments have been carried out for six different pool heights varying from h = 1.4 to 12 mm, encompassing thin liquid film, shallow pool, and deep pool regimes. Due to its wide usage, water has been chosen as the fluid of interest for droplet as well as for pool liquid. The study is focused on droplets impinging on the liquid pool at low Weber number ranging from 1 to 100. In order to characterize the post-impact perturbations in the liquid, background oriented schlieren (BOS) technique has been employed which offers real-time, non-intrusive whole-field measurements of the perturbations in the liquid pool. Measurements from BOS have been validated against the side-view projection of the impact. The transient variations of the air–water interface for different pool regimes and Weber numbers have been delineated. Results evince the formation of secondary wave at impact followed by the formation of primary wave after the crater retraction. The wave formation was faster and had higher amplitude in thin liquid regime for droplets with the same Weber number compared to the other regimes, but the perturbations were reduced through higher dissipation. The formation of the Worthington jet was seen in shallow and deep pool regimes for droplets with higher Weber number (We = 100), and its effect on the capillary wave is also discussed.

High-speed photography and background oriented schlieren techniques for characterizing tulip flame

High-speed photography and background oriented schlieren (BOS) techniques are used to characterize the tulip flame formation in half-open and closed-ends type channels. Flame propagation for a premixed mixture of Liquefied Petroleum Gas (LPG) and air in the equivalence ratio range of 0.8–1.0 are tested. The classical tulip flame is observed only in the closed channel. Refractive index and flow density are calculated numerically using Poisson’s equation and Gladstone–Dale relation. Pseudo-schlieren images, shadowgram, and density contours are plotted using the BOS technique, and these reconstructed fields matched well with the direct observations. Instantaneous velocity fields are calculated using consecutive images as the reference image for the BOS correlation method. The results show that the velocity during the early stages of the flame expansion is close to the laminar burning velocity of the premixed LPG-air flame. As the flame started to inverse, the flame tip close to the wall accelerated to approximately 1.16 m/s. But later the fully-formed tulip flame propagates at 0.25–0.3 m/s till the end of the chamber. A reverse flow in the burned gas region is observed after the tulip flame formation. However, BOS could not capture any acoustic changes in the unburned gas except for a high gradient backflow just behind the flame front.

Experimental Investigation of Transonic Shock Buffet on an OAT15A Profile

Self-sustained shock wave oscillations on airfoils, commonly defined as shock buffet, can occur under certain combinations of transonic Mach numbers and angles of attack (AoAs) due to the interaction between the shock and the separated boundary layer. To improve the understanding of this complex phenomenon, the flow over a supercritical profile (OAT15A) was experimentally investigated for a fixed Reynolds number of and numerous aerodynamic conditions within the ranges of and . Deformation and force measurements were used to assess the actual rigidity of the model and its interaction with the flow. The tracking of the shock location by means of background-oriented schlieren allowed for studying the shock features and the frequency content of the buffet flows. Furthermore, the inversion of shock motion and buffet onset, which are referred to as buffet boundaries, were estimated. The inversion of shock motion proved to be a necessary but not sufficient condition for buffet onset. The trends of the results showed a good agreement with the relevant literature cases. However, the buffet amplitude was smaller, and buffet onset occurred at considerably higher AoAs. The comparison of the literature results also revealed a general sensitivity of buffet features to both numerical and experimental parameters. For this reason, the influence of the boundary-layer suction at the vertical walls and the gap flow at the side windows on the buffet features was examined. The buffet frequencies and amplitudes were slightly affected, but the buffet boundaries appeared to be virtually insensitive to these factors. Given the large number of investigated aerodynamic conditions, these results are valuable for validation purposes of computational fluid dynamics simulations.

Estimating BOS Image Deformation With A Lightweight CNN

We introduce a Convolutional Neural Network (CNN) to post-process recordings obtained by means of Background Oriented Schlieren (BOS), a popular technique to visualize compressible/convective flows. To reconstruct BOS image deformation, we devised a lightweight network (LIMA) that has comparatively fewer parameters to train, allowing the deployment of the network on embedded GPU hardware. To train the CNN, we introduce a novel strategy based on the generation of synthetic images with random, irrotational displacement field that mimic those provided by real BOS recording. This allows us to generate a large number of training examples at minimal computational cost. To assess the accuracy of the reconstructed displacement, we consider test cases consisting of synthetic images with sinusoidal displacement as well as images obtained in a real experimental study of flow past and inside a heated hollow hemisphere. By comparing the prediction of the CNN with conventional post-processing techniques such as Direct Image Correlation (DIC) and Particle Image Velocimetry (PIV) cross-correlation, we show that LIMA gives more accurate and robust results for the synthetic example. When applied to the recordings from the real experiment, all methods provide consistent deformation fields. As they offer similar or better accuracy at a fraction of the computational costs, properly designed CNNs offer a valuable alternative to conventional post-processing techniques for BOS experiments.