To reduce the influence of interference fringes in gas detection based on tunable diode laser absorption spectroscopy (TDLAS) and improve measurement accuracy, an interference fringe filtering method based on a back propagation neural network (BPNN) was developed in this study. First, a set of numerical simulation data of interference fringes was generated by periodic sinusoidal functions with different frequencies and phases. The topology, input, and output parameters in the BPNN were determined and optimized by the simulation data. A varying temperature WMS experimental system with a dual optical path was built, by which signals and reference concentrations of CH4 in different concentration ranges were measured. The experimental results verified the excellent performance of the BPNN in suppressing interference fringes with different frequencies and phases. When the ratio of the fringe spectral range to the absorption line width was 1.37, the standard deviation of BPNN was reduced by 4.9 times, and the relative error was reduced by 4.5 times compared with the traditional least squares (LS) method. The obtained results sufficiently demonstrated that the BPNN can effectively reduce interference fringe noise in TDLAS.

View registration (VR) refers to the process of determining the orientations and locations of the images captured by one or more cameras. It represents a fundamental procedure underpinning a wide spectrum of applications, ranging from optical sensing, object identification, and image processing. Existing VR procedures typically rely on a set of images captured on a fixed calibration target (referred to as FVM, fixed view method, hereafter). This work reports the development of a new view registration procedure, referred to as the variable view method (VVM). The VVM procedure relies on a set of images taken on a calibration target at various orientations and/or distances. As shown in this work, both numerically and experimentally, the VVM can significantly improve the VR accuracy compared to the FVM, by more than 40% in the tests conducted here. Such enhanced accuracy is expected to benefit a range of applications, and a specific example is demonstrated involving three-dimensional velocity measurement based on particle imaging velocimetry.

Three-dimensional and three-component (3D3C) velocity measurements have long been desired to resolve the 3D spatial structures of turbulent flows. Recent advancements have demonstrated tomographic particle image velocimetry (tomo-PIV) as a powerful technique to enable such measurements. The existing tomo-PIV technique obtains 3D3C velocity field by cross-correlating two frames of 3D tomographic reconstructions of the seeding particles. A most important issue in 3D3C velocity measurement involves uncertainty, as the derivatives of the measurements are usually of ultimate interest and uncertainties are amplified when calculating derivatives. To reduce the uncertainties of 3D3C velocity measurements, this work developed a regularized tomo-PIV method. The new method was demonstrated to enhance accuracy significantly by incorporating the conservation of mass into the tomo-PIV process. The new method was demonstrated and validated both experimentally and numerically. The results illustrated that the new method was able to enhance the accuracy of 3D3C velocity measurements by 40%-50% in terms of velocity magnitude and by 0.6°-1.1° in terms of velocity orientation, compared to the existing tomo-PIV technique. These improvements brought about by the new method are expected to expand the application of tomo-PIV techniques when accuracy and quantitative 3D flow properties are required.

Recently, reconstruction integrating view registration (RIVR) has been demonstrated as an improved method to significantly enhance the accuracy of three-dimensional (3D) measurements in nonreactive flows. This work extended the RIVR method to 3D measurements of highly turbulent reactive flows with two specific goals. The first goal was to examine if the RIVR method can be effectively applied to highly turbulent flame structures, which display distinctively different spatial features from nonreactive flows. This examination of RIVR was performed specifically using two performance metrics, accuracy and spatial resolution. The second goal was to quantify the end benefits the RIVR can bring about on key flame properties involved in turbulence-chemistry interaction, such as flame surface density. The results demonstrated that the RIVR method can effectively enhance reconstruction accuracy of the thin flame front marked by CH radicals in 3D distribution. Compared to past methods, the RIVR method reduced the reconstruction errors by ~48% on average and improved the spatial resolution by ~26% on average. Such accuracy enhancement ultimately led to a ~15% improved accuracy on the determination of the 3D flame surface density as an indicator of the total burning rate.

This paper reports the development of a two-dimensional two states (2D2S) model for the analysis of thermal behaviors of Li-ion battery packs and its experimental validation. This development was motivated by the need to fill a niche in our current modeling capabilities: the need to analyze 2D temperature (T) distributions in large-scale battery packs in real time. Past models were predominately developed to either provide detailed T information with high computational cost or provide real-time analysis but only 1D lumped T information. However, the capability to model 2D T field in real time is desirable in many applications ranging from the optimal design of cooling strategies to onboard monitoring and control. Therefore, this work developed a new approach to provide this desired capability. The key innovations in our new approach involved modeling the whole battery pack as a complete thermal-fluid network and at the same time calculating only two states (surface and core T) for each cell. Modeling the whole pack as a complete network captured the interactions between cells and enabled the accurate resolution of the 2D T distribution. Limiting the calculation to only the surface and core T controlled the computational cost at a manageable level and rendered the model suitable for packs at large scale with many cells.

3D (three dimensional) flame measurements based on tomography consist of two steps: view registration (VR) to determine the orientations of the projections, and the subsequent tomography reconstruction. The practical errors in both steps affect the overall accuracy of the 3D flame measurements. Past work treated these two steps separately. This work described a method to treat both steps holistically, applied the new method to 3D flame measurements, and demonstrated that the new method can substantially improve the overall accuracy. Such accuracy improvement was achieved because considering these two steps holistically establishes a feedback mechanism between the VR and tomography step, so that each step can leverage the information from the other step. In the application to 3D measurements on highly turbulent flames, the new method was quantitatively examined by simultaneously performing a well-established planar measurement as a benchmark, and was demonstrated to reduce the measurement error by ~48% on average in terms of the overall reconstruction error when compared to past methods that treated VR and tomography separately.

Understanding the dynamics of fire events in ground vehicles is critical to improving crew survivability. To advance our understanding, four dimensional (4D) measurements are sorely needed to resolve both the temporal and spatial dynamics of fire events. However, there are several key challenges toward such measurements, including equipment requirements and optical access. 4D measurements, especially with sufficient temporal resolution, can be equipment intensive. Such equipment requirements are further compounded by the relatively hostile environments encountered in vehicular testbeds. Moreover, there is often very limited optical access available for obtaining such measurements within vehicular environments. This work describes the design and implementation of a customized fiber-based endoscope (FBE) setup to overcome these challenges in order to enable 4D flame measurements in a ground vehicle testbed located at the Army Research Laboratory, Aberdeen Proving Ground. Using a customized 9-to-1 FBE bundle, 4D imaging of relatively large-scale fire events was demonstrated at 500 Hz in a volume of 35 cm (width) x 35 cm (depth) x 29 cm (height), with a single camera located at a safe distance outside the vehicle. The measurements were then processed by volumetric tomography techniques to resolve the temporal dynamics and spatial structures of the target flame within the vehicle testbed. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

•The conventional open flow battery modules modified with a guided flow.•Maximum cell temperature difference found lower than 0.24 °C.•Cell temperature reduced up to 6.19 °C compared with open flow case.•The arrangement allows easier sub-module deployment for vehicular application.

Tomographic measurements involve two steps: view registration (VR) to determine the orientation of the projections and the subsequent tomography reconstruction. Therefore, the practical error in both steps impacts the overall accuracy of the final tomographic measurements. Past work treated these two steps separately. This work shows that the overall tomography accuracy can be enhanced substantially if these two steps are considered holistically because there is an opportunity for each step to leverage the information in the other step to improve the overall accuracy if they are considered holistically. Based on this recognition, this work has developed a new method called the reconstruction integration view registration (RIVR) method to implement such a holistic scheme. The key of this implementation involved the use of the Metropolis criterion to adjust the initial orientation provided by the traditional VR process dynamically. Both controlled experiments and accompanying numerical analyses were conducted to validate the RIVR method. Two sets of controlled experiments were conducted and analyzed, including a static uniform dye solution and turbulent flows, where the RIVR technique was demonstrated to significantly reduce the overall reconstruction error (by ∼37% and ∼35%, respectively) compared to past methods that treated VR and tomography separately.

This paper reports exhaust measurements performed using a nonintrusive acoustic technique on a JT15D-1A turbofan research engine. Using multiple streamwise displaced microphones, the integrated exhaust velocity and static temperature were measured simultaneously in engine flows with bypass Mach numbers up to 0.48. A Kiel pressure and K-type thermocouple probe was used to assess the accuracy of the acoustic measurements. The Integrated velocity and static temperature rms errors of 3.8m/s and 2.3K were identified. Using probe-based measurements at a low engine power setting, two calibration constants were identified for conversion from integrated flow parameters to mass flow and thrust. Applying the calibration constants to the acoustic measurements, 1.1kg/s and 200N rms errors in mass flow and thrust were observed. These promising results indicate that the acoustic technique may be used to characterize and monitor gas-turbine engine performance. To the authors' knowledge, this is the first time a nonintrusive acoustic technique has been used to characterize engine flows with Mach numbers greater than 0.3.

This erratum corrects information regarding the author listing in Appl. Opt.57, 420 (2018)APOPAI0003-693510.1364/AO.56.00420.

This Letter reports a hybrid three-dimensional (3D) visualization approach for turbulent flows at the kilohertz range. The approach, named scanning volumetric laser induced fluorescence (SVLIF), combines 3D tomography with scanning to significantly enhance spatial resolution of 3D measurements in a given domain (or equivalently, to enlarge the domain size under a given resolution) compared to past tomographic approaches. The SVLIF technique (1) divides a large measurement domain into smaller sub-domains, (2) performs 3D tomographic measurement in each sub-domain by scanning the excitation laser pulses across them consecutively, and (3) combines the measurements in all sub-domains to form a final measurement. This hybrid approach enables the conversion of temporal resolution into spatial resolution or domain size to optimize 3D measurements in a wider design space. In this work, the SVLIF was demonstrated and validated at a scanning rate of 1.86 kHz in a volume of 38.4 mm x 26.5 mm x 25.2 mm with 7.1 million voxels, representing a similar to 5 times enhancement in the number of voids or the domain size compared to past tomographic techniques. (C) 2018 Optical Society of America

Planar laser induced fluorescence (PUF) represents an indispensable tool for flow and flame imaging. However, the PUF technique suffers from limited spatial resolution or blurring in many situations, which restricts its applicability and capability. This work describes a new method, named SR-PUF (super resolution PLIF), to overcome these limitations and enhance the capability of PUF. The method uses PUF images captured simultaneously from two (or more) orientations to reconstruct a final PUF image with resolution enhanced or blurring removed, This paper reports the development of the reconstruction algorithm, and the experimental demonstration of the SR-PUF method both with controlled samples and with turbulent flows seeded with iodine vapor. Using controlled samples with two cameras, the spatial resolution in the best case was improved from 0.06 mm in the projections to 0.03 mm in the SR image, in terms of the spreading width of a sharp edge. With turbulent flows, an image sharpness measure was developed to quantify the spatial resolution, and SR reconstruction with two cameras can effectively improve the spatial resolution compared to the projections in terms of the sharpness measure. (C) 2017 Elsevier Ltd. All rights reserved

This paper reports the thermal management of a prismatic Li-ion battery pack consisting of a total of 18 cells based on reciprocation flow and active control. Both controlled experiments and accompanying analysis are reported to illustrate the effectiveness of reciprocating cooling flow combined with active control for regulating the cell temperature, reducing temperature non-uniformity, and minimizing parasitic power consumption. Experimentally, a platform with a 3 by 6 prismatic battery module was constructed to perform controlled tests on several competing cooling strategies, including unidirectional cooling flow, reciprocating cooling flow with constant period, and the actively controlled reciprocating cooling flow. The surface and core temperatures of the cells were monitored by the thermocouples during the tests. The major observations from these experiments were twofold. First, the reciprocating cooling flow is effective in reducing the maximum temperature rise and the temperature non-uniformity in the battery pack of practical size. Second, the active control of the reciprocating flow can further reduce both the temperature non-uniformity and the cooling power consumption with a minimal increased maximum temperature rise. Thus through the active control of reciprocating cooling flow, the battery pack can reach a more uniform temperature at the minimum parasitic energy consumption. Published by Elsevier Ltd.

This Letter reports the first direct comparison between two-dimensional (2D) and three-dimensional (3D) laser-induced fluorescence (LIF) applied to highly turbulent flames, with the goal of experimentally illustrating the capabilities and limitations of volumetric LIF (VLIF). To accomplish these goals, planar LIF (PLIF) and VLIF measurements were simultaneously performed on turbulent flames based on the CH radical. The PLIF measurements imaged a planar cross-section of the target flames across a 2D field-of-view (FOV) of 42  mm×42  mm. The VLIF measurements imaged the same region in the target flame with a 3D FOV of 42  mm×42  mm×5  mm, with 5 mm being the thickness of the measurement volume. The VLIF signals generated in this volume were captured by five intensified cameras from different perspectives, based on which a 3D tomographic reconstruction was performed to obtain the 3D reconstruction of the CH radical (as a marker of the flame front). The PLIF measurements were then compared to a cross-section of the VLIF measurement to demonstrate the feasibility and accuracy of instantaneous 3D imaging of flame topography and flame surface area in highly turbulent flames.

Recent results have experimentally demonstrated the feasibility of obtaining 3D (three-dimensional) combustion measurements using tomographic VLIF (volumetric laser-induced fluorescence), specifically of the CH radical, representing the flame surface. To elucidate the fundamental capabilities and limitations of the VLIF technique, this work reports an analysis of its performance in terms of signal level, size of the field of view (FOV) in 3D, and accuracy. Compared to the established PLIF (planar LIF) technique that uses a thin laser sheet to excite the target species in a plane, the VLIF technique uses a thick laser slab to excite the target species in a volume. As a result, the VLIF technique involves more performance metrics compared to PLIF, and the relationship between these metrics is also different from that in the PLIF technique. Therefore, both experimental and computational studies were conducted to analyze the performance metrics of VLIF. First, experiments were conducted on well-controlled flames to examine the relationship among excitation energy, signal level, and FOV in 3D. Second, based on these experimental data, numerical simulations were performed to benchmark the VLIF technique under a range of conditions. These results illustrate the relationship among signal level, 3D FOV, and accuracy and are expected to be valuable for the optimal design of the VLIF technique.

This paper reports the demonstration of instantaneous three-dimension (3D) measurements in turbulent flows at repetition rates up to 10 kHz using VLIF (volumetric laser induced fluorescence). The measurements were performed based on the LIF signal of iodine (I-2) vapor seeded in the flow. The LIF signals of I-2 vapor were generated volumetrically by a thick laser slab and then simultaneously captured by a total of seven cameras from different perspectives, based on which a 3D tomographic reconstruction was performed to obtain the 3D distribution of I-2 vapor concentration. Single-shot measurements obtained in a duration of hundreds of nanoseconds (limited by the pulse duration of the excitation laser) were demonstrated in a 50 x 50 x 50 mm(3) at a repetition rate up to 10 kHz. These measurements demonstrated the feasibility and potential of VLIF for resolving the 4D spatiotemporal dynamics of turbulent flows. Based on the experimental results obtained, this work also studied the VLIF signal level and its effects on the reconstruction accuracy under different the measurement conditions, illustrating the capabilities and limitations of performing high speed VLIF measurements. (C) 2017 Elsevier Ltd. All rights reserved.