arXiv subjects

Aamir Farooq

Publications and source records attributed to Aamir Farooq.

14 recordsLinked to original sources

arXiv2026

Thermodynamic Non-Uniformities Behind Incident and Reflected Shocks in a Single-Diaphragm Shock Tube

Shock tubes provide well-controlled high-temperature and high-pressure conditions for chemical kinetics studies, yet the region behind the reflected shock is seldom perfectly homogeneous. Axial and radial gradients arise from shock formation, attenuation, and the interaction of the reflected shock wave with the boundary layer, and these variations influence chemical kinetic measurements such as ignition delay time. The present study combines experimental diagnostics and numerical simulations to quantify these gradients in a single-diaphragm shock tube. A coupled RANS-LES framework implemented in CONVERGE CFD incorporates realistic diaphragm opening profiles and is validated using pressure histories and shock velocity profiles for argon, nitrogen, and carbon dioxide. The results show that incident shock attenuation strongly influences the thermodynamic state of the reflected-shocked region, with test gas-dependent differences: a nearly uniform core with modest axial gradients is maintained in argon, whereas substantial axial gradients due to reflected-shock and boundary-layer interactions is seen in nitrogen and carbon dioxide. The analysis provides a foundation for quantifying test-gas homogeneity in shock-tube experiments and potential extrapolation to improving interpretation of ignition data acquired under non-ideal flow conditions.

physics.flu-dyn↗

arXiv2026

Infrared photonics for healthcare: A roadmap for proactive and predictive health management

The field of infrared (IR) photonics is currently undergoing remarkable progress, moving rapidly towards practical sensing applications demanded by medical therapy and diagnostics (theranostics). The Developments can be divided into three main categories: (i) novel devices and measurement concepts including advanced updates of classical approaches that push medical sensing into the spotlight; (ii) new demonstrations of photonic integrated circuit (PIC-)based IR devices enabling highly miniaturized sensors for point-of-care application as well as medical and wellness wearables; and (iii) technologically-mature IR demonstrators that enable first medical sensing and treatment applications. This roadmap paper provides a consolidated overview of this highly dynamic and interdisciplinary research field with a focus on the major roadblocks that limit the widespread adoption of IR photonics in large-scale medical diagnostics. Special attention is given to the ambivalence between the molecular-level spectroscopic interpretation and a broader health-state assessment, highlighting the need for a common framework. Additionally, the paper discusses the critical importance of unified measurement standards, calibration protocols, and medical certification processes to ensure the validity of experimental results, reproducibility, and clinical trust, particularly when novel experimental techniques and AI algorithms are involved. Perspectives from major past and current contributors to application-oriented IR photonics will be provided.

physics.app-ph↗

arXiv2025

HAPS Communication Networks: A Tutorial-cum-Survey on Integration with Optical Atmospheric Sensing

High-Altitude Platform Stations (HAPS) are emerging as key enablers of future non-terrestrial networks (NTNs), supporting gigabit-class free-space optical (FSO) backhaul links while hosting laser-based sensing payloads. This tutorial and survey reviews recent advances in HAPS optical communication and integration with atmospheric remote sensing via shared optical links. Among several sensing techniques, Differential Absorption Lidar (DIAL) is identified as most promising due to its range-resolved sensitivity, spectral selectivity, and compatibility with HAPS constraints. A roadmap is outlined for implementing telecom-band DIAL on HAPS alongside high-throughput FSO systems. The paper analyzes architectural and atmospheric advantages of HAPS over terrestrial and satellite nodes, emphasizing spatial-temporal coverage, station-keeping ability, and support for compact laser payloads such as DIAL, in-situ sensors, and multispectral imagers. It highlights the feasibility of co-locating sensing and communication within a shared optical and power envelope, especially in the telecom C-band (1.53-1.57 um), enabling trace-gas retrieval (CO2, CH4, N2O, H2S, O3) while maintaining multi-Gbps downlinks. Suitable HAPS architectures (balloons, UAVs, airships) and use cases are identified where integrated sensing and communication (ISAC)-enabled HAPS outperform satellites and UAVs, including greenhouse gas monitoring, disaster response, air-quality mapping, and 6G NTN extensions. A literature survey for 2005-2025 shows HAPS publications have tripled since 2014, indicating rapid growth. The results confirm that optical hardware, favorable transmission windows, and active R&D are positioning HAPS as a persistent stratospheric layer for 6G ISAC communications and environmental observation.

physics.optics↗

arXiv2025

Fuelprop: Fuel property prediction from ATR-FTIR spectroscopic data

Synthetic fuels are crucial for decarbonizing the transportation sector. A significant challenge lies in the rapid and efficient characterization of these fuels. Chemometric methods using ATR-FTIR data offer a potential alternative to conventional techniques. This study expands the applicability and performance of chemometric models by providing an extensive ATR-FTIR spectral dataset and exploring various data enhancement strategies. Data enhancement was achieved by semi-supervised data generation, consistency enforcement through unsupervised data augmentation, and data imputation using synthetic spectra blending and pseudo-labeling. Models were trained on surrogate fuels and rigorously tested on real fuels, representing out-of-distribution testing conditions. We believe that this work will enhance the adoption of chemometric models for fuel characterization.

physics.chem-ph↗

arXiv2025

On the flow characteristics in the shock formation region due to the diaphragm opening process in a shock tube

The shock formation process in shock tubes has been extensively studied; however, significant gaps remain in understanding the effects of the diaphragm rupture process on the resulting flow non-uniformities. Existing models predicting the shock attenuation and propagation dynamics overlook critical diaphragm mechanics and their impact on shock behavior. Addressing this gap is vital for improving predictive capabilities and optimizing shock tube designs for applications in combustion kinetics, aerodynamics, and high-speed diagnostics. This study investigates the shock wave formation and propagation through combined experimental and numerical approaches over a range of driver-to-driven pressure ratios (Driver pressure: 9.4 - 24.2 bar of helium; Driven pressure: 100 Torr of argon). High-speed imaging captures the diaphragm opening dynamics, while pressure and shock velocity measurements along the entire driven section of the shock tube provide key validation data for CFD. Two-dimensional numerical simulations incorporate experimentally measured diaphragm opening profiles, offering detailed insights into flow features and thermodynamic gradients behind the moving shock front. Key parameters, including deceleration and acceleration phases within the shock formation region, shock formation distances, and times, have been quantified. A novel theoretical framework is introduced to correlate these parameters, enabling accurate predictions of shock Mach number evolution under varying conditions. This unified methodology bridges theoretical and experimental gaps, providing a robust foundation for advancing shock tube research and design.

physics.flu-dyn↗

arXiv2025

Rigorous Perturbation Bounds for the QX Decomposition for Centrosymmetric Matrices

Konrad Burnik suggests a structure-preserving QR factorization for centrosymmetric matrices, known as QX factorization. In this article, we obtain the explicit expressions for rigorous perturbation bounds of the QX factorization when the original matrix is perturbed, either norm-wise or component-wise. First, using the matrix-equation approach, weak rigorous perturbation bounds are derived. Then, strong rigorous perturbation bounds are obtained by combining the modified matrix-vector equation approach with the strategy for the Lyapunov majorant function and the Banach fixed-point theorem. The mixed and component-wise condition numbers and their upper bounds are also explicitly expressed. Numerical tests illustrate the validity of the obtained results.

math.NA↗

arXiv2024

Influence of Diaphragm Dynamics on Shock Wave Propagation in Double-Diaphragm Shock Tubes

Shock tubes have become indispensable tools for advancing research across diverse fields such as aerodynamics, astrophysics, chemical kinetics, and industrial applications. The present study examines shock velocity variations in double-diaphragm shock tubes and their impact on flow dynamics at the end of the shock tube. Experiments were conducted using helium as the driver gas at 30 bar and argon as the driven gas at 100 Torr, with two different mid-section pressures (4.8 and 18.8 bar). A previous study highlighted two mechanisms governing the shock velocity profile: a higher peak shock velocity in the case of lower mid-section pressure due to stronger shock heating of the mid-section gas, and a second stage of acceleration of the shock wave resulting from the higher mid-section pressure and delayed opening of the first diaphragm. Pressure histories of the mid-section between the two diaphragms and high-speed imaging of the diaphragm-opening process helped validate the mechanisms influencing shock velocity. Additionally, numerical simulations were performed (i) to understand the effect of diaphragm-opening dynamics on the experimental shock profiles and (ii) to quantify the axial temperature as well as pressure profiles within the shocked gas. The simulations revealed that while axial pressure variations were not significantly affected by shock velocity variation, there were large temperature gradients in the shocked gas, with up to 6.5 percent higher peak temperature in the case of lower mid-section pressure compared to its counterpart. These pivotal findings underscore the need to control the mid-section pressure to minimize temperature variations within the shocked gas region.

physics.flu-dyn↗

arXiv2024

Predicting Non-Ideal Effects from the Diaphragm Opening Process in Shock Tubes

Shock tubes are instrumental in studying high-temperature kinetics and simulating high-speed flows. They swiftly elevate the thermodynamic conditions of test gases, making them ideal for examining rapid chemical reactions and generating high-enthalpy flows for aerodynamic research. However, non-ideal effects, stemming from factors like diaphragm opening processes and viscous effects, can significantly influence thermodynamic conditions behind the shock wave. This study investigates the impact of various diaphragm opening patterns on the shock parameters near the driven section end-wall. Experiments were conducted using helium and argon as driver and driven gases, respectively, at pressures ranging from 1.32 to 2.09 bar and temperatures from 1073 to 2126 K behind the reflected shock. High-speed imaging captured different diaphragm rupture profiles, classified into four distinct types based on their dynamics. Results indicate that the initial stages of diaphragm opening, including the rate and profile of opening, play crucial roles in resulting incident shock Mach number and test time. A sigmoid function was employed to fit the diaphragm opening profiles, allowing for accurate categorization and analysis. New correlations were developed to predict the incident shock attenuation rate and post-shock pressure rise, incorporating parameters such as diaphragm opening time, rupture profile constants, and normalized experimental Mach number. The results emphasize the importance of considering diaphragm rupture dynamics in shock tube experiments to achieve accurate predictions of shock parameters.

physics.flu-dyn↗

arXiv2023

Methane and n-hexane ignition in a newly developed diaphragmless shock tube

Shock tubes have been routinely used to generate reliable chemical kinetic data for gas-phase chemistry. The conventional diaphragm-rupture mode for shock tube operation presents many challenges that may ultimately affect the quality of chemical kinetics data. Numerous diaphragmless concepts have been developed to overcome the drawbacks of using diaphragms. Most of these diaphragmless designs require significant alterations in the driver section of the shock tube and, in some cases, fail to match the performance of the diaphragm-mode of operation. In the present work, an existing diaphragm-type shock tube is retrofitted with a fast-acting valve, and the performance of the diaphragmless shock tube is evaluated for investigating the ignition of methane and n-hexane. The diaphragmless shock tube reported here presents many advantages, such as eliminating the use of diaphragms, avoiding substantial manual effort during experiments, automating the shock tube facility, having good control over driver conditions, and obtaining good repeatability for reliable gas-phase chemical kinetic studies. Ignition delay time measurements have been performed in the diaphragmless shock tube for three methane mixtures and two n-hexane mixtures at $P_5$ = 10 - 20 bar and $T_5$ = 738 - 1537 K. The results obtained for fuel-rich, fuel-lean, and oxygen-rich (undiluted) mixtures show very good agreement with previously reported experimental data and literature kinetic models (AramcoMech 3.0 [1] for methane and Zhang et al. mechanism [2] for n-hexane). The study presents an easy and simple method to upgrade conventional shock tubes to a diaphragmless mode of operation and opens new possibilities for reliable chemical kinetics investigations.

physics.flu-dyn↗

arXiv2022

High-Resolution Spectroscopy in the 11.6-15 μm Range by a Quasi-cw Difference-Frequency-Generation Laser Source

We report an approach for high-resolution spectroscopy using a widely tunable laser emitting in the molecular fingerprint region. The laser is based on difference-frequency generation (DFG) in a nonlinear orientation-patterned GaAs crystal. The signal laser, a CO2 gas laser, is operated in a kHz-pulsed mode while the pump laser, an external-cavity quantum cascade laser, is finely mode-hop-free tuned. The idler radiation covers a spectral range of 11.6 - 15 μm with a laser linewidth of 2.3 MHz. We showcase the versatility and the potential for molecular fingerprinting of the developed DFG laser source by resolving the absorption features of a mixture of several species in the long-wavelength mid-infrared. Furthermore, exploiting the wide tunability and resolution of the spectrometer, we resolve the broadband absorption spectrum of ethylene (C2H4) over 13 - 14.2 μm and quantify the self-broadening coefficients of some selected spectral lines.

physics.optics↗

arXiv2022

A Selective Benzene, Acetylene, and Carbon Dioxide Sensor in the Fingerprint Region

A mid-infrared laser-based sensor is designed and demonstrated for trace detection of benzene, acetylene, and carbon dioxide at ambient conditions. The sensor is based on a distributed feedback quantum cascade lase and a multidimensional liner (DFB-QCL) emitting near 14.84 um. Tunable diode laser absorption spectroscopy (TDLAS) and a multidimensional linear regression algorithm are employed to enable interference-free measurements of the target species. The laser wavelength was tuned over 673.8-675.1 cm-1 by a sine-wave injection current (1 kHz scan rate). Minimum detection limits of 0.11, 4.16, and 2.96 ppm were achieved for benzene, acetylene, and carbon dioxide, respectively. The developed sensor is insensitive to interference from overlapping absorbance spectra, and its performance was demonstrated by measuring known mixture samples prepared in the lab. The sensor can be used to detect tiny leaks of the target species in petrochemical facilities and to monitor air quality in residential and industrial areas.

physics.ins-det↗

arXiv2022

Dual-camera high-speed imaging of n-hexane oxidation in a high-pressure shock tube

Shock tubes are widely used in the study of chemical kinetics. Its benefits rely on the almost ideal shock-heating process that provides high temperatures and pressures to a chemical system for a limited test time. Just like any reactor, shock tubes are not immune to non-ideal effects. The study of conditions that might deviate experiments from ideal conditions is thus of the utmost importance. High-speed imaging has been proven to be a powerful bytool to analyze non-ideal / non-homogenous combustion in shock tubes. In this work, dual-camera high-speed imaging experiments were performed at 10, 15 and 20 bar in a high-pressure shock tube (HPST). An optical section was designed as an extension of the HPST which enabled simultaneous visualization from the endwall and the sidewall of the driven section of the shock tube. n-Hexane, a fuel with a negative temperature coefficient (NTC) behavior that has been identified as prone to non-homogenous ignition, is used as a test fuel. Reactive mixtures and thermodynamic conditions were selected to visually analyze ignition processes at the high-temperature, NTC and low-temperature regimes. Non-homogeneous ignition was observed mostly at the local maximum of the IDT, which is comprised by the high-temperature and NTC regions. Stoichiometric n-hexane mixture with high fuel loading (5% n-hexane) presented the highest deviation from constant volume chemical kinetic simulations. The inclusion of helium as a bath gas to mitigate preignition was tested and it showed to improve the susceptibility of the mixtures to develop reaction fronts. The modified Sankaran criterion for the identification of ignition regimes in shock tubes was tested and it showed an overall good agreement against the experimental observations.

physics.flu-dyn↗

arXiv2021

Multiplicative perturbation bounds for the Generalized block Cholesky downdating problem

The explicit expressions for the strong and the weak rigorous multiplicative perturbation bounds for the Generalized block Cholesky downdating problem are obtained. By bringing together the modified matrix-vector equation approach with the method of Lyapunov majorant function and the Banach fixed point theorem, we derived the strong rigorous multiplicative perturbation bounds. By using the matrix-equation approach the weak rigorous multiplicative bounds are presented. Numerical experiments are provided to illustrate the obtained results.

math.NA↗

arXiv2017

Absolute spectroscopy near 7.8 μm with a comb-locked extended-cavity quantum-cascade-laser

We report the first experimental demonstration of frequency-locking of an extended-cavity quantum-cascade-laser (EC-QCL) to a near-infrared frequency comb. The locking scheme is applied to carry out absolute spectroscopy of N2O lines near 7.87 μm with an accuracy of ~60 kHz. Thanks to a single mode operation over more than 100 cm^{-1}, the comb-locked EC-QCL shows great potential for the accurate retrieval of line center frequencies in a spectral region that is currently outside the reach of broadly tunable cw sources, either based on difference frequency generation or optical parametric oscillation. The approach described here can be straightforwardly extended up to 12 μm, which is the current wavelength limit for commercial cw EC-QCLs.

physics.ins-det↗