arXiv subjects

Touqeer Anwar Kashif

Publications and source records attributed to Touqeer Anwar Kashif.

5 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↗

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↗

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↗

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

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↗