Devesh Ranjan

The influence of gas molecular weight and specific heat ratio on entrainment and mixing is investigated for sonic jets in a supersonic crossflow. Five pure gases (argon, ethylene, carbon dioxide, helium, and nitrogen) spanning molecular weights of 4–44 g/mol and specific heat ratios of 1.24–1.66 are injected into a Mach 1.71 crossflow at momentum flux ratios from 1 to 6. Planar Mie scattering and particle image velocimetry provides measurements of velocity fields on the flow's plane of symmetry. The velocity fields provide information about mean and fluctuating velocities, as well as the jet bow shock geometry. Convective Mach number profiles are computed directly from the velocity data. Results show that compressibility effects grow faster for lower molecular weight gases, with peak convective Mach numbers reaching approximately unity for helium compared to higher values for heavier gases. Injectants with higher molecular weight and lower specific heat ratios demonstrate faster entrainment and mixing, as evidenced by decreased velocity fluctuations in the near field of the jet. The convective Mach number estimates explain these trends through compressible suppression of hydrodynamic instabilities. The velocity data and spatially resolved convective Mach number profiles for a large range of injectant properties provide unprecedented quantitative guidance for the design of systems dependent on supersonic injection and mixing.

Turbulent mixing in a supercritical CO $_2$ shear layer is examined using both experimental and numerical methods. Boundary conditions are selected to focus on the rarely studied near-critical regime, where thermophysical properties vary nonlinearly with respect to temperature and pressure. Experimental results are obtained via Raman spectroscopy and shadowgraphy, while numerical results are obtained via direct numerical simulation. The shear layer growth rate is found to be 0.2. Additionally, density profiles indicate a relaxation of density gradients between the mixed fluid and heavy fluid as the flow evolves downstream, which runs counter to existing supercritical shear layer data in the literature. The computational results identify significant anisotropy in the turbulence in the shear layer, which is discussed in terms of the development of regions of high density gradient magnitude. The Reynolds-averaged enstrophy budget at various streamwise locations indicates no significant dilatational or baroclinic contribution within the shear layer.

Turbulent/non-turbulent interfaces (TNTI) are evaluated for flows subject to the Rayleigh–Taylor instability (RTI). Experiments are conducted in a gas tunnel facility with air as heavy fluid and helium+nitrogen+air mixture as light fluid giving a low Atwood number of ≈0.1. Simultaneous velocity-density measurements are taken via particle image velocimetry and laser induced fluorescence. The nature of the TNTI on bubble front, as well as the change in mean quantities and turbulence statistics across this TNTI are investigated. The molecular mixing is studied relative to the TNTI. The TNTI shows a complex conditionally averaged volume fraction profile in its vicinity. In the external layer, the fluid is mostly pure heavy fluid, leading to no concentration gradients and a nearly zero measurement of the scalar dissipation. At the interface, there is a very large magnitude of scalar dissipation. In the adjustment layer, the scalar dissipation is nearly constant. These results challenge the conventional shape of profiles of turbulence statistics in RTI flows which are typically assumed parabolic. An alternate way to interpret the variation of turbulence statistics across the mixing width is to assume them to be nearly uniform in the core of the flow and be modulated by the location of the TNTI.

Gravity-driven granular flows along an inclined plane were considered with exposure to different levels of irradiation. Experiments were performed with average normal radiative heat fluxes of 400, 500, and 600 kW/m 2 , and the spatial and temporal free-surface velocity and temperature fields were measured. Granular flow behavior changed under different irradiation, primarily due to an increase in particle friction changing with increased temperatures. A transient, pseudo-two-dimensional heat and mass transfer model was developed to capture the relevant heat transfer mechanisms in these flows. The model considered irradiation penetration, effective conduction heat transfer, and non-uniform velocity profiles. The predicted steady state free-surface temperatures were well-correlated to experimental measurements with Pearson's correlation coefficients >0.9. The model predicted a large temperature gradient in the flow due to irradiation attenuation, flow velocity variations in depth, and low effective thermal conductivity. This study effectively captured the heat transport of the irradiated inclined plane granular flows.

Increasing thermal efficiency of power plants is an important step to reduce cost of produced electricity, an essential goal in achieving long-term stable economic growth. Working fluids at supercritical state (especially water, carbon dioxide, and helium) have been considered as the heat transfer media for Brayton cycle to reach this goal for decades. There is no phase change in supercritical cycles, as the cycle operates above the critical pressure of the working fluid, which results in reduction of the power plant size and cost due to elimination of condenser. It is well known that small variations in the fluid temperature and pressure near critical point, pseudo-critical region, can lead to significant change in the thermo-physical properties of the fluid. Acceptable analytical and numerical models for thermal and hydraulic resistance of these fluids in pipes are still being developed especially in turbulent flow and at high heat fluxes where property variation is significant. Therefore, further experimental investigations are required to better understand the thermal and hydraulic behavior of these fluids before it could be widely used in industrial applications. Heat transfer and pressure drop measurements for pseudo-critical C02 flow in a horizontal pipe is reported here at high Reynolds number (2x104 < Re < 105). Heat transfer measurements were carried out at several mass flow rates, inlet fluid temperatures, system pressures, and wall heat fluxes. Semi-local heat transfer coefficients are obtained to investigate the influence of these parameters on the forced convection heat transfer in the tube. The obtained heat transfer data is analyzed and compared with existing empirical correlations. Total pressure drop measurements were also conducted in some selected conditions. The effect of thermo-physical properties variations and heat fluxes in total pressure drop is analyzed by comparing the obtained data with existing correlations.

Abstract In a World of Diverse Challenges, Mechanical Engineers are Developing the Solutions. Their Contributions Have Never Been as Valuable As They are Today. “Almost everyone comes into contact with the products of mechanical engineering on a daily basis, but very few people—including some engineers themselves—can provide a succinct explanation of what mechanical engineering is. ”

The fluid mixing caused by variable-density instabilities is important in a wide variety of scenarios from ocean mixing and astrophysical phenomena to nuclear fusion techniques and atomic weapons. This thesis explores the mixing resulting from a specific instability known as the Blast Driven Instability (BDI). This work investigates the variable density mixing in an explosively driven environment due to the fluid instabilities at the material interfaces. Specifically, diverging Richtmyer-Meshkov (impulsive-acceleration environment) and Rayleigh-Taylor (variable-acceleration environment) instabilities (present in supernova and inertial confinement fusion) are studied using advanced high-speed diagnostics in carefully designed laboratory experiments. The BDI morphology is presented through a time development of Mie scattering images, and steps through the parameter space (varying density ratio and driver speed), highlighting the development of the structures that form during mixing. A scaling criterion is used to relate the two systems of vastly different spatiotemporal scales. Velocity fields in the BDI have been captured for the first time using the high temporal resolution PIV technique. Subsequent analysis of the dynamics of the instability from the velocity fields illustrates the distribution of kinetic energy, the transition to turbulence, and the characteristic growth of the instability are discussed. This study furthers understanding of how blast-driven instability pertains to supernova and inertial confinement fusion science. The morphology of the BDI has been characterized for the first time. This work steps through the parameter space covered in Mie scattering experiments, and how the different parameters contribute to development of structures and mixing. It also examines a scaling of the Atwood number for expanding predictive capabilities to other experimental conditions and simulations. The first collection of velocity fields acquired for the BDI are recorded, and subsequent analysis evaluating the distribution of kinetic energy throughout space and time for two density ratios from the overall parameter space, as well as the transition to turbulence, estimated from a Reynolds number calculated based on momentum mixing are all presented. This information is useful in advancing the development of models to predict physics of high energy density applications where experiments are not always readily available. This research has successfully demonstrated understanding for the time criteria defining regimes where the shock driven (Richtmyer-Meshkov instability) and the buoyancy driven (Rayleigh-Taylor instability) dominates through a parametric study of density variation (Atwood number) and driver speed (Mach number). All this furthers understanding of how the BDI pertains to SN and ICF. The technical objectives include but are not limited to: • Building a lab-scale facility in efforts to study hydrodynamic instabilities present in supernova explosions. • Observe the turbulent length scales necessary for mixing transition to occur after a critical Reynolds number is achieved. • Measure and quantify the growth model for the blast-driven mixing environment and understand the effects of varying the Atwood number and Mach number have on the interface. • Acquire measurements of velocity fields from the Blast Driven Instability for the first time to develop an understanding of the dynamics of the flow field. In this work, two PhD students were supported (theses submitted, students graduated), and one postdoctoral student was also supported. The following journal publications, conference publications, and other works in preparation for submission have all come out (in addition to several other conference presentations and invited talks): • Musci, B., Petter, S., Pathikonda, G., Ochs, B., & Ranjan, D. (2020). Supernova hydrodynamics: A lab-scale study of blast-driven instability using high-speed diagnostics. The Astrophysical Journal, 896(2), 92. • Musci, B., Olson, B., Petter, S., Pathikonda, G., & Ranjan, D. (2023). Multifidelity validation of digital surrogates using variable-density turbulent mixing models. Physical Review Fluids, 8(1), 014501. • Musci, B., Petter, S., Pathikonda, G., Ranjan, D. (2024). Morphological behavior of the BDI: sensitivity to Atwood and Mach numbers. Under preparation. • Petter, S., Musci, B., Pathikonda, G., Ranjan, D. (2024). The effect of density ratio on velocity dynamics in the blast driven instability. Under preparation. • Petter, S., Musci, B., Pathikonda, G., Ranjan, D. (2018). An experimental study of the blast driven Rayleigh-Taylor and Richtmyer-Meshkov instabilities: Preliminary results. International workshop on the physics of compressible turbulent mixing (IWPCTM-16) conference proceedings. • Petter, S., Musci, B., Pathikonda, G., Ranjan, D. (2023). Experimental investigation of blast driven turbulence. International symposium of shock waves (ISSW 34) conference proceedings.

In this work, a new solver, FPSolve, is developed to study fiber orientation kinetics using the Fokker–Planck (FP) equation. The solver employs the finite-volume method. The FP equation is discretized on unstructured cubed-sphere grids using the centered differencing scheme (CDS) or a blend of the CDS and the upwind differencing scheme. Time integration is performed using a second-order two-stage explicit Runge–Kutta scheme. Different shape factors and rotational diffusion coefficients are implemented to study suspensions in dilute to semiconcentrated regimes. The verification of the solver is performed for the fiber orientation in simple shear flow up to a Peclet number of 1 0 5 . Grid independence analysis is presented to show the second-order accuracy of FPSolve. It is demonstrated that the solver does not need stabilization by upwinding. Simulations for semiconcentrated suspensions are performed using the model of Ferec et al. (2014). Time-accurate solutions of the FP equation with explicit time stepping for this model are presented for the first time.

Abstract This paper presents the design, implementation, and impact of a simultaneous curricular intervention in freshman and senior capstone design courses at an undergraduate Mechanical Engineering program. The two primary objectives of this intervention were to i. Enhance students' understanding of the design process, emphasizing the importance of end-users and stakeholders, and ii. To create an opportunity for students to be rewarded for learning and teaching their peers. This study lays the foundation for a long-term longitudinal study to understand further the impact of peer mentorship and socio-technical projects from freshman to senior years. Published literature indicates that undergraduates teaching other undergraduates is one of the most effective methods for achieving both cognitive and attitudinal goals of undergraduate education. The paper will present the benefits and challenges associated with engaging seniors and freshmen while solving an authentic design challenge through surveys and focus groups. These results will help develop the framework to build vertical integration within the curriculum for effectively teaching engineering design.

This paper investigates the multilayer Rayleigh–Taylor instability (RTI) using statistically stationary experiments conducted in a gas tunnel. Employing diagnostics such as particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF), we make simultaneous velocity–density measurements to study how dynamics and mixing are linked in this variable density flow. Experiments are conducted in a newly built, blow-down three-layer gas tunnel facility. Mixing between three gas streams is studied, where the top and bottom streams are comprised of air, and the middle stream is an air–helium mixture. Shear is minimized between these streams by matching their inlet velocities. The four experimental conditions investigated here consist of two different density ratios (Atwood numbers 0.3 and 0.6), each investigated at two instability development times (or equivalently, two streamwise locations), and all experiments are with the same middle stream thickness of 3 cm. The growth of the middle layer is measured using laser-based planar Mie scattering visualization. The mixing width is found to grow linearly with time at late times. Various quantitative measures of molecular mixing indicate a very high degree of molecular mixing at late times in the multilayer RTI flow. The vertical turbulent mass flux $a_y$ is calculated. In addition to mostly negative values of $a_y$ , typical of buoyancy-dominated flows due to negative correlation between velocity and density fluctuations, positive regions are also observed in profiles of $a_y$ due to entrainment and erosion at the lower edge of the mixing region. Global energy budgets are calculated for the multilayer RTI flow at late times and it is found that the majority of potential energy released has been dissipated due to viscous effects, and a large value of mixing efficiency ( $\sim$ 60 %) is observed.

Fulfilling the role of ammonia as a viable energy vector and clean alternative fuel for combustion systems requires an improved understanding of its fundamental chemical kinetics. In this work, autoignition delay time (IDT) measurements of ammonia/oxygen/argon (NH3/O2/Ar) mixtures behind reflected shock waves were reported at a temperature range of 1180–1941 K, a pressure range of 11–20 atm, and equivalence ratios of 0.5, 1.0, and 2.0. The effect of fuel concentration on the IDT was investigated for stoichiometric mixtures containing 1–22% NH3 by mole fraction. A combination of diagnostics, including pressure and chemiluminescence from excited hydroxyl radicals (OH*) near 307 nm at the sidewall and the endwall, and direct absorption of a diode laser emission near 2.2 μm, were used to provide a consistent determination of the autoignition event. The experimental data were compared with predictions by several kinetic models applicable to ammonia oxidation from literature to assess their performance. Large variations were observed among model predictions; the model by Mathieu and Petersen (2015) was found to produce the best agreement with the experimental data at low fuel concentrations, and the model by Mével et al. (2009) reproduced the experimental data the best at high fuel concentrations. However, none of these models is capable of accurate prediction of the IDT measurements across the entire range of fuel concentration investigated; most models shift from being over-reactive to under-reactive as the fuel concentration increases. Reaction path and sensitivity analyses performed with selected kinetic models revealed the importance of reactions related to NH3 and N2HX species in controlling the autoignition, especially at high fuel concentrations. The reliable shock tube IDT data from this study highlighting the fuel concentration effect could benefit the validation and refinement of kinetic models.

Mixing of fluids in a coaxial jet is studied under four distinct viscosity ratios, $m=1$ , $10$ , $20$ and $40$ , using highly resolved large-eddy simulations (LES), particle image velocimetry and planar laser-induced fluorescence. The accuracy of predictions is tested against data obtained by the simultaneous experimental measurements of velocity and concentration fields. For the highest and lowest viscosity ratios, standard RANS models with unclosed terms pertaining to viscosity variations are employed. We show that the standard Reynolds-averaged Navier–Stokes (RANS) approach with no explicit modelling for variable-viscosity terms is not applicable whereas dynamic LES models provide high-quality agreement with the measurements. To identify the underlying mixing physics and sources of discrepancy in RANS predictions, two distinct mixing modes are defined based on the viscosity ratio. Then, for each mode, the evolution of mixing structures, momentum budget analysis with emphasis on variable-viscosity terms, analysis of the turbulent activity and decay of turbulence are investigated using highly resolved LES data. The mixing dynamics is found to be quite distinct in each mixing mode. Variable viscosity manifests multiple effects that are working against each other. Viscosity gradients induce additional instabilities while increasing overall viscosity decreases the effective Reynolds number leading to laminarization of the turbulent jet, explaining the lack of dispersion and turbulent diffusion. Momentum budget analysis reveals that variable-viscosity terms are significant to be neglected. The scaling of the energy spectrum cascade suggests that in the TLL mode the unsteady laminar shedding is responsible for the eddies observed.

High-speed experiments studying the Blast Driven Instability are used to validate Reynolds-Averaged Navier-Stokes (RANS) and Large eddy simulation (LES) turbulent mixing models. The work helps to elucidate the three mixing regimes of the instability and shows that the LES can successfully capture, at lease to the first order, the complex physics therein. The work also highlights the limitations of RANS models for transitioning instabilities and how LES results remain highly sensitive to the characterization of initial conditions.