About me

Welcome to my website. I am a Graduate Research Assistant at University of Maryland. After I obtained a Bachelor's degree from Aerospace Engineering department at Cairo University and finished my Master's coursework, I worked for four years (2012-2016) at a consulting company, Optumatics LLC, specialized in Computational Fluid Dynamics (CFD), combustion and software development, where I worked on multi-disciplinary research and development projects. During that period, I obtained a Master's degree in Aerospace Engineering from Cairo University.

I moved to the USA in 2017, and I started my doctoral studies at Kansas State University, where I worked with Dr. James Chen (Now at SUNY Buffalo) on a Morphing Continuum approach to hypersonic flow simulations. After one year, I transferred my PhD program to the department of Mechanical Engineering at the University of Maryland. I am currently working with Prof. Arnaud Trouvé on computational modeling of the coupling between solid fuel thermo-chemical decomposition and flame propagation, particularly in wildfire scenarios. We also develop advanced combustion and radiation models for more accurate numerical predictions of different fire phenomena.

Research interests: Combustion, Aero-thermo-dynamics, Turbulence, Turbomachinery, Multiscale Numerical Simulations and High Performance Computing.

Research Work

Solid biomass thermo-chemical degradation and flame probagation

A robust Lagrangian-Eulerian model is developed to provide a multiphase computational description of a porous solid fuel bed (particles) in a fire problem. The model describes in particular: the thermal decomposition of the solid fuel through drying, pyrolysis and char oxidation reactions; the corresponding release of fuel vapors that supply the gas-phase combustion processes; the modification of the flow due to particle drag; the gas-to-solid heat transfer associated with convective and radiative processes; and the modified transport of radiation due to particle emission and absorption effects. The video shows preliminary results of a small-scale fire spread over a non-charring material that features shrinkage in size. Ongoing work focuses on validating the current model against experimental observations, implementing more advanced descritpion of the burning characteristics of porous biomass particles particularly the oxidation of char and the transition from smoldering to flaming modes of combustion. This would allow us to study in a great detail the sensitivity of fire spread to multiple controlling physical parameters.

Unstable flame structure and gas-to-liquid thermal feedback pool fires

Fine-grained Large Eddy Simulations (LES) of a canonical 30-cm-diameter methanol pool fire configuration are performed in order to evaluate the ability of current fire models to predict the flame structure and the rate of heat transfer to the liquid fuel surface. Great attention is paid to the design of the computational grid and to the control of spatial resolution. Less attention is paid to the LES model formulation: except for the treatment of radiation for which we consider both a simplified model using a prescribed global radiant fraction and a more advanced model based on the Weighted-Sum-of- Gray-Gases approach. Consistent with experimental observations, the simulated flames feature a strong instability characterized by the cyclic formation of thin boundary layer flames at the liquid pool surface and vortex rings at the burner rim that grow into large puffs and determine the structure of the entire flame. Equally encouraging results are obtained for the mean radiative and convective heat fluxes near the pool surface; quantitatively, simulations are found to overestimate the intensity of the thermal feedback by 25%. These results suggest that provided that the computational grid is fine enough, current fire models can be used to predict the gas-to-fuel thermal feedback.

Simulations of thermo-chemical degradation of solid biomass particles under oscillatory heating conditions

This study provides a fundamental understanding of the burning process of biomass fuel particles under oscillatory heat flux conditions that represent flapping flames or fluctuations in the irradiation above natural vegetation beds. These conditions were typically observed in field-scale tests of forest fires. The results of the numerical simulations suggest that the particles experience a quasi-linear response in the case of fluctuating irradiation or fluctuating local gas temperature; this linear response means that the effects of oscillations can essentially be ignored. However, a non-linear response is obtained in the case of unsteady convective heating, when the local gas velocity fluctuates in phase with the local gas temperature; this non-linear response leads to a net effect of the oscillations in the form of an augmented heat transfer, higher temperatures, higher values of the fuel mass loss rate, and shorter burnout times.

Boltzmann-Curtiss description for flows under translational non-equilibrium

The Boltzmann–Curtiss formulation describes gases allowing both rotational and translational degrees-of-freedom and forms morphing continuum theory (MCT). The first-order solution to Boltzmann–Curtiss equation yields a stress tensor that contains a coupling coefficient that is dependent on the particles number density, the temperature, and the total relaxation time. A new bulk viscosity model derived from the Boltzmann–Curtiss distribution is employed for shock structure and temperature profile under translational and rotational nonequilibrium. Numerical simulations of argon and nitrogen shock profiles are performed in the Mach number range of 1.2–9. This study, when comparing with experimental measurements and direct simulation Monte Carlo (DSMC) method, shows a significant improvement in the density profile, normal stresses, and shock thickness at nonequilibrium conditions than NS equations.

Simulations of hypersonic flow at Mach 8 around a sphere cone using a morphing continuum approach

A non-equilibrium flow solver is developed in openFoam CFD library as part of the extension of a set of fluid governing equations called (MCT), introduced originally by A. Eringen for micropolar continuum fluids and later derived by J. Chen and coworkers from statistical mechanics and adopted for turbulence and hypersonic simulations.

Transonic axial flow compressors with tandem rotor blades

Tandem rotor blades have the potential to improve the performance of transonic axial flow compressors by providing higher pressure ratio per stage which means a significant reduction in the total weight of the compressor as less stages are used to achieve a target pressure ratio. The pressure rise is obtained by flow diffusion through a shock wave on the front blade followed by flow turning on the aft blade. The performance of an optimum tandem rotor design with same inflow characteristics of the reference transonic rotor ‘NASA Rotor 37’ is investigated numerically. The simulations show that large improvements in the flow turning and diffusion are obtained without flow separation. The tandem design shows 17% increase in the total pressure ratio and 2% increase in the rotor adiabatic efficiency at the design point relative to the baseline rotor.

Analysis of hypersonic flows using Ideal Dissociating Gas (IDG) model

Ideal Dissociating Gas (IDG) model is adopted in a shock-tunnel problem to analyze the flow relaxation behind shocks, the non-equilibrium expansion in the nozzle and the flow behind an oblique shock.

Incompressible Navier-Stokes Solver and Iterative Methods

This study compares the convergence of different classical algorithms, such as Jacobi, Gauss-Seidel (GS), Alternating Direction Implicit (ADI), Successive Over-Relaxation (SOR) and its line variants, and V cycle Multi Grid, using a dveloped Fortran-90 code running on a single processor. More details can be found here and here.

Taylor-Green vortex at different convective schemes

Dissipative error of different convective schemes available in OpenFoam is examined in a DNS simulation of the classical Taylor-Green vortex problem. The plots compare the predicted evolution of the kinetic energy and enstrophy with the exact solution.

Decay of homogeneous isotropic turbulence in LES

The classical isotropic box turbulence problem is used to examine different LES subgrid-scale models. The plots show the predicted grid resolved kinetic energy (GS) and the total kinetic energy (GS+SGS) and the data by Comte-Bellot and Corrsin (CBC). The results show that the Dynamic K-equation model over predicts the SGS kinetic energy.

Education

University of Maryland
PhD in Mechanical Engineering (2018 - Present)
Advisor: Prof. Arnaud Trouvé

Cairo University
BSc and MSc in Aerospace Engineering (2011, 2015)

Publications

Peer-reviewed Journal Articles

1. R. Taher, M. M. Ahmed , Z. Haddad, C. Abid (2021). Poiseuille-Rayleigh-Bénard mixed convection flow in a channel: Heat transfer and fluid flow patterns. International Journal of Heat and Mass Transfer 180, 121745

2. M. M. Ahmed , A. Trouvé (2021). Large eddy simulation of the unstable flame structure and gas-to-liquid thermal feedback in a medium-scale methanol pool fire. Combustion and Flame 225, 237-254

3. M. M. Ahmed , A. Trouvé (2021). Simulations of the unsteady response of biomass burning particles exposed to oscillatory heat flux conditions. Fire Safety Journal 120, 103059.

4. M. M. Ahmed, M. I. Cheikh, J. Chen (2020). Boltzmann–Curtiss Description for Flows Under Translational Nonequilibrium. Journal of Fluids Engineering 142 (5).

5. M. Mohsen, F. M. Owis, A. A. Hashim (2017). The impact of tandem rotor blades on the performance of transonic axial compressors. Aerospace Science and Technology 67, 237-248.

Technical Conferences

1. M. M. Ahmed, J. M. Chen (2019). Verification and Validation of a Morphing Continuum Approach to Hypersonic Flow Simulations. In AIAA Scitech 2019 Forum (p. 0891).

Get In Touch

Feel free to contact me via email or phone

  • Address 3106 J.M. Patterson Building 4356 Stadium Dr. College Park, MD 20742-3031 (USA)

  • Phone +1-785-317-7060

  • Email mmahmed@umd.edu