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20 August 2019


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Dr. Charbel Habchi, Assistant Professor at the Department of Mechanical Engineering at Notre Dame University-Louaize (NDU), recently completed his Habilitation à Diriger des Recherches (HDR) – “accreditation to supervise research” – one of the highest qualifications available in academia. He discusses his research, the process of acquiring his HDR, and his next steps at NDU in this exclusive interview:

Could you tell us a bit about your position here at NDU?

I am an Assistant Professor of Mechanical Engineering at NDU since 2015. I teach courses in the Thermofluids track for undergraduate and graduate students. I am continuously supervising Ph.D. students in collaboration with colleagues from French and Canadian universities and I have obtained several research grants from National and International sources. Some of my research topics are related to fundamental sciences in heat and mass transfers other to applied mechanical engineering and physics. For instance, I participated in an international project on plasma characterization in tokamaks (nuclear fusion reactors) in collaboration with Dr. Ghassan Antar from AUB, and researchers from CEA (Commissariat of Atomic Energy in France). In this project we designed and implemented a probe into a tokamak to study plasma edge turbulence, and are currently working on another probe to characterize UV radiation. In parallel, and every summer semester, I work in a research lab abroad as visiting researcher in order to endure existing collaborations or to build new ones. The institutions I have visited are CalTech (USA), IMT Lille Douai, and Polytech Angers (France). At NDU, I always like to motivate talented students to participate in my research activities, and I have had great experiences with them. Three of these students are already pursuing their Ph.D. studies in reputable Universities.

Could you tell us about the process of acquiring an HDR? What goes into it, how long does it take, was it cumulative (multiple smaller research projects) or monographical (one large research project), and what was the defense like etc.?

The Doctoral School in which I was registered as HDR student is the Science for Engineers of Angers University. The first step in the HDR procedure is to choose an HDR director who must be a Professor or HDR from this same University. My HDR director was Professor Thierry Lemenand the head of BEMS Department at Polytech Angers. Then the candidate should submit his curriculum vitae and HDR proposal to the Doctoral School who forward it to two or three anonymous Professors. These Professors will review the application and send their comments back to the Doctoral School. The Academic Council of the doctoral school, which meets twice a year, validates the HDR application based on the reviewer’s comments. Once validated applicants may register at the University as HDR candidate and will have a student status during this period. The registration process is very similar to that of a Ph.D. thesis; however, the HDR student has 1 or 2 years to prepare for their defense. The HDR report should include a summary of the curriculum vitae, list of publications, teaching experience and proficiency and the core of the report is a summary of the research projects. This report must show an excellence in self-conducted and well-structured research topics including supervision of Ph.D. students and earning grants. At the moment of my HDR defense I had published 47 journal papers and 48 conference proceedings. I had co-supervised 4 Ph.D. students who successfully defended their theses between 2015 and 2019, and have 3 other Ph.D. theses in progress. In addition, I have obtained 9 research grants from National and International institutions.


The HDR could be either cumulative or monographical, though cumulative HDR are more common, especially in engineering. Once the report is finalized by the HDR student, a committee of at least 6 professors should be formed where 3 serve as readers and 3 as examiners. In my case, I had a total of 7 committee members from France, Belgium and Lebanon. The defense is very similar to that of a Ph.D. thesis where the HDR student presents their research for one hour followed by questions and recommendations. In addition to showing well systematized research background, the HDR candidate must include their perspectives on research and development, and the ideas they are going to work on in future.


What did your research cover?

Enhancement of transport phenomena in fluid flows is crucial in many applications ranging from house hold machines such as blenders and mixers to chemical reactors, heat exchangers in power plants and tokamaks in nuclear fusion devices. Transport phenomena in fluid flows could be classified as:

  • Mass transfer or mixing
  • Heat transfer or convection
  • Momentum transfer

My research was to design and analyze new intensified systems in which these transport phenomena are enhanced. deep and focused analysis on the flow structure and its correlation to transport phenomena by using numerical simulations (Computational fluid dynamics (CFD)) and experimental methods.
Under this framework, my research was classified in three main axes as discussed below (though these segments are fairly technical):

Axis 1: Passive enhancement of mixing and heat transfer
In this research axis I use passive methods to enhance mixing and heat transfer for different flow regimes ranging from creeping to turbulent flows. In the first method we use inserts such as helical tapes and screen mixers. We studied these geometries using chemical probe method and numerical simulations, respectively, in turbulent flow regimes. In the second method we use rigid vortex generators (VG) consisting of different shapes and geometries in order to enhance the heat and mass transfer processes in laminar and turbulent flow regimes. Some optimization studies were also performed on the rolling angle and angle of attack. These flow configurations are studied using both numerical and experimental techniques, namely particle image velocimetry (PIV) and laser Doppler velocimetry (LDV). Finally for creeping flows, we use the split and recombine (SAR) static mixers to enhance the mixing process for very low Reynolds numbers and highly viscous fluids. We proposed recently a new flow configuration named Double SAR in which double splitting and recombination is performed to further enhance the mixing process.

Axis 2: Dynamic enhancement of mixing and heat transfer
In axis 1, rigid VG are used to achieve good mixing and heat transfer. In axis 2 we propose a pioneer method consisting on the use of flexible VG which could oscillate under the periodic forces of the fluid due to vortex shedding caused by shear layer instabilities. First, I have developed a fluid-structure interaction (FSI) numerical solver under OpenFoam with strong two way coupling. This solver was successfully validated against results from the open literature. We then studied the heat transfer and mixing enhancement obtained by these flexible VG using numerical simulations in 2D and 3D configurations. The results show the great advantage of using such dynamic systems relative to the passive methods studied in axis 1. Moreover, we recently studied auto-adaptive VG which can auto-adapt their angle of attack depending on the flow rate. This method is beneficial for economizing pressure losses when low rates of heat transfers are required.

Axis 3: Magneto hydrodynamic flows
Axis 3 is devoted for magneto hydrodynamic flows (MHD) which is divided into two categories: the first is for electrolytes and the second for plasma. In the first part I study the effect of generating streamwise vortices and coherent structures by using electromagnetic forcing instead of VG (as in axes 1 and 2) in order to reduce the pressure losses caused by the presence of VG in the flow core. Moreover, I studied experimentally and numerically the flow dynamics in rotating flow instabilities and in two dimensional turbulence. These studies are fundamental to better understand the dynamics of atmospheric and ocean currents and polar vortices on Jupiter and Saturn for instance. In the second part, we aim to enhance the ablative or pulsed laser propulsion which is based on the principle of momentum transfer by laser to target. We enhance the plasma plume expansion by using an axial magnetic field which leads to enhance the thrust due to plasma confinement. For this end, we use numerical and experimental techniques to study the plasma plume expansion. Numerical simulations are based on computing the enthalpy equation and the Langmuir probe is used to perform the experimental studies.

What are your next steps after acquiring this academic qualification?

Now that I have acquired my HDR, I will be working on my progress at NDU by self-directing new Ph.D. students in collaboration with colleagues from different disciplines.
My vision is to establish a research lab starting with two primary experimental benchmarks which could be used to accomplish my goals for research and development. The two main experimental facilities I would like to build are the following:

  • An experimental benchmark which could be easily adapted to analyze the flow structure and heat transfer in different flow configurations such as heat exchangers and chemical reactors in which vortex generators, elastic flaps, static mixers, phase change material or nanoparticles could be implemented to enhance the energetic performances. This is a very interesting topic since it will serve first to validate the numerical studies and then to move to another level where the intensified flow configurations could be tested in real practical situations. This would be very beneficial for many petrochemical, pharmaceutical, and mechanical engineering industries that can test their systems using this benchmark.
  • An experimental facility to analyze and enhance the thermal comfort of neonates (newborns), especially preterm babies. This is partially possible now since at NDU we have a climatic chamber and I have obtained a donated Caleo infant incubator from Drager (German) and PrimeMedical (Lebanon) in addition to an AUF-CNRSL grant. We are currently building our thermal manikin representing a neonate of 32 gestational weeks. This project is not only crucial for the design and optimization of medical devices such as incubators, but also for textile industries where the clothing effect on thermal comfort could be studied.


What does having an HDR professor mean for NDU’s research, not to mention its students?

First of all, acquiring an HDR degree permits me to self-direct or co-direct Ph.D. theses. This will open the door for earning more grants to fund these Ph.D. projects which is beneficial for NDU students aiming to pursue higher educational degrees. These Ph.D. projects will be in collaboration with French institutions and universities in which the students will work in advanced research labs and meet well known scientists from different fields. Moreover, while preparing for my HDR, I was able to step back and organize my research activities into well-defined topics and axes and this will have a great impact on my future developments.

What would you say to NDU students who aspire to the same level of academic excellence?

I would like to encourage NDU students to enjoy learning, and believe that it is going to help them build their future. Let them feel how their education can help harness and refine their skills; how they can use their imagination to develop new methods and problem solving techniques. All accompanied by practical lab experience and internships, of course. Even after graduation, this learning journey will continue in different forms. I always advise my students to read scientific magazines and journals to broaden their vision of the world, and enrich their sense of scientific curiosity, which is beneficial for both their professional and for aspiring to higher academic excellence.


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