Radulescu Gasdynamics Laboratory

Just came back from a week trip to Taiwan, where I was invited by Professor Shouyin (Ian) Yang of the National Formosa University and Professor Ming-Hsun Wu of the National Cheng Kung University. with Shouyin (Ian) Yang and Ming-Hsun Wu at NCKU Had and amazing time, gave two talks.  The first at NCKU on Detonation dynamics and stability: Insights from Fickett’s toy model . and the second as a keynote lecture at the 29th National Conference on Combustion and Energy, Taiwan:   A detonation paradox: The influence of turbulent diffusive processes in controlling the burning rate in detonations. Lab tours at NCKU: and and took in some of the architecture in Tainan. Thanks again to Ian and Ming-Hsun for this wonderful opportunity.

Our recent paper was just published in Combustion and Flame .  The idea is simple.  At the molecular collision level, chemical reactions liberating energy give rise to energetic particles. These highly energetic particles may trigger other reactions locally, before their translational kinetic energy can be equilibrated with the bulk.  Such hotspots have been theorized nearly 60 years ago by Prigogine and co-workers.  Our molecular dynamic simulations confirm that large modifications to the reaction rate can be anticipated for highly exothermic reactions.

Maxime La Flèche has now graduated with a Masters thesis on Shock-Flame interactions in a Hele-Shaw cell.  Awsome thesis from a great student, who now went to work for Pratt and Whitney.  Some highlights from his thesis : "Dynamics of Blast Wave and Cellular H2-Air Flame Interaction in a Hele-Shaw Cell" Experiments of flames in a Hele-Shaw cells permit to visualize the flame dynamics in nearly 2D geometry.  A shock can be generated at the center of the cell after the flame ignition via capacitor discharge or via thin shock tube discharge.  This permits to study how the flame deforms from the shock acceleration (Richtmyer Meshkov instability) and the subsequent interaction with the expansion wave following the lead shock (Rayleigh-Taylor instability). The problem is also conducive to numerical reconstruction for probing the dynamics of the flame reversal and further deformation.

Discoveries are best made when trying to solve paradoxes. And there is such a paradox in detonation theory - which has stalled progress in the last 20 years. Current simulations of inviscid detonations predict the incorrect trends for detonation limits.  Simply put, simulations cannot predict experiments unless one uses the poor numerical resolution available to researchers of the 1980-1990's. The evidence points to the lack of dissipation in Euler models, effects which would otherwise account for burning of large amounts of gas on time scales relevant to the detonation propagation. Of course, other culprits may participate: incorrect knowledge of chemical rates and relaxation rates behind the non-steady shocks of detonations, three-dimensional effects absent in 2D simulations, etc... The paradox is ongoing. My paper summarizes this paradox. I now bitterly remember having attempted to publish the results of simulations showing this effect back in 2006 for the combust...

I am extremely enthusiastic to report that Brian Maxwell, my first PhD student has now started as an assistant professor at Case Western Reserve University. 

Our paper on detonation dynamics with constant lateral strain rate is now published .  This is the first time, in my opinion, where a meaningful comparison with the first principles predictions of the steady ZND model can be made.  Cellular detonations are more robust that laminar ones:  the turbulent cellular structure helps in their propagation. More to come from Qiang Xiao on this subject.

dit Cabane à Sucre, bien sûr! et qui dit Cabane à Sucre, dit Oreilles de Criss' et un copieux repas en bonne compagnie du DRDL. et une petite promenade santé en campagne.

Brian Maxwell just published  his results of DDT modelling with the Linear Eddy Model, where compressible dynamics of reactive gases and turbulence are explicitly taken into account.  Nicely done! Here, a detonation passes through a perforated plate, generating a turbulent flame headed by a shock. For a while, the reaction front propagates as a CJ deflagration while getting ready for transition.  Pressure fronts build up internally, favouring local auto-ignition events within the turbulent structure. We now have a tool to predict DDT in turbulent flames where compressible gas dynamics play an important role (always!).

July 2017, She-Ming Lau-Chapdelaine and I attended the International Symposium on Shock Waves in Nagoya Japan.   Shem presented his paper on shock reflections in low-gamma gases, which earned him a best paper award!  We showed how the slip line jets forward with decreasing gama and increase in shock Mach number.  Clearly relevant for Mach reflections in detonations.

From left, Maxime La Fleche, Nick Sirmas, She-Ming Lau-Chapdelaine, me, Willstrong Rokotoarison, Qiang Xiao, Wentian Wang, Brian Maxwell and Ramki Murugesan Fun times in and out of the technical sessions: Julian and John Lee with Jenny Chao mike drop? awsome Edyta the karaoke-queen Road trip on the way back, pit stop in Magog, for 70% done Steak-Frites (WW)

Today marks the final submission of Nick's seminal PhD thesis on shock instability in gases with non-elastic collisions.  cheers to the Radulescu-Sirmas (Sirmas-Radulescu?) instability! very proud of this guy! The shock instability captured via molecular dynamics in gases with inelastic collisions (bottom) can only be recovered at the continuum scale if molecular fluctuations are included in a Navier Stokes (middle).  Absence of viscous terms (top) cannot dissipated the fluctuations, and unphysical mixing ensues.   

Thanks to Scott Jackson and Maha Manoubi, all the past proceedings of the International Colloquium on the Dynamics of Explosions and Reactive Systems since 1999 are now available online .

The dynamics of cellular detonations are simpler to illustrate using Lagrangian trackers. The video was obtained from a simulation using James Quirk's AMRITA by Bijan Borzou.

Earlier in the week, some of us were in Waterloo, ON, at the Canadian Section of the Combustion Institute yearly technical meeting , where we presented progress in our reactive gasdynamics work. Wentian, Bijan, Nick, Matei, Brian and Mohamed Thermal ignition revisited with molecular dynamics: role of fluctuations in activated collisions N. Sirmas & M.I. Radulescu Nick Sirmas, in true form, presented his study on the role of molecular fluctuations in self-ignition in gases. He tackled thermal ignition problem with molecular dynamics. He found that non-equilibrium effects during thermal runaway occurs via hot-spots.  These are spontaneously generated by molecular fluctuations, and preferred in systems with high activation energies and heat release.  Reactive collisions are preferred in the neighborhood of previous reactive collisions, as common sense dictates.  The result is that at sufficiently low temperatures, hot spots are responsible for drastic r...