Radulescu Gasdynamics Laboratory

 Bowling - pizza party with Brian Maxwell's group: Samaneh, Omar, Ramki and our own: Kevin (and Gabrielle), Hongxia, Aliou, Farzane and Samir visiting us from KAUST.

Many great things realized in 2023, but the best were the successful thesis defences and graduation of three students: Ramki Murugesan defended his PhD thesis  Translational Non-Equilibrium Effects in Reactive Dynamics of Detonations Willstrong Rakotoarison defended his PhD thesis Chapman - Jouguet deflagrations and their transition to detonations Sarthak Lalchandani defended his Masters thesis  Modelling of Quasi Steady Detonations with Inert Confinement Congratulations guys.  You rock!

We just published an article suggesting this is the case, provided the gases are sufficientlty insensitive to not have transited prior. It is the flame's last resort for transition.   Our paper.

Congratulations to Hongxia Yang and her co-authors Aliou Sow and Wentian Wang for winning the best paper award for "Pressure evolution from head-on reflection of high-speed deflagration in hydrogen mixtures" presented at 10th International Conference on Hydrogen Safety (ICHS 2023), Québec City, September 19-21, 2023. Her paper can be found at here . The paper worked out in closed form the pressure signature of a hydrogen explosion on a wall. Well done!

In a cylindrical thin channel, ignite a flame at its center. The flame grows radially until it barely reaches the end wall. In the annulus separating the flame and the wall, there is a layer of non-reacted compressed gas. Then ignite a detonation in that layer. The spinning detonation has all the required ingredients to be relevant to RDE’s, being weakly confined by combustion products. The end wall can be made rough, porous etc to extend to more and more realistic scenarios.

Ok, we haven't updated this blog for a while, although much has happened. But happy to report on today's nice group hike on Wolf Trail. We celebrate the end of the year, and the graduation of Sarthak. Sad Aliou could not make.

Qiang Xiao defended successfully his PhD thesis on the Dynamics of Detonations with Lateral Strain, showing that, gauged with respect to the steady ZND model, detonability in enhanced with increasing instability.   Congratulations Qiang!   Thesis details coming soon...

I have now published a summary of the shock change-equations and their use for evolution equations of shocks.  The relations relate the shock speed, acceleration and curvature to the flow derivatives behind the shock, controlling the shock motion. Physics of Fluids 32, 056106 (2020); https://doi.org/10.1063/1.5140216 For example, the relation between the shock speed, acceleration and curvature with the rate of expansion of the gas behind the shock, essential in modelling of reactive gas dynamics along a particle path, is 1 ρ D ρ D t = 2 ( 3 M w 2 + 1 ) M ̇ w ( γ + 1 ) + c 0 κ ( M w 2 − 1 ) ( 2 + M w 2 ( γ − 1 ) ) M w ( M w 2 − 1 ) ( γ + 1 ) 2 . or for a strong shock: γ − 1 S w ρ 0 S ̇ w D ρ D t = 6 + 2 ( γ − 1 ) ( γ + 1 ) S w 2 κ S ̇ w

Combustion and Flame 215 (2020) 437–457 We now show that unstable detonations with long reaction zones, such as H2/O2/Ar, can be very well predicted by the ZND model, which neglects the cellular structure. This may come as a surprise, since the cellular structure takes very high amplitude perturbations, and the transverse waves are among the strongest of all detonations, i.e., they are reactive. The good agreement with ZND predictions is likely because of their very long reaction zones, as compared to the induction zones, as shown by their ZND profiles: The cellular structure locally changes the induction zone length along the front, as shown in the photos (Fig. 12) above, but leaves the much longer reaction zone non-affected.  Since the global divergence competes with the net rate of energy release in dictating the eigenvalue solution, and the latter is weakly affected by the cellular structure, then the ZND works well.

The Noble-Abel Stiffened Gas model e ( p , v ) = p + γ p ∞ γ − 1 v − b + q , is a simple extension of the perfect gas model to treat compressible flows in dense media, liquid and solids and obtain closed form analytical solutions. What started as an example in a gas dynamics course taught in Fall 2019, is now a complete description of the gasdynamics in closed form, now published in Physics of Fluids . Find here your favourite analytical expressions for Riemann variables, expansion and shock jump conditions, isentropes, detonations and deflagrations and the solution to the Riemann problem, as an example. Phys. Fluids  32 , 056101 (2020);  doi: 10.1063/1.5143428 Phys. Fluids  31 , 111702 (2019);...

It is the time of the year to reminisce over the highlights that made this year memorable. The first is of course the graduation of She-Ming Lau-Chapdelaine , with a PhD thesis on  Viscous Triple Shock Reflections Relevant to Detonation Waves, and Detonation Dynamics Predicted by the Fickett Model Shem impressed the members of his Jury (Ashwin Chinnayyaa, Bruno Savard and James McDonald), who nominated him for a thesis prize. Apparently, they weren't the only ones impressed, as he was invited to be the selected valedictorian speaker at the Convocation ceremony where students of the University of Ottawa received their diplomas. He has now left to take a job as Assistant Professor at the Royal Military College of Canada in Kingston. Congratulations Shem, we will miss you!

Happy holidays to all from our research group at the University of Ottawa! From left, Hongxia Yang (PhD), Farzane Zangene (PhD), Willstrong Rakotarison (PhD), me, Kevin Cheevers (MASc), Aliou Sow (postdoc), Qiuang Xiao (PhD) and Ramki Murugesan (PhD).

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.