The Hrenya Research Group performs theoretical, simulation, and experimental work on a variety of complex flows involving soft matter.  The emphasis is on flows involving solid particles.  Current thrusts include cohesion, polydispersity, and stability of such systems.  Below is a collage of images from some recent work; further details and references can be found below.


Janine Galvin flies aboard NASA's KC-135 to study wetted particle-wall collisions in a microgravity environment.  To achieve microgravity, the plane flies in parabolic trajectories such that ~2g is experienced in the upward portion of the path (during which experiment is readied), and ~0g is experienced in the downward portion (during which experiment is conducted).  Due to the associated difficulties, 40 consecutive parabolas are flown per day, for 4 days in a row.  The plane is thus coined the "vomit comet".


In this work, the desktop toy known as Newton's cradle (left) is used as inspiration for an experimental setup (right, with Carly Donahue) designed to probe collisions between dry or wetted particles [2-4]. Results from the dry systems shed light on fundamental differences between 2-particle and multi-paritcle collisions, and their associated models.  In the wetted counterpart, particles are coated with a thin layer of viscous liquid prior to collision (center).  This "Stokes' cradle", so coined due to the dominance of Stokes flow in the liquid layer, initially led to all outcomes (fully separated, fully agglomerated and "reverse" Newton's cradle) except the traditional Newton's cradle outcome.  A theoretical analysis revealed surprising physics, which eventually led to the experimental discovery of the elusive Newton's cradle outcome.

Jia-Wei Chew performing riser experimens at PSRI, Chicago (left). Gustavo Joseph, Joe Kozlowski, and Christine Hrenya at the CU lab with a fluidized bed used for segregation experiments (center).  Christine Hrenya in the foreground of the Valmont Power Facility in Boulder, CO (right).
This DOE NETL-sponsored project is targeted at the developement, verification, and validation of first-principles models for polydisperse, gas-solid flows, with an ultimate goal of improving the efficiency of gasifiers with solids feedstocks (coal, biomass, etc.)  This work, which is a collaborative effort between Colorado, Iowa State, Princeton, and PSRI, involves a combination of theory, simulation, and experiments [5-10].  The resulting models are being incorporated into the open-source MFIX software, for availability to the greater research community.

Molecular dynamics (MD) simulations are a powerful tool for understanding particulate  flows, since particle properties (e.g., elasticity), flow conditions (e.g., shearing walls) and physical interactions (e.g., cohesion) are straightforward to integrate.  Moreover, because the paths of individual particles are traced in both space and time, MD is useful for simulating the behavior of a specific system or unit operation, and also for the testing and/or extraction of transport coefficients for continuum models (add ref).  We have used such simulations extensively in the study of polydisperse systems [11-19], cohesive systems [20-22], and Knudsen effects [23-24].


The surface of moons and planetary bodies are covered with a layer of fine particulates known as regolith.  When landing a spacecraft , the particles are ejected from this surface due to exhaust gases from the spacecraft.  Relative to Earth, the lack of gravity and air resistance cause the particles to travel at extremely high speeds and over long distances, thereby sandblasting any nearby equipment.  In fact, back-of-the-envelope calculations indicate that such an ejected particle cloud may orbit the Moon entirely before settling back onto the surface.  For this project performed in collaboration with NASA, we are using MD simulations to aid in the development of continuum models for particulate material with extreme size disparities.


Christine Hrenya (3rd from left) participates in a workshop at NASA Kennedy Space Center.



[1]    A. A. Kantak, J. E. Galvin, D. J. Wildemuth, R. H. Davis, Low-velocity collisions of particles with a dry or wet wall, Microgravity Sci. Technol. 17 (2005) 18-25.
[2]    C. M. Donahue, C. M. Hrenya, A. P. Zelinskaya, K. J. Nakagawa, Newton's cradle undone: Experiments and collision models for the normal collision of three solid spheres, Phys. Fluids 20 (2008) 11.
[3]    C. M. Donahue, C. M. Hrenya, R. H. Davis, K. J. Nakagawa, A. P. Zelinskaya, G. G. Joseph, Stokes’ cradle: Normal three-body collisions between wetted particles, Journal of Fluid Mechanics 650 (2010) 479-504.
[4]    C. M. Donahue, C. M. Hrenya, R. H. Davis, Stokes’s cradle: Newton’s cradle with liquid coating, Physical Review Letters, 105 (2010) art. no. 034501.
[5]    V. Garzó, J. W. Dufty, C. M. Hrenya, Enskog theory for polydisperse granular mixtures. I. Navier-stokes order transport, Phys. Rev. E 76 (2007) art. no. 031303.
[6]    V. Garzó, C. M. Hrenya, J. W. Dufty, Enskog theory for polydisperse granular mixtures. Ii. Sonine polynomial approximation, Phys. Rev. E 76 (2007) art. no. 031304.
[7]    R. B. Rice, C. M. Hrenya, Clustering in rapid granular flows of binary and continuous particle size distributions, Phys. Rev. E 81 9.
[8]    C. M. Hrenya, Extraction of transport coefficients from molecular dynamics simulations: A perspective, Industrial & Engineering Chemistry Research 49 (2010) 5304-5309.
[9]    C. M. Hrenya, Kinetic theory for granular materials: Polydispersity, Computational gas-solids flows and reacting systems: Theory, methods and practice, edition, IGI Global, Hershey, PA, 2010,
[10] J. W. Chew, J. Wolz, C. M. Hrenya, Axial segregation in bubbling gas-fluidized beds with gaussian and lognormal distributions of Geldart Group B particles, AIChE Journal (in press)
[11] R. Clelland, C. M. Hrenya, Simulations of a binary-sized mixture of inelastic grains in rapid shear flow, Phys. Rev. E 65 (2002) art. no. 031301.
[12] S. R. Dahl, R. Clelland, C. M. Hrenya, The effects of continuous size distributions on the rapid flow of inelastic particles, Phys. Fluids 14 (2002) 1972-1984.
[13] S. R. Dahl, C. M. Hrenya, V. Garzó, J. W. Dufty, Kinetic temperatures for a granular mixture, Phys. Rev. E 66 (2002) art. no. 041301.
[14] S. R. Dahl, R. Clelland, C. M. Hrenya, Three-dimensional, rapid shear flow of particles with continuous size distributions, Powder Tech. 138 (2003) 7-12.
[15] S. R. Dahl, C. M. Hrenya, Size segregation in rapid, granular flows with continuous size distributions, Phys. Fluids 16 (2004) 1-13.
[16] S. R. Dahl, C. M. Hrenya, Size segregation in gas-solid fluidized beds with continuous particle size distributions, Chem. Eng. Sci. 60 (2005) 6658-6673.
[17] H. Iddir, H. Arastoopour, C. M. Hrenya, Analysis of binary and ternary granular mixtures behavior using the kinetic theory approach, Powder Tech. 151 (2005) 117-125.
[18] J. E. Galvin, S. R. Dahl, C. M. Hrenya, On the role of non-equipartition in the dynamics of rapidly-flowing, granular mixtures, J. Fluid Mech. 528 (2005) 207-232.
[19] R. B. Rice, C. M. Hrenya, Characterization of clusters in rapid granular flows, Phys. Rev. E 79 (2009) art. no. 021304.
[20] M. W. Weber, D. K. Hoffman, C. M. Hrenya, Discrete-particle simulations of cohesive granular flow using a square-well potential, Granular Matter 6 (2004) 239-254.
[21] M. W. Weber, C. M. Hrenya, Square-well model for cohesion in fluidized beds, Chem. Eng. Sci. 61 (2006) 4511-4527.
[22] M. W. Weber, C. M. Hrenya, Computational study of pressure-drop hysteresis in fluidized beds, Powder Tech. 177 (2007) 170-184.
[23] J. E. Galvin, C. M. Hrenya, R. D. Wildman, On the role of the knudsen layer in rapid granular flows, J. Fluid Mech. 585 (2007) 73-92.
[24] C. M. Hrenya, J. E. Galvin, R. D. Wildman, Evidence of higher-order effects in thermally-driven, rapid granular flows, J. Fluid Mech. 598 (2008) 429-450.