Curtis Hamman Center for Turbulence Research Department of Mechanical Engineering Stanford University Building 500, Room 500F Stanford, CA 94305 Email: cwhamman@stanford.edu Website:   http://www.stanford.edu/~cwhamman/

Current Research

Turbulent flows with variations of density arise in many different areas of science and technology. Combustion of the fuel in a car engine, urban air pollution/smog, convection in stellar interiors, and a draft of cold air from a window are but a few examples. My research seeks to improve our physical understanding of and computational algorithms for turbulent mixing in accelerated variable-density shear flows. Here are a few samples of my current research.

• Effects of Shear on Unstably Stratified Convection

When a fluid is heated from below and cooled from above, a regular pattern of convection cells may develop as the hot fluid rises and cool fluid falls. When such convective motions grow from random initial conditions in an infinite horizontal layer, patches of convection rolls will develop with no preferred orientation. But, when subject to a uniform mean shear flow, the rolls readily align themselves in the direction of the mean wind. Too much shear, however, can break-up these convection rolls leading to decreased heat transfer as the thermal plumes connecting the hot lower wall and cold upper wall are torn apart. Part of my thesis research is devoted to understanding the mechanisms that lead to thermal plume breakup and investigating methods to control these large-scale organized convective motions in turbulent shear flow.

Rayleigh-Bénard Convection Without Shear	Rayleigh-Bénard Convection With Shear



• Thermal Convection in Three-Dimensional Turbulent Shear Flow

Thermal convection in the presence of a mean shear flow occurs in many geophysical and engineering applications, e.g. drifting clouds in the atmospheric boundary layer and coolant in automobile radiators. The mean flow is often steady and largely directed in a single direction. But, whenever a force is applied to a boundary layer in a direction perpendicular to the streamwise direction, a cross-flow may develop leading to three-dimensional mean-flow skewing. Coolant turning along curved serpentine passages in automobile radiators or cloud rolls turning in response to mountains ranges or synoptic-scale pressure gradients are examples of such three-dimensional turbulent boundary layers. When a rapid shift in the mean wind direction occurs, the vertical transport of momentum and heat flux is sharply reduced compared to the equilibrium value. Eventually, the turbulence and heat transfer levels recover as the flow adjusts to the new wind direction. Identifying the mechanisms that suppress turbulence and heat transfer in response to mean flow skewing is another part of my thesis research.

View my poster on the Stability of Impulsively Started Vortex Rolls to learn more.

Download this movie [high-res (103M), low-res (15M)] showing subharmonic transition to turbulence in a zero-pressure gradient flat plate boundary and this movie [high-res (122M), low-res (44M)] showing fundamental transition to turbulence that Taraneh Sayadi, Parviz Moin and I made recently. This is a classic example of a two-dimensional turbulent shear flow since the mean flow velocity and mean vorticity are everywhere orthogonal, i.e. the mean flow statistics are only a function of the streamwise position and distance from the wall. This work is to be presented at the upcoming 2011 APS DFD Conference.

• Sound Propagation in Boiling Turbulent Shear Flows

Vapor bubbles liberated during local boiling not only generate large sound pressures as they pulsate, grow and collapse, but also display a distinct tonal dependence characteristic of the local multiphase environment, much like the familiar fugue of water boiling on a hot stove or the sound of steak sizzling on a grill. Even at relatively low bubble void fractions, the strong variations in mixture sound speed and density are known to strongly affect the propagation of sound in heat exchanger ducts. The propagation of sound through a wire-wrapped nuclear fuel bundle carrying an inhomogeneous multiphase mixture was examined. Pressure excitations applied at one end of the bundle propagate through this complex geometry and interact with the bubbly flow. The bubbly mixture was found to generate purely flow-induced waveguides owing to variations in sound speed and geometry that act to support sound propagation along the fuel bundle in low impedance regions with amplified intensity and frequency relative to the original forcing.

View my poster on the Acoustic Detection of Boiling in Nuclear Reactors to learn more.

Writings on Energy, Physics and Society

Recently, I wrote a few short articles on energy, physics and society that are posted on-line at the Stanford PH240 and PH241 websites. Below I've posted links to those articles along with a brief excerpt. I enjoyed writing and learning about these topics and hope to post more writing samples in the future.

• Energy for Plastic

By multiplying the net heat of combustion for plastic (in J/kg) by the mass of plastic produced (in kg/year), upper and lower estimates for the total hydrocarbon feedstock energy (in J/year) used in the plastics industry were found. When added to the reported estimates for the non-feedstock energy used in the production of plastics, estimates of the energy for plastic were found and compared with the global and U.S. energy consumption. World-wide between 2.5% and 3.6% of oil and natural gas consumption was used as feedstock for plastics in 2008. The amount of hydrocarbon feedstock energy diverted to chemically produce plastic materials in the U.S. was between 1.1 x 10¹⁸ J and 1.8 x 10¹⁸ J in 2008, which is between 3.1% and 5.1% of the combustible energy content of U.S. oil and gas production. The energy-intensive processes used to synthesize key petrochemicals, produce plastics, and manufacture plastic products consumed between 1.4 x 10¹⁸ J and 2.2 x 10¹⁸ J in 2008. Total U.S. primary energy consumption was about 1.0 x 10²⁰ J in 2008. Based on these estimates, the energy for plastic, including both hydrocarbon feedstock energy, energy used to refine feedstocks into base plastics, and the energy to manufacture plastic products, was between 2.5% and 4.0% of the total U.S. primary energy consumption in 2008.

• Space Heating

The energy consumed in homes is directed to several end-use functions, including space heating, air-conditioning, water heating, refrigerators and appliances. Space heating using electricity, natural gas, fuel oil, kerosene, or liquid petroleum gases accounted for about 4.54 × 10¹⁸ J in 2005. This figure does not include wood fuel used for space heating, which itself amounted to 4.54 × 10¹⁷ J. Then, if all wood fuel was used for space heating, then about 4.99 × 10¹⁸ J were used for space heating. This estimate does not include energy derived directly from solar space heating, coal, nuclear or other sources except that which is included in the metered electricity delivered to U.S. households. Then about 43% of home energy consumption and 4.7% total U.S. primary energy consumption was due to space heating in 2005.

• Virtual Nuclear Testing

On the road to disarmament, the U.S. nuclear weapons labs face an identity crisis. They are tasked with maintaining the technical competence needed to support the nation's nuclear deterrent even as the size of that stockpile tends toward zero. Some argue that this limit is achievable by investing in surveillance and remanufacture capabilities preceded by only one last-time modification of the physics package. Others argue that only those skills needed to remanufacture defective weapons according to their original specifications should be retained, akin to curatorship of antique nuclear weapons, while others are left to atrophy. Stockpile Stewardship, on the other hand, has sought to sustain the nuclear enterprise, even as the physical weapons are dismantled, by the infusion of scientifically challenging problems and experimental facilities to support a virtual testing ground. The limits of computation are, however, real. After more than a decade under stewardship, some now claim that the only way to maintain the long-term safety and reliability of the stockpile is to end the moratorium and resume underground nuclear testing and weapon design. Institutional patronage is apparently at odds with public policy.

• Technetium of the Stars

Nuclear reactions inside stars synthesized the majority of the chemical elements, including carbon, oxygen and other star-stuff found on Earth. Optical telescopes are unable to peer into stellar cores to observe nucleosynthesis directly. Instead, measurement of star light emitted from the stellar surface provides an indirect observational technique to infer the conditions in the stellar interior. Models of stellar evolution, convection, and nuclear reaction networks are often required to evaluate such processes of stellar nucleosynthesis. To illustrate these mechanisms, a brief history of the first observations of the unstable element technetium in stars are reviewed. Estimates of the relevant stellar and nuclear burning timescales are presented that suggest such stars continuously synthesize technetium via slow neutron capture and beta decay.

Education

• M.S. Mechanical Engineering. Stanford University, Stanford, CA, June 2009. Advisor: Professor Parviz Moin.
• B.S. Mechanical Engineering, Physics minor, with Honors and Research Distinction. University of Utah, Salt Lake City, UT, May 2007. Advisors: Professors Robert M. Kirby and Joseph C. Klewicki.

Refereed Journal Publications

1. C.W. Hamman, J.C. Klewicki and R.M. Kirby, ''On the Lamb vector divergence in Navier-Stokes Flows,'' Journal of Fluid Mechanics, Vol. 610 pp. 261-284, 2008.
2. C.W. Hamman, R.M. Kirby and M. Berzins, ''Parallelization and scalability of a spectral element channel flow solver for incompressible Navier-Stokes equations,'' Concurrency and Computation: Practice and Experience, Vol. 19:10, pp. 1403-1422, 2007.

Conference Proceedings

1. C.W. Hamman and P. Moin, Stability of impulsively started vortex rolls, DOE CSGF Annual Fellow's Conference, July 2011.
2. C.W. Hamman and P. Moin, Overall Winner DOE CSGF Fellows Poster Contest for Acoustic Detection of Boiling in Nuclear Reactors, June 2010.
3. J. Christophe, S. Moreau, C.W. Hamman, J.A.S. Witteveen and G. Iaccarino, Uncertainty quantification for trailing-edge noise of a Controlled-Diffusion airfoil, In the Proceedings of the 2010 CTR Summer Program.
4. C.W. Hamman and P. Moin, Acoustic Detection of Boiling in Nuclear Reactors, Received Best Poster Award, Stanford Thermal-Fluids Sciences Affiliates Conference, February 2010.
5. C.W. Hamman and P. Moin, Effects of Shear on Heat Transfer in Nuclear Reactors, Stanford Thermal-Fluids Sciences Affiliates Conference, February 2010.
6. C.W. Hamman and P. Moin, Effects of Shear on Unstably Stratified Convection, 62nd Annual Meeting of the American Physical Society, Division of Fluid Dynamics, Minneapolis, MN, November 2009.
7. C.W. Hamman and P. Moin, The Sound of Boiling, DOE CSGF Annual Fellow's Conference, July 2009.
8. C.W. Hamman, Atomic Power for Petascale Computing, Received Best Poster Award (1st Place), Stanford High Performance Computing Conference, August 2008.
9. C.W. Hamman, Towards Predictive Simulation for Nuclear Energy Applications, Stanford High Performance Computing Conference, August 2008.
10. C.W. Hamman and P. Moin, Fundamental Research Needs for Nuclear Energy Simulation, Stanford Thermal-Fluids Sciences Affiliates Conference, February 2008.
11. C.W. Hamman, R.M. Kirby and J.C. Klewicki, On the Lamb vector divergence as a momentum field diagnostic employed in turbulent channel flow, 59th Annual Meeting of the American Physical Society, Division of Fluid Dynamics, Tampa Bay, FL, November 2006. [.pdf | .ps]

Invited Lectures

1. C.W. Hamman, Predictive Parallel Performance Models for Petascale Platforms, Stanford High Performance Computing Seminar Series, May 2009.

Center for Turbulence Research Department of Mechanical Engineering Stanford University

Email:   cwhamman@stanford.edu