Curtis HammanCenter for Turbulence Research
Department of Mechanical Engineering
Building 500, Room 500W
Stanford, CA 94305
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.
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 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 (88M) showing both subharmonic and fundamental transition to turbulence in a zero-pressure gradient flat plate boundary 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 was presented at the 2012 APS DFD Conference and was awarded the Milton Van Dyke Award in the Gallery of Fluid Motion.
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.
Detailed lectures notes on turbulence and computing with annotated references:
A few short articles on energy, physics and society that I wrote 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.
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.
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.
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.
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.