Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
Browse
5 results
Search Results
Item Increasing Helicity towards Dynamo Action with Rough Boundary Spherical Couette Flows(2022) Rojas, Ruben; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamo action is the process through which a magnetic field is amplified and sustained by electrically conductive flows. Galaxies, stars and planets, all exhibit magnetic field amplification by their conductive constituents. For the Earth in particular, the magnetic field is generated due to flows of conductive material in its outer core. At the University of Maryland, our Three-meter diameter spherical Couette experiment uses liquid sodium between concentric spheres to mimic some of these dynamics, giving insight into these natural phenomena. Numerical studies of Finke and Tilgner (Phys. Rev. E, 86:016310, 2012) suggest a reduction in the threshold for dynamo action when a rough inner sphere was modeled by increasing the poloidal flows with respect to the zonal flows and hence increasing helicity. The baffles change the nature of the boundary layer from a shear dominated to a pressure dominated one, having effects on the angular momentum injection. We present results on a hydrodynamics model of 40-cm diameter spherical Couette flow filled with water, where torque and velocimetry measurements were performed to test the effects of different baffle configurations. The selected design was then installed in the 3-m experiment. In order to do that, the biggest liquid sodium draining operation in the history of the lab was executed. Twelve tons of liquid sodium were safely drained in a 2 hours operation. With the experiment assembled back and fully operational, we performed magnetic field amplification measurements as a function of the different experimental parameters including Reynolds and Rossby numbers. Thanks to recent studies in the hydrodynamic scale model, we can bring a better insight into these results. Torque limitations in the inner motor allowed us to inject only 4 times the available power; however, amplifications of more than 2 times the internal and external magnetic fields with respect to the no-baffle case was registered. These results, together with time-dependent analysis, suggest that a dynamo action is closer than before; showing the effect of the new baffles design in generating more efficient flows for magnetic field amplification. We are optimistic about new short-term measurement in new locations of the parameter space, and about the rich variety of unexplored dynamics that this novel experiment has the potential to reach. These setups constitute the first experimental explorations, in both hydrodynamics and magnetohydrodynamics, of rough boundary spherical Couette flows as laboratory candidates for successful Earth-like dynamo action.Item Inertial waves in a laboratory model of the Earth's core(2011) Triana, Santiago Andres; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A water-filled three-meter diameter spherical shell built as a model of the Earth's core shows evidence of precessionally forced flows and, when spinning the inner sphere differentially, inertial modes are excited. We identified the precessionally forced flow to be primarily the spin-over inertial mode, i.e., a uniform vorticity flow whose rotation axis is not aligned with the container's rotation axis. A systematic study of the spin-over mode is carried out, showing that the amplitude dependence on the Poincaré number is in qualitative agreement with Busse's laminar theory while its phase differs significantly, likely due to topographic effects. At high rotation rates free shear layers concentrating most of the kinetic energy of the spin-over mode have been observed. When spinning the inner sphere differentially, a total of 12 inertial modes have been identified, reproducing and extending previous experimental results. The inertial modes excited appear ordered according to their azimuthal drift speed as the Rossby number is varied.Item Rotating, hydromagnetic laboratory experiment modelling planetary cores(2009) Kelley, Douglas H.; Lathrop, Daniel P.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes a series of laboratory experiments motivated by planetary cores and the dynamo effect, the mechanism by which the flow of an electrically conductive fluid can give rise to a spontaneous magnetic field. Our experimental apparatus, meant to be a laboratory model of Earth's core, contains liquid sodium between an inner, solid sphere and an outer, spherical shell. The fluid is driven by the differential rotation of these two boundaries, each of which is connected to a motor. Applying an axial, DC magnetic field, we use a collection of Hall probes to measure the magnetic induction that results from interactions between the applied field and the flowing, conductive fluid. We have observed and identified inertial modes, which are bulk oscillations of the fluid restored by the Coriolis force. Over-reflection at a shear layer is one mechanism capable of exciting such modes, and we have developed predictions of both onset boundaries and mode selection from over-reflection theory which are consistent with our observations. Also, motivated by previous experimental devices that used ferromagnetic boundaries to achieve dynamo action, we have studied the effects of a soft iron (ferromagnetic) inner sphere on our apparatus, again finding inertial waves. We also find that all behaviors are more broadband and generally more nonlinear in the presence of a ferromagnetic boundary. Our results with a soft iron inner sphere have implications for other hydromagnetic experiments with ferromagnetic boundaries, and are appropriate for comparison to numerical simulations as well. From our observations we conclude that inertial modes almost certainly occur in planetary cores and will occur in future rotating experiments. In fact, the predominance of inertial modes in our experiments and in other recent work leads to a new paradigm for rotating turbulence, starkly different from turbulence theories based on assumptions of isotropy and homogeneity, starting instead with inertial modes, which are the linear eigenmodes of any rapidly rotating fluid.Item Seasonal and Interannual Ocean-Atmosphere Variability in the Tropical Atlantic: Observed Structure and Model Representation(2008-08-03) Chang, Ching-Yee; Carton, James A.; Nigam, Sumant; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tropical Atlantic is clamped by the South America (the Amazon) in the west and Africa (the Sahel) in the east. These two regions have been undergoing significant climate/environment changes for decades. In order to use climate models to study the impacts of these changes, climate models need to be able to well simulate the seasonal climate of the tropical Atlantic sector. The first part of this dissertation focuses on the representation of the seasonal cycle in the CCSM3 coupled atmosphere-ocean model. CCSM3 SST has a north-south dipole pattern of bias centered at the latitude of the thermal equator, resembling the observed pattern of interannual climate variability in boreal spring. Along the equator in boreal spring CCSM3 exhibits striking westerly winds at the surface, reminiscent of the pattern of climate variability in boreal summer. The westerly winds cause deepening of the eastern thermocline that keeps the east warm despite enhanced coastal upwelling. Next, a comparison is made with a simulation using historical SST to force the atmospheric model (CAM3) in order to deduce information about the origin of bias in CCSM3. The patterns of bias in CAM3 resemble that in CCSM3, indicating that the source of the bias in CCSM3 may be traced to difficulties in the atmospheric model. The next chapter presents a modeling study of the origin of the westerly wind bias CAM3 by using a steady-state linearized atmospheric model. The results indicate that underestimation of rainfall over the eastern Amazon region can lead to the westerly bias in equatorial Atlantic surface winds. They suggest that efforts to reduce coupled model biases, especially seasonal ones, must target continental biases, even in the deep Tropics where ocean-atmosphere interaction generally rules. The fourth chapter investigates the relationship between the two predominate modes of Tropical Atlantic interannual variability. The leading modes of Tropical Atlantic SST variability in boreal spring and summer are shown to be related, with the spring meridional mode leading into summer equatorial mode. The presence of a meridional mode with warm SST anomalies in the southern tropics in spring leads to a warm phase equatorial mode in summer, and vice-versa. This modal linkage occurs independently of climate variability in other ocean basins (e.g., ENSO). Atmospheric diabatic heating associated with a meridional shift of the Inter-Tropical Convergence Zone plays an important role in this relationship. The identification of this relationship enhances the prospects for prediction of boreal summer rainfall over the Guinea Coast of equatorial Africa.Item Liquid sodium model of Earth's outer core(2004-08-27) Shew, Woodrow Lee; Lathrop, Daniel P.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Convective motions in Earth's outer core are responsible for the generation of the geomagnetic field. We present liquid sodium convection experiments in a spherical vessel, designed to model the convective state of Earth's outer core. Heat transfer, zonal fluid velocities, and properties of temperature fluctuations were measured for different rotation rates and temperature drops across the convecting sodium. The small scale fluid motion was highly turbulent, despite the fact that less than half of the total heat transfer was due to convection. Retrograde zonal velocities were measured at speeds up to 0.02 times the tangential speed of the outer wall of the vessel. Power spectra of temperature fluctuations indicate a well defined knee characterizing the convective energy input. This frequency is proportional to the ballistic velocity estimate. In the context of Earth's outer core, our observations imply a thermal Rayleigh number Ra=10^22, a convective velocity near 10^-5 m/s, and length and time scales of convective motions of 100 m and 2 days.