Drazen Fabris

Observed 1970-2005 cooling of summer daytime temperatures in coastal California.

Abstract This study evaluated 1950–2005 summer [June–August (JJA)] mean monthly air temperatures for two California air basins: the South Coast Air Basin (SoCAB) and the San Francisco Bay Area (SFBA). The study focuses on the more rapid post-1970 warming period, and its daily minima temperature Tmin and maxima temperature Tmax values were used to produce average monthly values and spatial distributions of trends for each air basin. Additional analyses included concurrent SSTs, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) sea level coastal pressure gradients, and GCM-downscaled average temperature Tave values. Results for all 253 California National Weather Service (NWS) Cooperative Observer Program (COOP) sites together showed increased Tave values (0.23°C decade−1); asymmetric warming, as Tmin values increase faster than Tmax values (0.27° versus 0.04°C decade−1) and thus decreased daily temperature range (DTR) values (0.15°C decade−1). The spatial distribution of...

MODEL-BASED CONTROL OF CAVITY OSCILLATIONS, PART II: SYSTEM IDENTIFICATION AND ANALYSIS

Experiments using active control to reduce oscillations in the flow past a rectangular cavity have uncovered surprising phenomena: in the controlled system, often new frequencies of oscillation appear, and often the main frequency of oscillation is split into two sideband frequencies. The goal of this paper is to explain these effects using physics-based models, and to use these ideas to guide control design. We present a linear model for the cavity flow, based on the physical mechanisms of the familiar Rossiter model. Experimental data indicates that under many operating conditions, the oscillations are not self-sustained, but in fact are caused by amplification of external disturbances. We present some experimental results demonstrating the peak-splitting phenomena mentioned above, use the physics-based model to study the phenomena, and discuss fundamental performance limitations which limit the achievable performance of any control scheme.

Length dependence of current-induced breakdown in carbon nanofiber interconnects

Current-induced breakdown is investigated for carbon nanofibers (CNF) for potential interconnect applications. The measured maximum current density in the suspended CNF is inversely proportional to the nanofiber length and is independent of diameter. This relationship can be described with a heat transport model that takes into account Joule heating and heat diffusion along the CNF, assuming that breakdown occurs when and where the temperature reaches a threshold or critical value.

Electrothermal Analysis of Breakdown in Carbon Nanofiber Interconnects

To elucidate the observed current capacity behavior, a model is developed that takes into account heat transport through the entire carbon nanofiber interconnect test structure and breakdown location. The model also includes variations in contact location with the support material. The resulting predicted heat dissipation and current capacity are completely consistent with experimental data.

Closed-Loop Control in Cavities with Unsteady Bleed Forcing

An experimental investigation aimed at controlling multiple acoustic modes in a cavity has been conducted. Closed-loop control is used to suppress and/or enhance individual resonant modes in the cavity at Mach numbers ranging from 0.2 to 0.55. The energy distribution among the resonant acoustic modes is shown to depend on cavity depth. A connection between the various Rossiter modes was established using narrow bandpass filters to control specific resonant modes. When the second Rossiter mode is suppressed, then the first and third modes are enhanced and vice versa. Furthermore, suppressing the second Rossiter mode caused the fourth mode to be suppressed also. Enhanced energy in a mode indicates that it exists for a greater percentage of the time of the data record. Multiple acoustic modes could be suppressed simultaneously with the appropriate configuration of the feedback control system. Introduction Acoustic resonances in aircraft weapons bay cavities can reach sound pressure levels of 170 dB at Mach numbers as low as 0.5. Such large pressure fluctuations result in early structural failures within the aircraft and stores, create high drag on the aircraft, and generate unpredictable loads on the stores during separation. Systematic investigations of the nature of the acoustic resonances within aircraft cavities have been conducted since the 1950’s’Z2. Nomenclature C, = Pressure coefficient (p-p,)/9 D = Cavity depth f = Frequency L = Cavity length M = Freestream Mach number PI = Wind tunnel static pressure 9 = Dynamic pressure % pU2 St = Strouhal number, fL/U U = Freestream speed W = Cavity width In practical applications the high sound pressure levels are commonly reduced using passive devices such as serrated spoilers at the upstream edge of the cavity and ramps at the downstream wall. However, the performance of passive sound suppression devices are known to be sensitive to Mach numbe? limiting the range of their effectiveness, and enhancing certain acoustic tones at off-design conditions. Ideally the cavity control system should adapt to changing flight conditions to maintain a minimum sound pressure level over the entire flight envelope. The potential for active control of weapons bay cavities was recognized by the U.S. Air Force several years ago and led to the formation of the Active Robust ConTrol of Internal Cavities (ARCTIC) group. This consortium of Department of Defense laboratories, industries, and universities was formed in 1996 as the ’ Professor, Associate Fellow AIAA T Currently, Assistant Professor, Mechanical Engineering Department, Santa Clara University, Santa Clara CA. Copyright

Application of Carbon Nanotubes to Thermal Interface Materials

Improvements in thermal interface materials (TIMs) can enhance heat transfer in electronics packages and reduce high temperatures. TIMs are generally composed of highly conductive particle fillers and a matrix that allows for good surface wetting and compliance of the material during application. Two types of TIMs are tested based on the addition of carbon nanotubes (CNTs): one mixed with a commercial TIM product and the other only CNTs and silicone oil. The materials are tested using an in-house apparatus that allows for the simultaneous measurement of temperature, pressure, heat flux, and TIM thickness. Results show that addition of large quantities of CNTs degrades the performance of the commercial TIM, while the CNT-silicone oil mixtures showed improved performance at high pressures. Thickness and pressure measurements indicate that the CNT-thermal grease mixtures are more compliant, with a small increase in bulk thermal conductivity over the range of tested pressures. [DOI: 10.1115/1.4003864]

Tunneling between carbon nanofiber and gold electrodes

In a carbon nanofiber (CNF)-metal system such as a bridge between two gold electrodes, passing high current (current stressing) reduces the total resistance of the system (CNF resistance RCNF plus contact resistance Rc) by orders of magnitude. The role of current stressing is modeled as a reduction in the interfacial tunneling gap with transport characteristics attributed to tunneling between Au and CNF. The model predicts a reduction in Rc and gradual disappearance of the nonlinearity in the current-voltage (I-V) characteristics as Rc decreases. These results are consistent with measured I-V behavior.

Impacts of Climate Change in Degree Days and Energy Demand in Coastal California

An analysis of 1970―2005 observed summer daily maximum and minimum temperatures in two California air basins showed concurrent daytime coastal cooling and inland warming. To study the impacts of these results on energy consumption, summer cooling degree day (CDD) and winter heating degree day (HDD) trends were analyzed via these temperatures. The 2 m level air temperatures consisted of data from 159 locations in California, each with daily minimum and maximum values. Primary data sources included Cooperative Weather Station Network sites, first order National Weather Service stations, and military weather stations. An analysis of the CDD and HDD data has been undertaken for California, in general, and the San Francisco Bay Area and South Coast Air Basin, in particular, as the source of data for an analysis of energy-demand trends. Regional climate fluctuations have considerable effects on surface temperatures, which in turn affect CDD and HDD values. An asymmetric increase in summer CDD values between coastal and inland regions of California was found during the last 35 years, while winter HDD values showed decreases in most of California. In general, coastal areas experienced decreases of CDD, while inland regions experienced increases. The summer asymmetric increases in CDD is attributed to intensified sea breeze flows, which suggests increases in cold marine air intrusions over coastal land masses due to an increased regional sea breeze potential, which ventilates coastal areas, helps reduce maximum temperatures, and contributes to CDD decreases. An analysis of energy demands in the two air basins supports these climatological findings. .

Temperature Modeling for Carbon Nanofiber Breakdown

Carbon nanofibers (CNF) are proposed for electrical interconnect applications because of their relatively large current capacity and ability to form well-aligned one-dimensional structures. It is experimentally determined that nanofibers that are suspended between two electrodes breakdown at or near the nanofiber center. Based on published property values a simple model is used to calculate the temperature and quantify the effect of heat generation at the CNF/electrode interface on the nanofiber temperature. The model has the capability to separately account for the substrate temperature and the temperature at the CNF/electrode junction. It is determined that the CNF reaches a temperature at which carbon oxidation is likely to occur.Copyright

Thermoreflectance Measurement of Temperature and Thermal Resistance of Thin Film Gold

To improve performance and reliability of integrated circuits, accurate knowledge of thermal transport properties must be possessed. In particular, reduced dimensions increase boundary scattering and the significance of thermal contact resistance. A thermoreflectance measurement can be used with a valid heat transport model to experimentally quantify the contact thermal resistance of thin film interconnects. In the current work, a quasi-steady state thermoreflectance measurement is used to determine the temperature distribution of a thin film gold interconnect (100 nm) undergoing Joule heating. By comparing the data to a heat transport model accounting for thermal diffusion, dissipation, and Joule heating, a measure of the thermal dissipation or overall thermal resistance of unit area is obtained. The gold film to substrate overall thermal resistance of unit area beneath the wide lead (10 μm) and narrow line (1 μm) of the interconnect are 1.64 × 10−6 m2 K/W and 5.94 × 10−6 m2 K/W, respectively. The thermal resistance of unit area measurements is comparable with published results based on a pump-probe thermoreflectance measurement.