Decades of research have fostered a greater understanding of the environmental controls that drive tropical cyclone (TC) intensity change, yet the community has achieved only small improvements in intensity forecasting. Numerous environmental factors impact TC intensity, such as vertical wind shear and sea surface temperatures (SSTs), but little research has focused on establishing if SST change under the TC, or SST gradients, influence these intensity changes. This study investigated three methods to compute SST gradients. The first method calculated the SST change within fixed distances along the track. In the second and third methods, the SST was calculated over the distance traversed by the TC in two separate six-hour periods. By examining 455 24-hour weakening episodes in the eastern North Pacific, this study revealed that the first SST gradient method explained the highest 24-hour weakening variance for TCs located within SSTs at or lower than 26.5 degrees C.

京都大学 / 新制・課程博士 / 博士(地球環境学) / 甲第24766号 / 地環博第238号 / 新制||地環||46(附属図書館) / 京都大学大学院地球環境学舎地球環境学専攻 / (主査)教授 小林 広英, 准教授 落合 知帆, 准教授 深町 加津枝 / 学位規則第4条第1項該当 / Doctor of Global Environmental Studies / Kyoto University / DFAM

While the standard wind product (L2B) available operationally in near-real time from SeaWinds on QuikSCAT is only 25 km in resolution, QuikSCAT data can be enhanced to yield a 2.5 km ultra-high resolution (UHR) product. The latter can be used to help estimate Tropical Cyclone (TC) characteristics such as TC eye center and wind radii. Two studies are conducted in this thesis, in which QuikSCAT UHR wind product's effectiveness in estimating these TC characteristics is evaluated. First, a comparison is made between the analyst's choice of eye location based on UHR images and interpolated best-track position. In this analysis, the UHR images are divided into two categories, based on the analyst's confidence level of finding the eye center location. In each category, statistical error quantities are computed. UHR images within the high confidence category can provide, for a given year and basin, mean error distance as small as 19 km with a 10 km standard deviation. Second, a visual comparison of QuikSCAT's performance in estimating wind radii is made. QuikSCAT's performance is gauged against H*wind dataset and the Extended Best-Track (EBT) dataset. Results show that QuikSCAT UHR data yields a correct 34-kt wind radius most of the time regardless of the TC category when compared to both H*wind and EBT, whereas the 50- and 64-kt wind radii visual estimates do not always agree with H*wind and EBT. A more sophisticated method is also implemented to automatically estimate wind radii based on a model fit to QuikSCAT data. Results from this method are compared with EBT wind radii. Wind radii obtained from QuikSCAT model fit are generally highly correlated with EBT estimated wind radii. These two studies show that QuikSCAT UHR wind products are helpful in estimating TC eye location and wind radii, thus improving TC forecasting and analysis.

Helicopter turboshaft engines may ingest large amounts of foreign particles (most commonly sand/dust), which can cause significant compressor blade damage and even engine failure. In many helicopters, this issue is mitigated by separating the particles from the intake airstream. An effective device for engine air-particle separation is the vortex tube separator (VTS), which uses centrifugal forces in a vortical flow to radially filter foreign particles from a duct with an annular exit. Dozens or hundreds of these devices are linked together on a shared manifold known as a VTS array. There is a distinct lack of scientific literature regarding these arrays, which likely feature significantly more complex flowfields than singular VTSs due to aerodynamic interactions between the devices. The research presented in this dissertation identifies and explains flow features unique to arrays by means of an experimental investigation downstream of various VTS configurations in a wind tunnel. Mean PIV flowfields reveal that the VTS array rapidly generates a strong central recirculation zone while a single VTS does not, implying the existence of axial flow gradients within associated separators that could affect filtration efficiency. The key factor here is the global swirl intensity, which is increased in array flows due to high angular momentum contributions from separators that are radially distant from the duct center. A preliminary momentum integral model is constructed to predict the onset of recirculation in VTS flows. Analysis is then extended to the unsteady flowfield, where it is shown that VTS-generated turbulence contains only low levels of anisotropy. Spectral proper orthogonal decomposition is conducted on the array flow; it reveals the existence of low-frequency harmonic behavior composed of back-and-forth pumping motions downstream of the central VTS. Additionally, a unique precession motion is found in the same region at a slightly higher frequency. Similar precessing vortex cores have been shown to reduce separation efficiency in other cyclone separators. Both of these coherent structures may be associated with the central recirculation zone and may interfere with VTS array filtration given their timescales relative to potential particle relaxation timescales. This dissertation opens the door for future experimental and computational studies of fluid and particle dynamics in VTS flows with the goal of improving VTS array-specific design philosophies. / Doctor of Philosophy / Vortex tube separators (VTSs) help protect helicopter engines by filtering harmful particles (sand, dust, snow, ash, sea spray, etc.) they would otherwise ingest. This is done by creating a vortex in which centrifugal forces eject particles outwards, separating them from the main airstream. These devices are effective when dozens are grouped together into VTS arrays, but little is understood of the complex air and particle dynamics that result from the many interacting vortices both in and around such arrays. This dissertation describes an early effort to study these aerodynamics and open the door for subsequent particle dynamics research. A laser-based measurement technique called particle image velocimetry is used to determine flow velocities downstream of a VTS array placed in a wind tunnel. When velocities are averaged together over time, they reveal a central recirculation zone (a known feature of intensely swirling flows) downstream of the VTS array that vanishes when only a single separator in the array is active. A mathematical model is developed to predict such recirculation. It demonstrates that a VTS array comprises many separators that are far from the center of the duct they are contained within, and these contribute greatly to the overall swirl intensity. Other data analysis techniques are used to investigate the instantaneous velocity flowfield, which differs significantly from averaged quantities. One such technique is spectral proper orthogonal decomposition, which extracts so-called "coherent structures" from the flow - correlated high-energy motions that exist at certain frequencies and may not be visible in the raw data. This analysis finds two interesting structures at the very center of the duct, possibly associated with the recirculation zone: a back-and-forth pumping motion at a very low frequency (and some of its harmonic frequencies), and a "precessing" (unsteadily rotating) vortex at a slightly higher frequency. These motions, as well as the central recirculation zone itself, are impactful because they may affect the filtration process within the VTS upstream of where they were measured. Such effects will be investigated in future experiments and, if confirmed, may influence the design of VTS arrays.

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This thesis includes a numerical comparison of different turbulence models and particle models in terms of convergence time and physical accuracy. A cyclone is used as the computational domain. Cyclones are common devices for separating two or more substances. The work is divided into an experimental part and a numerical part. In the experiments, characteristics of the cyclone were measured. This data is then used to evaluate different numerical modeling approaches. The numerical part consists of two parts, namely single phase flow and multiphase flow, where different modeling aspects are examined and presented. Furthermore, important parameters that characterize a cyclone, such as pressure drop and separation efficiency, are calculated. The separation efficiency, i.e. how much dust that actually goes to the dust bin, is calculated for two different types of dust. The software used for the numerical simulations has been Star-CCM+.