This thesis presents investigations of diversity at three genes (class I, DQA and DQB) of the Major Histocompatibility Complex (MHC) in cetaceans. The MHC genes encode for proteins that are crucial for initiating an immune response by binding invading pathogens in vertebrates. It has been acknowledged that a high diversity at these genes results in the ability to recognise a wider range of pathogens, therefore functional diversity is important for the survival of a species. Furthermore this diversity has been created under the influence of selection, which can reveal interesting contrasts with neutral markers about the history of selection of populations and species. The diversity at two genes (DQA and DQB) in natural populations of two contrasting species of cetaceans has been investigated in more detail. The species selected included both sub-species of Hector’s dolphin, the Hector’s dolphin (Cephalorhynchus hectori hectori) and the Maui’s dolphin (Cephalorhynchus hectori maui), as well as the long-finned pilot whale (Globicephala melas). These species were chosen, because both Hector’s dolphin sub-species contrast with the pilot whale species in regards to their population size, abundance, population structure and life history. For example both sub-species of Hector’s dolphin have small population sizes and only inhabit coastal areas around New Zealand, whereas the pilot whale is an abundant, pelagic dolphin species. In Chapter 2 the expression of class II MHC genes (DQA and DQB) was demonstrated for the first time for a cetacean species, the Hector’s dolphin. Using available information from the bottlenose dolphin (Tursiops truncatus), I also designed primers to investigate class I MHC. Fragments of MHC genes were amplified from cDNA, which was derived from blood samples of two Hector’s dolphins. These dolphins were the subject of a temporary live capture, presenting a unique opportunity for blood collection. No evidence was found for duplication of both MHC class II loci, but cloning suggested a minimum of three copies of class I genes within the genomic DNA. However, the expression of all class I genes was uncertain, since only one allele could be isolated from cDNA. Functionality for all three genes (class I, DQA and DQB) was supported by the evidence for balancing selection having operated on these genes, indicated by a higher ratio of non-synonymous to synonymous substitutions. In Chapter 3, a combination of single-strand conformation polymorphism (SSCP) and direct sequencing was used to describe DQA and DQB diversity in the Hector’s and Maui’s dolphin. Genetic samples for the Hector’s dolphin were available from previously collected stranding and biopsy samples (n = 233), representing three populations from the South Coast of New Zealand and the sub-species on the West Coast of the North Island of New Zealand. For the Hector’s dolphin of the South Island, a surprisingly large number of alleles at both loci (DQA = 4, DQB = 6) were found, considering their small population size and compared to other cetacean populations with larger population sizes. The Maui’s dolphin has been classified as critically endangered with less than 100 dolphins, but showed a relatively high nucleotide diversity for DQB ( = 4.5%). This diversity was based on only three alleles that have been retained in the sub-species, representing the most divergent of all six alleles. All populations showed strong geographic differentiation at both loci (DQA: FST=0.252; DQB: FST=0.333), with the greatest differentiation between the South Island population and the North Island Maui’s dolphin. Comparison to mitochondrial and microsatellite diversity suggested influence of stochastic genetic drift, although the pressure of balancing selection acting on DQB over an evolutionary time period was also evident by a higher ratio of non-synonymous to synonymous substitutions (dN/dS=5.9) and by a pattern of trans-specific allele sharing within the family of Delphinidae. In Chapter 4 similar methods were used to describe DQA and DQB in pilot whales using genetic samples from the long-finned pilot whale that were available from five mass-strandings from around New Zealand (n = 237). A larger number of alleles than for the Hector’s dolphin were found at both loci (DQA= 8; DQB= 8), although their large population size and pelagic abundance raises the expectation of an even greater number of alleles. The overall differentiation between mass-strandings was low, but significant for both loci (DQA: FST =0.012, DQB: FST =0.014). The differentiation of all strandings was greatest for the Golden Bay mass-stranding at DQA, but deviation from Hardy-Weinberg equilibrium at DQB suggested either sub-structure within mass-strandings (Wahlund effect) or the presence of null alleles. As for the Hector’s dolphin and other mammalian species, the influence of balancing selection acting on DQB over a long evolutionary time period was evident by a higher ratio non-synonymous to synonymous substitutions (dN/dS=9.3) and by a pattern of trans-specific allele sharing within the family of Delphinidae. Overall, diversity is surprisingly similar between these two cetacean species despite different life history characteristic, but low compared to domesticated ungulate species, such as the cow. If low MHC diversity is a general feature of cetaceans, due to the marine environment as suggested previously or rather a side effect of short-term demographic forces remains speculative. A standardised nomenclature for the increasing number of MHC alleles from cetacean is proposed in this thesis to assist with future development of this research.

This thesis presents investigations of diversity at three genes (class I, DQA and DQB) of the Major Histocompatibility Complex (MHC) in cetaceans. The MHC genes encode for proteins that are crucial for initiating an immune response by binding invading pathogens in vertebrates. It has been acknowledged that a high diversity at these genes results in the ability to recognise a wider range of pathogens, therefore functional diversity is important for the survival of a species. Furthermore this diversity has been created under the influence of selection, which can reveal interesting contrasts with neutral markers about the history of selection of populations and species. The diversity at two genes (DQA and DQB) in natural populations of two contrasting species of cetaceans has been investigated in more detail. The species selected included both sub-species of Hector’s dolphin, the Hector’s dolphin (Cephalorhynchus hectori hectori) and the Maui’s dolphin (Cephalorhynchus hectori maui), as well as the long-finned pilot whale (Globicephala melas). These species were chosen, because both Hector’s dolphin sub-species contrast with the pilot whale species in regards to their population size, abundance, population structure and life history. For example both sub-species of Hector’s dolphin have small population sizes and only inhabit coastal areas around New Zealand, whereas the pilot whale is an abundant, pelagic dolphin species. In Chapter 2 the expression of class II MHC genes (DQA and DQB) was demonstrated for the first time for a cetacean species, the Hector’s dolphin. Using available information from the bottlenose dolphin (Tursiops truncatus), I also designed primers to investigate class I MHC. Fragments of MHC genes were amplified from cDNA, which was derived from blood samples of two Hector’s dolphins. These dolphins were the subject of a temporary live capture, presenting a unique opportunity for blood collection. No evidence was found for duplication of both MHC class II loci, but cloning suggested a minimum of three copies of class I genes within the genomic DNA. However, the expression of all class I genes was uncertain, since only one allele could be isolated from cDNA. Functionality for all three genes (class I, DQA and DQB) was supported by the evidence for balancing selection having operated on these genes, indicated by a higher ratio of non-synonymous to synonymous substitutions. In Chapter 3, a combination of single-strand conformation polymorphism (SSCP) and direct sequencing was used to describe DQA and DQB diversity in the Hector’s and Maui’s dolphin. Genetic samples for the Hector’s dolphin were available from previously collected stranding and biopsy samples (n = 233), representing three populations from the South Coast of New Zealand and the sub-species on the West Coast of the North Island of New Zealand. For the Hector’s dolphin of the South Island, a surprisingly large number of alleles at both loci (DQA = 4, DQB = 6) were found, considering their small population size and compared to other cetacean populations with larger population sizes. The Maui’s dolphin has been classified as critically endangered with less than 100 dolphins, but showed a relatively high nucleotide diversity for DQB ( = 4.5%). This diversity was based on only three alleles that have been retained in the sub-species, representing the most divergent of all six alleles. All populations showed strong geographic differentiation at both loci (DQA: FST=0.252; DQB: FST=0.333), with the greatest differentiation between the South Island population and the North Island Maui’s dolphin. Comparison to mitochondrial and microsatellite diversity suggested influence of stochastic genetic drift, although the pressure of balancing selection acting on DQB over an evolutionary time period was also evident by a higher ratio of non-synonymous to synonymous substitutions (dN/dS=5.9) and by a pattern of trans-specific allele sharing within the family of Delphinidae. In Chapter 4 similar methods were used to describe DQA and DQB in pilot whales using genetic samples from the long-finned pilot whale that were available from five mass-strandings from around New Zealand (n = 237). A larger number of alleles than for the Hector’s dolphin were found at both loci (DQA= 8; DQB= 8), although their large population size and pelagic abundance raises the expectation of an even greater number of alleles. The overall differentiation between mass-strandings was low, but significant for both loci (DQA: FST =0.012, DQB: FST =0.014). The differentiation of all strandings was greatest for the Golden Bay mass-stranding at DQA, but deviation from Hardy-Weinberg equilibrium at DQB suggested either sub-structure within mass-strandings (Wahlund effect) or the presence of null alleles. As for the Hector’s dolphin and other mammalian species, the influence of balancing selection acting on DQB over a long evolutionary time period was evident by a higher ratio non-synonymous to synonymous substitutions (dN/dS=9.3) and by a pattern of trans-specific allele sharing within the family of Delphinidae. Overall, diversity is surprisingly similar between these two cetacean species despite different life history characteristic, but low compared to domesticated ungulate species, such as the cow. If low MHC diversity is a general feature of cetaceans, due to the marine environment as suggested previously or rather a side effect of short-term demographic forces remains speculative. A standardised nomenclature for the increasing number of MHC alleles from cetacean is proposed in this thesis to assist with future development of this research.

This thesis presents investigations of diversity at three genes (class I, DQA and DQB) of the Major Histocompatibility Complex (MHC) in cetaceans. The MHC genes encode for proteins that are crucial for initiating an immune response by binding invading pathogens in vertebrates. It has been acknowledged that a high diversity at these genes results in the ability to recognise a wider range of pathogens, therefore functional diversity is important for the survival of a species. Furthermore this diversity has been created under the influence of selection, which can reveal interesting contrasts with neutral markers about the history of selection of populations and species. The diversity at two genes (DQA and DQB) in natural populations of two contrasting species of cetaceans has been investigated in more detail. The species selected included both sub-species of Hector’s dolphin, the Hector’s dolphin (Cephalorhynchus hectori hectori) and the Maui’s dolphin (Cephalorhynchus hectori maui), as well as the long-finned pilot whale (Globicephala melas). These species were chosen, because both Hector’s dolphin sub-species contrast with the pilot whale species in regards to their population size, abundance, population structure and life history. For example both sub-species of Hector’s dolphin have small population sizes and only inhabit coastal areas around New Zealand, whereas the pilot whale is an abundant, pelagic dolphin species. In Chapter 2 the expression of class II MHC genes (DQA and DQB) was demonstrated for the first time for a cetacean species, the Hector’s dolphin. Using available information from the bottlenose dolphin (Tursiops truncatus), I also designed primers to investigate class I MHC. Fragments of MHC genes were amplified from cDNA, which was derived from blood samples of two Hector’s dolphins. These dolphins were the subject of a temporary live capture, presenting a unique opportunity for blood collection. No evidence was found for duplication of both MHC class II loci, but cloning suggested a minimum of three copies of class I genes within the genomic DNA. However, the expression of all class I genes was uncertain, since only one allele could be isolated from cDNA. Functionality for all three genes (class I, DQA and DQB) was supported by the evidence for balancing selection having operated on these genes, indicated by a higher ratio of non-synonymous to synonymous substitutions. In Chapter 3, a combination of single-strand conformation polymorphism (SSCP) and direct sequencing was used to describe DQA and DQB diversity in the Hector’s and Maui’s dolphin. Genetic samples for the Hector’s dolphin were available from previously collected stranding and biopsy samples (n = 233), representing three populations from the South Coast of New Zealand and the sub-species on the West Coast of the North Island of New Zealand. For the Hector’s dolphin of the South Island, a surprisingly large number of alleles at both loci (DQA = 4, DQB = 6) were found, considering their small population size and compared to other cetacean populations with larger population sizes. The Maui’s dolphin has been classified as critically endangered with less than 100 dolphins, but showed a relatively high nucleotide diversity for DQB ( = 4.5%). This diversity was based on only three alleles that have been retained in the sub-species, representing the most divergent of all six alleles. All populations showed strong geographic differentiation at both loci (DQA: FST=0.252; DQB: FST=0.333), with the greatest differentiation between the South Island population and the North Island Maui’s dolphin. Comparison to mitochondrial and microsatellite diversity suggested influence of stochastic genetic drift, although the pressure of balancing selection acting on DQB over an evolutionary time period was also evident by a higher ratio of non-synonymous to synonymous substitutions (dN/dS=5.9) and by a pattern of trans-specific allele sharing within the family of Delphinidae. In Chapter 4 similar methods were used to describe DQA and DQB in pilot whales using genetic samples from the long-finned pilot whale that were available from five mass-strandings from around New Zealand (n = 237). A larger number of alleles than for the Hector’s dolphin were found at both loci (DQA= 8; DQB= 8), although their large population size and pelagic abundance raises the expectation of an even greater number of alleles. The overall differentiation between mass-strandings was low, but significant for both loci (DQA: FST =0.012, DQB: FST =0.014). The differentiation of all strandings was greatest for the Golden Bay mass-stranding at DQA, but deviation from Hardy-Weinberg equilibrium at DQB suggested either sub-structure within mass-strandings (Wahlund effect) or the presence of null alleles. As for the Hector’s dolphin and other mammalian species, the influence of balancing selection acting on DQB over a long evolutionary time period was evident by a higher ratio non-synonymous to synonymous substitutions (dN/dS=9.3) and by a pattern of trans-specific allele sharing within the family of Delphinidae. Overall, diversity is surprisingly similar between these two cetacean species despite different life history characteristic, but low compared to domesticated ungulate species, such as the cow. If low MHC diversity is a general feature of cetaceans, due to the marine environment as suggested previously or rather a side effect of short-term demographic forces remains speculative. A standardised nomenclature for the increasing number of MHC alleles from cetacean is proposed in this thesis to assist with future development of this research.

[pt] O presente trabalho analisa a transferência de calor e a perda de carga em escoamento turbulento através das regiões anulares lisas e aletadas. Os coeficientes de transferência de calor foram obtidos experimentalmente, utilizando-se a teoria de trocadores de calor. Também foram determinados os coeficientes de perda de carga associados a cada uma das geometrias analisadas. Os experimentos foram realizados em um trocador de calor bi-tubular concêntrico. Água (fluído quente) escoava através do tubo interno, enquanto ar (fluido frio) fluía através da região anular. Medindo-se as vazões e temperaturas na entrada e saída da seção de teste do trocador de calor, obtém-se a efetividade da mesma e, consequentemente, o coeficiente global de transferência de calor (médio). As condições de contorno adotadas foram de temperatura uniforme na superfície externa do tubo interno e de isolamento térmico na superfície externa do tubo interno e de isolamento térmico na superfície externa da região anular. Com o objetivo de obter-se condições de escoamento completamente desenvolvido, o trocador de calor possui um comprimento, a montante da seção de testes, de aproximadamente trinta diâmetros hidráulicos. Os coeficientes de transferência de calor e de perda de carga são apresentados de forma adimensional, como função do número de Reynolds do escoamento, os resultados obtidos para as regiões anulares lisas e aletadas são comparados entre si. O propósito de tal comparação foi o de estudar a influencia das aletas na perda de carga e na taxa de transferência de calor. No caso das regiões aletadas, verifica-se que a eficiência das aletas influencia a transferência de calor. Deste modo, realizou-se uma análise bi-dimensional de transferência de calor nas aletas para obter-se a eficiência das mesmas e, consequentemente, a eficiência das regiões aletadas. Também é mostrado que o desempenho térmico da região depende principalmente do número de Nusselt e da eficiência da mesma. Tais parâmetros são apresentados, em última análise, como funções do número de Reynolds e da geometria do problema. / [en] The present work is concerned with the turbulent heat transfer and pressure drop in smooth and finned annular ducts. Average heat transfer coeeficients have been obtained by means of the heat exchanger theory. In addition, friction factors have also been determined. The experiments were performed by utiling four double-pipe heat exchangers. The average heat transfer coefficients, for air flowing in the annular section, were fertmined by measuring the overall heat transfer coefficients of the heat exchangers. In order to aatain fully developed conditions, the heat exchangers had a starting length of 30 hydrualic diameters. The thermal boundary conditions consisted of uniform temperature on the inner surface, the outer surface being insulate. The heat transfer coefficients and friction factors are presented in dimensinless forms, as functions of the Reynolds number of the fow. The results for smooth and finned annular ducts were compared. The purpose of such comparison was to study the influence of the fins on the pressure drop and heat transfer rate. In the case of the finned annular ducts, it is shown that fin efficiency has some influence on the heat transfer rates. Then a two-dimensional heat transfer analysis was performed in order to obtain the fin efficiency and the annular region efficiency. It is also shown that the overall thermal performance of finned surfaces depends mainly of the Nusselt number and on the region efficiency. These parameters are presented as functiond of the Reynolds number of the flow and the geometry of the problem.

This master’s thesis deals with a design of Na-CO2 heat exchanger. There is a comparison of shell and tube heat exchanger with PCHE in the first part. Further the shell and tube heat exchanger with internally finned tubes was chosen as the basic conception. There was performed an optimization of construct and operations parameters for this concept. The optimization was performed on the basis of thermal and hydraulic calculations. Further there were performed calculations for ensuring of safe operation of the heat exchanger. The conclusion of the diploma thesis deals with comparison of the designed heat exchanger with similar projects.

Extensively migrating population of neural crest cells, which contributes to many tissues and builds up most of craniofacial vertebrate structures, has a crucial role in embryonic development of vertebrate body. The migratory pathways of neural crest cells are thought to be very conserved throughout the vertebrates and cranial neural crest migration is defined by progression of three migratory streams: trigeminal, hyoid and a common branchial stream. In this diploma thesis, migration of cranial neural crest was analysed using embryos of the Senegal bichir (Polypterus senegalus) and of sterlet (Acipenser ruthenus), which represent two basal-most lineages of extant ray-finned fishes. A combination of several techniques was used in both species in order to study cranial neural crest cells from their sites of origin to post- migratory stages and the pattern of migration was compared and discussed in revealed embryonic context. In the Senegal bichir the hyoid neural crest stream was shown to migrate first and it is also the most abundant; this heterochrony shift is apparently related to formation of external gills, which in bichir are situated on the hyoid arch only. In sterlet, neural crest cells migrate in a classic pattern of three progressive streams but their dynamics and patterning is influenced by...

La prédiction du bruit émis par les unités extérieures des pompes à chaleur, provenant en partie des batteries à ailettes, constitue un enjeu industriel important. L’obstacle majeur réside dans l’impossibilité de réaliser le calcul acoustique d’une batterie à ailettes avec les outils de simulation classiques (Éléments Finis de Frontière) à cause du trop grand nombre d’éléments nécessaires (près de 600 ailettes par mètre). Cette thèse a pour objectif d’exploiter la périodicité des batteries à ailettes afin de pouvoir calculer leur bruit rayonné à partir du rayonnement d’une seule ailette. La propagation des vibrations est décrite sous la forme d’une décomposition en ondes, autorisant la réalisation séparée des calculs pour chaque onde. Le déplacement en tout point de la structure est calculé à partir du déplacement d’un seul élément unitaire, obtenu par la Méthode des Éléments Finis (FEM). Un élément unitaire acoustique est spécialement défini, comprenant des baffles rigides destinés à représenter l’influence des autres ailettes de la structure. Le rayonnement de cet élément est calculé pour chaque onde en utilisant la méthode des Éléments Finis de Frontière (BEM), puis dupliqué spatialement en tenant compte de la nature propagative des différentes ondes. Pour chaque structure étudiée, la comparaison avec le calcul direct de la structure complète montre une très bonne concordance avec des temps de calcul fortement réduits. L’influence de plusieurs paramètres sur le comportement de la structure a également été identifiée, mettant ainsi en évidence certains phénomènes particuliers propres aux structures périodiques. Dans la dernière partie de cette thèse, une validation expérimentale de l’approche périodique est proposée. Les résultats numériques sont comparables aux mesures vibratoires et acoustiques réalisées sur les batteries à ailettes. / The prediction of noise from outdoor units of heat pumps, partly coming from finned coils, is an important industrial issue. The major obstacle to the study of finned coils lies in the inability to perform the acoustic calculation with classic simulation tools (Boundary Element Method) because of the huge number of required elements (about 600 fins per meter). The goal of this thesis is to exploit the periodicity of finned coils in order to calculate the total noise radiated from the radiation of a single fin, and thus get rid of the number of fins composing the structure. The vibrations propagation is described as a wave decomposition, allowing the realization of separate calculations for each wave. The displacement at any point of the structure is calculated from the displacement of a single unit element, obtained with the Finite Element Method (FEM). An acoustic unit element is especially defined, comprising rigid baffles representing the influence of other fins of the structure. The radiation of this component is calculated for each wave using the Boundary Element Method (BEM), then spatially duplicated taking into account the propagation of the different waves. For each considered structure, a comparison with the direct calculation of the complete structure shows very good agreement with greatly reduced computation times. The influence of several parameters on the behaviour of the structure was also identified, thus highlighting some particular phenomena specific to periodic structures. In the last part of this thesis, an experimental validation of the periodic approach is proposed. Numerical results are close to vibratory and acoustic measurements done on the finned coils.

Flow past circular cylinders with stepwise discontinuities in diameter was investigated experimentally and numerically for the diameter ratio D/d = 2 and three Reynolds numbers, Re = 150, 300, and 1050. The investigation was focused on the vortex shedding phenomena occurring in the wake of the cylinders. In the first series of experimental and numerical studies, the flow development past a single step cylinder was investigated. The single step cylinder model is comprised of a small diameter cylinder (d) attached coaxially to a large diameter cylinder (D). The results show that three distinct spanwise vortex cells form in the step cylinder wake: a single vortex shedding cell in the wake of the small cylinder (the S-cell) and two vortex shedding cells in the wake of the large cylinder, one in the region downstream of the step (the N-cell) and the other away from the step (the L-cell). Due to the differences in vortex shedding frequencies between the three cells, complex vortex connections occur in two vortex-interaction regions located between the adjacent cells. The region at the boundary between the S-cell and the N-cell is relatively narrow and its spanwise extent does not fluctuate significantly. In this region, vortex dislocations manifested as half-loop connections between two S-cell vortices of opposite sign. In contrast, the region at the boundary between the N-cell and the L-cell exhibits a transient behavior, with large scale vortex dislocations causing cyclic variation in the extent of N-cell vortices. For Re = 300 and 1050, small scale streamwise vortices forming in the wake complicate the vortex dynamics within the adjacent S-cell and L-cell. There is no significant Reynolds number effect on the average spanwise extent of the vortex cells and the two transition regions between neighboring cells. Finally, formation of N-cell vortices is linked to downwash fluctuations near the step. The flow development past a dual step cylinder was studied experimentally for Re = 1050. The dual step cylinder model is comprised of a small diameter cylinder (d) and a large diameter cylinder (D) mounted at the mid-span of the small cylinder. The experiments were completed for a range of large cylinder aspect ratios 0.2 ≤ L/D ≤ 17. The flow development is highly dependent on the aspect ratio of the large cylinder, L/D. The results identify four distinct flow regimes: (i) for L/D = 17, three vortex shedding cells form in the wake of the large cylinder, one central cell and two cells of lower frequency extending over about 4.5D from the large cylinder ends, (ii) for 7 < L/D ≤ 14, a single vortex shedding cell forms in the wake of the large cylinder, whose shedding frequency decreases with decreasing L/D, (iii) for 2 ≤ L/D ≤ 7, vortex shedding in the wake of the large cylinder is highly three-dimensional, with vortices deforming in the near wake, (iv) for 0.2 ≤ L/D ≤ 1, only small cylinder vortices are shed in the wake and can form vortex connections across the wake of the large cylinder.

This experimental study explores the heat transfer from heated bare and finned tubular surfaces to particulates in packed bed cross flow. The results from this experiment will be used to help select the type of particulates that will be used. Additionally, these results will assist in estimating heat transfer in prototype and commercial particle to fluid heat exchangers (PFHX). This research is part of larger effort in the use of particulates in concentrating solar power technology. These solid particles are heated by concentrated sunlight to very high temperatures at which they are a suitable heat source for various thermal power and thermochemical cycles. Furthermore, one of the advantages of this concept is the ability to store thermal energy in the solid particles at relatively low cost. However, an important feature of any Particle Heat Receiver (PHR) system is the PFHX, which is the interface between the solar energy system and the thermal power or chemical system. In order to create this system material data is needed for the design and optimization of this PFHX. The paper focuses on the heat transfer properties of particulates to solid surfaces under plug flow conditions. The particulates will be evaluated for three grain sizes of sand and two grain sizes of proppants. These two materials will be tested at one, five and ten millimeters per second in order to see how the various flow rates, which will be required for different loads, will affect the heat transfer coefficient. Finally the heat transfer coefficient will also be evaluated for both finned and non-finned heat exchangers to see the effect that changes in the surface geometry and surface area have on the heat transfer coefficient. The heat transfer coefficient will help determine the appropriate material that will be used in the PHR system.

Flow past circular cylinders with stepwise discontinuities in diameter was investigated experimentally and numerically for the diameter ratio D/d = 2 and three Reynolds numbers, Re = 150, 300, and 1050. The investigation was focused on the vortex shedding phenomena occurring in the wake of the cylinders. In the first series of experimental and numerical studies, the flow development past a single step cylinder was investigated. The single step cylinder model is comprised of a small diameter cylinder (d) attached coaxially to a large diameter cylinder (D). The results show that three distinct spanwise vortex cells form in the step cylinder wake: a single vortex shedding cell in the wake of the small cylinder (the S-cell) and two vortex shedding cells in the wake of the large cylinder, one in the region downstream of the step (the N-cell) and the other away from the step (the L-cell). Due to the differences in vortex shedding frequencies between the three cells, complex vortex connections occur in two vortex-interaction regions located between the adjacent cells. The region at the boundary between the S-cell and the N-cell is relatively narrow and its spanwise extent does not fluctuate significantly. In this region, vortex dislocations manifested as half-loop connections between two S-cell vortices of opposite sign. In contrast, the region at the boundary between the N-cell and the L-cell exhibits a transient behavior, with large scale vortex dislocations causing cyclic variation in the extent of N-cell vortices. For Re = 300 and 1050, small scale streamwise vortices forming in the wake complicate the vortex dynamics within the adjacent S-cell and L-cell. There is no significant Reynolds number effect on the average spanwise extent of the vortex cells and the two transition regions between neighboring cells. Finally, formation of N-cell vortices is linked to downwash fluctuations near the step. The flow development past a dual step cylinder was studied experimentally for Re = 1050. The dual step cylinder model is comprised of a small diameter cylinder (d) and a large diameter cylinder (D) mounted at the mid-span of the small cylinder. The experiments were completed for a range of large cylinder aspect ratios 0.2 ≤ L/D ≤ 17. The flow development is highly dependent on the aspect ratio of the large cylinder, L/D. The results identify four distinct flow regimes: (i) for L/D = 17, three vortex shedding cells form in the wake of the large cylinder, one central cell and two cells of lower frequency extending over about 4.5D from the large cylinder ends, (ii) for 7 < L/D ≤ 14, a single vortex shedding cell forms in the wake of the large cylinder, whose shedding frequency decreases with decreasing L/D, (iii) for 2 ≤ L/D ≤ 7, vortex shedding in the wake of the large cylinder is highly three-dimensional, with vortices deforming in the near wake, (iv) for 0.2 ≤ L/D ≤ 1, only small cylinder vortices are shed in the wake and can form vortex connections across the wake of the large cylinder.