Gap Ratio Research Articles

Cross flow over two heated cylinders in tandem arrangements at subcritical Reynolds number using large eddy simulations

This study analyses the heat transfer and flow characteristics of cross-flow over two heated infinite cylinders in a tandem (in-line) configuration. Non-isothermal Large Eddy Simulations (LES) using the dynamic Smagorinsky model were conducted at a fixed Reynolds number of 3,000 (based on the free stream velocity and the cylinder diameter). A range of cylinder gap ratios (1.0≤L/D≤5.0) was investigated (in increments of 0.25) with two different Prandtl numbers Pr=0.1 and 1.0. Results show that the flow structures vary according to the order of the patterns: (i) Extended body regime: without attachment for low L/D (1.0-1.25) where cylinders behave as a single bluff body with top–bottom vortex shedding, (ii) Shear layer reattachment regime: with reattachment for moderate L/D (1.5-3.75) where the detached shear layer from the upstream cylinder reattaches to the downstream cylinder, and (iii) Co-shedding regime: for high gap ratios (3.75≤L/D≤5.0) a phenomenon called “jumping”, where the two cylinders behave as isolated bluff bodies. Furthermore, it was observed that the average Nusselt number of both cylinders experience a drastic variation at a critical spacing ratio (between 3.75≤L/D≤4.0). For L/D≤3.0, the average Nusselt number of the upstream cylinder was found to be higher than that of the downstream one. However, for spacing ratios L/D>3.0, the average Nusselt number was similar for both cylinders. For the downstream cylinder, the maximum Nusselt number was located at the separation angle and was found to be independent of the spacing ratio.

Experimental investigation of the vortex-induced vibration of a circular cylinder near a flat plate

The vortex-induced vibration (VIV) of a near-wall cylinder involves complex interactions between the cylinder wake and wall boundary layer. In this study, the VIV of a circular cylinder with a relatively low mass ratio m* = 43.6 located near a flat plate was experimentally investigated with 3360 ≤ Re ≤ 10,800, 3.0 ≤ U* ≤ 9.6, and 0 ≤ S* ≤ 2.5, where Re is the Reynolds number based on the cylinder diameter, U* is the reduced velocity, and S* is the gap ratio between the cylinder and flat plate. The cylinder vibration, vortex shedding mode, and pressure fluctuation on the flat plate for different S* and U* values were studied in detail. Based on qualitative and quantitative analyses of the near wake of the cylinder using smoke-wire flow visualisation and particle image velocimetry, a classification of vortex shedding modes according to the behaviours of the induced secondary vortex from the flat plate was proposed. The cylinder collided with the flat plate for 0.1 ≤ S* ≤ 0.5, and the effect of the flat plate was almost negligible at S* = 1.5 and completely disappeared at S* = 2.5. In addition, the vibrating cylinder significantly influenced the pressure on the flat plate, particularly in the area below it. Both the time-averaged and fluctuating pressure coefficients Cp‾ and Cp′ presented their extrema at S* = 0.3 when the vibrating cylinder collided with the plate.

Gap ratio effects on the coherent structures surrounding a near-wall square cylinder

Turbulent flow around a square cylinder offset at different heights from an upstream rough wall are investigated using time-resolved particle image velocimetry. The Reynolds number based on the freestream velocity and cylinder height was 12750 and the incoming turbulent boundary layer thickness was 7.2 times the cylinder height. The examined gaps beneath the cylinder were 0.0, 0.3, 0.5, 1.0, 2.0, 4.0, and 8.0 times the cylinder height. The results show that the mean recirculation length remains relatively constant for cases with a gap size greater than or equal to 1.0 times the cylinder height. Flow acceleration through the gap under the cylinder is strongest with a gap size of 2.0 times cylinder height. Downstream of the cylinder, the vertical velocity fluctuations contribute strongly to the production of turbulent kinetic energy by interacting with mean wall-normal velocity gradient in the vertical direction. The dominant frequency embedded in the incoming turbulent boundary layer persists in the wake of the cylinder when the gap size is less than 1.0 cylinder height. Kelvin-Helmholtz vortices originating from the top leading edge of cylinder occur at Strouhal numbers of 2.77-3.11 for the cases with gap ratio ranging from 0.3-8.0 cylinder heights, while spectral migration towards lower frequencies is observed for cases with gap sizes equal to 0.3 and 0.5 cylinder heights. The spatial trajectory of top Kelvin-Helmholtz vortices extends further away from the cylinder as the gap size increases. Spectral proper orthogonal decomposition analysis shows that the von Kármán vortex shedding structures possess the highest spectral energy for all offset cases despite regular vortex shedding being disrupted when the gap size is 0.3 times the cylinder height.

Numerical simulations of flow around dual tandem circular cylinders under a strong axial magnetic field

This work is dedicated to understand the magnetohydrodynamic (MHD) flow around two identical tandem circular cylinders confined in a duct under external axial magnetic field. The influences of magnetic field (represented by the Hartmann number Ha), inlet velocity of conducting fluid (represented by the Reynolds number Re), and the gap ratio (L/d, the ratio of the distance between the centers of two cylinders to the diameter of the circular cylinder) on the flow regimes, vortex shedding frequency, pressure coefficients, drag coefficients, and pressure drop are investigated. The simulations are conducted in the parameter ranges 2≤L/d≤8, 180≤Re≤1440, and 101≤Ha≤1818, respectively. Four flow modes are observed, namely, no vortex shedding, single body, reattachment regime, and vortex shedding (VS) modes, with various Ha/Re2 and L/d values. For L/d>5, the flow field presents the VS flow model, a vortex street with synchronous periodic shedding appears behind the two cylinders, and the vortex shedding frequency matches well with that of the single one. The pressure coefficient shows different tendencies because of the arrangement of two circular cylinders. It can be found that the mean drag coefficients for the upstream circular cylinder and the downstream circular cylinder vary with Re/Ha0.8. For Re=361, Ha=504 and Re=722, Ha=1212, the flow field remains almost unchanged, which implies the balance between inertial force and Lorentz force because the inertial force would promote the flow, while the electromagnetic force leads to the reverse effect. Furthermore, the effects of magnetic field and inlet velocity of metal fluid on pressure drop ΔP between inlet and outlet can be fitted as a linear relation ΔP∼ReHa.

Aspect ratio and interference effects on flow over two rectangular cylinders in tandem arrangement

Flow past twin tandem rectangular cylinders for various gap ratios (L* = 1.0–8.0) and aspect ratios (B* = 0.3–4.0) at a Reynolds number of 150 was investigated via 2D direct numerical simulation. Results showed that based on the variation trend of fluid force with B*, three types of gaps spacing are categorized, i.e., narrow gap (L* = 1.0, 2.0), medium gap (L* = 3.0, 4.0), and wide gap (L* = 6.0, 8.0). Higher B* reduces the fluctuating force of downstream cylinder (DC) for narrow gaps, but amplifies that of the DC for wide gaps. For medium gaps, variation of fluid forces with B* < 1 is in line with that of wide gaps, while it resembles that of narrow gap type at B* > 1. Moreover, gap flow is classified into three flow regimes, i.e., extended body, reattachment, and co-shedding. Wake flow structures of DC show 2S vortex type at narrow gaps for all B* cases, whereas they transfer from synchronized C(2S) to 2S type when B* > 0.3 for medium and wide gaps. Proper orthogonal decomposition analysis captures the spatial distribution and temporal evolution of coherent structure hidden in flow field; thus, it well explains the fundamental mechanism that fluid force, gap flow and wake vortex change with B* and L*.

Can internet development accelerate the green innovation efficiency convergence: Evidence from China

The development of internet has become an important driver that aids in promoting technological innovation and environmental governance. Using Chinese city-level data for the period 2004–2018, the urban green innovation efficiency (GIE) was calculated based on the super-Epsilon-based measure model under metafrontier (M-Super-EBM). Based on this, the convergence characteristic of China's urban GIE was then investigated. This study further probed whether the development of internet has accelerated the green innovation efficiency convergence. The Empirical results revealed that China's urban GIE appeared an upward trend during the sample period. East China had the largest GIE and technology gap ratio (TGR) index. Moreover, the existence of σ convergence, β convergence, and stochastic convergence reference to China's urban GIE was validated. Club convergence analysis results showed that, the whole sample can be divided into 3 groups with 172, 111 and 1 cities, respectively. Internet development significantly promotes the China's urban GIE. The estimated results remained valid after a range of robustness checks. Results imply that, it is of great significance to develop internet infrastructure and promote the growth of GIE.

Technology gap ratio decomposition in sugarcane farming in Indonesia

Indonesia's population and income growth have led to a higher demand for raw sugar and its derivative products; for this reason, sugarcane production must be improved, especially in central production areas. However, efforts are constrained by the disparity in resource endowment and technology. This study aimed to analyse technical, technological, and managerial gaps in sugarcane farming in different areas of East Java. The study used primary data collected through a survey during the 2019/2020 sugarcane planting season in Malang, Kediri, and Mojokerto Regencies. A structured questionnaire was used to collect data on inputs and outputs. A meta-frontier data envelopment analysis (meta-DEA) approach was used to measure the technical efficiency of sugarcane farming in each research area. The decomposition using meta-DEA showed that, on average, the meta-technical inefficiency of sugarcane farmers was caused mainly by managerial gaps, as the average managerial gap inefficiency (MGI) was more significant than the technological gap inefficiency (TGI). Malang regency showed the largest TGI and MGI, indicating that sugarcane farmers in Malang regency have lower managerial skills for decision-making and a lower level of production technology compared to the other two regions. Based on the findings of this study, the government should improve technological innovation to mitigate the technology gap that was found in our research location.

Flow Past of a Drilling Riser System with Auxiliary Lines in Laminar Flow

Vortex-Induced Vibration (VIV) has emerged as a crucial problem that may arise during a drilling operation, which could result in riser failure. Engineers focus on drilling riser's flow characteristics and controls with the intention of improving the drilling environment. Auxiliary lines or control rods are usually placed around the main drilling riser to suppress the VIV. However, the issue related to the flow past drilling riser system with auxiliaries has yet to be resolved. The flow characteristics around a riser system with auxiliaries were analysed in this research. The simulations were conducted using Computational Fluid Dynamics (CFD) software, Altair AcuSolve. This work focuses on the influence of various gap ratios (G/D) and diameter ratios (d/D) on the vortex interaction. A main cylinder with six auxiliary lines was modelled with G/D of 0 to 2.0 and d/D of 0.10 to 0.60 to simulate the riser system. The simulations were carried out at Reynold Number of 200 in the laminar flow regime. The results revealed that the hydrodynamic forces decreased when d/D and G/D increased. The vortex shedding was significantly reduced for auxiliary lines with G/D between 0.3 and 1.4. The numerical simulation results indicated that the vortex interaction in the wake region was, and the hydrodynamic forces were reduced due to the auxiliary lines configurations. The findings of this study are intended to contribute a new CFD simulation result for a better prediction of VIV on a drilling riser with auxiliary lines.

Passive Flow Control around Two Side‐by‐Side Square Cylinders Using a Control Plate

This article is concerned with the numerical analysis of reduction of fluid forces (drag and lift) and suppression of vortex shedding around two side‐by‐side square cylinders using the lattice Boltzmann method. For this purpose, a passive flow control methodology is adopted in which a flat plate (termed as the control plate) is placed at the upstream position of cylinders. The gap ratio (g) between the cylinders and the plate is varied from 0 to 4 while the height of the control plate is varied from 0.2 to 0.8 at fixed Reynolds number of 150. Different flow patterns are found in this study depending on the gap ratios and different heights of the control plate. These flow patterns are single bluff body flow, flip‐flopping flow, and antiphase vortex shedding flow patterns. The narrow gap ratio (g = 0.5) is found to have more impact in terms of force reduction and flow control as compared to larger gap ratios. This article also shows that the fluid flow parameters such as the mean drag coefficient, the Strouhal number, and rms values of drag and lift coefficients of both cylinders reduce considerably due to the control plate as compared to without the control plate. In this regard, it is found that the mean drag coefficient of upper cylinder reduces upto 45.94%, the Strouhal number of both cylinders reduces upto 18.66%, while a maximum reduction of 83.47% and 95.39% is observed in root‐mean‐square values of drag and lift coefficients of upper cylinder, respectively. Another important finding of this article is that the control plate does not suppress forces and control flow at all gap ratios and heights of control plate considered in this article. In some cases, the flow‐induced forces were found to be increasing as compared to without the control plate, and this generally happened at higher gap ratios.

The 3-D study on flow around two wall-mounted cylinders in side by side arrangement

To investigate the characteristics of flow over two finite-height cylinders in side-by-side arrangement, 3-D numerical simulations are performed using the CFD technology for gap ratios (G = S/d, where d is the diameter of the cylinders, and S is the separation gap between the cylinders) between 1 and 4 at a Reynolds number of 100. The height-to-diameter ratio (h/d = 8, where h is the height of the cylinder) is fixed at 8. The drag and lift coefficients of the cylinders, as well as the Strouhal number are presented and compared with those of their 2-D counterparts. Our re?sults show that the wake of the cylinders indicates a transition from the anti-phase state to the in-phase state at large gap ratios, accompanied by a decrease in the lift coefficients of the cylinders.

Flow structures around a circular cylinder with bilateral splitter plates and their dynamic characteristics

Flow structures around a circular cylinder with bilateral splitter plates and their dynamic characteristics are numerically investigated at a low Reynolds number of 100. The two splitter plates with the same length as the half of cylinder diameter are symmetrically placed beside the cylinder. In this work, three gap ratios of G/D = 0, 0.5 and 1.0 (G is the gap distance between the cylinder surface and the near end of plate, D is the diameter of circular cylinder) and inclination angle (α, between splitter plate and wake centerline) ranging from 0° to 90° are examined. According to flow characteristics, five vortex shedding regimes are identified, including "2S" (two vortices are alternatively shed per cycle) regime, single-sided regime, wake-flapping regime, steady regime and irregular regime. Apart from the main vortices, some special vortices such as subordinate vortices, tip vortices and persisted vortices are recognized. Additionally, local "2S″ and "4S" (four vortices are shed per cycle) regimes are observed behind the cylinder and the plates, respectively. The dynamic mode decomposition (DMD) results show that the first mode possesses the majority of energy of the flow field. As α increases, the splitter plates contribute to the lift reduction, especially when a gap is introduced. By contrast, the bilateral plates lead to a significant growth in the drag coefficient when α ≥ 15°.

GAP Ratio, Posisi Devisa Neto, Biaya Operasional Pendapatan Operasional dan Profitabilitas pada Perbankan di Indonesia

This research intends to examine the factors that influence the profitability of the banking industry since, as a sector that influences economic circumstances, the banking industry has witnessed fluctuations in profitability over the previous decade. This form of study employs quantitative secondary data sources. The population of this research consisted of all state-owned banks from 2010 to 2021. Multiple linear regression test and also hypothesis testing are employed in this study. Using descriptive quantitative statistics, the technique of analysis explains the relationship between the dependent and independent variables. The study's findings give empirical proof that gap ratio and BOPO have a negative impact on ROA. Meanwhile, PDN has no impact on ROA. This study also demonstrates the validity of the theory of asset-liability management (ALMA); maximizing earnings via risk-managing the structure of the business's financial situation and the anticipated income theory; increasing bank liquidity and optimize bank earnings by offering efficient long-term lending.&#x0D; Keywords: Gap Ratio; Net Open Position (NOP); Operating Cost of Operating Income (BOPO); Profitability (ROA)

Strongly overdoped La2−xSrxCuO4 : Evidence for Josephson-coupled grains of strongly correlated superconductor

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) with $x\ensuremath{\ge}0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at ${T}_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at ${T}_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at ${T}_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to ${T}_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general.

Agrifood system participation and production efficiency among smallholder vegetable farmers in Northern Ghana

AbstractFormalized marketing arrangements between smallholder farmers and produce buyers are gradually replacing spot market transactions in developing countries due to the rapid structural transformation of agrifood systems. This study examines the impact of agrifood system participation on the total value of vegetable production and technical efficiency (TE), using a sample of 423 vegetable farmers from Northern Ghana. We employ propensity score matching and W. Greene's selection bias‐corrected stochastic production frontier methods to correct for observable and unobservable selection bias issues, respectively. We further use a metafrontier model to derive technology gap ratios (TGRs) and meta‐TE for agrifood system participants and nonparticipants. The results reveal that agrifood system participants are about 50% more productive than nonparticipants. In addition, participants have higher meta‐TE (58% vs. 55%) and TGR (98% vs. 94%) than nonparticipants. Variables such as farmer group membership, extension visits, mobile phone ownership, irrigation and road access are the notable correlates of smallholder farmers' participation in agrifood systems. The total value of vegetable production is significantly influenced by fertilizer, agrochemicals, seeds, irrigation, soil fertility, and location‐fixed effects. [EconLit Citations: D24, Q12, Q18]

Impact of “Three Red Lines” Water Policy (2011) on Water Usage Efficiency, Production Technology Heterogeneity, and Determinant of Water Productivity Change in China

This research evaluates the effects of the Three Red Lines policy on water usage efficiency (WUE), production technology heterogeneity, and water productivity change in 31 Chinese provinces between 2006 and 2020. SMB-DEA, Meta-frontier analysis, and Malmquist-Luenberger index (MLI) techniques were employed for estimation. Results revealed that the mean WUE (2006-2020) in all Chinese provinces was 0.52, with an improvement potential of 48%. Shanghai, Beijing, Shaanxi, and Tianjin were the best performers. The WUE scores before (2006-2011) and after (2012-2020) water policy implementation were 0.58 and 0.48, respectively; on average, there was more than a 9% decline in WUE after the implementation of the water policy. The eastern region has the most advanced water utilization technology as its technology gap ratio (TGR) is nearly 1. The average MLI (2006-2020) score was 1.13, suggesting that the MLI has increased by 12.57% over the study period. Further technology change (TC) is the key predictor of MLI growth, whereas efficiency change (EC) diminished from 2006 to 2020. The mean MLI score for 2006-2011 was 1.16, whereas the MLI Score for the period 2012-2020 was 1.10, indicating a modest decline following the implementation of the water policy. All three Chinese regions experienced MLI growth during 2006-2020, with TC the main change factor.

TARGETING THE POOR WOMEN-HEADED HOUSEHOLDS IN THE PURSUIT TO END POVERTY: A CASE STUDY OF ZUNHEBOTO DISTRICT OF NAGALAND IN NORTH EASTINDIA

The gender dimensions of poverty seemed to have grabbed the attention of many researchers and policy analysts in their study of gender and poverty aspects particularly on issues that target women in their pursuit to end poverty. Poverty situation in many developing countries like India is both an economic phenomenon as well as a social phenomenon. The age-old customs and traditions in India based on the socio-cultural and patriarchal setup bounded by traditional customary laws and practices puts Indian women in a subordinate position and thus increase their vulnerability. The paper estimates whether the incidence of poverty and deprivation is higher among the female-headed households than the male-headed households in Zunheboto District of Nagaland, in the North-eastern region of India. The study is based on a primary survey carried out across a sample of 160 households which were chosen based on purposive sampling. Using the specified measurement tools such as the Head Count Ratio, Poverty Gap Ratio, Gini Coefficient and the Adjusted Head Count Ratio, the findings of the study indicate a higher incidence of deprivation, vulnerability to and a higher intensity of poverty among the female-headed households vis-à-vis the male-headed households.

Two-dimensional multiband superconductivity of the optimally and uniformly Li-intercalated FeSe nanoflakes

We investigated the electrical transport properties of optimally Li+-intercalated FeSe nanoflakes. Using solid electrolyte gating, we obtained high and uniform electron doping up to ∼0.55 electrons per Fe sites in the FeSe nanoflakes, which exhibit a sharp superconducting transition at Tc ∼45.0 K. The temperature and magnetic-field dependent current-voltage characteristics clearly revealed the two-dimensional nature of the superconductivity in the intercalated FeSe nanoflakes. From the self-field critical current measurements, we found two distinct isotropic superconducting gaps with different gap ratios. These findings suggest that the highly electron-doped FeSe hosts two interband pairing channels between the hybridized electron pockets, in contrast to the case of pristine FeSe with electron and hole pockets.

Labor misallocation and productivity growth in Vietnamese manufacturing firms

PurposeThis research identifies the level of labor misallocation in Vietnamese manufacturing sector for the period 2005–2019. The paper also examines the effects of labor misallocation on productivity in Vietnamese manufacturing firms controlled by industry- and firm-level factors.Design/methodology/approachThe level of labor misallocation and efficiency gains in total factor productivity (TFP) are assessed using Vietnam's annual enterprise survey data for the period 2005–2019 and Hsieh and Klenow (2009) productivity decomposition framework.FindingsThe results indicate four main points. Firstly, labor misallocation tends to increase from 2005 to 2019. Secondly, labor misallocation by firm ownership and technology level is found to be highest in state-owned enterprise and low-tech industries, whereas foreign direct investment and high-tech firms have lowest labor misallocation. Labor misallocation in small- and medium-sized enterprises is higher than in large-sized enterprises and is equivalent to overall sample. Thirdly, labor misallocation decreases productivity in manufacturing firms. The firm-level factors such as bigger technology gap, external capital, firm scale and poor liquidity ratio decrease productivity in manufacturing firms. Whereas firm-level factors such as Vietnam's accession to the WTO, reasonable corporate tax structure, capital intensity, human capital and firm age increase productivity of manufacturing firms. The industry-level factors such as FDI horizontal, forward and supply backward spillovers promote productivity from foreign firms to domestic ones. Meanwhile, only backward linkages reduce productivity of firms. Finally, by difference-in-differences (DID) method, the result indicates foreign firms have higher average labor productivity than domestic firms before or after Vietnam's accession to the WTO. After joining WTO, the average labor productivity of foreign firms is increased by 854 million VND while the average labor productivity of domestic firms is increased by 895 million VND. The DID between the two groups (domestic firms and foreign firms) before and after Vietnam's accession to WTO is 41 million dong.Research limitations/implicationsThe main limitation of the study is that the market is assumed perfectly competitive. The model focuses on selective factors affecting labor productivity.Originality/valueThe focus of many previous international research papers was generally to look at the level of labor misallocation in developed countries. However, knowledge about labor misallocation is limited, particularly in the context of developing countries. This paper examines the level of labor misallocation by region, ownership, level of technology and firm size on productivity and the effect of misallocation on productivity in Vietnamese manufacturing firms.Peer reviewThe peer-review history for this article is available at: https://publons.com/publon/10.1108/IJSE-09-2021-0552.

Normal and abnormal thermalization indicators in a one-dimensional low-density Jaynes-Cummings Hubbard model with and without dipole-dipole interaction.

In the one-dimensional low-density Jaynes-Cummings Hubbard (JCH) model, we find that when the hopping strength is much smaller than the coupling strength, the average restricted energy gap ratio exhibits an abnormal statistical behavior that is neither a Poisson nor a Gaussian orthogonal ensemble. But the average half-chain entanglement entropy exhibits ergodicity, and the eigenstate thermalization hypothesis (ETH) is valid for the observable. These results are quite different from those of the standard JCH model. In addition, when the hopping and the coupling strengths are of the same order, quantum chaos still appears in the low-density JCH model, which is in contrast to the integrability of the one-dimensional hard-core bosons. Finally, the dipole-dipole interaction breaks the particle-hole symmetry and leads the abnormal statistical properties to be closer to those of the integrable system at the weak hopping strength limit, but the quantum chaos properties cannot be affected when the hopping strength is of the same order as the coupling strength. Our results demonstrate the counterintuitive behavior in the low-density JCH model and explain the physics behind them from the perspective of the energy spectrum.

Revisiting the reattachment regime: a closer look at tandem cylinder flow at

Tandem cylinder flow comprises several different flow regimes. Within the reattachment regime, the development of the gap shear layers is of utmost importance to the flow, but has received little attention so far. Through direct numerical simulations at$Re = 10^{4}$, for a gap ratio of 3.0, we have discovered that the shear layers are significantly altered with respect to a single cylinder. These differences include early onset of separation, crossflow stabilising, delayed transition to turbulence and little meandering of the transition region. Vortex pairing in the gap shear layers is reported for the first time. The interaction between the recirculating gap flow and the shear layers was investigated. Asymmetrical, large-scale gap vortices influence the position of transition to turbulence through direct contact and through secondary flows. The occurrence of transition in the gap shear layers has consequences for both the reattachment mechanism and the development of the downstream cylinder wake. The reattachment points are unsteady with large amplitude fluctuations on a fine time scale. Reattachment is seen to be a combination of impingement and modification of the upstream shear layers, which causes a double shear layer in the downstream cylinder near-wake. Buffeting by and interaction with the gap shear layers likely cause transition to turbulence in the downstream cylinder boundary layer. This leads to significant changes in the wake topology, compared with a single-cylinder wake.