A two-dimensional numerical study on the laminar flow past a circular cylinder rotating with a constant angular velocity was carried out. The objectives were to obtain a consistent set of data for the drag and lift coefficients for a wide range of rotation rates not available in the literature and a deeper insight into the flow field and vortex development behind the cylinder. First, a wide range of Reynolds numbers (0.01⩽Re⩽45) and rotation rates (0⩽α⩽6) were considered for the steady flow regime, where α is the circumferential velocity at the cylinder surface normalized by the free-stream velocity. Furthermore, unsteady flow calculations were carried out for one characteristic Reynolds number (Re=100) in the typical two-dimensional (2D) vortex shedding regime with α varying in the range 0⩽α⩽2. Additionally, the investigations were extended to very high rotation rates (α⩽12) for which no data exist in the literature. The numerical investigations were based on a finite-volume flow solver enhanced by multi-grid acceleration and the local grid refinement technique to achieve efficient computations and accurate numerical results. The predictions show that the rotation of the cylinder suppresses the vortex development in both the steady and the unsteady flow regimes and significantly changes the flow field close to the cylinder. For very low Reynolds numbers, the drag force is not affected by rotation and the lift force is a linear function of α. For higher Re in the steady flow regime, the drag force decreases with increasing rotational velocities even leading to negative values. The lift force is almost a linear function of the rotational velocity and nearly independent of Re for low rotational speeds of α<2. However, for higher α values and larger Reynolds numbers (Re>1), a progressive increase in the lift force is observed. A very interesting phenomenon was found in the unsteady flow regime at Re=100. For low rotation rates (α⩽2) the flow exhibits the behavior known from the literature, e.g., a linear increase of the mean lift coefficient with increasing α and the suppression of vortex shedding beyond a critical α value of about αL≈1.8. However, for α≈5, an unsteady periodic flow motion was found in the wake which is characterized by a frequency much lower than that known for normal vortex shedding. The change in the flow structure also leads to a distinct change in the mean lift coefficients which exhibits a linear relation of very high rotations rates and asymptotically converges to the values known from the potential flow theory.
To examine in detail the behavior of a new vortex shedding mode found in a previous investigation ͓Phys. Fluids 14, 3160 ͑2002͔͒, a two-dimensional numerical study on the laminar incompressible flow past a rotating circular cylinder in the Reynolds number range 60рReр200 and at rotational rates 0р␣р6 was carried out. The results obtained clearly confirm the existence of the second shedding mode for the entire Reynolds number range investigated. A complete bifurcation diagram ␣(Re) was compiled defining both kind of shedding modes. The unsteady periodic flow in the second mode is characterized by a frequency much lower than that known for classical von Kármán vortex shedding of the first mode. The corresponding Strouhal number shows a strong dependence on the rotational velocity of the cylinder, while only a weak dependence is observed for the Reynolds number. Furthermore, the amplitudes of the fluctuating lift and drag coefficients are much larger than those characterizing classical vortex shedding behind nonrotating or slowly rotating cylinders. Additionally, negative values for the mean drag denoting thrust are found within the second shedding mode.
The tourism industry represents a significant part of the global economy. However, tourism demand is very sensitive to crisis events, such as economic crises, epidemics, pandemics, natural disasters and terrorist attacks. Regarding this, the primary research goal of the paper is to analyze the impact of the Covid-19 disease on global tourism. During 2020, in order to prevent further spread of this contagious disease, almost all countries in the world have introduced a ban on movement, closing of borders and other important health measures. The review of the relevant literature provided in the paper shows that the introduced measures have a negative impact on the global hospitality, travelling and tourism industry.
The purpose of this paper is to thoroughly analyse the concept of multichannel strategy, focussing on retail, to enable the academic community and marketers to better understand its advantages and disadvantages. This paper presents a comprehensive literature review and financial data analysis. The authors have analysed the financial data of 88 retail companies in the 2007 to 2014 period, and have proven that the importance of multichannel strategy has grown with the emergence of e-commerce. The main hypothesis is that the multichannel concept dominates modern marketing channels because it is widely accepted and provides superior financial performance. Multichannel retailing is definitely a winning concept, if adequately implemented. However, wrongly implemented it can negatively influence business performance.
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