The free end effect on the near wake of a finite circular cylinder in a cross flow has been investigated experimentally. Three finite cylinders with aspect ratios (L=D) of 6, 10 and 13 were tested in a subsonic wind tunnel at a Reynolds number of 20 000. A hot-wire anemometer was employed to measure the wake velocity. Mean pressure distributions on the cylinder surface were also measured. The flow near the free end was visualized to observe the flow structure qualitatively in a circulating water channel. The experimental results from these finite cylinder (FC) models were compared with those of a two-dimensional circular cylinder. The flow past the FC free end shows a complicated three-dimensional wake structure. As the FC aspect ratio decreases, the vortex shedding frequency is decreased and the vortex formation region is elongated. The free end effect becomes dominant close to the FC free end. The three-dimensionality of the FC wake may be attributed mainly to the strong entrainment of irrotational fluids, caused by the downwash of counter-rotating vortices separated from the FC free end. The downwash flow is concentrated in the central region of the wake. A peculiar flow structure having a 24 Hz frequency component was observed near the free end using spectral analysis and cross-correlation of the velocity signals. This 24 Hz frequency component is closely related to the counter-rotating twin vortices formed near the FC free end.
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"Figure 12 shows the computed streamwise velocity profile behind the human body in X direction at Z = 1.15 m. The distance between this location and the head vertex of the human body in Y direction is 1.45 m, 5 times of the back width of the human (5D), which is far enough to ensure the wake and turbulence is sufficiently developed in three-dimension (Park and Lee 2000, 2002; Krajnovic 2011). Figure 12a indicates that the streamwise velocity is significantly large in the wake behind the human body (X ranges from 0.8 to 1.2 m). "
[Show abstract][Hide abstract]ABSTRACT: This article investigates the aerodynamic effects of human movement by experiment and numerical simulations. In the experiment, a life-size thermal manikin, a double-track orbit, and a trolley were used to realize human movement, and the velocity distribution of the induced airflow was measured. In the numerical simulations, dynamic meshing was used to simulate the human movement. The aerodynamic effects and flow fields under moving speeds of 0.5, 0.75, 1.0, 1.25, and 1.5 m/s were studied. The same timing relationship and tendency of the instantaneous velocity can be found between the measured and computed results, although the computed peak values are smaller than the measured ones. Apparent recirculation zones and vortices can be seen in the wake behind the human body in numerical simulations. The streamwise velocity profile and the structure of the wake depend on the profile of the human body and the moving speed. At each location, the nondimensional relative velocities of different moving speeds are substantially the same. The aerodynamic effects of human movements depend on the moving speed, moving distance, and spatial location. These results can be a good help for the studies on pollutant dispersion, control of air quality, and infectious diseases in indoor environment.
"These tests were followed by the relevant shaft tests. These tests (in sample EET – not reported in this paper) were conducted before commencing tests in Sample 2. It has been noted previously that end effects (reflecting non-plane-strain flow) can be significant for model pipe experiments in Newtonian fluids (Farell & Fedeniuk, 1988; Park & Lee, 2000; Afgan et al., 2007). For model pipe testing at high velocities in non-Newtonian fluids (such as a fluid-like kaolin slurry), there are no published studies of the implications of using a model pipe with a finite length. "
[Show abstract][Hide abstract]ABSTRACT: Submarine slides are a significant hazard to the safe operation of pipelines in the proximity of continental slopes. This paper describes the results of a centrifuge testing programme aimed at studying the impact forces exerted by a submarine slide on an offshore pipeline. This was achieved by dragging a model pipe at varying velocities through fine-grained soil at various degrees of consolidation, hence exhibiting properties spanning from the fluid to the geotechnical domains, relevant to the state of submarine slide material. To simulate the high strain rates experienced by the soil while flowing around a pipe in the path of a submarine slide, tests were conducted at pipe-soil velocities of up to 4.2 m/s. The changing density and shear strength of the samples were back-calculated from T-bar penetrometer test results. A hybrid approach combining geotechnical and fluid-mechanics-based components of horizontal drag resistance was developed. This approach provides an improved method to link the density and strength of the slide material to the force applied on the pipe. Besides fitting the present observations, the method provides an improved reinterpretation of similar data from the literature.
"However, a significant amount of fluctuating energy is observed for St % 0:0720:08, but it is hard to characterize it because of its high irregularity. This feature is in good agreement with what was found in Park and Lee (2000) where a dominant spectral component at St % 0:07 was found with a fluctuating energy that decreases with reducing h/d, even if it was not clearly detectable anymore already for a model with h/d ¼6. "
[Show abstract][Hide abstract]ABSTRACT: An experimental investigation on the flow features of the wake generated from a circular cylinder with finite height and placed vertically on a plane is presented. Through force measurements the mean drag coefficient is found to be roughly invariant by varying Reynolds number in a range between 6×104 and 11×104. As for the fluctuating forces, a dominant spectral component is clearly detected for the signals of the cross-flow force. A spectral contribution with roughly the same Strouhal number is detected from velocity signals acquired, through hot-wire anemometry, in proximity to the lateral wake boundary; its energy is found to decrease by moving the probe away from the wake and upwards. Simultaneous velocity measurements showed that these fluctuations can confidently be ascribed to an alternate vortex shedding. Subsequently, dynamic measurements of the pressure field over the lateral surface and the free-end of the model were carried out, which highlight that the spectral component connected to vortex shedding is found over the lateral surface, with maximum energy at an azimuthal position just before the separation of the shear layers. The fluctuating energy connected to vortex shedding decreases by moving towards regions immersed in the separated wake, and with increasing vertical coordinate; as a matter of fact, above about half model height an evident energy peak cannot be detected anymore. This feature highlights that a regular alternate vortex shedding occurs only for the lower half-span of the model and that the remaining part is dominated by the upwash generated by the flow passing over the free-end. From the spectral analysis of the pressure measurements carried out over the model free-end no evidence of the presence of the spectral component connected to the alternate vortex shedding is found, as expected. However, a significant fluctuating energy is observed at lower dominating frequencies.
Preview · Article · Jan 2012 · Journal of Fluids and Structures