Cylinders roughness effects

Schlicting (12) shows data on roughness effects on circular cylinders for Re below about 2500, there is no effect. For... 

Nu with intensity up to 0.01 (E2, R2). As /r increases further, but still /r < /rc, Nu increases roughly linearly, but more slowly. Similar effects have been observed for cylinders (MIO). For spheres at higher Re, the average Nusselt number increases linearly with /r for /r < /r (R2). Few reliable data are available for /r > /r. Figure 10.13 presents a tentative correlation for the effect of turbulence on the average Nusselt number for spheres. The ordinate is Nu/Nuo, the ratio of the Nusselt number at /r to the value in the absence of turbulence, while the abscissa is the ratio of /r, the intensity, to /r, the critical intensity. The value of Nuq was calculated from the correlations in Table 5.4 and from Eqs. (10-44) and (10-45). The correlation is divided into... 

A truncated cone of fairly small angle behaves roughly like the cylinder. Briscoe et al4 have quoted relationships for a number of indentor geometries and considered the effect of geometry in detail. 

Collection on porous filter media is perhaps the most efficient means of particle removal. Aerosol filtration is an effective means of air purification, while at the same time it has been widely used for sampling airborne material for mass and chemical composition determination. A wide variety of filter media is available, ranging from fibrous mats of relatively inert material to porous membranes. Fibrous mats and model filter arrays appear microscopically as stacks of overlaid cylinders, where the cylinders may be smooth or rough. In contrast, the membrane media are plastic films with microscopic holes of nearly uniform size nuclepore filters, for example, are produced of sheets of polyester, and the holes are introduced by neutron bombardment. 

It was found that to within experimental error, all of the observed discrepancies could be explained by two factors. The first factor is that the model did not exactly describe the physical situation in the experiment the wave tank had a single cylinder, whereas the calculation is for a series of cylinders. The second factor was the surprising result that the roughly 5% reflected wave from the wave tank significantly affected the experimental results due to modifications in the dynamic pressure fluctuations. In this instance a detailed examination of the model and experimental results has indicated that an experimental effect thought to be small could in fact cause noticeable deviations in the data measured. 

In the displacement of Cu from CuSO by Zn and Cd, and of Ag from AgNC>3 by Zn and Cu (24), rates were found as high as eight times the values expected by comparison with dissolution from smooth cylinders. This was ascribed to enhanced transport by local cell electrolysis, but much of the effect was probably due to roughness caused by the metal deposit. With Zn in 2. 5 x 10" M AgNOg (followed by radiotracer technique) the rate was not abnormal of course very little Agwas deposited. 

E. Achenbach, The Effect of Surface Roughness on the Heat Transfer From a Circular Cylinder to the Cross Flow of Air, Int. J. Heat Mass Transfer (20) 359-369,1977. 

A. Zhukauskas, J. Ziugzda, and P Daujotas, Effects of Turbulence on the Heat Transfer of a Rough Surface Cylinder in Cross-Flow in the Critical Range of Re, in Heat Transfer 1978, vol. 4, pp. 231-236, Hemisphere, Washington, DC, 1978. 

In the first experiment, TomUnson sealed fused silica fibers in a brass cylinder, heated them to about 1000°C, and observed their adhesion as he manipulated them inside the chamber. The adhesion was substantial. To prove that dry air had no effect on the adhesion, he circulated air through sulfuric add, calcium chloride towers, phosphorus pentoxides and roasted pumice to remove all moisture. He then found that the adhesion remained high for sfac weeks. But when the fibers were exposed to damp air, the adhesion was reduced to almost nothing in a few hours. His conclusion was that smooth silica adheres when dry but not much when wet. The reason for Stone s misleading observations, that the surfaces of glass beads are rough and become fiUed with water, will be eiqilained later in Chapter 7, as illustrated in Fig. 6.2. 

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