Roughness height

When the film thickness is of the order of roughness heights, the effects of roughness become significant which have to be taken into account in a profound model of mixed lubrication. The difficulty is that the stochastic nature of surface roughness results in randomly fluctuating solutions that the numerical techniques in the 1970s are unable to handle. As... 

Suppose rj i,/) is a sequence of random numbers produced by computer generation, which takes a Gaussian distribution of a known standard deviation cr, the target roughness heights z i,/) that obeys a given ACF can be written as... 

The model has been applied successfully to examine the performances of mixed lubrication, such as the effects of the roughness height distribution, the behavior of asperity deformation under different conditions of contact and sliding, and the transition of lubrication regime from hydrod5mamic to boundary lubrication. 

Figure 3.20. Spreading behavior of a single droplet impinging on non-flat surfaces of different roughness height (Re = 19060, We = 2119, AJD0= 2.8) (Reprinted from Ref. 389, 1995, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.)...

The Monin-Obukhov length L is not a parameter that is routinely measured. Colder (1972), however, established a relation between the stability classes of Pasquill, the roughness height zo (see Section VII,B), and L. The results of his investigation are shown in Fig. 4. Alternatively, the local wind speed and cloud cover measurements are used to estimate the Pasquill stability class (Table IV). In addition, Colder developed a nonogram for relating the gradient Richardson number R-, to the more easily determined bulk Richardson number / (, ... 

Fig. 4. Relationship between Monin-Obukhov length L and roughness height Zo for various Pasquill stability classes. From Myrup and Ranzieri (1976).

In the experiments used to draw Fig. 20.2, wind speeds were measured at different heights above the water surface. Since wind speed generally decreases when approaching the water surface, these experiments can be compared only if we find a means to transform the wind speeds to a standard height (usually 10 m). Mackay and Yeun (1983) use the standard boundary layer theory with a roughness height of 0.03 cm and a wind stress coefficient of 1.5 x 10 3 to get ... 

These two parameters combined with a pictorial description of the roughness pattern are sufficient to classify the roughness height, z0, and the displacement height, d, for each configuration. 

Treated surfaces involve fine-scale alternation of the surface finish or coating (continuous or discontinuous). They are used for boiling and condensing the roughness height is below that which affects single-phase heat transfer. 

Surface roughness Affects only if the surface roughness height profile is nonnegligible compared to D(l Affects only if the surface roughness height profile is nonnegligible compared to Dh... 

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