Surface pressure coefficient

The second part of computing building pressures involves the pressure coefficient for a particular spot on the building. The surface pressure coefficient, Cp, indicates the share of the wind kinetic energy that is transferred to the static pressure ... 

For a building with sharp corners, Cp is almost independent of the wind speed (i.e., Reynolds number) because the flow separation points normally occur at the sharp edges. This may not be the case for round buildings, w here the position of the separation point can be affected by the wind speed. For the most common case of the building with a rectangular shape, Cp values are normally between 0.6 and 0.8 for the upwind wall, and for the leeward wall 0,6 < C, < —0.4. Figure 7.99 and Table 7.32 show an example of the distribution of surface pressure coefficient values on the typical industrial building envelope. 

Values of Cp for simple building geometries may be obtained from the British Standards Institution or from Liddament. The following relationship between wind incident angle a, building side ratio, and average surface pressure coefficient is based on the database developed by Swami and Chandra ... 

FIGURE 7.99 Example of surface pressure coefficient values for a typical industrial building envelope. 

TABLE 7.32 Approximate Surface Pressure Coefficient Values for a Building with a Rooftop Vent... 

Principles of wind speed and surface pressure coefficient evaluation were covered earlier. 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

Pressure at a given location on the building surface is usually expressed as a pressure coefficient times a reference wind pressure at the building height without the building in place ... 

Under wind conditions, a complex system of pressures is set up on the external surfaces of the building, which will vary with wind speed and direction. Pressure coefficients Cp define the relationship according to the formula ... 

The importance of lipophilicity to bitterness has been well established, both directly and indirectly. The importance of partitioning effects in bitterness perception has been stressed by Rubin and coworkers, and Gardner demonstrated that the threshold concentration of bitter amino acids and peptides correlates very well with molecular connectivity (which is generally regarded as a steric parameter, but is correlated with the octanol-water partition coefficient ). Studies on the surface pressure in monolayers of lipids from bovine, circumvallate papillae also indicated that there is a very good correlation between the concentration of a bitter compound that is necessary in order to give an increase in the surface pressure with the taste threshold in humans. These results and the observations of others suggested that the ability of bitter compounds to penetrate cell membranes is an important factor in bitterness perception. 

The simplest way to predict the lipid/ water partition coefficient, Kiw, of a drug is based on measurements of the surface pressure, ttd, of the drug as a function of its concentration in the aqueous subphase (Gibbs adsorption isotherm). The Gibbs adsorption isotherm provides the air/water partition coefficient, Kaw, and the cross-sectional area, Ad of the drug and allows calculation of the lipid/water partition coefficient, K]w, according to Eq. (6) [59] ... 

Fig. 20.1. Correlation between the air/water partition coefficient, Kaw, determined from measurements of the surface pressure as a function of drug concentration (Gibbs adsorption isotherm) in buffer solution (50 mM Tris/HCI, containing 114 mM NaCI) at pH 8.0 and the inverse of the Michaelis Menten constant, Km obtained from phosphate release...

The remaining forces can be defined using pressures, coefficients, and surface areas ... 

Form drag and skin friction drag coefficients are obtained from the numerical results by integrating the distributions of surface pressure and vorticity ... 

One interesting feature of the functional form derived here is the direct relationship of the activity coefficients and composition between the micellar and surface psuedo-phases. This allows a comparison of nonideal interactions in the micelle and monolayer as modeled by their respective net interaction parameters. In principle, this form may also allow extension to more complicated situations such as the treatment of contact angles in nonideal mixed surfactant systems. Here, the functional form derived above depends on differences in surface pressures and these may be directly obtained from experimentally measured parameters under the proper conditions (30). 

Results for the various binary mixed surfactant systems are shown in figures 1-7. Here, experimental results for the surface tension at the cmc (points) for the mixtures are compared with calculated results from the nonideal mixed monolayer model (solid line) and results for the ideal model (dashed line). Calculations of the surface tension are based on equation 17 with unit activity coefficients for the ideal case and activity coefficients determined using the net interaction 3 (from the mixed micelle model) and (equations 12 and 13) in the nonideal case. In these calculations the area per mole at the surface for each pure component, tOj, is obtained directly from the slope of the linear region in experimental surface tension data below the cmc (via equation 5) and the maximum surface pressure, from the linear best fit of... 

By extending regular solution theory for binary mixtures of AEg in aqueous solution to the adsorption of mixture components on the surface (3,4), it is possible to calculate the mole fraction of AEg, Xg, on the mixed surface layer at tt=20, the molecular interaction parameter, 6, the activity coefficients of AEg on the mixed surface layer, fqg and f2s and mole concentration of surfactant solution, CTf=20 3t surface pressure tt=20 mn-m l (254p.l°C). The results from the following equations are shown in Table I and Table II. 

The physical properties of solvents greatly influence the choice of solvent for a particular application. The solvent should be liquid under the temperature and pressure conditions at which it is employed. Its thermodynamic properties, such as the density and vapor pressure, temperature and pressure coefficients, as well as the heat capacity and surface tension, and transport properties, such as viscosity, diffusion coefficient, and thermal conductivity, also need to be considered. Electrical, optical, and magnetic properties, such as the dipole moment, dielectric constant, refractive index, magnetic susceptibility, and electrical conductance are relevant, too. Furthermore, molecular... 

EXAMPLE 7.3 Suppression of Evaporation by Monolayers. The rate of evaporation is quantified by a parameter called the transport resistance r. For water with octadecanol monolayers at surface pressures of 10, 20, 30, and 40 mN m -1, ris about 1, 2, 3, and 4 s cm 1, respectively. This resistance drops off rapidly at lower pressures and approaches 2 10 3 s cm -1 for pure water. By considering the rate of water uptake as a diffusion problem, suggest how these r values are calculated from data collected in an experiment like that described above. Use the fact that Mr is dimensionally equivalent to the diffusion coefficient D divided by a length. 

Due to the chain architecture and the large size of the macromolecules, the wetting behaviour of polymer liquids can be different from that of simple liquids. The effect becomes particularly strong when the dimension of the liquid phase, e.g. film thickness and droplet diameter, approaches the dimension of the polymer coil. In addition to the spreading coefficient and the surface pressure effects, entropic elasticity of the polymer chain provides a strong contribution to the free energy for a constant volume V0=Ad ... 

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