A note on two-dimensional fence flows,with emphasis on wall constraint

An experimental investigation of the effects of wind tunnel blockage on the flow over two-dimensional surface mounted flat plates is reported. The blockage ratio was varied for a range of boundary layer thickness to fence height ratio ($\text{δ}\text{h}$) and wall friction to free stream velocity ratios ($\text{U}{}_{\mathrm{\tau }}\text{U}{}_1$). Measurements of fence surface pressures and the length of the downstream recirculation zone indicated that the effect of wall constraint was, in general, inseparable from the effects of the upstream flow characteristics. In contrast to the case of plates normal to a uniform free stream, blockage had a significant effect on the maximum front face pressures. This effect, like that on the base pressure, seemed to correlate with $\text{U}{}_{\mathrm{\tau }}\text{h}\text{ν}$ whereas the resultant variations of drag with blockage were dependent on $\text{δ}\text{h}$. Both the normalised length, $\text{R}{}_{\mathrm{<mtext>L</mtext>}}\text{h}$, of the recirculating region at zero blockage and its variation with blockage were also determined by $\text{δ}\text{h}$ only, but there was clearly no simple qualitative connection between $\text{R}{}_{\mathrm{<mtext>L</mtext>}}\text{h}$ and the base pressure, C_pp, as their often is for bluff bodies away from a boundary.Despite previous attempts in the literature to look for a correlation between fence drag and a single boundary layer parameter (Plate [1]; Good and Joubert [2]; Raju et al. [3], it is argued that any such correlation is probably marginal. The implications of these results for bluff body experiments of the ‘environmental’ kind are briefly discussed.     

Flow over spherical inflated buildings

The pressure distributions on three domes of different heights ($\text{h/c = 0.5, 0.37}\text{and}\text{0.25}$) have been measured in a boundary-layer wind tunnel in which the wind flow over a sparsely wooded area was simulated. The tests were made in the wall-law region of the boundary layer and the results are therfore first normalized using the undisturbed skin friction $\text{τ}{}_{\mathrm{<mtext>W</mtext>}}$.The tensions in inflated membranes of the same geometry were determined using the SAP IV finite-element program assuming that the effects of membrane weight and air friction are unimportant. The principal tensions then indicate where buckling would first take place and what would be the minimum internal pressure to just prevent it.The minimum internal gauge pressure to prevent buckling, which is shown to occur on the upwind side of the building, is 180 ($\text{h/c) τ}{}_{\mathrm{<mtext>W</mtext>}}$. This conclusion is compatible with the recommendations of the National Standards of Canada (1981) [1]. The location, orientation and magnitude of the largest principal tension indicate that rupture of the membrane would first occur near the top of the building.     

Numerical studies of flow through a windbreak

The pattern of flow through a porous windbreak has been investigated numerically using several well-known closure schemes (turbulence models). The shelter is included as a momentum extraction term in the streamwise momentum equation, for a fence having the value $\text{k}{}_{\mathrm{<mtext>r</mtext>}}\text{u}\text{|}\text{u}\text{|δ(x,0)s(z,H)}$ where k_r is the pressure-loss coefficient of the fence, ū is the local mean horizontal (x) velocity, δ(x,0) is the delta function and s(z,H) is a unit step function which is zero for heights (z) greater than the fence height, H. Previous experiments on neutrally stratified surface-layer flow through a porous fence were numerically simulated. Very good agreement with the observed velocity deficit in the near wake (x ? 15H where H = fence height) of the fence was obtained using a Reynolds-stress closure scheme. The predictions of the $\text{“k-?”}$ closure scheme (which includes turbulent kinetic energy and energy dissipation rate equations to estimate the eddy viscosity) and the simplest scheme tested, eddy viscosity $\text{K = K}{}_0\text{= ku}{}_{10}\text{z}$ eddy viscosity at all downwind distances equal to its value far upstream $\text{ku}{}_{10}\text{z}$ where k = von Karman's constant, $\text{u}{}_{\mathrm{1o}}\text{=}\text{friction velocity}\text{, z =}\text{height}$) were only slightly less satisfactory. Satisfactory estimates of the pattern of turbulent kinetic energy behind the fence were obtained. All simulations failed to predict the sharp speedup observed over the fence, and consequently yielded a slower rate of recovery towards equilibrium than observed. Attempts to improve prediction of the speed-over and the far wake by including corrections for mean streamline curvature were unsuccessful.Design aids for isolated windbreaks have been generated from the prediction of the second-order closure model. These give the velocity reduction to be expected in the near wake of the fence and the drag on the fence for a range of values of the fence pressure-loss coefficient, k_rmr.     

The behaviour of hot gases flowing within a staircase

This paper describes a detailed study of the movement of smoke in staircases using a one-fifth scale model of a five-storey staircase.The flow processes observed within the staircase were complex. The height ($\text{x}{}_{\mathrm{<mtext>s</mtext>}}$) of the nominal smoke base above the fire compartment (i.e., the height of the equivalent ideal smoke layer having the same static buoyant head as the gases in the stairwell) was found to be proportional to the fourth power of the ratio of the mass flow of air entrained within the stairwell ($\text{M}\text{dot}{}_{\mathrm{<mtext>e</mtext>}}$) to the mass flow of gases leaving the fire compartment ($\text{M}\text{dot}{}_{\mathrm{<mtext>cor</mtext>}}$)

$\text{i.e.}\text{X}{}_{\mathrm{<mtext>s</mtext>}}\text{ΔT}{}_{\mathrm{<mtext>cor</mtext>}}\text{=α(}\text{M}\text{?}{}_{\mathrm{<mtext>e</mtext>}}\text{M}\text{?}{}_{\mathrm{<mtext>cor</mtext>}}\text{)}{}^4$

where $\text{α = 4.9 × 10}{}^{\mathrm{?3}}\text{± 3 × 10}{}^{\mathrm{?4}}$.This equation can be solved simultaneously with an equation describing venting in the stairwell (also given in the paper) to give ($\text{M}\text{dot}{}_{\mathrm{<mtext>e</mtext>}}$), and hence the dilution of smoke within the staircase, and also the mass flow (and change in smoke concentration) of gases leaving the staircase, perhaps into an upper floor.These equations are suitable for incorporation into existing computer programs for predicting movement of smoke and fire gases, with the aim of increasing their usefulness as design aids for life safety in buildings.     

Essais d'aerateurs: Enseignements tires de 500 essais en eau claire effectues dans 200 stations d'epuration differentes—II ResultatsTests on aerators: Conclusions drawn from 500 clean water tests conducted in 200 different sewage treatment plants—II Data interpretation

The large number of aerator tests conducted in the field by the CEMAGREF teams has allowed developing a critical review of tests results by means of statistical analysis. Measurement quality is estimated from the variation of the transfer coefficient values from one sampling point to another. Individual transfer coefficient values $\text{(}\text{K}{}_{\mathrm{L}}\text{a)i}$ are compared with the average value of the test $\text{(}\text{K}{}_{\mathrm{L}}\text{a). (K}{}_{\mathrm{L}}\text{a)i/}\text{K}{}_{\mathrm{L}}\text{a}$ ratios correlating with one year's data are gathered in histogramms (Fig. 1).A χ² test is used for evaluating the fit of a Normal distribution on each histogram (Tables 1 and 2 results for two kinds of surface aerators). When the Normality is not rejected, the variances between distributions are compared by means of a Fisher test.$\text{(K}{}_{\mathrm{L}}\text{a)i/}\text{K}{}_{\mathrm{L}}\text{a}$ variations in a same run are independent of transfer coefficient when the K_La ranges 1–10 h^?1 and applied specific power ranges 15–70 W m^?3. Those variations allow evaluating the technique of measuring, the reliability of oxygen probes and the operator's experience (Table 3).In a well mixed tank and when the recommended procedure is correctly implemented the relative error in measuring the specific oxygen input (kg O₂ kWh^?1) is 5%. In less favorable cases it reaches 10%.Tables 4 and 5 show the reproductibility of repetitive run results in the same plant and of tests performed with the same aerator in different plants.The histograms of the performances of the main types of mechanical surface aerators (high speed vertical shaft turbine, slow speed vertical shaft turbine, horizontal shaft aerator) are presented (Figs 2, 4, 5). Relations between oxygenation capacity and specific power consumption are plotted in Figs 3 and 6. As to other aeration systems, fewer data are available. Table 6 summarizes the usual range of performances.In 1974, after a series of measurements performed on 72 different plants, CEMAGREF stated that the oxygenation capacities claimed by manufacturers were, on the average, 40% higher than actual results. Now, the performances predicted by manufacturers in France are generally very close to the measured values. A difference higher than 10% between predicted values and measured ones now occurs as an exception.     

Design against wind-induced vibration of multi-flue chimney stacks

Experimental study based on flow visualization, model oscillation testing and power spectral measurement has revealed the complexity of flow passing a multi-flue arrangement. The occurence of instability due to various aerodynamic excitation is possible. In the case of the sectional configuration having flues and structural members in a pentagonal arrangement, the oscillation induced by wind was found to be due to galloping at reduced wind velocity greater than $\text{V}{}_{\mathrm{<mtext>r</mtext>}}\text{= 16}$ with the amplitude of oscillation increasing with increase in wind velocity. With the application of slat devices to the chimneys, galloping oscillation was suppressed completely. As it is not yet possible to predict the response of a multi-flue chimney stack in wind with any certainty, it is useful to consider means of reducing any oscillatory tendency by original design. The design of the new chimney stack for the Royal United Hospital in Bath against the possibility of wind-induced vibration is discussed as an example.     

Ultimate strength of compressed stiffened plates and minimum stiffness ratio of their stiffeners

A theoretical investigation into the effectiveness of a stiffener against the ultimate strength of a stiffened plate is carried out. Series of the buckling analyses, the elastic large deflection analyses and the elastic-plastic large deflection analyses are performed by the analytical method and the finite element method on the stiffened plate under thrust. Experiments are also carried out on the stiffened plate under thrust to confirm the theoretical results.On buckling of a stiffened plate, it is well known that ther exxists a minimum stiffness ratio of a stiffener to the plate, $\text{λ}{}^{\mathrm{B}}{}_{\min }$, which gives the maximum limiting value of the buckling strength. Concerning the ultimate strength it was confirmed that there exists a significant stiffness ratio of a st stiffener to the plate, $\text{λ}{}^{\mathrm{U}}{}_{\min }$ similar to $\text{λ}{}^{\mathrm{B}}{}_{\min }$ for the buckling strength.It was also found that there are three typical collapse modes for the stiffened plate under thrust, that is: (1) MODE OO, overall collapse after overall buckling; (2) MODE LO, overall collapse after local buckling; (3) MODE LL, local collapse after local buckling.Approximatelmethods are proposed to evaluate the ultimate strength and $\text{λ}{}^{\mathrm{U}}{}_{\min }$ of a multi-stiffened plate under thrust.The effects of initial imperfections such as welding residual stresses and initial deflection on ultimate strength and $\text{λ}{}^{\mathrm{U}}{}_{\min }$ of a stiffened plate under thrust are also discussed.     

Time-averaged aerodynamic forces acting on a hemisphere immersed in a turbulent boundary

The time-averaged pressure distribution over the surface of a hemisphere immersed in a fully developed turbulent boundary layer developing along a smooth, plane wall has been measured for a number of hemispheres of differing diameter d in order to establish the relationship between the aerodynamic force acting on such a hemisphere and the characteristics of the boundary layer. It is found that the drag coefficients defined by $\text{C}{}_{\mathrm{<mtext>D</mtext><mtext>\tau </mtext>}}\text{= D/(}\text{1}\text{2}\text{πu}{}_{\mathrm{\tau }}{}^2\text{A}$ may be expressed as a function of u_τd/ν alone in the range $\text{d/δ ? 1.0}$, where D is the pressure drag, u_τ the shear velocity, ν the kinematic viscosity, A denotes the area of the hemisphere projected onto a plane normal to the main flow direction, and δ is the boundary-layer thickness.     

Analysis of atmospheric turbidity for daylight calculations

A large set of illuminance and irradiance data has been collected for four years at 15-minute intervals in San Francisco. This data set has been used to investigate the impact of atmospheric turbidity on daylight calculations. Existing predictive formulae for Linke turbidity, $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$, provide moderate agreement to measured values of $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$ when using nominal design values for the Ångström scattering coefficient, β, and precipitable water vapor, w. When average measured values for β and w are used, the agreement improves. We suggest the use of an illuminance turbidity, T_il, to calculate direct normal illuminance directly. We derive a simple approximate solution, T_il = 1 + 21.6 β. T_il appears to be a better parameter to describe atmospheric conditions since, unlike $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$, it is insensitive to air mass and thus solar altitude or time of day. We present and compare plots of T_il and $\text{T}{}_{\mathrm{<mtext>l</mtext>}}$ vs. solar altitude, time of day, and month. Finally, we examine and compare several alternative pathways to derive direct normal illuminance from irradiance and luminous efficacy (dependent on β and w), or directly from β.     

On the contact pressure of circular plates on elastic subgrade

A methematical model is presented based on combined methods for determining the contact pressure of circular plates on elastic subgrades. The proposed numerical approach considers two parameters of the subgrade simultaneously, the modulus of elasticity $\text{E}{}_{\mathrm{s}}$ and the modulus of subgrade reaction $\text{K}$. This approach can be utilized in the design of circular foundations such as oil and water towers, silos, chimneys and other structures of similar nature.     

Methyl chloride as a gas-tracer for measuring stream reaeration coefficients—I: Laboratory studies

A method is described for measuring stream reaeration rate coefficients using methyl chloride as a gas-tracer. Methyl chloride is a stable tracer that is capable of being determined to a lower detection limit of 1 × 10^?12 g by a gas chromatographic technique. Laboratory studies are reported, in which k₂ and the analogous transfer coefficient for methyl chloride, k_CH3Cl, are measured simultaneously at different temperatures in the range 5–35°C. The laboratory results indicate a temperature dependence for the transfer coefficient ratio, $\text{k}{}_{\mathrm{<mtext>CH</mtext>}}{}_3{}_{\mathrm{<mtext>Cl</mtext>}}\text{k}{}_2$, fitted empirically to an expression based upon the absolute rate model for diffusion giving

${}^{\mathrm{k}}\text{CH}{}_1\text{Cl}\text{K}{}_2\text{=4.323 exp(?}\text{530.97}\text{T}\text{)}$

where T is the absolute temperature.     

Terrain-dependence of longitudinal R.M.S. velocities in the neutral atmosphere

Values of the ratio of the longitudinal r.m.s. velocity σ to the friction velocity $\text{u}{}_{\mathrm{★}}$ are compared for a number of sites for which these quantities are adequately defined. Conflicting hypotheses concerning the variation of this ratio with terrain are examined in the light of some recent roughness-change measurements and theoretical results. Although the results of this re-examination are not conclusive, they tend to favour the traditional view that $\text{σ/u}{}_{\mathrm{★}}$ is constant in fully developed equilibrium flows which are not affected by roughness changes.     

A comprehensive study on the biological treatabilities of phenol and methanol—II the effects of temperature,pH, salinity and nutrients

The effects of temperature, pH, salinity, and nutrients on bacterial activities were investigated and evaluated using a statistical method. The substrate utilization rate coefficient (k) decreased as pH deviated from neutral and as salinity increased, and the unfavorable pH and salinity alleviated the temperature effect on k. The modified Arrhenius equation, $\text{k}{}_{\mathrm{<mtext>T2</mtext>}}\text{= k}{}_{\mathrm{<mtext>T1</mtext>}}\text{θ(}{}^{\mathrm{T2?T1}}\text{)}$, was not effective in describing the temperature effect on k: the temperature coefficient (θ) ranged between 1.0–1.4 depending on the temperature range, pH, salinity, and substance (phenol or methanol). The endogeneous respiration activity was affected by various environmental factors such as pH, temperature, and salinity; however, the cell decay coefficient (k_d) turned out to be correlated to a single parameter, k. Thus, k_d = 0.066 k^0.87 and k_d = 0.0115 k^0.634, where k and k_d are based on the unit of h^?1, were proposed for the prediction of cell decay coefficient for phenol and methanol acclimated activated sludge, respectively. In batch treatment of 770 mg l^?1 of phenol and 1000 mg l^?1 of methanol as TOC, nitrogen and phosphorus did not have any recognizable effect on k, while trace elements such as Fe²⁺, Mg²⁺, Mn²⁺, Ca²⁺, and Zn²⁺, etc. showed a slightly perceptible effect on it. The absence of extra-cellular nitrogen and phosphorus resulted in a greater cell yield; however, the cells in this condition decayed more rapidly than normal cells. The primary factor affecting the substrate decomposition rate in natural systems was pH: phenol decomposition resulted in a considerable decrease in pH so that the buffering capacity of the water was the most important factor, and methanol decomposition did not affect pH significantly so that the initial pH of the water was the most important factor. An initial lag phase was observed in 8 out of 115 phenol batch tests and 31 out of 66 methanol batch tests.     

La diffusion intraparticulaire dans la clinoptilolite application aux ions Na+ et NH4+Intracrystalline self-diffusion of ions in clinoptilolite ammonia and sodium cations studies

Studies on NH₄⁺ removal by mean of a zeolite: clinoptilolite, previously realised have led the authors to examine the exchange mechanisms between cations on the clinoptilolite grains.The use of clinoptilolite as conventional ion exchange resin (1, 2, 3, 4, 5, 6) for elimination of ammonia ion from waste waters or drinking water (7, 8, 12) poses a great number of difficulties, principally during regeneration.This text presents a brief theorical presentation of the material and the ion-exchange, especially in the case of isotopic exchange, allowing the introduction of the fundamental equation (2) and its approximate expression:

$\text{U(t)}\text{1?exp}\text{?}\text{D}\text{tπ}{}^2\text{r}{}^2{}_0{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$

with U(t) the exchanged fraction at time t.The experimental study concerns the NH₄⁺ cation exchange with Na⁺ ions on clinoptilolite.In this case, the curve u(t) and ln(l ? u²) vs time shows the non-applicability of the above formula (Fig. 5).Our subject was to examine the divergence causes and to propose for our regeneration problem a semi-empiric law which form is:

$\text{U(t)}\text{1?exp}\text{?}\text{β}\text{D}\text{tπ}{}^2\text{r}{}^2{}_0{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$

with

$\text{β=[1?U(t)]}\text{2}\text{3}$

.The validated relation (Fig. 7) for the Na⁺/NH₄⁺ exchanges lead us to find the D_Na⁺ and D_NH2⁺ coefficients consistent with the literature.Using the experimental results from AMES, we show that our formula can be applied to other exchange cases (e.g. Sr⁺/NH₄⁺) (Fig. 8). The form of the β relation seems coherent with the proposed film diffusion interpretation.     

Aerodynamic forces acting on a rectangular prism placed vertically in a turbulent boundary layer

An experimental investigation was carried out into the aerodynamic forces acting on a rectangular prism with a square cross-section, placed vertically in a turbulent boundary layer. Experimental data were collected to examine the effects on the aerodynamic forces due to variations of (1) the parameters characterizing the boundary layer, (2) the aspect ratio of the prism, and (3) the angle of attack of approaching flow. For flow with zero angle of attack, the pressure drag coefficient of the prism defined by $\text{C}{}_{\mathrm{<mtext>D</mtext><mtext>\tau </mtext>}}\text{= D/(}\text{1}\text{2}\text{?u}{}_{\mathrm{\tau }}{}^2\text{hw)}$ was found to be expressed by a power function hu_τ/ν in the range h/δ < 1.2?;1.4 for each aspect ratio $\text{h}\text{w}$, where D is the time-mean drag, u_τ is the shear velocity, ? is the density of fluid, h and w are the height and width of the prism, respectively, ν is the kinematic viscosity, and δ is the thickness of the boundary layer. It was also found that the drag coefficient is uniquely related to h/δ in the range of h/δ larger than 1.2–1.4, and the characteristics of the mean and fluctuating aerodynamic forces varying with the angle of attack are similar to those obtained from a two-dimensional rectangular prism. However, the magnitudes of the mean and fluctuating forces were found to be considerably smaller than those of the two-dimensional rectangular prism.