P.J. Richards

Appropriate boundary conditions for computational wind engineering models using the k-ε turbulence model

Abstract In computational models of wind engineering problems within the atmospheric surface layer the approach flow should normally be modelled as a homogeneous flow. Velocity and turbulence profiles associated with the k-e turbulence model are proposed which produce homogeneous conditions. These equations are discussed in the light of full-scale measurements at Silsoe using sonic anemometers. It is suggested that the model constants k = 0.42, Cμ= 0.013 and σe= 3.22 more closely match the data obtained. It is also shown that the cospectrum for the Reynolds stress exhibits a characteristic frequency no≈ u*/z which is consistent with the suggested profile equations.

Flow patterns and pressure field around a full-scale building

Abstract This paper describes measurements of the flow patterns and pressure field around a low-rise portal framed building (24 m long × 13 m span × 4 m eaves height, 10° roof pitch) on which extensive full-scale wind load measurements have been made. Flow patterns have been observed using smoke, generated from a smoke canister, nylon tufts attached to the surface of the building, and on one occasion the deposition of snow on and around the building. These techniques provide some quantitative information about the flow pattern are primarily qualitative, aiding an understanding of the three-dimensional flow field for comparison with wind-tunnel studies and computational fluid dynamics predictions. A technique, using static pressure probes, is described which provides more quantitative information on the pressure field around the building for a wind direction transverse to the line of the ridge. Measurements were made on the major axes of the building, extending to 50 m upstream, 10 m downstream, and 20 m to one side. Over 220 hours of recordings were made when the mean wind speed at building ridge height was greater than 8 m/s. Results of these pressure field measurements are presented and compared with predictions from computational fluid dynamics using a k-ϵ model. These results have implications for the siting of instrumentation in boundary layer wind tunnels where a major problem has been the location of the reference static pressure sensor. The static field measurements give an inidcation of suitable locations and possible levels of error in past work.

Wind loads on porous structures

The wind loads on porous structures depend strongly on the resistance to through flow, which may be characterised by the loss coefficient. It is shown that for round wire mesh screens the loss coefficient is related to the porosity (β). For other structures the loss coefficient is a function of the porosity and the construction. It is therefore suggested that it is useful to use an effective porosity (βe), which is the porosity of a round wire mesh screen with the same loss coefficient. It is shown that loads on porous structures are less than those on solid structures by a factor (1−βe). When porous structures are at an angle to the wind, the effective loss coefficient is reduced by a cos2(θ) factor, where θ is the angle between the wind and a normal to the surface. As a consequence the corresponding effective porosity increases and the loads decrease. These concepts are shown to match results from a number of sources.

The influence of Helmholtz resonance on internal pressures in a low-rise building

Abstract This paper deals with an investigation of the phenomenon of Helmholtz resonance under oblique wind flow, and an examination of the applicability of the quasi-steady approach to internal pressures in buildings with a dominant opening. Studies on a 1:50 scale model of the Texas Tech University (TTU) test building in a boundary layer simulation show that ‘Helmholtz resonance under oblique wind flow’ produces an extremely strong response in internal pressure fluctuations, in comparison with that obtained under normal onset flow. It is verified that ‘eddy dynamics over the opening’ rather than ‘freestream turbulence’ is responsible for the intense excitation at oblique flow angles, implying that even if the Helmholtz resonance frequency were to be in the tail of the freestream turbulence spectrum, severe excitation would still be possible.Experimental measurements of internal pressures for a range of opening situations also reveal that the quasi-steady approach is inapplicable in the prediction of peak internal pressures. Furthermore, it is demonstrated that while the provisions of the Australian/New Zealand wind loading code—AS/NZS1170.2:2002, which is based upon the quasi-steady method, is adequate as far as mean internal pressures are concerned, it however underpredicts peak internal pressures in some situations. In particular, for the range of situations studied, measurements indicated that peak pressures were up to 25% higher than the AS/NZS1170.2:2002 provisions, in the case of openings in the positive pressure and sidewall regions. It is also shown that for openings located in the sidewall region, peak internal pressures could be just as extremely positive as it can be negative. It is suggested that in the calculation of internal pressures, the AS/NZS1170.2:2002 provide for the use of local pressure factors K l , that are at present applied only to external pressure calculations. Secondly, the code should provide for internal pressure coefficients to be both negative and positive, when openings are located in sidewall regions. Finally, in order to account for the effects of additional fluctuations arising from Helmholtz resonance oscillations, the possibility of the use of an internal pressure factor K i should be explored.

Computational modelling of the transient response of building internal pressure to a sudden opening

Abstract This paper describes the computational and experimental modelling of the transient response of internal pressures to sudden openings in buildings and models. It is shown that the computer model correctly predicts both the Helmholtz frequency and the decay rate of internal pressure. A study of the flow patterns from the computational results also show that a vena-contracts forms in the oscillating flow following a sudden opening. It is thought that viscous losses may be important and new analytic models have therefore been suggested which have been used to obtain a match with the experimental results.

Computer modelling of downwind sails

Abstract An investigation into the use of Computational Fluid Dynamics to model downwind sails has been carried out to determine its place in the design process. The finite volume software package CFDS-FLOW3D was used to model the flow around a spinnaker/mainsail combination. The sail shapes used were those designed by North Sails NZ Ltd for the Whitbread 60 yacht Winston. Both uniform flow and logarithmic flow with wind twist can be modelled using this method. The sails were modelled in a uniform flow at apparent wind angles from 70° to 180 at intervals of 10. Force coefficients were calculated at each interval to compare with wind tunnel data at 90 apparent. Sail trim was carried out by altering the spinnaker pole and boom angles. The differences between the computed and wind tunnel results were 15% for lift and 3% for drag. Comparisons between uniform and logarithmic flow show that the logarithmic flow resulted in a decrease in the force coefficients of approximately 30%. This is comparable with wind tunnel tests. Flow visualisation, using the software package SeeFD, was also carried out to gain an understanding of the resulting flow. This investigation has shown that CFD is a useful tool for the design of downwind sails.

Computational and wind tunnel modelling of mean wind loads on the Silsoe structures building

Abstract Results from both wind-tunnel and computational modelling of mean wind loads on the Silsoe Structures Building are compared with full-scale data. It is shown that systematic differences exist between wind-tunnel data obtained at BRE (U.K.), UWO (Canada) and the University of Auckland (NZ). The Auckland wind-tunnel data is well correlated with the full-scale data but shows increasing scatter for low pressure regions. Numerical solutions obtained using the CFD package PHOENICS also show reasonable correlation with the full-scale data but exhibit greater scatter than obtained with wind-tunnel modelling.

Structure of the atmospheric boundary layer below 25 m and implications to wind loading on low-rise buildings

Abstract Measurements of instantaneous wind velocity have been made in the flow approaching a low-rise building at a position sufficiently far up-stream to be unaffected by the buildings pressure field. Analysis of the measurements show that the mean streamwise velocity profile is well represented by a log-law but that velocity spectra, espcially close to the ground, depart from the expected form in the inertial sub-range. The work illustrates, through spectral analysis, the requirement to relate surface pressures on a building to the approaching flow originating from a height corresponding to that of the streamline affecting the tapping point. An empirical form of the velocity spectrum is presented which gives a good representation of the measurements made and which has general application to wind engineering.

Full-scale wind load measurements point the way forward

Refinement of experimental technique has enabled more detailed analyses to be made of the full-scale measurements on the Silsoe Structures Building. This has shown the relevance of quasi-steady analysis in assessing rms and maximum and minimum pressure coefficients, and highlighted the effect of turbulence averaging on the mean pressure coefficient. Comparisons of the spectra of the approaching turbulent flow and of the surface pressure indicate the changing patterns that occur as the flow develops over the building. The results of the quasi-steady analysis have direct implications on wind tunnel modelling.

A re-examination of the characteristics of tropical cyclone winds

Abstract This paper describes a review of past measurements with respect to tropical cyclone winds, to show that convective instability in such storms results in enhanced turbulence levels. An examination of AS1170.2-1989 in the light of this shows that some provisions including turbulence intensities might be inadequate. Drawing on the past data, the authors recommend increases in turbulence intensities for tropical cyclone-affected regions. The enhanced turbulence characteristics of the tropical cyclone wind also means that the turbulence energy spectrum of the wind will not be adequately described by the ESDU model for neutral stability conditions. It is suggested that the spectral relations for unstable conditions be utilised. A re-examination of the mean and gust velocity profiles in the AS1170.2-1989 might also be warranted.