R.P. Hoxey

Bluff bodies in deep turbulent boundary layers : Reynolds-number issues

It is generally assumed that flows around wall-mounted sharp-edged bluff bodies submerged in thick turbulent boundary layers are essentially independent of the Reynolds number Re, provided that this exceeds some (2–3) × 104. (Re is based on the body height and upstream velocity at that height.) This is a particularization of the general principle of Reynolds-number similarity and it has important implications, most notably that it allows model scale testing in wind tunnels of, for example, atmospheric flows around buildings. A significant part of the literature on wind engineering thus describes work which implicitly rests on the validity of this assumption. This paper presents new wind-tunnel data obtained in the ‘classical’ case of thick fully turbulent boundary-layer flow over a surface-mounted cube, covering an Re range of well over an order of magnitude (that is, a factor of 22). The results are also compared with new field data, providing a further order of magnitude increase in Re. It is demonstrated that if on the one hand the flow around the obstacle does not contain strong concentrated-vortex motions (like the delta-wing-type motions present for a cube oriented at 45? to the oncoming flow), Re effects only appear on fluctuating quantities such as the r.m.s. fluctuating surface pressures. If, on the other hand, the flow is characterized by the presence of such vortex motions, Re effects are significant even on mean-flow quantities such as the mean surface pressures or the mean velocities near the surfaces. It is thus concluded that although, in certain circumstances and for some quantities, the Reynolds-number-independency assumption is valid, there are other important quantities and circumstances for which it is not.

Spectral models for the neutral atmospheric surface layer

Abstract Measurements of atmospheric turbulence spectra and cospectra at low heights in a rural atmospheric surface layer are presented. These show that at low frequencies the streamwise and transverse spectral densities vary in proportion to the square of the mean velocity. It is also shown that in the inertial sub-range the data is well matched by the Kolmogorov law with the Kolmogorov constant invariant with height. At intermediate frequencies the data exhibit spectral power-law regions with exponents that decrease with height. Spectral models are proposed which closely match these observed behaviours.

Full-scale measurements and computational predictions of wind loads on free-standing walls

Recent developments in the wind loading codes for the UK, Australia and Europe have introduced new, more onerous, pressure coefficient data for the design of free-standing walls. These data, derived from wind-tunnel studies conducted in the mid-1980s in the UK and Australia, have been called into question by various interested parties. In 1993, a research programme was initiated to undertake an independent, full-scale study of the wind pressures on free-standing walls in order to critically appraise the new data and to determine reliable design data. The full-scale, variable-geometry experimental facility which includes automatic, rapid, data-logging instrumentation is described. To supplement this full-scale work, CFD investigations in 2 and 3 dimensions have been undertaken at the University of Auckland using the PHOENICS finite volume code, Version 2.1, with a k−e turbulence model. Comparisons are presented which reveal that despite the simplicity of its structural form, the free-standing wall exhibits surprising aerodynamic effects which render it an excellent and highly challenging test case to model computationally.

The Silsoe Structures Building: Comparisons of pressures measured at full scale and in two wind tunnels

Abstract Two independent sets of full-scale wind pressure measurements have been completed on the Silsoe Structures Building in both its configurations (featuring a conventional sharp eaves and a modern curved eaves). Results are compared with those from two 1 : 100 scale measurement programmes conducted in two leading wind tunnels on models of the building with both the sharp and curved eaves. These comparisons highlight the importance of implementing certain wind tunnel procedures in order to obtain good predictions of full-scale mean pressure coefficients. However, areas of high negative pressure still tend to be under-estimated in wind-tunnel measurements and this is considered to be associated with a Reynolds number effect.

Full-scale testing to determine the wind loads on free-standing walls

Abstract Recent developments in the wind loading codes for the UK, Australia and Europe have introduced new, more onerous, pressure coefficient data for the design of free-standing walls. These data, derived from wind-tunnel studies conducted in the mid 1980s in the UK and Australia, have been called into question by various interested parties. In the summer of 1993, a 3 yr research programme was initiated to undertake an independent, full-scale study of the wind pressures on free-standing walls in order to critically appraise the new data and to determine; reliable design data. The full-scale, variable-geometry experimental facility which includes automatic, rapid, data-logging instrumentation is described. Results obtained in the first year of measurements are presented which reveal the significance of wall length, position along the wall, wind direction and the presence of return corners. Priorities for the future are also outlined.

Design code, full-scale and numerical data for wind loads on free-standing walls

Abstract Following the release in 1991 of Draft BS6399, Part 2: Code of practice for wind loading, concern arose over the appearance therein of significantly increased wind pressure loadings for free-standing walls. This paper reviews the data and compares them with data from other recognised design sources, both traditional and recently introduced. The results of full-scale wind pressure measurements on two long, 5 m high masonry walls are presented together with the results from a series of computational fluid dynamics analyses on a two-dimensional wall and comparisons are made with the data from the recognised design sources which are all based on wind-tunnel results. An unsatisfactorily diverse range of data is shown to exist in the design sources. A programme of full-scale measurements is considered to be required to provide an independent appraisal of the wind-tunnel data on which existing design data are based and to establish reliable and economic loading data for free-standing walls.

Wind loads on boundary walls: Full-scale studies

Recent revisions to the wind loading codes for the UK and Australia, and the recent draft European Standard, Eurocode 1, provide new, more onerous, pressure coefficient data for the design of boundary, or free-standing walls. This design information, although obtained from modern wind-tunnel tests, has been called into question by various interested parties. In 1993, a research programme was initiated to undertake an independent, full-scale study of the wind pressures on free-standing walls in order to critically appraise the new data and to determine reliable design data. The full-scale, variable-geometry experimental facility which includes automatic, rapid, data-logging instrumentation is described, and results are presented which show the strong dependencies loads have on wall length, wind direction, position along wall, the presence of a gap, and the presence of return corners. Despite the simplicity of its structural form, the free-standing wall produces surprising aerodynamic effects which have prompted complementary work: reference is made to collaborative research, both model-wind-tunnel testing and computational modelling, which are helping to resolve some of the outstanding questions.

The folly of using extreme-value methods in full-scale experiments

Abstract Extreme-value methods have been applied successfully to meteorological measurements of wind speed to predict extreme events which have a given probability of exceedence. Attempts have been made to extend the method to give extreme values of loads on buildings. Although this method may have application to stationary flows, it is demonstrated here that it is misleading to apply extreme-value methods to full-scale wind-load measurements where the flow is intrinsically non-stationary. This finding may also apply to many wind-tunnel studies where records are of insufficient length to satisfy the condition for stationarity or the flow properties are not controlled adequately.

Correction of wind-tunnel pressure coefficients for Reynolds number effect

Detailed measurements on a low-rise building made at full scale and on a wind-tunnel model show a significant Reynolds number effect associated with the separated flow region on the windward roof slope of the building. Other possible causes for the observations are explored but are systematically eliminated. Brief details of the experimental procedure are presented together with significant results which suggest generalised corrections for scale effects. It is also proposed that the sensitivity of the separated flow at the windward eaves to Reynolds number has a secondary effect on the flow pattern downstream. Further detailed measurements are required to enable the correction factors to be generalised for other types of structure.

The role of corner vortices in the design of structures

The sharp edges on bluff bodies (flat shapes) frequently cause the air flow around the bodies to separate and generate a vortex flow pattern which produces severe suction loadings in the separated flow region. This paper presents examples from full-scale flow studies around low-rise bluff bodied structures that illustrate the types of flow separation that can occur. The area of influence of the vortices generated from the edges of the buildings are assessed from surface pressure measurements; edge details can be used to mitigate against vortex generation.