Timothy A. Reinhold

Reduced drag coefficient for high wind speeds in tropical cyclones

The transfer of momentum between the atmosphere and the ocean is described in terms of the variation of wind speed with height and a drag coefficient that increases with sea surface roughness and wind speed. But direct measurements have only been available for weak winds; momentum transfer under extreme wind conditions has therefore been extrapolated from these field measurements. Global Positioning System sondes have been used since 1997 to measure the profiles of the strong winds in the marine boundary layer associated with tropical cyclones. Here we present an analysis of these data, which show a logarithmic increase in mean wind speed with height in the lowest 200 m, maximum wind speed at 500 m and a gradual weakening up to a height of 3 km. By determining surface stress, roughness length and neutral stability drag coefficient, we find that surface momentum flux levels off as the wind speeds increase above hurricane force. This behaviour is contrary to surface flux parameterizations that are currently used in a variety of modelling applications, including hurricane risk assessment and prediction of storm motion, intensity, waves and storm surges.

Hurricane Andrew's Landfall in South Florida. Part I: Standardizing Measurements for Documentation of Surface Wind Fields

Abstract Hurricane Andrews landfall in south Florida left a swath of destruction, including many failed anemometer recording systems. Extreme destruction led to exaggerated claims of the range of wind speed that caused such damage. The authors accumulated all available data from surface platforms at heights ranging from 2 to 60 m and reconnaissance aircraft at altitudes near 3 km. Several procedures were used to represent the various types of wind measurements in a common framework for exposure, measurement height, and averaging period. This set of procedures allowed documentation of Andrews winds in a manner understandable to both meteorologists and wind engineers. The procedures are accurate to ±10% for marine and land observing platforms, and boundary layer model adjustments of flight-level winds to the surface compare to within 20% of the nearest surface measurements. Failure to implement the adjustment procedures may lead to errors of 15%–40%. Quality control of the data is discussed, including tre...

Tropical Cyclone Destructive Potential by Integrated Kinetic Energy

Tropical cyclone damage potential, as currently defined by the Saffir-Simpson scale and the maximum sustained surface wind speed in the storm, fails to consider the area impact of winds likely to force surge and waves or cause particular levels of damage. Integrated kinetic energy represents a framework that captures the physical process of ocean surface stress forcing waves and surge while also taking into account structural wind loading and the spatial coverage of the wind. Integrated kinetic energy was computed from gridded, objectively analyzed surface wind fields of 23 hurricanes representing large and small storms. A wind destructive potential rating was constructed by weighting wind speed threshold contributions to the integrated kinetic energy, based on observed damage in Hurricanes Andrew, Hugo, and Opal. A combined storm surge and wave destructive potential rating was assigned according to the integrated kinetic energy contributed by winds greater than tropical storm force. The ratings are based...

Wind damage to envelopes of houses and consequent insurance losses

Abstract Examination of insurance claim files from Hurricanes Hugo and Andrew has revealed that most wind damage to houses is restricted to the envelope of the building. Rain entering the building then causes the insurance loss to be magnified by a factor ranging from two, at lower wind speeds, to nine at higher speeds. In wooded and urban areas near the coast, damage to buildings and their contents generally begins when the gradient wind speed reaches 40 m/s. There is a linear increase in the average insurance loss with wind speed until the gradient speed reaches about 70 m/s, at which point the average loss is approximately 12% of the insured value. Between 70 and 82 m/s (the upper limit observed in Hurricane Andrew) the average loss increases rapidly to 75%, although some small areas may experience losses over 90%. This rapid increase is associated with the loss of roof sheathing and damage to windows and doors. Probabilistic relationships are developed for expected insurance losses. These show that most hurricane-prone cities are more vulnerable to damage than inland cities, but South Florida represents an extreme risk. To reduce the vulnerability of future housing, it is recommended that envelopes be designed for the same probability of failure as the main structural system. A program to determine design loads and envelope component resistance is described. However, improvements in the wind resistance of the building stock will be slow and hurricane losses will remain high, unless large and aggressive retro-fitting programs are iniated.

Pressures on a surface-mounted rectangular prism under varying incident turbulence

Abstract The pressure and load coefficients obtained from two groups of eight pressure taps on the upper surface of a surface-mounted prism are characterized in terms of their mean, rms, peak, probability distribution, peak correlations and durations. The prism is a 1:50 scale model of the WERFL experimental building at Texas Tech University. Results obtained with flows generated over seven different wind tunnel floor-roughness configurations in the boundary layer wind tunnel at Clemson University cover a wide range of turbulence intensities. The results presented include the spatial variation of the peak pressure and peak load coefficients, and their variations with incident turbulence. The stochastic characteristics of the peak coefficients are also addressed here. The results reveal that the distribution of the peak coefficients is in general well established by the Extreme Value Type I (Gumbel) distribution. Conditional sampling is employed to study the duration as well as the space and space–time correlations of the peaks. Analysis of the peaks reveals that those with the larger magnitudes are generally of longer duration.

Importance of turbulence for the prediction of surface pressures on low-rise structures

Abstract Full/model-scale pressure coefficients at four different locations on the surface of a rectangular structure are compared in order to evaluate basic wind-tunnel simulation criteria. Analysis of field records from the Wind Engineering Research Field Laboratory (WERFL) are limited to those records for which the streamwise and lateral turbulence intensities closely matched those of the simulations. No attempt is made to scale the turbulence integral scale. Flow parameters and pressure data from field records are compared with those from two wind-tunnel model (scale 1:50) experiments. The first simulation features the conventional spire-roughness method, while for the second simulation small spires are added just upstream of the model location. The purpose of the addition of the small spires is to match the lateral turbulence intensities and to increase the small-scale turbulence content of the three components in the incident flow.

Effects of surface roughness element spacing on boundary-layer velocity profile parameters

Abstract Turbulent boundary-layer velocity profiles over rough surfaces depend on the size, shape and spacing of the roughness elements. In this study, simple theoretical equations to predict the aerodynamic roughness length and displacement height as functions of element spacing density are developed when cubes are used as roughness elements. These parameters are also evaluated from wind-tunnel tests for various element spacing densities. The experimental results agree well with the theoretical equations. Dependence of the shear velocity (or surface drag coefficient), turbulence intensity and longitudinal turbulent length scale on the element spacing density are also evaluated from the experiments.

Laser Doppler velocimeter measurements of separated shear layers on bluff bodies

Abstract Greater understanding of the mechanism of separation and reattachment of shear layers around bluff bodies and the separated regions associated with these shear layers is necessary to advance the knowledge of fluid–structure interactions. These mechanisms are particularly important in wind engineering in determining design pressures on structures. In almost all instances the maximum design pressure occurs in regions of separated flow. The details of the flow in these separated regions and the criteria for physical modeling of the flows in these regions are not well understood. Measurements have been conducted using a laser Doppler velocimeter (LDV) to examine the structure of separated shear layers and the associated regions of separated flow. A LDV is an important tool for this application. A LDV system is capable of measuring reversing flows and consequently high local turbulent intensities. Thermal anemometry does not allow measurements under these conditions. When making measurements with a LDV in air, it is necessary to seed the flow with particles. Seeding is a challenge in applying these measurement techniques to flows in boundary layer wind tunnels. Efforts to understand the basic mechanisms affecting pressure fluctuations and associated extreme pressures on a bluff body will be advanced by the ability to measure the flow in separated shear layers and within separated regions associated with these shear layers. The use of a LDV system provides a additional tool to explore this complex region.

Detailed simulation of pressures in separated/reattached flows

Abstract Mean and fluctuating pressure coefficients from four wind tunnel roughness configurations are investigated for three sets of pressure taps on the roof of a 1:50 scale model of the Texas Tech Wind Engineering Research Field Laboratory experimental building, and compared with the field data where possible. Pressure coefficients obtained from a set of pressure taps under the corner vortex are also investigated and presented. This investigation also includes the study of extreme pressure peaks and their duration, for individual pressure taps and area averaged pressures. Flow simulation requirements for wind tunnel experiments are introduced, which are considered essential for a realistic assessment of wind loads in critical areas.

Velocity-pressure correlation in stagnation and separation regions on surface-mounted prisms

Experiments are presented for a 1:50 scale model of the Wind Engineering Research Field Laboratory (WERFL) experimental building located on the Texas Tech University campus in Lubbock, Texas. The model was immersed in three different boundary layers generated in the Clemson University wind tunnel. These flows closely simulated the lowest 50 m of the atmospheric boundary layer at the WERFL site. The tests were conducted in order to study the velocity-pressure relations for the near-field and far-field flows. Observations were made with the flow normal to the roof edges for roof pressures along a line parallel to the flow, and for the roof-corner pressures with oblique azimuth angles. Additional results to study the far-field relation for stagnation flows were obtained in the Ruhr-University Bochum wind tunnel from a 1:350 scale model of the main building at the Eindhoven Technical University (ETU) in the Netherlands. Results of the nonlinear cross-bicoherence and linear cross-coherence between far-field and near-field velocity fluctuations and surface pressure fluctuations are discussed in detail. These coherences represent the contribution of the velocity fluctuations to the surface pressures according to Poissons equation.