Gust Strength Research Articles

Dimensional analysis of a transverse gust encounter

A dimensional analysis of transverse gusts was performed using the Buckingham Π theorem in an effort to standardize their characterization. This analysis identified three key terms that characterized transverse gusts: gust strength, gust width, and gust development rate. The strength and width terms currently exist in gust literature with varying amounts of general use and are commonly defined as the gust ratio, GR=v/U∞, and encounter width, We=w/c, respectively. Gust development rate is a new parameter to fully characterize and compare different velocity profiles and is similar to characterization metrics used in other unsteady flow research. For a single persistent or discrete gust the existing term is dimensionless pitch rate, Ω⁎=α˙c/2U∞, and for sinusoidal gusts the term is reduced frequency, k=πfc/U∞. These existing terms are therefore used as the dimensionless gust development rate. A short analysis of these terms demonstrates how all other desired information about a gust profile can be determined from these parameters. The set of dimensionless parameters allows for a more complete classifications of gusts based on strength and development rate. The parameters are applied to a selection of recent gust studies to show how the addition of a gust development rate term can help to identify areas of the gust field that are understudied.

Assessment of discrete wind gust parameters: Towards the worst-case scenario of a FSI application in form of an inflated hemisphere

The paper is a step towards the evaluation of the worst-case scenario caused by strong wind gusts impacting civil engineering air-inflated lightweight structures. These extreme events with short durations but high strengths are responsible for short-term highly instantaneous loads endangering the structural integrity of the design. For this purpose, a generic test case is defined which includes a discrete wind gust model, the approaching turbulent boundary layer and a flexible structure exposed to the resulting fluid flow. The simulation framework relies on a partitioned coupled solver for fluid–structure interaction extended by two source-term formulations which allow to inject the wind gusts as well as the background turbulence. To save CPU time, a part of the investigations is conducted for the rigid case as a physical meta-model. The particularly critical cases found in this way were examined for the case of the flexible structure. Under varying system parameters such as the strength, length and position of the gust the following objective functions are evaluated: Force coefficients, maximal deflections and local inner stresses. The worst case occurs for maximal gust strength and length, when the gust hits the membrane at half height. Furthermore, the effect of the superposition of the discrete gust with background turbulence is analyzed for two scenarios. The gust is first superimposed to different inflow turbulences of the same intensity leading to non-negligible deviations of force coefficients and deflections. Second, the level of the turbulence intensity is successively increased up to a factor of five showing only a minor effect on the flexible structure not generating a new worst case.

Using historical tropical cyclone climate datasets to examine wind speed recurrence for coastal Australia

Likelihood estimates of extreme winds, including those from tropical cyclones (TCs) at certain locations are used to inform wind load standards for structural design. Here, wind speed average recurrence intervals (ARIs) determined from TC climate data dating back to the 1970s in two quantile–quantile adjusted reanalysis datasets (ERA5 and BARRA [1990]), and best-track observations for context, were compared with Standardized ARIs (AS/NZS) across seven tropical and two subtropical Australian inland coastal regions. The novelty of this work lies in determining TC-wind speed ARIs from a range of datasets that are not typically used to evaluate this metric. Inherent differences between the data used to determine the Standard ARIs (large sample size allow for larger extrapolations; GEV function) and TC data ARIs (smaller sample size and less certain data; the more asymptotic Lognormal/Weibull functions are used) led to the use of different extreme value functions. Results indicated that although these are two distinct ways of determining design wind speeds, when they are considered equivalent, there was a moderate reproduction of the ARI curves with respect to the Standard in both reanalysis datasets, suggesting that similar analyses using climate model products can provide useful information on these types of metrics with some caveats. Trends in TC wind strength affecting coastal Australia were also analyzed, indicating a potential slight downtrend in tropical West coast TC wind strength and slight uptrend for tropical East coast TC wind strength, noting considerable uncertainty given the short time period and limitations of data quality including over longer time periods. Such trends are not only limited to the relationship between TC intensity and anthropogenic warming, but also to regional changes in TC frequency and track direction. This could lead to significant trends emerging in regional Australian TC wind gust strength before several decades of warming have occurred. It is hoped that climate models can provide both longer-term and a more homogenous base for these types of evaluations and subsequent projections with respect to climate change simulations.

Development of a Low-Turbulence Transverse-Gust Generator in a Wind Tunnel

This work demonstrates the viability of a novel transverse-gust generator that is capable of producing a controllable time-varying gust without increasing the turbulence level within a large region of the flow facility. The new gust-generator concept is based on a vortex-generator array () along one of the walls of a facility’s test section at a given streamwise location. Using such a device, a steplike gust and a sinusoidal gust with amplitudes of 5.7% of freestream velocity are demonstrated in a wind tunnel. The sinusoidal gust produces practically pure-harmonic oscillation in the freestream’s direction of at a frequency of 2 Hz for a freestream speed of . A simplified vortex-array model is shown to be a viable tool for the design of the new gust generator. This study is focused on demonstrating the concept of the gust generator while leaving the generator’s design optimization and the exploration of the limits of the gust strength and uniformity to future work.

Agent-based modelling of wind damage processes and patterns in forests

Powerful storms, consisting of strong gusts and winds, damage forests. Therefore, foresters need forest management strategies to reduce the damage risk. This paper focused on the damage patterns within the forest as the final results of multiple tree-wind dynamic interactions in time and space during a storm. Recent developments in computer technology allow for the possibility of simulating the complex and dynamic phenomena of damage during a storm but are extremely time consuming. To simplify the simulations without losing the crucial aspects of wind damage in forests, we introduced a computer simulation model using the agent-based modelling (ABM) technique, which capture the phenomena and interactions of individuals called ‘agents’. We created an ABM for forest wind damage simulation, coupling together an accepted understanding of wind gusts in forests, tree bending moments, and damage propagation. The model was tested with variations in three conditions: trees acclimated and unacclimated to their wind environment; three levels of gust strength; and three tree planting densities. The ABM was able to replicate damage patterns and demonstrate damage propagation within the forest and the effects of forest edges. The difference in the rate of damage in the forest between acclimated and unacclimated edges became similar with an increase in the gust intensity, and a decrease in tree density through a reduction in the shelter effect of the forest. The ABM could be improved in the future by parametrizing the variation in individual tree resistance, and the variation in gust and wind strength, as well as adding more information on local environmental conditions such as topography and soil variation, and storm characteristics such as duration and intensity.

A semi-analytical method for oblique gust - cascade interaction

A semi-analytical method is proposed for calculating the response of a linear cascade with spanwise mean flow subject to oblique gusts. It is developed based on the boundary value problems defined by Lloyd and Peake (AIAA paper 2008–2840). A gust strength parameter is introduced and the correct three-dimensional (3-D) response is obtained. The classic similarity rules are extended; the approach can be used to extend any 2-D methods to account for oblique gusts and 3-D mean flows. It is validated against analytical approximations for single-airfoil and cascade responses. The method is used to investigate the effect of gust angle αg on the unsteady lift and the sound field. It is found that as αg increases, the 2-D equivalent response varies slightly. However, the 3-D lift is amplified by factor 1/cosαg, and the spanwise phase variation increases. Cascade effects are also studied. The inter-blade phase angle (IBPA) is important even for very low solidity. As the solidity increases, the chordwise distribution of lift is no longer leading-edge dominant. Cascade effects are small only when the cascade blade count is lower than a limit. A statistics analysis reveals that Mach number is the most important parameter for determining this blade count limit, and frequency is the least important.

A numerical investigation of the airfoil-gust interaction noise in transonic flows: Acoustic processes

The sound produced by airfoil-gust interaction is a significant source of broadband noise in turbofan engines or contra-rotating open rotors (CRORs). There are competing mechanisms in this regime because of the presence of shocks that were seldom considered in the previous subsonic studies. A numerical investigation of airfoil-gust interaction noise at transonic speeds is undertaken in this work. By introducing vortical gust/synthetic turbulence to specified regions in the computational domain to interact with different elements in the flow field, it is shown that the dominant sound source is caused by leading edge-gust interaction. It is demonstrated that both streamwise and transverse disturbances interact with the near-field non-uniform mean flow and shocks can produce sound using a local gust injection method. The propagation of sound is significantly influenced by the presence of the shocks, and the far field radiation pattern is changed. We also study the effect of gust strength on the near and far field properties. The linearity is maintained for gust strength smaller than 1.0% of the mean flow velocity. Otherwise, the shocks may experience oscillations that will alter the near-field aerodynamics and far-field radiation.

UNMANNED ROTORCRAFT IN AGGRESSIVE ENVIRONMENT: AERODYNAMIC FLOW PERFORMANCE AGAINST WIND GUST

Unmanned rotary-wing aircrafts or rotorcrafts are often prone to diverse atmospheric turbulences, and undeniably, abrupt gusts are reckoned to be the most acquainted commotion of them. Time and again, gust turbulence have dictated being the regulating trigger for countless mishaps concerning micro aerial vehicles. Given that the core/main rotor provides principal lift along with governing directional control and dynamic stability for any rotorcraft, the demeanors of thrust or induced air-flow through the gyrating blades largely signify the barebones of its functionality. This paper presents an idiosyncratic approach towards reviewing the impact of wind gust on rotor induced aerodynamic flow performance of an unmanned rotorcraft. Artificial gusts have been generated through the inherent concept of forced pitched oscillation without the use of any conventional wind tunnel. Gust air-speed and the rotor induced air-speed are gauged through the same contraption to provide homogeneous quantitative valuation. Each rotor-halves have been assessed `root-to-tip' across designated span-wise positions against selected strengths of gust. Comparative analysis with normal atmospheric condition indicated gradual loss of cumulative thrust during hover (15-40%) and forward flight (10-30%) within the stipulated gust range and infers the risk of declining altitude. Protuberant imbalance of thrust across the rotor disk during hover indicated the likelihood of lackadaisical half-pitched rolling motion. However, during forward flight, the reduced discrepancy of thrust between the rotor halves signified gradual loss of cruising speed with regards to the increasing gust strength for the specific rotorcraft model. Experimental findings in this study exhibited prospect of appraisal with full-scale rotorcrafts.

Gust Response Analysis for Helicopter Rotors in the Hover and Forward Flights

Dynamic load due to gust for helicopter rotors directly affects the structural stress and flight performance. In case of gust, it may cause the loss of trust force or the increase of deflection for rotors. In current work, an effective coupled aeroelastic model based on a medium-deflection beam theory and a nonlinear unsteady aerodynamic model in the time domain were constructed. Three types of gust in vertical direction were added in the model. The dynamic response and structural load for helicopter rotors under three types of gust were calculated, respectively. Results indicated that when rotors suffer a gust in hover at downward direction, the thrust force on rotor disk would decrease significantly when the gust amplitude increases, which should be paid attention in the design. Among the three gust types with the same gust strength, the maximum instantaneous shear force due to impulse shape gust is the largest. When the rotors suffer a gust in a forward flight, the shear force at the root of rotors would increase with the gust strength first but then it decreases. More attention should be paid to the decrease of thrust force and the increase of structural load in a forward flight.

Cyclones, windstorms and the IMILAST project

By way of introduction to the TELLUS thematic cluster on outcomes of the IMILAST project (Intercomparison of MId-LAtitude STorm diagnostics), this paper presents the results of new research that is fundamental for the correct interpretation of IMILAST results. Specifically we investigated the mesoscale structure of cyclonic windstorms, and the representation of those windstorms in re-analysis data. The paper concludes with an overview of the project itself. Twenty-nine historic windstorms are studied in detail, using wide-ranging observational data, and on this basis a conceptual model of the life cycle of a typical windstorm-generating cyclone is developed. The model delineates three wind phenomena, the warm jet, the sting jet and the cold jet, and maps out the typical damage footprint left by each. Focussing on the boundary layer, the physical processes at work in each jet zone are investigated. These include the impact of near-surface stability and exposure on gust strength. Based on numerous cases, a generic description of the sting jet is provided, with many new features highlighted. This phenomenon looks to be unique in that exceptional gusts can be realised well inland because destabilisation is activated from above. We next investigate how well the widely-referenced ERA-Interim re-analysis, that has been a primary data source for IMILAST, can represent windstorms. In many ways, performance is suboptimal. Compared to a benchmark manually-analysed dataset, windstorm-generating cyclones generally do not deepen rapidly enough. In part, this is a resolution limitation. For one medium-sized cyclone, it is shown, using other models, that horizontal resolution of order 20 km or better is required to capture the most damaging winds. In the context of IMILAST, which has used data at resolutions ≥80 km, this is a fundamental result. For this and other reasons, caution is clearly needed when inferring storm behaviour and severity from model-based metrics.

Cyclones, windstorms and the IMILAST project

By way of introduction to the TELLUS thematic cluster on outcomes of the IMILAST project (Intercomparison of MId-LAtitude STorm diagnostics), this paper presents the results of new research that is fundamental for the correct interpretation of IMILAST results. Specifically we investigated the mesoscale structure of cyclonic windstorms, and the representation of those windstorms in re-analysis data. The paper concludes with an overview of the project itself. Twenty-nine historic windstorms are studied in detail, using wide-ranging observational data, and on this basis a conceptual model of the life cycle of a typical windstorm-generating cyclone is developed. The model delineates three wind phenomena, the warm jet, the sting jet and the cold jet, and maps out the typical damage footprint left by each. Focussing on the boundary layer, the physical processes at work in each jet zone are investigated. These include the impact of near-surface stability and exposure on gust strength. Based on numerous cases, a generic description of the sting jet is provided, with many new features highlighted. This phenomenon looks to be unique in that exceptional gusts can be realised well inland because destabilisation is activated from above. We next investigate how well the widely-referenced ERA-Interim re-analysis, that has been a primary data source for IMILAST, can represent windstorms. In many ways, performance is suboptimal. Compared to a benchmark manually-analysed dataset, windstorm-generating cyclones generally do not deepen rapidly enough. In part, this is a resolution limitation. For one medium-sized cyclone, it is shown, using other models, that horizontal resolution of order 20 km or better is required to capture the most damaging winds. In the context of IMILAST, which has used data at resolutions ≥80 km, this is a fundamental result. For this and other reasons, caution is clearly needed when inferring storm behaviour and severity from model-based metrics. Keywords: cyclone life-cycle, conceptual model, sting jet, cyclone tracking, windstorm footprint, satellite imagery, re-analyses, boundary layer, convective instability, evaporation (Published: 1 September 2015) Citation: Tellus A 2015, 67, 27128, http://dx.doi.org/10.3402/tellusa.v67.27128

Generation of vertical gusts in a transonic wind tunnel

This article reports on the qualification of a gust generator device in a transonic wind tunnel. A vanning apparatus has been installed in the contraction of the S3Ch transonic wind tunnel at the ONERA Meudon center in order to generate up and down air movements in the test section. The apparatus has been tested in a range of Strouhal number based on frequency and vane chord up to 0.15 and in a range of Mach number between 0.3 and 0.73. The amplitude of the gusts has been characterized by a fast-response two-hole pressure probe and phase-averaged PIV. The system delivers vertical velocity amplitude of 0.5 % of the freestream velocity at transonic speeds. For a constant vane oscillation angle, the gust strength is found to increase with the Strouhal and the Mach numbers. The gust exhibit a satisfying uniformity and a quasi-sinusoidal waveform. A simple dynamic point vortex model of the oscillating vanes and of the downstream wake has been developed in order to (1) compare the experimental results and (2) enrich the description of the flow induced by the gusts. In particular, the model is used to analyze the detrimental effect of the upper and lower walls. This simple unsteady model gives a valuable prediction of the amplitude of the gust obtained in the tunnel and the workable frequency range permitted by the present apparatus.

Transient cross-wind aerodynamic loads on a generic vehicle due to bora gusts

Experiments were carried out to investigate transient aerodynamic loads experienced by vehicles on bridges using a generic prism model. A van model was exposed to an air flow, which simulated gusty bora winds blowing at the Croatian coast. The mean side force and the overturning moment of the vehicle increase nearly linearly with increases in the gust velocity. Two major mechanisms determine transient aerodynamic loads on a vehicle: vortex shedding from the bridge and the vehicle, and wind gusting. For the side force, the predominant unsteady flow mechanism is the buffeting by vortices shed from the bridge deck. The overturning moment is simultaneously influenced by vortex shedding and by wind gusting. With increasing wind gust strength, the unsteady component of the overturning moment becomes predominantly influenced by the wind gusting, while vortex shedding effects become less exhibited. The repetition rate of wind gusting does not seem to have a significant influence on transient aerodynamic loads.

An Experimental High-Resolution Forecast System During the Vancouver 2010 Winter Olympic and Paralympic Games

Environment Canada ran an experimental numerical weather prediction (NWP) system during the Vancouver 2010 Winter Olympic and Paralympic Games, consisting of nested high-resolution (down to 1-km horizontal grid-spacing) configurations of the GEM–LAM model, with improved geophysical fields, cloud microphysics and radiative transfer schemes, and several new diagnostic products such as density of falling snow, visibility, and peak wind gust strength. The performance of this experimental NWP system has been evaluated in these winter conditions over complex terrain using the enhanced mesoscale observing network in place during the Olympics. As compared to the forecasts from the operational regional 15-km GEM model, objective verification generally indicated significant added value of the higher-resolution models for near-surface meteorological variables (wind speed, air temperature, and dewpoint temperature) with the 1-km model providing the best forecast accuracy. Appreciable errors were noted in all models for the forecasts of wind direction and humidity near the surface. Subjective assessment of several cases also indicated that the experimental Olympic system was skillful at forecasting meteorological phenomena at high-resolution, both spatially and temporally, and provided enhanced guidance to the Olympic forecasters in terms of better timing of precipitation phase change, squall line passage, wind flow channeling, and visibility reduction due to fog and snow.

Forecasting wind gusts in complex terrain

Wind gusts are calculated in a collection of simulated atmospheric flows in complex terrain. This study focuses on a region in West-Iceland during February to April 2007 which includes several windstorms. The atmospheric data is a subset in a large collection of realtime numerical simulations used for forecasting in Iceland. It is generated at horizontal resolutions of 9 and 3 km, and in two sensitivity tests at 1 km. The gust prediction method is based on turbulence kinetic energy, static stability and wind speed in the atmospheric boundary layer. The gust prediction method is implemented as post-processing. The calculated gust strength is compared with wind gust observations from several automatic weather stations. The estimated gusts are strongly dependent on the quality of the simulated flow and are on average well captured when the mean winds are correctly simulated. Maximum gusts in downslope windstorms are however frequently underestimated. The error is presumably related to an inadequate simulation of the downslope surface winds which are also too weak. The windstorms in the current study appear to be related to gravity wave activity aloft and are better reproduced at higher resolutions than at coarse resolution. There are cases of overestimated gusts on the upstream side of mountains, which may be related to an inadequate simulation of the upstream deceleration of the flow and overestimated surface winds. Gustiness in mountain wakes is frequently too great, which appears to be related to overestimated turbulence in the wakes.

Gust occurrence in simulated non-stationary winds

An investigation is presented of peak gust intensity in non-stationary winds of different durations, in which the non-turbulent component of the event is fully controlled. A typical downburst was simulated repeatedly using a technique that utilizes a parametric analytical model of the downburst's non-turbulent winds, and an amplitude-modulated Gaussian stochastic process for the turbulent component. The peak gust strength of each event was expressed as a peak factor in relation to the largest non-turbulent speed. The properties of the simulated downburst peak factors are compared with those of stationary events of varied duration. A parametric study in which the non-turbulent component of the wind was simulated using a sine wave, which can be related to a downburst through the use of a “storm period” (SP) parameter, was conducted. A range of SP and turbulence amplitude modulation factors were used, and the properties of the peak factors are discussed.

The use of a cloud resolving model in the development and evaluation of a probabilistic forecasting algorithm for convective gusts

AbstractA Cloud Resolving Model (CRM) is used to simulate the showery convective activity that occurred over the United Kingdom on 13 April 1999, which was notable for observations of strong gusts. Simulations based on radiosonde profiles from both Herstmonceux and Hemsby are shown to give good agreement with available observations for this case. In particular, there is good agreement with the observed cloud top heights, precipitation amount and type, and strength of convective gusts. An algorithm to predict the maximum convective gust strength is shown to be consistent with values measured from the model. Further analysis of the model gust strengths shows that they could be approximated to a Gaussian distribution and that the mean and standard deviation of the Gaussian can be related to the maximum gust strength and the mean wind. A simple algorithm for predicting the probability distribution of the gust strengths measured in the model gives reasonable qualitative agreement with the measured probabilities and it is suggested that this demonstrates the utility of the CRM in developing and evaluating probabilistic forecasting tools for convective weather. Copyright © 2003 Royal Meteorological Society

Estimating Downburst-Related Maximum Surface Wind Speeds by Means of Proximity Soundings in New South Wales, Australia

Abstract A regional climatology of strong wind gusts associated with thunderstorms is presented, and the ability to estimate gust strength from ambient conditions is tested. Strong wind events were selected for 10 stations in New South Wales, Australia, from anemograph records and coincident thunderstorm reports. Most events took place between midafternoon and late evening and during the warmer months of the year, which is broadly consistent with the occurrence of severe thunderstorms in general. One sounding-based index, designed to predict the strength of microbursts, proves to be of limited value in predicting the magnitude of strong convective gusts, even of short-lived gusts. A modified index that combines the microburst index with upper-level wind speed is more useful.