Advanced and Robust Analysis of Wind Effects
Advanced and Robust Analysis of Wind Effects
What the Science Tells Us
There are multiple ways in which respiratory particles are generated as follows:
After particles form, they undergo a complex transformation as they pass through the respiratory tract before they are respired. The outcome can be a process of deposition i.e. change in initial size distribution. This is a very important fact that needs to be considered when looking at transmissibility as the size of the particles has a direct correlation to their ability to be airborne for long periods of time. Wells who is considered by many to be one of the fathers of research in the area of airborne transmission of infectious diseases had published a paper that suggested particles 1000µm, 100µm, 10µm and 1µm in size could be airborne for 0.3s, 3s, 300s and 30,000s respectively, with particles being less than 10µm being of key interest when considering the transmissibility of airborne pathogen since they represent the majority of all particles expelled by the mouth. Bourouiba, L., et al. (2014) explains the dynamics of respiratory particles during a cough or sneeze in Figure 1, where larger particles fall to the ground, whereas the column of smaller particles continue to expand as they float away from the jet source, which would then be subjected to eventual mixing with ambient air.
Figure 1. Bourouiba, L., et al. Violent expiratory event: on coughing and sneezing, Journal of Fluid Mechanics, 745:563, 2014
At the outset of the pandemic, various countries had put in place social distancing measures to limit the spread of SARS-CoV-2 particularly in relation to community interaction within indoor spaces. It was quite interesting to see the different levels of social distancing countries were adopting between 1m and 2m. This could inexplicably be tied to the type of respiratory activity considered as the worst case, whether a heavy activity (i.e. cough/sneeze), moderate activity (i.e. talking) or light activity (i.e. breathing). Prof. Yuguo Li during a recent presentation as part of the International Workshop on Infectious Disease and Airflows around Human Body presented a curve that showed a relationship between dilution factor of total exhaled content and distance from the mouth of a two-stage jet (i.e. when a jet transitions from a jet to a puff) for different activity types with Dilution Factor of 100 being seen as an acceptable level of dilution as shown in Figure 2. This figure highlights that the social distancing measures put in place in the various countries could be seen as correct depending on what type of activity they are trying to prevent exposure to.
Figure 2. Dilution factors calculated based on the steady jet and the two-stage jet when the discharge orifice is 0.02m
While there is a decent level of understanding relating to the physics of human expiratory activities in the area of droplet size and droplet concentration, there is much less knowledge about the relationship between droplet virus concentration as a function of droplet size from real infecting agents expelled by infected individuals, as well as the probability of infection. These metrics are of key importance in understanding the actual spread of viral infections and in the development of appropriate infection risk criteria that could be used for indoor ventilation or air filtration modelling. Figure 3 shows an example of criteria for SARS-CoV-2 that could be used to drive proper modeling of indoor spaces from a ventilation/air filtration perspective, with more research being done as new variants materialise with different infection risk profiles. Furthermore, it should be noted that research needs to extend to the study of interpersonal variability as this knowledge could be critical in explaining the ‘super spreading’ capabilities of some individuals, hence the reason why criteria have been developed for the typical case for a given pathogen. This generally requires obtaining key parameters from real-life outbreaks, and there are many examples of such models being put together by researchers around the world before the pandemic, and even more so since the onset of the pandemic i.e. Stable, L., Buonanno, G. and Morawska, L. (2020), Miller et al. (2021), Azimi et al. (2021) and Kriegel et al. (2020).
Figure 3. Cortellessa, L., et al. 2021. Close proximity risk assessment for SARS-CoV-2 infection. Science of total Environment, 794: 148749
Modeling and Mitigation
Some would suggest that a quick fix is to open the window. In December 2021, the World Health Organisation (WHO) published a poster focusing on ventilation and its main narrative was that people should always have windows open to ensure adequate ventilation to reduce exposure risk. This concept is not considered a novel one, in fact, Florence Nightingale an English social reformer, statistician, and founder of modern nursing back in 1859 discussed the role of opening the window. She went on to say, “Always air from the air without, and that, too, through those windows, through which the air comes freshest”. The issue with natural ventilation is that it is only effective when the conditions are right i.e. not too hot, not too cold, not too noisy. In reality, most climates do not allow for these conditions to be right most of the time, hence the window remains closed and there is no ventilation, or minimal ventilation if you take into consideration that most buildings are not airtight or have been somewhat designed to ensure that a certain % of dwellings achieve effective natural ventilation through various means.
So then how can we improve indoor air quality without ventilation? There have been a number of studies in this area that focus on the issues associated with the mechanisms behind infection transmission, and there has been evidence in research that filter-based air cleaners do work if used properly. An example of a research paper in this area is one presented by Zhao, B, Liu, Y. and Chen, C. (2020). It should however be noted that air purifiers cannot eliminate the accumulation of CO2, VOCs, and other gaseous indoor-generated pollutants if there is no ventilation, and this can have negative impacts on human health and function within a space. Hence air purifiers are not an effective solution in isolation. Morawska, et al. (2020) provided some guidance as to what measures could be used to minimize the transmission of SARS-CoV-2 using a mixture of measures as presented in Figure 4, and this included sufficient and effective ventilation, avoiding air recirculation, particle filtration and air disinfection, and avoiding overcrowding.
Figure 4. Morawska, et al. How can airborne transmission of COVID-19 indoors be minimised?, Environment International, 142: 105832, 2020
Sufficient ventilation entails that there is enough ventilation, it is available everywhere and there is no air flow from person to person. There are few well known sources that talk about what is considered sufficient as follows:
However, most of the above standards do not take into consideration the number of people that will be present in a space, what activities they are conducting, and the risk of infection which varies for each pathogen and its variants. The only effective method to assess whether an indoor space is sufficiently ventilated is to use risk assessment models and tools. With this approach, it is possible to build a risk model, calculated the required ventilation, set up ventilation, and verify the outcomes. This can be done on a venue-to-venue basis since the conditions and risk models for each venue differ from one another. There are however limits observed when the individual risk of infection percentage is high such that in some cases required ventilation is higher than what mechanical systems can provide. An example of this is presented in Mikszewski et al. (2021), where the study indicates that with highly infectious viruses such as Adenovirus, TB, SARS-CoV-2, and Measles not even a high ventilation rate of 14 L/s/person was sufficient to maintain event reproduction below acceptable levels.
Therefore, disinfection can be added to the basket of mitigative measures, for example, Germicidal Ultraviolet (GUV) air disinfection, which is low energy, does not generate new pollutants, and is robust from a maintenance and cost point of view.
With the above risk model, the nature and extent of the above-mentioned mitigative measures tuned through detailed numerical analysis of airflow distribution and direction using advanced Computational Fluid Dynamics (CFD) analysis of a single room/dwelling, or an entire floor plate or building with multiple floor plates. Such numerical modeling can take a long time to solve, however with access to powerful computer clusters like the ones used at Windtech with over 10,000 CPU cores, these computationally intensive simulations can be done in a fraction of the time.
With all this in mind, it is possible to shift toward a performance-based design approach (i.e. away from a code-based design approach) by adopting advanced modeling techniques that consider key parameters that influence the design of indoor spaces from an air quality point of view.
If you have any questions relating to the content of this article or wish to speak to one of our experts in the area of air quality for both indoor or outdoor spaces, please reach out to our regional office via our Contact Us page.
To get regular updates on news and events, please follow us on our LinkedIn page.
References:
According to the International Energy Agency (IEA), renewable energy is expected to overtake coal as the primary means of electricity production by 2025, with solar energy expected to lead the surge. Currently, solar is already the cleanest and most abundant energy source, on top of being the cheapest in most countries.
Despite supply-side disruptions to solar panels and assemblages due to COVID-19, according to the IEA, global solar photovoltaic (PV) capacity additions to hit almost 107 gigawatts in 2020 – more than half of all renewable capacity additions for the year – indicating stable growth from 2019 numbers.
Figure 1. Proposed Sun Cable Solar Farm located in the Northern Territory, Australia
Large solar infrastructure projects are being rolled out all over the word on a scale never seen before with some solar farms (i.e. Sun Cable located in Australia) in development targeting between 17 to 20 gigawat-peak (GWp) of Electricity, part of which will be stored on-site in what is planned to be the world’s largest battery at 36 to 42 gigawatt-hours.
With the expansion of solar infrastructure projects some larger than ever before, developers, owners, operators are in the throes of learning from past experiences, in an industry that is rapidly changing with higher demands placed on engineered solutions to make projects of this size sustainable from a development and operational perspective.
Reliable wind loading information on solar panels is essential for the efficient large-scale deployment of solar infrastructure. However, only a limited amount of published data from wind tunnel studies is available to address aerodynamic loading information for these assets. Because of this knowledge gap, no provisions exist in the current building codes and standards for the design of PV mounting systems in areas that are vulnerable to extreme winds. As such, the only viable way to address this is through establishing the appropriate data on a project-to-project basis through quantitative methods using robust wind tunnel testing techniques.
Aaron Lefcovitch, Director of Windtech based out of our Singapore Office states “We are having an increase in enquiries relating to wind related issues for solar farms. Some clients are commissioning us to understand how they can secure their assets though the accurate prediction of wind loads on the panels themselves, which are inherently light, wind sensitive structures, some of which tend to exhibit complex dynamic behaviour under fluctuating wind loads. We develop scaled models placed in the wind tunnel to determine the underlying aerodynamic coefficients, then combine this data with an analysis of the regional wind climate, which is unique to the region where the solar farm is located. This is usually done during the design/development of a solar farm, however for some clients we need to get involved from a remedial perspective due the failure of the asset(s) under wind load. One thing becomes immediately apparent when embarking on a remedial study, which is the absence of a proper Performance-Based Design (PBD) approach which could have saved the asset from performance degradation or even downtime, and expensive repair bills.”
Figure 2. Al Noor 1.2GQ Solar Plant, Abu Dhabi
Windtech has been working on solar infrastructure projects for more than 15 years including some of the world’s largest solar farms built to date for example the Al Noor Solar Plant which covers an 8 square kilometre plot of land in Abu Dhabi and generates 1.2GW of electricity, enough to provide power to approximately 90,000 residences. Projects of this size need careful planning to be able to efficiently evaluate the aerodynamic coefficients in the wind tunnel as these farms tend to occupy a vast area with irregular edge profiles. It may be cost-prohibitive to test each and every panel at a scale that would large enough to not exhibit scaling effects (i.e. wind flow structures that would naturally depart away from reality if a model is too small). Hence to address this, Windtech Consultants has developed a robust and efficient methodology that accounts for the various locations within the cluster and edge configurations. Furthermore, provided the upstream terrain and panel geometry remains the same between solar farm sites, the underlying aerodynamic coefficients that were determined in the wind tunnel can be reused for subsequent sites with adjustments for the local directional wind speeds. Therefore, the client can potentially conduct one wind tunnel test for multiple sites rather than conduct a wind tunnel test for each site. Following this approach has resulted in a large uptick in the number of Solar Infrastructure tested in our facility as clients take advantage of the significant savings on the cost of wind tunnel testing. This is indeed true of both ground-mounted or water mounted solar farms in standard fixed or solar tracker configuration, each having its individual challenges.
Figure 3. Testing Effect of Wind Fence on Leading Edge Corner Panels for Al Noor 1.2GQ Solar Plant
Current in-house capabilities in solar include the determination of load cases for the purpose of structural design through the use of rigid model and/or aeroelastic test methods. These capabilities are supported by a wealth of experience in being able to reliably predict the appropriate design loads as well as any potential aerodynamic instabilities of a solar panel system.
Windtech is proud to be contributing towards performance-based wind design capabilities for the Solar Infrastructure industry. If you are working on any projects that could benefit from this capability, please reach out to our regional office via our Contact Us page.
To get regular updates on news and events, please follow us on our LinkedIn page.
This month Greenland Australia and builders Probuild have completed Sydney’s tallest residential tower, the $700-million mixed-use Greenland Centre in the city’s CBD. The tower was designed by award-winning Australian architectural firms BVN and Woods Bagot.
The building faced some unique challenges at the outset as pointed out by Andrew Johnson, the lead structural engineer from Arup Sydney, “It is the first building in the world that we are aware of where 45 stories have been grafted onto the top of an existing 25-story building, resulting in a slender tower of 10:1 aspect ratio integrating the existing structural frame and foundations into the redeveloped building.”
Andrew goes on to say that they were “constrained by the planning requirements of retaining the existing building structure—once the second tallest building in Sydney and completed in 1965—the structural design has integrated the original primary vertical steel structure into new composite columns, existing steel girders into the new low-rise floor structures, and augmented the deep pad footings founded on sandstone to support the higher gravity and wind loads of the new building.
Figure 1. The Model of the Greenland Centre (yellow) being tested in one of Windtech’s wind tunnel facilities in Sydney.
It was clear at the outset that the design constraints posed some interesting challenges from a structural design point of view. As such, there needed to be a collaborative approach adopted by all members of the design team. Tony Rofail, a Principal at Windtech Sydney Office quotes, “We needed to collaborate quite closely with the design team as we were embarking on a project that was atypical. Given the height, slenderness, and perceived structural dynamics of the tower, it was a given that it was bound for the wind tunnel. Testing of a preliminary massing of the tower in the wind tunnel very early in the concept design phase allowed the team from Arup to have a reliable indication of the lateral wind loads that they are up against for this very atypical structure to enable the selection of an appropriate structural system. The structural designer was then able to progress the structural design further prior to conducting a wind tunnel test of the final articulated design, where the actual wind loads came within 10% of the original assumptions and where the accelerations remained within the criteria for occupant comfort.”
Figure 2. The Completed Greenland Centre building at the corner of Pitt and Bathurst Streets, Sydney (image courtesy of the Urban Developer)
Another interesting feature of the development is the innovative balcony design put forward by the architects BVN and Woods Bagot. The unique glass-fronted ‘Sydney balconies’ conjure the idea of the quintessential verandah that can be used year-round. Windtech utilised a hybrid wind tunnel and Computational Fluid Dynamics (CFD) approach to test the concept, with remarkable results. This innovative design was demonstrated to have adequate wind comfort and natural ventilation under normal serviceability conditions, even at higher levels where wind tends to be more severe. Tony went on to say that “wind comfort and natural ventilation tend to compete with each other. However, we were able to demonstrate that this innovative design was able to achieve a healthy balance and gave the designers the confidence to create a seamless transition between the indoor and outdoor spaces which is a tricky proposition at these heights”.
Figure 3. Unique glass-fronted ‘Sydney balconies’ (image courtesy of the Urban Developer)
During the design process, Windtech had undertaken a diverse range of wind engineering studies for this development, including the following
We would like to congratulate the team made up of BVN, Woods Bagot, Arup, Probuild, Greenland, and many others for pulling off this unique and innovative award-winning project. It is truly an iconic Sydney development and will be for many years to come.
Figure 4. Pressure Contours for the northern aspect including pressures in the glazing behind the Sydney Balconies (Left Image: Outer Layer, Right Image: Inner Layer)
Figure 5. Wind Vector Diagram for one of the wind cases.
Figure 6. Smoke from BBQ location Number 3 with the effect of a weekly maximum westerly wind.
Figure 7. Smoke from BBQ location Number 4 with the effect of a weekly maximum westerly wind.
To enquire about our solutions relating to Wind Tunnel Testing and CFD Modeling, please reach out via our Contact us Page
To get regular updates on news and events, please follow us on our LinkedIn Page
Double skin façades are façade systems consisting of two layers, usually glass, wherein air flows through the intermediate cavity. This space (which can vary from 20 cm to a few meters) acts as insulation against extreme temperatures, winds, and sound, improving the building’s thermal efficiency for both high and low temperatures. One such example of a project with a double-skin facade is 30 St Mary Axe Building, “The Gherkin.”
The airflow through the intermediate cavity can occur naturally or be mechanically driven, and the two glass layers may include sun protection devices, such as blinds, either fully retractable or of a blade-type design.
While the concept of double-skin façades is not new, there is a growing tendency for architects and engineers to use them. Particularly in skyscraper design, they are favoured for their transparent façade, thermal and auditory comfort, reduced air conditioning costs, and elimination of the need for window-specific technologies.
Additionally, double skin façades are adaptable to cooler and warmer weather. It is this versatility that makes them so interesting: through minor modifications, such as opening or closing inlet or outlet fins or activating air circulators, the behaviour of the façade is changed.
In cold climates, the air buffer works as a barrier to heat loss. Sun-heated air contained in the cavity can heat spaces outside the glass, reducing the demand for indoor heating systems.
Windtech conducts detailed Computational Fluid Dynamics (CFD) and Wind Tunnel testing, to assist in understanding the internal temperature, solar, and ventilation rates within various double skin façade design makeups. The techniques lead to an overall detailed design in the material build-up in response to the achievable air-flow rates either mechanical or passive and the overall end reduction in solar/external temperature transfer.
Figure 1. Central Bank of Baghdad, Iraq (Image Courtesy of Zaha Hadid Architects)
Dr. Niall O’Sullivan a Technical Director at Windtech based out of the London Office indicated that “The new headquarters of the Central Bank of Iraq (CBI) comprises a podium and tower of 170 meters height and a gross internal area of 90,000 square metres. The structural exoskeleton frames the facade which is composed of an alternating pattern of open and closed elements, providing a variety of areas of light and shade within. The exoskeleton gradually opens and reduces as the tower rises, bringing greater lightness and views across the capitol. The project was designed by Zaha Hadid Architects and constructed by DAAX Construction”.
Windtech was responsible for the analysis of the mixed mode double skin façade design during the detailed design and construction documentation.
Dr. Niall O’Sullivan goes on to say “The detailed mixed-mode ventilation and solar thermal transfer CFD study was undertaken to investigate the solar transfer and the ventilation cooling rate prescribed in the current design. The simulation methodology including the material properties is included (in Figure 2). The simulations used the Conjugate heat transfer solvers available to calculate the solar and thermal transfer from the solid and fluid zones in the simulation domain. Through this, the internal building heat load could also be accounted for in the end simulations as well as the external solar transfer during the peak solar condition. All detailed glazing properties could be employed to define the solar loss through the façade build-up, where specific gas concentrations in each layer are defined to further the detail in the imposed thermal loss.”
Figure 2. Computational Methodology
Figure 3. DSF Final Glazing Specification and 0.46m/s Inflow cross-sectional contours of solar irradiance, Internal Temperature and Velocity.
The outputs of the computational study presented the solar irradiance, internal cavity temperature and ventilation velocity. A selection of one of these simulations is shown in Figure 3.
The end result of the study led to the qualification of the designs effectiveness and further design adjustments in the glazing specification and ventilation rate imposed to allow for the double skin façades internal temperature to be within effective levels to maintain the overall internal buildings required heating performance during the peak seasons. It is only possible through the detailed analysis of this type for effective double-skin façade designs to be assured and implemented.
Windtech experience in this area of design led to the end clients design specification being assured and validated prior to construction.
To enquire about our solutions relating to CFD Modeling, please reach out via our Contact us Page
To get regular updates on news and events, please follow us on our LinkedIn Page
We are pleased to announce the arrival of the new edition of “Wind Actions on Structures” under the new designation AS/NZS 1170.2:2021. This document supersedes AS/NZS 1170.2:2011. Windtech directors Tony Rofail and Dr Nicholas Truong have been key contributors to the significant updates and new content in this new edition being key members of the wind load sub-committee of Standards Australia/ Standards New Zealand.
The objective of this Standard is to provide wind actions for use in the design of structures subject to wind action. It provides a detailed procedure for the determination of wind actions on structures, varying from those less sensitive to wind action to those for which dynamic response are to be taken into consideration.
The objectives of this revision are to remove ambiguities and to incorporate recent research and experiences from recent severe wind events in Australia and New Zealand.
Windtech Directors Tony Rofail and Dr Nick Truong, based out of our Sydney HQ, played an integral role in the upgrade of the standard, with Tony chairing the committee responsible for the review of Section 6 (dynamic response of tall, slender structures). Below is a non-exhaustive list of changes that were brought about through the efforts of Windtech staff:
A more complete overview of the changes can be obtained by downloading a preview of AS/NZS 1170.2:2021 at the following link: AS/NZS 1170.2:2021 Preview
