Ka rere ngā mea katoa – everything flows

KEYWORDS:

• Fluid mechanics
• environmental sciences
• computational fluid dynamics
• sensing technology
• biomedical engineering
• industry
• gender balance
• Māori engagement

Everything flows – or ‘panta rhei’ in ancient Greek – is a saying credited to Heraclitus symbolising the fact that everything around us is in a constant state of flux. Indeed ‘since the earth is 75% covered with water and 100% covered with air, the scope of fluid mechanics is vast and touches nearly every human endeavour’ as written by Frank M. White in a classical textbook (White 2017). Fluid mechanics is the science of anything that flows and it underpins weather systems, ocean circulation, vehicle aerodynamics, vascular or pulmonary mechanics, rocket dynamics, air conditioning systems, riverbank erosion, and many more natural or applied science applications. For this reason, fluid mechanics is a fundamental component of the curriculum for many university programmes such as physics, civil/mechanical/chemical engineering, and applied/pure mathematics. Fluid mechanics is the subject matter glue which binds a very diverse academic community involving mathematicians, engineers, physicists, rheologists, and chemists alike.

Fluids in New Zealand, or FiNZ as we know it, is an informal group initially launched in 2013 with the express intention of providing a forum for this community to exchange and cross-fertilise ideas and tools, to break the silos between disciplines, and to leverage their strengths to create more scientific, economical, and societal impact. An inaugural workshop in 2013 attracted a modest crowd of 40 researchers from across New Zealand, coming for the most part from tertiary education institutions and Crown Research Institutes. Since then, FiNZ workshops have taken place annually near the end of January, hosted on University campuses: Canterbury, Auckland, Otago and Massey (Albany) to date. This special issue was drawn from selected papers presented at the 2020 workshop, which was hosted by Dr Emilia Nowak and her team at the Massey Albany Campus, and included around 70 participants. Among these researchers were many regulars from the initial years, but also a growing crowd of young researchers ranging from masters level to postdocs. Giving an opportunity to this young and emerging community to present their research work in a casual, critical yet supportive environment has been a key mission of the FiNZ workshops since their inception.

Another quote well-known to most fluid dynamicists, by Richard P. Feynman, states that ‘turbulence is the most important unsolved problem of classical physics’. Though turbulence research has made immense progress since, this quote still eloquently illustrates fluid mechanics as an untarnishable source of complex, stimulating, relevant, impactful, and fascinating problems. This special issue presents 11 contributions from New Zealand researchers and their international colleagues, showcasing the wide variety of subject matter involved in fluidics research.

Among these papers, there is a strong theme reflecting the current interest in understanding and managing our environment. Waite et al. (2021) numerically investigate in-stream remediation systems to clear rivers of undesirable dissolved pollutants such as nitrates. The system consists of a porous remediation unit placed within a river channel. Pollutant removal is modelled as a sink term in the pollutant transport equation. Using this numerical model, the authors are able to perform a parametric study and shed light on the effects of void fraction in the porous material or unit(s) size and arrangement. Staying with waterways, Russell and Vennell (2021) present a mathematical analysis describing the 3D flow of water around a bend in an open channel – relevant to river flows which contribute to material erosion and transport. Specifically, the model improves on previous work to find the depth-average velocity across the width of a channel. The same authors have also contributed a paper on the dynamics of the flow in a curved tidal channel (Russell and Vennell 2021). The authors show through a simple perturbation method that the channel curvature generates secondary circulations modulating the vertical profile of the primary flow and leading to an outward shift of the cross-channel velocity maxima. This shift is observed for a curved tidal channel of the Otago Harbour and measured to be approximately 40 m or 10% of the channel width.

Two papers are concerned with flows of air – otherwise known as wind! Wakes et al. (2021) used a combination of computational fluid dynamics (CFD) simulations and a machine learning algorithm to better understand the development of turbulent flow over foredunes and quantify the effect of geometric and wind parameters on the presence and strength of recirculation vortices. In this paper, the use of machine learning significantly reduces the number of simulations required to sample the parameter space to extract relevant trends and patterns. The methodology developed by the authors will help understand how the geomorphology of dunes is affected by wind. Pancholy et al. (2021) conducted CFD simulations based on the Reynolds-averaged Navier-Stokes (RANS) equations to model the flow of air in the presence of a non-uniform street canyon. The authors investigated both the flow structure through and around the canyon, and the effect of the canyon on pedestrian comfort based on the Extended Land Beaufort Scale. They found that non-uniform canyons are detrimental to pedestrian comfort in the step-up canyon configuration, suggesting uniform canyons are more appropriate from a design perspective.

Fluid mechanics also plays an important role in the development of measurement and sensor technologies which involve liquids. Post (2021) has studied the use of a phase-Doppler instrument to measure the drop size and velocity distributions in a field of spray. In the New Zealand context, such measurements are particularly important for understanding agricultural spray drift. The study focuses on how user settings on such instruments can affect measurement outcomes. Manickavasagam et al. (2021) used the linearised Stokes flow equation to compute the hydrodynamic loads on an array of fully-submerged, vibrating cantilever beams, as a model for the surface scanning process in multi-beam atomic force microscopes. The authors investigated the coupling effects of non-neighbouring beams on loads for a range of configurations. They found that as the spacing between the beams is sufficiently small so that the boundary layer of neighbouring beams overlap, the outer beams experience a magnified load for low Reynold numbers, while the load on the middle beam is increased for high Reynolds numbers.

The other papers cover a range of biomedical and industrial applications. Yazdi et al. (2021) have studied the flow of blood over grafts on the walls of arteries. To do this, they used a ‘phantom’ made of flexible polymer tubing, and particle tracers were observed to measure fluid flow. Their work demonstrates the importance of using pulsatile flow and compliant (flexible) materials when assessing such grafts. Kalyanaraman et al. (2021) investigated numerically the washout of a fluid through a porous medium, with permeability depending on porosity via the Kozeny-Carman law. The authors used a penalisation approach combining the Navier-Stokes' equations with the Brinkman form of Darcy's law in the porous domain and solved the resulting system numerically using the method of characteristics in time and the finite-element method in space. Simulations were conducted to model the washout of railway ballasts in two dimensions. Ramsay et al. (2021) analyse the conical diffuser – essentially, a pipe with an expanding cross-section. Conical diffusers have many uses, such as adjusting flow rates in pipework, or maximising the pressure in aeroplane turbine engine exhaust. This work models the effect of suction on the air flow through a diffuser, showing that this can reduce energy losses due to turbulence. Finally, we are delighted to include an invited paper (McQuarrie et al. 2021) from the group of Professor Wilcox, Director of Oden Institute for Computational Engineering and Sciences at the University of Texas at Austin, who has a strong New Zealand connection. The paper presents a new numerical methodology based on Operator Inference which blends machine learning and physical modelling to infer the complex dynamics of a system using a finite set of observed ‘snapshots’ of its evolution. The algorithm is applied successfully to dynamics of the rocket engine combustion.

Diversity

Given the range of situations in which fluid mechanics is important, one might expect the appeal of studying fluids to be universal. Yet, it would not escape any observer that the large majority of fluid mechanics researchers in New Zealand is Pākehā and male. In a way this is unexceptional because it reflects the academic disciplines from which fluids researchers are mostly drawn – in 2019, female made up around 45% of New Zealand's domestic tertiary students in maths and physics, and closer to 20% in engineering, despite there being 1.5 times as many female as male enrolled in tertiary education overall (Ministry of Education 2020). 1 in 6 NZers are Māori, and 1 in 12 are Pasifika, yet PhD participation rates for both groups are below 5% in these disciplines (Ministry of Education 2020). Low student participation rates (Turnbull et al. 2019) represent the start of the ‘leaky pipeline’ which sees females making up less than a third of the global scientific R&D workforce (UNESCO Institute for Statistics 2019). In New Zealand universities, science and engineering (along with education) had the largest gender performance pay gaps between 2003 and 2012, with males making up 70%–80% of the academic staff in these areas in 2012 (Brower and James 2020). For Māori and Pasifika, the statistics are much worse (McAllister et al. 2020).

Improving diversity in science is currently a focus for the New Zealand Government (Ministry of Business, Innovation and Employment [date unknown]), so it is an opportune time for the FiNZ community to reflect on our track record, and the work we have ahead of us. It is worth mentioning that FiNZ has strived to be inclusive and welcoming. Indeed, the community has some ethnic and cultural diversity, at least at the student level, reflecting that the study of fluid mechanics has brought people from around the globe to New Zealand. So our lopsided record to be detailed below probably reflects structural factors and unconscious biases. As a starting point, the authors (and indeed the Associate Editors) featured in this Special Issue unfortunately highlight our lack of diversity. This has prompted us to take a closer look at diversity within the FiNZ workshops, our major activity as an organisation.

Figure 1 shows the extent of the challenge, as well as some progress made in terms of gender representation. It is notable that the data are incomplete, and that gender representation is estimated, due to the way in which data have been collected and preserved – something to improve on immediately for 2022! Figure 1A shows that low proportions of females are participating in FiNZ meetings. In some areas, representation is lower than the fractions of students and academics in our disciplines (above). FiNZ has had a blind spot when it comes to those invited to chair sessions, and the proportion of female speakers has been low, although edging up over time (Figure 1B). The bar for acceptance for an oral presentation at FiNZ is not high, so encouraging participation and promoting a comfortable environment are important factors.

Figure 1. Gender representation A by role and B over time at FiNZ meetings 2013–2021. All data are estimated (i.e. not based on self-reporting). Columns are labelled with the total number in each category, with an asterisk (*) indicating data that are not available.

We do not have reliable data from FiNZ meetings regarding race, ethnicity, disability, sexual orientation, and the intersectionality of these identities. However, it is clear that Māori and Pasifika peoples are virtually absent. For Māori, this is particularly problematic due to the need for commitment to Te Tiriti o Waitangi. Perhaps even more significantly, exclusion of Māori and Pasifika represents a huge lost opportunity for the research discipline when one considers the great Pacific navigators, the concept of kaitiakitanga (environmental stewardship), and increasing interest from indigenous peoples in the cutting edge technologies developed and used in engineering and physical science laboratories.

We will now flag two areas in which we can take action: fluid mechanics education, and the structures around our FiNZ community.

There may be an illusion that cultural and societal context are far removed from the teaching of fluid mechanics. However, the culture of fluids research and teaching reflects the history of the discipline and the choices of those involved, choices that are influenced by their identity, position and society. As we consider change in this discipline, we face an opportunity to broaden that culture, and perpetuate diversity.

Historically, the study of fluid mechanics (and many similar academic subjects) has failed to offer an inclusive culture. While females have had some part in constructing our knowledge in the field, there is an ongoing failure to recognise this, which continues to limit inclusivity. Indeed, it is difficult to find robust histories on fluidics researchers who have been systematically disadvantaged. However, there are positive examples which can be emphasised in teaching, such as research into surface tension started by Agnes Pockels (1862–1935). Pockels grew up in Germany at a time when women were excluded from universities, and carried out experiments using grease and water in her sink (Mischnick 2011). Using physics textbooks borrowed from her brother's university library, she was able to invent a trough to study gas-liquid interfaces. Although she had no formal science education, she was awarded an honorary PhD from Carolo-Wilhelmina (Al-Shamery 2011). This story is continued in the USA with Katharine Burr Blodgett (1898–1979) who improved upon Pockels' work and helped to invent the Langmuir-Blodgett trough. Blodgett is ‘credited as the inventor of nonreflecting glass and held a number of patents for her work as well as publishing in academic journals’ (Horrocks 2011). Such examples are often overlooked.

As a concrete action, this Editorial will be circulated to all co-ordinators of tertiary fluids courses in NZ, along with encouragement to discuss the issues raised, and to develop and share course materials that could address this issue.

Within the FiNZ research community, we will initially commit to developing, publicising and adhering to best practices that have been developed for organisations in similar fields to our own. FiNZ is already employing a code of conduct for our annual meetings, and we will seek to ensure diverse representation in speakers, panels, organising committees and session chairs. Notably, we find that equity initiatives recently introduced in similar organisations recently have led to benefits in terms of diversity outcomes. Examples include the equity policy at the MacDiarmid Institute in the field of advanced materials science (The MacDiarmid Institute [date unknown]), the policy for providing financial support developed by ANZIAM in applied mathematics ([date unknown]) and work towards the Pleiades awards ([date unknown]) at the University of Auckland's Department of Physics (The Department of Physics, The University of Auckland [date unknown]). In committing to these developments, we anticipate that FiNZ is probably similar to informal groupings formed among our colleagues in similar disciplines, and we would encourage those colleagues to take similar steps.

Outlook

Where will fluid dynamics research in New Zealand head in the future? The capabilities of fluids researchers are well aligned with many of New Zealand's most prominent current and upcoming challenges – including competing in the America's Cup! Clearly environmental science is high on the agenda. A number of FiNZ regular attendees are involved in large, funded national science programmes in this space, such as the Deep South National Science Challenge and the Antarctic Science Platform (ASP). These feature research on topics such as the flow of Antarctic ice sheets and ice shelves, the development of state-of-the-art regional and global climate models, and the impact of sea level rise and storm surges on coastal communities. Aligned with the threat of climate change is the requirement to explore new methods for energy generation, storage and transmission. Fluid mechanics is widely important in this area, from turbines to batteries, from dams to tidal and wind technologies. Fluids research has also had a role in the most prominent news topic at present: virus transmission. Studies of generation, motion and survival of droplets have often featured in international media over the past year, and indoor air flows driven by open windows or air conditioning units have also been of interest. Finally, there is significant potential for collaboration between traditional Western approaches to fluid dynamics, and mātauranga Māori. For instance, Māori have strong interests and advocacy relating to protection of freshwater waterways, risk management for coastal assets and communities, and sources of kai moana. Although recent projects have initiated such collaborative efforts, e.g. through the National Science Challenges and the ASP, fluid dynamicists, with their toolkit of sensing and simulations capability, are ideally placed to co-produce valuable knowledge and useful, practical solutions to these challenges.

Acknowledgments

The authors thank everyone who has contributed to FiNZ, and particularly those who have assisted with data collection for this Editorial. The support of the University of Canterbury is gratefully acknowledged for publication of this special issue.