Thursday, May 15, 2025

Research reveals why next-generation engine noise grinds our gears

University of Bristol

Fig 1 — image:
Hot-wire measurement setup for a BLI configuration, featuring a ducted fan installed adjacent to a curved airframe surface. The setup enables high-resolution velocity field analysis to uncover the aerodynamic origin of the haystacking noise pattern and its link to perceived noise.
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Credit: Dr Feroz Ahmed

A breakthrough study has revealed why emerging electric aircraft engine technology sounds so annoying — and how to fix it.

Scientists at the University of Bristol in collaboration with the University of Salford have uncovered the root cause behind the particularly irritating noise produced by boundary layer ingesting (BLI) engines — a key technology in future electric and hybrid aircraft.

Building on earlier work that identified general noise sources in BLI systems, this latest research, published today in Nature npj Acoustics, dives deeper into the physics of aerodynamic sound generation, revealing how turbulent boundary layer flow interacts with rotating fan and duct components to produce two distinct and perceptually unpleasant acoustic signatures.

Key to this research is a fluid-mechanics-based assessment that uncovers the fundamental aerodynamic origin of two different types of broadband noise patterns, known as ‘haystacking’ — spectral features that affect how noise is perceived. In acoustics, ‘haystacking’ describes the effect of turbulent flow scattering tonal sound fields, causing the energy of a specific tone to be spread across a wider range of frequencies.

The study shows that at low thrust (during cruise), the weaker fan suction allows the airframe boundary layer flow to remain largely undisturbed. In this regime, flow ingestion is governed by airframe curvature-induced flow distortion, which exposes only the blade tips to low-momentum turbulent flow structures. Since the duct’s acoustic contribution dominates at low thrust, the primary noise-generating mechanism is the interaction between the turbulent flow and the duct’s internal acoustic field — resulting in duct haystacking.

At high thrust (during take-off), strong fan suction disrupts the airframe boundary layer flow, producing fan-induced flow distortion that draws in high-momentum, highly-unsteady turbulent flow structures across a larger portion of the blade span. This intense interaction between fan-induced distorted flow and rotating blades leads to fan haystacking, where the unsteady flow is repeatedly sliced by the rotating blades by a large portion of the rotating blade span.

Lead researcher Dr Feroz Ahmed, who was based in Bristol’s Faculty of Science and Engineering, while conducting this study, said: “These two hidden sound signatures — haystacking — make future embedded aircraft engines feel perceptually irritating, not just loud.

“By linking turbulent flow ingestion patterns to how people perceive noise, we are giving engineers the tools to design future aircraft that truly sound as quiet as they look.”

These insights offer a new roadmap for designing embedded engines that sound quieter, not just measure quieter – a vital step for improving public acceptance of future urban air mobility aircraft.

Using a high-fidelity wind tunnel setup that replicates real-world conditions, researchers gathered detailed flow and noise data across flight regimes using advanced instrumentation — including hot-wire anemometry, pressure sensors, and far-field microphones. This allowed them to isolate and connect each noise signature (haystacking) to both its aerodynamic cause and its impact on human perception.

The implications of this research are wide-ranging. These insights provide actionable design guidance for both large-scale transport aircraft — such as the Airbus ZEROe, ONERA NOVA, NASA/MIT Aurora D8, Airbus Nautilus, and MITSAX-40 — and for manufacturers of next-generation electric vertical take-off and landing (eVTOL) aircraft in the urban air mobility (UAM) sector, supporting efforts to meet the EU’s FlightPath 2050 goal of reducing aircraft noise by 65%. These findings could help design perceptually quieter engines for future electric aircraft and air taxis.

The team now plans to develop aerodynamic and acoustic control strategies to reduce both fan and duct haystacking. They are also looking to expand this analysis to other propulsion concepts involving turbulent flow ingestion, with the aim of shaping the future of quiet aviation.

BLI ducted fan test rig inside the anechoic chamber at the University of Bristol, featuring a ducted fan mounted on the curved S-plate and an overhead microphone array for aeroacoustic measurements.

Paper:

‘Aeroacoustics and psychoacoustics characterization of a boundary layer ingesting ducted fan’ by Feroz Ahmed, Carlos Ramos-Romero, Antonio J. Torija and Mahdi Azarpeyvand in Nature npj Acoustics.

Journal

npj Acoustics

Method of Research

Experimental study

Subject of Research

Not applicable

Article Title

Aeroacoustics and psychoacoustics characterization of a boundary layer ingesting ducted fan

Article Publication Date

15-May-2025

Recessive genes are subject to Darwinian selection

Radboud University Medical Center

As a group, carriers of recessive disorders are slightly less healthy and have a reduced chance of having offspring. This disadvantage is greatest for carriers of a recessive gene for intellectual disability, and reflected in a shorter school career and increased childlessness, according to research from Radboudumc published in Nature human behavior. Time to rewrite the textbooks?

Dominant mutation

Researchers from Radboudumc, Department of Human Genetics, demonstrated something remarkable in a 2014 publication in Nature. Contrary to expectations, inherited variants in genes were rarely responsible for intellectual disability. In the vast majority, the disability arose from a spontaneous mutation: a mutation that neither parent has, but suddenly appears in the child. It is a biological fact that about one hundred spontaneous mutations occur in the hereditary material of each child. Mutations that therefore do not originate from the parents. On average, only one of these one hundred mutations affects a gene. And only a fraction of those affects one of the hundreds of genes that can cause intellectual disability. But rare events still happen. Most intellectual disabilities arise in this way.

From dominant to recessive

“There was something else going on,” said Christian Gilissen, professor of Genome Bioinformatics and first author of the 2014 article. "Children receive a gene from both father and mother by default. When a single mutation in the child leads to intellectual disability, this is called a mutation in a dominant gene. Other mutations affect recessive genes. Having a single mutation in a recessive gene, in principle, has no effect on the carrier. A handicap only arises if the mutation is in both copies of the recessive gene. In our clinic we found very few of these double mutations in recessive genes in children with intellectual disabilities. Strange, because there are more recessive than dominant genes for this. So we wondered: where did all those recessive mutations go?"

More medical diagnoses

The question led to a long and extensive search, the results of which have now been published in Nature human behavior [link]. “We first investigated how often these recessive mutations occur in the population,” says Han Brunner professor of clinical genetics and, like Gilissen, involved in both publications. To do so, the data of more than 300,000 people included in the UK Biobank were examined. "We found that each person carries on average two mutations in any one of 1,900 recessive genes. According to textbooks, carriers of such recessive genes should not experience any disadvantage. However, we found that, as a group, they do have more medical diagnoses, and slightly less offspring. Thus, they are less likely to pass on their genes for recessive disorders."

Less education

Compared to other recessive genes, genes for intellectual disability were the most absent. Apparently, there is another aspect at play here. The UK Biobank data provide a clue. Gilissen: "Carriers of a recessive gene for intellectual disability go to school for a shorter time, which suggests that they have a lower level of education. Mind you, these are carriers, according to existing theory, are not affected at all by their recessive gene."

More likely to be childless

Thus, carriers of recessive genes for intellectual disability have reduced reproductive success and a shorter school career. Brunner: "In 1859, Darwin published On the Origin of Species by Means of Natural Selection. It was mainly about survival, about genes that make you strong and healthy, about natural selection. Twelve years later he wrote The Descent of Man, and Selection in Relation to Sex, in which he refined his theory. You can have fine genes, but if no one wants to have children with you, those genes will not be passed on. The principle of sexual selection. We're not demonstrating it rock solid here, but if I had to give a cause for these findings - based in part on a lot of sociological research - sexual selection is the most obvious explanation."

Rewriting textbooks

This research clearly shows that recessive carriers of diseases have a selection disadvantage at the group level. In the past, this has been pointed out for specific disorders. Here, for the first time this is demonstrated for recessive disorders in general. That may go into the textbooks. Brunner: "Our research indicates that our genes are still constantly changing, that the genetic landscape remains in motion, that evolution continues. The idea that in our modern age evolution has come to a creaking halt thanks to health care and other amenities is not true." Nor will it ever be, Gilissen suggests: “We will never be optimized for tomorrow's problem.”

-------

Paper in Nature human behaviour: Reproductive and cognitive phenotypes in carriers of recessive pathogenic variants - Hila Fridman, Gelana Khazeeva, Ephrat Levy-Lahad, Christian Gilissen, Han G. Brunner

Journal

Nature Human Behaviour

DOI

10.1038/s41562-025-02204-7

Method of Research

Data/statistical analysis

Subject of Research

People

Article Title

Reproductive and cognitive phenotypes in carriers of recessive pathogenic variants

Article Publication Date

15-May-2025

Amazon could survive long-term drought but at a high cost

University of Edinburgh

image:
Rainforest study area in north-eastern Brazil, showing rows of transparent panels to redirect water away from trees.
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Credit: Pablo Sanchez Martinez

The Amazon rainforest may be able to survive long-term drought caused by climate change, but adjusting to a drier, warmer world would exact a heavy toll, a study suggests.

The findings show that adjusting to cope with the effects of climate change could see some parts of the Amazon rainforest lose many of its largest trees.

This would release the large amount of carbon stored in these trees to the air, and reduce the rainforest’s immediate capacity to act as an important carbon sink, researchers say.

Parts of the Amazon are expected to become drier and warmer as the climate changes, but the long-term effects on the region’s rainforests – which span more than 2 million square miles – are poorly understood.

Previously, research has raised concerns that a combination of severe warming and drying, together with deforestation, could lead to lush rainforest degrading to a sparser forest or even savanna.

Now, findings from the world’s longest-running drought study in tropical rainforest have revealed some of the profound changes the Amazon could undergo in a drier world.

Over a 22-year period, a one-hectare area of rainforest in north-eastern Amazonian Brazil – roughly the size of Trafalgar Square – has been subjected to long-term drought conditions.

The experiment began in 2002, with thousands of transparent panels installed above the ground to redirect roughly half of the rainfall to a system of gutters, taking it away from the trees.

Analysis by a team co-led by scientists from the University of Edinburgh and the Federal University of Para, Brazil, shows that most of the study area’s largest trees died during the first 15 years of the experiment, after which the forest stabilised.

The team’s findings show that for the seven years after the large initial biomass losses the availability of water increased for the surviving trees. Tests on these remaining trees showed they were now no more drought-stressed than those in nearby rainforest not subjected to drought.

Overall, the area lost more than one-third of its total biomass – the trunks, branches, stems and roots where carbon is stored in living vegetation. Such widespread losses across the Amazon would see the rainforest release vast amounts of carbon, and greatly reduce its immediate capacity to act as a sink for emissions from human activities, the team says.

Having lost carbon through excess tree deaths during the first 15 years of the study, surviving trees in the area are now making slight carbon gains, the team says.

While the study area has less woody biomass than normal rainforests in the Amazon, it still has more than many dry forests and savannas. This indicates that the rainforest has some long-term resilience to the drier conditions it could experience due to climate change, but that this comes at a high cost.

The amount of biomass the Amazon could lose, and the time required for it to stabilise, may be underestimated, as the study only assessed the effects of soil drought, the team says.

Further research is needed to assess other likely impacts, such as changes to moisture in the air, temperature and the compounding effects of other climate-related factors such as storms or fires, they add.

The study, published in the journal Nature Ecology and Evolution, was carried out by a team led by Professors Patrick Meir of the University of Edinburgh and Antonio Carlos Lôla Da Costa of the Universidade Federal do Pará and the Museu Paraense Emílio Goeldi, Brazil. It also involved researchers from the Universities of Exeter and Cardiff, and CREAF in Spain. The research was supported by the Natural Environment Research Council (NERC), the Royal Society and the UK Met Office Newton Fund.

Lead author Dr Pablo Sanchez Martinez, of the University of Edinburgh’s School of GeoSciences, said: “Our findings suggest that while some rainforests may be able to survive prolonged droughts brought on by climate change, their capacity to act as both a vital carbon store and carbon sink could be greatly diminished.”

Professor Patrick Meir, of the University of Edinburgh’s School of GeoSciences, said: “Ecological responses to climate can have very large impacts on our environment, locally and globally; we cannot understand and predict them without long-term collaborative research of this sort.”

Journal

Nature Ecology & Evolution

Aerial shot of the Caxiuanã National Forest in north-eastern Brazil, where the study took place.
Credit
Paulo Bittencourt

Close-up of rainforest trees in the study area being monitored using scientific equipment.
Credit
Pablo Sanchez Martinez

Does adapting to a warmer climate have drawbacks?

A laboratory study of zebrafish shows that fish can be bred to adapt to warmer water temperatures without apparent trade-offs. But it's not clear that wild fish can adapt as quickly as their pampered laboratory kin

Norwegian University of Science and Technology

Adapting zebrafish to warmer water temperatures — image:
In a long term experiment designed to mimic evolution over seven generations, researchers specially selected a group of zebrafish to tolerate warmer water temperatures. Anna Andreassen, who did her PhD research at the Norwegian University of Science and Technology (NTNU) on this project, is shown holding some of the participants from the experiment. Photo: Thor Nielsen / NTNU
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Credit: Photo: Thor Nielsen / NTNU

Global warming is already very tough for animals in the wild, but it may be toughest for creatures like fish, whose body temperatures are controlled by the water temperatures around them.

Fish have to evolve to handle higher water temperatures, if they can’t move to areas with colder water. But what if adapting to warmer water has other unwanted consequences?

In a new publication in Nature Climate Change, researchers looked at zebrafish that they had specially bred over 7 generations to tolerate higher maximum water temperatures.

Very few research groups have been able to test how fish can evolve when facing climate change, because it takes thousands of fish and many years of careful experiments.

In the research world, an experiment of this size and scale is unique, said Fredrik Jutfelt, senior author of the paper and head of the Jutfelt Fish Ecophysiology Lab at the Norwegian University of Science and Technology (NTNU). Jutfelt also has a position at the University of Gothenburg.

"Very few research groups have been able to test how fish can evolve when facing climate change, because it takes thousands of fish and many years of careful experiments," he said. "That makes this work very important because we can finally start to understand how evolution may help fish adapt to warming waters.”

The study was designed to see if the higher heat tolerance would affect other aspects of the fish’s metabolism and ability to survive.

Surprising result

Much to their surprise, the researchers found that the fish that were bred to tackle warmer temperatures were also more tolerant to colder temperatures.

When we look at how well they reproduce, and how well they grow and perform, we didn't really see any trade-offs.

“What we normally expect, if we compare species in the wild that are warm adapted, is that they have lower cooling tolerance. And then we saw this higher cooling tolerance, which was surprising,” said Anna Andreassen, the first author of the new paper.

Andreassen was a PhD research fellow at NTNU in the Jutfelt group when she did this research, and is now a postdoc at the Technical University of Denmark.

What was even more surprising for the researchers was that increased heat tolerance didn’t affect other aspects of the fish’s metabolism or ability to reproduce or swim.

A wild population from India

Andreassen and her colleagues designed their experiments to compare three groups from a population of wild fish collected in India and brought back to NTNU in 2016.

One group from this population was specifically selected to be more tolerant to high temperatures. A second control group of “normal” zebrafish was allowed to reproduce without being selected for a specific heat tolerance. The third group from the same population was selected to be worse at tolerating heat.

After observing the change in warming tolerance, the researchers compared other traits among the three groups to see if being adapted to warmer temperatures gave fish a physiological or reproductive advantage or disadvantage compared to “natural” fish from the same original group.

Oxygen is key

One test involved seeing how efficiently the 2-cm-long fish used oxygen.

Just like with humans, higher temperatures increase the fish’s metabolism, so that individuals need more oxygen at higher temperatures.

One possible mechanism to explain why heat-tolerant fish do better in warmer waters might be that their bodies could use oxygen more efficiently than the controls or the fish with low warm tolerance.

“So one idea is that the (heat-tolerant) fish itself has some capacity to take up more oxygen than others,” Andreassen said.

Testing in a swim tunnel

So the researchers put the fish in a fish equivalent of a treadmill, a swim tunnel, where the researchers could adjust the water speed and simultaneously measure the oxygen consumption.

“We can measure how much oxygen they take up from the water while they're swimming at their maximum capacity,” Andreassen said. “It’s like putting them on little fish treadmills.”

But much to their surprise, they essentially found no difference in the use of oxygen among groups.

There was one exception. The group that was bred to be more sensitive to high temperatures didn’t do as well as the other groups when temperatures were highest.

In short, “we don't see that oxygen actually helps fish in tolerating warming in this case,” Andreassen said.

“And that is big in our field,” she said. “One very big hypothesis is that (global) warming itself might not limit the animals, but that oxygen becomes limited at high temperatures. But we didn’t see that.”

Swimming and having babies

The story was more or less the same when it came to other measures of what makes a successful life for a fish.

The researchers thought there might be some kind of trade-off for tolerating higher water temperatures. So they looked at the commonly accepted indicators of what makes a fish well-adapted to its environment.

“When we look at how well they reproduce, and how well they grow and perform, we didn't really see any trade-offs” for the fish that had evolved to tolerate warmer temperatures, Andreassen said.

That doesn’t mean there isn’t a trade-off, she said, but that the researchers didn't find one in the most commonly accepted measures of fitness that they tested.

Better in a warming world?

Global warming means higher average temperatures, but it can also cause severe heat waves. Researchers from all over the planet are scrambling to understand if plants and animals can handle this additional stress, and if so, how.

It took three years for researchers in the Jutfelt group to breed their heat-tolerant zebrafish. Could wild populations evolve on their own that rapidly?

Rachael Morgan, who also did her PhD with the same fish from the Jutfelt Lab concluded in a 2020 article that there was a low potential for zebrafish to be saved by evolving to tolerate higher temperatures. Nevertheless, Andreassen said, it’s important for biologists understand how these adaptations might actually come about.

“We’re trying to understand the physiology and what is actually changing in the body of the animals,” she said.

Jutfelt said it's important not to overlook the larger problems that come from the warming the planet is experiencing now.

“Even though the zebrafish didn’t show any adverse effects from developing a tolerance to higher water temperatures, climate change will nevertheless continue to pose unanticipated and dangerous challenges to all life on Earth,” he said.

Researchers from the Jutfelt Lab at the Norwegian University of Science and Technology (NTNU) traveled to India in 2016 to collect wild zebrafish. These animals were the basis for a unique study on how selecting fish for heat tolerance over seven generations would affect their evolution.

Credit

Photo: Fredrik Jutfelt/NTNU

Reference:
Anna H. Andreassen, Jeff C. Clements, Rachael Morgan, Davide Spatafora, Moa Metz, Eirik R. Åsheim, Christophe Pélabon & Fredrik Jutfelt. (2025) Evolution of warming tolerance alters physiology and life history traits in zebrafish. Nature Climate Change. https://doi.org/10.1038/s41558-025-02332-y

Journal

Nature Climate Change

DOI

10.1038/s41558-025-02332-y

Method of Research

Experimental study

Subject of Research

Animals

Article Title

Evolution of warming tolerance alters physiology and life history traits in zebrafish

Article Publication Date

14-May-2025

Researchers delve into incels’ rejection of work and study

McGill researchers’ analysis of online forum conversations finds that some incels offer an ideological rationale for not working or studying, one reinforced by peer pressure

TOXIC MASCULINTY LIVING IN MOM'S BASEMENT

McGill University

Researchers delve into incels’ rejection of work and study

McGill researchers’ analysis of online forum conversations finds that some incels offer an ideological rationale for not working or studying, one reinforced by peer pressure

The critically acclaimed Netflix drama Adolescence has put a spotlight on the culture and ideas of incels (involuntary celibates), an online subculture of people (mostly male and heterosexual), who define themselves as unable to find a romantic or sexual partner, largely due to their perceived unattractiveness.

By analyzing exchanges on online incel forums, McGill University researchers have gained insights into the negative attitudes toward labour force participation held by many young men who identify as incels. The researchers suggest that while some incels struggle with finding and retaining work because of various mental health challenges, for many members of the incel forum the choice not to work, study or train for a job is based on deeply held beliefs, and that these beliefs are often reinforced by other online forum participants.

This suggests there is a need for education and employment support programs that address not only the practical barriers to employment and study, but also the deep sense of alienation and social rejection felt by many in this group, the researchers said.

Unemployment as a commitment to incel identity

Recent surveys have shown that a disproportionally high percentage (up to 30 per cent) of incels are not in employment, education or training (NEET). This prompted researchers to analyze more than 1,200 online comments on employment on 171 discussion threads on a popular incel forum during two weeks in the fall of 2022.

“They use employment status to assess the degree of commitment of their peers to incel identity and often encourage other incels to embrace a life of unemployment and isolation. Many also share the belief that the absence of a female romantic partner makes working pointless,” said Eran Shor, a McGill sociologist and co-author of the study published in the journal Gender, Work & Organization.

High levels of internal policing help maintain inceldom

Members of the community participating in the discussions frequently encouraged others to retain their NEET status, something they praised as a confirming sign of “true” inceldom (a “truecel”). Those who talked about trying to improve their situation by working or studying were regularly labelled “fakecels.”

Only about one-quarter of forum members suggested that they or others should try to improve their situation by studying or finding work.

“This peer pressure makes unemployment a badge of honour rather than a problem to be solved. These online spaces can reinforce harmful ideas and discourage members from seeking help or changing their situation. So, understanding how these beliefs form is the key to finding ways to support and engage with these marginalized young men,” said Shor.

Need for a multi-pronged approach to reintegrating incels into society

The researchers suggest that encouraging incels to consider changing will require a multi-layered approach. They said there is a need for education, job training and placement programs. Interventions that focus on mental health and online community engagement will also be important, they said.

Shor added, "Long-term change will require challenging harmful narratives about masculinity, relationships, and success. Instead of punishing incels or banning their communities outright, helping them reintegrate into society will be more effective.”

The study

“Don't Work for Soyciety:” Involuntary Celibacy and Unemployment by AnnaRose Beckett-Herbert and Eran Shor was published in Gender, Work & Organization.

DOI: 10.1111/gwao.13248

There were no external sources of funding for the research.

Journal

Gender Work and Organization

DOI

10.1111/gwao.13248

Method of Research

Literature review

Subject of Research

People

Article Title

“Don't Work for Soyciety:” Involuntary Celibacy and Unemployment