Author: Rainer Groh

Wind energy is expected to play a central role in the global transition to low-carbon electricity. Over the past decades, the dominant design for wind turbines has been the single large rotor mounted on a tall tower. However, as turbines continue to grow in size, engineers face increasing challenges related to transportation, manufacturing, structural loads, and cost.

One alternative concept that has attracted growing interest is the multi-rotor wind turbine (MRWT). Instead of one very large rotor, a multi-rotor turbine uses many smaller rotors mounted on the same structure to achieve the same overall power output. This approach could offer several potential advantages: smaller blades are easier to manufacture and transport, maintenance can be simplified, and the system may continue operating even if one rotor fails. In some cases, interactions between the rotors may even slightly increase the total power generated.

Despite this promise, research on multi-rotor systems has been limited by a practical issue: the lack of suitable reference rotor designs that researchers can use for modelling and comparison. Most existing wind-turbine reference models were created for single-rotor systems and are not well suited to studying arrays of smaller rotors.

To address this gap, we have developed an open-source library of optimised wind-turbine rotors, covering rated powers from 100 kW to 1 MW. These rotor designs were created using aeroelastic optimisation methods, which consider both aerodynamic performance and structural behaviour. The optimisation process enabled the design of blades that balance energy production, structural safety, and material use.

A key feature of the library is that the designs are fully open and compatible with widely used simulation tools. This means researchers around the world can use the models directly in their own studies. The library also includes a method for interpolating between designs, allowing new rotor sizes to be generated between the predefined ratings.

By providing a shared set of reference designs, this work aims to make it easier to explore questions such as:

• How many rotors should a multi-rotor turbine have?
• How do rotor interactions affect performance and loads?
• Can multi-rotor systems reduce the cost of wind energy?

Ultimately, the goal is not to produce “ready-to-build” turbine blades, but rather to provide a flexible starting point for research and innovation.

Nature often solves engineering problems in ways that are both elegant and efficient. A striking example can be found in the beak of the hummingbird. When a hummingbird captures an insect, its beak snaps shut with remarkable speed. What is fascinating about this motion is that it does not rely purely on muscular force. Instead, the geometry of the beak itself helps store and rapidly release elastic energy.

Our work explores how this natural mechanism can inspire new morphing structures in engineering. In particular, we investigate snap-through, a mechanical instability in which a structure suddenly transitions from one stable shape to another. Many people have experienced this behaviour when pressing on a rubber cap that suddenly everts. The motion appears instantaneous because the structure slowly stores elastic energy and then releases it all at once.

The central question of a recently published paper was: how much does the shape of the hummingbird inspired structure matter?

Using computational models, we systematically varied the geometry—such as curvature and cross-sectional shape—and observed how these changes affected the speed and intensity of the snap-through motion.

What emerges is a clear message: geometry is not just a detail—it is the mechanism. Small geometric changes can dramatically alter how much elastic energy is stored before instability occurs, and therefore how fast the structure moves when it snaps. Some geometries allow the structure to accumulate energy efficiently and release it in an extremely rapid motion.

Interestingly, the geometries that produce the most effective snap-through behaviour resemble the shapes we observe in real hummingbird beaks. This suggests that biological systems may naturally exploit these mechanical principles.

Why does this matter for engineering? Because snap-through offers a way to generate fast motion without large motors or complex mechanisms. By carefully designing geometry, we can create structures that move quickly, reliably, and efficiently. Such ideas could inform the design of soft robotic grippers, deployable aerospace structures, or rapid mechanical switches.

More broadly, the study illustrates a recurring theme in structural mechanics: performance often emerges not from stronger materials, but from the intelligent use of geometry and instability. When we learn how to harness these effects—many of which nature already uses—we can design structures that are both simpler and more capable.

Moths are not usually thought of as noisy animals, but some species produce rapid bursts of ultrasonic clicks that interact with the echolocation systems of hunting bats. In ermine moths (Yponomeuta), these sounds originate from a small structure on the hindwing known as a tymbal. The mechanics behind how this tiny feature generates sound are surprisingly rich and provide an interesting example of how biological structures exploit mechanical instabilities.

The tymbal consists of a narrow band of ridges located along a natural fold in the wing. As the moth flaps, this region deforms under aerodynamic and inertial loads. Our work shows that the ridges do not simply bend smoothly. Instead, they undergo sequential snap-through buckling: each ridge flips between two stable shapes one after another. Every snap produces a short mechanical impulse, and the series of snaps during a wingbeat results in a burst of ultrasonic clicks.

To understand this process, we studied the geometry and motion of the wing and then constructed a simplified creased-shell analogue, inspired by the mechanics of origami. The model captures how a thin shell with discrete crease-like features can buckle in a controlled sequence. In the moth, the resulting vibrations propagate into a nearby thin region of the wing, which acts as a lightweight radiating surface that emits sound.

From a mechanics perspective, this is interesting because buckling is usually considered a structural failure. Here, however, it is deliberately built into the morphology and used as a functional mechanism. The ermine moth effectively couples the wing-beat motion during flight into a repeatable sequence of mechanical snap events that generate sound (and because the moth is deaf, it is none the wiser for it).

Beyond the biological insight, this system highlights how thin shells with patterned creases can be designed to trigger controlled sequences of instabilities. Such ideas may be useful in the design of lightweight acoustic devices, responsive structures, or engineered systems that harness snap-through behaviour rather than avoiding it.

Kirigami is the sister of the more well-known origami. While origami focuses on creating art out of folding paper, kirigami extends this to allow for cuts to be introduced. Both origami and kirigami have been researched extensively over the last decade to explore potential applications in structural engineering.

For example, when a parallel matrix of cuts is introduced into a sheet of paper and that sheet is then tensioned, the cells between the cuts buckle out-of-the-plane to form a pattern. This effect is currently being used for expandable packaging but also to create lightweight tension springs for space applications.

What is interesting is that depending on the ratio of cut length to vertical and horizontal cut spacing, different patterns can form. The cells either buckling anti-symmetrically, symmetrically or into a mixture of these two motifs (co-existence). These three behaviours lead to significantly varying tensional stiffness, which means that design approaches are needed to predict the buckling onset and stiffness correctly.

To date, the three regimes have largely been explored using computational methods, e.g. finite element analysis. These methods are very accurate but tend to be slower than analytical methods and require designers to have access to sophisticated modelling software. In a recent publication, one of my PhD students (Yuwen Du) and I have introduced an analytical model that correctly predicts buckling loads as well as the buckling modes into the three possible configurations.

With such a model it is now possible to rapidly design with this particular kirigami structure. In addition, the model has allowed us to delineate the energetic contributions that force the kirigami sheet to buckle into the anti-symmetric or the symmetric mode.

Aircraft wings are designed for the worst-case scenario encountered during a service lifetime. Generally, this is a severe gust that occurs only a handful of times but produces significant bending stresses at the wing root to require substantial reinforcement. Because the aircraft spends the majority of its lifetime not experiencing such a rare, extreme load case the wing is over-designed for much of its service life.

The additional reinforcement and mass required to sustain these rare load cases unnecessarily increases fuel burn during standard operation (99.9% of the time).

What if this extreme load case could therefore be removed from the aircraft’s life cycle?

For example, by deploying a spoiler or tab into the airflow on the top wing surface to disrupt the boundary layer, detach the flow, and thereby dump lift and reduce bending moments at the root of the wing.

Such a device is possible, but usually requires the integration of an active control system with sensors and actuators that also add weight and complexity to the structure and aircraft system. A more elegant solution is to use a passive device. For example, a structure that deforms as the wing bends during a gust and then triggers the spoiler to pop-out of the top wing surface. In a way, this would represent a “reflex” of the wing to protect itself from severe loading.

In previous work, we (PhD student Ed Wheatcroft and Bristol colleagues Mark Schenk, Alberto Pirrera and I) designed such a device (1, 2) and tested its performance in a wind tunnel (see above image). These wind tunnel experiments proved the concept that a deploying leading-edge spoiler (albeit using an actuator in this proof of concept) could effectively reduce the lift produced by the airfoil.

In our latest publication, the effectiveness of the spoiler is demonstrated on an aircraft-level study in collaboration with Airbus UK. To do this, an idealised full aircraft model was simulated and subjected to a number of different flight load cases, with the lift-reduction effect of the spoiler hard-wired into the simulation. The results demonstrated that the spoiler is capable of reducing the sizing wing root bending moment by up to 17% for the particular airframe considered.

Following these promising results, the next steps are to integrate a spoiler into a flexible wing model that can then be tested live in a wind tunnel.

A PhD student I am co-supervising (Abdirahman Sheik Hassan) has just published his first paper—a review on multi-rotor wind turbines.

To extract more and more energy from the wind, turbine blades are getting longer and longer. This scaling places significant challenges on the materials, manufacturing, testing, transportation, operation, and maintenance of the blades. With blades exceeding 100 m in length, glass-reinforced composites are increasingly being replaced with stiffer carbon fibre; the quality of resin infusion during manufacturing is increasingly difficult; and the transportation to the final site along roads is a major logistics operation.

One possible solution out of this bottleneck is to locate multiple smaller rotors on a single supporting frame, a so-called multi-rotor system. Not only would this approach remove many of the challenges listed above around manufacturing and transportation, but it would theoretically allow the use of more sustainable bio-based composites whose inferior mechanical properties make them unsuitable for longer blades. In addition, analytical scaling arguments suggest that the cost of energy from multi-rotors decreases favourably with the number of rotors employed.

Our review paper summarises the literature on multi-rotors from the past decades including aerodynamics, materials, structures, design, control, sustainability, reliability, and
maintenance. The paper also highlights open questions around the environmental impact benefits of multi-rotors, the characterisation of rotor–rotor inter-
action effects, and the investigation of aerodynamic noise, amongst others.

I have just published a new paper in the Proceedings of the Royal Society A with collaborators at IIT Hyderabad (India).

Thin-walled cylinders are used in various engineering applications ranging from fuselages in aircraft to fuel tanks in launch vehicle stages. When thin-walled cylinders are compressed, they are susceptible to buckling and this occurs at loads well below what is suggested by analysis. As a result, cylinders are designed with very conservative safety factors.

One way to improve the design process is to devise a non-destructive testing framework that can predict when a manufactured cylinder will buckle, which would then provide information on potential remedial measures. Such a methodology has recently been developed, where a cylinder is probed laterally from the side to measure its resistance to indentation, but the method is very dependent on the location of probing.

Buckling of cylinders is a local event, where a single dimple initiates a dynamic buckling sequence. Probing should occur at the weakest spot where the dimple initiates. However, this site is not known a priori as it depends on imperfections in manufacturing and loading.

Our paper showed that the dimple initiation site can be predicted from deformation measurements (using digital image correlation) before buckling occurs. The measured deformations are used to compute a bending energy measure which successfully reveals the presence of a developing dimple at approximately 60% of the critical buckling load.

This information can help to determine where to probe the cylinder to determine the buckling load in a non-destructive manner, i.e. without ever pushing the cylinder to the buckling point.