In large human blood vessels, such as the aorta, turbulence may occur as is known from the aerodynamics of aeroplanes and cars. While this is a well-known phenomenon in medicine, the exact processes are still poorly understood, although they have a bearing on various aspects of health. In a project funded by the Austrian Science Fund FWF, a team led by principal investigator Björn Hof is now exploring in greater detail what type of turbulence can occur in pulsating flows such as blood flow. In the process, they discovered a new type of turbulent flow that can also occur in smaller blood vessels.
The fact that turbulence in blood has been little studied so far can be explained by the fact that this chaotic form of flow is one of the most complex phenomena in physics. Because turbulence has a great influence on the flow resistance of gases and liquids, understanding it is important for many technical applications, for example in aviation or the construction of pipelines. Chaotic flows also occur in simple pipes or hoses, but whether this actually happens depends on a number of factors, such as the diameter of the pipe and the viscosity of the fluid.
“In the aorta, the largest blood vessel in the human body, the velocities are high enough for turbulence to develop, just as it would in a garden hose or a water pipe,” Hof explains, and he also notes that this is a fact known in medicine. Blood does not move around steadily in the human body, however, but flows in spurts induced by the pumping motion of the heart. There are more exotic types of instability and turbulence specifically associated with pulsating flows, which could also form in smaller blood vessels. Hof has now succeeded in demonstrating such a phenomenon in the context of an FWF-funded project at the Institute of Science and Technology Austria which will run until 2022.
Unstable during braking
In order to demonstrate this phenomenon, Hof's team conducted flow experiments with water in pipes. Tiny reflective particles were added to the water to make the currents visible and to be able to analyse them with cameras. The research team did not pump the water evenly through the pipe, but in a pulsating rhythm. It turned out that at small points of disturbance such as bends or branches rotating, helical flows occur which were previously unknown in blood vessels. “The particular feature of the instability we found is that it occurs during deceleration and disappears again during acceleration,” Hof explains.
Flow experiments with blood
The proof of the new effect was achieved with water, but Hof's research group was not content with that. “Blood is much more complex. It consists of 40 percent red blood cells, which have a great influence on the properties of the liquid,” Hof explains. The experiments were therefore repeated with pig's blood. In this case, detecting the turbulence was more difficult because blood is not transparent. But by measuring pressure distributions at the edge of the liquid, the researchers were able to detect the new form of flow also in blood.
“This result convinced us that the new instability can actually occur in human blood vessels. It differs significantly from well-known turbulence as found in garden hoses or pipelines,” Hof notes.
The team’s result is of medical interest because turbulence leads to shear stress on the vessel walls, where it can cause inflammation and subsequently thrombosis. In addition, perturbations in blood vessels trigger turbulence in the first place, which makes this a chicken-and-egg problem, as Hof explains. In further studies, Hof and his team now want to look at how turbulent flows affect the epithelium, the innermost cell layer of blood vessels.
In his work, Hof relies mainly on experiments. While pipes in particular can be simulated very efficiently on the computer, simulating realistic situations is more complex and thus significantly more expensive. “In real blood vessels there are complex fluid properties, elastic walls that expand and contract,” Hof notes, listing the challenges. And the trials with real blood also revealed the limits of the experiment, he says: “You can't simulate the entire complexity of the flow in such cases, and the experiments also become much more difficult.”
With the measuring methods available, it is not possible to look inside the blood, but only to observe individual blood cells at the edges. While it has proved possible to indirectly detect the turbulence seen in water also in blood, Hof has to concede: “There are many things that we unfortunately don't yet know, and we hope to draw the right conclusions from the combination of model calculations and experiments.” The project has been running since 2019 and is designed to last three years. It is being conducted in collaboration with partners from Germany and Switzerland. Hof, who initiated the project with the German partners, is the principal investigator of the Austrian team.
Why there isn’t more turbulence
After the discovery of the new helical turbulence, there is still one puzzling aspect that keeps Hof's team busy: vessels like the aorta are actually large enough also to allow the conventional kind of turbulence that occurs not only in pulsating flows. Why this does not seem to happen to the extent one would expect at the prevailing velocities is still unclear. “We want to investigate whether the waveform of pulsatile velocity makes it more likely or less likely for normal turbulence to occur in large blood vessels,” Hof announces. The team already has new results on this issue, which Hof is currently preparing for publication. He hopes that these findings could prove useful for technical applications, for example to reduce turbulence, and thus friction, in pipes.
Björn Hof is professor and head of the Nonlinear Dynamics and Turbulence Group at the Institute for Science and Technology Austria (IST Austria) in Klosterneuburg near Vienna. The physicist is interested in the emergence of turbulent flows and in self-organisation. The international research project “Instabilities in pulsating pipe flow in complex fluids” is co-funded by the Austrian Science Fund FWF to the amount of EUR 356,000 and runs until the end of 2022.
Duo Xu, Atul Varshney, Xingyu Ma, Baofang Song, Michael Riedl, Marc Avila, Björn Hof: Nonlinear hydrodynamic instability and turbulence in pulsatile flow, in: PNAS, May 2020