Research in mechanical engineering at BME

Interview with Gábor Stépán, Dean of the Faculty of Mechanical Engineering.

In the second half of the 19th century, the Budapest University of Technology and Economics was one of the first engineering institutions to get the rank of university, at the same time as the era's best institutions of technology in Europe. After this glamorous beginning, with world-famous scientist and inventors like Donát Bánki and Tódor Kármán teaching at the University, BME turned into an important center of higher education.

Mechanics - a basic subject that penetrates all sciences

Q: My first question concerns mechanics. Is it a very basic discipline or has it seen any recent innovations?
GS: The answer would be both. Mechanics is indeed a very basic subject, as one might note, it is more than three hundred and fifty years old. This is the discipline of describing mechanical systems which move and change in time. At this university, our faculty teaches mechanical engineering, and the fact that the name has the word "mechanical" in it also implies that it is indeed a basic subject. This might lead one to think that there is nothing new in it any more. This, however, is not true. In some sense, mechanics is considered to be a paradigm or a methodology that tells us how to approach different problems when we have processes which change in time. It was Newton's idea to use differential equations as a mathematical tool for describing them. Later on, all the other disciplines took it as a paradigm. We very often say that a new field becomes a science when it starts using differential equations: that is, when we start using the same methodology that mechanics used and still uses.
The first thing that comes to mind is of course fluid dynamics, which is very closely related to Newtonian mechanics, but later on, thermodynamics or electronics, electromagnetics as well as other fields started using the same method, and it is even used in chemical engineering processes today. In biology, we also witness efforts to (inevitably) combine the rules and laws which were established in mechanics, electromagnetism and chemical processes. In this sense, mechanics is a basic subject, it has provided us with a methodology.

To mention a third aspect, I would say that the discipline can shed new light even on Newtonian mechanics in a number of ways. As an example, I would mention the nonlinear oscillations or nonlinear vibrations. These fields originate in the nineteenth century, when the study of nonlinear differential equations began. Towards the end of the twentieth century - just a few decades ago -, we experienced some surprising discoveries with relation to deterministic systems. Natural systems are governed by deterministic rules and laws, where all the parameters and equations are deterministic, like in case of Newton's law. Still, if there are strong nonlinearities in a system, it can behave in a stochastic way. Such systems are distinguished from others by being called chaotic. This refers to the fact that even though there is a deterministic differential equation "behind the scenes", the appearance of the system is stochastic. So this is an example for a scientific field which is very new and important, and it affected all the other fields of physics, chemistry, biology, and engineering, too.

We fail to see the process

Q: What is your main area of research?
GS: It's dynamics, I would say. I am primarily interested in things which move, which change in time. There are two important things that I try to explain to my students at the beginning of my lecture series. First of all, if they want to be a bit more innovative and to see the world from a higher perspective, they always have to look at the processes in time. This is extremely important for mechanical engineers. We often say that, for example, if a civil engineer or an architect designs something and it starts moving, there is something wrong there. Because of course a building should be steady. On the other hand, if a mechanical engineer designs something and it won't move, that is a problem, too. So we have to design things which actually move. It is sad that we have problems with understanding this all around the world, and especially in Hungary. We refuse to think in terms of processes. And we rarely see charts and diagrams where there is time on the horizontal axis and we can see the change of certain quantities in the vertical axis. You may think about the unemployment rate, the inflation rate, the crime rate or other things which would be very important to see in time, how they change - even if we don't know exactly what is on the vertical axis or what is the range of the parameter or what is the unit there. But knowing whether something gets better or worse and how it changes in time gives us extremely important information. So that's the other point I try to teach to my students: look at all the things as processes which change in time, try to find a time scale. Maybe the change is very slow and we have to measure time in years. In mechanical engineering sometimes we have to measure the time in milliseconds or microseconds when something impacts another body, and travelling waves are running in a piece of steel, and so on. These are the fields which are interesting to me.

Q: In connection with motion, the different kinds of time-delayed systems are important fields of interest these days...
GS: This topic is especially interesting for my research group here at the Department of Applied Mechanics (or engineering mechanics as we call it), there are essential dynamical systems in which some kind of delay effect or memory effect shows up. It might seem contradictory that machines should have memory or they can have effects which refer to the past behaviour of the system. But this happens quite naturally. We might take the example of the vibrations which occur on machine tools and affect the quality of the work-piece. If the tool vibrates a little bit, the surface quality will not be perfect. Clearly, there is a large delay in the system in this case, as the past motion of the tool will affect the surface of the work-piece as it rotates around, and this in turn will cause variation in chip thickness. A similar effect occurs in case of the vibration of wheels. We could take these supermarket trolleys as another example of the phenomenon when the wheel starts doing this kind of "shimmy" or dancing motion. In these cases the contact region between the ground and the wheel behaves as a kind of memory. It will "remember" via the lateral contact forces between the wheel and the ground.

All these are quite apparent in the case of lateral vibrations of wheels and become very important when we develop ABS systems for cars and we try to optimize the braking of the wheels. As you may know, Hungary has some leading research in the field of ABS systems. But I could mention several other cases for delay effects where a human being is also present and cooperates with machines. It is well-known that we have delay effects due to the finite speed of information transmission in our neural system.