The mysteries of turbulent flow
– by Albrand Veldhuizen, 04/02/20
Despite being a common phenomenon that we witness every day all around us, turbulent flow is related to one of the oldest unsolved mathematical problems. Predicting its possibilities requires heavy computer simulations and even then we’re merely scratching the surface.
Laminar flow vs turbulent flow
In fluid dynamics we talk about two kind of flows: laminar and turbulent. A laminar flow is an easy and smooth flow where all the medium molecules are flowing perfectly in parallel to each other. Turbulent flow is the flow of a medium which has exploded into chaotic movements and vortexes.
The medium can be anything – water, air, smoke or any other gas or liquid.
Both laminar and turbulent flows are very common, and we see them happening around us every day in all forms… Smoke coming out of a cigarette. Waterfalls. The wind. A supercell cloud, popping up on a hot summer day.
Turbulent flow in the HVAC system
At H&H our most basic HVAC system uses air and water as a medium to carry energy. Both mediums manifest laminar and turbulent flows and, as you can guess, turbulent flow is the one that causes the most challenges.
Imagine a straight rectangular duct, running above the deck and crossing several cabins. Let’s say turbulent air is thundering through the duct. Vibrations and excessive noise will lead to complaints from the people occupying the cabins beneath the duct. Moreover, extra energy is needed as turbulent air gives much more resistance then laminar air.
Factors that influence turbulent flow
Knowing the source of a problem helps you understand how to solve it, and the same goes for turbulent flow. Understanding what is causing the flow to become excessively turbulent helps you avoid turbulent flow problems.
Every obstacle inside a pipe or duct, such as bends, appendages or constrictions, disturbs the flow and can cause it to become turbulent. While they are part of the system and cannot be eliminated, the trick is to reduce the resistance of the object as much as possible. An example is to use curved bends and constrictions which guide the flow.
The (inside) pipe roughness influences the flow of the medium, especially with small diameters. Rough surfaces cause friction, disturb the flow and eventually lead to turbulence. The bigger the pipe or duct, the less this effect on the medium.
The speed or velocity of the medium itself is also a factor. At high velocities the medium becomes turbulent. Think of it as a mountain stream – quiet and smooth on a summer day, but fierce in early spring when the snow is melting. There is a special equation to determine the speed at which the medium becomes turbulent. This calculation involves the dimensionless Reynolds number.
There are more factors that influence turbulence, like the medium’s density and viscosity. As our HVAC systems only work with water and air, these properties will always be the same.
The oldest unsolved problem in classical physics
Nowadays we have advanced computer programs like CFD (computational fluid dynamics) which simulate when a flow becomes turbulent and how it will manifest. This is not an exact science, however, and the calculations are based upon statistics. These suffice in our case of course but the point remains.
Turbulence or the Navier-Strokes equation is one of the seven Millennium Problems. Solving the exact mathematical equation will earn you a million dollars but rest assured it won’t be easy. The brightest minds in history have broken their heads on this topic, including Werner Heisenberg, who won the Nobel Prize for Physics for his contribution to quantum mechanics.
The story goes that Werner would have two questions when he met God. “Why quantum mechanics? And why turbulence?” He was supposedly pretty sure that God would be able to answer the first question….
Albrand Veldhuizen | Commissioning Engineer
Albrand has been working at Heinen & Hopman since 2006. He has worked himself up to the position of Commissioning Engineer and nowadays he is stationed at one of the largest yacht builders in Germany. During his many years working onboard numerous luxury yachts, he has developed a great expertise in HVAC systems for superyachts.