In today's post, I would like to introduce you to the theory of turbulence. It is important to know the basic issues of fluid mechanics because thanks to this we are able to verify our assumptions and simulation results.
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| Fluid flow mechanics |
At the outset, three types of fluid flow must be distinguished:
1. Laminar flow
2. Transitional flow
3. Turbulent flow
What are the main differences in laminar and turbulent flow?
"In simple terms, laminar flow is when every particle of fluid flows along one smooth path. The particles of the fluid do not interfere with one another, they don't mix or shift between layers. Turbulent flow is when the flow of a fluid is irregular. Particles in turbulent flow can move back and forth between layers, mixing and falling into whirlpool-like patterns of flow ".
If you want to learn more about the differences between these fluid movements, visit the link below:
As for the transient flows, there are 5 mechanisms that can cause the fluid movement to shift from laminar to turbuent. See pictures below :
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| https://www.comsol.com/blogs/which-turbulence-model-should-choose-cfd-application/ |
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| http://cfpl.ae.utexas.edu/simulations-of-turbulent-spots-and-wedges-over-textured-surfaces |
TS Waves (Tollmien – Schlichting wave)
In fluid dynamics, a Tollmien – Schlichting wave (often abbreviated T-S wave) is a streamwise unstable wave which arises in a bounded shear flow (such as boundary layer and channel flow). It is one of the more common methods by which a laminar bounded shear flow transitions to turbulence. The waves are initiated when some disturbance (sound, for example) interacts with leading edge roughness in a process known as receptivity. These waves are slowly amplified as they move downstream until they may eventually grow large enough that nonlinearities take over and the flow transitions to turbulence.
https://en.wikipedia.org/wiki/Tollmien%E2%80%93Schlichting_wave
Spanwise Vorticity Flow
In the case of surface flow for low Reynolds numbers with relatively high curvature in the cross-section parallel to the direction of undisturbed flow, a detachment bubble may be generated. In this case, the presence of a bubble is a trigger for the transition from laminar to turbulent motion.
Three-dimensional vortex breakdown
Such a transition (relaminarization) is a phenomenon which, in the case of aerodynamic problems, takes place as a result of high acceleration of the fluid in a given zone of the system. This is often the result of a large pressure gradient that occurs locally. Such acceleration reduces the height of the boundary layer, decreases the intensity of turbulence, decreases the friction coefficient and increases the aspect ratio. In short, it is the ratio of loss thickness to shoot loss thickness.
https://sites.me.ucsb.edu/~meiburg/pubs/Ruith_et_al_2003.pdf
https://ntrs.nasa.gov/api/citations/19900012428/downloads/19900012428.pdf
Turbulent spots
Turbulent spots are arrowhead shaped pockets of turbulent that form in the late stages of laminar to turbulent transition process (red circle in the schematic below). These spots increase in size as they travel downstream and form fully turbulent flow as they merge together. My research is looking at the formation and growth mechanisms of turbulent spot as well as interactions of millimeter scale surface textures with spots. If laminar to turbulent transition can be delayed using surface textures, then drag could be reduced.
http://cfpl.ae.utexas.edu/simulations-of-turbulent-spots-and-wedges-over-textured-surfaces
Fully Turbulent Flow
In fluid dynamics, turbulent flow is characterized by the irregular movement of particles (one can say chaotic) of the fluid. In contrast to laminar flow the fluid does not flow in parallel layers, the lateral mixing is very high, and there is a disruption between the layers. Turbulence is also characterized by recirculation, eddies, and apparent randomness. In turbulent flow the speed of the fluid at a point is continuously undergoing changes in both magnitude and direction.
Detailed knowledge of behavior of turbulent flow regime is of importance in engineering, because most industrial flows, especially those in nuclear engineering are turbulent. Unfortunately, the highly intermittent and irregular character of turbulence complicates all analyses. In fact, turbulence is often said to be the "last unsolved problem in classical mathemetical physics."
If U want to know more about transition from laminar to turbulent see link below
https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1120&context=honorstheses
If U want to read more posts about CFD tutorials and tips go links below
https://howtooansys.blogspot.com/2021/10/cfd-ansys-rivals-cfx-vs-fluent-in-cht.html
https://howtooansys.blogspot.com/2021/10/obstacles-to-overcome-duel-on-simscale_4.html
https://howtooansys.blogspot.com/2021/09/cht-nozzle-game-with-radiation-on.html
https://howtooansys.blogspot.com/2021/09/how-toeasy-mesh-cfd-models-in-ansys.html
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