The Importance of Fluid Flow Instabilities

Key Takeaways

• What fluid flow instabilities are.

• The types of fluid flow instabilities.

• Why understanding fluid flow instabilities is important. 

Fluid flow instabilities

Although there is not widespread consensus in the scientific community that changes in climate are the root cause of the increasing number of tornadoes and the severity of hurricanes, it is without a doubt that these events are increasing. And, the effects of these fluid flow instabilities can be significant, such as threatening the structural integrity of buildings, the flight of an airplane, or the steadiness of a ship. The potential for unstable fluid flow to be disruptive is not limited to situations where large structures or vessels are involved. In fact, the threat of fluid flow instabilities exists virtually everywhere fluid is in motion. 

What Are Fluid Flow Instabilities?

As fluid flow instabilities are so prevalent, it is difficult to find a single definition that can be easily applied broadly. However, the following may provide a useful viewpoint from which to understand these occurrences.

Fluid flow instabilities are not limited by the type of fluid flow. For example, these disturbances may occur in compressible or incompressible--constant density--fluids, including air or other gases. 

What Are the Types of Flow Instabilities?

As disturbances within fluid flow occur everywhere, there is a wide range of different conditions that may exist for each occurrence. However, these can be classified into distinct types that exhibit similar characteristics, such as:

• Kelvin-Helmholtz Instability
  A Kelvin-Helmhotz instability is a common occurrence where there is a shear change in velocity within a single fluid. An example of this is when a laminar fluid passes a structural inlet in a pipe.

• Rayleigh-Taylor Instability
  Another common fluid flow disruption is the Rayleigh-Taylor instability that happens when two fluids of differing density (with the heavier one on top) are under a constant downward pull as by gravity. This can also occur when the entire system is accelerating towards the higher density fluid.

• Richtmyer–Meshkov Instability
  If the fluid acceleration in a system where a higher density flows on top of a lighter one is due to a shockwave or similar event, then the disturbance is classified as a Richtmyer-Meshkov instability.

• Karman Vortex Street (von Kármán Vortex Street)
  When a fluid flow is interrupted by a solid object such that it creates vortices (as shown in the figure below) the resulting disruption is deemed a Karman vortex street.  

Example of von Kármán vortices from NASA Earth Observatory

This is a common formation around islands, as shown above, but also occurs within pipes where valves or other physical inlets are present. The result is the vortex shedding shown. 

Why Is Understanding Flow Instabilities Important? 

Fluid flow instabilities are natural occurrences. However, in order to understand and analyze these disturbances, fluid flow analysis equations and CFD techniques are utilized. These include the well-known Navier-Stokes and Bernoulli equations as well as quantities like the Reynolds number. The purpose of these analyses is to determine how the system will respond to hydrodynamic instability in the presence of these adverse fluid flow conditions. Of specific importance are the following:

• What conditions may cause hydrodynamic instability?
  The first question to answer concerning hydrodynamic instability is what conditions are likely to cause disturbances that may escalate into turbulence.

• Can hydrodynamic instability be avoided?
  If, as is often the case, hydrodynamic instability is a natural phenomenon, can the system be designed such that effects of instability can be mitigated? At least such that their impact is negligible.

• How will disturbances impact system integrity?
  In order to make mechanical design choices to minimize the impact of instability—for example, on the system’s physical integrity—it is necessary to analyze how the system will hold up structurally.

• How will disturbances impact system operation?
  The impact on the system’s ability to maintain operation is probably the most common question to answer by performing hydrodynamic instability analysis. Answers to this question will define what contingency plans need to be in place for the safety of any personnel, customers, and the system.

• Will instability subside naturally or should it be accelerated?
  In many cases, natural fluid flow instabilities cannot be avoided. However, they will typically subside after some turbulence interval. If so, the extent of the disturbance interval needs to be known, as it impacts how the system should be designed as well as operational procedures. 

The list above gives essential questions concerning the fluid flow instabilities that may be encountered. Failing to provide adequate solutions to the various types of fluid flow contingencies that may occur can have catastrophic results. Therefore, you should rely on advanced solver tools, such as Cadence’s Omnis and Pointwise, to deliver usable data to help you design your system. 

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