Helical Strakes with your Chimney?

Helical strakes are often used on chimneys to reduce vortex induced vibration (VIV). However, these strakes also have a significant drawback - they induce massive increases in drag and side forces that have significant structural implications for the chimney. Read on for a Computational Fluid Dynamics (CFD) study that compares a smooth cylindrical chimney to the same chimney with helical strakes.

CFD Simulation of a Helical Strakes ChimneyCFD Simulation of a Helical Strakes ChimneyVelocity magnitude contours

CFD Simulation of a Smooth ChimneyCFD Simulation of a Smooth ChimneyVelocity magnitude contours

Streamlines around a Helical Strakes ChimneyStreamlines around a Helical Strakes Chimney

Streamlines around a Smooth ChimneyStreamlines around a Smooth Chimney

Side Force

The side force (perpendicular to the flow direction and parallel to the ground) time history shows that the helical strakes chimney has a lower frequency of VIV than the smooth chimney. However, the average side-force magnitude for the helical strakes chimney is many times (~300%) higher than that of the smooth chimney.

Helical Strakes Chimney Side Force Time HistoryHelical Strakes Chimney Side Force Time History

Smooth Chimney Side Force Time HistorySmooth Chimney Side Force Time History

Drag Force

The drag force time history shows a massive increase in drag (~200%) for the helical strakes chimney compared to the smooth chimney.

Drag Time HistoryDrag Time HistoryCompares a helical strakes chimney to a smooth chimney

Conclusion

This set of CFD simulations does not account for fluid-structure interaction, so if the frequency of the VIV is similar to the natural frequency of the structure then you are likely to see larger amplitude side-force oscillations.

The average side forces are only ~1% of the drag forces for each chimney.

The chimney designer has to weigh carefully the need to reduce VIV with helical strakes over the associated large increases in drag.

Notes

  • Chimney: diameter (D) = 2 m, height = 20 m
  • Helical strakes: pitch = 10 m (5D), strake width = 0.2 m (0.1D)
  • Wind speed = 22 m/s (50 mph)
  • Simulations created and performed in Caedium Professional using the incompressible, transient RANS solver, and the k-omega SST turbulence model

Comments

Caedium in Architecture

I'm an architect.
I've been trolling through your blog trying to come across a sample of CFD analysis with Caedium on an architectural model, with no luck so far. (Got to page 14.)
My interest would be in airflow and heat loss/gain through a space and ducts & plenums. This analysis would be for sustainability related building performance on small cottages (which don't have large budgets). Particularly related to the effect of changing temperatures in associated surfaces such as floor slabs.
Incorporating the effect of radiation (sunlight) on a surface over time would also be good.
Can you direct me to articles that might illustrate that kind if capability?
Or is this still beyond the reach of the desktop tools?
Thanks

HVAC

In terms of external air flow around structures we have a number of examples:

However, I assume external air flow is not your focus. Based on your comment I see you are interested in HVAC in buildings and rooms. Caedium customers have successfully performed CFD simulations for such applications, e.g., Black Silver Solutions. For a given space you would typically model the inlets and outlets to the HVAC system, unless the duct work is your focus.

There's no inherent limitation in desktop CFD. All the major CFD vendors provide applications that work great on desktops - optimized for multi-core processors.

Caedium does not currently support radiation modelling, but is good for forced and natural (buoyancy driven) heat transfer.

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