Poor Assessment of CFD for Rooftop Solar Arrays
In the press release announcing the report "Rooftop Solar Arrays and Wind Loading: A Primer on Using Wind Tunnel Testing as a Basis for Code Compliant Design per ASCE 7 (pdf)," there is the phrase:
"It also addresses the applicability of Computational Fluid Dynamics (CFD), which is sometimes proposed as an alternative to testing."
With eager anticipation I read the report and found a quite damning assessment of CFD.
In assessing CFD the report says:
- "CFD is not currently in a state where it can be used reliably to estimate peak pressures on low-rise building roofs, let alone solar arrays on such buildings, and should not be used as a basis for design at this point in time."
- "The primary challenge is that peak pressures and peak loads at the component scale are not well estimated by this method. This is unsurprising, given the fact that turbulence models (such as Reynolds-Averaged Navier Stokes, RANS, models) have been developed to estimate the mean flow field."
I was disappointed that in making these claims the authors cite no references or comparisons with their wind tunnel data.
I have to think there is a fundamental misunderstanding here on the nature of RANS modeling. The averaging is based on the turbulent fluctuations around a notional mean velocity. The time scales of the fluctuations are assumed to be orders of magnitude smaller than the time scale of the developing flow, whether the flow is assumed to be steady or unsteady. Typically in wind tunnel testing you employ the same averaging to report the localized quantities, such as velocity and pressure. RANS will be able to predict peak wind loads just as you are able to capture the data from a wind tunnel. For instance RANS has no problem predicting the peak low pressure (suction peak) on an airfoil.
Interestingly the paper cites a number of issues that arise in using a wind tunnel, such as problems relating to:
- Blockage effects - restricted wind tunnel working area versus free air
- Reynolds number effects - scaled wind tunnel model versus full size
As I discussed previously the great complementary strength of CFD is that you can perform simulations at full scale in open air. Alas the authors make no mention of this feature of CFD and did not use CFD to validate their wind tunnel assumptions.
I have to believe that if CFD is useful for simulating complicated flows around aircraft and automobiles then it would also be useful in assessing relatively simple flows around low-rise building roofs with solar arrays. Roofs and solar arrays more accurately resemble flat plates than curved wings and car bodies do, and therefore turbulence models should be in their sweet zone given that a basic test of any turbulence model is to accurately model the flow over a flat plate.
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