= William H. Cabot
{c}
<William H. Cabot> coauthored the <dynamic subgrid-scale model> developed with <Massimo Germano>, <Ugo Piomelli> and <Parviz Moin>. Their 1990 report compared modeled unresolved stresses with data from <direct numerical simulation> and tested <large eddy simulation> of transitional and turbulent channel flow.<A dynamic subgrid-scale eddy viscosity model by Massimo Germano and others (1990)>[Author byline and Section3: numerical tests]{ref} A participant retrospective identifies Cabot as a Stanford research associate during the collaboration.<Thirty years of Dynamic Modeling by Ugo Piomelli and Parviz Moin>[Page3, account of the1990 summer program]{ref}
The numerical comparison illustrates an important issue in <turbulence> research: matching the objects being compared. A coarse simulation describes resolved motion, while a finely resolved reference contains additional fluctuations. The report filtered the reference data before some comparisons, so the comparison did not automatically count intentionally omitted small scales as errors in a resolved field.<A dynamic subgrid-scale eddy viscosity model by Massimo Germano and others (1990)>[Section3, Figures5–8 and discussion of filtered DNS]{ref}
For an original arithmetic illustration, fine-grid velocities $0$ and $2$ have mean $1$ and mean square $2$. The square of their mean is only $1$. Replacing both samples by the mean preserves that mean but loses a fluctuation contribution of $1$. This elementary difference is why an averaged velocity and an averaged momentum flux contain different information, and why <subgrid-scale stress> requires explicit treatment.
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