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We present a simple, new criterion to extract line plumes from the velocityfields, without using the temperature field, in a horizontal plane close to theplate in turbulent convection. The existing coherent structure detectioncriteria from velocity fields, proposed for shear driven wall turbulence, arefirst shown to be inadequate for turbulent convection. Based on physicalarguments, we then propose that the negative values of$\overline{\nabla_H}\cdot\overline{u}$, the horizontal divergence of thehorizontal velocity field extracts the plume regions from the velocity field.The $\overline{\nabla_H}\cdot\overline{u}$ criterion is then shown to predictthe total length of plumes correctly over a Rayleigh number range of $7.7\times10^5\le Ra_w\le 1.43\times 10^9$ and a Prandtl number range of $3.96\le Pr\le5.3$. The thickness of the plume region predicted by the present criterion isapproximately same as that obtained by a threshold of RMS temperaturefluctuations, $\sqrt{T'^2}=0.2 \Delta T$, where $\Delta T$ is the temperaturedrop across the fluid layer.

A criterion to detect line plumes from velocity fields in turbulent convection