| A systematic study of the thermal dissipation field and its statistical properties is carried out in turbulent Rayleigh-Benard convection. A local temperature gradient probe consisting of four identical thermistors is made to measure the normalized thermal dissipation rate &egr N(r) in two convection cells filled with water. The measurements are conducted over varying Rayleigh numbers Ra (8.9x10 8 &le Ra &le 9.3x109) and spatial positions r across the entire cell. It is found that &egrN( r) contains two contributions one is generated by thermal plumes, present mainly in the plume-dominated bulk region, and decreases with increasing Ra. The other contribution comes from the mean temperature gradient, being concentrated in the thermal boundary layers, and increases with Ra.From the measured instantaneous thermal dissipation rate &egr N(r, t), we construct a locally averaged thermal dissipation rate &egrtau( r, t) over a time interval tau and study the tau-dependence of the moments, &lang 3pt (r, t)&rangt, averaged over time t. A good scaling regime, in which &lang( 3it )p&rang goes as &lang( 3it )p&rang&sim tmip for all values of p up to 8, is found for all three dissipation contributions &egrtau(r, t) (i = x, y, z). The obtained exponents mui(p) at three representative locations in the convection cell are explained by a phenomenological model, which combines the effects of both buoyancy for velocity statistics and geometric shape of the most dissipative structures in turbulent convection. The experiment thus provides a complete physical picture about the thermal dissipation field and its statistical properties in turbulent convection. |
