Structure of a Turbulent Thermal Plume Rising along an Isothermal Wall

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Abstract
The weakly buoyant region of a turbulent thermal plume formed by a line heat source along the base of a vertical isothermal wall was investigated. The turbulence quantities: u′, v′, w′, t′, u′v′, u′t′, and v′t′ were measured, supplementing existing measurements of mean quantities in this flow. The plumes exhibited local similarity in terms of local plume thermal energy flux and height above the source. In the outer portions of the flow, the shear stress agreed with conventional mixing length models and the turbulent Prandtl number approached 0.5. Near the wall, however, turbulent stress and transport quantities do not approach zero when gradients in mean quantities are zero and eddy viscosity models do not properly represent the turbulence characteristics. Predictions of mean quantities were examined using a local similarity hypothesis and various eddy viscosity models. The theoretical results are in fair agreement with the measurements, in spite of the inaccurate representation of turbulence quantities near the wall. The theory indicates that local similarity for mean quantities is only approximately observed, and suggests the presence of systematic Grashof number effects that were not apparent in the measurements due to data scatter.

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