Reynolds number dependence of the large structure in a slightly heated turbulent boundary layer

Simultaneous temperature traces in a slightly heated boundary layer with zero pressure gradient were used in a study involving visual identification and conditional measurements of the coherent large structure. Ensemble averages of the products uv, (u)(theta), and (v)(theta) (u and v are the longitudinal and normal velocity fluctuations, and theta is the temperature fluctuation) were obtained using a combined X-wire and single cold wire arrangement along with a rake of cold wires. Using the visual method, the mean period normalized by outer layer parameters and the inclination of the back of the large structure are approximately independent of Rm, the momentum thickness Reynolds number, when Rm is greater than approximately 1000. The width of the region near the back of the large structure which is associated with a sudden change in temperature is equal to approximately 0.042 delta, and is approximately independent of Rm. The contribution of the large structure to the Reynolds shear stress decreases as Rm decreases; this contribution amounts to about 20% at Rm = 1000.