A two-dimensional rectangular cavity of aspect ratio one is studied experimentally using a laser-doppler velocimeter. The enclosure is air filled and consists of two vertical walls at unequal isothermal temperatures and two connecting horizontal walls with temperatures varying linearly between the two vertical surfaces. This study clearly defines the existence of periodic laminar flow regimes detectable at Ra numbers above 0.3 × 107. These periodic variations in velocity are induced by the upper and lower boundary conditions. The envelopes of vertical and horizontal velocity fluctuations are reported as a function of position for Rayleigh numbers of 0.25 × 107, 0.50 × 107, and 0.85 × 107. In addition, the effect of Ra number on frequency of flow is reported.

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