Abstract
The purpose of this study is to explore the effect of the branch-to-trunk area ratio upon the tendency of flow to become turbulent in symmetrically branched tubes. Velocity was measured with a laser Doppler anemometer in glass tubes which branched at 70° from a 25 mm diameter trunk. The branch-to-trunk area ratio of branches studied was .4, .6, .8, 1.0, 1.2, and 1.4. For branch-to-trunk area ratios of .4 to .8, the critical Reynolds number in the trunk at which the transition to turbulence in the branch occurred was relatively constant at approximately 2100. As the branch-to-trunk area ratio increased beyond .8, the critical Reynolds number decreased until, at a ratio of 1.4, the critical Reynolds number was 1200. Since the Reynolds number of transition in the trunk was found to be 2100, it is apparent that a branch-to-trunk area ratio of 1.4 caused a prominent reduction of the critical Reynolds number in the system. A branch-to-trunk area ratio less than .8 had no effect. The velocity profiles at a branch-to trunk area ratio of .4 showed acceleration of velocity in the branch; while at an area ratio of 1.4, the velocity was shown to decelerate. This explains the tendency for flow to be unstable in branches with area ratios greater than one. These observations suggest that the branch-to-trunk area ratio usually observed in patients at the aortic bifurcation is such that laminar flow is promoted.
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