

Article
Effect of surface tension on global modes of confined wake flows
Authors: 
Tammisola, O., Lundell, F., Söderberg, D. 
Document Type: 
Article 
Pubstate: 
Published 
Journal: 
Physics of Fluids 
Volume: 
23
014108 
Year: 
2011 
AbstractMany wake flows are susceptible to selfsustained oscillations, such as the wellknown von Kármán vortex street behind a cylinder that makes a rope beat against a flagpole at a distinct frequency on a windy day. One appropriate method to study these global instabilities numerically is to look at the growth rates of the linear temporal global modes. If all growth rates for all modes are negative for a certain flow field then a selfsustained oscillation should not occur. On the other hand, if one growth rate for one mode is slightly positive, the oscillation will approximately obtain the frequency and shape of this global mode. In our study, we first introduce surface tension between two fluids to the wakeflow problem. Then we investigate its effects on the global linear instability of a spatially developing wake with two coflowing immiscible fluids. The inlet profile consists of two uniform layers, which makes the problem easily parametrizable. The fluids are assumed to have the same density and viscosity, with the result that the interface position becomes dynamically important solely through the action of surface tension. Two wakes with different parameter values and surface tension are studied in detail. The results show that surface tension has a strong influence on the oscillation frequency, growth rate, and shape of the global modes. Finally, we make an attempt to confirm and explain the surfacetension effect based on a local stability analysis of the same flow field in the streamwise position of maximum reverse flow.

