VORTEX FLOWS
   THERMAL FLOWS
   NUMERICAL METHODS
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   AERODYNAMICS
     WAVES
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 FORCED WAKES  
 Benard von Karman Instability  
 
Our goal is to study efficient control strategies of naturally developed flow instabilities like the Bénard von Kármán (BvK) instability from bluff bodies (cylinders and plates). The BvK instability is generated in the wake of a circular cylinder of diameter d, at a critical Reynolds number of order 50. Reversed flow first occurs leading to the formation of two attached eddies in the near wake of the cylinder, thus breaking the upstream-downstream symmetry.
 
 

After that, for increasing Reynolds number, the flow is no longer symmetric about the centerline and stationary; it settles into a time periodic regime in which vortices are shed alternatively from the two sides of the cylinder, giving the von Kármán vortex street. The wake flow behind a solid cylinder, as well as the transverse hydrodynamic forces fluctuations originated by periodic vortex shedding can be supressed or greatly reduced. In a recent paper we proposed a method of control were we achieve a global modification of the flow field around a single cylinder by monitoring the pressure distribution. Normally, the average pressure in the near wake is smaller than the one near the front stagnation point. Thus, the idea is to compensate this pressure difference by decreasing the pressure at the front stagnation point.
Suction from Stagnation Point. The instability of the wake of a circular cylinder of diameter d, at supercritical Reynolds numbers (Re < 150) can be suppressed if suction is locally applied at the front stagnation point to decrease local pressure. Normally, the average pressure in the near wake is smaller than the one near the front stagnation point. Thus, the idea is to compensate this pressure difference by decreasing the pressure at the front stagnation point. Under suction, the front stagnation point bifurcates into a pair of stagnation points located symmetrically with respect to the flow centerline.
This effect produces a delay for the onset of the BvK instability. The method allows an accurate dynamic control of the BvK instability over a period of time greater than the inverse of the time growth rate of the instability at a given Reynolds number.
The method is applied to a row of 3 identical circular cylinders. If we modulate the instability and therefore the vortex emission over a specific temporal window and we get a spatial distribution of vortices like in the above figure (contours of absolute velocity). These vortices are advected downstream by the mean flow, thus resulting in a localized vortex distribution, in time and space, were local vorticity spread out by the effect of viscosity.

 Flat plate wake and effects of mechanical forcing
 
Bluff body wakes flows are spatially developing open flows and the wake of a thin flat plate fall into the class of noise amplifiers, the system is very sensitive to external noise, which can, in some situations, be amplified. A simple thin plate system consist of a fixed flat plate of length L with a small flap of length b at the trailing edge (supported by small ball-bearings), which can perform small oscillations when forced by an external shaker.
 
flap-airfoil
Two wake flow resonances can occur: (i) a so-called inertial resonance when the inverse of the forcing frequency (fo) matches the flight time of fluid particles along the flap and (ii) a viscous resonance when the forcing frequency of the flap equals one half of the vortex shedding frequency of the plate-flap system. Flow imaging through light scattering from smoke wire particles using a rapid ccd camera, give us the picture at left. We see the inner and outer structure of the forced wake. Overall wake response to different forcing regimes is obtained by systematic measurements of wake flow velocity profiles using thermal anemometry.
  Scanning the cross stream coordinate (y) with a hot wire probe, provides a detailed measurement of the wake dynamics during a time-limited forcing strategy. At low thickness (e) based Reynolds numbers, frequency response results display strong evidence of the spatio-temporal resonance of the near wake (using very small forcing amplitudes). The figure at the right, shows the energy fluctuations resonant map where the black circle indicates the inertial resonance (wide wake) and the black square indicates the viscous resonance (thin wake). The inertial resonance occurs when the dimensionless number F=bfo/U=1 (where U is flow velocity) The system also allows to propagate arbitrary forcing signals, like discrete number of bursts and wave packets through the flat plate wake that are recorded through synchronous hot wire measurements.
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  Universidad de Chile, Facultad de Ciencias Físicas y Matemáticas