FLOW CONTROL FOR INTERNAL FLOWS
(ECCOMAS CFD 2006)
Dionysios Skamnakis*, Michail Kefalakis*and Kyriacos Papailiou* *National Technical University of Athens
9, Iroon Polytechniou, 11525 Athens, Greece
Email: kpapail@ltt.ntua.gr Web page: www.ltt.mech.ntua.gr
ABSTRACT
Recognition of the usefulness of unsteady flow control has been gaining ground during the last twenty five years. Following the experimental proof of the advantages related to the use of pulsating jets for the reduction of the extend or even the
elimination of existing flow separation regions, attempts were made during the 90s to simulate the associated flow phenomena. Such attempts, which may be found in the literature (see for instance ref.[1]), are capable to reproduce with an adequate engineering approximation, existing experimental results. For doing this, well established two equation models are used for simulating the turbulence behaviour. This situation leaves open the field for improvements, using more sophisticated models, but, on the other hand, allows to evaluate the efficiency of flow control applied to a specific configuration. Such computations provide an evaluation of the influence of the pulsed jet amplitude, when the position and the frequency of the pulsed jet are specified. During the presentation, the methodology, which allows to optimally specify the position and the frequency of the pulsed jet will be outlined and discussed (ref.[2]). For this, the relation between vorticity and velocity fluctuations behaviour will be established in a context, which describes the amplification or damping of these quantities, as they travel downstream from the point of their emergence. The corresponding stability analysis on the basis of which the above discussion will be made, will be established (explained) first for laminar and, then for turbulent flows. Examples will be given for a duct and a compressor cascade,
covering internal and turbomachinery applications. Although, the analysis which will be presented is two dimensional, three dimensional flows can also be treated the approach being based on the same concepts.
The above discussion concerns the problem of vorticity amplification through the application of unsteady active flow control. The next problem considered is the one related to the destruction of vorticity, which allows the extend of the stall free
operating range of an axial compressor (ref.[3]). 3D time accurate calculations will be used in order to describe the break up of the tip clearance vortex, when the
REFERENCES
[1] Langley Research Center Workshop (2004) on CFD Validation of Synthetic Jets and Turbulent Separation Control. Organizers T. Gatski and Ch. Rumsey, March 29-31, 2004, Williamsburg, Virginia, USA.
[2] D. Skamnakis and K. Papailiou (2005), “Flow Stability Analysis and Exitation Using Pulsating Jets”, C.R. Mecanique 333 , pp 628-635 (2005).
[3] D. Skamnakis, Thesis on Active Flow Control (in preparation).
