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ECN publication
Inviscid stall model
Published by: Publication date:
ECN Wind Energy 1-9-2001
ECN report number: Document type:
ECN-RX--01-053 Conference Paper
Number of pages: Full text:
5 Download PDF  

Presented at: European Wind Energy Conference and Exhibition, Copenhagen, Denmark, 2-6 juli 2001.

Snel's boundary layer model [1,3] on rotational effects gave the first estimate of three-dimensional effects in stall, which have been valuable understanding rotor behaviour. The reason for an alternative model was to include the often observed and intuitively expected radial flow. We model the separated flow area and show that the separated air flows in a radial stream with vr as the dominant velocity. Furthermore, the new model is not based on boundarylayer theory: we use the full set of equations and do not use the property of boundary layers in which partial velocity gradients can be estimated with the ratio of differences, which is invalid in separated flow. We describe the separated flow on rotating blades without any effect of viscosity, which seems to be a paradox. However by studying the physics of flow separation this becomes clear. Separation occurs because the air is coming to a standstill in the main flow direction due to friction and the positive pressure gradient. Then, in 2d-flow, beyond the point of separation, a dead-water region is formed. Here the frictional forces are negligible and the accelerations and the pressure gradients are small. In 3d-flow, we also have the situation that the flow comes to a standstill in the chord-wise direction and separates due to the back-flow at a slightly larger chord-wise position. As in 2d-flow, near and beyond the stagnation line, the gradient of the chord-wise velocity normal to the wall is very small or even zero, so viscous effects and chord-wise accelerations are negligible which means that all chord-wise forces balance. The rotation has introduced a chord-wise Coriolis force so that the pressure gradient must balance the Coriolis force. The difference with the 2d-situation is that we have a chord-wise pressure gradient instead of the flat 2d-distribution and that we have a radial pressure gradient and a radial external force (the centrifugal force) which both accelerate the separated flow in the radial direction. For a complete overview on rotational effects we refer to [7].

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