Geometry effects on flow transition in multilouvered fins – onset, propagation, and characteristic frequencies

The effect of fin pitch, louver angle and thickness, and flow depth on the onset, propagation, and characteristic frequencies of instabilities in multilouvered fins is investigated. The initial instability appears in the wake of the exit louver. Subsequently, instabilities are established in the interior of the louver bank which spread upstream as the Reynolds number increases. It is shown that the interior instabilities are completely independent of the exit wake instability. On the other hand, the exit wake instability is not only dependent on the exit louver geometry but also on the internal geometry of the louver bank. Increasing louver thickness and angle increases the propensity of the flow to become unstable. Increasing fin pitch, louver angle, and thickness enhances the propagation of instabilities into the louver bank. It is shown that the characteristic frequencies in the interior of the louver bank, scale with the fin pitch rather than the louver thickness or louver pitch. The exit wake frequencies, on the other hand scale with the projected length of the exit louver in the flow direction. A simplified model for estimating the onset of instabilities in the interior of the louver bank is proposed.