LDA measurements of coherent flow structures and cross-flow across the gap of a compound channel with two half-rods

For high Re, the stream wise velocity root mean square vrms between the two half rods is higher near the walls, similar to common channel flow. As Re decreases, however, an additional central peak in vrms appears at the gap centre, away from the walls. The peak becomes clearer at lower P/D ratios and it also occurs at higher flow rates. Periodical behaviour of the span wise velocity across the gap is revealed by the frequency spectrum and the frequency varies with P/D and decreases with Re. The study of the stream wise velocity component reveals that the structures become longer with decreasing Re. As Re increases, these structures are carried along the flow closer to the gap centre, whereas at low flow rates they are spread over a wider region. This becomes even clearer with smaller gaps.