Abstract
A three-dimensional unsteady flow around the Ahmed body at a low Reynolds number ( Re) was numerically investigated for the cases of slant angles 𝜃 = 29° and 31°. The critical Reynolds numbers are determined as Re𝑐 = 1049 for 𝜃 = 29° and Re𝑐 = 1090 for 𝜃 = 31°. The oscillatory flows appearing near the critical Reynolds number have a single frequency of Strouhal number St ≈ 0.4, and St increases up to St ≈ 0.6 with the increase for Re. At certain large Re, the oscillation of the flow changes to the multi-frequency behavior. The time-averaged flows are mainly composed of a pair of longitudinal vortices shed from the sides of the slant surface and another pair of longitudinal vortices shed from the spanwise center of the slant surface. For the relatively small Re cases, the flows are separated from the slant surface, whereas for the relatively large Re, the separated flow re-attaches downstream and center of the slant surface. The value of Re where the re-attachment takes place is smaller for the case of 𝜃 = 29°. From the energy analysis based on the Reynolds-Orr equation, the dominant terms and their spatial distribution are investigated. The regions where the energy is strongly supplied from the time-averaged field to the deviated oscillation field appear along the two pairs of longitudinal vortices of the time-averaged field. It was found that the energy supply along vortices at spanwise centers becomes relatively large with the increase of Re.
