Search

Highlights • The three-dimensional dune wake are investigated by PIV. • The large-scale structure is responsible for the formation of separation region. • The intermediate-scale structure is more active near the separation region. • The intermediate-scale structure makes more contribution to Reynolds stresses. • The small-scale vortical structures decay quickly at the downstream. Abstract The three-dimensional wake structures behind a barchan dune model are investigated by the high-speed particle image velocimetry (PIV) at the Reynolds number of 5530. The measured velocity fields are first evaluated by time-averaged flow patterns and power spectra analysis. Then the one-dimensional orthogonal wavelet analysis is applied to reveal multi-scale turbulent structures and their corresponding second order statistics in the dune wake. The instantaneous multi-scale structures suggest that the large-scale structure is responsible for the formation of separation region. The intermediate-scale structure tends to be more active at the boundary of separation region, which may be due to the secondary vortex movement caused by the interactions between the wake and main flow. The appearance of small-scale vortices at the downstream of separation region indicates the breakdown process of large-scale vortices. The Reynolds stress and kinetic energy distribution suggest that the large-scale structures dominate the generation of turbulence in the dune wake. The lower kinetic energy contained in intermediate- and small- scale structures is considered to be related to smaller-scale interactions that reflect energy cascading phenomenon. The Reynolds shear stresses distributions suggest that the intermediate-scale structures also make significance in the separated boundary layer. As for small-scale structure, it is worthy to note that a region of high Reynolds shear stresses with an opposite sign is observed around the dune crest. The cross-correlation function of multi-scale structures suggest the quasi-periodicity of large-scale structure. The intermediate-scale structures also show a certain periodicity, however, such structures tend to be more unstable and the strength of them reduces as they travel downstream. When decreasing to small-scale, the vortical structures decays quickly at the downstream with larger convective velocity. This may indicate the breakdown process of large-scale vortices and the quick dissipation of small-scale vortices. [ABSTRACT FROM AUTHOR]

• The flow field structure and heat flux distribution around an isolated cylinder and tandem cylinders at different angles of attack (AoA) and inflow conditions are investigated in a Mach 6 low-noise wind tunnel. • Downstream of the recirculation region, the heat flux distribution of the isolated cylinder model forms a parallel stripe structure, while in the tandem cylinder model, it resembles a "Λ-shaped" distribution. • With the decrease in AoA , the "Λ-shaped" heat flux structure in the wake of the downstream tandem cylinders gradually disappears, and the stripe structure near the second cylinder is in a "shuttle-shaped" distribution. • With the increase in the spacing between the two tandem cylinders, the spanwise width of the "shuttle-shaped" distribution increases. • The low-heat flux area behind the second cylinder initially increases with an increase in the spacing, but when the spacing is too large, a high-heat flux stripe begins to appear in the centre of the low-heat flux region. • Placing a third cylinder on the wake centreline of the tandem cylinder has a slight impact on the flow, but its effect is enhanced when the third cylinder is located outside the recirculation region of the tandem cylinder. The flow field structure and heat flux distribution around an isolated cylinder and tandem cylinders on a flat plate at different angles of attack (AoA) and inflow conditions are investigated in a Mach 6 low-noise wind tunnel, using nano-tracer-based planar laser scattering (NPLS) techniques and the temperature sensitive paint (TSP) technique. The results reveal that, first, at AoA = 10°, the downstream cylinder will cause a horseshoe vortex formed by the upstream cylinder, which originally propagates parallel to the downstream, to expand to both sides. Subsequently, when the number of cylinders increases, the spanwise width of the affected area is increased further. In addition, a low-heat flux region caused by the recirculation region appears behind the cylinder; downstream of the low-heat flux region, the heat flux distribution of the isolated cylinder model forms a parallel stripe structure, while in the tandem cylinder model, it resembles a "Λ-shaped" distribution. A second key finding is that, with the decrease in AoA , the region of influence of the cylinder decreases. Moreover, the "Λ-shaped" heat flux structure in the wake of the downstream tandem cylinders gradually disappears, and the stripe structure near the second cylinder is in a "shuttle-shaped" distribution. Notably, with the increase in the spacing between the two tandem cylinders, the spanwise width of the "shuttle-shaped" distribution increases. The low-heat flux area behind the second cylinder initially increases with an increase in the spacing, but when the spacing is too large, a high-heat flux stripe begins to appear in the centre of the low-heat flux region. In addition, placing a third cylinder on the wake centreline of the tandem cylinder has a slight impact on the flow, but its effect is enhanced when the third cylinder is located outside the recirculation region of the tandem cylinder. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the results of an experimental study of the near-wake of a NACA 0012 airfoil placed upstream of a 90° bend section using hot-wire anemometry. The bend has a cross-section of 0.457m×0.457m with a mean radius-to-height ratio of 1.17, and the airfoil is of chord length 0.15m. Among the features studied are the effects of changes in the orientation of the airfoil with respect to the oncoming flow and the effects of the freestream velocity, namely, 10m/s, 15m/s and 20m/s corresponding to chord Reynolds numbers of 1, 1.5 and 2×105, respectively. Emphasis is placed on the boundary layer on the upper surface of the airfoil and the wake up to one chord length downstream. The parameters of interest are the mean and turbulence quantities and their variations in the streamwise, normal and spanwise directions. Results from the investigations indicated the sensitivity of the wake characteristics to the boundary layers on the airfoil. Measurements of mean velocity and streamwise turbulence intensity taken in the spanwise direction within the wake revealed wave-like variations which are linked to the presence of vortical structures in the wake. [ABSTRACT FROM AUTHOR]