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• 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]

As a potential method of reducing wall heat flux and friction drag in hypersonic vehicles, wall-seeping gas film (WSGF) can affect the hypersonic boundary layer stability. An experimental study investigating the WSGF effect on second-mode waves in hypersonic boundary layer downstream of the seepage zone was conducted in a Mach 6 hypersonic quiet wind tunnel under a unit Reynolds number of 5.86 × 10 6 m − 1 . The test model was a slender cone with a 7-degree half-angle. WSGF was realized by seeping out air through permeable stainless steel. Five seeping ratios, 0, 0.041%, 0.055%, 0.066% and 0.138% were studied. The flow structure visualized by schlieren technique showed that WSGF with seeping ratio of 0.055% ∼ 0.138% induced weak shock wave. WSGF thickened density boundary layer. The cross-correlation of wall fluctuating pressure measured by PCB sensor arrays between two adjacent measuring stations showed that WSGF had no significant impact on the propagation velocity of second-mode waves. The power spectrum density of wall fluctuating pressure indicated that the peak frequency of second-mode waves decreased at each measuring station as the seeping ratio increased. The peak frequency decreased along the streamwise direction without WSGF. However, the decreasing trend slowed down as seeping ratio increased to 0.041%. At seeping ratio of 0.055%, the peak frequency first remained almost constant (around 121.2 kHz) at first three measuring stations and then gradually decreased. Surprisingly, at higher seeping ratios of 0.066% and 0.138%, the peak frequency even exhibited an initial gradual increase followed by a slight decrease downstream. Additionally, WSGF made the second-mode waves amplitude increase fast downstream of the seepage zone and decrease sharply far downstream. The maximum amplification factor is approximately 0.85 and the minimum is -1.87. [ABSTRACT FROM AUTHOR]