Experimental study on the hypersonic boundary layer transition induced by tandem cylinders

The flow behind nine different arrays of cylinders is experimentally investigated via Particle Image Velocimetry (PIV) at a Reynolds number of Re ∼100 based on the diameter of the cylinders. Each array consists of a column of four cylinders in front and three in the rear. The horizontal distance between the two columns and the vertical distance between the cylinders within each column are varied for H/D=[2,4,8] and V/D=[2,4,6], resulting in nine different arrays denoted as mVnH, where m corresponds to V/D and n stands for H/D. The PIV measurements are conducted for 15 s at 200 Hz frequency, corresponding to 39 to 360 vortex shedding events for the wakes in this study. Then, proper orthogonal decomposition is applied to the velocity fields to analyze the flow dynamics. All arrays show unsteady flow, and based on their flow structures, they are classified in to three main categories of single bluff body (SBB), transitional (TR), and co-shedding (CS) flow. SBB characteristics can be seen for 2V2H and 2V4H arrays, but the latter has more steady vortex shedding as the H/D increases from 2 to 4. Then, 2V8H and 4V2H have an asymmetric flow with several vortex streets and act as an intermediary stage in the shift from SBB to CS flow structure when the distances are increased. The highest total kinetic energy values and widest probability density functions of the velocity components are observed for this group. The five remaining arrays in the CS group have symmetric flow, with three or five vortex streets present behind. However, based on the distances, the frequency and phase synchronization of the vortex streets change considerably, which might have an important effect on, for example, the heat transfer or the structural load of the cylinders.

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Flow structure and dynamics behind cylinder arrays at Reynolds number ∼100

Ghazijahani

Cierpka

2023

Physics of Fluids

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Supersonic cooling film flow evolution on a curved wall under hypersonic flow

Zhang,

Yi,

Liu

et al. 2024

Physics of Fluids

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Curvature plays a crucial role in evolving supersonic cooling film flow-field structures. Flow-field structural images were captured using nanotracer-based planar laser scattering,, and wall pressure values were obtained using experimentally validated numerical simulations. A supersonic cooling film is tangentially injected at the Mach number of Maj = 2.3 into a laminar boundary layer at a mainstream of Ma = 6. The supersonic cooling film inhibits mixing-layer instability on the convex curved wall (CV) and promotes it on the concave curved wall (CC). After increasing the total incoming pressure, the reduction ratio of static pressure (RSP) between the supersonic cooling film and the mainstream flow causes a delay in the position of the mixing-layer instability, smaller-scale vortex structures, and decreased flow velocity of the typical vortex structures on the CC and CV. The wall pressure increases for the CV and decreases for the CC, indicating that the supersonic cooling film suppresses the changes in wall pressure due to curvature. The supersonic cooling film suppresses the decrease in the impulses for bulk dilatation (Ip) due to convex curvature and the increase in Ip due to concave curvature. The growth rate of Ip on the CC increases from −15% to −8% and decreases on the CV from 31% to 12% in the bending impulse (IΦ) range of |IΦ| = 1.337–3.624 for a total inlet pressure of 0.5 MPa. Increasing the RSP could control the Ip values on curved surfaces more effectively. The results of this study can be applied to cooling the infrared optics window on hypersonic vehicles.

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Experimental study on influences of steps on hypersonic boundary layer transition at different angles-of-attack

Xu,

Ye,

et al. 2024

Physics of Fluids

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The influences of the forward-facing step (FFS) and backward-facing step (BFS) on the 7° half-angle conical boundary layer instability and transition are investigated at different angles-of-attack (AoAs) in a hypersonic quiet wind tunnel using the Nano-tracer-based Planar Laser Scattering techniques, Temperature-Sensitive Paints, and high-frequency pressure sensors. The results show that the FFS stabilizes the second mode instability but strongly destabilizes the crossflow instability. Conversely, the BFS destabilizes both the second mode and crossflow instabilities, yet its impact on crossflow instability is weaker compared to that of an FFS with an identical height. At a small AoA (AoA < 3° in this paper), the boundary layer transition is dominated by the second mode instability. For sharp cones, the transition is delayed on the windward side but promoted on the leeward side, resulting in a monotonically inclined transition front. In contrast, blunt cones exhibit localized depressions in the transition front on the leeward side. At large AoA, the transition process is dominated by the crossflow instability, resulting in heat flux stripes on the leeward side, making the transition front distribution more complex. Therefore, at small AoAs, the promotion effect of the BFS on the conical boundary layer transition is stronger than that of the FFS with the same height. Moreover, the FFS with a small height even exhibits a suppressive effect on the transition. However, as the AoA increases, low-frequency instability modes, such as crossflow instability, gradually become dominant. Consequently, the promoting effect of the FFS surpasses that of the BFS.

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Experimental study on the hypersonic boundary layer transition induced by tandem cylinders

Cited by 3 publications

References 36 publications

Flow structure and dynamics behind cylinder arrays at Reynolds number ∼100

Flow structure and dynamics behind cylinder arrays at Reynolds number ∼100

Supersonic cooling film flow evolution on a curved wall under hypersonic flow

Experimental study on influences of steps on hypersonic boundary layer transition at different angles-of-attack

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