Laminar boundary layer separation at a fin-body junction in a hypersonic flow

Abstract

Planar laser-induced fluorescence (PLIF) imaging was performed to visualize the fin bow shock, separation shock, viscous shear layer and recirculation region of the flowfield at the junction of a blunt fin and a flat plate. Making use of the temperature dependence of the PLIF technique, images were made sensitive to temperature to provide qualitative information on the flowfield. The PLIF technique was also used as the basis for a flow-tagging technique, making it possible to measure a velocity component and to demonstrate the reverse flow of the separated region. Flow visualisation of the plane of symmetry allowed determination of the point of boundary layer separation, the angle of the separation shock and the bow shock standoff distance. These parameters were compared with predictions made by computational fluid dynamic simulations of the flowfield. Good agreement between theory and experiment was achieved. Comparisons between theoretical and experimental velocity measurements showed good agreement.

Experimental Investigation on the Effects of Swept Angles on Blunt Fin-Induced Flow

“…Fox et al [5] used fluorescence imaging to investigate the flow upstream of a blunt fin in a hypersonic freestream with a transitional boundary layer, and the separation shock can be observed from the results, but the structures in the boundary layer can be barely distinguished. Houwing et al [6] used planar laser-induced fluorescence to visualize the fin bow shock, separation shock, viscous shear layer, and recirculation region of the flowfield at the junction of a blunt fin and a flat plate.…”

N2O molecular tagging velocimetry

“…Nitric oxide is a very stable MTV tag that has been used in both low-and high-temperature gas flows [23][24][25][26][27][28][29][30][31]. In low-pressure high-speed flows, NO can be naturally present (e.g., arc-heated tunnel) or directly added to the gas.…”

“…In low-pressure high-speed flows, NO can be naturally present (e.g., arc-heated tunnel) or directly added to the gas. When the NO is electronically excited along a line in this low quenching rate environment, the NO fluorescence lifetime is sufficient for the laser line displacement to be tracked in time [23][24][25]. In air flow without NO addition, a highly focused ArF laser beam can create photochemically produced NO along line [26].…”

Hydroxyl-Tagging-Velocimetry Measurements of a Supersonic Flow over a Cavity

“…However, Rayleigh scattering can be orders of magnitude weaker than either LIF or particle scattering. In confined aerodynamics facilities, the Rayleigh scattering signals are difficult to differentiate from the much stronger laser scattering from tunnel walls and particles in the flow [19]. The velocity component measured by Doppler-shift methods (i.e., LIF or Rayleigh scattering) is defined by the geometry/orientation of the laser beam/ sheet and the observer; in aerodynamic facilities with limited optical access, this can prevent measurement of the desired velocity feature [19].…”

“…In some molecular tagging methods, gas molecules (or atoms) are seeded into the flow and subsequently tagged with a laser beam; the seeded molecule can be electronically excited, vibrationally excited, ionized, or photodissociated to form a new molecule. Researchers have used a variety of molecular seeds in this technique, including acetone [20], biacetyl [21,22], nitric oxide [19,23,24], nitrogen dioxide [24,25], sodium [26], strontium [27], and tert-butyl nitrate [28]. Seeding a flow with molecules or atoms is often undesirable due to a variety of reasons (expense, seeding toxicity, corrosive behavior, etc.…”