Azimuthal organisation of turbulent structures in underexpanded impinging round jets

• The azimuthal organisation of the turbulent structures in underexpanded round jets impinging on a flat plate have been investigated using compressible large eddy simulation. The jet shear-layer region, as well as the region of the wall jets created on the plate after the impact are considered. The jets are characterized by a Nozzle Pressure Ratio of 4.03, a fully expanded Mach number of 1.56, and a Reynolds number of 6 × 104. The distance between the nozzle and the plate varies from 4.16r0 to 9.32r0. The jets generate acoustic tones due to a feedback mechanism. In this paper, the near pressure and density fields of the jets are analysed using Fast Fourier Transform on the nozzle exit plane, the plate, and an azimuthal plane. The amplitude and the phase fields on these sections at the tone frequencies are represented. Similar organisations of the turbulent structures are found in the jet shear layers and the wall jets. Thus, axisymmetric and helical arrangements of the structures in the shear layers lead to spiral and concentric distributions of the structures on the plate, respectively. In particular, for one of the jets, a spiral shape and concentric rings, associated with two tone frequencies generated simultaneously, are observed on the flat plate in the pressure and density phase fields. Moreover, the convection velocity of the turbulent structures on the plate is evaluated from the phase fields. Several diameters away from the jet axis, the velocities found for the present jets compare well with those found experimentally in the phase-averaged distributions of fluctuating pressure for impinging ideally expanded jets using fast-response Pressure-Sensitive Paint. Near the jet axis, given that the present jets are underexpanded, differences are observed, due to the presence of shock cell structures in the jets. Finally, the convection velocity of the turbulent structures on the wall are estimated from cross-correlations of radial velocity. The values obtained compare well with those determined from the phase fields.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Teknisk mekanik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Applied Mechanics (hsv//eng)

Nyckelord

Aeroacoustics

Fast Fourier transforms

Jets

Large eddy simulation

Nozzles

Paint

Reynolds number

Shear flow

Velocity

Compressible large eddy simulations

Convection velocity

Feedback mechanisms

Fluctuating pressures

Nozzle pressure ratio

Pressure sensitive paints

Shock cell structure

Turbulent structures

Plates (structural components)

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