A numerical study of shear layer characteristics of low-speed transverse jets

Iyer, Prahladh S.; Mahesh, Krishnan

doi:10.1017/jfm.2016.7

Cited by 52 publications

(90 citation statements)

References 35 publications

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“…Megerian et al (2007) investigated flush and elevated jet nozzles and reported that for low enough R self-sustained strong pure-tone oscillations are generated by rolling-up of the upper jet shear layer very close to the leading edge of the jet orifice. These observations were later confirmed by other experiments (Davitian et al 2010;Getsinger et al 2014;Gevorkyan et al 2018), as well as numerically (Iyer & Mahesh 2016) and theoretically (Regan & Mahesh 2017). On the other hand, JICF with low velocity of injection is less well described.…”

Section: Introductionsupporting

confidence: 73%

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Experiments on a jet in a crossflow in the low-velocity-ratio regime

Klotz

Gumowski²,

Wesfreid

2019

J. Fluid Mech.

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The hairpin instability of a jet in a crossflow (JICF) for low jet-to-crossflow velocity ratio is investigated experimentally for a velocity ratio range of R ∈ (0.14, 0.75) and crossflow Reynolds numbers Re D ∈ (260, 640). From spectral analysis, we characterize the Strouhal number and amplitude of the hairpin instability as a function of R and Re D . We demonstrate that the dynamics of the hairpins is well described by the Landau model, and hence that the instability occurs through Hopf bifurcation, similarly to other hydrodynamical oscillators such as wake behind different bluff bodies. Using the Landau model, we determine the precise threshold values of hairpin shedding. We also study the spatial dependence of this hydrodynamical instability, which shows a global behaviour.

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Section: Introductionsupporting

confidence: 73%

“…12 (open symbols) to compare with our data. We also included in the comparison the values reported by Iyer & Mahesh (2016) and Regan & Mahesh (2017). Since the authors used the Strouhal number St based on jet velocity, we multiply inferred values of St by the corresponding velocity ratio R. The resulting values are plotted in Fig.…”

Section: Spectral Analysismentioning

confidence: 99%

Experiments on a jet in a crossflow in the low-velocity-ratio regime

Klotz

Gumowski²,

Wesfreid

2019

J. Fluid Mech.

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“…For lower velocity ratios, Megerian et al (2007) noticed an instability transitioning from a convective to an absolute nature when the velocity ratio based on peak velocity is decreased below approximately 3.5. This behaviour was confirmed by direct simulations (Iyer & Mahesh 2016) and linear stability analysis (Regan & Mahesh 2017).…”

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confidence: 55%

Global linear analysis of a jet in cross-flow at low velocity ratios

et al. 2020

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The stability of the jet in cross-flow is investigated using a complete set-up including the flow inside the pipe. First, direct simulations were performed to find the critical velocity ratio as a function of the Reynolds number, keeping the boundary-layer displacement thickness fixed. At all Reynolds numbers investigated, there exists a steady regime at low velocity ratios. As the velocity ratio is increased, a bifurcation to a limit cycle composed of hairpin vortices is observed. The critical bulk velocity ratio is found at approximately R = 0.37 for the Reynolds number Re D = 495, above which a global mode of the system becomes unstable. An impulse response analysis was performed and characteristics of the generated wave packets were analysed, which confirmed results of our global mode analysis. In order to study the sensitivity of this flow, we performed transient growth computations and also computed the optimal periodic forcing and its response. Even well below this stability limit, at R = 0.3, large transient growth (10 9 in energy amplification) is possible and the resolvent norm of the linearized Navier-Stokes operator peaks above 2 × 10 6 . This is accompanied with an extreme sensitivity of the spectrum to numerical details, making the computation of a few tens of eigenvalues close to the limit of what can be achieved with double precision arithmetic. We demonstrate that including the meshing of the jet pipe in the simulations does not change qualitatively the dynamics of the flow when compared to the simple Dirichlet boundary condition representing the jet velocity profile. This is in agreement with the recent experimental results of Klotz et al. (J. Fluid Mech., vol. 863, 2019, pp. 386-406) and in contrast to previous studies of Cambonie & Aider (Phys. Fluids, vol. 26, 2014, 084101). Our simulations also show that a small amount of noise at subcritical velocity ratios may trigger the shedding of hairpin vortices.

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“…Iyer & Mahesh (2016) performed DNS reproducing the stability transition observed in the experiments of Megerian et al. (2007), which they explained by proposing that the upstream shear layer is a counter-current shear layer, which is identified across the reverse flow upstream, and the jet.…”

Section: Introductionmentioning

confidence: 92%

“…Huerre & Monkewitz (1985) showed that when a mixing layer is absolutely unstable, whereas if a mixing layer is convectively unstable. Iyer & Mahesh (2016) measured the mixing layer velocities of the JICF as the maximum and minimum (most negative) vertical velocities across the upstream shear layer of the turbulent mean flows. The values of and for and , respectively, which suggested that the stability characteristics for the JICF may be driven by the same mechanism that drives the stability of free shear layers.…”

Section: Introductionmentioning

confidence: 99%

Adjoint sensitivity and optimal perturbations of the low-speed jet in cross-flow

Regan

Mahesh²

2019

J. Fluid Mech.

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The tri-global stability and sensitivity of the low-speed jet in cross-flow are studied using the adjoint equations and finite-time horizon optimal disturbance analysis at Reynolds number $Re=2000$, based on the average velocity at the jet exit, the jet nozzle exit diameter and the kinematic viscosity of the jet, for two jet-to-cross-flow velocity ratios $R=2$ and $4$. A novel capability is developed on unstructured grids and parallel platforms for this purpose. Asymmetric modes are more important to the overall dynamics at $R=4$, suggesting increased sensitivity to experimental asymmetries at higher $R$. Low-frequency modes show a connection to wake vortices. Adjoint modes show that the upstream shear layer is most sensitive to perturbations along the upstream side of the jet nozzle. Lower frequency downstream modes are sensitive in the cross-flow boundary layer. For $R=2$, optimal analysis reveals that for short time horizons, asymmetric perturbations dominate and grow along the counter-rotating vortex pair observed in the cross-section. However, as the time horizon increases, large transient growth is observed along the upstream shear layer. When $R=4$, the optimal perturbations for short time scales grow along the downstream shear layer. For long time horizons, they become hybrid modes that grow along both the upstream and downstream shear layers.

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A numerical study of shear layer characteristics of low-speed transverse jets

Cited by 52 publications

References 35 publications

Experiments on a jet in a crossflow in the low-velocity-ratio regime

Experiments on a jet in a crossflow in the low-velocity-ratio regime

Global linear analysis of a jet in cross-flow at low velocity ratios

Adjoint sensitivity and optimal perturbations of the low-speed jet in cross-flow

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