scholarly journals Jet preferred mode vs shear layer mode

2020 ◽  
Vol 32 (6) ◽  
pp. 064106
Author(s):  
M. Mair ◽  
M. Bacic ◽  
K. Chakravarthy ◽  
B. Williams
Keyword(s):  
Shear Layer ◽  
Layer Mode

scholarly journals On self-sustained oscillations in two-dimensional compressible flow over rectangular cavities

2002 ◽  
Vol 455 ◽  
pp. 315-346 ◽  
Author(s):  
CLARENCE W. ROWLEY ◽  
TIM COLONIUS ◽  
AMIT J. BASU
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Stokes Equations ◽  
Boundary Layer Transition ◽  
Computational Domain ◽  
Two Dimensional ◽  
Mean Flow ◽  
Recirculating Flow ◽  
Layer Mode ◽  
Mach Numbers

Numerical simulations are used to investigate the resonant instabilities in two-dimensional flow past an open cavity. The compressible Navier–Stokes equations are solved directly (no turbulence model) for cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field, which is shown to be in good visual agreement with schlieren photographs from experiments at several different Mach numbers. The results show a transition from a shear-layer mode, primarily for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear-layer mode is characterized well by the acoustic feedback process described by Rossiter (1964), and disturbances in the shear layer compare well with predictions based on linear stability analysis of the Kelvin–Helmholtz mode. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number independent of Mach number. The wake mode oscillation is similar in many ways to that reported by Gharib & Roshko (1987) for incompressible flow with a laminar upstream boundary layer. Transition to wake mode occurs as the length and/or depth of the cavity becomes large compared to the upstream boundary-layer thickness, or as the Mach and/or Reynolds numbers are raised. Under these conditions, it is shown that the Kelvin–Helmholtz instability grows to sufficient strength that a strong recirculating flow is induced in the cavity. The resulting mean flow is similar to wake profiles that are absolutely unstable, and absolute instability may provide an explanation of the hydrodynamic feedback mechanism that leads to wake mode. Predictive criteria for the onset of shear-layer oscillations (from steady flow) and for the transition to wake mode are developed based on linear theory for amplification rates in the shear layer, and a simple model for the acoustic efficiency of edge scattering.

scholarly journals Role of Shear Layer Instability in Driving Pressure Oscillations in a Backward Facing Step Combustor

2016 ◽  
Author(s):  
Srinivas K. Kirthy ◽  
Santosh Hemchandra ◽  
Seunghyuck Hong ◽  
Santosh Shanbhogue ◽  
Ahmed F. Ghoniem
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Acoustic Pressure ◽  
Combustion Instability ◽  
High Amplitude ◽  
Open Loop ◽  
Hydrodynamic Modes ◽  
Pressure Oscillations ◽  
Layer Mode ◽  
Low Amplitude

This paper presents a global hydrodynamic stability analysis of flow fields in a backward facing step combustor, assuming weakly non-parallel flow. The baseline experiments in a ‘long’ combustor of length of 5.0 m shows the presence of two combustion instability states characterized by coherent low and high amplitude acoustic pressure oscillations. The analysis is performed for Propane-air mixtures at three values of ϕ = 0.63, 0.72 and 0.85 which correspond to quiet, low amplitude and high amplitude instability states in the long combustor experiments. Base flow velocity and density fields for the hydrodynamic stability analysis are determined from time averaged PIV measurements made after the length of the duct downstream of the step has been shortened to eliminate acoustic pressure oscillations. The analysis shows that the shear layer mode is self-excited for the ϕ = 0.72 case with an oscillation frequency close to that of the long combustor’s fundamental acoustic mode. We show from an analysis of the weakly forced, variable density Navier-Stokes equations that self-excited hydrodynamic modes can be weakly receptive to forcing — suggesting that the low amplitude instability in the long combustor is due to semi-open loop forcing of heat release oscillations by the shear layer mode. At ϕ = 0.85 the flow is hydrodynamically globally stable but locally convectively unstable. Spatial amplification of velocity disturbances by the convectively unstable flow causes high amplitude combustion instability in the long combustor case. These results show that combustion instability can be sustained by acoustic and hydrodynamic modes being either strongly coupled, resulting in fully closed loop forcing, or weakly coupled, resulting in semi-open loop forcing of the flame by a self-excited hydrodynamic mode.

Linear instability analysis of low- incompressible flow over a long rectangular finite-span open cavity

2016 ◽  
Vol 799 ◽  
Author(s):  
Qiong Liu ◽  
Francisco Gómez ◽  
Vassilios Theofilis
Keyword(s):  
Laminar Flow ◽  
Shear Layer ◽  
Three Dimensional ◽  
Linear Instability ◽  
Linear Amplification ◽  
Instability Analysis ◽  
Finite Span ◽  
Layer Mode ◽  
Linear Instability Analysis ◽  
Steady Laminar

TriGlobal linear instability analysis and direct numerical simulations have been performed to unravel the mechanisms ultimately responsible for transition of steady laminar flow over a long rectangular finite-span open cavity with dimensions $L$ : $D$ : $W$$=$ 6 : 1 : 2 to unsteadiness. The steady laminar three-dimensional flow loses stability at $\mathit{Re}_{D,cr}\approx 1080$ as a consequence of linear amplification of a travelling eigenmode that is qualitatively analogous to the shear-layer mode known from analyses of flow in spanwise-periodic cavities, but has a three-dimensional structure which is strongly influenced by the cavity lateral walls. Differences in the eigenspectrum of the present and the spanwise homogeneous flow configuration are documented. Topological changes exerted on the steady laminar flow by linear amplification of the unstable shear-layer mode are reminiscent of observations in experiments at an order of magnitude higher Reynolds number.

Symmetry breaking and instabilities of conical vortex pairs over slender delta wings

2017 ◽  
Vol 832 ◽  
pp. 41-72 ◽  
Author(s):  
Bao-Feng Ma ◽  
Zhijin Wang ◽  
Ismet Gursul
Keyword(s):  
Symmetry Breaking ◽  
Shear Layer ◽  
Dynamic Mode Decomposition ◽  
Stream Velocity ◽  
Dynamic Mode ◽  
Delta Wings ◽  
Similarity Parameter ◽  
Vortex Pairs ◽  
Wide Range ◽  
Layer Mode

An investigation of symmetry breaking and naturally occurring instabilities over thin slender delta wings with sharp leading edges was carried out in a water tunnel using particle image velocimetry (PIV) measurements. Time-averaged location, strength and core radius of conical vortices vary almost linearly with chordwise distance for three delta wings with $75^{\circ }$, $80^{\circ }$ and $85^{\circ }$ sweep angles over a wide range of angles of attack. Properties of the time-averaged vortex pairs depend only on the similarity parameter, which is a function of the angle of attack and the sweep angle. It is shown that time-averaged vortex pairs develop asymmetry gradually with increasing values of the similarity parameter. Vortex asymmetry can develop in the absence of vortex breakdown on the wing. Instantaneous PIV snapshots were analysed using proper orthogonal decomposition and dynamic mode decomposition, revealing the shear layer and vortex instabilities. The shear layer mode is the most periodic and more dominant for lower values of the similarity parameter. The Strouhal number based on the free stream velocity component in the cross-flow plane is a function of only the similarity parameter. The dominant frequency of the shear layer mode decreases with the increasing similarity parameter. The vortex modes reveal the fluctuations of the vorticity magnitude and helical displacement of the cores, but with little periodicity. There is little correlation between the fluctuations in the cores of the vortices.

ABSOLUTE INSTABILITY OF A SUPERCRITICAL SHEAR LAYER

2016 ◽  
Vol 26 (8) ◽  
pp. 815-826 ◽  
Author(s):  
Qing-fei Fu ◽  
Li-Jun Yang ◽  
Chao-Jie Mo
Keyword(s):  
Shear Layer ◽  
Absolute Instability

SHEAR LAYER AND WAVE STRUCTURE OVER PARTIALLY SPANNING CAVITIES

2013 ◽  
Vol 20 (1-2) ◽  
pp. 111-123
Author(s):  
Rajarshi Das ◽  
Heuy Dong Kim ◽  
Job Kurian
Keyword(s):  
Shear Layer ◽  
Wave Structure

Characteristics of Leeward Shear Layer Structure of Hollow-Cone Spray in Gaseous Crossflow

AIAA Journal ◽  
2021 ◽  
Vol 59 (1) ◽  
pp. 405-409
Author(s):  
Haibin Zhang ◽  
Shilin Gao ◽  
Bofeng Bai ◽  
Yechun Wang
Keyword(s):  
Shear Layer ◽  
Layer Structure ◽  
Hollow Cone ◽  
Hollow Cone Spray
Sign in / Sign up

Export Citation Format

Share Document