scholarly journals Investigation of Flow Around a Slender Body at High Angles of Attack

2019 ◽  
Vol 30 (1) ◽  
pp. 51-61
Author(s):  
Ibraheem AlQadi Ibraheem AlQadi
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
The Body ◽  
Slender Body ◽  
Frequency Mode ◽  
Dynamic Mode ◽  
High Frequency Mode ◽  
Wide Range ◽  
Low Frequency Mode ◽  
High Angles Of Attack

A numerical investigation of flow around a slender body at high angles of attack is presented. Large eddy simulation of the flow around an ogive-cylinder body at high angles of attack is carried out. Asymmetric vortex flow was observed at angles of attack of α = 55◦ and 65◦ . The results showed that the phenomenon is present in the absence of artificial geometrical or flow perturbation. Contrary to the accepted notion that flow asymmetry is due to a convective instability, the development of vortex asymmetry in the absence of perturbations indicates the existence of absolute instability. An investigation of the unsteady flow field was carried out using dynamic mode decomposition. The analysis identified two distinct unsteady modes; high-frequency mode and low-frequency mode. At angle of attack 45◦ the high-frequency mode is dominant in the frontal part of the body and the low-frequency mode is dominant at the rear part. At α = 55◦ , the highfrequency mode is dominant downstream of vortex breakdown. At α = 65◦ , the spectrum shows a wide range of modes. Reconstruction of the dynamical modes shows that the low-frequency mode is associated with the unsteady wake and the high-frequency mode is associated with unsteady shear layer.

scholarly journals Self-sustained hydrodynamic oscillations in lifted jet diffusion flames: origin and control

2015 ◽  
Vol 775 ◽  
pp. 201-222 ◽  
Author(s):  
Ubaid Ali Qadri ◽  
Gary J. Chandler ◽  
Matthew P. Juniper
Keyword(s):  
Stability Analysis ◽  
High Frequency ◽  
Passive Control ◽  
Diffusion Flames ◽  
Low Frequency ◽  
Frequency Mode ◽  
High Frequency Mode ◽  
Jet Diffusion Flames ◽  
Local Stability Analysis ◽  
Low Frequency Mode

We use direct numerical simulation (DNS) of the Navier–Stokes equations in the low-Mach-number limit to investigate the hydrodynamic instability of a lifted jet diffusion flame. We obtain steady solutions for flames using a finite rate reaction chemistry, and perform a linear global stability analysis around these steady flames. We calculate the direct and adjoint global modes and use these to identify the regions of the flow that are responsible for causing oscillations in lifted jet diffusion flames, and to identify how passive control strategies might be used to control these oscillations. We also apply a local stability analysis to identify the instability mechanisms that are active. We find that two axisymmetric modes are responsible for the oscillations. The first is a high-frequency mode with wavemaker in the jet shear layer in the premixing zone. The second is a low-frequency mode with wavemaker in the outer part of the shear layer in the flame. We find that both of these modes are most sensitive to feedback involving perturbations to the density and axial momentum. Using the local stability analysis, we find that the high-frequency mode is caused by a resonant mode in the premixing region, and that the low-frequency mode is caused by a region of local absolute instability in the flame, not by the interaction between resonant modes, as proposed in Nichols et al. (Phys. Fluids, vol. 21, 2009, article 015110). Our linear analysis shows that passive control of the low-frequency mode may be feasible because regions up to three diameters away from the fuel jet are moderately sensitive to steady control forces.

scholarly journals Inertial regimes in a curved electromagnetically forced flow

2017 ◽  
Vol 813 ◽  
pp. 860-881 ◽  
Author(s):  
J. Boisson ◽  
R. Monchaux ◽  
S. Aumaître
Keyword(s):  
Magnetic Field ◽  
Secondary Flow ◽  
High Frequency ◽  
Characteristic Length ◽  
Low Frequency ◽  
Frequency Mode ◽  
Square Duct ◽  
High Frequency Mode ◽  
Duct Geometry ◽  
Low Frequency Mode

We investigated experimentally the flow driven by a Lorentz force induced by an axial magnetic field $\boldsymbol{B}$ and a radial electric current $I$ applied between two fixed concentric copper cylinders. The gap geometry corresponds to a rectangular section with an aspect ratio of $\unicode[STIX]{x1D702}=4$ and we probe the azimuthal and axial velocity profiles of the flow along the vertical axis by using ultrasonic Doppler velocimetry. We have performed several runs at moderate magnetic field strengths, corresponding to moderate Hartmann numbers $M\leqslant 300$. At these forcing parameters and because of the geometry of our experimental device, we show that the inertial terms are not negligible and an azimuthal velocity that depends on both $I$ and $B$ is induced. From measurements of the vertical velocity we focus on the characteristics of the secondary flow: the time-averaged velocity profiles are compatible with a secondary flow presenting two pairs of stable vortices, as pointed out by previous numerical studies. The flow exhibited a transition between two dynamical modes, a high- and a low-frequency one. The high-frequency mode, which emerges at low magnetic field forcing, corresponds to the propagation in the radial $r$-direction of tilted vortices. This mode is consistent with our previous experiments and with the instability described in Zhao et al. (Phys. Fluids, vol. 23 (8), 2011, 084103) taking place in an elongated duct geometry. The low-frequency mode, observed for high magnetic field forcing, consists of large excursions of the vortices. The dynamics of these modes matches the first axisymmetric instability described in Zhao & Zikanov (J. Fluid Mech., vol. 692, 2012, pp. 288–316) taking place in an square duct geometry. We demonstrated that this transition is controlled by the inertial magnetic thickness $H^{\prime }$ which is the characteristic length we introduce as a balance between the advection and the Lorentz force. The key point here is that when the inertial magnetic thickness $H^{\prime }$ is comparable to one geometric characteristic length ($H/2$ in the vertical or $\unicode[STIX]{x0394}r$ in the radial direction) the corresponding mode is favoured. Therefore, when $H^{\prime }/(H/2)\approx 1$ we observe the high-frequency mode taking place in an elongated duct geometry, and when $H^{\prime }/\unicode[STIX]{x0394}r\approx 1$ we observe the low-frequency mode taking place in square duct geometry and high magnetic field.

A New Kind of Energy Transfer From the High-Frequency to Low-Frequency Mode in a Composite Laminated Plate

2010 ◽  
Author(s):  
Wei Zhang ◽  
Xiang-Ying Guo ◽  
Qian Wang ◽  
Cui-Cui Liu ◽  
Yun-cheng He
Keyword(s):  
Numerical Simulation ◽  
Natural Frequency ◽  
Thin Plate ◽  
High Frequency ◽  
Low Frequency ◽  
Frequency Mode ◽  
Rectangular Thin Plate ◽  
Chaotic Motions ◽  
Simply Supported ◽  
Low Frequency Mode

This paper focuses on the analysis on a new kind of nonlinear resonant motion with the low-frequency large-amplitude, which can be induced by the high-frequency small-amplitude mode through the mechanism of modulation of amplitude and phase. The system investigated is a simply supported symmetric cross-ply composite laminated rectangular thin plate subjected to parametric excitations. Experimental research has been carried out for the first time. The test plate was excited near the first natural frequency with parametric forces and the above mentioned high-to-low frequency mode has been observed, whose frequency is extremely lower than the first natural frequency. Theoretical job goes to analysis the above phenomenon accordingly. Based on the Reddy’s third-order shear deformation plate theory and the von Karman type equation, the nonlinear governing equations of the simply supported symmetric cross-ply composite laminated rectangular thin plate subjected to parametric excitations are formulated. The Galerkin method is utilized to discretize the governing partial differential equations into a two-degree-of-freedom nonlinear system. Numerical simulation is conducted to investigate this non-autonomous system subsequently. The results of numerical simulation demonstrate that there is a qualitative agreement between the experimental observation and the theoretical result. Besides, the multi-pulse chaotic motions are also reported in numerical simulations.

NONLINEAR INTERACTIONS BETWEEN TWO WIDELY SPACED MODES — EXTERNAL EXCITATION

1993 ◽  
Vol 03 (02) ◽  
pp. 417-427 ◽  
Author(s):  
S.A. NAYFEH ◽  
A.H. NAYFEH
Keyword(s):  
Natural Frequency ◽  
High Frequency ◽  
Analytical Study ◽  
Experimental Studies ◽  
Low Frequency ◽  
Harmonic Excitation ◽  
Frequency Mode ◽  
Large Contribution ◽  
High Frequency Mode ◽  
Two Degree Of Freedom

Recent experimental studies indicate that energy can be transferred from high- to low-frequency modes in structures with weak nonlinearity. In each of these experiments, a high-frequency mode was driven near its natural frequency but the response included a large contribution due to the first mode of the structure. In this paper, an analytical study of the response of a two-degree-of-freedom nonlinear system with widely spaced modes to a simple-harmonic excitation near the natural frequency of its high-frequency mode is presented. This system serves as a paradigm for the interaction of high- and low-frequency modes.

Optical Phonons of ZnS1–xSex and CdS1–xSex Mixed Crystals: Pressure Effects

1970 ◽  
Vol 24 (2) ◽  
pp. 187-192 ◽  
Author(s):  
J. R. Ferraro ◽  
S. S. Mitra ◽  
C. Postmus ◽  
C. Hoskins ◽  
E. C. Siwiec
Keyword(s):  
Low Frequency ◽  
Mixed Crystals ◽  
Pressure Effects ◽  
Frequency Mode ◽  
Optical Phonons ◽  
High Frequency Mode ◽  
Mode Behavior ◽  
Low Concentrations ◽  
Ir Transmission ◽  
Low Frequency Mode

The ir transmission of the optical phonons for the mixed crystals ZnS1– xSe x and CdS1– xSe x, at various concentrations of x, was studied as a function of pressure up to about 43 kbar. At all concentrations a two-mode behavior was observed. For both systems the high frequency mode exhibits a greater pressure dependence than the low frequency mode. The gap mode observed for the ZnS1– xSe x mixed crystals at low concentrations of Se is found to be pressure insensitive as contrasted to the Cd1- xSe x mixed crystals.

An Experimental Investigation of Energy Transfer from a High- Frequency Mode to a Low-Frequency Mode in a Flexible Structure

1995 ◽  
Vol 1 (1) ◽  
pp. 115-128 ◽  
Author(s):  
Pavol Popovic ◽  
Ali H. Nayfeh ◽  
Kyoyul Oh ◽  
Samir A. Nayfeh
Keyword(s):  
Energy Transfer ◽  
High Frequency ◽  
Power Spectra ◽  
Low Frequency ◽  
Excitation Amplitude ◽  
High Amplitude ◽  
Frequency Mode ◽  
High Frequency Mode ◽  
Beam Frame ◽  
Low Amplitude

The objective of the present article is to experimentally observe and characterize the transfer of energy from low-amplitude, high-frequency modes to high-amplitude, low-frequency modes. The subject of the study is a three-beam frame. The excitation amplitude is restricted to below 2 g peak. The authors have focused on observing, characterizing, and documenting the excitation of the first mode by high-frequency forcing. The energy-transfer processes are identified by power spectra and characterized further by frequency and amplitude sweeps. The energy-transfer routes observed in the experiment are subharmonic resonance of order one-half, combination resonance of the additive type, and interaction between widely spaced modes. In the latter route, an excitation at a frequency that is more than 100 times the first-mode frequency has been observed to excite the first mode.

Dynamics of a Pipe Conveying Pulsating Flow Near Criticality

2000 ◽  
Author(s):  
Robert J. McDonald ◽  
N. Sri Namachchivaya
Keyword(s):  
Flow Velocity ◽  
High Frequency ◽  
Global Bifurcation ◽  
Low Frequency ◽  
Conservative System ◽  
Pulsating Flow ◽  
Frequency Mode ◽  
Double Zero ◽  
Zero Eigenvalue ◽  
High Frequency Mode

Abstract In this paper, we study the dynamics of a simply supported pipe conveying pulsating flow near the flow velocity where the system first becomes unstable. We study a Galerkin discretization of this system which involves the first two modes of the pipe, near the flow velocity where the two degree of freedom Hamiltonian system possesses a nonsemisimple double zero eigenvalue and a pair of imaginary axis eigenvalues. The damping, as well as me parametric forcing due to the pulsating flow, are considered as perturbations to a conservative system. Using local bifurcation analysis and recently developed global bifurcation methods, we study the transfer of energy from the forced high frequency mode to the unforced low frequency mode. This transfer of energy causes unwanted vibrations in the mode associated with the double zero eigenvalue, due to the 0:1 resonance that this mode has with the high frequency mode.

scholarly journals Auxetic vibration behaviours of periodic tetrahedral units with a shared edge

2021 ◽  
Vol 8 (10) ◽  
Author(s):  
H. Tanaka ◽  
S. Asao ◽  
Y. Shibutani
Keyword(s):  
Degrees Of Freedom ◽  
Kinematic Model ◽  
Low Frequency ◽  
Slow Motion ◽  
Frequency Mode ◽  
Linear Elastic ◽  
Wide Range ◽  
Strain Relationship ◽  
Relationship Of ◽  
Low Frequency Mode

A very low-frequency mode supported within an auxetic structure is presented. We propose a constrained periodic framework with corner-to-corner and edge-to-edge sharing of tetrahedra and develop a kinematic model incorporating two types of linear springs to calculate the momentum term under infinitesimal transformations. The modal analysis shows that the microstructure with its two degrees of freedom has both low- and high-frequency modes under auxetic transformations. The low-frequency mode approaches zero frequency when the corresponding spring constant tends to zero. With regard to coupled eigenmodes, the stress–strain relationship of the uniaxial forced vibration covers a wide range. When excited, a very slow motion is clearly observed along with a structural expansion for almost zero values of the linear elastic modulus.

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