Dynamic Modeling of Air Cushion Vehicles

2007 ◽  
Author(s):  
M. Pollack ◽  
B. Connell ◽  
J. Wilson ◽  
W. Milewski
Keyword(s):  
Free Surface ◽  
Pressure Distribution ◽  
Dynamic Pressure ◽  
Strong Dependence ◽  
Low Frequency ◽  
Coupled System ◽  
Lumped Parameter Model ◽  
Mode Shapes ◽  
Resonant Frequencies ◽  
Air Cushion

The motion of an Air Cushion Vehicle (ACV) is a complex process involving the nonlinear dynamics of the ship, free surface waves, air cushion cavity, skirt, and air flow hydraulics (e.g. orifice behavior of the bag feed holes). The overall system is tightly coupled, with the loading of the ship dependent on the pressure field within the cavity, and the dynamics of the cavity dependent on the motion of the ship, free surface, and skirt. Principle excitation to the system is through the free surface motion and the fan flow. The large dimensions of the system introduce low frequency acoustic and mechanical resonances, which lead to complex and coupled system dynamics. The focus of this paper is on analytical modeling of an ACV and its physics to enable verification of a numerical model which is under development. The initial focus is on the dynamics of the air cushion cavity, with emphasis on its resonant frequencies and mode shapes. The mode shapes are important because they define the nature of the dynamic pressure distribution acting on the ship, and associated heave, pitch, and roll excitation. The strong dependence of the cavity resonant characteristics on the impedance of the skirt, which bounds the cavity, is first demonstrated by assessing limiting cases of a high impedance skirt (e.g. massive or rigid) and of a low impedance skirt (e.g. light or soft). The changes in resonant frequency and dynamic pressure distribution associated with the changes in skirt impedance are illustrated. Because the actual skirt impedance will lie between these two idealized cases, we also develop a lumped parameter model of the skirt dynamics. Initial parametric studies with this model, which investigate variations in the skirt mass, further demonstrate the strong dependence of the resonant frequencies and pressure distributions on the skirt impedance.

Self-Excited Oscillations in Combustors With Spray Atomisers

2000 ◽  
Author(s):  
M. Zhu ◽  
A. P. Dowling ◽  
K. N. C. Bray
Keyword(s):  
Strong Dependence ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Coupled System ◽  
Fuel Spray ◽  
Combustion Model ◽  
Unsteady Combustion ◽  
Low Frequency Oscillation ◽  
Monte Carlo Techniques ◽  
Excited Oscillation

Combustors with fuel-spray atomisers are susceptible to a low-frequency oscillation, particularly at idle and sub-idle conditions. For aero-engine combustors, the frequency of this oscillation is typically in the range 50–120Hz and is commonly caused ‘rumble’. In the current work, Computational Fluid Dynamics (CFD) is used to simulate this self-excited oscillation. The combustion model uses Monte Carlo techniques to give simultaneous solutions of the Williams’ spray equation together with the equations of turbulent reactive flow. The unsteady combustion is calculated by the laminar flamelet presumed pdf method. A quasi-steady description of fuel atomiser behaviour is used to couple the inlet flow in the combustor. A choking condition is employed at turbine inlet. The effects of the atomiser and the combustor geometry on the unsteady combustion are studied. The results show that, for some atomisers, with a strong dependence of mean droplet size on air velocity, the coupled system undergoes low frequency oscillations. The numerical results are analysed to provide insight into the rumble phenomena. Basically, pressure variations in the combustor alter the inlet air and fuel spray characteristics, thereby changing the rate of combustion. This in turn leads to local ‘hot spots’, which generate pressure fluctuations as they convect through the downstream nozzle.

scholarly journals Mathematical Model of Suspension Seat-Person Exposed to Vertical Vibration for Off-Road Vehicles

2019 ◽  
Vol 16 (2) ◽  
pp. 6773-6782
Author(s):  
S. Aisyah Adam ◽  
N. A. A. Jalil ◽  
K. A. Md Razali ◽  
Y. G. Ng ◽  
M. F. Aladdin
Keyword(s):  
Human Body ◽  
Degrees Of Freedom ◽  
Low Frequency ◽  
Coupled System ◽  
Vertical Vibration ◽  
Lumped Parameter Model ◽  
Model Parameters ◽  
Road Vehicles ◽  
Lumped Parameter ◽  
Variable Function

Off-road drivers are exposed to a high magnitude of vibration at low frequency (0.5-25Hz), that can cause harm and possibly attribute to musculoskeletal disorder, particularly low-back pain. The suspension seat is commonly used on an off-road condition to isolate the vibration transmitted to the human body. Nevertheless, the suspension seat modelling that incorporates the human body is still scarce. The objective of this study is to develop a mathematical modelling to represent the suspension seat-person for off-road vehicles. This paper presents a three degrees-of-freedom lumped parameter model. A curve-fitting method is used for parameter identification, which includes the constraint variable function (fmincon()) from the optimisation toolbox of MATLAB(R2017a). The model parameters are optimised using experimentally measured of suspension seat transmissibility. It was found that the model provides a reasonable fit to the measured suspension seat transmissibility at the first peak of resonance frequency, around 2-3 Hz. The results of the study suggested that the human body forms a coupled system with the suspension seat and thus affects the overall performance of the suspension system.  As a conclusion, the influence of the human body should not be ignored in the modelling, and a three-degrees degree-of-freedom lumped parameter model provides a better prediction of suspension seat transmissibility. This proposed model is recommended to predict vibration transmissibility for off-road suspension seat.

Free Vibration Investigation of Submerged Thin Circular Plate

2020 ◽  
Vol 12 (03) ◽  
pp. 2050025
Author(s):  
Xi Yang ◽  
Adil El Baroudi ◽  
Jean Yves Le Pommellec
Keyword(s):  
Free Surface ◽  
Free Vibration ◽  
Circular Plate ◽  
Plate Theory ◽  
Fluid Pressure ◽  
Three Dimensional ◽  
Coupled System ◽  
Mode Shapes ◽  
Thin Circular Plate ◽  
Benchmark Solutions

Free vibration of coupled system including clamped-free thin circular plate with hole submerged in three-dimensional cylindrical container filled with inviscid, irrotational and compressible fluid is investigated in this work. Numerical approach based on the finite element method (FEM) is performed using the Comsol Multiphysics software, in order to analyze qualitatively the vibration characteristics of the plate. Plate modeling is based on Kirchhoff–Love plate theory. Velocity potential is deployed to describe the fluid motion since the small oscillations induced by the plate vibration is considered. Bernoulli’s equation together with potential theory is applied to get the fluid pressure on the free surface of the plate. To prove the reliability of the present numerical solution, a comparison is made with the results in the literature, which shows a very good agreement. Then, different parameters effect including fluid density, fluid height, free surface wave, hole radius and hole eccentricity on the natural frequencies of the coupled system is discussed in detail. Some three-dimensional mode shapes of the submerged plate are illustrated. Furthermore, the obtained results can serve as benchmark solutions for the vibration control, parameter identification and damage detection of plate.

Self-Excited Oscillations in Combustors With Spray Atomizers

2000 ◽  
Vol 123 (4) ◽  
pp. 779-786 ◽  
Author(s):  
M. Zhu ◽  
A. P. Dowling ◽  
K. N. C. Bray
Keyword(s):  
Strong Dependence ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Coupled System ◽  
Fuel Spray ◽  
Combustion Model ◽  
Unsteady Combustion ◽  
Low Frequency Oscillation ◽  
Monte Carlo Techniques ◽  
Excited Oscillation

Combustors with fuel-spray atomizers are susceptible to a low-frequency oscillation, particularly at idle and sub-idle conditions. For aero-engine combustors, the frequency of this oscillation is typically in the range 50–120 Hz and is commonly called “rumble.” In the current work, computational fluid dynamics (CFD) is used to simulate this self-excited oscillation. The combustion model uses Monte Carlo techniques to give simultaneous solutions of the Williams’ spray equation together with the equations of turbulent reactive flow. The unsteady combustion is calculated by the laminar flamelet presumed pdf method. A quasi-steady description of fuel atomizer behavior is used to couple the inlet flow in the combustor. A choking condition is employed at turbine inlet. The effects of the atomizer and the combustor geometry on the unsteady combustion are studied. The results show that, for some atomizers, with a strong dependence of mean droplet size on air velocity, the coupled system undergoes low-frequency oscillations. The numerical results are analyzed to provide insight into the rumble phenomena. Basically, pressure variations in the combustor alter the inlet air and fuel spray characteristics, thereby changing the rate of combustion. This in turn leads to local “hot spots,” which generate pressure fluctuations as they convect through the downstream nozzle.

On a Rectangular Pressure Distribution of Oscillating Strength Moving Over a Free Surface

1977 ◽  
Vol 21 (01) ◽  
pp. 11-23
Author(s):  
Hsao-Hsin Chen
Keyword(s):  
Free Surface ◽  
Pressure Distribution ◽  
Hydrodynamic Instability ◽  
Low Frequency ◽  
Wave Theory ◽  
Hydrodynamic Coefficients ◽  
Low Frequencies ◽  
Air Bubble ◽  
Negative Damping ◽  
Heave Motion

Through the application of linearized water-wave theory, a solution of the free surface due to the disturbance of a moving, oscillatory pressure distribution of rectangular form is obtained. Such a solution provides the basis for formulating the wave resistance of the moving pressure distribution just cited, as well as the hydrodynamic coefficients of a captured-air bubble of rectangular footprint in heave motion. Numerical methods are developed to compute the aforementioned resistance and hydrodynamic coefficients, the numerical examples of which are also presented. Of special interest is the occurrence of negative effectivemass at low frequencies for the two forward speeds considered in the present work. Another interesting feature is the hydrodynamic instability and the negative damping that occurs in the low-frequency range when the Froude number is very high.

Structural-Acoustic Coupling Analysis of Two Cavities Connected by Boundary Structures and Small Holes

2005 ◽  
Vol 127 (6) ◽  
pp. 566-574 ◽  
Author(s):  
Chang-Gi Ahn ◽  
Hyoung Gil Choi ◽  
Jang Moo Lee
Keyword(s):  
Low Frequency ◽  
Coupled System ◽  
Expansion Method ◽  
Mode Shapes ◽  
Coupling Analysis ◽  
Theoretical Formulation ◽  
Modal Expansion ◽  
Acoustic Coupling ◽  
Coupling Characteristics ◽  
Small Holes

In some passenger vehicles, unexpected acoustic modes in the low-frequency range may be observed that cannot be explained by the conventional vibro-acoustic coupling analysis. It is because these methods only use the dynamic characteristics of a vehicle structure and its compartment cavity. However, some small holes or gaps existing at the boundaries between the compartment cavity and the trunk cavity of the vehicles change the modal characteristics of a coupled system. In this paper, a new analytical method is presented to investigate the structural-acoustic coupling characteristics of two cavities connected by small holes and in-between boundary structures. Small holes are modeled as an equivalent mass-spring-damper system in the analysis. A theoretical formulation for vibro-acoustic characteristics of this system is made, and the modal expansion method is used to obtain eigenvalues and their mode shapes. The validity of the proposed method is successfully examined by comparing the results of the analytical predictions with those of experiments.

scholarly journals Bulging Modes of Circular Bottom Plates in Rigid Cylindrical Containers Filled with a Liquid

1997 ◽  
Vol 4 (1) ◽  
pp. 51-68 ◽  
Author(s):  
Marco Amabili
Keyword(s):  
Free Surface ◽  
Liquid Surface ◽  
Dynamic Pressure ◽  
Ritz Method ◽  
Free Vibrations ◽  
Free Liquid Surface ◽  
Mode Shapes ◽  
Bottom Plate ◽  
Free Surface Waves ◽  
Theoretical Approaches

In this article the free vibrations of the bottom plate of an otherwise rigid circular cylindrical tank filled with liquid are studied, considering only the bulging modes (when the amplitude of the plate displacement is predominant with respect to that of the free surface). The tank axis is vertical, thus the free liquid surface is orthogonal to the tank axis. The liquid is assumed to be inviscid, and the contribution of the free surface waves to the dynamic pressure on the free liquid surface is neglected. Wet and dry mode shapes of the plate are assumed to be the same, so that the natural frequencies are obtained by using the nondimensionalized added virtual mass incremental (NA VMI) factors and the modal properties of dry plates. This simplifies computations compared to other existing theoretical approaches. NAVMI factors express the nondimensionalized ratio between the reference kinetic energy of the liquid and that of the plate and have the advantage that, due to their nondimensional form, they can be computed once and for all. Numerical results for simply supported and clamped bottom plates, as well as for supported plates with an elastic moment edge constraint are given. For more accurate results, and to exceed the limits of the assumed modes approach, the Rayleigh-Ritz method is applied and results are compared to those obtained by using the NAVMI factors and other existing methods in the literature.

Analysis of Dynamic Stiffness and Damping of Partial Porous Aerostatic Thrust Bearings

2009 ◽  
Vol 16-19 ◽  
pp. 596-600
Author(s):  
Ya Zhou Sun ◽  
Xue Mei Yu ◽  
Hai Tao Liu ◽  
Ying Chun Liang
Keyword(s):  
Pressure Distribution ◽  
Dynamic Pressure ◽  
Dynamic Stiffness ◽  
Low Frequency ◽  
Distribution Functions ◽  
Thrust Bearings ◽  
Supply Pressure ◽  
The Stability ◽  
Numerical Calculation Method ◽  
Stiffness And Damping

Porous materials have been successfully used in an aerostatic bearing. In this paper, the dynamic stiffness and damping of partial porous aerostatic thrust bearings are analyzed by numerical calculation method. Firstly, the pressure distribution function of the bearing is divided into the static and dynamic pressure distribution functions through minor perturbation method. Then, the static and dynamic pressure distribution functions are calculated by FEM. Finally, the dynamic stiffness and damping coefficients of the bearing are solved. The result indicates that the dynamic stiffness increases obviously with the increment of supply pressure and first increases then decreases with the increment of frequency, and that there is negative damping in the low frequency band and the supply pressure has a great impact on the stability of the bearing.

Tire Vibrations

1977 ◽  
Vol 5 (4) ◽  
pp. 202-225 ◽  
Author(s):  
G. R. Potts ◽  
C. A. Bell ◽  
L. T. Charek ◽  
T. K. Roy
Keyword(s):  
Natural Frequencies ◽  
Standing Waves ◽  
Resonant Mode ◽  
Mode Shapes ◽  
Resonant Vibrations ◽  
Resonant Frequencies ◽  
Radial Tire ◽  
Analysis Techniques ◽  
Thin Ring ◽  
Good Agreement

Abstract Natural frequencies and vibrating motions are determined in terms of the material and geometric properties of a radial tire modeled as a thin ring on an elastic foundation. Experimental checks of resonant frequencies show good agreement. Forced vibration solutions obtained are shown to consist of a superposition of resonant vibrations, each rotating around the tire at a rate depending on the mode number and the tire rotational speed. Theoretical rolling speeds that are upper bounds at which standing waves occur are determined and checked experimentally. Digital Fourier transform, transfer function, and modal analysis techniques used to determine the resonant mode shapes of a radial tire reveal that antiresonances are the primary transmitters of vibration to the tire axle.

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