Hovercraft Rigid Body Response to Free Field Airblast

As multiple axis vibration testing has become more widespread, it has become increasingly important to ensure the instrumentation is accurately portrayed in the instrumentation table. However, errors do occur. The method used in this paper to help uncover these errors is based on the condition that at low frequencies (below any resonant frequencies of the object being studied) the response is essentially rigid body. The spectral density matrix (SDM) at a low frequency, of many more than six response measurements, is decomposed using singular value decomposition (SVD). Under the assumption of rigid body response, it is assumed that the first six singular vectors are linear combinations of the six rigid body modes. The best linear fit is then calculated for this fit. The measurements are then removed one at a time, and the reduction in the fit error is calculated. It is assumed that if the removal of a measurement reduces the error significantly, that measurement is likely in error.

Download Full-text

The Influence of Towing Method on Planing Boat Accelerations in Waves

10.5957/attc-2017-0026 ◽

2017 ◽

Author(s):

Carolyn Judge ◽

Bill Beaver ◽

John Zseleczk

Keyword(s):

Rigid Body ◽

Digital Measurement ◽

Vertical Acceleration ◽

Regular Waves ◽

Towing Tank ◽

Analog Signals ◽

Hull Structure ◽

Calm Water ◽

Low Pass ◽

Body Response

TRACT Vertical acceleration measurements are often used to evaluate the “rigid body” response of a planing hull to hydrodynamic forces in waves. Unfortunately accelerometers respond to both the rigid body hull motions of interest and to unwanted vibrations, which if not addressed, produce artificially higher peak acceleration values (Riley, et.al, 2010). In full scale hulls, vibrations from the propulsors are telegraphed through the hull structure to the accelerometer. In towing tank models vibrations from the carriage are transmitted by the tow post through the hull and to the accelerometer. Historically, different methods have been used to eliminate the unwanted acceleration components including engineering judgement, electronic low-pass filtering of analog signals and postprocessing of digital measurement records using computational filtering techniques. This paper documents a study of the effects of different towing methods on planing boat model accelerations. A four foot long planing hull was tested in calm water and in waves using three different towing methods: - Traditional heave post with model towed at constant velocity - Self-propelled model mounted on a lightweight free-in-surge sub-carriage - String tow bridal with spring as proposed by Savitsky 2016 Tests were conducted in regular waves which made it possible to overlay accelerations peaks from a large number of nearly identical hull slams and make direct comparisons of the magnitude and shape of acceleration peaks measured with each towing method. Details of the three towing methods and the pros/cons of each are presented. The string tow method produced significantly cleaner acceleration THE 30th AMERICAN TOWING TANK CONFERENCE WEST BETHESDA, MARYLAND, OCTOBER 2017 2 records. The plots presented make a strong case for this simple and unconventional towing method and may encourage other towing tank facilities to experiment with it in the future.

Download Full-text

Rigid-Body Response of Base-Isolated Structures

Journal of the Structural Division ◽

10.1061/jsdeag.0006020 ◽

1982 ◽

Vol 108 (8) ◽

pp. 1806-1814

Author(s):

Iradj G. Tadjbakhsh ◽

Jonathan J. Ma

Keyword(s):

Rigid Body ◽

Isolated Structures ◽

Body Response

Download Full-text

Response of piles and casings to horizontal free-field soil displacements

Canadian Geotechnical Journal ◽

10.1139/t84-079 ◽

1984 ◽

Vol 21 (4) ◽

pp. 720-725 ◽

Cited By ~ 11

Author(s):

Peter M. Byrne ◽

Donald L. Anderson ◽

Walter Janzen

Keyword(s):

Rigid Body ◽

Free Field ◽

Earthquake Loading ◽

Field Soil ◽

Simple Method ◽

Laterally Loaded Pile ◽

Laterally Loaded

A simple method of analysis for predicting the response of piles and casings to horizontal free-field soil displacements is presented. Such free-field displacements can be induced in the soil by ice or earthquake loading and can damage piles or casings within the soil. The method involves a modification to the conventional laterally loaded pile problem to account for the load induced by the free-field deflections.The method is applied to analyze the response of a casing within a caisson-retained island subject to high ice loading. The results indicate that flexible casings essentially track with the soil and are subjected to low forces and moments, whereas stiff casings deflect as a rigid body and attract high forces and moments.

Download Full-text

Rigid Body Response of a Mach 2 Shock Train to Downstream Forcing

2018 Fluid Dynamics Conference ◽

10.2514/6.2018-3542 ◽

2018 ◽

Author(s):

Louis M. Edelman ◽

Mirko Gamba

Keyword(s):

Rigid Body ◽

Shock Train ◽

Body Response

Download Full-text

Derivation of Rigid Body Analysis Models From Vehicle Architecture Abstractions

Volume 9: Transportation Systems; Safety Engineering, Risk Analysis and Reliability Methods; Applied Stochastic Optimization, Uncertainty and Probability ◽

10.1115/imece2011-63613 ◽

2011 ◽

Author(s):

Rostyslav Lesiv ◽

Glen Prater ◽

Gary Osborne ◽

David Lamb ◽

Matthew Castanier

Keyword(s):

Rigid Body ◽

Three Dimensional ◽

Analysis Model ◽

Response Models ◽

Vehicle Architecture ◽

Physical Representation ◽

Cad Models ◽

Analysis Engine ◽

Analysis Models ◽

Body Response

Vehicle analysis models of every type have their basis in some type of physical representation of the design domain. Rather than describing three-dimensional continua of a collection of components as is done in detail-level CAD models, an architecture-level abstraction describes fundamental function and arrangement, while capturing just enough physical detail to be used as the basis for a meaningful representation of the design, and eventually, analyses that permit architecture assessment. The design information captured by the abstractions is available at the very earliest stages of the vehicle developing process, so the model itself can function as a “design space for ideas”. In this paper we describe a generalized process for analysis model extraction from vehicle architecture abstractions, and then apply that process to the specific case of rigid body response models. We also discuss implementation of a rigid body analysis engine that forms part of the analysis suite of a software package supporting all aspects of vehicle architecture design.

Download Full-text

Recovering the Free-Field Acoustic Characteristics of a Vibrating Structure from Bounded Noisy Underwater Environments

Sensors ◽

10.3390/s21165521 ◽

2021 ◽

Vol 21 (16) ◽

pp. 5521

Author(s):

Wei Lin ◽

Sheng Li

Keyword(s):

Rigid Body ◽

Free Field ◽

Sound Field ◽

Coupling Method ◽

Noisy Environments ◽

Acoustic Characteristics ◽

Acoustic Coupling ◽

Vibrational Behavior ◽

Measurement Surface ◽

The Difference

The vibrational behavior of an underwater structure in the free field is different from that in bounded noisy environments because the fluid–structure interaction is strong in the water and the vibration of the structure caused by disturbing fields (the reflections by boundaries and the fields radiated by sources of disturbances) cannot be ignored. The conventional free field recovery (FFR) technique can only be used to eliminate disturbing fields without considering the difference in the vibrational behavior of the structure in the free field and the complex environment. To recover the free-field acoustic characteristics of a structure from bounded noisy underwater environments, a method combining the boundary element method (BEM) with the vibro-acoustic coupling method is presented. First, the pressures on the measurement surface are obtained. Second, the outgoing sound field and the rigid body scattered sound field are calculated by BEM. Then, the vibro-acoustic coupling method is employed to calculate the elastically radiated scattered sound field. Finally, the sound field radiated by the structure in the free field is recovered by subtracting the rigid body scattered sound field and the elastically radiated scattered sound field from the outgoing sound field. The effectiveness of the proposed method is validated by simulation results.

Download Full-text