A Study of the Hydroelastic Instabilities of Supercavitating Hydrofoils

1960 ◽  
Vol 4 (04) ◽  
pp. 28-38
Author(s):  
Paul Kaplan ◽  
C. J. Henry
Keyword(s):  
Dynamic Instability ◽  
Experimental Studies ◽  
Surface Flow ◽  
Small Range ◽  
Equal Importance ◽  
Static Instability ◽  
Elastically Restrained ◽  
Density Ratios ◽  
Oscillating Foil ◽  
Stream Direction

Presented herein are the results of a theoretical study of the static and dynamic hydroelastic instabilities of rigid supercavitating hydrofoils on elastic supports. A two-dimensional theory is used to define the unsteady hydrodynamic force and moment acting on the oscillating foil, which is assumed to be elastically restrained in translation normal to the free-stream direction and in rotation about a prescribed axis which is normal to the plane of flow. All other motions are neglected. The effects of variation in the elastic and inertial properties, as well as the effect of varying the position of the upper surface flow-separation point on the possibility of either form of instability, are determined. Also, the effect of cavitation number over a small range near zero is hypothesized. The theory predicts that dynamic instability (bending-torsion flutter) is possible at the density ratios typical of supercavitating operation. This is in contrast to the results for fully-wetted flow, where the occurrence of flutter is unlikely at the structural-to-fluid density ratios typical of hydrodynamic operation. The flutter possible in supercavitated operation is also more severe than that indicated for fully-wetted flow. Furthermore, it is shown that for the supercavitating hydrofoil, static instability (torsional divergence) and dynamic instability are of equal importance which again differs from the results in fully-wetted flow where static instability was shown to be the more important practical problem. Recommendations are made for experimental studies to verify these theoretical results.

Analysis of Polygonal Vortex Flows in a Cylinder with a Rotating Bottom

2021 ◽  
Vol 11 (3) ◽  
pp. 1348
Author(s):  
A. Rashkovan ◽  
S.D. Amar ◽  
U. Bieder ◽  
G. Ziskind
Keyword(s):  
Experimental Studies ◽  
Free Surface Flow ◽  
Careful Analysis ◽  
Numerical Approach ◽  
Surface Flow ◽  
Initial Water ◽  
Eddy Simulation ◽  
Disk Rotation ◽  
Inner Radius ◽  
Liquid Flows

The present paper provides a physically sound numerical modeling of liquid flows experimentally observed inside a vertical circular cylinder with a stationary envelope, rotating bottom and open top. In these flows, the resulting vortex depth may be such that the rotating bottom disk becomes partially exposed, and rather peculiar polygon shapes appear. The parameters and features of this work are chosen based on a careful analysis of the literature. Accordingly, the cylinder inner radius is 145 mm and the initial water height is 60 mm. The experiments with bottom disk rotation frequencies of 3.0, 3.4, 4.0 and 4.6 Hz are simulated. The chosen frequency range encompasses the regions of ellipse and triangle shapes as observed in the experimental studies reported in the literature. The free surface flow is expected to be turbulent, with the Reynolds number of O(105). The Large Eddy Simulation (LES) is adopted as the numerical approach, with a localized dynamic Subgrid-Scale Stresses (SGS) model including an energy equation. Since the flow obviously requires a surface tracking or capturing method, a volume-of-fluid (VOF) approach has been chosen based on the findings, where this method provided stable shapes in the ranges of parameters found in the corresponding experiments. Expected ellipse and triangle shapes are revealed and analyzed. A detailed character of the numerical results allows for an in-depth discussion and analysis of the mechanisms and features which accompany the characteristic shapes and their alterations. As a result, a unique insight into the polygon flow structures is provided.

Numerical study of asymmetric and axisymmetric thermal jet with entropy generation concept

2021 ◽  
Vol 15 (1) ◽  
pp. 7628-7636
Author(s):  
D. Belakhal ◽  
Kouider Rahmani ◽  
Amel Elkaroui Elkaroui ◽  
Syrine Ben Haj Ayech ◽  
Nejla Mahjoub Saïd ◽  
...  
Keyword(s):  
Entropy Generation ◽  
Numerical Study ◽  
Experimental Studies ◽  
Three Dimensional ◽  
Variable Density ◽  
Current Investigation ◽  
Numerical Resolution ◽  
Different Temperatures ◽  
Density Ratios ◽  
The Impact

In the current investigation, numerical study of a thermal jet of asymmetric (rectangular and elliptical) and axisymmetric (circular) geometry was investigated with variable density to verify the impact of the ratio of density and geometry on the generation of entropy. The central jet was brought to different temperatures (194, 293 and 2110 K) to obtain density ratios (0.66, 1 and 7.2) identical to a mixture jet ((Air-CO2), (Air-Air) and (Air-He)), respectively. Solving the three-dimensional numerical resolution of the Navier Stocks for turbulent flow permanent enclosed on the turbulence model K-εstandard was made. The results acquired are compared with that carried out in previous experimental studies, where it was concluded that, the axisymmetric (circular) geometry increases the entropy generation.

Analytical and experimental studies on out-of-plane dynamic instability of shallow circular arch based on parametric resonance

2016 ◽  
Vol 87 (1) ◽  
pp. 677-694 ◽  
Author(s):  
Airong Liu ◽  
Hanwen Lu ◽  
Jiyang Fu ◽  
Yong-Lin Pi ◽  
Youqin Huang ◽  
...  
Keyword(s):  
Parametric Resonance ◽  
Dynamic Instability ◽  
Experimental Studies ◽  
Circular Arch ◽  
Out Of Plane

The behavior of a compressible silty fine sand

1999 ◽  
Vol 36 (1) ◽  
pp. 88-101 ◽  
Author(s):  
An-Bin Huang ◽  
Huai-Houh Hsu ◽  
Jia-Wei Chang
Keyword(s):  
Quartz Sand ◽  
Experimental Studies ◽  
Fine Sand ◽  
Silty Sand ◽  
Small Range ◽  
Cone Penetration ◽  
Fines Content ◽  
Cone Penetration Tests ◽  
Extensive Field ◽  
Tip Resistance

Publications associated with sands are often limited to clean (i.e., little fines content), uniform, uncemented silica or quartz sand. On the other hand, the importance of mineral content, particle shapes, as well as gradation to the behavior of sand has long been recognized. Although systematic studies of sands other than clean quartz sand have been limited, there is increasing attention being paid to sands with an appreciable fines content. Because of a major construction project, extensive field and laboratory experiments were performed on a silty fine sand from Mai-Liao, which is located on the central west coast of Taiwan. Results show that Mai-Liao Sand (MLS), a silty sand, can be significantly more compressible than clean quartz sand under static load. The particles of MLS have moderate strength, and significant crushing can be induced by triaxial shearing. As a result, MLS has low dilatancy and a relatively small range of peak friction angles. Cone penetration tests in MLS were conducted in a calibration chamber. Analyses of the data indicate that interpreting cone tip resistance in MLS using methods developed based on clean quartz sand without considering the differences of compressibility can be unrealistic. This paper documents results of the experimental studies on MLS.Key words: silty fine sand, strength, dilatancy, compressibility, crushing, in situ test.

Static instability of an elastically restrained cantilever under a partial follower force

AIAA Journal ◽  
1985 ◽  
Vol 23 (10) ◽  
pp. 1637-1639 ◽  
Author(s):  
Lech Tomski ◽  
Jacek Przybylski
Keyword(s):  
Follower Force ◽  
Static Instability ◽  
Elastically Restrained

Bow Wave Dynamics

2002 ◽  
Vol 46 (01) ◽  
pp. 1-15
Author(s):  
T. A. Waniewski ◽  
C. E. Brennen ◽  
F. Raichlen
Keyword(s):  
Experimental Studies ◽  
Free Surface Flow ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Surface Flow ◽  
Wave Profile ◽  
Towing Tank ◽  
Wave Dynamics ◽  
Bow Wave ◽  
Surface Disturbances

Experimental studies of air entrainment by breaking waves are essential for advancing the understanding of these flows and creating valid models. The present study used experimental simulations of a ship bow wave to examine its dynamics and air entrainment processes. The simulated waves were created by a deflecting plate mounted at an angle in a supercritical free-surface flow in a flume. Measurements of the bow wave geometry at two scales and also for a bow wave created by a wedge in a towing tank are presented. Contact line and bow wave profile measurements from the different experiments are compared and demonstrate the similarity of the flume simulations to the towing tank experiments. The bow wave profile data from the towing tank experiments were used to investigate the scaling of the wave with the flow and the dependence on geometric parameters. In addition, surface disturbances observed on the plunging wave are documented herein because of the role they play in air entrainment. The air entrainment itself is explored in Waniewski et al (2001).

Instability in Nonlinear Oscillation of Dielectric Elastomers

2015 ◽  
Vol 82 (6) ◽  
Author(s):  
Jian Zhu
Keyword(s):  
Nonlinear Oscillation ◽  
Electromechanical Coupling ◽  
Haptic Feedback ◽  
Dynamic Instability ◽  
Dielectric Elastomer ◽  
Dielectric Elastomers ◽  
Critical Amplitude ◽  
Dielectric Elastomer Actuators ◽  
Coupling Dynamic ◽  
Static Instability

A membrane of a dielectric elastomer oscillates when subject to AC voltage. Its oscillation is nonlinear due to large deformation and nonlinear electromechanical coupling. Dynamic instability in dielectric elastomers—the oscillation with an unbounded amplitude—is investigated in this paper. The critical amplitude of AC voltage for dynamic instability varies with the frequency of AC voltage and reaches a valley when the superharmonic, harmonic, or subharmonic resonance is excited. Prestretches can improve dielectric elastomer actuators' capabilities to resist dynamic instability. The critical deformation at the onset of dynamic instability can be much larger than that at the onset of static instability. Oscillation of dielectric elastomers can be used for applications, such as vibration shakers for haptic feedback, soft loudspeakers, soft motors, and soft pumps. We hope that the current analyses can improve the understanding of dynamic behavior of dielectric elastomers and enhance their stability and reliability.

Tongue and Groove Joint Stability Testing in Air

2010 ◽  
Author(s):  
Jesse W. Whitlow ◽  
Robert P. Tomko
Keyword(s):  
Dynamic Instability ◽  
Static Stability ◽  
Stability Testing ◽  
Joint Stability ◽  
Mass System ◽  
Tongue In Groove ◽  
Unstable Configuration ◽  
Key Variables ◽  
Flow Through ◽  
Static Instability

Dynamic and static stability testing was performed on a tongue-in-groove joint using air as the driving fluid. The testing was performed to evaluate the conditions that would produce oscillations that could result in sustained vibration of the test piece (dynamic instability) and to determine the conditions for which the joint would seal (static instability). The key variables that were adjusted in the test were the length of the gap, the width of the gap, and the position of the tongue within the groove. Four test configurations were evaluated with different gap widths and engagement lengths to produce L/go (tongue engagement length to groove gap width) ratios ranging from 1.2 to 42. The smallest L/go configuration, which was the most dynamically unstable configuration, was repeated with damping added to the spring-mass system. Each configuration was flow tested over a range of differential pressures and disk positions within the groove. Differential pressure, disk position, displacement due to disk vibration, disk acceleration, flow through the gap, and acoustic pressure in the gap were measured and recorded. Testing showed the most violent unstable behavior occurs when the disk is centered in the groove, while the behavior tends to be stable when the disk is near the downstream sealing surface of the groove.

scholarly journals Instability of a homogeneous flow from a lumped source in the presence of special boundary conditions on a free surface

2019 ◽  
Vol 213 ◽  
pp. 02074 ◽  
Author(s):  
Anastasia Shmyrova ◽  
Andrey Shmyrov
Keyword(s):  
Boundary Conditions ◽  
Vortex Flow ◽  
Hydrodynamic Instability ◽  
Experimental Studies ◽  
Spatial Separation ◽  
Surface Flow ◽  
Water Volume ◽  
Reverse Situation ◽  
Surface Active ◽  
Upper Boundary

In this work, experimental studies were carried out to investigate the structure of a surface flow and to understand potential mechanisms leading to the formation of this type instability. The surface flow was generated by feeding water through three sources: a lumped source with free upper boundary, a slot gap, and a lumped source for inducing a capillary-driven Marangoni flow. For flow visualization, a traditional light knife technique was used. The application of a method of spatial separation of the water volume into two isolated parts whose common surface remained unchanged and the realization of a reverse situation with a divided surface made it possible to study in detail the surface flow structure and to determine the conditions for the appearance of such hydrodynamic instability. It is shown that the formation of a vortex flow is caused by the interaction between the coordinate of the flow homogeneous along the transverse flow and the layer of a surface-active substance adsorbed at the interface. The obtained experimental results demonstrate the importance of setting different boundary conditions for potential and vortex velocity components of a convective flow in the region near the interface occupied by a surfactant.

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