Hydroelastic Instability of Low Aspect Ratio Control Surfaces

2004 ◽  
Vol 126 (1) ◽  
pp. 84-89 ◽  
Author(s):  
Michael E. McCormick ◽  
Luca Caracoglia
Keyword(s):  
Aspect Ratio ◽  
Special Class ◽  
Three Dimensional ◽  
Added Mass ◽  
Experimental Results ◽  
Low Aspect Ratio ◽  
Dimensional Effects ◽  
Aspect Ratios ◽  
Modified Equations ◽  
Control Surfaces

As the operational speeds of surface ships and submarines increase, so does the probability that unwanted vibrations caused by the hydroelastic instability (flutter) of the special class of hydrofoils called control surfaces. These include rudders and diving planes. By nature, these are thick symmetric hydrofoils having low aspect ratios. The 3-D tip effects become more pronounced as the aspect ratio decreases. In the present study, the added-mass and circulation terms of the 2-D flutter equations are modified to include three-dimensional effects. The modifications are performed by introducing quasi-steady coefficients to each term. The results predicted by the modified equations are found to compare well with experimental results on a towed rectangular foil having an aspect ratio of one.

A Method to Predict Rudder Stall Inception from Two-dimensional Airfoil Data

2014 ◽  
Vol 58 (01) ◽  
pp. 1-19
Author(s):  
Michael J. Hughes ◽  
Young T. Shen
Keyword(s):  
Aspect Ratio ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Theoretical Formulation ◽  
Stall Inception ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Flow Similarity ◽  
Naca 0015 ◽  
Naca 0012

The behavior of the force on a rudder changes significantly after the inception of stall, requiring different mathematical formulae to compute rudder forces prior-and poststall. Determining the inception angle at which stall occurs is important for predicting the rudder force on a maneuvering ship. A method to compute the inception angle of stall on a rudder is presented in this article. The theoretical formulation is based on a flow similarity approach, which relates three-dimensional rudder stall inception with two-dimensional airfoil data. Rudders are low-aspect ratio wings, and the three-dimensional lift is based on the low-aspect ratio wing theory. The two-dimensional airfoil stall data are obtained from National Advisory Committee for Aeronautics (NACA) reports. The derived theory is first validated with wind tunnel data from foils with a NACA 0015 profile of aspect ratios 1, 2, and 3. The theory is also validated with data from foils with a NACA 0012 profile and an aspect ratio of 2, 3, and 6.

Cross-Wind Influence on Low Aspect Ratio Wings at Low Reynolds Numbers

2013 ◽  
Author(s):  
Amir Karimi Noughabi ◽  
Mehran Tadjfar
Keyword(s):  
Aspect Ratio ◽  
Numerical Study ◽  
Three Dimensional ◽  
Micro Air Vehicles ◽  
Reynolds Numbers ◽  
Aerodynamic Characteristics ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Lift And Drag ◽  
Lift And Drag Coefficient

The aerodynamics of the low aspect ratio (LAR) wings is of outmost importance in the performance of the fixed-wing micro air vehicles (MAVs). The flow around these wings is widely influenced by three dimensional (3D) phenomena: including wing-tip vortices, formation of laminar bubble, flow separation and reattachment, laminar to turbulent transition or any combination of these phenomena. All the recent studies consider the aerodynamic characteristics of the LAR wings under the effect of the direct wind. Here we focus on the numerical study of the influence of cross-wind on flow over the inverse Zimmerman wings with the aspect ratios (AR) between 1 and 2 at Reynolds numbers between 6×104 and 105. We have considered cross-wind’s angles from 0° to 40° and angle of attack from 0° to 12°. The results show that lift and drag coefficient generally decrease when the angle of the cross-wind is increased.

scholarly journals Experimental Investigations of Three-Dimensional Effects on Cavitating Hydrofoils

1961 ◽  
Vol 5 (03) ◽  
pp. 22-43
Author(s):  
R. W. Kermeen
Keyword(s):  
Aspect Ratio ◽  
High Speed ◽  
Three Dimensional ◽  
Cavitating Flow ◽  
Experimental Investigations ◽  
Water Tunnel ◽  
Pitching Moment ◽  
Two Dimensional ◽  
Dimensional Effects ◽  
Aspect Ratios

An investigation in the high-speed water tunnel of the hydsrodynamic characteristics of a family of three-dimensional sharp-edged hydrofoils is described. Four rectangular plan-form, 6-deg wedge profiles with aspect ratios of 4.0, 2.0, 1.0 and 0.5 were tested over a range of cavitation numbers from noncavitating to fully cavitating flow. The effects of aspect ratio on the flow and cavity configurations and on the lift, drag and pitching moment are discussed. Where data were available the results have been compared with the two-dimensional case.

Calculation of the Rise and Pitch of a Three-Dimensional Low-Aspect-Ratio Flat Ship

1991 ◽  
Vol 35 (04) ◽  
pp. 314-324
Author(s):  
Todd McComb
Keyword(s):  
Aspect Ratio ◽  
Three Dimensional ◽  
Static Case ◽  
Equilibrium Flow ◽  
Low Aspect Ratio ◽  
Fast Speed ◽  
Flow Past

Using low-aspect-ratio flat ship theory, this paper defines a procedure to determine the position of a hull which is in equilibrium at some "fast" speed in terms of a given hull shape for the same hull at rest. This procedure is then used to find the equilibrium flow past a moving ship, when given the shape of the hull at rest. The method is then extended to find the hull configuration at various speeds based on either the configuration in the static case or at some other equilibrium speed, leading to a calculation of drag versus speed. Some general formulas and some simple examples are given.

Improving the Performance of a Turbine With Low Aspect Ratio Stators by Aft-Loading

2004 ◽  
Vol 128 (3) ◽  
pp. 492-499 ◽  
Author(s):  
Graham Pullan ◽  
John Denton ◽  
Eric Curtis
Keyword(s):  
Aspect Ratio ◽  
Three Dimensional ◽  
Secondary Flows ◽  
Loss Reduction ◽  
Low Aspect Ratio ◽  
Surface Pressure Distribution ◽  
Loaded Surface ◽  
Nozzle Guide ◽  
Nozzle Guide Vanes ◽  
Stage Efficiency

Experimental data and numerical simulations are presented from a research turbine with low aspect ratio nozzle guide vanes (NGVs). The combined effects of mechanical and aerodynamic constraints on the NGV create very strong secondary flows. This paper describes three designs of NGV that have been tested in the turbine, using the same rotor row in each case. NGV 2 used three-dimensional design techniques in an attempt to improve the performance of the datum NGV 1 blade, but succeeded only in creating an intense vortex shed from the trailing edge (as previously reported) and lowering the measured stage efficiency by 1.1% points. NGV 3 was produced to avoid the “shed vortex” while adopting a highly aft-loaded surface pressure distribution to reduce the influence of the secondary flows. The stage with NGV 3 had an efficiency 0.5% points greater than that with NGV 1. Detailed comparisons between experiment and computations, including predicted entropy generation rates, are used to highlight the areas where the loss reduction has occurred and hence to quantify the effects of employing highly aft-loaded NGVs.

Three-Dimensional Thermocapillary Convection in Open Cylindrical Annuli

2000 ◽  
Author(s):  
Bok-Cheol Sim ◽  
Abdelfattah Zebib
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Critical Frequency ◽  
Thermocapillary Convection ◽  
Three Dimensional ◽  
Time Dependent ◽  
Infinite Layer ◽  
Aspect Ratios ◽  
Temperature Oscillations ◽  
Good Agreement

Abstract Three-dimensional, time-dependent thermocapillary convection in open cylindrical containers is investigated numerically. Results for aspect ratios (Ar) of 1, 2.5, 8, and 16 and a Prandtl number of 6.84 are obtained to compare the results of numerical simulations with ongoing experiments. Convection is steady and axisymmetric at sufficiently low values of the Reynolds number (Re). Transition to oscillatory states occurs at critical values of Re which depend on Ar. With Ar = 1.0 and 2.5, we observe, respectively, 5 and 9 azimuthal wavetrains travelling clockwise at the free surface near the critical Re. With Ar = 8.0 and 16.0, there are substantially more, but pulsating waves near the critical Re. In the case of Ar = 16.0, which approaches the conditions in an infinite layer, our results are in good agreement with linear theory. While the critical Reynolds number decreases with increasing aspect ratio in the case of azimuthal rotating waves, it increases with increasing aspect ratio in the case of azimuthal pulsating waves. The critical frequency of temperature oscillations is found to decrease linearly with increasing Ar. We have also computed supercritical time-dependent states and find that while the frequency increases with increasing Re near the critical region, the frequency of supercritical convection decreases with Re.

Experimental Study on Flow-Induced Vibration of Floating Squared Section Cylinders With Low Aspect Ratio: Part I — Effects of Incidence Angle

2015 ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
Dênnis M. Gambarine ◽  
Felipe P. Figueiredo ◽  
Fábio V. Amorim ◽  
André L. C. Fujarra
Keyword(s):  
Aspect Ratio ◽  
Incidence Angle ◽  
Ocean Basin ◽  
Circular Cylinders ◽  
Structural Damping ◽  
Flow Induced Vibration ◽  
Double Frequency ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Elastically Supported

Experiments regarding flow-induced vibration on floating squared section cylinders with low aspect ratio were carried out in an ocean basin with rotating-arm apparatus. The floating squared section cylinders were elastically supported by a set of linear springs to provide low structural damping to the system. Three different aspect ratios were tested, namely L/D = 1.0, 2.0 and 3.0, and two different incidence angles, namely 0 and 45 degrees. The aims were to understanding the flow-induced vibration around single columns of multi-column platforms, such as semi-submersible and TLP. VIV on circular cylinders were also carried out to compare the results. The range of Reynolds number covered was 2,000 < Re < 27,000. The in-line and transverse amplitude results showed to be higher for 45-degree incidence compared with 0-degree, but the maximum amplitudes for squared section cylinders were lower compared with the circular ones. The double frequency in the in-line motion was not verified as in circular cylinders. The yaw amplitudes cannot be neglected for squared section cylinders, maximum yaw amplitudes around 10 degrees were observed for reduced velocities up to 15.

Separating and Reattaching Flow Structure in a Suddenly Expanding Rectangular Duct

1995 ◽  
Vol 117 (1) ◽  
pp. 17-23 ◽  
Author(s):  
G. Papadopoulos ◽  
M. V. O¨tu¨gen
Keyword(s):  
Aspect Ratio ◽  
Flow Structure ◽  
Three Dimensional ◽  
Side Wall ◽  
Stream Velocity ◽  
Rectangular Duct ◽  
Mean Flow ◽  
Wall Effects ◽  
Velocity Measurements ◽  
Aspect Ratios

The incompressible turbulent flow over a backward-facing step in a rectangular duct was investigated experimentally. The side wall effects on the core flow were determined by varying the aspect ratio (defined as the step span-to-height ratio) from 1 to 28. The Reynolds number, based on the step height and the oncoming free-stream velocity, was 26,500. Detailed velocity measurements were made, including the turbulent stresses, in a region which extended past the flow reattachment zone. Wall static pressure was also measured on both the step and flat walls. In addition, surface visualizations were obtained on all four walls surrounding the separated flow to supplement near-wall velocity measurements. The results show that the aspect ratio has an influence on both the velocity and wall pressure even for relatively large aspect ratios. For example, in the redevelopment region downstream of reattachment, the recovery pressure decreases with smaller aspect ratios. The three-dimensional side wall effects tend to slow down the relaxation downstream of reattachment for smaller aspect ratios as evidenced by the evolution of the velocity field. For the two smallest aspect ratios investigated, higher centerplane streamwise and transverse velocities were obtained which indicate a three-dimensional mean flow structure along the full span of the duct.

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