9,482 research outputs found
Statistical modelling for prediction of axis-switching in rectangular jets
Rectangular nozzles are increasingly used for modern military aircraft propulsion installations, including the roll nozzles on the F-35B vertical/short take-off and landing strike fighter. A peculiar phenomenon known as axis-switching is generally observed in such non-axisymmetric nozzle flows during which the jet spreads faster along the minor axis compared to the major axis. This might affect the under-wing stores and aircraft structure. A computational fluid dynamics study was performed to understand the effects of changing the upstream nozzle geometry on a rectangular free jet. A method is proposed, involving the formulation of an equation based upon a statistical model for a rectangular nozzle with an exit aspect ratio (ARe) of 4; the variables under consideration (for a constant nozzle pressure ratio (NPR)) being inlet aspect ratio (ARi) and length of the contraction section. The jet development was characterised using two parameters: location of the cross-over point (Xc) and the difference in the jet half-velocity widths along the major and minor axes (ÎB30). Based on the observed results, two statistical models were formulated for the prediction of axis-switching; the first model gives the location of the cross-over point, while the second model indicates the occurrence of axis-switching for the given configuration
Petiolate wings: effects on the leading-edge vortex in flapping flight
The wings of many insect species including crane flies and damselflies are petiolate (on stalks), with the wing planform beginning some distance away from the wing hinge, rather than at the hinge. The aerodynamic impact of flapping petiolate wings is relatively unknown, particularly on the formation of the lift-augmenting leading-edge vortex (LEV): a key flow structure exploited by many insects, birds and bats to enhance their lift coefficient. We investigated the aerodynamic implications of petiolation P using particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values of P = 1â3. The wings were driven using a mechanical device, the âFlapperatusâ, to produce highly repeatable insect-like kinematics. The wings maintained a constant Reynolds number of 1400 and dimensionless stroke amplitude Î* (number of chords traversed by the wingtip) of 6.5 across all test cases. Our results showed that for more petiolate wings the LEV is generally larger, stronger in circulation, and covers a greater area of the wing surface, particularly at the mid-span and inboard locations early in the wing stroke cycle. In each case, the LEV was initially arch-like in form with its outboard end terminating in a focus-sink on the wing surface, before transitioning to become continuous with the tip vortex thereafter. In the second half of the wing stroke, more petiolate wings exhibit a more detached LEV, with detachment initiating at approximately 70% and 50% span for P = 1 and 3, respectively. As a consequence, lift coefficients based on the LEV are higher in the first half of the wing stroke for petiolate wings, but more comparable in the second half. Time-averaged LEV lift coefficients show a general rise with petiolation over the range tested.This work was supported by an EPSRC Career Acceleration Fellowship to R.J.B. (EP/H004025/1)
The effect of aspect ratio on the leading-edge vortex over an insect-like flapping wing
Insect wing shapes are diverse and a renowned source of inspiration for the new generation of autonomous flapping vehicles, yet the aerodynamic consequences of varying geometry is not well understood. One of the most defining and aerodynamically significant measures of wing shape is the aspect ratio, defined as the ratio of wing length (R) to mean wing chord (). We investigated the impact of aspect ratio, AR, on the induced flow field around a flapping wing using a robotic device. Rigid rectangular wings ranging from AR = 1.5 to 7.5 were flapped with insect-like kinematics in air with a constant Reynolds number (Re) of 1400, and a dimensionless stroke amplitude of (number of chords traversed by the wingtip). Pseudo-volumetric, ensemble-averaged, flow fields around the wings were captured using particle image velocimetry at 11 instances throughout simulated downstrokes. Results confirmed the presence of a high-lift, separated flow field with a leading-edge vortex (LEV), and revealed that the conical, primary LEV grows in size and strength with increasing AR. In each case, the LEV had an arch-shaped axis with its outboard end originating from a focus-sink singularity on the wing surface near the tip. LEV detachment was observed for around mid-stroke at span, and initiated sooner over higher aspect ratio wings. At the larger, stronger vortex persisted under the wing surface well into the next half-stroke leading to a reduction in lift. Circulatory lift attributable to the LEV increased with AR up to AR = 6. Higher aspect ratios generated proportionally less lift distally because of LEV breakdown, and also less lift closer to the wing root due to the previous LEV's continuing presence under the wing. In nature, insect wings go no higher than likely in part due to architectural and physiological constraints but also because of the reducing aerodynamic benefits of high AR wings
Large-scale instabilities in a STOVL upwash fountain
The fountain flow created by two underexpanded axisymmetric, turbulent jets
impinging on a ground plane was studied through the use of laser-based
experimental techniques. Velocity and turbulence data were acquired in the jet
and fountain flow regions using laser doppler velocimetry and particle image
velocimetry. Profiles of mean and rms velocities along the jet centreline are
presented for nozzle pressure ratios of two, three and four. The unsteady nature
of the fountain flow was examined and the presence of large-scale coherent
structures identified. A spectral analysis of the fountain flow data was
performed using the Welch method. The results have relevance to ongoing studies
of the fountain flow using large eddy simulation techniques
A note on elastic surface waves
The structure of harmonically time-dependent free surface waves on a homogeneous, isotropic elastic half-space can be described by proceeding from the following assumptions: (1) the plane boundary is free of surface traction; (2) the Laimé potentials, and consequently all physical quantities, decay exponentially with distance away from the boundary. In the absence of further a priori assumptions, the resulting surface waves need be neither plane nor axially symmetric, and thus the derivation sketched here constitutes a generalization of the ones usually given in the textbook literature [e.g., Love, 1944; Ewing et al., 1957].
With reference to Cartesian coordinates Ï_1, Ï_2, Ï_3, the half-space under consideration occupies the region x_3â„0. The displacement vector u of a typical point has Cartesian components u_j, and the associated components of stress are denoted by Ï_(jk). The summation convention is used, Latin and Greek subscripts have the respective ranges 1, 2, 3 and 1, 2, and a subscript preceded by a comma indicates differentiation with respect to the corresponding coordinate
Applications of radio interferometry to navigation
Radio astronomy experiments have demonstrated the feasibility of making precise position measurements using interferometry techniques. The application of this method to navigation and marine geodesy is discussed, and comparisons are made with existing navigation systems. The very long baseline technique, with a master station, can use either an artificial satellite or natural sources as position references; a high-speed data link is required. A completely ship-borne system is shown to be feasible, at the cost of poorer sensitivity for natural sources. A comparison of Doppler, delay and phase-track modes of operating a very long baseline configuration is made, as that between instantaneous measurements and those where a source can be tracked from horizon to transit. Geometric limitations in latitude and longitude coverage are discussed. The characteristics of natural radio sources, their flux, distribution on the sky, and apparent size are shown to provide a limit on position measurements precision. The atmosphere and frequency standard used both contribute to position measurement uncertainty by affecting interferometric phase
On the Scale of the Nonlinear Effect in a Crack Problem
When crack problems are analyzed on the basis of nonlinear
theories, such as finite elasticity or deformation theory of
plasticity, it is inevitable that nonlinear effects will
predominate near a crack-tip, even if the loads are small. The
most favorable circumstance in this regard occurs when the
loads are so small that the zone of significant nonlinearity lies
within the region of validity of the near-tip approximation to
the global solution of the associated linearized crack problem.
This situation - called small-scale yielding for crack problems
in plasticity - permits simplifications in analysis which are
often decisive; see, e.g., Knowles (1977) and Rice (1968).
Insofar as we know, there are no analytical estimates
available of the level of load below which nonlinear effects are
guaranteed to be small-scale in the above sense. Indeed, even a
precise version of the question seems to be lacking. In the present
note we formulate and answer such a question for an
especially cooperative crack problem; that corresponding to
finite anti-plane shear of an infinite medium containing a
crack of finite length for an elastic material of Neo-Hookean
type. The associated boundary value problem is a linear one
for Laplace's equation and thus can be solved globally. Nevertheless,
there is a significant nonlinear effect of Kelvin type in
the stress field. We give a condition under which this nonlinear
response occurs on a small scale near the crack tips
Unstable Elastic Materials and the Viscoelastic Response of Bars in Tension
Some homogeneous elastic materials are capable of sustaining
finite equilibrium deformations with discontinuous
strains. For materials of this kind, the energetics of isothermal,
quasi-static motions may differ from those conventionally
associated with elastic behavior. When equilibrium states
involving strain jumps occur during such motions, the rate of
increase of stored energy in a portion of the body may no
longer coincide with the rate of work of the external forces
present. In general, energy balance now includes an additional
effect due to the presence of moving strain discontinuities. As
a consequence, the macroscopic response of the body may be
dissipative. This fact makes it possible to model certain types
of inelastic behavior in solids with the help of such "unstable"
elastic materials; see, for example, Abeyaratne and Knowles
(1987a,b,c)
A note on the driving traction acting on a propagating interface: Adiabatic and non-adiabatic processes of a continuum
An expression for the driving traction on an interface is derived for an arbitrary continuum undergoing an arbitrary thermomechanical
process which may- or may not be adiabatic
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