180,286 research outputs found
A new stall-onset criterion for low speed dynamic-stall
No full textThe Beddoes/Leishman dynamic-stall model has become one of the most popular for the provision of unsteady aerofoil data embedded in much larger codes. The underlying modeling philosophy was that it should be based on the best understanding, or description, of the associated physical phenomena. Even though the model was guided by the flow physics, it requires significant empirical inputs in the form of measured coefficients and constants. Beddoes provided these for a Mach number range of 0.3–0.8. This paper considers one such input for a Mach number of 0.12, where, from the Glasgow data, it is shown that the current stall-onset criterion, and subsequent adjustments, yield problematic results. A new stall criterion is proposed and developed in the best traditions of the model. It is shown to be very capable of reconstructing the Glasgow's data for stall onset both the ramp-up and oscillatory tests
Control of rotorcraft retreating blade stall using air-jet vortex generators
No full textA series of low-speed wind tunnel tests were carried out on an oscillating airfoil fitted with two rows of air-jet vortex generators (AJVGs). The airfoil used had an RAE 9645 section and the two spanwise arrays of AJVGs were located at x/c=0.12 and 0.62. The devices and their distribution were chosen to assess their ability to modify/control dynamic stall; the goal being to enhance the aerodynamic performance of helicopter rotors on the retreating blade side of the disc. The model was pitched about the quarter chord with a reduced frequency
(k) of 0.1 in a sinusoidal motion defined by a=15o+10sin_ t. The measured data indicate that, for continuous blowing from the front row of AJVGs with a momentum blowing coefficient (C μ) greater than 0.008, modifications to the stalling process are encouraging. In particular, the pitching moment behavior exhibits delayed stall and there is a marked reduction in the normal force hysteresis
Bifurcation analysis of surge and rotating stall in the Moore-Greitzer compression system
No full textA simple compression system model, described by a set of three ordinary nonlinear differential equations (the Moore-Greitzer model) is studied using bifurcation analysis to give a qualitative understanding of the presence of surge and rotating stall. Firstly, three parameter values are chosen and a reduced planar system is studied to detect the local bifurcations of pure surge modes. The global bifurcation diagrams are then completed with the help of the continuation software AUTO. A special feature of this 2D system is a set of parameter values where two Takens-Bogdanov points merge. As a next step, the interaction of surge and rotating stall modes is analysed using the same branch tracking technique. Several novel bifurcation scenarios are described. Two-parameter bifurcation maps are computed and a satisfactory agreement with experimental results is found. An explanation is given for the onset of deep surge, rotating stall, classic surge and the hysteresis effects experienced in measurements
Effect of dynamic stall on the aerodynamics of vertical-axis wind turbines
No full textAccurate simulations of the aerodynamic performance of vertical-axis wind turbines pose a significant challenge for computational fluid dynamics methods. The aerodynamic interaction between the blades of the rotor and the wake that is produced by the blades requires a high-fidelity representation of the convection of vorticity within the wake. In addition, the cyclic motion of the blades induces large variations in the angle of attack on the blades that can manifest as dynamic stall. The present paper describes the application of a numerical model that is based on the vorticity transport formulation of the Navier–Stokes equations, to the prediction of the aerodynamics of a verticalaxis wind turbine that consists of three curved rotor blades that are twisted helically around the rotational axis of the rotor. The predicted variation of the power coefficient with tip speed ratio compares very favorably with experimental measurements. It is demonstrated that helical blade twist reduces the oscillation of the power coefficient that is an inherent feature of turbines with non-twisted blade configurations
On the effects of leading edge vortex generators on an OA209 airfoil
No full textLeading edge vortex generators have been found to significantly increase the aerodynamic performance of an airfoil under dynamic stall conditions. However, the principle of operation of these devices is still unclear. Therefore static wind and water tunnel experiments as well as CFD simulations have been conducted on a rotary aircraft wing profile OA209. A POD analysis applied to the vector fields generated by PIV measurements showed that the vortex generators break larger
flow structures into small scale formations. They are able to decrease the impact of the dynamic stall vortex and decrease the undesirable aerodynamic fluctuations associated with dynamic stall
Numerical investigation of recess casing treatments in axial flow fans
No full textT he casing treatment technique for the axial fan has never been more significant
since its potential applications were recognized in gas turbines, tunnel ventilation
and many other industrial applications where the axial fan would benefit from the
casing treatment. In the last two decades experimental investigations were carried
out at Cranfield University to examine the influence of recess casing treatment on
stall margin, operating efficiency and flow field of a low-speed axial flow fan. They
showed more than 50% improvement in the stall margin with a negligible loss in the
efficiency. However, a little work has been done on the numerical simulation of casing
treatments due to its complexities, even though in recent years computational fluid
dynamics [CFD] analysis has been very active in the prediction of various phenomena
in turbomachinery.
This work presents numerical investigation of flow in a single axial-flow fan with
and without recess casing treatment. It involves the detailed effect of the recess
casing on stall margin improvement as well as its influence on global performance
parameters. The project offers a contribution to the understanding of the physical
processes occurring when approaching stall and the working mechanism by which
recess casing treatments improve stall margin. A Reynolds-Averaged Navier-Stokes
CFD code was used for the analysis using steady and unsteady simulations. The
numerical investigation of the overall performance, efficiency and work-input characteristics
of the fan were found to agree very well with the previously reported
experimental results.
The effect of casing treatment was investigated using two types of configurations,
vaneless and vaned casing. The vaneless casing treatment produced a sizeable stall
margin improvement with a measurable loss in both pressure rise and efficiency. The
recess was fitted later with vanes and was shown to offer both a further stall margin
improvement and an increase in the pressure rise coefficient without any significant
drop in efficiency at design conditions. The effect of number of vanes inside the
recess was also investigated by doubling and halving the number of vanes originally
adopted. The predicted results highlighted the importance of the vane inside the
casing. Unsteady simulations for the fan with solid and treated casing were carried out.
The solid casing simulated for a single blade passage as well as for the entire fan
containing all 27 blades highlighted the flow physics of the tip stall growth process,
as a large amount of radial flow injected from the hub at the blade suction side near
the trailing edge towards the outer casing and occupy this through a mechanism of
radial low momentum flow transport. This transport process is the main contributor
to the very large separation observed in the shroud region in addition to the locally
induced separation due to high blade loading and tip clearance. Although the examination
of the unsteady simulation of the recess treatment cavities does not offer
an image of large scale unsteady activity at the flow condition investigated, this
is on itself quite significant and enables the drawing of an important conclusion
namely that large casing treatments rely primarily on a steady-state flow process.
The corollary of this conclusion is of course that a steady-state simulation should
then be sufficient to capture the essential features of the recess treatment
Transitional separation bubbles and unsteady aspects of aerofoil stall
No full textA time-accurate solution method for the coupled potential flow and integral boundary-layer equations is used to study aerofoils near stall, where laboratory experiments have shown high-amplitude low-frequency oscillations. The laminar-turbulent transition model incorporates an absolute instability formulation, which allows the transition process in separation bubbles to be sustained in the absence of upstream disturbances, in agreement with recent direct numerical simulations. The method is demonstrated to capture large scale flow oscillations with Strouhal numbers and amplitudes comparable to experiments. The success of this particular physical model suggests that bubble bursting is primarily due to a potentialflow/ boundary-layer interaction effect, in which relatively simple models of boundary layer transition and turbulence suffice to describe the key phenomena
Exploring values, context and perceptions in contingent valuation studies: the CV Market Stall technique and willingness to pay for wildlife conservation.
No full textPublic preferences for conservation and environmental management may be identified in willingness to pay (WTP) studies. Normally part of a contingent valuation exercise, WTP studies elicit monetary estimates of non-market economic goods. This paper describes a new approach to WTP, the CV Market Stall, a technique that adds a discursive, qualitative dimension to contingent valuation. It is suggested that the CV Market Stall technique is a good method for exploring attitudes and responses to environmental project proposals. The flexible format, with an emphasis upon information provision, discussion and learning would also allow contingent valuation to be extended to much more complex and uncertain environmental issue
Pitching Airfoil Boundary Layer Investigations
No full textThe present paper describes an experiment performed in a transonic wind tunnel facility where a new test section has been developed especially for the investigation of the unsteady flow above oscillating airfoils under dynamic stall conditions. Dynamic stall is characterized by the development, movement and shedding of one or more concentrated vortices on the airfoils upper surface. The hysteresis loops of lift-, drag- and pitching moment are highly influenced by these vortices. To understand the very complicated unsteady flow involved, a detailed knowledge of the instantaneous flow fields is of crucial importance. With the application of the described measuring techniques it is expected to gain more insight into the problem. Results from these tests are ready for comparison with numerical data
Physical analysis of the coherent motion during hysteretic static stall of Naca foil using proper orthogonal decomposition
No full textAt static stall of airfoil, an hysteretic behavior (corresponding to an abrupt loss of lift) has been observed leading to a partially detached flow (called state I, upper increasing angle branch) and massively separated flow (called state II, lower decreasing angle branch). In order to predict the type of hysteretic loop that may occur or to quantify unsteady oscillatory flow in the near wake, experiments have been performed to explore laminar separation, transition and detachment of the boundary layer (Mittal and Saxena, 2000) but the keyphysical properties of the vortex shedding establishment during hysteretic loop is not clearly understood
- …
