Search results

This paper aims to propose a precise turbulence model for vehicle aerodynamics, especially for vehicle window buffeting noise.

Aiming at the fact that commonly used turbulence models cannot precisely predict laminar-turbulent transition, a transition-code-based improvement is introduced. This improvement includes the introduction of total stress limitation (TSL) and separation-sensitive model. They are integrated into low Reynolds number (LRN) k-ε model to concern transport properties of total stress and precisely capture boundary layer separations. As a result, the ability of LRN k-ε model to predict the transition is improved. Combined with the constructing scheme of constrained large-eddy simulation (CLES) model, a modified LRN CLES model is achieved. Several typical flows and relevant experimental results are introduced to validate this model. Finally, the modified LRN CLES model is used to acquire detailed flow structures and noise signature of a simplified vehicle window. Then, experimental validations are conducted.

Current results indicate that the modified LRN CLES model is capable of achieving acceptable accuracy in prediction of various types of transition at various Reynolds numbers. And, the ability of this model to simulate the vehicle window buffeting noise is greater than commonly used models.

Based on the TSL idea and separation-sensitive model, a modified LRN CLES model concerning the laminar-turbulent transition for the vehicle window buffeting noise is first proposed.

A Paper, describing the Research work and flight testing that went into the first American jet‐propelled fighter aeroplane, prepared by Mr Clarence L. Johnson, Chief Research Engineer, the Lockheed Aircraft Corporation, and presented before the Fifteenth Annual Meeting of the Institute of Aeronautical Sciences, New York, January 1947.

The purpose of this paper is to identify the implications of managerial herding for investors’ wealth and capital allocation across funds, and the critical role played by fund governance in monitoring herding incentives.

The author adopt the fund herding measure first proposed by Grinblatt et al. (1995) over the long sample period 1992-2007. Univariate and multivariate tests are then constructed to examine the relationship between managerial herding, performance, and investors’ sensitivities. OLS, fixed-effect panel data models are utilized to conduct the tests.

The author show that managers that do not herd have above-average managerial skills, trade less on noise, and significantly outperform herding managers. The author also illustrate that although fund herding could be used as a signal of managerial quality, underperforming herding funds manage to survive in equilibrium, indicating that investor flows do not adequately respond to the information content of a persistent herding behavior. Finally, the author demonstrate that better governance in the form of stronger managerial incentive schemes constitutes a significant deterrent against detrimental herding strategies, representing an effective monitoring device of the response of fund managers to poor flow-performance sensitivity.

The paper provides original evidence on the efficacy of external and internal governance in deterring wealth-reducing herding strategies. The author document that where more effective managerial incentives schemes are put in place by the management companies, fund managers are more likely to be better informed, resulting in fewer incentives to mimic the trading decisions of their peers.

The purpose of this paper is to identifying ways to reduce the effects of wing-vortex interaction by applying surface porosity on selected areas of the exposed surface. A number of papers recently have investigated the aerodynamic implication of free-stream vortices impinging upon airfoils.

The free-stream disturbance in these studies were represented by planting a vortex ahead of the wing or using some other disturbance invoking mechanism like von-Karman vortices in the wake of a cylinder or using a flipping plate to invoke a discrete vortex. In the present work, a well-defined method was used to germinate a system of controlled vortices of known strength, size and frequency ahead of the wing, and the impact of the subsequent interaction was studied with and without the presence of the surface porosity. The simulations tackled a number of cases when porosities of up to 20 and 22 per cent were applied to selected regions near the leading edge, with vortices of controlled strengths directed at the wing surface.

The results showed that the effects of large vortices spanning the entire lengths of the wing can indeed be damped when porosity is selectively applied at strategic regions.

Surface porosity application at strategic regions of a wing may dampen the effects of the unsteadiness of the incoming flow. This has profound implications on flight safety and structural damage prevention. Further implications could possibly be extended to UAV and wind turbines that operate at heavy gusting environment.

Implementation of this particular method resolves some of the issues arisen when an airplane encounters atmospheric turbulence.

THE Accidents Investigation Sub‐Committee of the Aeronautical Research Committee has issued a detailed technical report on the accident to the Junkers F.13‐type aeroplane G‐AAZK which occurred at Meopham, Kent, on July 21, 1930. The report, which fills ninety‐two pages, gives a complete account of the researches and technical investigations that were made at the instigation of the Sub‐Committee, much of which is of great technical interest. It is impossible here to do more than give a brief summary of the circumstances of the accident and the inquiries which led to the rejection of a number of theories of the cause, leading to the final conclusion that it was due to a phenomenon called “Buffeting,” which is defined as “an irregular oscillation of the tail unit, in which the tail‐plane bends rapidly up and down and the elevators move in an erratic manner.” It is caused by the eddies given off by the wings at large angles of incidence and is, the Sub‐Committee state, quite distinct from flutter, which, in the case of machines of the Junkers F.13‐type would develop only at speeds above 250 m.p.h.

During a routine benchmarking and scalability study of CFD codes for typical large-scale wind engineering runs, it was observed that the resulting loads for buildings varied considerably with the number of parallel processors employed. The differences remained very small at the beginning of a typical run, and then grew progressively to a state of total dissimilitude. A “butterfly-effect” for such flows was suspected and later confirmed. The paper aims to discuss these issues.

A series of numerical experiments was conducted for massively separated flows. The same geometry – a cube in front of an umbrella – was used to obtain the flowfields using different grids, different numbers of domains/processors, slightly different inflow conditions and different codes.

In all of these cases the differences remained very small at the beginning of a typical run, they then grew progressively to a state of total dissimilitude. While the mean and maximum loads remained similar, the actual (deterministic) instantiations were completely different. The authors therefore suspect that for flows of this kind a “butterfly effect” is present, whereby even very small (roundoff) errors can have a pronounced effect on the actual deterministic instantiation of a flowfield.

This implies that for flows of this kind the CFD runs have to be carried out to much larger times than formerly expected (and done) in order to obtain statistically relevant ensembles.

For practical calculations this implies running to much larger times in order to reach statistically relevant ensembles, with the associated much higher CPU time requirements.

This is the first time such a finding has been reported in the numerical wind engineering context.

Aircraft noise is dominant for residents near airports when planes fly at low altitudes such as during departure and landing. Flaps, wings, landing gear contribute significantly to the total sound emission. This paper aims to present a passive flow control (in the sense that there is no power input) to reduce the noise radiation induced by the flow over the cavity of the landing gear during take-off and landing.

The understanding of the noise source mechanism is normally caused by the unsteady interactions between the cavity surface and the turbulent flows as well as some studies that have shown tonal noise because of cavity resonances; this tonal noise is dependent on cavity geometry and incoming flow that lead us to use of a sinusoidal surface modification application upstream of a cavity as a passive acoustics control device in approach conditions.

It is demonstrated that the proposed surface waviness showed a potential reduction in cavity resonance and in the overall sound pressure level at the majority of the points investigated in the low Mach number. Furthermore, optimum sinusoidal amplitude and frequency were determined by the means of a two-dimensional computational fluid dynamics analysis for a cavity with a length to depth ratio of four.

The noise control by surface waviness has not implemented in real flight test yet, as all the tests are conducted in the credible numerical simulation.

The application of passive control method on the cavity requires a global aerodynamic study of the air frame is a matter of ongoing debate between aerodynamicists and acousticians. The latter is aimed at the reduction of the noise, whereas the former fears a corruption of flow conditions. To balance aerodynamic performance and acoustics, the use of the surface waviness in cavity leading edge is the most optimal solution.

The proposed leading-edge modification it has important theoretical basis and reference value for engineering application it can meet the demands of engineering practice. Particularly, to contribute to the reduce the aircraft noise adopted by the “European Visions 2020”.

The investigate cavity noise with and without surface waviness generation and propagation by using a hybrid approach, the computation of flow based on the large-eddy simulation method, is decoupled from the computation of sound, which can be performed during a post-processing based on Curle’s acoustic analogy as implemented in OpenFOAM.

Under this heading are published regularly abstracts of all Reports and Memoranda of the Aeronautical Research Council, Technical Reports and Translations of the United States National Aeronautics and Space Administration and publications of other similar Research Bodies as issued.

WITH the introduction of high‐powered propulsion systems, and paralleling their continued development, an accompanying increase in acoustical problems has arisen.

THE Deutsche Versuchsanstalt für Luft‐fahrt—known for short as the D.V.L.—at Berlin‐Adlershof, is the central establishment for aeronautical research in Germany. Hitherto the D.V.L. had as its special prerogative the testing of aircraft and aero‐engines for certificates of airworthiness. Since April 1, 1933, the Inspection Department has passed under the authority of the “Reichsamt für Flugsicherung” (Air Security Board), but it has not actually been separated from the D.V.L., continuing to have its premises at the D.V.L. establishment. (Further details of the Inspection Department are given later.)