Proposal for Future Work

The information obtained in this work does not give an indication of whether or not the turbulent event produced by a particle’s impact would eventually result in a fully turbulent boundary layer. Due to the nature of the experiment, namely determining whether or not a turbulent-spot-like flow structure is generated by a particle passing through an otherwise laminar shear layer, the measurement location was chosen as close as possible to the particle’s impact. The gathered data shows that this is insufficient to detect in what way the particle, when travelling through the laminar boundary layer, influences its “natural” transition to a intrusiveness of potential measurement equipment located further upstream.

Furthermore, two modifications are proposed as an upgrade to the current setup:

(1) A means should be provided to relate all particle impact measurements to a common time scale as this could alleviate the post-processing procedures considerably.

(2) In addition, a means should be provided to temporarily hold the rotation of the particle in order to allow for the flow field to recover its original state before a subsequent particle impact occurs.

On the other hand, it should be noted that the proposed method is quite suitable to investigate the influence of particle flux.

Having established a critical particle Reynolds number of a smooth spherical particle for a turbulent event to be generated, subsequent experiments should also include investigations on cylindrical particles of the dimensions as proposed by Hall [7], namely a length-to-diameter ratio, L/D, of 2.5, as this would provide for useful correlation of laboratory results to those partially derived from flight tests. In a final step, particle geometries that are more representative of actually occurring ice crystal shapes should be explored. It is worth noting this regarding that such experiments have already begun. Dabadie & Coisnon (2011) [15]

investigated the effect of both a cylinder (L/D ~ 3.3) and a sphere of the same diameter (6.0 mm) at otherwise identical experimental conditions on the shape factor of the local boundary layer profile. While both values were affected, the influence of the cylinder was considerably larger approaching values that are commonly considered turbulent.

This is expected, since also in a uniform flow filed, the critical value of a cylinder is considerably smaller, especially, when not being of infinite length.

It should be noted that the RAA may have the potential for a water channel experiment.

This would allow for an additional increase of the experiment’s dimensions at further reduced speeds, and thus for considerable alleviations with regard to applied measurement techniques and optional flow visualisation.

6 Conclusions

(1) The information on the mechanisms leading to the transition of a laminar boundary layer due to a small particle travelling through it is incomplete.

(2) The previously proposed critical particle parameters above which transition is triggered within the affected boundary layer have not been experimentally verified until today.

(3) Conventional laboratory methods are not likely to capture all facets of the naturally occurring mechanisms without appreciable efforts.

(4) An alternative method, which was developed by the authors specifically for this purpose and which is quite simple in terms of its complexity, has been successfully employed in this study.

(5) A particle travelling through an initially laminar boundary layer is capable of producing a turbulent event, which closely resembles the characteristics of a turbulent spot when compared to previously contamination originating from the particle’s wake above certain critical conditions.

(7) The critical Reynolds number of a smooth spherical particle for a turbulent event to be produced within the affected laminar boundary layer has been determined to be approximately in the order of 300.

Acknowledgments

This study has only been possible by cooperation with the University of Glasgow, who kindly provided their wind tunnel facility. effectiveness of hybrid laminar flow control aircraft”, PhD Thesis, Cranfield University, UK, 2002.

[3] Boeing, “Boeing 787 Dreamliner Livery Change Enhances Airplane Performance”, press release, Seattle, 10th Jul 2006, accessed 28th Feb 2013:

http://www.boeing.com/news/releases/2006/q3/0 60710d_nr.html.

[4] Young, T.M., “An investigation into potential fuel savings for 110–130 seat passenger transport aircraft due to the incorporation of natural laminar flow or hybrid laminar flow control on the engine nacelles”, J. Aero. Eng., Vol. 227(8), 2013. Flow Control Aircraft Operations, CEAS Aerospace Aerodynamics Research Conference, Cambridge, Paper 58, 10-12 June 2002.

[7] Hall, G.R., “On the Mechanics of Transition Produced by Particles Passing Through an Initially Laminar Boundary Layer and Estimated Effect on the Performance of X-21 Aircraft”, Northrop Corp., Contract-No.: N79-70656, 1964.

[8] Davis, R.E., Maddalon, D.V., Wagner, R.D.,

“Performance of Laminar Flow Leading Edge Test Articles in Cloud Encounter”, NASA CP-1987-2487, Langley Research Center, USA, 1987. Stream Particles,” 12th AIAA ATIO Conference, 17th-19th Sep, Indianapolis, USA, 2012.

[11] Schmidt, C. & Young, T.M. The Impact of Cirrus Cloud on Laminar Flow Technology, 7th AIAA Aviation Technology, Integration and Operations Conference (ATIO), 18-20 Sep, Belfast, Northern Ireland, 2007.

[12] Schmidt, C. & Young, T.M., “The impact of freely suspended particles on laminar boundary layers”, 47th AIAA Aerospace Sciences Meeting (ASM), 5-8 Jan, Orlando, Florida, USA, 2009.

[13] Schmidt C., “Laminar Flow Breakdown due to Particle Interaction”, AFRL-AFOSKR-UK-TR-2012-0035, Final Report, Contract-No.: EOARD 11-3044, 2012.

Conny Schmidt, Trevor M. Young, Emmanuel P. Benard, Lei Zhao

[14] Schmidt, C., Young, T.M., Benard, E.P., Atalay, S., “The influence of cirrus cloud on drag reduction technologies based on laminar flow”, CEAS/KAT net II Conference on Key Aerodynamic Technologies, 12th-14th May, Bremen, Germany, 2009.

[15] Dabadie, C., Coisnon, R., “Etude de la transition par particule”, Institut Supérieur de l'Aéronautique et de l'Espace, Dép.

Aérodynamique, Energétique et Propulsion, Toulouse, France, 2011.

[16] Schlichting, H. “Boundary Layer Theory”, 7th Edition, McGraw-Hill Book Company, 1979.

[17] Cantwell, B., Donald, C., Dimotakis, P.,

“Structure and entrainment in the plane of symmetry of a turbulent spot”, , J. Fluid Mech., Vol. 87, 1978, part 4, pp. 641-672.

[18] Zhao, L., “Experimental study of laminar to turbulent transition effect by flying particles”, MSc thesis, Dep. Aerospace Engineering, University of Glasgow, Glasgow, UK, 2010.

[19] Hall, G.R., “Interaction of the wake from bluff bodies with an initially laminar boundary layer”, AIAA Journal, Vol. 5, No. 8, pp. 1386-1392, 1965.

[20] Hall, G.R., “Interaction of the wake from bluff bodies with an initially laminar boundary layer”, AIAA-1966-126, Aerospace Sciences Meeting, 3rd, New York, N.Y., Jan 24-26, Paper 66-126, 1966.

[21] Petrie, H.L., Morris, P.J., Bajwa, A.R., Vincent, D.C., “Transition induced by Fixed and Freely Convecting Particles in Laminar Boundary Layers”, The Pennsylvania State University, Appl. Res. Lab., PA, USA, 1993.

[22] Sakamoto, H. & Haniu, H., “The formation mechanism and shedding frequency of vortices from a sphere in uniform shear flow”, J. Fluid Mech., Vol. 287, 1995, pp. 151-171.

[23] Ladd, D.M., Hendricks, E.W., “The effect of background particulates on the delayed transition of a heated 9:1 ellipsoid”, Exp. in Fluids, Vol. 3, 1985, pp. 113-119.

Abstract

Landing gear is known to be one of the main contributors to the noise generated from an aircraft on approach. The scope of this work is to carry out an initial exploratory investigation using an isolated wheel to aid in the understanding of the aerodynamics around a landing gear. The isolated wheel considered in this paper was modelled in free air initially to provide a baseline and thereafter with decreasing ground proximity. The effect of Yaw and Turbulence Intensity (Tu) with decreasing ground proximity were also modelled to understand how these variables affect the aerodynamic flow field around the wheel.

1 Introduction

The effect of ground proximity on the aerodynamics of an aircraft’s landing gear is a complex problem which has received little attention in the aircraft industry. Landing gear is a major contributor to the noise levels generated from aircraft on approach and results from unsteady variations in the flow field as the air interacts with the exposed components of the gear. The resultant affect of this noise produces significant disruption and discomfort to the many millions of people who live in the vicinity of airports.

One of the first studies to consider wheel aerodynamics was conducted by Fackrell [1]

who performed an experimental investigation using an isolated wheel in contact with the ground. This study which used a moving floor on a range of wheel profiles, showed that less drag is produced from an isolated rotating wheel than from a stationary wheel in contact with the ground. Local areas of separation over the top of the wheel were also found. The rotating wheel configuration in contact with the ground was also found to produce a much higher pressure at the upstream area of contact patch beneath the wheel. Based on [1], McManus and Zhang [2]

also carried out a computational study for a wheel in contact with the ground. This study was focused on the same wheel geometry in [1]

with lift and drag force coefficients of CD=0.48 and CL=0.35 being obtained from a stationary wheel in contact with the ground. Areas of separation were also identified on the top rear shoulders of the wheel as well as counter-rotating vortices on the lower half of the wheel.

These vortices were thought to be caused by the interaction between the flow over the top of the wheel and the main flow emerging from the sides. For the case of the rotating wheel, the flow also separated near the top of the wheel subsequently forming an arch shaped vortex.

Additionally for this configuration, the flow travelling from under the front of the wheel in contact with the ground formed another separated region, but this region was found to be smaller for the rotating wheel than the stationary wheel. More recently an experimental study was carried out by Zhang et al[3] on a single wheel