This study investigates how a particle travelling through an initially laminar boundary layer can lead to its breakdown to turbulence With increasing kerosene costs and an awareness of limited available oil reserves, laminar flow technologies are again being considered to realize the necessary efficiency increases of aircraft, and more detailed information on the operational issues is required. The adverse impact of flying through cirrus clouds has been simplified to the effect of a single particle on a laminar boundary layer over a zero-pressure gradient flat plate. First results indicate that the critical values could be substantially smaller than initially assumed

B Cantwell

DM Ladd

GC Lauchle

HL Petrie

English

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The Effect of a Particle travelling through a Laminar Boundary Layer on Transition

Schmidt, Conny

Young, Trevor M.

Bénard, Emmanuel

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BenardAbstract This study investigates how a particle travelling through an initiallylaminar boundary layer can lead to its breakdown to turbulence. With increasingkerosene costs and an awareness of limited available oil reserves, laminar flow tech-nologies are again being considered to realize the necessary efficiency increases ofaircraft, and more detailed information on the operational issues is required. Theadverse impact of flying through cirrus clouds has been simplified to the effect of asingle particle on a laminar boundary layer over a zero-pressure gradient flat plate.First results indicate that the critical values could be substantially smaller than ini-tially assumed.1 BackgroundLaminar flow technologies, which could substantially contribute to efficiency in-creases required for air transportation in order to cope with forecasted growth andsimultaneously limit the environmental impact due to air traffic, have been foundto be non-functional while flying through cirrus cloud. The first recognition of thisphenomenon was made by Northrop during its X-21 flight test program, which in-vestigated a full chord suction type Laminar Flow Control (LFC) system in the early1960s. Laminar flow was entirely lost when entering thick cirrus clouds and evenslightly degraded when light haze (leaving visibilities of up to 80 km) surroundedthe aircraft.Conny SchmidtUniversity of Limerick, Plassey, Co. Limerick, Ireland, e-mail: conny.schmidt@ul.ieTrevor M. YoungUniversity of Limerick, Plassey, Co. Limerick, Ireland, e-mail: trevor.young@ul.ieEmmanuel P. BenardUniversity of Glasgow, Glasgow G12 8QQ, Scotland, UK, e-mail: ebenard@eng.gla.ac.ukThe effect was attributed to the ability of the ice crystals contained in cirrusto produce turbulence within their wakes while travelling through the artificiallymaintained laminar boundary layer. Subsequent theoretical analysis in conjunctionwith the available flight test data resulted in the so-called “Hall criteria” [3], whichis presented in Figure 1 and suggests that there exist both a minimum ice crystaldiameter and a minimum crystal flux above which laminar flow is progressivelydegraded with increasing ice particle number concentration. Further dependenciesproposed were air density and viscosity (thus temperature or altitude), flight speed,leading edge geometry and the shape of the ice particle being investigated [2].Fig. 1 The “Hall Crite-ria”, valid for an altitudeof 40,000ft, a Mach numberof 0.75 and a cylindrical (ice)particle approximation witha length-to-diameter ratio of2.5, redrawn from [2]When correlating the effect to results from other experimental studies found inthe literature [4-7], additional parameters must be taken into account, especially ifdifferent flow media and particle materials are engaged. The density ratio betweenthe particulate and the continuous phase, for instance, is believed to play a role;however, this still might not complete the picture.Correspondingly, Ladd & Hendricks [4] assumed that the particles would tem-porarily stick to the surface acting as a roughness element. Petrie et al. [7] suspectedthe observed post-impingement spanwise fluctuations as representing a parameterwhich could have initiated transition. Furthermore, the effect of the no-slip con-dition on the particle itself, which most likely causes a temporary deformation ofthe velocity profile of the laminar boundary layer investigated, found to the authors’knowledge only very little consideration until today. Despite the values ranging from450 to 800, the order of magnitude of the critical Reynolds number for a sphericalparticle to trigger a turbulent event seems to be generally agreed.2 Experimental WorkSchmidt & Young [8] have shown that the effect can be re-created in a laboratorywind tunnel environment. Both the double pitot measurement approach as well assurface mounted hot films were capable of detecting premature transition caused bya particle cloud dispersed into the free stream; whereas, the latter was seen to pro-duce more accurate answers. Furthermore, these preliminary experiments indicatedthat substantially smaller particles than previously assumed could cause turbulentevents to be triggered.In order to gain a deeper insight into the problem, the flow field alterations causedby a single, freely-suspended, spherical particle entering a laminar boundary layerover a flat plate in low speed zero-pressure gradient conditions are under investiga-tion. As it was suspected that the vertical component of a freely falling particle couldexceed critical conditions before entering the boundary layer, a pendulum approachhas been applied to the particle to elude this problem. Both hot wire traverses andsurface mounted hot film measurements have been taken downstream of the regionimpacted by the particle.Fig. 2 Hot film data, showingturbulent structures in thesignal taken downstream ofthe particle impact at variousReynolds numbersThe hot film data obtained support the earlier speculation that the Reynolds num-ber for a particle to contaminate laminar flow could be considerably lower in com-parison to information published previously, as turbulent structures are apparent forReynolds numbers as low as 330 (see Figure 2). No effect could be determined fromthe hot wire traverses outside the laminar boundary layer, showing that the pen-dulum approach succeeded to suppress critical conditions within the free stream.Furthermore, when assembling the measurement curves of a traverse into a singlefigure, similar trends were evident compared to results published by Cantwell et al.[1] for an artificially generated turbulent spot.3 Numerical WorkIn order to analyse in more detail the differences of the critical values obtainedduring this work when compared to previously published information, a numericalstudy has been done in which the conditions of the experimental work of Petrie etal. [7] have been re-created.It was found that the streamwise velocity component of the near-neutrally buoy-ant spherical particles used adapted almost instantaneously to changes in the flowfield within the boundary layer when entering it. Thus, the major contribution to acritical particle Reynolds number originated from the particle’s transverse velocitycomponent, which in most cases had achieved its terminal value and was in the orderof the free stream speed. As such these particles were supercritical before reachingthe boundary layer and developed their wake in a nearly vertical direction. Thismight explain why the corresponding vortices could not be observed in the flowvisualisation of the original study [7]. Interestingly, in this numerical analysis, thelowest particle Reynolds number of all considered cases, for which the generationof a turbulent spot had been reported originally, was in the order of 300.4 Concluding RemarksThe information on how ice crystals occurring in cirrus cloud can lead to the tempo-rary loss of laminar flow technology’s benefits is not sufficiently complete to enablethe full potential of such systems to be realized in terms of optimized fuel planning.First experimental results give an indication that the Reynolds number, abovewhich a spherical particle impinging on a surface subjected by an air flow couldtrigger a turbulent event within a laminar boundary layer over that surface, is con-siderably smaller than previously thought. Whether this is due to the shear flowprevailing within the boundary layer or if other previously detected mechanismsplay an additional role is not clear at the moment and requires further investigation.Consideration of the flow field actually “seen” by the particle has led to the designof a new test facility, where the particles are held at a fixed position and the surfaceto be investigated is moved instead. This procedure, it is hoped, will provide foruseful information and circumvent the issues encountered when trying to re-createrealistic conditions in a laboratory experiment.References1. Cantwell, B., Donald, C., Dimotakis, P. Structure and entrainment in the plane of symmetryof a turbulent spot. J. Fluid Mech., Vol. 87, Part 4, 641–672 (1978)2. Davis, R. E., Maddalon, D.V., Wagner, R.D.: Performance of Laminar Flow Leading EdgeTest Articles in Cloud Encounter. NASA CP-1987-2487, Langley Research Center (1987)3. Hall, G.R.: On the Mechanics of Transition Produced by Particles Passing Through an Ini-tially Laminar Boundary Layer and Estimated Effect on the Performance of X-21 Aircraft.Northrop Corp., Contract-No.: N79-70656, (1964)4. Ladd, D.M., Hendricks, E.W.: The effect of background particulates on the delayed transitionof a heated 9:1 ellipsoid. Exp. in Fluids, Vol. 3, 113–119 (1985)5. Lauchle, G.C., Gurney, G.B.: Laminar Boundary Layer Transition on a Heated UnderwaterBody. J. Fluid Mech., Vol. 144, 79–101 (1984)6. Lauchle, G.C., Petrie, H.L., Stinebring, D.R.: Laminar Flow Performance of a Heated bodyin particle-laden water. Exp. in Fluids, Vol. 19, 305–312 (1995)7. Petrie, H.L., Morris, P.J., Bajwa, A.R., Vincent, D.C.: Transition induced by Fixed and FreelyConvecting Particles in Laminar Boundary Layers., The Pennsylvania State University, Appl.Res. Lab., PA, USA (1993)8. Schmidt, C. ,Young, T.M., The impact of freely suspended particles on laminar boundary lay-ers. 47th AIAA Aerospace Sciences Meeting (ASM), 5-8 Jan, Orlando, Florida, USA (2009)

D.C.: Transition induced by Fixed and Freely Convecting Particles in Laminar Boundary Layers., The Pennsylvania State University,

D.R.: Laminar Flow Performance of a Heated body in particle-laden water.

E.W.: The effect of background particulates on the delayed transition of a heated 9:1 ellipsoid.

G.B.: Laminar Boundary Layer Transition on a Heated Underwater Body.

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.

R.D.: Performance of Laminar Flow Leading Edge Test Articles in Cloud Encounter.

Structure and entrainment in the plane of symmetry of a turbulent spot.

The impact of freely suspended particles on laminar boundary layers.

The effect of a particle travelling through a laminar boundary layer on transition

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The effect of a particle travelling through a laminar boundary layer on transition

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