Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can supp ...

No AccessTechnical NoteStagnation Line Adjustment in Flat-Plate Experiments via Test-Section VentingMarco Ferro, Robert S. Downs III and Jens H. M. FranssonMarco FerroLinné Flow Centre, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden*Graduate Student, Linné Flow Centre, KTH Mechanics.Search for more papers by this author, Robert S. Downs IIILinné Flow Centre, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden†Postdoctoral Researcher, Linné Flow Centre, KTH Mechanics. Member AIAA.Search for more papers by this author and Jens H. M. FranssonLinné Flow Centre, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden‡Professor, Linné Flow Centre, KTH Mechanics; .Search for more papers by this authorPublished Online:21 Jan 2015https://doi.org/10.2514/1.J053603SectionsView Full TextPDFPDF Plus ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail About References [1] Tropea C., Yarin A. L. and Foss J. F. (eds.), Springer Handbook of Experimental Fluid Mechanics, Springer, New York, 2007, pp. 888–890, Chap. 12.3.4. CrossrefGoogle Scholar[2] Saric W. S., “Low-Speed Experiments: Requirements for Stability Measurements,” Instability and Transition, Springer, New York, 1990, pp. 162–174. CrossrefGoogle Scholar[3] Klingmann B. G. B., Boiko A. V., Westin K. J. A., Kozlov V. V. and Alfredsson P. H., “Experiments on the Stability of Tollmien–Schlichting Waves,” European Journal of Mechanics. B, Fluids, Vol. 12, No. 4, 1993, pp. 493–514. EJBFEV 0997-7546 Google Scholar[4] Hutchins N. and Marusic I., “Evidence of Very Long Meandering Features in the Logarithmic Region of Turbulent Boundary Layers,” Journal of Fluid Mechanics, Vol. 579, May 2007, pp. 1–28. doi:https://doi.org/10.1017/S0022112006003946 JFLSA7 0022-1120 CrossrefGoogle Scholar[5] Hafez S., Chong M., Marusic I. and Jones M., “Observations on High Reynolds Number Turbulent Boundary Layer Measurements,” Proceedings of the 15th Australasian Fluid Mechanics Conference, edited by Behnia M., Lin W. and McBain G., Univ. of Sydney Paper AFMC-00200, NSW, Australia, 2004. Google Scholar[6] Nagib H. and Hites M., “High Reynolds Number Boundary Layer Measurements in the NDF,” AIAA Paper 1995-0786, 1995. LinkGoogle Scholar[7] Chauhan K. A. and Nagib H. M., “On the Development of Wall-Bounded Turbulent Flows,” IUTAM Symposium on Computational Physics and New Perspectives in Turbulence, Springer, New York, 2008, pp. 183–189. CrossrefGoogle Scholar[8] Castillo L. and Walker D. J., “Effect of Upstream Conditions on the Outer Flow of Turbulent Boundary Layers,” AIAA Journal, Vol. 40, No. 7, 2002, pp. 1292–1299. doi:https://doi.org/10.2514/2.1818 AIAJAH 0001-1452 LinkGoogle Scholar[9] Chauhan K. A., Monkewitz P. A. and Nagib H. M., “Criteria for Assessing Experiments in Zero Pressure Gradient Boundary Layers,” Fluid Dynamics Research, Vol. 41, No. 2, 2009, Paper 021404. doi:https://doi.org/10.1088/0169-5983/41/2/021404 FDRSEH 0169-5983 CrossrefGoogle Scholar[10] Kay J. M., “Boundary-Layer Flow Along a Flat Plate with Uniform Suction,” Aeronautical Research Council Repts. and Memoranda 2628, Cambridge, MA, May 1948. Google Scholar[11] Bake S., Fernholz H. H. and Kachanov Y. S., “Resemblance of K-and N-Regimes of Boundary-Layer Transition at Late Stages,” European Journal of Mechanics. B, Fluids, Vol. 19, No. 1, 2000, pp. 1–22. doi:https://doi.org/10.1016/S0997-7546(00)00108-4 EJBFEV 0997-7546 CrossrefGoogle Scholar[12] Knobloch K. and Fernholz H. H., “Statistics, Correlations, and Scaling in a Turbulent Boundary Layer at Reδ2≤1.15×105,” IUTAM Symposium on Reynolds Number Scaling in Turbulent Flow, Springer, New York, 2004, pp. 11–16. CrossrefGoogle Scholar[13] Pook D. A. and Watmuff J. H., “Streak Generation in Wind Tunnels,” Physics of Fluids, Vol. 26, No. 7, 2014, Paper 073605. doi:https://doi.org/10.1063/1.4891180 CrossrefGoogle Scholar[14] Lindgren B. and Johansson A. V., “Evaluation of the Flow Quality in the MTL Wind-Tunnel,” Royal Inst. of Technology, TRITA-MEK-TR-2002:13, Stockholm, 2002. Google Scholar[15] Drela M., “XFOIL: An Analysis and Design System for Low Reynolds Number Airfoils,” Low Reynolds Number Aerodynamics, edited by Mueller T., Vol. 54, Lecture Notes in Engineering, Springer, Berlin, 1989, pp. 1–12. CrossrefGoogle Scholar[16] Fransson J. H. M., Matsubara M. and Alfredsson P. H., “Transition Induced by Free-Stream Turbulence,” Journal of Fluid Mechanics, Vol. 527, March 2005, pp. 1–25. doi:https://doi.org/10.1017/S0022112004002770 JFLSA7 0022-1120 CrossrefGoogle Scholar Previous article Next article

Over the last decade wind tunnel experiments and numerical simulations have shown that steady spanwise mean velocity gradients are able to attenuate the growth of different types of boundary layer disturbances if introduced in a controlled way. In this paper different techniques to setup the spanwise mean velocity variations are reviewed and their stabilizing effect leading to transition delay are quantified. This control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge.

Over the last decade wind tunnel experiments and numerical simulations have shown that steady spanwise mean velocity gradients are able to attenuate the growth of different types of boundary layer disturbances if introduced in a controlled way. In this paper some different techniques to setup the spanwise mean velocity variations are discussed and their stabilizing effect leading to transition delay are quantified. This control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge.

A novel type of surface roughness is deployed in a zero-pressure-gradient boundary layer with the goal of delaying the onset of laminar-to-turbulent transition for drag reduction purposes. This proof-of-concept experiment relies on forcing phase-triggered Tollmien-Schlichting (TS) waves across a range of initial amplitudes to produce amplified boundary-layer disturbances in a controlled and repeatable manner. Building on earlier work demonstrating attenuation of forced disturbances and delay of transition with spanwise arrays of discrete roughness and miniature vortex generators (MVGs), the present work seeks a roughness shape which might find success in a wider range of flows. Toward that end, streamwise-elongated humps are regularly spaced in the spanwise direction to form a wavy wall. By direct modulation of the mean flow, growth rates of the forced disturbances are increased or decreased, depending on the roughness configuration. Boundary-layer velocity measurements with hot-wire probes have been performed in a parametric study of the effects of roughness-field geometry and forcing amplitude on TS-wave growth and transition. The roughness field proves detrimental to passive flow control efforts in some configurations, while a reduction in the TS-wave amplitudes compared with the smooth-wall reference case is observed at other conditions. Substantial delays in the onset of transition are demonstrated when TS waves are forced with large amplitudes.

No AccessTechnical NotePassive Laminar Flow Control at Low Turbulence LevelsErica N. Lovig, Robert S. Downs and Edward B. WhiteErica N. LovigDepartment of Aerospace Engineering, Texas A&M University, College Station, Texas 77843*Undergraduate Research Assistant, Department of Aerospace Engineering. Student Member AIAA.Search for more papers by this author, Robert S. DownsDepartment of Aerospace Engineering, Texas A&M University, College Station, Texas 77843†Graduate Research Assistant, Department of Aerospace Engineering. Student Member AIAA.Search for more papers by this author and Edward B. WhiteDepartment of Aerospace Engineering, Texas A&M University, College Station, Texas 77843‡Associate Professor, Department of Aerospace Engineering; . Associate Fellow AIAA.Search for more papers by this authorPublished Online:22 Apr 2014https://doi.org/10.2514/1.J052363SectionsView Full TextPDFPDF Plus ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail About References [1] Kurian T., Fransson J. H. M. and Alfredsson P. H., “Boundary Layer Receptivity to Freestream Turbulence and Surface Roughness over a Swept Flat Plate,” Physics of Fluids, Vol. 23, No. 3, 2011, Paper 034107. doi:https://doi.org/10.1063/1.3562843. CrossrefGoogle Scholar[2] Saric W. S., Reed H. L. and White E. B., “Stability and Transition of Three-Dimensional Boundary Layers,” Annual Review of Fluid Mechanics, Vol. 35, Jan. 2003, pp. 413–440. doi:https://doi.org/10.1146/annurev.fluid.35.101101.161045 ARVFA3 0066-4189 CrossrefGoogle Scholar[3] Radeztsky R. H., Reibert M. S. and Saric W. S., “Effect of Isolated Micron-Sized Roughness on Transition in Swept-Wing Flows,” AIAA Journal, Vol. 37, No. 11, 1999, pp. 1370–1377. doi:https://doi.org/10.2514/2.635 AIAJAH 0001-1452 LinkGoogle Scholar[4] Reibert M. S., Saric W. S., Carrillo R. B. and Chapman K. L., “Experiments in Nonlinear Saturation of Stationary Crossflow Vortices in a Swept-Wing Boundary Layer,” AIAA Paper 1996-0184, 1996. LinkGoogle Scholar[5] Saric W. S., Carrillo R. B. and Reibert M. S., “Nonlinear Stability and Transition in 3-D Boundary Layers,” Meccanica, Vol. 33, No. 5, 1998, pp. 469–487. doi:https://doi.org/10.1023/A:1004368526215 Google Scholar[6] Carpenter A. L., Saric W. S. and Reed H. L., “Roughness Receptivity in Swept-Wing Boundary Layers: Experiments,” International Journal of Engineering Systems Modelling and Simulation, Vol. 2, No. 1/2, 2010, pp. 128–138. doi:https://doi.org/10.1504/IJESMS.2010.031877 CrossrefGoogle Scholar[7] Saric W. S., Carpenter A. L. and Reed H. L., “Passive Control of Transition in Three-Dimensional Boundary Layers, with Emphasis on Discrete Roughness Elements,” Philosophical Transactions of the Royal Society of London, Series A: Mathematical and Physical Sciences, Vol. 369, No. 1940, 2011, pp. 1352–1364. doi:https://doi.org/10.1098/rsta.2010.0368 PTRMAD 1364-503X CrossrefGoogle Scholar[8] Hunt L. E. and Saric W. S., “Boundary-Layer Receptivity of Three-Dimensional Roughness Arrays on a Swept-Wing,” AIAA Paper 2011-3881, 2011. LinkGoogle Scholar[9] Downs R. S., and White E. B., “Free-Stream Turbulence and the Development of Cross-Flow Disturbances,” Journal of Fluid Mechanics, Vol. 735, Nov. 2013, pp. 347–380. doi:https://doi.org/10.1017/jfm.2013.484 CrossrefGoogle Scholar[10] Muller B. and Bippes H., “Experimental Study of Instability Modes in a Three-dimensional Boundary Layer,” Fluid Dynamics of Three-Dimensional Turbulent Shear Flows and Transition, AGARD CP-438, 1989, pp. 1–15. Google Scholar[11] Hunt L. E., Downs R. S., Kuester M. S., White E. B. and Saric W. S., “Flow Quality Measurements in the Klebanoff-Saric Wind Tunnel,” AIAA Paper 2010-4538, 2010. LinkGoogle Scholar[12] Dagenhart J. R. and Saric W. S., “Crossflow Stability and Transition Experiments in Swept-Wing Flow,” NASA TP-1999-209344, 1999. Google Scholar[13] Downs R. S., Lovig E. N. and White E. B., “Experimental Investigation of the Crossflow Instability in Moderate Freestream Turbulence,” AIAA Paper 2012-2824, 2012. LinkGoogle Scholar Previous article Next article

Abstract The cross-flow instability that arises in swept-wing boundary layers has resisted attempts to describe the path from disturbance initiation to transition. Following concerted research efforts, surface roughness and free-stream turbulence have been identified as the leading providers of initial disturbances for cross-flow instability growth. Although a significant body of work examines the role of free-stream turbulence in the cross-flow problem, the data more relevant to the flight environment (turbulence intensities less than 0.07 %) are sparse. A series of recent experiments indicates that variations within this range may affect the initiation or growth of cross-flow instability amplitudes, hindering comparison among results obtained in different disturbance environments. To address this problem, a series of wind tunnel experiments is performed in which the free-stream turbulence intensity is varied between 0.02 % and 0.2 % of free-stream velocity, ${U}_{\infty } $ . Measurements of the stationary and travelling mode amplitudes are made in the boundary layer of a 1.83 m chord, $45{{}^\circ} $ swept-wing model. These results are compared to those of similar experiments at higher turbulence levels to broaden the current knowledge of this portion of the cross-flow problem. It is observed that both free-stream turbulence and surface roughness contribute to the initiation of unsteady disturbances, and that free-stream turbulence affects the development of both stationary and unsteady cross-flow disturbances. For the range tested, enhanced free-stream turbulence advances the transition location except when a subcritically spaced roughness array is employed.

The crossflow instability that arises in swept-wing boundary layers is sensitive to freestream turbulence and surface roughness. Although the effects of these disturbance sources comprise a wide body of existing research, there has been little work focusing on low to moderate levels of freestream turbulence (0.02% to 0.2%). Comparison of the results from low-turbulence wind tunnels suggests that freestream turbulence in this range may play a role beyond traveling mode initiation. Development of the stationary crossflow mode in moderate levels of freestream turbulence is measured in the boundary layer of a 45-degree swept wing. Compared with the baseline turbulence level of 0.02%, a reduction in the stationary disturbance amplitudes is observed for turbulence intensities as low as 0.05%. The uncertainties in these measurements are estimated using a Monte Carlo simulation approach.

A modified process yields lower levels of internal porosity for solder joints produced in reduced-gravity environments. The process incorporates both alternative materials and a modified procedure. The process provides the necessary cleaning action to enable effective bonding of the applied solder alloy with the materials to be joined. The modified process incorporates a commercially available liquid flux that is applied to the solder joint before heating with the soldering iron. It is subsequently heated with the soldering iron to activate the cleaning action of the flux and to evaporate most of the flux, followed by application of solder alloy in the form of commercially available solid solder wire (containing no flux). Continued heating ensures adequate flow of the solder alloy around and onto the materials to be joined. The final step is withdrawal of the soldering iron to allow alloy solidification and cooling of the solder joint.

Nomenclature U∞ = free-stream velocity U, V, W = streamwise, wall-normal, and spanwise velocities F = forcing function used in immersed boundary technique Udes = desired velocity α, β = constants used in immersed boundary technique k = maximum roughness height Rek = roughness based Reynolds number, (U(k)k/ν) Rex = Reynolds number Re’ = unit Reynolds number, U∞/ν ν = kinematic viscosity x, y, z = streamwise, wall-normal, and spanwise coordinates λk = spanwise roughness element spacing (32 mm) δ = boundary layer thickness, (x/Re’) 1/2

In-flight measurement of stationary crossflow vortices on 30-degree swept-wing models is described for two flight testing programs. Excitation of crossflow wavelengths is accomplished using spanwise arrays of Discrete Roughness Elements (DREs). Multielement, surface-mounted hotfilm sensor arrays are used for these measurements. As such measurements are typically limited by the small numbers of anemometer channels available on test aircraft, a novel approach for improving the spectral resolution of these measurements is developed and applied. The nominally stationary crossflow vortices actually meander in the spanwise direction during model yaw variations; removing these spanwise shifts from multiple instants in time produces densely sampled sets of data. A least-squares spectral analysis is applied to this unevenly spaced data to produce well-resolved wavelength spectra. The results of this spectral analysis are discussed in the context of linear stability theory calculations and the effectiveness of DREs at exciting crossflow disturbances is evaluated.

Computing amplitudes of periodic components in a measured signal is commonly encountered in data analysis. When this process is hampered by low-resolution data, it is sometimes possible to exploit certain qualities of the data to mitigate these limitations. In this work, spectral analysis of crossflow vortices measured in flight tests using multi-element hotfilm sensors is accomplished despite restrictive sensor counts. The vortices are nominally steady but subject to randomly changing phase shifts that can be computed to form well-resolved sets of data. The reliability and efficiency of this analysis are tested via Monte Carlo simulation and the uncertainties in detected wavelengths are quantified. This analysis technique is applied to in-flight measurements of crossflow instabilities in swept-wing boundary layers.

The Klebanoff-Saric Wind Tunnel is a low-speed, closed-return facility with lowdisturbance flow capabilities suitable for boundary-layer stability and transition studies. Previously known as the Arizona State University (ASU) Transition Facility or the ASU Unsteady Wind Tunnel, the tunnel was relocated to Texas A&M University in 2005. During its subsequent reconstruction, several component modifications were introduced, including more advanced acoustic treatments, pneumatic isolation, duct reshaping, and motor-drive alterations, to further enhance flow quality and experimental control. Details regarding the tunnel capabilities, flow quality improvements and tunnel calibration are provided. Freestream turbulence and acoustic measurements are given.

Boundary layer receptivity to physically realizable, sub-optimal disturbances has attracted research efforts using theoretical, experimental and numerical tools. Progress requires these approaches be used in a complementary fashion and reach mutual agreement. The present work compares results from each on the sub-optimal transient growth due to surface roughness. New experimental error analyses and comparison to direct numerical simulation (DNS) is shown for three-dimensional discrete roughness elements. Biorthogonal decomposition of DNS results to obtain the continuous spectrum distribution is presented and contrasted with optimal and linear receptivity cases. Realizable disturbances are shown to be sub-optimal and to evolve linearly over large portions of the measurement domain. A technique for performing the decomposition into continuous spectrum modes without pressure and wall-normal velocity measurements is demonstrated and is shown to be effective when the measurable disturbances are of sufcient accuracy.

Recent experiments on transient disturbances generated by three-dimensional roughness have used spanwise-periodic arrays of geometrically simple cylindrical roughness elements. Connecting these laboratory experiments to more realistic situations requires the study of surfaces with distributed roughness. This is accomplished in this work by numerically generating quasi-random rough surfaces and manufacturing these surfaces using rapid-prototyping technology. Measurements of the disturbances that the rough surface creates in a Blasius boundary layer are obtained for three test configurations corresponding to roughness-based Reynolds numbers of Re k = 164, 227 and 301. The two lower values give laminar flow; the highest value results in localized transition approximately 140 mm downstream of the leading edge of the roughness. All three configurations exhibit transient growth of steady disturbances. Unsteady fluctuations indicate that transition in the Re k =301 configuration is likely an example of a bypass transition mechanism in which the unsteady-disturbance growth outpaces the stabilizing relaxation of the steady flow. Measurements above the roughness surface in the Re k = 227 configuration provide a phenomenological model for distributed receptivity.

This paper documents a research effort on reduced gravity soldering of plated through hole joints which was conducted jointly by the National Center for Space Exploration Research, NASA Glenn Research Center, and NASA Johnson Space Center. Significant increases in joint porosity and changes in external geometry were observed in joints produced in reduced gravity as compared to normal gravity. Multiple techniques for mitigating the observed increase in porosity were tried, including several combinations of flux and solder application techniques, and demoisturizing the circuit board prior to soldering. Results were consistent with the hypothesis that the source of the porosity is a combination of both trapped moisture in the circuit board itself, as well as vaporized flux that is trapped in the molten solder. Other topics investigated include correlation of visual inspection results with joint porosity, pore size measurements, limited pressure effects (0.08 MPa - 0.1 MPa) on the size and number of pores, and joint cooling rate.

Experiments and simulations that consider how three-dimensional surface roughness affects laminar-to-turbulent transition have a history of mixed success. Two-dimensional roughness has been well understood as a generator of Tollmien‐Schlichting waves. Three-dimensional roughness has been less well understood but is now the subject of increased attention associated with the development of transient growth theory. Experiments that address the receptivity and growth of transient disturbances generated by roughness arrays and models of these processes are reviewed here. Additionally, an extension to random, distributed roughness that takes advantage of manufactured surfaces is described and implemented. The approach lends itself to rigorous comparisons between experimental and simulation data and theoretical models.