Roughness-induced transient growth in a flat-plate boundary layer

White, Edward B.; Reshotko, Eli

doi:10.2514/6.2002-138

Cited by 22 publications

(10 citation statements)

References 15 publications

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“…In this experiment, disturbance peaks are observed to approach 77 = 2.2 with increasing downstream distance. White and Reshotko (2002) suggested that movement of the peak would result if the peak location were a function of spanwise wavelength, but the current results show that this is not the case. Furthermore, recent Tumin confirm that the peak's location is not a strong function of wavelength (Tumin, personal communication).…”

Section: Roughness-array Experimentscontrasting

confidence: 48%

“…A optimal-filtering approach is used for random, distributed roughness patches. This approach is similar to what is suggested by Naguib et al (1996) and is described in detail for use with distributed roughness by White and Reshotko (2002). Two different approaches to moving hotwires within the measurement plane were considered.…”

Section: Hotwire Measurement Techniques and Data Analysismentioning

confidence: 99%

“…Movement of the Urms peak was noted by White and Reshotko (2002), who suggested that this behavior would occur if the 77 location of the disturbance maximum were a function of disturbance wavelength; movement of the Urms peak would then occur as increasingly long wavelength disturbances become more important. However, spectra from White and Reshotko's study were not sufficiently well resolved to verify this suggestion.…”

Section: Roughness-array Experimentsmentioning

confidence: 99%

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Experiments in Transient Growth and Roughness-Induced Bypass Transition

White¹

2005

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SummaryThe work reported here is a series of wind-tunnel experiments that establishes the basic behavior of roughness-induced disturbances that undergo transient algebraic growth in a laminar boundary layer. The experiments explore three aspects of these disturbances' behavior: their fundamental response to various configurations of surface roughness, their receptivity, and their behavior when the surface roughness amplitude is large. The experiments consist of hotwire measurements of the steady and unsteady disturbances created by isolated 3D roughness elements, spanwise arrays of 3D elements and patches of random, distributed roughness. Spanwise roughness arrays are used most extensively as these are found to produce disturbances best-suited for comparison with theoretical predictions.The results establish that surface roughness does create disturbances that undergo transient growth and that it does so in a manner consistent with the physical picture known as Landahl's lift-up mechanism. The observed growth is qualitatively similar to what is predicted by optimal-disturbance theory. However, theoretical predictions tend to overestimate the length over which disturbances grow and are thought to overestimate the total amount of transient growth.The difference between what is predicted by optimal disturbance theories and what is observed in the experiments is linked to receptivity. Receptivity is found to play two roles in the transient growth phenomenon: it determines initial disturbance amplitude and also distributes energy among the modes of the continuous disturbance spectrum. This second effect is associated with the initial-value-problem formulation of the transient growth problem and explains why some disturbances grow and decay in a manner very similar to what is predicted by optimal-disturbance theory while others only decay and others undergo a significant decay and then a weak phase of transient growth. Regardless of whether certain disturbances undergo transient growth or not, all of the roughness-induced disturbances studied in this experiment have energies that scale approximately as the square of roughness-based Reynolds number across a wide range of Reynolds numbers. At high values of the roughness-based Reynolds number (above approximately 250) transition occurs just aft of the arrays of 3D roughness elements. This transition does not appear to be connected with transient growth. Rather, it occurs because the element's wakes are unstable to a high-frequency Kelvin-Helmoholtz type instability mechanism. Transient growth of steady disturbances only occurs if this rapid transition connected with the unsteady disturbances does not occur.

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Section: Roughness-array Experimentscontrasting

confidence: 48%

Section: Hotwire Measurement Techniques and Data Analysismentioning

confidence: 99%

Section: Roughness-array Experimentsmentioning

confidence: 99%

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Experiments in Transient Growth and Roughness-Induced Bypass Transition

White¹

2005

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“…3. The front portion of the frame sits in a shallow recess in the flat plate surface that has been used to hold recessed sandpaper for similar experiments (White and Reshotko 2002). Here it allows a continuous interface between the plate and the frame while maintaining a suitably thick frame leading edge.…”

Section: Roughness Sheetmentioning

confidence: 99%

Experiments on the Receptivity of Transient Disturbances to Surface Roughness and Freestream Turbulence

White¹

2008

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The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering end maintaining the date needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to the Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid 0MB control number. PLEASE DO NOT RETURN YOUR SUPPLEMENTARY NOTES ABSTRACTTransient growth is a boundary-layer instability mechanism that leads to algebraic growth of disturbances generated by surface roughness and freestream turbulence. An earlier research program verified that stationary, roughness-induced disturbances undergo transient growth but that these disturbances are sub-optimal and depend critically on the details of the receptivity process. This project seeks to provide a more complete understanding of the receptivity of transient disturbances to regular and random surface roughness as well as freestream turbulence. This objective is pursued through three separate tracks. First, a technique is developed to permit a rigorous decomposition of measured steady disturbances across the continuous spectrum of Orr-Sommerfeld/Squire eigenmodes. Second, the receptivity and transient growth of steady disturbances generated by quasi-random distributed surface roughness is investigated. Third, transient disturbances generated by controlled freestream turbulence are to be investigated. SUBJECT TERMS SummaryTransient growth is a boundary-layer instability mechanism that leads to algebraic growth of disturbances generated by surface roughness and freestream turbulence. An earlier research program (AFOSR FA9620-02-1-0058, 12/01-11/04) verified that stationary, roughness-induced disturbances undergo transient growth but that these disturbances are sub-optimal and depend critically on the details of the receptivity process. This project seeks to provide a more complete understanding of the receptivity of transient disturbances to regular and random surface roughness as well as freestream turbulence. This objective is pursued through three separate tracks. First, a technique is developed to permit a rigorous decomposition of measured steady disturbances across the continuous spectrum of Orr-Sommerfeld/Squire (OS5) eigenmodes. Second, the receptivity and transient growth of steady disturbances generated by quasi-random distributed surface roughness is investigated. Third, transient disturbances generated by controlled freestream turbulence are to be investigated.This work was originally proposed as a 36-month project to be conducted at Case Western Reserve University. However, the PI, Pr...

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“…The most basic of these experiments on the behavior of stationary transient disturbances generated by surface roughness elements in zero-pressuregradient boundary layers (White and Reshotko 2002;White 2002;White and Ergin 2003) can and have been conducted with flat-plate and hotwire instrumentation system similar to a number of classical flat-plate transition studies. However, ongoing and proposed studies are intended to extend this work into the behavior of transient disturbances in curved-surface boundary layers with pressure gradients and high levels of freestream turbulence.…”

Section: Project Background and General Objectivesmentioning

confidence: 99%

Instrumentation for Wind Tunnel Transient Growth Studies

White¹

2004

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Roughness-induced transient growth in a flat-plate boundary layer

Cited by 22 publications

References 15 publications

Experiments in Transient Growth and Roughness-Induced Bypass Transition

Experiments in Transient Growth and Roughness-Induced Bypass Transition

Experiments on the Receptivity of Transient Disturbances to Surface Roughness and Freestream Turbulence

Instrumentation for Wind Tunnel Transient Growth Studies

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