scispace - formally typeset
Search or ask a question
Author

Owen Williams

Other affiliations: Princeton University
Bio: Owen Williams is an academic researcher from University of Washington. The author has contributed to research in topics: Boundary layer & Turbulence. The author has an hindex of 9, co-authored 23 publications receiving 205 citations. Previous affiliations of Owen Williams include Princeton University.

Papers
More filters
Journal ArticleDOI
28 May 2020
TL;DR: Robust principal component analysis is used to improve the quality of flow field data by leveraging global coherent structures to identify and replace spurious data points and to investigate PIV measurements behind a two-bladed cross-flow turbine that exhibits both broadband and coherent phenomena.
Abstract: Robust principal component analysis (RPCA) is a powerful technique from robust statistics that can be used to extract dominant coherent structures from flow fields corrupted with outliers and missing measurements. The effectiveness of RPCA on flows acquired experimentally and by simulation is demonstrated. In all cases, RPCA is able to de-noise these fields and vastly improve subsequent modal analysis, showing that RPCA can be used to robustly process particle image velocimetry flow fields.

60 citations

Journal ArticleDOI
TL;DR: In this article, the effects of compressibility, slip, and fluid inertia on the frequency response of particle-based velocimetry techniques for supersonic and hypersonic flows were examined.
Abstract: We examine the effects of compressibility, slip, and fluid inertia on the frequency response of particle-based velocimetry techniques for supersonic and hypersonic flows by solving the quasi-steady drag equation for solid, spherical particles. We demonstrate that non-continuum and fluid inertial effects significantly affect the particle response under all typical supersonic flow conditions. In particular, the particle frequency response obtained from a shock response test depends on the strength of the shock, decreasing with shock strength as non-continuum effects become more prominent. For weak disturbances, such as those typical of turbulence, the actual particle frequency response can therefore be much lower than that obtained from a typical shock response. The greatest variability in the response was found to occur at low supersonic Mach numbers. The results were found to be typical of solid particles used for velocimetry under a wide range of wind tunnel conditions, and so, previous particle frequency response analyses based solely on shock response tests may well have overestimated the response to turbulence.

46 citations

Journal ArticleDOI
TL;DR: In this article, the scaling and structure of a smooth, flat-plate turbulent boundary layer with a free stream Mach number of 7.5 was studied and correlation lengths and structure angles were found to be less sensitive to compressibility than indicated by previous studies based on density fields or mass-weighted statistics.
Abstract: Particle image velocimetry and filtered Rayleigh scattering experiments were performed over a range of Reynolds numbers to study the scaling and structure of a smooth, flat-plate turbulent boundary layer with a free stream Mach number of 7.5. The measurements indicate few, if any, dynamic differences due to Mach number. Mean and fluctuating streamwise velocities in the outer layer show strong similarity to incompressible flows at comparable Reynolds numbers when scaled according to van Driest and Morkovin. In addition, correlation lengths and structure angles based on velocity statistics were found to be less sensitive to compressibility than indicated by previous studies based on density fields or mass-weighted statistics, suggesting that the density and velocity fields obey different scaling. Finally, the boundary layer displays uniform momentum zones, with the number of these zones similar to incompressible boundary layers at comparable Reynolds numbers.

43 citations

Journal ArticleDOI
TL;DR: In this article, the effects of stable thermal stratification on turbulent boundary layers are experimentally investigated for smooth and rough walls, and the weakly stable and strongly stable regimes are delineated by the point where the turbulence no longer scales with the local wall shear stress, a significant departure from previous definitions.
Abstract: The effects of stable thermal stratification on turbulent boundary layers are experimentally investigated for smooth and rough walls. For weak to moderate stability, the turbulent stresses are seen to scale with the wall shear stress, compensating for changes in fluid density in the same manner as done for compressible flows. This suggests little change in turbulent structure within this regime. At higher levels of stratification turbulence no longer scales with the wall shear stress and turbulent production by mean shear collapses, but without the preferential damping of near-wall motions observed in previous studies. We suggest that the weakly stable and strongly stable (collapsed) regimes are delineated by the point where the turbulence no longer scales with the local wall shear stress, a significant departure from previous definitions. The critical stratification separating these two regimes closely follows the linear stability analysis of Schlichting ( Z. Angew. Math. Mech. , vol. 15 (6), 1935, pp. 313–338) for both smooth and rough surfaces, indicating that a good predictor of critical stratification is the gradient Richardson number evaluated at the wall. Wall-normal and shear stresses follow atmospheric trends in the local gradient Richardson number scaling of Sorbjan ( Q. J. R. Meteorol. Soc. , vol. 136, 2010, pp. 1243–1254), suggesting that much can be learned about stratified atmospheric flows from the study of laboratory scale boundary layers at relatively low Reynolds numbers.

31 citations


Cited by
More filters
Book ChapterDOI
01 Jan 1997
TL;DR: The boundary layer equations for plane, incompressible, and steady flow are described in this paper, where the boundary layer equation for plane incompressibility is defined in terms of boundary layers.
Abstract: The boundary layer equations for plane, incompressible, and steady flow are $$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

2,598 citations

ReportDOI
30 Jul 2004
TL;DR: In this paper, the authors investigated the effect of roughness on the near-wall drag-producing turbulent structures and proposed control strategies to reduce momentum loss in rough-wall boundary layers.
Abstract: : The objective of this project is to improve our fundamental knowledge of turbulent flows over rough surfaces. Specifically, we hope to investigate the manner in which roughness affects the near-wall drag-producing turbulent structures, and to what extent surface roughness affects the outer part of rough-wall boundary layers. Ultimately we hope to use this knowledge to propose control strategies to reduce momentum loss in rough-wall boundary layers.

298 citations

Journal ArticleDOI
TL;DR: In this paper, direct numerical simulations of turbulent boundary layers with nominal free-stream Mach number ranging from 0.3 to 12 were performed to assess the scalings with respect to the mean and turbulence behaviors as well as the possible breakdown of the weak compressibility hypothesis for turbulent boundary layer at high Mach numbers.
Abstract: In this paper, we perform direct numerical simulations (DNS) of turbulent boundary layers with nominal free-stream Mach number ranging from 0.3 to 12. The main objective is to assess the scalings with respect to the mean and turbulence behaviours as well as the possible breakdown of the weak compressibility hypothesis for turbulent boundary layers at high Mach numbers (M > 5). We find that many of the scaling relations, such as the van Driest transformation for mean velocity, Walz's relation, Morkovin's scaling and the strong Reynolds analogy, which are derived based on the weak compressibility hypothesis, remain valid for the range of free-stream Mach numbers considered. The explicit dilatation terms such as pressure dilatation and dilatational dissipation remain small for the present Mach number range, and the pressure–strain correlation and the anisotropy of the Reynolds stress tensor are insensitive to the free-stream Mach number. The possible effects of intrinsic compressibility are reflected by the increase in the fluctuations of thermodynamic quantities (p′rms/pw, ρ′rms/ρ, T′rms/T) and turbulence Mach numbers (Mt, M′rms), the existence of shocklets, the modification of turbulence structures (near-wall streaks and large-scale motions) and the variation in the onset of intermittency.

263 citations

Journal ArticleDOI
TL;DR: A direct numerical simulation database of high-speed zero-pressure-gradient turbulent boundary layers developing spatially over a flat plate with nominal freestream Mach number ranging from 2.5 to 14 and wall-to-recovery temperature ranging from 0.18 to 1.0 is presented.
Abstract: In this paper, we present a direct numerical simulation database of high-speed zero-pressure-gradient turbulent boundary layers developing spatially over a flat plate with nominal freestream Mach number ranging from 2.5 to 14 and wall-to-recovery temperature ranging from 0.18 to 1.0. The flow conditions of the DNS are representative of the operational conditions of the Purdue Mach 6 quiet tunnel, the Sandia Hypersonic Wind Tunnel at Mach 8, and the AEDC Hypervelocity Tunnel No. 9 at Mach 14. The DNS database is used to gauge the performance of compressibility transformations, including the classical Morkovin's scaling and strong Reynolds analogy as well as the newly proposed mean velocity and temperature scalings that explicitly account for wall heat flux. Several insights into the effect of direct compressibility are gained by inspecting the thermodynamic fluctuations and the Reynolds stress budget terms. Precomputed flow statistics, including Reynolds stresses and their budgets, will be available at the website of the NASA Langley Turbulence Modeling Resource, allowing other investigators to query any property of interest.

106 citations