Meredith Metzger

A comparative study of near-wall turbulence in high and low Reynolds number boundary layers

The present study explores the effects of Reynolds number, over three orders of magnitude, in the viscous wall region of a turbulent boundary layer. Complementary experiments were conducted both in the boundary layer wind tunnel at the University of Utah and in the atmospheric surface layer which flows over the salt flats of the Great Salt Lake Desert in western Utah. The Reynolds numbers, based on momentum deficit thickness, of the two flows were Rθ=2×103 and Rθ≈5×106, respectively. High-resolution velocity measurements were obtained from a five-element vertical rake of hot-wires spanning the buffer region. In both the low and high Rθ flows, the length of the hot-wires measured less than 6 viscous units. To facilitate reliable comparisons, both the laboratory and field experiments employed the same instrumentation and procedures. Data indicate that, even in the immediate vicinity of the surface, strong influences from low-frequency motions at high Rθ produce noticeable Reynolds number differences in the ...

The near-neutral atmospheric surface layer: turbulence and non-stationarity.

The neutrally stable atmospheric surface layer is used as a physical model of a very high Reynolds number, canonical turbulent boundary layer. Challenges and limitations with this model are addressed in detail, including the inherent thermal stratification, surface roughness and non-stationarity of the atmosphere. Concurrent hot-wire and sonic anemometry data acquired in Utahs western desert provide insight to Reynolds number trends in the axial velocity statistics and spectra.

Interactions within the turbulent boundary layer at high Reynolds number

Simultaneous streamwise velocity measurements across the vertical direction obtained in the atmospheric surface layer (Re_τ ≃ 5 × 10^5) under near thermally neutral conditions are used to outline and quantify interactions between the scales of turbulence, from the very-large-scale motions to the dissipative scales. Results from conditioned spectra, joint probability density functions and conditional averages show that the signature of very-large-scale oscillations can be found across the whole wall region and that these scales interact with the near-wall turbulence from the energy-containing eddies to the dissipative scales, most strongly in a layer close to the wall, z^+ ≲ 10^3. The scale separation achievable in the atmospheric surface layer appears to be a key difference from the low-Reynolds-number picture, in which structures attached to the wall are known to extend through the full wall-normal extent of the boundary layer. A phenomenological picture of very-large-scale motions coexisting and interacting with structures from the hairpin paradigm is provided here for the high-Reynolds-number case. In particular, it is inferred that the hairpin-packet conceptual model may not be exhaustively representative of the whole wall region, but only of a near-wall layer of z^+ = O(10^3), where scale interactions are mostly confined.

Scaling the near-wall axial turbulent stress in the zero pressure gradient boundary layer

Based upon high resolution LDA measurements over a range of momentum deficit thickness Reynolds numbers (Rθ=U∞θ/ν) from 1430 to 31 000, DeGraaff and Eaton [J. Fluid Mech. 422, 319 (2000)] propose a new mixed scaling for the near-wall region profile of the axial turbulent stress, u2¯. The present results support the validity of this scaling over an extended Reynolds number range 1000⩽Rθ⩽5×106.

Viscous sublayer flow visualizations at Rθ≂1 500 000

Plan view flow visualization experiments were conducted in the atmospheric surface layer that flows over the Great Salt Lake Desert at the U.S. Army Dugway Proving Ground, Dugway, Utah. Measurements were acquired on a nonconductive, polyethylene platform made flush with the desert floor. Surface conditions upstream of the measurement site were flat, devoid of vegetation, and because of the dried mud/clay/salt composition, essentially dust free. Local surface variations ranged between 1 and 3 mm, which corresponded to three to ten viscous units during the experiments. Flow visualizations were accomplished by continuously injecting theatrical fog through a tangential slit covering a smoke reservoir buried under the platform. During the visualizations, the atmospheric surface layer flow was near neutral thermal stability. Flow velocities at 2.0 m above the surface maintained directional constancy, with a magnitude of about 1.5 m/s. A single element hot‐wire probe positioned near y+=3.4 was used to measure th...

Statistical structure of the fluctuating wall pressure and its in-plane gradients at high Reynolds number

The fluctuating wall pressure and its gradients in the plane of the surface were measured beneath the turbulent boundary layer that forms over the salt playa of Utahs west desert. Measurements were acquired under the condition of near-neutral thermal stability to best mimic the canonical zero-pressure-gradient boundary-layer flow. The Reynolds number (based on surface-layer thickness, δ, and the friction velocity, u τ ) was estimated to be 1 × 10 6 ± 2 × 10 5 . The equivalent sandgrain surface roughness was estimated to be in the range 15≤ k s + ≤85. Pressure measurements acquired simultaneously from an array of up to ten microphones were analysed. A compact array of four microphones was used to estimate the instantaneous streamwise and spanwise gradients of the surface pressure. Owing to the large length scales and low flow speeds, attaining accurate pressure statistics in the present flow required sensors capable of measuring unusually low frequencies. The effects of imperfect spatial and temporal resolution on the present measurements were also explored. Relative to pressure, pressure gradients exhibit an enhanced sensitivity to spatial resolution. Their accurate measurement does not, however, require fully capturing the low frequencies that are inherent and significant in the pressure itself. The present pressure spectra convincingly exhibit over three decades of approximately −1 slope. Comparisons with low-Reynolds-number data support previous predictions that the inner normalized wall pressure variance increases logarithmically with Reynolds number. The wall pressure autocorrelation exhibits its first zero-crossing at an advected length that is between one tenth and one fifth of the surface-layer thickness. Under any of the normalizations investigated, the present surface vorticity flux intensity values are difficult to reconcile with low-Reynolds-number data trends. Inner variables, however, do yield normalized flux intensity values that are of the same order of magnitude at low and high Reynolds number. Spectra reveal that even at high Reynolds number, the primary contributions to the pressure gradient intensities occur over a relatively narrow frequency range. This frequency range is shown to be consistent with the scale of the sublayer pocket motions. In accord with low-Reynolds-number data, the streamwise pressure gradient signals at high Reynolds number are also characterized by statistically significant pairings of opposing sign fluctuations.

Mean momentum balance in moderately favourable pressure gradient turbulent boundary layers

Moderately favourable pressure gradient turbulent boundary layers are investigated within a theoretical framework based on the unintegrated two-dimensional mean momentum equation. The present theory stems from an observed exchange of balance between terms in the mean momentum equation across different regions of the boundary layer. This exchange of balance leads to the identification of distinct physical layers, unambiguously defined by the predominant mean dynamics active in each layer. Scaling domains congruent with the physical layers are obtained from a multi-scale analysis of the mean momentum equation. Scaling behaviours predicted by the present theory are evaluated using direct measurements of all of the terms in the mean momentum balance for the case of a sink-flow pressure gradient generated in a wind tunnel with a long development length. Measurements also captured the evolution of the turbulent boundary layers from a non-equilibrium state near the wind tunnel entrance towards an equilibrium state further downstream. Salient features of the present multi-scale theory were reproduced in all the experimental data. Under equilibrium conditions, a universal function was found to describe the decay of the Reynolds stress profile in the outer region of the boundary layer. Non-equilibrium effects appeared to be manifest primarily in the outer region, whereas differences in the inner region were attributed solely to Reynolds number effects.

Sensitivity Analysis of a Three-Dimensional Wind Tunnel Design

The present paper describes the design of the TreadPort Adaptive Wind Tunnel (TPAWT), which is a wind tunnel retrofit to the University of Utah Treadport Virtual Environment (VE). The TPAWT integrates haptic wind sensation with the preexisting VE, which includes a large tilting treadmill and a cave-like frontal visual display. Desired flow patterns at the center of the TPAWT (where the VE user stands) are generated by appropriately steering the inlet airstream from two sets of vents embedded in the side walls of the facility. Hypothetically, the addition of haptic wind sensation in the virtual environment will improve the sense of immersion in the virtual reality. The present study focuses on quantifying the sensitivity of the flow field inside the TPAWT to perturbations in boundary conditions at the vents. Results from three-dimensional and two-dimensional numerical simulations are compared to laboratory experiments in a 1:4 scale model of the TPAWT. The stability of the flow near the VE user is shown to be highly sensitive to the angle of the mean flow at the vent boundary.Copyright

Viscous wall region structure in high and low Reynolds number turbulent boundary layers

The present study explores Reynolds number dependence in the viscous wall region of turbulent boundary layers. Low Reynolds number experiments (R0=2,000) were conducted in the boundary layer wind tunnel at the University of Utah. High Reynolds number experiments 0^=4,000,000) were conducted in the atmospheric surface layer which flows over the salt flats of the Great Salt Lake Desert in western Utah. Velocity measurements are derived from a five element vertical rake of wires spanning the region 2<y<40. For some of the experiments, the high frequency surface pressure fluctuations were simultaneouslyj^acquired with a microphone flush mounted immediately upstream of the hot-wire array. In addition, visualizations of the viscous sublayer were performed using dual slit, tangential smoke injection. To facilitate reliable comparisons, both the laboratory and field experiments employed the same instrumentation. The results presented include longtime statistical profiles, space and space-time correlations, and length scale and flow structure correlation information from the visual data. Comparisons between the high and low Re results indicate that even in the immediate vicinity of the surface there are strong influences produced by the low frequency motions at high Rj. These influences are pronounced in the axial velocity and wall pressure statistics. Nomenclature 9 momentum thickness 8 boundary layer thickness v kinematic viscosity p density 1 length of hot-wire sensor y distance above surface x streamwise/axial distance u streamwise velocity U.. free stream velocity z0 aerodynamic roughness length f filter frequency co circular frequency 0) spanwise vorticity p static surface pressure iw wall shear stress Copyright 01996 by the American Institute of Aeronautics and AMronauUc*, Inc. All rights reserved. Re

Conceptual design of an adaptive wind tunnel for the generation of unsteady complex flow patterns

The present paper describes the conceptual design of a three-dimensional adaptive wind tunnel capable of generating complex, unsteady flow fields in a relatively compact physical domain. The design involves multiple, independently controllable vents located around the periphery of a semi-enclosed facility. Desired flow patterns at target areas within the facility are produced by actively steering the inlet flow via appropriately adjusting the magnitude and direction of the air flow entering from each vent. The present study is motivated by a desire to incorporate tactile wind sensation into CAVE-like virtual environments, thereby increasing the overall sense of immersion in the virtual reality. The present wind tunnel design concept may also have potential application to laboratory studies of such problems as unsteady aerodynamics. Results in the present study include examples of two flow patterns obtained from numerical simulations using Fluent. Results from a companion parametric study analyzing the sensitivity of the numerical solution to mesh size and tolerance are also provided. In addition, the feasibility of using a linear-based control strategy to generate prescribed flow patterns within the wind tunnel is discussed.Copyright