arXiv subjects

Gregory P. Bewley

Publications and source records attributed to Gregory P. Bewley.

At least 19 recordsLinked to original sources

arXiv2026

Effects of mean flow skew on turbulent shear layers. Part II. Experimental investigation

Planar turbulent mixing layers, formed by the interactions of two parallel streams with different velocities, have been studied far more than three dimensional (3D) turbulent mixing layers, in which the incoming streams are skewed, and not parallel. Yet many practical shear flows are 3D. Here, we develop and validate an experimental methodology to generate and characterize skewed turbulent mixing layers and to quantify how mean-flow skew modifies mixing layer dynamics. We introduce skew with a spanwise deflection of the mean flow using turning vanes mounted near the trailing edge of a splitter plate, and we use cross-wire anemometry to investigate the downstream evolution of the flow. Relative to the planar configuration, the skewed mixing layer exhibits systematic reductions in both mean and turbulent quantities, with deviations reaching approximately 40\%. Despite these quantitative differences, the fundamental characteristics of the mixing layer remain largely unchanged. Mean-velocity profiles collapse under similarity scaling, shear-layer thicknesses retain approximately linear downstream growth, and Reynolds-stress profiles preserve their characteristic near-Gaussian form. Townsend's structure parameter, which quantifies the efficiency of turbulent momentum transport, remains approximately invariant between the planar and skewed configurations, in contrast to skewed turbulent boundary layers, wherein comparable mean flow skewing reduces the parameter by approximately 30\%. These results indicate that mean flow skew modifies turbulent mixing layers quantitatively while exerting only a secondary influence on their underlying dynamics. This study establishes a controlled experimental framework and empirical benchmark for future investigations of three-dimensional free-shear turbulence.

physics.flu-dyn↗

arXiv2026

Experimental Evidence for Longitudinal Scaling Exponent Saturation in Shear Turbulence

The asymptotic behavior of velocity statistics in the tails of distributions and at high Reynolds numbers remains unresolved in turbulence. To investigate this behavior we measured the $n$th-order moments of the distributions of longitudinal velocity differences, $S_n(r) \equiv \langle [u(x+r)-u(x)]^n \rangle \sim r^{ζ_n}$, in turbulent shear layers at Taylor-scale Reynolds numbers up to $Re_λ\approx 1400$. We used a nanoscale hot-wire probe with a sensing length, $l_w$, that was about half the Kolmogorov scale, $η$. We obtained datasets that were up to $5\times 10^7$ integral timescales long, so that the statistics converged up to $n=14$. In the inertial range, the exponents, $ζ_n$, deviate from classical models and appear to saturate near $ζ_n \approx 2.2 \pm 0.1$ for $n \gtrsim 12$. The saturation in the exponents is supported by a collapse of the tails of the velocity-difference distributions, and by plateaus in their compensated moments. These results constitute the first experimental evidence for scaling exponent saturation in longitudinal velocity increments, and is consistent with a dominance of localized vortex filaments in turbulence.

physics.flu-dyn↗

arXiv2026

Experimental investigation of intermediate-dissipation range energy spectra in shear turbulence

The shape of the turbulent energy spectrum in the dissipation range, where viscous effects dominate, remains an open question despite decades of work. We report an experimental investigation of intermediate dissipation range energy spectra in turbulent shear layers at Taylor-scale Reynolds numbers, $Re_λ$, ranging from approximately 450 to 1500, which are among the highest achieved in shear flow experiments that resolved small scales. We generated turbulent shear layers in a wind tunnel and measured using nanoscale hot-wire probes with a sensing length $l_w \approx (0.2-0.5)η$ that was smaller than the Kolmogorov scale $η$ at all $Re_λ$. The measurements resolved wavenumbers up to $k_{max} η$ $\approx 17$ at the lowest $Re_λ$ and $k_{max} η$ $\approx 1$ at the highest $Re_λ$, where $k_{max}$ is the highest resolved wave number. In the range $0.1 \lesssim k η\lesssim 0.5$, the spectra collapse onto a universal stretched-exponential form, $E(kη) \sim $ exp$(-kη)^γ $, with $γ\approx 0.5$ independent of $Re_λ$. This value of stretching exponent, $γ$, is consistent with recent empirical and computational studies. The Reynolds-number invariance of $γ$ is strong evidence for universal scaling in the intermediate dissipation range of high-Reynolds-number shear turbulence.

physics.flu-dyn↗

arXiv2025

Tailwind turbulence: a bound on the energy available from turbulence for transit, tested in Kraichnan's model

We investigate the unconstrained minimum energy required for vehicles to move through turbulence. We restrict our study to vehicles that interact with their environment through thrust, weight and drag forces, such as rotorcraft or submersibles. For such vehicles, theory predicts an optimum ratio between vehicle velocity and a characteristic velocity of the turbulence. The energy required for transit can be substantially smaller than what is required to move through quiescent fluid. We describe a simple picture for how a flight trajectory could preferentially put vehicles in tailwinds rather than headwinds, predicated on the organization of turbulence around vortices. This leads to an analytical parameter-free lower bound on the energy required to traverse a turbulent flow. We test this bound by computationally optimizing trajectories in Kraichnan's model of turbulence, and find that the energy required by point-models of vehicles is slightly larger than but close to our bound. Finally, we predict the existence of an optimum level of turbulence for which power is minimized, so that turbulence can be both too strong or too weak to be useful. This work strengthens previous findings that environmental turbulence can always reduce energy use. Thus, favorable trajectories are available to maneuverable vehicles if they have sufficient knowledge of the flow and computational resources for path planning.

physics.flu-dyn↗

arXiv2022

Scaling in Decaying Turbulence at High Reynolds Numbers

The way the increment statistics of turbulent velocity fluctuations scale with the increment size is a centerpiece of turbulence theories. We report data on decaying turbulence in the Max Planck Variable Density Turbulence Tunnel (VDTT), which show an approach of the inertial range statistics toward a nontrivial shape at small scales. By correcting for the contributions of energy decay to the large-scale statistics with a model, we find the scaling exponent of the second-order velocity increment statistics to be independent of the Reynolds number and equal to $0.693\pm0.003$ for $2000\lesssim R_λ \lesssim 6000$. This is evidence of a universal inertial range at high Reynolds numbers.

physics.flu-dyn↗

arXiv2021

Universality in Decaying Turbulence at High Reynolds Numbers

A hallmark of fluid turbulence theory is the universal power law scaling of the velocity difference statistics between two points in space in the inertial range between the large energy injection scale and the small energy dissipation scale. Even at the highest Reynolds numbers available, laboratory and natural flows such universal power laws have not been convincingly demonstrated. Here we show for the decaying active grid turbulence of the Max Planck Variable Density Turbulence Tunnel that the velocity difference statistics at high Reynolds numbers do not exhibit a power law, but have a universal functional form independent of the Reynolds number. We separate this functional form from the power law exponent and discuss potential consequences for turbulence modelling.

physics.flu-dyn↗

arXiv2021

Oscillations Modulating Power Law Exponents in Isotropic Turbulence: Comparison of Experiments with Simulations

Inertial-range features of turbulence are investigated using data from experimental measurements of grid turbulence and direct numerical simulations of isotropic turbulence simulated in a periodic box, both at the Taylor-scale Reynolds number $R_λ\sim 1000$. In particular, oscillations modulating the power-law scaling in the inertial range are examined for structure functions up to sixth order moments. The oscillations in exponent ratios decrease with increasing sample size in simulations though, in experiments, they survive at a low value of $4$ parts in $1000$ even after massive averaging. The two data sets are consistent in their intermittent character but differ in small but observable respects. Neither the scaling exponents themselves nor all the viscous effects are consistently reproduced by existing models of intermittency.

physics.flu-dyn↗

arXiv2021

Turbulence explains the accelerations of an eagle in natural flight

Turbulent winds and gusts fluctuate on a wide range of timescales from milliseconds to minutes and longer, a range that overlaps the timescales of avian flight behavior, yet the importance of turbulence to avian behavior is unclear. By combining wind speed data with the measured accelerations of a golden eagle (Aquila chrysaetos) flying in the wild, we show that the eagle's accelerations can be explained by a linear interaction with turbulence for timescales between about 1/2 and 10 s. These timescales are comparable to those of typical eagle behaviors, corresponding to between about 1 and 25 wingbeats, and to those of turbulent gusts both larger than the eagle's wingspan and smaller than large-scale atmospheric phenomena such as convection cells. The eagle's accelerations exhibit power spectra and intermittent activity characteristic of turbulence, and increase in proportion to the turbulence intensity. Intermittency results in accelerations that are occasionally several times stronger than gravity, and much larger than the ones we experience while driving, for instance. These imprints of turbulence on the bird's movements need to be further explored to understand the energetics of birds and other volant lifeforms, to improve our own methods of flying through ceaselessly turbulent environments, and to engage airborne wildlife as distributed probes of the changing conditions in the atmosphere.

q-bio.QM↗

arXiv2021

How to Extract Energy from Turbulence in Flight by Fast Tracking

We analyze a way to make flight vehicles harvest energy from homogeneous turbulence by fast tracking in the way that falling inertial particles do. Mean airspeed increases relative to flight through quiescent fluid when turbulent eddies sweep particles and vehicles along in a productive way. Once swept, inertia tends to carry a vehicle into tailwinds more often than headwinds. We introduce a forcing that rescales the effective inertia of rotorcraft in computer simulations. Given a certain thrust and effective inertia, we find that flight energy consumption can be calculated from measurements of mean particle settling velocities and acceleration variances alone, without need for other information. In calculations using a turbulence model, we optimize the balance between the work performed to generate the forcing and the advantages induced by fast tracking. The results show net energy reductions of up to about 10% relative to flight through quiescent fluid and mean velocities up to 40% higher. The forcing expands the range of conditions under which fast tracking operates by a factor of about ten. We discuss how the mechanism can operate for any vehicle, how it may be even more effective in real turbulence and for fixed-wing aircraft, and how modifications might yield yet greater performance.

physics.flu-dyn↗

arXiv2019

Control of long-range correlations in turbulence

The character of turbulence depends on where it develops. Turbulence near boundaries, for instance, is different than in a free stream. To elucidate the differences between flows, it is instructive to vary the structure of turbulence systematically, but there are few ways of stirring turbulence that make this possible. In other words, an experiment typically examines either a boundary layer or a free stream, say, and the structure of the turbulence is fixed by the geometry of the experiment. We introduce a new active grid with many more degrees of freedom than previous active grids. The additional degrees of freedom make it possible to control various properties of the turbulence. We show how long-range correlations in the turbulent velocity fluctuations can be shaped by changing the way the active grid moves. Specifically, we show how not only the correlation length but also the detailed shape of the correlation function depends on the correlations imposed in the motions of the grid. Until now, large-scale structure had not been adjustable in experiments. This new capability makes possible new systematic investigations into turbulence dissipation and dispersion, for example, and perhaps in flows that mimic features of boundary layers, free streams, and flows of intermediate character.

physics.flu-dyn↗

arXiv2018

Experimental Study of the Bottleneck in Fully Developed Turbulence

The energy spectrum of incompressible turbulence is known to reveal a pileup of energy at those high wavenumbers where viscous dissipation begins to act. It is called the bottleneck effect. Based on direct numerical simulations of the incompressible Navier-Stokes equations, results from Donzis & Sreenivasan (2010) pointed to a decrease of the strength of the bottleneck with increasing intensity of the turbulence, measured by the Taylor micro-scale Reynolds number $R_λ$. Here we report first experimental results on the dependence of the amplitude of the bottleneck as a function of $R_λ$ in a wind-tunnel flow. We used an active grid in the Variable Density Turbulence Tunnel (VDTT) (see Bodenschatz et al. (2014)) to reach $R_λ$ > 5000, which is unmatched in laboratory flows of decaying turbulence. The VDTT with the active grid permitted us to measure energy spectra from flows of different $R_λ$, with the small-scale features appearing always at the same frequencies. We relate those spectra recorded to a common reference spectrum, largely eliminating systematic errors which plague hotwire measurements at high frequencies. The data are consistent with a power law for the decrease of the bottleneck strength for the finite range of $R_λ$ in the experiment.

physics.flu-dyn↗

arXiv2018

Reynolds number dependence of the structure functions in homogeneous turbulence

We compare the predictions of stochastic closure theory (SCT) with experimental measurements of homogeneous turbulence made in the Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for Dynamics and Self-Organization in Gottingen. While the general form of SCT contains infinitely many free parameters, the data permit us to reduce the number to seven, only three of which are active over the entire inertial range. Of these three, one parameter characterizes the variance of the mean field noise in SCT and another characterizes the rate in the large deviations of the mean. The third parameter is the decay exponent of the Fourier variables in the Fourier expansion of the noise, which characterizes the smoothness of the turbulent velocity. SCT compares favorably with velocity structure functions measured in the experiment. We considered even-order structure functions ranging in order from two to eight as well as the third-order structure functions at five Taylor-Reynolds numbers (Rl) between 110 and 1450. The comparisons highlight several advantages of the SCT, which include explicit predictions for the structure functions at any scale and for any Reynolds number. We observed that finite-Rl corrections, for instance, are important even at the highest Reynolds numbers produced in the experiments. SCT gives us the correct basis function to express all the moments of the velocity differences in turbulence in Fourier space. The SCT produces the coefficients of the series and so determines the statistical quantities that characterize the small scales in turbulence. It also characterizes the random force acting on the fluid in the stochastic Navier-Stokes equation, as described in the paper.

physics.flu-dyn↗

arXiv2017

Dissipative Effects on Inertial-Range Statistics at High Reynolds numbers

Using the unique capabilities of the Variable Density Turbulence Tunnel at the Max Planck Institute for Dynamics and Self-Organization, Göttingen, we report experimental result on classical grid turbulence that uncover fine, yet important details of the structure functions in the inertial range. This was made possible by measuring extremely long time series of up to $10^{10}$ samples of the turbulent fluctuating velocity, which corresponds to $\mathcal{O}\left(10^5\right)$ large eddy turnover times. These classical grid measurements were conducted in a well-controlled environment at a wide range of high Reynolds numbers from $R_λ=110$ up to $R_λ=1600$, using both traditional hot-wire probes as well as NSTAP probes developed at Princeton University. We found that deviations from ideal scaling are anchored to the small scales and that dissipation influences the inertial-range statistics at scales larger than the near-dissipation range.

physics.flu-dyn↗

arXiv2016

Extreme fluctuations of the relative velocities between droplets in turbulent airflow

We compare experiments and direct numerical simulations to evaluate the accuracy of the Stokes-drag model, which is used widely in studies of inertial particles in turbulence. We focus on statistics at the dissipation scale and on extreme values of relative particle velocities for moderately inertial particles (St < 1). The probability distributions of relative velocities in the simulations were qualitatively similar to those in the experiments. The agreement improved with increasing Stokes number and decreasing relative velocity. Simulations underestimated the probability of extreme events, which suggests that the Stokes drag model misses important dynamics. Nevertheless, the scaling behavior of the extreme events in both the experiments and the simulations can be captured by the same multi-fractal model.

physics.flu-dyn↗

arXiv2014

On the Relaxation of Turbulence at High Reynolds Numbers

Turbulent motions in a fluid relax at a certain rate once stirring has stopped. The role of the most basic parameter in fluid mechanics, the Reynolds number, in setting the relaxation rate is not generally known. This paper concerns the high-Reynolds-number limit of the process. In a classical grid-turbulence wind-tunnel experiment that both reached higher Reynolds numbers than ever before and covered a wide range of them ($10^4 < Re = UM/ν< 5\times10^6$), we measured the relaxation rate with the unprecedented precision of about 2\%. Here $U$ is the mean speed of the flow, $M$ the forcing scale, and $ν$ the kinematic viscosity of the fluid. We observed that the relaxation rate was Reynolds-number independent, which contradicts some models and supports others.

physics.flu-dyn↗

arXiv2014

Variable Density Turbulence Tunnel Facility

The Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany produces very high turbulence levels at moderate flow velocities, low power consumption and adjustable kinematic viscosity between $10^{-4} m^2/s$ and $10^{-7} m^2/s$. The Reynolds number can be varied by changing the pressure or flow rate of the gas or by using different non-flammable gases including air. The highest kinematic viscosities, and hence lowest Reynolds numbers, are reached with air or nitrogen at 0.1 bar. To reach the highest Reynolds numbers the tunnel is pressurized to 15 bar with the dense gas sulfur hexafluoride (SF$_6$). Turbulence is generated at the upstream ends of two measurement sections with grids, and the evolution of this turbulence is observed as it moves down the length of the sections. We describe the instrumentation presently in operation, which consists of the tunnel itself, classical grid turbulence generators, and state-of-the-art nano-fabricated hot-wire anemometers provided by Princeton University [Vallikivi et al. (2011) Exp. Fluids 51, 1521]. We report measurements of the characteristic scales of the flow and of turbulent spectra up to Taylor Reynolds number $R_λ\approx 1600$, higher than any other grid-turbulence experiment. We also describe instrumentation under development, which includes an active grid and a Lagrangian particle tracking system that moves down the length of the tunnel with the mean flow. In this configuration, the properties of the turbulence are adjustable and its structure is resolvable up to $R_λ\approx 8000$.

physics.flu-dyn↗

arXiv2014

Observation of the sling effect

When cloud particles are small enough, they move with the turbulent air in the cloud. On the other hand, as particles become larger their inertia affects their motions, and they move differently than the air. These inertial dynamics impact cloud evolution and ultimately climate prediction, since clouds govern the earth's energy balances. Yet we lack a simple description of the dynamics. Falkovich et al. describes theoretically a new dynamical mechanism called the "sling effect" by which extreme events in the turbulent air cause idealized inertial cloud particles to break free from the airflow (Falkovich G, Fouxon A, Stepanov MG 2002 Nature 419, 151). The sling effect thereafter causes particle trajectories to cross each other within isolated pockets in the flow, which increases the chance of collisions that form larger particles. We combined experimental techniques that allow for precise control of a turbulent flow with three-dimensional tracking of multiple particles at unprecedented resolution. In this way, we could observe both the sling effect and crossing trajectories between real particles. We isolated the inertial sling dynamics from those caused by turbulent advection by conditionally averaging the data. We found the dynamics to be universal in terms of a local Stokes number that quantifies the local particle velocity gradients. We measured the probability density of this quantity, which shows that sharp gradients become more frequent as the global Stokes number increases. We observed that sharp compressive gradients in the airflow initiated the sling effect, and that thereafter gradients in the particle flow ran away and steepened in a way that produced singularities in the flow in finite time. During this process both the fluid motions and gravity became unimportant. The results underpin a framework for describing a crucial aspect of inertial particle dynamics.

cond-mat.other↗

arXiv2012

The journey of hydrogen to quantized vortex cores

Nanoscale hydrogen particles in superfluid helium track the motions of quantized vortices. This provides a way to visualize turbulence in the superfluid. Here, we trace the evolution of the hydrogen from a gas to frozen particles migrating toward the cores of quantized vortices. Not only are the intervening processes interesting in their own right, but understanding them better leads to more revealing experiments.

cond-mat.stat-mech↗