3D-PTV measurements in a plane Couette flow

Genuine plane Couette flow is hard to realize experimentally, and no applications of modern spatially resolving measurement techniques have been reported for this flow so far. In order to resolve this shortcoming, we designed and built a new experimental facility and present our first results here. Our setup enables us to access the flow via 3D particle tracking velocimetry and therefore to obtain truly three-dimensional flow fields for the first time experimentally in plane Couette flow. Results are analyzed in terms of basic flow properties, and a clear distinction of flow regimes (laminar for Re < 320, transitional for 320 < Re < 400, and turbulent when Re > 400) could be made. Comparison with DNS data shows good agreement in the turbulent regime and builds trust in our data. Furthermore, vortical coherent structures are studied in detail with the additional help of kalliroscope imaging, and the typical vortex spacing is determined to be roughly one gap width. As a noteworthy result, we find that the onset of the turbulent regime coincides with the range of Reynolds numbers at which a distance of 100 wall units is comparable to the gap width.     

Experiments on the spreading of shear-free turbulence under the influence of confinement and rotation

From Lie-group (symmetry) analysis of the multi-point correlation equation Oberlack and Günther (Fluid Dyn Res 33:453–476, 2003) found three different solutions for the behavior of shear-free turbulence: (i) a diffusion like solution, in which turbulence diffuses freely into the adjacent calm fluid, (ii) a deceleration wave like solution when there is an upper bound for the integral length scale and (iii) a finite domain solution for the case when rotation is applied to the system. This paper deals with the experimental validation of the theory. We use an oscillating grid to generate turbulence in a water tank and Particle Image Velocimetry (PIV) to determine the two-dimensional velocity and out-of-plane vorticity components. The whole setup is placed on a rotating table. After the forcing is initiated, a turbulent layer develops which is separated from the initially irrotational fluid by a sharp interface, the so-called turbulent/non-turbulent interface (TNTI). The turbulent region grows in time through entrainment of surrounding fluid. We measure the propagation of the TNTI and find quantitative agreement with the predicted spreading laws for case one and two. For case three (system rotation), we observe that there is a sharp transition between a 3D turbulent flow close to the source of energy and a more 2D-like wavy flow further away. We measure that the separation depth becomes constant and in this sense, we confirm the theoretical finite domain solution.     

An LDV system for turbulence length scale measurements

An LDV system for making spatial correlation measurements of velocity fluctuations in turbulent nonreacting and reacting flows is presented. The LDV system is the dual beam type and consists of an elongated probe volume and a two-point optical fiber detector. Results are presented of the integral length scale measured in a nonreacting grid generated turbulent flow.A version of this paper was presented at the ASME Winter Annual Meeting of 1984 and printed in AMD, Vol. 66     

The influence of temperature on the measurements of Reynolds stresses in shear free turbulence near a wall

 Temperature changes have a significant influence on the measurements of Reynolds stresses in turbulent boundary layers. As compared to the spanwise velocity fluctuations the streamwise turbulence intensity is especially sensitive to temperature deviations. Although this is a general statement its importance is clearly elucidated in a shear-free turbulence near a solid wall, since the mixing due to turbulence production is minimized in this flow. A consequence of temperature influence on hot-wire measurements is that frictional heating from the wall has produced contradictory results in different experiments on shear-free turbulence. In the current paper, measurements of streamwise and spanwise turbulence intensities have been conducted at different wall temperatures, thereby simulating the contradictory results mentioned above. A simple model has been developed showing that the turbulence intensities are affected by both the rms. value of the temperature fluctuations and the correlation between fluctuating temperature and velocity. These correlations are measured and the developed model is used to explain deviations in earlier measurements on shear-free turbulence. Moreover, the individual magnitudes of the two correlations in the temperature correction are estimated and their individual importance is discussed. Received: 17 February 1997 / Accepted: 18 January 1998     

Pseudo turbulence in PIV breaking-wave measurements

 The particle image velocimetry (PIV) technique was employed to measure the instantaneous velocity distribution under nonbreaking and breaking water waves. The corresponding turbulence intensity was calculated by the ensemble average of repeated measurements. The pseudo turbulence found was large enough to affect the accuracy of the turbulence measurements. We follow Prasad et al.'s (1992) approach to demonstrate that the pseudo turbulence is related to the bias error, which is the discrepancy between the true position of the particle image and the position calculated from the pixel array data with inadequate pixel resolution. To reduce the bias error (or the pseudo turbulence), we first calculate it from a turbulence-free flow with the same experimental set-up as that used for the targeted experiments (i.e., we use the same size of field of view, seeding particles, seeding density, lens aperture, and laser wavelength in both experiments). Then we minimize the bias error from the turbulence measurements in the actual experiments. To demonstrate the procedure, the evolution of a breaking wave is investigated. Received: 30 January 1998/Accepted: 28 October 1999     

Velocity and turbulence measurements in combustion systems

A laser-Doppler velocimeter is used in the measurement of high-temperature gas flows. A two-stage fluidization particle generator provides magnesium oxide particles to serve as optical scattering centers. The one-dimensional dual-beam system is frequency shifted to permit measurements of velocities up to 300 meters per second and turbulence intensities greater than 100 percent.Exiting flows from can-type gas turbine combustors and burners with pre-mixed oxy-acetylene flames are described in terms of the velocity, turbulence intensity, and temperature profiles.The results indicate the influence of the combustion process on turbulence.List of Symbols A exit area of combustor or burner (m²) - A/F mass air-fuel ratio - D exit diameter of combustor or burner (m) - M mass flow rate of gases (kg/s) - N _D number of Doppler bursts used in each velocity measurement - Q volumetric flow rate at T _r (m³/s) - R exit radius of combustor or burner (m) - R _1/2 distance from centerline to radius where the velocity is one-half of the local centerline velocity (m) - Re exit Reynolds number based on cold flow, QD/A - r distance from centerline of flow (m) - T temperature (°C) - T _CL centerline temperature (°C) - T _r inlet (cold) air temperature of combustor or burner (°C) - T.I. turbulence intensity, - mean velocity (m/s) - U _i instantaneous velocity individually realized by LDV (m/s) - mean velocity at centerline of flow (m/s) - mean square velocity fluctuation (m²/s² - x distance along centerline downstream of exit (m) - absolute viscosity at T _r (kg/(ms)) - density at T _r (kg/m³)     

The degeneration of turbulence in a liquid with internal rotation

Viscous fluids which possess even a small degree of elasticity behave in quite a different way from viscous fluids in non-steady-state flow which is fairly fast. The presence of elasticity leads to a change in the nature of the damping of small scale (high frequency) turbulent eddies when the turbulence becomes degenerate [1]. The difference in the specific properties of viscoelastic fluids manifests itself in the behavior of these eddies.     

High-intensity turbulence measurements in a Taylor-Couette flow reactor

Turbulence-intensity measurements were made in a Taylor-Couette flow reactor consisting of two counter-rotating concentric cylinders designed for the purpose of studying turbulent premixed-flame propagation. In the annulus separating the two cylinders, a nearly homogeneous turbulent flow is generated. The intensities of turbulent velocity fluctuations in the annulus in both axial and circumferential directions were measured by using laser-Doppler velocimetry for a wide range of cylinder rotation rates, corresponding to low through high (120 cm/s) intensities relative to typical laminar flame speeds for lean methane-air mixtures. The experimental measurements indicate a linear relation between turbulence intensities and average cylinder surface speed and demonstrate the usefulness of the Taylor-Couette apparatus for studies of premixed-flame propagation in high-intensity turbulent flow.     

Hot-wire spatial resolution effects in measurements of grid-generated turbulence

We use grid-generated turbulence as a benchmark flow to test the effects of spatial resolution on turbulence measurements with hot wires. To quantify the spatial filtering, measurements of the turbulence statistics and spectra downstream of the grid were made using hot wires of varying length and compared to the results from a new nanoscale thermal anemometry probe, which has a sensing length of the order of, or smaller than, the Kolmogorov scale. In order to separate the effects of temporal and spatial filtering, a study was performed to ensure that the data were free of the artifacts of temporal filtering so that differences in the measurements could be wholly attributed to spatial filtering. An empirical correlation for the attenuation of the streamwise Reynolds stress due to spatial filtering is constructed, and it is shown that these grid turbulence results relate directly to the near-wall region of wall-bounded flows, where the effects of spatial filtering are most acutely felt. The effect of spatial filtering on the streamwise spectrum function is observed to extend to almost all wavenumbers, even those significantly lower than the length of the hot wire itself. It is also shown that estimates of the Kolmogorov scale are affected by spatial filtering when wires longer than the Kolmogorov length are used.     

Transverse velocity and temperature derivative measurements in grid turbulence

The same probe, comprising two parallel wires, is used to measure either velocity or temperature derivatives in shearless grid turbulence at a Taylor microscale Reynolds number of about 40. The aerodynamic interference of the probe affects the mean velocity when the transverse separation jy between the wires is smaller than about 3m, where m (ۂ.4 mm for the present experiments) is the Kolmogorov length scale, but not the mean temperature. Spectra and corresponding moments of transverse velocity and temperature derivatives are significantly but similarly affected when jyh3m, thus suggesting that this effect is more likely to be caused by electronic noise than aerodynamic interference. Indeed, after noise corrections are applied, the resulting derivative variances are brought into alignment with values inferred from two-point correlations with respect to y. Transverse derivative variances and their corresponding spectra satisfy isotropy closely but second-order structure functions satisfy it only when the separation is less than about 10m, i.e. the dissipative range scales.     

Turbulence statistics from optical whole-field measurements in particle-laden turbulence

Obtaining turbulence statistics in particle-laden flows using optical whole-field measurements is complicated due to the inevitable data loss. The effects of this data loss are first studied using synthetic data and it is shown that the interpolation of missing data leads to biased results for the turbulence spectrum and its derived quantities. It is also shown that the use of overlapping interrogation regions in images with a low image density can lead to biased results due to oversampling. The slotting method is introduced for the processing of particle image velocimetry (PIV) data fields with missing data. Next to this, it is extended to handle unstructured data. Using experimental data obtained by a dual-camera PIV/PTV (particle tracking velocimetry) system in particle-laden grid turbulence, the performance of the new approach is studied. Some preliminary two-phase results are presented to indicate the significant improvement in the statistics, as well as to demonstrate the unique capabilities of the system.An erratum to this article can be found at

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Velocity measurements of turbulence collapse in a linearly stratified jet

The behavior of a horizontal turbulent round jet␣in a linearly density-stratified fluid is studied and compared to the unstratified case using digital particle-tracking velocimetry (DPTV). Close to the nozzle, the stratified jet grows axisymmetrically at the same rate as an unstratified jet. At a critical distance downstream, the vertical growth is suppressed, while the lateral spreading increases. The self-similar nature of unstratified round jets is not preserved in the stratified case. The velocity field and turbulent stresses are affected by the stratification closer to the jet nozzle than is expected from the flow visualization observations. In particular, the vertical normal stress and shear stress are significantly reduced compared to the unstratified case. Received: 11 September 2000/Accepted: 28 March 2001     

Effect of rotation on a fiber-reinforced thermo-elastic under Green-Naghdi theory and influence of gravity

A general model of the equation of Green-Naghdi theory (G-N), which is applied to study the influence of reinforcement on the total deformation for an infinite space weakened by a finite linear opening thermal shock, is solved. We study the influence of reinforcement on the total deformation of a rotating thermo-elastic half-space and the mutual interaction under the influence of gravity. The material is homogeneous, isotropic and elastic half-space. The methodology applied here consists of the use of a normal mode analysis to obtain the exact expressions for the temperature, the displacement components, and the stress components. Some particular cases are also discussed in this context. The deformation of a body depends on the nature of the force applied as well as the type of boundary conditions. The variations of the variables considered with the horizontal distance are illustrated graphically. Comparisons are made with the results predicted by the type II and type III cases in the presence and in the absence of the effect of fiber reinforcement. It is found that reinforcement, rotation, and gravity have great effect on the distribution of the field quantities.     

The influence of rotation in slow viscous flows

We investigate the secondary streaming motion due to the slow rotation of an axisymmetric body in a rotating viscous fluid. We find in general that for all such bodies the velocities tend to zero at a distance $\text{0(E}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ from the body, where E is the (large) Ekman number. Four specific geometries are considered: sphere, spheroid, spherical cap and the double sphere; in all except the first case a small Rossby number has been assumed. The resultant translational force on the spherical cap, where there is no fore-aft symmetry, has been calculated. Further, separated flow in the last two cases can be displayed.     

Electromagnetic methods of turbulence measurements — shortcomings and advantage

Two major electromagnetic methods of turbulence measurements are discussed. Both methods are based on the measurement of the electrical field induced by the fluid motion in the presence of a magnetic field. In the first method a multielectrode potential difference probe (PDP) is used in the presence of an external magnetic field, while in the second one the source of the magnetic field is incorporated in the probe itself and the magnetic field is strongly localized around the probe. Shortcomings and advantages of both methods are discussed along with recent developments.This paper was presented at the 10th Symposium on Turbulence, University of Missouri-Rolla, Sept. 22–24, 1986.     

Three-dimensional magnetic resonance velocimetry measurements of turbulence quantities in complex flow

A magnetic resonance velocimetry (MRV) experimental technique based on magnetic resonance imaging and capable of measuring the turbulent Reynolds stresses in a 3D flow domain is described. Results are presented in backward facing step flow in a square channel with a Reynolds number of 48,000 based on step height and freestream velocity at the step. MRV results are compared to particle image velocimetry (PIV) measurements in the centerplane containing the streamwise and cross-stream axes. MRV and PIV mean velocity measurements show excellent agreement. MRV measurements for Reynolds normal stresses compare to within ±20% of the PIV results while results for the turbulent shear are less accurate.     

Wind tunnel measurements of the preferential concentration of inertial droplets in homogeneous isotropic turbulence

We describe an experimental setup aimed at studying turbulent-induced droplet collisions in a laboratory setting. Our goal is to reproduce conditions relevant to warm-rain formation in clouds. In these conditions, the trajectories of small inertial droplets are strongly influenced by the background air turbulence, and collisions can potentially explain the droplet growth rates and spectrum broadening observed in this type of clouds. Warm-rain formation is currently under strong scrutiny because it is an important source of uncertainty in atmospheric models. A grid at the entrance of a horizontal wind tunnel produces homogeneous isotropic turbulence at a Re _λ in the range of 400–500. Water droplets are injected from the nodes of the turbulence-inducing grid at a volume fraction (?) of 2.7?×?10^?5 and with sizes of 10–200?μm. A complex manifold-injection system was developed to obtain uniform water droplet seeding, in terms of both water content and size distribution. We characterize the resulting droplet-laden turbulent flow, and the statistics of droplet pairs are measured and analyzed. We found that the radial distribution function (RDF), a measure of preferential concentration of droplets that plays a key role in collision kernel models, has a large peak at distances below the Kolmogorov microscale of the turbulence. At very long separations, comparable with the integral length scale of the turbulence, these RDFs show a slow decay to the average probability given by the mean droplet number density. Consistent with this result, conditional analysis shows an increased local concentration of droplets within the inertial length scale (≈ 10–100 Kolmogorov lengths). These results are in good agreement with previous experiments that found clustering of inertial droplets with St?≈ 1 at scales on the order of 10η. Ultimately, our results support the hypothesis that turbulence-induced preferential concentration and enhanced settling can lead to significant increases in the collision probability for inertial droplets in the range 10–50?μm.