Maximum Turbulence Research Articles

Turbulent Structure of Water and Clay Suspensions with Bed Load

The effect on turbulence of adding coarse sand, carried as bed load, in flows of water and clay suspensions was studied. The results showed that the mean velocity of the flows carrying bed load was smaller and the turbulence intensity was greater than those in the flows without bed load. The presence of bed load resulted in higher Reynolds stress and larger velocity gradient, and thus greatly enhanced the energy transmitted from the mean motion to turbulence. In flows of clay suspension, turbulence developed in the lower flow with quite high turbulence intensity, while laminar flow occurred in the upper flow. However, with bed load the entire flow became turbulent. The position of maximum turbulence intensity was more distant from the bed. The average turbulence intensity increased with rate of bed‐load transport. The spectrum of turbulent energy shifted to higher frequencies in bed‐load‐carrying flows.

Prediction of the maximum turbulence intensities generated by grid-plate obstacles in explosion-induced flows

A method is presented to predict the maximum turbulence levels generated downstread of a grid-plateobstacle by an explosion-induced flow. From theoretical considerations, it is shown that the turbulence intensity, defined as the ratio of the root-mean-square (rms) turbulence velocity ú to the mean velocity of the flow U , is simply given by the equation u ′ / U = C Τ K where K is the pressure-loss coefficient and C T is a constant (equal to 0.58 in theory). Established techniquesfor the prediction of flow-pressure-loss coefficient K across obstacles in steady-state flows were used, and practical values of the constant C T were determined by using measured data of the peak turbulence downstream of grid plates in steady-state flows. The applicability of such steady-state flow concepts was supported by experimental evidence. Two practical values of C T were determined: 0.225 for “thin” or sharp-edged obstacles and 0.076 for “thick” or round-edged ones. The model was shown to predict available turbulence measurements in both transient (explosion-generated) and steady-state flows.

Coherent launch-site atmospheric wind sounder: theory and experiment

The coherent launch-site atmospheric wind sounder (CLAWS) is a lidar atmospheric wind sensor designed to measure the winds above space launch facilities to an altitude of 20 km. In our development studies, lidar sensor requirements are defined, a system to meet those requirements is defined and built, and the concept is evaluated, with recommendations for the most feasible and cost-effective lidar system for use as an input to a guidance and control system for missile or spacecraft launches. The ability of CLAWS to meet NASA goals for increased safety and launch/mission flexibility is evaluated in a field test program at Kennedy Space Center (KSC) in which we investigate maximum detection range, refractive turbulence, and aerosol backscattering efficiency. The Nd:YAG coherent lidar operating at 1.06 µm with 1-J energy per pulse is able to make real-time measurements of the three-dimensional wind field at KSC to an altitude of 26 km, in good agreement with our performance simulations. It also shows the height and thickness of the volcanic layer caused by the volcanic eruption of Mount Pinatubo in the Philippines.

The Flow Inside a Model Gas Turbine Combustor: Calculations

The present study assesses the ability of the k-ε turbulence model to calculate the flow inside gas turbine combustors. Results of calculations using a cylindrical system of coordinates, hybrid differencing, and a mesh with about 40,000 nodes are compared with velocity measurements of the flow inside a perspex model of can-type gas turbine combustor. The larger discrepancies between measurements and predictions were found in the primary region. The complexity of the flow near the primary jet impingement led to underprediction of the maximum negative axial velocity and turbulence kinetic energy by about 35 and 20 percent, respectively. The calculated results exhibited higher levels of momentum diffusion compared to the experiments and did not show the two contrarotating vortices created between the primary jets; no qualitative agreement with the azimuthal velocity downstream of the primary jets could be achieved. Despite these deficiencies, the model gave acceptable results in other regions of the combustor and correct prediction of the main features of the combustor flow was possible.

Influence of a Shroud on Swirler Flow Fields

The flow fields for swirlers with and without a shroud were measured using a twocomponent laser-Doppler velocimeter (LDV) system. The primary goal of this study is to investigate the effect of shrouds on swirler flow fields, in order to provide useful design information for the manufacture of gas turbine fuel nozzles, and to supply benchmark data for comparison with numerical predictions. As a result of the measurements, the radial distributions of three mean velocity components, turbulence intensity, and shear stresses were obtained at five locations (x/d = 0.1, 1, 2, 4, and 8) along the axis of the swirlers. The LDV system was operated in the 20 deg off-axis forward scatter mode with beam expanders and Bragg cell frequency shifting on both components. The flow was seeded by 1 μm mean diameter atomized particles of glycerol and water (50/50) mixture. Comparison of flow with and without the shroud showed that the jet diameter was much smaller, and the flow deceleration in the downstream direction was faster, due to the influence of the shroud, at the same supply pressure (750 mm H2O). As a result of the significant reduction in the swirl number due to the addition of the shroud, the recirculation zone disappeared. In addition to its influence on recirculation, the shroud caused a radially inward shift of the maximum mean and turbulence parameters at all axial locations. The anisotropy of turbulence increased as compared to the values for the swirler without the shroud.

Laminar-Transitional Convection From Repeated Ribs in a Channel

Velocimetry, heat transfer, and pressure drop experiments are reported for laminar/transitional air flow in a channel containing rectangular transverse ribs located along one channel wall. The geometry is intended to represent an array of low profile electronic packages. At fixed pumping power per unit channel volume, the heat transfer rate per unit volume is independent of rib-to-rib spacing and increases with decreasing wall-to-wall spacing. The fully developed, rib-average heat transfer coefficient is found to be linearly related to the maximum streamwise rms turbulence measured above the rib-tops. Linear correlations, in terms of a descriptor of the rms streamwise turbulence, are shown to unify heat transfer/pressure drop data for channels containing either two-or three-dimensional protrusions.

The Adiabatic Heat Transfer Coefficient and the Superposition Kernel Function: Part 1—Data for Arrays of Flatpacks for Different Flow Conditions

This paper describes an investigation of the forced convection heat transfer and pressure drop characteristics of a regular in-line array of flatpacks for several channel heights and inlet velocities. The work has both practical and theoretical interest since it relates to technical problems now faced by the electronics industry, and it embodies one of the most general heat transfer problems: non-uniform heat release from nonuniform geometries. To predict operating temperatures in situations where the wall temperature distribution is non-uniform, one must use superposition. Both the adiabatic heat transfer coefficient, had, and the superposition kernel functions, g*, are required. The problem can be solved using superposition directly (had and g*) or indirectly (using had and g* to calculate the correct value of hm). Either way the superposition data is required. This work presents the first full set of superposition data for flatpack arrays. Part 1 presents heat transfer and pressure drop results and part 2 presents a model for heat transfer that is based on the maximum turbulence fluctuations in the channel.

The Adiabatic Heat Transfer Coefficient and the Superposition Kernel Function: Part 2—Modeling Flatpack Data as a Function of Channel Turbulence

This paper describes an investigation of the forced convection heat transfer and pressure drop characteristics of a regular in-line array of flatpacks for several channel heights and inlet velocities. The work has both practical and theoretical interest since it relates to technical problems now faced by the electronics industry, and it embodies one of the most general heat transfer problems: nonuniform heat release from nonuniform geometries. To predict operating temperatures in situations where the wall temperature distribution is nonuniform, one must use superposition. Both the adiabatic heat transfer coefficient, had and the superposition kernel functions, g* are required. The problem can be solved using superposition directly (had and g*) or indirectly (using had and g* to calculate the correct value of hm). Either way the superposition data is required. This work presents the first full set of superposition data for flatpack arrays. Part 1 presents heat transfer and pressure drop results and part 2 presents a model for heat transfer that is based on the maximum turbulence fluctuations in the channel.

Characteristics of excited shear layers

Numerical simulations of the two dimensional turbulent mixing layer subjected to controlled excitation leading to turbulence suppression are carried out using a vortex-in-cell method. Details of the flow characteristics, in terms of the contours of vorticity, stream function and Reynolds stress are presented and discussed. The results of an experimental study of an axisymnetric mixing Layer subjected to high amplitudes of excitation are also presented. The experimental results support the numerical finding that at high amplitudes of excitation the maximum turbulence suppression occurs at a frequency higher than the maximally unstable frequency predicted by the linear theory.

Application of a Reynolds stress turbulence model to the compressible shear layer

Theoretically based turbulence models have had success in predicting many features of incompressible, free shear layers. However, attempts to extend these models to the high-speed, compressible shear layer have been less effective. In the present work, the compressible shear layer was studied with a second-order turbulence closure, which initially used only variable density extensions of incompressible models for the Reynolds stress transport equation and the dissipation rate transport equation. The quasi-incompressible closure was unsuccessful; the predicted effect of the convective Mach number on the shear layer growth rate was significantly smaller than that observed in experiments. Having thus confirmed that compressibility effects have to be explicitly considered, a new model for the compressible dissipation was introduced into the closure. This model is based on a low Mach number, asymptotic analysis of the Navier-Stokes equations, and on direct numerical simulation of compressible, isotropic turbulence. The use of the new model for the compressible dissipation led to good agreement of the computed growth rates with the experimental data. Both the computations and the experiments indicate a dramatic reduction in the growth rate when the convective Mach number is increased. Experimental data on the normalized maximum turbulence intensities and shear stress also show a reduction with increasing Mach number.

Endothelial Cell Morphology around Graded Stenoses of the Dog Common Carotid Artery

Graded stenoses (40-64% diameter reduction) were applied to dog common carotid arteries under anesthesia, and 27 days later the poststenotic velocity field was examined using transcutaneous pulsed Doppler velocimetry. Following in situ fixation, endothelial cell area, shape and orientation was measured by digitizing endothelial cell outlines in scanning electron micrographs. Endothelial cell area in the vicinity of the stenosis throat (within +/- 1 diameter) was reduced. No relation to stenosis hemodynamics was apparent. Cell area was increased 5 diameters downstream; in this region, there was turbulence due to jet break up. Cells were maximally rounded at the stenosis throat and gradually returned to upstream elongation by 5 diameters downstream. There was a region of flow separation and low velocity recirculation between the stenosis and the site of maximum turbulence, thus, downstream cell rounding was associated with flow separation. Subdivision of the experiments into four stenosis grades revealed little variation of the pattern of morphology between groups. It is possible that this is related to flow limitation with increasing grade. Elongation of endothelial cells through the entrance of the stenosis was apparent in only four experiments. Poststenotic dilatation of greater than 12% external diameter increase was noted in only four out of the sixteen experiments. In general, the relation of cell elongation to shear stress is supported, but this relationship does not fully explain endothelial cell shape at a stenosis.

Axial flow velocity patterns in a normal human pulmonary artery model: Pulsatile in vitro studies

It has been clinically observed that the flow velocity patterns in the pulmonary artery are directly modified by disease. The present study addresses the hypothesis that altered velocity patterns relate to the severity of various diseases in the pulmonary artery. This paper lays a foundation for that analysis by providing a detailed description of flow velocity patterns in the normal pulmonary artery, using flow visualization and laser Doppler anemometry techniques. The studies were conducted in an in vitro rigid model in a right heart pulse duplicator system. In the main pulmonary artery, a broad central flow field was observed throughout systole. The maximum axial velocity (150 cm s −1) was measured at peak systole. In the left pulmonary artery, the axial velocities were approximately evenly distributed in the perpendicular plane. However, in the bifurcation plane, they were slightly skewed toward the inner wall at peak systole and during the deceleration phase. In the right pulmonary artery, the axial velocity in the perpendicular plane had a very marked M-shaped profile at peak systole and during the deceleration phase, due to a pair of strong secondary flows. In the bifurcation plane, higher axial velocities were observed along the inner wall, while lower axial velocities were observed along the outer wall and in the center. Overall, relatively low levels of turbulence were observed in all the branches during systole. The maximum turbulence intensity measured was at the boundary of the broad central flow field in the main pulmonary artery at peak systole.

Turbulence structure in a shock wave/turbulent boundary-layer interaction

The boundary-layer turbulence structure in a two-dimensional 24-deg compression corner flow was investigated experimentally in the Princeton University 203 x 203-mm supersonic blowdown wind tunnel. The incoming conditions were M<» = 2.84, Re^/l = 6.5 x 10, and 60 = 26 mm. The results show that the maximum mass-flux turbulence intensity is amplified by a factor of about 5, whereas the mean mass flux increases by only 2 through compression. Probability density functions (pfd's) of the mass-flux fluctuations show that upstream the distributions are Gaussian-like and typical of a fully turbulent boundary layer. Behind the interaction, however, the distributions are bimodal, centering about a level indicative of the freestream mass flux and a lower level indicative of the mass flux near the wall. The unsteady shock motion does not appear to contribute signficantly to the turbulence amplification. It is suggested that the strong, bimodal mixing indicated in the pdf's is probably caused by the presence of large-scale motions associated with the instability of the inflectional velocity profiles observed downstream of the interaction. Space-time correlations of the mass flux and wall pressure were measured, as well as the level of intermittency of the boundary layer to further characterize the changes in the boundary-layer structure through the interaction.

The Effect of Inlet Turbulence Intensity on the Reattachment Process Over a Backward-Facing Step

Behavior of a separated shear layer over a backward-facing step and its reattachment is presented when a two-dimensional cavity or rod is installed upstream of the step in order to change local turbulence intensity in addition to grid turbulence in the free-stream. The reattachment length has a strong negative correlation with maximum turbulence intensity near the wall at the separation point. Turbulence in the entrainment region immediately downstream of the step plays an important role in determining the reattachment length.

Turbulent oscillatory flow over rough beds

Velocity measurements are presented for turbulent oscillatory flow over rough beds. Two components of velocity were measured with a laser-Doppler anemometer and the rough beds consisted of a single layer of sand, gravel or pebbles on a flat surface. Turbulence intensities showed significant variation during the course of the cycle. Maximum turbulence intensity propagated out from the bed at a more or less constant velocity for all beds. Variation of time-mean turbulence intensity with height was qualitatively similar to that observed in steady flows. Reynolds stress showed several interesting features. Near the bed, maximum Reynolds stress was in phase with one of the two peaks of turbulence intensity but further out it was in phase with the other, i.e. the phase of maximum Reynolds stress showed a 180° phase shift at a certain height above the bed. A related effect was seen in the time-mean eddy viscosity which was negative near the bed but positive further out. It is suggested that these effects are caused by the jets of fluid associated with vortex formation and ejection in oscillatory flow over rough beds. Maximum Reynolds stress was also significantly less than the horizontal force per unit area of bed obtained from the momentum integral. Eddy viscosity and mixing length were found to vary significantly during the course of the cycle. Variation with height of time-mean values of these variables showed similar trends, except in the near-bed region, to those observed in steady flow but derived values of the Karman constant were significantly lower. Non-dimensional defect velocity appeared to show dependence on a/ks as well as on y/δ in the outer layer away from the bed, even at high Reynolds numbers.

Airflow-terrain interactions through a mountain gap, with an example of eolian activity beneath an atmospheric hydraulic jump

The integration of atmospheric soundings from a fully instrumented aircraft with detailed sedimentary and geomorphic analyses of eolian features in the Ferris dune field of south-central Wyoming lends insight into the manner in which topography interacts with airflow to modify eolian activity. Topographically modified airflow results in zones of airflow deceleration, acceleration, and enhanced atmospheric turbulence, all of which influence the surface morphology and sedimentology. Extreme lateral confluence of prevailing airflow produces accelerated, unidirectional winds. These winds correlate with unusually continuous and elongate parabolic dunes that extend into a mountain gap (Windy Gap). Persistently heightened winds produced at the entrance to Windy Gap have resulted in a concentration of active sand dunes that lack slipfaces. Common development of a strongly amplified atmospheric wave analogous to a hydraulic jump in the gap contributes to the formation of a variety of eolian features that mantle the surface of Windy Gap and the Ferris dune field tail. Heightened, unidirectional winds in this zone promote grain-size segregation, the formation of elongated and aligned sand drifts, climbing and falling dunes, elongate scour streaks, and parabolic dunes that have low-angle (< 20/sup 0/) cross-stratification. Deflation of bedrock and loose sediment has been enhanced in the zonemore » of maximum turbulence beneath the hydraulic jump.« less

Relaxation effects observed for turbulent flow over a wavy surface

Measurements are presented for different flow rates of the time-averaged wall shear stress and of the root-mean-square value of the turbulent fluctuations along a small-amplitude sinusoidally shaped solid surface. The stresses are found to have a variation along the wave surface which is also sinusoidal. The influence of flow rate and of wavelength on the amplitude and phase angle can be correlated by using a wave-number α+ made dimensionless with wall parameters.It is found that for α+ &gt; 10−2 a frozen-turbulence assumption can be made whereby the influence of the wave-induced variation of the mixing length can be ignored. For α+ &lt; 10−4 the flow can be described by assuming the Reynolds stresses are given by an equilibrium assumption. The relaxation from this equilibrium condition is characterized by a sharp change in the phase angle for 6 × 10−4 &lt; α+ &lt; 10−3.This relaxation is associated with physical processes in the viscous wall region which are not yet understood. It is argued that these are principally related to the wave-induced variation of the pressure gradient.The wave-induced variation of the turbulent fluctuations in the wall shear stress also indicate a relaxation in that the maximum turbulence intensity is located in a region of favourable pressure gradient.

Hot-wire Investigation of an Unseparated Shock-VVave/Turbulent Boundary-Layer Interaction

Detailed hot-wire measurements of the longitudinal component of the mass-flow fluctuations have been made for an adiabatic fully attached shock-wave/turbulent boundary-layer interaction. The shock wave is induced by an 8-deg compression corner at a Mach number of 2.85 and a unit Reynolds number of 6.3 X10 m ~ l . The data indicate that the absolute mass-flow turbulence intensity increases abruptly through the shock wave, and continues to increase with further distance downstream. When nondimensionalized by the local freestream massflow rate the maximum turbulence intensity actually exceeds the upstream equilibrium value at the last measurement station. Measurements of the probability density of the fluctuations reveal only minor difference between the boundary-layer behavior upstream and downstream of the shock. A significant dip in the kurtosis was observed near the point of maximum intensity, which also coincided with the onset of intermittency.

Effects of imperfect spatial resolution on measurements of wall-bounded turbulentbx shear flows

The effects of imperfect spatial resolution on hot-film and hot-wire measurements of wall-bounded turbulent shear flows were studied. Two hot-film probes of different length were used for measurements of fully developed turbulent channel flow in a water tunnel. In the near-wall region significant effects of spanwise spatial averaging due to finite probe size were found for a probe 32 viscous units long. The maximum turbulence intensity attained a 10% lower value than that for a probe about half as long, and the zero-crossing of the skewness factor was shifted away from the wall. This could be attributed to spatial averaging of narrow low-speed regions. Results for different Reynolds numbers, but with the same sensor length in viscous units, showed that Reynolds-number effects are small, and that much of the reported discrepancies for turbulence measurements in the near-wall region can be ascribed to effects of imperfect spatial resolution. Also the number of events detected with the variable-interval time-averaging (VITA) technique was found to depend strongly on the sensor length, especially for events with short duration.

Turbulence originating from convectively stable internal waves

A theory has recently been developed for the generation of turbulence by unstable tides and gravity waves (Lindzen, 1981). Omitted from this theory was consideration of the possibility that turbulence could be generated by gravity waves which were not convectively unstable (i.e., breaking). The possibility of such generation was suggested by a result of McComas and Bretherton (1977) showing that internal gravity waves are indeed unstable to other gravity waves with shorter wavelengths. The present paper estimates the maximum turbulence which might be generated by such a process and shows that even this turbulence would not greatly alter earlier results.