Stanley J. Kleis

Modification of grid-generated turbulence by solid particles

The effects of almost neutrally buoyant plastic particles and heavy glass particles on grid-generated turbulence were studied experimentally in a water flow facility. From measured velocities of both the solid and liquid phases, drag and slip velocities of the particles and energy spectra and dissipation rates of the liquid phase were estimated. A monotonic increase in the dissipation rate of the turbulence energy with particle concentration was observed. The increase in energy dissipation rate for suspensions of glass particles was about twice that of suspensions of plastic particles. The increase in dissipation was larger than that predicted by a simple model based on the slip velocities between the phases

A ‘turbulent spot’ in an axisymmetric free shear layer. Part 2

The mechanics of a spark-induced coherent structure (called a ‘spot’) in the turbulent mixing layer of a 12.7 cm diameter incompressible air jet has been investigated through phase-locked measurements at three streamwise stations. Phase averages have been obtained from 200 realizations of X-wire (time-series) data after these are optimally time-aligned with respect to one another through an iterative process of maximization of cross-correlation of individual realizations with the ensemble average. Realizations that are grossly out of alignment owing to turbulence-induced distortions have been rejected; the rejection ratio increases with increasing radial position. Data include phase-average time series of background turbulence intensities, coherent and background Reynolds stresses, vorticity and intermittency at different transverse positions. Spatial distributions of these properties over the extent of the spot have been presented as contour maps. The computed pseudo-stream-functions have been compared with the phase-average streamlines inferred from the measured distributions of the velocity vector. Comparison with the phase-average intermittency contours show that the pseudo-stream-functions are reliable and, even though the integration involved produces smoothed-out stream functions, are most useful in deducing the structure dynamics and its convection velocity. The spark-induced spot is an elongated large-scale coherent vortical structure spanning the entire thickness of the mixing layer, which moves downstream at a convection velocity of about 0.68 U e . The dynamics of the turbulent mixing layer spot, whose signature is buried in the large-amplitude background fluctuations, is much more complicated than that of the boundary-layer spot. The spot transports jet-core fluid outwards at its front and entrains ambient fluid primarily at its back; the outward-momentum transport dominates the inward transport. The Reynolds stress contribution by the spot structure is noticeably larger than that due to the background turbulence. The coherent structure vorticity is significantly modified by the structure-induced organization of the background Reynolds stress at the locations of ‘saddle points’ of the latters distribution. The vorticity, intermittency and other turbulence measures, zone averaged over the extent of the spot, compare well with the time-average values, thus suggesting that the spark-induced ‘spot’ is probably not different from a naturally occurring large-scale coherent structure.

Initial streamwise vorticity formation in a two-stream mixing layer

The characteristics of the initial formation of streamwise vortices and their relation to the spanwise structures are experimentally investigated at several phases of the initial roll-up and pairing in a two-stream mixing layer at high Reynolds numbers (up to Re0 = 1.25 x lo3). The spanwise structures were stabilized by acoustically exciting the mixing layer at the natural instability frequency and its first subharmonic. No artificial spanwise forcing was applied. Quantitative information was obtained through conditional sampling with the forcing signal as the phase reference. Time traces of the three velocity components were recorded by hot-wire anemometry at a grid of locations on planes normal to the flow at four successive streamwise locations which include the roll-up and first pairing of the spanwise structures. From these measurements, three-dimensional distributions of ensemble-average vorticity components were computed and analysed. The results show that the spanwise structures remain mostly two-dimensional from roll-up to the end of the first pairing, except for small oscillations in curvature of their axes. Concentrated streamwise vortices evolve at locations corresponding to small ‘kinks’ in the spanwise rollers and first appear at the beginning of the pairing process. Their subsequent development bears many similarities with the results of smallperturbation simulations. They expand laterally, forming counter-rotating rib vortices that grow almost exponentially in the braid region. In the core region, a complex but organized streamwise vorticity pattern emerges. The pattern consists of layered streamwise vortices whose orientation suggests that the oscillation of the spanwise roller core is TI radians out of phase with that of the streamwise vortex lines that join the rib vortices. The peak streamwise vorticity and circulation in the core region increase until the completion of the spanwise structure pairing when the layered pattern collapses. Measurements at large spanwise locations conducted downstream show that the ‘kink‘ is not the only location of streamwise vorticity generation: streamwise vortices develop at nearly periodic spanwise locations other than that of the initial ‘kink’. The initial spacing between streamwise vortices agrees with previous stability analysis. At the end of the spanwise structure pairing, isolated pairings between streamwise vortices take place, but a global doubling in spanwise wavelength does not occur.

Reproducibility and variability of digital thermal monitoring of vascular reactivity

Background:  Previous studies demonstrated that digital thermal monitoring (DTM) of vascular reactivity, a new test for vascular function assessment, is well correlated with Framingham Risk Score, coronary calcium score and CT angiography. This study evaluates the variability and reproducibility of DTM measurements. We hypothesized that DTM is reproducible, and its variability falls within the accepted range of clinical diagnostic tests.

Sensitivity of Digital Thermal Monitoring Parameters to Reactive Hyperemia

Both structural and functional evaluations of the endothelium exist in order to diagnose cardiovascular disease (CVD) in its asymptomatic stages. Vascular reactivity, a functional evaluation of the endothelium in response to factors such as occlusion, cold, and stress, in addition to plasma markers, is the most widely accepted test and has been found to be a better predictor of the health of the endothelium than structural assessment tools such as coronary calcium scores or carotid intima-media thickness. Among the vascular reactivity assessment techniques available, digital thermal monitoring (DTM) is a noninvasive technique that measures the recovery of fingertip temperature after 2-5 min of brachial occlusion. On release of occlusion, the finger temperature responds to the amount of blood flow rate overshoot referred to as reactive hyperemia (RH), which has been shown to correlate with vascular health. Recent clinical trials have confirmed the potential importance of DTM as an early stage predictor of CVD. Numerical simulations of a finger were carried out to establish the relationship between DTM and RH. The model finger consisted of essential components including bone, tissue, major blood vessels (macrovasculature), skin, and microvasculature. The macrovasculature was represented by a pair of arteries and veins, while the microvasculature was represented by a porous medium. The time-dependent Navier-Stokes and energy equations were numerically solved to describe the temperature distribution in and around the finger. The blood flow waveform postocclusion, an input to the numerical model, was modeled as an instantaneous overshoot in flow rate (RH) followed by an exponential decay back to baseline flow rate. Simulation results were similar to clinically measured fingertip temperature profiles in terms of basic shape, temperature variations, and time delays at time scales associated with both heat conduction and blood perfusion. The DTM parameters currently in clinical use were evaluated and their sensitivity to RH was established. Among the parameters presented, temperature rebound (TR) was shown to have the best correlation with the level of RH with good sensitivity for the range of flow rates studied. It was shown that both TR and the equilibrium start temperature (representing the baseline flow rate) are necessary to identify the amount of RH and, thus, to establish criteria for predicting the state of specific patients cardiovascular health.

Fractal properties of isovelocity surfaces in high Reynolds number laboratory shear flows

The fractal properties of isovelocity surfaces are studied in three high Reynolds number (Rλ≊2.0×102–3.2×103) laboratory shear flows using the standard box‐counting method. The fractal dimension D=−d(log Nr)/d(log r) was estimated within the range of box sizes r from several Kolmogorov scales up to several integral scales (Nr is the number of boxes with size r required to cover the line intersection of an isovelocity surface). The inertial subrange was of particular interest in this investigation. Measurements were carried out for external intermittency factors γ≊0.6–1.0. The data were processed using threshold levels U±2.5u’ (U and u’ denote mean and rms values of longitudinal velocity). Over the parameters studied, no wide range of constant fractal dimension was found. On the other hand, the accuracy of constant fractal dimension approximation with D≊0.4 over the inertial subranges was shown to be similar to that of the Kolmogorov [Dokl. Akad. Nauk SSSR 30, 301 (1941)] ‘‘two‐thirds law.’’

Influence of vortex-pairing location on the three-dimensional evolution of plane mixing layers

Detailed three-dimensional measurements of the first vortex pairing of a large plane mixing layer reveal excitation of several three-dimensional instability modes. Time evolution in three-dimensional space (x,y,z, t) shows how the two-dimensional rollers become three-dimensional as they approach each other and that the linear growth of at least two instability waves leads to a spanwise periodic pairing. The results are based on phase-locked measurements made in three-dimensional spatial grids, with a mesh spacing of 8.5% of the fundamental instability wavelength. Spanwise-uniform, periodic acoustic excitation stabilizes the most probable two-dimensional natural features - roll-up and first pairing. The second subharmonic is added to study the effect of alternate streamwise pairing locations on the three-dimensional characteristics of vortex pairing. Velocities are measured using hot-wire anemometry, and the coherent structures are reconstructed from the ensemble-averaged vorticity field

New Indices of Endothelial Function Measured by Digital Thermal Monitoring of Vascular Reactivity: Data from 6084 Patients Registry

Background. Endothelial function is viewed as a barometer of cardiovascular health and plays a central role in vascular reactivity. Several studies showed digital thermal monitoring (DTM) as a simple noninvasive method to measure vascular reactivity that is correlated with atherosclerosis risk factors and coronary artery disease. Objectives. To further evaluate the relations between patient characteristics and DTM indices in a large patient registry. Methods. DTM measures were correlated with age, sex, heart rate, and systolic and diastolic blood pressure in 6084 patients from 18 clinics. Results. DTM vascular reactivity index (VRI) was normally distributed and inversely correlated with age (r = −0.21, p < 0.0001). Thirteen percent of VRI tests were categorized as poor vascular reactivity (VRI < 1.0), 70 percent as intermediate (1.0 ≤ VRI < 2.0), and 17 percent as good (VRI ≥ 2.0). Poor VRI (<1.0) was noted in 6% of <50 y, 10% of 50–70 y, and 18% of ≥70 y. In multiple linear regression analyses, age, sex, and diastolic blood pressure were significant but weak predictors of VRI. Conclusions. As the largest database of finger-based vascular reactivity measurement, this report adds to prior findings that VRI is a meaningful physiological marker and reflects a high level of residual risk found in patients currently under care.

DOUBLE-HELICAL PAIRING IN PLANE MIXING LAYERS

Double-helical pairing is identified in the second vortex pairing of a spanwise-unforced plane mixing layer. Phase-locked measurements using two-dimensional (2D) periodic acoustic excitation reveal that as the rollers of the second pairing approach each other, three-dimensional (3D) modes become amplified in each roller. Rollers then revolve around each other, forming a double helix with spanwise wavelength on the order of the streamwise lengthscale. Some spanwise areas exhibit a more advanced pairing stage than others, producing “branched” vortex patterns.

Core instabilities and “bridging” in the first pairing of plane mixing layers

Core instabilities and “bridging” in the first pairing of transitional plane mixing layers are identified experimentally through phase-locked measurements. Spanwise-uniform acoustic excitation is employed to stabilize the two-dimensional (2D) instability modes. The three-dimensional (3D) instability modes undergo amplification during pairing of the spanwise rollers. The most amplified 3D instability modes are associated with upstream perturbations generated by small imperfections in the splitter-plate surface that produce a kink [e.g., as in the graph of tanh(z)] and a bend [e.g., as in the graph of sech(z)] in the rollers. These perturbations produce core dynamics, bridging phenomena, and “lambda” structures.