G. J. Brereton

Response of a turbulent boundary layer to sinusoidal free-stream unsteadiness

The forced unsteadiness was sinusoidal and was superimposed locally on an otherwise-steady mainstream, beyond a turbulent boundary layer which had developed under constant-pressure conditions. The local response of the boundary layer to the free-stream effects was studied through simultaneous measurements of the u- and v-components of the velocity field

Nucleation in small capillary tubes

Abstract In this paper, the classical theory of nucleation is extended to describe heterogeneous nucleation in small capillary tubes, of diameters of less than 100 μm. The effect of confinement within progressively finer capillary tubes is to increase the required superheat or applied tension before nucleation is observed and is particularly pronounced when the liquid has been pre-compressed under high pressure. Thus, by reducing the tube diameter, the nucleation thresholds or concentrations of gases which remain dissolved in carrier liquids transported along capillaries can be raised. The extended theory requires a pair of experimentally-determined coefficients which account for physico-chemical effects on nucleation at the tube surface. It then yields results for capillary tubes of a range of diameters in good agreement with: (i) experimental measurements of the degree of supersaturation of water with oxygen which can be achieved without nucleation during decompression; and (ii) measurements of the temperature at which nucleation of pure water is observed.

Aqueous Oxygen A Highly O2-Supersaturated Infusate for Regional Correction of Hypoxemia and Production of Hyperoxemia

BACKGROUND High levels of hyperoxemia may have utility in the treatment of regional tissue ischemia, but current methods for its implementation are impractical. A catheter-based method for infusion of O2, dissolved in a crystalloid solution at extremely high concentrations, ie, 1 to 3 mL O2/g (aqueous oxygen [AO]), into blood without bubble nucleation was recently developed for the potential hyperoxemic treatment of regional tissue ischemia. METHODS AND RESULTS To test the hypotheses that hypoxemia is correctable and that hyperoxemia can be produced locally by AO infusion, normal saline equilibrated with O2 at 3 MPa (30 bar; 1 mL O2/g) was delivered into arterial blood in two different animal models. In 15 New Zealand White rabbits with systemic hypoxemia, AO was infused into the midabdominal aorta at 1 g/min. Mean distal arterial PO2 increased to 236+/-113 and 593+/-114 mm Hg on 1-hour periods of air and O2 breathing, respectively, from a baseline of 70+/-10 mm Hg (P<.01). In contrast, infusion of ordinary normal saline in a control group (n=7) had no effect on arterial PO2. No differences between groups (P>.05) in temporal changes in blood counts and chemistries were identified. In 10 dogs, low coronary blood flow in the circumflex artery was delivered with a roller pump through the central channel of an occluding balloon catheter. Hypoxemic, normoxemic, and AO-induced hyperoxemic blood perfusates (mean PO2, 52+/-4, 111+/-22, and 504+/-72 mm Hg, respectively) were infused for 3-minute periods in a randomized sequence. Short-axis two-dimensional echocardiography demonstrated a significant decrease (P<.05) in left ventricular ejection fraction compared with baseline physiological values with low-flow hypoxemic and normoxemic perfusion but not with low-flow hyperoxemic perfusion. CONCLUSIONS Intra-arterial AO infusion was effective in these models for regional correction of hypoxemia and production of hyperoxemia.

A Frequency-Domain Filtering Technique for Triple Decomposition of Unsteady Turbulent Flow

A new technique is presented for decomposing unsteady turbulent flow variables into their organized unsteady and turbulent components, which appears to offer some significant advantages over existing ones. The technique uses power-spectral estimates of data to deduce the optimal frequency-domain filter for determining the organized and turbulent components of a time series of data. When contrasted with the phase-averaging technique, this method can be thought of as replacing the assumption that the organized motion is identically reproduced in successive cycles of known periodicity by a more general condition

Dynamic response of boundary‐layer turbulence to oscillatory shear

The temporal response of a well‐developed turbulent boundary layer to the superposition of oscillatory shear has been measured experimentally, over a wide range of frequencies. The response is primarily a periodic organization in magnitude of components of the turbulent velocity field at the forcing frequency. Oscillatory production of turbulence arises predominantly as a modulation of the mean production process in the parent boundary layer. Close to the wall, the relative phases of response of components of turbulent kinetic energy indicate that temporal redistribution of turbulent kinetic energy is driven by robust coherent motions of the underlying mean flow. The local directions of redistribution deduced from these measurements indicate a wall impingement (splatting) effect, consistent with characterizations from numerical simulation.

Aqueous Oxygen Attenuation of Reperfusion Microvascular Ischemia in a Canine Model of Myocardial Infarction

Uncorrected microvascular ischemia may contribute to left ventricular impairment during reperfusion after prolonged coronary artery occlusion. Attenuation of such ischemia in microvessels with impaired erythrocyte flow may require delivery of oxygen at high levels in plasma. Intraarterial infusion of aqueous oxygen (AO) can be used in a site specific manner to achieve hyperoxemic levels of oxygenation in the perfusate. With this new approach, the hypothesis was tested that reperfusion microvascular ischemia can be attenuated.After a 90 min coronary balloon occlusion in a canine model, AO hyperoxemic intracoronary perfusion was performed for 90 min after a 30 min period of autoreperfusion. Control groups consisted of normoxemic reperfusion, both passive (autoreperfusion) and active (roller pump). A significant improvement in left ventricular ejection fraction (p < 0.05) at 2 hr of reperfusion was noted only in the AO hyperoxemia group (17 ± 6% by two dimensional echo), without a significant reduction in the improvement 1 hr after termination of treatment. During AO hyperoxemic perfusion, ECG ST segment isoelectric deviation normalized, and frequency of ventricular premature contractions was significantly reduced, in contrast to the autoreperfusion control group (p < 0.05). Microvascular blood flow, measured as the ischemic/normal left ventricular segment ratio by radiolabeled microspheres immediately after AO hyperoxemic perfusion, was double the value of the autoreperfusion control group at 2 hr of reperfusion (p < 0.05).We conclude that reperfusion microvascular ischemia is attenuated by intracoronary AO hyperoxemic perfusion and acutely improves left ventricular function in this model.

Stochastic estimation as a statistical tool for approximating turbulent conditional averages

In this paper, extensions to conventional stochastic estimation techniques are presented, whereby uncertainties in individual estimates may be deduced. Test applications to time series of velocity measurements in a turbulent boundary layer confirm the fidelity of the uncertainty estimation procedure and illustrate how the optimal choice of stochastic estimation model can be strongly dependent on the event upon which the average is conditioned. They also demonstrate how stochastic estimations may be refined to yield more accurate descriptions of particular coherent motions, and how they can reveal the existence of rare events, different in statistical character to their more frequent counterparts, which might otherwise be undetected by conventional stochastic estimation.

Exact solutions for some fully developed laminar pipe flows undergoing arbitrary unsteadiness

New representations of exact solutions are presented for time-unsteady, fully developed laminar pipe flows of constant-property Newtonian fluids. They extend the well-known steady Hagen-Poiseuille solutions to flows that are initially steady or stationary and subsequently undergo transients with arbitrary time unsteadiness. In these unsteady-flow solutions, each Hagen-Poiseuille result is reexpressed as the momentary steady-flow relation for one variable as a function of the other, together with an additive functional of the other’s time derivative. The functional expressions are convolution integrals over the entire history of the transient and represent the memory of the pipe-flow Navier-Stokes equations of earlier values of transient quantities.

The interdependence of friction, pressure gradient, and flow rate in unsteady laminar parallel flows

Solutions to the Laplace-transformed Navier–Stokes equations are developed that describe transients in fully developed channel and pipe flow. The relative ease with which inverse Laplace transforms can be carried out numerically makes it straightforward to find the form of new expressions relating flow rate, pressure gradient, and wall friction, for flows of arbitrary unsteadiness in time. In particular, expressions for flow rates and cumulative throughflows are derived in terms of wall shear stress and pressure–gradient histories, together with a channel-flow counterpart to Zielke’s pipe-flow friction law [J. Basic Eng. 90, 109 (1968)] expressing wall shear stress as a functional of flow-rate history. It is shown how these results can be expressed as the unsteady counterparts to well-known steady-flow relationships. The relative importance of unsteady effects to quasisteady ones is determined by a dimensionless parameter of the form (1/U)(∂U/∂t)R2/ν, where R is the span of the duct. When departures from ...

An adaptive turbulence filter for decomposition of organized turbulent flows

A new decomposition has been developed in which turbulent processes in shear flows may be represented as a combination of organized and more random turbulent motions. Each component is modeled as a summation of its characteristic eddies, of strength that varies in time and space as a function of the entire process. The contribution of all turbulent eddies of the more random component are estimated with an adaptive turbulence filter, which recognizes this component as the orthogonal partner to organized motion, with a power density spectrum of appropriate shape. The decomposition recovers organized motion from time and space series of data in a physically meaningful way, and can be used to characterize interaction between coherent and more random motions. It also provides an estimate for the turbulence in shear flows that are too complex for a meaningful average motion to be identified.