High Shear Gradients Research Articles

Plasma Transforming Growth Factor-β1 (TGF-β1) Levels Increase with Age and Aortic Stenosis Progression and Low-Dose Galunisertib Attenuates Disease Progression in Mice

Objective: Aortic stenosis (AS) is an age-related degenerative valvular disease that ultimately leads to heart failure and sudden death. Platelet-derived TGF-β1 is a major source of plasma TGF-β1 and contributes to AS progression in LDLR -/-ApoB 100/100(LA100), a mouse model that spontaneously develops AS that simulates human pathology, including fibrosis and high shear gradient. Although risk factors, such as hypercholesteremia and hyperglycemia, are like those for coronary artery disease, lowering cholesterol and glucose has not been successful in stopping progression, and there is no biomarker for early detection of AS. In this study, we measured plasma TGF-β1 levels as a function of age in LA100 mice and human AS patients and tested a new approach of partial inhibition of TGF-β signaling to inhibit AS progression in LA100 mice, while limiting cardiotoxicity. Methods: We measured plasma TGF-β1 levels in controls, severe AS patients, and LA100 mice before they developed AS (2-months of age), when they developed moderate AS (1-year), and when they developed severe AS (2-years). LA100 mice at different stages of AS were treated with low-dose galunisertib, an inhibitor of the TGF-β receptor, ALK5. Results: Plasma levels of total TGF-β1 were significantly higher in AS patients (n=34) compared to healthy controls (n=22) and patient controls (n=47) (p<0.001 for AS patients vs. healthy controls and p=0.049 for AS patients vs. patient controls). Age was positively correlated with total TGF-β1 levels in AS patients (r=0.475; p=0.005), but not patient or healthy controls (r=0.12, p=0.43). LA100mice developed AS with aging and plasma TGF-β1 levels in LA100mice (n=21-52; ages 2 months-2 years) were significantly higher than those in age-matched WT mice (n=23); (p<0.0001). Plasma TGF-β1 levels correlated with age in LA100 mice (R=0.6, p=<0.0001). Low-dose oral galunisertib was administered at a concentration of 1 µM in drinking water, giving an average daily dose of ~0.06 mg/kg, compared to a human cancer treatment dose of ~4.2 mg/kg daily. Galunisertib treatment: a) halted AS progression at all stages in LA100 mice compared to vehicle-treated control mice (p<0.05, n=9-13), b) did not change plasma TGF-β1 levels, c) inhibited TGF-β1 signaling as shown by decreased Smad2 phosphorylation in valve cells compared to vehicle-treated animals. An age-dependent increase in neutrophil to lymphocyte ratio (NY/LY), a marker of inflammation, was observed in LA100 mice compared to control mice (n=12-31; p<0.01), and the NY/LY ratio correlated with total TGF-β1 plasma levels (R=0.16; p=0.020). Galunisertib treatment reduced NY/LY ratios in LA100 mice (p=0.03). Conclusions: 1. Increases in plasma TGF-β1 levels may be a valuable surrogate marker of AS progression. 2. Low-dose galunisertib attenuates AS progression in the LA100 mouse model without producing the cardiotoxicity associated with higher doses and thus may warrant further study in human AS patients.

Turbulence Properties of a Deep‐Sea Hydrothermal Plume in a Time‐Variable Cross‐Flow: Field and Model Comparisons for Dante in the Main Endeavour Field

AbstractA large eddy simulation is used to study a high temperature hydrothermal vent plume in a stratified and tidally modulated crossflow, in order to identify its turbulence and mixing characteristics. The model parameters and source conditions that are comparable to the vertical velocity and refractive index fluctuations measured 20 m above the Dante sulfide mound in the Main Endeavour vent Field are a heat transport of 50 MW over a cross‐sectional area of 4 × 4.5 m2. With these model source conditions and output results taken at 20 m above the source with 1 Hz sampling, the shear production of turbulent kinetic energy (TKE), the vertical transport of TKE, and the buoyancy production/dissipation are quantified showing that shear production dominates. Similarly, thermal variance production, and its vertical transport, is also quantified showing that the advective term dominates. Because of enhanced entrainment of ambient water into the plume during strong crossflows, all mean and turbulent quantities show tidally modulated values. Assuming steady state, the dissipation rates are evaluated. During strong crossflows, the tilting of the vertical velocity contours and isotherms plays a critical role in the stability of the plume and in creating high shear and thermal gradients on the upstream side of the plume center axis. These dissipation rates are used to quantify the refractive index fluctuations, and given the high thermal dissipation quantity it is the main contributing factor in acoustic forward scatter.

Rheology and Flow of Complex Fluids: Fundamental and Experimental Aspects

The study of the rheological behavior of complex fluids in relation to their molecular structure is a primary tool to predict their flow behavior. Such study is often conducted under high shear gradients equivalent to those present in the processes. The ultimate goal of this paper is to introduce the fundamental aspects of shear rheology. The experimental aspects include showing how to establish flow curves (stress versus shear gradients or viscosity versus shear gradients) necessary for any numerical simulation of the flows.

Skin Friction Measurements Using Oil Film Interferometry in a 3-D Supersonic Flowfield

The application of oil film interferometry (OFI) for the measurement of global skin friction on the surface beneath a complex three-dimensional supersonic flow is explored in this study. The flowfield is produced by a fin-generated oblique shock interacting with a compressible turbulent boundary layer. Measurements were obtained by modifying a comprehensive acquisition and processing software, previously developed for two-dimensional, low-speed flows, to measure skin friction distributions underneath such flows. This approach required the local flow direction on the surface, and therefore traditional oil flow visualization was coupled with OFI measurements. A cross-correlation-based algorithm was incorporated to extract flow direction from sequences of oil flow visualization images. Photogrammetry was used with both surface oil flow and OFI to ensure proper spatial overlap for the two measurements. Results from this study demonstrate the capability of the approach in a complex, supersonic flow field that contains regions of both separation and reattachment concomitant with a wide range of skin friction values. The skin friction results show excellent homogeneity, standard deviation of 5.1% of the mean, in the undisturbed boundary layer upstream of the interaction and capture well-resolved high shear gradients in the interaction region. A comparison to legacy data from a similar interaction shows matching trends within the developed flow.

VWF Cleavage Products Inhibit Shear-Induced Self-Association

BackgroundVon Willebrand factor (VWF) self-association is key to hemostasis in high-shear regions of the vasculature. Further, in pathologic small vessel thrombosis, VWF can also self-associate, even on an intact endothelium in the absence of vessel injury (Dong et al. Blood, 2002, 100:40339). VWF self-association preferentially occurs between large multimers, which are more sensitive to shear-induced unfolding. This process is regulated by ADAMTS13 proteolysis, which converts circulating ultra-large VWF multimers to smaller, less-adhesive multimers(Sadler. Proc Natl Acad Sci USA, 2002, 99(18):11552-4). ADAMTS13 not only cleaves large multimers, in the process it also generates small VWF cleavage fragments. This process is sometimes excessive, such as in von Willebrand disease (VWD) type 2A, in which large amounts of cleavage products may be present. However, the role of these small VWF fragments in regulating VWF function has not been studied. Here, we propose that small VWF fragments inhibit VWF self-association by competitively inhibiting self-association sites without providing functional substrates to propagate VWF strand formation.MethodsDevice: We developed a technique based on a device recently used by the Diamond laboratory to visualize VWF self-association in vitro (Zhu et al. Biorheology, 2015, 52:303; Herbig and Diamond. J. Thromb. Haem. 2015, 13:1699). The device consists of a PDMS microfluidic channel of width 60 µm with a 30 µm square pillar in the center. Flow through the channel produces high shear gradients next to the block. The inlet shear rate was set at 5000 s-1.VWF fragment preparation: Purified plasma VWF was incubated with purified recombinant ADAMTS13 (with a biotin tag) at a ratio of 5:1 at 37°C in 1.5 M urea for 16 hr. Recombinant ADAMTS13 was then removed with streptavidin-coated beads, and urea was removed with a PD10 column. This produced fragments corresponding to N- and C-termini VWF dimers.Assay: VWF fragments were mixed with purified plasma VWF (5 µg/mL) at a molar ratio (fragment to VWF monomer) of 1:1 and 5:1 and the mixture was immediately perfused into the channel. Results were compared to those from samples that contained either only fragments or only purified VWF (30 µg/mL). A total sample volume of 200 µL was perfused over 10 minutes, and VWF strand formation was monitored in real-time with differential interference contrast and immunofluorescence microscopy.Patient Samples: Samples from patients with type 2A VWD and thrombotic thrombocytopenic purpura (TTP) were obtained from consented adults under a protocol approved by the University of Washington Institutional Review Board. A total of 200 µL of sample was perfused in the channel over 10 minutes.ResultsWe were able to visualize VWF strand formation induced by shear stress alone when plasma was perfused through the channel. The strands formed from the initial adhesion of VWF to the pillar and subsequent self-association of VWF with the adherent VWF. Strands formed at physiologic VWF concentration and shear stress. The addition of small VWF fragments at 1:1 and 5:1 ratios delayed the formation and elongation of VWF strands and reduced the final extent of strand formation by approximately 20% and 60%, respectively. Compared to normal, strand formation was more pronounced in the absence of small VWF fragments (TTP) and less pronounced in the presence of larger amounts of small VWF fragments (VWD 2A).SummaryWe present a microfluidic assay to visualize in real-time VWF strand formation by generating elongational flow (shear gradients). VWF strands thus formed were reduced in the presence of small VWF fragments generated by ADAMTS13 both in a purified system and in patient samples. This result suggests that the hemostatic defect associated with type 2 VWD is a consequence not only of the deficiency of large multimers, but also of the presence of excessive quantities of proteolytic fragments, and that the profound thrombotic phenotype of TTP results not only from excess ULVWF, but also from the deficiency of smaller fragments. DisclosuresNo relevant conflicts of interest to declare.

A Microengineered Matrix to Decouple the Biophysical and Biochemical Mechanisms of Blood Cell Interactions with Thrombi and Vascular Wall Matrices

Biochemical aspects of cellular process are well characterized, but more recently, it has been shown that cells dynamically sense and respond to biophysical cues such as substrate stiffness and geometrical constraints; physical cues even direct cell differentiation and stem cell lineage (Discher et al, Science, 2005). In hematology, we know that platelets are shear activated and attenuate force based on substrate stiffness, and that endothelial cells align with flow and are activated by shear stress. Blood cells pass through, and interact with, biological matrices such as fibrin clots and the vascular wall, but the physical and biochemical aspects of these interactions are indistinguishable from one another in vivo. As such, there is a gap in knowledge as to how blood cells respond to matrices as they transit through them.To decouple the physical and biochemical interactions of blood cells and biological matrices, we sought to recreate the physical geometry of a fibrin network in a controlled, non-biological, in vitro microfluidic system. To this end, we designed a two-part microfluidic device comprised of an array of micron sized pillars (~1 µm diameter, 3 µm height, and 2 µm gap between pillars) overlaid with a microfluidic channel (Fig 1). The dimensions of the pillars are on the order of the diameter of fibrin fibers and the mesh size of a fibrin gel (Okada et al, J. Biol. Chem, 1985), while channels of various dimensions can be bonded over the pillar array to represent various biological scenarios. Standard microfluidic processes cannot produce pillars with the feature sizes reported herein, so electron beam lithography was used to create the mold from which the elastomeric pillars are made.The biophysical interaction of platelets flowing through fibrin mesh (absent biological factors) was recreated by a pillar array oriented perpendicular to the direction of flow in a 6 µm tall channel. When washed platelets are perfused through the system, they adhere to the pillars, aggregate, and form an occlusive mass that extends to the edges of the array (Fig 2A). Platelet adhesion initiates exclusively at the pillars and aggregation propagates to the extents of the channel area perpendicular to flow, resulting in channel occlusion and flow cessation. These findings show that in the absence of platelet agonists and biological ligands, platelets are activated by the shear environment afforded by the presence of fibrin fibers. Thus, in addition to the biochemical players in clot formation, the geometry of the fibrin mesh plays a role in platelet adhesion, and clot propagation. As expected, passive adsorption of fibrinogen and collagen to the pillar surfaces enhances platelet aggregation, as evidenced by a decrease in time to channel occlusion from 10 min to 2 min and 6 min, respectively. With thus we see the synergistic effect of biophysical and biochemical factors in clot propagation. This novel microfluidic system both separates biophysical and biochemical aspects of clot formation and allows researchers to specify the precise location and extent of clot formation in vitro.Platelets are not the only blood cells to interact with and react to physical barriers. Red blood cell (RBC) deformation has been historically studied in single cell assays, SEM studies of fixed clots, and more recently after RBCs have passed through a filtration system comprised of either beads or long slits (Deplaine et al, Blood, 2010); however, real time visualization of RBC deformation in geometries representative of biological matrices has remained elusive. The deformation (and possible fragmentation) that RBCs undergo when passing through the physical challenges of a fibrin matrix or the interendothelial slits of the spleen can be visualized in our system: an array of pillars overlaid by a 3 µm channel. Our findings visually suggest that red blood cells are able to deform through the matrix with little effect on their membranes, and that exposure to high shear gradients alone does not cause cell fragmentation (Figure 2B). The ready deformation and transit of healthy RBCs in our system confirms recent computational studies of RBC filtration by the spleen (Pivkin et al, PNAS, 2016). Further studies will give insight into the deformation and transit of sickled cells and malaria infected RBCs in physical matrices. Overall, our microfluidic studies give novel insight into the biophysical aspect of blood cell interactions with biological matrices. [Display omitted] [Display omitted] DisclosuresLam:Sanguina, LLC: Equity Ownership.

ESTIMATION DYNAMIC VISCOSITY OF THE LUBRICANT MATERIALS IN TRIBOLOGICAL CONTACT

There is presented the example of the separate determination of the dynamic viscosity of structuredboundary lawyers in the lubricant material of non-hydrodynamic nature in the period ofstart-up and the general dynamic effective viscosity at stabilization of tribological parametersin the contact. General legitimacy established during the long-term investigations of mineraland synthetic oils for hydro-mechanical transmissions is increasing effective viscosity of theadsorption boundary layers, reduction the gradient of shear stress of lubricant materials on theactivated by friction contact surfaces and obtaining properties of non-Newtonian fluids. It wasdetermined that the high shear gradients oil layer during breaking-in and increasing the temperaturein contact zone caused decreasing of the general dynamic effective viscosity of lubricantmaterials at operating time.

Velocity measurement of near-wall flow over inclined and curved boundaries by extended interfacial particle image velocimetry

Particle image velocimetry (PIV) measurements near a curved boundary usually require efforts to deal with low tracer density, high shear gradient and wall reflection. To resolve these difficulties, we presented a near-wall measurement technique named interfacial PIV (IPIV) that could return a tangential velocity component and a tangential component of wall gradient [Nguyen TD, Wells J, Nguyen C. Wall shear stress measurement of near-wall flow over inclined and curved boundaries by stereo interfacial particle image velocimetry. International Journal of Heat and Fluid Flow 2010;31(3):442–9]. In this paper, we introduce an extension of IPIV to measure the tangential and wall-normal velocity components. This extension allows IPIV to obtain three velocity components when IPIV is applied to stereoscopic PIV images of flows over inclined and curved boundaries. The performance of IPIV is validated against a particle image distortion (PID) technique using synthetic images generated from a direct numerical simulation velocity field of a turbulent flow over a wavy wall. Results show that IPIV is more accurate than PID in the near-wall velocity measurement. Practical applications of IPIV to experimental images of open flume tests with a wavy wall and a backward-facing step are described.

Conductive Composites Based on Metallocene Isotactic Poly(propylene): Preparation and Properties

AbstractSummary: The research deals with the preparation and the further comprehensive characterization of metallocene polypropylene‐based composite materials by incorporation of carbon black nanoparticles. Composites containing up to 10 wt% of carbon black were prepared by direct melt mixing in a single screw extruder Brabender Extrusiograph type 30/25D with attached static mixer at melt temperature of 200 °C and a screw speed of 30 rpm, according to a two‐step process. Some composites were treated with 3 wt% maleic anhydride grafted polypropylene (MAH‐PP). The rheological behaviour of the miPP nanocomposites was determined by cone/plate rheological measurements at 180 °C. The composites were characterized by SEM for morphological details and uniaxial stress‐strain measurements for determining the mechanical parameters. Electric conductivity of injection molded plates from these composites was investigated. The different miPPs studied are ranked in an ascending order according to their increasing molecular weight concerning the magnitude of their rheological parameters. The maleic anhydride compatibilizer leads to lower viscosity values even at high shear gradients and to better homogenization of the nanofiller in the polymer matrix. The processing conditions, carbon black concentration and viscosity of the virgin polymer have an impact on the final conductivity of the miPP/carbon black composites.

Role of Shear Stress in the Blister Formation of Cerebral Aneurysms

The development of cerebral aneurysms is related to hemodynamic stress. To elucidate the role of shear stress in the blister formation of cerebral aneurysms. Among 82 aneurysms detected during catheter-based 3D rotational angiography (3DRA), 4 aneurysms enlarged with blister formation during a mean follow-up period of 10.1 month. Three of these 4 aneurysms were analyzed in this study. The regions of blister formation were characterized by comparing 3DRA before and after blister formation, and computational fluid dynamic simulations were performed based on the aneurysm geometry before blister formation. The spatially averaged shear magnitude was lower in the aneurysm region (0.97 ± 0.39 Pa) than in the parent artery (2.75 ± 0.92 Pa). The spatially averaged shear magnitude of the blister-forming area was extremely low (0.48 ± 0.12 Pa), and the shear magnitude dropped precipitately to subphysiological levels, resulting in a high shear gradient near the border of the blister-forming area. These data suggest that low shear magnitude may trigger the progression of cerebral aneurysms and that blister formation is associated with high shear gradient in the large region of low shear magnitude on the aneurysm wall.

Synthesis of Temperature Responsive Poly(N-isopropylacrylamide) Using Ultrasound Irradiation

Ultrasound was employed to synthesize poly(N-isopropylacrylamide) [poly(NIPAM)] either as aqueous solutions or microgels in the absence of a chemical initiator. Poly(NIPAM) of different microstructures can be readily prepared via ultrasound irradiation by varying the reaction temperature. At a preparation temperature of 20 degrees C, poly(NIPAM) was formed in aqueous solutions, whereas, at a higher preparation temperature (beyond the lower critical solution temperature of approximately 32 degrees C), poly(NIPAM) microgels were formed. In addition, the high shear gradients generated by the acoustic cavitation process aid to control or vary the molecular weights of poly(NIPAM) formed in aqueous solutions. The swelling behavior of poly(NIPAM) at different concentrations of sodium dodecyl sulfate (SDS) was also studied. An increase in the transition temperature and hydrodynamic size of the particles was attributed to SDS binding to the polymer network through hydrophobic interactions. Light scattering data reflected the formation of larger microgels at low cross-linker concentrations. The encapsulation of rhodamine B within the microgels was achieved by sonicating the monomer containing the dye. The subsequent release of the dye was consistent with Fickian diffusion and the diffusion coefficient of the dye was estimated as 4.0 x 10(-12) and 3.6 x 10(-11) m(2) s(-1) at 20 and 40 degrees C, respectively. From the diffusion coefficients, the viscosities of the polymer samples at 20 and 40 degrees C were determined using the Stokes-Einstein equation to be 77 and 10 cP, respectively.

Shear field induced diffusion and molecular weight fractionation during polymer processing

AbstractIt has been hypothesized that a high shear gradient induced during polymer processing could cause an increased migration of oligomer to the surface, thereby leading to “plateout” on mold and roll tooling. To test this hypothesis, poly(ethylene terephthalate) (PET) and poly(ethylene‐co‐cyclohexylenedimethylene terephthalate) (PETG) were extruded into film, and the molecular weight probed as a function of the depth from the surface by incremental milling. GPC analysis showed an 18% and 14% higher concentration of cyclic oligomer on the surface of these films as compared with the center for PET and PETG respectively. These results compare favorably with predictions from a theoretical shear field model based on a Convected Maxwell fluid. Results from this study should lead to a better understanding of how oligomer buildup at the die wall may relate to roll plateout, die drooling, die lubrication, and possibly even melt fracture. Polym. Eng. Sci. 44:1283–1294, 2004. © 2004 Society of Plastics Engineers.

DETERMINATION OF PHASE VELOCITIES IN MULTI-PHASE FLOWS IN HOPPERS USING DUAL PHOTON GAMMA-RAY TOMOGRAPHY

The temporal and spatial variations of the relative fluxes in a binary mixture flow consisting of particles of different size but almost equal density are investigated using a dual photon gamma-ray transmission technique. Experiments are carried out using the dual photon gamma-ray technique in a mass flow hopper containing a binary mixture of 20% per weight turnip seeds (fine phase) and 80% per weight ABS particles coarse phase of size ratio 2.11:1 and of nearly equal densities. Measurements are taken in a ‘snap-shot’mode to identify the solids fraction of the individual components in the mixture and a statistical analysis is applied to the solids fraction values to calculate self-diffusion velocities for each of the individual components of the mixture. Significant phase separation is observed within zones of high bulk shear gradients such as close to the hopper walls and close to the hopper outlet. Large interstitial voidage changes observed during the initial transient stage of flow are also found to give rise to significant self-diffusion velocities.

Sensitivity of the shear gage to normal strains

This paper documents an experimental study that was conducted to demonstrate the sensitivity of the shear gage to the presence of normal strains. The shear gage is a specially designed strain gage rosette that measures the average shear strain in the test section of notched specimens such as the losipescu, Arcan and compact shear specimens. These specimens can have complicated stress states with high shear and normal strain gradients. To evaluate the sensitivity of the shear gage to normal strains, shear gages were tested on an Arcan specimen. The Arcan specimen is a notched specimen that can be loaded in pure shear (90 deg), pure tension (0 deg) and at intermediate 15- deg increments. The shear modulus for an aluminum specimen was determined at each of these loading angles. It was found that the gages display nearly zero sensitivity to normal strains (ɛx,ɛy). Moire interferometry was used to document the shear and normal strain distributions in the test section and to provide an independent method for determining the average shear strain. These results reinforce the robust nature of testing with the shear gage.

Shear stress gradients remodel endothelial monolayers in vitro via a cell proliferation-migration-loss cycle.

Wall shear stress has been implicated in the genesis of atherosclerosis because a strong correlation exists between the location of developing arterial lesions and regions where particular gradients in stress occur. Studying the behavior of endothelial cells in such regions may contribute to our understanding of the disease etiology. We report the detailed migratory history of endothelial cells subjected to large shear stress gradients caused by a surface protuberance in an in vitro model system. The history of cell migration, cell division, and cell loss from the surface was continuously monitored in confluent human umbilical vein endothelial cell monolayers for 48 hours after the onset of flow. Individual cells were tracked using time-lapse video microscopy. In contrast to a uniform laminar flow field in which cells were observed to continually rearrange their relative position with no net migration, in a disturbed flow field there was a net migration directed away from the region of high shear gradient. This organized migration pattern under disturbed flow conditions was accompanied by more than a twofold increase in cell motility. In addition, cell division increased in the vicinity of the flow separation (maximum shear stress gradient of 34 dyne/cm2 per mm) whereas cell loss was increased upstream and downstream in the regions where the shear gradient diminishes. These data suggest a steady cell proliferation-migration-loss cycle and indicate that local shear stress gradient may play a key role in the morphological remodeling of the vascular endothelium in vivo.

The diurnal cycle of the upper equatorial ocean: Turbulence, fine‐scale shear, and mean shear

Physical processes within the diurnal cycle of the upper ocean in the Pacific Equatorial Undercurrent are investigated based on observations at 0°/140°W of April 1987. The cycle extended below the surface mixed layer into the pycnocline, where nighttime maxima and daytime minima occurred in turbulent dissipation rates ε, fine‐scale shear variance, and in “large‐scale,” low‐frequency zonal shear, buoyancy frequency, and gradient Froude number. The time‐of‐day average e varied by a factor of 40 at 20 m, near the base of the mixed layer. Its night/day ratio decreased with increasing depth, the diurnal signal extending to 45–60 m, compared to nighttime mixed layer depths of 10–25 m. The daily variation of ε extended throughout the layer of high shear and high gradient Froude number Fr, above the Undercurrent core, being cut off where the large‐scale Fr dropped below 1. The diurnal cycle of ε remained unaffected by changes in the large‐scale shear and stratification. The nighttime fine‐scale shear variance in the 5–10 m vertical wavelength band exceeded daytime minima by a factor of 2 on average, the daily variation decaying rapidly with increasing depth. Turbulent overturns were smaller than 5 m, so that we suspect that the fine‐scale shear was related to internal waves. Fine‐scale shear was large enough to be a factor in local shear instability. A zone some 20 m thick below the mixed layer showed weak nighttime maxima in the large‐scale, low‐frequency zonal shear, stratification, and Froude number. We suspect that these variations were a product of turbulent mixing. They could become a discriminative tool in a future comparison of our observations with upper ocean models. The application of such models is encouraged by indications of a local control of the upper ocean heat budget by the surface heat flux. In apparent response to a weaker Undercurrent, stronger stratification, and weaker trade winds, mean and nighttime dissipation rates were lower in 1987 than in November 1984. The night/day ratio, however, was larger in 1987 owing to the absence of daytime high‐ε events.

Transient rheology in a new type of couette apparatus. Application to blood

By using our new air-bearing viscometer different types of transient flow can be studied; in the present work this viscometer was specifically applied to non-Newtonian blood suspensions. To observe the influence of both the aggregation and the deformation of red blood cells (RBCs), different concentrations of fibrinogen and dextran were used: the suspended RBCs were simply washed or rigidified with diamide. From the data three rheological parameters were determined, two of which are relative to the behavior at low and at high shear gradients, respectively. Their values were related to the theory of Taylor, and the internal viscosity of RBCs was estimated to be around 3 cp.

A new pulsed-wire probe for measuring the Reynolds stresses in the plane containing the main shear direction of a turbulent shear flow

The design, manufacturing procedures and results of testing a new pulsed-wire probe are presented. This probe is used for measuring Reynolds stresses in highly turbulent low speed air flows containing high mean shear gradients. Pulsed wire probes of the standard type suffer from measurement errors when the pulsed wire is oriented in or at 45 degrees to the main shear direction of such flows. This is due to the shearing of the thermal cloud giving the time of flight information which is used for measuring the instantaneous velocity. The new probe design proposed is designed in a fashion to avoid these 'gradient effects'; it is furthermore shown that aerodynamic interference effects have also been minimised. A detailed description of the manufacturing process is given.

The effects of non-Newtonian viscoelasticity and wall elasticity on flow at a 90 degrees bifurcation.

To study the fundamentals of hemodynamics in arteries, the flow parameters: pulsatility, elasticity and non-Newtonian viscoelasticity were considered in detail in a 90 degrees-T-bifurcation of a rigid and elastic model. The velocity distribution 2.5 mm behind the bifurcation in the straight tube was measured with a laser-Doppler-anemometer. The fluid used was an aqueous glycerine solution and a viscoelastic Separan mixture. Flow visualization studies were done with a sheet of laser light in the plane of the bifurcation. The velocity distribution was measured for both steady and pulsatile flows with a laser-Doppler-anemometer in a backward scattered way. From the velocity measurements the shear gradients were calculated. Substantial differences were found in the flow behavior of Newtonian and non-Newtonian fluids, especially behind the bifurcation in the main tube, where secondary flows and flow separation started. Also, differences due to the elastic and rigid wall could be seen. Very high shear gradients were found in the flow between main flow and the separation zone which can lead to a damage of the blood cells.

Structure and properties of linear polyethylene crystallized by rapid cooling of melts: 3. The effect of shear

This paper investigates the relationship between the mechanical performance of an injection moulded product and the flow and temperature characteristics within the mould during manufacture. This has been achieved by devising a laboratory experiment involving production of a thin moulded rod. We have shown that the modulus in linear polyethylene is anomalously high when the polymer melt is rapidly solidified whilst being exposed to a high shear gradient. We have related this observation to the morphology of the solid product.