Laminar Region Research Articles

Experimental investigations on single-phase convection and steam-water two-phase flow boiling in a vertical rod bundle

Single-phase convective heat transfer and steam-water two-phase flow boiling heat transfer experiments have been carried out with a vertical seven-rod bundle at low flow conditions near atmospheric pressure. With respect to single-phase experimental results, new correlations were developed to predict convective heat transfer coefficients of laminar flow and Reynolds numbers at the transition from the laminar region to the turbulent region. Two-phase experimental results show that the rod bundle geometry enhanced heat transfer compared with circular tubes. An obvious mass velocity effect on the flow boiling heat transfer coefficient was observed. This conclusion indicates that forced convection, instead of nucleate boiling heat transfer, dominated two-phase flow boiling heat transfer under the experimental conditions. In addition, two-phase flow boiling heat transfer coefficients in the vertical rod bundle increased first and then decreased with the quality increases for a given mass velocity. By taking the enhancement factor of forced convection and the suppression factor of nucleate boiling into account, a new correlation in terms of Boiling number, Reynolds number and Martinelli number was developed to be successful in predicting predict two-phase flow boiling heat transfer coefficients.

Convective heat transfer coefficient and pressure drop of water-ethylene glycol mixture with graphene nanoplatelets

In the present work, we report the convective heat transfer coefficient and pressure drop of water-ethylene glycol mixture seeded with graphene nanoplatelets under laminar, transition and turbulent flow regions. Sodium deoxycholate was used as the surfactant to prepare stable nanofluid dispersions. Thermophysical properties of nanofluids were measured experimentally. Experimental investigations on the convective heat transfer coefficient and pressure drop were performed in a tube-in-tube counter flow heat exchanger using nanofluid as the hot fluid and chilled water as the cold fluid. The effects of nanofluid inlet temperature on the convective heat transfer coefficient and pressure drop were investigated for different mass flow rates. The enhancement of convective heat transfer coefficient was found to increase with respect to Reynolds number, graphene loading and inlet temperature. The maximum enhancement of convective heat transfer coefficient is observed to be ∼170% at 0.5vol%inthe turbulent region. The pressure drop increment of the nanofluid is predominant in the laminar region as compared to turbulent region. The enhancement of pressure drop is moderate in the turbulent region which favours these nanofluids to be used in the thermal systems for different engineering applications.

Design and Analysis of Passive Y-Type Micromixers for Enhanced Mixing Performance for Biomedical and Microreactor Application

In this paper, endeavor has been made to design and analyze different Y-type micromixers by characterizing the mixing and flow behavior in ultra-low Reynolds number region. The effects of different geometric and flow parameters on the mixing and pressure drop are studied through computational fluid dynamics (CFD) simulations using COMSOL Multiphysics software. The parameters investigated are obstacle geometry, obstacle arrangement, obstacle depth, aspect ratio (AR), entrance angle ([Formula: see text], Reynolds number (Re) and obstacle packing factor (OPF). The simulation results reveal that rectangular shaped obstacles in staggered arrangement gives the best mixing. Increasing obstacle depth and OPF increases both mixing and pressure drop whereas increasing entrance angle enhances mixing but has negligible effect on pressure drop. Also both the mixing and pressure drop performance enhances with decreasing AR and lower Reynolds number gives better mixing in the lower laminar flow region.

Analysis of impedance in a molten carbonate fuel cell

Analysis of AC impedance in a molten carbonate fuel cell (MCFC) has been diversified among researchers regardless of the convenience of its measuring. The insufficient understanding of mass transfer processes at the electrodes may be a reason. In this work, the AC impedance analysis was carried out with 100cm2 class MCFCs, and simulation based on an equivalent circuit and reported kinetic parameters were conducted. From the cell, two of high and low frequency half circles were observed which represented liquid and gas phase mass-transfer effects, respectively. The simulation successfully represented that the liquid phase mass-transfer was due to the concentration of active species in the cathode while the gas phase mass-transfer was mostly attributed to the gas flow rate in the anode. For the gas flow rate effect, the mass transfer resistance was a function of boundary layer thickness because the flow at both electrodes was fully in the laminar region.

A comprehensive deposition velocity model for slurry transport in horizontal pipelines

Transportation of solids in form of slurries has become one of the most important unit operations in industries across several disciplines. In fact, the need is more pronounced in industries that are very important for human survival such as food processing, pharmaceuticals and energy (coal, oil and gas). A lot of work has been done in the past 30 years in understanding the factors affecting the deposition velocity of solids in slurries. Experimental observation and theoretical predictions pointed to mixture velocity and solid/fluid properties especially rheology of the resulting slurry to be the most important factors that dramatically affect particle motion and patterning. This paper presents a critical deposition velocity model and a “stability flow map” for complex rheology slurries. The critical deposition model utilizes a more robust generalized two-parameter rheology model to account for any given slurry rheology. The “stability flow map” demarcates the different flow patterns that may be observed at different mixture velocities and rheologies. On this map, the homogeneous slurries are predicted at low rheology and high mixture velocity, whereas heterogeneous slurries (with a concentration gradient) predicted at high rheology (yield stress effects). Sensitivity analysis was conducted on critical Reynolds number, particle density, carrier fluid density, generalized flow behavior index and pipe diameter. It was observed that increase in shear thinning behavior, particle density, pipe diameter and particle diameter led to a decrease in the laminar region and an increased unstable region. The model showed good performance when tested on glass and stainless steel beads test data available in open literature. Preliminary simulation with this map may help engineers select flowline size and carrier fluid rheology for a given type of solid particle.

Analysis of Turbulence Characteristics in the Laminar Sub-Layer Region of a Perturbed Turbulent Boundary Layer

Turbulent boundary layer plays an important role for generation of aerodynamic drag. Shear force and pressure force due to the presence of boundary layer separation from the body surface contribute to the total drag. Studies of drag reduction due the the boundary layer effect are continuously performed by many researchers. Present study is intended to evaluate the behaviour of the laminar sub-layer in a turbulent boundary layer using a hot-wire anemometer system. The study was conducted in a low-speed wind tunnel at a Reynolds number based on the momentum thickness of approximately Reθ = 1000. A smooth-flat plate and a plate with a single transverse square groove was used in the study of the boundary layer characteristics. The groove size of 10 mm x 10 mm was cut transversally across the test plate. The results show that no significant differences in the streamwise mean velocity, steamwise turbulence intensity, and velocity signals for the smooth-and grooved-wall cases. For the the energy spectra for the two cases, however, show significant differences.

Experimental study on thermal performance of MWCNT nanocoolant in Perodua Kelisa 1000cc radiator system

Latest advances in nanotechnology have allowed development of new applications of nanofluids in vehicle engine cooling. In this study, the heat transfer enhancement of vehicle radiator by using water/ethylene glycol based multi-walled carbon nanotube (MWCNT) nanocoolant was studied experimentally. A car engine system (Proton Kelisa 1000cc) was utilized to test the heat transfer performance using this nanocoolant as a substitute to the conventional coolant. Different nanoparticle volume concentrations (0.1%, 0.25%, 0.50%) in 50:50 water/ethylene glycol have been prepared and tested. Liquid flow rate has been regulated in the range of 2, 4 and 6l/min and the experimental analysis was carried out in laminar flow region. Our experimental results revealed that the average heat transfer coefficient is directly proportional to the volume concentration of nanofluids and Reynolds number. In addition, the maximum average heat transfer coefficient enhancement was found to be 196.3% for 0.5% nanoparticle volume concentration compared with base fluid.

Experimental study of second-mode instability growth and breakdown in a hypersonic boundary layer using high-speed schlieren visualization

Visualization experiments are performed to investigate the development of instability waves within the boundary layer on a slender cone under high Mach number conditions. The experimental facility is a reflected-shock wind tunnel, allowing both low (Mach-8 flight equivalent) and high-enthalpy conditions to be simulated. Second-mode instability waves are visualized using a high-speed schlieren set-up, with pulse bursting of the light source allowing the propagation speed of the wavepackets to be unambiguously resolved. This, in combination with wavelength information derived from the images, enables the calculation of the disturbance frequencies. At the lower-enthalpy conditions, we concentrate on the late laminar and transitional regions of the flow. General characteristics are revealed through time-resolved and ensemble-averaged spectra on both smooth and porous ceramic surfaces of the cone. Analysis of the development of individual wavepackets is then performed. It is found that the wavepacket structures evolve from a ‘rope-like’ appearance to become more interwoven as the disturbance nears breakdown. The wall-normal disturbance distributions of both the fundamental and first harmonic, which initially have local maxima at the wall and near $y/{\it\delta}=0.7$–0.75, exhibit an increase in signal energy close to the boundary-layer edge during this evolution. The structure angle of the disturbances also undergoes subtle changes as the wavepacket develops prior to breakdown. Experiments are also performed at high-enthalpy ($h_{0}\approx 12~\text{MJ}~\text{kg}^{-1}$) conditions in the laminar regime, and the visualization technique is shown to be capable of resolving wavepacket propagation speeds and frequencies at such conditions. The visualizations reveal a somewhat different wall-normal distribution to the low-enthalpy case, with the disturbance energy concentrated much more towards the wall. This is attributed to the highly cooled nature of the wall at high enthalpy.

Experimental study of thermal conductivity and convective heat transfer enhancement using CuO and TiO2 nanoparticles

Heat transfer characteristics of CuO/water and TiO2/water nanofluids were measured in a shell and tube heat exchanger under laminar and turbulent flow condition. In shell and tube heat exchanger, water based CuO and TiO2 based nanofluids with 0.02%, 0.04% and 0.06% volume fraction have been used as working fluids for different flow rates of nanofluids. This enhancement was investigated with regard to various factors; concentration of nanoparticles, types of base fluids, sonication time and temperature of fluids. The effects of Nusselt number and Reynolds number of suspended nanoparticles, on the heat-transfer characteristics were investigated. The experimental results have been compared to those of Dittus–Boelter correlation, Xuan & Li and Gnielinski correlation for the laminar and turbulent flow region. Low thermal conductivity is a major concern in the development of the energy-efficient heat transfer fluid. In this paper, the effect of CuO and TiO2 nanoparticles on the thermal conductivity of base fluids like monoethylene glycol and water was studied. The nanofluids showed an enhancement in effective thermal conductivity, and overall heat transfer coefficient. The thermal conductivity measurement has been made for different concentrations and temperatures. As the concentration of nanoparticles increased, there was a significant enhancement in thermal conductivity and overall heat transfer due to more interaction between particles. It was also observed an improvement in thermal conductivity of base fluid as temperature increases.

Turbulence collapse in a suction boundary layer

Turbulence in the asymptotic suction boundary layer is investigated numerically at the verge of laminarisation using direct numerical simulation. Following an adiabatic protocol, the Reynolds number$Re$is decreased in small steps starting from a fully turbulent state until laminarisation is observed. Computations in a large numerical domain allow in principle for the possible coexistence of laminar and turbulent regions. However, contrary to other subcritical shear flows, no laminar–turbulent coexistence is observed, even near the onset of sustained turbulence. High-resolution computations suggest a critical Reynolds number$Re_{g}\approx 270$, below which turbulence collapses, based on observation times of$O(10^{5})$inertial time units. During the laminarisation process, the turbulent flow fragments into a series of transient streamwise-elongated structures, whose interfaces do not display the characteristic obliqueness of classical laminar–turbulent patterns. The law of the wall, i.e. logarithmic scaling of the velocity profile, is retained down to$Re_{g}$, suggesting a large-scale wall-normal transport absent in internal shear flows close to the onset. In order to test the effect of these large-scale structures on the near-wall region, an artificial volume force is added to damp spanwise and wall-normal fluctuations above$y^{+}=100$, in viscous units. Once the largest eddies have been suppressed by the forcing, and thus turbulence is confined to the near-wall region, oblique laminar–turbulent interfaces do emerge as in other wall-bounded flows, however only transiently. These results suggest that oblique stripes at the onset are a prevalent feature of internal shear flows, but will not occur in canonical boundary layers, including the spatially growing ones.

Dynamics of the gas flow turbulent front in atmospheric pressure plasma jets

In this paper, dynamic characterizations of the turbulent flow field in atmospheric pressure plasma jets (APPJs) are investigated by focusing on the effect of different APPJ parameters, such as gas flow rate, applied voltage, pulse repetition frequency, and time duration of the pulse. We utilize Schlieren photography and photomultiplier tubes (PMT) as a signal triggering of an intensified charge coupled device (ICCD) and also a high speed camera to examine the formation of the turbulent front and its dynamics. The results reveal that the turbulent front will appear earlier and closer to the tube nozzle by increasing the gas flow rate or the applied voltage amplitude. However, the pulse time duration and repetition frequency cannot change the dynamics and formation of the turbulent front. Further investigation shows that every pulse can excite one turbulent front which is created in a specific position in a laminar region and propagates downstream. It seems that the dominating mechanisms responsible for the formation of turbulent fronts in plasma jets might not be ion momentum transfer.

Schlieren imaging investigation of the hydrodynamics of atmospheric helium plasma jets

This work investigates the hydrodynamic characteristics of a coaxial double-ring electrode helium plasma jet by means of a “Z-type” Schlieren imaging system. The Schlieren images and visual optical photographs made show that a transition point from a laminar region to a turbulent region exists for gas flow without plasma when the helium flow rate exceeds a certain value. After plasma ignition, the laminar region shrinks with voltage increases, and the maximum length of the plasma plume is confined to the laminar region. The heat transfer equation and the spectral broadening of the He I 667.8 nm were used to estimate the increased gas temperature in the plasma jet, and the change in gas velocity by ionic momentum transfer was found by application of a double sphere collision model. As a result, gas heating is considered to be the dominant factor for the earlier onset of turbulence after plasma ignition, whereas the role of ion momentum transfer to neutral gas molecules is comparatively weak. The hydrodynamic behaviors of the plasma jet at the impact region for organic glass and silicon substrates are also researched. The ionization front propagates along the organic glass surface and contracts at the impact point on the silicon surface. More visible vortices are observed from Schlieren images with silicon substrates than with organic glass substrates. Possible mechanisms related to the different treatment effects are discussed.

Numerical simulation and wind tunnel tests investigation and validation of a morphing wing-tip demonstrator aerodynamic performance

This paper presents the results obtained from the numerical simulation and experimental wind tunnel testing of a morphing wing equipped with a flexible upper surface and controllable actuated aileron. The technology demonstrator is representative of a real aircraft wing tip section, and it was developed following a complex, multidisciplinary design process. The model was fitted with a composite material upper skin whose shape can be morphed, as a function of the flight condition, by four electrical actuators placed inside the wing structure. The optimizations were performed with the aim of controlling the extent of the laminar flow region, and the resulting shapes were scanned using high-precision photogrammetry. The numerical simulations were performed using Computational Fluid Dynamics (CFD) and included a model for predicting the laminar-to-turbulent flow transition over the entire wing surface. The analyses included cases with three aileron deflection angles and angles of attack situated within five degrees range. The CFD results were compared with infrared thermography measurements in terms of transition location, surface pressure measurements and balance loads measurements acquired during subsonic wind tunnel tests performed at the National Research Council Canada.

Improving the rate of solar heat recovery from nanofluids by using micro heat exchanger

This work investigates the possibility of improving the solar heat recovery from copper–water nanofluids using a micro heat exchanger. This micro heat exchanger consisted of double brass tubes with a micro annular space. The effect of nanoparticles addition to the base fluid and its flow rate on the heat transfer characteristics and heating rate for both nanofluids in solar simulator section and water in shell side of micro heat exchanger were studied. The nanofluids was forced to flow in the tube side, while the fluid in the shell side was pure water. The effect of copper (Cu) weight fraction in nanofluids and the flow rate of nanofluids on the heat transfer coefficient (h), overall heat transfer coefficient (U), thermal resistance (R), and heating rate for both nanofluids in heating section and cold water in micro heat exchanger were studied. Four different concentrations of nanofluids in the range 0.01–0.1 wt.% had been used. The flow rate of nanofluids was changed in laminar region using a control valve. The temperature of heated nanofluids varied from 22 to 73°C. The heat transfer and overall heat transfer coefficients of the nanofluids 0.1 wt.% were found to increase by 49.9 and 42.08%, respectively, when it was compared with pure water. Also to be increased by increasing the nanofluid flow rate. Also, it was observed that the weight fraction has no significant effect on the final temperature of the nanofluid exit from heating section.

Layers and internal waves in uniformly stratified fluids stirred by vertical grids

Laboratory experiments in which uniformly stratified fluids are stirred by horizontally moving vertical grids, or arrays of vertical rods, are reviewed to examine their consistency and to compare their findings, particularly those relating to the generation of layers. Selected experiments are of three types, those in which (a) turbulence spreads from a horizontally confined region where it is continuously generated by an oscillating grid; (b) grid stirring is maintained throughout a rectangular tank; or (c) a ‘cloud’ of turbulence decays after a short period of horizontally localized grid mixing. In all the experiments turbulence is generated over the full vertical extent of the tank. In the experiments of types (a) and (c) layers of comparable scale are observed to intrude into the ambient fluid from the turbulent region. In the type (b) experiments, layers form only when the time interval between the passage of the grid through the stratified fluid is sufficiently long, allowing turbulence to decay substantially between grid strokes. Two mechanisms are found to be dominant in the production of layers. In experiments of type (a) and (c) overturning eddies in the turbulent region of scale significantly larger than the Ozmidov length scale collapse and spread, intruding and forming layers in the adjoining laminar region. Internal shear waves propagating ahead of the intrusions have a vertical wavelength that is approximately twice the layer height. In type (b) experiments, layers are formed through a process described by Holford & Linden (Dyn. Atmos. Oceans, vol. 30, 1999a, pp. 173–198): the bending of vortices shed by the grid bars. The height of the layers is approximately half the vertical wavelength of internal shear waves that travel at the speed of the grid. It is proposed that the flow field of the shear waves bends the vortices, resulting in diapycnal mixing that forms the layers. The relationship of layers and internal shear waves in the experiments is therefore as follows: in type (a) and (c) experiments internal waves are generated by layers intruding from the turbulent region into the quiescent stratified region, but in experiments of type (b) internal waves drive and dictate the height of the layers; layers are generated as a consequence of vortex bending by internal waves. There is insufficient evidence to establish whether zigzag instability, either of vortex pairs or of vortex streets shed by a moving grid, accounts for the layers in any of the three types of grid experiments. The Phillips and Posmentier instability may reinforce layers formed by other processes. The generation of pancake vortices or vortical mode motion is left for later review.

Effect of rib roughness pitch on thermal and thermo-hydraulic performance of a solar air heater roughened artificially with arc rib having gap

This experimental study on a solar air heater having absorber plate roughened artificially by providing roughness in the form of arc ribs having gap was carried out in the Department of Mechanical Engineering, Punjab Agricultural University, Ludhiana, India to study the effect of relative roughness pitch on thermal and thermohydraulic performance as well as to compare the performance of arc rib with gap roughened solar air heater with that of continuous arc rib roughened solar air heater. The roughness geometry parameters included relative roughness height of 0.043, angle of attack of 30 degree, relative gap position of 0.80, gap-width equal to the width of the rib and five values of relative roughness pitch ranging from 4 to 12 for flow Reynolds number range of 2000 to 16,000. The Nusselt number and friction factor were found to be more for relative roughness pitch value of 10 as compared to other values of relative roughness pitch. Thermo-hydraulic performance of solar air heaters roughened by arc with gap and continuous arc roughness geometries were found to be 1.91 times and 1.78 times respectively as compared to that of solar air heater having smooth absorber plate due to generation of turbulence in laminar sublayer region. However, improvement in thermo-hydraulic performance of solar air heater roughened by arc with gap geometry over continuous arc rib roughened solar air heater was attributed to generation of a region of turbulence on downstream side of the gap.

Equivalent circulating density modeling of Yield Power Law fluids validated with CFD approach

A numerical and experimental analysis is conducted on the flow of Newtonian and non-Newtonian fluids in annuli. A numerical model is presented that accurately estimates the annular frictional pressure losses with and without the inner pipe rotation. The numerical model is validated using a CFD software. Experiments are conducted at a 27 m long flow loop using various fluids that can be characterized as Yield Power Law (YPL). The results of the experiments are compared with the results obtained with the numerical model. Today, the most drilling fluids show YPL behavior. The proposed numerical model can calculate the equivalent circulating density while circulating YPL fluids accurately. The model is validated with a CFD software and compared with experimental results, the published experimental results and with the slot approximation. The results obtained from the numerical model shows good agreement with the experiments and also with the experimental results from the literature. The comparisons between the models indicate that the slot approximation can result in large errors especially when the diameter ratio is low, meaning the diameter of the inner pipe is significantly smaller compared to outer pipe. The numerical model is coupled with a stability criterion that determines the onset and offset of the transitional flow between laminar and turbulent regions of YPL fluids. Various degrees of eccentricity are analyzed in terms of pressure profile and flow stability with the proposed method. This study contributes to a better understanding of flow in annuli. The results obtained from this study are useful to predict the transition and the annular frictional pressure loss profiles more accurately than existing methods. Potential applications include risk avoidance and optimized operations.

HOMOCLINIC ORBITS IN TRANSITIONAL PLANE COUETTE FLOW

Recent studies on wall-bounded shear flow have emphasized the significance of the stable manifold of simple nonlinear invariant solutions to the Navier-Stokes equation in the formation of the boundary between the laminar and turbulent regions in state space. In this paper we present newly discovered homoclinic orbits of the Kawahara and Kida(2001) periodic solution in plane Couette flow. We show that as the Reynolds number decreases a pair of homoclinic orbits move closer to each other until they disappear to exhibit homoclinic tangency.

Modeling of high pressure steam condensation in inclined horizontal tubes of PAFS in APR+

ABSTRACTA Single-tube COndensation exPeriment (SCOP) was performed at Korea Advanced Institute of Science and Technology (KAIST) to simulate the condensation phenomena of Passive Auxiliary Feedwater System (PAFS), which is introduced in Advanced Power Reactor Plus. SCOP tests were conducted for two pipes of different diameters [22.6 and 44.8 mm inside diameter] with 8.2 m length and 2.5–4 degrees of the inclined angles of the upper and lower section of each tube. The experiment were performed at 1.5–6.5 MPa of steam pressure and 0.12–0.45 kg/s of steam flow. Based on the Kim and No correlation (KAIST, 2000), we developed a new turbulent–condensation correlation for the turbulent region of the liquid film in the inclined tube. We introduced the factors to account for the different inclined angle effect, different diameter effect. The Nusselt correlation is adopted for condensation near the tube inlet where the laminar region of the liquid film is dominant due to the small amount of condensed water. The present correlation was compared with the Shah and RELAP5 condensation model using the SCOP data. The root mean square errors of the present correlation, Shah model, and RELAP5 model are 17.41%, 57.88%, and 43.07%, respectively: the present correlation can predict well the condensation characteristics in comparison with other correlations in PAFS.

Optic nerve head and intraocular pressure in the guinea pig eye

The guinea pig is becoming an increasingly popular model for studying human myopia, which carries an increased risk of glaucoma. As a step towards understanding this association, this study sought to characterize the normal, developmental intraocular pressure (IOP) profiles, as well as the anatomy of the optic nerve head (ONH) and adjacent sclera of young guinea pigs. IOP was tracked in pigmented guinea pigs up to 3 months of age. One guinea pig was imaged in vivo with OCT and one with a fundus camera. The eyes of pigmented and albino guinea pigs (ages 2 months) were enucleated and sections from the posterior segment, including the ONH and surrounding sclera, processed for histological analyses - either hematoxylin and eosin (H&E) staining of paraffin embedded, sectioned tissue (n = 1), or cryostat sectioned tissue, processed for immunohistochemistry (n = 3), using primary antibodies against collagen types I–V, elastin, fibronectin and glial fibrillary acidic protein (GFAP). Transmission and scanning electron microscopy (TEM, SEM) studies of ONHs were also undertaken (n = 2 & 5 respectively). Mean IOPs ranged from 17.33 to 22.7 mmHg, increasing slightly across the age range studied, and the IOPs of individual animals also exhibited diurnal variations, peaking in the early morning (mean of 25.8, mmHg, ∼9 am), and decreasing across the day. H&E-stained sections showed retinal ganglion cell axons organized into fascicles in the prelaminar and laminar region of the ONHs, with immunostained sections revealing collagen types I, III, IV and V, as well as elastin, GFAP and fibronectin in the ONHs. SEM revealed a well-defined lamina cribrosa (LC), with radially-oriented collagen beams. TEM revealed collagen fibrils surrounding non-myelinated nerve fiber bundles in the LC region, with myelination and decreased collagen posterior to the LC. The adjacent sclera comprised mainly crimped collagen fibers in a crisscross arrangement. Both the sclera and LC were qualitatively similar in structure in pigmented and albino guinea pigs. The well-organized, collagen-based LC of the guinea pig ONH is similar to that described for tree shrews and more similar to the human LC than that of other rodents that lack collagen. Based on these latter structural similarities the guinea pig would seem a promising model for investigating the relationship between myopia and glaucoma.