Mean Aperture Research Articles

Transport and deposition of weakly inertial particles in closed channel flows at low Reynolds number

An analytical approach is developed to show how particles sparsely distributed in steady and laminar channel flows can be transported for long distances or conversely deposited inside the channel due to the relative importance between the flow-induced drag and gravity forces. More precisely, we establish a rather simple particle trajectory equation which demonstrates that when particles’ inertia is negligible, their behavior is characterized by the channel geometry and by a dimensionless number W that relates the ratio of the particles sedimentation terminal velocity to the flow mean velocity. The proposed particle trajectory equation is verified by comparing its predictions to particle tracking numerical simulations taking into account particle inertia and fully resolving equations. The equation is shown to be valid under the conditions that flow inertial effects are limited. Based on this trajectory equation, we build a regime diagram that can predict the behavior of particles entering closed channel flows. This diagram, by relating W to the ratio of the channel mean aperture to its total length, enables to forecast if the particles entering the flow will be either deposited or transported along the channel. The influence of the channel geometry on the particle behavior is then investigated by considering channels with straight and sinusoidal walls. In particular, the effect of the corrugation amplitude, of the asymmetry and of the phase lag between the walls on the extent of the transport and deposition zones is evaluated and verified against numerical experiments. Firstly, it is shown that the regime diagram for straight channels can be used for wavy channels with in-phase walls. Secondly, it is found that increasing the phase lag between the two walls and/or the walls corrugation amplitude leads to an increase of both the transport and sedimentation zones. Finally, it is demonstrated that increasing the lower wall corrugation amplitude relatively to the upper wall corrugation enhances particle transport.

The behaviors of LNAPL migration and entrapment in a rough-walled vertical fracture

The migration of light non-aqueous phase liquids (LNAPLs) and their entrapment in unsaturated vertical fractures exhibit complex behaviors, such as upward LNAPL migration that follows water level rise. In this study, behavior of LNAPL migration and entrapment in a vertical rough-walled fracture was investigated with optical measurement. A series of experiments were preformed, including fracture drainage, LNAPL infiltration and accumulation in an unsaturated fracture, and upward LNAPL migration with water level rise. Experimental results indicate that the fracture aperture significantly influences the LNAPL migration path and entrapped blobs distribution. The LNAPL-filled aperture follows a normal distribution, with the mean aperture ranging 0.253 mm to 0.287 mm, and the standard deviation around 0.13 mm. The characteristics of the entrapment blob morphology indicate that 80% of the individual blob areas are smaller than 20 mm2. The irregular structures of discrete blobs increase the ‘LNAPL-water’ interface, which will exacerbate contamination.

Three‐dimensional resistor network modeling of the resistivity and permeability of fractured rocks

AbstractThe resistivity and permeability of 3‐D fracture networks have been modeled as the fractures within these networks are incrementally opened. In these models, the ratio of the rock resistivity to that of the fluid is 104, and the rock permeability is 1 × 10−18 m2. The changes in both resistivity and permeability depend on characteristics of the network itself such as fracture density, as well as the aperture distribution within individual fractures. In dense fracture networks where the density constant α is equal to 30, a percolation threshold can be defined in terms of mean fracture aperture, below which both resistivity and permeability are close to their matrix values. At the percolation threshold, a change in mean aperture of 0.02 mm can change the permeability by 4 orders of magnitude and resistivity by a factor of 4. The percolation threshold does not necessarily occur at the same aperture for different flow directions, so fracture networks near their percolation threshold commonly show anisotropy in both resistivity and permeability. Most sparse networks (α equal to 0.3 or less) do not percolate no matter how open the fractures are. On the other hand, many fracture networks with an intermediate value of α (equal to around 3) will percolate only in one or two directions. This can lead to very strong anisotropy in both resistivity and permeability (up to a factor of 160 and 109, respectively), with anisotropy increasing as the aperture of the fractures increases.

The Effect of Side Screen Attached to Flat-board Debris-flow Breaker on Damage Reduction

이 논문에서는 바닥스크린의 토석류 피해저감 기능을 더욱 극대화하기 위하여 바닥스크린에 측면스크린을 부착하여 그 효과를 파악함으로써 측면스크린의 적정 순간격을 도출하고자 하였다. 그 결과, 바닥스크린에 측면스크린을 부착함으로써 작은 입경의 공급재료가 소량 측면스크린을 통과하였다. 그 대신에 바닥스크린의 상단면부에는 토석류의 선단부를 구성하던 큰 입경의 토석들이 측면스크린에 의해 규제된 유로폭 내에서 다량 포착된 후, 이 두터운 조립질층의 공극을 작은 입경의 계상재료가 메워 세립화되는 것으로 나타났다. 이러한 세립화는 결국 바닥스크린의 상단면부로부터 하단면부로의 토사 투과를 차단하여 결국 바닥스크린 하단면부에서의 퇴적량은 감소하였다. 한편, 이러한 바닥스크린의 기능 향상은 측면스크린의 순간격이 계상재료의 평균입경 및 최대입경에 일치할 경우 가 동일하게 나타났다. 그러나 스크린 부재의 순간격이 클수록 시공비 절감에 유리할 것으로 추정되며, 따라서 바닥스크린에 부착하는 측면스크린의 순간격은 계상재료의 최대입경과 동일하게 하는 것이 바람직할 것으로 판단된다.The objectives of this study were to evaluate effects of the flat-board debris-flow breaker (FDB) equipped with side screen and to derive an optimal slit aperture of the side screen for maximizing its positive effects. The results showed that only a few small-sized materials were passed through side screen, causing storage at both sides of FDB. Instead, a massive amount of large-sized materials was trapped on the upper surface of FDB regulated by side screen, and this coarse layer was filled with fine sediment. This fining may not arrow passage of even fine materials from upper to lower surface of FDB. Meanwhile, concerning the damage reduction by debris flows, there was no functional difference between the mean and maximum grain-size slit apertures. However, because a larger slit aperture of the screens has an advantage in reducing construction costs, the slit aperture of FDB should be matched to the maximum grain size of bed materials.

Relating electrical resistivity to permeability using resistor networks

We use resistor network models to explore the relationship between electrical resistivity and permeability in fractures filled with an electrically conductive fluid. The fracture aperture distribution is determined by generating fracture surface pairs that are constructed based on characteristics measured on rock samples. We use these to generate and solve resistor networks with variable hydraulic and electrical resistance. The aperture is incrementally increased, to analyse the changes in both properties as a fault is opened. At small apertures, electrical conductivity and permeability increase moderately with aperture until the fault reaches its percolation threshold. Above this point, the permeability increases by four orders of magnitude over a change in mean aperture of less than 0.1 mm, while the resistivity decreases by up to a factor of 10. The permeability increases at a greater rate than the conductivity, and therefore the percolation threshold can be defined in terms of the matrix to fracture resistivity ratio, M. The value of M at the percolation threshold, MPT, varies with the ratio of rock to fluid resistivity, the fault spacing, and the fault offset but is always less than 10. Greater M values are associated with fractures above their percolation threshold and therefore open for fluid flow.

Scale and size effects on fluid flow through self-affine rough fractures

A permeability estimation of a single rough fracture remains challenging and has attracted broad attention because of its fundamental importance. This study examines the effects of surface roughness on fracture flow and proposes a new, triple-effect permeability estimation model that takes surface and hydraulic tortuosity other than surface roughness factor (SRF) into account according to the functioning patterns. Due to the scale effect on hydraulic and surface tortuosities, it can also be reformulated into a scaling aperture-permeability equation for a self-affine fracture. Results indicate that tortuosity effects are scaled by 4(H-1) (H is the Hurst exponent) with the mean aperture and that the local SRF, as expected, is stationary at the measurement scale of the mean aperture. In addition, based on the scaling equation and the size effect of self-affine objects, flow regimes are examined and three flow regimes are then identified. Consequently, different permeability models for these regimes are established by fabricating fundamental and well-defined physical properties. The estimation results from these models are in excellent agreement with results from lattice Boltzmann simulations.

RECOVERY OF FOVEAL PHOTORECEPTOR INTEGRITY AFTER VITRECTOMY IN EYES WITH AN IMPENDING MACULAR HOLE WITH VITREOMACULAR TRACTION SYNDROME.

To identify factors associated with the recovery of foveal photoreceptor disruption in eyes with an impending macular hole (MH) with vitreomacular traction syndrome after surgery. This study comprised 33 consecutive patients who underwent vitrectomy for Stage 1 impending macular hole with disrupted photoreceptor inner segment/outer segment (IS/OS) layer and were followed up for a minimum of 1 year after surgery. Preoperative optical coherence tomography (OCT) parameters were compared between eyes that achieved complete restoration of the IS/OS layer (Group A) and those that did not (Group B). Postoperative serial changes in best-corrected visual acuity (BCVA) and IS/OS disrupted length were also investigated. Smooth and symmetric foveolar contour was restored in 29 eyes (87.9%). Complete recovery of IS/OS disruption was observed in 11 of 33 cases (33.3%, Group A). Group A exhibited a larger percentage of foveal pseudocysts (54.5% vs. 13.6%, P = 0.033) and a smaller mean aperture size (102.1 ± 182.1 μm vs. 241.5 ± 163.8 μm, P = 0.031) than Group B. Postoperatively, Group A revealed a significantly better visual outcome than Group Be, which was the same as Group B, but with the four eyes that developed a full-thickness macular hole excluded. Restoration of the foveal photoreceptor layer was more likely to occur in eyes with a foveal pseudocyst and smaller aperture size.

Heat as a tracer for understanding transport processes in fractured media: Theory and field assessment from multiscale thermal push‐pull tracer tests

AbstractThe characterization and modeling of heat transfer in fractured media is particularly challenging as the existence of fractures at multiple scales induces highly localized flow patterns. From a theoretical and numerical analysis of heat transfer in simple conceptual models of fractured media, we show that flow channeling has a significant effect on the scaling of heat recovery in both space and time. The late time tailing of heat recovery under channeled flow is shown to diverge from the behavior expected for the classical parallel plate model and follow the scaling for a simple channel modeled as a tube. This scaling, which differs significantly from known scalings in mobile‐immobile systems, is of purely geometrical origin: late time heat transfer from the matrix to a channel corresponds dimensionally to a radial diffusion process, while heat transfer from the matrix to a plate may be considered as a one‐dimensional process. This phenomenon is also manifested on the spatial scaling of heat recovery as flow channeling affects the decay of the thermal breakthrough peak amplitude and the increase of the peak time with scale. These findings are supported by the results of a field experimental campaign performed on the fractured rock site of Ploemeur. The scaling of heat recovery in time and space, measured from thermal breakthrough curves measured through a series of push‐pull tests at different scales, shows a clear signature of flow channeling. The whole data set can thus be successfully represented by a multichannel model parametrized by the mean channel density and aperture. These findings, which bring new insights on the effect of flow channeling on heat transfer in fractured rocks, show how heat recovery in geothermal tests may be controlled by fracture geometry. In addition, this highlights the interest of thermal push‐pull tests as a complement to solute tracers tests to infer fracture aperture and geometry.

Nanoscale grain boundary channels in fracture cement enhance flow in mudrocks

AbstractHydrocarbon production from mudrock or shale reservoirs typically exceeds estimates based on mudrock laboratory permeability measurements, with the difference attributed to natural fractures. However, natural fractures in these reservoirs are frequently completely cemented and thus assumed not to contribute to flow. We quantify the permeability of nanoscale grain boundary channels with mean apertures of 50–130 nm in otherwise completely cemented natural fractures of the Eagle Ford Formation and estimate their contribution to production. Using scanning electron imaging of grain boundary channel network geometry and a digital rock physics workflow of image reconstruction and direct flow modeling, we estimate cement permeability to be 38–750 nd, higher than reported permeability of Eagle Ford host rock (~2 nd) based on laboratory measurements. Our results suggest that effective fracture‐parallel mudrock permeability can exceed laboratory values by upward of 1 order of magnitude in shale reservoirs of high macroscopic cemented fracture volume fraction.

Relating permeability and electrical resistivity in fractures using random resistor network models

AbstractWe use random resistor network models to explore the relationship between electrical resistivity and permeability in a fracture filled with an electrically conductive fluid. Fluid flow and current are controlled by both the distribution and the volume of pore space. Therefore, the aperture distribution of fractures must be accurately modeled in order to realistically represent their hydraulic and electrical properties. We have constructed fracture surface pairs based on characteristics measured on rock samples. We use these to construct resistor networks with variable hydraulic and electrical resistance in order to investigate the changes in both properties as a fault is opened. At small apertures, electrical conductivity and permeability increase moderately with aperture until the fault reaches its percolation threshold. Above this point, the permeability increases by 4 orders of magnitude over a change in mean aperture of less than 0.1 mm, while the resistivity decreases by up to a factor of 10 over this aperture change. Because permeability increases at a greater rate than matrix to fracture resistivity ratio, the percolation threshold can also be defined in terms of the matrix to fracture resistivity ratio, M. The value of M at the percolation threshold, MPT, varies with the ratio of rock to fluid resistivity, the fault spacing, and the fault offset. However, MPT is almost always less than 10. Greater M values are associated with fractures above their percolation threshold. Therefore, if such M values are observed over fluid‐filled fractures, it is likely that they are open for fluid flow.

A novel software and conceptual design of the hardware platform for intensity modulated radiation therapy.

To deliver high quality intensity modulated radiotherapy (IMRT) using a novel generalized sparse orthogonal collimators (SOCs), the authors introduce a novel direct aperture optimization (DAO) approach based on discrete rectangular representation. A total of seven patients-two glioblastoma multiforme, three head & neck (including one with three prescription doses), and two lung-were included. 20 noncoplanar beams were selected using a column generation and pricing optimization method. The SOC is a generalized conventional orthogonal collimators with N leaves in each collimator bank, where N = 1, 2, or 4. SOC degenerates to conventional jaws when N = 1. For SOC-based IMRT, rectangular aperture optimization (RAO) was performed to optimize the fluence maps using rectangular representation, producing fluence maps that can be directly converted into a set of deliverable rectangular apertures. In order to optimize the dose distribution and minimize the number of apertures used, the overall objective was formulated to incorporate an L2 penalty reflecting the difference between the prescription and the projected doses, and an L1 sparsity regularization term to encourage a low number of nonzero rectangular basis coefficients. The optimization problem was solved using the Chambolle-Pock algorithm, a first-order primal-dual algorithm. Performance of RAO was compared to conventional two-step IMRT optimization including fluence map optimization and direct stratification for multileaf collimator (MLC) segmentation (DMS) using the same number of segments. For the RAO plans, segment travel time for SOC delivery was evaluated for the N = 1, N = 2, and N = 4 SOC designs to characterize the improvement in delivery efficiency as a function of N. Comparable PTV dose homogeneity and coverage were observed between the RAO and the DMS plans. The RAO plans were slightly superior to the DMS plans in sparing critical structures. On average, the maximum and mean critical organ doses were reduced by 1.94% and 1.44% of the prescription dose. The average number of delivery segments was 12.68 segments per beam for both the RAO and DMS plans. The N = 2 and N = 4 SOC designs were, on average, 1.56 and 1.80 times more efficient than the N = 1 SOC design to deliver. The mean aperture size produced by the RAO plans was 3.9 times larger than that of the DMS plans. The DAO and dose domain optimization approach enabled high quality IMRT plans using a low-complexity collimator setup. The dosimetric quality is comparable or slightly superior to conventional MLC-based IMRT plans using the same number of delivery segments. The SOC IMRT delivery efficiency can be significantly improved by increasing the leaf numbers, but the number is still significantly lower than the number of leaves in a typical MLC.

Numerical Simulation of non-Newtonian Fluid Flows through Fracture Network

We present a numerical simulation of non-Newtonian fluid flow in a twodimensional fracture network. The fracture is having constant mean aperture and bounded with Hurst exponent surfaces. The non-Newtonian rheology behaviour of the fluid is described using the Power-Law model. The lattice Boltzmann method is employed to calculate the solutions for non-Newtonian flow in finite Reynolds number. We use a constant force to drive the fluid within the fracture, while the bounceback rules and periodic boundary conditions are applied for the fluid-solid interaction and inflow outlflow boundary conditions, respectively. The validation study of the simulation is done via parallel plate flow simulation and the results demonstrated good agreement with the analytical solution. In addition, the fluid flow properties within the fracture network follow the relationships of power law fluid while the errors are becoming larger if the fluid more shear thinning.

Investigating Aperture-Based Permeability and Capillary Pressure in Rock Fractures

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170819, “Investigating Aperture-Based Stress-Dependent Permeability and Capillary Pressure in Rock Fractures,” by Da Huo, SPE, Boxiao Li, SPE, and Sally M. Benson, SPE, Stanford University, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed. Laboratory measurements of fracture-aperture distributions have been made by use of computed-tomography (CT) scanning under various conditions of effective stress. The stress-dependent aperture-distribution data demonstrate that increasing stress results in two effects: The mean aperture will decrease and the variance of aperture distribution will increase. By analyzing the stress-dependent mean and variance of aperture distributions, this paper provides a more straightforward method for estimating stress-dependent permeability and capillary pressure in rock fractures. Experimental Setup A Berea sandstone is cut to create a fracture in the middle of the core. The sample is moderately homogeneous, with little porosity variation. The fracture surface is very rough and has large aperture variations. The length of the sample is 8 cm, and the radius is 5 cm. The porosity of the sample is 0.22. The rock sample is wrapped in a sleeve and then placed in an aluminum core holder. Water is injected around the sleeve to create the confining stress. Two high-accuracy pressure transducers are used to measure pressure at the inlet and outlet of the core, and electric heaters maintain the temperature of the core. Pore pressure is kept constant at atmospheric pressure, and temperature stays constant at 50°C. For permeability measurements, a dual-pump system is used to inject water into the core. A pressure-regulating pump is connected to the outlet of the core to maintain 2.07 MPa at the downstream end of the core throughout the experiment. The experimental setup is shown in Fig. 1. Permeability is measured with the steady-state method. Fluid is injected at three different rates while the pressure drop across the core is measured. On the basis of core size, fluid viscosity, slope of pressure drop, and fluid-injection rate, Darcy’s law is used to calculate permeability. The intact-core permeability is measured at different stress levels and is subtracted from the fractured-core permeability. Permeability is calculated using a mean aperture of 0.1 mm. The core holder is placed in a medical X-ray CT scanner. Five repeated scans are taken and averaged to reduce the uncertainty of the CT scanning.

自仿射粗糙割理中流体渗流的分形定律

The surfaces geometric controlling effects on fluid flow through a single cleat in coal reservoir were investigated in detail, by coupling the fractal theory, governing equation of fracture flow and numerical simulations. At first, we proposed a multiple-effect fracture-permeability model according to the resistant ways on the fracture flow from the surface geometries, in which, apart from the stationary local roughness effect, the hydraulic tortuosity and surface tortuosity are physically taken into consideration. Based on the new proposed model, a fractal fracture-permeability equation was established according to the self-affine property of the cleats internal surfaces, with the observed scale being its mean aperture. The new fractal fracture-permeability model indicates that: the effects from hydraulic and surface tortuosities are both scaling-invariant to the mean aperture, but they possess different tortuous fractal dimension; while the local roughness effect is stationary at long range. The performance of the new fractal fracture-permeability model were verified by lattice Boltzmann methods, and the numerical experiments show that: in a rough cleat, the velocity profile is a skewed or norm distribution model other that the parabolic one; except for the frictional effect, the surface rough geometry will bring about eddy effect, that will speed up the loss of pressure due to the counterpresssure distribution near the rough surfaces.

Hydraulic sealing due to pressure solution contact zone growth in siliciclastic rock fractures

AbstractThermo‐hydro‐mechanical‐chemical simulations at the pore scale are conducted to study the hydraulic sealing of siliciclastic rock fractures as contact zones grow driven by pressure dissolution. The evolving fluid‐saturated three‐dimensional pore space of the fracture results from the elastic contact between self‐affine, randomly rough surfaces in response to the effective confining pressure. A diffusion‐reaction equation controls pressure solution over contact zones as a function of their emergent geometry and stress variations. Results show that three coupled processes govern the evolution of the fracture's hydraulic properties: (1) the dissolution‐driven convergence of the opposing fracture walls acts to compact the pore space; (2) the growth of contact zones reduces the elastic compression of the pore space; and (3) the growth of contact zones leads to flow channeling and the presence of stagnant zones in the flow field. The dominant early time compaction mechanism is the elastic compression of the fracture void space, but this eventually becomes overshadowed by the irreversible process of pressure dissolution. Growing contact zones isolate void space and cause an increasing disproportion between average and hydraulic aperture. This results in the loss of hydraulic conductivity when the mean aperture is a third of its initial value and the contact ratio approaches the characteristic value of one half. Convergence rates depend on small‐wavelength roughness initially and on long‐wavelength roughness in the late time. The assumption of a characteristic roughness length scale, therefore, leads to a characteristic time scale with an underestimation of dissolution rates before and an overestimation thereafter.

An in Vivo 3D Computed Tomographic Analysis of Femoral Tunnel Geometry and Aperture Morphology Between Rigid and Flexible Systems in Double-Bundle Anterior Cruciate Ligament Reconstruction Using the Transportal Technique

The aim of this study was to compare femoral tunnel length, femoral graft-bending angle, posterior wall breakage, and femoral aperture morphologic characteristics between rigid and flexible systems after double-bundle (DB) anterior cruciate ligament (ACL) reconstruction using the transportal (TP) technique. We evaluated 3-dimensional computed tomography (3D-CT) results for 54 patients who underwent DB ACL reconstruction using the TP technique with either a flexible system (n= 27) or a rigid system (n= 27). The femoral tunnel length, femoral graft-bending angle, posterior wall breakage, femoral tunnel aperture height to width (H:W) ratio, aperture axis angle, and femoral tunnel position were assessed using OsiriX Imaging Software and Geomagic Qualify 2012 (Geomagic, Cary, NC). The mean anteromedial (AM) femoral tunnel length of the flexible group was significantly longer than that of the rigid group (P= .009). The mean femoral graft-bending angles in the flexible group were significantly less acute than those in the rigid group (AM, P < .001; posterolateral [PL], P= .003]. Posterior wall breakage was observed in both groups (P= 1.00). The mean H:W ratios in the rigid group were significantly larger (more elliptical) than those of the flexible group (AM, P < .001; PL, P= .006). The mean aperture axis angle of the PL femoral tunnel in the rigid group was more parallel to the femoral shaft axis than that in the flexible group (P < .001). There were no significant differences in femoral tunnel position between the 2 groups. The AM femoral tunnel length and the AM/PL femoral graft-bending angle of the flexible system were significantly longer and less acute than those of the rigid system. However, the aperture morphologic characteristics of the AM/PL femoral tunnel and the aperture axis angle of the PL femoral tunnel in the rigid system were significantly more elliptical and closer to parallel to the femoral shaft axis than those of the flexible system. Level III, retrospective comparative study.

Examination of the properties of IMRT and VMAT beams and evaluation against pre-treatment quality assurance results

This study aimed to provide a detailed evaluation and comparison of a range of modulated beam evaluation metrics, in terms of their correlation with QA testing results and their variation between treatment sites, for a large number of treatments. Ten metrics including the modulation index (MI), fluence map complexity, modulation complexity score (MCS), mean aperture displacement (MAD) and small aperture score (SAS) were evaluated for 546 beams from 122 intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans targeting the anus, rectum, endometrium, brain, head and neck and prostate. The calculated sets of metrics were evaluated in terms of their relationships to each other and their correlation with the results of electronic portal imaging based quality assurance (QA) evaluations of the treatment beams. Evaluation of the MI, MAD and SAS suggested that beams used in treatments of the anus, rectum, head and neck were more complex than the prostate and brain treatment beams. Seven of the ten beam complexity metrics were found to be strongly correlated with the results from QA testing of the IMRT beams (p < 0.00008). For example, values of SAS (with multileaf collimator apertures narrower than 10 mm defined as ‘small’) less than 0.2 also identified QA passing IMRT beams with 100% specificity. However, few of the metrics are correlated with the results from QA testing of the VMAT beams, whether they were evaluated as whole 360° arcs or as 60° sub-arcs. Select evaluation of beam complexity metrics (at least MI, MCS and SAS) is therefore recommended, as an intermediate step in the IMRT QA chain. Such evaluation may also be useful as a means of periodically reviewing VMAT planning or optimiser performance.

Numerical method for estimating void spaces of rock joints and the evolution of void spaces under different contact states

To determine the void spaces of rock joints under different normal stresses and shear displacements, we mainly introduce a numerical method which was developed based on equivalent void space derived from composite topography. The new method requires the 3D surface data of rock joints, and the normal closure data of the compression test under different shear displacements, while in conventional methods, some disparate materials are inserted between the joint surfaces or special equipments are needed for the measurement of the void space of rock joints without shearing occurs. To apply the technique, a modified 3D box counting method that considers the self-affine fractal property of void spaces was employed to calculate the 3D fractal dimension of the void space. Specially designed experiment was conducted on a cylindrical specimen of artificial joints to explore aperture distribution, and the correlations between void space characteristics, 3D fractal dimension and mean aperture, and normal stress under different shear displacements. The present study focuses on the introduction of the new method for estimating void spaces of rock joints, while the void spaces model obtained contains the combined surfaces roughness and aperture information of rock joints under different normal loads and shear displacements is promising in investigating the mechanical and hydraulic properties during the loading process.

Morphology of the shell of Happiella cf. insularis (Gastropoda: Heterobranchia: Systrophiidae) from three forest areas on Ilha Grande, Southeast Brazil

We conducted a study on shell morphology variation among three populations of Happiella cf. insularis (Boettger, 1889) inhabiting different areas (Jararaca, Caxadaco, and Parnaioca trails) at Vila Dois Rios, Ilha Grande, Angra dos Reis, state of Rio de Janeiro, Brazil. Linear and angular measurements, shell indices representing shell shape, and whorl counts were obtained from images drawn using a stereomicroscope coupled with a camera lucida. The statistical analysis based on ANOVA (followed by Bonferroni's test), Pearson's correlation matrix, and discriminant analysis enabled discrimination among the populations studied. The variable that most contributed to discriminate among groups was shell height. Mean shell height was greatest for specimens collected from Jararaca, probably reflecting the better conservation status of that area. Good conservation is associated with enhanced shell growth. Mean measurements were smallest for specimens from Parnaioca, the most disturbed area surveyed. Mean aperture height was smallest for specimens from Parnaioca, which may represent a strategy to prevent excessive water loss. Discriminant analysis revealed that the snails from Jararaca differ the most from snails collected in the two other areas, reflecting the different conservation status of these areas: shells reach larger sizes in the localities where the humidity is higher. The similarities in shell morphology were greater between areas that are more similar environmentally (Caxadaco and Parnaioca), suggesting that conchological differences may correspond to adaptations to the environment.

SU‐D‐18A‐05: Assessing Elekta MLC Tracking Performance

Purpose:To investigate the technical feasibility of using Elekta MLCs in tracked radiotherapy, specifically to show the capability of an Agility MLC to follow model tumor trajectories using object tracking. The system performance of the MLC is to be assessed via dosimetric experiments and temporal response measures.Methods:The system observer in the tracking cascade consisted of a camera serving for this proof of concept; Images of a moving object were acquired continuously and decorrelated for the position information, which was fed forward to the RT computer controlling the MLC via an interface provided by Elekta. The 1D motion (in the direction of the leaves) was induced by a CIRS motion controller. Additionally, the aperture position was observed by means of the light field of the linac. Calculating the phase between aperture and object yielded the mean aperture lag for the current setup. The object was restricted to sinusoidal motion with a period of 10s. For dosimetry, GafChromic radiosensitive film was irradiated with a total dose of 1000MU using an aperture size of 40mm and a motion range of 30mm.Results:Object tracking can substantially reconstruct the geometric dose response of a static target. In the dynamic case, dose is smeared out into the legs of the static distribution, leading to a reduced plateau and increased FWHM of 5mm, compared to the static width of 38mm. The time‐lag between object and aperture was determined to be approximately 300ms for the current set‐up.Conclusion:We demonstrated a tracking experiment performed on a clinical Elekta linear accelator for the first time. Observed profile variations show the dosimetric impact of tracked delivery. The determined lag is a valuable descriptor for a future tracking cascade employing predictor filters. The performed experiments are generic and possible predecessors for future applications with MR‐Linac or ultrasound probes.Conflict of interest: this project is partly funded by Elekta.