Computational Fluid Dynamic Modelling And Simulation .

Computational Methods and Experimental Measurements XIII127Computational fluid dynamic modelling andsimulation evaluation of the plume evacuationdevice efficienciesF. Farshad1, H. Rieke1, L. C. LaHaye2 & S. C. Nulu112University of Louisiana at Lafayette, USAVision Pro LLC, USAAbstractThe purpose of our work has been to evaluate the fluid flow dynamics of distaland proximal handheld plume evacuation devices used during LASIK eyesurgery using Computational Fluid Dynamic (CFD) Modelling.Fluid flow dynamics studies using CFD simulations were conducted on aproximal plume evacuator, LAHayeSIK surgical device, and on the VISX StarS3, which is a distal large volume plume evacuation device. The resulting datawas compared and analyzed with experimental data.CFD results show that the proximal plume evacuation system generated auniform laminar airflow velocity of 0.94 m/s across the corneal surface ascompared to 1.3 m/s reported by the distal evacuation system. Flow profilesindicate high shear regions resulting in vortex formations, for the large volumedistal evacuator.The CFD simulations conducted to determine the airflow profiles generatedby the two surgical plume evacuation devices concur with data obtained fromexperiments. Flow patterns simulated by the CFD modeling, indicate that theproximal plume evacuation devices generate a gentle laminar airflow profilesover the stromal surface. On the other hand, the distal large volume plumeevacuators generate multiple regions of varying air flow velocities contributingto ineffective plume capture.Keywords: computational fluid dynamics, fluent, LAHayeSIKTM, LASIK, plumeevacuation, CFD simulation.WIT Transactions on Modelling and Simulation, Vol 46, 2007 WIT Presswww.witpress.com, ISSN 1743-355X (on-line)doi:10.2495/CMEM070131

128 Computational Methods and Experimental Measurements XIII1IntroductionOne of the ultimate goals in performing any surgical procedure is to minimizeless than desirable outcomes arising from both infectious and noninfectiouscontaminants entering the surgical field (LaHaye et al., [1, 2]). Although notnormally thought of as such, plume smoke is a by-product contaminate ofexcimer laser surgery. The complexities of plume formation and its rapiddynamic movements both vertically and laterally impose problems that presentundesirable outcomes and present health related issues for the surgeon, patient,and nursing staff. In excimer refractive surgery a laser’s accuracy, effectiveness,and reproducibility can be directly affected by how well the surgical operatingenvironment is managed. The authors emphasize that managing the microclimateof the stromal bed during the excimer refractive procedures is the only importantavenue where substantial improvements can be made through advancements indesign for better outcomes and fewer health risks. Air flow dynamics generatedby plume evacuation systems can have direct and indirect influences onrefractive outcome. A direct effect of LASIK plume smoke is the masking effectcreated as the plume hangs just over the ablating stromal bed, blockingsubsequent excimer pulses which can cause a measurable difference in theresulting ablation (Duffey, [3]). Researchers contend that the plume particlesfalling back onto the on the ablating stromal bed creates additional beammasking (Noack et al., [4]) and may be a contributing factor to the “Sands of theSahara” syndrome (Dell, [5]).Research has demonstrated that plume vapor condensation with precipitationcontributes to visible fluid accumulation on the surface of the stromal bed duringablation. This additional regional accumulation of fluid can interfere with beametching to cause a decrease in transmission of energy to the stroma throughincreased reflection and absorption of incident laser energy. This resulted inundesirable ablation, such as central islands, “hot and cold” spots, and undercorrections (Oshika et al., [7]). Most corporate and physician-based nomogramsare based on a certain portion of the excimer beam being blocked by plumeparticles (Maguen and Machat, [8], Duffey, [3]). Laser manufacturers add pulsesto the nomograms based on outcome averages to “compensate” for plumemasking attributes of their systems. The “blanket” method of re-mediating thenumerous problems associated with plume appears to have little logic. Themany potential problems associated with plume should direct one to solving theunderlying issue by removing the cause and therefore the effect as opposed tosimply adding additional laser pulses in an attempt to compensate.Since the adoption of excimer refractive procedures some 15 years ago, theindustry has several alternatives for plume management. The options include; (1)no plume management; (2) operating room ventilation fan generated room airblown across the surgical field; (3) laser integrated distal plume evacuation; (4)laser integrated devices which combine blown air and distal plume evacuation;and (5) handheld proximal plume evacuation devices. Because of the potentialhealth hazards associated with plume, we have chosen to compare systems thatWIT Transactions on Modelling and Simulation, Vol 46, 2007 WIT Presswww.witpress.com, ISSN 1743-355X (on-line)

Computational Methods and Experimental Measurements XIII129are, in theory, designed to only remove plume. Experimental analysis andComputational Fluid Dynamics were used to comprehend the dynamics of theplume generated during the LASIK surgery. Two types of plume evacuationdevices, the proximal plume evacuation device – LAHayeSIK Surgical Systemand the distal plume evacuation system – VISX Star S3 are compared for theireffectiveness in design to remove plume generated during LASIK surgery. OurCFD results of this study emphasize modern computational techniques like CFD,which can be used with a great effect in determining the best design techniquefor medical equipment design.2CFD simulation of the proximal LAHayeSIK plumeevacuation systemThe flow domain for the proximal plume evacuation system is identified as thepath inside and outside the handpiece in the vicinity of the stromal surface,where the plume particles travel under the influence of the plume evacuationforce. Figure 1 shows the plume flow domain of the proximal plume evacuationsystem that is to be modeled.Figure 1:Solid model (domain) of the LAHayeSIK surgical device.The solid model of the domain is created in FLUENT’s GAMBIT using thebasic geometrical tools such as edges, faces, and volumes. The design of thesolid model incorporates all the nuances in the model and the exactmeasurements of angles and distances. The model is then meshed using variousmeshing strategies to come up with the best quality mesh. The mesh can be usedby FLUENT to solve the numerical equations without any divergence problems.Figure 2 shows the meshed model of the plume evacuation function of theLAHayeSIKTM surgical device. The meshed model from GAMBIT is then set upinside the FLUENT console after performing a grid check for negative volumesand inconsistent meshing. Thus, the solution is setup with the requiredparameters and boundary conditions and is iterated for convergence with aconstant monitoring of the solution using the residuals. The convergence criteriaare set to be 10-6 and once the residuals reach this prescribed value, the solutionis said to have converged and the data is post processed and the results analyzed.WIT Transactions on Modelling and Simulation, Vol 46, 2007 WIT Presswww.witpress.com, ISSN 1743-355X (on-line)

130 Computational Methods and Experimental Measurements XIIIFigure 2:Meshed model of the plume evacuation flow domain of theLAHayeSIK surgical device.Figure 3:Solid model of the plume evacuation flow domain of the distalplume evacuation surgical device.3CFD simulation of the distal VISX Star S3 plumeevacuation systemThe flow domain for the distal plume evacuation system is identified as the pathin the vicinity of the stromal surface from the evacuation tube, where the plumeparticles travel under the influence of the suction force. Figure 3 presents theplume flow domain that has been modeled. To better capture the effects of facialfeatures such as the nose, setting of the eyes, and the evacuation tube’s influenceon the flow of room air. A modeled face is in the surgical position as the subjectthat under goes surgery. Notice the considerably sharp features of the nose andreasonably deep-set eyes in figure 3. The model is then meshed using variousmeshing strategies to come up with the best quality mesh, which can be used byFLUENT to solve the numerical equations without any divergence problems.The plume evacuation function of the VISX surgical device was constructed as amesh model. The meshed model from GAMBIT is then exported to the solver,which is FLUENT in the present CFD analysis.4 CFD simulation resultsThe CFD results obtained in this simulation indicate some important flowaspects of the plume under the evacuation field generated by the plumeWIT Transactions on Modelling and Simulation, Vol 46, 2007 WIT Presswww.witpress.com, ISSN 1743-355X (on-line)

Computational Methods and Experimental Measurements XIII131evacuation devices. The proximal plume evacuation system shows a moreeffective evacuation influence on the microclimate over the corneal surface andthus assists in the onsite plume removal without giving the plume particles alonger time of travel. It is noted that the longer the plume particles are in thevicinity of the corneal surface, the more chances for plume masking and othercomplications. The proximal plume evacuation technique allows a quick 360o onsite plume removal technique along the circumference of the cornea with highvelocity gradients created just above the corneal surface. The design of the portsis such that low and uniform velocity fields are generated over the cornealsurface itself, thereby reducing the risk of over dehydrating the cornea.Figure 4:Flow path of the plume particles which are situated at height of theplume channel of the LAHayeSIK surgical device. Simulationresults show that all the plume is captured by the sevenstrategically placed ports.Figure 4 shows the trajectories of the plume particles that are generated underthe influence of the evacuation force of the LAHayeSIK plume evacuationfunction. The particles are created at a distance away from the corneal surfaceand the seven ports at a height of 2 cm from the corneal surface. The simulationshows that the entire plume is being captured by the seven strategically placedports on the circumference of the handpiece.Simulating the particle paths to evaluate the velocity functions with a certaindirection over the surface is a common practice to either create a set of pointsand plot their velocities or simulate the paths of the particle trajectories. InFLUENT, rakes are used to serve the same purpose. Rakes are a predeterminednumber of points between two specified endpoints.Figures 5 and 6 simulate the actual flow path of the plume particles that aregenerated during the actual surgery. These simulations show that not only theentire plume is effectively captured by the seven ports but also the flow pathsindicate that there are no vortices or turbulent behavior during the plume travel.Contour maps show the velocity profiles over specified cross sections of afluid domain (Figs. 7 and 8). These maps play an important role in indicating thechange in velocities along the radial direction of the device. Figure 7 presents acontour map of simulated velocities generated by the plume evacuation functionof the LAHayeSIK surgical device at the corneal height. It shows lowWIT Transactions on Modelling and Simulation, Vol 46, 2007 WIT Presswww.witpress.com, ISSN 1743-355X (on-line)