Multi-zone Technique Research Articles

Comparison of Octree and Omni-Tree Cartesian Grid for Civil-Plane High-Lift Model Simulations

This comparative study is motivated by recent developments dealing with transport aircraft high-lift aerodynamics. Different than these developments Cartesian grids are used to achieve octree and omni-tree data structure. In this paper, the computation of complex flowfield is carried out using the cell-center finite volume method and dual-time stepping scheme. Based on the normal momentum equation a curvature-corrected symmetry technique has been developed to build the wall boundary treatment. A face-to-face algorithm is used in combination with the multizone technique, which can precisely establish the information exchange of flowfields in the interfaces among different zones. Compared with wind-tunnel experiments the above methods have been verified with two test cases involving, respectively, a takeoff configuration and a landing configuration. The focus is on the comparison of the octree and omni-tree data structure in Cartesian grid generation. The computed results indicate preliminarily that the omni-tree data structure is of higher efficiency, allows Cartesian grids to be adapted in an arbitrary manner, and offers the potential of a dramatic reduction of the total number of grids with the solution accuracy almost unaffected.

A modified tracer-gas decay model for ventilation rate measurements in long and narrow spaces

Ventilation is essential to the health and comfort of occupants in enclosed spaces. However, it is difficult to accurately measure the ventilation rates in large, long, and narrow spaces such as aircraft cabins and train compartments. This study has proposed a modified tracer-gas-concentration decay method that combines the multi-zone technique with the genetic algorithm to determine ventilation rate in such spaces. To validate the proposed method, the investigation utilized both the modified decay method and the traditional decay method to numerically measure the ventilation rate in an MD-82 aircraft cabin by computational fluid dynamics technique. The results showed that the modified tracer-gas-concentration decay method can significantly improve the accuracy and reliability of the ventilation rate measurements in such a large, long, and narrow space. The modified tracer-gas-concentration decay method was also used to experimentally measure the ventilation rate in an actual MD-82 aircraft cabin. Although no exact ventilation rate could be measured, but compared with the traditional tracer-gas-concentration decay method, the ventilation rate determined by the modified method was much closer to that obtained by the constant tracer-gas-concentration method, which is considered to be the most accurate measurement.

Study of Civil-Plane High-Lift Model Using Omni-Tree Cartesian Grids

We developed a CFD code applies the omni-tree Cartesian grid to simulate the flow fields around one civil-plane high-lift model using the multi-zone technique. With a face-to-face algorithm, the flow control equations are solved numerically, in combination with the cell-center finite volume method and dual-time stepping scheme. The computed results are in good agreement with the experimental data and show preliminarily that numerical method is effective.

Numerically analyzing more efficiently high-lift aerodynamics of wing/body model with omni-tree Cartesian grids

A modified Cartesian grid method is introduced to simulate the flow fields around wing/body high-lift configurations by using a multi-zone technique. With a face-to-face algorithm, the Euler equations are solved numerically, in combination with the cell-center finite volume method and dual-time stepping scheme. The multi-zone technique is employed to simplify the great difficulties in grid generation. The face-to-face algorithm is constructed to precisely establish the information exchange of flow fields on the interface between zones. The numerical results are presented and compared with experimental data.

Numerical Simulations for Transport Aircraft High-Lift Configurations Using Adaptive Cartesian Grid Methods

Computations of complex flow fields around three-dimensional high-lift configurations using an adaptive Cartesian grid method and a cell-center finite-volume method are carried out and compared to wind-tunnel experiments. Because of complexities in both configuration and flowfield, there are great difficulties in numerical simulation of flowfields around transport aircraft high-lift configurations with leading slats and trailing flaps. In this paper, a modified Cartesian grid method is introduced to simulate the complex flow using the adaptive grid technique. The main emphasis is given to a multizone technique and a face-to-face algorithm and its applications. The multizone technique is employed to simplify the great difficulties in grid generation. The face-to-face algorithm is constructed to precisely establish the information exchange of flowfields on interfaces between zones. We solve Euler equations with a conventional algorithm, which includes a cell-center finite-volume method and an explicit four-stage Runge-Kutta time-stepping scheme in combination with a dual-time stepping approach. For three three-dimensional high-lift configurations, the numerical results are presented and analyzed. The computational results are in good agreement with the available experimental data show that the present method is feasible and effective for solving the flow around complex three-dimensional high-lift systems.

Multizone decomposition for simulation of time-dependent problems using the multiquadric scheme

This paper discusses the application of the multizone decomposition technique with multiquadric scheme for the numerical solutions of a time-dependent problem. The construction of the multizone algorithm is based on a domain decomposition technique to subdivide the global region into a number of finite subdomains. The reduction of ill-conditioning and the improvement of the computational efficiency can be achieved with a smaller resulting matrix on each subdomain. The proposed scheme is applied to a hypothetical linear two-dimensional hydrodynamic model as well as a real-life nonlinear two-dimensional hydrodynamic model in the Tolo Harbour of Hong Kong to simulate the water flow circulation patterns. To illustrate the computational efficiency and accuracy of the technique, the numerical results are compared with those solutions obtained from the same problem using a single domain multiquadric scheme. The computational efficiency of the multizone technique is improved substantially with faster convergence without significant degradation in accuracy.

The Relationship between Rabbit Corneal Opacity and Immunohistochemical Expression of Heat Shock Protein 72/73 and c-fos after Excimer Laser Photorefractive Keratectomy

To evaluate the etiology of corneal opacities, a study was done on the immunohistochemical staining for heat shock protein (HSP) 72/73 and c-fos after myopic excimer laser photorefractive keratectomy (PRK). New Zealand white rabbits (60 eyes) were divided into four groups: –6-, –9- and –15-dpt operation groups and a control group (only corneal epithelium removed). Using an excimer laser (VisX 20/20, Sunnyvale, Calif., USA), myopic PRK was performed on the rabbit’s cornea by a single or multizone technique followed by an immunohistochemical stain to detect the expression of HSP 72/73 and c-fos at 6, 24, 48 h, 1 week and 8 weeks. Corneal epithelia were healed within 3 days in all groups, and 8 weeks after the operation, the mean density of corneal opacity was 0.83 (–6-dpt operation group), 1 (–9-dpt operation group) and 1.67 (–15-dpt operation group). All groups exhibited HSP 73 in corneal epithelium and stroma. However, the maximal expression of c-fos and HSP 72 appeared in corneal epithelium at 24 and 48 h, respectively, after the operation in the –15-dpt group. These results suggest that the expressions of HSP 72 and c-fos in rabbit corneal epithelium are significant factors in relation to corneal opacity after excimer laser PRK.

Multizone photorefractive keratectomy for high and very high myopia: Long-term results

Purpose: To compare 2 year results of multizone excimer laser photorefractive keratectomy (PRK) in eyes with myopia (spherical equivalent [SE]) from 6.00 to 10.00 diopters (D) with those in eyes with myopia from 10.25 to 25.75 D.Setting: Wellington Hospital and Laser Vision Harley Street, London, England.Methods: Of the 281 PRK cases with an SE of at least −6.00 D, 59 eyes had a minimum follow-up of 2 years. Twenty-six of these had an SE from −6.00 to −10.00 D and 33, from −10.25 to −25.75 D. The single-pass, multizone technique was used to create ablation zones at 5.0, 5.5, and 6.0 mm. Postoperatively, patients received a minimum of 4 weeks of fluorometholone 0.1%; those exhibiting regression after corticosteroids were discontinued were restarted on a tapering regimen.Results: The refraction stabilized 3 months postoperatively in the −6.00 to −10.00 D group, and the final mean SE refraction was a small undercorrection. Two years postoperatively, 88.5% of eyes had a visual acuity of 20/40 or better, and 77.0% were within ±1.00 D of the intended correction. In eyes with myopia greater than 10.00 D, regression continued during the 2 years and refraction did not stabilize. Forty-two percent had an uncorrected acuity of 20/40 or better, and 48.0% were within ±1.00 D of the intended correction. The incidence and severity of haze were higher in the group with myopia greater than 10.00 D. The overall incidence of complications was low.Conclusion: Two years after multizone PRK, refractive and visual acuity results inyes ith myopia from 6.00 to 10.00 D were good. Results in eyes with myopia of more than 10.00 D were not satisfactory, and refraction had not stabilized.

Treatment of halos after photorefractive keratectomy.

Some patients, despite a good visual outcome after photorefractive keratectomy (PRK), have severe problems with halos. Ten eyes with a near-plano refraction after PRK were treated again with PRK to reduce halos. The goal of the second surgery was to enlarge the diameter of the treatment zones without changing the refraction obtained by the first surgery. The treatment zone was enlarged from 5 or 6 mm to 7 mm. In eyes in which a multizone technique was used in the first PRK, the amount of correction was increased in the transition zone. However, the refractive power in the central 4 or 5-mm zone was not altered. This result was obtained by calculating the amount of desired increased correction on a large zone (6 or 7 mm) and then for the same amount on a smaller zone (4 or 5 mm). The latter, which would produce changes of curvature in the central zone, is subtracted from the overall treatment. Only the part of the treatment where the laser beam increased from 4 or 5 mm to 6 or 7 mm was delivered to the eye. In this process, only the peripheral zones were modified. The central 4 or 5 mm zone was deepened but its curvature and the refraction remained the same. A decrease of symptoms was noticed by seven of the ten patients. In all eyes, the refraction obtained by the initial surgery was not significantly changed by the second surgery. The best results were obtained in milder myopic patients who had their first surgery with a small diameter ablation zone. Halos can be reduced by repeating the PRK and by increasing the treatment zone without changing the refraction obtained by the first PRK.

Excimer laser photorefractive keratectomy for extreme myopia

Purpose: To determine the efficacy, predictability, and safety of excimer laser photorefractive keratectomy (PRK) in treating very high myopia.Setting: Laser Vision Harley Street, London, England.Methods: Eighty-eight patients whose spherical errors ranged from −6.00 to −20.00 diopters (D) had PRK with the VISX 20/20B excimer laser and a multizone ablation technique. The patients were divided into two groups of 44: those with errors up to −10.00 D and those with errors above −10.25 D. Visual acuity, manifest refraction, corneal haze, and topography were examined at 1 week and 1, 2, 3, 6, and 12 months postoperatively.Results: Minimum follow-up was 1 year. Predictability of results was greater in Group 1 than in Group 2 patients: 90.0% were within 1.00 D of intended correction and 70.0% were within 2.00 D, respectively. The average for both groups was 85.0% within 2.00 D and 63.6% within 1.00 D of intended correction. No serious complications or overcorrections were seen in either group. Preoperative astigmatism ranged from 1.50 to 5.50 D. Postoperatively, 82.0% of patients had no residual astigmatism; in the other 18.0%, it ranged from 0.50 to 2.50 D (83.0% had 1.00 D or less). Results with the multizone technique were more successful than those reported with the single- or dual-zone ablation techniques.Results: High myopia was treated with reasonable success using the VISX 20/20B excimer laser system and a multizone technique. Comparable studies using single- and dual-zone approaches appear to have less successful outcomes.

Analysis of load and non-load-carrying fillet welds using HBIEs

In order to obtain the accurate stress intensity factors of crack problem, a finer mesh is usually required in boundary element analysis since the stress (or displacement) solution is not accurate enough, especially near a crack tip. In this study, hypersingular boundary integral equations (HBIEs), combined with a multi-zone technique and traction singular quarter-point element, are implemented to evaluate the accurate boundary stress components especially near the crack tips. The boundary stresses are then used to calculate stress intensity factors by a stress extrapolation method. Numerical results are presented for an edge cracked plate and the stress intensity factors estimated by other methods are compared. Moreover the sensitivity to element mesh and stability to crack lengths are investigated. Finally, the stress intensity factors of load-carrying and non-load-carrying fillet welds are calculated, and the effects of weld size and attachment thickness on the stress intensity factors are analyzed.

Multizone/multipass photorefractive keratectomy: Six month results

A multizone/multipass photorefractive keratectomy (MP-PRK) technique was used to treat 315 eyes with the 193 nm VISX 20/20 excimer laser. Algorithms were developed to treat the full range of myopia, including 170 low myopic eyes (−1.0 to −6.0 diopters [D]), 105 moderately myopic eyes (−6.0 to −10.0 D), and 40 highly myopic eyes (-10.0 to -27.0 D). Preoperative mean myopia was -6.69 D; astigmatism equal to or greater than −0.5 D (mean −1.18 D) was present in 193 eyes. At one month postoperatively, most eyes were slightly hyperopic (mean +0.8 D); this regressed to a mean of −0.16 D. At six months, 95.5% of low myopes, 84.8% of moderate myopes, and 59.5% of high myopes were within 1.0 D of emmetropia; 78.4% of high myopes were within 2.0 D of the intended correction. There was no loss of 20/40 best corrected visual acuity (BCVA) at six months; a significant gain in 20/25 BCVA was seen in the moderate and high myopia groups (P < .05). At six months, uncorrected visual acuity was 20/25 in 89.4% of low myopes, 75.0% of moderate myopes, and 25.7% of high myopes. Mean haze value was 0.48 at one month and was not significantly different among the three groups (0.44, 0.52, 0.51, respectively). A control group treated with the one-pass multizone technique had a significantly higher amount of haze: 1.05 (P < .01). The data presented compare favorably with those obtained from other centers. Based on these results, we believe that MP-PRK is a safe, reliable technique to correct most low, moderate, and high myopia.