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AIAA-2003-0686

AIR FLOW REGIMES IN OPERATING THEATRES FOR ENERGY EFFICIENT PERFORMANCE

Ramiz Kameel†, Essam E. Khalil‡

Mechanical Engineering DepartmentFaculty of EngineeringCairo University, Egypt

ABSTRACTOne of the main objectives of ideal air distribution in heating, ventilation, and air

conditioning systems is to create proper combination of temperature, humidity, and air motion in the occupied zone of the air-conditioned room. Recent advances in air conditioning technologies and applications had led to better performance, development and modifications to air flow pattern in air conditioned rooms design for ultimate comfort, Berglund 1, Hosni et al. 2 and Medhat 3.

The healthcare requirements take different views depending on the air conditioning applications. The healthcare in residential applications is completely different than those in the medical applications, due to the nature of applications, and occupants. Pioneers in the medical applications enforced many restrictions to the comfort criteria requirements to form health criteria. These criteria were accumulated and compiled to healthcare standards such as ASHRAE standard for the residential and commercial applications and the National Health Service (NHS) standard for the healthcare applications.

Optimization of airflow regimes, heat transfer for comfort and hygiene in operating theatres is one of the main targets of HVAC engineers. This optimization considers the optimum conditions besides reaching the energy efficient design. Most of operating theatres operate on 100% fresh air principles; typically requiring excessive fresh air-cooling loads particularly in hot and humid climates.

Recent development of experimental measuring techniques for air temperature, relative humidity, velocities, and turbulence intensities in flow regimes had aided better understanding of flow phenomena, heat transfer and turbulence interactions. Extensive efforts are exerted to adequately predict the air velocity and turbulence intensity distributions in the room and to reduce the energy requirements and noise to ultimately produce quite and energy efficient air conditioning systems.

The present work fosters a mathematical approach to numerically predict the flow regimes, heat transfer and relative humidity in surgical operating theatres of different internal loads conditions; the present work also utilizes the 3DHVAC program, Khalil 4-6, Kameel 7,8, and Kameel and Khalil 9-11. Full three-dimensional solves of the governing conservation equations of mass, momentum, energy, and contaminant age was utilized in the present work with the k-ε turbulence model closure. It was found that energy efficiency depends drastically on the inside personnel, surgery staff members and the sources of the heat in the operating zone.

† Research Associate, M.Sc‡ Professor of Mechanical Engineering, M. ASME, ASHRAE, Associate Fellow AIAA

Copyright © 2003 by the American Institute of Aeronautics andAstronautics, Inc. All rights reserved.

41st Aerospace Sciences Meeting and Exhibit6-9 January 2003, Reno, Nevada

AIAA 2003-686

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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1. INTRODUCTION

Since the early sixties, many studies and researches concerned of the indoor air quality (IAQ) of healthcare applications. Hospital air conditioning resumes a more important role than just the promotion of comfort. In many cases, proper air conditioning is a factor in patient therapy; insome instance, it is the major treatment. Studies show that patient in controlled environments generally have more rapid physical improvement than do those in uncontrolled environments (ASHRAE, 1999) 12. Indoor air quality is more critical in health care facilities than in most other indoor environments due to many dangerous microbial and chemical agents present and due to the increased susceptibility of the patients, especially immunosuppressed persons (Healthy Buildings, 2000) 13.

The basic requirements in earlier studies were to achieve the aseptic conditions inside the healthcare applications and to control hazardous emissions for patients, personnel and visitors. The earlier studies were concerned of the optimum design of the ventilation system inside the hospitals to maintain the desired comfort and hygiene conditions, regardless the energy efficient performance.

Operating suite is the most critical area inside the hospital that requires special caring. The systems serving the operating rooms, including cystoscopic and fracture rooms, require careful design to minimize the concentration of airborne organisms. The largest amount of the bacteria found in the operating theatre comes from the surgical team and is a result of their activities during surgery. During an operation, most members of surgical team are in the vicinity of the operating table, creating the undesirable situation of contaminating in this highly sensitive area.

Studies of operating room air distribution devices and observation of installations in industrial clean rooms indicate the delivery of the air from the ceiling, with a downward movement to several exhaust inlets located

on opposite walls, is probably the most effective air movement pattern for maintaining the concentration of contamination at an acceptable level. Completely perforated ceilings, partially perforated ceilings, and ceiling-mounted diffusers have been applied successfully, Pfost 14. Surgical operating theatre suites are typically in use no more than 8 to 12 hr/day (excepting trauma and emergency departments). For energy conservation, the air conditioning system should allow a reduction in the air supplied to some or all the operating rooms when possible. Positive space pressure must be maintained at reduced air volumes to ensure sterile conditions. The time that required for an inactive room to become usable again must be considered. Consultation with the hospital surgical staff will determine the feasibility of this feature, ASHRAE 1999 12. A separate air exhaust system or special vacuum system should be provided for the removal of anesthetic trace gases, NIOSH 15. Medical vacuum systems have been used for removal of non-flammable anesthetic gases, NFPA standard 99 16. One or more outlets may be located in each operating room to permit connection of the anesthetic machine scavenger hose. Although good results have been reported from air disinfection of operating theatres by irradiation, this method is seldom used. The reluctance to se irradiation may be attributed to the need for special designs for installation, protective measures for patients and personnel, constant monitoring of lamp efficiency, and maintenance.

1.1. Purpose of Present WorkMost of operating theatres operate on

100% fresh air principles; typically requiring excessive fresh air-cooling loads particularly in hot and humid climates. That principle of operation consumes high energy. The present work is devoted to critically analyze the energy efficiency of the operational conditions and due to the presence of the surgery team for the proposed operating

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)1(S)grad-V(Div eff, ΦΦ =Φ⋅ΓΦρ

theatre configuration; this is carried out to enhance designer knowledge in peruse of influence of the surgery team on the energy efficiency.

Numerical computations were performed to predict the airflow characteristics utilizing the 3DHVAC program that was developed earlier by Khalil 4-6, and modified by Kameel 7-8, and Kameel and Khalil 9-11. 3DHVAC program capabilities were critically examined to adequately predict the airflow characteristics inside a complicated surgical operating theatre by Kameel 8, and Kameel and Khalil 17-19.

2. THEATRE CONFIGURATION

A typical operating theatre configuration, is shown in Figure 1, and is proposed here as a base investigation case. A proposed case study adopts the Health Technical Memorandum (HTM) developed by the National Health Service (NHS) standard of UK 20. The room dimensions are 6.0 m in length (L), 5.0 m in width (WD), and 3.0 m in height (H). The operating table has a length of 2.0 m and a width of 1.0 m, and is1.0 m high. For the proposed design case study, ceiling square perforated supply air grilles were located at the center of the room with total dimensions 1.8 m x 1.8 m (9 modules of 0.6 m x 0.6 m with absolute filter banks), as shown in Figure 1.

The exhaust ports were located on the left and right opposite walls, Figure 1. The extract air port dimensions were 0.6 m x 1.8 m. The lower port center point is located at 0.7 m from the floor. The upper extract port center-point is located at 1.7 m from the floor. The distance from the lower level of the light pendent to the floor was kept as 2.0 m, Kameel 8.

3. NUMERICAL METHODS

3.1. Mathematical EquationsComputational Fluid Dynamics (CFD) is

used to model airflow by numerically solving equations based on fundamental physical principles. Three time averaged

velocity components in X, Y, and Z coordinate directions were obtained by solving the governing equations using a "SIMPLE Numerical Algorithm" [Semi Implicit Method for Pressure Linked Equation] described earlier in the work, Spalding and Patankar 21, Launder and Spalding 22, and Khalil 23. The turbulence characteristics in the present work were represented by standard low-Reynolds k-εmodel to account for normal and shear stresses and near-wall functions. Fluid properties such as densities, viscosity and thermal conductivity were obtained from references. The present work made use of the Computer Program 3DHVAC, which was developed, by Khalil 4-6 and modified later by Kameel 7,8, and by Kameel and Khalil 9-11,17-19. The program solves the differential equations governing the transport of mass, three momentum components and energy in three-dimensional configurations. The different governing partial differential equations are typically expressed in a general form as:

The effective diffusion coefficients and source terms for the various differential equations are listed in Table 1. The Computational Fluid Dynamics (CFD) model utilizes the following approximations in calculating the turbulence quantities, such as isotropic turbulence and the Boussinesq eddy viscosity concept.

Table 1. Values of Φ, ΓΦ,eff, and SΦ for Partial Differential EquationsΦ ΓΦ,eff SΦ

Continuity 1 0 0X-momentum U µ - ∂P/∂x+ρgx

Y-momentum V µ - ∂P/∂y+ρgy

Z-momentum W µ - ∂P/∂z+ρgz+ρgβ∆tH-equation H µ / σH SH

RH-equation RH µ / σRH SRH

τ-age equation τ µ / στ ρµ = µlam + µ t

µ t = ρ Cµ k2 / ε

G = µ [2{(∂U/∂x)2 +(∂V/∂y)2 +(∂W/∂z)2}+(∂U/∂y + ∂V/∂x)2

+(∂V/∂z + ∂W/∂y)2 +(∂U/∂z + ∂W/∂x)2]C1 = 1.44, C2 = 1.92, Cµ = 0.09σH = 0.9, σRH = 0.9, στ = 0.9, σk = 1.0, σε = 1.3H-equation represents the Energy Equation.

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3.2. Boundary ConditionsThe solution of the governing equations

can be realized through the specifications of appropriate boundary conditions. The values of velocity, temperature, kinetic energy, and its dissipation rate should be specified at all boundaries.

3.2.1 WallsA non-slip condition at all solid walls is

applied to the velocities. The logarithmic law of the wall (wall function) of Launder and Spalding 22 was used here, for the near wall boundary layer.

3.2.2. Air Supply InletsAt inlets, the air velocity was assumed to

have a uniform distribution; inlet values of the temperature were assumed to be of a constant value and uniform distribution. The kinetic energy of turbulence and its dissipation rate are commonly estimated as follow.kin = 3 (0.5 (Iin Uin)

2),εin = Cµ (kin)

1.5 / le,

3.2.3. Initial Guessed ValuesAll velocity components were set as zeros

initially, and temperatures were assumed to be equal to the steady state value of the comfort condition. The kinetic energy and its dissipation are estimated in the following manners.kinitial = 11E-5, εinitial = Cµ (kinitial)

1.5 / c ⋅ d,

3.2.4. Light PendentThe outer surface of the light pendent is

represented by solid surface with the application of wall treatment to the full-scale pendent. The heat dissipated from the light pendent was considered to be equivalent to 100 W/m2, transferred from the lower surface of the pendent only.

3.2.5. Surgery TeamThe surgery team is represented

physically by a blockage of 1.8 height and square cross section (0.3 x 0.3 m). Heat emission rates are positioned in the human to represent sensible heat and evaporative

heat losses. For computations of relative humidity, a source of water vapor was introduced in the relevant positions of the team to yield the appropriate respiratory losses. The heat loss due evaporation from skin was neglected relative to the sensible heat loss. The heat loss is 130 W/m2/person, (According to ASHRAE 12 this heat generation equivalent to the heavy load). The water vapor source/person is 0.03 kg water vapor/ kg dry air.

The sensible heat that results from skin was calculated from the ASHRAE 12

formulae (neglect radiative loss).

The water vapor source that results from respiratory losses was calculated from the ASHRAE 12 formulae as follows:

3.3. Numerical ProcedureThe Computer Program, 3DHVAC of

Kameel and Khalil 9-11,17-19, was used to solve the time-independent (steady state) conservation equations together with the standard k-ε model as Launder and Spalding 22, and the corresponding boundary conditions. The numerical solution grid divided the space of the surgical operating theatre into discritized computational cells (80 x 60 x 30 grid nodes) using the modified hyperbolic equation of Kameel and Khalil 24, as in equation 4. The discrete finite difference equations were solved with the SIMPLE algorithm, Patankar 25. Solution convergence criteria, was applied at each iteration and ensured the summations of normalized residuals were less than 0.1% for flow, 1% for k and ε, and 0.1 for energy.

( ) ( )( ) ( ) )4(]

2/Cosh/2/Sinh

)5.0n/I(Cosh/)5.0n/I(Sinh

1[L5.0LX

11

i1i1

i1kk

αβα−αβ−α

++= −

(2))h1/f)/(R t-(t) t-(thfSH cclclaskaclccl +==

)3( W0.20- t0.0000650.0277W aaex +=

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3.4. Models ValidationPrevious comparisons between measured

and predicted flow pattern, turbulence characteristics, and heat transfer was reported earlier in the open literature utilizing the present models. The predictions of flow and turbulence characteristics are in general qualitative agreement with the corresponding experiments and numerical simulations published by others, for further detail review, Kameel and Khalil 26-28.

4. PARAMETRIC CASES

The present investigation compares four basic cases of different surgery team simulation in the same operating theatre. For all cases the inlet supply discharge velocity was kept as 0.287 m/s and the air temperature was kept as 284 oK and the room temperature was taken as 296 oK. Table 2 and figure 1 represent the parametric cases under consideration.

Table 2: Case StudiesCase Surgery Staff No. Light Pendent

1 - No2 2 Persons No3 2 Persons Yes4 6 Persons Yes

4. RESULTS

For further enhancement and to understand the global effect of the surgery staff on the internal environment of the surgical operating theatre, it should investigate the local effect of the surgery staff. The following results will be presented in sections near the human object, as shown in the figure 1. The human object face was supposed to be look toward the center of the operating table. So the position A will represent the effect of the front of the human and position B will represent the effect of the back.

Figure 2 represents the normalized W-velocity component (ϖ = Wp/Wo) along the room height at section A-A for cases 1 and

2. The figure represents that the airflow between the two levels (Z/H = 0.4 and 0.6) in case 2 appeared directed upward, that due to the presence of the heat generated from the human object. The same figure represents the normalized temperature (θ = Tp-Ts/Tr-Ts) along the room height at position A for cases 1 and 2. The area between the two curves in the temperature results represents the additional heat from the human body, which translated in terms of increasing in the local temperature.

In the same way, the results of the normalized relative humidity (φ = RHp-RHr/RHs-RHr) along the room height at position A are presented. The respiratory simulation of the surgery staff has a pronounced influence on the relative humidity distribution. In case 2, the maximum relative humidity occurred at the level of the breathing. In case 1, the maximum values occurred near the supply diffusers, due to the supplied humid air.

Figure 3 represents the normalized W-velocity component (ϖ = Wp/Wo) along the room height at position B for cases 1 and 2. The figure represents an interesting result, the airflow at that section in case 1 has two recirculation zones, between two levels (Z/H = 0.7 and 1.0) and between two levels (Z/H = 0.1 and 0.4). But in case 2 the presence of the body object shares to strong the downward airflow. From the details of the case 2, it can observe the size decreasing of the recirculation zones. From the temperature results, in figure 3, it can observe the influence of the added heat from the surgery staff on the temperature distribution. On the other hand, the effect of the respiratory of the surgery staff is limited on the relative humidity distribution at position B in case 2. From the two figures 2 and 3, it can conclude the pronounced local effect of the surgery staff.

Figure 4 shows the comparison between the case 3 and case 4. The comparison between the two cases 3 and 4 is presented

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at the vertical line that passes across the center of the ceiling and the floor, position C, as shown in figure 4.

This comparison was carried out to represent the effect of the surgery staff members increasing. The airflow distribution is affected by the added heat from the extra staff members. The velocity results in the figure 4 shows an upward airflow in the vicinity of the operating table. The upward flow is a result of the buoyancy effect due to the extra heat flux around the operating table. And the temperature results represent the effect of the added heat in the vicinity of the operating table. On the other hand, figure 4 shows the relative humidity results. The relative humidity value in the vicinity of the lower surface of the light pendent in case 4, because of two reasons:

� The presence of the six members of the surgery staff aided to increase the relative humidity in the air.� The upward airflow aided to transfer the water vapor vertically in the vicinity of the vicinity of the light pendent.

5. DISCUSSION AND CONCLUSION

It is found the internal energy consumed in the operating theatre will depend on the thermal load, so the supply conditions were fixed to detect the thermal distribution

From results, one can observe that the energy added to the supply air (for cooling) will consumed in area over the operating table (operating area). On the other hand, the other parts of the room stated at the return conditions, which is relatively hot to the supply conditions. From these results one can observe, in the present operating theatre, that theatre doesn’t efficient in energy. Actually, the design permits reasonable conditions in the operating area, but on account the energy efficiency.

From the present work, it observed the HVAC systems of surgical operating theatre can perform its duties to maintain the operating area in the required proper conditions, on account the energy consumption. Indeed, the healthcare applications especially the operating theatre concerns in the first place to save the life of patients and working staff. But that doesn’t give the permission to engineers to design a relatively low energy efficient HVAC systems.

Finally, it can observed that the operating area is the optimum zone in the whole theatre, that can lead to consider all other zones in the theatres are “Dead Zones” relative to the operating zone. In the surgeries that need large surgery staff or preformed for two patients simultaneously, the most of them will lay in the “Dead Zones”, unless the design of HVAC system was changed.

6. RECOMMENDATIONS

From the present work, we can introduce the following recommendations to increase the global energy efficiency in surgical operating theatres.

� It is preferable to remove any objects in the downstream of the supply air, such as, the light pendent, that will enhance the performance of the supply air and airflow regimes and that will reflect on the energy consumption of the HVAC system.� It is preferable to maintain the operating area under the supply diffuser with the lowest thermal load that will improve the energy efficiency. � The surgery staff members should be decreased to possible number to decrease the losses in the energy.� It should study the distribution of the surgery staff members around the operating table and the influence of staff movement in the next researches.

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REFERENCES

1. Berglund, L. G., 1998, Comfort and Humidity, ASHRAE Journal, pp. 35-41, 1998.

2. Hosni, M. H., Tsai, K., and Hawkins, A. N., 1996, Numerical Predictions of Room Air Motion, Fluids Engineering Division Conference, Vol. ASME, pp. 745-751.

3. Medhat, A. M., 1993, Air Conditioning Flow patterns in Enclosures, M.Sc., Thesis, Cairo University.

4. Khalil, E. E., 1994, Three-Dimensional Flow Pattern in Enclosures, Interim Report, Egyptalum, Egypt.

5. Khalil, E. E., 1999, Fluid Flow Regimes Interactions in Air Conditioned Spaces, Proc. 3 rd Jordanian Mech. Engineering Conference, Amman, May 1999.

6. Khalil, E. E., 2000, Computer Aided Design For Comfort In Healthy Air Conditioned Spaces, Proceedings of Healthy Buildings 2000, Finland, Vol. 2, Page 461-466.

7. Kameel, R., 2000, Computer Aided Design of Flow Regimes in Air Conditioned Spaces, M.Sc. Thesis, Cairo University.

8. Kameel, R., 2002, (In progress), Computer aided design of flow regimes in air-conditioned operating theatres, Ph.D. Thesis work, Cairo University.

9. Kameel, R., and Khalil, E. E., 2000, Computer Aided Design of Flow Regimes in Air Conditioned Spaces, Proc. ESDA2000 ASME 5 th Biennial Conference on Engineering Systems Design & Analysis, Montreaux 2000.

10. Kameel, R., and Khalil, E. E., 2000, Fluid Flow and Heat Transfer in Air Conditioned Spaces, International Conference of Energy Systems, 2000, ICES, 2K. Page 188 –200, Amman, 25th

– 28th, Sept. 2000.11. Kameel, R., and Khalil, E. E., 2001,

Numerical Computations Of The Fluid Flow And Heat Transfer In Air-

Conditioned Spaces, NHTC’01 1592, 35th National Heat Transfer Conference, June 10-12, 2001, Anaheim, California.

12. ASHRAE, Fundamentals, 1999, published by ASHRAE, Atlanta.

13. Healthy Building, 2000, Workshops, Workshop 22: IAQ in Hospitals, 2000

14. Pfost, J. F., 1981, A re-evaluation of laminar airflow in hospital operating rooms, ASHRAE Transactions, Vol. 87(2): Page, 729-739.

15. NIOSH, 1975, Elimination of waste anesthetic gases and vapors in hospitals, Publication No., NIOSH 75-137, MAY, U.S. Dept. of Health, Education, and Welfare, Washington, DC.

16. NFPA, 1996, Standard for health care facilities. ANSI/NFPA standard 99-96.

17. Kameel, R., and Khalil, E. E., 2001, Operating parameters affecting air quality in operating theatres: a numerical approach, Clima 2000/Napoli 2001 World Congress – Napoli (I), 15-18 September 2001.

18. Kameel, R., and Khalil, E. E., 2001, Air quality appraisal in air-conditioned operating theatres: numerical analysis, Clima 2000/Napoli 2001 World Congress – Napoli (I), 15-18 September 2001.

19. Kameel, R., and Khalil, E. E., 2001, Air quality appraisal in air-conditioned spaces: numerical analyses, Proc.4th IAQVEC Conference, Changsha, China, Page 287-297.

20. NHS Estates, 1994, Health Technical Memorandum 2025: Ventilation in Healthcare, remises. HMSO: London.

21. Spalding, D. B., and Patankar, S. V., 1974, “A Calculation Procedure for Heat, Mass and Momentum Transfer in Three Dimensional Parabolic Flows”, Int. J. Heat & Mass Transfer, 15, pp. 1787.

22. Launder, B. E., and Spalding D. B., 1974, “The Numerical Computation of Turbulent Flows”, Computer Methods App. Mech., pp. 269-275.

23. Khalil, E. E., 1978, “Numerical Procedures as a tool to Engineering

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Design”, Proc. Informative 78, Yugoslavia.

24. Kameel, R., and Khalil, E. E., 2002, Generation of the grid node distribution using modified hyperbolic equations, 40th Aerospace Sciences Meeting & Exhibit, Reno, Nevada, AIAA-2002-656, 12-15 January 2002.

25. Patankar, S. V. 1980, Numerical heat transfer and fluid flow, 1980 Hemisphere, WDC.

26. Kameel, R., and Khalil, E. E., 2002, Experimental investigations of airflow regimes in air-conditioned operating theatres, Roomvent 2002-122,2002

27. Kameel, R., and Khalil, E. E., 2002,Predictions of flow, turbulence, heat transfer and humidity patterns in operating theatres, Roomvent.2002-120,2002

28. Kameel, R., and Khalil, E. E., 2002,Predictions of turbulence behavior using k-ε model in operating theatres, Roomvent 2002-121,2002

Figure 1: Layout of Theatre Configuration

NOMENCLATURE

c constantd Distance to nearest sidewall.fcl clothing area factor; (taken ~ 1.3),H Room Heighthc convective heat transfer coefficient,

W/(m2⋅K),Iin Intensity of disturbance at air inlet.k Turbulence kinetic energy, m2/s2.le Dissipation length at air inlet.p Pressure, Pascal.Rcl thermal resistance of clothing, (m2⋅K)/W,SΦ Source term of entity Φ, Φ = U, V, W, …SH sensible heat; W/m2,ta ambient air temperature oCtcl clothing temperature oC,ts skin temperature oC,U,V,W Instantaneous components of velocity in

three directions, m/s.Wa humidity ratio of inhaled (ambient) air,

kg (water vapor)/ kg (dry air), Wex humidity ratio of exhaled (ambient) air,

kg (water vapor)/ kg (dry air),X,Y,Z Coordinate directions.

ε Turbulence dissipation rate.Φ Dependent variable.ΓΦ,eff Effective diffusion coefficient.

Γ Molecular viscosity.µ Absolute viscosity of air, kg/ms.ρ Density of air, kg/m3.

σ Effective Prandtl number.φ Normalized Relative Humidity

= (RHp-RHr/RHs-RHr)

θ Normalized Temperature= (Tp-Ts/Tr-Ts)

ϖ Normalized W velocity = (Wp/Wo)

Subscriptsi Inner room (related to return outlets).i,j,k Denoting Cartesian coordinate direction

takes the values of axes X, Y, Z.o Outlet Supply (related to supply).t Turbulent property.

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Figure 2: Numerical Predictions obtained at Position A

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Figure 3: Numerical Predictions obtained at Position B

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Figure 4: Numerical Predictions obtained at Position C