CFD modeling for the conservation of the Gilded Vault Hall in the Domus Aurea

Original article

CFD modeling for the conservation of the Gilded Vault Hallin the Domus Aurea

Sabatino Albero a, Carlo Giavarini a,*, Maria Laura Santarelli a, Antonello Vodret b

a CISTeC—Centro Interdipartimentale di Scienza e Tecnica per la Conservazione del Patrimonio Storico-Architettonico,Università di Roma “La Sapienza”, via Eudossiana 18, 00184 Rome, Italy

b Soprintendenza per i Beni Archeologici di Roma, Ministero per i Beni e le Attività Culturali, Piazza S. Maria Nova 53, 00186 Rome, Italy

Received 11 February 2003; accepted 6 August 2003

Abstract

A work was carried out to simulate the virtual microclimate conditions in the Hall with the Gilded Vault in the Domus Aurea. Acomputational fluid dynamic (CFD) code based on the finite volumes method solved all the equations needed to predict the detailedtemperature and humidity distribution as well as the air movement in each part of the Hall. The simulation was used to suggest actions for theconservation of the Hall and of the monument as a whole.© 2004 Elsevier SAS. All rights reserved.

Keywords: CFD; Domus Aurea; Hypogeum; Microclimate; Conservation

1. Research aims

The first purpose of the research project is to establish areliable simulation model, capable of providing a virtualrepresentation of the physical reality at a specific site andmoment. The model will clearly show what happens in awell-defined portion of the monument (e.g. a small decoratedarea of a wall) when we introduce changes, such as theclosing of one or more openings.

The “Gilded Vault Hall”, originally enriched with goldenpictorial and plaster decorations, is considered as a firstexample to implement the simulation model through experi-mental measurements and visual observations. The final pur-pose of the work is to suggest the conservation approachesneeded to preserve the decoration and masonry. Followingthis work, the same procedure will be extended to otherportions of the Domus Aurea.

2. Introduction

A previous paper [1] reported the main studies and planscompleted by the Soprintendenza Archeologica di Roma

(SAR) in collaboration with Interdepartmental Centre ofScience and Technology for the Conservation of Historical-Architectural Heritage (CISTeC) for the conservation of theDomus Aurea and public access.

Commissioned by Nero between 64 and 68 AD, the Do-mus Aurea was the most extensive of the imperial residencesin Rome; it was included afterwards in the foundations of theTrajan thermae and was, consequently, buried [2]. The part ofthe monument now open to the public includes a number ofimpressing halls, some of them still containing considerableremains of the rich original decoration.

A monitoring program was planned to measure a numberof environmental parameters, including air temperature (Ta),relative humidity (RH) and airflow velocity (v). It was con-firmed that the extent of ventilation determined the correctconservation of the most delicate frescoes, through its influ-ence on the internal hygrometric conditions. In spite of theclosure of various air passageways, the deterioration of thepictorial decoration and masonry remains a major concern.Considering the complexity of the building and its vulner-ability to external elements, the data measured by the instru-ments in a number of points were not sufficient for a detailedanalysis of the full impact from various environmental pa-rameters in many portions of the decorated halls. Hence theneed for a fluid dynamic simulations, based on computa-tional fluid dynamic (CFD) code. The Gilded Vault Hall was

* Corresponding author.E-mail address: [email protected] (C. Giavarini).

Journal of Cultural Heritage 5 (2004) 197–203

www.elsevier.com/locate/culher

© 2004 Elsevier SAS. All rights reserved.doi:10.1016/j.culher.2003.08.001

selected as a case study, both for its importance and particularplace inside the Domus Aurea (Fig. 1).

3. The Gilded Vault Hall

Located in a central place of the Colle Oppio pavilion, theHall with the Gilded Vault (referred to as the Hall in the

following) is one of the most outstanding parts of the DomusAurea. The dimensions of the floor are 10.0 × 9.8 m; themaximum height is 9.6 m. Only a portion of the vault deco-ration is still visible, but badly damaged (Fig. 2). It was inbetter condition in XVI century, when it was used as a modelby a number of artists [2]. Before the nineteenth centuryexcavations, the Hall was full of earth and debris, so that theground level reached the base of the vault and was a good

Fig. 1. Layout of the Domus Aurea.

Fig. 2. Efflorescence on the vault of the Hall.

198 S. Albero et al. / Journal of Cultural Heritage 5 (2004) 197–203

point of observation for the artists who entered the Domusfrom the ceilings. The south side of the Hall (originallyfacing the Domus central court) is still closed by a non-excavated ground frontage. Two doors (each of them 2.6 mwide and 3.6 m height) face each other in the two longerwalls (No. 1 and 2 in Fig. 3). Airflows enter through thesedoors from the public entrance of the Domus Aurea or (on theopposite side), from the “criptoportico” and from the open-ings in the Octagonal Hall [1]. On the north wall there is anopening (2.25 m2) near the vault, connected to an adjacentside passage (No. 5 in Fig. 3). Two other openings in thevault, made by early explorers (artists and “treasure hunters”)were connected with the above Colle Oppio gardens (Nos.3 and 4 in Fig. 3); these openings are now closed. However,the closure of the central and bigger opening is about 2 minside the vault (No. 3 in Fig. 3) and influences the fluiddynamics of the Hall.

4. The microclimate inside the Gilded Vault Hall

Typical average monthly values of the relative humidity(RH), air temperature (Ta) and airflow velocity (v) inside theGilded Vault Hall during a year (2000) are shown in Fig. 4,while Fig. 5 indicates the external values of the same param-eters. Fig. 6 shows the average values for the airflow velocityin the Hall during the 12 months of the year. The mixing ratio(MR) inside the Hall is compared with the external MRvalues in the following Fig. 7 [3]. The reported data showthat, in the Hall, an evaporation/condensation cycle is nevergenerated, but there is always a slow phenomenon of evapo-ration from the masonries, which absorb humidity from thefloor and from the gardens covering the Domus Aurea. Thediffused evaporation should generate effloresces on the ma-sonries of the Hall; however, this phenomenon is only presentin some portions of the walls and of the ceiling (Fig. 2).

The fluid dynamic approach (CFD Simulation) [4] hasbeen introduced for a better evaluation of the air movementsand of the RH and Ta distributions.

Fig. 3. Geometrical CFD model of the Gilded Vault Hall.

Fig. 4. Mean monthly RH and Ta values inside the Hall.

Fig. 5. Mean monthly RH and Ta values outside the Domus Aurea.

199S. Albero et al. / Journal of Cultural Heritage 5 (2004) 197–203

5. The CFD simulation

The CFD code is based on the finite volumes principle tosolve the momentum, mass and energy equations needed to

predict the detailed temperature and humidity distribution aswell as the air movement within the Hall. The selectedsoftware (STAR-CD, Adapco, UK) allows us to obtain atridimensional evaluation of the climate inside the Hall. Upto now the only known application of this particular methodfor monument conservation concerns the Mole Antonellianain Turin [5] and the caves of Lascaux in France [6]. The fullscale geometrical CFD model contains an air volume of813.10 m3 divided into 151,283 cells, each representing a20 × 20 × 20 cm3 volume. The boundary conditions for thesimulation (Ta, RH and v) are given by the instrumentationinstalled inside the Hall; the average values are consideredfor each season. The air is characterised by the values ofspecific heat (Cp), density, molecular weight, viscosity, ther-mal conductivity and by the humidity content (expressed askilograms of moisture per kilogram of dry air).

6. Simulation outputs

The collected data makes it possible to consider any pe-riod of the year. For sake of simplicity we describe here thesituation related to the colder month (January) and thewarmer one (August) for the year 2000. More data andsimulations are reported elsewhere [7].

6.1. January simulation

If we consider as boundary conditions for the simulationthe average measured values v = 0.1 m s–1, Ta = 9.7 °C andRH = 92%, the outputs for the airflow (Fig. 8) show that thehigher air velocities are near the doors (v = 0.28 m s–1). Theair enters by the west door and rises to the ceiling of the vault,then it falls and exits the east door; the average value of thesimulated airflow velocity in this area is 0.2 m s–1. The

Fig. 8. Outputs of the airflow velocity distribution in January, considering the average value of the measured airflow velocity.

Fig. 6. Mean monthly values of the airflow velocity inside the Hall.

Fig. 7. Mixing ratio values inside the Hall and outside the Domus Aurea.


simulation also shows that the air moves along the south partof the vault, where some decay phenomena are visible(Fig. 9). The RH outputs show maximum values (93.4%)near the north wall opening and average values (92.5%) onthe vault and at centre of the Hall; the RH outputs near thewalls are 90.3%. The simulation shows that the higher hu-midity on the vault is due to the presence of the northopening. The average Ta decreases from 9.7 to 9.1 °C, goingfrom the upper to the lower portions of the Hall. On the basisof the experimental data collected by the instruments, othersimulations are possible. When the measured maximumvalue of the airflow velocity (v = 0.47 m s–1) is considered asboundary condition, the simulation gives higher values of theair velocity between the two doors and through the northopening. RH and Ta outputs show a homogeneous distribu-tion; however, the values are higher respect to the previoussimulation. The influence of the north opening on the micro-climate of the Hall is now clearly emphasized.

6.2. August simulation

Similar simulations can be carried out for the summermonth of August. In this case the average data considered asboundary conditions are RH = 99.5%, Ta = 18.9 °C andv = 0.1 m s–1. The outputs show the highest airflow betweenthe two doors, with no special influence on the vault; theaverage velocity for this flow is v = 0.12 m s–1, with amaximum value (v = 0.27 m s–1) near the east door. Thesimulation also shows the presence of airflow (v = 0.22 m s–1)from the north opening to the floor. The temperature is moreconstant than in January (average Ta = 19.2 °C), while the RHoutputs increase from 99.3% to 100%, from the ceiling to thefloor. Changing the boundary conditions for the airflow inletand considering the maximum value (v = 0.73 m s–1), theoutputs (Fig. 10) reveal the presence of an appreciable air-flow (coming from the north opening) near the vault (averagev = 0.55 m s–1), while in the rest of the Hall the airflow is

Fig. 9. Biological attack and salt efflorescence in the southern part of the vault.


Fig. 10. Outputs of the airflow velocity distribution in August (maximum airflow velocity inlet).

Fig. 11. Outputs of the airflow velocity distribution in August, considering the closure of the north opening (maximum airflow velocity inlet).

Fig. 12. Outputs of the simulation implemented on the basis of the best conditions suggested by the literature [8–11] and previous experiences.


generally lower (v = 0.2 m s–1). The temperature is lesshomogeneous and the average value is now 19 °C; RH issimilar the previous simulation. In conclusion, the Augustsimulations show that the vault is less influenced by theairflow than in January, if we except some effects due to thehigher air velocity.

7. Suggested actions for the conservation

The CFD approach can be applied to study the conserva-tion of the Hall. Other simulations can be carried out with thesame boundary conditions of the previous ones, but consid-ering now the closure of the north wall opening in order tolimit the turbulence in the vault and improve the conservationof the vault itself (Fig. 11). A more general action willinclude the reduction of the airflow through the doors in orderto reduce the turbulence on the adjacent walls and to reduceevaporation from the walls and formation of efflorescence.

On the basis of previous studies on the conservation ofhypogeum structures [8–11] and considering the above men-tioned monitoring and simulation data, we can consider anew simulation with the following boundary conditions:RH = 92%, Ta = 17 °C, v = 0.2 m s–1. These values can besuggested as conditions for maintaining a conservative mi-croclimate inside the Domus. In fact, these values are sug-gested by the necessity of maintaining high humidity valuesand constant air temperature to conserve the frescoes, limit-ing the possibility of salt crystallisation. Low airflow veloci-ties produce low turbulence and low variations of humidityand temperature; at the same time it allows air replacement,which could be useful to avoid problems related to the pres-ence of visitors. To study the influence of these conditionsnew simulations were done (Fig. 12) confirming such as-sumption. In conclusion, a better control of the Domus Aureaentrance is necessary (e.g. partial closure or double doorinstallation). This action will be beneficial both for theGilded Vault Hall and for the monument as a whole; in fact, itwill help to maintain temperature and consistent humiditythroughout the Domus, limiting the direct contact with theexternal climate.

Acknowledgements

The authors thank Prof. P. Oliaro and Prof. E. Cafaro(University of Turin) for the useful suggestions, Dr. A. Mas-sobrio (CD Adapco Italy), TECNO.EL and Prof. C. Nardi-nocchi (University of Rome La Sapienza).

References

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CFD modeling for the conservation of the Gilded Vault Hall in the Domus Aurea

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