(Microsoft Word - \317 \346\323\307\343 \346\335\355\325\341 19-26)


 
 

  

Al-Khwarizmi 

Engineering 

Journal 
 

           Al-Khwarizmi Engineering Journal, Vol. 16, No. 4, December, (2020) 

P. P. 19- 26  

 

 

Impact of using Double Layers Perforated Liners on the Acoustic 

Treatments of the Combustor Systems 

 
Wisam S. Hacham*                  Faisal S. Hiji** 

*Department of Mechatronics Engineering/ Al-Khwarizmi college of Engineering / University of Baghdad 

**
 
Hull University, Hull, HU6 7RX, United Kingdom 

*Email: wisam@kecbu.uobaghdad.edu.iq 

**Email: f.s.hiji-2017@hull.ac.uk 

 
(Received 13 August 2020; accepted 25 October 2020) 

https://doi.org/10.22153/kej.2020.10.002 

 

 

Abstract 

 
The response of the combustor’s liner to the air-flow that passes through it is the key reason for the combustion 

chamber’s noise, hence, the instabilities of the chambers that decrease the mechanical efficiency of such sections by 

increasing its mechanical vibrations, which increases the failure rate that is created during the origination of the cracks 

that spread by the shakes that are produced by a series of high-level frequencies. Accordingly, any work debating the 

impact of the context of the liners in the combustion chamber can provide grasping for the combustion noise generated 

by the undesirable vibrations, and benefits the industrial firms to design an ideal production procedure which increases 

the lifespan of the combustor. The goal of this work is to examine the influence of the acoustic treatment using a double 

layers cylindrical perforated liner on the acoustic transmission loss of gas turbines. The liners layout influences on this 

acoustic parameter were examined using experimental data gained by the insertion of full scale single and double 

patterns of perforated liners into the unique large scale acoustic wind tunnel at the acoustic research center at Hull 

University. The experimental tests, using the designed acoustic signals, were established firstly without existing air 

cross-flow velocity (i.e., 0m/s), then by employing air with velocities interval 5m/s ranging between 10m/s to 25m/s. 

MATLAB script was used to simulate, analyze, and figure out the data collected by the insertion of single and double 

layer patterns of the perforated liner. The experiments demonstrated that, if the perforation acoustic treatments are 

applied, the acoustic transmission loss will decrease particularly obviously at higher level frequencies. Furthermore, the 

results exposed that the air tunnel with the insertion of the liner with double layers reveals an improved perforation 

acoustic treatment with a percentage of about 19 % in most cases. 

 
Keywords: Acoustic treatment, Acoustic wind tunnel, Perforated liner, Sound transmission loss (STL). 
 

 

1. Background 
 

The mechanical instabilities inside combustion 

systems such as gas turbines are extensively 

common. This phenomenon usually damages such 

systems, reducing their mechanical efficiency, and 

the lifespan of their components [1]. The more 

efficient turbines are the engines that have 

premixed combustion technology, which has 

combustion stability and low carbon emission. 

Consequently, a stable operating engine has 

smooth and stable combustion during the 

operation periods for the combustors. 

Different techniques are used to control the 

combustion instability for combustors, such as the 

fuel supplying optimization method by scheduling 

the fuel injection [2]. Further, using an adjustment 

strategy for a combustor’s length. Furthermore, 

the installation of a passive damping device such 

as a Helmholtz resonator, quarter- or half-wave 

tube, or the perforated acoustic liner(s) as applied 



Wisam S. Hacham                           Al-Khwarizmi Engineering Journal, Vol. 16, No. 4, P.P. 19- 26 (2020) 
 

20 

in this work. However, using the first two 

approaches be contingent on the weight and space 

limitations of the combustors [3].  

The perforated liner is used as an acoustic 

damper by absorbing the pressure fluctuations 

inside the combustion chambers, where using the 

liner as a damper involves vortex generation over 

the border of its perforations [4]. 

Nowadays, there are respective studies that 

have revealed outstanding summaries on the 

progress, status, and challenges of the application 

of the perforated liner as an acoustic damper for 

different combustor systems [5]. However, most 

of those studies have been primarily focused on 

the simulation and experimental characterization 

of the single-layer liners.  

To reduce an operating temperature of metal-

wall below the true- or engineering-strength 

borders, the thermal loads on the combustor's wall 

of the aero-engines, to ensure in other gas-

turbines, is reduced by applying multi-perforated 

liners [6]. Furthermore, a double-layers liner is 

thought to supply better patterns for an acoustic 

transmission loss TL than the single-layer liner 

[5]. Therefore, the existing literature of this field 

of knowledge needs more researches to enrich it; 

the focus should be oriented to cylindrical double-

layers perforated liners with the context of wind-

tunnel experimental tests instead of simulation 

and numerical analysis. 

The sound transmission loss (STL) is the 

parameter used to count the amount of sound 

energy that is traveling via the acoustic treatment 

devices, furthermore, it is an accumulation for the 

amount of the sound acoustic pressure wave 

intensity that propagates outwards from the sound 

source, where the intensity of the sound wave can 

be reduced by increasing the range of wave 

attenuation and/or wave spreading [7]. 

Consequently, to decrease the power of sound 

crossing inside any wind-tunnel, it is prevalent to 

stratify single and/or double layers of perforated 

liners. Stratifying such liners with different 

perforation styles, percentage of holes, and sizes 

in the acoustic treatment will impact the amount 

of transmission loss of sound [8]. 

There are prime dilemmas that emerge with the 

use of an acoustic treatment in the installation of 

the wind-tunnel, the tuning is one of which, where 

the unwanted level of frequency of sound pressure 

fluctuation is most effectively macerated or 

attenuated to the required frequency, by making 

the sound frequency of the resonant is as close to 

the required frequency as much as manageable, 

through changing specifications concerning the 

geometry of the liner such as the percentage of its 

porosity (i.e., numbers and size of holes), and its 

plate thickness [8]. 

A provided controlled air-stream that is 

produced by the wind-tunnel can be used to help 

examine the effects of the movement resistance or 

drag force, which arises from the air on any object 

impacted by the air-stream, where it does not 

matter whether the stationary selected model can 

withstand the high force of the wind pressure [9], 

or whether or not the model that is being tested is 

styled by a technique permits it to easily move 

through a stream of air or not as it conducted in 

this experimental work. 

Calculating STL for different frequencies 

assists in showing some parameters such as the 

sound transmission loss distribution [10]. The 

perforated liners are typically tubular cylinders in 

a single layer, double layer, or multiple layers, 

which have slots or holes with small diameters 

created before operating and assembling the 

cylinder itself [11]. The size of layers and their 

numbers help figure out the durability of the liner 

and its efficiency to withstand strong vibrations. 

However, a liner with a single-layer can be 

applied for long operating time conditions, while 

a multiple-layers liner has a priority when 

operating in high heat-load conditions. 

Meanwhile, they are all used in combustor 

techniques to absorb the power of the incident 

acoustic waves.  

The sound transmission loss indicates the total 

intensity reduction of a sound wave-front power 

through sound noise propagation away from the 

source. Furthermore, as common in most acoustic 

applications, the intensity reduction as a sound 

wave-front propagates through a certain design 

[3].  

This experimental work attempts to enrich the 

existing available works by performing 

experimental investigations through a wind-tunnel 

with the context of single- and double-layers 

cylindrical perforated liners. A brief search shows 

that most of the available researches focus on the 

characterization of cylindrical liners that consist 

of a single-layer, using computational and 

numerical simulations and analyses. This work 

addresses a different approach to enrich the 

existing available literature of this field of 

knowledge. The significance of this work is to 

illustrate how increasing the liner layers can 

eventually reduce harmful vibration and 

ultimately raise the efficiency of combustors, 

through analyzing some key physical parameters 

like layout combinations of the single- and 

double-layers perforated liners and also the 

velocity of the air-flow, by investigating and 



Wisam S. Hacham                           Al-Khwarizmi Engineering Journal, Vol. 16, No. 4, P.P. 19- 26 (2020) 
 

21 

optimizing the double-layers perforated liners 

through acoustic experimental investigations. 

Therefore, these concepts will provide the 

performance-enhancing of the issues of the 

combustion chamber design. 

 

 

2. Methods 
2.1 Experimental Methodology 
 

Two strategies that have been experimentally 

attempted during this work are the acoustic 

experiments using the wind tunnel, which has 

been arranged alongside the single-and double-

layers cylindrical perforated liner. 

In the same procedures and setups for the large 

scale acoustic wind-tunnel, the experimental tests 

with the presence of both full-scale double- and 

single-layer perforated liner patterns, which are 

(inner4+outer2) and (inner1), respectively, as 

shown in Table 1, which was performed using a 

noise generator, loudspeaker, amplifier, four 

microphones, a computer connected to data 

acquisition apparatus and the sound card, and a 

single processing unit. 

The specifications of the acoustic treatment 

that have been employed in the experimental tests 

are shown in Table (1). It can be seen that the 

outer and inner layers of the double liner were 

manufactured in different porosity distributions, 

sizes, and thicknesses. The arrangements and 

configurations of the perforated liners are used in 

this work are illustrated in Figures (1) and (2). 

After starting-up of the tunnel, the sucked air by 

the fan of the tunnel is brought up to the required 

velocity and will be guided using an external vent 

to pass it through the intake partition to the 

perforated liners that provide a terminus for the 

echoic across its walls while it moves out from 

this partition (i.e., removing the sound noise from 

the fan itself and from the flow of air which 

travels past the fan into the main partitions of the 

tunnel). Then the sound will be generated and 

added as a white noise signal to the air-stream as 

it passes through the position of the loudspeakers, 

which connects to a noise generator via an 

amplifier, before it propagates as a sound wave 

downstream towards the rest of the tunnel's 

straight section. The loudspeakers were installed 

in different sizes to provide different ranges of 

frequency at this partition. Then, the stream of air 

that accompanied the sound waves passes through 

the main full-scale perforated liner, which is 

clamped securely either side in place far enough 

downstream of the tunnel to permit the plane 

sound waves to travel through a fully developed 

flow. Consequently, the sound was terminated and 

removed again before exiting the tunnel through 

the tunnel's rear vent. 

The experimental tests were firstly started by 

collecting and taking readings for the condition of 

no airflow (i.e., 0m/s), using Agilent data capture 

software installed in the computer available for 

this purpose. Then the velocity was set by 

measurements for the cross-flow velocity, starting 

at 10m/s up at intervals of 5m/s to a maximum of 

٢٥m/s. Likewise, the program of Agilent data 
capture was used to take 30 readings for each 

stream of airflow. 
 

 

 
 
Fig. 1. Illustrates the layout of a single-layer 

perforated liner pattern utilized at this work. 

 

 
 
Fig. 2. Illustrates the layout of a double-layers 

perforated liner pattern utilized at this work. 



Wisam S. Hacham                           Al-Khwarizmi Engineering Journal, Vol. 16, No. 4, P.P. 19- 26 (2020) 
 

22 

2.2 Theoretical Methodology 
 

The performance of the transmission loss for 

certain acoustic materials differs greatly with the 

level of the frequency applied, thus, STL can be 

marked as the ratio of the level of a sound power 

transmitted through the material during an 

acoustic treatment to the level of an incident 

sound power [12]. 

STL � 20log
 ��
�                                         … (1) 
TF � ����                                                          … (2) 
where the transmission factor is represented by 

TF, while the transmitted and incident sound 

powers are represented by (Pt) and (Pi), 

respectively. In general, the decibels (dB) scale 

mathematically terms the measurement of the 

sound transmission loss. The sound powers are 

demonstrated in Figure (3). 
 

 

3. Results 
 

Using the four microphones installed on the 

single-and double-layers perforated liner, the 

procedure of the experimental tests illustrated in 

detail above was performed and carried out to 

acquire the sound transmission loss against the 

level of the frequency applied. Then, the test's 

data was processed to gain the pertinent diagrams. 

The data of sound transmission loss against the 

level of the frequency were plotted in Figures (4) 

to (8). The presented results are the average of 

thirty scenarios of the experimental tests with and 

without the context of the cross-velocity of 

airflow through the single- and double-layers 

perforated liners, which are figured out as 

illustrated in Table (1). 
 
 

4. Discussion 
 

As shown in Figure (4), it is clear that the 

magnitude of the sound transmission loss is highly 

dependent on the level of the frequency applied. 

Additionally, it can be recognized that at 225Hz 

the liner decreases the level of incident sound 

power by 10dB. Furthermore, at roughly 660Hz 

and above, the tunnel's liner did not decrease the 

level of the incident sound power at all. 

Furthermore, it can be accepted that the results 

have outlandish dicrotic notches, particularly 

recognized at frequency levels less than 260Hz. 

Basically, there is a lack of satisfaction with this 

behavior, especially in case of no presence of the 

air crossflow. This behavior can be attributed to 

the rate at which sound power decays in a liner 

during the sound attenuation process for the 

intensity of the acoustic wave either by sound 

power absorption or by the sound power 

reflection.  

The hole (acoustic treatment) decreases the 

effective sound transmission loss, particularly 

distinguished at a higher level of the frequencies 

[12], as shown in Figures (4) to (8). As the hole 

(acoustic treatment) gets larger, the liner's effect 

was not introduce dramatically as at the initial 

perforation holes, but was especially introduced at 

the higher level of the frequencies. 

 

 
Fig. 3. demonstrates the sound powers through the perforated liners. The (Ptransmitted), (Pabsorbed), (Preflected), and 

(Pi) represents the transmitted, absorbed, reflected, and incident sound powers, respectively. 

 
Table 1, 

Illustrates the characteristics of the double-layers and single-layer perforated liner. 

                                                  Liner  Perforations 

Cylinder 
Internal Diameter 

(mm) 

Thickness 

(mm) 

External Diameter 

(mm) 

Length 

(mm) 
NO.

* Diameter 

(mm) 

INNER 1 160.65 1.6 163.85 524  3402
 

0.725-0.875 

INNER 4 162.14 2 166.14 524  2256
 

0.775-0.975 

OUTER 2 176.6 1.6 179.8 524  384
 

3.525-3.675 

* Refers to a number of rows and a (holes/row) for each liner. For inner1 they are 54 and 63, respectively. For inner4 
they are 47 and 48, respectively. For outer2 they are 16 and 24, respectively. 

Pincident 

Preflected 

Pabsorbed 

Ptransmitted 



Wisam S. Hacham                           Al-Khwarizmi Engineering Journal, Vol. 16, No. 4, P.P. 19- 26 (2020) 
 

23 

 
The style of staggering used in acoustic treatments 

is the most widespread style. Additionally, its 

structural strength more than other patterns of the 

perforation as concluded by [18]. Furthermore, 

the increase of the perforation orifices chamfering 

decreases the effective thickness of the hole (i.e., 

the edge profile of the hole), as concluded by [19]. 

The data presented in Figures (4) to (8) clearly 

show an obvious slow-down pattern in the decay 

of the sound transmission loss of the acoustic 

power with an increase in the magnitude of the air 

cross-flow velocity. The decay level of this 

parameter gradually rises above the level of slow-

down of the 10m/s tests. Consequently, the larger 

the velocity of air cross-flow, the stronger rise in 

decay style, thus, for this work, it can be 

concluded that the sound transmission loss for the 

10m/s is preferable. 

Finally, at least it is clear that the air-tunnel 

with the presence of the double-layers liner shows 

better sound transmission loss results. This may 

depend on the number of the perforation holes and 

the style of the distribution of those holes at the 

same liner or may be due to the layers thickness 

and/or the gap thickness between the two layers 

used. It concludes that using a liner with the 

context of double-layers may produce a better 

result than those gained by applying a liner with 

the context of a single-layer. 

 
 

 

5. Conclusions 
 

The sound transmission loss helps on how to 

determine and improve the acoustic properties of 

acoustic materials, and it can be a perfect scale 

with which to inspect the efficacy of a tunnel in 

decrease of sound energy transmission, using the 

small perforation holes that extremely decrease 

the transmission loss parameter. 

Depending on the findings of this work, it can be 

stated that it is needful to use a typical perforated 

liner to efficiently remove or control the sound 

noise that is originated from the dynamical 

vibrations to reach the noise to a specific level of 

the dB scale. Additionally, increasing the velocity 

of the air cross-flow leads to at least the slowing-

down in the decay which occurs in the sound 

transmission loss magnitude. Moreover, in terms 

of the sound transmission loss, the air-tunnel 

revealed better results with the presence of the 

liner composed by double-layers. 

 

 

 

 
 
Fig. 4. The acoustic transmission loss versus frequency. The data were collected with air cross-velocity equal to 

(0m/s) using the perforated liners. 

 

 

 

 

 

 

 



Wisam S. Hacham                           Al-Khwarizmi Engineering Journal, Vol. 16, No. 4, P.P. 19- 26 (2020) 
 

24 

 
 

Fig. 5. The acoustic transmission loss versus frequency. The data were collected with air cross-velocity equal to 

(10m/s) using the perforated liners. 

 

 

 
 

Fig. 6. The acoustic transmission loss versus frequency. The data were collected with air cross-velocity equal to 

(15m/s) using the perforated liners. 

 

 
Fig. 7. The acoustic transmission loss versus frequency. The data were collected with air cross-velocity equal to 

(20m/s) using the perforated liners. 



Wisam S. Hacham                           Al-Khwarizmi Engineering Journal, Vol. 16, No. 4, P.P. 19- 26 (2020) 
 

25 

 

 

 
Fig. 8. The acoustic transmission loss versus frequency. The data were collected with air cross-velocity equal to 

(25m/s) using the perforated liners. 

 

 

6. References 
 
[1] Kim, D., Jung, S. and Park, H., 2018. Design 

of acoustic liner in small gas turbine 

combustor using one-dimensional impedance 

models. Journal of Engineering for Gas 

Turbines and Power, 140(12), p.121505. 

[2] Yoon, M.G. and Kim, D., 2017. Acoustic 
transfer function of a combustion system with 

premixing chamber. Journal of Mechanical 

Science and Technology, 31(12), pp.6069-

6076. 

[3] Lawn, C., 2016. The acoustic impedance of 
perforated plates under various flow conditions 

relating to combustion chamber liners. Applied 

Acoustics, 106, pp.144-154. 

[4] Wang, J., Rubini, P. and Qin, Q., 2017. 
Application of a porous media model for the 

acoustic damping of perforated plate 

absorbers. Applied Acoustics, 127, pp.324-

335. 

[5] Lahiri, C. and Bake, F., 2017. A review of bias 
flow liners for acoustic damping in gas turbine 

combustors. Journal of Sound and 

Vibration, 400, pp.564-605. 

[6] Zhao, D. and Li, X.Y., 2015. A review of 
acoustic dampers applied to combustion 

chambers in aerospace industry. Progress in 

Aerospace Sciences, 74, pp.114-130. 

[7] Tran, H., Du, W., Ghetmiri, S.A., Mosleh, A., 
Sun, G., Soref, R.A., Margetis, J., Tolle, J., Li, 

B., Naseem, H.A. and Yu, S.Q., 2016. 

Systematic study of Ge1− xSnx absorption 

coefficient and refractive index for the device 

applications of Si-based 

optoelectronics. Journal of Applied 

Physics, 119(10), p.103106. 

[8] Zhao, D. & Morgans, A., 2009. Tuned Passive 
Control of Combustion Instabilities Using 

Multiple Helmholtz Resonators. Journal of 

Sound and Vibration, pp. 744-757. 

[9] Sirjani, E., Sameni, A., Moosavi, A.A., 
Mahmoodabadi, M. and Laurent, B., 2019. 

Portable wind tunnel experiments to study soil 

erosion by wind and its link to soil properties 

in the Fars province, Iran. Geoderma, 333, 

pp.69-80. 

[10] Li, Y., Sun, C., Jiang, Y. and Feng, F., 2019. 
Scaling Method of the Rotating Blade of a 

Wind Turbine for a Rime Ice Wind Tunnel 

Test. Energies, 12(4), p.627. 

[11] Dai, X. and Aurégan, Y., 2016. Acoustic of a 
perforated liner with grazing flow: Floquet-

Bloch periodical approach versus impedance 

continuous approach. The Journal of the 

Acoustical Society of America, 140(3), 

pp.2047-2055. 

[12] Heckl M., The tenth Sir Richard Fairey 
memorial lecture: Sound transmission in 

buildings, J. Sound Vib. 77(2): 165-189, 

1981. 

[13] Lahiri, C., 2014. Acoustic performance of 
bias flow liners in gas turbine combustors, 

Berlin: Technical University of Berlin. 

[14] Temiz, M. A., Arteaga, I. L. & Hirschberg, 
A., 2015. Sound absorption measurements 

for micro-perforated plates: the effect of edge 

profile, Maastricht: Euronoise 2015.



�� 4، ا���د����16 ا���ارز
� ا����	�� ا����
                                                  و	�م 	��ي ����      �� ، 26 -19)2020( 
 

26 

 

 

 

 

 

��3/�ر.���ت ا�2�ز� ��01 ا	/��ام .-�,�ت 
*دو(� 
)'&� %�$ ا��!�#" ا�!� ��   
 

                **�7!6 ���5 ��4             *و	�م 	��ي ����
��اد*��� ھ���� ا	������و���/ 
��� ا	����� �ا	&%ارز "/ !� ��   

 **!� �� ھ�ل ، ھ�ل ، HU6 7RX ، ا	����� ا	�()�ة **
wisam@kecbu.uobaghdad.edu.iq 	-�,�ا ا/	�(�و�":   * 

 hull.ac.uk-f.s.hiji@2017ا	-�,� ا/	�(�و�": **
 

 

 ا��8��
 

ا	�?�ر ا	�<��" 	=%>�ء  ا	(" ���; ��: ا	���5-���9ت ا	�(�85 دا01 �5�7 ا/1(�اق 5" ا	�)�
�ت ا	(%ر����� و��2 ا	ا	�%اء ��2  ا	(�ا01�(-� ,
 "	�)	��ا/����ر ، وز,�دة اھP)از ا	��%��ت، وز,�دة  ��ل ,Oدي إ	L ا�&�Kض 5" أداء ا	��(ك �Hم ا�(�Fار ا/@(�اق. وھDا ,��2 أنA-� "5 ا	ا/@(�اق و

��-A ا�(�Qر ا	(8FQ 	��)�ك  ا	�������" R���	ي ,)�ث اD	از اP2 ا/ھ( � ��TU� ��5 L	دي إO, يD	اء ا�!Vا W���, �,�F� ن أيX5 ،"	�)	��(�ددات �H	��. و
2H �	%), ي ,��2 أنD	ا/@(�اق، وا R�Y< :��  ازاتP)ا/ھ	ا ���F)	ا L�H ر%Z��	 ��H��?	ت ا�
�Q	وا �����	ت ا���)Y�	ا ��K, ، ,��2 أن���7%ب 5��	ا ���

 ��H "5 ز,�دة L	دي إO� ")	ا L�Z�	ت ا/@(�اقا�
 FZ-� أ�9%ا���  Pدو!� ا	��ف ا	�<��"  2 ھDا ا	��0 ھ% ا	()TU� 2  8F�� ا�(&�ام ����9ت ان .  )�
��1رة  �� 0 �� ا	()TU� 2  8F��ات ��%,2 ا	-���9ت L�H ا	&?�<_ ا	?%���،  2 1^ل @��ب @�[ ا	?%�" 	(%ر����ت ا	��ز.  ا/�-��ث�F5 L�Hان 

���(&�ام �8K ا	�,�ح ا	?%�" ا	�-�� ا	)�Y  ` ���ق H���ت ا	-ا	?%�"  ا/ر��ل ����H )?%ل	(" �� ا	ت ا����-	2 ا  ��دو!� وا�P�	د,� وا�K	ا �-FZ�	���9 ا
٠��ون ���H (ا	��9ق،   �K�)&  85�� ت�H�� 2 م / ث) و ` ا�(&�ام��. �� إ��Qء ا	(�Yرب ���(&�ام �8K م / ث ٥و�0f�K م / ث  ٢٥م / ث إ	L  �١٠(�اوح 

�)�ث g	 ھ�ل P
�  "5 hH%� 2  �,�K	ا ��?%�	,�ح ا�	ما���	ه اD` ھ�ل !���
V .���%?	ا�(&�ام  &9%ط�ت ،ا �� MATLAB  0 ور���ة و�)��
�(�	
 k(<أو .�-FZ  �!دوP وا@�ة و �F-ق ط��,�ح  ` ��	8 اK� ام�&)��� h�U إذا �� ا�(&�ام ا	���	�Y ا	?%����(�Yرب ا	!��` ا	-����ت ا	(�Y,-�� ا	(" �� !���� 

A�FZ)	ام ا�&)���، X5ن ا	(TU�� 	�� درا �ً� ا	AFZ، و ` ز,�دة @�Y ا	��	��، و/ ���� ��H ا	(�ددات ا	K��ل ا	?%�" ا/ر��ل  �� 0، �5%ف ,(�-A 5" ا�&�Kض 
��H )�ل	ھ% ا ��
�0�Q �1ص ��H ا	(�ددات اL�Hn. أو>)ا	AFZ ا/�(�ا<" اnو	"  p<ا واD%ا<"، وھ�	,�ح ا�	8 اK� أن R>�)�	ا k  ���9-	ق ا��� ` 

  .ا	?%�" ا/ر��لا	�Pدو!� ,�R>�)� "9 أ@ 2  0=5��F5 ]ان