Acoustic and Flow Aspects of Novel Synthetic Jet Actuator

Round 1

Reviewer 1 Report

The manuscript was about the acoustic characteristics of different synthetic jet arrays. The topic itself is interesting and as the authors said that synthetic jets can be used for internal cooling of electric circuits, therefore, the research outcome can be useful.

The manuscript is considerably well-written with some minor typing and formatting mistakes were spotted. Specific comments are given below.

1) The only major concern about the experimental setup was the experiments were not conducted in an acoustic chamber. Although the authors confirmed that the background noise is 10dB lower than the noise generated by the synthetic jet, how could the authors ensure that the background noise remained constant throughout the test? For example, if there suddenly a lorry passed-by or even just someone walked pass the laboratory area, the background noise level will be affected. The question here is how could the authors confirm that the background noise level always maintained 10dB below the SPL of the synthetic jet? Did the author continuously measuring and monitoring the background noise level throughout each set of the experiment. If so, the authors should provide a graph to show the background noise level and time relation.

2) It is a bit unclear about the frequency of measurement of the NI card used in the experiment. Did the authors measure the signal outputs by all the 16 synthetic jet actuators simultaneously or the signal outputs by the actuators are measured in series? What is the maximum sampling frequency of the NI card used in the experiment? 

3) A few words usually use in informal writing have been spotted in the manuscript. For example, line 55 'did not' not didn't', line 64 'were not' not 'weren't', line 197 'does not' not 'doesn't' etc.

4) In line 59 and line 111, the authors should use the symbol - to represent to.

5) The caption in line 129 should be centered.

 

Author Response

Dear Reviewer,

thanks for so positive feedback on our paper. Below we present the answers to the objections and doubts that arose while reading the manuscript.

Reviewer: The manuscript was about the acoustic characteristics of different synthetic jet arrays. The topic itself is interesting and as the authors said that synthetic jets can be used for internal cooling of electric circuits, therefore, the research outcome can be useful.
The manuscript is considerably well-written with some minor typing and formatting mistakes were spotted. Specific comments are given below.
The only major concern about the experimental setup was the experiments were not conducted in an acoustic chamber. Although the authors confirmed that the background noise is 10dB lower than the noise generated by the synthetic jet, how could the authors ensure that the background noise remained constant throughout the test? For example, if there suddenly a lorry passed-by or even just someone walked pass the laboratory area, the background noise level will be affected. The question here is how could the authors confirm that the background noise level always maintained 10dB below the SPL of the synthetic jet? Did the author continuously measuring and monitoring the background noise level throughout each set of the experiment. If so, the authors should provide a graph to show the background noise level and time relation.

Authors response: The measurements were conducted in a laboratory room much bigger than the tested synthetic jet actuator, no sound reflections were observed. The sound pressure level (SPL) of the background was measured before, during and after each measurement of the generator’s SPL. The SPL of the background was at least 10dB lower than the noise generated by the SJ during all measurements. In accordance with ISO 3746: 2010, it is allowed to measure the SPL without an anechoic chamber, including background noise corrections. But if the difference in the SPL of tested equipment and background noise (measured with tested equipment switched off) is 10dB or more then the background noise correction is equal to zero. The SPL was measured automatically with LabView software without personnel in the laboratory room.

Reviewer: It is a bit unclear about the frequency of measurement of the NI card used in the experiment. Did the authors measure the signal outputs by all the 16 synthetic jet actuators simultaneously or the signal outputs by the actuators are measured in series? What is the maximum sampling frequency of the NI card used in the experiment?

Authors response: The NI-USB-6211 card has 16 Bit and can record 250kS/s per all three channel (multichannel maximum). Three devices were connected to the NI card: the anemometer, the thermometer and the sound level meter. The sampling frequency was set automatically depending on the synthetic jet actuator working frequency. A constant number of 400 samples per one cycle was set. For every single measurement, data of 100 cycles were recorded. For example, measurements of the synthetic jest actuator working at 200 Hz were performed at 80kS/s frequency. The velocity measurement for one geometrical case was performed at a single orifice assuming similar velocity profiles at all orifices with the same diameter. This assumption had been checked in the auxiliary experiment from which it follows that the velocity profiles are similar. At line 121-128 proper comment was added.

Reviewer: A few words usually use in informal writing have been spotted in the manuscript. For example, line 55 'did not' not didn't', line 64 'were not' not 'weren't', line 197 'does not' not 'doesn't' etc.

Authors response: It has been corrected in line 55, 64, 231, 257, 288.

Reviewer: In line 59 and line 112, the authors should use the symbol - to represent to.

Authors response: This is a mistake from our native language. It has been corrected in line 59, 118, 246, 289.

Reviewer: The caption in line 129 should be centered.

Authors response: Line 131 has the format that complies with the templates of the publisher.  

Author Response File: Author Response.pdf

Reviewer 2 Report

This experimental work to investigate the acoustic and flow aspects of the so-called novel synthetic jet actuator (SJA) with fins inside the cavity is a good step toward the development of SJA using 3 types of enclosure design and 2 models of the loudspeaker. However, some critical issues must be resolved before further proceed for the publication, and some of them are listed below. 

1. At line 14: '..optimal solution in those regards.' - What do you really mean by this statement? Please state clearly. 
2. At line 17: ' ( or the lack of them)' - What do you really mean by this statement? Please state clearly. 
3. In Table 1: ,,a'' and ,,b'' should be corrected or, modified. 
4. The sentence 'The power ..............0.25% measured value.' should be re-edited or rewritten. Check the grammar. 
5. Please replace the symbol '÷' with a conventional and well-known symbol. 
6. At line 117: What is 'd' here? Please introduce it in the text. 
7. At line 122: The term 'test stand' is introduced here before referring to the figure. Please correct it. 
8. From Table 1, we can see that the basic difference between the two loudspeakers is only the resonant frequency, so far. If there are any differences in their characteristics, please mention clearly. A comparison should not be made by referring the type only. 
9. In Figure 4: Either 'Urms.c' or 'U0.c' should be used. 
10. At lines 169-171: The sentence should be modified and re-written. 
11. So far, '...the pressure gradient........diaphragm.' implies that the loudspeaker's diaphragm generates an asymmetrical pressure gradient. Please provide clear evidence and justify your answer.
12. In discussion: The 1st paragraph should be modified and re-written. This kind of presentation is not good for general readers. Moreover, the grammar should be checked carefully.  
13. As the results state that the loudspeaker has a significant influence on the performance of SJA. It implies that other types of loudspeakers may have effects. Then how the authors made a chocie for optimal configuration only based on these two types. Justify.   
14.   As the authors mentioned, the fin arrangement has a negative influence on U/SPL ratio, then, how the optimal configuration is decided. 

Author Response

Dear Reviewer,

thanks for so positive feedback on our paper. Below we present the answers to the objections and doubts that arose while reading our article.

Reviewer: This experimental work to investigate the acoustic and flow aspects of the so-called novel synthetic jet actuator (SJA) with fins inside the cavity is a good step toward the development of SJA using 3 types of enclosure design and 2 models of the loudspeaker. However, some critical issues must be resolved before further proceed for the publication, and some of them are listed below. 
At line 14: '..optimal solution in those regards.' - What do you really mean by this statement? Please state clearly. 

Authors response: In our case, optimal means the actuator with the lowest SPL and the highest velocity. The parameter used for the choice of the optimal solution is described in lines 19-20.

Reviewer: At line 17: ' ( or the lack of them)' - What do you really mean by this statement? Please state clearly. 

Authors response: We investigated the actuator with the classic fins, the pin fins arranged on a rectangular grid and without the fins in the cavity. The lack of fins refers to the third investigated case.
The sentence has been corrected to make it clearer.

Reviewer: In Table 1: ,,a'' and ,,b'' should be corrected or, modified. 

Authors response: We corrected the quotation marks so that the marking was the same as the text, e.g. in line 103 or 184. Now, it is “a” and “b”.

Reviewer: The sentence 'The power ..............0.25% measured value.' should be re-edited or rewritten. Check the grammar. 

Authors response: The sentence has been corrected - line 111 and 112.

Reviewer: Please replace the symbol '÷' with a conventional and well-known symbol. 

Authors response: This is a mistake from our native language. It has been corrected in line 59, 118, 246, 289..

Reviewer: At line 117: What is 'd' here? Please introduce it in the text. 

Authors response: Both symbols ‘x’ and ‘d’ were not described. This has been corrected in line 136 and 137.

Reviewer: At line 122: The term 'test stand' is introduced here before referring to the figure. Please correct it. 

Authors response: The sentence “The test stand is presented in Figure 2” was removed in line 147 the appropriate reference has been added in line 142

Reviewer: From Table 1, we can see that the basic difference between the two loudspeakers is only the resonant frequency, so far. If there are any differences in their characteristics, please mention clearly. A comparison should not be made by referring the type only. 

Authors response:  The loudspeakers have different Thiele/Small parameters. The parameters of loudspeakers are presented below in table 1.

Table 1. Thiele/Small parameters  of loudspeakers

	STX W.18.200.8.FGX	STX M.18.200.8.MCX
SPL (2.83V/1m)	89	92
Compliance of the driver's suspension	1.08 mm/N	0.45 mm/N
Mass of the diaphragm/coil	16.5g	13.5g

The basic difference is resonant frequency and the SPL resulting from this frequency (Fig. 1, please see attachment). Most loudspeakers have limited bandwidth of SPL at low frequency (below 100-150Hz for 8” loudspeaker).

So if we use a loudspeaker with the lowest resonant frequency, the SPL generated by the loudspeaker will be lowest. This was proved experimentally in the manuscript. At line 93 proper comment was added.

Reviewer: In Figure 4: Either 'Urms.c' or 'U0.c' should be used. 

Authors response: The text been corrected in line 183. The 'U_0.c' is the symbol used In the paper.

Reviewer: At lines 169-171: The sentence should be modified and re-written. 
So far, '...the pressure gradient........diaphragm.' implies that the loudspeaker's diaphragm generates an asymmetrical pressure gradient. Please provide clear evidence and justify your answer.

Authors response: The sentence has been changed in lines 208-211. The original sentence was, indeed, an over-interpretation. It is more likely that the asymmetry of the velocity profile results from the synthetic jet actuator geometry, for example the close proximity of the bottom of the cavity to the orifice or the interaction with fins.

Reviewer: In discussion: The 1st paragraph should be modified and re-written. This kind of presentation is not good for general readers. Moreover, the grammar should be checked carefully.

Authors response: Major modifications in the paragraph have been made in order to make it more comprehensible.

Reviewer: As the results state that the loudspeaker has a significant influence on the performance of SJA. It implies that other types of loudspeakers may have effects. Then how the authors made a choice for optimal configuration only based on these two types. Justify. 

Authors response: The number of investigated loudspeaker models is, indeed, limited and it is possible that different loudspeakers would show better performance in studied application, however, there is a good reason for such choice. For the potential real-life application of the device, we proposed it is important that it is built with abundant and readily available components and there is a problem with a variation of resonant frequencies in the available loudspeaker catalog. There are a lot of loudspeakers with resonant frequencies in the range of 25-40Hz and quite a small number of models with a higher resonant frequency. In other words: we had chosen a couple of loudspeakers that represent the two most popular types. Even with a small number of investigated loudspeakers, we proved that the loudspeaker with lower resonant frequency provides lower SPL.

Reviewer: As the authors mentioned, the fin arrangement has a negative influence on U/SPL ratio, then, how the optimal configuration is decided. 

Authors response: As it was mentioned in the conclusion, the highest U_0.c/SPL ratio was obtained in case 2a. It is the optimal configuration in the sense we adopted throughout the paper. The appropriate sentence was added a line 261.

Author Response File: Author Response.pdf