Jet Velocity and Acoustic Excitation Characteristics of a Synthetic Jet Actuator
Round 1
Reviewer 1 Report
Please see the attached file.
Comments for author File: 
Comments.pdf
Author Response
We thank the editor and reviewer for their comments. All comments have been addressed and have helped to improve the paper.
- The most important geometrical parameters are introduced in text. However, in my opinion, more dimensions can be present in Figs. 1 and 2 to get an immediate idea about actuator size from the picture (piezo diameter, cavity length, etc...).
We agree with the reviewer that including dimensions would certainly improve the manuscript. As recommended, the main dimensions are now added to Figures 1 and 2 in addition to the text.
- What was the electrical connection of the piezoelements? Probably parallel connection was used. Do they operate in phase? We had an experience with piezoelements in parallel connection (keeping right polarity), that some of them operate in anti-phase, some with phase shift…
Yes, the piezoelectric elements were all operated in phase through parallel connection. The polarity of operation was checked beforehand (as published in previous work, Feero et al. 2015), and we did not experience any lag or anti-phase operation in this setup. This point has now been clarified in the revised manuscript (lines 183-184).
- Lines 199, 396-409, please check correct usage of terms “resonance frequency” and “eigen
frequency” (or “natural frequency”). In my opinion the term “eigen frequency” comes from analysis of “eigen modes” of cavity, which is analysis without external excitation of the cavity. While the term “resonance frequency” is used for frequencies of external excitation giving maximal displacement/velocity amplitudes of a system. Theoretically the values of those frequencies are different, but can be close at certain conditions.
We agree. The text has been modified to use ‘natural frequency’ for values given by simulations and ‘resonance frequency’ for values obtained by external excitation in experiments. One of the main findings of this work is that the experimentally acquired resonance frequencies closely match the predicted values for natural frequencies (Table 1). Hence, the simulations were validated and used to show the acoustic pressure distributions.
- 4 and 7, maybe it would be nice to display a value of evaluated Helmholtz frequency in the graphs (e.g. by a vertical line).
This is a great suggestion. Thank you. Figures 4 and 7 have been updated in the revised version to show the theoretical value of the Helmholtz frequency.
Author Response File: Author Response.pdf
Reviewer 2 Report
The paper presents some interesting results on the acoustic behaviors of multiple synthetic jet actuators. Using experimental and simulation methods, different acoustic behaviors were oberved based on the different isolation of the actuator cavities. Before publication, there are two issues
1. More details of the similation are needed. For example, whether different domain size and time step size can affect the simulation result? In the validation, only the peak frequency was used. How about the whole frequency band and acoustic pressure distribution ?
2. Whether the acoustic behaviors of large-aspect-ratio cavity change if the actuators have different phase or frequency among them?
Author Response
We thank the editor and reviewer for their comments. All comments have been addressed and have helped to improve the paper.
- More details of the similation are needed. For example, whether different domain size and time step size can affect the simulation result? In the validation, only the peak frequency was used. How about the whole frequency band and acoustic pressure distribution ?
The domain size (i.e., the number of mesh elements) was determined after performing a grid independency test. During this test, the mesh size was refined incrementally until the obtained results from simulations were found to match the experimentally observed results and did not change with further refinement of mashes. This point is explained in lines 207 to 212 in the manuscript.
Time step is not a factor in these acoustic mode shape simulations, as they are frequency domain simulations and do not vary in the time domain. For validation, only the natural frequencies (eigenvalues) can be extracted from acoustic simulations and compared to peak frequencies (resonant frequencies) from experiments. The simulations do not give frequency bands as was possible with the measurements. On the other hand, acoustic pressure distribution was not attained experimentally as it would require surface pressure measurements at various locations over the cavity walls, which is not possible with this setup. In fact, the matching of the jet velocity measurements, in Figure 6, with the acoustic pressure distribution is further validation of the simulations.
- Whether the acoustic behaviors of large-aspect-ratio cavity change if the actuators have
different phase or frequency among them?
It is fair to assume that the acoustic behaviour of the cavity will change if a different phase or frequency is externally imposed on the piezoelectric elements. However, in this work, all the piezoelectric elements were excited simultaneously (through parallel connection) and operated in phase. This technique is followed to obtain the maximum output of the synthetic jet actuator. Any external phase mismatch or frequency shift may degrade the output jet velocity, which is not in the scope of this work.
Author Response File: Author Response.pdf
