Fluidic actuator performance variation via internal ...

Proceedings of the 17th International Conference on Computational and Mathematical Methods in Science and Engineering, CMMSE 2017 4–8 July, 2017.

Fluidic actuator performance variation via internal dimensions modifications Masoud Baghaei1 , Josep M Bergada2 and David Del Campo1 1 2

Physics Department, UPC-ESEIAAT Colon 7-11, Terrassa, Spain

Fluid Mechanics Department, UPC-ESEIAAT Colon 7-11, Terrassa, Spain emails: [email protected], [email protected], [email protected]

Abstract When aimed to modify the downstream vortex shedding of a given bluff body, whether any road vehicle or wing profile, the use of Active Flow Control (AFC) appears to be an efficient technology. Among the different (AFC) methodologies the use of periodic forcing is ment to have better efficiency since it requires less energy to activate the shear layer, the reason behind this efficiency lies on the fact that periodic forcing interacts with the shear layer natural instabilites. In the present paper, one of the devices widely emloyed to generate pulsating flow, is carefully studied via 3D-CFD and using OpenFOAM. Initially the base flow is being determined and compared with previous experimental results, in a second step several internal dimensions of the fluidic actuator are being modified to characterize the output frequency and amplitude variations, among the conclusions obtained it is found that a given fluidic actuator is capable of generating several output frequencies and amplitudes when modifiying some internal dimensions while maintaining a constant incoming flow Reynolds number. Key words: Fluidic actuators, Active flow control, Computational fluid dynamics.

1

Introduction

One of the newest technology to modify the lift and drag of a given body is via injecting or sucking flow to or from the boundary layer nearby the separation point. The technique which seems to be more effective, mostly because it involves the use of the smallest amount of energy, is the use of periodic forcing, the main advantage of using periodic forcing is that

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Fluidic actuator performance variation via internal dimensions modifications

the injected flow interacts with the shear layer natural instabilities, and threrefore deeply activates the flow. To generate pulsating flow, two main types of fluidic oscillators are being used, the Zero Net Mass Flow (ZNMF)fluidic oscillators and the Fluidic actuators (FA). The former consist of a membrane located inside of an open chamber, the membrane moves back and forward and so a pulsating flow is being generated, with positive and negative velocities alternating every half cycle, as a result, the net mass flow at every cycle is null. The later generates a sinusoidal outgoing flow and has the advantage of having no moving parts, this particular advantage is very handy when designing long lasting and reliable systems. Original (FA) designs goes back to the 60s and 70s, left nearly unchanged for over 45 years. Their possible output frequency ranges from several Hz to KHz and the flow rate is usually of a few dm3 /min. Among their applications in flow control, it is worth to mention their use in combustion control [1, 2, 3], mixing enhancement [4], flow separation in aerofoils [5], boundary layer control on hump diffusers used in turbomachinery [6], flow separation control on stator vanes of compressors [7], drag reduction on trucks [8] and cavity noise reduction [9]. It appears that fluidic actuators have the potential of being much widely used in the near future, and according to the authors there is the need of better understanding their behaviour in order to further improve their performance. Regarding the fluidic actuators design two main groups exist, the one based on Coanda effect [10], and the one based on a jet mixing chamber, also called vortex oscillators [11]. The former group had an early application as pressure, temperature and flow measuring devices [12, 13, 14], the latter group has recently been applied as a flow control device [15]. To push forward (FA) boundaries several new designs have been recently created. Uzol and Camci [16], studied experimentally and via (CFD) a fluidic oscillator based on two elliptical cross-sections placed transversally and an after-body located in front of them. Such configuration was in fact proposed by Bauers patent [17, 18]. The device operates at frequencies of around 30 Hz and under laminar flow. The relation frequency versus Reynolds number was found to be perfectly linear. Huang and Chang [19] performed a deep experimental study on a V-shaped fluidic oscillator. Playing with the dimensions and the internal oscillator circular cavity, they defined the regimes under which oscillation was generated and they proved that frequencies from few Hz to several KHz could be obtained by modifying oscillator parameters. Additionally, an analysis of the streamline patterns behind the oscillator was also presented. Khelfaoui et al [20], presented an experimental and numerical analysis of non-symmetrical mini and micro oscillators. They found a linear relationship between the actuator frequency and the feedback channel volume, and noticed that above a certain input pressure choked flow appeared. Gebhard et al [21] studied a micro-oscillator operated with water, finding a linear relationship between the output frequency and the input volumetric flow. Raman and Raghu [9] evaluated the decrease of a cavity tone by using fluidic oscillators. The main acoustic frequency was reduced by over 10 dB, concluding that fluidic

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excitation is a candidate in noise control applications. A numerical simulation of a two dimensional fluidic oscillator by using Navier-Stokes equations in laminar and incompressible flow, was performed by Nakayama et al [22]. They were able to visualize the periodical flow movement and measured the temporal axial and tangential fluid velocities, oscillation frequency being of 40Hz. Gregory and Raghu [23], created a fluidic oscillator based on Coanda effect but driven by piezoelectric devices. One of the main interesting performances of such device is that the oscillating frequency can be decoupled from the input flow and pressure differential. Frequency just depends on input electrical signal, being the oscillator able to work at a range of velocities which goes up to sonic conditions. The present paper will introduce a numerical evaluation of a fluidic actuator previously studied in [24, 25, 26, 27]. In these previous studies, and extensive CFD model including the analysis of several turbulent models in order to find out which one was the most appropriate, was undertaken. Besides, they performed an experimental study obtaining a good agreement between experimental and CFD results. In the present paper, experimental results obtained in [24, 25], will be compared with the new CFD calculations. A discussion regarding how different fluidic actuators internal parts and dimensions may affect its performance will be carried on. The authors main aim is to give to the reader some hints to be able to modify a given oscillator to fulfil a particular application.

2

Numerical problem definition and boundary conditions

The three dimensional fluidic actuator considered in the present paper is depicted in figure 1, its thickness was of 3.25mm. The incoming flow enters the actuator mixing chamber (2) through the flattered pipe located on the left hand side of the figure (1), on both sides of the mixing chamber there are the feedback channels (3), their function is to allow transporting fluid from the downstream mixing chamber site to the upstream one and vice-versa, the fluid leaves the actuator alternatively through one of the two exit surfaces located on both sides of the external chamber (4). Notice that a second fluidic actuator with a buffer zone (5) is also presented, the idea behind this second configuration is evaluating the efect of the outlet boundary conditions onto the pulsating flow. The mesh employed for the present simulation had a total of 2242000 cells, the grid used was structured and care was taken to obtain a very small y+ in all directions, in fact the maximum y+ respectively obtained in x, y, and z directions was, 1.8, 4.7 and 1.2 for a Reynolds number of 16034. Boundary conditions employed were, fluid velocity at the entrance and absolute pressure 1.01978 ∗ 105 P a at the output, Dirichlet boundary conditions were set to all walls. A range of different input velocities from 0.758 to 1.23 m/s were studied, its minimum and maximum Reynolds number associated was 8711 and 16034. The fluid employed was water and it was considered as incompressible. Fluid dynamic viscosity was chosen as 0.001003 Kg/(m ∗ s) and fluid density was 998.2 Kg/m3 . The characteristic length was chosen to be the inlet

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Fluidic actuator performance variation via internal dimensions modifications

width, which value was 2.55 ∗ 10−3 m. The turbulence model used was the SpalartAlmaras DDES, which is a hybrid LES model. OpenFOAM version 3.0, was employed for all 3D simulations, finite volumes approach was employed. Inlet turbulence intensity was set to 0.05% in all cases; PISO was used as a solution method, being the time step of 10−6 s, spatial discretization was set to second order. The CFD model designed had a probe covering one of the two actuator exits, at this section the frequency and amplitude associated to the temporal mass flow were measured. The frequencies obtained were compared with the ones experimentally obtained in [24, 25], table 1 compares both results. Notice that the difference is minimum giving confidence to the CFD simulations undertaken. Table 1: Comparison experimental and CFD results. Reynolds number Frequency [Hz], ref. [24, 25] Frequency [Hz], present paper Difference %

(a)

8711 12.9 12.98 0.62%

11152 15.5 15.87 2.3%

13593 18.7 19.41 3.7%

16034 21.8 22.7 4.1%

(b)

Figure 1: Fluid actuator general view and its different parts, (a) original fluidic actuator (b) fluidic actuator with buffer zone. As previously stated, and in order to characterize the possible effect of the boundary conditions on the flow performance, a fluidic actuator with a buffer zone was generated, for this particular case the outlet boundary conditions were maintained the same as in the original case but the outlet was located at the end of the buffer zone, the total number of cells used in this new model was of 2854500. A single Reynolds number of 16034 was studied, the frequency obtained from this particular buffer zone model increased by 2.6% versus the one obtained with the original case. The authors have considered that the effect of buffer zone is pretty negligible and the rest of the cases will be studied without it. Finally and in order to compare the effect of the mesh on the results obtained, the original fluidic actuator was modelled via using 4.4 million cells, almost twice as much as the ones

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employed initially. The simulation was done at the maximum Reynolds number Re=16034. The values of maximum x+, y+ and z+ obtained for this new mesh were of 1.8; 4.7 and 0.6 respectively. The frequency obtained when using this extremely dense mesh was of 22.83Hz which involves an increase of 0.57% versus the original case. Understanding that at lower Reynolds numbers the differences will be even smaller and considering that the time required to simulate the (FA) with 4.4 million cells is 74% higher than the one needed to perform the simulation with 2242000 cells, it can be concluded that using 2.2 million cells is precise enought for the cases under study.

3

Internal dimensions modifications

Figure 2 introduces the three modifications considered in this paper, the mixing chamber inlet width (a), was increased and decreased respectively by 64% and 114%, nine different positions were considered. The outlet width (b), increase and decrease was of 82%, again nine different positions were evaluatd. Regarding the outlet angle reduction (c), it was of 36.5%, the angle increase was of 100%, as in the previous two cases, nine different angles were simulated. In what follows the results obtained when performing these modifications are presented for a single Reynolds number, Re=16034.

Figure 2: Fluidic actuator mixing chamber internal dimensions modifications.

4

Results

In the present section, the main performance characteristics of a fluidic actuator having some internal dimensions modified will be presented. For all cases, the frequency from the pulsating mass flow measured at one of the two outlets and the amplitude of such pulsating flow will be presented. Some pictures will be introduced for each case to better understand the flow behaviour inside the actuator.

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Fluidic actuator performance variation via internal dimensions modifications

4.1

Outlet width modification

Fiure 3 presents the variation of fluidic actuator non dimensional output frequency and amplitude, whenever the outlet width is being modified. It is interesting to realize that as the width decreases the pulsating frequency increases, increasing as well the output amplitude, and vice-versa. This increase in amplitude is explained when observing that the maximum fluid velocity increases with the width decrease. The temporal mass flow amplitude and the fluid velocity amplitude go hand by hand. Figure 4 shows the velocity magnitudes inside the (FA) for the highest and lowest outlet widths evaluated in this study, notice the difference in velocity at the (FA) output. From figure 3 it must be realized that the change in frequency versus the original actuator one, for the cases evaluated, is about ±10% while the amplitude is suffering an increase of nearly 60% and a decrease of about 40%.

(a)

(b)

Figure 3: Fluid actuator performance when modifying the outlet width. (a) Frequency variation (b) Amplitude variation.

(a)

(b)

Figure 4: Fluid actuator internal field velocity magnitude, (a) Maximum outlet width (b) Minimum outlet width.

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4.2

Outlet angle modification

The variation of non dimensional output frequency and amplitude when the mixing chamber outlet angle is modified, is presented in figure 5. Notice that when the angle decreases, see figures 2, 5 and 6, the output frequency tends to increase, notice as well from figure 6b), that these small angles tend to direct the flow alternatively through the feedback channels, therefore explaining why frequency increases. On the other hand, as the angle increases, the flow is being directed towards the (FA) outlet, jeopardizing the degrees of freedom of the fluid, and so minimizing the output frequency and amplitudes. Notice that the angles variation studied, affected the output frequencies by approximately ±10%, the amplitude was affected by +11% and minus 50%. Clearly, these two modifications already presented, affect much deeply the output amplitudes than the frequencies.

(a)

(b)

Figure 5: Fluid actuator performance when modifying the mixing chamber outlet angle. (a) Frequency variation (b) Amplitude variation.

(a)

(b)

Figure 6: Fluid actuator internal field velocity magnitude, (a) Maximum outlet angle (b) Minimum outlet angle.

4.3

Mixing chamber inlet width variation

The last modification to be introduced consist of changing the mixing chamber inlet width. The first thing to be realized when seeing figures 7 and 8, is that the inlet width modifi-

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Fluidic actuator performance variation via internal dimensions modifications

cation, generates a completely different pattern onto the outgoing flow. If the inlet width overcomes a certain minimum or maximum values, there is no pulsating flow at the outlet, the flow simply goes straight fom inlet to outlet. In fact, for small values of inlet width, the incoming jet borders impinge onto the walls and create a flow stream which goes from left to right,upstream to downstream, along both feedback channels at the same time, preventing any feedback from downstream to upstream. Notice that this feedback is imprescindible to generate pulsating flow inside the mixing chamber. It is interesting to ralize that an increase of the inlet width is capable of producing a frequency increase of about 40%, while making the output amplitude to decrease nearly a 30%. This tendency is completely different than the one obtained with the previous two modifications, and it has mostly do to with the fact that for the present modification, when high widths are considered, the mixing chamber incoming jet, just suffers a slight wavering inside the chamber, causing at the (FA) exit a small variation of amplitude.

(a)

(b)

Figure 7: Fluid actuator performance when modifying the mixing chamber inlet width, (a) Frequency variation (b) Amplitude variation.

(a)

(b)

Figure 8: Fluid actuator internal field velocity magnitude, (a) Maximum inlet width (b) Minimum inlet width.

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5

Conclusions

A given Fluidic actuator has been studied via 3D-CFD and when modifiying its internal parameters, such as mixing chamber inlet and outlet widths, and outlet angle. When outlet width or outlet angle are being decreased, flow output frequency and amplitude increases and vice-versa. The modification of the inlet width produces quite an oposite effect, as inlet width increses the freqeuncy increases generating a decrease of the output amplitude.

Acknowledgements The present paper presents part of the results obtained thanks to a competitive research project number FIS0016-77849-R founded by Spanish economy ministry.

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Fluidic actuator performance variation via internal dimensions modifications

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