Open access peer-reviewed chapter

Relationships between Large-Scale Coherent Motions and Bursting Events in a Turbulent Boundary Layer

By Yasuhiko Sakai, Kouji Nagata and Hiroki Suzuki

Submitted: October 21st 2010Reviewed: May 14th 2011Published: July 27th 2011

DOI: 10.5772/17578

Downloaded: 1500

1. Introduction

A turbulent boundary layer (TBL) over a flat rigid surface can be seen in many industrial and environmental flows. Therefore, for the prediction and control of turbulence and design of better industrial apparatus, it is of great importance to elucidate the spatiotemporal structure of the TBL. In TBLs, it is well known that coherent motions associated with the bursting events exist near the wall. In the outer turbulent/nonturbulent (potential) interface of the TBL, large-scale coherent motions exist, which have the size of order of the boundary-layer thickness. The latter structures are called `bulge' and `valley' and are unsteady in space and time (Robinson 1991). Therefore, it is of importance to clarify the spatiotemporal structures of these coherent motions in order to elucidate the structure of the TBL. However, it is difficult to study the entire structure of the TBL by a single-point measurement using an I- or X-type hot-wire probe. In addition, previous studies (Ichijo & Kobashi 1982, Kobashi et al. 1984, Thomas & Bull 1983) have suggested that these coherent motions are closely associated with wall static pressure fluctuations. Therefore, the simultaneous measurement of multipoint instantaneous velocities and wall static pressure fluctuations is appropriate for clarifying the spatiotemporal structure of the TBL.

In this chapter, we will show the experimental results on the relationships between the bursting events occurring near the wall and the large-scale bulge and valley structures in the outer turbulent/nonturbulent interface of the TBL, and the spatiotemporal structures of the coherent motions in a zero-pressure-gradient TBL. With regard to the experiments, multipoint instantaneous streamwise velocities and instantaneous wall static pressures are simultaneously measured using a combination of a rake consisting of 23 I-type hot-wire probes, which covers the entire TBL, and a microphone pressure sensor. The velocity signals are analysed by the KL (Karhunen-Loéve) expansion (also known as the proper orthogonal decomposition (POD) method). The flow field is reconstructed by using either lower- or higher-order modes to investigate the coherent motions in the TBL. The bursting events are detected by applying the VITA (Variable Interval Time Average) technique to the instantaneous velocity signals in the original flow. The large-scale bulge and valley structures are detected by using the newly proposed conditional sampling method, which is applied to both the original flow and the reconstructed flow by using the KL expansion. Further, to show the future research direction, the experimental data on the statistical properties of TBL affected by the disturbances in the outer mainstream are demonstrated. The disturbances are generated using the bi-plane square grid. The effect of the disturbance on the velocity statistics is discussed.

2. Analytical methods

2.1. VITA technique

The VITA (Variable Interval Time Average) technique (Blackwelder & Kaplan 1976, Blackwelder & Haritonidis 1983, Chen & Blackwelder 1978, Osaka et al. 1986) is one of the conditional mean methods used to detect coherent motions in turbulent flows. The VITA technique is appropriate for detecting the bursting events in a TBL since the bursting events accompany the large velocity fluctuations. Following the general VITA technique (Blackwelder & Haritonidis 1983), we define the variable-interval time average of the fluctuating streamwise velocity u˜as

va(t;x,T)=1TtT/2t+T/2u˜(t')2dt'[1TtT/2t+T/2u˜(t')dt']2E1

where Tis the averaging time. Note that vaapproaches the conventional time-averaged result as Tincreases. The detection criterion is established by introducing a threshold value for the VITA variance signal. Thus, the detection function αu(t)is defined as (Blackwelder & Haritonidis 1983, Chen & Blackwelder 1978, Osaka et al. 1986)

αu(t)={1;va(t)Kurms2andddt[u˜(t)u¯]>00;otherwise,E2

where u¯is the time-averaged mean velocity, urmsis the rms (root mean square) value of velocity fluctuations, and Kis the threshold value, which is constant. In general, the selection of Kand Tis important for detecting the bursting events using the VITA technique. In this study, we chose K=1andT+=Tuτ2/ν=20, on the basis of a previous study (Osaka et al. 1986), where uτis the friction velocity and νis the kinematic viscosity. We have carefully verified that the detected burst frequency is in a good agreement with previously published data (Blackwelder & Haritonidis 1983, Osaka et al. 1986).

2.2. Methods for detecting large-scale coherent motions

The large-scale coherent motions in the outer turbulent/nonturbulent interface of the TBL are detected using the following method. The first probe from the top of the rake, which is placed near the turbulent/nonturbulent interface of the TBL (y/δ=1.23; yand δare the distance from the wall and the thickness of the boundary layer, respectively), is used to detect the large-scale coherent motions since these motions pass through this probe. The detection of the motions depends on both uand u/taty/δ=1.23. Note that the direction of the rotation is based on the configuration shown in Figs. 1 and 2; the flow is from the left to the right along the x-direction. Therefore, the mean spanwise vorticity is in the clockwise direction.

Figure 1.

Experimental setup

Figure 2.

Schematic of bulge structure and bursting event (not to scale)

2.2.1. Detection of clockwise large-scale coherent motions

The large velocity fluctuation at y/δ=1.23with acceleration is associated with the `bulge' structure, which is the large-scale (δ-scale) vortical structure rotating in the same direction as the mean vorticity (i.e. in the clockwise direction). Therefore, we detect the bulge structure by using the following conditions:

[u˜(t)u¯]2K'urms2andddt[u˜(t)u¯]>0E3

where K'is the threshold value, which is constant. In this study, from a careful inspection of the detected signals, K'has been chosen equal to 3.

2.2.2. Detection of anticlockwise large-scale coherent motions

The large velocity fluctuation at y/δ=1.23with deceleration is associated with the `valley' structure, which is the large-scale (δ-scale) structure at the edge of the bulges (Robinson 1991) rotating in the opposite direction of the mean vorticity (i.e. in the anticlockwise direction). Therefore, we detect the valley structure by using the following conditions:

[u˜(t)u¯]2K'urms2andddt[u˜(t)u¯]<0E4

It has been verified that the value of K'=3is appropriate for detecting the valley structures.

2.3. KL expansion

The KL(Karhunen-Loéve)expansion, which is also known as the proper orthogonal decomposition (POD) method and is an extension of the general Fourier series (Davenport & Root 1958, Lumley 1967, 1981), is the representation of a stochastic process as a linear combination of orthogonal functions. The basis functions of the expansion are defined such that the coefficients are uncorrelated (independent) with each other and are arranged in the descending order according to the magnitude of their eigenvalues. The sum of all the eigenvalues corresponds to the sum of the mean square values in the entire region where the KL expansion is applied. Here, the KL expansion is applied for the simultaneous measurement of the streamwise velocities at 23 points in the TBL. Therefore, there are 23 modes (n=1~23) in the KL expansion. The lower modes are expected to extract the turbulence structures in the near-wall region (i.e., the inner region), where turbulence energy is larger, and the higher modes are expected to extract the turbulence structures from the outer region to the outside of boundary layer, where turbulence energy is smaller. In this study, the sum of the first three modes (lower modes) retains 65% of the total energy; therefore, the flow field reconstructed by using the lower modes (n=1~3) is considered to represent the energetic coherent motions. The modes from 4 to 23, which contain the rest of the 35% energy, are regarded as the higher modes in this work.

3. Experiments

3.1. Experimental apparatus

Experiments are carried out in an Eiffel-type wind tunnel. The length, height, and span of the test section of this tunnel are 2,600, 300, and 520 mm, respectively. The TBL is developed along a smooth flat plate (Fig. 1) placed horizontally in the wind tunnel. The plate is 2,100 mm in length, 8 mm in thickness, and 520 mm in span and is made of Bakelite in order to reduce the heat loss from the hot-wire probe in near-wall measurements. The leading edge of the plate has a parabolic shape. To generate a fully developed TBL in a short distance from the leading edge, transition was promoted by using a tripping wire of 1.0 mm diameter placed 50 mm downstream from the leading edge of the plate, as shown in Fig. 1. The zero-pressure-gradient condition is achieved by adjusting the height of the test section. The coordinate system is as follows: the x-axis is along the streamwise direction starting from the leading edge, the y-axis is in the vertical direction, and the z-axis is along the spanwise direction (see Fig. 1).

3.2. Velocity and wall static pressure measurements

A constant-temperature hot-wire anemometer (CTA) is used for the velocity measurement. The rake consists of 23 I-type probes, which covers the entire TBL at x=1,884mm; the sensors of the rake cover a length of y=0.1~48.5mm. The wires are made of tungsten. The length of the sensor is l=1mm and its diameter is d=5μm, giving an aspect ratio ofl/d=200. By comparing the data obtained from a single I-type probe with those from the rake of I-type probes, it has been confirmed that there is no interference between the probes, which are placed with 2.2 mm apart from each other.

To measure the instantaneous static pressure at the wall, the 1/8 inch microphone pressure sensor (Brüel & Kjær Type 4138) is used. The sensor is mounted flush to the wall surface at the measurement position (x=1,884mm) through a pressure hole of D=0.6mm diameter. The frequency response of the pressure sensor has been checked; both the phase difference φand the magnitude ratio Mbetween the input signal and the output signal from the sensor exhibit flat responses up to 7 kHz. These results suggest that the frequency response limit of the pressure sensor is 7 kHz. Note that nondimensional diameter of the sensing area in this study isD+=Duτ/ν=18. Previous studies (Gravante et al. 1998, Schewe 1983) have shown that whenD+<19~20, the spatial resolution is sufficient to resolve the essential structures of the static pressure fluctuations at the wall. Therefore, in addition to the frequency response, the spatial resolution is sufficient to measure the static pressure fluctuations at the wall. In fact, we have confirmed that whenD+<18, the power spectra of wall static pressure fluctuations are identical.

3.3. Experimental conditions and data sampling method

The mean speed of the uniform flow is set at U0=10.9m/s. The unit Reynolds number is Re=U0/ν=7.38×105m-1, and the Reynolds number based on the momentum thickness at the measurement point (x=1,884mm) isReθ=2,820. Note that the spatial resolution of the hot-wire probe in the near-wall measurement is sufficient for Reθ<5,000(Robinson 1991). The δand uτat the measurement point (x=1,884mm) are 41 mm and 0.453 m/s, respectively. We have confirmed that there is no pressure gradient in the streamwise direction, and the mean flow is two-dimensional. The sampling frequency and sample size are 10 kHz and 229,376, respectively, and are sufficient to obtain reliable statistics: these parameters are based on the performance of the A/D converter (Microscience ADM-688PCI) and the computer (Dell Dimension 8200) used. The output voltage signals from the hot-wire anemometer and the pressure sensor are converted into 12-bit digital data by the A/D converter and recorded on the hard disc of the computer. The A/D converter and the digital output board are PCI interface boards and are synchronized through master-slave connections. The time lag between the output of the clock signal from the master unit and the input of the signal into the slave unit is only 125 ns; therefore, very fast multichannel sampling is achieved. To confirm this point, we input the same wave pattern generated by the function generator into the whole channels simultaneously, and compare the output signals. The result showed that the output signals are the same and no discernible time lag is observed. Statistical processing of the digitized data was carried out using the computer.

4. Results and discussion

4.1. Reliability of measurements

The vertical profile of streamwise mean velocity is shown in Fig. 3, where U+ = U/u andy+=yuτ/ν. The log-law region can be seen for30<y+<200. Figures 4 and 5 show the power spectrum of streamwise velocity fluctuations normalized by the squared rms value of the velocity fluctuations, urms2, and the power spectrum of the wall static pressure fluctuations normalized by the squared rms value of the wall static pressure fluctuations, prms2, respectively. Figure 4 shows that the power spectrum of the longitudinal velocity fluctuations collapses irrespective ofy+. The spectrum also shows the -1 and -5/3 decay regions (Perry et al. 1986). On the other hand, the power spectrum of wall static pressure fluctuations (Fig. 5) has the -1, -7/3, and -5 decay regions, and the result agrees with that of previous experiments carried out by Gravante et al. (1998) and Löfdahl (1996). In addition, turbulence intensities and probability density functions (not shown) are in good agreement with those found in previous studies. Therefore, it is confirmed that the present measurements of velocity and wall static pressure fluctuations are highly accurate.

Figure 3.

Vertical profile of streamwise mean velocity

Figure 4.

Power spectrum of streamwise velocity fluctuations

Figure 5.

Power spectrum of wall static pressure fluctuations

4.2. Relationships between bursting events and large-scale coherent motions

The clockwise and anticlockwise large-scale coherent motions are detected using the method proposed in the subsection 2.2, and the results are shown in Figs. 6 and 7, respectively. In Figs. 6 and 7, the upper and lower graphs show the conditionally averaged time histories of the normalized velocity, u˜(τ)'=(u˜(τ)u¯)/urms, and the conditionally averaged time histories of normalized wall static pressure, p˜(τ)'=(p˜(τ)p¯)/prms, respectively. Here, u˜(τ)and p˜(τ)are the ensemble-averaged values of conditionally sampled velocity and wall static pressure, respectively, and u¯and p¯are the ordinary (non-conditional) time-averaged velocity and wall static pressure, respectively. In Figs. 6 and 7, τis the time lag from the detection time at which the instantaneous velocity signal takes the maximum or minimum value at the detection point ofy/δ=1.23, and is normalized by using U0 andδ. Figure 6 shows that when the clockwise large-scale coherent motion passes, the velocity increases outside the TBL (y/δ>1) and decreases inside the TBL (y/δ<1). The result suggests the presence of a rotational flow whose center is located near the turbulent/nonturbulent interface: this structure corresponds to the bulge (Kobashi et al. 1984). It is also observed from Fig. 6 that the wall static pressure decreases when the clockwise large-scale coherent motion passes. It is presumed that measured decrease in the wall static pressure is associated with the low pressure in the clockwise large-scale coherent motion (Robinson 1991).

Figure 6.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure corresponding to the bulge structure

Figure 7.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure corresponding to the valley structure

On the other hand, Fig. 7 shows that when the anticlockwise large-scale coherent motion passes, the velocity decreases outside the TBL (y/δ>1) and slightly increases inside the TBL (y/δ<1). The detected signals suggest the valley, which exists at the edge of the bulges. It has been reported (Robinson 1991, Thomas & Bull 1983) that there is an inflow from the free-stream flow into the turbulent region, and the inflow has an instantaneous velocity vector that is against the mean flow. Therefore, it is expected that the decrease in the velocity in the outer region in Fig. 7 is associated with the inflow from the free-stream flow. The wall static pressure increases when the anticlockwise large-scale coherent motion passes. These results are in agreement with the model for the structure of a TBL proposed by Thomas & Bull (1983).

The VITA technique is applied to the raw signals to detect the bursting events. Figure 8 shows the ensemble-averaged time histories of conditionally sampled velocity

Figure 8.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure. The detection is based on the VITA technique

u˜(τ)'=(u˜(τ)u¯)/urmsand of conditionally sampled wall static pressurep˜(τ)'=(p˜(τ)p¯)/prms. Here, u˜(τ)and p˜(τ)are the conditionally averaged velocity and wall static pressure by the VITA technique, respectively. The solid and broken lines in Fig. 8 denote u˜(τ)'andp˜(τ)', respectively. τis the time lag from the detection time at which the instantaneous velocity u˜(t)is the same as the ordinary (non-conditional) time-averaged velocity u¯at each measuring point. Figure 8 shows the cycle of events of ejection (u˜(τ)'<0) atτ<0, which is the upward motion of a low-speed fluid lump, and sweep (u˜(τ)'>0) atτ>0, which is the downward motion of a high-speed fluid lump (Osaka et al. 1986). The wall static pressure exhibits a rapid increase when the burst occurs above the measurement point (Kobashi et al. 1984). Considering the results shown in Figs. 6 and 7 together with the result in Fig. 8, it is concluded that the bulge structure, which is accompanied by the pressure decrease, cannot coexist with the bursting event, which is accompanied by the pressure increase. On the other hand, the valley structure, which is accompanied by the pressure increase, can coexist with the bursting event.

4.3. Flow field reconstructed by using lower modes (n=1 ~ 3)

Using the method proposed in the subsection 2.2, the clockwise and anticlockwise large-scale coherent motions are detected in the flow field reconstructed by using the lower modes (n=1~3) of the KL expansion, and the results are shown in Figs. 9 and 10, respectively. Figures 9 and 10 show that both the clockwise large-scale coherent motion (i.e. bulge), which is accompanied by the pressure decrease, and the anticlockwise large-scale coherent motion (i.e. valley), which is accompanied by the pressure increase, exist in the flow field reconstructed by using the lower modes. Comparing these results with those shown in Figs. 6 and 7, which are obtained by applying the method to the original flow field (n=1~23), it is found that these structures are more clearly observed in the flow field reconstructed by using the lower modes and they are observed even near the wall.

Figure 11 shows the ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the lower modes (n=1~3) and the detection by the VITA technique is applied to the original flow field (n=1~23) aty/δ=0.003. It is observed from Fig. 11 that the velocity inside the TBL (in the reconstructed flow) increases and the velocity outside the TBL (in the reconstructed flow) decreases when the bursting event occurs. These velocity patterns are similar to those during the anticlockwise large-scale coherent motion (i.e. valley). Thus, the results suggest that the bursting event and valley can coexist at the same longitudinal location in the flow reconstructed by using the lower modes (n=1~3).

Figure 9.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the lower modes (n=1~3) and the detection method of the clockwise bulge structure is applied.

Figure 10.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the lower modes (n=1~3) and the detection method of the anticlockwise valley structure is applied.

Figure 11.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the lower modes (n=1~3). The detection is based on the VITA technique applied to the original flow field (n=1~23) at y/δ=0.003

4.4. Flow field reconstructed by using higher modes (n=4 ~ 23)

Using the method proposed in the subsection 2.2, the clockwise and anticlockwise large-scale coherent motions are detected in the flow field reconstructed by using the higher modes (n=4~23) of the KL expansion, and the results are shown in Figs. 12 and 13, respectively. Figure 12 (for the bulge) shows a velocity variation near the wall. However, the velocity hardly changes near the wall in Fig. 13 (for the valley). These results suggest that in the flow field reconstructed by using the higher modes, only the clockwise large-scale coherent motion (i.e. bulge) exists near the wall and the anticlockwise large-scale coherent motion (i.e. valley) does not influence the flow field near the wall. These results also suggest that the turbulent kinetic energy in the bulge structure is larger than that in the valley structure. This result is consistent with the fact that the valley is generated by the bulge structures.

Figure 14 shows the ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the higher modes (n=4~23) and the detection by the VITA technique is applied to the original flow field (n=1~23) aty/δ=0.003. Figure 14 shows that when the bursting events are detected, the velocity in the outer region does not change in the reconstructed flow. This result suggests that in the flow field reconstructed by using the higher modes, the large-scale coherent motions are weakly associated with the bursting events.

Figure 12.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the higher modes (n=4~23) and the detection method of the clockwise bulge structure is applied.

Figure 13.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the higher modes (n=4~23) and the detection method of the anticlockwise valley structure is applied.

4.5. Effect of freestream turbulence on the Turbulent Boundary Layer

In the industrial flows such as those in turbomachines and heat exchangers, inflow streams often contain the turbulent eddies generated by the upstream structures. These free stream turbulence influences the large scale coherent structure and the turbulence characteristics of the TBL. In this subsection, some experimental data on the statistical properties of TBL affected by the freestream turbulence generated by the square grid are demonstrated.

The experiments are performed by using the experimental apparatus shown in the subsection 3.1. The turbulence-generating grid is installed at 100mm upstream from the leading edge of the plate. The square-mesh biplane grid M10 is used. M10 was constructed from square-sectioned aluminium rods. The mesh size M and the rod thickness d are 310-2m and 610-3m, respectively, and therefore, its solidity is = 0.36. The mean speed of free stream flow when the grid is installed is set at U0 = 10.1m/s. The unit Reynolds number is Re = U0/ = 6.84 105m-1 and the Reynolds number based on the momentum thickness at the measurement point (x=1,880 mm) is Re = 3,000. When the grid is not installed, the flow condition is adjusted to realize Re = 3,000 at the same measurement point (x=1,880 mm). The instantaneous streamwise velocity is measured by the hot-wire CTA with the I-type probe. The wire is made of tungsten. The length of sensor is 1.0 mm and its diameter is 5m, giving an aspect ratio of l/d = 200. The output signals from the CTA are digitized and recorded on the hard disc using the data recording unit (KEYENCE WR-60). The sampling frequency and sample size are 20 kHz and 524,288(=219), respectively.

The vertical profiles of y+ dU+/dy+ in both cases with and without the grid are shown in Fig.15, where the result by the Direct Numerical Simulation (DNS) in the case of Re = 1,410 (Spalart 1988) is also depicted for the comparison. Note that in the logarithmic region the value of y+ dU+/dy+ should be constant, i.e., y+ dU+/dy+ = 1/. From Fig.15, it is found that the logarithmic region for the case with the grid expands to the outer side of the TBL in comparison with the case without the grid. Figure 16 shows the vertical profiles of the velocity fluctuation rms value urms normalized by the friction velocity u, i.e.,urms+=urms/uτ. It is found that urms+in the case with the grid increases in comparison with the case without the grid. However it should be noted that in general the free-stream disturbance does not always increaseurms+, and in some cases urms+decreases (Hancock & Bradshow 1989, Nagata et al. 2011). The effects of free-stream disturbance on the characteristics of the TBL are still open question. The future works are expected to elucidate the relation between the free-stream disturbance and the flow dynamics of the TBL.

Figure 14.

Ensemble-averaged time histories of conditionally sampled velocity and wall static pressure when the velocity is reconstructed by the higher modes (n=4~23). The detection is based on the VITA technique applied to the original flow field (n=1~23) at y/δ=0.003

Figure 15.

The vertical profiles of y+dU+/dy+

Figure 16.

The vertical profiles of urms+

5. Conclusions

Multipoint instantaneous streamwise velocities and instantaneous wall static pressures have been simultaneously measured by using a combination of a rake consisting of 23 I-type hot-wire probes and a microphone pressure sensor in a fully developed, zero-pressure-gradient turbulent boundary layer (TBL) in a wind tunnel. The KL(Karhunen-Loéve)expansion, the VITA technique and the newly proposed method for the detection of large-scale coherent motions were applied to examine the relationships between large-scale coherent motions and the bursting events. Further, the statistical properties of the TBL affected by the freestream turbulence generated by the square grid were investigated.

The main results obtained in this chapter are summarized as follows:

  1. The structure of the TBL inferred from the present experiment is in agreement with the TBL model proposed by Thomas & Bull (1983).

  2. The bursting events and valley structure (both accompanied by an increase in wall static pressure) can coexist at the same longitudinal location, whereas the bursting events and bulge structure cannot coexist with each other.

  3. In the flow field reconstructed by using the lower modes, both the bulge structure, which is accompanied by bursting events, and the valley structure, which is not accompanied by the bursting events, exist. On the other hand, in the flow field reconstructed by using the higher modes, the large-scale coherent structures are weakly associated with the bursting events.

  4. The grid turbulence introduced into the outer mainstream has the effect to enlarge the logarithmic region to the outer side of turbulent boundary layer.

Acknowledgments

This study was partially supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology through Grants-in-Aid (Nos. 22360076 and 22360077).

© 2011 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike-3.0 License, which permits use, distribution and reproduction for non-commercial purposes, provided the original is properly cited and derivative works building on this content are distributed under the same license.

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Yasuhiko Sakai, Kouji Nagata and Hiroki Suzuki (July 27th 2011). Relationships between Large-Scale Coherent Motions and Bursting Events in a Turbulent Boundary Layer, Wind Tunnels and Experimental Fluid Dynamics Research, Jorge Colman Lerner and Ulfilas Boldes, IntechOpen, DOI: 10.5772/17578. Available from:

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