separation control using synthetic vortex generator jets in axial compressor cascade

7/28/2019 Separation Control Using Synthetic Vortex Generator Jets in Axial Compressor Cascade

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Acta Mech Sin (2006) 22:521–527

DOI 10.1007/s10409-006-0042-5

R E S E A R C H PA P E R

Separation control using synthetic vortex generator jets in axialcompressor cascade

Xinqian Zheng · Sheng Zhou · Anping Hou ·

Zhengli Jiang · Daijun Ling

Received: 16 June 2006 / Revised: 3 July 2006 / Accepted: 7 August 2006 / Published online: 17 November 2006© Springer-Verlag 2006

Abstract An experimental investigation conducted in

a high-speed plane cascade wind tunnel demonstratesthat unsteady flow control by using synthetic (zero mass

flux) vortex generator jets can effectively improve the

aerodynamic performances and reduce (or eliminate)

flow separation in axial compressor cascade. The Mach

number of the incoming flow is up to 0.7 and most

tested cases are at Ma = 0.3. The incidence is 10◦ at

which the boundary layer is separated from 70% of the

chord length. The roles of excitation frequency, ampli-

tude, location and pitch angle are investigated. Prelimi-

nary results show that the excitation amplitude plays a

very important role, the optimal excitation location is

just upstream of the separation point, and the optimalpitch angle is 35◦. The maximum relative reduction of

loss coefficient is 22.8%.

Keywords Flow control · Compressor · Synthetic jet

The project supported by the National Natural Science

Foundation of China (10477002 and 50476003) and the Ph.D.Innovative Foundation of Beihang University.The English text was polished by Yunming Chen.

X. Zheng (B) · S. Zhou · A. HouNational Key Laboratory of Aircraft Engine,Beihang University,Beijing 100083, Chinae-mail: [email protected]

Z. Jiang · D. LingChinese Gas Turbine Establishment, Jiangyou,Sichuan 621700, China

1 Introduction

Flow separation greatly constrains the improvement of

machine performances involving flow and many effec-

tive control methods have been developed to shift the

separation point downstream or eliminating the separa-

tion entirely [1]. It is well known that the strong adverse

pressure gradient and the decreased momentum of the

boundary layer are two critical factors underlying the

boundary layer separation. Therefore, almost all sepa-

ration control methods, however different they may be,

are focused on accelerating the low momentum of fluid

against the adverse pressure gradient.

Suction is the first method ever proposed by Pra-ndtl for separation control. The basic principle is to

remove decelerated fluid near a surface and deflect the

high-momentum free-stream fluid towards the surface.

Blowing is another effective method to add momen-

tum directly. Seifert et al. [2] found that the injected

mass flow can be reduced by an order of magnitude

for a given level of mixing by using oscillating jets

instead of steady ones. They demonstrated that unsteady

control methods are much more effective than steady

ones and can be realized at very low level of power

input.

For the methods of suction, blowing and oscillat-ing jets, the routing of transporting air may present a

prohibitive addition of complexity and weight. How-

ever this problem could be avoided with synthetic jet,

which is characterized by zero net mass flux and non-

zero momentum flux and does not require a complex

system of pumps and pipes [3]. This technique is very

efficient because the low momentum fluid is sucked

into the device during the suction period of the cycle,

whereas a high momentum wall-jet is superimposed on



522 X. Zheng et al.

the separating velocity profile during the blowing period

of the cycle. The spanwise vorticity is enhanced dur-

ing both periods [4]. In addition, the method of vor-

tex generator jets (VGJs) for separation control has

been developed for many years [5–7]. Blowing from

“small, skewed, and pitched holes” is utilized in this

method to create streamwise vortices similar to those

created by solid vortex generators. The oscillating flowof synthetic jets and the compound angle injection of

VGJs are combined to produce synthetic VGJs. Volino

[8] used synthetic VGJs to control separation on low-

pressure turbine airfoils at a very low Reynolds number

(Re = 25, 000) in the case of the low-speed incoming

flow.

In axial compressors, the adverse pressure gradient

is generally strong. Since the adverse pressure gradient

associated with the diffusion becomes stronger at high-

loading levels in modern designs, an unsteady separa-

tion is inevitable [9]. This has a negative impact on stall

margin, efficiency and pressure rise capability and evenleads to rotating stall or surge. But, only a few studies

have been conducted to control the separation in axial

compressors by using an unsteady excitation. Culley [10]

used the embedded injection to control the separation

on stator vanes in a low-speed axial multistage compres-

sor. Zheng [11] showed that the disordered unsteady

separated flow could be effectively controlled by the

periodic suction and blowing in a wide range of inci-

dence, resulting in enhancement of the time-averaged

aerodynamic performances of the axial compressor cas-

cade.

In the present study, the synthetic vortex genera-tor jets (VGJs) are used to control separation on the

axial compressor blades in the plane cascade wind tun-

nel experiments. The boundary layer of the test case

is turbulent since the Reynolds number on the chord

exceeds 6.0 × 105, which is a typical Reynolds num-

ber in the real axial compressors. The high incidence of

10◦, which denotes the high blade loadings and where

the separation of the turbulent boundary layer occurs,

is chosen for all the test cases. The Mach numbers of

the incoming flows are from 0.3 to 0.7. Thus, the flow

velocity for tests is approximately in tune with that of

real high-pressure compressors. Detail tests are con-

ducted with 0.3 Ma. There are many parameters that

can be varied in a synthetic VGJs study, such as jet

location, jet velocity, jet oscillating frequency, and jet

angle. The roles of these parameters are investigated

and discussed preliminarily. To the authors’ knowledge,

this is believed to be the first application of synthetic

VGJs to the separation control in the axial flow com-

pressors, especially in a high-speed experimental

facility.

2 Experimental apparatus and procedures

2.1 Cascade wind tunnel

Experiments were carried out in the plane cascade wind

tunnel located in China Gas Turbine Establishment. The

available experimental Mach number ranges from 0.3 to

2.0. The wind tunnel with a length of 19 m consists of agate valve, a quick start valve, a pressure regulator valve,

a stabilizing section, a nozzle section, an experimental

section, an air-collector, an air-injector, an air-suction

system, etc. The sizes of the experimental section are

300 mm (width)×160 mm (height). The pressured, dried,

and cleaned air is stored in nine containers with the total

cubage of 1,000 m3. The maximum flow flux is 22 kg/s

and the stable continuous working time exceeds 4 min

The start-up time is about 3 s. The pressure fluctuation is

less than 0.3%. At the entrance of experimental section,

the mean velocity is uniform to within 0.5%. The free

stream turbulence intensity (FSTI) is 0.3±0.05%.

2.2 Test blades

Six blades were prepared to ensure that the third blade,

i.e. the test blade, would not be influenced by the side-

wall. The span-to-chord ratio of 2 was chosen to

ensure two-dimensional flow along the span wise center-

line of the blades where all measurements were made.

The blade-root profile of the first stage rotor of a high-

pressure compressor, which is a high turning angle pro-

file (42.83◦), was chosen as the experimental blade in

order to facilitate the separation. The chord length of ablade is 80.6 mm and the solidity is 1.5.

The synthetic VGJs are produced from the cavities.

The blade material is 2Cr13. Four cavities of diameter

3.6 mm are drilled through the blade span at four loca-

tions, i.e. 15, 25, 50, and 70% of the chord length b, as

shown in Fig. 1. One end of each cavity is plugged by

plastic clay, and another end is attached to the narrow

end of a funnel. A high-power loudspeaker of diame-

ter 200 mm is attached by screw to the wide end of the

funnel. The joints are sealed with silicone to prevent air

from leakage. Inside the speaker, a piston is driven by

electromagnetic force to move back and forth, sucking

and blowing the air alternatively to generate a synthetic

jet. The speaker is driven by a 500 W audio amplifier

(HuSan PB2500) which is driven by a function genera-

tor (GFG-813). The function generator was set to output

a sine wave in the present study. Small holes for syn-

thetic VGJs are drilled into the suction surface along

four span-wise lines at 15, 25, 50, and 70% of the chord

length. Each VGJ hole extends from the suction surface

into the cavity at a pitch angle γ . The VGJ holes are



Separation control using synthetic vortex generator jets in axial compressor cascade 523

Fig. 1 Drawing of the suction side with cavity and VGJ holes.a Full blade, b Cross-section of VGJ holes

0.8 mm in diameter (0.99% of b) and are spaced 5.0 mm

(6.2%of b).Nineholesareinalinefrom37.5to62.5%of

span. The VGJ holes were separately drilled at 35◦, 50◦,

90◦ pitch angles γ for three test cases. Three test blades

were needed. In fact, each test blade had a backup and

there were six test blades totally.

2.3 Measurement and data processing

Total pressure p∗1 was measured by total pressure rake

inside the stabilizing section. Static pressures were mea-

sured in 21 afore-cascade static pressure holes drilled

in several passages located at the central part of the

cascade and their area-averaged value was taken as

the afore-cascade static pressure p1. At the location

19 mm behind the cascade and 50% of the blade height,

total pressures were measured at 28 points (the dis-

tance between consecutive points is 2 mm) by 3-hole

steady probes of the model PS682A in the circumfer-

ential direction and within one pitch space, and their

area-averaged value was taken as the post-cascade totalpressure p∗2. Thus, the benefit to the dynamic perfor-

mance due to the synthetic VGJs with no mass addition

is quantified by a total pressure loss coefficient. The

conventional definition of loss coefficient for a blade

passage is

= ( p∗1 − p∗

2)/( p∗

1 − p1).

The velocity of synthetic jets was measured at the

narrow end of the funnel by a one-dimensional hot-wire

Fig. 2 Time trace of jet velocity and speaker input voltage, f e = 100Hz, P = 35 W, sine wave

anemometer and data acquisition was implemented by

the data acquisition system TSI-IFA300. Data were col-

lected for 6.5536 s at a 40 kHz sampling rate (218 sam-

ples). All raw data were saved. The high sampling rateprovides an essentially continuous signal and the long

sampling time results in low uncertainty in statistical

quantities. Inaccuracy in velocity measurement is esti-

mated to be lower than 0.5 m/s.

Typical variation of jet velocity versus time is shown

in Fig. 2, in which the maximal velocity of the synthetic

jet V jet is about 83 m/s. Since one-dimensional anemom-

eter was used, both the blowing velocity and reversal

suction velocity are shown as positive value. For brevity,

the experiment results about the effects of various para-

meters, such as excitation frequency, excitation intensity

and excitation waveform, on the jet velocity will not bepresented and discussed here. The detail, can be found

in Ref. [12].

3 Results

3.1 Effect of excitation frequency and jet velocity

In order to indicate the improvement of aerodynamic

performance by synthetic VGJs,the relative reduction of

loss coefficient versus the excitation frequency is shown

in Fig. 3(a) for three pitch angles. Let us first focus on

the curve of γ = 35◦. When the excitation frequency is

1,690Hz, the prominent positive effect is obtained, and

the maximum relative reduction of loss coefficient δ()

is 22.8%. In addition, aerodynamic performance is also

improved when the excitation frequencies are 710 Hz

and about 40 Hz although the positive effect decreases

lightly. The inflexions of the curve are at 710 Hz and

1,690 Hz and the positive effect decreases sharply at

other excitation frequencies. The relations of δ() ver-



524 X. Zheng et al.

Fig. 3 a The increase of loss coefficient versus frequency; b Jetvelocity (and speaker impedance) versus frequency. Ma = 0.3,i = 10◦, L = 0.7b

sus f e exhibit similar well-regulated variation in the cases

of γ = 50◦ and 90◦. However, the positive excitation

effect decreases. When γ = 50◦ the positive effect is the

smallest in the three cases. The effect of pitch angle will

be further analyzed later.

What is the reason behind the best excitation effect at

1,690 and 710 Hz? Figure 3(b) shows the peak velocity

of the synthetic jet versus the excitation frequency. The

strength of jet was measured at the VGJ output after

the speaker was coupled to the blade. The dashed line

is the speaker impedance versus frequency. It is wellknown that the maximum impedance is corresponding

to the resonance frequency of the loudspeaker [12]. It is

just at 710 and 1,690 Hz that there exist two resonance

frequencies and the maximum jet velocity is obtained.

At the resonance frequencies of 710 and 1,690 Hz, the

relative excitation amplitudes ¯ A are 0.495 and 0.53 (the

free-steam velocity is about 102 m/s), respectively. Cor-

respondingly, the maximum positive effects are obtained

at the resonance frequencies. The positive effect

decreases rapidly (even no positive effect is obtained)

at non-resonance frequencies where the jet velocity (or¯ A) decreases. It is obvious that the jet velocity (or ¯ A)

plays a significant role and the jet velocity must be suffi-

ciently high to exceed a threshold value. The jet velocity

instead of the frequency itself affects the control results.

The frequency role can not be visualized in the present

experiments since the jet velocity is dependent on the

resonance characteristics of the loudspeaker. In order to

investigate the frequency role, it is necessary to devise

a fluidic actuator, the jet velocity of which is indepen-

dent on the frequency. Of course, optimal analysis of

individual parameters based on reliable numerical sim-

ulations is also an economical and effective method.

Furthermore, the positive effect is also obvious within

the low-frequency domain (about 40 Hz) although the

jet velocity ( ¯ A = 0.33) is lower than that correspond-

ing to 710Hz ( ¯ A = 0.495) or 1,690Hz ( ¯ A = 0.53).

The authors would like to guess the underlying phys-

ical mechanism. Excitation at lower frequency wouldproduce larger scale vortex-structures during the inter-

action between the jets and the main flow since the mass

flux of jets is larger in each outward pulse. Larger scale

vortex-structure generated by synthetic VGJs might

increase the mixing of streamwise momentum more

effectively.

The main difference between steady methods (such

as suction or blowing) and unsteady methods is that the

excitation frequency plays a role on the separation con-

trol in the latter. Many researches show that the excita-

tion frequency is correlated with the instability of shear

layer. Zheng et al. [11] found that the optimal excita-tion frequency was nearly equal to the characteristic

frequency of vortex shedding and the effective excita-

tion frequency spaned a wide spectrum in axial com-

pressor cascades. Wu et al. [13] showed that the largest

lift increase was attained when the excitation frequency

was half of the vortex shedding frequency over a post-

stall airfoil. Amitay et al. [14] investigated the effect of

the actuation frequency on the coupling between the

synthetic jets and the crossflow over a stalled airfoil in

detail andthe experiment results showedthat there were

two distinct frequency bands in the response of the sepa-

rated flow to the actuation. All of these researches indi-cated that the excitation frequency plays an important

role. In particular, the flow in axial compressors is more

complicated than the flow over an airfoil. All unsteady

vortexes move randomly with different scales and

frequencies and constitute a highly nonlinear multi-

frequencies system. There are more challenges and

opportunities in investigating the essential role of exci-

tation frequency by using synthetic VGJs in axial com-

pressors, which is our next object.

3.2 Effect of excitation location

Figure 4a illustrates the oil flow of unexcited case of

Ma = 0.3 and i = 10◦. The oil cumuli denote the separa-

tion region and the separation line is obvious at about

70%b. The boundary layer is turbulent from the lead-

ing edge since the Reynolds number on chord is up to

0.6 × 106, surpassing the transition Reynolds number.

Figure 4b shows the oil flow of typical excited case when

the excitation location is at 0.7b, just at the separation

line. By comparing Fig. 4a and b, it can be seen that the




Fig. 4 Oil flow visualization a unexcited case and b excited case. f e = 710Hz, ¯ A = 0.495, L = 0.7b, γ = 35◦, Ma = 0.3, i = 10◦

separation is eliminated entirely and the aerodynamicperformance is improved. When the location is at the

other three positions, no positive effect is obtained. It is

obvious that the excitation location plays also an impor-

tant role.

We guess that the boundary layer close to the sepa-

ration point is sensitive to external disturbances. Just

around this specific local region, the boundary layer

is receptive to unsteady excitations. The basic steady

flow has a high velocity component upwards in the local

region near the separation point, which will amplify the

effect of any nonlinear streaming and thus make local

oscillations an effective actuator in delaying the separa-tion on the blade. This amplification mechanism, which

cannot be found in attached flows, will enhance the pri-

mary entrainment and rolling-up coalescence inherent

in boundary layer, resulting in a reduced separation

region or even eliminating the separation entirely.

3.3 Effect of pitch angle

The pitch angle effect on the performance has been

given in Fig. 2. The further discussion is presented in

this section. One of the effects of blade separation is sig-

nificant widening and deepening of the blade wake. Cir-

cumferential surveys of the downstream total pressure

recover coefficient distribution across the blade pitch

are used as an indicator of the degree of this separa-

tion and the improvement by excitation. The compari-

son between unexcited flow wake and excited flow wake

for the three pitch angles is shown in Fig. 5. The loss due

to the viscous dissipation and the separation is serious

in the case of large incidence (i = 10◦). The increase

of total pressure by synthetic VGJs occurs only in the

Fig. 5 Wake total pressure recover coefficient for the three pitchangle a γ = 35◦, b γ = 50◦, and c γ = 90◦. f e = 710Hz, ¯ A = 0.495,L = 0.7b, Ma = 0.3, i = 10◦

boundary layer on the blade suction side, whereas the

main flow and the boundary layer on the blade pressure

side exhibit little change.

Synthetic VGJs generate streamwise vortices to inter-

act with the main flow and enhance cross-stream mix-

ing [6]. The strength of the streamwise vortices plays a

key role and is a strong function of the jet velocity and

the pitch angle. The vortex is strongest at γ = 35◦ and

weakest at γ = 50◦. The stronger the vortex, the better

the positive effect. VGJs are typically configured with

a low pitch angle (30◦–45◦) and aggressive skew angle

(45◦–90◦) [7]. However, the positive effect is also obvi-

ous at γ = 90◦ by using synthetic VGJs although the



526 X. Zheng et al.

Fig. 6 Wake total pressure recover coefficient at Ma = 0.7.γ = 35◦, f e = 710Hz, ¯ A = 0.47, L = 0.7b, i = 10◦

positive effect is slightly weaker than that of γ = 35◦, at

least in our experiment.

3.4 Effect of compressibility

The experiments are mostly conducted at Ma = 0.3.

In order to investigate the effect of compressibility, a

few additional experiments were also carried out at

Ma = 0.5 and 0.7. Figure 6 presents the comparison

between unexcited flow wake and excited flow wake

at Ma = 0.7. Even the Mach number is up to 0.7, which

is near the real velocity at blade-root of high-pressure

compressor, the positive effect is also obtained, and the

relative reduction of loss coefficient δ() is 5.1%. Com-

pared with the condition of Ma = 0.3 (see Fig. 5a, the

δ() is 18%), the positive effectdecreases moderately atMa = 0.7. The relative excitation amplitude ¯ A decreases

from 0.495 to 0.21 when the main flow velocity increases

from about 102 m/s (Ma = 0.3) to 238 m/s (Ma = 0.7).

It could be predicted that the positive effect would be

larger with higher jet velocity. In addition, the widening

and deepening of the wake at high Mach number lead to

the increase of loss coefficient, which is another reason

that the relative reduction of loss coefficient decreases

at high Mach number.

4 Conclusions

Active separation control is successfully demonstrated

on axial flow compressor blades by using synthetic vor-

tex generator jets. The experiments were performed in

a high-speed plane cascade wind tunnel by using a typi-

cal blade-root shape under high-loading conditions (i =

10◦), where the separation from the suction side of the

blade occurred at about 70% of chord length from the

leading edge. The experiment results showthat the aero-

dynamic performancecan be substantially increased and

the maximum relative reduction of loss coefficient is

22.8%. The separation is almost eliminated and the total

pressure recovery coefficient is enhanced only in bound-

ary layer on the blade suction side instead of an entire

improvement along the pitch space.

The role of some parameters, such as excitation fre-

quency, amplitude, location, and pitch angle, are stud-ied at Ma = 0.3. Unfortunately, the role of excitation

frequency is not demonstrated since the jet velocity

is dependent on the frequency in the present experi-

ment. The positive effect is obtained at the resonance

frequency of loudspeaker where the jet velocity is max-

imal and exceeds the threshold. The results show that

the jet amplitude plays an important role. In order to

investigate the role of frequency and take advantage

of an instability mechanism, it is necessary to devise a

new fluidic actuator, the jet velocity of which is indepen-

dent on the frequency. The test were carried out at four

excitation locations, i.e. L = 15, 25, 50, 70%b, and pos-itive effect was obtained at L = 70%b only where the

incipient flow separation occurred. The positive effect

reaches the maximaum at γ = 35◦ and the minimum at

γ = 50◦ in the three cases of pitch angle with respect

to the main flow (i.e., 35◦, 50◦, 90◦). The effect of com-

pressibility was also investigated. Even when the Mach

number is up to 0.7, which is near the real velocity at

blade-root of high-pressure compressor, positive effect

is also obtained, and the relative reduction of loss coeffi-

cient is 5.1%. It can be predicted that the positive effect

would be larger with higher jet velocity. These results

lead the authors to conclude that the application of thesynthetic vortex generator jets for the separation control

on axial compressor blades shows great promise.

Acknowledgments The authors are deeply grateful to Jian Cui,Yang Gu, Zhiwei Liu, Shenghong Peng, Hui Wang, Jian Yang,Hong Li, and the entire 301SB crew at the China Gas TurbineEstablishment for their efforts to make this experiment possible.The author Xinqian Zheng would also like to thank Prof. YajunLu, Qiushi Li, Wei Yuan and Liyun Ren for their great support.In addition, this works are greatly supported by National KeyLaboratory of Diesel Engine Turbochargering Technology.

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separation control using synthetic vortex generator jets in axial compressor cascade

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