[American Institute of Aeronautics and Astronautics 48th AIAA Aerospace Sciences Meeting Including...

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Northrop Grumman Case 09-2367 Dated 12/14/09

1

Experimental Investigation of Actuators for Flow Control in

Short Inlet Ducts

John C. Vaccaro1, Yossef Elimelech

2 and Michael Amitay

3

Rensselaer Polytechnic Institute, Troy, New York, 12180, USA

An experimental investigation of flow control actuation concepts on a short inlet duct was

conducted. Concepts tested were steady blowing from both a two-dimensional jet and a

spanwise varying jet. Experiments were run at an inlet Mach number of 0.43 on a very

short inlet with a length to diameter ratio of 1.5. Spanwise varying jet actuation was found

to be more beneficial compared to two-dimensional blowing in that the secondary flow

structures could be manipulated. An optimal jet location was determined to be at the

position of maximum spanwise velocity. However, the spanwise varying actuator could only

manipulate the location of the secondary flow structures, not mitigate them. Both actuators

resulted in a significant decrease in the spectral content of the total pressure at the AIP,

especially around the centerline.

Nomenclature

σ = standard deviation

AIP = aerodynamic interface plane

DAIP = aerodynamic interface plane hydraulic diameter, [m]

h = jet slit height, [mm]

L/D = inlet length-to-diameter ratio

= mass flow entering inlet, [kg/s]

= mass flow through control jets, [kg/s]

= mass flow ratio

Po = total pressure, [Pa]

PR = pressure recovery

Pxx = power spectral density (PSD) of the pressure signal, [Pa2/Hz]

I. Introduction

HE inlet to an aircraft propulsion system is typically designed to supply flow to the compressor with minimal

pressure loss, distortion, or unsteadiness. Otherwise, the overall system performance will be reduced, which

can result in stall or surge of the compressor and a catastrophic failure of the engine. Attractive to aircraft designers

are compact inlets, which implement curved flow paths to the compressor face. While the inlet length that is

required to avoid separation and its associated losses may not be a significant design driver for some vehicles, in

other configurations (such as Unmanned Air Vehicles, UAV’s), the inlet geometry typically drives the overall

vehicle length.

Low length-to-diameter ratio inlets have high degree of centerline curvature, which inevitably causes flow

separation. Furthermore, such a rapid centerline curvature and the fact that the velocity is not uniform across the

inlet (due to the duct walls) result in cross-stream pressure gradients and the onset of secondary flows. The

combined effects may result in pressure non-uniformity (i.e., distortion) and total pressure loss at the inlet exit.

Technologies such as passive or active flow control that can alter these distorted baseline flows may allow the

design of efficient low length-to-diameter ratio inlets and lead to a significant overall system benefit.

1 Graduate Research Assistant, MANE Department, 110 8

th Street, Troy, NY 12180

2 Post-doctoral scholar, MANE Department, 110 8

th Street, Troy, NY 12180

3 Associate Professor, MANE Department, 110 8

th Street, Troy, NY 12180, and Associate Fellow, AIAA

T

48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition4 - 7 January 2010, Orlando, Florida

AIAA 2010-862

Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.



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The application of flow control devices for inlets has been investigated since the late 1940s using vortex

generator vanes to re-energize the boundary layer to prevent flow separation. In the early 1970s it was demonstrated

that vortex generators can result in restructuring of the development of the secondary flow and thus improving

engine face distortion. This suggests that in order to effectively improve the inlet performance, separation control

alone is not sufficient and a global manipulation of the secondary flow must be implemented. This approach calls

for the use of flow control actuators to alter the pressure loss and distortion of the base flow. While many studies

explored active flow control in inlets, very few implemented closed-loop controllers that incorporate unsteady

actuation.

In the last two decades different control techniques have been experimentally implemented on low L/D inlets:

vortex generators, steady blowing, unsteady blowing, and more. Lockheed Martin and NASA implemented both

micro-vane type vortex generators and vortex generator jets on a duct with a length to diameter ratio of 2.5 at an

inlet flight Mach number of 0.6 (Hamstra et al., 2000). In their study, an array of 36-micro vanes was shown to have

beneficial effect on pressure recovery, with somewhat mitigated results with the VGJ. Luers (2003) conducted a

parametric study on an aggressive inlet using pulsating injection at the separation line at a flight Mach number of 0.6

with very promising results.

The present manuscript is a comparative study on the effect of flow control actuation systems on a low aspect

ratio short duct with a length to diameter ratio of 1.5. All experiments were performed at an inlet Mach number of

0.43. Base flow characteristics of this inlet include two locations of flow separation, one on the upstream lower

surface, and a second on the downstream upper surface. An earlier effort (Vaccaro, et al. 2009) looked into flow

control actuation at both these locations. The study showed that the simple steady blowing actuation system being

employed was not optimal. Therefore, the present study investigates different concepts of actuation by studying the

effect of two different control principles: steady blowing and spanwise variation of blowing through five Coanda-

type injectors. In order to simplify the problem, actuation is applied only on the upstream lower surface. Following

the understanding of the effect of actuation on the flow around the upstream turn, the effect of flow control on the

downstream turn will be investigated. Quantification of actuator performance will be based both on the total

pressure recovery (PR) and pressure unsteadiness at the AIP and along the lower duct surface.

II. Experimental Setup

A. Wind tunnel facility

A high subsonic inlet duct facility was designed and built at Rensselaer Polytechnic Institute (Vaccaro et. al.,

2009a and 2009b). The facility can achieve Mach numbers of up to 0.5 and mass flow rates of up to about 2.3 kg/s.

The duct has length-to-diameter ratio of 1.5 with a rectangular cross-section (114.3 mm wide by 88.9 mm high) at

the inlet and a square cross section (114.3 mm wide by 114.3 mm high) at the outlet from the duct. Figure 1

presents the design layout of the facility, which includes a blower, a diffuser, a settling chamber, a contraction, an

inlet duct and a diffuser. The air from the blower enters a diffuser section, followed by a settling chamber. The

settling chamber incorporates a honeycomb and a screen to condition the air. The flow transitions from the settling

chamber to the test section through a contraction section, which has a standard fifth order polynomial curvature and

it is made from 3.175 mm thick stainless steel. The contraction starts from the settling chamber (1.22 m x 1.22 m)

and reduces to the inlet. The settling chamber to inlet cross-section area ratio is 142:1. The Mach number and mass

flow rate are measured at the end of the contraction, at a constant cross-sectional area region. Finally, a diffuser is

placed to minimize total pressure head before exhaustion to the room.

Figure 1. Experimental facility



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B. Inlet Duct

The inlet duct is a low aspect ratio short duct. It has a length-to-diameter ratio of 1.5 and starts with a

rectangular cross section of 88.9 mm x 114.3 mm and ends with a square cross section of 114.4 mm x 114.4 mm

(Figure 2). The walls of the test section are made of optical-grade Lexan, which was chosen for its optical qualities

to enable PIV measurements. The inlet is fully instrumented with static and high frequency pressure transducers

both along the duct’s upper and lower surfaces and at discrete locations across the AIP (Figure 3). The low surface

is equipped with eight high frequency pressure transducers (Kulite model XTL-140, denoted by numbers in Figure

2) and 82 static pressure ports, read by four Scanivalve DSA3217 (16 channels each) pressure scanners.

Figure 2. Inlet duct schematic

Figure 3. AIP measurement domain

C. 2-D Control Jet Actuator

The 2-D control jet actuator has one continuous slit (Figure 4a), which spans 101.6 mm and has a height of

0.5mm. The jet is a Coanda-type injector where the jet is issued parallel to the surface just upstream of the upstream

turn on the lower surface.

D. Spanwise Varying Jet Actuator

The spanwise varying actuator (Figure 4b) is comprised of five independently addressable jets. All five jets are

Coanda-type injectors and can be combined to form a continuous 101.6mm x 0.5mm exit slit (if all are activated at

the same exit velocity). The actuator is symmetrical with respect to the centerline, with the center jet having a

spanwise length of 30.48mm while the jets next to the center jet have a spanwise length of 19.5mm each and the

outer most jets have a length of 16mm.



4

Figure 4. Jet schematics for rotary valve actuator and spatially varying jet actuator

III. Results and Discussion

An earlier effort (Vaccaro et al., 2008 and 2009) has shown that flow control effectiveness is highly dependent

on the momentum coefficient (defined as the jet momentum divided by the inlet momentum), as well as on the

blowing ratio (defined as the jet velocity divided by the inlet velocity). Tests were performed for two different slit

heights, h = 0.5mm and h = 1mm, and for a given momentum coefficient, the performance enhancement was larger

when a thinner slit was used (i.e., larger blowing ratio at the same momentum coefficient). As a result, the current

experiments focus only on actuators with a 0.5mm slit height. Analysis of the flow control performance is

conducted using the time-averaged pressure recovery (PR) and a spectral analysis at specific points along the duct’s

lower surface and at the AIP (emphasis on the AIP lower half).

A. Steady Blowing from Two-Dimensional Control Jet – Effect of Mass Forcing

1. Time-averaged analysis at the AIP

As discussed earlier, the base flow of in the inlet is characterized by two locations of separations: one on the first

upstream lower surface bend and another on the downstream upper surface. These separations lead to areas of

reduced pressure recovery at the AIP. To address this phenomenon a local pressure recovery is introduced as the

local total pressure measured by each sensor at the AIP divided by the total pressure entering the inlet:

(1)

Figure 5 presents color maps of the local pressure recovery at the AIP. The marks in these color maps show the

actual locations of the pressure sensors (see Figure 3) for convenience. The local pressure recovery of the base flow

shows large areas of total pressure loss on the upper and lower portions of the duct while high pressure recovery is

present in the center of the duct (i.e., the core flow). Three-dimensional flow structures that evolve due to the

curvature in conjunction with the pressure gradient induced by the sidewalls (Tournier 2005) are seen in the base

flow (Figure 5a). These structures induce velocity up from the lower surface at the centerline and toward the lower

surface along the sidewalls. Evidence of these structures can be seen in Figure 5a where the pressure recovery is

higher along the lower sides of the sidewall. The induced velocity of the counter rotating vortices brings the high

momentum of the core flow closer to the lower surface at the sidewalls and at centerline the induced velocity

introduces low momentum fluid into the core flow. Flow control, via the two-dimensional control jet, is used to

suppress the flow separation at the bottom of the duct (i.e. the upstream turn) and the evolution of the secondary

flow structures, where the forcing level is quantified by the mass flow ratio, defined as the mass flow of the control

jets divided by the mass flow entering the inlet:



5

(2)

Using the 2-D control jet, the effect of mass forcing was studied. Using a mass forcing level of 0.53% (Figure

5b) results in an improvement over the baseline case at the centerline; however, the secondary flow structures are

still dominant in the lower corners of the duct (Figure 5b). Higher levels of mass forcing has little effect on the

pressure recovery at the AIP, as the averaged pressure recovery increased only by 1% from all different mass forcing

levels. All actuated cases yield the same qualitative flow structures (Figure 5b-d), suggesting that a two-

dimensional control jet can only affect centerline reattachment can not mitigate the formation of secondary flow

structures.

Figure 5. Effect of mass forcing on the pressure recovery at the AIP for the 2-D control jet actuator, (a)

baseline, (b) , (c) , and (d)

2. Spectral Analysis of the Pressures Measured at the AIP

The instantaneous pressure signals measured at the AIP were examined by conventional spectral analysis

methodologies implemented in MATLAB. Figure 6a-d shows color contours of the pressure signal standard

deviation normalized by the inlet total pressure for the baseline case and the three mass forcing cases presented in

Figure 5. While red areas reflect pressure undulations up to 10% of the inlet’s total pressure, the deep blue depicts



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areas of zero pressure undulations. High values of the pressure standard deviation can be seen in the baseline case in

the lower half of the AIP (Figure 6a). In the lower portion of the AIP, the two-dimensional control jet helps to

diminish the pressure fluctuations caused by the separating shear layer (Figure 6b-d). Actuation is more effective

toward the core flow region than close to the surface.

In order to explore the frequency that are responsible for the pressure unsteadiness, the cumulative PSD was

calculated for Kulites 26 and 30 and is presented in Figure 7. For the baseline cases, the PSD throughout the

frequency domain is higher farther from the lower wall. Note that at both locations there is a jump in the PSD ~ 350

Hz, which corresponds to the shedding frequency of the separated mixing layer. When flow control is applied, there

is a reduction of the PSD throughout the domain and the jump at 350 Hz is not present, suggesting that flow

separation was mitigated. Furthermore, for all forcing levels, the effect of flow control is more pronounced on

Kulite 26, which is located closer to the center of the duct.

To depict the effect of actuation on the separated mixing layer, color maps of the cumulative power spectral

density in a frequency window between 330 Hz – 370 Hz are provided in Figures 8a-d. The baseline case (Figure

8a) is characterized by high value of cumulative power spectral density, especially on the upper and lower portions

of the AIP, while the forced cases clearly show a considerable reduction of pressure undulations at this frequency

range. The cumulative power spectral density contours are quite similar for all forcing levels in this frequency

window (Figure 8b-d). Yet, the overall standard deviation of the signal is lowest for the largest mass forcing case.

Figure 6. Normalized pressure standard deviation at the AIP for the 2-D control jet actuator, (a) baseline, (b)

, (c) , and (d)



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Figure 7. Spatial dependence of cumulative PSD for two dimensional slit actuator.

Figure 8. Normalized pressure standard deviation between a 330 Hz – 370Hz frequency window at the AIP

for the 2-D control jet actuator, (a) baseline, (b) , (c) , and (d)



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The spanwise pressure variations caused by flow control actuation seen in the time-averaged pressure data

(Figure 5b-d) are also present in the unsteady component of the signal. This is evident by comparing the spectral

content of two pressure sensors located at the lower half of the AIP (Figure 9). For the base flow the cumulative

power spectral densities, at both locations, are similar (there is a larger jump in the PSD at 350 Hz for Kulite 27).

When actuation is applied, there is a significant reduction of the PSD at both spanwise locations (compared to the

corresponding baselines), where the PSD is lower at the centerline location. Furthermore, at both locations the jump

at the cumulative PSD is suppressed.

Figure 9. Comparison on the effect of high forcing level at AIP for Kulites 27 and 31.

The effect of the forcing level on the cumulative spectral density is presented in Figure 10. For all three forcing

levels the effect of actuation is a reduction in the cumulative PSD. Cumulative PSD is significantly reduced

(compared to the baseline) in all forced cases and the effect is similar.

Figure 10. Effect of forcing on the spectral content at AIP locations: (a) Kulite 27 (b) Kulite 31.



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B. Spanwise Varying Actuator

1. Time-averaged analysis at the AIP

As was shown in the previous section, the two-dimensional control jet does not reduce the losses at the off-

centerline locations (Figure 5); therefore, the effect of a spanwise varying jet (Figure 4) was investigated. Figure

11a-d shows pressure recovery distributions at the AIP for various spanwise forcing distributions. Figure 11a shows

the baseline case, which features the total pressure reduction caused by flow separation of the upper and lower

surfaces as well as the secondary flow structures. Figure 11b presents the pressure recovery distribution for the case

where the all the control mass flow ( ) is through the center jet. The pressure recovery is improved along

the centerline, where all the forcing is concentrated, while the secondary flow structures are forced into the lower

corners. Figure 11c shows the case where all the control mass flow is through the inner jets. This case has similar

trends as the case presented in Figure 11b, but with better performance (i.e., better pressure recovery). It is worth

noting that here the control jets are at the locations where the spanwise velocity is at its maximum (by means of

symmetry). This might be the reason why the inner jet configuration was found to perform better than the other

configurations. Finally, the control mass flow is through the two outer most jets (Figure 11d), where the forcing

causes the two secondary flow vortices to reside near the center of the duct. The pressure recovery is increased at

the locations of the jets; however, the separation along the centerline is not suppressed, and actually worsens the

pressure recovery at the lower centerline. It is to be noted that the average pressure recovery at the AIP is not

significantly improved under any of the actuation schemes.



10

Figure 11. Spanwise varying actuator pressure recovery at the AIP for the spanwise varying actuator,

. (a) baseline, (b) center jet, (c) inner jets, and (d) outer jets.



11

2. Unsteady AIP Pressure

The unsteady total pressure at the AIP was calculated from the pressure data and is presented in Figure 12. The

baseline case was discussed earlier and is presented here for reference. When flow control is activated, the standard

deviation of the pressure in the lower separation region is reduced for the center jet case and the inner jets cases

(Figure 12b and c, respectively). Note that the effect is slightly larger when the inner jets are used, which

correspond to the enhanced reattachment at the centerline. When the outer jets are activated (Figure 12d), the

standard deviation is spatially altered yet still high (similar to the baseline case), which may be due to the fact that

the outer jets do not have the ability to reattach the flow along the centerline.

Figure 12. Normalized pressure standard deviation at the AIP for the spanwise varying actuator,

. (a) baseline, (b) center jet, (c) inner jets, and (d) outer jets.

The power spectral density was integrated over a frequency window of 330 Hz – 370 Hz (around the shedding

frequency) and is presented in Figures 13a-d. For all actuation schemes, flow control reduces the energy content

about the 350 Hz frequency. Both the center jet case and the inner jets case (Figure 13b and c, respectively) show

large reductions of the cumulative power spectral density in this frequency window, which further proves that the

separation is suppressed by actuation. The outer jet actuation also shows reduction in the pressure unsteadiness in

this frequency range but not to the same extent (Figure 13d). Interestingly, comparison of Figure 12d Figure 13d

shows that the outer jets do reduce the frequencies in the 330 Hz – 370 Hz range, yet the standard deviation of the

entire signal is increased.



12

Figure 13. Normalized pressure standard deviation between a 330 Hz – 370Hz frequency window at the AIP

for spanwise varying actuator, . (a) baseline, (b) center jet, (c) inner jets, and (d) outer jets.

To further explore the effect of the spanwise varying actuator, the power spectral density of the centerline Kulite

number 27 was calculated and is presented in Figure 14. Figure 14a shows the spectra of the baseline case as well as

the three forcing schemes (center jet, inner jets, and outer jets). Without flow control, there is a distinct peak in the

spectrum, which corresponds to the shedding frequency of the separated mixing layer. When control is applied, the

peak at ~350 Hz is either diminished (when the control jet or the inner jets are used) or reduced (when the outer jets

are activated. This is also evident from the cumulative PSD, shown Figure 14a.



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Figure 14. Effect of spanwise forcing variation on spectral content of Kulite 27; (a) PSD, and (b) cumulative

PSD.

3. Surface Pressure Measurements

In addition to the total pressure measurements at the AIP, surface pressure data were acquired. The focus in this

section is on the unsteady surface data (note that the effect of flow control on the steady surface pressure was

discussed in details in our previous work, Vaccaro et al., 2009a and b). The effect of the spanwise varying jet on the

spectra (left column), as well as on the cumulative PSD (right column) on- and off-centerline (Kulites 7s and 8s), is

presented in Figure 15. The unsteady baseline flow is quite two-dimensional with a distinct peak at 350 Hz (note the

second peak at 4 Hz, which corresponds to the structural resonance of the facility). When the outer jets are activated

(Figure 15a), the spectrum along the centerline is different than off-centerline, suggesting that the placement of

actuation can have a three-dimensional effect on the frequency content of the flow field. When forcing is applied

through the inner jets (Figure 15b), the flow control makes the surface’s cumulative power spectral density quite

uniform. The center jet case (Figure 15c) has also some three-dimensionality but not as severe as the outer jet case.

Note that for all three actuation schemes there is an increase in the cumulative PSD (compared to the baseline),

except for the out jets actuation off-centerline (Kulite 8, Figure 15a). This is still under investigation.

These results demonstrate the ability of spanwise varying forcing to manipulate the secondary flow structures.

However, it is clear that spanwise forcing alone is inefficient to eliminate these structures, which dominate the AIP

and cause undesirable total pressure loss and unsteadiness.

(a)

(b)



14

Figure 15. Spanwise surface effects on the spectral content along the surface (Kulites 7s and 8s) for the

spanwise varying jet actuator: (a) center jet, (b) inner jets, and (c) outer jets.

IV. Conclusion

The flow control actuation, via a two-dimensional steady Coanda jet and spanwise varying jet, was tested and

quantified by both the time-averaged pressure recovery and its spectral content at the AIP and the lower wall of a

low length-to-diameter ration inlet duct. Applying a two-dimensional steady jet was shown to have effect on the

duct centerline but could not improve off centerline pressure recovery. This was due to the fact that two-

dimensional steady blowing could not manipulate the three-dimensional flow structures, which develop in the

curved duct. It did, however, reduce the standard deviation of the pressure field in that region. The most effective

actuation scheme (in terms of improved pressure recovery and reduction in unsteady pressure magnitude) was when

two jets, placed at quarter of the duct’s width from each sidewall, were used. We believe that it is due to the fact

that the two jets were applied at the location of the maximal spanwise velocity. Although the spanwise forcing did

alter the location of the three-dimensional structures, it did not totally diminish them. Future work will include the

introduction of steady and unsteady vortex generator jets along with a tangential blowing jet to introduce counter

vorticity that hopefully will minimize the formation and the growth of these three-dimensional structures.

(a)

(b)

(c)



15

Acknowledgments

This work is supported by Northrop Grumman Corporation (program manager is Ms. Florine Cannelle). The

help of Brian Belley, Joseph Vasile, Wasif Khan, Sam Langendorf, and Kristen Gurekovich in designing and

assembling the experimental facility is greatly appreciated.

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