NASA Contractor Report 178249 

t 

# 

TURBULENCE CONTROL OF AN AIRBORNE LASER PLATFORM 

Mohamed Gad-el -Ha k 

FLOW INDUSTRIES, INC. 

F1 ow Research Company 

Kent, Washington 

Contract NAS 1- 18213 

March 1987 

AIIBBGPNE (BASA-CR-178249) LASE& ZLdgEGEE TOREULEICE Final CCIZGOZ Technical Oh' AB Y87-20503 

lieport, 1 Jan, - 30 JUn. Is66 (PlOU 

0 &€search8 he,) 55 p CSCL 20D Unclas 

63/34 45369 

s 

National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton,Virginia 23665 

. 
a 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

PHASE I SBIR - FINAL TECHNICAL REPORT 

PROJECT TITLE 

CONTRACT NUMBER 

ISSUED BY 

TECHNICAL MONITOR 

CONTRACTOR 

PRINCIPAL INVESTIGATOR : 

DURATION 

PROJECT SUMIARY 

Turbulence Control on An Airborne Laser Platform 

NAS1-18213 

NASA-Langley Research Center 

John C. Lin 

Flow Industries, Inc. 

Mohamed Gad-el-Hak 

1 January 1986 - 30 June 1986 

An active flow control device to generate large-scale, periodic structures 

in a turbulent shear flow is developed. 

device may be used on airborne laser platforms to reduce or eliminate optical 

distortion caused by the turbulence in the aircraft's boundary layer. 

jet issuing from a spanwise slot is used to collect the turbulent boundary 

layer for a finite time and then release all of the flow instantaneously in 

one large eddy that convects downstream. 

measurements are used together with pattern recognition algorithms to demonstrate the viability of the flow control method. 

channel is used as a test bed. The instantaneous velocity signal is used to 

compute important statistical quantities of the random velocity field, such as 

the mean, the root-mean-square, the spectral distribution, and the probability 

density function. When optimized for a given boundary layer, it is shown that 

the cyclic jet will produce periodic structures that are similar to the random, 

naturally occurring ones. 

events near the wall of the plate. 

structures in both the outer and inner regions of a turbulent boundary layer. 

Together with adaptive optics, the 

A cyclic 

Flow visualization and hot-film probe 

A flat plate towed in a water 

These structures seem to trigger the onset of bursting 

Thus, the present device generates periodic 

1 

L- 
. 

I 

TABLE OF COIoTEmS 

Page 

Project Summary 

Table of Contents 

1. Introduction 

2. Large-Eddy Generating Device 

3. Experimental Approach 

3.1 Towing Tank 

3.2 Flat Plate 

3.3 Large-Eddy Generating Device 

3.4 Visualization Methods 

3.5 Velocity Measurements 

3.6 Pattern-Recognition Algorithms 

Periodic Injection in the Absence of Boundary Layer Flow 

Periodic Injection into a Turbulent Boundary Layer 

Effects of Artificial Large-Scale Structures on Bursting 

4. 

5. 

6. 

7. Spectral Analysis 

8. Probability Distribution 

9. Summary 

References 

1 

2 

3 

5 

12 

12 

12 

12 

17 

17 

18 

20 

25 

33 

39 

45 

49 

51 

2 
1. IRTRDDUCTION 

Short-wavelength laser propagation from an airbofne platform is hampered - 

by optical distortion caused by beam scattering as it passes through the aircraft boundary layer and shear layer flows. 

by the small-scale size of the turbulence in these regions (Chernov, 1967). 

The large-scale structures in the flow are random, and their arrival time at a 

particular point in the boundary layer is not a deterministic variable, making 

it very difficult to devise corrective measures for reducing or eliminating 

the optical distortion. 

The scattering is mostly caused 

A typical airborne platform has a blunt turret that causes the flow to 

separate in its leeward side. At high subsonic Mach numbers, turbulence and 

flow separation result in a high level of density fluctuations which adversely 

affect the electromagnetic radiation transmitted from the aircraft. 

the optical transmission quality of the flow field, the unsteadiness in the 

turbuient fiow reghiis r;;i;st be redwed by using passive or active flow control 

devices. 

quantify the beam degradation due to the flowfield around a cylinder-hemisphere 

body. 

the density fluctuations and turbulent correlation lengths in the shear layer. 

Their results, when incorporated into the optical propagation theory, predicted 

very low intensity in the farfield. 

To improve 

de Jonckheere et al. (1982) conducted a wind tunnel experiment to 

Holographic interferometry and hot-wire anemometry were used to obtain 

de Jonckheere and Chou (1982) developed scaling relationships to allow 

extrapolation of wind tunnel test results to full-scale flight conditions. 

a later paper, de Jonckheere and Chou (1983) considered classical active flow 

control techniques including boundary layer suction and blowing. 

improvement in the optical transmission quality of the flow field was achieved. 

Moreover, the classical control methods represented an unacceptable level of 

power drain from the aircraft. 

In 

No net 

Smith et al. (1985) tested several fairing configurations surrounding the 

The passive fairings caused some reduction in the unsteadiness 

While such’passive flow conblunt turret. of the separated flow region behind the turret. 

trol methods do not represent significant power drain, their overall effectiveness is perhaps limited. 

power requirements, are obviously needed for significantly improving the 

optical transmission quality of the flow field around the airborne platform. 

Innovative active flow controi devices, with modest 

3 
By producing suitable periodic structures as the dominant feature of the 

flow.field, the possibility of developing adaptive optics to compensate for 

the optical distortion is increased. The present investigation is aimed at 

eLploring the feasibility of generating large-scale, periodic structures in a 

highly turbulent flow field. 

ture will be investigated. 

two-dimensional jet issuing from the wall of a boundary layer to collect the 

turbulence for a finite time and then release the collected fluid instantaneously in one large eddy that convects downstream once every period. 

second method involves a turret-like device to divert the airplane's boundary 

layer away from the path of the laser beam. 

will then be controlled by subharmonic perturbation. 

experiments were conducted using a flat plat towed in a water channel to test 

the first active control method. The control of the free shear layer using 

subharmonic perturbation will be the subject of a subsequent report. 

Two methods for altering the turbulence strucThe first active flow control method is a cyclic, 

The 

The resulting free shear layer 

In the present paper, 

The concept of large-eddy generation is detailed in Section 2. Description 

of the experimental setup is given in Section 3. 

the results of the flow visualization and fast-response velocity-probe measurements conducted to demonstrate the concept of artificial generation of large 

eddies in a turbulent boundary layer. Finally, a brief summary is presented 

in Section 9. 

Sections 4 through 8 contain 

4 
2. LARGE-EDDY GBMERATI#G DEVICE 

Two different ideas for generating andlor reinforcing the large-scale 

eddies in turbulent shear flows are sought. The first method will alter an 

existing turbulent boundary layer by creating larger eddies in a controlled 

manner. The second method uses a turret-like device to divert the laser beam 

away from the airplane's boundary layer. In its stead, a free shear layer of 

smaller thickness is formed and a means of controlling its eddies and dynamics 

is proposed. 

element of control over the eddies within the flow field. 

In the past, similar techniques have been shown to impose an 

A large eddy generating device is sought for altering an existing turbulent 

boundary layer. 

within the boundary layer for a finite time period and then release it instantaneously in one large eddy to pass downstream. 

period, the flow downstream of the device would be free of the existing 

turbulence. 

and the cycle would then repeat itself. 

In principle, the device would collect the turbulent fluid 

Thus, during the collection 

This period would be followed by the passage of one large eddy, 

An idealization of this concept, which utilizes a deformable airfoil, is 

illustrated in Figure 1. 

the airfoil is presumed flush with the surface. 

edge rises above the turbulent boundary layer of thickness 6, but the trailing edge remains attached to the surface. 

is entrapped between the airfoil and the surface. 

leading edge continues to rise, although at a decreasing rate, such that all 

of the incoming turbulent fluid is collected. 

deform, i.e., alter its camber, so that separation would not occur on the upper 

surface, hence ensuring a new, thin boundary layer at its trailing edge. 

soon as the limits of the deformation are reached and separation is imminent, 

the airfoil would quickly resume its initial flush position, thus releasing 

all of the collected fluid. 

At the beginning of the cycle, the upper surface of 

At time t > 0, its leading 

Thus, the turbulent boundary layer 

As time progresses, the 

Ideally, the airfoil would also 

As 

One practical means of realizing the above idealized concept is to use a 

small, two-dimensional, cyclic jet as shown in Figure 2. The jet width d is 

typically much less than the boundary layer thickness, the jet spanwise length 

W is of the order of the laser beam diameter, and the jet velocity varies 

during the period P. 

layer will be entrapped and form a large eddy. 

boundary layer must result since it is deprived of much of its turbulence. 

As the fluid issues from the jet, the exieting boundary 

Meanwhile, downstream a thinner 

In 

5 
00 RELEASED TURBULENCE U 

(LARGE-SCALE, PERIODIC STRUCTURE) 

COMING TURBUENT 

OUNDARY LAYER . 

a. Beginning of Cycle. Airfoil Flush with Surface. 

The Generated Large Eddy from the Previous 

Cycle is seen Downstream 

W 

U 

FREQUENCY f 

COLLECTED TURBULENCE DEFORMABLE CAMBER 

b. Airfoil at Mid-Cycle, with the Entrapped Turbulence 

Figure 1. An Idealized Device for Generating Large Eddies 

6 

t 

~ ~~ ~~ ~~ 

~ ~~ 
TIME 

P 

U 

-DFigure 2. Large Eddy Generating Device Consisting of an Oscillating 

Jet of Fluid which Periodically Blocks the Normal Passage of 

the Turbulent Boundary Layer 

7 
the early stages of the cycle, the jet velocity could increase at a sufficient 

rate to contain most of the turbulence. 

no further velocity increase is possible. 

ately and the collected turbulence would be swept downstream as one large eddy 

and the cycle would begin again. 

downstream would have a very thin layer over it during most of the period, 

followed by the passage of one large eddy which is exactly periodic. This 

method could be further enhanced by using suction over part or all of the 

distance L in Figure 2. 

could further reduce or eliminate the thin boundary layer during the period 

that the turbulence is being collected upstream by the jet. 

Eventually, a point is reached where 

The jet would be shut off iuunediConsequently, the protected area of size D 

When phase locked with the pulsating jet, the suction 

Techniques similar to the method described above have been successfully 

used for generating and/or controlling the large eddies in turbulent boundary 

layers. 

to produce large eddies in boundary layers with zero and adverse-pressure 

gradients. 

were able to decrease the natural separation and improve the diffuser's performance. Alternatively, thin ribbons could be placed in the boundary layer and 

oscillated. Corke (1981) and Bushnell (1983) have shown that, at zero angleof-attack without oscillation, a thin ribbon placed away from a flat plate and 

parallel to it leads to a reduction in the skin friction of the boundary layer. 

Corke (see his Figure 26) has also found that at negative attack angles, these 

devices enhance the large-eddy structure; thus, oscillating these devices in a 

pitching mode would suggest that a series of large and smaller eddies would be 

produced. However, it is felt that the pulsating jet has greater practical 

advantage because it is easier to control and has no external moving parts. 

In addition, by properly synchronizing the jet ' s amplitude, angle and acceleration, a great degree of controllability would appear to be achievable with the 

proposed jet. 

(1985; 1986a; 1986b) has led to a pulsating jet device that controls the 

formation and growth of the large bound vortices on delta wings and similar 

1 ifting surfaces, 

Viets (1980) used an asymmetrical rotating cam embedded in the wall 

By using this device in a wide angle diffuser, Viets et al. (1981) 

Similar developmental research by Gad-el-Hak and Blackwelder 

The second method of interfering with the boundary layer on the plate is 

to divert it away from the path of the laser beam as illustrated in Figure 3. 

An aerodynamic window (Russell, 1974; Christiansen et al., 1975) may be 

contained inside the turret-like device shown in the figure. It is assumed 

8 
PERTURBATION 0 \rL U BY INJECTION 

(FREQUENCY fl 7 

’ ARGE-SCALE, \ 

PERIODIC STRUCTURES 

d 

t 

Figure 3. A Diversion Fairing Used to Divert the Laser Beam Away from 

the Airplane’s Boundary Layer. The Insert Shows a Small 

Perturbation Introduced at the Lip of the Fairing to Control 

the Resulting Free Shear Layer 

9 
that a region of size D is to be protected from the normal boundary layer so 

that radiation emitted through this region will not suffer distortion by the 

turbulence. Thus, a fairing of height H, which is larger than the boundary 

layer thickness 6, is erected around the region D. 

layer to pass by the side of the turret-like device, thus providing an undisturbed path for the laser beam. 

from the problem, a turbulent free shear layer will be formed at the lip of 

the fairing, thus providing another source of distortion. However, this free 

shear layer can perhaps be readily controlled, as described below. 

This forces the boundary 

Although the boundary layer is eliminated 

In an uncontrolled environment, Brown and Roshko (1971; 1974) have shown 

that a free shear layer flow consists of vortical eddies having length scales 

comparable to the layer thickness. 

that these eddies form by an amalgamation process called pairing, which usually 

consists of the merging of two neighboring eddies to form a single larger eddy. 

Ho and Huang (1982) have shown that whenever a subharmonic perturbation is 

added to the flow field, amalgamations of 3,4,...,10 vortices can be accomplished during one cycle, which leaves a more stable flow field consisting of 

larger vortices that are relatively inactive; i.e. they do not participate in 

any further amalgamations for a considerable distance downstream. 

(1978) used this method to generate and control the eddies formed behind a 

splitter plate in a mixing layer. 

Winant and Browand (1974) further showed 

Oster et al. 

To control the growth and development of the mixing layer forming at the 

top of the proposed diversion-fairing device, a subharmonic disturbance may be 

introduced at the lip of the fairing. 

indicate that eddies of size U,/f (where € is the forcing frequency) can 

be generated over a fairly broad range of frequencies. 

(1984) have shown that a mixing layer can be controlled even in the presence 

of a noisy background by simply increasing the amplitude of the subharmonic 

perturbation. Thus, large-scale, periodic structures may be artificially 

generated in the free shear layer at the lip of the diversion fairing. 

structures will replace the random ones occurring naturally. 

radiating out of the turret-like device will encounter a series of periodic 

structures as the dominant feature of the flow field, and the possibility of 

developing adaptive optics to compensate for these organized structures is 

then greatly increased. 

The results of Oster et al. (1978) 

Also, Ho & Huerre 

These 

A laser beam 

10 
One possible problem with the diversion-fairing device applied to an 

airborne platform would be its directional sensitivity. 

changes its direction of motion, the free shear layer flow and its natural 

shedding frequency would change. 

periodicity of the large-scale structures. 

plate at the top of the turret-like structure, its directional sensitivity 

will be greatly reduced. 

of the free shear flow under consideration. In contrast to two-dimensional 

and axially-symmetric flows, perturbing such a complex three-dimensional flow 

may not be straightforward and very little is available in the open literature 

on how to handle the additional complications introduced by the geometry. 

Some research along these lines is obviously needed. 

Whenever the airplane 

This will make it difficult to control the 

However , by adding a small splitter 

A second important issue is the three-dimensionality 

In the present paper, only the first method of artificially generating 

large eddies in a turbulent boundary layer is considered. 

free shear layer using subharmonic perturbations will be the subject of a 

subsequent report. 

The control of the 

11 
3. EXPERIMENTAL APPROACE 

3.1 Tawing Tank 

Turbulent and laminar boundary layers were generated by towing a flat plate 

in a water channel that is 18 meters long, 1.2 meters wide and 0;9 meter deep. 

The towing tank, shown in Figure 4, has been described by Gad-el-Hak et al. 

(1981) and Gad-el-Hak (1986~). 

carriage that rides on two tracks mounted on top of the tank. 

the carriage was supported by an oil film to ensure avibrationless tow, having 

an equivalent freestream turbulence of about 0.1 percent. 

towed by two cables driven through a reduction gear by a 1.5-hp Boston Ratiotrol 

motor. The towing speed was regulated within an accuracy of 0.1 percent. 

system was able to achieve towing speeds between 5 and 140 cm/s for the present 

study. 

20 cm/s. 

The flat plate was rigidly mounted under a 

During towing, 

The carriage was 

The 

However, most of the runs reported here were conducted at a speed of 

3.2 Flat Plate 

A unique, zero-pressure-gradient flat plate was constructed for the present 

investigation. 

of glass-reinforced polycarbonate plate 6 mm thick, glued to a stainless steel 

frame designed for minimum obstruction to the flow. The plate has an elliptic 

nose at the leading edge and an adjustable lifting flap at the trailing edge. 

To avoid leading-edge separation and premature transition, the flap is adjusted 

so that the stagnation line near the leading edge is located on the working 

surface of the plate. 

microns make it one of the most controlled test beds available for boundary 

layer research. 

working surface for towing speeds in the range of 5 to 80 cm/s. 

to generate a fully developed turbulent boundary layer. The trips are brass 

cylinders 0.32 cm in diameter and 0.25 cm high placed 5 cm downstream of the 

leading edge and having their axes perpendicular to the flat plate. 

The l-meter by 2meter structure, shown in Figure 5, is made 

The plate's smooth surface and flatness to within a few 

A laminar boundary layer can be obtained over the entire 

Trips are used 

3.3 Large-Eddy Generating Device 

The large-eddy generating device considered in the present investigation 

consists of a two-dimensional, spanwise, cyclic jet, shown schematically in 

12 

. 
Figure 4. Flow Research 18-Meter Towing Tank 

13 
FLOW r) 

Figure 5. Photograph of the Zero-Pressure-Gradient Flat Plate 

14 
Figure 2. 

turbulent boundary layer for a finite time and then release all of the flow 

instantaneously in one large eddy that convects downstream. 

parameters of this jet are its physical size d and W; amplitude of velocity 

U a frequency f; angle 0; and acceleration of jet velocity dU./dt. 

of the jet d is dictated by the boundary layer thickness, i.e., the jet must be 

smaller than the boundary thickness 6. 

the laser beam diameter. 

and cannot necessarily be examined independently. 

and ranges can be established; e.g., the frequency f of the artificial perturbation should match the mean frequency of the naturally occurring large-eddy 

structures. 

initial range of exploration uses a velocity amplitude of the order of U, and a 

reduced (or nondimensional) jet frequency in the range of 0.1 5 f6/Uai 1.0. 

The jet angle 8 is greater than 90' as defined in Figure 2; thus, several angles 

in this range should be explored. 

this angle was fixed at 0 = 135O. 

variation of the jet velocity) may not necessarily be sinusoidal; consequently, 

forward and rearward facing ramp functions, triangular functions, etc., should 

be explored. 

As mentioned in Section 2, the periodic jet is used to collect the 

The primary 

The width j' 3 

Its spanwise width W is determined by 

The other four parameters are clearly interrelated 

Nevertheless some guidelines 

Therefore, f must be of order U,/6 (Kovasznay et al., 1970). The 

In the initial phase of this investigation, 

The optimum acceleration (i.e. temporal 

TO accomplish the desired degree of control of the cyclic jet, the system 

shown schematically in Figure 6 was used. 

to pressurize a stainless-steel tank containing water. 

secondary fluid was controlled by using a pressure regulator for the air's 

supply line and a ball valve driven by a stepping motor for the secondary 

fluid's conduit. 

tions of the experimental apparatus using an APPLE I1 microcomputer. 

example, a slow opening followed by fast closing, a synunetric cycle followed 

by a given delay before the starting of a new cycle, etc. 

secondary flow passed through a small conduit to an internal reservoir directly 

behind a llmn-wide, 15-cm-long slot that was built into the flat plate. 

reservoir ensured that the ejection speed was uniform to within 2 percent 

along the entire spanwise slot. 

of the plate's leading edge. 

The laboratory's air supply was used 

The flow rate of the 

The duty cycle of the valve was controlled within the limitaFor 

The controlled 

This 

The slot vas located at x = 62 cm downstream 

15 
W 

-I> 

JJ 

<< to> 

9 Na 

-W 

5s 

p3 

1 

3 

0 

J 

LL 

f 0 

65 B 

W 

* 

U 

U 

w 

rc 

0 

16 
.- 

I 

3.4 Visualization Wethods 

To visualize the large eddies and near-wall events in the natural and 

perturbed turbulent boundary layers, both fluorescent dye and hydrogen-bubble 

techniques were used (Gad-el-Hak, 1986a; 1986b). 

the boundary layer through a spanwise slot 0.15 mm wide and 15 cm long and 

located 77 cm downstream of the leading edge of the flat plate. 

was milled at a 45' angle inclined toward the plate's trailing edge to minimize the flow disturbance. Dye was also introduced into the primary flow by 

mixing minute amounts of fluorescent dye with the secondary fluid in the 

pressure tank. 

wire having a diameter of 50 microns and a length of 10 cm. 

placed at different locations either parallel to or perpendicular to the flat 

plate to obtain top views or side views, respectively, of the flow field. To 

create time lines, a standard circuit was employed to supply intermittently 

100 volts to the wire, which acted as a cathode. 

The dyes were seeped into 

The dye slot 

The hydrogen bubbles were generated using a stainless-steel 

The wire was 

Both the fluorescent dyes and the hydrogen bubbles were illuminated using 

a sheet of laser light projected in the desired plane. 

degree of freedom in observing the large-eddy structures and the bursting 

events, because both the tracer and the light location could be controlled 

within the limitations of the experimental apparatus. In order to generate a 

sheet of light, a 5-watt argon-ion laser (Spectra Physics Model 164) was used 

with a mirror mounted on an optical scanner having a natural frequency of 

720 Hz (General Scanning, Inc.) and driven by a sine-wave signal generator of 

the desired frequency. 

to the inverse of the shutter speed of the camera. 

approximately 1 mm thick, which was sufficient to resolve the large structure 

within the turbulent regions. 

flow was used to visualize side views of the large-eddy structures, and a 

horizontal sheet near the surface of the plate was used to obtain top views of 

the flow field. 

This provided an extra 

The frequency of the sine wave was usually set equal 

The light sheets were 

A vertical sheet of laser light parallel to the 

3.5 Velocity MeasurePents 

Miniature boundary-layer hot-film probes (TSI Model 1260) were used in the 

present investigation to measure the longitudinal mean and fluctuating velocities. 

0.25 mm. 

The probe diameters were 0.025 mm, and their sensing lengths were 

A probe traverse powered by a stepping motor and controlled through 

' 17 
an APPLE 11 microcomputer was used for surveying the boundary layers. 

tional statistical quantities, such as the mean, the root mean square, the 

spectral distribution, the autocorrelation coefficient, and the probability 

density function, were computed from the velocity signals. 

Conven3.6 Pattern-Recognition Algorithms 

The purpose of the present investigation is to determine the effectiveness 

of the periodic injection of secondary fluid on the large-eddy structures that 

dominate the outer portions of a turbulent boundary layer. 

initial visualization experiments indicated that the artificial eddies affect 

the low-speed streaks and bursting events that occur very near the wall. 

further investigate this important finding, two pattern-recognition algorithms 

were used to more objectively detect low-speed streaks and bursts. 

Rowever, the 

To 

A novel pattern-recognition technique that utilizes the streamwise velocity 

signal from three hot-film probes was developed to detect low-speed streaks 

(Gad-el-Hak et al., 1984). The probes were located at y = 10 at the same 

streamwise position with a spanwise separation of approximately 20 v/u . the total array spanned 40 u/u which is approximately half the average lowspeed streak spacing. A pattern recognition algorithm identified a low-speed 

region whenever all three signals were less than the local mean velocity by 

one-half the root-mean-square (rms) value, and the velocity measured by the 

middle probe was less than the velocity on either side. 

of each low-speed region could thus be identified, and the streaks' average 

length and frequency computed. 

+ 

Thus, T 

T' 

The beginning and end 

To test this algorithm, experiments were first carried out using a rake of 

+ twelve velocity probes located at y = 10 but having a significantly broader 

*spanwise extent. Low-speed streaks were readily identified using the isovelocity contours computed from the rake's output, and their spacing was in reasonable agreement with results obtained from visualization experiments conducted 

under similar conditions. The results of the algorithm were compared with the 

low-speed streaks obtained from the rake. 

detected algorithm picked out the streaks exceedingly well, and that the 

streaks meander in the spanwise direction so that often a single streak 

crosses the probes two or more times, resulting in multiple detections. 

The comparison showed that the 
The second pattern-recognition algorithm employed in the present investigation was a burst detection scheme using the variable-interval time-averaging 

(VITA) technique developed by Blackwelder & Kaplan (1976). 

probe located at y 

the number of bursts that occur near the wall and recorded their intensities. 

A single hot-film 

+ = 20 was used for burst detection. The program counted 

19 
4. PERIODIC IIRTECTION IN THE ABSENCE OF BOUNDARY-LAYER FLOW 

The flow rate from the cyclic spanwise slot was controlled using the 

pressure regulator and the ball valve shown schematically in Figure 6. 

typical example of the ejection of water, mixed with fluorescent dye, in the 

absence of boundary-layer flow (the flat plate was held stationary in the water 

channel) is shown in the photographs in Figure 7. 

light perpendicular to the spanwise slot illuminated the dyed fluid issuing 

from the slot. 

acceleration rate of 150 cm/s2 (total acceleration time = 0.3 s), followed by 

a deceleration from 45 to 0 cm/s at a rate of 450 cm/s2 (total deceleration 

time = 0.1 s). The leading edge of the flat plate is located to the left of 

the side view shown in the figure, and the time in seconds measured from the 

starting cycle is shown under each photograph. 

flow forms a clockwise-rotating vortex as it comes out of the 45" sharpleading-edge slot. 

evolution of this vortex. 

and the deceleration rate also affect the flow from the spanwise slot. 

A 

A vertical sheet of laser 

The flow speed increased linearly from 0 to 45 cm/s at an 

It is clear that the secondary 

The presence of the wall of the flat plate influences the 

The maximum injection speed, the acceleration rate, 

The instantaneous injection velocity of the cyclic jet can be recorded by 

placing a hot-film probe just outside the spanwise slot. Typical time record 

is shown in Figure 8 for a periodic injection having maximum injection speed 

of 140 cm/s, acceleration rate of 350 cm/s2 and deceleration rate of 560 cm/s . The sensitive element of the hot-film probe was placed parallel to the spanwise 

slot at a distance of 0.5 mm (0.5d) from the two-dimensional orifice. 

clear from the 15 injection cycles shown in Figure 8 that excellent repeatability is achieved. 

2 

It is 

A closeup of the same run is repeated in Figure 9a for three injection 

cycles. 

tion speed, the flow is less turbulent as shown in Figure 9b. 

case, the duty cycle of the ball valve was symmetric and both the acceleration 

and deceleration rates were 667 cm/s . cycles was also programed in this case. 

The flow from the slot is clearly turbulent. At lower maximum injecIn the latter 

2 A larger delay between successive 

In summary, the flow rate from the spanwise slot can be controlled within 

the limitations of the experimental apparatus. 

the acceleration and deceleration rates, and the delay time between successive 

cycles can be changed to optimize the simulation of large-eddy structures in a 

given turbulent boundary layer. 

The maximum injection speed, 

20 
t = 0.28s 

t = 0.38s 

t = 0.34s 

t = 0.41 s 

t = 0.44s t = 0.47s 

Figure 7. Periodic Injection from Spanwise Slot. Flat Plate is Stationary and 

Maximum Injection Speed = 45 cmls 

21 
ORGINAL PAGE IS 

@" ' a '#'P&R QUMnV 

t = 0.47 s 

-~ 

t = 0.69s 

t = 0.75 s t = 0.81 s 

t = 0.87s t = 1.19s 

Figure 7. Periodic Injection from Spanwise Slot. Flat Plate is Stationary and 

Maximum Injection Speed = 45 cmls (Cont.) 

22 
IFI -- 

' 23 
100 

50 

0 

0 0.4 0.8 1.2 1.6 2.0 

t (SI 

a. Time ON = 0.4 s; Time OFF = 0.25 s; Maximum Injection Speed = 140 cmls 

40 

30 

Uj(t) 20 

(crnls) 

10 

0 

1 I I I I I I I I I 

0 0.2 0.4 0.6 0.8 1 .o 

b. Time ON = 0.06 s; Time OFF = 0.13 s; Maximum Injection Speed = 40 cmls 

t (SI 

Figure 9. Instantaneous Injection Velocity from Spanwise Slot 

24 
5. PERIODIC IZWECTION IWl'O A TuRBuLaRT BOUNDARY LAYER 

The aim of the present investigation is to develop a method of artificially 

generating large-scale, periodic structures in a turbulent shear flow. To test 

whether or not these artificial structures simulate the random, naturally 

occurring ones, flow visualization and fast-response probe measurements were 

conducted . At a freestream speed of 20 cm/s and at the two stations considered in the 

present investigation x = 67 cm and x 100 cm, the momentum thickness Reynolds 

numbers were R = 455 and 632, and the displacement thickness Reynolds numbers 

were R 

dinal velocity profiles of the turbulent boundary layer agreed well with those 

for the zero-pressure-gradient flat plate (Purtell, et al., 1981). 

profiles always had well-defined linear, logarithmic and wake regions. 

8 

= 585 and 813, respectively. The mean and root-mean-square longitu- 8" 

The mean 

In the presence of a turbulent boundary layer, the periodic injection from 

the spanwise slot was visualized using a vertical sheet of laser light parallel 

to the freestream direction. In Figure 10, the turbulent boundary layer flow 

was from left to right and the secondary fluid from the slot was premixed with 

minute amounts of fluorescent dye. The flow was visualized just downstream of 

the injection slot. The parameters for the periodic injection were: maximum 

z injection speed = 20 cm/s, acceleration rate = 50 cm/s , and deceleration 

rate = 80 cm/s . from the start of a typical cycle is indicated under each photograph. As shown 

in the figure, large-eddy-like structures are formed downstream of the spanwise 

slot. The shear flow in the boundary layer interacts in a unique way with the 

clockwise-rotating vortex issued periodically from the slot to form the structures marked by the dye. 

Two consecutive structures are separated by a distance that depends on the 

ratio of the injection speed to the freestream speed and on the reduced jet 

frequency f8/Um. 

to the freestream speed and the nondimensional frequency is 0.18. 

2 The freestream speed was Ua = 20 cm/s, and the time in seconds 

These structures stretch while convecting downstream. 

In the present example, the maximum injection speed is equal 

An apparently different large-eddy structure is artificially generated when 

the reduced jet frequency is increased, as shown in Figure 11. In this case, 

the freestream speed and the maximum injection speed are the same as the case 

depicted in Figure 10, but the acceleration and deceleration rates are 

increased such that the reduced frequency is increased three folds to 

' 25 
ORIGINAL PAGE IS 

OF POOR QUALITY 

v) 

8 

0 

II 

c 

cy 

1 

v) 

Q) 

2 

II 

Y 

26 
v) 

0 

7 

0 

II 

c 

Y 

u, 

v) 

LD 

0 

9 

II 

c 

ORIGINAL PAGE IS 

OF POOR QUALm 

v) 

3 

0 

I1 

Y 

N 61 

v) 

27 
f6/UW = 0.58. 

picted in Figure 10 (opening time = 0.4 s, and closing time = 0.25 s), while it 

is symmetric in the case shown in Figure 11 (opening time = closing time = 0.1 SI. 

The two consecutive eddies are closer to each other in the latter case. 

The duty cycle of the ball valve is asymmetric in the run deA different perspective of the flow field is achieved by dying both the 

incoming turbulent boundary layer flow as well as the cyclic, secondary fluid 

from the spanwise slot. 

of the injection slot near the leading edge of the plate, and a lighter color 

dye marks the periodic flow from the spanwise slot. 

eddy structures are visualized in the first photograph at t = 0 (origin of 

time coincides with the start of the cyclical injection). 

introduced into the natural boundary layer such that the maximum injection 

speed = U, = 20 cm/s and the reduced frequency fS/U, = 0.1. 

phenomenon is associated with this relatively slow injection of secondary fluid. 

At the end of the opening cycle (fourth frame in Figure 12 at t = 1.05 s), the 

injected fluid reaches approximately the same height as that of the natural 

boundary layer. 

action between the natural and artificial eddies results in the large intrusion 

of the lighter dye into the potential flow region. 

region is almost twice that of the natural boundary layer thickness. 

In Figure 12, a dark yellow dye is introduced upstream 

The natural, random largeA periodic jet is 

An interesting 

However, at t = 1.45 s (last frame in Figure 121, the interThe thickness of this 

Hot-film probes were used in the present investigation to record the 

instantaneous streamwise velocity components. 

in Figure 13 for the natural and perturbed turbulent boundary layers. 

free- stream speed in all three runs was U, = 20 cm/s, and the probe was 

located at x = 100 cm. At y/6 = 0.6, the natural boundary layer has a mean 

velocity of 17.92 cm/s and the typical random fluctuations shown in the top 

record in the figure. 

velocity is increased to 19.21 cm/s and the negative spikes (marked by the 

arrows in the middle record) indicate the passing of natural, large-eddy 

structures by the fixed probe position. 

a mean frequency of about 0.8 Hz. 

spanwise slot at a period corresponding to the mean natural period, the 

velocity records shown in the bottom of Figure 13 result. 

film probe is located 38 cm (126) downstream of the injection slot, yet the 

negative spikes resulting from the passing of the artificial eddies by the 

Three such records are depicted 

The 

When the probe is raised to y/6 = 0.8, the mean 

These eddies occur at random but have 

When secondary fluid is introduced from the 

Note that the hot28 
ORIGINAL PAGE 1s 

OF POOR QIJALITY 

icn 

u II 

I- il 

9 

LL 

.,: .4 

'1 7 

I' 

29 
NATURAL B.L.; yld = 0.6 

1 .o 

U(0IUw 

0.9 

0.8 

1.1 

1 .o 

U(t)lUw 

0.9 

0.8 

NATURAL B.L.; yld = 0.8 

1.1 r PERTURBED B.L.; yld = 0.8 

I 

U(t)lUW 

2 3 4 

t (5) 

0 1 

Figure 13. Instantaneous Streamwise Velocir/ Records. 

U, = 20 cmls; x = 100 cm 

30 
fixed probe are still perfectly periodic. 

between the velocity signature of the natural eddies and the artificial ones. 

Although this does not seem to be important for the particular application 

sought by the present investigation, the injection parameters can further be 

optimized to produce the desired spike's configuration. 

Some differences exist, however, 

The repeatability of the artificial structures is evident from the 

instantaneous streamwise velocity record depicted in Figure 14. 

injection speed in this run is twice the ambient flow speed. 

recorded by the probe at this position is 18.48 cm/s, slightly lower than 

that recorded at the same position in the absence of cyclic perturbations 

(19.21 cm/s). 

but some slight changes in their amplitude are observed and are probably 

caused by their interactions with the random background turbulence. It is 

also interesting to note that, when the boundary layer is perturbed, the 

artificially generated periodic structures dominate the flow and the naturally 

occurring random eddies are no longer observed visually or with a hot-film 

probe. 

(bursting, entrainment, etc.) generating the large'eddy structures in the 

first place to produce periodic eddies. 

The maximum 

The mean speed 

The negative spikes shown in Figure 14 are perfectly periodic 

Apparently the periodic excitations interact with whatever mechanism 

In summary, cyclic injection from a spanwise slot generates periodic 

structures that appear visually to resemble the random, naturally occurring 

large eddies. 

similar to the natural large-eddy structures. 

periodic structures dominate the flow and the naturally occurring random 

eddies are no longer observed. 

The velocity signature of the artificial structures is also 

The artificially generated 

31 
1.1 I 

1 .o 

U(t)/Ua, 

0.9 

0.8 

I I I I 

0 5 10 15 20 

I 

25 

Figure 14. Instantaneous Streamwise Velocity Records. Twenty One Large 

Eddy Structures were Artificially Generated from the Spanwise 

Injection Slot. U, = 20 cmls; x = 100 cm; y/d = 0.8; 

Maximum Injection Speed = 40 cmls 

'3 2 
6. EFFECTS OF ARTIFICIAL LARGE-SCALE STRUCTURES ON BURSTING 

The purpose of the device considered in the present study is to produce 

periodic structures that are similar to the random, naturally occurring, largeeddy structures. 

airborne laser platforms to reduce or eliminate optical distortion caused by 

the turbulence in the aircraft's boundary layer. In Section 5, the similarity 

between the artificial (periodic) structures and the natural (random) ones has 

been established. 

device was discovered during the present experiments. 

dic large-eddy structures trigger the generation of bursting events near the 

wall of the flat plate. That is, not only the large eddies that characterize 

the outer parts of a turbulent boundary layer become periodic, but also the 

near-wall bursts, which randomly occur in a natural boundary layer, seem to be 

occurring periodically. The present section establishes this result. We start 

with a brief recap of the bursting phenomenon. 

Together with adaptive optics, such device may be used on 

An unexpected additional benefit of the large-eddy generating 

It seems that the perioThe wall region of a turbulent boundary layer is dominated by a sequence 

of intense organized motions that are collectively called the bursting phenomenon (Kline et al., 1967). According to at least one school of thought, the 

process begins with a pair of elongated, steamwise, counter-rotating vortices. 

These vortices exist in a strong shear and induce low- and high-speed regions 

between them. 

randomly in space and time. The low-speed regions appear to grow downstream, 

lift up and develop inflectional velocity profiles. At approximately the same 

time, the interface between the low- and high-speed fluid begins to oscillate. 

The low-speed region lifts up away from the wall as the oscillation amplitude 

increases, resulting in chaotic breakdown of the streak. Since this latter 

process occurs on a very short time scale, Kline (1978) called it a 'burst'. 

+. .. 

Both the vortices and the accompanying eddy structures occur 

A side view of a naturally occurring bursting event is shown in Figure 15. 

The freestream speed is 20 cm/s and the wall region at x = 100 cm is visualized 

using fluorescent dye seeping from a spanwise slot just upstream of the observation station. 

parallel to the flnw directinn, and the flow is from left to right. The time 

measured from an arbitrary origin is indicated underneath each photograph. 

The apparent accumulation of dye is caused by the formation of a low-speed 

region, and the lifting and breakup of this region is evident in the photographic sequence shown in the figure. 

The dye is excited with a vertical sheet of laser light 

33 
ORIGINAL PAGE IS 

OF 

34 
As shown by Gad-el-Hak & Hussain (19861, bursting events can also be 

induced artificially by withdrawing near-wall fluid from two minute holes 

separated in the spanwise direction or by pitching a miniature delta wing that 

is flush-mounted to the wall. 

like vortex and low-speed streak that resemble naturally occurring structures. 

The resulting sequence of events that occur et a given location can then be 

uniquely controlled, thus allowing detailed examination via phase-locked 

measurements. 

Either of these two actions generates a hairpinIn contrast to the above method of inducing bursts by simulating wall 

events, in the present investigation the passing of an artificially induced, 

periodic large-eddy structure seems to trigger a bursting event near the 

wall. 

Figures 16 and 17. 

freestream speed is 20 cm/s, and the observation station is 100 cm downstream 

of the leading edge. For the case depicted in Figure 16, the maximum injection speed is 45 cm/s, the ball valve's duty cycle is symmetric with opening 

time = closing time = 0.1 s. 

opening (0.1 s) and lower maximum injection speed (20 cm/s) are the parameters 

of the run shown in Figure 17. In both cases, it is evident that the passing 

of the artificially generated large-eddy structure triggers a burst-like event 

in the wall region. 

still photographs were taken, the bursting event was periodic , having exactly 

the same frequency as the structure that induced it. 

This is shown convincingly in the visualization results depicted in 

In both figures, the flow is from left to right, the 

A slow closing (closing time = 1 s), fast 

- 

As shown in the cine films from which these selected 

The pattern-recognition algorithms described in Section 3.6 were used to 

establish the similarity between the artificially induced wall events and the 

naturally occurring ones. The streak detector searches for low-speed regions 

near the wall relative to the background mean velocity. 

searches for periods of high acceleration without necessarily larger fluctuations than the background turbulence. 

(19761, such periods are associated with events characterized by a high degree 

of coherence in time and in the direction normal to the wall and having a 

conditionally-averaged Reynolds shear stress that is an order of magnitude 

greater than its conventionally-averaged value. 

based on the kinematics and indirectly on the dynamic properties of the 

organized structure. 

The burst detector 

As shown by Blackwelder & Kaplan 

The detection scheme is then 

' 35 
ORIGINAL PAGE IS 

OF POOR QUALITY 

t = 0.0s t = 0.06 s 

t = 0.20 s t = 0.145 

I) FLOW 

t = 0.42 s t = 0.58s 

2 cm 

Figure 16. A Bursting Event Triggered by the Passing of an Artificial Large Eddy Structure 

36 
e 

m 

Q) 

8 

II 

c 

m 

9 

0 

II 

c 

ORIGINAL PAGE IS 

OF POOR QUALln 

37 
*. 

Whenever an artificially generated large-eddy structure passed on top of 

the velocity probe located very near the wall, a low-speed streak and a burst 

were detected by the two pattern-recognition algorithms; thus further 

confirming the visualization results depicted in Figures 16 and 17. 

In sumnary, the cyclic injection from a spanwise slot seems to generate 

periodic structures similar to the random, naturally-occurring large eddies. 

These structures in turn appear to trigger periodic bursting events near the 

wall, even at stations far downstream of the large-eddy generating device. 

Thus, the present device generates periodic structures in both the outer and 

inner regions of a turbulent boundary layer. 

of the artificial large-eddy structures and the spanwise periodicity of the 

bursting events are important issues that have not been investigated in the 

present preliminary study. 

The degree of two dimensionality 

38 
7. SPECTRAL ANALYSIS 

The autocorrelation and spectrum of the longitudinal velocity fluctuations 

are two statistical quantities useful in analyzing the intermittent signal from 

a hot-film probe located near the outer portion of a turbulent boundary layer 

and the effects of the large-eddy generating device. 

the autocorrelation coefficient is the correlation between the values of the 

streamwise velocity fluctuations, u, at two different times nondimensionalized 

using the mean square: 

At a given spatial location, 

where the overbar denotes a time average. 

correlation coefficient is the power spectral density: 

The Fourier transform of the autowhich is the normalized spectral distribution of the squared intensity of the 

longitudinal velocity fluctuations (Hinze, 1975). These two functions provide 

a convenient method of determining length scales in the flow field and may 

provide clues to periodic and/or other structures embedded in the random 

velocity fluctuations. 

The effects of the - large-eddy generating device on the dimensional spectral 

distribution [S'(f) = u S(f)] and the autocorrelation coefficient are eluciated 

by comparing these statistical quantities for a natural boundary layer (Figure 18) 

and a perturbed one (Figure 19). In both cases, the freestream speed is 20 cm/s 

and the probe is located at x = 67 cm and y/6 = 0.6. 

in Figure 19, the maximum injection speed is 40 cm/s and the cyclic jet is on for 

0.1 s and off for 0.4 s. 

a frequency of 2 Hz. 

tion coefficient with a period of 0.5 s, exactly equal to the period of the 

cyclic jet. 

2 

For the perturbed case shown 

A pronounced spectral peak is evident in Figure 19 at 

Also, a corresponding oscillation appears in the correlaSimilar comparison is made further downstream at x = 100 cm and y/6 0.8, 

and this is shown in Figures 20 and 21 for the natural case and the perturbed 

case, respectively. The maximum injection speed for the perturbed case is 

39 
1 00 

10-2 

S' p (cm Is) 

10-6 

I I I 

10-1 1 00 101 102 

Figure 18. Spectrum and Autocorrelation for Natural Boundary Layer. 

U, = 20 cmls; x = 67 cm; yld = 0.6 

40 
. 

\ 

\ t 

. 

t10- sl 

10-1 10" 

-0.6 - 

I I I I 

2.0 -1.0 ; 0 1 .o 

T (SI 

Figure 19. Spectrum and Autocorrelation for Perturbed Boundary Layer. 

U, = 20 cmls; x = 67 cm; y/d = 0.6 Maximum Injection 

Speed = 40 cmls 

41 
10-6 

10-1 1 00 101 102 

f(Hzl 

-0.2 

-0.6 

- 

- 

I I I I I I I I 

c 

Figure 20. Spectrum and Autocorreltion for Natural Boundary Layer. 

U, = 20 cmlsec; x = 100 cm; yld = 0.8 
. 1 00 

I I I 

10-1 1 00 101 102 1 03 

10-61 

f(Hz) 

1.0 & 1 

-0.6 1 

1 2 3 4 -1 .o 

0 

. Figure 21. Spectrum and Autocorrelation for Perturbed Boundary Layer. 

U, = 20 cm/s; x = IO0 cm; yld = 0.8 

Maximum Injection Speed = 40 cmls 

43 
unchanged (40 cm/s), but the ball valve opens more slowly in this latter comparison (opening time e: 1 s; closing time = 0.25 8). 

the spectrum of the natural boundary layer (Figure 20). 

position, the mean frequency of the natural large-eddy structures is 0.9 Hz. 

Two other peaks are observed in the same figure, corresponding to the first 

and second harmonics. In the perturbed case, all three peaks are much more 

pronounced and a well-defined period of 1.25 s is observed in the autocorrelation coefficient. It is interesting to note that the oscillations in the 

autocorrelation coefficient are symmetric in the case of fast opening and slow 

closing ball valve (Figure 19) and quite asymmetric when the ball valve opens 

slowly and close fast (Figure 21). 

A slight peak appears in 

At this higher 

As evident from comparing the area under the spectral distribution curves 

for the natural and perturbed boundary layers, the mean square of the steamwise 

velocity fluctuations is increased by the introduction of the cyclic secondary 

fluid into the turbulent boundary layer. 

turbulence level is a penalty associated with making the large-eddy structures 

periodic. More work is needed to investigate this point, specially whether or 

not further optimization of the device will lead to better management of the 

level of turbulence. 

This means that an increase in the 

, 

. 
8. PROBABILITY DISTRIBUTION 

A useful statistical quantity that may shed some light on the effects of 

the large-eddy generating device on the flow field is the probability density 

function of the steamwise velocity fluctuations (Lumley, 1970; Tennekes & 

Lumley, 1972). In Figure 22, comparison is made between the probability 

density function for the natural boundary layer and the perturbed one. 

abscissa in the figure is the streamwise velocity, and the ordinate is a 

normalized probability density function such that the area under the curve is 

unity. 

and the hot-film probe is located at x 

wise injection slot) and y/6 = 0.6. 

and the duty cycle of the ball valve consists of fast opening (0.1 s) followed 

by slow closing (0.4 8). 

i.e., the mean velocity is slightly lower than the most probable velocity. 

The distribution in the perturbed case is more broad, however, consistent with 

the higher turbulence levels. 

The 

For both cases depicted in the figure, the freestream speed is 20 cm/s 

67 cm (5 cm downstream of the spanThe maximum injection speed is 40 cm/s 

Both probability distributions are slightly skewed, 

The negative skewness of the perturbed case is more pronounced in the case 

shown in Figure 23. 

the ball valve consists of slow opening (1 s) followed by fast closing (0.25 SI. 

At higher probe position, y/6 = 0.8, the turbulence level is lower and the 

probability distribution is narrower, as shown in Figure 24. 

parameters in both the natural and perturbed cases are the same as those for the 

runs depicted in Figure 23. 

In here x 100 cm, y/6 = 0.6, and the duty cycle of 

All the other run 

In summary, it appears that the large-eddy generating device results in a 

broader, more negatively skewed probability distribution. Both these effects 

are a result of the higher turbulence levels generated by the cyclical injection 

and are consistent with the instantaneous velocity records depicted in Figures 13 

and 14. 

45 
NATURAL B.L. 

-A_- PERTURBED B.L. 

Figure 22. Effects of Periodic Injection on Probability Density Function. 

U, = 20 cmls; x = 67 cm; yld = 0.6 

46 

24 
0.7 

0.6 

NATURAL B.L. 

PERTURBED B.L. 

---- 1 0.4 

14 16 18 20 10 12 

U (cmls) 

Figure 23. Effects of Periodic Injection on Probability Density Function 

U, = 20 cmls; x = 100 cm; y/d = 0.6 

47 
Figure 24. Effects of Periodic Injection on Probability Density Function. 

U, = 20 cmls; x = 100 cm; y/d = 0.8 

48 
9. SUMUARY 

An active flow control device to generate large-scale, periodic structures 

in a turbulent boundary layer was considered in the present experimental 

investigation. Together with adaptive optics, the device may be used on airborne laser platforms to reduce or eliminate optical distortion caused by the 

turbulence in the aircraft's boundary layer. 

were artificially generated by cyclically injecting secondary fluid from a 

spanwise slot. 

to establish the feasibility of the flow control device. 

flow was visualized using either hydrogen bubbles shed from a stainless-steel 

wire or fluorescent dyes injected either from the large-eddy generating device 

or from a second, downstream spanwise slot. Both the hydrogen bubbles and the 

fluorescent dyes were illuminated using sheets of laser light projected in the 

desired plane. Hot-film probes were used to record the longitudinal velocity 

fluctuations. Important statistical quantities, such as the mean, the rootmean-square, the autocorrelation, the spectrum, and the probability distribution were computed from the instantaneous velocity records to assess the 

effectiveness of the flow control device. 

Periodic large-eddy structures 

A flat plate towed in a water channel was used as a test bed 

The boundary layer 

A high degree of control of the cyclical injection was achieved by using a 

pressure regulator and a computer driven ball valve. Within the limitations 

of the experimental apparatus, the maximum injection speed from the spanwise 

slot, the acceleration and deceleration rates of secondary fluid's injection, 

and the delay time between successive cycles can be changed to optimize the 

simulation of large-eddy structures in a given turbulent boundary layer. 

The periodic structures generated from the spanwise injection device were 

shown to be similar to the random, naturally occurring, large-eddy structures. 

The negative spikes resulting from the passing of the artificial eddies by a 

fixed hot-film probe were still perfectly periodic at a distance 126 downstream of the large-eddy generating device. 

between the velocity signature of the natural eddies and the artificial ones. 

Although this may not be important for the particular application sought by 

the present research, the injection parameters can further be optimized to 

produce the desired spike's configuration. 

structures dominate the flow, and the natural random eddies are no longer 

Some differences existed, however, 

The artificially generated periodic 

, 49 
observed visually or with a hot-film probe. Apparently the periodic excitations interact with whatever mechanism generating the random large eddies in 

the first place to produce periodic eddies. 

At a given location in the boundary layer, the injection device resulted 

in slightly lower mean velocity but higher turbulence levels. 

tral peak and a corresponding oscillation in the autocorrelation coefficient 

appeared at the injection frequency. 

bution corresponded to the first and second harmonics of the perturbation 

frequency. The probability distributions were more broad and more negatively 

skewed in the case of the perturbed boundary layer as compared to the distributions in a natural turbulent boundary layer. 

A dominant specTwo other peaks in the spectral distriIn addition to removing the inherent randomness in large-eddy structures, 

the periodic large eddies generated from the spanwise slot seemed to trigger 

the generation of bursting events near the wall of the flat plate. Thus, both 

the large eddies and bursts occurred periodically in the perturbed turbulent 

boundary layer, even at stations far downstream of the large-eddy generating 

device. 

capable of producing periodic structures in both the outer and inner regions 

of a turbulent boundary layer. 

remain to be addressed by future research. 

dimensionality of the artificial structures, the spanwise periodicity of the 

bursting events, and methods to reduce the high turbulence levels introduced 

by the control device. 

the control of a free shear layer using subharmonic perturbations, as was 

briefly discussed in Section 2. 

In other words, the device considered in the present investigation is 

There are, however, some important issues that 

For example, the degree of two 

Future research will also be conducted to investigate 

. 

50 
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-- 

52 
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63, p. 237. 

The Mechanism of 

- 

53 
Standard Bibliographic Page 

.. Report No. 

NASA CR- 178249 

2. Government Accession No. 

'. Author(s) 

Mohamed Gad-el -Ha k 

b. Performing Organization Name and Address 

Flow Research Company/Flow Industries, Inc. 

21414 68th Avenue S. 

Kent, WA 98032 

Large Eddy Structures; Turbulence Control ; 

Cyclic Jet; Random Eddies; Periodic 

~ 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, DC 20546 

L5. Supplementary Notes 

Langley Technical Monitor: John C. Lin 

SBIR Phase I Final Report 

Unclassi fied-unl imi ted 

3. Recipient's Catalog No. 

._ - _- 

19. Security Classif.(of this report) 20. Security Classif.(of thi, page) 21. No. of Pages 

5. Report Date 

22. Price 

March 1987 

6. Performing Organization Code 

Unclassified 

6. Performing Organization Report No. 

Flow Res. Rept. No. 365 

Uncl assi fied 54 A04 

10. Work Unit No. 

11. Contract or Grant No. 

NAS 1- 18213 

13. Type of Report and Period Covered 

Contractor Report 

14. Sponsoring Agency Code 

505-60-31 

16. Abstract 

An active flow control device to generate large-scale, periodic structures in 

a turbulent shear flow is developed. Together with adaptive optics, the device may 

be used on airborne laser platforms to reduce or eliminate optical distortion cause 

by the turbulence in the aircraft's boundary layer. 

spanwise slot is used to collect the turbulent boundary layer for a finite time and 

then release all of the flow instantaneously in one large eddy that convects downstream. Flow visualization and hot-film probe measurements are used together with 

pattern recognition a1 gorithms to demonstrate the viability of the flow control 

method. A flat plate towed in a water channel is used as a test bed. The instanta 

neous velocity signal is used to compute important statistical quantities of the 

random velocity field, such as the mean, the root-mean-square, the spectral distribution, and the probability density function. 

boundary layer, it is shown that the cyclic jet will produce periodic structures 

that are similar to the random, naturally occurring ones. 

trigger the onset of bursting events near the wall of the plate. 

device generates periodic structures in both the outer and inner regions of a 

turbulent boundary layer. 

A cyclic jet issuing from a 

When optimized for a given 

These structures seem to 

Thus, the present 

17. Key Words (Suggested by Authors(s)) I 18. Distribution Statement 

Flow Control; Airborne Laser Platform; I 

Eddies. 

Subject Category 34 

For sale by the National Technical Information Service, Springfield, Virginia 22161 

Langley Form 63 (June 1985)