Received: 17 March 1997/Accepted: 31 December 1997

K. B. ChunTest & Development TeamSamsung Motors Technology CenterSan d50, Kongse-ri, Kihung-eup, Yongin-city, Kyungki-do, 449-900,Korea

H. J. SungDepartment of Mechanical EngineeringKorea Advanced Institute of Science and Technology373-1, Kusong-dong, Yusong-ku, Taejon, 305-701, Korea

Correspondence to: H. J. Sung

Experiments in Fluids 25 (1998) 133—142 ( Springer-Verlag 1998

Visualization of a locally-forced separated flow over a backward-facing stepK. B. Chun, H. J. Sung

Abstract A laboratory water channel experiment was made ofthe separated flow over a backward-facing step. The flow wasexcited by a sinusoidally oscillating jet issuing from a separ-ation line. The slit was connected to a cavity in which water wasforced through a rigid pipe by a scotch-yoke system. TheReynolds number based on the step height (H) was fixed atRe

H\1200. The forcing frequency was varied in the range

0.305OStHO0.955 at the forcing amplitude A

0\0.3. Time-

averaged flow measurements were made by a LDV system,especially in the recirculating region behind the backward-facing step. To characterize the large-scale vortex evolutiondue to the local forcing, flow visualizations were performedby a dye tracer method with fluorescent ink. The vortexamalgamation process was captured at the effective forcingfrequency (St

H\0.477) for laminar separation. This vortex

merging process enhances flow mixing, which leads to theshortening of the reattachment length.

List of symbolsA0 forcing amplitude, A0{(Q

&03#%$[Q

6/&03#%$)/U0

f forcing frequency, Hzg slit width, g\2.0^0.1 mmH step height, H\20 mmQ total velocity measured at (x/H, y/H)\([0.02, 1.01),

m/sRe

HReynolds number based on H and U0, Re

H{U0H/l

StH

reduced forcing frequency, Strouhal number,St

H{fH/U0

Sth reduced forcing frequency based on the momentumthickness, Sth{fh/U0

U streamwise time-mean velocity, m/su@ streamwise fluctuation velocity, m/sU0 free-stream velocity, m/s

(u@2)1/2 r.m.s. intensity of streamwise velocity fluctuation,m/s

xr reattachment length, mxr0 reattachment length at A0\0, mx, y distance of streamwise and vertical, respectively, m

Greek symbolscp forward-flow time fractiond boundary layer thickness, mmd* displacement thickness, mmk viscosity, Ns/m2h momentum thickness, mm/ phase of local forcing, °

1IntroductionFlow with separation and reattachment has long been a subjectof fundamental fluid dynamics research. The presence ofa separated flow, together with a reattaching flow, gives riseto increased unsteadiness, pressure fluctuations, structurevibrations and noise. Also, they enhance heat and masstransfer and augment mixing. In particular, reattaching flowscause large variations of local heat transfer coefficients (Vogeland Eaton 1985). Thus, control of separated and reattachingflows is an essential issue in practical applications.

A literature survey reveals that there have been manyattempts to control or lessen the unfavorable behavior asso-ciated with separated and reattaching flows. The methodof an oscillating separation edge was applied by Nagib et al.(1985) and Roos and Kegelman (1986). Use of sound waves toinfluence the reattchment process was examined by severalresearchers, and the relevant flow geometry, forcing methodand effective reduced frequency are summarized in Table 1. Asa feasible technique, the introduction of a local forcing in thevicinity of the separation edge has been contemplated (Hasan1992; Kiya et al. 1993; Sigurdson 1995; Chun and Sung 1996).These experimental efforts utilized a small-amplitude localizedjet flow close to the separation edge. The jet flow containeda well-defined single-frequency pulsation. It was demonstratedthat, by means of a small localized perturbation near theseparation edge, the overall characteristics of the separated andreattaching flows were altered significantly.

Recently, Chun and Sung (1996) made a wind tunnelexperiment to control the separated and reattaching flow over

133

Table 1. Summary of otherexperiments Case Flow geometry Forcing method Effective reduced frequency

Nagib et al. (1985) Backward-facing step Oscillating flap StH\0.06

Bhattacharjee et al. (1996) Backward-facing step Acoustic forcing StH\0.35, Sth\0.007

Roos and Kegelman (1986) Backward-facing step Oscillating flap StH\0.22

Hasan (1992) Backward-facing step Acoustic forcing Sth\0.012Kiya et al. (1993) Blunt circular cylinder Acoustic forcing Sth\0.012Obi et al. (1993) Diffuser Acoustic forcing St

H\0.026

Sigurdson (1995) Blunt circular cylinder Acoustic forcing StD\2.5

Chun and Sung (1996) Backward-facing step Acoustic forcing StH\0.28, Sth\0.011

The definitions of St are: StH\fH/U

=, Sth\fh/U

=and St

D\fD/U

=.

Fig. 1. Configuration of test section and flow visualization

a backward-facing step. They showed that the unfavorablebehavior associated with separated and reattaching flows canbe reduced by the introduction of a small, localized perturba-tion in the vicinity of the separation edge. The local forcing wasproduced by a single-frequency sinusoidal disturbance atthe separation edge through a thin slit. The effect of localforcing on the flow structure was scrutinized by altering theforcing amplitude (0OA0O0.07) and forcing frequency(0.305OSt

HO0.955). They clarified that the local forcing

mechanism is effective for controlling the separated andreattaching flows. A small localized forcing near the separationedge enhanced the shear-layer growth rate and produceda large rolled-up vortex at the separation edge.

In an effort to investigate the evolution and dynamicbehavior of the large-scale vortices in the controlled separatedflows, a flow visualization study has been made in a waterchannel. Although numerous studies of the basic flow overa backward-facing step have been conducted in wind tunnels,flow visualizations in water tunnels are relatively scarce.The present water-channel flow visualization provides anunderstanding of vortex merging phenomena due to thelocal forcing for laminar separation. A dye tracer methodwith fluorescent ink was employed. To measure time-averaged flow quantities in the recirculating region, a two-component LDV system was utilized. For an effective flowvisualization, the Reynolds number based on the stepheight was fixed at Re

H\1200. The forcing amplitude was

also fixed at A0\0.3 due to the practical constraint ofthe present forcing system. The vortex amalgamation processwas clearly observed at an effective forcing frequency. Theinitial rolled-up vortex was split into two vortices due to thelocal blowing, which merged downstream. These amalgama-tions of the rolled-up vortices are shown to invigorate flowmixing.

2Experimental apparatus and procedure

2.1Water channelExperiments were performed in a recirculating open waterchannel. The water channel, in which a diffuser, a contraction

and a honeycomb were placed in sequence, was designed toprovide a high-quality flow in the test section. The turbulenceintensity was less than 0.7% at Re

H\1200, where the Reynolds

number was defined based on the step height (H) of the testsection and the free-stream velocity (U0). As shown in Fig. 1,the dimensions of the inlet open channel were 250 mm inwidth, 250 mm in depth and 1200 mm in length in thestreamwise direction.

2.2Test section and local forcingA test rig of the backward-facing step was immersed andaligned with the uniform approach flow. As illustrated in Fig. 2,the test section was placed within the open flow stream. Thestep height was H\20 mm and the spanwise width of thebackward-facing step was 250 mm. Since the aspect ratio was12.5, the two-dimensional flow assumption is valid to a reason-able accuracy in the central portion of the test section(Brederode and Bradshaw 1978). The length of the test sectionplate was 300 mm long, which was deemed sufficient toaccommodate the reattachment point. The front of the testsection was located 200 mm downstream of the entrance of theinlet channel, and the test section was placed 50 mm apart

134

Fig. 2. Test section and local forcing system

Table 2. Initial boundary layer conditions at x/H\[0.2, ReH\1200

d d* h d*/h

A0\0 (no forcing) 11.25 2.93 1.36 2.15A0\0.3, St

H\0.477 8.00 2.10 1.09 1.92

Table 3. Summary of uncertainty estimates

Measured quantity Uncertainty

xr/H ^0.002U/U0 ^0.02(u@2)1/2/U0 ^0.01

from the bottom wall. The blockage ratio of the cross-sectionalarea of the test section to the channel was less than 12%.

The local forcing was introduced by a sinusoidally oscillat-ing jet through a spanwise thin slit along the separation line. Asseen in Fig. 2, the slit was connected to a square cavity in whichwater was forced through a rigid pipe with a piston-and-cylinder system driven by a scotch-yoke mechanism. Theforced flow was passed through two screens (1 mm and and0.5 mm in mesh sizes) and a honeycomb (2 mm in cell size and10 mm in length) to achieve a regulated forced flow. Thescotch-yoke mechanism with varying strokes produced differ-ent forcing frequencies and forcing levels (Kiya et al. 1993).Special care was exercised to minimize air bubbles inside theforcing system. The forcing amplitude (A0) was defined asthe ratio of the difference of total velocity (Q) caused bylocal forcing to the mean free-stream velocity (U0), i.e.,A0\(Q

&03#%$[Q

6/&03#%$)/U0. The total velocity Q, which was

equal to (U2]V2)1/2, was measured at the position (x/H, y/H)\([0.02, 1.01). A coordinate system and a cross-sectional viewof the test section are illustrated in Fig. 2. The x axis is taken inthe main flow direction and the y axis in the vertical direction.The origin is located at a position on the bottom of the step.The time-mean velocity components in the x and y directionsare denoted by U and V, the fluctuating velocity componentsby u@ and v@, respectively.

2.3Flow visualization and instrumentationThe specific gravity of the dye used was 1.006. The dye wasinjected at x/H\[3 into the boundary layer along thecenterline through a 0.5 mm inner diameter tube. Since theinjection velocity was very low, the initial shear layer was notdisturbed. A 4 W Argon-ion laser was used as a light source. Tomake a light sheet and to prevent the light scattering, 4 mirrors,a collimator and a cylindrical lens were installed. A detaileddiagram of the flow visualization setup is shown in Fig. 1.Photographs were taken with an ASA-3200 film by a camera(Nikon FX-2). The exposure time was varied from 1/125 to1/250 s. The moving image was captured by a CCD camera andwas recorded in an S-VHS video recorder. A macro lens was

used to take enlarged photographs. This image was recordedby an image printer and stored in a 586 computer after beingscanned by a 1200 DPI scanner.

A three-beam, two-color LDV system (TSI 9100-8) was usedto measure the velocity components in the recirculating region.A dichroic color separator was employed to accomplish thecolor separation in transmitting optics. The beam expanderincluded in this system had a 2.27]beam expansion, whichimproved the signal-to-noise ratio about five times in signalpower compared to the system without a beam expansion.A frequency shifter was included in the system to shift a singlecolor beam after two colors were separated by a dichroictransmitting optics. The receiving optics had a compactdichroic color splitter and a photomultiplier system. Theprocessor was interfaced to a 586 computer by an A/Dconverter. Each beam had a Bragg cell shifted by 20 kHz toeliminate the ambiguity of flow directions. Two channels wereoperated simultaneously with the beams aligned at ]45° and[45° to the tunnel axis for the measurements of U]V andU[V components of velocity. The backward scattered lightfrom particles passing through the measuring volume wasdetected by multiplier tubes. Signals from the photomultipliertubes were processed in counters that performed eightperiodicity checks on the signals and digitized the signals with2 ns resolution.

In order to find the reattachment length (xr), the forward-flow time fraction (cp) in the vicinity of the wall (y/H\0.05)was measured by a split film probe (TSI model-1288). A cut-offfrequency was fixed at 10 Hz. This was because a pseudo-periodic low frequency flapping motion can be detected, wherethe normalized frequency is about 0.2 Hz. A sampling fre-quency was fixed at 20 Hz. The overall uncertainty intervalswere calculated for the 95% confidence limits using themethod of Abernethy et al. (1985). These are summarized inTable 3.

3Results and discussionAs mentioned earlier, the effect of local forcing on theturbulent separated flow over a backward-facing step has been

135

Fig. 3. Normalized reattachment xr/xr0 against forcing frequency StH

Fig. 4. Distributions of the forward-flow time fraction cp for threeforcing cases, Re

H\1200

Fig. 5. Distributions of U/U0 and (u@2)1/2/U0 at x/H\[0.02

scrutinized in the wind tunnel experiment (Chun and Sung1996). The main aim of the present experiment is to delineatethe salient vortex amalgamation process through a flowvisualization in water. Accordingly, in the present study, thegeneral flow features over a backward-facing step by localforcing are reviewed.

At first, the reattachment variations by local forcing areinspected. The time-mean reattchment position (xr) wasdefined as the point where the forward-flow time fractionis equal to cp\0.5 near the bottom wall (y/H\0.05). Thenormalized reattachment length xr/xr0 is displayed in Fig. 3as a function of the local forcing frequency (St

H\fH/U0).

Here, xr0 denotes the time-mean reattachment lengthwithout local forcing (A0\0). The forcing frequency wasvaried in the range 0.305OSt

HO0.955. The lower limit

of the forcing frequency was set by the practical constraints.Therefore, no experimental data were available in thepresent experiment when St

Hwas lower than St

H\0.305.

As seen in Fig. 3, the effect of local forcing on the reat-tachment length is substantial. At a particular forcingfrequency, i.e., at St

H:0.477, the reattachment length is

reduced significantly. However, as StH

increases furtherSt

HP0.7, the reattachment length is larger than that of the

unforced flow (A0\0), i.e., xr/xr0P1. This experimental findingis consistent with the previous assertion (Chun and Sung1996).

Distributions of the forward-flow time fraction cp onthe surface are represented in Fig. 4. The point of cp\0.5corresponds to the reattachment point (xr). It is shown thatthe reattachment length (xr/H\2.63) of the forced flow ofA0\0.3, St

H\0.477 is much shorter than that of A0\0

(xr/H\7.64). For the unforced flow (A0\0), a separationpoint is seen near the corner (x/H\1.0). This suggests theexistence of a secondary recirculation near the corner sec-tion (Chun and Sung 1996). However, for the forced flows,no secondary flows are detected near the corner region.The disappearance of the secondary separation bubble isbrought about by the formation of larger and stronger

vortices immediately downstream of the separation edgeby the forcing, which will be demonstrated by flow visualiz-ation.

The effect of local forcing on the initial boundary layer isshown in Fig. 5. Two cases are selected to distinguish the flowstructures by local forcing. One is the case of no forcing(A0\0). The other is the case of A0\0.3, St

H\0.477, which

yields a minimum reattachment length in Fig. 3. Similarly tothe previous case of air flow, the turbulence intensity (u@2)1/2/U0by local forcing is very large near the separation edge(1.1Oy/HO1.3). However, the effect of local forcing on themean velocity profile (U/U0) is insignificant. As remarked inChun and Sung (1996), the shear layer in the vicinity of thesharp separation edge is modified by local forcing, which givesrise to large increases in entrainment close to the separationedge. The detailed initial boundary layer conditions are listedin Table 2.

136

Fig. 6a, b. Profiles of U/U0 abd (u@2)1/2/U0 for two forcing cases Fig. 8. Location of points of maximum turbulence level

Fig. 7. Location of points of 10% free-stream velocity

Bhattacharjee et al. (1986) stated that the most effectivenon-dimensional forcing frequency (St

H) is between 0.2 and

0.4. Roos and Kegelman (1986) found the natural instabilityfrequency of the shear layer to be at St

H\0.4 for laminar

separation. The reduced forcing frequency in the presentexperiment is St

H\0.477, which is slightly larger than those

above. However, when the momentum thickness (h) near theseparation edge is used, i.e., Sth\fh/U

=, the forcing frequency

is Sth\0.025, which is larger than the data in the literatureSth\0.011 (Eaton and Johnston 1980; Battachrjee et al. 1986;Hasan 1992; Kiya et al. 1993). This may be caused by the factthat the present Reynolds number is lower (Re\1200) thanthat of the other experiments.

In order to see the effect of local forcing on the time-averaged flow, a detailed measurement was made at Re

H\1200

for two cases (A0\0 and A0\0.3, StH\0.477). Contrary to the

air flow case, the measurement in the recirculating regionbehind a backward-facing step was possible by using thepresent two-component LDV system. In the prior wind tunnelexperiment (Chun and Sung 1996), the hot-wire anemometerwas employed, i.e., the profiles in the recirculating region couldnot be measured. As seen in Fig. 6a, a relatively large effect bylocal forcing is displayed on the development of the separatedflow (1Ox/HO6). In the near-region of separation (x/H\0),the mean velocity profiles are only slightly modified by localforcing. However, significant changes are detected in the shearlayer region (1Ox/HO3). After the reattachment (x/H\3),the flow is gradually redeveloped. Figure 6b represents thetime-averaged turbulence energy levels. The influence of localforcing on the turbulence energy levels is pronounced in therecirculating region. As opposed to the mean velocity profiles

in Fig. 6a, after the reattachment (x/HP3), the turbulenceintensity is recovered rapidly. This suggests that the amalga-mation of the rolled-up vortices induced by local forcing canincrease turbulent intensity levels within the separated shearlayer.

Locations of the 10% free-stream velocity levelare displayed in Fig. 7 as a function of the downstreamposition. The rate of shear layer growth is substantiallychanged by local forcing and the curvature toward the wallis closely connected with the shrinkage of the reattachmentlength due to an increase in entrainment into the recirculatingregion. Figure 8 represents the distribution of maximumturbulent intensity at the streamwise locations. The maximumvalue of turbulence intensity appears in the recirculatingregion. This is attributed to the entrainment of turbulent fluidtransported from the reattachment region. It is known thatstrong enhancement of the rolled-up vortices by local forcingcontributes to the increase of turbulence levels in the recir-culating region. A rapid decay of turbulent intensity down-stream of the reattachment region is seen. As mentioned

137

Fig. 9a–f. Instantaneous flow visualizationsfor five forcing cases, Re

H\1200

earlier, these experimental findings are consistent with theearlier air flow case (Chun and Sung 1996).

Now, the results of flow visualization are exhibited. Globalpictures of the separated flow behind a backward-facingstep are displayed in Fig. 9 for six local forcing cases(0OSt

HO0.822). The pictures were taken by a snapshot. When

A0\0 in Fig. 9a, the initial boundary layer near the separationedge propagates downstream up to x/H\3. For x/HP4, theflow begins to flap. This is caused by the shear layer instability.It is seen that a large-scale vortex is formed near x/H\7, whichproduces a reattachment region (x/H\7\8). When the localforcing is perturbed at A0\0.3 and St

H\0.477, the vortex

structure is significantly changed in the recirculating region.A pair of counter rotating vortices is clearly displayed. Due tothe local forcing at the separation edge, amalgamation of therolled-up vortices is observed in the recirculating region(0Ox/HO0.3). This modified vortex structure propagatesdownstream, causing a large increase in entrainment close to

the separation edge and resulting in an increase of turbulencelevels. This process causes a substantial reduction of thereattachment length. For St

H\0.550, the large-scale vortices

are produced by amplification of the local forcing. As StH

increases further (StH\0.650 and St

H\0.717), the local forcing

effect is gradually attenuated. The vortices do not merge. Thisgives a long reattachment length, as compared with the case ofSt

H\0.477. For a high local forcing frequency (St

H\0.822),

the initial vortex merging is no longer present. As seen inFig. 9f, the disturbed flow by local forcing cannot penetrateinto the shear layer, but it is convected downstream along thedividing streamline. As a result, the reattachment length is notshortened.

A sequence of pictures for one period of local forcing(0°O/O360°) is presented in Fig. 10 for three differentforcings. These cases, respectively, are exemplary of thequalitatively distinct conditions: no forcing (A0\0); theminimum reattachment length case (A0\0.3, St

H\0.477); and

138

Fig. 10. Time evolution of large-scale vortices, ReH\1200

the case where xr/xr0 is larger than 1 (A0\0.3, StH\0.822). For

A0\0, the flow at the separation edge flaps downstream due tothe shear layer instability at x/H\4—5. A closer inspection nearthe corner step discloses that a corner flow exists near x/HO1.Moreover, a weak flow circulation is clearly displayed withinthe recirculating region (0Ox/HO4). After the reattachment,the large-scale vortices are seen to redevelop downstream.When the effective local forcing is perturbed (A0\0.3,St

H\0.477), the rolled-up vortex due to local forcing is seen

at 0Ox/HO1. As time elapses, the rolled-up vortex mergeswith the prior one and they convect downstream. However,when the forcing condition is A0\0.3, St

H\0.822, the recir-

culating region is not influenced by the local forcing. Thedisturbed flow simply propagates downstream along thedividing streamline.

To see the vortex amalgamation process in detail, the timeevolution of the vortex structure at A0\0.3, St

H\0.477 is

demonstrated in Fig. 11 for two periods of local forcing(0°O/O720°). An initial rolled-up vortex, which is generatedby the local issuing jet through a thin slit, is split into two

vortices with different rotations. This splitting may be causedby the shear layer force imbalance near the separation edge.The shear layer balance is destroyed by the local forcing, whichis very strong at the instant of local blowing. These splitrolled-up vortices propagate downstream. As time elapses,the vortex amalgamation proceeds as follows: the lowerclockwise vortex grows gradually in the recirculating region;the upper vortex, which is rotating counter-clockwise, isformed as a counterpart to the lower one to satisfy circulation.As a result, its strength is weaker than that of the lower vortex.The upper vortex merges with the prior lower vortex, which isconvected toward the bottom wall at x/H\1.5 by the upperone. The new lower vortex is convected downstream. Theamalgamation of the rolled-up vortices can increase flowmixing. Accordingly, the turbulence level is higher than that ofthe unforced flow. When the local forcing promotes thesevortex pairings, the reattachment length is shortened(xr/H\2—3).

An enlarged view of the initial rolled-up vortex formationis displayed in Fig. 12. A closer inspection of the flow near

139

Fig. 11. Vortex amalgamation process due to local forcing at A0\0.3, StH\0.477

the separation edge reveals that, in the local suction stage(0°O/O90°), a rolled-up vortex has been generated from theinlet wall layer at the separation edge. As time proceeds, thisrolled-up vortex flows downstream. However, when the localblowing commences (/P105°), the inlet rolled-up vortex isinfluenced by the local issuing jet. It is seen that a jet flow due

to the local blowing penetrates into the initial rolled-up vortex.A new rolled-up vortex formation is shown, which is then splitinto a pair of vortices, i.e., the upper and lower vortex. Asstated above, the upper vortex merges with the previous lowervortex, which increases flow mixing in the recirculating region.However, as the forcing frequency is further increased in

140

Fig. 12. Detailed evolution ofinitial rolled-up vortex at A0\0.3,St

H\0.477

Fig. 13 (A\0.3, StH\0.822), there is no pairing process. The

initial weak vortex is not merged with the local blowing, but itpropagates downstream.

4ConclusionsAs an extension of the prior wind tunnel experiment, a waterchannel experiment has been made of the separated flowover a backward-facing step. The main objective of the presentflow visualization study was to delineate the vortex amalgama-tion process due to local forcing. The local forcing wasintroduced by a sinusoidally oscillating jet, which was drivenby a scotch-yoke system. The overall flow characteristicswere consistent with those of the wind tunnel experiment.It was found that the reattachment length has a minimumat an effective forcing frequency (A0\0.3, St

H\0.477). The

effective reduced forcing frequency based on the momentumthickness was obtained at Sth\0.025, where the low Reynoldsnumber flows with laminar boundary layers upstream ofseparation were dealt with. The vortex amalgamation processwas captured in the present flow visualization. Due to thelocal forcing at the separation edge, the initial rolled-upvortex was split into two vortices with different rotations.The upper vortex merged with the prior lower vortex, whilethe new lower one was convected downstream. Amalgamationof the rolled-up vortices increased flow mixing, which re-sulted in shortening the reattachment length. However, fora higher local frequency, no vortex pairing occurred. Thedistributed flow by local forcing could not penetrate into theshear layer, but propagated downstream along the dividingstreamline. Accordingly, the reattachment length was notshortened.

141

Fig. 13. Detailed evolution of initial rolled-upvortex at A0\0.3, St

H\0.822

ReferencesAbernethy RB; Benedict RP; Dowdell RB (1985) ASME measurement

uncertainty. J Fluids Eng 107: 161—164Bhattacharjee S; Scheelke B; Troutt TR (1986) Modifications of vortex

interactions in a reattaching separated flow. AIAA J 24: 623—629Brederode V; Bradshaw P (1978) Influence of the side walls on the

turbulent center-plane boundary-layer in a squareduct. J Fluids Eng100: 91—96

Chun KB; Sung HJ (1996) Control of turbulent separated flow overa backward-facing step by local forcing. Exp Fluids 21: 417—426

Eaton JK; Johnston JP (1980) Turbulent flow reattachment: Anexperimental study of the flow and structure behind a backward-facing step. Report MD-39, Thermoscience Division, MechanicalEngineering, Stanford University

Hasan MAZ (1992) The flow over a backward-facing step undercontrolled perturbation: laminar separation. J Fluid Mech 238:73—96

Kiya M; Shimizu M; Mochizuki O; Ido Y; Tezuka H (1993)Active forcing of an axisymmetric leading-edge turbulent

separation bubble. AIAA paper (AIAA-93-3245), July 6—9,Orlando, FL

Kiya M; Shimizu M; Mochizuki O; Ido Y (1993) The forced turbulentseparation bubble. 9th Symposium on Turbulent Shear Flows,August 16—18, Kyoto, Japan

Nagib HM; Reisenthel PH; Koga DJ (1985) On the dynamical scaling offorced unsteady separated flows. AIAA Shear Flow Control Confer-ence (AIAA-85-0553), March 12—14, Boulder, Colorado

Obi S; Ohizumi H; Aoki K; Masuda S (1993) Turbulent separationcontrol in a plate asymmetric diffuser by periodic perturbation. 2ndInt Symp on Engineering Turbulence Modeling and Measurements,May 31—June 2, Florence, Italy

Roos FW; Kegelman JT (1986) Control of coherent structures in re-attaching laminar and turbulent shear layers. AIAA J 24: 1956—1963

Vogel JC; Eaton JK (1985) Combined heat transfer and fluid dynamicmeasurements downstream of a backward-facing step. ASME J HeatTransfer 107: 922—929

Sigurdson LW (1995) The structure and control of a turbulentreattaching flow. J Fluid Mech 298: 139—165

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