Hansen, Martin Otto Laver

Okulov, Valery

Velte, Clara Marika

The current work describes the experimental parametric study of streamwise vortices generated in a boundary layer by a rectangular vane (commonly named vortex generator) mounted perpendicularly to the wall and at an angle to the oncoming flow. Stereoscopic Particle Image Velocimetry measurements have been conducted in cross-planes to obtain a full picture of the wake for each measured case. It is observed that this seemingly simple configuration produces a complicated vortex system consisting of 4 vortices: the tip vortex, a horseshoe vortex system consisting of two sleeves and a secondary vortex generated by the interaction between the tip vortex and the wall. Depending on the height and angle of the vane, some structures are enhanced and others are suppressed. Comparing the corresponding vorticity and velocity fields, a strong correlation between the two is found. Stream-wise vorticity induced velocity with the observed strong correlation found is not at all evident. This has previously been shown to be true for the primary (tip) vortex, but not for the remaining secondary structures.<br/

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Downloaded from orbit.dtu.dk on: Dec 17, 2017SPIV investigations of correlation between streamwise vorticity and velocity in thewake of a vortex generator in a boundary layerVelte, Clara Marika; Okulov, Valery; Hansen, Martin Otto LaverPublished in:Proceedings of 10th International Symposium on Particle Image Velocimetry - PIV13Publication date:2013Document VersionPeer reviewed versionLink back to DTU OrbitCitation (APA):Velte, C. M., Okulov, V., & Hansen, M. O. L. (2013). SPIV investigations of correlation between streamwisevorticity and velocity in the wake of a vortex generator in a boundary layer. In Proceedings of 10th InternationalSymposium on Particle Image Velocimetry - PIV13 TU Delft.10TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY - PIV13Delft, The Netherlands, July 1-3, 2013SPIV investigations of correlation between streamwise vorticity and velocity inthe wake of a vortex generator in a boundary layerClara M. Velte1, Valery L. Okulov2 and Martin O.L. Hansen21 Department of Mechanical Engineering, Technical University of Denmark, Nils Koppels Alle´ B403, 2800 Kgs. Lyngby,Denmark, cmve@dtu.dk2 Department of Wind Energy, Technical University of Denmark, Nils Koppels Alle´ B403, 2800 Kgs. Lyngby, DenmarkABSTRACTThe current work describes the experimental parametric study of streamwise vortices generated in a boundary layer by a rectangularvane (commonly named vortex generator) mounted perpendicularly to the wall and at an angle to the oncoming flow. StereoscopicParticle Image Velocimetry measurements have been conducted in cross-planes to obtain a full picture of the wake for each measuredcase. It is observed that this seemingly simple configuration produces a complicated vortex system consisting of 4 vortices: the tipvortex, a horseshoe vortex system consisting of two sleeves and a secondary vortex generated by the interaction between the tip vortexand the wall. Depending on the height and angle of the vane, some structures are enhanced and others are suppressed. Comparing thecorresponding vorticity and velocity fields, a strong correlation between the two is found. Stream-wise vorticity induced velocity withthe observed strong correlation found is not at all evident. This has previously been shown to be true for the primary (tip) vortex, butnot for the remaining secondary structures.IntroductionIn recent years, the use of vortex generators (VGs) in aerodynamics has become a widely used method for suppressing the separationof a boundary layer from various surfaces, including airfoils, turbine blades, nozzles, diffusers, etc. The use of VGs for preventing flowseparation in diffusers was originally proposed in 1974 by Taylor [1]. A VG typically represents a short vane arranged perpendicularto the surface, at a certain angle of attack relative to the oncoming flow, so that the circulation flow past the VG forms a longitudinal tipvortex. In the presence of a VG, the flow separation is significantly delayed or even eliminated due to redistribution of the longitudinalmomentum of the moving medium. This approach to form a large scale vortex motion in the boundary layer for re-energizing the flowis one example of passive control of flow separation. The positive effect was studied and confirmed for various VG configurations andrelated vortex flow regimes. However, the vortex structures induced in the VG wake are still insufficiently studied and identified. Formore effective simulation and control of VG-induced vortex flows, this gap must evidently be closed.We have studied the flow structures downstream of an elementary VG representing a simple rectangular flat plate by the method ofStereoscopic Particle Image Velocimetry (SPIV) [2] and observed, in addition to the primary longitudinal tip vortex, some additionalsecondary vortex structures in several regimes. The presence of secondary vortices leads to perturbations and deformations of theprimary vortex [2] and influences its position [3, 4]. Therefore, it was expedient to study the vortex structure behind the VG in moredetail for a wider range of flow regime parameters.The present investigation was originally aimed at detecting and explaining the appearance of stable multi-vortex structures in the wakeof a rectangular VG or small vane in a near-wall boundary layer [5]. The resulting wakes, which were mapped in regimes, showed,in themselves, interesting results. However, when comparing the stream-wise velocity and vorticity, they displayed quite similardistributions and are hence connected. The current work aims to show the existence of this correlation and discuss the implications ofthis result.1. MethodThe experiments were carried out in a low Reynolds number closed loop wind tunnel with an 8:1 contraction ratio and a test sectionof cross-sectional area 300×600 mm with length 2 m. The test section had optical access through the top and bottom walls as well asthrough the sidewall opposite to the wall with the attached vortex generator. The wind tunnel speed was obtained by measuring thepressure drop across an orifice plate. The turbulence intensity at the inlet from laser doppler anemometry (LDA) measurements hasbeen found to be 13%. The boundary layer thickness at the position of the vortex generator has been estimated from LDA measurementsto be approximately δV G =25 mm.A single rectangular vane was mounted on one of the test section walls with a variable angle of attack to the oncoming flow. The vortexgenerator was positioned on a vertical wall in the center of the test section with its trailing edge 750 mm downstream of the inlet gridwhen it is at zero angle to the mean flow. In order to easily and accurately alter the device angle, the vortex generator was attached toa pin which could be accessed from outside the test section through a hole in the test section wall. This pin was in turn attached to apointer arm placed over a protractor indicating the relative angle of the actuator to the mean flow direction. The protractor had a radiusof 200 mm and grading for integer values of each degree.Stereoscopic PIV measurements were performed in cross planes 10 vane heights (h) downstream of the trailing edge of a singlerectangular vane in a first round. The vane had a thickness of about 1 mm. In order to avoid reflections from the wall and the vortexgenerator within the wavelength band of the camera filters, these areas were treated with a fluorescent dye, Rhodamine 6G, mixed withmatt varnish to obtain a smooth surface and to ensure that the dye stayed attached. The cameras were in turn equipped with band-passfilters designed to only pass light with wavelengths close to that of the laser light so that unwanted reflections were not recorded by thecameras. The angle of attack relative to the oncoming flow direction was varied from 9◦ to 54◦ with increments of 3◦. In addition, theVG height was varied from 5 to 25 mm with increments of 5 mm. The vane length (l) was always set to twice its height (l = 2h). Witha free-stream velocity of U∞ =1 m/s and a VG height of between 5-25 mm, the Reynolds number varied between about 300 and 1 700.The SPIV equipment was mounted on a rigid stand and included a double cavity NewWave Solo 120XT Nd-YAG laser (wavelength532 nm) capable of delivering light pulses of 120 mJ. The pulse width, i.e. the duration of each illumination pulse, was 10 ns.The light-sheet thickness at the measurement position was 2 mm and was created using a combination of a spherical convex and acylindrical concave lens. The equipment also included two Dantec Dynamics HiSense MkII cameras (1344×1024 pixels) equippedwith 60 mm lenses. Both cameras were mounted on Scheimpflug angle adjustable mountings. The seeding, consisting of DEHS(diethyl-hexyl-sebacin-esther) droplets with a diameter of 2.3 µm, was added to the flow downstream of the test section in the closed-circuitwind tunnel in order to facilitate a homogeneous distribution of the particles before they enter the test section. The two cameras wereplaced in the forward scattering direction. The angle of each respective camera to the laser sheet was 45◦. The f-numbers of thecameras were set to 2.8, yielding a depth of field which is small but sufficient to cover the thickness of the laser sheet and keeping allilluminated particles in focus while still attaining sufficient scattered light from the tracer particles. A calibration target was alignedwith the laser sheet. This target had a well-defined pattern, which could be registered by the two cameras to obtain the geometricalinformation required for reconstructing the velocity vectors received from each camera to obtain a full description of all three velocitycomponents in the plane. Calibration images were recorded with both cameras at five well-defined stream-wise positions throughout thedepth of the laser sheet in order to capture the out-of-plane component in the reconstructed coordinate system of the measurement planeunder consideration. A linear transform was applied to these images for each camera to perform the reconstruction. This procedure wasexecuted both previous to and after the conduction of the measurements to ensure that no drift had occurred. The images were processedusing Dantec DynamicStudio software version 2.0. Adaptive correlation was applied using refinement with an interrogation area sizeof 32×32 pixels. Local median validation was used in the immediate vicinity of each interrogation area to remove spurious vectorsbetween each refinement step. The overlap between interrogation areas was 50%. For each measurement position, 500 realizationswere acquired. The recording of image maps was done with an acquisition rate of 1.0 Hz, ensuring statistically independent realizationsbased on the convection velocity U∞ =1.0 ms−1 and the mesh size d =0.039 m, yielding a time scale of t = d/U∞ =0.039 s. Thevelocity vector maps contain 73×61 vectors. The linear dimensions of the interrogation areas (x,y)=(1.55,1.04) mm can be comparedto the Taylor microscale and the Kolmogorov length scale estimated to λ f ≈9 mm and η≈0.5 mm from LDA measurements [6].After analysis of the initial measurements, a second campaign was initiated where the most interesting cases were investigated morein-depth by mapping the downstream evolution of the wake in a number of planes covering both the near and the far wake. In the currentstudy, measurements at positions 3h and 10h downstream of the trailing edge have been utilized. The experimental setup represented aslightly modified system described elsewhere [2] and further details can also be found in [5].2. Results2.1 Wake regimesFrom water channel visualizations, the basic vortex structures were identified and subsequently schematically depicted in Figure 1(c.f. [5]). A circular flow past the plate oriented at the attack angle results in separation of the primary vane tip vortex P from theupper edge of the plate. In addition, a pressure distribution around the leading edge creates a weaker horseshoe-like vortex with twoarms occurring on opposite sides of the VG plate and passing downstream from the excess-pressure (HP) and -suction (HS) sides ofthe plate. The existence of these clearly pronounced horseshoe vortices was also observed in the flow past fixed ends of blades in axialturbines [5], where these vortices form a system of interblade forces. In the case of VGs, the HP and HS horseshoe vortices are stronglyinfluenced by the primary tip vortex and their behavior is thus somewhat different from that in the axial turbines.At an early stage, the primary vortex P sweeps the HS horseshoe vortex underneath itself and drives it to the side of HP vortex, whichdevelops independently of the (P, HS) pair. It was established that the horseshoe vortex does not appears in all flow regimes. Theformation of this vortex depends on the angle of attack and the relative VG height compared to the boundary layer thickness. Forcertain values of these parameters, perturbations introduced by the vane are insufficiently large and the resulting side arms cannot beidentified as vortices with individual cores and only appear as vorticity clouds on the surface. Thus, two typical configurations ofprincipal vortex structures consisting of one (P) or three (P, HS, HP) can appear in the wake flow downstream of the VG blade.In addition to the principal vortex structure described above, a secondary vortex system S can appear downstream of the VG as a resultof the interaction of the primary tip vortex with the boundary layer. A physical mechanism of this process was described long ago [7],while imaging and identification of the primary and secondary vorticity was reported quite recently [4]. In the regimes where only theprimary longitudinal vortex is well distinguished, the formation of a secondary vortex is also well illustrated by the results of our SPIVmeasurements [2] in the wind tunnel, see Figure 2. As the angle of attack is increased, the intensity of the tip vortex grows and itsaction upon the boundary layer increases. As a result, the region of vorticity concentration in the boundary layer on the right growsand eventually pinches off and forms an additional discrete vortex, D. This result agrees with the description of the mechanism of asecondary vortex structure in [7].Thus, the vortex structure downstream of the VG can represent various numbers and combinations of the primary and secondary vortexstructures. Taking into account the complex dynamics of the flow, with the possible interaction and merger of vortices with eachother, it is impossible to predict their number and mutual arrangement. For this reason, we have carried out a complete parametricinvestigation of the development of multi-vortex regimes in a wake past a model VG. The flow regimes were changed by varying thegeometric parameters of the VG, including the attack angle β and height h/δ of the vane, where δ is the local boundary layer thickness.As a result, three stable regimes have been established for the vortex system downstream of the VG. Figure 3a shows examples of thedistribution of the axial component of vorticity in these regimes with increasing number of vortices in the wake. Figure 3b presents amap of the possible regimes of flow past the VG.Figure 1: Sketch of vortex structures observed around a wall-mounted vane.Figure 2: Generation of secondary vortex structures by a VG with increasing angle of attack.2.2 Connection between stream-wise velocity and vorticityIn the process of the previous investigations, the stream-wise velocity and vorticity fields were investigated. It was observed, bothin the near wake and in the stable far wake of the vortex generator, that these display a clear coupling. Figures 4 and 5, display thestream-wise vorticity (left column) and corresponding stream-wise velocity (right column) fields for vane angles 21◦, 33◦, 48◦ and 54◦at 3h and 10h downstream of the vane trailing edge, respectively. At both positions, the two quantities are seen to be correlated. Thiswas previously know to be true for the primary vortex [2], but not for the secondary vortex structures.3. Summary and conclusionsThe emergence of secondary structures is observed to be highly dependent on the geometric attributes of the vortex generator, andcan result in a range of various far wake states. However, independently of which state is considered, the stream-wise velocity andvorticity fields display a directly visible correlation for all flow structures, including the secondary ones. The direct relation betweenthese variables was previously known to exist for the primary vortex, but not for the secondary structures. This is an important resultfor understanding the vortex generator wake, due to the coupling between the secondary and the primary velocity components. Thisknowledge can be implemented in models of the wake of VGs, e.g., in boundary layers, but is also interesting from a more fundamentalpoint of view to describe and control vortical motions behind VGs.Figure 3: Regimes of the different wake vortex structures produced behind the vane.REFERENCES[1] Taylor HD, Research Department Report No. R-4012-3 (Unitged Aircraft Co., East Hartford, 1947).[2] Velte CM, Hansen MOL and Okulov VL, “J. Fluid Mech.” 619 (2009) pp. 167–177.[3] Kramer W, Clercx HJH and van Heijst GJF, “Phys. Fluids” 19 (2007), 126603.[4] Harris DM, Miller VA and Williamson CHK, “Phys. Fluids” 22 (2010), 091106.[5] Velte CM, Okulov VL and Naumov IV, “Techn. Phys. Letters” 38 (2012), pp. 379-382.[6] Schmidt JJ, Experimental and numerical investigation of separated flows. PhD thesis, Tehcnical University of Denmark, Kgs.Lyngby (1997).[7] Harvey JK and Perry FJ, “AIAA J.” 9 (1971) pp. 1659–1660y/h [−]21°−2 −1 0 1 20123 21°−2 −1 0 1 20123y/h [−]33°−2 −1 0 1 20123 33°−2 −1 0 1 20123y/h [−]45°−2 −1 0 1 20123 45°−2 −1 0 1 20123y/h [−]54°x/h [−]−2 −1 0 1 20123 54°x/h [−]−2 −1 0 1 20123Figure 4: Correlation between streamwise vorticity (left column) and streamwise velocity (right column) in the near wake at x = 3h.y/h [−]21°−2 −1 0 1 20123 21°−2 −1 0 1 20123y/h [−]33°−2 −1 0 1 20123 33°−2 −1 0 1 20123y/h [−]48°−2 −1 0 1 20123 48°−2 −1 0 1 20123x/h [−]54°y/h [−]−2 −1 0 1 20123 54°x/h [−]−2 −1 0 1 20123Figure 5: Correlation between streamwise vorticity (left column) and streamwise velocity (right column) in the far wake at x = 10h.

SPIV investigations of correlation between streamwise vorticity and velocity in the wake of a vortex generator in a boundary layer

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SPIV investigations of correlation between streamwise vorticity and velocity in the wake of a vortex generator in a boundary layer

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