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Analysis of a Jet-Controlled High-Lift Hydrofoil with a Flap Shin Hyung Rhee, Sung-Eun Kim (Fluent Inc., USA), Haeseong Ahn, Jungkeun Oh, Hyochu] Kim (Res. Tnst. of Marine Systems Eng., Seoul Nat'] Univ., Korea) ABSTRACT A jet-controlled high-lift hydrofoil with a flap is investigated using both experimental and computational methods. Experiments are being carried out in a cavitation tunnel to measure forces and moment acting on the hydrofoil, and surface pressure distribution. The measured data show the feasibility of such a device for marine applications. Computational studies have also been carried out in parallel with the measurements. The computational results are analyzed in terms of global and local quantities using available experimental data. The present computational results compare well with the well-known experimental data for circulation control flows. The results for flow around a hydrofoil with a blown flap further validate the concept behind the proposed device. The results of the study demonstrate the applicability of the technology to the design of practical control surfaces. INTRODUCTION =, _ ~ It is widely acknowledged that one of the most efficient ways to increase lift is boundary layer and flow control. The technology has been investigated both experimentally and numerically. However, most of the studies focused on aerospace applications, and efforts targeted for marine applications are rare. With the growing interest in low-soeed maneuverability which has become increasingly Important In design process or surface ships and under-water vehicles, studies of high-lift devices for marine applications are warranted. Besides mechanical flaps or slats or spoilers, internal blowing systems for boundary layer control (BLC) is known to be most efficient for lift production. Internally blowing BLC systems are characterized by the use of fluid jets ducted from within the control surface. BLC utilizes the added momentum due to the injected fluid to delay flow separation at critical locations on a lifting surface, augmenting lift much beyond what is possible without the injection. For application to marine vehicle control, the concept of a blown flap has several advantages over other power devices (Wilson and von Kerczek, 1979~; (1) it is mechanically relatively simple, (2) the flap size could be chosen to provide adequate performance without blowing at high speed, (3) powered operation at low speeds could provide a wide range of available control force, (4) the flap could be used as a plain device without power, if fluid pumping system fails, and vice versa, and (5) water exhausted from the ship could be ducted into a high-pressure plenum that would act as a supply for blowing. Also in terms of hydrodynamic performance, the blown flap is the most attractive high-lift scheme of all those considered in Wilson and van Kerczek (1979~. Many experimental studies have been done for BLC airfoil with a blown flap in the aerodynamics field, where substantial lift- augmentation was demonstrated (e.g., Attinello, 1961~. Similar studies with marine applications in mind are, however, hardly found. Studies of circulation control (CC) wings also have been carried out, and some of them were intended for application to marine vehicle control surfaces (e.g., Englar and Williams, 1971~. Yet most studies focused on short take-off and landing (STOL) and vertical take-off and landing (VTOL) capabilities and lift augmentation for transport aircraft (Abramson and Rogers, 1983; Englar and Huson, 1983; McLachlan, 1989; Englar et al., 1993~. A number of computational studies for CC flows are available in literature. Dvorak and his colleagues used interactive approaches of potential and viscous flow solvers (Dvorak and Kind, 1979; Dvorak and Choi, 1983) in early days. Reynolds- Averaged Navier-Stokes (RANS) equations were solved later with algebraic and two-equation turbulence models for jets of small and intermediate momentum (Shrewsbury, 1985; Berman, 1985; Pulliam et al., 1985; Linton, 1994~. All of them
showed reasonable and promising results, although limited to relatively mild conditions. Quite recently, computational studies for unsteady flows around more practical geometry (Liu et al., 2001) and for jets of higher momentum were reported using sophisticated turbulence models (Slomski et al., 2002~. These studies provide valuable insights into the physical and numerical aspects of the flows. Studies for BLC hydrofoil using a blown flap, however, have not been reported in the literature, to the authors' knowledge. The present study is concerned with experimental and computational investigation of a jet-controlled high-lift hydrofoil with a flap, i.e., BLC hydrofoil using a blown flap. The primary objective is to understand the lift increase phenomena by such a device, which can be implemented in many of already deployed surface ships and under-water vehicles. To this end, an extensive experimental study is underway at the Research Institute of Marine Systems Engineering (RIMSE) of Seoul National University (SNU). In parallel, a concurrent and complementary computational investigation has been conducted at Fluent Inc. The global quantities, such as lift, drag and momentum, and local flow measurements, such as surface pressure, are measured for a range of flow conditions. Computational fluid dynamics (CFD) study reinforces the validity of the technology and provides insights for optimum design of such devices. CFD study of CC flows for two types of geometry and conditions were also carried out to verify the selection of appropriate numerical schemes and turbulence models. The present paper is organized as follows. The experimental study is described in the next section along with the experimental facility, equipments, set-up, and conditions, followed by computational method employed for the present study. The CFD validation for CC flows is presented first in the section for comparison and validation, and the experimental and CFD study results and comparison for BLC flow follow. Lastly, some concluding remarks are made. EXPERIMENTAL METHOD AND VERIFICATION The experiments are being carried out in the cavitation tunnel of RIMSE. The geometry of two- dimensional (2D) hydrofoil section was taken from NACA 0021 airfoil, which is close to a typical rudder section, and a 0.25c flap, where c is the chord length, was installed in a way that the jet slot maintains its height at 0.0088c. The blowing system is accommodated inside the morel. Figure 1 shows the hydrofoil section with a flap. The jet flow is supplied by a non-contact type Labyrinth connector and kept at a constant rate using a pressurized chamber outside the tunnel. The air in the chamber was compressed, so that fluctuations due to water supplied are suppressed. A three-component load cell was manufactured and installed inside the hydrofoil for force measurements. Prior to being assembled, the load cell was calibrated statically for each component of force. Calibrations were also carried out after the assembly of the load cell. The calibration results showed good linearity, negligibly small hysteresis, and reasonably small interference between force components. The voltage signals for each force component are digitized sequentially through an A/D converter and stored. One thousand data points per second were recorded for 20 seconds. Each data set was divided into several subsets and only the ones with the standard deviation less than 10-3 were taken. For surface pressure measurements, 28 pressure taps were placed on the hydrofoil surface with a cosine spacing distribution. A scannivalve was connected to the pressure taps by vinyl tubes and pressure transducers were used to get the total head corresponding to surface pressure. ~ Her Id- -S~n~s~r ,,, i '~5_ N ,;E, 1 I/ _ Figure 1. Foil section with a flap (dimension in mm). 1.2' no` n7` .,'_ _ _ _ _ L _ _ _ _ ___ _ _ I _ 0 5 10 15 20 25 a Figure 2. Lift coefficient vs. angle of attack for NACA 0021 without a flap. In order to verify the appropriateness of the experimental set-up and measurement accuracy, a
NACA 0021 airfoil without a flap was built, and then forces and surface pressure were measured. Figure 2 shows sectional lift coefficient, Car, at angle of attack, cz, between 5 and 30 degrees. Although the present measurements show a delayed stall, which is attributed to possible three-dimensional (3D) effects, the agreement is fairly good up to a=20 degrees. Figure 3 shows the surface pressure coefficients at cx=0 degree. Also shown for comparison are Euler and RANS solutions from FLUENT. The comparison is commendable and the experiment is shown to capture the boundary layer effect near the trailing edge correctly, which verifies that the present pressure measurement system is reliable. The non-dimensionalized momentum of jet, i.e., jet momentum coefficient, Cal, is defined as C =mVj/] pV2c (1) where m is jet mass flow rate, Vje, is averaged jet velocity through the slot, and pa and Van is free stream density and velocity, respectively. 0.5 c) 0 1< D ' ' O Present ~ ' , Euler solution (FLUENT) ~ , ~ , - - - - -, RANS k-m poludon (FLUENT) _~ -1 - -1 0 0.05 U.1 U.1) U.2 U.~) x (m) Figure 3. Surface pressure coefficients vs. chord length for NACA 0021 without a flap at a=0 degree. Based on the experience obtained through this preliminary study, the experimental set-up and measurement techniques were refined, and experiments are being carried out for BLC hydrofoil using a blown flap. Some of the results for global quantities were presented and demonstrated the feasibility and potential of the device (Akin et al., 2000~. It was evident there that the Coanda effect works favorably, i.e., the flow speed increases and pressure decreases on the suction side, and consequently separation is suppressed, resulting in lift increase. COMPUTATIONAL METHOD CFD study was carried out using FLUENT. FLUENT solves the Reynolds-averaged Navier- Stokes equations. The k-co SST (Menter, 1994), k-w hereafter, and Reynolds stress transport (Kim, 2001), RSTM hereafter, turbulence models are used for turbulence closure in the present study. The k-co model is one of the most widely used turbulence models for external aero- and hydrodynamics. RSTM is the most advanced turbulence models for engineering applications and has shown better potential to predict the key features of the present flow than other models. FLUENT employs a cell- centered finite-volume method along with a linear reconstruction scheme that allows use of computational elements with arbitrary polyhedral shape. Convection terms are discretized using a second order accurate upwind scheme, while diffusion terms are discretized using a second order accurate central differencing scheme. For transient flow calculations, time derivative terms are discretized using a first order backward implicit scheme. The velocity-pressure coupling and overall solution procedure are based on a SIMPLE type segregated algorithm adapted to unstructured grid. The discretized equations are solved using pointwise Gauss-Seidel iterations, and algebraic multi-grid method accelerates the solution convergence. More detailed description of numerical method is available in Kim et al. (1998~. CFD RESULTS AND COMPARISON DATA Both CC and BLC flows are challenging to any CFD codes: different Reynolds number flow regimes, due to the difference in length and velocity scales between the foil and jet slot, should be considered in a single flow field; a turbulence model that can properly take wall jet effects into account is required, and; stagnation point and pressure gradient near the jet, which are largely correlated with surface pressure, should be accurately predicted. In the present section, CFD results for both CC and BLC flows are presented. CC flow simulations serve as verification tests of the present physical modeling and numerical schemes. CC flow results are validated against the well-known experimental data for elliptic foil geometry (Kind and Maul, 1968; Englar, 19711. BLC flows are considered for a range of parameters and the results are compared with data obtained from the present experimental study. Circulation Control Flows around Elliptic Airfoils First, CC flow around a 15-percent pure elliptic airfoil, Englar case hereafter, is considered (Englar, 1971). An upper surface tangential slot with a height of 0.00125c is placed at 0.924c from the
leading edge. The computational domain is oval- shaped with extent -2 < x/c < 6 and -2.5 < y/c < 2.5 . The computational mesh shown in Figure 4 consists of 50,984 quadrilateral cells and the first cell spacing off the solid surface is approximately I in terms of wall y+. Computational conditions are set to reproduce one of the experimental conditions in Englar (1971), i.e., chord based Reynolds number Re = V~c/v = 5.48xlO , where v is the kinematic viscosity, cx=3 degrees and COO. 138. Incompressible air flow was assumed. Based on c of 0.2032 m, m of 0.084 kg/s is supplied at the end of the plenum shown in Figure 4, while Vet =39.4 m/s is imposed on the front and side boundaries. No-slip condition is imposed on solid surfaces, and zero-static-pressure condition on exit boundary. it. ~ _ a_ _ Figure 4. Computational mesh for Englar case. reproduced. Two CFD results are so close to each other and hard to discern one from the other, suggesting that k-a) model performs sufficiently well for this type of flows, i.e., boundary layer with clearly detaching wall jets. Aeon tic; ec~loo~ on -2 l -4 ~ . _~. :! , , . 1 6` f . . r ~ Presents k-m -- ~ -- Present,,RSTM O Englar 61971) -v , ., . I,,,, I,,,, I,,, _ _ 0 o.os 0.1 0.15 0.2 x (m) Figure 5. Foil surface pressure coefficient vs. chord length for Englar case. ~ ~42 1~87e+Ce 7~ 14~ I..'. 1~ : S.: T: , j . 1. 9~1 .8,~1 . B.~, 5~g1 4.~1 , 2~: 7.3300i: 1 : ~~ aY~ty ~6 em $, Cat za =2 Table 1. Cl for Englar case. Figure 6. Velocity magnitude contours for Englar k-co RSTM Measured case: k-m solution. C, 1.986 1.946 1.944 Both the global, i.e., sectional lift coefficient Cl = lip/ 2 REV 2 ~ and local, i.e., pressure coefficient Cp = pressure/2 pV2, quantities are considered for validation. No adjustments were done to either ~ or Car, since two-dimensionality was already assured in the measured data. Table 1 presents predicted Car obtained using both k-m and RSTM. Also presented is the measured value (Englar, 19711. Both results agree well, less than 2.2% difference, with the experimental data. Cp predictions also compare well with experimental data as shown in Figure 5. Peak pressures and slopes of the Cp curves are well : ~ . . . ~ Path Lines =~ Dy P+sure Bert Mar 26 ~ 12 E!JI 6.~(Z~, setled, sst~ Figure 7. Pathlines colored by Cp for Englar case: k-m solution.
Figure 6 shows contours of velocity magnitude, q = ,/u2 +v2, where u and v are x- and y-components of velocity in the Cartesian coordinate system, around the trailing edge. The jet flow detaches smoothly and clearly from the trailing edge, which is not round enough to produce strong Coanda effect, and the flow field resembles that of BLC flows. This is the reason why rounded trailing edges produce larger lift and are preferred for CC airfoil studies (Kind and Maul, 1968; Englar, 1971; Abramson and Rogers, 1983; Englar et al., 1993~. Pathlines colored by Cp around the foil are shown in Figure 7. The Coanda effects, i.e., relocated stagnation point and increased pressure difference between suction and pressure sides, are clearly displayed. Figure 8. Computational mesh for K&M case. Table 2. C/ for K&M case. . . . . . k-co RSTM Measured C, 2.27 2.60 2.71 The second case for CC flow is turbulent flow around a 20-percent elliptic foil with circular trailing edge that creates 0.00067c high slots on the upper and lower surfaces at 0.962c from the leading edge, K&M case hereafter (Kind and Maul, 1968~. The computational domain is larger than that of Englar case with extent ~ < x/c < 11 and -5 < y/c < 5, to accommodate thicker airfoil and encompass larger jet influenced region. The computational mesh shown in Figure 8 consists of 48,600 quadrilateral cells and the first cell spacing off the solid surface is approximately I in terms of wall y+. Computational conditions are set to the experimental conditions, i.e., Re = 7.5x 10 , oc=-0.6 degree and C~=0.094 from the upper slot only. As for Englar case, incompressible air flow was assumed. Based on c of 0.372 m, m of 0.076 kg/s is supplied at the end of the plenum, while VOO =29.45 m/s is imposed on the front and side boundaries. No-slip condition is imposed on solid surfaces, and zero- static-pressure condition on exit boundary. - ~G - - -8 . . . . . . . . . . . . . . . . I . . . . . . . . . . . 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 x (m) Figure 9. Foil surface pressure coefficient vs. chord length for K&M case. A. -1A15~9` _~.~t~ _1~ _~*K _~`q: _~*= '_~.~4-0C"'''''' 4.~x - .85 - QC''''' ; _~-.~ . .. ~ P.IbLiO65~P~ - =~8~ .,, ~ ~ , `54', 2~,,~ .~ ~ "~'r2i°.~_. .................................. ^pt0, ~ ~_ _ Figure 10. Pathlines colored by Cp for K&M case: k-m solution. _~.~x -1~ _1.5~X =~X ~X =.~X ~ -3~,... .^ - - _~X _~x ...~,,,,....~... _4~0C . . - ~X [' ~ /' '\ ~ : ~ Pbthtm8$~P~ - ~8tl ~t01 FLLIENT6.~ ~j R~ Figure 11. Pathlines colored by Cp for K&M case: RSTM solution.
Again Car and Cp are considered for validation. No adjustments were done to either ~ or Cat. Table 2 presents predicted Car obtained using both k-w and RSTM. Also presented is the measured value (Kind and Maul, 19681. The RSTM prediction is again quite good, i.e., 4.2% difference, although not as close to the measured value as for the Englar case. Of interest here is, however, the fact that, unlike the Englar case, k-6o model prediction is much lower than the experimental data. Similar behavior was also observed by Slomski et al. (2002) with a family of k- £ models. They found that isotropic, eddy-viscosity based turbulence models are inadequate for flows with curved wall jets and separation. Although k-m models generally show better performance than k-£ models for boundary layer with crossflow and streamwise vortices (Kim and Rhee, 2002), they suffer from the lack of fidelity as far as curved wall jets are concerned. This is supported by Figure 9, which shows Cp comparison on the hydrofoil surface. The pressure side Cp is low and flat after x=0.3m and it contributes to the low Car prediction. The reason of this low Cp can be found in the behavior of the pathlines around the foil shown in Figures 10 and 1 1. In the k-co model solution, the jet remains attached to the foil surface further around the round trailing edge, and this suction reduces Cp in this region and eventually C', although not as severe as in the k-£ model results reported by Slomski et al. (2002~. This behavior warrants a special care when CC flows with rounded trailing edges are to be simulated. However, as observed by the Englar case, the k-m model seems to perform quite reasonably, as long as there is a clear jet detachment physically at the trailing edge. Boundary Layer Control Flow around a Jet- Controlled Hydrofoil with a Flap Experimental and computational conditions, measured and computed Car and Cp, and flow field analysis using CFD results for BLC hydrofoil using a blown flap are described and discussed in this section. Based on the authors' experience and related study (Kerwin et al., 1972), and considering marine vehicles' operating conditions, two As, i.e., 0- and 10 degrees, and three C,u's' i.e., 0, 0.16 and 0.64, are considered with one flap angle, &20 degrees. Computational conditions are indicated by combinations of numbers for ~x, 3, and C, hereafter, i.e., (10,20,0.64) case indicates that oc=10 degrees, ~20 degrees, and CJU=0.64. Re based on the original chord length, 0.25m, is l.91xlO5. Two non-zero C,~`'s corresponding to the chord length can be reproduced with mof 6.6 kg/s and 13.2 kg/s at the end of the plenum, and Van =l m/s is imposed on the front and side boundaries. No-slip condition is imposed on solid surfaces, and zero-static-pressure condition on exit boundary. The computational domain is C- shaped with extent -2 < x/c < 5 and -4 < y/c < 4 . . . . ~ _ . . . Figure 12. Computational mesh for (10,20,0.64) case. The computational mesh shown in Figure 12 is for (10,20,0.64) case and consists of 198,892 quadrilateral and triangle cells. Boundary layers are placed around the solid surface with the first cell spacing equal to approximately l in terms of wall y+. Circular sub-domain around the hydrofoil is generated and triangle cells fill the remaining region of the sub-domain. The region outside this circular sub-domain is also filled with appropriately growing triangle cells using the sizing function capability available in GAMBIT, a Fluent pre-processor. Computational meshes for other cases, i.e., other As, are generated simply by rotating the circular sub- domain and remeshing the region outside the sub- domain. In this way, mesh quality around the hydrofoil is maintained even with different As. -7. 0 0.0s of. 0.~5 0.2 0.2s x (m) Figure 13. Foil surface pressure coefficient for (10,20,0.64) case obtained by k-m and RSTM models.
In order to have confidence in the turbulence model selected and mesh quality, the influence of turbulence models on the solution and mesh dependence were investigated with (10,20,0.64) case. As expected from the already discussed CC flow results, both k-m and RSTM models perform equally well for this type of flows. Car values show 2% difference from each other, i.e., 4.33 (k-Go) and 4.24 (RSTM). Cp's on the hydrofoil surface are also close to each other as shown in Figure 13. For BLC hydrofoil using a blown flap, which is discussed in the present section, therefore, only k-w model is employed. -1 -7.5 _ _ _ ~ // lo/ I_ ·,` . . . . . . . . . . . . . . . . . -1V 0.05 0.1 0.15 0.2 0.25 x (m) Figure 14. Foil surface pressure coefficients for (10,20,0.64' case obtained on coarse and fine meshes. Mesh dependence test was done using two meshes, and computed results on the meshes were compared. The fine mesh is shown and discussed above. The coarse mesh is generated in the same domain, but with larger first cell spacing, y+~30, and less cell numbers, 46,002 cells. Figure 14 shows one of the comparisons, i.e., Cp on the hydrofoil surface for (10,20,0.64J case, where two curves show close agreement. Car comparison between two solutions also shows less than 3.7% difference, i.e., 4.33 (fine) and 4.17 (coarse). The fine mesh was used for computations and presentations. Table 3. Car for BLC cases. . Computed Measured 0.56 1.20 2.09 1.48 2.20 (0,20,0) 0.62+0.022 (0,20,0.16) 1.97 (0,20,0.64) 2.96 (10,20,0) 1.47+0.008 (10,20,0.16) 3.08 (10,20,0.64) 4.33 3.33 As for CC flow cases, both global and local quantities are considered for analysis and discussion. Cl's obtained from experiments and computations are compared in Table 3. Note that computed C`'s for (0,20,0J and (10,20,0) cases display unsteady oscillations, which are unavoidable with 20 degree flap angle, and are presented by mean values plus/minus fluctuations. Measured Cats for the same cases show mean values only. Unlike CC flow cases, differences between computed and measured values are large. Reasons of these large differences seem to be (a) increased three-dimensional flow and tunnel wall effect with jet flow and flapped geometry, and (b) inconsistency in C,u due to inadequate jet ejection system. Nevertheless, it is interesting to see that (a) lift augmentation is more efficient at ~0 degree than at o`=lO degrees, i.e., 4.8 VS. 2.95 times from C~=0 to C~=0.64, (b) lift augmentation is larger between COO and CFO.I6 than between C,~O.16 and C'O.64, and (c) unsteady oscillations in lift are removed by BLC. {;ofiou~s clt T~t~ YE (~4 -I ~~ ~ ~ = fLU~r6.~[,= ~~81" 8~. ~, Figure 15. Turbulent eddy viscosity contours for (0,20,0) case at a certain instant. o -2 4 -6 0 0.05 0.1 0.15 0.2 0.25 x (m) l l l l l 0~ ' I I LO O ~ O _ ~ ~ it;, <) ~ ~ ~ ~ l l l l 1 1 1 1 t00006~ i' , r-----~---------~----------~---- ~ 1 , Ill , , , 1 1 1 1 l l l Ill 1 1 1 1' I 1 1 1 1 , , , 1 1 I I 1 1 1 _ 1 , I ~ O ' Masured ' ' | I ~ Computed ~ , l l l ~ 1 l 1 1 1 \ l ,,,.i.,.,i,,,,i,,,~ i,... Figure 16. Surface pressure coefficients for (0,20,0.16) case. The unsteadiness in (0,20,0) and (10,20,0) cases are confirmed by transient mode computations with time step size 0.001. Figure 15 shows turbulent eddy viscosity contours for (0,20,0) case at a certain instant where shed vortices are clearly displayed.
Removing unsteadiness in the flow field is one of the advantages of using BLC for a flapped hydrofoil, because high frequency unsteadiness is a main cause of vibration, noise, and control problems. 2.5 Or _ r ~7.5 . ~00000 100 ~ ~ I I IU ,,,,, ~ .,, . i,,,, 6,,, . ~ · .,, 0.05 0.1 0.15 0.2 0.25 x (m) Figure 17. Surface pressure coefficients for ¢0,20,0.64) case. 2 _ o -2 -6 1 ~ 1 1 ~0 ' ~- 'I ' O O J ~ ; . ;,0 ~ r I ~ T 1 1 1 1 1 1 u~ 1 _ _ _ _ J _ _ _ _ _ _ _ _ _ _ _ _ . i~ ~ M= / ~ Computed . - , , ,...~....~....~....~.... -8( 0.05 0.1 0.15 0.2 0. 25 x (m) Figure 18. Surface pressure coefficient for (10,20,0.16> case. 2.5 o -2. Sac 5 7 ~ 1 ~ ~ 1 W00g 0 O.' '= 0~0 ' ~ , o04 O Q 0 , oo ,/ ,° / ! . ~ , , , , ~ . - i- ~ ~ M88~ j / , Computed ~ I 10 /,,,, i, ... ~ ., .. I .......... ~ 0 0.05 0.1 0.15 0.2 0. 25 x (m) Figure 19. Surface pressure coefficient for (10,20,0.64) case. ' Cp comparisons on the hydrofoil surface are presented in Figures 16 through 19. As for Car, differences are larger than that shown in the validations for CC flow cases. Suction side pressure peaks and the overall pressure difference between suction and pressure side are smaller in the measured data. The differences between measured and computed Cp's increase consistently with increasing ax and Cal. Having seen the good agreement shown for verification measurements and CC flow validations, there appears to be non-negligible amount of uncertainties and errors in the measurements, especially the jet ejection system. More rigorous verification and improvement of the measurement system are under way. Pa Lass =~ by Pr~ssuls coor~aort pir~g-7.4~ol ~ ~ Mer 29, 20132 FLUENTLY t~ `~t~ ache ~~ Figure 20. Pathlines colored by Cp for (0,10,0) case. | Path Lines Id by Pressum (;O~QIlt led Mix) 20 ~ FLU~T ~ ~ ~3. se~t" , Figure 21. Pathlines colored by Cp for (0,109O.16) case. Flow field alteration is another advantage of employing BLC hydrofoil using a blown flap. Figures 20 through 22 show the pathlines colored by Cp around the hydrofoil. The undulating and recirculating flow field due to the 20 degree deflected flap is removed by BLC. Increasing circulation, i.e., shifting down jet flow in the wake, and larger difference in suction and pressure side CptS7 which augment lift, are evident. This also can help the wake flow straighten up and eventually reduce noise and signature.
n~cines=~ tyP~m =~8~ M8t29.~ . ___ ~T ~ O (= ~~ eta Figure 22. Pathlines colored by Cp for (0, 20, 0.64J case. CONCLUDING REMARKS A jet-controlled high-lift hydrofoil with a flap is investigated using both experimental and computational methods. The experiments are being carried out in a cavitation tunnel to measure forces and moment and surface pressure distribution on the hydrofoil. The measured data, although not final, prove the feasibility of such a device operating in water flow. In addition to that, a substantial amount of know-how has been accumulated in the course of the experimental program, which would improve the on-going experiments. CFD studies have been carried out simultaneously for the CC flow around a trailing edge, and the present problem, i.e., BLC flow around a hydrofoil with a blown flap. The computational results are analyzed with global and local quantities and validated using available experimental data. The present computational method reproduces the well- known experimental data for CC flow quite well. The results for BLC hydrofoil flow using a blown flap reinforce the validity of the technology. Also the CFD study identifies the high frequency and small amplitude unsteadiness in the cases of deflected flap without blowing, closely capturing the flow field in the wake modified due to the flap. Another important finding from the present CFD study is that k-m model performance is largely comparable to that of RSTM's for some of the less severe cases. The results of the study demonstrate the applicability of the technology to the design of practical control surfaces. Also it is found that the device functions more efficiently at smaller ~ and Cal, which is desirable especially for low-speed maneuvering. However, some improvements are needed. Jet ejection system needs to be improved so that more reliable data can be obtained. Rigorous uncertainty analysis is warranted for both experimental and computational studies. As for the future work, it would be interesting to consider unsteady fluctuating jet ejection with less jet momentum. The effects on propeller wake, where usually control surfaces are located, and hull/control- surface juncture flow should also be taken into account. ACKNOWLEDGMENT The experimental portion of the present study is being supported by RIMSE and the Brain Korea 21 project. REFERNCES Abramson, J., and Rogers, E.O., "High-Speed Characteristics of Circulation Control Airfoils," AIAA Paper 83-0265, 1983. Ahn, H., Oh, J., and Kim, H., "An Experimental Evaluation of the Coanda Effect on a Submerged Flapped Wing," Proc. 4th International Conf. On Hydrodynamics, Yokohama, Japan, 2000. Attinello, J.S., "Design and Engineering Features of Flap Blowing Installations," in Boundary Layer and Flow Control, Ed. G.V.Lachman, Pergamon Press, New York, Vol.l, 1961. Berman, H.A., "A Navier-Stokes Investigation of a Circulation Control Airfoil," AIAA Paper 85- 0300, 1985. Dvorak, F.A., and Choi, D.H., "Analysis of Circulation Controlled Airfoils in Transonic Flow," J. Aircraft. Vol.20~ No.4, 1983, p.331- 337. Dvorak, F.A., and Kind, R.J., "Analysis Method for Viscous Flow Over Circulation-Controlled Analysis," J. Aircraft~ Vol.16~ No.l, 1979, pp.23-28. Englar, R.J., "Two-Dimensional Subsonic Wind Tunnel Tests of Two 15-Percent Thick Circulation Control Airfoils," David Taylor Naval Ship Research and Development Center. Technical Note AL-211 Aug. 1971. , , Englar, R.J., and Huson, G.G., "Development of Advanced Circulation Control Wing High Lift Airfoils " AIAA Paper 83-1847 1983. , , Englar, R.J., Smith, M.J., Kelley, S.M., and Rover III, R.C., "Development of Circulation Control Technology for Application to Advanced Subsonic Transport Aircraft," AIAA Paper 93- 0644, 1993. Englar, R.J., and Williams, R.M., "Design of a Circulation Control Stern Plane for Submarine Applications," David Taylor Naval Ship
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