Full-Scale Wind Tunnel Study of the Seaglider Underwater Glider

Transcription

1 Full-Scale Wind Tunnel Study of the Seaglider Underwater Glider Laszlo Techy, Ryan Tomokiyo, Jake Quenzer, Tyler Beauchamp, Kristi Morgansen Department of Aeronautics & Astronautics, University of Washington Seattle WA, UWAA Technical Report Number UWAATR-21-2 September 21 Department of Aeronautics and Astronautics University of Washington Box 3524 Seattle, Washington PHN: (26) FAX: (26) URL:

2 Contents 1 Introduction 2 2 Experimental Setup Test Configurations Test Regime Conventions and Nomenclature Results Aerodynamic Coefficients for Standard Seaglider Aerodynamic Coefficients for Seaglider with 1.5 m Wings Effects of the Wings Drag Comparison of Seaglider Tail Section versus Ogive Tail Section Parasite Drag and Dynamic Pressure Drag Contribution from the CT-cell Static Longitudinal Stability Directional Stability Summary 2 5 Appendix A: Configuration Appendix B: Configuration Appendix C: Configuration Appendix D: Configuration Appendix E: Parameters 34

3 Full-Scale Wind Tunnel Study of the Seaglider Underwater Glider Laszlo Techy, Ryan Tomokiyo, Jake Quenzer, Tyler Beauchamp, Kristi Morgansen Department of Aeronautics & Astronautics, University of Washington Seattle WA, September 21 Abstract This report describes full-scale wind tunnel tests of the Seaglider underwater vehicle that were conducted on June 3, 21 in the University of Washington Kirsten Wind Tunnel. The goal of the wind tunnel study was to identify aerodynamic force-and moment coefficients of the glider. The identified parameters are important for 1) online performance calculations; 2) high-fidelity simulations; 3) aiding future glider designs; 4) data post-processing. Several different glider configurations have been tested, including two different tail sections and two different set of wings. The glider s original concave tail section was interchangeable with a newer design that featured an ogive profile. The glider model was also tested both in the presence and in the absence of the conductivity-temperature (CT) cell to measure drag contribution from the CT-cell alone, and to be able to compare results across the tail sections. During the test it was observed that the Seaglider lacks static longitudinal stability at the typical range of operating Reynolds numbers. The lack of static aerodynamic stability is counter-balanced by the moment resulting from the offset between center of gravity and center of buoyancy. The document presents aerodynamic coefficients for the glider at typical operating speeds. 1 Introduction The Seaglider is a buoyancy-driven underwater glider originally developed for oceanographic research. The vehicle was designed at University of Washington as a collaboration between the Applied Physics Lab and the School of Oceanography [2]. From early design stages significant emphasis has been placed on the aerodynamic properties of the vehicle to maximize performance characteristics, such as range and endurance. The streamlined profile of the vehicle outer skin was selected based on hydrodynamic study of axisymmetric bodies in axial flow [5]. The shape that was adopted for the vehicle based on this study has the ability to maintain laminar flow over more than 8% of the surface area at speeds as high as 7 m/s [2]. Even though this streamlined shape necessitated a sophisticated pressure hull design, the anticipated performance improvements were large enough to make this design effort worthwhile. The Deepglider is a variant of the Seaglider currently under development at University of Washington. The Deepglider s maximum depth is 6 m, at which depth extreme stresses are exerted on the pressure hull. This necessitated the departure from the tapered Seaglider design, and a technologically simpler and more robust cylindrical shape was adopted. Drop-tests have been carried out at the saltwater tank facility of the Seaglider Fabrication Center to test different outer skin shapes [6]. The drop-tests revealed that the original Seaglider shape had the largest drag-coefficient of all tested models, and that an ogive nose section and tail section performed significantly better. Flow-visualization studies conducted in the Low-Speed Wind Tunnel at the University of Washington Aerodynamics Laboratory (UWAL) during early development stages of the glider showed that the flow separates just behind the station of maximum radius on the original Seaglider fairing [2]. It was noted in [6] that the ogive tail prolongs the area of attached flow due to its convex, gently-tapered shape, explaining some of the performance gain compared to the original Seaglider tail featuring a concave profile. Identification of the aerodynamic force-and moment coefficients of the Seaglider is important for several reasons. 1) The parameters can be used in performance studies of the vehicle to help identify optimal flight conditions. Estimates of these parameters are presently used in on-line guidance calculations on the glider computer to determine 2

4 desired pitch mass location and buoyancy setting as a function of pilot-specified mission parameters, such as duration of the dive and desired depth. More accurate knowledge of these parameters improves the precision of the glider s navigation algorithm. 2) The aerodynamic parameters are necessary for the development of high-fidelity flight simulations of the glider. Such simulations can be used to study and validate the efficacy of different motion planning strategies. 3) Comparing the drag coefficient across the different glider configurations helps determine minimum-drag skin profiles and may guide the design of future gliders. 4) The parameters are currently used during post-processing of the glides and to visualize the glider s path in the horizontal plane. The paths are reconstructed using dead-reckoning based on the estimated speed and heading angle. Accurate knowledge of the speed is desired for the accuracy of such plots. The full-scale wind-tunnel tests described in this document were conducted on June 3, 21 in the University of Washington Kirsten Wind Tunnel to accurately measure the aerodynamic properties of the Seaglider. A set of vehicle configurations and geometries were tested in the tunnel, including different wings and fairing profiles. In addition to the identification of the desired force-and moment coefficients, the following observations were made: The drag coefficient for the Seaglider with the CT-cell is 47.7 % larger than without the CT-cell. The original Seaglider shape (tapered nose-section, concave tail section, 1 m wings) lacks static longitudinal stability in the typical operating regime. It was observed that the aerodynamic center of the vehicle shifts aft with increasing dynamic pressure, and the glider becomes stable between.22 < q <.66 psf corresponding to.32 < v <.56 m/s in water. This speed is currently outside of the glider s operating regime, although may be accessible using larger buoyancy engines. The lack of static aerodynamic stability is counter-balanced by the moment couple resulting from the offset between center of gravity and center of buoyancy. This metacentric height helps recover stability, explaining why the glider performs stable glides in experiments. With the larger, 1.5 m, wings the glider is aerodynamically stable in the typical Seaglider operating regime. At the glider s typical operating speed the parasite drag coefficient shows variation with dynamic pressure and does not stay constant. The functional dependence suggested in [2] was confirmed in the wind tunnel experiments. The magnitude of the measured yawing moments is very small, indicating that the glider is marginally directionally stable. The drag coefficient for the original Seaglider configuration with the concave tail section is 12.4% larger than for the configuration with the newly designed ogive tail section. Although the CT-cell was removed from the Seaglider, the oxygen sensor was left attached by mistake, leaving this test inconclusive as the increased drag may come from the oxygen sensor. UWAATR

5 2 Experimental Setup The full-scale wind-tunnel tests of the Seaglider and modified configurations were conducted in a Reynolds number matched test regime at University of Washington Kirsten Wind Tunnel. The tunnel is a subsonic, closed circuit, double return wind tunnel that has a test section with a rectangular 8 x 12 cross-section that is 1 feet long. Two sets of diameter seven-bladed propellers move the air up to 2 MPH through the test section. Additional information on the tunnel can be found in [1]. 2.1 Test Configurations Test Mount. The Seaglider was mounted in a cantilevered configuration in the wind tunnel with a custom machined aluminum mount piece (see Figure 1). This was the only non-destructive way of mounting the outer skin of an otherwise fully operational Seaglider, and also lent the least amount of aerodynamic interference when compared with other strap-down or side-braced methods. Note that in this setup the antenna mast had to be removed. The measured drag is thus slightly less than the true value, but this difference is arguably small compared to the full body-wing contributions. Metal tape was used to seal unused bolt holes as well as the venting hole at the tip of the forward fairing (Fig. 3b) to prevent airflow through the inside of the body. Flow through the body has two effects: 1) increased skin friction, and 2) reduced dynamic pressure at the stagnation point. In normal operation the vehicle outer skin and the internal pressure hull are fitted tightly together. Although the gap between the fairing and the hull is flooded, there is only minimal fluid motion inside the fairing. The glider could not be mounted with the pressure hull inside the fairing, and letting air flow through the empty cavity would have caused large measurement error. The holes were sealed based on these considerations. The Seaglider test configuration matrix is displayed in Figure 2. (a) Seaglider on the test mount. (b) Test mount. Figure 1: Cantilevered test mount that was used during the wind tunnel study. Configuration 1: The first configuration of interest was the full-scale Seaglider shown in Figures 3a-3b. The configuration consisted of the original Seaglider forward and aft fairing, CT-cell and 1 m wings. A detailed study of this configuration is presented in Section 3.1. Configuration 2: Configuration 2 consisted of the standard Seaglider forward and aft fairings, but the 1 meter span wings were replaced with the larger 1.5 meter span wings as shown in Figures 3c-3d. The fastener pattern of the wings were made identical, making this swap possible. The profile of the 1.5 m wings follows the same shape as the 1 m wings, so the only aerodynamic difference was the added span. The effect of larger wings is increased lift and pitching moment coefficient. The glider is aerodynamically stable with the larger wings at the typical operating speed. Configuration 3: The third configuration, consisted of the Seaglider forward and aft fairing as well as the CT-cell, with no wings attached. As seen in Figures 3e-3f, the fastener holes were sealed with metal tape to prevent through flow across the body. The merit of this study was to obtain coefficients for the body only. The effect of different wing geometries can then be easily studied, and incorporated into models of the vehicle. UWAATR

6 Configuration Nose Tail CT-cell Wings Configuration 1 Seaglider Seaglider Present 1 m Configuration 2 Seaglider Seaglider Present 1.5 m Configuration 3 Seaglider Seaglider Present Off Configuration 4 Seaglider Ogive Absent 1 m Figure 2: Test configuration matrix. Configuration 4: Configuration 4 consisted of the standard Seaglider forward fairing attached to an ogive aft section. This configuration did not have external sensors that would abrupt the flow (see Figures 3g-3h). The ogive aft faring is compatible with the earlier design for antenna appendage, therefore the same mounting mechanism could be used. Due to some imperfection, the Seaglider forward fairing and the ogive tail section did not match completely smoothly, and a small gap was observed along the upper surface. Metal tape was used to smooth the transition between the two fairing pieces. Configuration 1b: The last configuration that was tested was the same as configuration 1, except the CT-cell removed. For this Configuration only a q-sweep was performed with zero angle of attack. The goal of this study was to compare parasite drag in the presence and absence of CT-cell (Configuration 1 vs. Configuration 1b). 2.2 Test Regime The Seaglider s typical speed is about 3 cm/s in water, corresponding to approximately 12.7 mph in air. This speed is close to the wind tunnel s low-speed boundary, where stabilizing the tunnel dynamic pressure is overly difficult. Stabilizing at a given propeller RPM was found to be significantly easier, speeding the test procedure. The lowest RPM value where the tunnel could stabilize was 3 RPM. From there the RPM was increased by increments of 1 RPM up to 6 RPM maximum, spanning the typical operating regime of the Seaglider. A few additional data points were added to provide data for future glider designs that may feature larger buoyancy-engines and hence increased operating speeds. Table 1 shows selected RPM values and the corresponding stabilized dynamic pressure, speed in water, speed in air and Reynolds number associated with each RPM. At these data points an α-sweep and a β-sweep were conducted that ranged from -2 to 2 by increments of 2. This process was carried out for Configuration 1 - Configuration 4, as described in Section 2.1. RPM q nom (psf) m/s in water mph in air Re normalized by V 1/ , , , , , , Table 1: Dynamic pressure, q nom, and speed values for selected RPM. UWAATR

7 (a) Configuration 1. (b) Configuration 1. (c) Configuration 2. (d) Configuration 2. (e) Configuration 3. (f) Configuration 3. (g) Configuration 4. (h) Configuration 4. Figure 3: Configurations tested during the wind tunnel study. UWAATR

8 2.3 Conventions and Nomenclature The vehicle body frame is a right-handed coordinate frame fixed at the center of buoyancy with the x-axis pointing forward through the nose along the fuselage reference line, the y-axis pointing through the right side of the vehicle, and the z-axis pointing down. The angle of attack, α, is the angle between the fuselage reference line and the projection of the relative wind onto the body x-z plane. The angle of attack is positive when the relative wind is on the underside of the vehicle. The sideslip angle, β, is measured to the relative wind vector from the same projection. The sideslip angle is positive when the relative wind is on the right side of the vehicle. The wind axes are obtained from the body axes by a negative (down) rotation through the angle of attack about the y-axis, followed by a positive (right) rotation through the sideslip angle about the z-axis (Figure 4). The two frames are related by the proper rotation matrix cos α cos β sin β sin α cos β R w/b = cos α sin β cos β sin α sin β sin α cos α that takes vectors from the body frame to the wind frame. The relative wind in the wind frame has only x-axis component, but the aerodynamic forces can, in general, have components in all three directions. Figure 4: Definition of aerodynamic angles. The components of relative wind, v, in the body frame are u, v and w. Image courtesy of Craig Woolsey. The aerodynamic forces in the wind frame are written as D X F w A = S = R w/b A b Y A b, L ZA b where XA b, Y A b and Zb A are the aerodynamic forces along the x, y and z axes, respectively, expressed in the body frame. The aerodynamic moments acting about the center of gravity and expressed in the wind frame are written as [l, m, n] T. All forces and moments in this report are expressed in the wind frame. In terms of wind-axes components the aerodynamic forces acting on the vehicle can be written as drag, D = q S C D lift, L = q S C L sideforce, S = q S C S rolling moment, l = q S b C l pitching moment, m = q S c C m yawing moment, n = q S l C n Here q is the dynamic pressure, S is a characteristic area, b is the wing-span, c is the mean aerodynamic chord of the wings, and l is the vehicle length. In this document the vehicle volume, V, to the two-third power will be used as characteristic area, and V to the one-third power as characteristic length, unless otherwise noted. The effects of Mach number and compressibility are clearly negligible for this study, the non-dimensional aerodynamic coefficients UWAATR

9 therefore depend on the aerodynamic angles α and β only. The moments are positive when they correspond to a positive right-hand rotation about the corresponding wind frame axis. We follow standard aircraft convention when assuming quadratic dependence for the drag coefficient, and linear dependence for the rest of the coefficients [3] [4]. C l (β) = C lβ β, C L (α) = C Lα α (1) C m (α) = C mα α, C D (α, β) = C D + C Dα α 2 + C Dβ β 2 (2) C n (β) = C nβ β, C S (β) = C Sβ β. (3) The glider is symmetric about the x-z plane, hence the rolling moment, yawing moment, and sideforce are zero when the sideslip angle is zero. Other than the science sensors, the glider is also symmetric about the x-y plane. Although the forces acting on the CT-cell are significant, we assume zero pitching moment at zero angle of attack. In the above definition of the forces and moments, the effect of the body, the wings and the rudder are all treated together. The wind-tunnel study was performed for configurations with the standard 1 m wings, with the 1.5 m wings and in the absence of wings (Configuration 1, 2 and 3, respectively). This allowed to measure the body and the wing contributions separately. The longitudinal forces and moments can be split into body and wing contributions as: [ D = D b + D w = qv 2/3 (CDb + C Dbα α 2 + C Dβ β 2) + S w ( V 2/3 CDw + C Dwα α 2) ] (4) L = L b + L w = qv 2/3 [ C Lbα α + S w [ m = m b + m w = qv 2/3 c ] V C 2/3 Lw α α l cb/acw S w C mbα α c m C V 2/3 Lwα α c (5) ], (6) where S w is the planform area of the wings, l cb/acw is the distance between the center of buoyancy and the mean aerodynamic center of the wings, and c m is a non-dimensional coefficient. Remark 2.1. Note that the wings are symmetric, hence the zero-lift pitching moment is zero. Since the aerodynamic coefficients for the wing are referenced about the mean aerodynamic center, the pitching moment for the wing is zero. Ideally, the moment contribution then comes from the wing lift force alone, defined by the wing volume ratio l cb/acw S w. V 2/3 c The coefficient c m is used to account for the discrepancy in the measured values. The dependence of these non-dimensional coefficients on the aerodynamic angles are first-order (quadratic for C D ) approximations of more complicated functional relationships. In reality, the force-and moment coefficients also vary as a function of Reynolds number. As pointed out in [2], the parasite drag at such low Reynolds numbers varies with dynamic pressure to the three-fourth power. Using this alternative functional dependence, one may write L = ql 2 C a α (7) D = ql 2 ( C b q 1/4 + C c α 2). (8) Remark 2.2. Note that in the above equations the vehicle length squared is used as characteristic area to keep with the notation of [2]. Remark 2.3. Also note that the coefficient C b is now dimensional. The wind tunnel tests confirmed this form of functional dependence. Nevertheless, the definitions in equations (1)- (6) are widely accepted, hence we present the non-dimensional coefficients for both equations (1)-(6) and (7)-(8). The coefficients for equations (1)-(6) are given at the Seaglider s typical operating speed, 27 cm/s. UWAATR

10 3 Results 3.1 Aerodynamic Coefficients for Standard Seaglider For typical operating speed of 27 cm/s the following aerodynamic coefficients were identified for the Seaglider. C l (β) = C lβ β, C L (α) = C Lα α C m (α) = C mα α, C D (α, β) = C D + C Dα α 2 + C Dβ β 2 C n (β) = C nβ β, C S (β) = C Sβ β. C Lα C D C Dα C Dβ C Sβ C lβ C mα C nβ using deg using rad Table 2: Configuration 1, 27 cm/s, SG aerodynamic coefficients. [ D = D b + D w = qv 2/3 (CDb + C Dbα α 2 + C Dβ β 2) + S w 1m ( V 2/3 CDw + C Dwα α 2) ] [ L = L b + L w = qv 2/3 C Lbα α + S ] w 1m V C 2/3 Lw α α [ ] m = m b + m w = qv 2/3 l cb/acw S w c C mbα α c m C V 2/3 Lwα α. c C Lbα C Lwα C Db C Dw C Dbα C Dwα C mbα c m using deg using rad Table 3: Configuration 1, 27 cm/s, body and wing aerodynamic coefficients. L = ql 2 C a α D = ql 2 ( C b q 1/4 + C c α 2). C a C b C c using deg using rad Table 4: Configuration 1, 27 cm/s, coefficients used in [2]. UWAATR

11 3.2 Aerodynamic Coefficients for Seaglider with 1.5 m Wings For typical operating speed of 27 cm/s the following aerodynamic coefficients were identified for the Seaglider with 1.5 m wings. C l (β) = C lβ β, C L (α) = C Lα α C m (α) = C mα α, C D (α, β) = C D + C Dα α 2 + C Dβ β 2 C n (β) = C nβ β, C S (β) = C Sβ β. C Lα C D C Dα C Dβ C Sβ C lβ C mα C nβ using deg using rad Table 5: Configuration 2, 27 cm/s, SG aerodynamic coefficients. D = D b + D w = qv 2/3 [ (CDb + C Dbα α 2 + C Dβ β 2) + S w 1.5m [ L = L b + L w = qv 2/3 C Lbα α + S w 1.5m V [ 2/3 m = m b + m w = qv 2/3 c ] C Lw α α l cb/acw S w C mbα α c m C V 2/3 Lwα α c ( V 2/3 CDw + C Dwα α 2) ] ]. C Lbα C Lwα C Db C Dw C Dbα C Dwα C mbα c m using deg using rad Table 6: Configuration 2, 27 cm/s, body and wing aerodynamic coefficients. Remark 3.1. Note that the parasite drag coefficient for the 1.5 m wings is negative, which is clearly not possible. The measured drag force at zero angle of attack was smaller for the Configuration 2 than for Configuration 3, which is presumably measurement error. Due to the extreme low test-speeds, the measured forces and moments were on the order of the tunnel s reported measurement accuracy. The reported accuracy might be overly conservative, and placing error bars on the same order of magnitude as the actual data is meaningless, especially in view of the qualitatively correct data plots for C D (α). Repeatability tests could be performed in the future to estimate measurement accuracy at these low test speeds and to obtain a second data set for drag coefficient. L = ql 2 C a α D = ql 2 ( C b q 1/4 + C c α 2). C a C b C c using deg using rad Table 7: Configuration 2, 27 cm/s, coefficients used in [2]. UWAATR

12 3.3 Effects of the Wings The first three configurations were tested to determine the effect the wings had on the performance of the Seaglider. Figures 5-7 show the plots for C L vs. α, C D vs. α, and C m vs. α for the Configuration 1 - Configuration 3. Figures 5 and 6 show that the main effect of the 1.5 meter wings is increased lift coefficient and that the glider becomes stable in the typical operating regime Data Points C D =.9289 α C L -- Volume 2/3 -.2 C D -- Volume 2/ Data Points C L =.7251 α (a) C L vs. α (b) C D vs. α Data Points C m = α Data Points C S = β.5.2 C m -- Volume C S -- Volume 2/ β (deg) (c) C m vs. α (d) C S vs. β Figure 5: Configuration 1 - Seaglider with 1 m wings - 5 RPM - 27 cm/s in water UWAATR

13 Data Points C D =.1144 α C L -- Volume 2/ C D -- Volume 2/ Data Points C L =.9754 α -1 (a) C L vs. α.1.5 (b) C D vs. α.6.4 Data Points C m = α.4.3 Data Points C S = β.2.2 C m -- Volume -.2 C S -- Volume 2/ β (deg) (c) C m vs. α (d) C S vs. β Figure 6: Configuration 2 - SG-174 with 1.5 m wings - 5 RPM - 27 cm/s in water UWAATR

14 Data Points C L = α.16 Data Points C D = α C L -- Volume 2/3 -.1 C D -- Volume 2/ (a) C L vs. α.6 (b) C D vs. α Data Points C m = α Data Points C S = β.5.2 C m -- Volume C S -- Volume 2/ β (deg) (c) C m vs. α (d) C S vs. β Figure 7: Configuration 3 - Seaglider with no wings - 5 RPM - 27 cm/s in water UWAATR

15 3.4 Drag Comparison of Seaglider Tail Section versus Ogive Tail Section Figure 8 shows the drag comparison between the Configuration 4 (ogive tail section) and Configuration 1b (Seaglider tail section). The drag force for the ogive tail section is somewhat larger (Figure 9), but the drag coefficient is smaller due to the larger volume and surface area. The characteristic area for the non-dimensionalization was V 2/3 for Configuration 1b and V 2/3 OG for Configuration 4 (see Appendix E for parameters). The drag coefficient for Configuration 1b is 12.4% larger than for Configuration 4 on average across the collected data points. To make the drag measurements comparable, the CT-cell was removed from the Seaglider, since the ogive tail section did not have the CT-cell. However, by mistake, the dissolved oxygen sensor was accidentally left on the glider for Configuration 1b. This may be the source of significant drag increase and may account for the 12.4% increased drag compared to the ogive tail section C D -- Volume 2/ Seaglider OGIVE Re -- Volume 1/3 Figure 8: C D vs. Re for Configuration 4 and Configuration 1b. The characteristic length for computing Reynolds number, Re, was V 1/3. The characteristic area for computing the surface area was V 2/3 for Configuration 1b and for Configuration 4. V 2/3 OG UWAATR

16 1-1 Seaglider OGIVE Drag (lb) Re -- Volume 1/3 Figure 9: Drag vs. Re for Configuration 4 and Configuration 1b. The drag force is 1.4% larger for Configuration 4 than Configuration 1b on average. Note that Configuration 4 has larger surface area, however, the oxygen sensor present in Configuration 1b is absent in Configuration 4. UWAATR

17 3.5 Parasite Drag and Dynamic Pressure The drag polar equation D = qs ( C D + C Dα α 2) is commonly used in the aircraft literature. In reality, the parasite drag coefficient also shows variation with Reynolds number. The equation suggested in [2] is given by D = qs (C b q 1/4 + C c α 2) Figure 1a shows the wind-tunnel data for drag force against dynamic pressure. The plot confirms that at the Seaglider s operating speed the parasite drag varies with dynamic pressure to the three-fourth power. At larger speeds outside of the standard Seaglider operating regime the parasite drag varies linearly with dynamic pressure as shown in Figure 1b Drag (lb) Drag (lb) Data Points.1 Data Points Drag = q Drag =.9123 q 3/ q (psf) (a) Parasite drag in the Seaglider s typical operating regime q (psf) (b) Parasite drag outside the operating regime. Figure 1: Drag vs. q for Seaglider with and without CT-cell. UWAATR

18 3.6 Drag Contribution from the CT-cell The final configuration that was tested in the wind tunnel (Configuration 1b) was the original Seaglider fairing without the CT-cell. Parasite drag coefficients were compared across Configuration 1 and 1b to obtain the drag contribution form the CT-cell. Figure 11 shows the C D versus Reynolds number plot for the two configurations. The parasite drag for the Seaglider with the CT-cell is on average 47.7% larger than without the CT-cell C D -- Volume 2/ With CT Cell Without CT Cell Re -- Volume 1/3 Figure 11: C D vs. Re for Seaglider with and without CT-cell. UWAATR

19 3.7 Static Longitudinal Stability The wind tunnel tests revealed that the glider lacks static longitudinal stability, indicated by positive pitching moment coefficient in the typical operating regime. Positive pitching moment coefficient results in a positive (nose-up) moment when the angle of attack is positive, which further increases the angle of attack. For positive α, a negative restoring moment would be required for static longitudinal stability. The moment coefficient plots presented in Figure 14, Figure 2, Figure 26 and Figure 32 show that at the lowest test speed the moment coefficient is a monotonically increasing function of α crossing the abscissa at the origin (no moment at zero angle of attack). As the wind speed increases the curves shift toward the origin; eventually they cross over the abscissa and lose their monotonic property. Beyond that point the curves have negative slope and an inflexion point at the origin. This indicates that the glider becomes stable beyond a certain flow speed, which, however, is currently outside of the operating regime. The glider is longitudinally stable with the larger, 1.5 m, wings in the current operating regime. Since the wings are mounted behind the center of gravity, the wing lift force acting with the moment arm between the wing mean aerodynamic center and vehicle center of gravity provides a restoring moment. This is also true for the shorter wings, however, since the body itself is aerodynamically unstable, the restoring moment from the short, 1 m span, wings is not sufficient to counter the body effects (observe these trends in Figure 14, Figure 2 and Figure 26). The Seaglider with the ogive aft fairing and the 1 m wings is also unstable in the current operating regime. The reason that the Seaglider performs stable glides in experiments is the restoring moment resulting from the offset between the center of buoyancy and center of gravity. Compared to this moment, the magnitude of the aerodynamic moment is small. Knowing the vertical CG-CB separation and other vehicle parameters, it is simple matter to perform a static equilibrium calculation and estimate the pitch angle variation compared to a stationary case. For example, the destabilizing aerodynamic moment resulting from 5 angle of attack may be easily countered by a gravity torque from a pitch angle increase of less than 1. UWAATR

20 3.8 Directional Stability The magnitude of the measured yawing moment was very small compared to other aerodynamic forces and moments. The bare hull is symmetric about any plane containing the fuselage reference line, hence the bare hull is directionally unstable for the same reasons it is longitudinally unstable. The vertical stabilizer provides the required restoring moment and ensures stable glides. The yawing moment coefficients presented in Figure 15, Figure 21, Figure 27 and Figure 33 are very small in magnitude, and on occasions not symmetric in sideslip angle. The reason for this is that the yawing moments calculated from C n were on the order of 1 2 in-lb, whereas the measurement accuracies for the wind tunnel are.1 lb for the forces and.2 in-lb for yawing moment. The anomalies in the yawing and rolling moment graphs is likely to be caused by the measurement accuracy of the wind tunnel and the small test speeds. Relatively small yawing moment coefficients imply that there is small restoring capability in the presence of sideslip angle, and hence the glider might be close to being neutrally stable in yaw angle. UWAATR

21 4 Summary This report described full-scale wind tunnel tests for the Seaglider that were conducted in the University of Washington Kirsten Wind Tunnel. Aerodynamic coefficients were presented for different vehicle geometries at typical operating speeds. The following observations were also made: 1. The drag coefficient for the Seaglider with the CT-cell is 47.7 % larger than without the CT-cell. The CT-cell was known to be a major drag contributor, however the extent of this contribution was unexpected. Housing all the science sensors inside the fairing could significantly reduce drag and improve efficiency in future gliders. 2. The Seaglider lacks static longitudinal stability in the typical operating regime. The lack of static aerodynamic stability is counter-balanced by the moment couple resulting from the offset between center of gravity and center of buoyancy. This metacentric height helps recover stability, explaining why the glider performs stable glides in experiments. Larger wings could help recover aerodynamic stability, and also improve glide performance. 3. The magnitude of the measured yawing moments is very small, indicating that the glider is marginally directionally stable. 4. The measured drag force for the newly designed ogive tail section (Configuration 4) is on average 1.4% larger than for the original Seaglider configuration (Configuration 1b). By mistake the dissolved oxygen sensor was left on the Seaglider for the drag comparison; the ogive tail section of Configuration 4 was completely smooth without any obstructions. The drag coefficient for Configuration 1b is 12.4% larger than the drag coefficient for Configuration 4. The outcome of this test remains inconclusive, as the 12.4% increased drag coefficient for Configuration 1b versus Configuration 4 may come from the oxygen sensor. UWAATR

22 References [1] Anon. Technical guide for the Kirsten Wind Tunnel. Technical report, University of Washington Aeronautical Laboratory, Seattle, WA, 22. [2] C. C. Eriksen, T. J. Osse, R. D. Light, T. Wen, T. W. Lehman, P. L. Sabin, J. W. Ballard, and A. M. Chiodi. Seaglider: A long-range autonomous underwater vehicle for oceanographic research. Journal of Oceanic Engineering, 26(4): , 21. Special Issue on Autonomous Ocean-Sampling Networks. [3] S. F. Hoerner. Fluid-dynamic Lift: practical information on aerodynamic and hydrodynamic lift. Brick Town, NJ, [4] S. F. Hoerner. Fluid-dynamic Drag: practical information on aerodynamic drag and hydrodynamic resistance. Bakersfield, CA, [5] R. M. Hubbard. Hydrodynamics technology for an Advanced Expendable Mobile Target (AEMT). Technical Report 813, University of Washington, Applied Physics Lab, Seattle, WA, 198. [6] N. Pelland. Analysis of potential fairing shapes through photography of scale-model freefall. Technical Report DeepGlider II Technical Report, University of Washington, School of Oceanography, Seattle, WA, 29. UWAATR

23 5 Appendix A: Configuration m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C D --Volume 2/ Figure 12: Configuration 1 C D (α) C L --Volume 2/ m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s Figure 13: Configuration 1 C L (α). UWAATR

24 m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C m --Volume Figure 14: Configuration 1 C m (α) m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C n --Volume β (deg) Figure 15: Configuration 1 C n (β). UWAATR

25 m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C l --Volume β (deg) Figure 16: Configuration 1 C l (β) C S --Volume 2/ m/s.24 m/s m/s.325 m/s m/s.862 m/s -.5 β (deg) Figure 17: Configuration 1 C SF (β). UWAATR

26 6 Appendix B: Configuration m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C D --Volume 2/ Figure 18: Configuration 2 C D (α). 1.5 C L --Volume 2/ m/s.24 m/s m/s.325 m/s.561 m/s.862 m/s -1.5 Figure 19: Configuration 2 C L (α). UWAATR

27 C m --Volume m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s Figure 2: Configuration 2 C m (α) C n --Volume m/s m/s.267 m/s m/s.561 m/s.862 m/s -.4 β (deg) Figure 21: Configuration 2 C n (β). UWAATR

28 C l --Volume m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s β (deg) Figure 22: Configuration 2 C l (β) C S --Volume 2/ m/s m/s.267 m/s m/s.561 m/s.862 m/s -.4 β (deg) Figure 23: Configuration 2 C SF (β). UWAATR

29 7 Appendix C: Configuration m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C D --Volume 2/ Figure 24: Configuration 3 C D (α) C L --Volume 2/ m/s m/s.267 m/s.325 m/s m/s.862 m/s -.5 Figure 25: Configuration 3 C L (α). UWAATR

30 C m --Volume m/s.24 m/s m/s.325 m/s m/s.862 m/s -5 Figure 26: Configuration 3 C m (α) m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C n --Volume β (deg) Figure 27: Configuration 3 C n (β). UWAATR

31 m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C l --Volume β (deg) Figure 28: Configuration 3 C l (β) C S --Volume 2/ m/s.24 m/s m/s.325 m/s m/s.862 m/s -.5 β (deg) Figure 29: Configuration 3 C SF (β). UWAATR

32 8 Appendix D: Configuration m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C D --Volume 2/ Figure 3: Configuration 4 C D (α). 1.5 C L --Volume 2/ m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s -1.5 Figure 31: Configuration 4 C L (α). UWAATR

33 m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s C m --Volume Figure 32: Configuration 4 C m (α). C n --Volume m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s β (deg) Figure 33: Configuration 4 C n (β). UWAATR

34 .6.5 C l --Volume m/s.24 m/s.267 m/s.325 m/s.561 m/s.862 m/s β (deg) Figure 34: Configuration 4 C l (β) C S --Volume 2/ m/s m/s m/s.325 m/s m/s.862 m/s -.5 β (deg) Figure 35: Configuration 4 C SF (β). UWAATR

35 9 Appendix E: Parameters Parameter Symbol SI units English units Kinematic Viscosity of Water ν w e-6 m 2 /s 1.279e-5 ft 2 /s Kinematic Viscosity of Air ν a m 2 /s 1.64e-4 ft 2 /s Density of Water ρ w 1,25.6 kg/m slugs/ft 3 Density of Air ρ a kg/m 3.24 slugs/ft 3 Bare Hull Volume V e-2 m ft 3 Bare Hull Volume of SG Nose with Ogive Tail V OG e-2 m ft 3 Total Vehicle Length l 1.8 m ft Bare Hull Displaced Mass m w kg slugs Distance from Nose to Wing Aero. Ctr. x ac 1.26 m ft Distance from Nose to C.B. x cg.9 m ft Distance between Wing Aero. Ctr. and C.B. l cb/acw.358 m ft Planform Area of 1.5m Wing S w1.5m m ft 2 Planform Area of 1m Wing S w1m.176 m ft 2 MAC of 1m Wing c 1m.1743 m.5717 ft MAC of 1.5m Wing c 1.5m.1526 m.58 ft Table 8: Parameters. UWAATR