Experimental Investigation of Effect of the Sail with Leading Edge Fillet on Flow around a Submarine

ISSN 0976-2612, Online ISSN 2278–599X, Vol-7, Special Issue-3-April, 2016, pp1-8

http://www.bipublication.com

h Article Researc

Experimental Investigation of Effect of the Sail with Leading Edge Fillet on

Flow around a Submarine

Mohsen Rahmany, Amir Hamzeh Farajollahi and Mojtaba Dehghan Manshadi

Mechanic & Aerospace Engineering

Department, Malek Ashtar University of Technology, Esfahan, Iran Email: a.h.farajollahi@gmail.com

ABSTRACT

Because of connecting the various appendages of submarine to the main body the vortices have been created that disrupt the flow uniformity and make the undesirable features such as vortex formation to flow. Vortices that have been created due to the connectivity of sail to the body of submarines have a significant impact on non-uniformity of submarine wake at location of the propeller disc. In present research the use of hot wire anemometer has created vertical flow field in back of the two sails in an experimental model of standard submarines in a wind tunnel. Sails have a same cross-section and height, but one simple and the other has a fillet at the leading edge. The vortical flow field in the form of a horseshoe vortex at downstream of sail has been obtained at four locations. The results of re-search have specified the formation of a horseshoe vortex on the body of submarine model due to the effect of con-nectivity sail to body. The amount and intensity of the vortex flow has considerably reduced in the sail with fillet on leading edge. In addition, increasing space from sail to downstream increases the amount of axial speed at the center of vortex but the range the vortex covers is smaller. Results have clearly shown the symmetry flow around sail of a submarine model.

Keywords: Sail; Fillet, Vortical Flow, Submarine, Wind Tunnel

1

Submarine is a underwater vehicle consists of the original body and the number of other side com-ponents. Main body of a submarine as well as their side parts is the flow line components, but when these components are assembled on the main body they disturbed the line flow and the phenomenon of complex flow. Since the horse-shoe vortex is so strong, it often leads to the pro-duction of acoustic. Wake caused by the main body, appendages of the submarine and horse-shoe vortex interact complicatedly, so the pro-peller flow is characterized by turbulent pulse and vortex movement, which results in the se-rious non-uniform wake at the submarine

in-creases and gradually gets unstable and enters a transitional phase. Turbulent boundary layer is formed next from a transitional phase that passes a flatland body to back and continued the growth and eventually at the stern have been separated from surface and as a result at the end of body a small wake is formed that will spread at down-stream flow [2].As it is mentioned before, the flow around a submarine with appendages is more complex than a bare body. This flow is specified usually by the thick boundary layers, vortex flow structures produced at the location of fittings section lateral to body. Complex flow around one submarine at yaw angle along with sail has been shown in figure 1.

Figure 1. Complex flow around one submarine at yaw

angle along with sail [3].

This flow field is surrounding by vortex separa-tions [3]. Some from these vortex structures that have been seen in this shape are consisting of: 1- cross-flow separation vortices 2- horseshoe vor-tex 3- the sail tip vorvor-tex 4- the sail root vorvor-tex. The formed vortices due to the effect of separa-tion transverse flow is taken into account the main vortex created while maneuvering one submarine. By increasing angle of attack, the speed of transverse flow leads to separate the flow at opposite side of flow and let us that flow leaves the boundary layers of the body and gen-erated vortex transfers to downstream. Cross-flow separation vortices are one common

3D flow separation from body of 3D object in flow at angle. A conventional 3D separation from the body of a 3D object is the flow line at an an-gle. Horseshoe vertices and sail root are resulting of flow at the junction of the body and sail. Flow passing through a submarine occurs by interact-ing vertices. Vertices flow around the body and appendages have an important influence on the properties of the submarine maneuver.

Figure 2 shows schematically how to form horseshoe vertices around sail on the body of submarines. Flow of boundary layer on the body of submarines has been blocked by the sail. Pre-senting the sail in opposite of flow causes to create the opposite pressure gradient. In this case, in upstream sail and boundary layer transverse vortex has been made on the body. Transverse vortex that has been blocked by the sail in direc-tion to forward is forced to rotate and turn the sail. Finally horseshoe vertex (vortex that its axis

is along to the model axis, ωx) transverse vertex

rotation will be formed in two sides of sail. In both sides of sail speed of cross-flow increases because of airfoil shape of sail and this cross-flow inside horseshoe vertex has been drawn and the longitudinal vortices component

(ωx) is expanded so that it flows in down hand of

the sail [4].

Different solutions have been studied to decrease the effects of horseshoe vortex. For example, filling out the location of connecting lateral parts to body (making muscle) is one way that can re-duce reverse gradient pressure in location of connecting sail to body and reduce vortices in longitudinal vertical wake [5].

Figure 2.Form of horseshoe vortices around sail of

Phenomenology of different vortices formed around the body and lateral sections of one sub-surface has drawn attention of the researchers. The research of Divenport and his colleagues (1992) on the fillet created at the front of the sail suggests that adding the fillet improves the flow quality [6]. In this study, the detection has been made by the oil has been a considerable separa-tion in the flow around the sail. Addisepara-tionally, to measure average speed near the fillet and in the symmetry plane, there are not back flows and it has been said that the fillet removes separation related to the leading edge of the sail clearly with reduced adverse pressure gradient. Liu and his colleagues (2011) studied the effects of a partic-ular vortex generator on reducing the sail horse-shoe vortex [1]. Their study suggests that the horseshoe vortex produced in the opposite direc-tion reduce the adverse effects up to 50 percent. In addition, the Reynolds number of subsurface model is effective on the effect of generator so that the impact of the generators is higher in high Reynolds numbers.

Experimental study of Saeidi Nezhad and his colleagues (2013) on the effect of lateral angle of submarine on horseshoe vertex indicates that at zero angle of attack horseshoe vertex is generated on model surface at location of connecting sail to the body [7]. In yaw angles because of presenting transverse flow on model, horseshoe vertex loses its symmetry in both sides of the sail and is in-clined to pro-side of flow.

Zhang and his colleagues (2005) studied the ef-fects of adding the fillet to the sail and appen-dages at the end of the submarine with RANS equations and described the mechanism obtained from the point of view of vortex [8]. Results of their studies indicate that adding the bend im-proves wake quality on the blade plate.

In this paper, two sails with a same cross-section and height are tested and the direct impact of fil-let on leading edge of sail has been studied on developing attacking formed behind it. It has been done by other researchers to test oil but here

we are using a hot wire vortex flow-meter. The area both sails are airfoil NACA 0018 and their height is 4cm. In figure 3, the image of the sail has been seen.

Figure 3. Shapes of the sails used in this study (left simple sail and right fillet sail at leading edge).

2.

The experiments in the present work are con-ducted in a closed loop subsonic wind tunnel with a test section size of 370×280×1200 mm. The axial fan of the tunnel provides air flow with maximum speed of about 30 m/s and turbulence intensity level of 0.25 % in the test section. The suboff model was designed by David Taylor Re-search Center [9] and the different experiments have been performed in wind tunnels and tension reservoirs [10]. A suboff bare hull submarine with a length (L) of 0.687 m and a maximum diameter (D) of 0.08 m is considered as the base model (figure 4).

figure 5, the place and how to install trip strip has been shown on suboff nose.

To place a submarine model within the test room as well as modeling the tests in pitch maneuver has been designed and built wake mechanism and angling the model. In figure 6, a schematic of settling the model has been displayed at angle of attack and angle mechanism seen in figure 7.

Main part of the holder is consisting of an iron pipe bearing the weight model and by the circular plate that is located on land at the bottom. The mechanism of the pitch angle is performed by changing a lever length. Blockage ratio of model plus bracket model in a wind tunnel test section is at an angle of zero to 4.9 percent.

Figure 4.Profile of sub off submarine model without appendages.

Figure 5.Submarine model with trip strip placed in nose of the model.

Figure 6.Suboff model and hot wire probe set in the wind tunnel at angle of attack.

To measure the boundary layer, a 1D probe of hot wire anemometer is used. This probe that the sensor of hot wire anemometer has been welded into probe tip to the base, before doing the test at the same angle is calibrated in the test room (without presenting the model). To move the probe from the transmission system three com-ponents is used with the accuracy at 0.01 mm. In order to do the tests of measuring speed of boun-dary layers, at first the model surface has been covered by very little colorless adhesive tapes near the surface of model for reducing heat transfer and electricity between hot wire anemo-meter sensor and model. Then the hot wire ane-mometer sensor becomes close to the model sur-face to some extent that the sensor bases becomes quite to the model surface. The probe moves near the surface so small that it does not burn probe. Then the first place of data mining is equal to a specified distance (the distance of sensor from the tip of probe). All tests of the research on speed 16 m/s. Reynolds number based on the in-tended model and speed is Re=106. Displacer device of hot wire probe and hot wire anemome-ter device are made of Frasnjsh Saba Manufac-turing Company.

2. RESULT AND DISCUSSION

In the study, the effects of fillet in the attacking edge on flow and velocity distribution around of the model on the top face of the model have been investigated at angles of attack 0 and 5 compared by the results. To verify the accuracy of speed, results on suboff model are compared with past results. To validate the results of the boundary layer the results of the research have been used by David Taylor Research Center.

The boundary layer profile in the location X/L = 0.9 is measured to compare with Huang and Liu result [10]. Figure 8 shows the boundary layer velocity profile in location X/L = 0.9 on the

lee-ward symmetry plane (φ=180°) for suboff model

at zero pitch angle and Re = 106. The result of

Huang and Liu is also shown in this figure. Comparing the present results with the Huang and Liu experiments [10] shows that the trend of the velocity profile of the boundary layer are very well predicted by present work.

U_x/U₀

(Y -R 0 )R m a x

0 0.2 0.4 0.6 0.8 1 1.2

0 0.5 1 1.5 2 Present Huang et.al.

Figure 8. Boundary layer profile in location X/L=0.9

at zero amgle of attack and Re=106 in comparison with [10].

To make dimensionless vertical distance two pa-rameters of maximum radius model Rmax and

lo-cal radius model R0 and dimensionlessness

direc-tion the speed is along with the axis Ux from a

reference speed U0. Good consistency is specified

between the results of Huang and these observa-tions. The reason of differences in the results up-per parts of the boundary layer (Y-R0)/R≥5 max

is resulting from the difference of the reference rate U0. Generally, comparing the results of tests

according to different Reynolds numbers show that the trip strip on the nose of the submarine model in tests of research results in an increase in the Reynolds number.

shape and makes that flow experiences ripples in its path so confused and vortex flow uniformity are again reinforced. The vortex is seen at the bottom contour continuously growing and strengthening because horseshoe vortex, root of sail and transverse vortex are gradually merged

and have formed a strong vortex. In addition, at the angle of attack 5 degree vortex height is greater than the angle of attack 0 degree as well as it covers from the surface to the tip sail.

Figure 9. Contours of flow around the model with simple sail configuration in different location at zero degree

AOA (right) and five AOA (left).

In figure 10, the contour of the two sails is compared at angle of attack 0 degree. With regard to the fig-ure, the effect of reducing vortex fillet is clearly visible. Presenting the fillet on both scales tip vortices and the vortex horseshoe and sail influenced roots and weaken them. The reason of weakening the tip of vortex sail in the presence of fillet is that when it hits the top of sail, the vortices are formed and by the end of the little sail are merged, i.e. in the final pages of the sail and then it cannot be said that the tip vortex to top sail is exactly what sets up and in the mentioned plates some effects of the vortices formed at the junction of the sail to the body (the sail root vortex and horseshoe vortex) is also evident in the sail tip vortex. Therefore, the fillet by reducing power of vertex formed at the junction of the sail to the body weakens the sail of tip vortex.

and X/L= 0.75 leads to lose complete tip vortices sail.

Figure 10.Contours of flow around the mosel with two sail configuration sail with fillet (left) and simple

sail or sail without the fillet (right) at zero degree AOA in different location.

Figure 11.Contours of flow around the mosel with two sail configuration sail with fillet (left) and simple sail or sail

2

In present study, the effect of leading edge fillet of a sail on the surrounding flow contours in a model of axial symmetry in the flow in line of attack 0° and 5° and Reynolds number Re=106is experimentally performed in ultrasound wind tunnel. The evaluation of contour‘s results on different plates during the model shows that the fillet has the good effects on flow and reduces the vortices. This result is quite experimental that Divonport and his colleagues [6] noted in the in-troduction are consistent and corroborated. The results of measuring by a hot wire anemometer show that:

 Moving from the middle to the end of model (stern), the size of the vortices on the surface of body model gets less but at the stern of subma-rine (end plates) it again strengthens because of the curved surface of the vortex.

 The presence of sail on the side of body leads to make the horseshoe vortex generated at the junc-tion of the sail to body.

 Fillet of leading edge in the sail in addition to reduce the horseshoe vortex and cross vortices also reduces tip vortices.

 Fillet impact on reducing the power of leading edge vortices, the angle of attack is greater than angle of attack of 5°.

Finally, it has been proposed that other research-ers do similar tests for a condition that all junc-tion places of the sail fillet are to the body and compare the results. It has been expected that it is the perfect fillet, fillet far more effective than leading edge fillet to reduce the vortices.

REFERENCES

[1] Liu Zhi-Hua, Xiong Ying, Wang Zhan-Zhi,

Wang Song and TU

Cheng-xu.(2011)Experimental Study on Ef-fect of a New Vortex Control Baffler and Its Influencing Factor, Chinese Ocean Engi-neering Society and Springer-Verlag Berlin Heidelberg

[2] Fidler J. E. and Smith C. A.(1978)Methods for Prediction Submersible Hydrodynamic Characteristics, Nielsen Engineering & Re-search incorporated, Report: NCSC TM-238-78.

[3] Sreenivas K., Hyams D. and Mitchell D.(2003) Physics Based Simulation of Rey-nolds number Effects in Vortex Intensive Incompressible Flows, NATO Research and Technology Organisation. Rue Ancelle, France.

[4] Gorski J. J.(2001) Marine Vortices and Their Computation, NSWC, Carderock, 9500 MacArthur Boulevard, West Bethesda, MD 20817-5700, USA.

[5] Simpson R. L.(2001)Junction Flows, Ann. Rev. Fluid Mech., Vol. 33, pp. 415- 443. [6] William J. Devenport, Roger L. Simpson,t

Michael B. Dewitz,J and Naval K. Agar-wal.(1992) Effects of a leading-edge fillet on the flow past an appendage-body Junction, AIAA JOURNAL.

[7] Saeidi Nezhad, A., Dehghan, A. A., Dehg-han Manshadi, M., Kazemi Esfeh, M., (2012) Experimental Investigtion of the Vortex Structure on a Submmersible Model, Modares Mechanical Engineering, Vol. 13, pp. 98-109.(In Persian).

[8] Zhang, N., Shen, H. C. and Yao, H. Z.(2005) Validation of Numerical Simulation on Re-sistance and Flow Field of Submarine and Numerical Optimization of Submarine Hull Form, Journal of Ship Mechanics, 9(1): 1~13. (in Chinese).

[9] Groves, N.C., Huang, T.T., and Chang, M.S., (1989)Geometric Characteristics of DARPA SUBOFF Models (DTRC Model Nos. 5470 And 5471), Report DTRC/SHD-1298-01, March.