Very little unclassified information has been published concerning the roll-yaw hydrodynamic instability of modern high-speed submarines. In particular, it appears that there is little understanding of the fluid-flow mechanism involved in the fairwater (sail)-hull interaction in a coordinated underwater turn. This article analyzes the results of smoke-flow studies conducted on a 1/75 scale model of the SSN 585 SKIPJACK submarine simulating a rolling-yawing turn.
It is obvious from studying photographs of the several new Russian attack submarines that they are attempting to solve the well-known “snap- roll” maneuvering problem. Modern submarines cannot maneuver underwater with great abandon like an F-16 fighter plane. One reason is that their hull crush-depth is only 4-6 hull lengths away and another reason is that if, in a melee situation, a modern high-speed sub pilot tries to turn too sharply at too high a speed, he might find himself in a snap-roll, banging from his seat belt and with a loss of several hundred feet in depth at a markedly slowed speed.
With our limited number of subs, we should be addressing this problem so that our attack subs can out-maneuver the other side in shallow waters as well as deep water.
Although the SKIPJACK was the first nuclear attack boat to utilize the new body of revolution hull design as pioneered by , the ALBACORE (AGSs-569) the same basic hull form has been used on both the 637 class and the 688 class designs, deviating only in length, i.e. fineness ratio, to accommodate more equipment and a larger reactor in the case of the 688 boats. A considerable body of aeronautical data exists from the study of bodies of revolution, as applied to airships and missiles, which has been useful to apply to the Modern submarine shape.
A simplified analogy of the body of revolution hull form might be useful at this point: We have all seen the wing-tip vortices — usually in wet weather — generated by aircraft, particularly when taking off or landing. If one were to simply eliminate all the wing between each tip, and then join the two symmetrical wing tips together, as a body of revolution, it can be seen that this also would generate two vortices rolling up inwardly toward each other at any time that the body of revolution was inclined to the free-stream flow.
These two vortices are relatively harmless on an airship, blimp, or missile, but their interaction with the submarine sail appears to be the root cause for the inability of the modern submarine to maneuver underwater with the same sort of stability as airplanes in the atmosphere.
To verify the simulation of wind-tunnel submarine data vs. full-scale data (in water), a drag coefficient vs. REYNOLDS NUMBER (Cf vs. Nr) plot was obtained for the wind tunnel model and compared with the data obtained from the David Taylor Model Basin (DTMB) model tests. Since data from full-scale sea trials has been in good agree-ment with the DTMB model data, this would appear to be a good comparIson.
Before discussing the wind tunnel results on the SKIPJACK model a few thoughts about laminar flow, turbulent flow, separated flow and Reynolds number for submarines might clarify what was observed.
The fully immersed streamline bodies that are typical of modern submarines produce very little wake, and their drag, or resistance to forward motion, is composed almost entirely of skin-friction drag. And this drag, for any given hull shape, will be dependent on REYNOLDS NUMBER, or the ratio of inertia forces to viscous forces, for any body sliding through the sea. REYNOLDS NUMBER is basically a scaling factor which is important so that one can test models and correlate their data with the full-scale desired results. REYNOLDS NUMBER is also important because it helps to define the demarcation between the very low drag created by LAMINAR FLOW of the water next to sub hull and the 300-400J higher drag of the TURBULENT FLOW next to the hull.
The layer of water next to a modern submarine hull, called the boundary layer, normally will be less than 1/2 inch in thickness from the bow past amidships.
In the study of the fluid dynamics about a moving submarine hull, the predominately TURBULENT FLOW boundary layer over the hull is generally easier to control than a laminar boundary layer. The story of the common golf ball can be useful to illustrate how this comes about:
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If one were to take a perfectly smooth golf ball and wallop it down the fairway with one of your best “250 yard” drives, you would be sorely disappointed to find the smooth-surfaced ball travelling only about half that distance! It is true. Note in fig.2a how the laminar airflow passes over the ball in smooth layers but when these layers reach the backside of the ball they can no longer adhere to the ball’s surface so they SEPARATE and form a large drag-producing separated wake. Now if one were to rough up the surface of the ball with small 1/8 inch dimples, it is easy to see, fig. 2b., that this will create a high energy TURBULENT layer of air next to the ball. This turbulent boundary layer has a little more energy in it so that when it sees the back side of the ball, it continues around the dimpled surface just a little further before it finally separates away. This leaves a smaller drag-producing wake than the smooth ball.
SEPARATED FLOW must be avoided at all costs on a submarine if only because or the resulting severe wake noise. Thus, a proper design should utilize a basic body of revolution and clever control plane design and placement to create a separation-free underwater vehicle that is quieter and faster.
FLOW STUDIES:
The test model was photographed in four different positions which are of interest in examining a coordinated undersea turning maneuver in the lateral plane.
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In all or the above photographs the submarine is yawed towards the camera and rolled into the camera.
FIGURE 3:
The low-drag hull design is evident with the flow remaining attached over more than 80S or the body with variations occurring only at the sail and the stern planes. The former is the result or the sail pressure distribution (remember the sail’s shape is exactly that or a short wing attached to the hull) while the latter is due to the influence or the stern and rudder planes.
FIGURE 4:
The sail is now developing considerable side force as a result or an effective angle-of-attack of 10 deg. In addition the hull is also developing a side-force as evidenced by the twin vortices which are rolling up inwards (in the classical manner or a lifting body or revolution) towards the low-pressure area at the near-side hull centerline. This side-force is necessary to counteract the centrifugal force or the sub as it progresses through its turning maneuver.
However, the most significant observation here is the manner in which the upper vortex core interacts with the downwash at the trailing edge or the sail. It would appear that the sail’s flow-field is attempting to pull the upper vortex away from its normal path and over to the top decking behind the sail. Note that the sail is now developing its maximum amount or side-force or “lift” which has created a large area of low pressure on the viewer’s side of the sub.
FIGURE 5:
With the same yaw angle as before but with a 20 deg. roll angle in addition, the moving of the sail into the region of the twin vortex cores (which are independent of the roll angle, being formed only as a result of the considerable side-force generated by the bull) appears to have caused both vortices to suddenly shift their position on the hull just aft of the sail. This violent flow separation should cause a significant rear pressure shift that would cause a stern-squatting motion with loss of depth and speed.
FIGURE 6:
This very startling flow study accentuates the unsteady flow phenomenon which is characteristic of this maneuver. It is evident that the sail/sail-plane pressure field, in moving further into the bow-generated vortex field, appears to be creating a violent separation on the lower mid section of the hull. Note that the phenomenon observed in Figs 4 & 5 is an unsteady flow field oscillating at a very low frequency less than 5.
DISCUSSION:
From the above flow studies, it is quite evident that the relatively large sail employed on all U.S. Navy attack submarines has a strong, negative influence on the hydrodynamic flow field that creates the forces generating an underwater turn. With a sail height over 60% of the bull diameter, the sail rolling-moment alone — at 20 knots can be several MILLION foot-pounds. Meanwhile, any upward shift of the bull center of pressure, due to the above vortex instability, would add another 500,000 to 1 million foot-pounds of rolling moment, seriously degrading the transverse metacentric stability of the sub.
On the other band, the Russian VICTOR, ALPHA, and AKULA class boats all have less prominent sails — have planes placed deep in the bow, and their sail height appears to be less than 40% or hull diameter — and the latter two classes have the sail blended into the hull with extensive fairings apparently designed to minimize the sail’s influence on the hull flow-field.
Additionally, the above flow separation will result in a higher pressure on the upper rear or the hull which will, in turn, tend to rotate the stern down. This will cause a further shift in the bow-generated vortex which will decrease speed, increase depth (since the sail side-force vector points downward) and further aggravate the degraded attitude or the sub. If, on the other hand, power is increased to counteract the loss or speed and stern-heavy attitude, a possible result would be a complete “barrel-roll”, — which should make things interesting for the crew.
Materially changing the sail shape and size on existing 637 and 688 class boats should not be such a difficult task. For example, the addition or a trailing edge flap on the rear or the sail with an appropriate control system might be sufficient to counteract the above flow difficulties to allow all of our attack boats to not only outmaneuver the opposition — at any speed — but also to freely maneuver in shallow waters where smaller subs have an advantage today. Reducing the size or the sail and fairing it into the hull will also provide further quieting or our existing attack boats. The sail-planes have finally been moved down to the hull and moved forward on the most recent 688 class boats and experimental work should continue in this area so that existing boats can be modified to maneuver not only quickly but also quietly.
It is believed that a number or experiments were conducted on the ALBACORE in the late 1950’s with a sail-flap and other control surfaces. Perhaps we should take another look at this data and its applicability towards making our attack boat fleet more effective against the more numerous Russian. Today’s attack submarines need not be saddled with the clumsy maneuvering ability of a Navy blimp . Although slow, quiet stealth has always been an important advantage for our sub fleet, it does not appear prudent to ignore the possibility of underwater “dog fights” in combat. This ability may become even more important as the Soviet attack subs become very quiet as well.
Henry E. Payne III
IN REMEMBRANCE
CAPT. CHARLES A. GOODING, USN (RET.)
MRS. R. A. (SUNNY) PETERSON
