Siberian Support for Submarine Forces
The Siberian Branch of the Russian Academy of Sciences (SB RAS) has always paid serious attention to the defense capacity of the country. Intense research aimed at developing and improving high-velocity submarines has been pursued for many years at various SB RAS institutes. Close cooperation between M. A. Lavrentiev, the founder and the first Chairman of the Siberian Branch, with the Navy authorities and leading designers of shipbuilding was preserved and enhanced by his successors.
The mathematical models allowed choosing optimal regimes of submarine motion and the depth of its submersion for different hydrological conditions for the purpose of maneuvering secrecy. The recommendations of Siberian researchers were highly appreciated by the Navy High Command; they were used to form tactical and engineering requirements to submarines and to perform design and development activities at the leading shipbuilding plants of the country
The Soviet submarine forces faced revolutionary changes in the late 1950s. The transition to nuclear power engineering caused a drastic improvement in submarine displacement, velocity, voyage autonomy, security, armament power, etc. To solve these problems, large-scale scientific support and organizational activities were required. As a result, Academicians M. A. Lavrentiev , L. I. Sedov, and A. P. Aleksandrov founded the Scientific Council for Problems of High-Velocity Hydrodynamics under the auspices of the Presidium of the USSR Academy of Sciences. In addition to academic researchers, the Council included leading specialists from the Navy, from various ministries and departments. People directly involved in research and design were regularly invited to report on their activities at the meetings of this Council. The Council was a good example of close interaction between researchers, designers, and military specialists.In the late 1950s, the Institute of Hydrodynamics headed by M. A. Lavrentiev was assigned the task of studying the possibility of drastic reduction of the total drag of submarines and optimization of their strength characteristics. Light autonomous models (including large-scale ones) that could surface under the action of buoyancy forces were used for experimental studies. The drag of bodies moving in water at actually reachable velocities is smaller than their total displacement by a factor of tens; therefore, even small positive or negative buoyancy is sufficient for a streamlined body to acquire a significant velocity of surfacing or submerging along its axis. The use of autonomous surfacing and submersing models offered a number of advantages: comparatively low cost, realistic external turbulence of the incoming flow, absence of vibrations, and large velocities of rather large models. Such studies, however, required a water area of sufficient depth near the shore. Some time later, Lake Issyk-Kul’ in Kyrgyzstan and the Black Sea (the Sukhumi Department of the Academy of Sciences), were found to be adequate areas
The Siberian Branch of the Russian Academy of Sciences, which was founded approximately at the same time, paid serious attention to defense issues from the very beginning. A lot of work was performed for the submarine forces of the country, including the activities within the framework of the Resolution on the Developing of High-Velocity Submarines, which was signed by the Soviet government in 1959. Close cooperation between M. A. Lavrentiev , the founder and first Chairman of the Siberian Branch, with the Navy authorities and leading designers of shipbuilding was preserved and enhanced by his successors. For many years, the Siberian Branch had a Coordination Council for Navy Problems, whose members were outstanding Siberian researchers, representatives of leading research institutes and of Navy authorities, as well as of leading shipbuilding plants.
As a whole, the studies performed by Siberian researchers for improving the performance of domestic submarines can be classified into three basic groups:
– increasing the submarine speed and maneuverability through drag reduction by means of controlling the boundary layer and using principally new coatings of the submarine hull;
– improving the strain and strength characteristics of the hull by using new promising materials;
– studying the hydrodynamic wake behind the ship moving in a stratified medium.
Let us consider each of these aspects and summarize the basic research results.
Bubbles against turbulence
In the early 1940s, Soviet scientists predicted a possibility of decreasing shear stresses by two orders of magnitude by means of saturating the turbulent boundary layer with a dense sheet consisting of bubbles. In the 1960s and 1970s, considerable reduction of shear stresses and, as a consequence, of the total drag of a flat plate and of axisymmetric bodies in bubbly flows was experimentally validated at the Institute of Hydrodynamics of the Siberian Branch of the USSR Academy of Sciences. The publications of these results in the mid-1970s aroused a keen interest, first of all, of American and Japanese researchers. Detailed reproduction of our experiments confirmed the results. However, the results obtained in different countries and on different test facilities revealed some principal contradictions. They showed that it was necessary to elucidate the mechanisms of bubble sheet interaction with turbulence in the boundary layer.A significant role in organizing and performing investigations by Siberian researchers for the Navy needs belongs to the Department of Applied Problems under the auspices of the Presidium of the USSR Academy of Sciences with an affiliated office in Novosibirsk, which was headed by G. S. Migirenko, and also to the Navy Research Department assigned to the Institute of Hydrodynamics
The studies performed at the Institute of Hydrodynamics showed that vortex structures, under the action of local pressure gradients, collide with small bubbles, absorb them, and take them away from the wetted surface. While colliding with large-size bubbles, vortex structures are destroyed, thus reducing the loss of energy necessary for their further development and, consequently, reducing shear stresses. A detailed study of specific features of the flow around various bodies in the case of negative and positive pressure gradients along the wetted surface allowed the researchers to understand not only the character of interaction of the bubble sheet with vortex structures near the body surface in different flow conditions, but also the reason for experimental discrepancies.
The understanding of interaction mechanisms provided the basis for optimizing the body contour in order to use this phenomenon most efficiently. Saturation of the turbulent boundary layer by bubbles is currently recognized, in the above-mentioned countries, as one of the most promising methods of decreasing the total drag of boats.
Polymer additives against turbulence
In the early 1970s, researchers at the Institute of Hydrodynamics started to study the effect of adding small high-molecular polymer additives to the flow, so as to reduce turbulent friction. The results of experiments on polymer additives available at that time aroused hope that the problem of drastic reduction of the drag of boats could be easily solved. Owing to close collaboration with chemists, production of a domestic high-molecular polymer (polyethylene oxide) was organized in an extremely short time. In terms of its properties, this polymer material was equivalent to American analogs.
High-Reynolds-number experiments with polymer additives revealed an 80 percent reduction of the total drag of an axisymmetric body at a zero angle of attack as well as significant suppression of high-frequency pressure oscillations on the wall. A large series of experimental activities was performed to improve the system of ejection of polymer solutions. The comprehensive research of Russian scientists resulted in creating a laboratory submarine equipped with a system for storage, preparation, and ejection of polyethylene oxide solutions.
As the polymers used to reduce the drag were rather expensive, the Institute of Thermophysics SB RAS, in the 1980s, performed some studies aimed at increasing the profitability of using these polymers. The results obtained determined the most promising direction of further activities on the targeted use of high-molecular additives: soft disintegration of their macromolecules before injecting them into the flow.
Compliant coatings against turbulence
The proposal to use compliant surfaces for drag reduction was put forward in the 1950s. The problem was that experiments demonstrated that compliant surfaces could either reduce or enhance the drag. Thus, it was necessary to develop a physical model that could give an adequate description of this phenomenon and indicate an algorithm for choosing a compliant surface to decrease the drag force. Such a model, based on wave interference, was created at the Institute of Thermophysics SB RAS. Using this model, the researchers managed not only to find the physically well-grounded properties of a compliant coating, which would reduce the drag force by decreasing turbulent friction, but also to propose a method for choosing such a coating.
Siberian scientists have recently developed compositions and production technologies for single-layer coatings that underwent several series of tests both in Russia and abroad, in the USA and in Great Britain. The analysis of the results confirmed the efficiency of the proposed numerical method of choosing coatings and hydrodynamic conditions for drag reduction; in addition, some ways for coating improvement that could retain the simple production technology were proposed. Compliant coatings for drag reduction are fairly attractive from the viewpoint of cost efficiency, which supports stable interest in their development and investigation.
The idea of using simultaneously a compliant coating, polymer additives, and saturation by bubbles for near-wall turbulence control appeared owing to the above-described achievements in studying this phenomenon. On the other hand, the areas of effective application of these control methods are rather different. The compliant surface responds to long-wave disturbances (more than 1000 viscous scales). Small high-molecular polymer additives suppress microscale vortex turbulence at a level of less than 100 viscous scales. A flowing near-wall screen composed of microbubbles can destroy powerful long-wave fluctuations directed toward the wall from the turbulent core and external flow. The combined use of these control methods is expected to reduce near-wall turbulence generation almost in the entire spatial spectrum.
Appropriate experiments with wide-range variations of hydrodynamic conditions and parameters have shown that there exists a positive interaction zone where the total efficiency of turbulent friction reduction is greater than the sum of individual efficiencies of these methods.
It means that combining various methods of near-wall turbulence control is one of the most important and promising research directions in this field.
Material for the future
In the mid-1960s, the Institute of Hydrodynamics SB RAS performed experiments to study the strength properties of titanium alloys which were expected to replace the structural steel used for submarine hulls. Being almost twice as light as steel, titanium has practically the same ultimate strength. Still, some problems remained to be solved. Thus, one of the specific features of titanium alloys is their creep, which means unlimited deformation in the case of standard temperature and loads even lower than the yield point. In terms of elastoplastic characteristics, titanium can be considered an isotropic material that behaves identically under tension or compression. In terms of creep characteristics, however, titanium is substantially anisotropic and behaves differently under tension or compression. An experiment was performed with two identical samples cut out from a metal sheet in the same direction. One sample was subjected to tension, while the other one was subjected to compression under identical stress. The former sample went to ruin in a month and a half. The latter, being under loading, was still intact for about two years, after which the experiment was terminated.
Irreversible deformations are known to be accompanied by accumulation of fatigue leading to material failure. This circumstance requires determining the stress-strain state of every structural element with the purpose of determining the place and time of its fracture. Anisotropy of titanium alloys under tension or compression substantially complicates the strength calculations of titanium structures; therefore, in addition to the experiments, the institute performed theoretical research so as to identify the stress-strain state of submarine structural elements with due allowance for plastic strains and the creep phenomenon.
Important results were also obtained in the experiments aimed at studying the effect of the ambient temperature on deformations of titanium alloys in the temperature range from – 50 to 100 ° С.
Numerical and experimental investigations of the strain and strength properties of structures made of promising composite materials have been performed for many years.
Moving in the wake
Together with the development of submarines there appeared the problems of their detection and camouflage. Acoustic methods of submarine detection led to the upgrading of submarines, which substantially reduced the noise generated by the boats. Non-acoustic methods of submarine detection developed in different directions.
Researchers from the Siberian Branch of the Russian Academy of Sciences focused their efforts on studying hydrodynamic field disturbances induced by a submarine moving at high velocity at a certain depth. In this case the major disturbances are hydrodynamic perturbations of the flow around the submarine and the turbulent wake behind it. These flows are rather versatile because of differences in fluid density over the depth (stratification), which exist in seas and oceans due to variable temperature and salinity. Such a fluid contains internal waves with the maximum amplitudes at a large depth and with a wide range of wave lengths and propagation velocities. It is recording these waves that seems the most promising factor for detecting submarines moving at a large depth under a free surface.
Attempts to solve this problem empirically were not successful, because small perturbations of hydrodynamic fields had to be identified on the background of strong external interferences. For this reason, the main role was given to mathematical modeling of the swirl behind the boat which consisted of the turbulent wake and the field of internal waves generated by this wake and the submarine hull. In parallel with that work, a lot of independent experiments were performed; their results were used for developing, testing, and correcting the mathematical models.
Specific features of transformations of the turbulent wake behind various bodies were studied both with and without propulsion, in homogeneous and stratified media. Mathematical models of the turbulent wake and of the internal waves generated by this wake made it possible to calculate the characteristics of these phenomena. Reliability of the models was validated by results of laboratory and full-scale experiments. Mathematical modeling made it possible to estimate the parameters of the turbulent wake and of the internal waves generated by this wake in the real sea.
The studies performed justified the use of simplified mathematical models when calculating the wake at large distances from the body, where two-dimensional Euler equations could be used to analyze the internal waves. A linear theory of internal waves generated by the turbulent wake was constructed for a number of stratifications important for practice. An approximate diffusion model was developed for calculating the characteristics of the turbulent wake proper. The use of physically well-grounded simplifications of the mathematical model reduced the time of numerical calculations at least by an order of magnitude, which allowed such calculations to be performed by onboard submarine computers.
The researchers demonstrated that layers with drastic density gradients actually act as waveguides for internal waves moving away from the hull; these waveguides allow disturbances to propagate large distances in the transverse direction. As a result, internal waves generated by the submarine are also visible on the free surface in the form of a field of transverse velocities in a narrow wake. While interacting with moderate wind-induced roughness (up to a wind force of four), this wake can be detected by location systems mounted on airplanes or satellites.
The results of investigations performed at the Siberian Branch of the Russian Academy of Sciences demonstrated the possibility of submarine detection by means of real swirl-detecting technical devices. Calculations by the techniques proposed made it possible to formulate requirements to sensitivity, space and time resolution, and inherent noise level of equipment for submarine detection on the basis of several indicators, namely internal waves from the submarine hull and turbulent wake, fluctuations of velocity and density (temperature, salinity) in the wake, and radioactivity level. The mathematical models allowed choosing optimal regimes of submarine motion and the depth of its submersion for different hydrological conditions for the purpose of maneuvering secrecy.
The recommendations of Siberian researchers were highly appreciated by the Navy High Command; they were used to form tactical and engineering requirements to submarines and to perform design and development activities at the leading shipbuilding plants of the country.
Sarkisov A. A. (ed.) Role of Russian Science in Creating Domestic Submarine Forces [in Russian], M.: Nauka. 2008. P. 176—186.
The paper includes illustrations from the book “Role of Russian Science in Creating Domestic Submarine Forces” (2008) and photographs made by R. Akhmerov