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Paper presented at the Technology Common to Aero and Marine Engineering: Proceedings of an international conference, January 26–28, 1988

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Keywords: vortex

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Proceedings Papers

Publisher: Society of Underwater Technology

Paper presented at the Technology Common to Aero and Marine Engineering: Proceedings of an international conference, January 26–28, 1988

Paper Number: SUT-AUTOE-v15-033

... cross-flow plane tunnel production logging agreement body

**vortex**forward fin flowfield reynolds number**vortex**production control tunnel model water tunnel aerospace & defense hydrogen bubble technique**vortex**location water tunnel technique reservoir surveillance calibre fin...
Abstract

Understanding the vortex patterns on the leeside of a body of revolution is a major problem in missile aerodynamics Above incidences of about 5 ° the boundary layer separates from the leeside of the body along a separation line and hence introduces circulation into the flow Up to about 25 ° to 35 ° incidence the circulation will concentrate itself around two symmetric vortices in the cross-flow plane The interaction of these vortices with downstream surfaces such as controls can induce large variations in local angle of attack causing control problems. In an effort to improve aerodynamic prediction codes British Aerospace (BAe) has had a continuing interest in vortex-dominated flowfields. As an aid to understanding the qualitative aspects of such flowfields a water tunnel has been found to be extremely useful in flow visualization. The high density and low diffusivity of the working fluid enable dense flow tracers with high reflectivities, such as dyes or particles to be used successfully BAe have amassed over the years data in the form of flow visualization photographs concerned primarily with the characteristics of vortices shed from bodies of revolution and subsequent interactions with downstream surfaces Studies have included the effects of curved flowfields on body vortices, the interaction of body vortices with intakes (Beaman, 1986) and of course, the effects of lifting panels on body vortex development (Deane, 1978) Some of the more qualitative results of the studies have assisted in the development of aerodynamic prediction methods (Beaman, 1986) However, concern has been expressed as to the validity of water tunnel techniques in representing flow at higher Reynolds numbers Reynolds numbers are orders of magnitude lower in the water tunnel than in wind tunnels or free flight, although at the high angles of incidence normally considered when investigating vortex effects and with the supersonic wing sections usually used in missile technology, it is expected that separation on a wing is fixed at the sharp leading edge and hence the wing flow will be well represented For the body, however, no such prominent feature fixes the separation position which, as is well known, can change significantly with Reynolds number As a step towards validating the use of water tunnels in simulating the flow around missile configurations at moderate angles of attack BAe have undertaken a comparative study in which reliable wind tunnel data on vortex locations have been reproduced in a water tunnel. The derivation of vortex locations from wind tunnel data can be a difficult problem to solve In many cases separation points are derived from surface pressure distributions, which can give rise to large uncertainties due to the interpretation required Vortex locations are generally derived from flow visualization in a wind tunnel, for instance vapour screen techniques which may require varying degrees of interpretation and tend not to be the primary objective of the tests with a consequent decrease in accuracy and scope BAe were therefore fortunate in performing some work for the Royal Aircraft Establishment, Bedford, that included the analysis of flowfield data around a body of revolution collected for the specific purpose of investigating vortex locations and strengths

Proceedings Papers

Publisher: Society of Underwater Technology

Paper presented at the Technology Common to Aero and Marine Engineering: Proceedings of an international conference, January 26–28, 1988

Paper Number: SUT-AUTOE-v15-073

... conrad tube dnft angle strut lloyd vorhce submanne probe expenment inadence campbell underwater technology vorticity vestiga te freestone probe locahon curved flow turn rate experiment

**vortex**revolution Experimen ts to in vestiga te the vorticity shed by a body of...
Abstract

Lloyd (1983) described SLWIM, a new kind of mathematical model being developed at the Admiralty Research Establishment, Haslar, to predict the manoeuvnng qualities of deeply submerged submarines. The model uses a simulation of the changing flow around the manoeuvring submarine to predict the time history of the forces and moments experienced by the boat. These are then used to drive the standard equations of motion and predict the trajectory of the vehicle in response to movements of the control surfaces A related model called NUSIM is also being developed in parallel at ARE Haslar and forms the subject of another chapter in this volume (Tinker, 1988). The main advantage of these models over conventional derivative based mathematical models is that they require no ad hoc model test data to provide information on the hydrodynamic characteristics of the particular submarine being simulated Predictions can therefore be made at an early stage in the design process without the need to build expensive physical models and test them over a long period of time. The flow around a manoeuvring submarine is dominated by the vortices which are shed from the appendages and the hull (Fig 1) The characteristics of the appendage vortices may be predicted using lifting line or lifting surface theory The body vortices are influenced by the incidence and the rate of turn of the vehicle, and the timely prediction of their characteristics seems to be beyond the scope of any existing theory The SUBSIM program therefore makes use of empirical data to represent their effects The NUSIM program includes a prediction of the body vortices, but this is time consuming and is at present confined to rectilinear flow (1 e at a simple angle of incidence) There exists a considerable mass of data on body vortices in the aeronautical literature Most of these data relate to the flow around missile-like bodies with pointed noses and blunt tails at supersonic speeds, although there are some subsonic data for airships Virtually all of these data seem to have been obtained in rectilinear flow, and any effects of flow curvature caused by manoeuvring have been ignored. This dearth of information on submanne-like bodies (with blunt noses and pointed tails) in curved flow demanded that suitable experiments be conducted to provide empirical data for SWIM and validation of future developments of NUSIM Lloyd and Campbell (1986) described development work leading to suitable experiments and gave(Fig. 1 is available in full paper) some preliminary results Since that tune further experiments have been completed and a more detailed and sophisticated method of analysis has been developed. This chapter discusses the interpretation of selected results in detail and describes some of the difficulties encountered in the analysis. The experiments were carried out in the Manoeuvring Tank at ARE(H) They were performed by the Wolfson Unit for Marine Technology and Industrial Aerodynamics (Southampton University) working under contract to ARE(H)

Proceedings Papers

Publisher: Society of Underwater Technology

Paper presented at the Technology Common to Aero and Marine Engineering: Proceedings of an international conference, January 26–28, 1988

Paper Number: SUT-AUTOE-v15-119

... Sarpkaya and Isaacson (1981) state that wave diffraction is important when D/¿1 > 02 This chapter, however, will consider the case where the wavelength is larger than the cylinder diameter and where the effects of fluid viscosity, leading to flow separation and

**vortex**shedding, are important A critical...
Abstract

The flow around circular cylinders has been studied by aeronautical engineers almost since powered flight began. There are numerous applications that can be cited, and the problems that have been examined closely range from the forces on the bracing wires and struts of early biplanes to the wind loading of space rockets on the launch pad Although a circular cylinder is geometrically extremely simple, its associated flow field is very complex and can be viewed as an extensive series of transitions between different flow states. In recent years it has become a focus for work in computational fluid dynamics and is often employed as a test case for various numerical approaches However, the circular cylinder still holds a number of secrets, as will be demonstrated by reference to problems in marine technology. The circular cross-section tube is a key element in offshore structures, where it may be exposed to a steady current flow or to waves. In many locations it may also experience a flow environment which is a combination of waves, with a directional spread, and a current. An important phenomenon observed in a steady water flow, but not found in air flow, is that a flexible circular cylinder may experience in-line oscillations due to the motion-m, regular shedding of synmetric vortices. This was first documented by Wootton et al (1969) and occurs when the structural damping is small and when the ratio of the mass of the cylinder to the mass of the fluid displaced is also small However, most of the research into circular cylinders in water flow, and the majority of the new results, relate to interaction with free surface waves When the wavelength, ¿, is less than or comparable to the diameter, D, of the cylinder, the loading can be estimated from diffraction theory, assuming the flow to be rotational Sarpkaya and Isaacson (1981) state that wave diffraction is important when D/¿1 > 02 This chapter, however, will consider the case where the wavelength is larger than the cylinder diameter and where the effects of fluid viscosity, leading to flow separation and vortex shedding, are important A critical question then is how do the various flow regimes and transitions observed in steady flow apply to a circular cylinder in waves? FLOW REGIMES IN STEADY FLOW Over the range of Reynolds number between 10 4 and l0 7 several important changes occur in the flow field, and it is known that these changes can be explained in terms of variations in the boundary layer development Figure 1, reproduced from Schewe (1983), shows drag coefficient plotted against Reynolds number At Reynolds numbers of the order of 105 the boundary layer at separation is laminar, and transition occurs within the free shear layers prior to their rolling up into the vortices that form the vortex street wake This regime is known as the sub-critical since it precedes the Reynolds number at which the drag coefficient falls rapidly.

Proceedings Papers

Publisher: Society of Underwater Technology

Paper Number: SUT-AUTOE-v15-177

... The normal way to calculate wave-induced motions and loads on ships and large volume structures is to use potential theory Free surface effects are included, but

**vortex**shedding is usually neglected in theoretical procedures This is generally an appropriate assumption. An exception is resonance...
Abstract

The normal way to calculate wave-induced motions and loads on ships and large volume structures is to use potential theory Free surface effects are included, but vortex shedding is usually neglected in theoretical procedures This is generally an appropriate assumption. An exception is resonance roll motion of ships and barges Eddymaking damping is then of equal importance as the wave radiation damping. In practical calculations viscous roll damping is accounted for by empirical formulae. This chapter presents a rational theoretical approach that includes both the effect of free surface waves and vortex shedding Forced harmonic roll oscillation of an infinitely long horizontal cylinder will be studied The method can in principle be applied to any forced body mode The body has to have sharp corners so that the separation points are well defined No boundary layer calculation is then needed A fundamental assumption of the method is that vorticity is concentrated in thin free shear layers. The theory is based on a time-step integration method where we in each time-step have to solve a potential flow boundary value problem outside the thin free shear layers. Two different cases are treated Rigid free surface Free surface waves generated by the body are allowed to propagate When the free surface is rigid, an "image-body" moving with opposite phase and with the same amplitude as the real body has to be introduced At each tune-step we have to solve a Fredholm's integral equation of the second kind By dividing both the body surface and the free shear layers into line-elements with dipole and source distribution over the body and dipole distribution over the free shear layers, the integral equation may be represented by a linear equation system where the unknowns are the fluid velocity potentials at the body element midpoints. In the case of a moving free surface we also have to represent the part of the free surface close to the body by linear elements In the far field the influence of the body is represented by the sum of a source and a horizontal dipole satisfying the linearized time-dependent free surface condition. The problem is treated as an initial value problem with the velocity potential on the free surface given at each time-step, and the fluid particle velocity on the free surface as the additional unknown variable in our problem The equivalence to the Kutta condition is that we require the potential lump at the separation points to be continuous and that vorticity is shed parallel to the body surface on one of the sides of the free shear layers at the separation points. To start up the time simulation a discrete vortex with a given position and circulation is introduced into the fluid in the first time-step. The position and strength of this vortex are calculated on the basis of a pure potential theory calculation as described by Rott (1956) and Pullin (1978).