Recent progress on the study of asymmetric vortex flow over slender bodies

The investigations of forebody vortex flow and its flow control have great importance in both academic field and engineering application areas. A large number of papers and many review papers have been published. However in this research field of forebody asymmetric vortices, three problems such as tip perturbation effect, Reynolds number effect and flow instability are less studied and thus not understood completely. So many researches are still working on the issues in recent years. The present paper attempts to provide a review of recent research progress on first two problems. The first problem is mainly concerned with how the vortex flow evolves after tip perturbation; how to solve the problem of repeatability and reproducibility of wind tunnel testing data; how to develop a conception of active flow control technique with tip perturbation based on the study of vortex flow response to tip perturbation. For the second problem one is mainly concerned that how the asymmetric vortices are developed with the increase of Reynolds number; how to classify the vortex flow patterns in different Reynolds number regimes; how to develop an appropriate boundary layer transition technique to simulate flows at high Reynolds number in the convention wind tunnels. Finally, some important ques- tions that deserve answers are proposed in the concluding remarks.

INFLUENCE OF NOSE PERTURBATIONS ON BEHAVIORS OF ASYMMETRIC VORTICES OVER SLENDER BODY

The influence of nose perturbations on the behaviors of asymmetric vortices over a slender body with a three-caliber ogive nose is studied in this paper. The tests of a nose-disturbed slender body with surface pressure measurement were conducted at a low speed wind tunnel with subcritical Reynolds number of 1×105 at angle of attack α=50°. The experiment results show that the behaviors and structure of asymmetric vortices over the slender body are mainly controlled by manual perturbation on the nose of body as compared with geometrical minute irregularities on the test model from the machining tolerances. The effect of the perturbation axial location on asymmetric vortices is the strongest if its location is near the model apex. There are four sensitive circumferential locations of manual perturbation at which bistable vortices over the slender body are switched by the perturbation. The flowfield near the reattachment line of lee side is more sensitive to the perturbation, because the saddle point to saddle point topological structure in this reattachment flowfield is unstable. Various types of perturbation do not change the perturbation effect on the behaviors of bistable asymmetric vortices.

ASYMMETRIC VORTICES FLOW OVER SLENDER BODY AND ITS ACTIVE CONTROL AT HIGH ANGLE OF ATTACK

The studies of asymmetric vortices flow over slender body and its active control at high angles of attack have significant importance for both academic field and engineering area.This paper attempts to provide an update state of art to the investigations on the fields of forebody asymmetric vortices.This review emphasizes the correlation between micro-perturbation on the model nose and its response and evolution behaviors of the asymmetric vortices.The critical issues are discussed, which include the formation and evolution mechanism of asymmetric multi-vortices;main behaviors of asymmetric vortices flow including its deterministic feature and vortices flow structure;the evolution and development of asymmetric vortices under the perturbation on the model nose;forebody vortex active control especially discussed micro-perturbation active control concept and technique in more detail.However present understanding in this area is still very limited and this paper tries to identify the key unknown problems in the concluding remarks.

AN ANALYTIC SOLUTION ON HYPERSONIC FLOW OVER AN ARBTTRARY SLENDER BODY WITH NEAR POWER-LAW PROFILE

On the basis of a self-similar solution as well as of the assumption of the 'Transserse Motion',a general linear theory on hypersonic flow over a general slender body is set up in this paper By means of this theory, the problem concerned can he put into a universal system of O.D Eqs .which can be integrated manerically in adyance

EXPERIMENTAL STUDY OF FLOWFIELD STRUCTURE AROUND AN OGIVE-CYLINDER

The flowfield structure and their aerodynamic characteristics over an ogive cylinder were studied by means of flow visualization and surface pressure measurement in a water tunnel and a wind tunnel. The existence of multi asymmetric vortices over long slender bodies was experimentally confirmed at large angles of attack and in the subcritical Reynolds number range. The spatial 3 D characteristics of the multi vortices system were analyzed and a physical model was developed. The topological structure of different patterns in cross flow plane was studied and the mechanism governing the formation of asymmetric vortices and multi vortices was discussed from the viewpoint of stability of the topological structure. It was concluded that the maximum in the sectional side force distribution curve are not caused by the shedding of higher position vortex, but by the cross over to the symmetric plane of the lower position vortex.

UNSTEADY SUPERCAVITATING FLOW PAST CONES

Based on integral equation method, the study of unsteady supercavitating flow past cones is presented. The shape and length of supercavity are calculated respectively using the finite difference time discretization method. The characteristics of the shape and length of supercavities, which vary with the cone＇s angle and cavitation number, are investigated respectively, the varied features of some supercavity scales are analyzed when the flow field is perturbed periodically. The curves relationship between cavity length and cavitation number, which are based respectively on present method and other theories, are discussed and compared. It is obviously shown that the supercavity changes have two characteristics： retardance and wave. These results obtained would be useful in the case of design and analysis of cavitator under water.

Effect of multiple rings on side force over an ogive-cylinder body at subsonic speed

Experiments and computations were performed over an ogive-cylinder body having an lift-to-drag ratio of 16 at a diameter Reynolds number of 29000. The side force on the slender body augments with increasing angles of attack for the case without a ring. This increase was mainly due to the increase in the asymmetry of the existing vortex pair in the wake of the body. Attempts were made to completely reduce the existing side force at the angle of attack ranging from 35° to 45°.Three rectangular cross-sectioned circumferential rings having a height of 3% of the local diameter were placed at axial distances of 2.5, 3.5 and 4.5 times the base diameter from the tip of the body so as to reduce the side force. The results obtained indicate that inclusion of three rings completely alleviated the side force on the slender body at the angle of attack ranging from 0° to 45°. The presence of rings was found to alter the growth of the vortices that helped in the reduction of the side force. Computations performed were in reasonable agreement with the experiments.

SYMMETRIC VORTICAL STRUCTURE AND ITS TOPOLOGICAL STABILITY

Using time dependent compressible laminar Navier-Stokes equations with a finite volume method incorporating a third-order-accurate discretization scheme, the flow structures around a slender at certain incidences are numerical simulated and typical crossflow patterns are presented. At incidence 10°, these vortical configurations are different at dissimilar axial locations though they are symmetric. At 35°, the symmetric vortical structures still maintain over the slender, yet their interaction at afterbody is intense than that at the forehody since the two vortices have fully developed downstream. The unstable topological structure of trajectory of saddle-to-saddle points and multiple limit cycle are further discussed in topological stability theory. These structures easily produce bifurcation with perturbation. The results support the view of hydrodynamic instability of vortices flow field.

RESEARCH ON THE HYDRODYNAMIC STABILITY OF FIBRE SUSPENSIONS

The stability of wall bounded fibre suspensions was studied. The linear stability analysis was performed applying the flow stability theory and slender body theory. The results of numerical analysis show that fibres and their hydrodynamic interactions reinforce the flow stability. Investigation of fibre orientation and vorticity in the suspension revealed the mechanisms behind the instability. Drag reduction properties in the transition regime were also presented. The experiments using dye emission and PIV techniques verified theoretical results.