The subject of flow control is broadly introduced in this first chapter, leaving much of the details to the subsequent chapters of the book. The ability to actively or passively manipulate a flow field to effect a desired change is of immense technological importance, and this undoubtedly accounts for the fact that the subject is more hotly pursued by scientists and engineers than any other topic in fluid mechanics. In this chapter classical tools of flow control are emphasized, leaving the more modern strategies to the following chapter. Methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation, or noise suppression are considered. The treatment is tutorial at times, making the material accessible to the advanced graduate student in the field of fluid mechanics. Emphasis is placed on external boundary-layer flows although applicability of some of the methods reviewed for internal flows will be mentioned. An attempt is made to present a unified view of the means by which different methods of control achieve a variety of end results. Performance penalties associated with a particular method such as cost, complexity, or trade-off will be elaborated.

Introduction to Flow Control

This paper presents the results of experimental studies to control the base pressure from a convergent nozzle under the influence of favorable pressures gradient at sonic Mach number. An active control in the form of four micro jets of 1 mm orifice diameter located at 900 intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region was employed to control the base pressure. The area ratio (ratio of area of suddenly expanded duct to nozzle exit area) studied are 2.56, 3.24, 4.84 and 6.25. The L/D ratio of the sudden expansion duct varies from 10 to 1. From the results, an important aspect to be noted here is that, unlike passive controls the favorable pressure gradient does not ensure augmentation of the control effectiveness for active control in the form of micro jets. To study the effect of micro jets on the quality of flow in the enlarged duct wall pressure was measured and it is found that the micro jets do not disturb the flow field in the duct rather the quality of flow has improved due to the presence of micro jets in some cases.

Sonic Under Expanded Flow Control With Micro Jets

Aerodynamic characteristics of external store configurations used on interceptor aircraft were investigated experimentally and computationally. Balance measurements, flow visualization, and static pressure measure- ments were made in a low-speed wind tunnel. The experimental and computational data revealed the global structure of the flow around a basic configuration. Six groups of models were used in the present study. This article discusses the results for the first and second group of models. The incompressible flow over a limited number of models was computed by solving the Navier-Stokes equations. The solution method is based on the Galerkin finite element discretization of space and the fractional step discretization of time. Reasonably good agreement was observed between the experimental and computational static pressure distributions on a basic geometry. The computational data, which revealed details of reverse flow regions, supported and supplemented the experimental data. using the code employed in the present study are presented in Refs. 5-7. The scope of the study was limited to the aero- dynamic characteristics of the isolated carriage, i.e., inter- ference with the aircraft body was not investigated. The experimental and computational data revealed the global structure of the flow around the carriage. Balance measure- ments showed that aerodynamically more suitable forms of the carriage geometry could be obtained by modifying its major drag-producing features. Surface flow visualization in- dicated locations of vortical flow regions whose existence was also supported by contour plots of the measured static pres- sure. Despite differences in the Reynolds numbers of the computed and measured flows, reasonably good agreement was observed between the experimental and computational static pressure distributions on the basic carriage geometry. The computations also exhibited details of reverse flow re- gions and, thus, both supported and supplemented the ex- perimental data that did not include velocity measurements. The geometry of the basic carriage used in the present study is not axisymmetric. There is a lack of data in the literature on studies of external stores that have nonaxisymmetric ge- ometries. References 8 and 9 present data for bodies having square and rectangular cross sections. Studies of the flow around axisymmetric bodies show that there exist four char- acteristic flow regimes10"12: "vortex-free," "symmetric vor- tex," "asymmetric vortex," and "wake-like" flow regimes are observed with increasing angle of attack. Crossflow separation of the boundary layer on an axisymmetric body gives rise to formation of vortices on the lee side. Reference 10 states that the nonlinear lift force due to these vortices can be much larger than the lift predicted by the linear (small perturbation) theory. References 13-15 describe studies that investigate the effect of afterbody modifications on the drag of axisymmetric bodies. Interference between the flowfields of stores and air- craft bodies is investigated by Refs. 1 and 2.

Aerodynamic characteristics of external store configurations at low speeds

This paper presents the experimental results on the flow characteristics of a suddenly expanded flow from the convergent nozzle for sonic under expanded case. In the present study micro jets were used to investigate the wall pressure in the enlarged duct. Accordingly an active control in the form of four micro jets of 1 mm orifice diameter located at 90 0 intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region was employed. The area ratio (ratio of area of suddenly expanded duct to nozzle exit area) studied are 2.56, 3.24, 4.84 and 6.25. The length-to-diameter (i.e. L/D) ratio of the sudden expansion duct was varied from 10 to 1. To study the effect of micro jets on the quality of flow in the enlarged duct wall pressure was measured and from the results it is found that the micro jets do not disturb the flow field in the duct. From the results, it is also seen that for L/D in the range L/D = 10 and 8 the flow remains oscillatory mostly for all the area ratios. However, these oscillations are suppressed gradually with the increase in the area ratio, also for all the L/D in the range 3 to 6. The nozzle pressure ratio (NPR) was varied from 1 to 3, however, in the present paper results are presented for under expanded case to ascertain the effectiveness of the micro jets under the influence of favorable pressure gradient (i.e. Pe/Pa = 1.5). The present study explicitly reveals that, the wall pressure in a suddenly expanded axi-symmetric duct can be controlled by employing micro jets.

The effect of micro jets on wall pressure for sonic under expanded flow

In the present study the experiments were conducted to control the base pressure from a convergent-divergent
nozzle at low supersonic Mach numbers to assess the effectiveness of active control mechanism in the form of
micro jets at different expansion level. The parameters considered in the present study are the diameter ratio,
length to diameter ratio (L/D), Nozzle Pressure Ratio (NPR), and the Mach number. The diameter ratio selected
for the present study are 1.6, 1.8, 2.2, and 2.5. Experiments were conducted for nozzle pressure ratio (NPR)
from 3 to 11. The L/D ratio of the enlarged duct was varied from 10 to 1, and results are presented for L/D 4, 3,
2, and 1. The Mach numbers of the present studies are 1.1, 1.2, 1.4, and 1.5. The results show that the Micro jets
are very effective and are able to raise the base pressure value to a considerable level under the influence of
favorable pressure gradient except at lower NPR 3. At NPRs 5 and 7 for some cases the trends differ due to the
level of expansion, nature of waves present in the base region, relief available to the flow, L/D ratio of the
enlarged duct and the Mach numbers. It is seen that most of the cases exhibit similar behavior for the L/Ds in
the range 4 and 3, which means; that the back pressure has not adversely influenced the flow field in the base
region as well as in the duct. The minimum duct length required for the flow to be attached is L/D = 2, even
though in some cases flow is attached with duct wall. With this it can be stated that the micro jets can be an
alternative for the for base pressure control.
Keywords - Base pressure, Length-to-diameter ratio, Mach number, Nozzle pressure ratio, Sudden expansion

Control of Suddenly Expanded Flow at Low Supersonic Mach Numbers

A method has been proposed of controlling separated base flows as a means of reducing aerodynamic drag. A series of steps attached to the base of the body of interest is used to control flow separation by forming a system of captive vortices on the steps. Kentfield's wind tunnel experiments seemed to confirm this concept for axisymmetric bodies at low subsonic Mach numbers and low Reynolds numbers. He reported drag reductions of as much as 56% using stepped afterbodies as compared to identical bodies with flat bases. The paper presents results obtained from further investigations of this stepped base concept in conjunction with both fin- and spin-stabilised configurations at high subsonic,

James A. Kidd

Papers

Drag Reduction by Controlling Flow Separation Using Stepped Afterbodies