Review of numerical simulations for high-speed, turbulent cavity flows

Abstract

High speed flows inside cavities are encountered in many aerospace applications including weapon bays of combat aircraft as well as landing gear. The flow field inside these cavities is associated with strong acoustic effects, unsteadiness and turbulence. With increasing emphasis on stealth operation of unmanned combat air vehicles and noise concerns near airports, cavity flows attracted the interest of many researchers in aerodynamics and aeroacoustics. Several attempts were made using wind tunnel experimentation and computational fluid dynamics analyses to understand the complex flow physics associated with cavity flows and alleviate their adverse effects via flow control. The problem proved to be complex, and current research revealed a very complex flow with several flow phenomena taking place. With the aid of experiments, CFD methods were validated and then used for simulations of several cavity configurations. The detached-eddy and large-eddy simulation methods proved invaluable for these studies and their application highlights the need for advanced turbulence simulation techniques in aerospace. The success of these methods and a summary of the current status of the experimental and computational progress over the past twenty years is summarised in this paper.

Introduction

Research into flows over weapons bays, more generally termed cavity flow, started as early as the 1940's. Experimental studies were commissioned by military companies such as English Electric [1], [2], [3] and Boeing [4] to study flow over weapons bays installed in aircraft such as the Canberra and the B47, and to study the impact of the cavity environment on stores and sensitive equipment contained within it.

The main focus of cavity research is still from the aerospace industry relating to understanding the mechanisms that drive the flow in a weapons bay. At high subsonic and supersonic speeds, deployment of stores is very difficult as the cavity environment becomes particularly harsh. Depending on the geometry of the weapons bays, problems could arise in relation to ejection of the stores through the turbulent air at the cavity opening or high intensity acoustic noise could damage the stores and the surrounding cavity structures. Therefore many military aircraft began loading stores onto pylons under the wings. This causes other issues, as external stores can account for up to 30% of the total aircraft drag and are also subjected to aerodynamic heating. In an attempt to overcome the store release problem, aircraft such as the F102 (Fig. 1(a)) had rigs installed to forcefully push the store through the cavity opening and away from the aircraft. However, such mechanisms were prone to jamming and so were not reliable. Therefore, effort was directed towards implementing flow control devices to make the cavity environment more suitable for internal store carriage and release. Modern military aircraft, such as the X-45 Uninhabited Combat Air Vehicles (UCAV) shown in Fig. 1(b), have put even more emphasis on internal store carriage due to the need for stealth characteristics.

Cavity flow research, however, is not exclusively military related. For example, an active area of research involves reducing the far-field noise produced by undercarriage wells when the landing gear is deployed. Around airports, the noise produced by these cavities equals that produced by the engine. Also, the automotive industry studies cavity flows in relation to sunroofs and open windows and door cavities, with the aim of reducing drag and the noise produced.

Since the first experimental studies, a large body of research has been performed in an attempt to better understand the complex fluid dynamics and aeroacoustics that are created when exposing a cavity to the free-stream flow. The Engineering Sciences Data Unit (ESDU) has produced summarising data sheets for the industrial community [7], [8], [9], [10], with both experimental and Computational Fluid Dynamics (CFD) results included. The paper provides a review of the literature in this area and additionally summarises an extensive work programme to add to the understanding of cavities and the influence of flow control devices. The full ‘effects of the control devices on the cavity flow-field are not fully understood and so understanding how simple devices, such as a spoiler at the leading edge, alter the cavity shear layer and cavity environment is key for developing more effective control devices. Finally, a set of computer animations are available on the Publisher's web pages to give interested readers further insight in the turbulent flow encountered in transonic cavity flows. A list of the available animations along with brief descriptions is presented as an appendix.