Openings (cavities) in the aerodynamic surfaces of vehicles can create noise and vibrations. Techniques are explored to control or delay the onset of such events.

This research has generated data that have been included in design guidelines used in industry, and influenced design practices. Ensuring the safe carriage and release of new stores on military aircraft is an expensive process, typically involving many hours of flight tests and release trials. Our results have helped to increase the reliability of, and confidence in, computational tools that are increasingly used to reduce the extent of such flight trials. Our research on reducing or eliminating undesirable pressure fluctuations arising in some cavities has revealed promising techniques to lessen the noise produced by the vehicle, avoid structural damage, and allow safer release of stores from military aircraft.

Key Facts

    • Cavities occur in many engineering situations, including car door gaps, gaps between train carriages, aircraft wheel-wells, and weapons bays.
    • The flow over such features naturally creates strong pressure fluctuations that cause noise and increased drag at low speeds and potentially damaging vibrations at high speeds. This can cause particular problems for military aircraft when trying to release stores from internal bays.
    • This can be avoided by modifying the behaviour of the flow over a cavity either with changes to the vehicle’s geometry (passive techniques) or by forcing the flow with actuators (active techniques), the latter requiring more complex systems but with the potential for better effects.
    • The Aeromechanical Systems Group at Shrivenham has so far focused on improving the understanding of cavity flows, and developing passive techniques to reduce the adverse effects of pressure fluctuations for high-speed applications. Wind-tunnel tests have been conducted, underpinned by advanced laser-based measurements and computational modelling, to gain insight into the cavity-flow phenomena and their control.

Why the research was commissioned

There are two aerodynamic problems associated with releasing stores from internal bays on aircraft. One is ensuring that the stores separate safely, without striking the aircraft. Assuring this typically involves expensive flight trials for each store/aircraft combination. The other problem is that high-speed flows over cavities (i.e. cut-outs or bays in a vehicle’s surface) can lead to extremely high pressure fluctuations that cause intense noise and structural damage (to aircraft and store). This research sought to address the first problem by increasing the understanding of the cavity flow environment and thereby improving safe release. The second problem was addressed via flow control techniques.

Why Cranfield?

The researchers of Cranfield University’s Aero-mechanical Systems Group have more than 15 years of experience in aero-acoustic measurements and corresponding data analysis, and specific expertise in the physics and control of cavity-flow resonance. Their past and current studies include the formulation and validation of cavity-flow models and the development of passive and active techniques for reducing the pressure fluctuations induced by cavity flows.

Facilities used

The transonic wind tunnel at Shrivenham has been used as it is a suitable facility for researching cavity flow resonance at high speed. Other wind tunnels are used for testing larger models at lower speeds. A range of diagnostic equipment including dynamic pressure transducers, hot-wire, pitot-probe and laser-Doppler anemometers, particle image velocimetry and other types of flow visualization, are used for capturing the flow and acoustic characteristics relevant to this type of study. The Cranfield supercomputer cluster has been used for the computational work.

Cavity flow pressure

Results of transonic windtunnel flow-visualisation tests on representative bodies in the exit plane of a cavity