S. J. Lawson

Assessment of Passive Flow Control for Transonic Cavity Flow Using Detached-Eddy Simulation

The implementation of internal store carriage on stealthy military aircraft has accelerated research into transonic cavity flows. Depending on the freestream Mach number and the cavity dimensions, flows inside cavities can become unsteady, threatening the structural integrity of the cavity and its contents (e.g., stores, avionics, etc.). Below a critical length-to-depth ratio, the shear layer formed along the cavity mouth has enough energy to span across the opening. This shear layer impacts the downstream cavity corner and the generated acoustic disturbances propagate upstream, causing further instabilities near the cavity front. Consequently, a self-sustained feedback loop is established. This extreme flow environment calls for flow control ideas aiming to pacify the cavity by breaking the feedback loop and controlling the breakdown of the shear layer. This is the objective of the present work, which aims to assess changes of the cavity geometry and their effect on the resulting flow using detached-eddy simulation. For the cases computed in this work, quantitative and qualitative agreement with experimental data has been obtained. All of the devices tested achieved similar reductions in overall sound pressure level in the rear half of the cavity; however, a slanted aft wall provided the largest noise reduction in the front half of the cavity.

Computational Fluid Dynamics Analyses of Flow over Weapons-Bay Geometries

Detached-eddy simulations for the M219 experimental cavity geometry and the 1303 uninhabited combat air vehicle cavity geometry are presented. First, results from three computations are presented with the aim of studying the effect of imposing synthetic velocity fluctuations at the inflow boundary. Both methods employed (synthetic-eddy method and fluctuations from a precursor large-eddy simulation) increased the high-frequency content in the boundary layer upstream of the cavity. Averaged profiles showed little change in the streamwise velocity; however, the profiles of normal velocity were noticeably different. The influence of the upstream boundary layer on the cavity flowfield means that the full aircraft geometry needed to be modeled. Consequently, advanced multiblock topologies had to be used to properly represent the planform of the uninhabited combat air vehicle and all the details of the cavity, including doors and hinges, while sliding meshes were needed to insert the store into the uninhabited combat air vehicle configuration. Results with an empty cavity were encouraging for such complex configurations. Visualizations using the Q criteria revealed the added turbulent content due to the door leading edges and the door hinges. The addition of a store in between the doors had little effect close to the front wall. However, averaged flowfields showed that the proximity of the shear layer to the apex of the store deflected it downward into the cavity and restricted its growth. Outside the cavity, shedding was observed from the sting and force balance.

Understanding Cavity Flows using Proper Orthogonal Decomposition and Signal Processing

Computational Fluid Dynamics (CFD) is increasingly being used to analyse complex flows. However, to perform a comprehensive analysis over a given time period, a large amount of data is provided and therefore a method for reducing the storage requirements is considered. The Proper Orthogonal Decomposition (POD) is a widely used technique that obtains low–dimensional approximate descriptions of high–dimensional processes. To demonstrate the potential for reduction in data storage, and the potential use of POD in CFD, the cavity flow case is used. This case is a challenge for CFD due to its unsteady nature and high frequency content. The POD modes were constructed using flow–field snapshots taken at regular intervals. Spatial POD modes for the cavity case showed that the modes came in pairs with a 90° phase shift. The lower modes represented the large dynamics of the shear layer and the higher modes the small scale turbulent structures. Reconstructions of the flow–fields showed that the very large dynamics could be represented with as few as 11 modes. However, approximately 101 modes (85% of the flow energy) were needed to approximate the frequency spectra below 1 kHz. Therfore a reduction of 70% in disk storage would be achieved over storing the complete set of flow–field snapshots produced by CFD.

Assessment of Flow Control Devices for Transonic Cavity Flows Using DES and LES

Since the implementation of internal carriage of stores on military aircraft, transonic flows in cavities were put forward as a model problem for validation of CFD methods before design studies of weapon bays can be undertaken. Depending on the free-stream Mach number and the cavity dimensions, the flow inside the cavity can become very unsteady. Below a critical length-to-depth ratio (L/D), the flow has enough energy to span across the cavity opening and a shear layer develops. When the shear layer impacts the downstream cavity corner, acoustical disturbances are generated and propagated upstream, which in turn causes further instabilities at the cavity front and a feedback loop is maintained. The acoustic environment in the cavity is so harsh in these circumstances that the noise level at the cavity rear has been found to approach 170 dB and frequencies near 1 kHz are created. The effect of this unsteady environment on the structural integrity of the contents of the cavity (e.g. stores, avionics, etc.) can be serious. Above the critical L/D ratio, the shear layer no longer has enough energy to span across the cavity and dips into it. Although this does not produce as high noise levels and frequencies as shorter cavities, the differential pressure along the cavity produces large pitching moments making store release difficult. Computational fluid dynamics analysis of cavity flows, based on the Reynolds-Averaged Navier—Stokes equations was only able to capture some of the flow physics present. On the other hand, results obtained with Large-Eddy Simulation or Detached-Eddy Simulation methods fared much better and for the cases computed, quantitative and qualitative agreement with experimental data has been obtained.

DES for UCAV Weapon Bay Flow

This paper demonstrates the Detached–Eddy Simulation approach for the computation of flows around uninhabited combat air vehicles. This new family of aircraft may feature weapon bays to enhance stealth characteristics and improve aerodynamic performance. However, during operation with the bay open, a highly energetic flow-field develops that can dramatically change the aerodynamics of the aircraft. For this reason detailed CFD analyses are needed to provide insight in the change of loads encountered when weapon bays are exposed. In contrast to previous studies where idealised, isolated cavities are used as model problems, a realistic aircraft geometry is used in this work.

Influence of stores on the flow inside UCAV weapon bays

Detached-eddy simulations for the 1303 UCAV geometry are undertaken aiming to investigate the flow physics of the interaction of a store with the flow inside a UCAV weapons bay. Advanced multi-block topologies had to be used to properly represent the planform of the UCAV and all the details of the weapon bay, including doors and hinges, while sliding meshes were needed to insert the store into the UCAV configuration. Results with an empty bay were encouraging for such complex configurations. Flow visualisation revealed the added turbulent content due to the door leading edges and hinges. The addition of a store in between the doors had little effect close to the front wall of the bay. However averaged flow-fields showed that the proximity of the shear layer to the apex of the store deflected it downwards into the bay and restricted its growth.