Cavity resonances in ducts is a classical problem that has involved researchers from very different fields over time. More recently the aerospace community became engaged again due to resonances found within the flow path of aero-engines, for example in bleed cavities in the low pressure compressor section. These resonances can lead to problems with the structural integrity of upstream components, and thus warrant investigation. The paper compares the results of the same cavity geometry within two wind tunnels of different dimensions and operational setups. The intention of the study is to isolate the Rossiter modes from any other geometric modes that are due to the wind tunnel's test section geometry. This isolation allows the modification of the model of Rossiter and the cavity depth model of East to improve the predictive capability over a larger range of Mach numbers and for deeper cavities. The experiments were performed for an operating range from low subsonic to transonic Mach numbers. The analysis focuses on modifying the constants in both the Rossiter model and the cavity depth model proposed by East along with investigating the phenomenon of mode switching for Rossiter modes. The analyses show a good repeatability of the data set between the two wind tunnels displaying strong resonances at similar operating points. An independence from Reynolds number of the acoustic frequency generation is demonstrated within the operating range. A mode switching behavior is identified with the shallowest and deepest cavity showing multiple mode transitions within the operating range.

Flow over open cavities can give rise to resonances where the acoustic response in the cavity couples with the shear layer oscillations. In turbomachinery, there are several cavities in which such resonance may occur, for example the bleed cavities in the intercompressor duct. This work is a study of flow induced cavity resonance in a rectangular Helmholtz type cavity with variable depth and at a range of Mach numbers. The phenomenon is studied by means of experimental test data, computational fluid dynamics and analytical models. It is identified that the cavity during resonance can respond with both plane modes as well as higher order antisymmetric modes in the cavity. It is shown that the shear layer locks in to the cavity response with integer periods when the cavity responds with antisymmetric modes and half-integer periods when the cavity responds with symmetric modes. It is also observed that there are specific ranges in cavity resonance frequencies which strongly interact with the shear layer instability and that the resonance may switch between different resonance modes even at steady flow condition.

 Cavity resonances in ducts is a classical problem that has involved researchers from very different fields over time. More recently the aerospace community became engaged again due to resonances found within the flow path of aero-engines, for example in bleed cavities in the low pressure compressor section. These resonances can lead to problems with the structural integrity of upstream components, and thus warrant investigation. 

This study expands the previous data set of an open box cavity with variable depth (D) and a rectangular opening with length (L), effectively testing L/D = [4, 2, ½] ratios. This parameter was indicated by Rossiter to be a strong predictive parameter in frequency generation. 

The paper compares the results of the same cavity geometry within two wind tunnels of different dimensions and operational setups. The intention of the study is to isolate the Rossiter modes from any other geometric modes that are due to the wind tunnel’s test section geometry. This isolation allows the modification of the model of Rossiter and the cavity depth model of East in order to improve the predictive capability over a larger range of Mach numbers and for deeper cavities. 

One wind tunnel has an open loop continuous flow operation where the Mach number is set by changing the outlet static pressure. The second wind tunnel has a blow-down operation where both Mach and Reynolds numbers can be set independently by pulling a downstream vacuum and setting the upstream total pressure. The experiments were performed for an operating range from low subsonic to transonic Mach numbers. The analysis focuses on modifying the constants in both the Rossiter model and the cavity depth model proposed by East along with investigating the phenomenon of mode switching for Rossiter modes. 

The analyses show a good repeatability of the data set between the two wind tunnels displaying strong resonances at similar operating points. The new proposed constants for the Rossiter model when using an L/D = 2 are 𝛾𝛾 = 0.15, 𝜅𝜅 = 0.58. For the deeper cavity with L/D = 0.5 the constants proposed are 𝛾𝛾 = 0.15, 𝜅𝜅 = 0.9. The modification of the cavity depth model by East to better predict the mode above Mach 0.3 is proposed by adjusting the constant B = 2.5 and leaving A = 0.65. An independence from Reynolds number of the acoustic frequency generation is demonstrated within the operating range. A modeswitching behavior is identified with the shallowest and deepest cavity showing multiple mode transitions within the operating range and a near constant Rossiter mode 1 for L/D=2. 

Resonance in aerospace is a phenomenon that engineers have been trying to predict and avoid for a long time. Acoustic resonance is only a part in this field. When it was previously studied, it was mostly in connection with long slender gaps at the fuselage of aircrafts. Lately it has become a focus in the development of highly efficient aero engines. Bleed systems in the compressor part of engines are needed but not easy to place aerodynamically. Additionally, these bleed systems have complex geometries. These geometries coupled with the operational range of modern aircraft from low to high subsonic Mach numbers can create unwanted acoustic resonances.This paper is part of project study of these resonances. Here the bleed geometry is simplified to an open box cavity that is studied experimentally in order to measure its acoustic behavior in low to high subsonic flow. The experimental data is compared to theoretical prediction models to create a baseline for future studies. The results show a good agreement between Rossiter prediction and experiments for a shallow cavity of L/D=4. Deeper cavities with a length to depth ratio of one and 0.5 represent more organ pipe resonance phenomena. This is especially governed by the geometry of the cavity itself and the height of the test section. All cavities experience a shift in modes depending on the operating point. This mode shift pattern is similar for deeper cavities. However, the operating range can be divided into four sections in which a mode shift occurs for all cavities.

As bypass-ratio in modern aero engines is continuously increasing over the last decades, the radial offset between low pressure compressor (LPC) and high pressure compressor (HPC), which needs to be overcome by the connecting s-shaped intermediate compressor duct (ICD), is getting higher. Due to performance and weight saving aspects the design of shorter and therefore more aggressive ducts has become an important research topic. In this paper an already aggressive design (with respect to current aero engines) of an ICD with integrated outlet guide vane (OGV) is used as a baseline for an aerodynamic optimization. The aim is to shorten the duct even further while maintaining it separation free. The optimization is broken down into two steps. In the first optimization-step the baseline design is shortened to a feasible extent while keeping weak aerodynamic restrictions. The resulting highly aggressive duct (intermediate design), which is shortened by 19 % in axial length with respect to the baseline, shows separation tendencies of low momentum fluid in the strut/hub region. For the second step, the length of the optimized duct design is frozen. By implementing new design features in the process of the optimizer, this optimization-step aims to eliminate separation and to reduce separation tendencies caused by the aggressive shortening. In particular, these features are: a nonaxisymmetric endwall contouring and parametrization of the strut and the OGV to allow for changes in lift and turning in both blade designs. By comparison of the three designs: Baseline, intermediate (separating flow) and final design, it can be shown, that it is possible to decrease length of the already aggressive baseline design even further, when adding a nonaxisymmetric endwall contouring and changes in blade shape of the strut and OGV. Flow separation can be eliminated while losses are kept low. With a more aggressive and therefore shorter duct the engine length and weight can be reduced. This in turn leads to lighter aircrafts, less fuel consumption and lower CO2 and NOx emissions.