1,720,969 research outputs found
Measurements of flow around a flap side edge with porous edge treatment
No full textWind-tunnel experiments were performed to investigate a flap side-edge vortex, which is a contributor to airframe noise. The flowfield investigation showed that the peak turbulent stresses were contained in the shear layer that rolled up to form the flap side-edge vortex. The wake from the main element was also entrained by the side-edge vortex. The near-field pressure fluctuations where the turbulent shear layer impinged on the flap side edge were broadband in nature from a Strouhal number of 10 to 50. Hot-wire measurements on the downstream vortex identified a broadband instability centered around a Strouhal number of 13.2. A porous side-edge treatment was applied to the half-span flap to modify the flap side-edge flowfield. The effect of applying a porous side edge was to reduce the Reynolds stresses contained within the vortex and the shear layer that formed it. The porous material also had the effect of displacing the vortex further away from the flap surface. This led to a reduction in the broadband pressure perturbations measured at the flap side edge. Compared with the accuracy of the measurements of the aerodynamic forces, the aerodynamic impact of the porous flap side edge was almost negligible
The noise generated by a landing gear wheel with hub and rim cavities
No full textWheels are one of the major noise sources of landing gears. Accurate numerical predictions of wheel noise can provide an insight into the physical mechanism of landing gear noise generation and can aid in the design of noise control devices. The major noise sources of a 33% scaled isolated landing gear wheel are investigated by simulating three different wheel configurations using high-order numerical simulations to compute the flow field and the FW-H equation to obtain the far-field acoustic pressures. The baseline configuration is a wheel with a hub cavity and two rim cavities. Two additional simulations are performed; one with the hub cavity covered (NHC) and the other with both the hub cavity and rim cavities covered (NHCRC). These simulations isolate the effects of the hub cavity and rim cavities on the overall wheel noise. The surface flow patterns are visualised by shear stress lines and show that the flow separations and attachments on the side of the wheel, in both the baseline and the configuration with only the hub cavity covered, are significantly reduced by covering both the hub and rim cavities. A frequency-domain FW-H equation is used to identify the noise source regions on the surface of the wheel. The tyre is the main low frequency noise source and shows a lift dipole and side force dipole pattern depending on the frequency. The hub cavity is identified as the dominant middle frequency noise source and radiates in a frequency range centered around the first and second depth modes of the cylindrical hub cavity. The rim cavities are the main high-frequency noise sources. With the hub cavity and rim cavities covered, the largest reduction in Overall Sound Pressure Level (OASPL) is achieved in the hub side direction. In the other directivities, there is also a reduction in the radiated sound
Active control with dielectric barrier discharge actuators applied to high-lift devices
No full textAn experimental investigation examined the capability of dielectric barrier discharge (DBD) actuators to control a high-lift device system. Aerodynamic tests investigated the potential of utilising the actuator to control the flap side-edge vortex flow field. Acoustic tests examined the attenuation of slat noise with a DBD actuator. The sparse knowledge related to the control of a vortex flow field with a DBD actuator necessitated a more fundamental study that used a NACA 0015 wing. From this study, it was shown that the application of control resulted in a more diffused tip vortex. The actuator's ability to control the evolving vortex flow field was weakly dependent on the Reynolds number but strongly dependent on the angle of attack. Consequently, a DBD actuator was applied to a flap side edge. However, it was concluded that the actuator had no discernible effect on the flow field due to its addition of momentum being too low to destabilise the formation of the flap side-edge vortex. The slat research concerned the attenuation of the leading-edge component of high-lift device noise. At an angle of attack of two degrees, several tonal noise components with broadband content appeared in the slat noise spectrum. These noise features were successfully suppressed with a DBD actuator operating in open-loop control. For closedloop control, a quasi-static feedback controller was implemented. Comparable levels of performance were obtained for both control methods with more than a 20 dB reduction achieved in the dominant tonal noise feature. The research conducted shed new light on the application of DBD actuators to control the high-lift device system. However, further research is needed if the device is to be utilised to control flap side-edge flow field. The attenuation of slat tonal noise with broadband content was achieved with the actuator
Active flow separation control by a positional based iterative learning control with experimental validation
No full textA novel iterative learning control (ILC) algorithm was developed and applied to an active flow control problem. The technique uses pulsed air jets to delay flow separation on a two-element high-lift wing. The ILC algorithm uses position-based pressure measurements to update the actuation. The method was experimentally tested on a wing model in a 0.9 m × 0.6 m low-speed wind tunnel at the University of Southampton. Compressed air and fast switching solenoid valves were used as actuators to excite the flow, and the pressure distribution around the chord of the wing was measured as a feedback control signal for the ILC controller. Experimental results showed that the actuation was able to delay the separation and increase the lift by approximately 10%–15%. By using the ILC algorithm, the controller was able to find the optimum control input and maintain the improvement despite sudden changes of the separation position
Measurement of flow around a flap side with porous edge treatment
No full textWind tunnel experiments were performed to investigate a flap side-edge flowfield. A porous side-edge treatment was applied to a half span flap in an attempt to reduce the flap side-edge noise. Measurements taken were forces, on-surface pressures, particle image velocimetry, hotwire anemometry and on-surface microphones. Oil flow was performed to visualise the on-surface flow. Three potential acoustic sources were identified. The first two sources were the turbulent shear layer reattaching on the side-edge and on the upper surface. A mid-frequency broadband hump was measured by an on-surface microphone at the point of reattachment of the turbulent shear layer on the flap side edge. The third source was a low frequency instability in the vortex due to non-linear vortical interactions upstream of the flap. This instability was measured by a hotwire in the downstream vortex and by an on-surface microphone in the main element flap cove. The effect of the porous side-edge was to reduce the magnitude of vorticity in the turbulent shear layer and the vortex. It was most noticeable in reducing the mid frequency broadband hump. It also had the effect of displacing the vortex further away from the flap surface. This reduced the magnitude of the low frequency perturbations from the unstable vortex that interacted with the solid surface
Numerical simulations of single and tandem wheels for aerodynamic loads prediction
No full textThe aim of this work is to improve the accuracy and effciency of unsteady aerodynamic loads prediction of landing gears in fight conditions, as part of the UK ATI ALGAAP (Advanced Landing Gear Aero-loads and Aero-noise Prediction) project. Delayed Detached-Eddy Simulations (DDES) with the Spalart-Allmaras turbulence model are performed to obtain the desired prediction improvement, both with the fully-turbulent in flow and with laminar in flow and fixed transition. The reference geometries for the current simulations are generic scaled landing-gear wheels in single and tandem configurations, which have been experimentally tested within the project. An experimental database, consisting of mean and unsteady aerodynamic loads, on-surface pressures and velocity fields from particle image velocimetry, is used for CFD validation. The results show the importance of modelling the transition in order to reproduce the experimental data in the transitional regime and to correctly capture the physical flow features. The proposed high-effciency DDES simulations improve the accuracy of the results with respect to the standard DDES model both on single and tandem wheels. The discrepancy between simulations and experiments on the total mean drag coefficient of tandem wheels is within 7% at zero angle of attack and up to 15% at higher angles of attack.</p
Investigation into noise emitted by bluff bodies with large roughness
No full textA set of wind tunnel experiments were performed to study the effect of large surface roughness on circular cylinder noise, with the goal of improving landing gear noise predictions. Roughness increases vortex shedding noise levels, and shifts the peak to a lower Strouhal number. The noise levels in the fall-off range also increase, but no significant change in the fall-off rate is observed. The decrease of the vortex shedding peak frequency has been associated with early detachment caused by the effect of roughness on the TBLs, which is in agreement with previous experimental studies with smaller roughness. The high frequency range of the spectrum revealed a broadband, Strouhal-based peak, which is caused by roughness noise generated on the upstream face of the cylinder. The peak Strouhal number is well predicted by Howe's model using the maximum outer velocity around the cylinder. Cylindrical roughness presents a weaker roughness noise peak, but higher noise levels for higher frequencies, and is thought to be caused by sharp edge separation. A bluff body roughness noise model has been developed based on the model of Howe and a Green's function tailored to the bluff body geometry, calculated using the Boundary Element Method. The application to rough circular cylinders using a at wall (ZPG) TBL model shows good agreement with experiments for downstream observers, but the model overpredicts the levels in over-head observers. The disagreement is thought to be due to inaccuracy of the at wall TBL model. The transition from smooth regime to rough regime was studied experimentally by partially covering the cylinder with distributed roughness in spanwise uniform configurations. Transition regarding vortex shedding happens mainly when roughness is added or removed around the separation region. The results agree with the fact that roughness changes the separation location by perturbing the TBL close to separation. Sparse and dense two-dimensional roughness on a circular cylinder, studied using CFD, have similar effects than distributed roughness regarding the vortex shedding peak level and frequency
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