| 1. |
- Landman, Drew, et al.
(författare)
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Hybrid Design for Aircraft Wind-Tunnel Testing Using Response Surface Methodology
- 2007
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Ingår i: Journal of Aircraft. - : American Institute of Aeronautics and Astronautics (AIAA). - 0021-8669 .- 1533-3868. ; 44:4, s. 1214-1221
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Tidskriftsartikel (refereegranskat)abstract
- A response surface methodology approach to wind-tunnel testing of high-performance aircraft is being investigated at the Langley Full-Scale Tunnel. An exploratory study was completed using a newly developed response surface methodology design in an effort to better characterize an aircraft’s aerodynamic behavior while simultaneously reducing test time. This new design called a “nested face-centered design” was developed when classic designs were found to have inadequate prediction qualities over a cuboidal design space with factors at five levels. A 19% scale modified X-31 aircraft model was chosen for evaluation of the new response surface methodology design based on its nonlinear aerodynamic behavior at high angle of attack that is representative of modern fighter aircraft and due to a substantial preexisting data base. A five-level nested fractional factorial design, augmented with centerpoints and axial points, produced regression models including pure cubic terms for the characteristic aerodynamic forces and moments over a cuboidal design space as a function of model position and control surface deflections. Model adequacy and uncertainty levels were described using robust statistical methods inherent to response surface methodology practice. Comparisons to baseline data and sample lateral–directional and longitudinal aerodynamic characteristics are given as validation of the new design.
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| 2. |
- Lim, H. D., et al.
(författare)
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Short-time proper orthogonal decomposition of time-resolved schlieren images for transient jet screech characterization
- 2020
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Ingår i: Aerospace Science and Technology. - : Elsevier BV. - 1270-9638 .- 1626-3219. ; 107
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Tidskriftsartikel (refereegranskat)abstract
- Short-time Proper Orthogonal Decomposition (POD) is proposed as an image-based technique to study the transient jet screech characteristics of moderately under-expanded supersonic jets emanating from a circular baseline and two bevelled nozzles. Time-resolved schlieren imaging of turbulent flow structures were performed with an ultrahigh-speed schlieren setup. Short-time POD was performed by systematically sampling image-series with a short time delay, performing PODs and applying spectral analyses on the first POD mode coefficients, and plotting the peak frequencies from the resulting PSDs into a peak frequency-occurrence count histogram. The results are in good agreement with the near-field noise spectra and wavelet transform analysis of the microphone measurements, which revealed intermittent jet screech occurrences at St=0.25 for both baseline and 30 degrees bevelled jets, while none was detected for the 60 degrees bevelled jet. In particular, the occurrence counts of the frequency bins is proposed as a suitable parameter to characterize the intermittent nature of jet screech, with the frequency bin revealing the jet screech frequency if present. The present study demonstrates the advantage of short-time POD analysis on time-resolved schlieren images over traditional image-based POD methods, which includes computational gains from parallelization, the ability to handle much larger datasets and revealing insights into a transient flow and noise phenomenon.
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| 3. |
- Lim, H.D., et al.
(författare)
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Visual-hull based 3D reconstruction of shocks in under-expanded supersonic bevelled jets.
- 2018
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Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 99, s. 458-473
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Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional shock structures produced by Mach 1.45 supersonic bevelled jets were digitally reconstructedbased on schlieren photography and a voxel-based visual hull technique. By taking advantage of the strong edgefeatures commonly found in schlieren images of shock waves, the proposed technique demonstrates the possibilityof performing shock wave reconstruction in supersonic jet applications without prior knowledge of theglobal density or velocity field. Semi-synthetic camera parameters were introduced as a method to circumventcamera calibration issues faced in the reconstruction procedure. This is key to achieving accurate and highresolutionreconstructed shock waves for both axisymmetric and asymmetric test cases with an average of 2.5%error when validated against raw schlieren images. When applied to bevelled jets with non-uniform nozzle exitgeometries, an additional assumption was made to address the problem of schlieren line-of-sight blockage by thenon-conventional nozzle, and reconstruction errors were found to be larger near regions of poorer shock wavecontrast. Current results indicate that the technique is robust and fast during image calibration and processing,with accuracy of reconstructed shock waves in both conventional and non-conventional nozzles strongly dependenton shock wave contrast. Compared to existing techniques that can be used to reconstruct 3D shockstructures, the proposed technique has the advantage of being totally non-intrusive as compared to point orparticle-based measurements, requires significantly less computation than tomographic methods, offers highresolution reconstruction even with limited camera resolution and projected schlieren views, and is easy and costeffective to implement.
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| 4. |
- Mariani, Raffaello, Assistant Prof. 1981-, et al.
(författare)
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On the application of non-standard rainbow schlieren technique upon supersonic jets
- 2020
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Ingår i: Journal of Visualization. - : Springer Nature. - 1343-8875 .- 1875-8975. ; 23:3, s. 383-393
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Tidskriftsartikel (refereegranskat)abstract
- Abstract: A quantitative rainbow schlieren study was conducted on an over-expanded jet at nozzle pressure ratio of 2.8, based on two different schlieren set-ups: the standard z-type and a single-mirror schlieren set-up. The technique used a single, weak focal-length lens placed in the field of view of the system to provide the calibration information required for the extraction of the quantitative data. In the case of the single-mirror set-up, the calibration image required further post-processing procedures to take into account the double refraction experienced by the light. Density gradients were calculated using Abel transform and compared to validated reference data. Results indicate that the single-mirror set-up is able to improve prediction of the density gradient field as compared to the standard z-type schlieren, due to its inherent property of higher sensitivity. The study has shown that the single-mirror set-up performs on average better than the standard z-type system, yielding an overall averaged error of ± 20%, with localized values as low as ± 5% where the shock cell structure is clearly defined, with respect to the validated reference data. At the same time, both systems perform poorly in regions where the flow structure displays poor image contrast. Graphic abstract: [Figure not available: see fulltext.].
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| 5. |
- Wei, X. F., et al.
(författare)
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Experimental Investigations of Screech Mitigation and Amplification by Beveled and Double-Beveled Nozzles
- 2022
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Ingår i: Journal of Aerospace Engineering. - : American Society of Civil Engineers (ASCE). - 0893-1321 .- 1943-5525. ; 35:4
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Tidskriftsartikel (refereegranskat)abstract
- An experimental study was conducted to investigate and compare the effects of beveled and double-beveled circular nozzles on supersonic jet screech at a nozzle pressure ratio of 5. In particular, near- and far-field microphone measurements and schlieren visualizations were utilized to look into selected acoustic and flow features associated with jet screech radiation. Results show that beveled nozzles eliminate jet screech by producing asymmetric shock structures and instability waves that are mismatched in phase and amplitude. In contrast, double-beveled nozzles produce symmetric shock structures and amplify screech intensity, even when jet mixing effects have been significantly enhanced. It is further observed that amplified jet screeches produced by double-beveled nozzles are highly unsteady and undergo nonperiodic and stochastic temporal variations. Last but not least, double-beveled nozzles also significantly impact screech peak noise locations and confer different changes along different measurement planes. The present study demonstrates that not all beveled-type nozzles are able to mitigate jet screech, with nonoptimal designs amplifying it instead.
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