Either single elements of the actuator can be addressed or models for the expected optical distortion can be assumed. Iterative methods are expected to be time-consuming due to the large parameter space. Usually an iterative approach is chosen, which does not require an additional sensor beside the imaging camera. However, image based wide-field correction requires a CCD-camera and a more complex optimization strategy. There are also sensor-less concepts in terms of FFT-based aberration correction of low spatial frequencies and by using the second moment of the image Fourier transform. the intensity is maximized and the spot shape is improved (e.g. Wavefront sensor-less adaptive optics is commonly used with a pinhole and a single photodetector allowing single light spot correction i.e. a transmission measurement is not performable, then a sensor based closed-loop setup cannot be applied, except a Fresnel guide star approach is applicable. If the optical access for a wavefront sensor is not available, i.e. for removing turbulence effects or distortion correction based on affine transformation algorithms. Beside active in-situ correction of disturbed wavefronts, also post-processing approaches can be applied to improve the image quality, e.g. Beside interferometric methods and holography-based modal wavefront sensing, the Hartmann-Shack wavefront sensor is a widely spread tool. Several methods exist to determine the wavefront of a light wave. The drawback of intrinsic aberration effects in a measurement system or from a measurement object can be compensated by applying spatial light modulators. Aberrations can also influence the point spread function (PSF) for high-resolution confocal microscopy. In microscopy for biomedical applications, aberrations mainly result from different cell layers. Another field of application is ophthalmology in order to get sharp retina-images within the human eye or effectively perform vision correction. Adaptive optics is used in astronomy for compensating atmospheric turbulence. Our approach offers a new way to reduce static or slowly changing wavefront distortions in a fluid flow measurement setup in which a wavefront sensor is not applicable.Īdaptive optics has developed to an established method in many research fields of applied optics. The proposed method allows for the reduction of systematic measurement uncertainties in particle image velocimetry. In this work we outline a sharpness metric based aberration correction with a deformable mirror, applied for the first time to particle image velocimetry. The sharpness metric is used as an indicator for wavefront aberrations in order to correct low-order Zernike modes that influence the image quality of particle image velocimetry. MethodsĪ combination of sharpness metric image evaluation and iterative optimization is demonstrated. The usage of a wavefront sensor can be hindered by disturbing light reflexes or scattering. Such distortions can occur at fluctuating phase boundaries during flow measurement and result from the accompanied refractive index changes. The former is attributable to boundary layer instabilities, while the latter possibly originates in part from spontaneous GW emission from the baroclinic wave.Optical distortions can significantly deteriorate the measurement accuracy in particle image velocimetry systems. Results indicate GW activity in both annulus configurations close to the inner cylinder wall and within the baroclinic wave. Possible regions of GW activity are identified by the horizontal velocity divergence and a modal decomposition of the small-scale structures of the flow. In both experiments, the model is initialised with a baroclinically unstable axisymmetric state established using a two-dimensional version of the code, and a low-azimuthal-mode baroclinic wave featuring a meandering jet is allowed to develop. The latter configuration is more atmosphere-like, in particular since the Brunt Vaisfila frequency is larger than the inertial frequency, resulting in more realistic GW dispersion properties. Experiments were performed using a classic laboratory configuration as well as using a much wider and shallower annulus with a much larger temperature difference between the inner and outer cylinder walls. ABSTRACT A finite-volume model of the classic differentially heated rotating annulus experiment is used to study the spontaneous emission of gravity waves (GWs) from jet stream imbalances, which may be an important source of these waves in the atmosphere and for which no satisfactory parameterisation exists.
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