Tomáš Vít

Hybrid synthetic jets as the nonzero-net-mass-flux synthetic jets

The “hybrid synthetic jet” combines the zero-net-mass-flux synthetic jet and fluidic pumping through a valveless pump. No fluid is supplied from an external source (blower or compressor). Hot-wire experiments on a demonstration model confirm that the hybrid synthetic jet exhibits a higher extrusion volume flow rate than the ordinary synthetic jet. For the tested configuration, the increase is by 25% at the 1.30 ratio of the extrusion and suction volume fluxes.

General temperature field measurement by digital holography

This paper presents a digital holographic method for measurement of periodic asymmetric temperature fields. The method is based on a modified Twyman-Green setup having double sensitivity. For measurement only one precisely synchronized and triggered digital camera is used. The periodicity and self-similarity of each cycle of the measured phenomenon combined with the precisely synchronized camera capture allow one to obtain data later used for three-dimensional (3D) measurement. The reconstruction of 3D temperature field is based on tomographic approach.

Design status of ASPIICS, an externally occulted coronagraph for PROBA-3

The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The proposed PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), with its novel design, will be the first space coronagraph to cover the range of radial distances between ~1.08 and 3 solar radii where the magnetic field plays a crucial role in the coronal dynamics, thus providing continuous observational conditions very close to those during a total solar eclipse. PROBA-3 is first a mission devoted to the in-orbit demonstration of precise formation flying techniques and technologies for future European missions, which will fly ASPIICS as primary payload. The instrument is distributed over two satellites flying in formation (approx. 150m apart) to form a giant coronagraph capable of producing a nearly perfect eclipse allowing observing the sun corona closer to the rim than ever before. The coronagraph instrument is developed by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent improvements and design updates of the ASPIICS instrument as it is stepping into the detailed design phase.

Heat and Mass Transfer Caused by a Laminar Channel Flow Equipped With a Synthetic Jet Array

Low Reynolds number laminar channel flow is used in various heat/mass transfer applications such as cooling and mixing. A low Reynolds number implies a low intensity of heat/mass transfer processes since they rely only on the gradient diffusion. To enhance these processes, an active flow control by means of synthetic (zero-net-mass-flux) jets is proposed. The present study is experimental, in which a Reynolds number range of 200―500 is investigated. Measurement has been performed mainly in air as the working fluid by means of hot-wire anemometry and the naphthalene sublimation technique. Particle image velocimetry (PIV) experiments in water are also discussed. The experiments have been performed in macroscale at the channel cross sections (20 × 100) mm and (40 × 200) mm in air and water, respectively. The results show that the low Reynolds number channel flow can be influenced by an array of synthetic jets. The effect of synthetic jets on the heat transfer enhancement is quantified. The stagnation Nusselt number is enhanced by 10―30 times in comparison with the nonactuated channel flow The results indicate that the present arrangement can be a useful tool for heat transfer enhancement in various applications, e.g., cooling and mixing.

Identification of the Temperature Field in Pulsatile Impinging Flow

The presented paper shows the possibility of using holographic interferometry and hot‐wire anemometry in the research of heat transfer from impingement pulsatile flow. The intensity of heat transfer in the case of impingement flow is often measured with glue‐on heat flux sensors, or by indirect methods such as naphthalene sublimation. All these methods have a response time too long for measuring instant values of the heat transfer coefficient on a surface cooled/heated by impingement pulsatile flow. This shortcoming should be overcome by using CTA glue‐on probes or, preferably, by using optical methods such as holographic interferometry. It is necessary to employ a special holographic setup with double sensitivity instead of the commonly used Mach‐Zehnder type of holographic interferometer in order to attain the parameters sufficient for the studied case. This setup is not light efficient like the Mach‐Zehnder type but has double sensitivity. The results from the holographic interferometry experiments wil...

Synthetic and continuous jets impinging on a circular cylinder

ABSTRACT Synthetic and continuous water jets impinging onto an electrically heated circular cylinder were experimentally investigated. The slot nozzle width was 0.36 mm, the cylinder diameter was 1.2 mm, and the cylinder-to-nozzle spacing related to the slot width was 5–21. Two optical methods were used: qualitative laser-induced fluorescence (LIF) visualization and laser Doppler vibrometry (LDV) measurements. Simultaneously with the optical experiments, the overall convective heat transfer from the circular cylinder was evaluated. The LDV quantified the velocity of the oscillating piezo-driven diaphragm at frequencies from 30 to 68 Hz. A majority of the study was performed at the near-resonant frequencies from 46 to 49 Hz. For all investigated jets, the Reynolds numbers based on the nozzle width ranged from 36 to 171. The LIF visualization revealed a dominant flow separation occurring on the windward cylinder side. This result is attributed to the effect of the miniscales, a relatively small ratio of the nozzle width to the cylinder diameter, and low Reynolds numbers. An increase in the Reynolds number changes the flow pattern from a steady jet-flow separation to a vortex shedding wake-flow regime. The heat transfer experiments were validated in a natural convection regime. An enhancement of the average Nusselt numbers by 4.2–6.2 times by means of the synthetic jets was quantified by comparison with the natural convection regime. A correlation for the average Nusselt number was proposed for both the continuous and synthetic jets.

Dynamic interferometric measurement with extended unambiguity range in flow measurement

This paper reports on a new approach to measure dynamic processes in fluid mechanics using interferometry with extended dynamic range. A key factor is the use of two wavelengths and the recording of interferograms from both wavelengths in one frame. Phase map evaluation is based on the Fourier transform. The difference between the obtained phase fields creates a synthetic phase whose dynamic range also covers large changes of the measured quantity.

The mounting system of lenses in ASPIICS coronagraph

This article describes the mounting system of lenses in a coronagraph ASPIICS (Association of Spacecraft for Polarimetric Imaging Investigation of the Corona of the Sun). ASPIICS is developed and produced in cooperation of twenty partners from seven countries. It is a part of the ESAs PROBA-3 mission, which includes a formation flight of a pair of satellites at orbit. Coronagraph itself consists of three objectives, where the last one is composed by one objective tube for each lens plus holder and Lyot stop. To achieve high accuracy of mounting of the individual lenses, it was necessary to achieve tight geometric and dimensional tolerances for manufacturing of the objectives barrels. In order to minimize the stress and to prevent the displacement of the lens from ideal position during a temperature change on orbit, an athermal solution was proposed. This is achieved by inserting a Teflon ring of a suitable thickness between the lens surface, objective barrel and the spring washer with a precisely defined contact force. It was necessary to find a suitable technological process of manufacturing, because of the specific behavior of PTFE during turning and complex design of other parts. All parameters of mounting system were repeatedly verified by a thermomechanical analysis in FEM software, based on tests of real parts.

Multi-wavelength digital holography for shape measurement of grinded surfaces with ultimate accuracy

The entry of CNC machining processes into optics brought the possibility of nearly arbitrary shape generation. Obviously the measurement of the generated shape increasingly gains the importance, because the generation has to be performed in an iterative manner as the required precision increases. Often mid spatial frequency error is neglected because it is not an easy task to be measured. Unfortunately those unwanted residual deviation in a shape left after grinding could dramatically complicate a subsequent polishing procedure. Mid spatial frequency content if not controlled well could spoils significantly the performance of the optical system. Elimination of mid spatial residuals originated in grinding process is nearly impossible or very difficult by sub-aperture polishing. Hence it is important to measure the grinded surface with sufficient lateral resolution. Tactile probes (usually used for shape measurement of grinded surfaces) can measure with sufficient lateral resolution but only at the expense of time. Interferometer based techniques fail when applied to scattering surfaces due to speckles. The authors have proposed multi-wavelength multi-directional digital holography – the method perfectly suitable for grinded surfaces shape measurement. Naturally, reconstructed phase maps are affected by speckle noise implying significant errors in the calculation of the shape of the surface. In order to reduce the effect of speckle noise and hence to increase the sensitivity of the measurement of the grinded surface shape, we propose to apply windowed digital holography. This paper describes the principle of the windowed digital holography and the way of straightforward application of the method in shape measurement of grinded surfaces.

Heat and mass transfer measurement using method of digital holographic tomography

A digital holographic interferometry (DHI) for 3D measurement of temperature distributions in moving fluid is presented in this paper. The measurement uses digital holographic setup for measurement of a flow of fluid propagated through an orifice and tomographic approach for 3D reconstruction of the flow. The periodic character of the flow and synchronization between the digital camera and external trigger driving the phenomenon allows us to measure phenomena with much higher frequency when compared to frame rate of the digital camera. Furthermore one can capture a large number of the flow projections from different viewing directions which are later used for 3D tomographic reconstruction of the whole temperature field of the flow. The measurement results are verified and compared with hot wire method (CTA) in the paper.