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Comparison of synthetic jet actuators based on sharp-edged and round-edged nozzles
Kordík, Jozef ; Trávníček, Zdeněk
Axisymmetric synthetic jet actuators based on a loudspeaker and on two types of flanged nozzles were tested and compared experimentally. The first type of the nozzle was a sharp-edged circular hole. The second one had a special design with fillets at inner and outer nozzle exit and with a small step in the middle of the nozzle. The function of the step was to prevent the flow reattachment during the extrusion stroke. The actuators with the two types of nozzles were operated at resonance and were compared first qualitatively using a simple phase locked flow visualization. Then the hot-wire anemometer was used to measure velocity \ndistributions along nozzle axis and velocity profiles at the nozzle exit. Comparison of the nozzles was based on evaluation of the characteristic velocity and integral quantities (volumetric, momentum, and kinetic energy fluxes). It was found out that these quantities, which were evaluated at the nozzle exit, differ substantially for both nozzles. On the other hand the velocity flow field in farther distances from the nozzle exit area did not exhibit such prominent differences.
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What can be done with microbubbles generated by a fluidic oscillator?
Tesař, Václav
Recently discovered possibility of generating sub-millimeter bubbles by means of pulsating the gas inlet by a fluidic oscillator has opened ways into many interesting and advantageous engineering applications. The paper, by presenting a survey of existing possibilities and their advantages, aims at providing to readers an inspiuration for their own developments.
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PIV and LIF study of flow and thermal fields of twine plumes in water
Broučková, Zuzana ; Trávníček, Zdeněk
Flow and thermal fields of a pair of plane plumes in water are investigated using PIV and LIF experiments. The plumes are generated from thermal line sources (electrically heated cylinders, diameter D = 1.21 mm). Either continuous or pulsating heating were used with the same heating input power. For a continuous heating, low frequency oscillations were identified and the natural frequency was evaluated as 0.5 Hz. Pulsating heating was used at frequency of 0.25 Hz. The maximum time-mean velocity magnitude at the continuous and pulsating heating were commensurable, approximately 0.007 m/s. Temperature fields reasonably agree with these findings.
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Active control of the wake behind the cylinder
Broučková, Zuzana ; Flídr, Erik ; Šafařík, P. ; Trávníček, Zdeněk
Active control of the flow around a cylinder using synthetic jet (SJ) array was experimentally investigated. The prevailing method was flow visualization. Modification of the wake was investigated at two values of a dimensionless momentum coefficient (Cμ = 0.21 and 1.02). Various α and Cf parameters were tested (where α is the angle between SJs and the main flow direction and Cf is the frequency ratio). The control moves separation point more downstream. Increasing frequency ratio Cf causes refining of the vortex structure. When SJs are placed at the front or rear stagnation points, wake tends towards the symmetrical pattern.
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Temperature and pressure correlation for volume of gas hydrates with crystal structures sI and sII.
Vinš, Václav ; Jäger, A. ; Hielscher, S. ; Span, R. ; Hrubý, Jan ; Breitkopf, C.
The temperature and pressure correlations for the volume of gas hydrates forming crystal structures sI and sII developed in previous study [Fluid Phase Equilib. 427 (2016) 268], focused on the modeling of pure gas hydrates relevant in CCS, were revised and modified for the modeling of mixed hydrates in this study. A universal reference state at temperature of 273.15 K and pressure of 1 Pa is used in the new correlation. Coefficients for the thermal expansion together with the reference lattice parameter were simultaneously correlated to both the temperature data and the pressure data for the lattice parameter. The new correlation is in good agreement with the experimental data over wide temperature and pressure ranges from 0 K to 293 K and from 0 to 2000 MPa, respectively. The new correlation provides significantly better agreement with the experimental data for sI hydrates compared to the original correlation. The results of both correlations are comparable in case of sII hydrates.
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Investigation of droplet nucleation in CCS relevant systems – Design and testing of a CO2 branch of the mixture preparation device.
Vinš, Václav ; Čenský, Miroslav ; Hykl, Jiří ; Hrubý, Jan
A unique in-house designed experimental apparatus for investigation of nucleation of droplets in CCS relevant systems is being developed by our team. The nucleation is measured with the help of a rapid pressure drop within an expansion chamber. In this study, a CO2 branch representing an important part of the mixture preparation device (MPD) was designed, assembled and tested. MPD is intended for preparation of CO2-rich mixtures in a sense of accurate setting of flow rates of individual gaseous components through the experimental section with the expansion chamber. In the CO2 branch, the saturated liquid CO2 is pressurized by a supercritical pump to the pressures of up to 340 bar. The required mass flow of the supercritical CO2 is accurately tuned with a help of a set of stainless steel capillary tubes. Needed lengths of the capillary tubes were determined with a help of a one-dimensional numerical model allowing prediction of both isothermal and adiabatic flow conditions.
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Intermitence factor evaluation by bypass boundary layer transition in flows on rough surface on flat plate
Skála, Vladislav ; Antoš, Pavel
Bypass boundary layer transition in flows on rough surface on flat plate was investigated experimentally. It was measuered cases with combination of roughness of surface by different free stream turbulence intenzity. Hot wire anemometry technique was used. Measuerement were made on flat plate in closed wind tunnel. Flat plate was covered by different roughness foils (sandpapers). Adverse pressure gradient was set by diffuser in tested section of wind tunnel. Grid turbulence of free stream was controlled by grid. Hot wire anemometry technique was used, intermitency factor was evaluated.
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Development of Boundary Layer on a Heated Flat Plate
Antoš, Pavel ; Kuznetsov, Sergeii
An experimental study of boundary layer on a flat plate with heat transfer was done. Temperature and velocity boundary layer development was studied on a uniformly heated flat plate with zero pressure gradient by means of hot-wire anemometry. Several levels of heat flux at the wall were used. The difference between the wall temperature and free-stream temperature was set up to 180 K. Measurements were performed in the closed-circuit wind tunnel with zero pressure gradient at free-stream velocity of 3-5 m/s and turbulence intensity of 0.8 percent. The velocity and the temperature profiles of mean values and fluctuations are presented.
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On the recirculation zone suppression behind HUMP profile using the DBD actuator
Procházka, Pavel P. ; Uruba, Václav
Previously, the DBD (dielectric barrier discharge) plasma actuator was used in rectangular channel to modify the properties of the boundary layer in spanwise and in streamwise orientation. The actuator was redesigned for using on the surface of the Glauert-Goldschmied body in different position to influence the point of the separation and the reattachment point as well as the total extent of the separation bubble. The most intensive effect occurs when the actuator takes effect in the point of separation. Further downstream, the ionic wind produced by DBD causes complex coherent structures in the wake for spanwise orientation in both direction. The effect of streamwise orientation is studied also. Actuator is operated in steady regime to produces continuous ionic wind as well as in unsteady regime when the vortex street is generated. The properties of vortex street is given by modulation parameters (frequency and duty cycle). The effect of these parameters on the wake is evaluated. This experiment is realized in the perspex channel connected to the blow-down wind tunnel. The HUMP profile is flush-mounted to the bottom side. The wire electrode of the actuator is situated in x/L = 0.63, 0.66, 0.69 and 0.72 of the chord length. The time-resolved PIV (Particle Image Velocimetry) is used as a main measurement technique. The flow field behind the profile is captured in longitudinal plane as well as in cross-section planes using 3D PIV. The results based on statistical quantities will be presented in this paper. More, next part will be devoted to the decomposition analysis of the flow dynamics (BOD, OPD).
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