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Mechanically modulated nozzles
Tesař, Václav
There are applications in which jet-generating nozzles are requested to have variable properties, responding to a mechanical input action. The paper studies nozzles with a needle - a component with tapered end inserted into the nozzle exit channel. Axial movement of the needle varies the cross-section area available for the flow. Author made laboratory and computational investigations of the simplest shape: a sharp-point cone inserted into a sharpedged, constant-diameter exit channel. Presented examples from accumulated data discuss pressure measurements: overall pressure loss, pressure distribution on the cone surface, and on surfaces of the exit channel.
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Proposal for an electro/fluidic transducer based on wall-jet separation from a heated curved surface
Tesař, Václav
Transducers for converting electric input signals into a change of flow rate in a fluidic device are increasing in importance – and yet there is no fully satisfactory design solution. Standard two-stage approach is to perform in the first stage a conversion into a motion of a movable (or deformable) mechanical component. Then in the second stage the mechanical effect acts on the fluid flow. Mechanical components complicate manufacturing, have tendency to get stuck or worn – or, in the case of elastic support, to endanger reliability by breakages (e.g. due to material fatigue). Their inertia limits the frequency range. Author proposes an apparently not yet applied idea of transducer containing no moving or deformed parts. The electric input heats a surface to which an air jet is attached, causing its separation. Preliminary experiments show that relatively small heating suffices to change substantially the flowfield — and the reduction of the attachment wall to a thin metal foil can make the frequency range of responses quite acceptable.
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Induced fluid rotation and bistable fluidic turn-down valves (a survey)
Tesař, Václav
Paper discusses engineering applications of mutual interaction between fluid flow rotation in adjacent cavitiesthat communicate via a window through which is transferred moment momentum. The transfer may be merely byshear stress on the interface or enhanced by some flow crossing the boundary which is in the window. The first case of zero-time-mean mass transport through the window has led to methods of measuring the wall shear stress – a quantity of high importance in theoretical fluid mechanics – by measuring the speed of induced rotation in an adjacent vortex chamber. The case with momentum transfer by a non-zero flow became the starting point of development of fluidic valves combining two properties otherwise mutually incompatible: bistability on one side and turning-down the flow on the other side.
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Colliding wall-jets on a cylindrical surface
Tesař, Václav ; Peszyński, K.
Paper discusses aerodynamics and potential engineering applications of an unusual and in literature practically unknown fluid flow configuration, with two wall-jets attached to a cylindrical surface colliding head-on and generating a single jet directed away. Applications are seen in pneumatic sensors, particularly operating at low Reynolds numbers. Performed experimental investigation combined with numerical flowfield computations revealed several interesting aspects of this flowfield. The most interesting among them is the discovery of symmetry-breaking existence of three different stable flow regimes. This opens a possibility for fluidic tristable amplifiers and control systems operating with ternary logic.
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Fluidic oscillator with bistable turn-down amplifier
Tesař, Václav ; Smyk, E. ; Peszyński, K.
Search for new principles of no-moving-part fluidic oscillators suitable for operation at low frequency – and yet compact – has led to a design based on the amplifier configuration combining bistability (capability to remain in two alternative stable regimes) with capability to turn down the fluid flow passing through it. The feedback was arranged in a manner of recently introduced oscillators with resonator channel. Tests revealed the expected non-constant Strouhal number - but, rather strange, no dependence was found of oscillation frequency on the channel length.
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Control of flow separation by vestigial "synthetic jet"
Tesař, Václav ; Pavelka, Miroslav ; Smyk, E. ; Peszyński, K.
A useful aerodynamic control action may be causing a separation of flow from the wall past which the fluid flows. Several current research activities aim at doing so by oscillating zero-time-mean flow generated in a nozzle located in the wall. The effect is often described as “flow control by synthetic jet”, although it is obvious that the vortex rings generated in the outflow phase of the oscillation are deformed by the controlled flow and also are carried too far away so that they cannot form the classical synthetic jet configuration of vortex rings. Authors describe experiments with such a control applied successfully in an axisymmetric fluidic valve. Presented is a hypothesis explaining the separation mechanism.
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Identification of coherence in turbulent flows by local correlations
Tesař, Václav ; Něnička, Václav
Coherent structures are not included in practically useful models of turbulence due to scarcity of data upon which the modelling could be based. The problem is indistinct, vague, and continuously varying boundary of each structure, making difficult even identification of its very existence. The introduced identification method operates with a pair of flow visualisation images, computing local correlation coefficients in sequentially interrogated small image parts. The coherence is revealed by high local values of the coefficient. There are two variants, a scalar and vector approaches. The method is shown applied to an experiment in which the structures play an important role.
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Atomiser with excitation by a fluidic oscillator
Tesař, Václav ; Hykl, Jiří
Paper summarises results obtained in preliminary feasibility study of a novel pneumatic atomiser for generation o fine droplet spray. The new feature is pulsating the air supply by means of a no-moving-part fluidic oscillator. Tests involved recording by high-speed camera and optical measurements of the droplet size by laser light scattering and suggested a promising future potential necessitating, however, further development and optimisation.
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Preventing microbubble conjunctions by gas flow excited by a fluidic oscillator
Tesař, Václav
Recently demonstrated was efficient generation of desirable small-size microbubbles in liquids by applying oscillation to the air flow supplied into the aerator - preferably by means of a simple and inexpensive fluidic oscillator. It remained unexplained what the oscillation actually does with the bubbles. In this paper was established that microbubbles are produced in a process consisting of several discrete steps, in each growing in size. Oscillation of the gas flow can eliminate this growth and keep the resultant bubbles small. Two alternative mechanisms were identified than can suppress the growth.
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Mechanism of Multiple Microbubble Conjunctions
Tesař, Václav ; Šonský, Jiří
In typical generation of microbubbles by blowing the gas through tiny holes in an aerator, the size of bubbles is much larger than hole exit diameter. Explanation for this discrepancy was found by analysing high-speed camera images of bubble generation. Primary microbubbles, of initial size corresponding to the aerator exits, tend to remain near the location of their formation at the aerator exit - and coalesce there with subsequently formed other primary microbubbles. Typically, this coalescence is repeated several times before the increased final size and position of the resultant larger bubble reaches the limit for separation and ascent.
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