scholarly journals Control of a Round Jet Intermittency and Transition to Turbulence by Means of an Annular Synthetic Jet

Actuators ◽  
2021 ◽  
Vol 10 (8) ◽  
pp. 185
Author(s):  
Zuzana Antošová ◽  
Zdeněk Trávníček
Keyword(s):  
Reynolds Number ◽  
Active Control ◽  
Flow Regimes ◽  
Velocity Profiles ◽  
Synthetic Jet ◽  
Transition To Turbulence ◽  
Jet Flows ◽  
Maximal Effect ◽  
Hot Wire ◽  
Round Jet

This paper deals with active control of a continuous jet issuing from a long pipe nozzle by means of a concentrically placed annular synthetic jet. The experiments in air cover regimes of laminar, transitional, and turbulent main jet flows (Reynolds number ranges 1082–5181). The velocity profiles (time-mean and fluctuation components) of unforced and forced jets were measured using hot-wire anemometry. Six flow regimes are distinguished, and their parameter map is proposed. The possibility of turbulence reduction by forcing in transitional jets is demonstrated, and the maximal effect is revealed at Re = 2555, where the ratio of the turbulence intensities of the forced and unforced jets is decreased up to 0.45.

An Experimental and Analytical Investigation of Rectangular Synthetic Jets

2009 ◽  
Vol 131 (12) ◽  
Author(s):  
Gopi Krishnan ◽  
Kamran Mohseni
Keyword(s):  
Flow Field ◽  
Turbulent Jets ◽  
Velocity Profiles ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Empirical Modeling ◽  
Transition To Turbulence ◽  
Two Dimensional ◽  
Limited Region ◽  
Velocity Decay

In this paper the flow field of a rectangular synthetic jet driven by a piezoelectric membrane issuing into a quiescent environment is studied. The similarities exhibited by synthetic and continuous turbulent jets lead to the hypothesis that a rectangular synthetic jet within a limited region downstream of the orifice be modeled using similarity analysis just as a continuous planar jet. Accordingly, the jet is modeled using the classic two-dimensional solution to a continuous jet, where the virtual viscosity coefficient of the continuous turbulent jet is replaced with that measured for a synthetic jet. The virtual viscosity of the synthetic jet at a particular axial location is related to the spreading rate and velocity decay rate of the jet. Hot-wire anemometry is used to characterize the flow downstream of the orifice. The flow field of rectangular synthetic jets is thought to consist of four regions as distinguished by the centerline velocity decay. The regions are the developing, the quasi-two-dimensional, the transitional, and the axisymmetric regions. It is in the quasi-two-dimensional region that the planar model applies, and where indeed the jet exhibits self-similar behavior as distinguished by the collapse of the lateral time average velocity profiles when scaled. Furthermore, within this region the spanwise velocity profiles display a saddleback profile that is attributed to the secondary flow generated at the smaller edges of the rectangular orifice. The scaled spreading and decay rates are seen to increase with stroke ratio and be independent of Reynolds number. However, the geometry of the actuator is seen to additionally affect the external characteristics of the jet. The eddy viscosities of the synthetic jets under consideration are shown to be larger than equivalent continuous turbulent jets. This enhanced eddy viscosity is attributed to the additional mixing brought about by the introduction of the periodic vortical structures in synthetic jets and their ensuing break down and transition to turbulence. Further, a semi-empirical modeling approach is proposed, the final objective of which is to obtain a functional relationship between the parameters that describe the external flow field of the synthetic jet and the input operational parameters to the system.

Transitory Behavior of Synthetic Jets

2005 ◽  
Author(s):  
Michael Amitay ◽  
Florine Cannelle
Keyword(s):  
Reynolds Number ◽  
Steady State ◽  
Input Signal ◽  
Mass Flux ◽  
Reynolds Numbers ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Hot Wire ◽  
Stroke Length ◽  
Zero Net Mass Flux

The transitory behavior of an isolated synthetic (zero net mass flux) jet was investigated experimentally using PIV and hot-wire anemometry. In the present work, the synthetic jet was produced over a broad range of length- and time-scales, where three formation frequencies, f = 300, 917, and 3100Hz, several stroke lengths (between 5 and 50 times the slit width) and Reynolds numbers (between 85 and 408) were tested. The transitory behavior, following the onset of the input signal, in planes along and across the slit was measured. It was found that the time it takes the synthetic jet to become fully developed depends on the stroke length, formation frequency and Reynolds number. In general, the transients consist of four stages associated with the merging of vortices in both cross-stream and spanwise planes that grow in size, which lead to the pinch off of the leading vortex before the jet reaches its steady-state.

Rapid Transition to Turbulence in Pipe Flows Accelerated From Rest

2003 ◽  
Vol 125 (6) ◽  
pp. 1072-1075 ◽  
Author(s):  
David Greenblatt ◽  
Edward A. Moss
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Laminar Boundary Layer ◽  
Pipe Flow ◽  
Velocity Profiles ◽  
Transition To Turbulence ◽  
Acceleration Phase ◽  
Pipe Flows ◽  
Rapid Transition ◽  
Rapid Deceleration

Rapid transition to turbulence in a pipe flow, initially at rest, was achieved by temporally accelerating the flow and then sharply decelerating it to its final Reynolds number. The acceleration phase was characterized by the growth of a laminar boundary layer close to the wall. The subsequent rapid deceleration resulted in inflectional velocity profiles near the wall, followed immediately by transition to turbulence. The time taken to transition was significantly less than the time to transition in a pipe flow monotonically accelerated to the same Reynolds number. Transition is intrinsically different to that observed in oscillatory pipe flows, but is qualitatively similar to pipe flows decelerated to rest.

Transition to turbulence in an oscillatory flow over a rough wall

2016 ◽  
Vol 792 ◽  
pp. 67-97 ◽  
Author(s):  
Marco Mazzuoli ◽  
Giovanna Vittori
Keyword(s):  
Reynolds Number ◽  
Incompressible Flow ◽  
Oscillatory Flow ◽  
Unsteady Flows ◽  
Flow Regimes ◽  
Rough Wall ◽  
Transition To Turbulence ◽  
Rigid Spheres

A study of the oscillatory incompressible flow close to a wall covered with fixed rigid spheres is carried out by numerical means to provide information on unsteady flows over a rough wall. The simulations are carried out for two bottom configurations, characterized by different values of the diameter of the spheres and different values of the Reynolds number for a total of 10 cases. Three different flow regimes are identified as functions of both the Reynolds number and the diameter of the spheres. The force exerted by the flow on the spheres is discussed also in relation to the different flow regimes.

Low Reynolds Number Flow Control Over an Airfoil Using Synthetic Jet Actuators

2010 ◽  
Author(s):  
Sebastian D. Goodfellow ◽  
Serhiy Yarusevych ◽  
Pierre Sullivan
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Flow Visualization ◽  
Low Reynolds Number ◽  
Excitation Frequency ◽  
Synthetic Jet ◽  
Hot Wire ◽  
Synthetic Jet Actuators ◽  
Jet Actuators ◽  
Low Reynolds

The influence of periodic excitation from synthetic jet actuators, SJA, on boundary layer separation and reattachment over a NACA 0025 airfoil at a low Reynolds number is studied. All experiments were performed in a low-turbulence recirculating wind tunnel at a Reynolds number of 100000 and angle of attack of α = 0°. Mounted just below the surface of the airfoil, the SJA consists of four (32.77mm diameter) piezo-electric ceramic diaphragms positioned in a single row. Flow visualization and hot wire tests were conducted with the SJA outside of the airfoil to characterize the exit flow. Results from flow visualization show a vertical jet pulse accompanied by two counter rotating vortices being produced at the exit of the simulated slot, with the vortices shed at the excitation frequency. Based on flow visualization results, the length scales of successive vortices were used to estimate the exit velocities. Hot-wire measurements determined the maximum jet velocity for a range of excitation frequencies (f = 50Hz–2.7kHz) and voltages (Vp–p = 50V–300V), which was used to characterize the excitation amplitude in terms of the momentum coefficient (cμ). With the SJA installed in the airfoil, preliminary flow visualization results show a reattachment of the boundary layer and a significant reduction in wake width.

scholarly journals Characteristics of the circular subsonic laminar jets flow with an initial Poiseuille profile

2021 ◽  
Vol 2119 (1) ◽  
pp. 012021
Author(s):  
V V Lemanov ◽  
V I Terekhov ◽  
K A Sharov ◽  
A A Shumeiko
Keyword(s):  
Experimental Data ◽  
Reynolds Number ◽  
Velocity Profile ◽  
Satisfactory Agreement ◽  
Initial Conditions ◽  
Transition To Turbulence ◽  
Hot Wire ◽  
Air Jet ◽  
Laminar Jets ◽  
Jet Axis

Abstract In this work, the experimental data are compared with the version of the “strong” jet (Re ≫ 1) of the exact Landau-Squire solution. The experiments were performed for a submerged air jet flowing out of a tube with a diameter of D = 3.2 mm and a length of more than 100D at a Reynolds number equal to Re = 436. The initial conditions in the jet are the Poiseuille velocity profile, the level of velocity pulsations is less than 1%. Measurements were carried out using a hot-wire anemometer. It is shown that satisfactory agreement with theory is achieved at distances from the tube starting from x/D = 5.6 and up to the zone of transition to turbulence (x/D > 35). Turbulence along the jet axis will increase from 1% to 2.5%, while in the mixing layers it increases to 4.7%.

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