Self-sustained Oscillatory Flow Research Articles

Numerical Investigation of Hypersonic Unsteady Flow Around a Spiked Blunt-body

The obvious unsteady flow characteristics of the flow field around the spiked blunt-body have negative effect on the drag reduction of vehicle and the thermo-protection of the head. The hypersonic self-sustained oscillatory flow was numerical simulated, and the flow structure and mechanism of the three process of one cycle: collapse, inflation and withhold, were discussed in detail. The correspondence between the drag coefficient curve and the different flow structure was obtained, which will provide basis to the drag reduction and thermo-protection of hypersonic blunt-body vehicle through flow control in the future.

Supersonic mixing enhanced by cavity-induced three-dimensional oscillatory flow

In this study, a novel wall-mounted cavity having a three-dimensional shape is proposed for enhancing supersonic mixing. This device induces an oscillatory secondary flow that effectively enhances mixing. To demonstrate the device performance, we experimentally compare supersonic mixing fields in three ducts without any devices, with a rectangular cavity, and with the newly proposed cavity (new device). In the experiments, time-dependent pressure measurements and oil-flow surface visualization are carried out. The experimental results show that the newly proposed cavity induces not only self-sustained flow oscillation but also secondary flow, both of which effectively enhance mixing. The jet issuing from the injector is also visualized for each duct by a planar laser-induced fluorescence (PLIF) technique. The PLIF visualizations reveal that mixing is enhanced far more rapidly in the duct with the newly proposed cavity than in the other ducts and that the jet penetration in the duct with the newly proposed cavity is much higher. These results are attributed to the large-amplitude jet fluctuation due to the oscillatory secondary flows induced by the newly proposed cavity.

Application of the Time-Domain Boundary Element Method to Analysis of Flow-Acoustic Interaction in a Hole-tone Feedback System with a Tailpipe

This paper is concerned with a mathematical model of a simple axi- symmetric silencer-like model, consisting of a hole-tone feedback system equipped with a tailpipe. The unstable shear layer is modeled via a discrete vortex method, based on axisymmetric vortex rings. The aeroacoustic model is based on the Powell- Howe theory of vortex sound. Boundary integrals are discretized via the boundary element method; but the tailpipe is represented by the exact (one-dimensional) so- lution. It is demonstrated though numerical examples that this numerical model can display lock-in of the self-sustained flow oscillations to the resonant acoustic oscillations.

Investigation of Pre-Stall Behavior in an Axial Compressor Rotor—Part I: Unsteadiness of Tip Clearance Flow

To investigate the pre-stall behavior of an axial flow compressor rotor, which was experimentally observed with spike-type stall inception, systematic experimental and whole-passage simulations were laid out to analyze the internal flow fields in the test rotor. In this part, emphases were put on the analyses of experimental results and the predicted results from steady simulations and unsteady simulations, which converged to equilibrium solutions with nearly periodic fluctuations of efficiency. The objective was to uncover the unsteady behavior of tip clearance flow and its associated flow mechanism at near-stall conditions. To validate the steady simulation results, the predicted total characteristics and spanwise distributions of aerodynamic parameters were first compared with the measured steady data, and a good agreement was achieved. Then, the numerically obtained unsteady flow fields during one period of efficiency fluctuations were analyzed in detail. The instantaneous flow structure near casing showed that tip secondary vortex (TSV), which appeared in the previous unsteady single-passage simulations, did exist in tip flow fields of whole-passage simulations. The cyclical motion of this vortex was the main source of the nearly periodic variation of efficiency. The simulated active period of TSV increased when the mass flow rate decreased. The simulated frequency of TSV at flow condition very close to the measured stall point equaled the frequency of the characteristic hump identified from the instantaneous casing pressure measurements. This coincidence implied that the occurrence of this hump was most probably a result of the movement of TSV. Further flow field analyses indicated that the interaction of the low-energy leakage fluid from adjacent passages with the broken-down tip leakage vortex (TLV) was the flow mechanism for the formation of TSV. Once TSV appeared in tip flow fields, its rearward movement would lead to a periodic variation in near-tip blade loading, which in turn altered the strength of TLV and TSV, accordingly, the low-energy regions associated with the breakdown of TLV and the motion of TSV, thus establishing a self-sustained unsteady flow oscillation in tip flow fields.

Numerical simulations and physical analysis of an overexpanded reactive gas flow in a planar nozzle

A numerical investigation of the overexpanded reactive gas flow in the ATAC nozzle of rectangular section, equipped with an H2 injection, is carried out. This study is aiming at reproducing the reacting phenomena occurring when air engulfed in the separated region at the tip of the extension meets the H2 rich overexpanded flow. Both steady Reynolds Averaged Navier Stokes (RANS) and unsteady Delayed Detached Eddy Simulation (DDES) approaches are used to model turbulence and compared to each other. The effect of the chemical nature of the mixture (either frozen or reacting) on the flow dynamics is discussed. The computations are compared to wall pressure measurements and flowfield visualizations by instantaneous Planar Laser Induced Fluorescence of OH (PLIF-OH) and OH spontaneous emission. A self-sustained flow oscillation at a frequency of around 1kHz is found numerically, in good agreement with the unsteady wall pressure measurements. An analysis of the space–time characteristics of the propagating disturbances contributes to a better understanding of this phenomenon.

Flow Rectification and Reversal Mass Flow in Printed Periodical Microstructures

The present paper aims at the study of flow rectification, reversal mass transport and flow characterization occurring in two-dimensional channels with designed asymmetric microstructures periodically placed in the streamwise direction using the lattice Boltzmann method. The present contribution deals with microchannels containing nozzle-diffuser-like structures (grooves), which promote net flow in a preferential direction for flows with time-dependent alternating direction. Different geometries are analyzed by varying the half-angle ø, which defmes the groove. The influence of ø on the flow rate ratio Qnet, and on flow rectification efficiency ε is presented for low-to-moderate-Reynolds-number flows. It is shown that geometries with large angles exhibit poor flow rectification during the steady state regime. In general, for all geometries studied, the flow rectification occurs in the diffuser direction for subcritical flows (stationary flows) and in the nozzle direction for supercritical flows (self-sustained oscillatory flows). Grooves with ø=11.31° and ø= 26.56° perform with the highest rectification efficiency but only at high driving body forces.

Numericl Analysis Supersonic Cavity Unsteady Flow

The computational analysis was performed of self-sustained oscillatory flow over the open cavity driven by a shear layer at flight Mach 5.0 condition with the solution of the Reynolds-averaged Navier-Stokes equations with a two-equation turbulence model. The self-sustained oscillation cycle of the open ramp cavity was got by simulation. It is found that the self-sustained oscillation feature of the cavity was complex flow. The shear layer rolls up and forms a vortex that grows in strength as the fluid enters the cavity. The amplitude of the pressure oscillation on the aft wall is much higher than that at the other wall due to the mass entrainment and ejection mechanism along the aft wall. This periodic mass addition and expulsion could be critical to the fuel and air mixing and flame-holding in the scramjet engine applications.

Numerical study of the hole-tone feedback cycle based on an axisymmetric formulation

A method for simulating the hole-tone feedback cycle (Rayleigh's bird-call), based on an axisymmetric discrete vortex method, is described. Evaluation of the sound generated by self-sustained flow oscillations is based on the Powell–Howe theory of vortex sound combined with a thin-plate boundary element theory, to account for scattering from the end plate. A model for acoustic feedback is developed. Several numerical examples are presented and discussed. These examples give an understanding of the relative strengths of contributions from monopole, dipole and quadrupole source terms. It is found that the contribution from monopole sources is largest, followed by the dipole contribution. The quadrupole contribution is quite small in comparison. The effect of acoustic feedback is carefully studied. The acoustic feedback velocity components are too small to alter the fundamental hole-tone dynamics. Still, their effect can be seen on the sound pressure frequency spectra. It is found that they do not alter the peak at the fundamental hole-tone frequency f0, but reinforce the peaks at higher harmonics 2f0, 3f0, … , and certain combination frequency peaks.

Heat transfer enhancement in self-sustained oscillatory flow in a grooved channel with oblique plates

The aim of this work is to investigate the conjugate heat transfer in periodic mounted obstacles channel with oblique plates as vortex generators installed at the rear obstacles on the opposite wall. Special attention will be paid to the analysis of flow evolution and heat transfer enhancement in the intermediate and low Reynolds number range without recourse to turbulent flow. Various physical arrangements are considered as plate length, tilt angle and Reynolds number in order to investigate their influence on the thermal and flow characteristics in the steady state as well as in the self-sustained oscillatory flow.

Pressure drop characteristics of flow in a symmetric channel with periodically expanded grooves

Pressure drop characteristics of flow in a periodically grooved channel are investigated experimentally. It is well known that a self-sustained oscillatory flow occurs from a steady-state flow at a certain critical Reynolds number in such grooved channels. The oscillatory flow enhances fluid mixing and leads to an increase in pressure drop. We measure the pressure drop with a pressure transducer. It is found that the pressure drop increases near the critical Reynolds numbers where the two- and three-dimensional oscillatory flows occur. In addition, the three-dimensional flow is confirmed by flow visualization.

Certain peculiarities of the solutocapillary convection

This paper presents experimental results on solutocapillary Marangoni convection, an effect that occurs in a thin horizontal layer of the inhomogeneous solution of a surface-tension-active agent (surfactant) either near the free upper boundary of the layer or near the surface of an air bubble injected into the fluid. A procedure using interferometry is developed for simultaneously visualizing convective flow structures and concentration fields. A number of new phenomena are observed, including the deformation and rupture of the liquid layer due to a surfactant droplet spread over its surface; bubble self-motion (migration) toward higher surfactant concentrations; self-sustained convective flow oscillations around stationary bubbles in a fluid vertically stratified in concentration; and the existence of a threshold for a solutal Marangoni flow in thin layers. A comparison of solutocapillary and thermo-capillary phenomena is made.

Self-sustained oscillations between two tandem cylinders at Reynolds number 1,000

This study focuses on the self-sustained oscillatory flow characteristics between two tandem circular cylinders of equal diameter placed in a uniform inflow. The Reynolds number (ReD), based on the cylinder diameter, was around 1,000 and all experiments were performed in a recirculating water channel. The streamwise distance between two tandem cylinders ranged within 1.5 ≤ Xc/D ≤ 7.0. Here Xc denotes the center-to-center distance between two tandem cylinders. For all experiments studied herein, quantitative velocity measurements were performed using hot-film anemometer and the LDV system. The laser sheet technique was employed for qualitative flow visualization. The wavelet transform was applied to elucidate the temporal variation and phase difference between two spectral components of the velocity signals detected in the flow field. The remarkable finding was that when two tandem circular cylinders were spaced at a distance within 4.5 ≤ Xc/D ≤ 5.5, two symmetrical unstable shear layers with a certain wavelength were observed to impinge onto the downstream cylinder. The responding frequency (fu), measured between these two cylinders, was much higher than the natural shedding frequency behind a single isolated cylinder at the same ReD. This responding frequency decreased as the distance Xc/D increased. Not until Xc/D ≥ 6.0, did it recover to the natural shedding frequency behind a single isolated cylinder. Between two tandem cylinders, the Strouhal numbers (Stc = fu Xc/Uc) maintained a nearly constant value of 3, indicating the self-sustained oscillating flow characteristics with a wavelength Xc/3. Here Uc is the convection speed of the unstable shear layers between two tandem cylinders. At ReD = 1,000, the self-sustained oscillating characteristics between two tandem circular cylinders were proven to exhibit a sustained flow pattern, not just a sporadic phenomenon.

Numerical investigation of fluid flow and heat transfer characteristics in sine, triangular, and arc-shaped channels

Time dependent Navier-Stokes and energy equations have been solved to investigate the fluid flow and heat transfer characteristics in wavy channels. Three different types of two dimensional wavy geometries (e.g. sine-shaped, triangular, and arc-shaped) are considered. All of them are of single wave and have same geometric dimensions. Periodic boundary conditions are used to attain fully developed flow. The flow in the channels has been observed to be steady up to a critical Reynolds number, which depends on the geometric configuration. Beyond the critical Reynolds number a self-sustained oscillatory flow has been observed. As a result of this oscillation, there is increased mixing between core and the near-wall fluids, thereby increasing the heat transfer rate. For the same geometric dimensions, flow becomes unsteady at relatively lower Reynolds number in the arc-shaped channel. .

Synchronization of self-sustained thermostatic oscillations in a thermal-hydraulic network

Flow and temperature oscillations occur under normal conditions in a thermal system with thermostatic control. We present experimental results of the synchronization of multiple oscillators in the secondaries of a thermal-hydraulic network. The test facility is composed of three secondary loops with heat exchangers that exchange heat with a primary heating loop on one side and a primary cooling loop on the other. A thermostatic controller senses the temperature at the outlet of a heat exchanger and modulates the flow rate in that loop. The flow valve is partially closed if the temperature goes above an upper limit and is completely opened if it falls below a lower limit. As a consequence a self-sustained flow and temperature oscillation is set up in that secondary. The frequency of the oscillation depends on the dead-band between the upper and lower temperature limits. Coupled oscillators are set up by the simultaneous action of multiple controllers on different branches. Frequency locking, phase synchronization as well as phase slips are observed to occur due to thermal-hydraulic coupling between the controllers. The phenomenon is a function of the detuning between them which is altered by changing the dead-band of the controllers.

Two-dimensional and three-dimensional self-sustained flow oscillations in interrupted fin heat exchangers

A numerical analysis of three-dimensional flow structures in a nominally two-dimensional fin geometry is presented. A sinusoidal louvred fin is considered. The heat transfer enhancement is achieved by combining boundary layer interruptions and vortical structures induced by the corrugation of the base fin. The fin shape and pitch, as well as flow conditions, are representative of typical automotive application. A wide ranging values of Reynolds number are investigated, spanning the steady laminar regime, the unsteady periodic laminar flow, and the chaotic transitional flow. Two- and three-dimensional numerical solutions are compared, looking for the onset of three-dimensional instabilities. At low values of the Reynolds number, up to the steady-unsteady flow transition, the flow is two-dimensional. As soon as unsteady oscillation appears, the simulation results show three-dimensional flow structures, even in a nominally two-dimensional geometry. The typical longitudinal vortex size is evaluated. In the periodic unsteady regime, fully three-dimensional computations yield time-averaged Nusselt number and friction factor significantly higher than those predicted by two-dimensional models. Furthermore, these flow structures induce an early transition from the periodic regime to the chaotic regime. In the chaotic regime, however, the heat transfer enhancement due to the three-dimensional flow structures is much lower.

Transition of the flow in a symmetric channel with periodically expanded grooves

Transition of the flow in a periodically grooved channel is numerically investigated for periodicity indices m = 1 up to 6 by assuming the two-dimensional and fully developed flow field, where m is defined as a number of grooves in which the flow repeats periodically. Critical Reynolds numbers for the onset of a self-sustained oscillatory flow from a steady-state flow are evaluated by numerical simulations. It is found that the bifurcations occur at the critical Reynolds numbers as a result of Hopf bifurcation, and a period in the streamwise direction of the oscillatory flow is twice as long as the groove pitch of the channel. In addition, flow visualization with the aluminum dust method is carried out to confirm the results obtained from the numerical simulations. The experimental results are in good agreement with the numerical ones.

Effect on Forced Oscillation Flow in Periodically Grooved Channel under Low Re Number Flow in the Parallel Plate

Ocean Thermal Energy Conversion (OTEC) usually uses plate type heat exchangers. For the improvements of OTEC efficiency, it is important to improve the heat exchanger efficiency. There are some methods for improving of plate type heat exchanger. As a typical method, there are the method of arranging grooves to a channel, and the method of giving the forced oscillation to a flow. By the former method, since a self-sustained oscillatory flow is generated in high Reynolds number, heat transfer enhancement is expectable. By the later method, since the resonance phenomenon appears between self and forced oscillations in the high Reynolds number flow, heat transfer is improved. However, few investigations were carried out for heat transfer enhancement on the low Reynolds number flow. Therefore, we carried out numerical calculations in grooved channel flow with forced oscillation at the low Reynolds number, and evaluated the influence of heat transfer enhancement and pressure drop. In addition, we carried out visualization experiments in the same model, and compared with the numerical solutions.

FULLY DEVELOPED FLOW STRUCTURES AND HEAT TRANSFER IN SINE-SHAPED WAVY CHANNELS

Fully developed unsteady fluid flow and heat transfer are studied numerically in a sine-shaped wavy channel by solving two-dimensional Navier-Stokes and energy equations. The flow in the channel has been observed to be steady up to a critical Reynolds number which depends on the geometric configuration. Beyond the critical Reynolds number a self-sustained oscillatory flow has been observed. The effect of variations of minimum height, amplitude and wavelength are studied. Decreasing channel height and increasing amplitude cause the flow to become more unstable and thereby increase friction factor and heat transfer, but variation of wavelength has minimal effect. The frequencies of oscillations are reported as well

波状壁流路内自立振動流による物質移動促進(O.S.2-2 内部流の実験と計算,OS2:乱流中の渦構造および流れの制御)

波状壁流路内自立振動流による物質移動促進(O.S.2-2 内部流の実験と計算,OS2:乱流中の渦構造および流れの制御)

EFFECT OF ASPECT RATIO ON CONVECTION ENHANCEMENT IN WAVY CHANNELS

Pressure drop and heat transfer characteristics are investigated in the fully developed region of three-dimensional wavy channels whose aspect ratios (width over height) range from one to infinity. Numerical simulations show that Nusselt numbers and friction factors increase with decreasing aspect ratios, in such a way that global performances improve. In all the channels considered, friction factors always increase with the Reynolds number, while Nusselt numbers significantly increase only above the critical value of the Reynolds number at which self-sustained flow oscillations begin. In turn, the critical value of the Reynolds number decreases with the aspect ratio, down to a minimum that is reached asymptotically.