Guang Hong

A piezoelectrically actuated micro synthetic jet for active flow control

The synthetic jet actuator (SJA) is a low power, highly compact microfluidic device which has potential application in boundary layer flow control. In recent work we have shown how synthetic jets work without cross flow and how effectively they modify the flow structure in the boundary layer under an adverse pressure gradient. This paper describes the piezoelectric synthetic jet actuator used in our experiments. The experimental set-up for flow control using this type of actuator is detailed. The results obtained show a significant enhancement of the jet effectiveness by forcing the boundary layer flow at the natural instability frequency. The actuators must have sufficient velocity output to produce strong enough vortices if they are to be effective for flow control. The forcing effect can occur at a frequency lower than the driving frequency of the actuator when used without cross flow. The forcing frequency appears to be an important parameter in synthetic jet boundary layer flow control.

An experimental and numerical study of synthetic jet flow

The flow generated by a synthetic jet actuator with a circular orifice is investigated experimentally and computationally. The synthetic jet establishes itself much more rapidly than the steady jet, primarily because of turbulent dissipation. The oscillatory nature of synthetic jet flow also gives rise to a much greater entrainment of ambient fluid compared with the case of a steady jet. Finally, self-similarity seems to be established when the oscillations introduced by the actuator are reduced to negligible levels

Investigation of the Calmed Region Behind a Turbulent Spot

Measurements are presented of the calmed region behind triggered wave packets and turbulent spots under a controlled diffusion adverse pressure gradient in a wind tunnel. Similar measurements are also presented from the stator blades of an axial flow compressor, where turbulent spots are induced by the passing of rotor wakes. The purpose is to gain an appreciation of turbulent spot behavior under a strong adverse pressure gradient as a foundation for the more accurate modeling of spots and their environment in predictions of transitional boundary layer flows. Under an adverse pressure gradient the calmed region behind the spot is extensive; its interaction with the surrounding boundary layer is complex and is dependent on whether the surrounding natural boundary layer is laminar or turbulent. Some insights are gleaned concerning the behavior of the calmed region, which will subsequently be used in attempts to model the calmed region. Although these fundamental investigations of the calmed region have been extensive, much remains to be understood.

Investigation to Charge Cooling Effect of Evaporation of Ethanol Fuel Directly Injected in a Gasoline Port Injection Engine

ABSTRACT Ethanol direct injection plus gasoline port injection (EDI+GPI) is a new technology to make the use of ethanol fuel more effective and efficient in spark ignition engines. It takes the advantages of ethanol fuel, such as its greater latent heat of vaporization than that of gasoline fuel, to enhance the charge cooling effect and consequently to increase the compression ratio and improve the engine thermal efficiency. Experimental investigation has shown improvement in the performance of a single cylinder spark ignition engine equipped with EDI+GPI. It was inferred that the charge cooling enhanced by EDI played an important role. To investigate it, a CFD model has been developed for the experimentally tested engine. The Eulerian-Lagrangian approach and Discrete Droplet Model were used to model the evolution of the fuel sprays. The model was verified by comparing the numerical and experimental results of cylinder pressure during the intake and compression strokes. Mesh density and time step sensitivities have been tested. The verified model was used to investigate the charge cooling effect of EDI in terms of spatial and temporal distributions of cylinder temperature and fuel vapor fraction. Compared with GPI only, EDI+GPI demonstrated stronger effect on charge cooling by decreased in-cylinder temperature. The cooling effect was limited by the low evaporation rate of the ethanol fuel due to its lower saturation vapor pressure than gasolines in low temperature conditions.

Effectiveness of Synthetic Jets Enhanced by Instability of Tollmien-Schlichting Waves

The control effectiveness of synthetic jets on flow separation in an adverse pressure gradient boundary layer was investigated experimentally in a low speed wind tunnel. Of particular interest was the enhancement of the control effect by the TollmienSchlichting (T-S) instability of the boundary layer flow to be controlled, at lower forcing frequencies. In our experiments, the forcing amplitude and forcing frequency of the synthetic jet actuator were varied over a set range. This paper reports the results of the synthetic jets generated with a forcing frequency of 100 Hz, one of the most effective frequencies, and low forcing voltage of ±7.5V. The displacement thickness Reynolds number in the measurement region was between 800 and 1300. The mean velocity and turbulence profiles with the synthetic jet on and off are used to demonstrate that flow separation is effectively resisted by synthetic jets driven by low power at T-S frequency. The spectra of velocity are used to analyze the interaction between the synthetic jets and the T-S waves in the boundary layer flow.

Time Series Estimation of Convective Heat Transfer Coefficients

Abstract In this work a novel methodology for the estimation of convective heat transfer coefficients is presented. The convective heat transfer coefficients are estimated using novel and existing time series methods, and are compared with those obtained from the standard ‘ratio of means’ approach, and those given in the CIBSE Guide. The time series methods are used to estimate an impulse response function of the heat flux at the surface to the temperature difference through the air boundary layer to the bulk air within the enclosure. The impulse response function displays the time-delayed response of the surface heat flux to the air-surface temperature difference, and thus indicates the speed of the process. The area under the estimated response function yields the ‘gain’ between the surface heat flux and the air-surface temperature difference, or the convective heat transfer coefficient. This methodology is employed to estimate the convective heat transfer coefficient of the ceiling within an experimental building, in a convective heating environment. The variation of the convective heat transfer coefficient across the boundary layer is shown, from which the thickness of the boundary layer is estimated. A methodology for the estimation of the hourly variation of the convective heat transfer coefficient is also presented, and is employed to investigate the time varying nature of the convective heat transfer coefficient.

Diesel Smoke Transient Control Using a Real-Time Smoke Sensor

A novel real time smoke sensor is described, which is mounted in the exhaust manifold and detects the smoke by virtue of the natural electrical charge which is carried on the smoke. The somewhat obscure origin of the charge on the smoke is briefly considered, as well as the operation of the sensor itself. The use of the sensor as part of a feedback control shows that it can be very effective in reducing smoke puffs. Copyright

Numerical Investigation to Forcing Frequency and Amplitude of Synthetic Jet Actuators

elimination of the laminar separation bubble in a boundary layer with an adverse pressure gradient. The numerical results of the synthetic jet actuator’s effectiveness were compared to identify the significance in changes made by varied forcing frequency at fixed forcing amplitude and by varied forcing amplitude at a fixed forcing frequency. Consistent with the wind-tunnel experiments, large eddy simulation results showed that the effectiveness of the synthetic jet actuator depends more on the forcing frequency than on the forcing amplitude. They support the inference from the experiments that instability due to adverse pressure gradient to seed the nonfrictional Kelvin– Helmholtz instability in a short laminar separation bubble be a source for enhancing the frictional Tollmien– Schlichting instability triggered by a synthetic jet actuator to resist the laminar separation. To make a micro synthetic jet actuator effective, all the instabilities in the base flow and generated by the actuators should be used. Understanding of the optimal uses of these instabilities is challenging.

Modeling of Boundary Layer Transition in Turbulent Flows by Linear Combination Integral Method

Transitional boundary layer parameters in zero and variable pressure gradient flows, typical of turbomachinery applications, are predicted using an integral method of the linear combination type. The code used is that of Dey and Narasimha and the turbulent layer is calculated by a lag-entrainment method. The predictions of test data represent an improvement upon earlier methods; although reasonable agreement is obtained for these low Reynolds number test cases further refinement of predictive correlations to account for free-stream turbulence effects on laminar boundary layers and transition inception is indicated.The transitional parameters are found to be particularly sensitive to the initial conditions selected for the turbulent layer. Techniques are identified for overcoming this sensitivity and for adequately representing the transition region. Free-stream turbulence effects are quite strong, particularly on the velocity profile of the laminar layer. Modifications to laminar methods are advocated to account for the strong effects on the velocity profile and the early formation of turbulent spots.Copyright

Synthetic jet actuators for flow control

The synthetic jet actuator is a novel means of applying flow control that lends itself to incorporation into micro- electro-mechanical systems. Experimental and computational studies of the flow generated by this actuator are presented and discussed. The synthetic jet is seen to resemble a steady turbulent jet. Self-similarity is, however, achieved closer to the orifice in the case of the synthetic jet, and this is consistent with the observation from the numerical simulations that a main vortex is trapped close to the orifice. This main vortex merges with the secondary vortex that is generated at the circumference of the orifice during each cycle of oscillation of the actuator membrane. The effect of synthetic jets on wake flows and flames has also been examined. Preliminary results of these investigations suggest that synthetic jets can significantly reduce drag and enhance mixing.