Effect Of Jet Flow Research Articles

Numerical Simulation of Active Control on Tip Leakage Flow in Axial Turbine

In an effort to reduce the blade tip clearance leakage in turbine designs, this article aims to numerically investigate the effects of active jet-flow injected from the blade tip platform upon the blade tip clearance flow. A CFD code integrated with dense-correction-based 3D Reynolds-averaged Navier-Stokes equations together with the well-proven Reynolds stress model (RSM) is adopted. The variation of specific heat is taken into consideration. The effects of jet-flow on the tip clearance flow are simulated at two tip clearance heights, each with five differently located injection holes and three jet-flow mass rates. In this way, the article elucidates the mechanism of active injection control, analyzes the topological tip clearance flow structure, and, finally, calculates the turbine efficiency considering the effects of the jet-flow. The results show that the mass rate of the third jet-flow turn to be the optimum in maximizing the turbine efficiency. The jet-flows through combined holes could increase the efficiency by 0.35% with loose tip clearance, and by 0.3% with tight tip clearance. The number of saddle points is equal to that of nodes near the injection hole, which satisfies the singularity point distribution law.

“Fluid plug” microfluidic valve for low Reynolds number fluid flow selector units

A novel no-moving-part valve was developed mainly for applications in microchemistry, where Reynolds numbers tend to be extremely low. The valve of very small size and working with viscous biological fluids was designed for operating at Re values between 10 and 100. In this range, the inertial jet flow effects used in standard fluidics could not be relied upon and a qualitatively new operating principle was introduced: the control fluid forms a "plug" at the entrance of the collector, precluding an entry of the main fluid flow. In particular, an array of new valves was used in a selector unit to allow a single reagent flow into a reactor from several available sources. The flow visualization validated the assumptions on which the design is based and compared well with numerical flowfield computations.

Comprehensive Study on Tip Vortex with Lateral Jet Flow

Comprehensive numerical and experimental investigations of tip vortical characteristics were conducted for lateral tip jet flow over a fixed wing as a step to reduce blade vortex interaction noise. The tip vortex of a NACA0012 blade was measured and visualized for the fundamental study of tip vortical flow, and the results were compared with numerical data as a validation of numerical solvers. Three-dimensional compressible Euler/Navier-Stokes codes were used to calculate the effect of jet flow from the tip of an OLS (modified BHT 540) fixed blade at various freestream velocities and jet conditions. The results show that the jet flowing from the wing tip can diffuse the tip vortex enlarging the core size of tip vortex and weakening its strength. When applied to the blade vortex interaction phenomena, this enlarged and weak vortex can produce a lower pressure gradient on the blade surface, which means that the jet flow can effectively reduce blade vortex interaction noise.

Effects of jet flow on jet noise via an extension to the Lighthill model

The Lighthill formalism for jet noise prediction is extended to accommodate wave transport by the mean jet flow. The extended theory combines the simplicity of the Lighthill approach with the generality of the more complex Lilley approach. There is full allowance for ‘flow-acoustic’ effects: shielding, as well as the refractive ‘cone of (relative) silence’. A source term expansion yielda a convected wave equation that retains the basic Lighthill source term. This leads to a general formula for power spectral density emitted from unit volume as the Lighthill-based value multiplied by a squared ‘normalized’ Green's function. The Green's function, referred to a stationary point source, delineates the refraction dominated ‘cone of silence’. The convective motion of the sources, with its powerful amplifying effect, also directional, is accounted for in the Lighthill factor. Source convection and wave convection are thereby decoupled, in contrast with the Lilley approach: this makes the physics more transparent. Moreover, the normalized Green's function appears to be near unity outside the ‘cone of silence’. This greatly reduces the labour of calculation: the relatively simple Lighthill-based prediction may be used beyond the cone, with extension inside via the Green's function. The function is obtained either experimentally (injected ‘point’ source) or numerically (computational aeroacoustics). Approximation by unity seems adequate except near the cone and except when there are coaxial or shrouding jets: in that case the difference from unity will quantify the shielding effect. Further extension yields dipole and monopole source terms (cf. Morfey, Mani, and others) when the mean flow possesses density gradients (e.g. hot jets).

Jet propulsion for quiet aircraft

A novel method of jet noise reduction has been subjected to bench and wind tunnel tests. This patented method uses very small nozzles arranged on struts protuding from the upper surface of an aircraft wing. Bench tests included various arrangements of nozzles on each strut; tests with heated air; tests of noise power and directivity versus number of struts, strut spacing, and location of struts on a simulated wing; and tests of discharge coefficients and velocity coefficients. Wind tunnel tests included induced aerodynamic effects of upper-surface blowing, effects of tunnel flow on discharge and velocity coefficients of the nozzles, acoustic effects of tunnel flow on the jet noise, and effects of jet flow on aerodynamic noise. A full-scale test of aerodynamic noise was made using dummy struts on a sailplane. Preliminary design of a quiet research test vehicle was prepared and its aural detectability was predicted (on the basis of the test data) to be significantly superior to other approaches. Predicted noise levels for 1500-ft flyover were shown to be less than 25 dB (1/3-octave-band SPL) and were dominated by aerodynamic noise of the aircraft wing. [This work was supported by USAFFDL and USAFAPL.]