The effects of periodic excitation on the evolution of a turbulent jet were studied experimentally. A short, wide-angle diffuser was attached to the jet exit, and excitation was introduced at the junction between the jet exit and the diffuser inlet. The introduction of high-amplitude periodic excitation at the jet exit enhances the mixing and promotes attachment of the jet shear layer to the diffuser wall. Vectoring is achieved by applying the excitation over a fraction of the circumference of the circular jet, enhancing its spreading rate on the excited side and its tendency to reattach to that side. Static-deflection studies demonstrate that the presence of the wide-angle diffuser increases the effectiveness of the added periodic momentum caused by a favorable interaction between the excitation, the jet shear layer, and the diffuser wall

/pdf/periodic-excitation-for-jet-vectoring-and-enhanced-spreading-3ihnlz8bvp.pdf

Periodic Excitation for Jet Vectoring and Enhanced Spreading

The effects of periodic excitation on the evolution of a turbulent jet were studied experimentally. A short, wide-angle diffuser was attached to the jet exit and excitation was introduced at the junction between the jet exit and the diffuser inlet. The introduction of high amplitude periodic excitation at the jet exit enhances the mixing and promotes attachment of the jet shear-layer to the diffuser wall. Vecotring is achieved by applying the excitation over a fraction of the circumference of the circular jet, enhancing its spreading rate on the excited side and its tendency to reattach to that side. Static deflection studies demonstrate that the presence of the wide-angle diffuser increases the effectiveness of the added periodic momentum due to a favorable interaction between the excitation, the jet shear-layer and the diffuser wall. This point was further demonstrated by the evolution of a wave packet that was excited in the jet shear-layer. Strong amplification of the wave packet was measured with a diffuser attached to the jet exit. The turbulent jet responds quickly (10-20 msec) to step changes in the level of the excitation input. The response scales with the jet exit velocity and is independent of the Reynolds number. Jet deflection angles were found to be highly sensitive to the relative direction between the excitation and the jet flow and less sensitive to the excitation frequency. The higher jet deflection angles were obtained for a diffuser length of about two diameters and for diffusers with half-angles greater than 15 degrees.

/pdf/periodic-excitation-for-jet-vectoring-and-enhanced-spreading-14yazjyx4x.pdf

We present the development of miniature multihole pressure probes and a novel neural-network-based calibration algorithm for them. Seven-hole probes of tip diameters as low as 0.035 in. (0.9 mm) were successfully fabricated with high tip surface quality. Any of the typical probe tip geometries, i.e., hemispherical, conical, or faceted, could be fabricated. The miniature probes were calibrated and tested in a wind tunnel. A backpropagation-based neural-network calibration algorithm was developed for these probes, with flexibility in network architecture design and network self-optimization capabilities. In the feedforward mode the algorithm yields computational speeds an order of magnitude higher than those typically achieved by similar accuracy interpolation algorithms. The new algorithm has prediction accuracies of 0.28 deg in the flow angles and 0.35% in the velocity magnitude

Miniature Multihole Pressure Probes and Their Neural-Network-Based Calibration

Multihole pressure probes are frequently used to gather e ow data, sometimes in highly three-dimensional e ow. If the e ow is reversed, no method currently exists for determining whether the multihole probe is facing the right direction for data acquisition, and that the e ow is as such. A seven-hole probe is rotated in yaw by 360 deg, and rules are formulated to determine if a seven-hole probe is in a reversed e ow. The current study allows seven-hole probes to be used for gathering data in steady reversed e ows and gives an indication whether a pressure probe is insufe cient for the measurement instrument, as in the case of highly recirculating unsteady turbulent e ows. An application of the technique thus developed is also given.

Examining the Effect of Flow Reversal on Seven-Hole Probe Measurements

Moving ground systems with rotating wheels have been used in wind tunnel tests during the last decades. Several studies on the effects of rotating wheels and the importance of wheel aerodynamics have been published. It is well known that both the local flow field and the global aerodynamic forces are affected by rotation of the wheels. Different studies indicate that the most significant effect from rotating the wheels is interference effects between the rear wheels and the underbody and vehicle base [1], [2]. A detailed flow field investigation around the wheels in close proximity to the vehicle has been performed on a passenger car in the Volvo Aerodynamic Wind Tunnel. Two omnidirectional 12-hole pressure probes were traversed in a number of planes close to the wheels. Effects of changing different parameters such as ground simulation and rim geometry were investigated. The local flow field has been scrutinised and related to the global aerodynamic properties of the vehicle. A clear dependency on ground simulation as well as a dependency on wheel geometry was found in the local flow field. All configurations showed lower global drag with moving ground activated. Generally a good correlation with numerical results was observed.

Detailed Flow Studies in Close Proximity of Rotating Wheels on a Passenger Car

The development of a nearly omnidirectional pressure probe for three-velocity-component and pressure measurements is described, focusing on the techniques employed in probe fabrication, calibration, and frequencyresponse study. The probe tip is a sphere with 18 pressure ports properly distributed over the spherical surface. The device eliminates the velocity directionality limitations of current multihole probes and constitutes a rugged tool for use in complex three-dimensional e ow mapping. An automated calibration system was used to generate a probe calibration database of approximately 10,000 individual probe orientations. Least-squares-based and neural-network-based calibration algorithms were developed. The reliability of the calibration procedures and algorithms is demonstrated e rst. Then probe utility is demonstrated in a e owe eld with e ow reversal, downstream of a backward-facing step. Last, a study of the probe frequency response is presented. I. Introduction T HE design, evaluation, and optimization of complex aerodynamicgeometriesinvolveextensivewind-tunneltestingand/or computationally intensive numerical simulations. Even in the latter case, high-quality experimental wind-tunnel results with minimal, quantie able errors are still necessary for code-validation purposes. Moreover, in aerodynamic testing facilities where large volumes of data need to be acquired in tight schedules, downtime due to poor instrumentation performance is highly undesirable. Such facilities include industrial testing wind tunnels, as well as high-productivity computational e uid dynamics code validation facilities. 1 In such environments, e ow measurement techniques such as laser Doppler velocimetry and particle image velocimetry, although powerful, usually require painstaking efforts toward their successful usage. Costly components; complex setups; troublesome e ow seeding requirements; lack of e exibility, ruggedness, and mobility; and ease of misalignment often render such techniques impractical. Moreover, in testing of complex three-dimensional geometries, accessibility of the entire e owe eld around the model is an essential issue. When employing optical techniques, large sections of the e owe eld are obstructed optically by the presence of the model. To access such regions, repositioning of the instrumentation setup is necessary, a time-consuming process with the associated potential pitfalls. Multihole pressure probes 2‐7 in many cases have provided the easiest-to-use and most cost-effective method for three-component e ow velocity measurements in research and industry environments. However, even with expanded measurement capabilities of such instruments, the current pressure probe cone gurations and techniques have a limited range of velocity inclinations that they can measure. The velocity inclination is indicated as the cone angle µ in Fig. 1. Let µmax be the maximum cone angle that can be measured reliably by the probe. In other words, a probe with a µmax of 40 deg can accurately measure any velocity vector that is contained within a cone with its apex at the probe tip, its axis along the probe axis, and with an apex included angle of 80 deg (Fig. 1). Measurable cone angles as high as 75 deg are not uncommon for seven-hole probes. 7 Following this reasoning, an omnidirectional probe is a probe that has a cone angle of 180 deg (360 deg included angle ); i.e., it can measure any velocity vector regardless of its orientation. The present work describes the development of a nearly omnidirectional probe that can measure up to cone angles of 170 deg

Development of a Nearly Omnidirectional Velocity Measurement Pressure Probe

Development of a nearly-omni-directional, three-component velocity measurement pressure probe

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Development of a Nearly Omnidirectional Velocity Measurement Pressure Probe

Development of a nearly-omni-directional, three-component velocity measurement pressure probe