LES investigation on the dependence of the flow through a centrifugal pump on the diffuser geometry

Large-Eddy Simulation is utilized to investigate the rotor–stator interaction within a centrifugal pump. Comparisons are presented across diffuser geometries for two values of the flow-rate. Decreasing the incidence angle on the diffuser blades at off-design is found the main source of higher pressure rise and lower overall values of turbulent kinetic energy within the pump, resulting in efficiency improvement. The impact on the second-order statistics of the flow is especially significant. However, the values of the pressure fluctuations acting on the diffuser blades, defining fatigue loads on them and cavitation phenomena, are found especially affected by the rotor–stator clearance. Results show that at reduced flow-rates the rotation of the diffuser blades around their mid camber is a better option than rotating them around their leading edge. They also suggest that at larger flow-rates the increased incidence on the diffuser blades causes pressure side separation and large shear layers populating the diffuser channels, not affecting substantially the region of interface between impeller and diffuser, but having detrimental effects on the performance of the latter. The rotation of the diffuser blades around their leading edge should be preferred when the pump operates at flow-rates larger than the design one, avoiding decreasing the rotor–stator gap, thus resulting in smoother rotor–stator interaction and lower pressure fluctuations.     

LES study on the influence of the diffuser inlet angle of a centrifugal pump on pressure fluctuations

Large-Eddy Simulations are conducted on a centrifugal pump at design and reduced flow-rates for three diffuser geometries, to investigate the effect of changing the diffuser inlet angle on the overall performance and the pressure fields. In particular, pressure fluctuations are investigated, which affect the unsteady loads acting on the pump, as well as vibrations, noise and cavitation phenomena. The considered modification of the diffuser geometry is targeted at decreasing the incidence angle at the off-design flow-rate by rotating the stationary blades of the pump around their leading edge. Results are compared against those of an earlier study, where the same modification of the diffuser inlet angle was achieved by increasing also the radial gap between impeller and diffuser, whose blades were rotated relative to their mid camber location. The comparisons across cases demonstrate that the radial gap between the trailing edge of the impeller blades and the leading edge of the diffuser blades has a more profound influence on pressure fluctuations, compared to the angle of incidence on the diffuser blades of the flow coming from the impeller.     

A study on the flow field and local heat transfer performance due to geometric scaling of centrifugal fans

Scaled versions of fan designs are often chosen to address thermal management issues in space constrained applications. Using velocity field and local heat transfer measurement techniques, the thermal performance characteristics of a range of geometrically scaled centrifugal fan designs have been investigated. Complex fluid flow structures and surface heat transfer trends due to centrifugal fans were found to be common over a wide range of fan aspect ratios (blade height to fan diameter). The limiting aspect ratio for heat transfer enhancement was 0.3, as larger aspect ratios were shown to result in a reduction in overall thermal performance. Over the range of fans examined, the low profile centrifugal designs produced significant enhancement in thermal performance when compared to that predicted using classical laminar flow theory. The limiting non-dimensional distance from the fan, where this enhancement is no longer apparent, has also been determined. Using the fundamental information inferred from local velocity field and heat transfer measurements, selection criteria can be determined for both low and high power practical applications where space restrictions exist.     

A study on the flow field and local heat transfer performance due to geometric scaling of centrifugal fans

Scaled versions of fan designs are often chosen to address thermal management issues in space constrained applications. Using velocity field and local heat transfer measurement techniques, the thermal performance characteristics of a range of geometrically scaled centrifugal fan designs have been investigated. Complex fluid flow structures and surface heat transfer trends due to centrifugal fans were found to be common over a wide range of fan aspect ratios (blade height to fan diameter). The limiting aspect ratio for heat transfer enhancement was 0.3, as larger aspect ratios were shown to result in a reduction in overall thermal performance. Over the range of fans examined, the low profile centrifugal designs produced significant enhancement in thermal performance when compared to that predicted using classical laminar flow theory. The limiting non-dimensional distance from the fan, where this enhancement is no longer apparent, has also been determined. Using the fundamental information inferred from local velocity field and heat transfer measurements, selection criteria can be determined for both low and high power practical applications where space restrictions exist.     

Numerical simulation of the flow field in the vicinity of an axial flow fan

The main purpose of the current investigation is the development and evaluation of a numerical model used to simulate the effect of an axial flow fan on the velocity field in the vicinity of the fan blades. The axial flow fan is modeled as an actuator disc, where the actuator disc forces are calculated using blade element theory. The calculated disc forces are expressed as sources/sinks of momentum in the Navier–Stokes equations solved by a commercially available computational fluid dynamic (CFD) code, Flo⁺⁺. The model is used to determine the fan performance characteristics of an axial flow fan as well as the velocity fields directly up‐ and downstream of the fan blades. The results are compared with experimental data. In general, good agreement is obtained between the numerical results and experimental data, although the fan power consumption, as well as radial velocity downstream of the fan blades, is underpredicted by the fan model. Copyright © 2001 John Wiley & Sons, Ltd.     

The onset of flow-rate limitation and flow-induced oscillations in collapsible tubes

Experiments were mounted to investigate the onset in a ‘Starling resistor’ of collapsible-tube oscillation, at the lowest possible Reynolds number so as to facilitate matched numerical simulations. The protocol adopted was to set pressure outside the tube and inside the tube at the upstream end, constant and equal to each other, then to progressively lower the downstream pressure past the point of tube collapse and, when this occurred, of oscillation onset. The working fluid was a glycerine/water mixture, and the silicone-rubber tube was suspended horizontally in air. Measurements were made of pressures and flow-rates and of the cross-sectional area at the approximate location of maximum oscillation; separately, the cross-sectional area of the tube in relation to transmural pressure was measured. Parameters varied in the flow experiments were the length of rigid pipe downstream of the collapsing tube, and the fluid viscosity. The pressure/flow-rate coordinates of both the point of peak flow-rate achieved before flow-rate limitation, and the point of oscillation onset, were satisfactorily independent of the pipe length downstream. Both points occurred at flow-rates that decreased with increasing fluid viscosity, so that the corresponding Reynolds numbers decreased more so. Oscillation did not break out below a Reynolds number of about 290 unless there was external mechanical agitation of the apparatus. The amplitude of oscillation decreased progressively towards zero at this point as viscosity was raised. After the flow-rate peak, flow limitation causes a local flow-rate minimum. When oscillation occurred, it started just before this minimum, and died away at the minimum.     

福建沿海地区地微动的谱结构特征

在福建沿海地区的福州、泉州、莆田等地进行了场地地微动的测试 ,利用快速富里叶变换 (FFT)方法对观测到的地微动信号进行频谱分析。结果表明 ,覆盖层厚度大、有厚层的软弱夹层淤泥的场地 ,其整体刚度小 ,地微动谱能量相当分散 ,频带宽 ,峰值频率低 ,卓越频率一般为 1～ 5Hz ;覆盖层薄、土层刚度 (Vs)大的场地 ,地微动的峰值频率也大 ,卓越频率一般为8～ 11Hz,且常呈单峰形态。软土对高频地微动信号有滤波作用 ,对低频信号起放大作用 ,而硬土层则相反.     

A film-based wall shear stress sensor for wall-bounded turbulent flows

In wall-bounded turbulent flows, determination of wall shear stress is an important task. The main objective of the present work is to develop a sensor which is capable of measuring surface shear stress over an extended region applicable to wall-bounded turbulent flows. This sensor, as a direct method for measuring wall shear stress, consists of mounting a thin flexible film on the solid surface. The sensor is made of a homogeneous, isotropic, and incompressible material. The geometry and mechanical properties of the film are measured, and particles with the nominal size of 11 μm in diameter are embedded on the film’s surface to act as markers. An optical technique is used to measure the film deformation caused by the flow. The film has typically deflection of less than 2% of the material thickness under maximum loading. The sensor sensitivity can be adjusted by changing the thickness of the layer or the shear modulus of the film’s material. The paper reports the sensor fabrication, static and dynamic calibration procedure, and its application to a fully developed turbulent channel flow at Reynolds numbers in the range of 90,000–130,000 based on the bulk velocity and channel full height. The results are compared to alternative wall shear stress measurement methods.     

利用子波分析对平壁湍流猝发现象的研究

利用槽道湍流直接数值模拟的数据库,采用子波分析的方法。对平壁湍流猝发现象的多尺度特性进行了研究,在不同惊讶上对猝发平均周期进行了统计,并利用局部标度指数研究了猝发过程的奇异性。     

Communication between the buffer layer and the wall in a turbulent channel flow

Direct numerical simulations obtained in large computational domains of a fully developed turbulent channel flow up to the Karman number 1100 are analyzed to determine the scaling of the spanwise correlation coefficients and the effect of the outer eddies. The local fluctuating velocity field is narrow-band-pass and low-pass filtered along the streamwise wavenumber. The spanwise correlations of the narrow-band passed signals in the low buffer layer adequately provide length scales and signatures of the active structures. The low-pass filtering is used to investigate the relative role of the outer eddies. The impact of the active and passive eddies on the wall is analyzed separately through the cross-correlations of the filtered velocity field with the wall shear stress fluctuations. Characteristic length-scales resulting from the analysis of the velocity field differ depending on the quantity and some are related to the conventional streak spacing but not all. The quasi-streamwise vortex paradigm, for the most part, allows the interpretation of these characteristics, but fails in some cases.     

Investigation of flow separation in a transonic-fan linear cascade using visualization methods

An extensive experimental study into the nature of the separated flows on the blade suction surface of modern transonic fans is described in this paper. The study was a subtask of a larger experimental effort focused on blade flutter excited by flow separation in the blade tip region. The tip sections of airfoils on transonic fan blades are designed for precompression and consequently they differ from sections on the rest of the blade. The blade tip section was modeled by a low aspect ratio blade and therefore most of the blade tested was exposed to the secondary flow effects. The aim of this work was to supply reliable data on flow separation on transonic fan blades for validation of future analytical studies. The experimental study focused on two visualization techniques: surface flow visualization using dye oils and schlieren (and shadowgraph) flow visualization. The following key observations were made during the study. For subsonic inlet flow, the flow on the suction surface of the blade was separated over a large portion of the blade, and the separated area increased with increasing inlet Mach number. For the supersonic inlet flow condition, the flow was attached from the leading edge up to the point where a bow shock from the upper neighboring blade imposed on the blade surface. Downstream, there was a separated flow region in which air flowed in the direction opposite the inlet flow. Finally, past the separated flow region, the flow reattached to the blade surface. For subsonic inlet flow, the low cascade solidity resulted in an increased area of separated flow. For supersonic flow conditions, the low solidity resulted in an improvement in flow over the suction surface.     

Edge waves under initial stress

The frequency equation giving the phase velocity of a wave at the edge of a thick plate under initial stress is obtained. Some particular cases are discussed to derive (a) the velocity of edge waves in a thin plate and (b) the velocity of Rayleigh waves in a plate of infinite thickness under initial stress. The results are compared with those for zero initial stress.     

Analysis of unsteady gas–liquid flows in a rectangular tank: Comparison of Euler–Eulerian and Euler–Lagrangian simulations

We study the dynamics of gas–liquid flows experimentally and computationally in a rectangular bubble column where the gas source is introduced at the corner. The flow in this reactor is complex and inherently unsteady in nature. The two-dimensional liquid phase velocity field is calculated by an Eulerian approach solving the unsteady Reynolds Averaged Navier Stokes equations. The conservation equations are closed using a two parameter turbulence model. The two-way coupling was accounted for by adding source terms in the conservation equations of the continuous phase to take into account the interaction with the dispersed phase. Bubble tracking is achieved through a Lagrangian approach. Here the equations of motion are solved taking into account the drag, pressure, buoyancy and gravity forces. The time-averaged flows along with the variables which characterize turbulence are analyzed for a wide range of gas flow-rates using Euler–Lagrangian simulations. These simulation predictions are validated with Euler–Eulerian simulations where the gas-phase distribution is captured as a void fraction and PIV experiments. The motion of bubbles induces turbulence in the flow. The applicability of two parameter models for turbulence like the standard k–ε model on time-averaged flow properties is addressed. From the results of the time averaged velocity field, turbulence intensity, turbulent viscosity and gas hold-up profiles, it is concluded that the Euler–Lagrangian model is applicable at lower gas flow-rates. The Euler–Eulerian approach was found to be valid at lower as well as higher gas flow-rates.     

Experimental study of the three-dimensional flow field in cross-flow fans

High-resolution PIV measurements of the flow field inside cross-flow fans have been performed in planes normal and parallel to the fan axis, both outside and inside the impeller. The well known difficulties in obtaining the optical access inside the impeller have been overcome by allowing the internal flow planes to be illuminated by the laser light sheet or shot by the CCD camera through the moving blade vanes. Measurements have been performed in two cross-flow fans having the same two-module impeller but casing geometries based on very different design concepts. PIV data in planes normal to the rotor axis show a strong correlation between vorticity distribution and turbulent shear stresses inside the eccentric vortex of each fan. Furthermore, they provide useful elements to explain the very different performance of the two fans evidenced by their characteristic curves. Measurements in planes parallel to the impeller axis show that wide three-dimensional recirculation structures develop near the casing end walls at the discharge of the fans. These mean flow structures are responsible for the backflow into the end portions of the impeller of part of the discharged fluid, which is then transported axially by the eccentric vortex towards the rotor central disc before being discharged once again outside the impeller. In the case of cross-flow fans including few rotor modules, the existence of significant axial velocity components inside the eccentric vortex can alter substantially the flow picture, common in the current literature, resulting from 2-D numerical models or measurements performed in a single transverse plane of the fan.     

Application of the SAS turbulence model to predict the unsteady flow field behaviour in a forward centrifugal fan

In the current study, the unsteady flow in a centrifugal fan is carried out using Computational Fluid Dynamics calculation based on the Scale Adaptive Simulation (SAS) approach to model the turbulence phenomenon. The SAS concept is based on the introduction of the von Karman length scale into the turbulence scale equation. The information provided by the von Karman length scale allows SAS models to dynamically adjust to resolved structures in an Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation, which results in a Large Eddy Simulation-like behaviour in unsteady regions of the flow field. At the same time, the model provides standard RANS capabilities in stable flow regions. The introduction of the von Karman length scale is based on the reformulation of Rottas's equation for the integral length scale. To validate the numerical results, the overall performances of the fan and the wall pressure fluctuations computed upon the volute casing surface are compared with the unsteady measured data.     

Unsteadiness of the shock wave structure in attached and separated compression ramp flows

Wall pressure fluctuations have been measured upstream of the corner-line in several two dimensional, adiabatic, compression ramp flows. The nominal freestream Mach number was 3 and the Reynolds number, based on boundary layer thickness, was 1.4 million. The measurements show that the shockwave structure is unsteady in both separated and attached flows, resulting in a region in which the wall pressure signal is intermittent. Statistical properties of this intermittent region, and of the separated flow, are presented and correlated with results from other studies.     

Flow-rate limitation in a uniform thin-walled collapsible tube,with comparison to a uniform thick-walled tube and a tube of tapering thickness

Previous experiments on a tapered-thickness tube showed qualitatively different behaviour from that exhibited by a uniform thick-walled tube. To understand whether the taper or the thinner wall was responsible, similar aqueous flow-limitation experiments were conducted on a uniform thin-walled tube of the same material, with all other experimental set-up the same. As in the thick tube, there was a dramatic reduction in flow-rate when collapse and flow limitation started, but during external pressure reduction, the limited flow-rate progressively increased, so that as in the tapered-thickness tube, there was little flow-rate increase when collapse ceased. Hysteresis was thus a prominent feature of the relationship between flow-rate and pressure drop along curves of constant upstream transmural pressure. Flow-rate limitation was mainly accompanied by large-amplitude self-excited oscillation for both increasing and decreasing external pressure, to an even greater extent than in the tapered-thickness tube. Clusters of points sharing the same pair of upstream transmural pressure and upstream driving pressure values were found, indirectly implying as in the tapered-thickness tube that the flow-limited flow-rate for a given pressure drop was not uniquely determined by upstream transmural pressure. Negative effort dependence was observed in all three tubes, but in the thin tube, as in the tapered-thickness tube, it was obscured for some values of upstream transmural pressure where low-frequency single-collapse-per-cycle oscillations occurred. Thus, the qualitatively unique properties of the tapered-thickness tube appear to be confined to the relative lack of hysteresis, and the oscillatory regime in which collapse ceased before the downstream end. The rest of the observed behaviours seem to be characteristic simply of more compliant tubing.     

Response of the streaks, the active and passive eddies in an unsteady channel flow

Spanwise space–time correlations of the wall shear stress and the longitudinal velocity fluctuations in the low buffer layer of an unsteady channel flow are reported. The imposed amplitude is 20% of the centerline velocity and the imposed frequency covers a large range going from the quasi-steady limit to the bursting frequency of the corresponding steady flow. The unsteady spanwise correlation coefficient is investigated both through its own modulation characteristics (amplitude and phase shifts) and those of the resulting streak spacing. A good correspondence is found between the modulation of the streak spacing and that of the ejection period. The data is further analyzed by temporal filtering of the wall shear stress and streamwise velocity fluctuations. It is shown that the large outer-layer structures play a “passive” role in the unsteady response of the near wall turbulence. The inner wall eddies, in return, are amply responsible for the unsteady reaction of both the turbulent wall shear stress and the streamwise velocity intensities in the buffer layer.     

Flow driven by a stamped metal cooling fan – Numerical model and validation

This paper presents a numerical fluid flow model for the stamped metal cooling fans popularly employed in electric motors. An experimental system is constructed to measure the performance of the cooling fan. The agreements between model prediction and experimental data are reasonably good. Parametric studies with the numerical model indicate that the viscous heating in the fluid and the variation of the air density have negligible effects on the fan performance. The blade edge thickness affects the flow driving capability of the fan. With various pressure differentials, three flow regimes are recognized. The first is the axial component dominated flow. In the second regime, the flow has a forward axial flow and a backward leakage flow. The third one is the leakage flow dominated regime, when the pressure differential across the fan is large.