A technique has been developed to compensate pressure readings from arrays of highly sensitive membrane-type pressure sensors for deflections caused by acceleration normal to the plane of the membrane using a single inertial measurement unit. By normalizing the fourth-order unsteady Kirchoff-Love equation, it can be shown that inertial body pseudoforces and applied surface pressure elicit a similar and additive response from the sensors. Inertial effects arising from linear and angular acceleration as well as angular velocity may therefore be converted to 'pseudopressures' and eliminated by means of a simple linear compensation process which can be calibrated using only gravity. To demonstrate, signals from a conventional six-axis inertial measurement unit (including three orthogonal components each of angular velocity and linear acceleration) are used to provide an approximation of the acceleration of the sensing dies within a seven-channel distributed array of ultra-low pressure sensors. Applying the proposed correction reduces the maximum full-scale uncertainty of the measurements by as much as 50%.
An experimental analysis of cylinders with diﬀerent porous coating conﬁgurations was performed to understand how these aﬀect near-wake vortex formation mechanics. Five coating conﬁgurations were produced using metallic foam to evaluate diﬀerent vortex formation parameters, namely the vortex shedding frequency, base pressure, formation length, wake thickness, vortex strength, and vorticity ratio. Existing scaling rules proved unable to describe all the vortex shedding patterns observed with less than six parameters, thus a new scaling was produced. By considering that immediately after detaching from a surface vortical structures will behave similarly unless disturbed, it was possible to reduce the scaling to wake velocity components alone. Results showed the new scaling to collapse the description of the near-wake from the initial six parameters to only four: three vortex relative speeds and vortex shedding frequency. Vorticity losses were shown to correlate with the ratio between vortex rotational speed and detaching shear layer speed with good agreement across all tested conﬁgurations. This validates the description developed of the vorticity losses in the near-wake, based on previous work describing the existence of two ﬂuctuating wake length scales: formation and diﬀusion lengths. In order to study the eﬀect of porous coatings on the coupling between drag and vortex shedding frequency, these were measured when sandpaper coatings and splitter plates were applied instead of metallic foams. Results showed the vortex shedding to be decoupled from drag unlike what had been observed for the metal foam coatings. Similar tests were performed on coating conﬁgurations with porous coatings removed, with results illustrating the combined eﬀect of porous coatings over base pressure and formation length. The well-established capability of porous coatings to decrease the unstable wake oscillations largely responsible for aerodynamic noise is likely to be related to the suppression of the diﬀusion length ﬂuctuations.
The streamwise velocity component in fully-developed turbulent channel flow is studied for two very rough surfaces and a smooth surface at comparable Reynolds numbers. One rough surface comprises sparse and isotropic grit with a non-Gaussian distribution. The other is a uniform mesh consisting of twisted rectangular elements which form a diamond pattern. The mean roughness heights (± the standard deviation) are, respectively, about 76 ± 42 and 145 ± 150 wall units. The mean velocity profile over the grit surface exhibits self-similarity (in the form of a logarithmic law) within the limited range of 0:03 ≤ y/h ≤ 0.05, but the profile over the mesh surface exhibits only a small region with a slope tangential to log-law slope scaled on outer variables. However, the mean velocity deficit and higher moments (up to the fourth order) all exhibit some degree of outer scaling over both surfaces. The distinction between self-similarity and outer similarity is clarified and the importance of the former is explained. Spatial correlations show that the dominant large-scale features are very large quasi-streamwise structures with circulation in the cross-flow plane, similar to those found in smooth-wall internal and external flows. However, in the present case, the spanwise length scales are considerably larger. © Springer Science+Business Media B.V. 2010.
Proceedings of the IUTAM Symposium held at the Royal Geographical Society, 19-22 September 2006, hosted by Imperial College, London, England.
A flow meter (1) comprising a sampling tube (3) through which fluid may flow and a sensor arrangement (9, 25, 27, 39, 41, 43, 44, 45, 46, 47), wherein the sampling tube (3) comprises a first hollow section (51, 53) having a first internal cross-sectional area (A1) and a second hollow section (55, 57) having a second internal cross-sectional area (A2) being less than the first internal cross-sectional area (A1); and the sensor arrangement (9, 25, 27, 39, 41, 43, 44, 45, 46, 47), is for measuring the difference between stagnation and static pressures (P01, P02, P1, P2) within the second hollow section (55, 57).
Trailing vortices have been repeatedly shown to exhibit a remarkably robust self-similarity independent of the Reynolds number and upstream boundary conditions. The collapse of the inner-scaled circulation profiles of a trailing vortex has even been previously demonstrated for the cases of highly unsteady and turbulent vortex systems, as well as for vortices which were incompletely developed. A number of factors which contribute to and may artificially promote this self-similarity are discussed. It is shown that the amplitude of vortex “wandering” (or the random modulations in the vortex trajectory) observed in some experimental measurements are of sufficient amplitude to cause any arbitrary finite and axisymmetric flow structure to collapse with an idealized trailing vortex when scaled on inner parameters. It is further shown that, for the case of an incompletely developed wing-tip vortex, similarity in the outer core region may be an artefact of the rate of roll-up of the vortex sheet. Great care must, therefore, be taken when interpreting experimental measurements of vortex flows.
A direct numerical simulation of a Batchelor vortex has been carried out in the presence of freely-decaying turbulence, using both periodic and symmetric boundary conditions; the latter most closely approximates typical experimental conditions, while the former is often used in computational simulations for the purposes of numerical convenience. The higher-order velocity statistics were shown to be strongly dependent upon the boundary conditions, but the dependence could be mostly eliminated by correcting for the random, Gaussian modulation of the vortex trajectory commonly referred to as 'wandering' using a technique often employed in the analysis of experimental data. Once corrected for this wandering, the strong peaks in the Reynolds stresses normally observed at the vortex centre were replaced by smaller local extrema located within the core region but away from the centre. The distributions of the corrected Reynolds stresses suggested that the formation and organization of secondary structures within the core is the main mechanism in turbulent production during the linear growth phase of vortex development.
J Song, S Fan, William Lin, L Mottet, H Wooward, M Davies Wykes, R Arcucci, D Xiao, J Debay, H ApSimon, E Aristodemou, David Birch, Matteo Carpentieri, F Fan, M Herzog, G Hunt, R Jones, C Pain, D Pavlidis, Alan Robins, C Short, P Linden (2018)Natural ventilation in cities: the implications of fluid mechanics, In: Building Research & Information46(8)pp. 809-828
Taylor & Francis
Research under the Managing Air for Green Inner Cities (MAGIC) project uses measurements and modelling to investigate the connections between external and internal conditions: the impact of urban airflow on the natural ventilation of a building. The test site was chosen so that under different environmental conditions the levels of external pollutants entering the building, from either a polluted road or a relatively clean courtyard, would be significantly different. Measurements included temperature, relative humidity, local wind and solar radiation, together with levels of carbon monoxide (CO) and carbon dioxide (CO2) both inside and outside the building to assess the indoor–outdoor exchange flows. Building ventilation took place through windows on two sides, allowing for single-sided and crosswind-driven ventilation, and also stack-driven ventilation in low wind conditions. The external flow around the test site was modelled in an urban boundary layer in a wind tunnel. The wind tunnel results were incorporated in a large-eddy-simulation model, Fluidity, and the results compared with monitoring data taken both within the building and from the surrounding area. In particular, the effects of street layout and associated street canyons, of roof geometry and the wakes of nearby tall buildings were examined.
The measurement of vortex flows with particle-image velocimetry (PIV) is particularly susceptible to error arising from the finite mass of the tracer particles, owing to the high velocities and accelerations typically experienced. A classical model of Stokes-flow particle transport is adopted, and an approximate solution for the case of particle transport within an axisymmetric, quasi-two-dimensional Batchelor q-vortex is presented. A generalized expression for the maximum particle tracking error is proposed for each of the velocity components, and the importance of finite particle size distributions is discussed. The results indicate that the tangential velocity component is significantly less sensitive to tracking error than the radial component, and that the conventional particle selection criterion (based on the particle Stokes number) may result in either over- or under-sized particles for a specified allowable error bound. Results were demonstrated by means of PIV measurements carried out in air and water using particles with very different properties.
For measuring three components of velocity in unknown flow fields, multi-hole pressure probes possess a significant advantage. Unlike methods such as hot-wire anemometry, laser-Doppler velocimetry and particle-image velocimetry, multi-hole pressure probes can provide not only the three components of local velocity, but also static and stagnation pressures. However, multi-hole probes do require exhaustive calibration. The traditional technique for calibrating these probes is based on either look-up tables or polynomial curve fitting, but with the low cost and easy availability of powerful computing resources, neural networks are increasingly being used. Here, we explore the possibility to further reduce measurement uncertainty by implementing neural-network-based methods that have not been previously used for probe calibration, including supervised and unsupervised learning neural networks, regression models and elastic-map methods. We demonstrate that calibrating probes in this way can reduce the uncertainty in flow angularity by as much as 50% compared to conventional techniques.
The streamwise velocity component is studied in fully-developed turbulent channel flow for two very rough surfaces and a smooth surface at comparable Reynolds numbers. One rough surface comprises sparse and isotropic grit with a highly non-Gaussian distribution. The other is a uniform mesh consisting of twisted rectangular elements which form a diamond pattern. The mean roughness heights (+/- the standard deviation) are, respectively, about 76 (+/- 42) and 145 (+/- 150) wall units. The flow is shown to be two-dimensional and fully developed up to the fourth-order moment of velocity. The mean velocity profile over the grit surface exhibits self-similarity (in the form of a logarithmic law) within the limited range of 0.04 < y/h < 0.06, but the profile over the mesh surface does not, even though the mean velocity deficit and higher moments (up to the fourth order) all exhibit outer scaling over both surfaces. The distinction between self-similarity and outer similarity is clarified and the importance of the former is explained. The wake strength is shown to increase slightly over the grit surface but decrease over the mesh surface. The latter result is contrary to recent measurements in rough-wall boundary layers. Single- and two-point velocity correlations reveal the presence of large-scale streamwise structure with circulation in the plane orthogonal to the mean velocity. Spanwise correlation length scales are significantly larger than corresponding ones for both internal and external smooth-wall flows.
As performance improvements of compressors become more difficult to obtain, the optimization of stator well structure to control the reverse leakage flow is a more important research subject. Normally, the stator well can be considered as two rotor–stator cavities linked by the labyrinth seal. The flow with high tangential velocity and high total temperature exited from the stator well interacts with the main flow, which can affect the compressor aerodynamic performance. Based on the flow mechanisms in the basic stator well, four geometries were proposed and studied. For geometry a and geometry b, seal lips were attached to the rotor and stator inside downstream rim seal while impellers were positioned in the cavities for geometry c and geometry d. Leakage flow rates, tangential velocities, and pressure distributions in the cavities were analyzed using validated method of computational fluid dynamics. In the current study, where ω = 8000 rpm, π = 1.05–1.30, the maximum reductions of leakage flow rate for geometry a and geometry b are 7.9% and 15.9%, respectively, compared to the baseline model. In addition, the rotating impellers in the downstream cavity for geometry c contribute to a more significant pressure gradient along radial direction, reducing the leakage flow as much as 46%. Although the stationary impellers in the upstream cavity for geometry d appear to have little effect upon the leakage, these impellers can be used to adjust the tangential velocity of ejected flow from the stator well to the mainstream.
A constant-temperature anemometer has been developed which uses a single high-fidelity speaker driver as a combined signal and power amplifier. Owing to its small size and simplicity of construction, the anemometer is well suited for applications requiring a large number of channels (such as hot-wire rakes) as well as applications requiring the embedding of instrumentation within confined experimental models (such as reduced-scale wind turbine blades). The anemometer is shown to have performance characteristics similar to those of a commercial anemometer when used under its design conditions. An operating bandwidth as high as 10 kHz can be achieved, which is greater than most available time-resolved digital particle-image velocimetry systems and is shown to be sufficient to track large-scale turbulence structures in channel flow.
A generalized calibration process is presented for multi-hole, pressure-based velocity probes which is independent of the number of holes and probe geometry, allowing the use of probes with large numbers of holes. The calibration algorithm is demonstrated at low speeds with a conventional seven-hole pressure probe and a novel nineteen-hole pressure probe. Because the calibration algorithm is independent of probe configuration, it is very tolerant of data corruption and imperfections in the probe tip geometry. The advantages of using probes with large numbers of holes is demonstrated in a conventional wing wake survey. The nineteen-hole probe offers a higher angular sensitivity than a conventional seven-hole probe, and can accurately measure velocity components even when an analytical calibration scheme is used. The probe can also provide local estimates of the diagonal components of the cross-flow velocity gradient tensor in highly vortical flows.
A two-component micropillar system has been developed for use in flow characterization and control applications. It is demonstrated that the piezoresistive elements in a conventional pressure sensing die can be used to measure moments directly applied to the die with reasonable sensitivity. The concept is then demonstrated by bonding a small, rigid pillar to the centre of a commercially-available, off-the-shelf doped silicon pressure sensing membrane with integrated piezoresistive bridge elements. The signal response of the system is then calibrated against aerodynamic loading, and a typical sensitivity of 70 mV/mNm is demonstrated. The functionality of the micropillars in flow control applications is verified by recessing the pillar below a flat surface on which a model boundary layer is developed. By processing the signals through the membrane bridge-arms independently, directionally-resolved forces may also be obtained.
The aerodynamic characteristics of an aft-body, in-line mounted, boundary layer ingesting, electric ducted fan, propulsion installation system has been investigated through experimental and computational analysis. A modular wind-tunnel model allows variation in the geometry of the propulsion installation system to be assessed, in combination with fan speed. Various experimental measurement techniques, including LDA, seven-hole-probe and surface pressures are employed. The propulsion installation system has also been investigated using RANS CFD and comparison with experimental data is presented. An investigation of the boundary conditions for efficiently representing the fan in CFD is described. Initial results show reasonably good agreement between CFD and experiment, in terms of velocity profiles and surface pressures, but highlight remaining differences for cases exhibiting flow separation.
The calibration of directional velocity probes can require significant facility time and resources, especially if carried out in situ. The techniques of design of experiments are therefore applied in order to formally optimize the selection of calibration points. A model is proposed for a generalized directional velocity probe, and this model is used to generate an approximate, polynomial response surface model which is shown to agree well with measurements from both multi-sensor hot-wire probes and multi-hole pressure probes, in a variety of geometries. The process of D-optimality is then applied based on this response surface model, and a typical probe is calibrated accordingly. The probe is then used to scan the wake of a vortex generator, in order to test the efficacy of the reduced calibrations. D-optimal calibration points are shown to offer a significant improvement in data fidelity over conventional rectangular grids, and minimal additional uncertainty is incurred after a 25-fold reduction in the number of calibration points.
A novel approach has been considered for the formal process of calibrating multiple hole pressure probes for use in wind tunnels. Rather than determining the attitude angles of a probe and subsequently flow angularity for a fixed probe, either by linear interpolation between sample points or through the use of piecewise functional fits, the outputs from the probe are mapped as continuous functions across the angular test space, using a set of sample points derived from Optimal Design of Experiments. This offers the potential of more accurate probe calibrations across a wider range of flow onset angles, with fewer sample points than currently used for the same purpose. Proof-of-concept tests using a five-hole probe have indicated that the approach is viable, while examination of fits to legacy data from prior tests indicates that the approach is easily extendable to probes with an arbitrary number of holes, and to multiple hot-wire installations.
There has been much recent interest in the combined structural and aerodynamic properties of porous metal foams, but there does not yet appear to be a consensus on the aerodynamic behaviour of these foam materials. A comprehensive analytical and experimental study with special attention to scaling was carried out in order to examine the flow around cylinders coated with porous metal foam and characterize the effects upon the mechanisms governing shear layer separation, vortex shedding and wake formation. Results have yielded a correlation between the distance separating the detaching shear layers and the vorticity losses in the near-wake. It seems that it is the coating configuration, rather than geometry, that influences vortex shedding, and therefore foams affect the flow in a similar way as shrouds or bleeding systems.
The nature of turbulence within wing-tip vortices has been a topic of research for decades, yet accurate measurements of Reynolds stresses within the core are inherently difficult due to the bulk motion wandering caused by initial and boundary conditions in wind tunnels. As a result, characterization of a vortex as laminar or turbulent is inconclusive and highly contradicting. This research uses several experimental techniques to study the effects of broadband turbulence, introduced within the wing boundary layer, on the development of wing-tip vortices. Two rectangular wings with a NACA 0012 profile were fabricated for the use of this research. One wing had a smooth finish and the other rough, introduced by P80 grade sandpaper. Force balance measurements showed a small reduction in wing performance due to surface roughness for both 2D and 3D configurations, although stall characteristics remained relatively unchanged. Seven-hole probes were purpose-built and used to assess the mean velocity profiles of the vortices five chord lengths downstream of the wing at multiple angles of attack. Above an incidence of 4 degrees, the vortices were nearly axisymmetric, and the wing roughness reduced both velocity gradients and peak velocity magnitudes within the vortex. Laser Doppler velocimetry was used to further assess the time-resolved vortex at an incidence of 5 degrees. Evidence of wake shedding frequencies and wing shear layer instabilities at higher frequencies were seen in power spectra within the vortex. Unlike the introduction of freestream turbulence, wing surface roughness did not appear to increase wandering amplitude. A new method for removing the effects of vortex wandering is proposed with the use of carefully selected high-pass filters. The filtered data revealed that the Reynolds stress profiles of the vortex produced by the smooth and rough wing were similar in shape, with a peak occurring away from the vortex centre but inside of the core. Single hot-wire measurements in the 2D wing wake revealed the potential origin of dominant length-scales observed in the vortex power spectra. At angles above 5 degrees, the 2D wing wake had both higher velocity deficits and higher levels of total wake kinetic energy for the rough wing as compared to the smooth wing.