Unsteady flow

The unsteady flow features over the suction surface can be adequately represented by linearized perturbation calculations, where the disturbance flow associated with t[r]

69134 Ecully Cedex, France Correspondence should be addressed to Xavier Carbonneau; xavier.carbonneau@isae.fr This study concerns a 2.5 pressure ratio centrifugal compressor stage consisting of a splittered unshrouded impeller and a vaned diffuser. The aim of this paper is to investigate the modifications of the flow structure when the operating point moves from peak efficiency to near stall. The investigations are based on the results of unsteady three-dimensional simulations, in a calculation domain comprising all the blade. A detailed analysis is given in the impeller inducer and in the vaned diffuser entry region through time-averaged and unsteady flow field. In the impeller inducer, this study demonstrates that the mass flow reduction from peak efficiency to near stall leads to intensification of the secondary flow effects. The low momentum fluid accumulated near the shroud interacts with the main flow through a shear layer zone. At near stall condition, the interface between the two flow structures becomes unstable leading to vortices development. In the diffuser entry region, by reducing the mass flow, the high incidence angle from the impeller exit induces a separation on the diffuser vane suction side. At near stall operating point, vorticity from the separation is shed into vortex cores which are periodically formed and convected downstream along the suction side.

The unsteady flow features over the suction surface can be adequately represented by linearized perturbation calculations, where the disturbance flow associated with t[r]

u| t=0 = u 0 on Ω, (1.1) where the unknowns are the velocity u and pressure p of the fluid. This system models the unsteady flow of an incompressible viscous fluid in a saturated rigid porous medium, in the case where the pressure p presents high variations. Indeed, in this case, the coefficient α de- pends on these values in an exponential way. We refer to [16] for details on the way of deriving this model. Depending on the specific initial boundary value, problem under consideration has mixed boundary conditions where the pressure is described on a part of the boundary and the normal component of the velocity on the rest of the boundary.

The application of an optimization procedure to such problems is faced to the following difficulties: first, the choice of the optimization algorithm is conditioned by the huge computational time of the unsteady-flow simulation, and second, it is necessary to consider several objectives concurrently. Typically, the improvement of the single time-averaged performance is usually not satisfactory for realistic ap- plications. Secondly, sensitivity analysis is tedious in the context of unsteady flows, due to the backward integration of the adjoint equation, which requires the storage, or partial storage / partial re-computation, of the unsteady solution.

Very recently, Shahsavari et al. (2017) have shown the results of field experiments aimed at investigating the impact of a single flush on the sediment bed of a combined sewer trunk in the municipality of Paris. The objective of the present paper is to show the preliminary results of the validation of a 1-D unsteady flow numerical model for the analysis of the cleaning performance of the performed flush. The analysis includes the comparison of experimental and model results concerning the evolution of both hydraulic and sediment-related parameters during the flush experiment.

[2] Bayeul-Lainé A. C., Dockter A., Bois G., Simonet S., 2011 6-9 July, Numerical simulation in vertical wind axis turbine with pitch controlled blades, IC-EpsMsO, 4 th , (Athens, Greece), pp 429-436 ISBN 978-960-98941-7-3 [3] Bayeul-Lainé A. C., Bois G., Simonet S., 2012 27 February-2 March, “Spectral analysis of unsteady flow simulation in a small VAWT”, 14 th International on Transport Phenomena and

* Correspondence: samer.majdalani@umontpellier.fr; Tel.: +33-4-67-14-90-59 Received: 10 February 2019; Accepted: 25 March 2019; Published: 28 March 2019



Abstract: In this paper, we studied water transport under an unsteady flow regime in an experimental channel (4 m in length; 3 cm in width). Our experiments implicated some measuring requirements, specifically, a water level (WL) detection technique that is able to measure WL in a range of 2 cm with a precision of 1 mm. The existing WL detection techniques could not meet our measurement requirements. Therefore, we propose a new measurement method that combines two approaches: An “old” water contact technique (float) with a “new” remote non-contact technique (infrared sensor). We used an extruded polystyrene (XPS Foam) that needed some adequate treatment before using it as float in experimental measurements. The combination of IR-sensors with treated float foam lead to a sensitive measurement method that is able to detect flat and sharp flow signals, as well as highly dynamic variations of water surface level. Based on the experimental measurements of WL and outflow at the channel output, we deduced a loop rating curve that is suitable with a power law adjustment. The new measurement method could be extended to larger scale applications like rivers and more complicated cross section geometry of irregular shape.

6.1 Introduction A large quantity of work has been devoted to the improvement of measurement methods, which are essential tools for the study of physical mechanisms. Nowadays, most of the conventional methods in fluid mechanics, such as hot wire probes or microphones, are able to give an accurate point-wise time-resolved characterization of a given physical quantity of a flow. When the need for flow characterization goes beyond a single point, it is possible to use arrays of sensors, but this has material limitations toward the spatial extent and resolution of the characterization, and may be too intrusive to get a global flow field measurement. Alternatively, a single sensor may be displaced to a large number of locations, yielding an arbitrarily dense set of measurements, which are however uncorrelated due to their non- simultaneity. Another quite widely used option consists of using Particle Image Velocimetry (PIV), that classically yields the two or three instantaneous displacement components, by acquiring and processing two images of the seeded flow separated by a very short time interval. Due to technical constraints related to illumination (usually with pulsed lasers) and camera imaging, the technique can still be considered as characterized by a trade-off between accuracy and temporal resolution. Indeed, low frame rate lasers have a high energy per pulse that guarantees a high signal-to-noise ratio in the images and a good measurement accuracy, but such traditional PIV systems cannot resolve the unsteady flow behavior. On the other hand, high frame rate systems (Time-Resolved PIV, TR-PIV) can characterize frequencies up to 10 kHz, however at the cost of a much lower signal-to-noise ratio, possibly hindering the accuracy unless some specific advanced processing are used (see for instance Jeon et al. (2014); Lynch and Scarano (2013); Yegavian et al. (2016)). Besides, it is worthwhile noticing

This paper discusses the results of a numerical study of transient flow in a rotated square tube bank with pitch-to-diameter ratio 2:1, in the range 30 ≤ Re ≤ 3000. Both stimulated and spontaneously-induced transient behav- iour are considered: An initial span-wise sinusoidal disturbance applied up- stream is found to be amplified downstream, so there is a net gain. When the appropriate feed-back mechanism is provided, a stable transient behaviour is obtained, with alternate vortices being shed from each cylinder, while the wake switches in a serpentine fashion. The cross-wise velocity oscillates at frequency, f, corresponding to a Strouhal number of around 0.3, in good agreement with experimental data. The stream-wise oscillations occur at 2f. The induced response is essentially independent of the amplitude and fre- quency of the applied disturbance, including the case of no excitation, or spontaneous transient behaviour. Quantitative details of pressure drop, lift, drag and heat transfer are provided, as well as stream-line plots illustrating the main features of the flow.

zero contribution to F 1 and F 2 loads. In a similar way, residual modes give a contribution to both F 1 and F 2 . Thus, the application of Proper Orthogonal Decomposition to the wall pressure field has highlighted that the main phenomenon responsible for the unsteady load acting on the nozzle is mainly contained in the first two energetic POD modes. For such a present flow configuration, POD is then a powerful mathematical proce- dure to isolate the physical contribution of the wall pressure field to the unsteady load. The main difference between both configura- tions holds in the structured and organized phenomena which are perfectly identified for the configuration A, and less stressed for the

We finally consider a volumetric im- age formation model based on the fact that pixel intensities result from an integral measurement of the concentration of tracer in the corresponding v[r]

Introduction Centrifugal compressors for the aeronautical industry have to simultaneously achieve high pres- sure ratio and high efficiency at design operating point, while minimizing the element size. In this context the trend has been toward high speed impeller associated with a vaned diffuser to achieve high pressure recovery in a reduced space. Nowadays, through the objectives of ACARE 2020, they are also required to operate over a large range of mass flow rates with an acceptable efficiency. At low mass flow rates, where generally the maximum pressure ratio is reached, the compressor system sta- bility is limited by the onset of instability known as rotating stall and surge phenomena. Rotating stall is characterized by the presence of one or several cells rotating around the annulus. Surge exhibits large amplitude oscillations of pressure through the compressor system (Greitzer (1976)).

Radius Ratio 3.2.A Frequency Spectrum of Total & Static Pressure Ratio 3.2.B Frequency Spectrum of Tangential & Radial Flow Angle 3.2.C Frequency Spectrum of Total[r]

Reference (DNS) 10 & degrees of freedom Our method (Red-LUM- based data- assimilation) 6 degrees of freedom Theoretical bound (optimal from 6-d.o.f. linear decomposition) 6 degr[r]

Case 10: P' Figure 6-14 Contours of Mach number and P', showing the effect of mean flow Mach number on the spatial distribution of the static pressure disturbance. The higher Mach number in case 10 decreases the speed of the upstream traveling disturbance, thus increasing the number of peaks per splitter chord.

Consequently, a steady state approximation to the Navier-Stokes equations will never fully converge, the error locations being predominantly near the shedding location[r]

The aim of previous studies presented in 2010 (Bayeul- Lainé A.C. and al [2-3]) was to give some numerical results like contours of pressure, velocity fields and power coefficients, compared to relative steady blades for a blade speed ratio of 0.4, a wind velocity of 8 m/s and for four blade stagger angles. The commercial code Star CCM+ 5.02.010 was used to make calculations. The benefit of rotating elliptic blades was shown: the performance of this kind of turbine was very good and better than those of classical VAWTs for some specific initial blade stagger angles between 0 and 15 degrees. It was shown that each blade’s behaviour has less influence on flow stream around next blade and on power performance. The maximum mean numerical coefficient was about 32%.

surface with suction effects. The model under consideration analyzes an unsteady flow along an accelerated non-conducting vertical flat surface of infinite extent with heat source and radiation through a porous medium in a rotating system. The solution of the PDEs involved in the model is found with the help of Laplace transform. The effect of various parameters on the flow field are analysed and discussed by ploting graphs against boundary layer co-ordinat. Also the variation in Nusselt number & skin-friction are shown by numerical values in tables.

TORQUES FOR CASES b* WITH THREE BLADES AND FOR DIFFERENT BLADE STAGGER ANGLES The benefit of rotating blades was confirmed in the first part of this paper. In this second part, results for four different blade stagger angles were presented . Only global results were resumed here. Figure 38 shows power coefficient with azimuth angle of blade 1 for the four different angles studied. Figures 39-42 give torques in each case. These results show a high unsteady flow for high angle (-30 and 30 degrees) and a relatively small power coefficient but, on the contrary, results for angle of 0 and 15 degrees are very good : a good periodicity is observed and good mean power coefficients are obtained as showing in figure 43.