Auswahl der wissenschaftlichen Literatur zum Thema „Laminarizace“

It is demonstrated experimentally and theoretically that through the use of an active (feedback) controller one can dramatically modify the nature of the flow in a toroidal thermal convection loop heated from below and cooled from above. In particular, we show how a simple control strategy can be used to suppress (laminarize) the naturally occurring chaotic motion or induce chaos in otherwise time-independent flow. The control strategy consists of sensing the deviation of fluid temperatures from desired values at a number of locations inside the loop and then altering the wall heating to either counteract or enhance such deviations.

Predictions of the isothermal, incompressible flow in the cavity formed between two corotating plane disks and a peripheral shroud have been obtained using an elliptic calculation procedure and a low turbulence Reynolds number k–ε model for the estimation of turbulent transport. Both radial inflow and outflow are investigated for a wide range of flow conditions involving rotational Reynolds numbers up to ∼106. Although predictive accuracy is generally good, the computed flow in the Ekman layers for radial outflow often displays a retarded spreading rate and a tendency to laminarize under conditions that are known from experiment to produce turbulent flow.

Airflow in fully developed turbulent state between two parallel plates was accelerated through a linearly converging section, and then it flowed into a parallel-plate channel again. The Reynolds number 2hum/ν was 10,000 and the acceleration parameter K in the accelerating section was 8 × 10−6. Fluctuations of streamwise velocity as well as time-mean velocity profiles were measured at ten traversing stations located along the test channel by a hot-wire anemometer. It was found that the flow, partly laminarized in the accelerating section, continued to laminarize in the first part of the downstream parallel-plate section and then the reversion to turbulence occurred in the way similar to the case of natural transition in a pipe, where the transition proceeds through a regime of the so-called turbulent slug flow.

The research on three dimensional (3-D) wind turbine blades has been introduced (Sutrisno, Prajitno, Purnomo, & B.W. Setyawan, 2016). In the current experiment, the 3-D wind turbine blades would be fitted with helicopter-like blade tips and additional fins to the blade hubs to demonstrate some laminarizing features. It was found that additional helicopter-like blade tip to the turbine blade creates strong laminar flows over the surface of the blade tips. Supplementary, finned hub, fitted to the blade body, creates rolled-up vortex flows, weakens the blade stall growth development, especially for blades at high-speed wind. A proposed mathematical form of modified lifting line model has been developed to pursue further 3-d blade development study of 3-d wind turbine blade. Rolled up vortex effects, developed by finned of the base hub, has been acknowledged could demolish the turbulent region, as well as laminarize the stall domain to intensify the induced wind turbine blade lift.

AbstractFully developed plane channel flow rotating in the spanwise direction has been studied analytically and numerically. Linear stability analysis reveals that all cross-flow modes are stable for supercritical rotation numbers, $Ro\gt R{o}_{c} $, where $R{o}_{c} $ will approach 3 from below for increasing Reynolds number. Plane Tollmien–Schlichting (TS) waves are independent of rotation and always linearly unstable for supercritical Reynolds numbers. Direct numerical simulation (DNS) of the laminarization process reveals that the turbulence is damped when $Ro$ approaches $R{o}_{c} $. Hence, the laminarization is dominated by linear mechanisms. The flow becomes periodic for supercritical Reynolds numbers and rotation rates, i.e. when $Ro\gt R{o}_{c} $ and $Re\gt R{e}_{c} $. At such rotation rates, all oblique (cross-flow) modes are damped and when the disturbance amplitude becomes small enough, the TS modes start to grow exponentially. Secondary instabilities are initially blocked by the rotation since all cross-flow modes are linearly stable and the breakdown to turbulence will be strongly delayed. Hence, the TS waves will reach extremely high amplitudes, much higher than for typical turbulent fluctuations. Eventually, the extreme-amplitude state with TS-like waves will break down to turbulence and the flow will laminarize due to the influence of the rapid rotation, thus completing the cycle that will then be repeated. This flow is strongly dominated by linear mechanisms, which is remarkable considering the extremely high amplitudes involved in the processes of laminarization of the turbulence at $Ro\geq R{o}_{c} $ and the growth of the unstable TS waves.

The one equation Spalart–Allmaras (SA) turbulence model in an extended modular form is presented. It is employed for the prediction of crosswind flow around the lip of a 90 deg sector of an intake with and without surface roughness. The flow features around the lip are complex. There exists a region of high streamline curvature. For this, the Richardson number would suggest complete degeneration to laminar flow. Also, there are regions of high favorable pressure gradient (FPG) sufficient to laminarize a turbulent boundary layer (BL). This is all terminated by a shock and followed by a laminar separation. Under these severe conditions, the SA model is insensitive to capturing the effects of laminarization and the reenergization of eddy viscosity. The latter promotes the momentum transfer and correct reattachment prior to the fan face. Through distinct modules, the SA model has been modified to account for the effect of laminarization and separation induced transition. The modules have been implemented in the Rolls-Royce HYDRA computational fluid dynamic (CFD) solver. They have been validated over a number of experimental test cases involving laminarization and also surface roughness. The validated modules are finally applied in unsteady Reynolds-averaged Navier–Stokes (URANS) mode to flow around an engine intake and comparisons made with measurements. Encouraging agreement is found and hence advances made towards a more reliable intake design framework.

A caracterização do escoamento dentro da camada limite noturna é muito complexa, devido a fraca intensidade da turbulência. Nessa situação, o escoamento apresenta um caráter bi-estável, ou seja, ele apresenta um equilíbrio quente, o que ocorre quando a temperatura próxima à superfície se aproxima da temperatura dos níveis mais altos, e um equilíbrio frio, quando a temperatura do ar fica próxima à temperatura da superfície, isto está envolvido com o colapso da turbulência. Nos últimos anos tem-se utilizados muito de experimentos numéricos para o estudo e a representação da transição entre os escoamentos turbulento-laminar, devido aos mesmos serem controlados e portanto livre de fenômenos em escalas maiores que as turbulentas. Esse trabalho traz como objetivo realizar uma simulação numérica para verificar a transição do escoamento de um fluído, dentro de um canal fechado, sobre diferentes gradientes de temperatura. Para a realização do mesmo, utilizou-se o software CFD (do inglês \textit{computer fluid dynamics}) de código livre OpenFOAM$^{\circledR}$, juntamente com o solver \textit{buoyantPimpleFoam}. O modelo de turbulência adotado durante a simulação foi o LES (do inglês \textit{Large Eddy Simulation}). A simulação ocorreu por por um período de $3600$ s, sem a variação de temperatura entre as placas. Logo após foi acrescentado os gradientes de temperatura ($0$ K, $3$ K, $5$ K, $7$ K e $10$ K) e seguiu-se a simulação por mais $3600$ s. Observando o gráfico dos perfis de velocidade do escoamento até o centro do domínio, é possível identificar que o fluido encontra-se turbulento, e logo após o resfriamento ocorre o processo de laminarização do escoamento. Isto se deve a velocidade do fluido ser baixa ($0,1$ m s$^{-1}$) e ao gradiente de temperatura ser alto para essa velocidade. Conclui-se que é possível destruir a turbulência utilizando um gradiente de temperatura, de maneira a laminarizar o escoamento. Porém, ainda que isto ocorra, a turbulência não é eliminada por completa.

The effect of centrifugal force on flame propagation velocity of stoichiometric propane–, kerosene–, and n-octane–air turbulent premixed flames was numerically examined. The quasi-turbulent numerical model was set in an unsteady two-dimensional (2D) geometry with finite length in the transverse and streamwise directions but with infinite length in the spanwise direction. There was relatively good comparison between literature-reported measurements and predictions of propane–air flame propagation velocity as a function of centrifugal force. It was found that for all mixtures the flame propagation velocity increases with centrifugal force. It reaches a maximum, then falls off rapidly with further increases in centrifugal force. The results of this numerical study suggest that there are no distinct differences among the three mixtures in terms of the trends seen of the effect of centrifugal force on the flame propagation velocity. There are, however, quantitative differences. The numerical model is set in a noninertial, rotating reference frame. This rotation imposes a radially outward (centrifugal) force. The ignited mixture at one end of the tube raises the temperature and its heat release tends to laminarize the flow. The attained density difference combined with the direction of the centrifugal force promotes Rayleigh–Taylor instability. This instability with thermal expansion and turbulent flame speed constitute the flame propagation mechanism towards the other tube end. A wave is also generated from the ignition zone but propagates faster than the flame. During propagation the flame interacts with eddies that wrinkle and/or corrugate the flame. The flame front wrinkles interact with streamtubes that enhance Landau–Darrieus (hydrodynamic) instability, giving rise to a corrugated flame. Under strong stretch conditions the stabilizing equidiffusive-curvature mechanism fails and the flame front breaks up, allowing inflow of unburned mixture into the flame. This phenomenon slows down the flame temporarily and then the flame speeds up faster than before. However, if corrugation is large and the inflow of unburned mixture into the flame is excessive, the latter locally quenches and slows down the flame. This occurs when the centrifugal force is large, tending to blowout the flame. The wave in the tube interacts continuously with the flame through baroclinic torques at the flame front that further enhances the above mentioned flame–eddy interactions. Only at low centrifugal forces, the wave intermingles several times with the flame before the averaged flame propagation velocity is determined. The centrifugal force does not substantially increase the turbulent flame speed as commented by previous experimental investigations. The results also suggest that an ultracompact combustor (UCC) with high-g cavity (HGC) will be limited to centrifugal force levels in the 2000–3000 g range.

This thesis deals with ways to suppress turbulent flow in pipelines. In the first part various methods of laminarization are presented, when the turbulent flow is transformed into laminar flow, including the results of experiments published by the authors. The next part presents the results from CFD. The calculations were performed for one of the methods mentioned in the first part and the results were compared with each other. In addition, several options have been suggested to improve the original method.

Hybrid, Implicit Large Eddy Simulations (ILES) for an idealized aero engine intake in a crosswind is performed. The ILES zone is smoothly blended to a near wall Reynolds Averaged Navier-Stokes (RANS) zone. The flow has a region of high favourable pressure gradient (FPG) where the streamwise acceleration parameter (KS) is found to be greater than 3×10−6. This is sufficient to laminarize the boundary layer (BL). As a consequence, the turbulence in the boundary is severely suppressed and this interacts with a shock causing separation and distortion at the engine fan face. This is known to be undesirable for aero engines. The separated shear layer reenergizes turbulence and this promotes reattachment. The calculation in the RANS zone has been enhanced with a novel three-component RANS model and this is used in the hybrid RANS/ILES framework. Simulations also consider the modelling of roughness. The turbulent statistics and the engineering relevance of these are also discussed in this work. Broadly, encouraging agreement is found with measurements. Substantial accuracy improvements are found relative to standard RANS model simulations. The accuracy of the hybrid simulations is also contrasted with pure ILES and the critical need for the RANS layer shown for modest grids.

The one equation Spalart Allamaras (SA) turbulence model in an extended modular form is employed for the prediction of cross wind flow around the lip of a 90 degree sector of an intake with and without surface roughness. The flow features around the lip are complex. There exists a region of high streamline curvature. For this the Richardson number would suggest complete degeneration to laminar flow. Also there are regions of high favourable pressure gradient (FPG) sufficient to laminarize a turbulent boundary layer (BL). This is all terminated by a shock and followed by a laminar separation. Under these severe conditions, the SA model is insensitive to capturing the effects of laminarization and the reenergization of eddy viscosity which promotes the momentum transfer and correct reattachment prior to the fan face. Through distinct modules, the SA model has been modified to account for the effect of laminarization and separation induced transition. The SA modules have been implemented in Rolls-Royce HYDRA Computational Fluid Dynamic (CFD) solver. They have been validated over a number of experimental test cases involving laminarization and also surface roughness. The validated modules are finally applied in unsteady RANS mode to flow around an engine intake and comparisons made with measurements. Encouraging agreement is found and hence advances made towards a more reliable intake design framework.

The effect of centrifugal force on flame propagation velocity of stoichiometric propane-, kerosene-, and n-octane-air turbulent premixed flames was numerically examined. The quasi-turbulent numerical model was set in an unsteady two-dimensional geometry with finite length in the transverse and streamwise directions but with infinite length in the spanwise direction. There was relatively good comparison between literature-reported measurements and predictions of propane-air flame propagation velocity as a function of centrifugal force. It was found that for all mixtures the flame propagation velocity increases with centrifugal force. It reaches a maximum then falls off rapidly with further increases in centrifugal force. The results of this numerical study suggest there are no distinct differences among the three mixtures in terms of the effect of centrifugal force on the flame propagation velocity. There are, however, quantitative differences. The numerical models are set in a non-inertial, rotating reference frame. This rotation imposes a radially outward (centrifugal) force. The ignited mixture at one end of the tube raises the temperature and its heat release tends to laminarize the flow. The attained density difference combined with the direction of the centrifugal force promotes Rayleigh-Taylor instability. This instability with thermal expansion and turbulent flame speed constitute the flame propagation mechanism towards the other tube end. A wave is also originated but propagates faster than the flame. During propagation the flame interacts with eddies that wrinkle and/or corrugate the flame. The flame front wrinkles interact with streamtubes that enhance Landau-Darrieus (hydrodynamic) instability, giving rise to a corrugated flame. Under strong stretch conditions the stabilizing equidiffusive-curvature mechanism fails and the flame front breaks up, allowing inflow of unburned mixture into the flame. This phenomenon slows down the flame temporarily and then the flame speeds up faster than before. However, if corrugation is large and the inflow of unburned mixture into the flame is excessive, the latter locally quenches and slows down the flame. This occurs when the centrifugal force is large, tending to blowout the flame. The wave in the tube interacts continuously with the flame through baroclinic torques at the flame front that further enhances the above mentioned flame-eddies interactions. Only at low centrifugal forces the wave intermingles several times with the flame before the averaged flame propagation velocity is determined. The centrifugal force does not substantially increase the turbulent flame speed as commented by previous experimental investigations. The results also suggest that an ultra-compact combustor (UCC) with high-g cavity (HGC) will be limited to centrifugal force levels in the 2000–3000g range.