Wake behind a discontinuous cylinder: Unveiling the role of the large scales in wake growth and entrainment
- Autores: Venkata Subba Rao Mandava
- Directores de la Tesis: Francesc Giralt Prat (dir. tes.), Joan Herrero Sabartés (codir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: Josep Anton Ferré Vidal (presid.), Josep M. Bergadà Grañó (secret.), Joan Llorens Llacuna (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencia, Materiales e Ingeniería Química por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
- español
Un estudio de la literatura existente indica que existe una falta de acuerdo con respecto al mecanismo de arrastre en la interfaz turbulenta/no-turbulenta (TNTI) en flujos libres, con varios estudios que encuentran diferencias en la importancia relativa de los mecanismos de nibbling y engulfment. Esto sugiere una dependencia significativa del flujo y que al controlar el desarrollo de estructuras a gran escala, se puede controlar la medida en que cada uno de estos mecanismos contribuye a la tasa de arrastre general. La hipótesis que motiva este estudio es que los vórtices mejorados que están alineados con la cizalladura media aumentarán la tasa de arrastre según las observaciones de la estructura del flujo y el arrastre en las regiones de autoconservación de las estelas lejanas. Probamos esta hipótesis con la configuración de cilindro discontinuo (DC), que consta de segmentos de cilindro de 5D de largo (siendo D el diámetro del cilindro) separados por espacios de 2,5D de ancho. Se utilizaron medidas de velocimetría de imagen de partículas (PIV) y anemometría de hilo caliente (HWA) para analizar el flujo en dos números de Reynolds, Re=4000 y 10000, para x/D<180. También se llevaron a cabo simulaciones de remolinos grandes (LES) para las estelas de CC y de cilindro continuo infinito (CC) a Re=10.000. La configuración de DC se diseñó para desencadenar el desprendimiento de vórtices de herradura (HSV) en la región de estela muy cercana con la intención de ilustrar el papel que estos HSV tridimensionales, previamente identificados en la región de estela lejana de CC, juegan en el proceso de arrastre. en estelas turbulentas. Se descubrió que la estela de CC crece y se propaga en la dirección transversal con una tasa mucho más rápida que la de CC, hasta aproximadamente x/D ≈ 50. Antes de esta ubicación, la tasa de crecimiento mejorada causada por el HSV alineado con cizalla llevó a un ancho de estela de aproximadamente 3 veces el de la estela CC, con un déficit de velocidad promedio máximo de aproximadamente la mitad.
- català
Un estudi de la literatura existent indica que hi ha una manca d'acord pel que fa al mecanisme d'arrossegament en la interfície turbulent/no-turbulent (TNTI) en fluxos lliures, amb diversos estudis que troben diferències en la importància relativa dels mecanismes de mordisqueig i d'embolic. Això suggereix una dependència significativa del flux i que controlant el desenvolupament d'estructures a gran escala, es pot controlar fins a quin punt cadascun d'aquests mecanismes contribueix a la taxa d'arrossegament global. La hipòtesi que motiva aquest estudi és que la millora dels vòrtexs alineats amb la cisalla mitjana augmentarà la taxa d'arrossegament en funció de les observacions de l'estructura del flux i l'arrossegament a les regions autopreservades de les esteles llunyanes. Vam provar aquesta hipòtesi amb la configuració del cilindre discontinu (DC), que consta de segments de cilindre de 5D de llarg (amb D és el diàmetre del cilindre) separats per buits d'amplada de 2,5D. Es van utilitzar mesures de Velocimetria d'imatge de partícules (PIV) i Anemometria de fil calent (HWA) per analitzar el flux a dos nombres de Reynolds, Re = 4.000 i 10.000, per a x/D<180. També es van realitzar grans simulacions de remolí (LES) tant per a les esteles de CC com per a les esteles de cilindre continu infinit (CC) a Re=10.000. La configuració de DC es va idear per desencadenar l'eliminació de vòrtexs de ferradura (HSV) a la regió molt propera de l'estela amb la intenció d'il·lustrar el paper que aquests HSV tridimensionals, identificats prèviament a la regió llunyana de CC, tenen en el procés d'enrotllament. en estels turbulents. Es va trobar que l'estela de CC creixia i s'estenia en la direcció transversal amb una velocitat molt més ràpida que la de l'estela CC, fins a uns x/D≈50. Abans d'aquesta ubicació, la velocitat de creixement millorada causada pel LED HSV alineat amb cisalla. a una amplada d'estela d'unes 3 vegades la de l'estela CC, amb un dèficit màxim de velocitat mitjana que era aproximadament la meitat.
- English
A study of the existing literature indicates that there is a lack of agreement regarding the mechanism of entrainment at turbulent/non-turbulent interface (TNTI) in free flows, with various studies finding differences in the relative importance of the nibbling and engulfment mechanisms. To summarize the several important observations in the literature, first, the coherent structures (CS) within turbulent shear flows play an important role in setting the details of the TNTI and the rate of entrainment. Second, coherent structures develop and evolve depending on the initial and boundary conditions of the flow. These suggest a significant flow dependence and that by controlling the development of large-scale structures, one can control the extent to which each of these mechanisms contribute to the overall entrainment rate. The hypothesis motivating this study is that enhancing vortices that are aligned with the mean shear will increase the rate of entrainment based on observations of flow structure and entrainment in the self-preserving regions of far wakes, as reported by Vernet et al. 1999 and Kopp et al. 2002. The key question, then, is how to do that? How to generate dominant shear aligned large scale structures in the near wake with high vorticity under turbulent conditions? We developed what we call a discontinuous cylinder wake, which consists of cylinder segments 5D long (with D being the diameter of the cylinder) separated by gaps of width 2.5D. Hereinafter, we will refer to this geometry as a discontinuous cylinder (DC) configuration. Our first working hypothesis is that it is possible to generate a three-dimensional (3-D) wake behind a cylindrical shape by substituting the solid cylinder (hereafter, continuous cylinder; CC) with regularly alternating discontinuous cylinder segments, each shedding horseshoe–like vortices that resemble the horseshoe vortices (HSV) or double roller (DR) flow patterns, which are aligned with the mean shear in far wake flows. Flow between two adjacent cylindrical pieces would split towards the two low pressure regions located behind them causing an extra input of momentum that would enhance both the kinking and the rotational energy of the shed horseshoe-like vortices. The second hypothesis is that, in the mid and far wake regions, these horseshoe vortices, shed behind each cylindrical segment, enhance the transfer of momentum from the mean flow, both laterally and vertically and, thus, result in a faster recovery of wake velocity with respect to the external free flow. The third and final hypothesis is that the DR that would evolve from the 3-D horseshoe vortices shed by the DC, as they are further stretched and randomized while convected downstream into the mid and far wake regions, would contribute more to the turbulent kinetic energy than those observed in the CC wake.
We carried out Particle Image Velocimetry (PIV) and Hot-Wire Anemometry (HWA) measurements as well as Large Eddy Simulations (LES) to analyze/characterize the wake growth and entrainment rates in the near and far wake of the DC. In order to establish the proper comparisons and to test the current setup, measurements and calculations were also performed for the continuous cylinder wake. PIV and HWA measurements data were used to obtain length scales of wake half-width as well as the distribution of velocity-defect. Numerical (LES) data was used to characterize the instantaneous shape of large-scale structures in the near wake of the DC. The momentum transfer into the wake region from the free stream flow region and from the free stream flow in the gap region between cylinder segments was also analyzed with the numerical data.
The instantaneous iso-surfaces of pressure confirmed the generation of DRs or HSV’s early in the near wake of the DC. The dominant flow structures in the DC wake are shown to be quasi-periodic, three-dimensional horseshoe vortices, which are fully formed by about x/D = 3. This structure contrasts with the periodic quasi-two-dimensional von Kármán vortices in the near wake of the CC. Following the vortex formation region of both wakes, the DC wake grows at faster rate than the CC wake, although the rate of initial development of the quasi-two-dimensional Kármán vortices is significantly higher within x/D < 3. However, the Kármán vortices decay quickly because of the lack of shear alignment whereas the HSV then develop a high rate over a significantly longer spanwise distance. So, while the wakes have approximately the same thickness at x/D = 4, the DC wake is substantially wider by x/D = 8 and maintains a higher growth rate until about x/D=50. Further downstream, beyond x/D=50, the wakes grow at the same rates, consistent with the two dimensional self-preserving solution.
However, the DC wake is significantly wider, such that, at x/D=50, the mean velocity half width is almost 3 times larger than that for the CC at the same physical location. At this same point, the maximum velocity defect for the DC is about half that for the CC even though the maximum velocity defect is significantly greater in the cylinder base region. The velocity-defect at x/D=50 in the DC wake was similar to the velocity-defect in the CC wake at around x/D=127. These results indicate a much greater entrainment rate for the discontinuous cylinder with fully formed horseshoe vortices in the near wake region, where flow already approaches self-preservation, but with a rate that is higher than in the fully developed far wake region. A simple scaling analysis indicates that the lateral entraining motions and the vertical stretching of the HSV can explain the growth rate of the DC wake.
Since the kinetic energy associated with the fluctuations of the near-wake, which are originated by the DRs or HSV structures, is high, the flow system was suitable to clarify the role of these velocity patterns in the entrainment process of wake flows. The rotation associated with the z-vorticity at the top of the HSV entrains fluid towards the horizontal centerline plane (y = 0). The counterrotating motion of the two legs of the HSV entrain fluid laterally from the free stream flow in the gap region between the cylinder segments and, together with the inflow at front of the HSV, forms a jet-like upstream motion at their back and towards the top/bottom edge of the wake. The high energy upstream jet motion in turn contributes to the vortex stretching and eventually to the vertical growth of the wake. The mean and instantaneous velocity and vorticity results showed that the entrainment process is the result of linked lateral rotating motions and vertical outward jet-like motions and the rear of the DR. The overall process, the increase in the wake spreading and momentum transfer rates observed in the DC wake, was caused, and dominated by the presence of highly stretched DR or HSV’s, which originated early in the near wake with high energy levels.
The current findings have several implications beyond the possible development of engineering tools to increase (or suppress) turbulent mixing rates. First, with respect to self-preserving flows, it is well known that the virtual origin of self-preserving plane turbulent wakes is variable for different bluff bodies or for similar bluff bodies but in different laboratory settings. The implication from the current work is that enhancing shear-aligned vorticity in the initial conditions should decrease the position of the virtual origin, i.e., enhancing wake thickness and reducing velocity defect. Second, for other turbulent shear flows, enhancing shear aligned vorticity should have similar effects. This would include plane turbulent jets and plane mixing layers, which, like plane turbulent wakes, tend to have primary instabilities which induce the formation of vortices that are orthogonal to the mean shear. This could also include the effects of trips for the transition of turbulent boundary layers. However, examination of such details remains for future work.
Vernet, A., Kopp, G. A., Ferré, J. A. & Giralt, F. 1999 Threedimensional structure and momentum transfer in a turbulent cylinder wake. J. Fluid Mech., 394, 303-337.
Kopp, G. A., Giralt, F. & Keffer, J. F. 2002 Entrainment vortices and interfacial intermittent turbulent bulges in a plane turbulent wake. J. Fluid Mech., 469, 49-70.
- español
