J. Jovanović

LDA measurements in the near-wall region of a turbulent pipe flow

This paper presents laser-Doppler measurements of the mean velocity and statistical moments of turbulent velocity fluctuations in the near-wall region of a fully developed pipe flow at low Reynolds numbers. A refractive-index-matched fluid was used in a Duran-glass test section to permit access to the near-wall region without distortion of the laser beams. All measurements were corrected for the influence of the finite size of measuring control volume. Measurements of long-time statistical averages of all three fluctuating velocity components in the near-wall region are presented. It is shown that the turbulence intensities in the wall region do not scale with inner variables. However, the limiting behaviour of the intensity components very close to the wall show only small variations with the Reynolds number. Measurements of higher-order statistical moments, the skewness and flatness factors, of axial and tangential velocity components confirm the limiting behaviour of these quantities obtained from direct numerical simulations of turbulent channel flow. The comparison of measured data with those obtained from direct numerical simulations reveals that noticeable discrepancies exist between them only with regard to the flatness factor of the radial velocity component near the wall. The measured v ’ flatness factor does not show the steep rise close to the wall indicated by numerical simulations. Analysis of the measured data in the near-wall region reveals significant discrepancies between the present LDA measurements and experimental results obtained using the hot-wire anemometry.

Interpretation of the mechanism associated with turbulent drag reduction in terms of anisotropy invariants

A central goal of flow control is to minimize the energy consumption in turbulent flows and nowadays the best results in terms of drag reduction are obtained with the addition of long-chain polymers. This has been found to be associated with increased anisotropy of turbulence in the near-wall region. Other drag reduction mechanisms are analysed in this respect and it is shown that close to the wall highly anisotropic states of turbulence are commonly found. These findings are supported by results of direct numerical simulations which display high drag reduction effects of over 30% when only a few points inside the viscous sublayer are forced towards high anisotropy.

On the Mechanism Responsible for Turbulent Drag Reduction by Dilute Addition of High Polymers: Theory, Experiments, Simulations, and Predictions

Turbulent drag reduction by dilute addition of high polymers is studied by considering local stretching of the molecular structure of a polymer by small-scale turbulent motions in the region very close to the wall. The stretching process is assumed to restructure turbulence at small scales by forcing these to satisfy local axisymmetry with invariance under rotation about the axis aligned with the main flow. It can be shown analytically that kinematic constraints imposed by local axisymmetry force turbulence near the wall to tend towards the one-component state and when turbulence reaches this limiting state it must be entirely suppressed across the viscous sublayer. For the limiting state of wall turbulence, the statistical dynamics of the turbulent stresses, constructed by combining the two-point correlation technique and invariant theory, suggest that turbulent drag reduction by homogeneously distributed high polymers, cast into the functional space which emphasizes the anisotropy of turbulence, resembles the process of reverse transition from the turbulent state towards the laminar flow state. These findings are supported by results of direct numerical simulations of wall-bounded turbulent flows of Newtonian and non-Newtonian fluids and by experiments carried out, under well-controlled laboratory conditions, in a refractive index-matched pipe flow facility using state-of-the art laser-Doppler anemometry. Theoretical considerations based on the elastic behavior of a polymer and spatial intermittency of turbulence at small scales enabled quantitative estimates to be made for the relaxation time of a polymer and its concentration that ensure maximum drag reduction in turbulent pipe flows, and it is shown that predictions based on these are in very good agreement with available experimental data.

Experimental investigations of turbulent drag reduction by surface-embedded grooves

Consideration of near-wall turbulence in the functional space that emphasizes the level of anisotropy of the velocity fluctuations not only provides an understanding of the causative physics behind remarkable effects of turbulent drag reduction, but also leads to the logical design of a surface topology which is shown experimentally to be capable of producing a significant reduction of viscous drag which far exceeds what has been achieved so far.

Statistical interpretation of the turbulent dissipation rate in wall-bounded flows

Statistical analysis was performed for interpreting the dissipation correlations in turbulent wall-bounded flows. The fundamental issues related to the formulation of the closure assumptions are discussed. Using the two-point correlation technique, a distinction is made between the homogeneous and inhomogeneous parts of the dissipation tensor. It is shown that the inhomogeneous part contributes half of the dissipation rate at the wall and vanishes remote from the wall region. The structure of an analytically derived equation was analysed utilizing the results of direct numerical simulations of turbulent channel flow at low Reynolds number.

Anisotropy-invariant mapping of turbulence in a flow past an unswept airfoil at high angle of attack

Turbulence investigations of the flow past an unswept wing at a high angle of attack are reported. Detailed predictions were carried out using large-eddy simulations (LES) with very fine grids in the vicinity of the wall in order to resolve the near-wall structures. Since only a well-resolved LES ensures reliable results and hence allows a detailed analysis of turbulence, the Reynolds number investigated was restricted to Re c =10 5 based on the chord length c. Admittedly, under real flight conditions Re c is considerably higher (about (35-40) 10 6 ). However, in combination with the inclination angle of attack α=18 deg this Re c value guarantees a practically relevant flow behavior, i.e., the flow exhibits a trailing-edge separation including some interesting flow phenomena such as a thin separation bubble, transition, separation of the turbulent boundary layer, and large-scale vortical structures in the wake. Due to the fine grid resolution applied, the aforementioned flow-features are predicted in detail. Thus, reliable results are obtained which form the basis for advanced turbulence analysis. In order to provide a deeper insight into the nature of turbulence, the flow was analyzed using the invariant theory of turbulence by Lumley and Newman. Therefore, the anisotropy of various portions of the flow was extracted and displayed in the invariant map. This allowed us to examine the state of turbulence in distinct regions and provided an improved illustration of what happens in the turbulent flow. Thus, turbulence itself and the way in which it develops were extensively investigated, leading to an improved understanding of the physical mechanisms involved, not restricted to a standard test case such as channel flow but for a realistic, practically relevant flow problem at a moderate Reynolds number.

Theoretical and experimental investigations of a wide-range thermal velocity sensor

This paper describes a two-wire thermal velocity sensor operating with an electrically heated upstream wire and a downstream wire acting as a resistance thermometer. The sensor can be operated in such a way that the phase shift between the periodic (sinusoidal) voltage that drives the first wire and the detected second signal is controlled by a combination of convection, diffusion and the finite thermal response time of the wires. This yields an effective operating range of 0.05?m?s-1?U?25?m?s-1 and corresponds to a bandwidth of 1-500. This wide velocity range is confirmed theoretically and experimentally for a number of sensors. The paper provides detailed analytical and numerical investigations which are well verified by experiments. The resultant design work also aimed at a sensor that can be manufactured industrially and can therefore be produced at low cost. Our immediate application is the measurement of flow velocities (or volume flow rates) in slowly changing unidirectional flows. This sensor design is briefly described and the description involves the mechanical part of the sensor as well as the electronics. Calibration of the first automatically manufactured sensors is shown. It is demonstrated that their performance is as good as those of the sensors that the authors have designed and built during their initial development work.

On unified boundary conditions for improved predictions of near-wall turbulence

A novel formulation of the wall boundary conditions relying on the asymptotic behaviour of the Taylor microscale λ and its relationship to the homogeneous part of the viscous dissipation rate of the kinetic energy of turbulence e h =5ν q 2 /λ 2 , applicable to near-wall turbulence, is examined. The linear dependence of λ on the wall distance in close proximity to the solid surface enables the wall-closest grid node to be positioned immediately below the edge of the viscous sublayer, leading to a substantial coarsening of the grid resolution. This approach provides bridging of a major portion of the viscous sublayer, higher grid flexibility and weaker sensitivity against the grid non-uniformities in the near-wall region. The performance of the proposed formulation was checked against available direct numerical simulation databases of complex wall-bounded flows featured by swirl and separation.

Near-wall behaviour of statistical properties in turbulent flows

Abstract Many technically relevant flows are wall-bounded flows at high Reynolds numbers. The knowledge of the near-wall behaviour of turbulence is important for the correct modelling of these flows. The influence of the Reynolds number on turbulence quantities results from the imposed boundary conditions at the edge of a boundary layer or on the axis of a channel or a pipe flow. This has often been assumed not to affect the near-wall region. However, experimental and numerical results show a Reynolds number dependence of turbulence intensity very close to the wall. In this work the low-Reynolds number effects in turbulent wall-bounded flows were investigated experimentally using the LDA measuring technique. A new method is explained how to eliminate the influence of the limited spatial resolution of the LDA measuring technique.

Experimental in situ investigations of turbulence under high pressure

In tube injection systems applied in high‐pressure processing of packed biomaterials and foods, the pressure‐transmitting medium is injected into the vessel to increase the pressure up to 1000 MPa, generating a submerged liquid‐free jet. The presence of a turbulent‐free jet during the pressurization phase and its positive influence on the homogeneity of the product treatment has already been examined by computational fluid dynamics investigations. However, no experimental data have supported the existence and properties of turbulent flow under high‐pressure (HP) conditions up to 400 MPa. This contribution presents the development of two experimental setups: HP‐laser Doppler anemometry and HP‐hot wire anemometry. For the first time the time‐averaged velocity profiles of a free jet during pressurization up to 300 MPa at different Reynolds numbers (Re) have been obtained. In this article, the dependence of the velocity profiles on the Re is discussed in detail. Moreover, the relaminarization phenomenon of the turbulent pipe flow most likely caused by the compressibility effects and viscosity changes of the pressure‐transmitting medium is examined.