Mechanical Engineering - Research Publications
Permanent URI for this collection
Search Results
1 - 9 of 9
-
ItemThe effect of aspect ratio and divergence on the turbulence structure of boundary layers( 2007)The effect of the aspect ratio of a turbulent boundary layer on the mean flow, broadband turbulence intensities and Reynolds shear stress has been studied. The aspect ratio (AR) is defined as the boundary layer thickness divided by the boundary layer width, i.e. the effective wind tunnel width. Measurements have been taken in a nominally zero pressure gradient layer at a single station for three different aspect ratio settings, AR=1/4, AR=1/7, and AR=1/13. The measurements show that the turbulent quantities were unaffected when the aspect ratio was increased from AR=1/13 to AR=1/7. However at AR=1/4 there appears to be a slight increase in the broadband turbulence intensities and Reynolds shear stress.
-
ItemOn the streamwise evolution of turbulent boundary layers in arbitrary pressure gradients(Cambridge University Press, 2002)A new approach to the classic closure problem for turbulent boundary layers is presented. This involves, first, using the well-known mean-flow scaling laws such asthe log law of the wall and the law of the wake of Coles (1956) together with the mean continuity and the mean momentum differential and integral equations. The important parameters governing the flow in the general non-equilibrium case are identified and are used for establishing a framework for closure. Initially closure is achieved here empirically and the potential for achieving closure in the future using the wall-wake attached eddy model of Perry & Marusic (1995) is outlined. Comparisons are made with experiments covering adverse-pressure-gradient flows in relaxing and developing states and flows approaching equilibrium sink flow. Mean velocity profiles, total shear stress and Reynolds stress profiles can be computed for different streamwise stations, given an initial upstream mean velocity profile and the streamwise variation of free-stream velocity. The attached eddy model of Perry & Marusic (1995) can then be utilized, with some refinement, to compute the remaining unknown quantities such as Reynolds normal stresses and associated spectra and cross-power spectra in the fully turbulent part of the flow.
-
ItemSimilarity law for the streamwise turbulence intensity in zero-pressure-gradient turbulent boundary layers( 1997)A similarity relationship is proposed to describe the streamwise broadband-turbulence intensity in a zero-pressure-gradient boundary layer. The formulation is applicable to the entire region of the flow beyond the viscous buffer zone and is based on the attached eddy hypothesis, the Reynolds-number-similarity hypothesis and the assumed existence of Kolmogorov eddies with a universal inertial subrange. Experimental data of the authors and those from various published works covering a large Reynolds number range are investigated in light of this formulation.
-
ItemEvolution and structure of sink-flow turbulent boundary layers( 2001)An experimental and theoretical investigation of turbulent boundary layers developing in a sink-flow pressure gradient was undertaken. Three flow cases were studied, corresponding to different acceleration strengths. Mean-flow measurements were taken for all three cases, while Reynolds stresses and spectra measurements were made for two of the flow cases. In this study attention was focused on the evolution of the layers to an equilibrium turbulent state. All the layers were found to attain a state very close to precise equilibrium. This gave equilibrium sink flow data at higher Reynolds numbers than in previous experiments. The mean velocity profiles were found to collapse onto the conventional logarithmic law of the wall. However, for profiles measured with the Pitot tube, a slight ‘kick-up’ from the logarithmic law was observed near the buffer region, whereas the mean velocity profiles measured with a normal hot wire did not exhibit this deviation from the logarithmic law. As the layers approached equilibrium, the mean velocity profiles were found to approach the pure wall profile and for the highest level of acceleration Π was very close to zero, where Π is the Coles wake factor. This supports the proposition of Coles (1957), that the equilibrium sink flow corresponds to pure wall flow. Particular interest was also given to the evolutionary stages of the boundary layers, in order to test and further develop the closure hypothesis of Perry, Marusic & Li (1994). Improved quantitative agreement with the experimental results was found after slight modification of their original closure equation.
-
ItemApplication of the wavelet transform in turbulence( 1997)Traditionally, Fourier transforms have been used to elicit the scale-based behaviour of the turbulent motion and one speaks synonymously of its wavenumber components with scales (large scales are associated with small wavenumbers and vice-versa). Although, this approach is theoretically correct, many workers have questioned its appropriateness on the grounds that a Fourier mode represents a wave like disturbance which is global in the physical domain, whereas an eddy is a disturbance with finite spatial extent. Consequently, a more appropriate scheme should involve a local decomposition of the velocity field which is more reminiscent of eddy like phenomena. In this paper we have explored the feasibility of the wavelet transform as an analyzing tool in deducing the turbulence spectrum.
-
ItemApplication of the attached eddy hypothesis for the evolution of turbulent boundary layers( 1997)The wall-wake attached eddy model of Perry & Marusic [1] is incorporated in a new approach to the classic closure problem for turbulent boundary layers recently proposed by Perry, Marusic & Jones [2]. This involves using the well known mean-flow scaling laws such as Prandtl's law of the wall and the law of the wake of Coles together with the mean continuity and the mean momentum differential and integral equations. The important parameters governing the flow in the general non-equilibrium case are identified and are used for establishing a framework for closure.
-
ItemNew evolution equations for turbulent boundary layers in arbitrary pressure gradients( 1997)A new approach at looking at the classic closure problem for turbulent boundary layers is presented. This involves using the well known mean-flow scaling laws such as Prandtl's law of the wall and Coles' law of the wake together with the mean momentum integral and differential equations. The important parameters governing the flow in the general non-equilibrium case are identified and are used to formulate the closure hypothesis. Once the mean flow field has been determined, relevant turbulence quantities can be computed using a coherent structure eddy model based on the attached eddy hypothesis.
-
ItemOn the validity of Taylor's hypothesis in wall turbulence( 1997)The validity of Taylor’s hypothesis of frozen turbulence has been the issue of much debate, especially when applied to flows with strong shear and high turbulence intensities. In the past, Taylor’s hypothesis was used by various researchers for the quantitative interpretation of the structure angle of the eddies on the basis of double-velocity correlations (eg. Alving et al. [1]) or velocity-wall pressure or velocity-wall shear stress correlations (e.g. Brown & Thomas [2] , Rajagopalan & Antonia [6]. In light of the ambiguity associated with Taylor’s hypothesis, naturally, there are resultant uncertainties in terms of the measured structure angle. Subsequently there is a need to investigate how do these uncertainties effect the structure angle measurements and as well as to examine the validity of Taylor’s hypothesis when applied to two-point double-velocity correlation measurements in an anisotropic shear flow.
-
ItemNew evolution equations for turbulent boundary layers in arbitrary pressure gradients(Indian Academy of Sciences, 1998)A new approach to the classical closure problem for turbulent boundary layers is presented. This involves using the well-known mean-flow scaling laws such as Prandtl's law of the wall and the law of the wake of Coles together with the mean continuity and the mean momentum differential and integral equations. The important parameters governing the flow in the general non-equilibrium case are identified and are used for establishing a framework for closure. Initially, closure is done here empirically from the data but the framework is most suitable for applying the attached eddy hypothesis in future work. How this might be done is indicated here.