Problem Of Turbulent Boundary Layer Research Articles

Speed-up algorithm based on parallel projection correlation technique for planar PIV: Accuracy and limitation

Implementation of advanced experimental techniques such as High-speed Time-resolved planar Particle Image Velocimetry is associated with big data processing and implies time-consuming operations. The Parallel Projection Correlation (PPC) technique is proposed in this paper for computation speed-up. Despite the fact that this technique reduces the calculation time, it has been revealed that its accuracy strongly depends on the interrogation window size. To show the limitations of PPC, three synthetic test cases are considered in this investigation: one-dimensional uniform flow, linear shear flow and turbulent boundary layer flow. Accuracy analysis is carried out for different interrogation window sizes. PPC is applied together with the iterative multigrid window-deformation technique (WIDIM). It is demonstrated that the maximum size of interrogation window that can be used for the shear flow analysis is 16 × 16 pix. The temporal filter applied at the initial and intermediate iterations of WIDIM is proposed to overcome this limitation enabling to use 64 × 64 pix interrogation windows for high-gradient shear flow. Finally, the results of turbulent boundary layer problem obtained by PPC are discussed in terms of accuracy of the streamwise velocity and turbulence intensities.

Smart Panel Technology for Broadband Noise Reduction

The possibility of a broadband noise reduction by piezoelectric smart panels is demonstrated. A smart panel is basically a plate structure on which piezoelectric patches with electrical shunt circuits are mounted and sound absorbing material is bonded on the surface of the structure. Sound absorbing material can absorb the sound transmitted at the mid frequency region effectively while the use of piezoelectric shunt damping can reduce the transmission at resonance frequencies of the panel structure. To be able to reduce the sound transmission at low panel resonance frequencies, piezoelectric damping is adopted. A resonant shunt circuit for piezoelectric shunt damping is composed of resistor and inductor in series, and they are determined by maximizing the dissipated energy through the circuit. The transmitted noise reduction performance of smart panels was tested in an acoustic tunnel. Sound absorbing material and air gap reduces the sound transmitted at the mid-frequency region effectively while the use of piezoelectric shunt damping works at the low frequency region. As a result, a remarkable noise reduction of 9-15dB was achieved in broadband frequency. Double panel exhibits better noise reduction than other panels. This approach was also applied to the structural acoustic problem of turbulent boundary layer (TBL) induced sound radiated from a panel, and the possibility of TBL noise reduction was demonstrated. The hybrid approach of smart panels is a promising technology for noise reduction over broad frequency band.

Modelling of surface waves breaking effects in the ocean upper layer

In this paper, we have studied the general dynamic structure of the turbulent upper layer produced by wave breaking and turbulent diffusion of wave kinetic energy. The turbulent model is of the k-ε group model. The TKE budget contains an extra term, which describes the turbulent diffusion of wave kinetic energy. We suggest a parameterization of the momentum flux and the energy flux induced by the wave breaking. The analysis is focused at the upper part of the Ekman layer where the constant friction approach is valid. It is shown that this layer is formed of three sublayers. The nearest layer to the ocean surface is the wave-turbulent sublayer. The turbulent energy production by the wave breaking significantly exceeds the mean shear effect, and the turbulent diffusion of the wave kinetic energy dominates in the range of depths where the wave motion continues to be vigorous. Below this layer, the wave effect does not play a significant role because the turbulent diffusion of turbulent kinetic energy begins to exceed the wave effect in the TKE budget. However, the turbulence of this layer influences the dynamics of the entire Ekman layer beneath. Below the wave-turbulent diffusive layer, there is a transitional turbulent diffusive layer where the turbulent diffusion still exceeds the mean shear contribution in the TKE budget but the wave motion effect becomes insignificant. The source of turbulence in this layer is the turbulent kinetic energy flux from the upper wave-turbulent layer. The turbulent kinetic energy and the dissipation rate decay with depth following power laws. Theoretical prediction is consistent with data observed in the ocean. Following after the transition turbulent diffusive layer, there is a layer where the mean shear production of turbulent energy dominates. In terms of the classic steady turbulent boundary layer problem, this layer, in steady or quasi-steady cases, corresponds to the logarithmic turbulent sublayer where the “wall turbulence” laws are valid. However, all turbulent characteristics are controlled by the wave breaking mechanism and the turbulence dynamics in the wave-turbulent layer and the turbulent diffusive layer. We may conclude that the suggested model gives an adequate description of the natural ocean turbulence for the constant friction layer of the upper ocean.

Numerical solution of the laminar boundary layer equations for power-law fluids

The power-law fluid boundary layer problem is formulated analogous to a turbulent flow problem. The deviation from Newtonian rheology is accounted for by an effective viscosity replacing the so called turbulent eddy viscosity. A numerical solution procedure for turbulent boundary layer problem is adapted to laminar boundary layer flows of power-law fluids. The transformed momentum equation is written as a set of first order equations, which is solved by the implicit Keller Box finite-difference scheme for parabolic problems. This method has second-order accuracy in all variables and permits arbitrary net spacings. Newton's method is used to solve the resulting system of nonlinear difference equations. Special attention is devoted to the extra nonlinearity introduced via the effective viscosity. The application of an incomplete linearization scheme, typical for turbulent flow computations, is compared with an alternative full linearization. Numerical results are presented for the similarity boundary layer flow along a flat plate, and for the non-similar boundary layer along a cylinder surface. The results are compared with other available numerical and series expansion solutions. The rate of convergence for the usual incomplete application of Newton's method is compared with the convergence of the fully linearized scheme. It was observed that the latter led to substantial savings in computational effort, except for some similarity boundary layers of pseudoplastic fluids.

On the frictional resistance of oscillating rough and smooth surfaces

The frictional resistance of oscillating rough and smooth surfaces is presented. The turbulent boundary layer problem associated with the harmonic oscillations of the surface is solved analytically by using a two-layer time invariant eddy viscosity m

Turbulent boundary layer on a flat plate in a stream of dissociating gas

This paper gives a theoretical solution of a turbulent boundary layer problem of dissociating gas on a flat plate. A half-empirical Prandtl-Kármán turbulent theory is used to solve this problem.