Slip-velocity of large neutrally-buoyant particles in turbulent flows

We discuss possible definitions for a stochastic slip velocity that describes the relative motion between large particles and a turbulent flow. This definition is necessary because the slip velocity used in the standard drag model fails when particle size falls within the inertial subrange of ambient turbulence. We propose two definitions, selected in part due to their simplicity: they do not require filtration of the fluid phase velocity field, nor do they require the construction of conditional averages on particle locations. A key benefit of this simplicity is that the stochastic slip velocity proposed here can be calculated equally well for laboratory, field, and numerical experiments. The stochastic slip velocity allows the definition of a Reynolds number that should indicate whether large particles in turbulent flow behave (a) as passive tracers; (b) as a linear filter of the velocity field; or (c) as a nonlinear filter to the velocity field. We calculate the value of stochastic slip for ellipsoidal and spherical particles (the size of the Taylor microscale) measured in laboratory homogeneous isotropic turbulence. The resulting Reynolds number is significantly higher than 1 for both particle shapes, and velocity statistics show that particle motion is a complex non-linear function of the fluid velocity. We further investigate the nonlinear relationship by comparing the probability distribution of fluctuating velocities for particle and fluid phases

Image Grey Scale Based Volume Fraction Measurements of Solid Granular Particles

Fluidized beds are formed inside containers where the flow of fluid from the bottom of the container causes solid particles to be suspended in the fluid due to friction. As the terminal velocity increases the solid phase eventually gets fluidized and starts to behave in a more fluid-like manner. Typical industrial applications of fluidized beds range from energy production to the chemical industry. Fluidized bed combustion (FBC) has proved to be an efficient and low emission technology due to its intensely turbulent mixing of the solid and fluid phases. Further insights into this complex process are the topic of much currect research. Experimental measurements are required to validate the computational models, and for example, measurement integrated simulations. This work examines a relatively thin, almost 2 dimensional from the camera\u92s point of view, lab-scale fluidized bed partially filled with granular sand particles. While particle image velocimetry (PIV) can be used to measure the solid phase velocity, the volume fraction is hard to measure accurately. A novel volume fraction determination method for the solid phase is proposed in this work